Non-enzymatic amperometric detection of hydrogen ...

Journal of Colloid and Interface Science 470 (2016) 117–122

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Non-enzymatic amperometric detection of hydrogen peroxide in human blood serum samples using a modified silver nanowire electrode Balamurugan Thirumalraj, Duo-Han Zhao, Shen-Ming Chen ⇑, Selvakumar Palanisamy Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, ROC

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Article history: Received 18 January 2016 Revised 19 February 2016 Accepted 19 February 2016 Available online 21 February 2016 Keywords: Silver nanowires Screen printed carbon electrode Hydrogen peroxide Non-enzymatic sensor

a b s t r a c t In this paper, we report a highly sensitive amperometric H2O2 sensor based on silver nanowires (AgNWs) modified screen printed carbon electrode. The AgNWs were synthesized using polyol method. The synthesized AgNWs were characterized by scanning electron microscopy, UV–vis spectroscopy and X-ray diffraction techniques. The average diameter and length of the synthesized AgNWs were found as 86 ± 5 and 385 nm, respectively. Under optimum conditions, the AgNWs modified electrode shows a stable amperometric response for H2O2 and was linear over the concentrations ranging from 0.3 to 704.8 lM. The non-enzymatic sensor showed a high sensitivity of 662.6 lA mM 1 cm 2 with a detection limit of 29 nM. The response time of the sensor was found as 2 s. Furthermore, the AgNWs modified electrode exhibited a good recovery of H2O2 (94.3%) in the human blood serum samples. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction An accurate and reliable detection of hydrogen peroxide (H2O2) in its low levels has received substantial attention to the readers in the analytical community, due to the importance of H2O2 in differ-

⇑ Corresponding author. E-mail address: [email protected] (S.-M. Chen). http://dx.doi.org/10.1016/j.jcis.2016.02.049 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.

ent areas including paper bleaching, food processing, clinical diagnostics, environmental analysis, minerals processing and fuel cells [1–3]. Different analytical methods such as titrimetry [4], Photometry [5], fluorimetry [6], chemiluminescence [7], fluorescence [8], spectrophotometry [9] and electrochemical methods [10] have been used for the determination of H2O2. However, due to the technical drawbacks (low sensitivity, selectivity, time consuming, and complicated instrumentation) of the traditional titrimetry, photometry and chemiluminescence methods, the electrochemical

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methods are widely used for the sensitive and selective determination of H2O2 [11]. Furthermore, the electrochemical methods are simple, fast, sensitive, and cost effective compared to the aformentioned traditional methods [11]. The iron redox enzymes are widely used as a selective probe for the detection of H2O2 owing to its high specificity. [12–15]. However, the stability, complicated immobilization procedure and high cost of the redox enzymes leads to the less attention in the fabrication of biosensors [16]. Hence, non-enzymatic H2O2 sensors based on carbon nanomaterials, dyes, transition metals, metal oxides, metal phthalocyanines, metal porphyrins and redox polymers are an alternative choice for sensitive detection of H2O2 [11,17,18]. On the other hand, one dimensional micro and nanomaterials have received much attention to the researchers from different disciplines including energy and electronic devices, sensors and biosensors [19–21]. In particular, one dimensional silver nanowires (AgNWs) have been widely used as a sensitive probe for detection of H2O2 due to their unique size dependent optical, electrical and thermal properties [22–24]. So far, different methods have been adopted for the synthesis of AgNWs including wet chemical [25], chemical reduction [26], polyol [27,28] and solvothermal synthesis [29] and so on. Compared to other methods, polyol synthesis of AgNWs is a simple and effective method for the mass production of AgNWs and high quality of AgNWs can be synthesized within short time [27]. In the present study, we have used AgNWs modified electrode as a selective probe and advanced electrocatalyst for reduction of H2O2. In addition, the AgNWs modified electrode could detect the H2O2 in sub-millimolar levels at lower overpotential than that of previously reported non-enzymatic H2O2 sensors (Table T1). Herein, we report a selective and sensitive amperometric detection of H2O2 using AgNWs modified screen printed carbon electrode (SPCE). The AgNWs modified electrode shows an enhanced catalytic activity and lower overpotential for H2O2 than that of unmodified SPCE. The selectivity, stability, repeatability, reproducibility and practicality of the fabricated sensor have also been studied and discussed in detail. 2. Experimental 2.1. Materials and method Silver nitrate, sodium chloride and ethanol were purchased from Sigma–Aldrich. Ethylene glycol (EG), isopropyl alcohol (IPA) and polyvinylpyrolidone (PVP) with Mw 40,000 were obtained from Aldrich. Hydrogen peroxide was obtained from Wako pure chemical industries. Human blood serum sample was collected from valley biomedical, Taiwan product & services, Inc. The supporting electrolyte 0.05 M pH 7 solution (PBS) was prepared by using 0.05 M Na2HPO4 and NaH2PO4 solutions in doubly distilled water. All chemicals were analytical grade and used without further purification. All the solutions were prepared using doubly distilled water without any further purification. This study was reviewed and approved by the ethics committee of Chang-Gung memorial hospital through the contract no. IRB101-5042A3. Cyclic voltammetry and amperometry i–t experiments were done using CHI1025B electrochemical work station. Scanning electron microscopy (SEM) was performed using Hitachi S-3000 H electron microscope. An energy-dispersive X-ray (EDX) spectrum was recorded using HORIBA EMAX X-ACT that was attached with Hitachi S-3000 H scanning electron microscope. Ultraviolet–visible (UV) spectroscopy was performed by a Hitachi U-3300 UV spectrophotometer. X-ray powder diffraction (XRD) was performed by PANalytical’s newest X-ray diffraction system based on the fully renewed X’Pert3 platform. A conventional three electrode system consists of screen printed carbon electrode (SPCE) as a working

electrode, a saturated Ag/AgCl as a reference electrode and platinum as a counter electrode respectively. Electrochemically active surface area of the AgNWs modified SPCE was found 0.046 cm2, which was calculated using CV as reported previously [30]. 2.2. Preparation of AgNWs The AgNWs were synthesized by polyol method with little modification of previously reported literatures [28]. Briefly, 40 mL of 0.56 M PVP was dispersed in IPA, at the same time 0.15 M NaCl solution was prepared using EG solution. First, PVP in IPA was heated to 140 °C for five minutes, and then 0.1 mL of NaCl in EG solution was injected into the PVP mixture. After the two minutes of reaction, 1.5 M AgNO3 solution (10 mL) was slowly added into the reaction mixture. The reaction was carried out until the color of the solution turn to grey (in our case 90 min as an optimum time), the color change indicates the formation of AgNWs. The obtained AgNWs solution were diluted with IPA and centrifuged at 6000 rpm to remove the PVA from the solution. The purified AgNWs were dried at hot oven and re-dispersed in ethanol for further use. The schematic illustration for the synthesis of AgNWs is shown in Scheme 1. 2.3. Fabrication of AgNWs modified electrode The AgNWs dispersion was prepared by dispersing of 1 mg/mL of AgNWs in ethanol and sonicated for 15 min using ultrasonicator with the frequency of 40 kHz and ultrasonic power of 150 W. About 8 lL of as prepared AgNWs dispersion was drop casted on the SPCE and dried at room temperature. The resulting AgNWs modified SPCE was further used for the electrochemical studies and stored in PBS at room temperature when not in use. 3. Results and discussion 3.1. Characterization of AgNWs Fig. 1 shows the SEM images of AgNWs in lower (A) and higher magnification (B). It can be seen that a well-defined and large quantity of uniform NWs were observed and the average diameter and length of the AgNWs were 86 ± 5 and 385 nm, respectively. On the other hand, the SEM and AFM images (C and D) of AgNWs clearly shows that only NWs are appeared and does not shows any nanoparticles (NPs) on the NWs, which indicates that all AgNPs are changed into NWs. The quantitative results of the corresponding EDX pattern of AgNWs (Fig. 1B inset) show that only the metallic Ag, which indicates the presence of Ag. According to the early studies, it is believed that the NWs are formed by growing from as formed nanoparticles in the solution [38]. The mechanism for the AgNWs formations by polyol method is well documented and the as synthesized AgNWs are in good agreements with the previous literatures [27,28]. The synthesized AgNWs was characterized by UV–vis spectroscopy and XRD. Fig. 2 shows the typical UV–vis spectrum (A) and XRD profile (B) of as prepared AgNWs. It is well-known that the nanoforms of Ag with different morphologies show an unique optical properties due to the surface Plasmon response [31]. As shown in Fig. 2A, the two distinct absorption peaks were observed at 349 and 385 nm, which is due to the quadrupole resonance excitation and the transverse Plasmon resonance of the AgNWs [32]. In addition, it is reported early that AgNWs with diameter about 80 nm are mostly showed the UV–vis absorption spectra for the quadrupole resonance excitation and the transverse Plasmon resonance at 349 and 385 nm [28]. The obtained results are in good agreement with the SEM and AFM images of AgNWs. The XRD


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Scheme 1. Schematic illustration for the synthesis of AgNWs (Note: the color and size is not to the scale and used only for illustration).

Fig. 1. SEM images of AgNWs at lower (A) and higher (B) magnifications. AFM images of AgNWs at lower (C) and higher (D) magnifications. The inset of B shows the quantitative results of selected areas from SEM images of AgNWs.

pattern of as synthesized AgNWs exhibited four distinct diffraction peaks (2h) at 38.1°, 44.2°, 64.3°, and 77.5°, which is corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of the FCC crystal structure (JCPDS File 04-0783). The ratio of intensity between the (1 1 1) and (2 0 0) peaks is 3.2, which is higher than the reported theoretical ratio of 2.5 [28]. The result clearly reveals that the AgNWs are greatly enhanced the (1 1 1) plane of the Ag. 3.2. Electrocatalytic activity of AgNWs The electrocatalytic activity of as synthesized AgNWs towards H2O2 was investigated by CV. Fig. 3 depicts the CV response of bare (a) and AgNWs (c) modified electrodes in N2 saturated PBS at a scan rate of 50 mV/s. The bare and AgNWs modified electrodes did not show any apparent response in PBS. At the same conditions, bare electrode (curve b) shows a broad peak at 0.75 V for the presence of 200 lM H2O2. On the other hand, AgNWs modified

electrode (curve d) shows an enhanced reduction peak current response at the potential of 0.465 V for the presence of 200 lM H2O2. In addition, the observed current response of AgNWs was 6 folds higher than those observed at bare electrode, while the reduction peak potential was 285 mV lower than un-modified electrode. The result indicates the high electrocatalytic activity of AgNWs towards the reduction of H2O2, which is possibly due to the unique properties of AgNWs. The electrochemical behavior of AgNWs modified electrode was further investigated in PBS containing 200 lM H2O2. Fig. S1 shows the CV response of AgNWs modified electrode in 200 lM H2O2 containing N2 saturated PBS at different scan rate from 20 to 300 mV s 1 (a–n). It can be seen that the reduction peak current of H2O2 increases with increasing the scan rates, while the reduction peak potential was shifted towards negative direction upon increasing the scan rates from 20 to 300 mV s 1. The cathodic peak current of H2O2 had a linear dependence over the square root of

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scan rates from 20 to 100 mV s 1 with the correlation coefficient of 0.9891 (Fig. S1 inset). The result suggests that electrochemical reduction of H2O2 is diffusion controlled electrochemical process at slow scan rates (20–100 mV s 1). However, at higher scan rates the cathodic peak current of H2O2 was not linear with the square root of scan rates, which clearly suggests that the electrochemical reduction of H2O2 at AgNWs modified electrode is controlled by a mixed diffusion–adsorption process [42]. In order to further verify the electrochemical process, log scan rate vs. log current response was plotted and the slope value was found as 0.654 with the correlation coefficient of 0.9946 (figure not shown). The obtained slope value (0.654) clearly indicates that the electrochemical reduction of H2O2 at AgNWs modified electrode is controlled by mixed diffusion–adsorption process., since slope value should be 1.0 for pure adsorption controlled process and 0.5 for pure diffusion controlled process [42]. The effect of pH on the reduction peak current of H2O2 was investigated using AgNWs modified electrode by CV. The obtained CV results are summarized in Fig. S2. Fig. S2 shows the calibration plot for reduction peak current response of 200 lM H2O2 in different pH (pH 3–9). It can be seen that a maximum reduction peak current response of H2O2 was observed in pH 7 than that of other pH. The reduction peak current response of H2O2 decreased when the pH was above or below 7. This is may be due to the high catalytic activity of AgNWs in pH 7; hence the pH 7 was used as an optimum pH for all other experiments. 3.3. Amperometric determination of H2O2

Fig. 2. Typical UV–vis spectrum (A) and XRD pattern (B) of as prepared AgNWs.

Fig. 4A shows the amperometric i–t response of AgNWs modified SPCE in the presence of different concentration additions of H2O2 from 0.3 to 1004.8 lM into the constantly stirred N2 saturated PBS at a working potential of 0.15 V. It can be seen from Fig. 4A (inset) that a stable and well defined response was observed for each addition of different concentration of H2O2 into the constantly stirred N2 saturated PBS. Notably, AgNWs modified electrode shows a slight response for the addition of 0.3 lM of H2O2 and shows a sharp response for the addition of 0.5, 1 and 5 lM H2O2. This indicates the high catalytic ability of AgNWs modified electrode. The response time of the sensor was calculated as 2 s, which indicates the fast electrocatalytic reduction of H2O2 by AgNWs. As shown in Fig. 4B, the AgNWs modified electrode exhibits a stable amperometric current response for H2O2 in the linear concentration ranging from 0.3 to 704.8 lM. The sensitivity (slope/electrode active surface area) of the sensor was calculated as 6.626 lA lM 1 cm 2. The detection limit of the sensor was estimated to be 0.029 lM based on S/N = 3, where standard deviation (n = 3) is 0.0033 lA. In order to show the advancement and novelty of the proposed sensor, the analytical performance of the developed sensor need to be compared with previously reported sensors. The proposed H2O2 sensor shows higher sensitivity, low LOD and wider linear response range for H2O2 that of previously reported H2O2 sensors, as shown in Table 1 [22–24,33–41]. In particular, the sensor shows an advanced analytical performance when compared with previously reported H2O2 sensor using AgNWs modified electrodes [34,36]. Hence, the AgNWs modified electrode can be further used for sensitive detection of H2O2. 3.4. Selectivity of the sensor

Fig. 3. Cyclic voltammetric response of bare (a) and AgNWs (c) modified SPCEs in the absence of 200 lM H2O2 in N2 saturated PBS at a scan rate of 50 mV/s. At the same conditions, cyclic voltammetric response of bare (b) and AgNWs (d) modified SPCEs in the presence of 200 lM H2O2.

The selectivity of the AgNWs modified electrode towards H2O2 was investigated in the presence of potentially interfering compounds such as dopamine (DA), ascorbic acid (AA), glucose, epinephrine (EP) and uric acid (UA). The selectivity of the proposed AgNWs modified electrode was investigated using amperometry. Fig. 5A shows the amperometric i–t response of AgNWs modified

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Fig. 5. (A) Amperometric i–t response obtained at AgNWs modified SPCE for the addition of 5 lM of H2O2 (a) and 200 lM of glucose (b), UA (c), DA (d) and EP (e) into the constantly stirred N2 saturated PBS at the working potential of 0.15 V. (B) Amperometric i–t response of AgNWs modified SPCE for addition of 50 lM of H2O2 (a) into the constantly stirred N2 saturated PBS and the background current response up to 1800 s; working potential = 0.15 V.

The negative working potential ( 0.15 V) is more beneficial for selective detection of H2O2 and less interference effect of other compounds. The result confirmed that AgNWs has a high selectivity towards the detection of H2O2 and can be used for the selective detection of H2O2 in real samples.

Fig. 4. (A) Amperometric i–t response obtained at AgNWs modified SPCE in the presence of different concentration additions of H2O2 into the constantly stirred N2 saturated PBS. Working potential = 0.15 V. Inset shows the enlarged amperometric response of AgNWs for the addition of 0.3 lM (a), 0.5 lM (b), 1 lM (c) and 5 lM (d) of H2O2 into the constantly stirred N2 saturated PBS at the working potential of 0.15 V. (B) Calibration plot for amperometric current response vs. [H2O2].

SPCE for the addition of 5 lM of H2O2 (a) and 200 lM additions of glucose (b), UA (c), DA (d) and EP (e) into the constantly stirred N2 saturated PBS at the working potential of 0.15 V. It can be seen that a well-defined amperometric response was observed for the addition of 5 lM of H2O2, while the interfering compounds did not show any obvious response on the AgNWs modified electrode.

3.5. Determination H2O2 in human blood serum samples In order to evaluate the practical ability of the fabricated sensor, the AgNWs modified electrode was evaluated for the detection of H2O2 in human blood serum samples using amperometry. An appropriate known concentration of H2O2 containing different human serum samples were spiked into the PBS for the real sample analysis. The standard addition method was used for the calculation of recovery of H2O2 in human serum samples and the recovery of H2O2 is listed in Table ST1. The modified electrode shows a recovery of H2O2 in human serum samples were in the range between 92.6% and 97.2% with an average recovery of 94.3%. The

Table 1 Comparison of electroanalytical performance of AgNWs modified electrode with previously reported similar modified electrodes for H2O2 determination. Modified electrode

Method of detection

AgNWs/CS/GCE GR/AgNWs/GCE PVP–AgNWs/GCE GNs/Ag/GCE AgNPs/GCE AgNWs/PtE SiNWs/AgNPs/GCE AgNPs/DNA/GCE CNT/AgNPs/GCE ZnONRs/AgNPs/GCE RGO/AgNPs/GCE Ag/GCE AgNWs/SPCE

Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry Amperometry

Eapp (V) 0.35 0.50 0.63 0.40 0.4 0.2 0.45 0.45 0.45 0.55 0.40 0.2 0.15

Sensitivity (lA mM 31.5 21 22.43 – 54.0 9.45 8.964 773.0 – 152.1 – 104.53 662.6

1

cm

2

)

LR (lM)

LOD (lM)

Response time (s)

Ref.

8.0–1350 1 20.0–3620 100.0–40,000 4.0–60.0 0.5–3000 0.2–20,000 2.0–2500 9.0–9000 0.9–983.0 100.0–8000 5.0–12,000 0.3–704.8

2.0 1.0 2.3 28.0 1.3 0.2 0.2 0.6 1.6 0.9 7.1 0.5 0.029

2.0 3.0 2.0 2.0 – 5.0 4.0 4.0 3.0 30.0–40.0 2.0 5.0 2.0

22 23 24 33 34 35 36 37 38 39 40 41 This work

Abbreviations: Eapp – working or applied potential; LR – linear response range; LOD – limit of detection; NWs – nanowires; CS – chitosan; GCE – glassy carbon electrode; GR – graphene; PVP – polyvinylpyrolidone; GNs – graphene nanosheets; NPs – nanoparticles; PtE – platinum electrode; CNT – carbon nanotubes; NRs – nanorods; RGO – reduced graphene oxide.

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result validates that the proposed sensor has a satisfactory recovery towards H2O2 in human serum samples. 3.6. Stability, precision and accuracy of the sensor The operational stability of the sensor was evaluated by amperometry and the results are shown in Fig. 5B. The sensor retains its 98.9% of its initial response H2O2 after continues run up to 1800 s, which indicates the good operational stability of the sensor. The sensor lost its 9.6% of its initial sensitivity to H2O2 after the storage (3 days) of the AgNWs modified electrode in PBS containing 200 lM H2O2 at room temperature. The stability of the AgNWs modified electrode was evaluated using CV and the experimental conditions are similar as of in Fig. 3. The result indicates the satisfactory storage stability of the AgNWs modified electrode towards H2O2. The precision and accuracy of the sensor was evaluated by CV. The RSD of 2.1% was found for three AgNWs modified electrodes to the detection of 200 lM H2O2. On the other hand, the RSD about 3.6% was observed for the 10 measurements of 200 lM H2O2 using a single AgNWs modified electrode. These results indicate a good precision and accuracy of the AgNWs modified electrode towards the detection of H2O2.

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4. Conclusions In conclusion, an amperometric H2O2 sensor has been developed using AgNWs modified electrode. The SEM, AFM, UV–vis spectrum and XRD pattern observations confirmed the formation of high quality AgNWs. Compared with unmodified electrode, AgNWs modified electrode showed an enhanced catalytic activity and lower overpotential for the detection of H2O2. The fabricated non-enzymatic H2O2 sensor shows excellent analytical features for H2O2 such as lower LOD, fast response time (2 s), ultra-high sensitivity along with appropriate linear response range. In addition, the fabricated AgNWs modified electrode had a high selectivity towards H2O2 in the presence of potentially interfering compounds. The good recovery of H2O2 in the human blood serum samples further confirmed that the AgNWs modified electrode can be used for the sensitive detection of H2O2 in biological samples.

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Acknowledgments This project was supported by the National Science Council and the Ministry of Education of Taiwan (Republic of China). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.02.049.

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