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Chemically modified flexible strips as electrochemical biosensors† Raju Thota and V. Ganesh* A flexible and disposable strip sensor for non-enzymatic glucose detection is demonstrated in this work. The strips are prepared by using chemical modification processes followed by a simple electroless deposition of copper. Essentially, polyester overhead projector (OHP) transparent films are modified with a monolayer of 3-aminopropyltrimethoxysilane (APTMS) and polyaniline (PANI) conducting polymer. Later, nanostructured copper is deposited onto this modified film. Scanning electron microscope (SEM) and X-ray diffraction (XRD) studies are used for the structural, morphological and crystallinity characterization of the modified films. Electrochemical techniques, namely cyclic voltammetry (CV) and chronoamperometry (CA), are employed for the non-enzymatic detection of glucose. These studies clearly reveal the formation of homogeneous, close-packed spherical Cu particles converged into uniform film that exhibits a good catalytic activity towards the oxidation of glucose. The Cu/PANI/ APTMS/OHP sensor displays a remarkable enhancement in the oxidation current density, a very high sensitivity value of 2.8456 mA cm2 per mM, and a linear concentration range from 100 mM to 6.5 mM associated with glucose detection. Detection limit is estimated to be 5 mM and the response time of the sensor is determined to be less than 5 s. For comparison, similar studies are performed without PANI, namely Cu/APTMS/OHP films for glucose detection. In this case, a sensitivity value of 2.4457 mA cm2 per mM and a linear concentration range of 100 mM–3 mM are estimated. The higher performance

Received 10th April 2014 Accepted 11th June 2014

characteristics observed in the case of Cu/PANI/APTMS/OHP are attributed to the synergistic effects of the conducting polymer acting as an electron facilitator and the nanostructured Cu films. These

DOI: 10.1039/c4an00646a

disposable, flexible and low-cost strip sensors have also been applied to the detection of glucose in

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clinical blood serum samples and the results obtained agree very well with the actual glucose level.

1. Introduction Ever-increasing demands for home-based systems for regular monitoring of blood and urine impel researchers to design and develop alternative biosensor materials. Diabetes, for instance, has become a worldwide public health problem. Diagnosis and management of diabetes require periodic monitoring of blood glucose levels.1–3 Glucose detection is also very important in the food processing industry, clinical diagnostics, bio-fuel cells and biotechnology.4–9 Generally, electrochemical biosensors such as amperometric glucose sensors are typically used in monitoring processes.2,10,11 These sensors usually employ an enzyme called glucose oxidase (GOx) as a crucial constituent in order to catalyze the oxidation of glucose to gluconolactone and produce hydrogen peroxide (H2O2) as a by-product.11–13 Concentration of glucose is indirectly estimated with the help of electrochemical responses associated with H2O2 concentration. These enzymeElectrodics and Electrocatalysis (EEC) Division, Council of Scientic and Industrial Research–Central Electrochemical Research Institute (CSIR–CECRI), Karaikudi, 630006, Tamilnadu, India. E-mail: [email protected]; [email protected] † Electronic supplementary 10.1039/c4an00646a

information

(ESI)

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available.

See

DOI:

modied electrodes generally demonstrate good selectivity and high sensitivity.14–17 However, due to the intrinsic characteristics of the enzymes, the activity of GOx can easily be affected by temperature, humidity and pH values. Moreover, enzyme-based sensors also suffer from unsatisfactory reproducibility, complicated immobilization processes, stability, high cost and loss of enzymatic activity.18–20 In order to solve these problems, non-enzymatic detection of glucose is proposed. Most enzymeless sensors are essentially based on usage of either nanomaterials or conducting polymers. In general nanomaterials possess conspicuous features and characteristics in terms of physical and chemical properties when compared to their bulk materials.21–23 An increasingly important strategy in recent years is to employ these nanomaterials in the fabrication of non-enzymatic glucose detection systems. In addition, owing to their unique electroactive properties and ease of preparation, conducting polymers have also attracted a great deal of attention in the construction of non-enzymatic glucose sensors.24–26 Conducting polymers also act as excellent substrate for the immobilization of biomolecules and are involved in rapid electron transfer reactions.27,28 Among various conducting polymers studied recently, polyaniline (PANI) is an attractive

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and important conducting polymer due to its ease of preparation, high electrical conductivity, good environmental stability and ability to act as a mediator for electron transfer reactions, thereby amplifying the signal-to-noise ratio.29,30 In the case of nanomaterials, most reports on non-enzymatic glucose detection are primarily based on Cu, Ni, Au, Pt and their oxides.5,14,24,31–40 In addition nanoparticles dispersed carbon nanotubes (CNTs) were also employed as electrode materials.18,19,41,42 Coating technologies, including electrodeposition, electroless deposition, inkjet printing, screen printing and vapour deposition methods, are widely employed for the preparation of nanostructured metal coatings on conductive solid supports.43–45 Adhesion of these metal coatings on the substrates can be enhanced via gas plasma treatment,46–48 chemical etching,49 ion beam bombardment50,51 and surface modication with gra polymerization.52–54 All these reports are essentially based on solid metal electrodes or metals/semiconductor-deposited glass electrodes, and they have been employed for glucose sensing, either as such or upon appropriate functionalization. On the other hand, exible and nonconductive substrates are rarely used. Sensing platforms based on exible plastic substrates represent the next phase of technological advancement, and the principal challenges reside in reducing the weight and cost of such devices. According to recent World Health Organization (WHO) reports, more than 200 million people globally suffer from diabetes.55 Thus the development of improved, low-cost, technologically advanced glucose-sensing devices embedded in plastic or fabric would denitely have a huge benecial impact on future generations. Smart (exible) fabrics could also be used as sensors for

Scheme 1

personalized health care and for monitoring human sweat and motion. Development of smart bandages using exible substrates is another emerging area in the biosensor eld. These exible substrates are not only used in biosensors but also in solar cells, supercapacitors and batteries, among other applications. In the current work, a non-conductive, exible polyester OHP lm is explored for non-enzymatic glucose detection aer appropriate functionalization using a conducting polymer and electroless deposition of copper. Flexible and non-conductive polyester OHP transparent lms were chemically modied with a self-assembled monolayer (SAM) of 3-aminopropyltrimethoxy silane (APTMS) followed by a gra oxidative polymerization of aniline leading to the formation of PANI (Scheme 1). Next, electroless deposition of copper was performed on such lms and this assembly was explored as a exible electrode material for non-enzymatic glucose sensing by monitoring the direct oxidation of glucose using electrochemical techniques. Electroless deposition is usually a less costly technique and requires no complex instrumentation. Structural and morphological characterizations were performed via SEM and XRD analyses. For comparison similar experiments were also carried out using modied lms without PANI in order to investigate the role of conducting polymers in the sensing process. Copper is a cheap metal and its nanostructured assembly can easily be controlled by electroless deposition on exible substrates that could be used as a disposable sensor. This particular exible electrochemical sensor was tested in a blood sample analysis and in selectivity studies for glucose detection.

Pictorial representation of methodology followed in this work to fabricate copper-coated flexible polyester OHP strips.

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2.

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Experimental section

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2.1. Materials Commercially available polyester OHP lms of 100 mm thickness were purchased from Desmat, India. Aniline (99.5% purity), 3-aminopropyltrimethoxysilane (APTMS) (99% pure), hydrazine (98% purity), stannous chloride (98% purity), palladium chloride (98% pure), CuSO4$5H2O (99% purity), D-(+) glucose of 99% purity, dopamine hydrochloride, uric acid and diethyl ether of 99% purity were procured form Sigma Aldrich. LAscorbic acid was purchased from Sisco Research Laboratories (SRL, Mumbai, India) and ethanol was received from Otto Inc. Mumbai, India. Acids namely HCl and H2SO4 were purchased from Fisher Scientic. NaOH and H2O2 were obtained from Merck. All these chemicals were used as such without further purication unless otherwise mentioned. 2.2. Preparation of chemically modied OHP lms Polyester OHP lms (1 cm  5 cm) were ultrasonically cleaned in acetone and ethanol for about 5–10 min sequentially followed by sonication in millipore water and nally dried with N2 gas. Further the lms were dipped into a freshly prepared piranha solution (H2SO4 and H2O2 in 3 : 1 v/v ratio) for about 2– 3 min. Aer that the lms were cleaned with plenty of water followed by ultrasonication in an ethanol and water mixture for about 20 min and dried with N2 gas. Then the lms were cut into small pieces of pre-dened geometric area (1 cm  1 cm) and used as strips for further experiments and characterization. Subsequently the pre-cleaned lms were immersed in a 2% APTMS solution in an ethanol and water (95 : 5 v/v) mixture for about 20 h. Then the modied lms were removed, washed with ethanol and water, and cured under vacuum at 30  C for 24 h. Furthermore, oxidative co-polymerization of aniline on APTMS/ OHP strips was carried out by immersing these lms into an aqueous solution of 0.5 M H2SO4 consisting of 0.2 M aniline and 0.2 M (NH4)2S2O8 at a reaction temperature between 0  C and 5  C along with stirring for about 5 h. Formation of a greencoloured lm on those strips (Fig. 1b) indicates that polyaniline (PANI) in emeraldine salt form was obtained as a result of graing, and these strips are called PANI/APTMS/OHP. Aer the reaction, the lm was washed thrice with plenty of water,

Photographic images of (a) bare OHP films without surface modification, (b) PANI/APTMS/OHP, and (c) Cu/PANI/APTMS/OHP films, respectively.

Fig. 1

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methanol and diethyl ether; and nally dried under vacuum at 30  C for 24 h. Modications at each and every step were analyzed using FTIR spectroscopy. 2.3. Electroless deposition of copper on chemically modied OHP strips Both APTMS/OHP and PANI/APTMS/OHP lms were subsequently modied with copper by using the electroless deposition technique. Before that, PANI/APTMS/OHP strips were soaked in 1 M H2SO4 for 1 h in order to obtain a protonated PANI layer which was reduced to the leucoemeraldine base form of PANI by immersion in hydrazine for about 1 h. Initially PANI/ APTMS/OHP strips were sensitized by soaking in 2.5 g L1 of SnCl2 dissolved in 9.25 g L1 HCl solution for 1–2 min and then activated by immersing in 1 g L1 PdCl2 consisting of 3.7 g L1 HCl aqueous solution for 3–5 min. Then the strips were removed and thoroughly rinsed with millipore water. Finally, electroless deposition of Cu was carried out on these strips by dipping them in an electroless Cu aqueous bath containing 7 g L1 CuSO4$5H2O, 25 g L1 potassium sodium tartrate, 4.5 g L1 sodium hydroxide and 9.25 g L1 formaldehyde. Deposition was performed for about 20 min at room temperature; aer the deposition the lms were washed well with plenty of water and dried under N2 atmosphere. For comparison, electroless deposition of Cu was also carried out in APTMS/OHP in order to investigate the role of the conducting polymer in the electron transfer process and, in turn, the sensing experiments. Structural, morphological and phase purity of these modied strips was analyzed using SEM and XRD studies. Scheme 1 shows the pictorial representation of the modication strategies adapted in the present work to fabricate exible and disposable strips for non-enzymatic glucose detection. 2.4. Glucose sensing experiments Electrochemical techniques such as cyclic voltammetry (CV) and chronoamperometry (CA) were used to investigate the redox behaviour of the modied lms and to perform glucose sensing experiments. A conventional three-electrode system, namely, a Pt foil as a counter electrode, Ag/AgCl as a reference electrode and Cu deposited exible polyester OHP lms as working electrodes were used for this study. Prior to the experiments, Pt foil was cleaned by dipping in conc. HNO3 for a couple of minutes and washed well with millipore water and the Ag/AgCl electrode was cleaned in millipore water. Initially CV experiments were carried out in 0.1 M NaOH aqueous solution at a xed scan rate of 50 mV s1 between 1.0 V and 1.0 V to study redox properties of the chemically modied lms. Glucose sensing experiments were also performed in 0.1 M NaOH solution using CV and CA by incremental addition of glucose. Cu/PANI/APTMS/OHP and Cu/APTMS/OHP strips were used for the study. CA experiments were done by measuring the current with respect to time at a xed potential for each and every incremental addition of glucose. Concentration of glucose was varied from 100 mM to 6.5 mM. Moreover, stability and reproducibility of these exible electrodes were also studied. Finally, the feasibility of employing these chemically modied polyester

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OHP strips for real sample analysis was explored by measuring the glucose content in blood serum collected from several patients; these samples were obtained from our Institute clinic and proper permissions and approval were obtained. All experiments were performed at room temperature.

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2.5. Instruments FTIR spectra were recorded using a Bruker Optik GmbH spectrometer. SEM studies were performed using Hitachi model S3000-H, which also has an EDAX facility attached. XRD analysis was carried out using an XPERT-PRO multipurpose X-ray diffractometer procured from the Netherlands, using Cu ˚ within 10–90 degrees Ka radiation with a wavelength of 1.540 A of a 2q range. Electrochemical studies were carried out using an EG&G Instruments Electrochemical Impedance Analyzer Model 6310 (Princeton Applied Research, USA). Electrochemical experiments and analysis of data were carried out using echem soware provided by EG&G. All other parameters are mentioned in the respective diagram.

3.

Results and discussion

Surface modication of non-conductive, exible polyester OHP strips was carried out using a three-step process as shown in Scheme 1. During the rst step polyester OHP lms were dipped in piranha solution for a few minutes to clean and etch the surface. (Caution: this solution reacts violently with glass and extreme care should be taken while handling.) This pre-treatment procedure leads to the formation of free terminal –OH and –COOH groups on the lm surface resulting in hydrophilic characteristics of the treated lms, which is conformed by contact angle measurements. In the second step these terminal functional groups on the surface react with silanes to form –O–Si bonds56 leading to APTMS SAM formation on the OHP lm. These functional group changes and the structural morphology during SAM formation are analysed by using FTIR and SEM studies. During the polymerization step, the terminal –NH2 functional groups on SAM of the APTMS-modied surface are converted to radical cations (–NH2c+) that act as nucleation sites, thereby initiating the growth of PANI on APTMS/OHP lms during the oxidative gra polymerization.56,57 Electroless deposition of Cu is performed in the nal step. Such modied, Cu-coated exible strips were explored for the detection of glucose. For comparison, similar strips without PANI, prepared by using a simple two-step process were also studied. Fig. 1 shows the photographs of bare OHP (a), PANI/APTMS/OHP (b), and Cu/PANI/APTMS/OHP (c), respectively. It can be clearly seen that the colourless OHP strips turn to green upon modifying the substrate with SAM of the APTMS followed by the PANI layer formation that nally turns into brown on coating with Cu. These visual colour changes apparently indicate the occurrence of surface modication processes on the exible strips. Structural and morphological characterizations of those strips at all modication steps were carried out using SEM, XRD and FTIR analyses.

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3.1. SEM analysis Fig. 2 shows SEM images of exible polyester OHP lms before and aer surface modication steps. The SEM image of unmodied bare polyester OHP lm is shown in Fig. 2a, and other images display the corresponding SEM images obtained at each and every surface functionalization steps. It can be seen from these images that bare OHP lms display no distinguishable structural features even aer the SAM formation using APTMS (Fig. 2b). Both of these strips were essentially nonconductive and the corresponding SEM images show typical contrast differences due to electrostatic charge accumulation on the surface. Formation of the APTMS monolayer results in a highly homogeneous and fairly uniform structure for APTMS/OHP sheets. Next, aer formation of these PANI layers (Fig. 2c), the exible strips become conductive and the colour changes from none to green, indicating the formation of the emeraldine form of PANI. The conductivity value, measured using impedance spectroscopy using a four-probe method, was 2.4  103 S cm1 for the PANI/APTMS/OHP lms. The corresponding images obtained were sharper, displaying very distinct structural features. Formation of uniform globular microstructures with dense and platelet morphology indicates the presence of conductive PANI layers. Further electroless deposition of Cu was carried out on these strips without and with PANI layers; corresponding SEM images are displayed in Fig. 2d and e, respectively, with magnied versions shown as insets. Conductivity of these exible strips was increased further aer the Cu deposition, which was nearly the same as metallic conductivity. The colour of these exible strips changed from green to brown, showing the presence of Cu as a result of the deposition process, which is also conrmed by EDX analysis (see ESI, Fig. S1†). SEM images of the Cu-deposited exible, conducting OHP lms showed formation of closely spaced spherical particles. In the case of OHP strips without PANI layers, viz. Cu/APTMS/OHP, Cu particles aggregate to form a non-uniform, clustered structure consisting of larger Cu particles with cauliower-like morphology, as shown in inset. In contrast, OHP strips coated with PANI layers, namely Cu/ PANI/APTMS/OHP, displayed beautiful uniform, spherical and high-density particles of Cu aer the deposition. Detailed analysis of these images suggests that these particles are clusters of even smaller Cu particles. Sphere size varied from 80 to 100 nm  8 nm. High-density Cu particles were formed due to the higher loading of Pd during the surface activation process involved in electroless deposition of Cu that arises from formation of reduced PANI layers (leucoemeraldine form). Morever, EDX spectra recorded for these Cu deposited exible strips clearly indicate the presence of Cu (see ESI; Fig. S1†), and quantitative analysis of these spectra suggested that the amount of Cu loaded in the case of PANI/OHP is much higher than for OHP strips without PANI layers. 3.2. X-ray diffraction (XRD) studies Crystalline structure, phase purity and orientation of the surface functionalized exible OHP strips were analyzed using

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Fig. 2 FESEM images of (a) bare OHP strip, (b) SAM of APTMS immobilized OHP, (c) PANI/APTMS/OHP, (d) Cu/APTMS/OHP, and (e) Cu/PANI/ APTMS/OHP strips, respectively. Insets show magnified version of (d) and (e) images.

XRD studies. Fig. 3 shows thin-lm XRD pattern of bare OHP strips before modication (A) and aer modication with Cu deposits in the absence of PANI layers (B), and then with Cu deposits in the presence of PANI layers (C), respectively. It can be noted from these diffraction patterns that Cu-deposited exible OHP strips clearly reveal the formation of characteristic peaks for copper possessing a face-centered cubic crystalline structure, in addition to the diffraction peaks arising from the bare OHP lms. Cu-deposited lms (Fig. 3B and C) show peaks at 43.4 , 50.6 and 74.2 corresponding to (111), (200) and (220) planes (b, c, and d in Fig. 3B and C), respectively, and this matches with the fcc structure of copper [JCPDS 85-1326]. These peaks are clearly visible in the magnied region shown in the insets. This also indicates that the resultant Cu lms on exible OHP strips were of pure fcc copper. The relative intensities ratio of these peaks suggests the growth of a standard polycrystalline Cu along the (111) plane in both cases. On the other hand, bare OHP lms (Fig. 3A) before any surface modication display a distinct prominent peak at 25.9 arising from the presence of crystalline carbon material (a in Fig. 3A). Since the thickness values of formed PANI layers and Cu deposits aer chemical functionalization are so thin when compared to the thickness of bare OHP lms, the peaks due to the underlying substrate (i.e., OHP sheets) were apparent and clearly visible. XRD studies

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clearly showed the formation of crystalline fcc phase of Cu lms deposited onto OHP sheets. The average crystallite size of these Cu deposits was calculated using the following Scherrer formula: L¼

0:9l b cos q

where L is the average crystallite size; 0.9, the Scherrer constant; l, the wavelength of radiation used; b, the full width at half maximum, in radians, of the corresponding diffraction peak; and q, the diffraction angle (peak position). Using the previous formula, the average crystallite size of Cu was calculated to be 28.4  0.7 nm for Cu/PANI/APTMS/OHP lms when compared to 42.7  1.2 nm for Cu/APTMS/OHP lms, which is in close agreement with observations made using SEM analysis. 3.3. Fourier transform infrared spectroscopic (FTIR) measurements Fig. 4 shows the FTIR spectra of exible OHP strips before and aer chemical functionalization processes. These spectra were recorded for the lms in transmittance mode. Peak formation at 1264 cm1 and 1063 cm1 in the case of APTMS/OHP (Fig. 4b) denotes the stretching vibrations of Si–CH2 and Si–O–Si functional groups, respectively. In addition, the peaks observed at

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OHP lms. The FTIR spectrum of the PANI/APTMS/OHP strips (Fig. 4c) displayed a couple of new peaks at 1592 cm1 and 1508 cm1 corresponding to the characteristic stretching vibrations of C]C from quinoid, and C]C from benzenoid forms of the PANI layers, respectively, along with the peaks noted in the former case. For purposes of comparison, a similar FTIR spectrum, recorded for bare OHP strips before surface modication, was also shown in Fig. 4a, which does not show any characteristics except a broad peak beyond 3400 cm1 indicating the presence of –OH groups on the surface arising from the pretreatment method employed. From these studies it is clear that OHP strips were chemically modied with APTMS and PANI layers, with subsequent deposition of Cu, as was also evident from SEM and XRD studies. Furthermore, these surface-functionalized exible strips were characterized using electrochemical studies and explored as possible sensing matrices for glucose detection. 3.4. Electrochemical characterization of functionalized, exible OHP lms

Fig. 3 X-ray diffraction pattern of (A) bare OHP, (B) Cu/APTMS/OHP, and (C) Cu/PANI/APTMS/OHP strips. Insets display an enlarged views of (B) and (C) XRD spectra. Here “a” denotes the underlying substrate peak, while “b”, “c” and “d” represent XRD peaks arising out of (111), (200), and (220) planes of Cu.

It is well-known that the surface immobilized layer oen imparts fast electron transfer characteristics to the underlying electrode.11,24–26 Depending on the nature of the functionalities within the immobilized layer, it is possible to mediate and tune the electron transfer across the interface. In this context, it is critical to understand the electrochemical characteristics in terms of redox behaviour of the modied lms. Specically, in this work chemically modied exible OHP strips were employed as electrodes. Cyclic voltammetry (CV), the most popular electrochemical technique, was used to investigate the electron transfer and redox properties of Cu-coated exible OHP strips. Fig. 5 shows the CVs of Cu/PANI/APTMS/OHP (a) and Cu/ APTMS/OHP (b) in 0.1 M NaOH aqueous solution at a xed potential sweep rate of 50 mV s1. For comparison, the CV of Cu foil (c) recorded under similar conditions is also shown in the

Fig. 4 FTIR spectra recorded in transmittance mode for (a) bare OHP, (b) APTMS/OHP, and (c) PANI/APTMS/OHP strips, respectively. 1

1

2850–2920 cm and around 3050–3150 cm correspond to the stretching vibrations of C–H and N–H groups. These observations indicate the formation of the APTMS monolayer on the

4666 | Analyst, 2014, 139, 4661–4672

Fig. 5 Cyclic voltammograms of (a) Cu/PANI/APTMS/OHP, (b) Cu/ APTMS/OHP, and (c) Cu foil recorded in 0.1 M NaOH aqueous solution at a fixed scan rate of 50 mV s1. In these CVs, A1, A2, A3, and C1, C2, denote the formation of respective oxidation and reduction peaks corresponding to redox reaction associated with Cu deposits.

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gure. Two striking features were noted: (i) formation of redox peaks in all cases indicating the existence of redox transitions, and (ii) in contrast to the Cu foil, the two Cu-coated exible strips showed 8–10 times greater current density, suggesting the possibility of using these exible strips for electrocatalytic applications. There are three different oxidation peaks denoted as A1, A2 and A3, in addition to two reduction peaks labeled C1 and C2, respectively. It is noteworthy that the Cu foil also displayed two reduction peaks while the other peaks were barely visible. In the case of Cu/PANI/APTMS/OHP (Fig. 5a), the oxidation peaks, namely A1, A2 and A3, were observed at 0.43 V, 0.17 V, 0.13 V vs. Ag/AgCl, whereas the reduction peaks C1 and C2 occurred at 0.65 V and 0.95 V vs. Ag/AgCl, respectively. Similarly, in the case of Cu/APTMS/OHP electrodes without PANI layers, oxidation peaks were noted at 0.33 V, 0.03 V and 0.27 V vs. Ag/AgCl, and reduction peaks were formed at 0.63 V and0.90 V vs. Ag/AgCl, respectively. These redox peaks were attributed to the following oxidation state transitions associated with Cu: A1 / Cu(0) to Cu(I), C1 / Cu(II) to Cu(I) A2 / Cu(0) to Cu(II) and Cu(I) to Cu(II), C2 / Cu(I) to Cu(0) A3 / CuII oxide to hydroxide

The possible mechanism proposed for these transitions based on the redox reactions of Cu follows:32,39,58–61 2Cu + 2OH 4 Cu2O + H2O + 2e (peak A1)

(1)

Cu + 2OH 4 CuO + H2O + 2e (peak A2)

(2)

Cu2O + OH4 2CuO + H2O + 2e (peak A2)

(3)

CuO + H2O / Cu(OH)2 (peak A3)

(4)

In addition, the effect of scan rate on this redox behaviour of Cu-coated OHP exible strips was investigated over a wide range of scan rates starting from 5 mV s1 to 500 mV s1 in 0.1 M NaOH aqueous solution within a xed potential range. The redox peak currents increased with scan rate, and the change in redox peak current with respect to increase in scan rate is linear and passes through the origin. The latter relationship suggests that Cu is indeed surface conned within the immobilized layers, and that the overall electron transfer process is predominantly controlled by the diffusion process. Further, these Cu-coated, OHP exible strips were employed as a potential sensor matrix for glucose detection. 3.5. Glucose-sensing experiments using Cu-coated OHP exible strips From the electrochemical characterization of chemically modied exible OHP strips, it is understood that electron transfer across the interface can be tuned based on the redox reaction of Cu present within the modied lms. The This journal is © The Royal Society of Chemistry 2014

electrochemical redox characteristics of Cu on these exible strips were further explored in glucose-sensing experiments in an alkaline medium. These studies were carried out by monitoring the electrochemical oxidation of glucose on the chemically modied OHP strips. Fig. 6a and b show the CV responses of Cu/APTMS/OHP and Cu/PANI/APTMS/OHP electrodes, respectively, upon addition of various glucose concentrations. Glucose concentration was varied from 0.5 mM to 3.5 mM. In these gures, the arrow indicates direction of increasing concentrations added to the electrolytic medium, and the insets display corresponding changes in the oxidation peak current density. These CVs were recorded in 0.1 M NaOH aqueous solution at a xed potential sweep rate of 50 mV s1. For comparison, CVs of the respective modied OHP electrodes recorded under similar conditions before adding glucose are also shown in Fig. 6. It is clear that the addition of glucose results in signicant enhancement of the oxidation current density in both cases when compared to the CV of exible electrodes without glucose. Although there is a small increment (almost negligible) in the current density observed for the A1, A2, and A3 and C1 and C2 peaks described earlier, the formation of a new oxidation peak due to adding glucose seems clear. This new peak was observed at 0.75 V in the case of Cu/APTMS/OHP and 0.55 V for Cu/PANI/ APTMS/OHP, suggesting an overall voltage gain (positive energy) of about 0.2 V due to the formation of PANI layers that aid in mediating the electron transfer process. Importantly, both electrodes resulted in systematic change in the oxidation current density due to increasing glucose concentrations. Insets in the gures show the increase in oxidation current density with respect to 0.5 mM incremental addition of glucose in both cases. The plot shows a typical linear change; in the case of Cu/APTMS/OHP (Fig. 6a), saturation is reached at 3.0 mM glucose, in contrast to Cu/PANI/APTMS/OHP (Fig. 6b) for which a perfect linear relation was noted. It is also noteworthy that the oxidation current density is higher for the glucose addition in the former case. For comparison, the oxidation current density for the 3.5 mM glucose addition to Cu/APTMS/OHP (Fig. 6a) was determined to be 15.3 mA cm2 compared to 5 mA cm2 before the glucose addition, resulting in 3-fold enhancement in the current density. In the case of Cu/PANI/APTMS/ OHP (Fig. 6b), for the 3.5 mM glucose addition, oxidation current density was calculated at 13.5 mA cm2, compared to 2.4 mA cm2 before glucose addition, indicating a signicant 6-fold enhancement in the oxidation current density. This is attributed mainly to synergistic effects arising from the presence of PANI layers and the resultant structural morphology of Cu deposits. Basically, PANI helps in the formation of uniform highly dense nanoparticles of Cu, as revealed by SEM studies. During electroless deposition of Cu on PANI-coated strips, the nucleation starts primarily at porous structures of the polymer matrix, which also controls the growth of these nanoparticles that plays a crucial role in electrocatalytic behavior by obtaining the higher current density and in shiing the potential to a comparatively lower value. These CV studies clearly demonstrate the

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Fig. 6 CVs in 0.1 M NaOH aqueous solution at a scan rate of 50 mV s1 for (a) Cu/APTMS/OHP and (b) Cu/PANI/APTMS/OHP electrodes towards addition of various glucose concentrations. In these figures, arrows indicate direction of increasing glucose concentrations. Glucose concentration was varied from 0.5 mM to 3.5 mM with increments of 0.5 mM. Insets show plot of oxidation peak current density change corresponding to glucose concentration. CV recorded under similar conditions before glucose addition is also shown for comparison purposes.

potential utility of Cu-coated, exible OHP strips in glucose detection. 3.6. Chronoamperometry studies Glucose detection experiments were also carried out using chronoamperometry studies performed in a constantly stirred 0.1 M NaOH aqueous solution in order to explore the possibility of using these exible electrodes for in vitro glucose sensing in real blood serum samples. Studies were carried out by measuring the amperometric response of change in oxidation current density with respect to time due to glucose oxidation at a xed potential. Glucose concentration was varied from 100 mM to 6.5 mM. Fig. 7a shows a typical chronoamperometric plot of the Cu/PANI/APTMS/OHP electrode showing response to successive additions of glucose. The oxidation current density

for each incremental addition of glucose was measured with respect to time at a xed potential of 0.6 V. As seen in the gure, every addition of glucose leads to a spike formation followed by a steady-state current, indicating a systematic increase in oxidation current density values with respect to increasing glucose additions. Corresponding changes in the current density values were plotted against the changes in glucose concentration; the respective plot in Fig. 7b displays a linear relation. The linear detection range for this particular exible electrode ranged from 100 mM to 6.5 mM with slight deviation observed at higher concentrations. The regression equation based on these data was found to be: J (mA cm2) ¼ 2.2021 + 2.8456C (mM), with R2 ¼ 0.9923, where J represents the oxidation current density and C denotes glucose concentration.

Fig. 7 (a) Typical chronoamperometric plot measured for Cu/PANI/APTMS/OHP strips in a stirred 0.1 M NaOH aqueous solution for successive

increments of glucose at a fixed potential of 0.6 V vs. Ag/AgCl. Glucose concentration was varied from 100 mM to 6.5 mM. (b) Plot of glucose oxidation current density (obtained from steady-state current of [a]) vs. glucose concentration. Linear regression equation used for fitting the data points is also provided.

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Using this equation, sensitivity was estimated at 2.8456 mA cm2 per mM, and Icat, 2.2021 mA cm2. Similarly, chronoamperometric plots and corresponding changes of oxidation current density values with respect to glucose addition for the Cu/APTMS/OHP electrode were also recorded at a xed potential of 0.8 V (see ESI, Fig. S2†). For this electrode, the estimated linear detection ranged from 100 mM to 3 mM, beyond which it attained a steady-state increase in current, indicating a negligible change in oxidation current density on higher glucose concentrations. Sensitivity was calculated at 2.4457 mA cm2 per mM. Response time for these strips to achieve the steady-state response was estimated at 3–5 s. Comparing the two exible electrodes, the Cu/PANI/APTMS/ OHP strips were found to be more sensitive; they exhibited a higher current density, and there was a gain of 0.2 V when compared to Cu-coated OHP strips without PANI layers, suggesting that the former is a better glucose sensor. Sensitivity, linear range of detection, current density and detection limit are much better than several other previously studied non-enzymatic sensor electrodes (see Table 1). As seen in Table 1, both exible electrodes analysed here are better glucose sensors, indicating the possibility of employing them as cheap, disposable strip sensors. 3.7. Interference studies and real sample analysis The selectivity of this particular exible strip-based glucose sensor was investigated by testing these electrodes for glucose sensing in a competitive environment, that is, in the presence of other interfering species, namely ascorbic acid (AA), uric acid (UA) and dopamine (DA). In a biological environment, levels of glucose are at least 30 times greater than the levels of these molecules. Fig. 8 shows the chronoamperometric plot obtained for the Cu/PANI/APTMS/OHP electrode in 0.1 M NaOH aqueous solution at a xed potential of 0.6 V vs. Ag/AgCl. Incremental addition of 0.5 mM glucose was carried out along with at least 10 times lower concentrations of AA (0.05 mM), UA (0.05 mM)

Fig. 8 Chronoamperometric curve obtained for Cu/PANI/APTMS/ OHP electrode in a stirred 0.1 M NaOH aqueous solution at a fixed potential of 0.6 V vs. Ag/AgCl showing effect of interference species in glucose-sensing experiments.

and DA (0.05 mM), respectively. As seen in Fig. 8, initial addition of glucose results in higher oxidation current density. Subsequently, the addition of interfering species that led to a very negligible current change (almost none) with further addition of greater glucose concentrations results in furthermore increase in oxidation current density, indicating that the Cu-coated exible OHP strips are indeed selective to glucose. Interestingly, a mere 10 times greater concentration of glucose compared to interfering species displayed a remarkable and signicant enhancement in oxidation current density. In order to quantify the effect of interfering species, a parameter called the current ratio for a xed concentration of mole ratio between glucose and other species was determined. For a constant 10 : 1 mole ratio of glucose versus the other species, the current ratio was calculated to be 3.34% for AA, 3.18% for UA and 2.0% for DA,

Comparison of electrode materials employed in non-enzymatic glucose detection: applied potential, sensitivity, linear range, and detection limita

Table 1

Type of electrodes

Applied potential (V vs. Ag/AgCl)

Sensitivity (mA mM1 cm2)

Linear range (mM)

Detection limit (mM)

Reference

Cu/PANI/APTMS/OHP (exible strip) Cu/APTMS/OHP (exible electrode) Cu NBs/carbon SPE Inkjet printed CuO NPs/Ag/Si Cu NPs/ZnO/ITO Cu NPs/SWCNT Cu nanoclusters/MWCNTs/GCE Cu nanocubes/MWCNTs CuO/MWCNTs/Ta plate CuO-graphene/GCE Cu–CuO/C CuO nanoowers/Cu

0.6 0.8 0.6 0.6 0.8 0.65 0.65 0.55 0.55 0.59 0.75 0.5

2.8456 2.4457 4.4333 2.7625 0.0006 0.256 0.253 1.096 2.19 1.36 0.598 0.7893

0.1–6.5 0.1–3.0 0.01–1.13 0.05–18.45 0.001–1.53 0.00025–0.5 0.0007–3.5 Up to 7.5 0.2–3.0 0.002–4 Up to 3.0 0.000095–3.13

5.0 0.1 10 0.5 0.2 0.25 0.21 1.0 0.8 0.7 5.0 70

This work This work 34 40 35 41 5 42 43 38 44 45

a

NBs, nanobelts; SPE, screen-printed electrode; NPs, nanoparticles; MCNTs, multi-walled carbon nanotubes; SWCNT, single-walled carbon nanotube; Ta, Tantalum.

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Table 2 Comparison of glucose values determined from human blood serum samples using chemically modified flexible polyester OHP strips (reported in this work) and clinical lab methods

Sample

Clinical (mM)

Sensor (mM)

RSD (%)

Recovery (%)

1 2

8.0 7.4

7.9 7.29

2.1 2.9

98 98.6

respectively. These observations suggest that the disposable sensor is very efficient and the impact of those species on glucose sensing is negligible. Moreover, reproducibility of this sensor was analyzed by following two different methods: (1) three exible strips of Cu/PANI/APTMS/OHP were fabricated and used to measure the amperometric response to increased glucose; and (2) a single Cu/PANI/APTMS/OHP exible strip was analyzed ve times. The relative standard deviation (RSD) was estimated to be less than 3%, and to be 1.2% in the latter case, suggesting that glucose-sensing results are highly reproducible. In addition, the long-term stability of this exible sensor was evaluated by measuring the oxidation current response for glucose added to 0.1 M NaOH aqueous solution every 7 days over a period of more than 50 days. From these experiments, it was found that the oxidation current density for glucose was decreased to 98% of its original value, and that it fell to 96% even aer 50 days. Besides, there was no obvious potential shi and no change in detection limit, linear range of concentration, sensitivity, and so on for glucose detection. These results convincingly proved that Cu-coated exible OHP strips as prepared in the present work are highly stable, reproducible, and possess long-term stability as sensors. In an attempt to explore this exible sensor for practical applications, the disposable Cu-coated polyester OHP strips were analyzed for measurement of glucose in human blood serum samples of patients obtained from the clinic of our Institute through appropriate permissions and approvals. Two serum samples consisting of various blood glucose levels were collected. About 200 mL of serum sample were added to 10 mL of 0.1 M NaOH aqueous solution, and the chronoamperometric current response was measured at 0.6 V using the Cu/PANI/APTMS/OHP strip (see ESI; Fig. S3a and b† show plots for two samples). Initially known concentrations of glucose were added; oxidation current density before and aer each glucose level increment were recorded. Glucose present in the given serum sample was used to develop a

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calibration graph based on current density and known concentrations of glucose. The calculated values were compared with the clinical lab reports and comparisons appear in Table 2. For the rst sample, 7.9 mM glucose was estimated from this exible sensor, compared to the lab report value of 8.0 mM. For the second sample, 7.29 mM glucose was calculated, and the lab report indicated 7.4 mM glucose. These studies show that the values obtained for real samples using exible Cu-coated OHP strips using amperometric measurements are in good agreement with the lab reports (almost 98–99%), indicating that these strips could be used for glucose detection.

3.8. Mechanism of glucose sensing Based on the experimental observations and results obtained for glucose detection using chemically modied, exible OHP strips, a plausible mechanism is proposed. The precise mechanism for glucose oxidation using metallic electrodes in alkaline medium is difficult to conrm. However, the most generally accepted mechanism involves the presence of metal oxyhydroxide, which plays a catalytic role in oxidizing glucose.59,62–65 Interestingly, glucose oxidation on these exible strips occurs aer the peak potential of Cu redox couple (A3), suggesting the active participation of this redox couple in the oxidation process. Moreover, the addition of glucose results in increasing oxidation current density and decreasing reduction current density, leading to enhancement in the ratio of anodic and cathodic peak currents, which in turn indicate catalytic activity for glucose oxidation. The proposed mechanism in this case involves a direct reaction between glucose and copper oxyhydroxide (CuOOH) formed on the exible electrode surface due to the presence of Cu in a dilute alkaline solution. Oxidation of glucose begins with de-protonation that isomerises further to form enediol on the catalytically active sites of Cu(OOH), which eventually leads to formation of gluconolactone by reforming CuO. Later the substance undergoes hydrolysis to form gluconic acid. At increasingly anodic potentials owing to the presence of abundant OHads species, the rate of glucose oxidation is being accelerated by the fast dissociation of water. The reactions involved in the oxidation of glucose follow: These results clearly suggest that the chemically modied, exible polyester OHP strips could well be used as disposable glucose-detection sensors.

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4. Conclusions In this work, exible, disposable and low-cost sensors for glucose detection are proposed and demonstrated. Essentially these exible sensors are made up of copper coated onto chemically modied polyester OHP sheets. Structural, morphological, crystallinity and electrochemical characterizations of these electrodes were performed by using microscopic (SEM), XRD, CV and CA techniques. Based on the experimental results obtained, it was shown that both exible electrodes, Cu/ PANI/APTMS/OHP and Cu/APTMS/OHP, could well be used for glucose detection. Among the two proposed sensors, Cu/PANI/ APTMS/OHP displayed a much superior performance in terms of higher oxidation current density, higher sensitivity, and a wide linear range of detection when compared to Cu/APTMS/ OHP strips. These exible sensors are also demonstrated to be useful for sensing glucose in real blood samples with an impressive response time of less than 5 s. Principal advantages of this exible sensor include ease in fabricating the sensor matrix, ease of use, high selectivity, low cost, possibility of mass production and disposal aer use at low cost; and they could easily be coated onto fabrics and plastics. In addition, this proposed sensor could also be used to detect glucose in clinical, agricultural and food industry activities.

Acknowledgements We are grateful to the Central Instrumentation Facility (CIF) of CSIR–CECRI, Karaikudi, for providing various characterization facilities. RT acknowledges CSIR, India, for the junior research fellowship (JRF) in his PhD program.

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Chemically modified flexible strips as electrochemical biosensors.

A flexible and disposable strip sensor for non-enzymatic glucose detection is demonstrated in this work. The strips are prepared by using chemical mod...
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