sensors Review

Optical Fibre Sensors Using Graphene-Based Materials: A Review Miguel Hernaez 1, *, Carlos R. Zamarreño 2,3, *, Sonia Melendi-Espina 4 , Liam R. Bird 4 , Andrew G. Mayes 1 and Francisco J. Arregui 2,3 1 2 3 4

*

School of Chemistry, Faculty of Science, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK; [email protected] Department of Electrical and Electronic Engineering, Universidad Publica de Navarra, Pamplona 31006, Spain; [email protected] Institute of Smart Cities, Universidad Publica de Navarra, Pamplona 31006, Spain School of Mathematics, Faculty of Science, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK; [email protected] (S.M.-E.); [email protected] (L.R.B.) Correspondence: [email protected] (M.H.); [email protected] (C.R.Z.); Tel.: +44-160-359-1679 (M.H.); +34-948-168-445 (C.R.Z.)

Academic Editor: W. Rudolf Seitz Received: 30 November 2016; Accepted: 12 January 2017; Published: 14 January 2017

Abstract: Graphene and its derivatives have become the most explored materials since Novoselov and Geim (Nobel Prize winners for Physics in 2010) achieved its isolation in 2004. The exceptional properties of graphene have attracted the attention of the scientific community from different research fields, generating high impact not only in scientific journals, but also in general-interest newspapers. Optical fibre sensing is one of the many fields that can benefit from the use of these new materials, combining the amazing morphological, chemical, optical and electrical features of graphene with the advantages that optical fibre offers over other sensing strategies. In this document, a review of the current state of the art for optical fibre sensors based on graphene materials is presented. Keywords: optical fibre sensors; graphene; graphene oxide; reduced graphene oxide; carbon materials; thin films; nanostructured coatings

1. Introduction Graphene (G), a two-dimensional carbon material with one-atom-thickness, has become a trending topic in different scientific fields, such as physics, chemistry and materials science, since Novoselov and Geim reported its successful isolation in 2004 [1]. Its outstanding properties make it an ideal candidate for several applications, such as fabrication of field effect transistors, transparent conductive films, clean energy devices or graphene-polymer nanocomposites with enhanced properties. However, the development of a method for the production of high-quality graphene in large quantities is essential to further exploit its full potential. In this regard, the use of graphene oxide (GO) and reduced graphene oxide (rGO) is a compromise between the interesting properties of graphene, and the synthesis price and complexity. Consequently, GO and rGO can be good substitutes of graphene in many applications. In particular, graphene-based materials (G, GO and rGO) have been widely used for sensing applications in the last few years due to their high specific surface area, high electronic mobility and low electrical noise. A wide range of chemical sensors, biosensors and gas sensors have been developed using graphene materials [2–6]. Among all other sensing strategies, optical fibre sensors have achieved a high impact in the last decades because they offer several advantages over electronic sensors [7–9]. One of their main features is that the optical fibre itself can act as both the transmission medium and the transducer, hence allowing remote sensing and multiplexing. Additionally, optical fibre sensors are light and small, Sensors 2017, 17, 155; doi:10.3390/s17010155

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resistant to harsh environments and high temperatures, biocompatible, immune to electromagnetic fields and electromagnetically passive [10–12]. These features make them particularly suitable for some specific applications, such as biosensing, health and medicine applications, offshore applications and sensing in harsh or flammable environments [8,11,13,14]. To date, publications on optical fibre sensors based on graphene materials are limited. However, the unique optical, chemical and morphological properties of graphene combined with the benefits of optical fibre sensing schemes are attracting a growing interest in the scientific community. The increase in the number of publications observed in the last few years is a clear indication of this fact. This review presents a comprehensive summary of the research on optical fibre sensors based on graphene and its derivatives, including experimental and theoretical studies. The document is structured in the following sections: first, a brief introduction to graphene materials is presented, paying special attention to the different synthesis methods: micromechanical exfoliation, epitaxial growth on SiC substrates, chemical vapour deposition, unzipping of carbon nanotubes, liquid phase exfoliation of graphite, and reduction of exfoliated graphene oxide. The next section is focused on the different optical fibre sensors found in the bibliography, classified by sensing technology (interferometry, surface plasmon resonance, fibre Bragg gratings, absorption and fluorescence). Finally, the conclusions of this review are summarized. 2. Graphene Materials 2.1. Graphene Discovery Graphene is the two dimensional form of carbon in which carbon atoms are arranged in a honeycomb crystal lattice such that each atom is joined to three others by sp2-bonding. The use of three σ-electrons in carbon-carbon bonding results in a system of delocalized π-electrons perpendicular to the honeycomb plane giving rise to graphene's exceptional electrical properties [15,16]. The name graphene was introduced by Boehm, Setton and Stumpp in 1986 [17]. For several decades, efforts had been made to produce a single sheet graphene. It was in 2004 that Andre Geim and Konstantin Novoselov reported its successful experimental isolation [1,18]. Authors used a surprisingly simple technique called the “adhesive tape method”. It involved peeling layers of graphite using adhesive tape and then folding and peeling the tape several times to make gradually thinner layers of graphite, ultimately leading to a single layer of carbon. The thinned down graphite was then transferred onto an oxidised silicon substrate and individual small highly oriented pyrolytic graphite domains were identified by means of optical microscopy [1]. Since its discovery, graphene has attracted much attention due to its fascinating structural, optical, mechanical and electrical properties [1,6–8], which make it an ideal candidate for sensing applications. Additionally, it shows huge potential as a chemical sensing material due to its large surface area [19], sensitivity to changes in the carrier concentration of the transverse Hall resistivity [20], single molecule adsorption detection [21] and ambipolar electric field effect [22], among other properties. 2.2. Synthesis Methods of Graphene Different graphene production processes have been reported, which can be classified in different categories depending on the physical or chemical procedures employed. Figure 1 shows the most common techniques for the production of graphene, which include micromechanical exfoliation (Scotch™ tape method) [1,18], epitaxial growth on SiC substrates [23,24], chemical vapour deposition (CVD) [25,26], unzipping carbon nanotubes [27,28], liquid phase exfoliation of graphite [29–31] and thermal & chemical reduction of exfoliated graphene oxide [22,32,33]. Each of these methods has its own advantages as well as limitations depending on its target application.

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Figure 1. Summary of the main graphene synthesis methods.

Figure 1. Summary of the main graphene synthesis methods.

2.2.1. Micromechanical Exfoliation

2.2.1. Micromechanical Exfoliation

As previously mentioned this is an amazingly simple method developed by Novoselov, et al.

As previously mentioned an amazingly simple method developed by Novoselov, et al. [1]. [1]. Micromechanical exfoliationthis hasishigh utility for producing single-layer sp2-conjugated domains Micromechanical haselectronic high utility for producing sp2-conjugated domains with high quality exfoliation structural and properties at up tosingle-layer millimetre size, and is therefore ideal with high quality structural properties up to millimetre size, and is therefore ideal for for producing grapheneand forelectronic fundamental physicsatresearch and proof-of-concept devices [34]. However, the manualforpeeling of highly oriented pyrolytic and the subsequent use of producing graphene fundamental physics research and graphite proof-of-concept devices [34]. However, microscopy to identify single-layer domains are labour-intensive and time-consuming due to the low the manual peeling of highly oriented pyrolytic graphite and the subsequent use of microscopy to throughput of microscopy techniques and the fact thatand monolayer domains are thethe minority among identify single-layer domains are labour-intensive time-consuming dueinto low throughput of many few- or many-layer platelets [18]. microscopy techniques and the fact that monolayer domains are in the minority among many few- or many-layer platelets [18]. 2.2.2. Epitaxial Growth on SiC Substrates

2.2.2. The Epitaxial Growth onofSiC Substrates epitaxial growth graphene on a SiC substrate involves the fabrication of a graphene film by thermal decomposition a preparedon SiC surface in temperature of up of to 1450 °C for film The epitaxial growth on of graphene a SiC substrate involvesconditions the fabrication a graphene up to 20 min [23]. This method is suitable for fabricating graphene-containing electronic components, by thermal decomposition on a prepared SiC surface in temperature conditions of up to 1450 ◦ C for since SiC is compatible with these applications [35], and produces films that are electrically up to 20 min [23]. This method is suitable for fabricating graphene-containing electronic components, continuous at a millimetre scale [23]. However, this method of producing graphene has limited since SiC is compatible with these applications [35], and produces films that are electrically continuous applications. The requirement for high temperatures means that this is an energy intensive process. at a millimetre scale [23]. However, this method of producing graphene has limited applications. Additionally, it is difficult to transfer graphene epitaxially grown on SiC due to the strong binding The requirement for high temperatures means that this is an energy intensive process. Additionally, between the deposited layer and the substrate [36].

it is difficult to transfer graphene epitaxially grown on SiC due to the strong binding between the deposited layerVapour and theDeposition substrate [36]. 2.2.3. Chemical chemical vapour deposition (CVD) of graphene films involves the decomposition of a fluid 2.2.3. The Chemical Vapour Deposition

at high temperature to form a film on a substrate. Evaporated Ni film on SiO2/Si wafers or copper vapour for deposition (CVD) of graphene the decomposition a fluid at foils The are chemical ideal substrates graphene synthesis [37,38]. films CVD involves can be used as a relatively of highhigh temperature to form a film on a substrate. Evaporated Ni film on SiO /Si wafers or copper throughput production method [39] and it has been demonstrated that the deposited graphene film foils 2

are ideal substrates for graphene synthesis [37,38]. CVD can be used as a relatively high-throughput

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production method [39] and it has been demonstrated that the deposited graphene film can be transferred from the original substrate to a wide range of other substrates [38,40]. Consequently, this production method is potentially suitable for applications where a graphene film is required on a flexible or polymeric substrate that could not withstand high-temperature processing. The quality of the film deposited onto a substrate depends on the temperature: while achievable at temperatures as low as 300 ◦ C [41], higher temperatures are generally correlated with a more continuous crystalline structure. Therefore, like epitaxial growth the need for high temperatures makes this an energy intensive process. 2.2.4. Unzipping of Carbon Nanotubes It is possible to ‘unzip’ a one-dimensional carbon nanotube (CNT) (i.e., break a continuous line of bonds along its length or in a helical pattern) to produce a two-dimensional graphene nanoribbon. The procedure for producing CNTs from graphite electrodes using the arc discharge method is well established [42]. Various methods of unzipping single- and multi-walled CNTs (SWCNTs and MWCNTs respectively) have been reported, including: suspension first in concentrated sulphuric acid and then in potassium permanganate in mild conditions [27], argon plasma etching [43], and mechanical sonication in an organic solvent [44]. Although the unzipping of CNTs produces graphene nanoribbons approximately 10–20 nm in width rather than continuous sheets, it is possible to use these ribbons to produce arrays [43]. 2.2.5. Liquid Phase Exfoliation of Graphite The liquid-phase exfoliation of graphite involves the dispersion of graphite flakes in a solvent, ultrasonication of the dispersion to separate individual graphitic layers, and separation of single-layer graphene from remaining multi- and few-layer graphene and from the solvent. This final stage can be achieved using centrifugation or sedimentation. Single-layer graphene can be identified using microscopic and spectroscopic techniques [30,31]. In selecting a solvent, it is necessary to minimise the interfacial tension between the graphite and the liquid in order to minimise the aggregation of single-layer graphene. Surfactants can be used to improve the dispersibility of graphene in the solvent [31], however this may lead to the introduction of heteroatoms to the graphene plane. Although the sonication process tends to produce small flakes with an area of at most 1 µm2 [45], these flakes have high utility in solution processing [30]. 2.2.6. Reduction of Exfoliated Graphene Oxide Most of the previously mentioned synthesis methods are unsuitable for commercial-scale graphene production. Fortunately, graphene oxide (GO) is a graphene precursor that can be easily produced at large scale by strong oxidation of graphite using acids via the Hummers’ method [46]. This method enables the exfoliation of GO from bulk graphite at low temperature and in a short period of time. It involves preparing a water-free mixture of powdered graphite, sodium nitrate, sulphuric acid and potassium permanganate, followed by filtration and centrifugation. GO, an oxidized form of graphene, is decorated by hydroxyl and epoxy functional groups on the hexagonal network of carbon atoms with carbonyl and carboxyl groups at the edges [47,48]. In addition to being easier to produce than pristine graphene, the oxygen-containing functional groups of GO give hydrophilicity, which can be very important for the large-scale uses of graphene as it enables its dispersion into some solvents for film deposition [49]. However, the presence of oxygenated functionalities in GO significantly diminishes its electrical conductivity compared to pristine graphene due to the disruption to the conjugated π-electron system [50]. Consequently, for some applications it is essential to remove some oxygen-containing functional groups by means of reduction, in order to partially restore the valuable properties of graphene. The material derived from the reduction of GO is called partially reduced graphene oxide (rGO) or chemically converted graphene (CCG). Several

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reduction methods have been reported [32,49,51–54]. They result in different properties of the obtained rGO. The reduction rate will influence the performance of the final application.

•

Thermal reduction of exfoliated graphene oxide

The thermal reduction is believed to be a green method because no hazardous reductants are used. GO can be reduced by thermal treatment and the process is named thermal annealing reduction. The key parameters in this reduction method are the annealing temperature and the annealing atmosphere. This process requires heating up to 1000 ◦ C under vacuum [55] or inert atmosphere [56]. Nevertheless, in hydrogen atmosphere the reduction can be carried out at much lower temperatures due to its high reduction ability [57,58]. Thermal annealing is an effective reduction method, however, due to the temperatures required, it is very energy intensive. In addition, some applications need the deposition of a GO thin film on a specific substrate, such as polymers, therefore this approach cannot be used to reduce GO films deposited on substrates with low melting-points.

•

Chemical reduction of exfoliated graphene oxide

Chemical reduction of GO involves its reaction with different chemical reducing agents. Hydrazine and its derivatives (hydrazine hydrate and dimethylhydrazine) have been accepted as the best reducing agents [21,22,33,49]. The GO reduction is achieved by the addition of the reducing agent to the GO dispersion, obtaining agglomerated graphene-based nanosheets due to the increased hydrophobicity. A significant issue is the dangerousness of these reductants, being toxic, hazardous, explosive and not environmentally benign. Consequently, continuous research has been focused on the development and optimization of eco-friendly reducing agents for GO reduction. Electron transfer reactions have been demonstrated to partially reduce graphene oxide in reactions involving alcohols [59], vitamin C [60], and in high-pH solvents [61].

•

Other reduction methods of exfoliated graphene oxide

A diverse range of alternative methods for the reduction of exfoliated graphene oxide have been proposed, including electrochemical and photolysis-based processes. The examples given are low-temperature methods with minimal heteroatom contamination of the reduced graphene oxide. The photolysis of graphene oxide by UV light, resulting in an order of magnitude improvement in conductivity, has been demonstrated to proceed quickly when catalysed by TiO2 or ZnO [62]. It has also been shown that UV photolysis can be used for the partial reduction of isolated solid graphene oxide [63,64] and for graphene oxide in an aqueous suspension [65]. By contrast, the photolysis of graphene oxide films using lasers has been demonstrated using the relatively simple technique of depositing a graphene oxide film onto a DVD and using the laser of a DVD drive to produce a freestanding film with high conductivity (1738 Sm−1 ) [66]. Numerous methods have been developed to synthesise graphene, however high yield and cost-effective production of defect-free graphene at large-scale is not widely available, which is crucial for real-world applications. The use of GO and rGO achieves a compromise between partial recovery of the conjugated electron system, high scalability of production and suitability for solution processing, making them ideal candidates for commercial applications. Consequently, research efforts in the field of optical fibre sensors have mainly focused on the use of GO and rGO as sensing coatings. 3. Optical Fibre Sensors Using Graphene-Based Materials 3.1. Interferometry Based Optical Fibre Sensors Using Graphene-Based Coatings Optical fibre interferometers use the interference between two beams that propagate through different optical paths (of a single fibre or different fibres). If one of the optical paths is affected

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by external perturbations, the interference will be also affected. Interferometric signals give huge temporal and spectral information. For this reason, the measurand can be quantitatively determined through different properties of the optical signal such as wavelength, phase, intensity, frequency or Sensors 2017, 17, 155 6 of 23 bandwidth [67]. There are two main groups of optical fibre interferometers: Fabry-Perot (FPI) and Sensors 2017, 17,(MZI). 155 6 of 23 Mach-Zehnder bandwidth [67]. There are two main groups of optical fibre interferometers: Fabry-Perot (FPI) and MZIs have been widely used for optical fibre sensing applications due to their flexible Mach-Zehnder bandwidth [67].(MZI). There are two main groups of optical fibre interferometers: Fabry-Perot (FPI) and configurations. The(MZI). early twofor independent the reference armdue (isolated from external MZIs have been MZIs widelyhad used optical fibrearms, sensing applications to their flexible Mach-Zehnder variations) andhave theThe sensing arm tooptical the variations of the externalarm medium). incident configurations. early MZIs(exposed had arms, the reference (isolated from external MZIs been widely usedtwo forindependent fibre sensing applications due to An their flexiblelight is split into both arms by a fibre then recombined by another fibre coupler to obtain variations) and the armcoupler (exposed to the variations thereference external medium). Anfrom incident light the configurations. Thesensing early MZIs had two and independent arms,ofthe arm (isolated external is split into both arms by a fibre coupler and then recombined by another fibre coupler to obtain the interference signal. The two-arm scheme was replaced by a more versatile in-line scheme. In this variations) and the sensing arm (exposed to the variations of the external medium). An incident light interference signal. The two-arm scheme was replaced by a more versatile in-line scheme. In this new split into both arms byfibre a fibre coupler andofthen by another fibre thefibre newisgeneration of optical MZIs a part therecombined beam guided through thecoupler core oftoanobtain optical generation ofsignal. optical fibre MZIs a part of the beam through thein-line core and of anthen optical fibre interference The two-arm scheme was replaced by a more versatile scheme. Inre-coupled this newis to is coupled to cladding modes of the same fibre by anguided intercalated element, coupled to cladding modes of theasame fibre an element, re-coupled to the generation of fibre MZIs partelement. of thebybeam guidedin-line through the and core ofthe an optical fibre is and the core mode byoptical another intercalated Inintercalated these MZIs boththen reference arm core mode by another intercalated element. In these in-line MZIs both and the then reference arm and the coupled to cladding modes of the same fibre by an intercalated element, re-coupled to the the sensing arm have the same physical length. However, as the cladding mode beam has a lower sensing armbyhave the same physical length.InHowever, as the cladding mode beamarm has and a lower core mode another intercalated element. these in-line MZIs both the reference the effective refractive index than the core mode beam they have different optical lengths due to the effective refractive index thanphysical the corelength. mode However, beam theyashave lengths due to the sensing arm have the same the different cladding optical mode beam has a lower modal dispersion [67]. Different configurations of MZIs can be found depending on the coupling modal dispersion Different of MZIs found depending on the coupling effective refractive[67]. index than theconfigurations core mode beam they can havebedifferent optical lengths due to the strategy used, such as[67]. long period crystal fibres, core mismatch, tapering, strategy used, such as long periodgratings, gratings, photonic photonic crystal fibres, core mismatch, fibre tapering, etc. etc. modal dispersion Different configurations of MZIs can be found depending onfibre the coupling In Figure 2 schematic representation of an optical fibre MZI can be found. In Figureused, 2a schematic representation of an photonic optical fibre MZIfibres, can be found. strategy such as long period gratings, crystal core mismatch, fibre tapering, etc. In Figure 2a schematic representation of an optical fibre MZI can be found.

Figure 2. 2.Schematic ofaaMZI-optical MZI-opticalfibre fibre sensor. Figure Schematicrepresentation representation of sensor. Figure 2. Schematic representation of a MZI-optical fibre sensor.

Some authors haveused used graphene-based materials in MZI configurations to obtain Some authors have graphene-based materials in MZI configurations to optical obtainfibre optical Some authors have used an graphene-based materials in MZI configurations to obtain fibre sensors. Yao et al. [68], report ammonia sensor based on a graphene/microfibre hybridoptical waveguide. fibre sensors. Yao et al. [68], report an ammonia sensor based on a graphene/microfibre hybrid sensors. Yao et al. [68], report an on ammonia sensor basedofonthe a graphene/microfibre hybrid waveguide. The sensing mechanism relies the modification graphene conductivity because of the waveguide. The mechanism sensing mechanism relies on the modification of the graphene conductivity because The sensing relies on the modification of the graphene conductivity because of light the adsorption of ammonia. Consequently, the effective refractive index of the device and the of the adsorption of ammonia. Consequently, the effective refractive index of the device and the light adsorption of ammonia. Consequently, effective refractive index of the device the light transmitted along it are very sensitive tothe ammonia concentration. A sensitivity of ~6and pm/ppm was transmitted along it are very sensitive to ammonia concentration. A sensitivity of ~6 pm/ppm transmitted along are very sensitive to ammonia concentration. A sensitivity of ~6 pm/ppm was was obtained using thisitapproach. obtained using this approach. obtained using this approach. Tan et al. presented a refractometer that involved the deposition of a graphene overlay onto the al. presented a refractometer that involved deposition ofof a(Figure graphene onto the the Tan Tan et al. a refractometer involved the deposition a graphene overlay onto surface ofetapresented photonic crystal fibre (PCF)that segment in athe fibre-based MZI 3) overlay [69]. This sensor surface photonic crystal fibre(PCF) (PCF) segment aa fibre-based MZI (Figure 3)of [69]. This sensor surface of aofphotonic crystal fibre segment in1.33 fibre-based MZI (Figure 3)17.5 [69]. This sensor achieved a asensitivity of 9.4 dB/RIU for RIs betweenin and 1.38 and a sensitivity dB/RIU for achieved a sensitivity of 9.4 dB/RIU for RIs between 1.33 and 1.38 and a sensitivity of 17.5 dB/RIU for RIs between 1.38 and achieved a sensitivity of 1.43. 9.4 dB/RIU for RIs between 1.33 and 1.38 and a sensitivity of 17.5 dB/RIU for RIs between 1.43. RIs between 1.381.38 andand 1.43.

Figure 3. (a) Schematic representation of the refractive index sensing element formed by the

Figure 3. (a)3.Schematic representation of the refractive index sensing element formed by theby deposition Figure (a) representation the (b) refractive sensing element formed deposition of aSchematic graphene overlay onto anof MZI; Change index in intensity of the interference vs. RIthe for deposition overlay of a graphene overlay ontoChange an MZI;in (b)intensity Change in intensity of the interference vs. for of a two graphene onto an MZI; the interference vs. with RI forpermission twoRIseparate separate trials (diamonds and(b) triangles) conducted oneof week apart. Reprinted two separate trials (diamondsconducted and triangles) oneReprinted week apart. Reprinted with permission trialsfrom (diamonds and triangles) oneconducted week apart. with permission from [69]. [69]. from [69].

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A similar similar device device using using GO GO as as sensitive sensitive coating coating has has been been recently A recently reported reported by by Dash Dash et et al. al. [70]. [70]. When the RI RI of of the analyte analyte is changed changed fromcoating 1.3310 to to 1.3715, the wavelength wavelength ofby theDash dip shifts shifts from A the similar device using GO as sensitive has been recently reportedof et al. [70]. When the is from 1.3310 1.3715, the the dip from 1544.4the nmRI to1553 1553 nm (wavelength sensitivity of212 212nm/RIU) nm/RIU) and the intensity of of the dip also changes When of thenm analyte is changed from 1.3310 to 1.3715, and the the wavelength the dip dipalso shifts from 1544.4 nm to (wavelength sensitivity of intensity of changes from −78.16 to −83.43 dBm (intensity sensitivity of 130 dB/RIU). This sensitivity is higher than the 1544.4 nm to 1553 (wavelength sensitivity of 212ofnm/RIU) and theThis intensity of theisdip also changes from − 78.16 to −nm 83.43 dBm (intensity sensitivity 130 dB/RIU). sensitivity higher than the previously reported results basedon onsimilar similarconfigurations configurations without any coating[71,72]. [71,72]. from −78.16reported to −83.43results dBm (intensity sensitivity of 130 dB/RIU). This sensitivity is higher than the previously based without any coating An FPI consists of two parallel optical mirrors separated by a certain distance. Interference previously reported results based on similar configurations without any coating [71,72]. An FPI consists of two parallel optical mirrors separated by a certain distance. Interference occurs occurs due to the multiple of reflected andseparated transmitted beams at the two mirrors. In the An FPImultiple consists of twoadditions parallel optical mirrors byat a the certain distance. Interference due to the additions of reflected and transmitted beams two mirrors. In the case of case of optical fibres, FPI sensors can be classified into extrinsic and intrinsic. Extrinsic FPIs use the occurs due to the multiple additions of reflected and transmitted beams at the two mirrors. In the optical fibres, FPI sensors can be classified into extrinsic and intrinsic. Extrinsic FPIs use the reflections reflections from an external cavity formed outside the fibre,inas shown Figure 4. can Thisbe cavity can be case ofanoptical fibres, FPIformed sensors can bethe classified extrinsic and intrinsic. Extrinsic FPIs use the from external cavity outside fibre, asinto shown Figure 4. in This cavity built using built an space and aor diaphragm or a coating made of a sensitive material. Intrinsic FPIs reflections from an cavity thea fibre, as shown in Figure 4. This can be an airusing space andair a external diaphragm aformed coatingoutside made of sensitive material. Intrinsic FPIscavity sensors have sensors have reflecting components within the fibre itself [67]. built using an air space and a diaphragm or a coating made of a sensitive material. Intrinsic FPIs reflecting components within the fibre itself [67]. sensors have reflecting components within the fibre itself [67].

Figure 4. Schematic representation an extrinsic Fabry-Perot interferometer onofthe tip of an Figure 4. Schematic representation of an of extrinsic Fabry-Perot interferometer on the tip an optical fibre. optical fibre. Figure 4. Schematic representation of an extrinsic Fabry-Perot interferometer on the tip of an optical fibre.

Li and co-workers from Beihang University (China) have developed in the last few years a wide variety of sensors based on FPI usingUniversity aUniversity G diaphragm. Figure 5a shows thein schematic diagram these Li co-workers from Beihang (China) have developed the years Liand and co-workers from Beihang (China) have developed in thelast lastfew few yearsaof awide wide FPI sensors which include a zirconia ferrule, a standard single modethe fibre (SMF) and a multi-layer variety of based on using Figure 5a schematic diagram of variety ofsensors sensors based onFPI FPI usingaaG Gdiaphragm. diaphragm. Figure 5ashows shows the schematic diagram ofthese these graphene diaphragm. The adiaphragm, adhered to the zirconia by van derand Waals forces, acts FPI sensors which aastandard single mode (SMF) aa multi-layer FPI sensors whichinclude include azirconia zirconiaferrule, ferrule, standard singlesubstrate modefibre fibre (SMF) and multi-layer as a lightdiaphragm. reflector. When this deviceadhered was tested as zirconia azirconia temperature sensor [73], the variation ofacts the graphene The to substrate by der Waals forces, graphene diaphragm. Thediaphragm, diaphragm, adhered tothe the substrate byvan van der Waals forces, acts cavity length was approximately 352 nm/°C in theas temperature range from[73], 20 °Cthe to variation 60 °C. Thisof effect as aa light reflector. When was aa temperature sensor the as light reflector. Whenthis thisdevice device wastested tested as temperature sensor [73], the variation of the ◦ Cin ◦ Cto ◦ C.This is induced bywas the approximately thermal deformation of the graphene diaphragm. However, the intensity andeffect phase cavity length 352 the range from 60 cavity length was approximately 352 nm/°C nm/ in thetemperature temperature range from20 20°C to 60°C. This effect shifts at common temperatures featured periodic appearance evenHowever, due athe narrow isis induced bybythe thermal deformation of the diaphragm. However, thetointensity andthermal phase induced the thermal deformation of agraphene the graphene diaphragm. intensity and fluctuation (Figure 5b). temperatures This group has used similar devices to develop sensors pressure [74], shifts at common temperatures featured a periodic appearance even due to toa afor narrow thermal phase shifts at common featured a periodic appearance even due narrow thermal adhesion energy [75] and thermal coefficient [76] to of graphene. A similar approach was fluctuation (Figure 5b). This group has for fluctuation (Figure 5b). This groupexpansion hasused usedsimilar similardevices devices todevelop developsensors sensors forpressure pressure[74], [74], used in [77] to obtain an thermal optical acetylene detector with level A detection of acetylene adhesion energy [75] and and thermalfibre expansion coefficient [76] oflow graphene. A similar approach wasa adhesion [75] expansion coefficient [76] of graphene. similar approach wasand used lower limit of 119.8 ppb. used indetection [77] to obtain an optical fibre acetylene detector withlevel low detection level detection of acetylene a in [77] to obtain an optical fibre acetylene detector with low of acetylene and aand lower lower detection limit of 119.8 ppb. detection limit of 119.8 ppb.

Figure 5. (a) Schematic diagram of the FP sensor; (b) Reflection spectra of the FP sensor for different temperatures. Reprinted with permission from [73]. Reflection spectra of the FP sensor for different Figure Figure5.5.(a) (a)Schematic Schematicdiagram diagramof ofthe theFP FPsensor; sensor;(b) (b) Reflection spectra of the FP sensor for different temperatures. Reprinted with permission from temperatures. Reprinted with permission from[73]. [73].

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3.2. 3.2. Surface Surface Plasmon Plasmon Resonance Resonance Optical Optical Fibre Fibre Sensors Sensors Using Using Graphene-Based Graphene-Based Coatings Coatings Over Over the the past past two two decades, decades, surface surface plasmon plasmon resonance resonance (SPR) (SPR) based based sensors sensors have have attracted attracted the the attention their potential applications in the fieldfield of physical, chemical and attention of ofmany manyresearchers researchersdue duetoto their potential applications in the of physical, chemical biomedical sciences [78–84]. A surface plasmon is is a atransverse and biomedical sciences [78–84]. A surface plasmon transversemagnetic magnetic (TM) (TM) polarized polarized electromagnetic wave excited by p-polarized light. Due to the exponential decay electromagnetic wave excited by p-polarized light. Due to the exponential decay of of the the plasmon plasmon electric the metal-dielectric metal-dielectric interface. electric field, field, it it is is strongly strongly localized localized at at the interface. When When aa plasmon plasmon is is excited, excited, an an absorption peak (SPR) at a determined wavelength (resonance wavelength, λ SPR ) is produced [79]. absorption peak (SPR) at a determined wavelength (resonance wavelength, λSPR ) is produced [79]. In In the the case case of of optical optical fibre-based fibre-based SPR SPR sensors, sensors, the the device device shown shown in in Figure Figure 66 is is typically typically used used to to excite excite aa surface surface plasmon. plasmon. The The cladding cladding from from aa small small portion portion of of the the fibre fibre is is removed removed and and this this unclad unclad portion portion is is coated coated with with aa thin thin layer layer of of metal. metal. The The light light from from aa polychromatic polychromatic source source is is coupled coupled into into the the fibre from one one end endand andthe thespectrum spectrumofofthe thetransmitted transmitted power other is collected. to fibre from power at at thethe other endend is collected. DueDue to the the SPR, a peak at λ SPR is obtained in the transmitted spectrum. This λ SPR shows a strong dependence SPR, a peak at λSPR is obtained in the transmitted spectrum. This λSPR shows a strong dependence on on refractive index of sensing the sensing medium around the metal this scheme, great thethe refractive index of the medium around the metal layer. layer. Using Using this scheme, a great avariety variety of can sensors can be just obtained just by depositing onto the metal thin-film is of sensors be obtained by depositing onto the metal thin-film a material thataismaterial sensitivethat to the sensitive to the chemical compound or physical property of interest [85,86]. chemical compound or physical property of interest [85,86].

Figure Figure 6. 6. Schematic Schematicrepresentation representation of of aa SPR-based SPR-based optical optical fibre fibre sensor. sensor.

In the last few years, some studies that include graphene materials in optical fibre SPR-based In the last few years, some studies that include graphene materials in optical fibre SPR-based sensors have been published. In these studies, graphene materials can play different roles. Some sensors have been published. In these studies, graphene materials can play different roles. Some authors have used them as SPR supporting materials instead of the typically used gold and silver authors have used them as SPR supporting materials instead of the typically used gold and silver layers or in addition to them. In other cases, the authors have utilized graphene-based coatings as the layers or in addition to them. In other cases, the authors have utilized graphene-based coatings as the sensitive material, which reacts to any variation of the target analyte. In the next paragraphs, some sensitive material, which reacts to any variation of the target analyte. In the next paragraphs, some examples of these sensors are introduced. examples of these sensors are introduced. Kim et al. used graphene in a SPR sensor as replacement material for gold or silver [87,88]. A Kim et al. used graphene in a SPR sensor as replacement material for gold or silver [87,88]. multi-layered graphene film was synthesized by (CVD) on a Ni substrate and transferred on the A multi-layered graphene film was synthesized by (CVD) on a Ni substrate and transferred on the sensing region of an optical fibre. The graphene coated SPR sensor showed a good sensitivity when sensing region of an optical fibre. The graphene coated SPR sensor showed a good sensitivity when used to analyse the interaction between structured DNA biotin and Streptavidin. used to analyse the interaction between structured DNA biotin and Streptavidin. In [89], the authors presented a theoretical study of an SPR biosensor for detection of bonding In [89], the authors presented a theoretical study of an SPR biosensor for detection of bonding between adenine and thymine or between guanine and cytosine (DNA hybridization). They selected between adenine and thymine or between guanine and cytosine (DNA hybridization). They selected gold as SPR-generating layer and introduced a multilayer graphene structure on its surface. The gold as SPR-generating layer and introduced a multilayer graphene structure on its surface. proposed sensor seemed to be more sensitive than conventional biosensors without the graphene The proposed sensor seemed to be more sensitive than conventional biosensors without the graphene layer. Additionally, the sensitivity linearly increased with the increase in the number of graphene layer. Additionally, the sensitivity linearly increased with the increase in the number of graphene layers. In particular, an improvement of 25% in the sensitivity was achieved by adding 10 G layers to layers. In particular, an improvement of 25% in the sensitivity was achieved by adding 10 G layers to the conventional gold thin film SPR biosensor. This improvement is mainly due to the better the conventional gold thin film SPR biosensor. This improvement is mainly due to the better adsorption adsorption of DNA molecules on G than on gold. Fu et al. [90] proposed a similar approach to of DNA molecules on G than on gold. Fu et al. [90] proposed a similar approach to demonstrate demonstrate theoretically the enhancement in sensitivity of a SPR refractive index sensor with G theoretically the enhancement in sensitivity of a SPR refractive index sensor with G layers onto a gold layers onto a gold SPR-supporting layer. SPR-supporting layer. In [91], the authors simulated the performance of a photonic crystal fibre (PCF) SPR-based In [91], the authors simulated the performance of a photonic crystal fibre (PCF) SPR-based refractive index sensor. A silver layer deposited onto the inner surface of one of the PCF holes acted refractive index sensor. A silver layer deposited onto the inner surface of one of the PCF holes acted as as SPR supporting layer and a G layer deposited onto the silver coating was used to inhibit its SPR supporting layer and a G layer deposited onto the silver coating was used to inhibit its oxidation oxidation (see sensor cross section in Figure 7a). The analyte, a liquid with high RI, was infiltrated (see sensor cross section in Figure 7a). The analyte, a liquid with high RI, was infiltrated into the into the deposited channel hole and the fibre cores. The proposed sensor showed a maximum RI deposited channel hole and the fibre cores. The proposed sensor showed a maximum RI sensitivity sensitivity of 3000 nm/RIU and an average RI sensitivity of 2390 nm/RIU in the sensing range of 1.46 to 1.49 (see Figure 7b). This sensitivity is slightly lower than the achieved by other works using a similar structure with a single gold layer [92].

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of 3000 nm/RIU and an average RI sensitivity of 2390 nm/RIU in the sensing range of 1.46 to 1.49 (see Figure 7b). This sensitivity is slightly lower than the achieved by other works using a similar Sensors 2017, 17, 155 9 of 23 structure with a single gold layer [92].

Figure 7.7. (a) (a) Cross-section Cross-section of Loss spectrum of of thethe Figure of the the SPR-based SPR-basedsensor sensorproposed proposedinin[91]. [91].(b)(b) Loss spectrum fundamental mode mode by by increasing increasing analyte under a Creative Commons fundamental analyteRI RIfrom from1.46 1.46toto1.49. 1.49.Published Published under a Creative Commons AttributionLicense License (CC-BY). (CC-BY). Attribution

Mishra and co-authors carried out a detailed study about the fabrication and characterization of Mishra and co-authors carried out a detailed study about the fabrication and characterization of SPR-based fibre optic gas sensors using rGO, carbon nanotubes (CNTs) and poly(methyl SPR-based fibre optic gas sensors using rGO, carbon nanotubes (CNTs) and poly(methyl methacrylate) methacrylate) (PMMA) [93]. Probes with a silver SPR-supporting layer were coated with different (PMMA) [93]. Probes with a silver SPR-supporting layer were coated with different sensitive materials sensitive materials (rGO, CNTs, rGO-CNTs and rGO/CNT/PMMA hybrid nanocomposite) in order (rGO, CNTs,the rGO-CNTs and rGO/CNT/PMMA hybrid nanocomposite) order toammonia, achieve the to achieve best performance sensor for the detection of several gases in (methane, best performance sensor for the detection of several gases (methane, ammonia, hydrogen hydrogen sulphide, chlorine, carbon dioxide, hydrogen, and nitrogen). Among all the sulphide, tested chlorine, carbon hydrogen, and nitrogen). Among all the tested possibilities, the selectivity sensor based possibilities, thedioxide, sensor based on rGO/CNT/PMMA hybrid nanocomposite showed a high on hybrid a high selectivity methane. The to rGO/CNT/PMMA methane. The sensitivity of nanocomposite this device was showed optimized by varying the to concentration of sensitivity rGO/CNT of this device was optimized by varying the concentration of rGO/CNT in PMMA. The optimum in PMMA. The optimum doping concentration was 5 wt. % and the maximum sensitivity was doping 0.33 concentration 5 wt. and thegas maximum sensitivity in thegroup rangedeveloped for methane nm/ppm in thewas range for% methane concentrations fromwas 10 to0.33 100 nm/ppm ppm. The same gas from 10 to on 100SPR ppm.using The same groupsupporting developed layer an ammonia sensor based on SPR an concentrations ammonia sensor based a copper and rGO/PMMA hybrid using a copper supporting layer and rGO/PMMA hybrid composite, obtaining a maximum sensitivity composite, obtaining a maximum sensitivity close to 1 nm/ppm in the same range of ammonia close to 1 nm/ppm concentration [94]. in the same range of ammonia concentration [94]. particularcase case SPR is localized surface plasmon resonance (LSPR). very thin AAparticular of of SPR is localized surface plasmon resonance (LSPR). WhenWhen a veryathin dielectric dielectric coating that includes metal nanoparticles (typically gold or silver) is deposited onto a coating that includes metal nanoparticles (typically gold or silver) is deposited onto a waveguide, a resonant coupling theelectromagnetic incident electromagnetic wave the surface of the awaveguide, resonant coupling between thebetween incident wave and the and surface of the thin-film is produced. Thisinresults in an electromagnetic wave the metal-dielectric interface isthin-film produced. This results an electromagnetic wave at the at metal-dielectric interface that that causes causes appearance of a sharp absorption peak at a determined wavelength in the transmitted appearance of a sharp absorption peak at a determined wavelength in the transmitted spectrum [95,96]. spectrum Nayak[95,96]. et al. developed different refractive index sensors using graphene oxide encapsulated et al. developed different[97] refractive index sensors using graphene oxide gold Nayak nanoparticles (GOE-AuNPs) and graphene oxide-encapsulated silverencapsulated nanoparticles gold nanoparticles (GOE-AuNPs) [97] and graphene oxide-encapsulated silver nanoparticles (GOE(GOE-AgNPs) [98] as LSPR generating materials. The main benefits of encapsulating the nanoparticles AgNPs) [98] as LSPR generating materials. The main benefits of encapsulating the nanoparticles in GO are the control of the inter-particle distance, preventing aggregation, the enhancement ofinthe GO are the control of the inter-particle distance, preventing aggregation, the enhancement of the colloidal stability and the prevention of the oxidation of the AgNPs, avoiding their direct contact colloidal stability and the prevention of the oxidation of the AgNPs, avoiding their direct contact with with the aqueous medium. The variation in the absorbance of the LSPR peak when the device was the aqueous medium. The variation in the absorbance of the LSPR peak when the device was immersed in aqueous solutions with refractive indices between 1.34 and 1.38 produced sensitivities of immersed in aqueous solutions with refractive indices between 1.34 and 1.38 produced sensitivities 2.288 ∆A/RIU and 0.9406 ∆A/RIU for GOE-Au NPs and GOE-AgNPs, respectively. of 2.288 ΔA/RIU and 0.9406 ΔA/RIU for GOE-Au NPs and GOE-AgNPs, respectively.

3.3. Fibre Bragg Gratings Sensors Using Graphene-Based Coatings 3.3. Fibre Bragg Gratings Sensors Using Graphene-Based Coatings Fibre gratings consist of a periodic perturbation of the properties of the optical fibre, generally of Fibre gratings consist of a periodic perturbation of the properties of the optical fibre, generally the refractive index of the core (Figure 8). In Fibre Bragg gratings (FBGs), this alteration produces a of the refractive index of the core (Figure 8). In Fibre Bragg gratings (FBGs), this alteration produces coupling of light from the forward-propagating mode of the optical fibre to a backward propagating a coupling of light from the forward-propagating mode of the optical fibre to a backward propagating mode. at aa specific specificwavelength wavelengththat thatdepends depends the period FBG mode.This This coupling coupling occurs occurs at onon the period of of thethe FBG andand thethe effective mode. As Asaaconsequence, consequence,a avariation variation either these parameters effectiveindex indexof of the the propagating propagating mode. inin either of of these parameters produce a change in the coupling wavelength (Bragg wavelength, λB) that can be measured. As strain and temperature have a direct influence on the mentioned parameters, many sensing schemes based on FBGs have been developed to measure these signals [99–107].

with a material whose structure varies in the presence of a particular chemical compound (analyte). When the concentration of this analyte changes, the deformation of the coating produces an axial strain. The FBG stretches or shrinks under such strain and therefore the spectral pattern of reflected light changes, producing a shift in reflected wavelength [108]. If the fibre core of the FBGs is covered by the cladding layer, it is less sensitive to the variations in the surrounding medium. To overcome Sensors 2017, 17, 155 10 of 24 this issue, the cladding is etched in order to expose the propagating modes of the core to the surrounding medium [109]. A schematic representation of an etched FBG (eFBG) coated with a sensitiveathin-film shown in Figure 8. These(Bragg deviceswavelength, have been used for can chemical applications such produce change inisthe coupling wavelength λB ) that be measured. As strain as refractive index sensing [106], gas sensors or relative humidity sensors [110] and also for and temperature have a direct influence on the [108] mentioned parameters, many sensing schemes based biosensing applications [109,111,112]. on FBGs have been developed to measure these signals [99–107].

Figure 8. Schematic representation of an eFBG coated with a sensitive thin film. Figure 8. Schematic representation of an eFBG coated with a sensitive thin film.

In the last few years, an increasing number of studies about optical fibre sensors based on FBGs FBGs can be also used forthat chemical biosensing. For this purpose, FBGs aresection, coated have been focused on devices includesensing coatingsand made of graphene-based materials. In this with a material whose structure varies in the presence of a particular chemical compound (analyte). different examples of these structures are presented. WhenThe the Sood concentration of this deformation of the coating an axial strain. group from theanalyte Indian changes, Institutethe of Science (Bangalore, India) produces has intensively studied The FBG stretches or shrinks under such strain and therefore the spectral pattern of reflected light this type of sensor. They have developed different sensing schemes based on etched FBGs including changes, producing a shift in reflected wavelength [108]. If the fibre core of the FBGs is covered by gas detectors, physical sensors and biosensors. In particular, they have enhanced the typical the cladding it is less sensitive the variations in the surrounding To overcome this sensitivity of layer, bare FBGs to strain and to temperature by coating with rGO anmedium. etched FBG. These sensors issue, the cladding is etched in order to expose the propagating modes of the core to the surrounding showed a sensitivity to strain of 5.5 pm/με and a sensitivity to temperature of 33 pm/°C (5 and 3 times medium [109]. A schematic representation of an etched FBG (eFBG) coated with a sensitive thin-film better than bare FBGs). The resolutions obtained with these sensors were about 1 με for strain is shown in Figure 8. These devices have been used for chemical applications such as refractive measurements and 0.3 °C for temperature measurements [113]. indexFurthermore, sensing [106], sensors or relativedifferent humidity sensors based [110] and alsoconfiguration. for biosensing thegas same group[108] has developed biosensors on this In applications [109,111,112]. [109], they presented an etched FBG coated with aminophenylboronic acid (APBA)-functionalized the exhibited last few years, increasingtonumber of studies about optical fibreasensors onBragg FBGs rGO In that highan sensitivity glucose. These sensors showed linear based shift in have been focused on devices that include coatings made of graphene-based materials. In this section, wavelength with the concentration of a glucose solution in the range of 1 nM to 10 mM, covering the different examples these structures presented. clinical range of theofestimated averageare glucose concentration in red blood cells, which enables them The Sood group from the Indian Institute of Science (Bangalore, India) has intensively studied to be used in detection of diabetes. They have also designed and characterized biosensors for proteins this sensor. They haveAdeveloped different sensing schemes based on etched FBGs including CRPtype [111]ofand concanavalin [114] detection using this sensing scheme. gas detectors, physical sensors and biosensors. In particular, they have enhanced theettypical sensitivity Other research groups have also exploited the mentioned mechanism. Wang al. have recently of bare FBGs to strain and temperature by coating with rGO an etched FBG. These sensors a developed a relative humidity sensor consisting of a tilted FBG coated with a GO thin film showed obtaining ◦ sensitivity tosensitivity strain of 5.5 and a sensitivity to temperature of 33 pm/ C (5 to and 3 times a maximum of pm/µε 0.129 dB/%RH in the relative humidity range from 10% 80% [115].better than Zhang bare FBGs). The resolutions obtained with these sensors were about 1 µε for strain measurements et al. studied the features of a graphene-coated microfibre FBG (GMFBG) as an ammonia and 0.3 ◦Figure C for temperature measurements sensor. 9 shows the experimental[113]. setup used in this study [116]. Different diameters of Furthermore, the for same has developed different of biosensors based on~50 thisppm, configuration. GMFBGs were tested NHgroup 3 gas with the concentrations 0 ppm, ~10 ppm, and ~100 In [109], they presented an etched FBG coated with aminophenylboronic acid (APBA)-functionalized ppm. To further demonstrate the enhancement effect by graphene, bare microfibre FBGs (MFBGs) rGO exhibited high sensitivity glucose. sensors showed a linear are shiftshown in Bragg wavelength withthat different diameters were alsoto tested. TheThese results of these experiments in Figure 10. It with the concentration of a glucose solution in the range of 1 nM to 10 mM, covering the clinical range can be concluded that MFBGs without the graphene cladding were almost not sensitive to gas of the estimated glucose concentration in red blood cells, enables used in adsorption whileaverage GMFBGs showed a maximum sensitivity of which 6 pm/ppm forthem 10 µto mbediameter detection of diabetes. They have also designed and characterized biosensors forwere proteins [111] microfibres. These results also indicate that the GMFBG with a smaller diameter moreCRP sensitive and concanavalin A [114] detection using this sensing scheme. to the gas concentration alteration. Other research groups have also exploited the mentioned mechanism. Wang et al. have recently developed a relative humidity sensor consisting of a tilted FBG coated with a GO thin film obtaining a maximum sensitivity of 0.129 dB/%RH in the relative humidity range from 10% to 80% [115]. Zhang et al. studied the features of a graphene-coated microfibre FBG (GMFBG) as an ammonia sensor. Figure 9 shows the experimental setup used in this study [116]. Different diameters of GMFBGs were tested for NH3 gas with the concentrations of 0 ppm, ~10 ppm, ~50 ppm, and ~100 ppm. To further demonstrate the enhancement effect by graphene, bare microfibre FBGs (MFBGs) with different diameters were also tested. The results of these experiments are shown in Figure 10. It can be concluded that MFBGs without the graphene cladding were almost not sensitive to gas adsorption while GMFBGs showed a maximum sensitivity of 6 pm/ppm for 10 µm diameter

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microfibres. These results also indicate that the GMFBG with a smaller diameter were more sensitive to the 2017, gas concentration alteration. Sensors 17, 155 11 of 23

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Figure 9. 9. Schematic representation of the experimental setup used in the characterization characterization of of an an Figure Schematic representation representation of of the the experimental experimental setup setup used used in in the the Figure 9. Schematic characterization of an ammonia sensor sensor consisting consisting of of a graphene-coated eFBG. eFBG. Reprinted with with permission from from [116]. ammonia ammonia sensor consisting of aa graphene-coated graphene-coated eFBG. Reprinted Reprinted with permission permission from [116]. [116].

Figure 10. Spectral shifts of the GMFBGs and MFBGs for different concentrations of NH3. Reprinted Figure 10. Spectral of the the GMFBGs GMFBGs and and MFBGs MFBGs for for different different concentrations concentrations of of NH NH33.. Reprinted Figure 10. Spectral shifts shifts of Reprinted with permission from [116]. with permission from [116]. with permission from [116].

3.4. Absorption-Based Optical Fibre Sensors Using Graphene-Based Coatings 3.4. Graphene-Based Coatings Coatings 3.4. Absorption-Based Absorption-Based Optical Optical Fibre Fibre Sensors Sensors Using Using Graphene-Based FBGs/LPGs and optical fibre interferometers require complex fibre optic pre-processing or FBGs/LPGs and optical fibre require complex fibre optic pre-processing or FBGs/LPGs andat optical fibre interferometers interferometers require optic or sophisticated control the micrometre level respectively. In complex contrast, fibre optical fibrepre-processing sensors based on sophisticated control at the micrometre level respectively. In contrast, optical fibre sensors based on sophisticated at thesuch micrometre level respectively. contrast, optical fibre sensors based light intensitycontrol monitoring, as transmission, absorptionInor reflection, are relatively simple to light intensity monitoring, such as transmission, absorption or reflection, areare relatively simple to on light intensity monitoring, such as transmission, absorption or reflection, relatively simple implement. On the other hand, these systems are vulnerable to light intensity variations, light source implement. On On the other hand, thesethese systems are vulnerable to light variations, light source to implement. hand, systems aresource vulnerable tointensity light light instabilities, micro the andother macro bending or external coupling. In intensity order to variations, overcome these instabilities, micro and macro bending or external source coupling. In order to overcome these source instabilities, micro and macro bending or external source coupling. In order to overcome undesired effects these systems use complex detection algorithms and systems with referenced undesired effects thesethese systems use use complex detection algorithms and referenced these undesired effects complex detection algorithms andsystems systemswith with referenced measurements, which are notsystems associated with the light intensity variations produced by the selected measurements, which are not associated with the light intensity variations produced by the selected measurements, which are not associated with the light intensity variations produced by the selected analyte [117,118]. analyte [117,118]. analyte [117,118]. Absorption-based optical fibre sensors rely on the fact that the selected target or transducer must Absorption-based optical fibre fibre sensors sensors rely rely on on the the fact fact that selected target or must Absorption-based optical that the the selected or transducer transducer must modify the intensity of the light propagated through the optical fibre core astarget a function of the selected modify the intensity of the light propagated through the optical fibre core as a function of the selected modify the intensity ofintensity the light loss propagated through the optical fibre core as a function of the selected measurand. Here, the can be described using the Beer-Lambert law: measurand. described using using the the Beer-Lambert Beer-Lambert law: law: measurand. Here, Here, the the intensity intensity loss loss can can be be described I1 = I0e−αl (1) I1 = I0e−αl (1) −αl I = I e (1) 1 0 I0 and I1 are the excitation and transmitted light where l is the length of the fibre sensitive region, where l is the length of the fibre sensitive region, I0 and I1 are the excitation and transmitted light intensities and α, the absorption coefficient, can be expressed as α = Cε/loge where C is the molar intensities andlength α, the of absorption coefficient, can beI0 expressed = Cε/loge and where C is the molar where l is the the fibre sensitive region, and I1 areas theα excitation transmitted light concentration and ε is the molar absorptivity. The absorbance (A), is proportional to the length of the concentration ε is absorption the molar absorptivity. Thebe absorbance is =proportional to the of the intensities andand α, the coefficient, can expressed(A), as α Cε/loge where C length is the molar fibre sensitive region and the analyte concentration and can be obtained from Equation (1) and fibre sensitive and region the analyte concentration and can be from Equation (1) and concentration ε is and the molar absorptivity. The absorbance (A),obtained is proportional to the length of expressed as: expressed as: A = log(I1/I0) = εCl (2) A = log(I1/I0) = εCl (2) Optical fibre geometry is generally modified in this type of sensor in order to enhance light Optical fibre geometry is generally modified in this type of sensor in order to enhance light interactions with the selected measurand or transducer. Some typical examples are the cases of Uinteractions with the selected measurand or transducer. Some typical examples are the cases of Ubent, side polished, cladding removed, microstructured, tapered fibres and so on. Concerning these bent, side polished, cladding removed, microstructured, tapered fibres and so on. Concerning these

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the fibre sensitive region and the analyte concentration and can be obtained from Equation (1) and expressed as: A = log(I1 /I0 ) = εCl (2) Optical fibre geometry is generally modified in this type of sensor in order to enhance light interactions Sensors 2017, 17,with 155 the selected measurand or transducer. Some typical examples are the cases 12 ofof 23 U-bent, side polished, cladding removed, microstructured, tapered fibres and so on. Concerning these intensity-based intensity-based measurements, the general idea is based on the fact that the optical properties of the target target or or transducer transducer are are altered altered as as aa function function of the selected magnitude magnitude or analyte concentration. concentration. More More specifically, specifically, the the optical optical absorption absorption of of graphene graphene layers layers can can be be considered considered proportional proportional to to the the number number of of layers layers with with little little or or no no perturbation perturbation between between adjacent adjacent layers. layers. Graphene also exhibits a quite flat flat response from 300 to 2500 nm with an absorption peak near 270 nm as shown in Figure 11 when it response from 300 to 2500 nm with an absorption peak near 270 nm as shown in Figure 11 when it is is compared compared with with other other transparent transparent conductors. conductors.

Figure 11. 11. Transmittance of different different transparent transparent conductors: conductors: Graphene, (ITO), Figure Transmittance of Graphene, indium indium tin tin oxide oxide (ITO), ZnO/Ag/ZnO, TiO 2 and arc discharge single-walled nanotubes. Reprinted from [119] with ZnO/Ag/ZnO, TiO22/Ag/TiO /Ag/TiO 2 and arc discharge single-walled nanotubes. Reprinted from [119] with permission from from Nature NaturePhotonics. Photonics. permission

The two-dimensional hexagonal shape structure of graphene, large surface area and high The two-dimensional hexagonal shape structure of graphene, large surface area and high electron electron mobility enable it to adsorb easily different kinds of gas molecules, volatile organic mobility enable it to adsorb easily different kinds of gas molecules, volatile organic compounds (VOCs) compounds (VOCs) and biological species [120]. Molecule adsorption on graphene’s surface can and biological species [120]. Molecule adsorption on graphene’s surface can modify the electrical modify the electrical conductivity and alter the complex refractive index value, which results in conductivity and alter the complex refractive index value, which results in variations of the absorption variations of the absorption spectrum. Thus, the detection and concentration measurement of spectrum. Thus, the detection and concentration measurement of different compounds is performed different compounds is performed by simply measuring the absorption spectrum [121]. by simply measuring the absorption spectrum [121]. The next paragraphs will focus the attention on the utilization of thin graphene-based coatings, The next paragraphs will focus the attention on the utilization of thin graphene-based coatings, fabricated onto diverse optical fibre sensing schemes, as transducers for the detection of physical fabricated onto diverse optical fibre sensing schemes, as transducers for the detection of physical magnitude or property changes such as temperature or UV radiation as well as chemical and magnitude or property changes such as temperature or UV radiation as well as chemical and biological biological compounds: relative humidity, ethanol, ammonia, glucose or DNA. These applications, compounds: relative humidity, ethanol, ammonia, glucose or DNA. These applications, together with together with the sensing characteristics of the obtained devices are also summarized in Table 1. the sensing characteristics of the obtained devices are also summarized in Table 1. Graphene-based thin-films can be exploited for the fabrication of optical fibre sensors. Ambient Graphene-based thin-films can be exploited for the fabrication of optical fibre sensors. Ambient temperature, humidity or ultra violet light radiation among others have a great impact on the temperature, humidity or ultra violet light radiation among others have a great impact on the conductivity of graphene, leading to variations of the effective refractive index and having a conductivity of graphene, leading to variations of the effective refractive index and having a measurable measurable effect on the transmitted optical power. Fast response all-fibre graphene assisted effect on the transmitted optical power. Fast response all-fibre graphene assisted temperature temperature sensors have been obtained using microfibre [122] and side-polished [123] optical fibre sensors have been obtained using microfibre [122] and side-polished [123] optical fibre structures structures with sensitivities of 0.134 dB/°C and 0.1018 dB/°C respectively for a wide range of with sensitivities of 0.134 dB/◦ C and 0.1018 dB/◦ C respectively for a wide range of temperatures. temperatures. The fabrication of UV light exposure sensors has been explored by means of the The fabrication of UV light exposure sensors has been explored by means of the utilization of highly utilization of highly birefringent fibre covered with graphene oxide (GO) [124] and tapered SMF birefringent fibre covered with graphene oxide (GO) [124] and tapered SMF microfibres in contact with microfibres in contact with methylene-blue functionalized reduced graphene oxide (MB-rGO) [125]. methylene-blue functionalized reduced graphene oxide (MB-rGO) [125]. Light intensity modulation of Light intensity modulation of rGO at high relativity humidity range (70%–95%) was also studied in [126] using side-polished single mode fibres (SMF) obtaining a sensitivity of 0.31 dB/%RH and a response time faster than 0.13% RH/s. Carbonyl and carboxylic acid functional groups present in GO show better affinity than graphene to capture ethanol or benzene molecules in aqueous solutions. Several authors have explored this advantage in order to develop ethanol sensors using diverse optical fibre architectures,

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rGO at high relativity humidity range (70%–95%) was also studied in [126] using side-polished single mode fibres (SMF) obtaining a sensitivity of 0.31 dB/%RH and a response time faster than 0.13% RH/s. Carbonyl and carboxylic acid functional groups present in GO show better affinity than graphene to capture ethanol or benzene molecules in aqueous solutions. Several authors have explored this advantage in order to develop ethanol sensors using diverse optical fibre architectures, such as tapered multimode fibres (MMF) [127–130], or U-bent fibres [131]. The combination of high surface area with the hydrophilic and hydrophobic properties of GO and rGO respectively is also very interesting for the detection of gaseous species and VOCs. In particular, Kavinkumar et al. [132] studied the role of Sensors 2017, 17, 155 13 of 23 functional groups in the absorption properties of cladding-removed multimode fibres (CRMMF) coated ◦ C, GO ◦ withfibres GO (CRMMF) and rGO (heated at 110 , and heated at 220 GOheated exposed to) when gaseous 110(heated 220 ) when coated with GO and rGO at 110 °C, GO110C, , and at 220 °C, GO220 ammonia, and ammonia, methanol ethanol (see Figure These(see results evidence good device sensitivity, exposedethanol to gaseous and 12). methanol Figure 12). These results evidence goodbut minimal selectivity in the response. device sensitivity, but minimal selectivity in the response.

Figure 12. 12. Change in output intensity at at gas concentrations Figure Change in output intensity gas concentrationsbetween between00and and500 500ppm ppmof of(a) (a)ammonia ammonia (b) ethanol and (c) methanol for GO and rGO (GO and GO ). Reprinted from [132]. 110 110 and GO 220220). Reprinted from [132]. (b) ethanol and (c) methanol for GO and rGO (GO

High to sensitive and selective VOCs detection has been also explored in [133] by Some and coworkers by means of the fabrication of GO, rGO and mixed GO/rGO thin-films onto the end tip of plastic optical fibres (POF) as it is represented in Figure 13. Some other works have explored the affinity of graphene and graphene-based materials for the detection of more complex molecules. For example, Qiu and co-workers explored glucose and double

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High to sensitive and selective VOCs detection has been also explored in [133] by Some and required inby this field of inthe order to establish a good linear sensitivity relationship co-workers means fabrication of GO, rGO and mixed GO/rGO thin-films and ontoguarantee the end tipthe of selectivity of the fabricated with the selected analyte. plastic optical fibres (POF) asdevices it is represented in Figure 13.

Figure 13. 13. Comparative Comparative plots plots of of the the sensing sensing responses responses of of GO GO and and rGO rGO to to eight eight different different vapours vapours at at aa Figure 500 ppb ppb concentration concentration level. level. Reprinted Reprinted from from [133]. [133]. 500

3.5. Fluorescence-Based Optical Fibre Sensors Using Graphene-Based Coatings Some other works have explored the affinity of graphene and graphene-based materials for the Fluorescence or phosphorescence measuring to asexplored fluorescence henceforward, detection of more complex molecules. For example,systems, Qiu andreferred co-workers glucose and double have been established as an important group of optical fibre sensors. This technique been strand DNA (ds-DNA) detection by means of the fabrication of a single-layer graphene ontohas tapered traditionally used analytical or respectively. biochemical However, applications due to itsis excellent POF (TPOF) [134] andintapered MMFchemistry (TMMF) [135] further research required performance, time saving, fast response, cost effectiveness, highguarantee sensitivity and specificity in this field in including order to establish a good linear sensitivity relationship and the selectivity of and good reproducibility. The detection mechanisms are based on fluorescence lifetime or intensity the fabricated devices with the selected analyte. measurements using modulated, pulsed or continuous excitation light sources [136]. The 3.5. Fluorescence-Based Optical Fibre Sensors Graphene-Based Coatings fluorescence of the active material can be Using enhanced or quenched as a function of the presence and concentration of the target molecule. Concerning the fluorescence intensity, it is important to take Fluorescence or phosphorescence measuring systems, referred to as fluorescence henceforward, into account the excitation andgroup the efficiency the sensors. active material, which is has the been ratio have been established as anintensity important of opticaloffibre This technique between the emitted and absorbed photons in the active material. In the case of optical fibre traditionally used in analytical chemistry or biochemical applications due to its excellent performance, fluorescence-based sensors, the optical is also used to transmit the fluorescence signal to including time saving, fastwhere response, cost fibre effectiveness, high sensitivity and specificity and the detector, it is also important to consider an optimal design to couple the maximum fluorescence good reproducibility. The detection mechanisms are based on fluorescence lifetime or intensity emission into using the fibre, such as in the of microstructured Moreover, measurements modulated, pulsed or case continuous excitation lightoptical sourcesfibres [136]. [137]. The fluorescence photodegradation of the material, photobleaching after long timepresence exposure to concentration the excitation of the active material can active be enhanced or quenched as a function of the and source or self-quenching at high target concentrations are some of the drawbacks of these devices. of the target molecule. Concerning the fluorescence intensity, it is important to take into account the Thus, as in the case of absorption-based sensors, a referenced signal is required in order to avoid excitation intensity and the efficiency of the active material, which is the ratio between the emitted and undesiredphotons power fluctuation [138]. absorbed in the activeeffects material. In the case of optical fibre fluorescence-based sensors, where GO is fluorescent over a broad range of wavelengths, its heterogeneous electronic the optical fibre is also used to transmit the fluorescence signalowing to theto detector, it is also important to structure. Blue to green fluorescence emission can be obtained as a function of the light excitation consider an optimal design to couple the maximum fluorescence emission into the fibre, such as in wavelength with a redshift optical of the fluorescence maximum photodegradation intensity with the of increase of excitation the case of microstructured fibres [137]. Moreover, the active material, wavelength above 400 nm as shown in Figure 14. photobleaching after long time exposure to the excitation source or self-quenching at high target concentrations are some of the drawbacks of these devices. Thus, as in the case of absorption-based sensors, a referenced signal is required in order to avoid undesired power fluctuation effects [138]. GO is fluorescent over a broad range of wavelengths, owing to its heterogeneous electronic structure. Blue to green fluorescence emission can be obtained as a function of the light excitation wavelength with a redshift of the fluorescence maximum intensity with the increase of excitation wavelength above 400 nm as shown in Figure 14.

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Figure 14. 14. Fluorescence Fluorescence intensity intensity excitation-emission excitation-emission map map of of GO; GO; strong strong signals signals indicated indicated by by dashed dashed Figure white lines are due to scattering of excitation light and its second order. Reprinted from [139]. white white lines lines are are due due to to scattering scattering of of excitation excitation light light and and its its second second order. order.Reprinted Reprintedfrom from[139]. [139].

In particular, particular, GO GO fluorescence fluorescence can be be obtained obtained by by inducing inducing aa bandgap bandgap using two two preferred preferred In In particular, GO fluorescence cancan be obtained by inducing a bandgap using using two preferred routes. routes. One consists of producing graphene ribbons and graphene quantum dots (GQDs) while the the routes. One consists of producing graphene ribbons and graphene quantum dots (GQDs) while One consists of producing graphene ribbons and graphene quantum dots (GQDs) while the other is other is based on chemical and physical treatments, such as oxygen plasma treatments [119,140,141]. other isonbased on chemical and physical treatments, as oxygen treatments [119,140,141]. based chemical and physical treatments, such assuch oxygen plasmaplasma treatments [119,140,141]. While While intrinsic GO fluorescence is interesting, much more needs to be understood about its properties properties While intrinsic GO fluorescence is interesting, needs be understood about its intrinsic GO fluorescence is interesting, muchmuch moremore needs to betounderstood about its properties in in relation relation to to quenching quenching by by external external (analyte) (analyte) mediators, mediators, measurement measurement conditions conditions etc. etc. before before this this in relation to quenching by external (analyte) mediators, measurement conditions etc. before this property property can can be be fully fully exploited exploited for for sensor sensor development. development. property can be fully exploited for sensor development. Partially rGO rGO (prGO) (prGO) with with adsorbed adsorbed fluorescent fluorescent rhodamine rhodamine 6G (Rh6G) (Rh6G) molecules (Type (Type 1) 1) was was Partially Partially rGO (prGO) with adsorbed fluorescent rhodamine 6G 6G (Rh6G) molecules molecules (Type 1) was 2+ ions, dopamine (DA) and single-strand DNA coated onto etched MMF for the detection of Cd 2+ ions, dopamine (DA) and single-strand DNA coated onto onto etched etched MMF MMF for for the the detection detection of of Cd Cd2+ coated ions, dopamine (DA) and single-strand DNA (ssDNA) [142]. Additional processing treatments were required required to to enhance enhance the the selectivity selectivity of of the the (ssDNA) [142]. [142].Additional Additionalprocessing processing treatments were (ssDNA) treatments were required to enhance the selectivity of the prGO prGO with adsorbed Rh6G molecules to the specific analyte, such as nitrate pre-immersion in Type prGOadsorbed with adsorbed Rh6G molecules to the specific aspre-immersion nitrate pre-immersion Type with Rh6G molecules to the specific analyte,analyte, such as such nitrate in Type 2insensor 2 sensor sensor (to (to make make the the device device not not absorbable absorbable to to ions ions and and immune immune to to DA DA agglomeration), agglomeration), or Na Na++ + 2 or (to make the device not absorbable to ions and immune to DA agglomeration), or Na functionalization + functionalization in in Type Type 33 sensor sensor (to form form COO COO-Na Na+ bindings). bindings). Fluorescence Fluorescence measurement measurement results results functionalization in Type 3 sensor (to form COO- Na+(to bindings). Fluorescence measurement results from the obtained from the obtained devices are shown in Figure 15. In particular, lower detection limits of 1.2 nM, 1.3 from theare obtained in Figure 15. In particular, lower detection of 1.2 nM, 1.3 devices shown devices in Figureare 15.shown In particular, lower detection limits of 1.2 nM, 1.3limits µM and 1 pM were μM and 1 pM were achieved for cadmium ion, DA and ssDNA respectively. μM and 1for pMcadmium were achieved forand cadmium DA and ssDNA respectively. achieved ion, DA ssDNAion, respectively.

Figure 15. Fluorescence spectra spectra of Type 11 (a), (a), Type 22 (b) (b) and and Type Type 33 (c) (c) sensor sensor when when immersed immersed in in 200 200 Figure 15. Fluorescence Fluorescence spectraofofType Type 1 (a),Type Type 2 (b) and Type 3 (c) sensor when immersed in 2+ (red curve), 10 mM DA (yellow curve) and 100 nM (green curve).(d–f), Histograms: the μM Cd 2+ (red μM µM Cd2+Cd (red curve), 10 mM DADA (yellow curve) 200 curve), 10 mM (yellow curve)and and100 100nM nM(green (green curve).(d–f), curve).(d–f), Histograms: the fluorescentrestoration restorationratio ratio of Type (d), Type (e)Type and 3.Type Type 3. Reprinted Reprinted from [142] under aa fluorescent restoration ratio Type 11 (d), (e) and 3. from [142] under fluorescent ofof Type 1 (d), TypeType 2 (e) 22and Reprinted from [142] under a Creative Creative Commons CC-BY license. Creative Commons CC-BY license. Commons CC-BY license.

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Table 1. Summary of graphene-based optical fibre sensors. Sensors 2017, 17, 155

Detection Mechanism

Material

MZI

GO

MZI

G

MZI

G

Detection Mechanism

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Dynamic Optical Fibre Sensitivity Analyte Range Configuration Table 1. Summary of graphene-based optical fibre sensors. 212 nm/RIU PCF MZI RI 1.33–1.37 Optical Fibre Dynamic 130Sensitivity dB/RIU Material Analyte Configuration

MZI

PCF MZI GO

MZI

2-arms MZI G

SPR

MZI Cu + rGO/PMMA SPR

SPR

Ag + SPR rGO/CNT/PMMA NPsLSPR

G

Uncladded MMF

Cu + rGO/PMMA Ag +

Uncladded MMF rGO/CNT/PMMA GOE-AuMMF NPs Uncladded

LSPR

GOE-Au

LSPR

GOE-Ag NPsLSPR Uncladded GOE-AgMMF NPs

Bragg wavelength

Bragg rGO wavelength EtchedrGO FBG Bragg rGO rGO wavelength Etched FBG Bragg APBA-rGO wavelength APBA-rGO Bragg Etched FBG Dendrimers-GO wavelength Bragg antiCRP-GO Dendrimers-GO wavelength Etched FBG Bragg p-doped G wavelength antiCRP-GO Bragg power Etched FBG GO at 1557 nm Bragg D-shaped polymer p-doped G PMMA-G wavelength FBG

Bragg wavelength Bragg wavelength Bragg wavelength Bragg wavelength Bragg wavelength

Absorption

GO

Bragg power at 1557 nm

GO

Bragg wavelength

PMMA-G

MFBG

Absorption

GO

TaperedGO MMF

Reflectance

Reflectance Absorption Absorption

GO

Absorption

Absorption

GO

Absorption

GO

Absorption

GO, rGO

Absorption Absorption Absorption Absorption Absorption

Absorption Absorption Absorption Absorption

Tilted FBG

Absorption Graphene/PANI

GO-rGO rGO rGO

Absorption Absorption Absorption

GO GO

Tapered MMF GO, rGO

Tapered MMF Graphene/PANI GO-rGO

U-bent MMF rGO rGO

CRMMF rGO

Side-polished GO MMPF Graphene GO

POF

MB-rGO

Side-polished SMF

Fluorescence

prGO Tapered SMF

RI

PCF MZI

PCFAmmonia MZI 2-arms MZI Ammonia Uncladded MMF Uncladded Methane MMF Uncladded MMF RI Uncladded RI MMF

Range

1.38–1.43 1.33–1.37

nm/RIU 17.5212dB/RIU

40–360 ppm 1.38–1.43

~6 17.5 pm/ppm dB/RIU

RI RI Ammonia

40–360 ppm

10–100 ppm

Ammonia

10–100 ppm

Methane10–100 10–100 ppm ppm

EtchedGlucose FBG

CR Protein

[69]

[70]

[68] [69]

0.4 s -

[68]

-

[94]

[94]

-

-

[93] [93]

2.288 ΔA/RIU 2.288 ∆A/RIU

-

-

[97] [97]

RI

1.34–1.38 1.34–1.38

0.940 ∆A/RIU 0.940 ΔA/RIU

-

-

[98] [98]

-

-

[113] [113]

-

-

NO2

pm/μє 5.55.5 pm/µ ◦C pm/°C 33 33 pm/

0.5–3 ppm

-

0.5–3 ppm Glucose

-

1A nM–10 mM Lectin Con 500 pM 0.01–100

Erythrocyte

-

-

-

6.3 pm/CRP - order magnitude

-

1 pm/ppm

-

6.3 pm/CRP

0.01–100 mg/L Relative order 10%–80%HRmagnitude 0.129 dB/%RH Humidity

Erythrocyte Ammonia MFBG

-

0–100 ppm

Aqueous Tapered MMF Relative HumidityEthanol10%–80%HR Aqueous Tapered MMF 5%–80% Ethanol Ammonia Aqueous0–100 ppm Tapered MMF 5%–40% Ethanol Aqueous Aqueous Ethanol U-bent MMF 5%–100% Ethanol Aqueous EthanolEthanol, 5%–80% CRMMF methanol, 0–500 ppm ammonia Side-polished Aqueous Ethanolammonia 5%–40% 0%–1% vol. MMPF

1 pm/ppm 6 pm/ppm

[143]

[143] [109]

[109] [114] [111]

-

[114]

[144]

-

[111]

-

[115]

-

[144] [116]

-

20–30 s

[129]

0.02–0.0275 ∆R/∆C 1

19–25 s

[130]

0.129 dB/%RH 6 pm/ppm 1 0.829 ∆A/∆C 0.44–0.0925 ∆A/∆C 1

0.02–0.0275 0.26, 0.2 and1 0.32 ∆R/∆C counts/ppm

1 0.829 4.1∆A/∆C ∆A/∆vol. 1

POF VOCs 0.44–0.0925 Aqueous Ethanol 5%–100% Side-polished 1 ∆A/∆C Temperature −7.8–77 °C 0.134 dB·°C−1 SMF Tapered SMF Temperature 30–80 °C 0.1018 °C−1 Ethanol, methanol, 0.26, 0.2dB· and 0–500 ppm Side-polished ammonia Humidity 70%–95% RH0.32 counts/ppm 0.31 dB/%RH SMF Tapered POF Glucose 1%–40% vol. 1 ammonia 0%–1% vol. 4.1 ∆A/∆vol. Tapered MMF DS-DNA 5–400 μM 0.0475 ∆A/μM HB fibre UV light 2–4 mW ~0.12 ∆A/mW VOCs 0.03–12.77 Tapered SMF UV light ~0.235 dB/mW mW Temperature −7.8–77 ◦ C 0.134 dB·◦ C−1 Cd2+ ions, DA, ◦ Etched MMF Temperature ssDNA 30–80 C

6 to 20 6 to min

20 min

1 nM–10 mM

Lectin Con A CR Protein 500 pMmg/L

Tilted FBG

-

0.4 s -

nm/ppm 0.330.33 nm/ppm

Etched FBG

D-shaped polymer FBG

~1 nm/ppm

-

[70]

Ref.

1.34–1.38 1.34–1.38

NO2 Etched FBG

~6 pm/ppm

~1 nm/ppm

Response Time

Ref.

RI

Strain and Strain and Etched FBG TemperatureTemperature Etched FBG

130 dB/RIU

Response Time

0.1018

dB- ·◦ C−1

-

[115]

-

[116]

15–40 s

20–30 1–2 s

[127,128]

s

19–25 s -

15–40 s

24–71.8 s -

[127, [145] 128]

[131]

-

0.13% RH/s 24–71.8