Ag Nanorods-Oxide Hybrid Array Substrates: Synthesis, Characterization, and Applications in Surface-Enhanced Raman Scattering.

sensors Review

Ag Nanorods-Oxide Hybrid Array Substrates: Synthesis, Characterization, and Applications in Surface-Enhanced Raman Scattering Lingwei Ma 1 , Jianghao Li 1 , Sumeng Zou 1 and Zhengjun Zhang 2, * 1

2

*

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China; [email protected] (L.M.); [email protected] (J.L.); [email protected] (S.Z.) Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Correspondence: [email protected]; Tel.: +86-10-6279-7033

Received: 12 July 2017; Accepted: 13 August 2017; Published: 17 August 2017

Abstract: Over the last few decades, benefitting from the sufficient sensitivity, high specificity, nondestructive, and rapid detection capability of the surface-enhanced Raman scattering (SERS) technique, numerous nanostructures have been elaborately designed and successfully synthesized as high-performance SERS substrates, which have been extensively exploited for the identification of chemical and biological analytes. Among these, Ag nanorods coated with thin metal oxide layers (AgNRs-oxide hybrid array substrates) featuring many outstanding advantages have been proposed as fascinating SERS substrates, and are of particular research interest. The present review provides a systematic overview towards the representative achievements of AgNRs-oxide hybrid array substrates for SERS applications from diverse perspectives, so as to promote the realization of real-world SERS sensors. First, various fabrication approaches of AgNRs-oxide nanostructures are introduced, which are followed by a discussion on the novel merits of AgNRs-oxide arrays, such as superior SERS sensitivity and reproducibility, high thermal stability, long-term activity in air, corrosion resistivity, and intense chemisorption of target molecules. Next, we present recent advances of AgNRs-oxide substrates in terms of practical applications. Intriguingly, the recyclability, qualitative and quantitative analyses, as well as vapor-phase molecule sensing have been achieved on these nanocomposites. We further discuss the major challenges and prospects of AgNRs-oxide substrates for future SERS developments, aiming to expand the versatility of SERS technique. Keywords: surface-enhanced Raman scattering (SERS); Ag nanorods-oxide hybrid array substrates; oblique angle vapor deposition (OAD); SERS sensitivity; stability; reusability; qualitative and quantitative analyses; vapor-phase molecule sensing

1. Introduction Since the discovery of surface-enhanced Raman scattering (SERS) in the 1970’s [1], this vibration spectroscopic phenomenon has attracted enormous attention both in experimental study [2–4] and theoretical calculation [5–7]. Compared with normal Raman signals, the Raman scattering cross-sections of molecules can be enhanced by many orders of magnitude when they are adsorbed on the rough surfaces of noble metal (Au, Ag, and Cu) nanostructures [8–10]. This remarkable enhancement is aroused from two types of mechanisms, i.e., electromagnetic (EM) and chemical (CM) enhancements. The EM enhancement originates from the strongly amplified electric field at metal surface that is capable of generating localized surface plasmon resonance (LSPR), which depends significantly on the shape, size, and separation of metallic nanostructures [11–13]. The Raman

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signals of molecules in close proximity to the electromagnetic field are dramatically amplified, with an enhancement factor (EF) of ~104 –107 [14–16]. While the CM enhancement is caused by the charge transfer between adsorbed molecules and metal surfaces, and always contributes to an EF of 10–102 [17–19]. Benefiting from the superior sensitivity and specificity, SERS has been widely implemented in the detection and analysis of molecules at extremely low concentrations, which exhibits tremendous opportunity for biological [20,21], chemical [14,22], clinical [23,24], environmental [25,26], and security sensing applications [27,28]. Recently, nanofabrication approaches based on oblique angle vapor deposition (OAD) [29–31] and anodic aluminum oxide (AAO)-templated growth [32–34] have been developed to produce uniform and large-area Ag nanorods (AgNRs) arrays as SERS-active substrates. As the combination of OAD and substrate rotation, the glancing angle deposition (GLAD) [35,36] technique is employed to produce well-designed nanostructures such as vertical nanopillars [37], L-shaped AgNRs [38], zigzag columns [39] and spirals [40,41]. Such nanostructures could promote the generation of “hot spots” that are crucial for SERS enhancement. AgNRs substrates with optimal morphology could generate a SERS EF as high as 109 [42], and have been utilized successfully for the determination of chemical molecules [29,33,37], bacteria [43,44], viruses [45,46], amino acids [47], uranyl ion [48], polychlorinated biphenyls [32,34], and so forth. Nevertheless, the development of the SERS technique requires substrates that can not only provide giant enhancement, but also are robust, stable, and easy and relatively inexpensive to fabricate and store. Unfortunately, AgNRs substrates suffer from some intrinsic drawbacks. First, they have a low melting point of ~100 ◦ C [49,50], which causes their thermal instability and as a result, deteriorates their SERS performance at high-temperature conditions. Meanwhile, the highly active surfaces of AgNRs are prone to oxidize/sulfurate in air [51–53], and are readily corroded by external etchants [54–56], leading to a severe decrease in SERS response. The high costs of SERS detections based on Ag nanostructures also restrict the universality of the SERS technique. To overcome these inevitable limitations, covering AgNR arrays with thin metal oxide layers has been proposed as a strategy to solve the above problems [49,50,56–61]. These oxide materials include Al2 O3 , TiO2 , SiO2 , HfO2 , and so on. Taking advantages of the excellent stability and multi-functions of oxide layers, AgNRs-oxide hybrid array substrates provide superior SERS sensitivity and reproducibility, high thermal stability, long-term activity in air, corrosion resistivity, and intense chemisorption of target molecules. These advantages contribute to recyclable and cost-effective SERS substrates for both qualitative and quantitative analyses. This review introduces the synthesis of AgNRs-oxide arrays as versatile SERS substrates, summarizes their structural, physical, and chemical properties, and highlights their practical applications. We further discuss the major challenges and prospects of AgNRs-oxide substrates for future SERS developments. 2. Fabrication of AgNRs-Oxide Hybrid Array Substrates 2.1. Fabrication of AgNR Arrays AgNR arrays are synthesized based on OAD technique. OAD is a physical vapor deposition technique in which the vapor atoms are deposited at a large incident angle θ (>70◦ ) with respect to the substrate normal [31,37]. The growth of AgNRs is controlled by shadowing effect and surface diffusion, and their morphology can be readily tailored by tuning the deposition conditions such as incident angle, growth time, growth rate, and substrate temperature. Typically, AgNRs were prepared on Si wafers in an electron-beam system at high vacuum level. During deposition, the incident angle of vapor flux was set at ~86◦ off the substrate normal. The deposition rate was fixed at 0.75 nm/s to a desirable thickness read by a quartz crystal microbalance (QCM) [49,57]. As shown in Figure 1A, the resulted AgNRs are of cylindrical shape, uniformly distributed and well-separated. The detailed deposition procedure can be found in previous publications [30,31,37,58].

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Figure 1. 1. (A) (A) Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) image image of of aa pristine pristine AgNRs AgNRs substrate substrate deposited deposited via via Figure ◦ , the deposition rate of 0.75 nm/s to a total thickness of the OAD method at the incident angle of 86 the OAD method at the incident angle of 86°, the deposition rate of 0.75 nm/s to a total thickness of 500 nm; nm; (B) (B) SEM SEM image image of of AgNRs AgNRs coated coated with with an O33 layer layer by by 55 atomic atomic layer layer deposition deposition (ALD) (ALD) 500 an Al Al22O cycles; (C) Transmission electron microscopy (TEM) images of AgNRs coated with Al O layers 3 cycles; (C) Transmission electron microscopy (TEM) images of AgNRs coated with Al2O23 layers byby 1, 1, 2, 3, and 5 ALD cycles, respectively (Reprinted with permission from [49]); (D) TEM images of 2, 3, and 5 ALD cycles, respectively (Reprinted with permission from [49]); (D) TEM images of AgNRs AgNRsand before andcoating after coating with2 SiO for different reaction times (Reprintedwith withpermission permission 2 layers before after with SiO layers for different reaction times (Reprinted from [58]. Copyright (2011) American Chemical Society); (E) TEM image of a single AgNR with TiO TiO22 from [58]. Copyright (2011) American Chemical Society); (E) TEM image of a single AgNR with capping primarily at the nanorod tip (top), and electron diffraction pattern of multiple AgNRs (bottom) capping primarily at the nanorod tip (top), and electron diffraction pattern of multiple AgNRs (Reprinted with permission from [50]. Copyright (2014) AIP Publishing LLC). (bottom) (Reprinted with permission from [50]. Copyright (2014) AIP Publishing LLC).

2.2. Fabrication Fabrication of of AgNRs-Oxide AgNRs-Oxide Hybrid Hybrid Array Array Substrates Substrates with with Different Different Oxide Oxide Layers Layers 2.2. Several approaches approaches have have been been exploited exploited to to produce Because Several produce oxide oxide layers layers over over AgNR AgNR arrays. arrays. Because SERS is is aa highly highly localized localized effect effect that that depends depends significantly significantly on on the the distance distance between between metal metal surfaces surfaces SERS and target molecules [62–64], the oxide shells should be thin enough so as not to eliminate the SERS and target molecules [62–64], the oxide shells should be thin enough so as not to eliminate the SERS enhancement. In atomic layer deposition (ALD)(ALD) holds great in the oxide formation enhancement. Inthis thisregard, regard, atomic layer deposition holdspotential great potential in the oxide of AgNRs-oxide hybrid substrates. ALD is a unique thin film growth technique by means of sequential formation of AgNRs-oxide hybrid substrates. ALD is a unique thin film growth technique by means self-limiting reactions of gaseous precursors It is capable preparing high-quality of sequential surface self-limiting surface reactions of gaseous[65–67]. precursors [65–67]. of It is capable of preparing films with precise thickness control at the atomic scale, excellent conformality independent of the high-quality films with precise thickness control at the atomic scale, excellent conformality substrate geometry, low defect density, and large-scale uniformity. As an example, Ma et al. independent of the substrate geometry, low defect density, and large-scale uniformity. Ashave an deposited Al O over AgNRs using trimethylaluminum (TMA) and water as ALD precursors [49]. 2 3 example, Ma et al. have deposited Al2O3 over AgNRs using trimethylaluminum (TMA) and water as High precursors purity N2 was as theNcarrier and purge gas. Typically, one complete reaction cycle ALD [49].adopted High purity 2 was adopted as the carrier and purge gas. Typically, one

complete reaction cycle consisted of four steps: (1) TMA reactant exposure; (2) N2 gas purging;

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consisted of four steps: (1) TMA reactant exposure; (2) N2 gas purging; (3) water vapor exposure; and (4) N2 gas purging. This reaction cycle was repeated for different times so as to control the Al2 O3 thickness. TiO2 and HfO2 have also been successfully deposited over AgNRs by ALD [57,60], with similar reaction cycles but different precursors and reaction time. It is worth noticing that a low-temperature ALD process at 50 to 80 ◦ C is required to avoid the coarsening and fusion of the underneath AgNR arrays during oxide growth. Besides, based on the hydrolysis reaction of tetraethyl orthosilicate (TEOS), uniform and conformal SiO2 layers have been coated onto AgNRs to form AgNRs-SiO2 core-shell nanostructures [58]. The shell growth made porous SiO2 more compact, and its thickness was tailored by varying the coating time. Moreover, Huang et al. proposed to cover AgNRs with a thin TiO2 layer directly through OAD fabrication [50]. To be specific, after AgNRs deposition, they changed the evaporation material from Ag to TiO2 , and did the deposition again at the same substrate orientation. Because this coating approach only requires the OAD system instead of many complicated apparatuses, it is very efficient and cost-effective and thus promising for real-world fabrication. 3. Characterization of AgNRs-Oxide Hybrid Array Substrates 3.1. Morphology of AgNRs-Oxide Hybrid Array Substrates Figure 1B shows the scanning electron microscope (SEM) image of the AgNRs-oxide substrate. Due to the ultrathin feature of oxide layer, AgNRs-oxide arrays reveal no visible morphology variation compared with the pristine AgNRs. To have a better observation of the oxide shells, high-resolution transmission electron microscope (HRTEM) analysis is employed to provide a visual evidence, which is also applied to investigate oxide thickness growth. As illustrated in Figure 1C,D, the oxide layers fabricated by ALD and hydrolysis reaction are amorphous in structure and of different thickness, uniformly and conformally wrapping AgNRs. When the reaction time is very short, the as-prepared oxide layers possess a few pinholes; as the reaction continues, the shells become thicker and more compact. HRTEM results also demonstrate that the growth thickness increases linearly with the ALD cycle/reaction time [49,58], which is beneficial for us to precisely control the layer thickness to sub-nanometer scale. The thickness of Al2 O3 , TiO2 , and HfO2 shells fabricated by ALD is about 0.6–0.8 nm per ALD cycle [49,57,60], and that of SiO2 grown by hydrolysis reaction is approximately 0.27 nm/min [58]. For TiO2 capping prepared by the OAD method in Figure 1E, it is about 5 nm thick and mainly located on the top surfaces of AgNRs, and has both amorphous and crystalline regions [50]. 3.2. SERS Sensitivity and Reproducibility of AgNRs-Oxide Hybrid Array Substrates It has been long recognized that the SERS enhancement of metal nanostructures depends strongly on the distance between metal surfaces and adsorbed molecules. We therefore investigate the coating effect on the sensitivity of SERS substrates. The results in Figure 2a clearly present that the SERS efficiency of AgNR arrays decreases to ~65% and ~50% after ~0.7 nm (1 ALD cycle) and ~1.5 nm (2 ALD cycles) oxide coating, and declines monotonously with further increasing the oxide thickness [49]. We should note that, due to the ultrathin feature of oxide layers, the strong SERS enhancement of AgNRs is well maintained. All hybrid substrates exhibit satisfactory SERS EFs on the order of 107 [49,57,60], confirming the remarkable sensitivity of AgNRs-oxide nanocomposites. A low relative standard deviation (RSD) value with Raman signals of ~5% [56] (see Figure 2b) indicates that the AgNRs-oxide substrates are uniform in structure and of good reproducibility for SERS measurements, which is also a prerequisite for quantitative analysis.

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−6 Figure 2. (a) Raman spectra of 5of×510 blue (MB) molecules adsorbed the uncoated Figure 2. (a) Raman spectra × 10−6M M methylene methylene blue (MB) molecules adsorbed on theon uncoated AgNRs AgNRs substrate (0 cycle) andand on AgNR arrays withAlAl O layers by 1, 2, 3, and 5 ALD cycles substrate (0 cycle) on AgNR arrayscoated coated with 2O 3 layers by 1, 2, 3, and 5 ALD cycles 2 3 5 M dipicolinic M− dipicolinic acid (Reprinted with permission from [49]); SERSintensity intensity distribution of of 1 ×110×−5 10 (Reprinted with permission from [49]); (b)(b) SERS distribution acid −1 band from 20 randomly −6 M methylene − 1 selected spots over the AgNRs substrate coated with an (DPA) at 1010 cm Figure 2. (a) Raman spectra of 5 × 10 blue (MB) molecules adsorbed on the uncoated (DPA) at 1010 cm band from 20 randomly selected spots over the AgNRs substrate coated with layer by(01 cycle) ALD and cycleon (Reprinted withcoated permission from Al2O3substrate AgNR arrays with Al 2O3 [56]. layersCopyright by 1, 2, 3,(2015) and 5 American ALD cycles an AlAgNRs 2 O3 layer by 1 ALD cycle (Reprinted with permission from [56]. Copyright (2015) American Chemicalwith Society). −5 M dipicolinic acid (Reprinted permission from [49]); (b) SERS intensity distribution of 1 × 10 Chemical Society).

(DPA) at 1010 cm−1 band from 20 randomly selected spots over the AgNRs substrate coated with an 3.3. Thermal Stability of AgNRs-Oxide Hybrid Array Substrates Al2O3 layer by 1 ALD cycle (Reprinted with permission from [56]. Copyright (2015) American 3.3. Thermal Stability of AgNRs-Oxide Hybrid Array Substrates Chemical Society). High-temperature SERS detection is a vital part for routine applications, which can be utilized for monitoring many in situ reactions, such thermal crystallization structural variations [69,70], and High-temperature SERS detection is aasvital part for routine[68], applications, which can be utilized for 3.3.chemical Thermal reactions Stability of[52,71] AgNRs-Oxide Hybrid Array Substrates at elevated temperatures. For pristine AgNR arrays, their structure begins monitoring many in situ reactions, such as thermal crystallization [68], structural variations [69,70], and to change at a very low temperature of 50 °C, and collapses completely at 100 °C [49,50]. It is thereby High-temperature SERS detectiontemperatures. is a vital part for routine applications, which their can bestructure utilized for chemical reactions [52,71] elevated pristine AgNR arrays, begins highly demanded to at improve the thermal stability For of AgNRs-based sensors both in morphology ◦ ◦ monitoring many in situ reactions, such as thermal crystallization [68], structural variations [69,70], and to change at a very low temperature of 50 and collapses completely at°C) 100is much C [49,50]. is thereby robustness and SERS sensitivity. Since the C, melting point of oxides (1700–2700 higherIt than chemical reactions [52,71] at elevated temperatures. For pristine AgNR arrays, their structure begins that of silver to (960improve °C), covering AgNR arrays with oxide layers might besensors effective both to address this highly demanded the thermal stability of AgNRs-based in morphology to change aillustrated very low temperature of 50the °C,AgNRs and collapses completely 100 °CAl It nm) is thereby ◦[49,50]. issue. ItatisSERS in Figure 3a that substrate coated(1700–2700 by at 1-cycle 2O3is (~0.7 are robustness and sensitivity. Since the melting point of oxides C) much higher than highly demanded to improve the thermal stability of AgNRs-based sensors both in morphology robust in morphology at 200 °C, but melt partly at 300 and 400 °C. For AgNRs coated by 2-cycle Al 2O3 ◦ that of silver (960 C), covering AgNR arrays with oxide layers might be effective to address this issue. robustness and SERS Since thechange melting point of oxides (1700–2700 °C)300 is much higher than of ~1.5 nm thick, nosensitivity. obvious structural is observed after being heated at and 400 °C [49]. It is illustrated Figure 3a that the AgNRs substrate coated bymight 1-cycle Al O3 (~0.7 nm) are robust 2 thatThe of morphology silverin(960 °C), covering AgNR arrays with oxide layers be effective to address this robustness also leads to the stabilized SERS performance at elevated temperatures, ◦ ◦ in morphology at 200 C,inbut melt atAgNRs 300arrays and 400 C.with For ~1.6 AgNRs coated 2-cycle Al issue. It is illustrated Figure 3a that substrate coated bynm 1-cycle Oby 3 (~0.7 nm) are2 O3 of demonstrated in Figure 3b–c. Aspartly such,the AgNR coated TiO 2 Al or 2HfO 2 shell also ◦ [49]. ~1.5 robust nmsustain thick, no obvious structural change isat observed after heated atcapping 300 and in morphology at 200 °C, but melt partly 300 and 400 °C. being For Moreover, AgNRs coated by 2-cycle Al 3 their morphology and SERS efficiency at 300–400 °C [57,60]. by the400 top2OC surfaces of AgNRs with high melting-temperature TiO 2 of ~5 nm, the Ag mass transport from tips to of ~1.5 nm thick, no obvious structural change is observed after being heated at 300 and 400 °C [49]. The morphology robustness also leads to the stabilized SERS performance at elevated temperatures, is slowed down. Theyalso preserve well coated at 100 performance °C, while theyat coarsen some extent Thesides morphology robustness leads their to theshapes stabilized SERS elevated temperatures, demonstrated in Figure 3b,c. As such, AgNR arrays with ~1.6 nm TiO2toor HfO 2 shell also at 200 °C [50]. These results suggest that the oxide coating/capping functions as a2 barrier to protect demonstrated in Figure 3b–c. As such, AgNR arrays coated with ~1.6 nm TiO or HfO 2 shell also ◦ sustain their morphology and SERS efficiency at 300–400 C [57,60]. Moreover, by capping the top AgNRs both in morphology and SERSatsensitivity against high temperatures, and covering sustain their morphology and stiffness SERS efficiency 300–400 °C [57,60]. Moreover, by capping the top surfacesthe ofentire AgNRs withofhigh melting-temperature TiO ~5 nm, the Ag mass transport from tips to 2 of these surfaces is especially useful toTiO achieve surfaces of AgNRs withAgNRs high melting-temperature 2 of ~5 nm,goals. the Ag mass transport from tips to

sides is slowed down. They preserve their shapes well at 100 ◦ C, while they coarsen to some extent sides is slowed down. They preserve their shapes well at 100 °C, while they coarsen to some extent ◦ C [50]. These results suggest that the oxide coating/capping functions as a barrier to protect at 200 at 200 °C [50]. These results suggest that the oxide coating/capping functions as a barrier to protect AgNRs both in morphology stiffness and SERS sensitivity andcovering covering the AgNRs both in morphology stiffness and SERS sensitivityagainst against high high temperatures, temperatures, and entirethe surfaces of AgNRs is especially useful to achieve these goals. entire surfaces of AgNRs is especially useful to achieve these goals.

Figure 3. Cont.

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Figure 3. (a) images of of AgNR with Al Al2O 1 and 2 ALD cycles Figure 3. SEM (a) SEM images AgNRarrays arrays coated coated with 3 layers by by 1 and 2 ALD cycles after after 2O 3 layers ◦ C; −MB 6 Madsorbed annealing 300 andand 400400 Raman × −610M AgNRs substrates coated annealing atSEM 300 °C;of Raman spectra 5 5× 10 on AgNRs substrates coated with Figure 3.at(a) images AgNRspectra arraysofofcoated with AlMB 2O3 adsorbed layers by on 1 and 2 ALD cycles after −6 Mcycles, 3 layers by (b) 1 ALD cycle and (c) 2 ALD cycles, before/after annealing at 200, 300, and 400 °C, Al with Al22OO layers by (b) 1 ALD cycle and (c) 2 ALD before/after annealing at 200, 300, and annealing at 300 and 400 °C; Raman spectra of 5 × 10 MB adsorbed on AgNRs substrates coated with 3 ◦ respectively (Reprinted with permission from [49]). O3 respectively layers by (b)(Reprinted 1 ALD cycle andpermission (c) 2 ALD cycles, before/after annealing at 200, 300, and 400 °C, 400Al2C, with from [49]). respectively (Reprinted with permission from [49]). 3.4. Temporal Stability of AgNRs-Oxide Hybrid Array Substrates

3.4. Temporal Stability of AgNRs-Oxide Hybrid Array Substrates 3.4. Temporal Stability of AgNRs-Oxide Array Substrates The effect of oxide coating on theHybrid temporal stability of AgNRs in air is investigated as a function The effect of see oxide coating on the temporal stability of arrays, AgNRsdue in air is investigated as a function of shelf time, details in Figure 4 [49]. For pristine AgNR to the highly active surfaces, The effect of oxide coating on the temporal stability of AgNRs in air is investigated as a function of shelf see details indrastically Figure 4 [49]. pristine AgNR arrays, due tostorage the highly active surfaces, the time, SERS activity drops in air,For which declines by half after 10-day time and is about of shelf time, see details in Figure 4 [49]. For pristine AgNR arrays, due to the highly active surfaces, the SERS activity drops drastically in the air,other which declines half are after 10-day storage is about one order smaller after 50 days. On hand, when by AgNRs uniformly wrappedtime withand oxides, the SERS activity drops drastically in air, which declines by half after 10-day storage time and is about the protective shells50 could suppress surface with air, accordingly the shelf life oxides, is one order smaller after days. On thetheir other hand,reactions when AgNRs areand uniformly wrapped with one order smaller after 50 days. On the other hand, when AgNRs are uniformly wrapped with oxides, dramatically increased. Specifically, the AgNRs substrate coated with ~0.7 nm Al 2 O 3 presents a slight the protective shells could suppress their surface reactions with air, and accordingly the shelf life is the protective shells could suppress their surface reactions with air, and accordingly the shelf life is signal decrease afterSpecifically, 50 days, which can be substrate explained coated by the with pinhole-containing feature of the dramatically increased. the ~0.7nm nmAl Al2O a slight 2O 3 presents dramatically increased. Specifically, theAgNRs AgNRs coated with Al ~0.7 3 presents a slight ultrathin oxide shell. While the substrates coatedsubstrate by ~1.5 nm or thicker 2O3 remain almost constant signal decrease after 50 days, which can be explained by the pinhole-containing feature of the ultrathin signal decrease after 50 days, whichtest can be explained bya the of the in SERS response during the whole period. As a result, thinpinhole-containing but compact coatingfeature layer could oxide shell. While the substrates coated by ~1.5 nm or thicker Al O remain almost constant in SERS 2 3 ultrathin oxidepassivate shell. While the substrates ~1.5 nm their or thicker 2O3 remain almost constant sufficiently the internal Ag NRscoated so as by to stabilize SERSAl activity under atmospheric response during the whole test period. As a result, a thin but compact coating layer could sufficiently in conditions SERS response during the whole period. has As been a result, a thin compact2 substrates coating layer for a long period. Similartest conclusion verified on but AgNRs-TiO [57].could passivate the internal Agthe NRs so as to activity atmospheric for a sufficiently passivate internal Agstabilize NRs so their as to SERS stabilize their under SERS activity underconditions atmospheric long period. Similar conclusion has been verifiedhas onbeen AgNRs-TiO substrates [57]. conditions for a long period. Similar conclusion verified2on AgNRs-TiO 2 substrates [57].

Figure 4. The normalized Raman intensities of 5 × 10−6 M MB Raman peak at 1622 cm−1 on the uncoated AgNRs substrate and on AgNR arrays coated with Al2O3 layers by 1, 2, 3, and 5 ALD cycles, as a function of the measurement time (Reprinted with permission from [49]). −6 M MB Raman peak at 1622 cm−1 on the uncoated Figure Thenormalized normalized Raman of 5of× 510× Figure 4. 4.The Ramanintensities intensities 10−6 M MB Raman peak at 1622 cm−1 on the 3.5.AgNRs Chemical stabilityand of AgNRs-oxide hybrid arraywith substrates substrate on AgNR arrays coated Al2O3 layers by 1, 2, 3, and 5 ALD cycles, as a uncoated AgNRs substrate and on AgNR arrays coated with Al2 O3 layers by 1, 2, 3, and 5 ALD cycles, function of the measurement time (Reprinted with permission from [49]). To function as measurement reliable SERS time sensors, the chemical stability is of essential as a function of the (Reprinted with permission from [49]). importance especially at erosive environments. Because chloride ions [54,72,73], strong oxidants [54,56], and acidic

3.5. Chemical stability of AgNRs-oxide hybrid array substrates 3.5. Chemical Stability of AgNRs-Oxide Hybrid Array Substrates To function as reliable SERS sensors, the chemical stability is of essential importance especially function as reliable SERS sensors, the chemical stability is of essential importance especially at To erosive environments. Because chloride ions [54,72,73], strong oxidants [54,56], and acidic at erosive environments. Because chloride ions [54,72,73], strong oxidants [54,56], and acidic solutions [61,74]

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canatoms severely etch silver atoms and cause the shape transformation of Ag 5a cansolutions severely [61,74] etch silver and cause the shape transformation of Ag nanostructures (see Figure Figureto5aimprove as an example), it is crucial to improve theagainst chemical stability We of AgNRs as nanostructures an example), it (see is crucial the chemical stability of AgNRs corrosion. find that against corrosion. We find that after Al 2 O 3 deposition, the chemical stability of coated substrates is if after Al2 O3 deposition, the chemical stability of coated substrates is substantially improved, even substantially improved, even if the oxide shell is sub-nanometer and has a few pinholes [56]. As the oxide shell is sub-nanometer and has a few pinholes [56]. As shown in Figure 5b–d, the chemically shown Figure 5b–d, the chemically inert Al2O3 shell prevents the internal AgNRs from direct inert Al2 Oin 3 shell prevents the internal AgNRs from direct contact with external etchants such as NaCl contact with external etchants their such as NaCl and H 2O2, retaining sufficiently their morphology and and H2 O2 , retaining sufficiently morphology and SERS efficiency. Additionally, for strong acidic SERS efficiency. Additionally, for strong acidic media where Al2O3 will dissolve quickly, ultrathin media where Al2 O3 will dissolve quickly, ultrathin HfO2 film has been implemented to protect AgNRs HfO2 film has been implemented to protect AgNRs from failure [61]. This kind of substrate possesses from failure [61]. This kind of substrate possesses the acid-resistant property, making it applicable in the acid-resistant property, making it applicable in acid solutions of practical environments. acid solutions of practical environments.

Figure 5. 5. SEM images substratecoated coatedwith with1-cycle 1-cycleAlAl 23O3 Figure SEM imagesofof(a) (a)pristine pristineAgNRs AgNRsand and (b) (b) the the AgNRs AgNRs substrate 2O after being merged (c) reflectance reflectancespectra spectravariations variations the AgNRs after being mergedinina a3030mM mMNaCl NaClsolution solution for for 33 h; h; (c) ofof the AgNRs substrate coated with 1-cycle Al O within a 12 h NaCl erosion time; (d) SERS performance of AgNRs 2 3 substrate coated with 1-cycle Al2O3 within a 12 h NaCl erosion time; (d) SERS performance of AgNRs coated with 1-cycle Al O before/after NaCl (30 mM, 3 h) and H O (2.2%, 0.5 h) immersion, using 2 3 2 2 coated with 1-cycle Al2O3 before/after NaCl (30 mM, 3 h) and H2O2 (2.2%, 0.5 h) immersion, using −6−6M 4-aminothiophenol (4-ATP) as probing molecules (Reprinted with permission from [56]. 1 ×1 10 × 10 M 4-aminothiophenol (4-ATP) as probing molecules (Reprinted with permission from [56]. Copyright (2015) American Copyright (2015) AmericanChemical ChemicalSociety). Society).

4. Applications AgNRs-OxideHybrid Hybrid Array Array Substrates Substrates 4. Applications ofofAgNRs-Oxide Reusable SERSSubstrates Substrates 4.1.4.1. Reusable SERS Given that SERSsubstrates substratesare aregenerally generally made made of reused, thethe Given that SERS of noble noblemetals metalsand andare arenot notreadily readily reused, costly preparationand and disposable disposable property hinder the the universality of SERS technique. For costly preparation propertyseriously seriously hinder universality of SERS technique. this reason, it is significant to develop recyclable SERS substrates. One feasible way is the direct For this reason, it is significant to develop recyclable SERS substrates. One feasible way is the direct degradation of adsorbed molecules from substrate surfaces SERS identification. For AgNR degradation of adsorbed molecules from substrate surfaces afterafter SERS identification. For AgNR arrays arrays coated with photocatalytic material TiO 2 [75–77], the self-cleaning ability is realized through coated with photocatalytic material TiO2 [75–77], the self-cleaning ability is realized through ultraviolet ultraviolet (UV) light-induced decomposition of organic molecules adsorbed on the substrate, i.e., (UV) light-induced decomposition of organic molecules adsorbed on the substrate, i.e., subsequent to subsequent to SERS measurements, the substrate can be purified by UV illumination and be reused SERS measurements, the substrate can be purified by UV illumination and be reused for further SERS for further SERS analysis [57]. As revealed in Figure 6a, four “detection-UV cleaning” circulations are analysis [57]. As revealed in Figure 6a, four “detection-UV cleaning” circulations are carried out on carried out on the AgNRs substrate coated with ~2 nm TiO2. Strong target signals are observed during the AgNRs substrate coated with ~2 nm TiO2 . Strong target signals are observed during SERS sensing,

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Sensorsalmost 2017, 17,vanish 1985 8 of 17three and they after UV irradiation. More importantly, the results from the subsequent circulations show that the Raman intensities are fully recovered at each detection step, which suggests SERS sensing, and they almost vanish after UV irradiation. More importantly, the results from the that the AgNRs-TiO2 structure is capable of enduring multiple UV irradiations with considerable subsequent three circulations show that the Raman intensities are fully recovered at each detection robustness. Another point note is that the molecule degradation capability not only comes from step, which suggests thatto the AgNRs-TiO 2 structure is capable of enduring multiple UV irradiations the photocatalysis of TiO , it also benefits from between Ag and TiO 2 with considerable robustness. Another point to the noteintensive is that the interaction molecule degradation capability not2 that optimizes the separation of photo-excited carriersfrom andthe as intensive a result facilitates degradation only comes from the photocatalysis of TiO2charge , it also benefits interactionthe between Ag efficiency (see Figure 6b) [78–80]. and TiO 2 that optimizes the separation of photo-excited charge carriers and as a result facilitates the degradation efficiencyis(see Figureway 6b) [78–80]. Thermal annealing another to detach molecules from adsorbed surfaces, and accordingly Thermal annealing is another way toSERS detachsubstrates. molecules from adsorbed surfaces, and accordingly it might be helpful to clean and regenerate By virtue of the high melting-temperature it might be helpful to clean and regenerate SERS substrates. By virtue of the high melting-temperature HfO2 [81], the AgNRs substrate coated by ~1.6 nm HfO2 represent good thermal stability and HfO2 [81], the AgNRs substrate coated by ~1.6 nm HfO2 represent good thermal stability and morphological robustness at temperatures up to 400 ◦ C [60]. After SERS detection, the regeneration morphological robustness at temperatures up to 400 °C [60]. After SERS detection, the regeneration of AgNRs-HfO2 can be achieved by heating the substrate on a hot plate within several seconds. of AgNRs-HfO2 can be achieved by heating the substrate on a hot plate within several seconds. This This process processleads leads to thermal the thermal of adsorbed and the substrate for to the releaserelease of adsorbed moleculesmolecules and refreshes the refreshes substrate for subsequent subsequent measurements. From Figure 6c,d,that onethe sees that substrate the hybrid substrate maintains its SERS measurements. From Figure 6c–d, one sees hybrid maintains its SERS efficiency efficiency well during 30 “detection-heating” cycles, demonstrating the remarkable stability well during 30 “detection-heating” cycles, demonstrating the remarkable stability and recyclability and recyclability of AgNRs-HfO substrate. As a consequence, reusability eliminate the single-use of AgNRs-HfO 2 substrate.2 As a consequence, reusability could eliminatecould the single-use shortcoming of conventional SERS substrates, in which high for SERS measurements are are shortcoming of conventional SERS substrates, in way whichthe way the costs high costs for SERS measurements substantially reduced practicabilityof ofthe the SERS SERS technique substantially reduced andand thethe practicability techniqueisisextended. extended.

−−66 M Figure (a) Raman spectra 5 ×1010 onon thethe AgNRs substrate coated with with Figure 6. (a)6.Raman spectra of 5of× M MB MBmolecules moleculesadsorbed adsorbed AgNRs substrate coated 2 in four “detection-UV cleaning” cycles; (b) the schematic for the photocatalytic mechanism ~2 nm TiO ~2 nm TiO2 in four “detection-UV cleaning” cycles; (b) the schematic for the photocatalytic mechanism of AgNRs-TiO2 hybrids (Reprinted with permission from [57]); (c) Raman spectra of 1 × 10−6 M MB −6on of AgNRs-TiO M MB 2 hybrids (Reprinted with permission from [57]); (c) Raman spectra of 1 × 10 the AgNRs substrate coated with ~1.6 nm HfO2 measured in multiple “detection-heating” cycles and on the AgNRs substrate coated with ~1.6 nm HfO measured in multiple “detection-heating” cycles 2 (d) the 1623 cm−1 peak intensity variations in 30 cycles (Reprinted with permission from [60]. − 1 and (d) the 1623 cm peak intensity variations in 30 cycles (Reprinted with permission from [60]. Copyright (2016) American Chemical Society). Copyright (2016) American Chemical Society).

4.2. Qualitative and Quantitative SERS Analyses

4.2. Qualitative and Quantitative SERS Analyses

To make SERS technique a practical and reliable analysis tool, both qualitative and quantitative

To makeare SERS technique a practical reliableIt analysis boththat qualitative abilities required for desirable SERSand substrates. has been tool, reported Al2O3 has and highquantitative affinity abilities are required for desirable SERS substrates. It has been reported that Al2 O3 has high affinity to carboxyl (–COOH) functional groups, ascribed to the strong polar interaction [82,83]. Therefore,

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to carboxyl (–COOH) functional groups, ascribed to the strong polar interaction [82,83]. Therefore, we adopt AgNRs-Al2O3 substrates to enhance the absorbability and correspondingly SERS detection efficiency of carboxylic acids. Dipicolinic acid (DPA) is a commonly used biomarker for the we adopt AgNRs-Al to enhance the absorbability and correspondingly SERS detection 2 O3 substrates recognition of bacterial spores [83,84],acid so its sensitive and quantitative probing is particularly efficiency of carboxylic acids. Dipicolinic (DPA) is a commonly used biomarker for the recognition important. 7a shows thesensitive Raman and spectra of DPA by employing AgNRs important. coated with 1-cycle of bacterial Figure spores [83,84], so its quantitative probing is particularly Figure 7a Al 2O3 as SERSspectra platform, withby theemploying limit of detection (LOD) with down to 10-8Al M. Meanwhile, the partial shows thethe Raman of DPA AgNRs coated 1-cycle O as the SERS platform, 2 3 least the squares (PLSR) Figure 7b exhibits good predictability the with limitregression of detection (LOD)[85,86] downmodel to 10−8inM. Meanwhile, the partial least squares within regression −8 −5 concentration 1 ×7b 10exhibits to 1 × 10 M,predictability which provides a calibration for the quantification (PLSR) [85,86] ranging model infrom Figure good within the concentration ranging from −5 M, which In10 addition to analytes thata can directlyfor react Al2O3 surfaces, forDPA. thoseInwho have 1of×trace 10−8DPA. to 1 × provides calibration thewith quantification of trace addition no or weakthat interaction withreact oxides pyridine, and cyanide the pinholeto analytes can directly with(such Al2 Oas foracridine, those who have no or [87–89]), weak interaction with 3 surfaces, containing AgNRs-Al 2O3 acridine, arrays provide a channel to anchor them directly ontoAgNRs-Al Ag surfaces oxides (such as pyridine, and cyanide [87–89]), the pinhole-containing 2 Othrough 3 arrays Al2O3 pinholes. Fortoinstance, the LOD of NaCN onsurfaces the pinhole-containing AgNRs-Al 2O3instance, substratethe is provide a channel anchor them directly onto Ag through Al2 O3 pinholes. For as lowofas 1 ppb, analysisAgNRs-Al also reveals satisfactory predictability [56]. These LOD NaCN onand the quantitative pinhole-containing is asPLSR low as 1 ppb, and quantitative 2 O3 asubstrate results that AgNRs-oxide are appropriate to detect a variety of that molecules in both analysissuggest also reveals a satisfactory substrates PLSR predictability [56]. These results suggest AgNRs-oxide qualitative and quantitative substrates are appropriate to manners. detect a variety of molecules in both qualitative and quantitative manners.

Figure 7. (a) (a) SERS SERSspectra spectraofofdipicolinic dipicolinic acid (DPA) on the AgNRs substrate coated with 2O 3 Figure 7. acid (DPA) on the AgNRs substrate coated with an Al2an O3Al layer −4 to 1 × 10 −8 M; (b) The concentration dependence − 4 − 8 layer by 1 ALD cycle, with concentrations from 1 × 10 by 1 ALD cycle, with concentrations from 1 × 10 to 1 × 10 M; (b) The concentration dependence −1asasa afunction −41to of DPA peak intensity at 1010 cm 10−4 × cm−1 functionofofDPA DPAconcentrations concentrationsranging rangingfrom from1 1× × 10to −8 − 8 M. The illustrates the actual DPADPA concentrations versus theirtheir predicted values between 1× 110× 10 M.inset The inset illustrates the actual concentrations versus predicted values between −5 to 1 10−81 M partialthe least squares (PLSR) model (Reprinted with permission 110× 10−5× to × with 10−8 the M with partial leastregression squares regression (PLSR) model (Reprinted with from [56]. Copyright American permission from [56].(2015) Copyright (2015)Chemical AmericanSociety). Chemical Society).

From a more realistic perspective, AgNRs-oxide substrates have been adopted to sense food From a more realistic perspective, AgNRs-oxide substrates have been adopted to sense food antiseptics [61]. It is known that food antiseptics with appropriate amount could inhibit bacteria and antiseptics [61]. It is known that food antiseptics with appropriate amount could inhibit bacteria extend food’s shelf life; whereas excessive addition might be harmful for human health [90,91]. and extend food’s shelf life; whereas excessive addition might be harmful for human health [90,91]. Hence, quantitative analysis of antiseptics based on SERS is of great value. Potassium sorbate (PS) Hence, quantitative analysis of antiseptics based on SERS is of great value. Potassium sorbate (PS) and and sodium benzoate (SB) are common food antiseptics. Because they only work in acidic sodium benzoate (SB) are common food antiseptics. Because they only work in acidic media [92,93], media [92,93], the AgNRs-HfO2 substrate with acid resistance is applicable for their identification [61]. the AgNRs-HfO2 substrate with acid resistance is applicable for their identification [61]. The LODs of The LODs of these two antiseptics are both 300 μg/L, which are much lower than their dosage these two antiseptics are both 300 µg/L, which are much lower than their dosage standard in food. standard in food. Therefore, the SERS substrate meets the demand of identifying PS and SB in Therefore, the SERS substrate meets the demand of identifying PS and SB in practice. Moreover, the practice. Moreover, the PLSR relationship between the concentrations and SERS spectra of a series of PLSR relationship between the concentrations and SERS spectra of a series of PS solutions performs PS solutions performs quite well. As for the mixture of PS and SB, even if some of their characteristic quite well. As for the mixture of PS and SB, even if some of their characteristic peaks overlap, the peaks overlap, the two respective PLSR models of them are both accurate and reliable (see Figure 8). two respective PLSR models of them are both accurate and reliable (see Figure 8). That is to say, the That is to say, the Raman peaks corresponding to SB in spectra do not interfere the quantification of Raman peaks corresponding to SB in spectra do not interfere the quantification of PS molecules, and PS molecules, and vice versa. vice versa.

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Figure 8. (a) The black, blue, and red lines represent the Raman spectra of PS solution, mixture Figure (a) The black, and red lines represent Ramanwith spectra PS solution, mixture solution8.containing PS andblue, SB molecules, and SB solution the measured SERSofsubstrates, respectively; solution containing PS andmark SB the molecules, and peaks SB solution measured with SERS to substrates, the pink and blue rectangles characteristic of PS and SB that are employed calibrate respectively; the pink and blue rectangles mark the characteristic peaks of PS and SB are PLSR model; (b) PS concentration predicted by the PLSR model established with Ramanthat spectra employed to calibrate PLSR model; (b) PS concentration predicted by the PLSR model established corresponding to PS characteristic peaks; (c) difference between predicted PS concentration of each with Raman spectra corresponding to PS characteristic peaks; (c)the difference predicted PS mixture and average predicted PS concentration of solution with same PSbetween concentration; (d) PS concentration mixture and with average predicted PS concentration of solution with model the same PS concentration of in each the test solution different compositions predicted by the PLSR of PS concentration; (d) PS concentration in the test solution with different compositions predicted by the (Reprinted with permission from [61]). PLSR model of PS (Reprinted with permission from [61]).

4.3. Vapor-Phase Molecule Sensing 4.3. Vapor-Phase Molecule Sensing Aside from revealing the feasibility of SERS determination in liquids, AgNRs-oxide arrays also Aside from revealing the feasibility of SERS determination in liquids, AgNRs-oxide arrays also provide real-time monitoring of vapor-phase molecules at ultralow concentrations [60]. To capture provide real-time monitoring of vapor-phase molecules at ultralow concentrations [60]. To capture and sense target gases, the AgNRs-HfO2 substrate is placed in a homemade gas detection system and sense target gases, the AgNRs-HfO2 substrate is placed in a homemade gas detection system presented in Figure 9a. 2-naphthalenethiol (2-NAT) is selected as the model gas, and high purity presented in Figure 9a. 2-naphthalenethiol (2-NAT) is selected as the model gas, and high purity N2 N2 is utilized as the carrier gas. During gas detection, N2 is injected into the analyte solution, and is utilized as the carrier gas. During gas detection, N2 is injected into the analyte solution, and target target molecules in the vapor phase are carried out together with the N2 flow and are captured by the molecules in the vapor phase are carried out together with the N2 flow and are captured by the AgNRs-HfO2 platform. SERS spectra are recorded simultaneously during gas passing. It is shown in AgNRs-HfO2 platform. SERS spectra are recorded simultaneously during gas passing. It is shown in Figure 9b that the 2-NAT signals ascend continuously with the gas flowing until the saturation of the Figure 9b that the 2-NAT signals ascend continuously with the gas flowing until the saturation of the substrate surface. The LOD of 2-NAT is down to 20 ppb, verifying the effectiveness of AgNRs-HfO2 substrate surface. The LOD of 2-NAT is down to 20 ppb, verifying the effectiveness of AgNRs-HfO2 substrate for gas detection. Furthermore, we explore the renewability of AgNRs-HfO2 arrays during substrate for gas detection. Furthermore, we explore the renewability of AgNRs-HfO2 arrays during gas recognition, i.e., after SERS measurement, the substrate is heated on a hot plate at 250 ◦ C for 30 s gas recognition, i.e., after SERS measurement, the substrate is heated on a hot plate at 250 °C for 30 s to desorb molecules. The results in Figure 9c reveal that the Raman intensities of 600 ppb 2-NAT to desorb molecules. The results in Figure 9c reveal that the Raman intensities of 600 ppb 2-NAT at at 1379 cm−1 peak escalate with vapor passing and after each annealing treatment, the substrate −1 1379 cm peak escalate with vapor passing and after each annealing treatment, the substrate is free is free of 2-NAT while it is totally recovered in the following “vapor exposure-thermal cleaning” of 2-NAT while it is totally recovered in the following “vapor exposure-thermal cleaning” cycles. We cycles. We believe this highly robust and versatile SERS platform could act as a recyclable sensor believe this highly robust and versatile SERS platform could act as a recyclable sensor for in situ for in situ monitoring of complex gases from realistic environments, such as air pollutants [94,95], monitoring of complex gases from realistic environments, such as air pollutants [94,95], explosives [96,97], explosives [96,97], volatile organic compounds [98,99], and chemical warfare agents [100,101]. volatile organic compounds [98,99], and chemical warfare agents [100,101].

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1 peak intensity −1 − Figure 9. 9. (a) (a)Schematic Schematicof ofthe thegas gassensing sensingdevice; device;(b) (b)SERS SERSspectra spectraand andthe the1379 1379 cm Figure cm peak intensity of of 600 ppb 2-NAT theAgNRs-HfO AgNRs-HfO substrate as a function of gas flow time. 1379 cm peak 2 substrate as a function of gas flow time. (c) cm−−11 peak 600 ppb 2-NAT onon the 2 intensity variations of 600 ppb 2-NAT during the repetition of “vapor exposure-thermal cleaning” cycles intensity variations of 600 ppb 2-NAT during the repetition of “vapor exposure-thermal cleaning” on theon substrate (Reprinted with permission from [60]. Copyright (2016) American ChemicalChemical Society). cycles the substrate (Reprinted with permission from [60]. Copyright (2016) American

Society).

5. Conclusions 5. Conclusions This review provides insights for the synthesis, characterization, and applications of AgNRs-oxide hybrid array substrates as SERS sensing platforms, allcharacterization, in order to emphasize our understanding and This review provides insights for the synthesis, and applications of AgNRsutilization ofarray the SERS technique. AgNR arrays are prepared basedtoonemphasize the OAD our method, and oxide oxide hybrid substrates as SERS sensing platforms, all in order understanding shells are readily ontotechnique. AgNRs byAgNR diverse approaches with controllable thickness and excellent and utilization ofcoated the SERS arrays are prepared based on the OAD method, and uniformity. characterization of these nanocomposites is detailed with in aspects of shellthickness growth, SERS oxide shells The are readily coated onto AgNRs by diverse approaches controllable and sensitivity, and reproducibility, as well as thermal, and chemical stability. By virtue the excellent uniformity. The characterization of thesetemporal, nanocomposites is detailed in aspects of of shell ultrathinSERS thickness, uniformity, and stability ofasoxide AgNRs-oxide possess large growth, sensitivity, and reproducibility, well layers, as thermal, temporal,substrates and chemical stability. SERS EFs,of outstanding reproducibility, and excellent the reusability, quantification, By virtue the ultrathin thickness, uniformity, and stability. stability In of addition, oxide layers, AgNRs-oxide substrates and gas sensing have been on the substrates, which great potential forIn theaddition, identification possess large SERS EFs, achieved outstanding reproducibility, andhold excellent stability. the of trace analytes in real systems. reusability, quantification, and gas sensing have been achieved on the substrates, which hold great Although substrates proved their extraordinary advantages, they are potential for theAgNRs-oxide identificationarray of trace analyteshave in real systems. still in their infancy for practical applications, and several challenges areadvantages, yet to be addressed. Although AgNRs-oxide arraySERS substrates have proved their extraordinary they are First, hybrid arrays possess satisfactory and reliability exceeding the still inAgNRs-oxide their infancy for practical SERS applications, andreproducibility several challenges are yet to be addressed. performance of commonly which are crucial for biosensing. The qualitative First, AgNRs-oxide hybrid used arraysnanoparticles, possess satisfactory reproducibility and reliability exceeding and the performance of commonly used which are crucial for biosensing. The qualitative and quantitative detection of DPA, an nanoparticles, anthrax biomarker, has been achieved on the AgNRs-Al O substrate. 2 3 quantitative detection of DPA, an anthrax biomarker, has beenAgNRs-oxide achieved on arrays the AgNRs-Al 2O3 As for the SERS-based immunoassay and nucleic acid detection, offer potent substrate. As for theto SERS-based and nucleic acid detection, AgNRs-oxide arrays offer capture substrates bind SERSimmunoassay nanotags. The close proximity of the substrate–nanotag interface potent capture substrates to bind SERS nanotags. The close proximity of the substrate–nanotag

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provides additional plasmonic coupling that further increases the SERS enhancement. It is thus urgent to demonstrate the SERS determination of bacteria, viruses, nucleic acids, and proteins on AgNRs-oxide arrays before the substrates can be routinely applied. Second, surface functionalization [102–104] should be realized on AgNRs-oxide substrates to further boost their widespread employments. Third, because different oxide materials possess various unique features according to their chemical or physical properties, further exploration of the diverse functions of AgNRs-oxide nanocomposites is still in great demand, which is essential for developing multifunctional SERS sensors in certain conditions. Moreover, aside from oxides, combining AgNR arrays with other attractive materials, e.g., graphene [105–107] and MoS2 [108,109], is another advisable method for future SERS developments. Overall, along with the growing study, deeper understanding, and better optimization of AgNRs-oxide nanostructures, these substrates hold great promise as affordable and portable SERS sensors, and would open up a new era for practical SERS applications. Acknowledgments: The authors are very grateful to the financial support by the National Basic Research Program of China (973 Program, Grant No. 2013CB934301), the National Natural Science Foundation of China (Grant No. 51531006 and No. 51572148), the Research Project of Chinese Ministry of Education (Grant No. 113007A), and the Tsinghua University Initiative Scientific Research Program. Author Contributions: Lingwei Ma and Zhengjun Zhang wrote the manuscript, performed the works and discussions; Jianghao Li and Sumeng Zou helped in preparation and modification of the manuscript. Conflicts of Interest: The authors declare no conflicts of interest.

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