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Non-Enzymatic Glucose Sensor Based on Au/ZnO Core-Shell Nanostructures Decorated With Au Nanoparticles and Enhanced With Blue and Green Light Cheng-Liang Hsu, Yu-Jui Fang, Ting-Jen Hsueh, Sin-Hui Wang, and Shoou-Jinn Chang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11257 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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The Journal of Physical Chemistry

Non-Enzymatic Glucose Sensor Based on Au/ZnO Core-Shell Nanostructures Decorated with Au Nanoparticles and Enhanced with Blue and Green Light

Cheng-Liang Hsu,*,† Yu-Jui Fang,† Ting-Jen Hsueh,‡ Sin-Hui Wang,§ Shoou-Jinn Chang§

†

Department of Electrical Engineering, National University of Tainan, Tainan 700, Taiwan.

‡

National Nano Device Laboratories, Tainan 741, Taiwan

§

Institute of Microelectronics & Department of Electrical Engineering Center for Micro/Nano

Science and Technology Advanced Optoelectronic Technology Center National Cheng Kung University, Tainan 701, Taiwan.

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ABSTRACT

Au/ZnO core-shell nanostructures decorated with Au nanoparticles were synthesized on ITO/glass substrate. The investigated sensor contains 2-D, 1-D, and 0-D nanostructures to provide a large surface-area-to-volume ratio, the catalytic quantum effect and avoid the issues inherent in heterojunction interface barriers. The sensitivities of the fabricated glucose sensors in the dark, and under blue and green LED illumination were 3371.9, 4410.9, and 4157.8 µA/cm2mM-1, respectively. The achieved sensitivities are higher than previous reports on Au nanostructure sensors by 2~100 times. Further, the blue and green LED illumination respectively enhanced the sensitivity and CV glucose sensing currents by ~30.8% and ~23.3%, and ~27% and ~35%. The detection limits of the glucose sensor in the dark and under visible illumination were the same at ~0.5µM. Moreover, these visible light illumination enhancements are attributed to the localized surface plasmon resonance effect.


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Introduction

Diabetes mellitus is a growing public-health issue approaching global epidemic proportions. This has prompted a strong demand for prompt and precise blood glucose measurement. In response, glucose sensors have attracted wide attention, due to their ability to accurately measure the glucose level of human blood. Currently, such sensors account for more than 85% of the market-share in the biomedical sensing field.1 Up to now, enzymatic glucose sensors have been the most popular product, primarily because the sensitivity and selectivity of early non-enzymatic glucose sensors were poor.2-4 The majority of enzymatic glucose sensors use glucose oxidase (GOD), which provides excellent sensing properties compared with other enzymes.2-4 However, the immobilized GOD glucose sensor has drawbacks, namely poor reproducibility, long-term activity decay and stability problems. In addition, the operation temperature, varying pH, ionic detergents and toxic chemicals all affect the performance of GOD. In the recent decade, however, the sensitivity of non-enzymatic glucose sensors have undergone great progress via the application of nanostructure (NSs) materials, which have considerable surface-area-to-volume ratio for increasing the working electrode area of the sensor.5 The sensing and catalyst materials of non-enzymatic glucose sensors have used a variety of metals for the NSs, including Au, Pt, Pd, Ni, Cu, Ag, and Co.6-12 These metal NSs glucose sensors display outstanding sensing performance, long-term stability and good reproducibility compared with enzymatic glucose sensors.6-12 Among the aforementioned NS metals, Au NSs have attracted much attention due to their absorbance of visible light and subsequent production of the localized ACS Paragon Plus Environment


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surface plasmon resonance (LSPR) phenomenon.13 In particular, Au nanoparticles (NPs), as compared with Au one-dimensional and two-dimensional NSs, feature a much stronger LSPR effect, which in turn enhances biosensor measurement.13 In this study, nanowire- and nanosheet-structures of ZnO NSs were hydrothermally grown on ITO/glass substrate. The Au was then sputtered onto the ZnO NSs to form Au/ZnO core-shell NS structures, after which the sample was decorated with Au NPs by photochemical synthesized for enhanced non-enzymatic glucose sensing. The glucose sensing properties of Au/ZnO core-shell NSs decorated Au NPs (Au NPs/Au/ZnO NSs) were further improved by blue and green LED illumination. Therefore, non-enzymatic glucose sensors with high sensitivity and selectivity can be low cost and effectively mass produced. The Au NPs/Au/ZnO NSs fabrication process and the sensing properties of the non-enzymatic glucose sensors are discussed in detail.

Experimental Section

The schematic in Figure 1 illustrates the synthesis and fabrication steps of the proposed Au NPs/Au/ZnO NSs electrode. The ITO/glass substrate (Corning® EAGLE XG) was ultrasonically cleaned by acetone and deionized (DI) water. ZnO nanowires and nanosheets were subsequently grown on the ITO/glass substrate via the hydrothermal method, which involved immersing the ITO/glass substrate in a solution of 0.06 M hexamethylenetetramine (C6H12N4) and 0.06 M zinc nitrate hexahydrate (Zn(NO3)2·6H2O). Afterwards, the specimens were placed in a sealed autoclave and heated at 95°C for 6 hours. 15, 20, and 40 nm-thick Au films were deposited onto the ZnO ACS Paragon Plus Environment

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nanowire and nanosheet ITO/glass samples using direct-current (DC) sputtering. The samples were then annealed in a furnace under Ar flow at 350 °C for 5 min. For Au NPs synthesis, hydrogen tetrachloroaurate(III) (HAuCl4•4H2O, 99.99%) was diluted with ethanol in a beaker, for which the HAuCl4 : ethanol ratios were 1:1000, 1:5000, 1:10000. The Au/ZnO core-shell NS samples were then immersed in the solution, and subsequently placed in a Kinsten KVB-30D UV box and exposed UV irradiation (380 nm, 100 W) for 5 minutes. This diluted solution absorbed UV light and deposited the Au NPs onto the surface of the Au/ZnO core-shell NS samples. Finally, the remaining chlorine of these samples was evaporated by annealing in air at 350 °C for 5 minutes. The surface morphologies and element composition ratios of the samples were measured with field emission scanning electron microscopy (FE-SEM, JEOL-7000F). A 254 nm Xe lamp as an excitation source was employed for photoluminescence (PL) measurements in a Jobin Yvon-Spex fluorolog-3 spectroscopy system. An electrochemical analyzer (Autolab PGSTAT101) was used for cyclic voltammetry and amperometetric analyses of the glucose-sensing measurements. The counter electrode and reference electrode were composed of Pt wire and Ag/AgCl, respectively. The illumination peaks of the blue and green LEDs were correspondingly ~420 and ~540 nm according to electroluminescence (EL) spectrum measurement.

Results and discussion

15, 20, and 40 nm-thick Au films were deposited on the ZnO NSs, and afterward the Au/ZnO core-shell NSs were immersed in three different diluted HAuCl4 solutions (ratios of 1:1000, 1:5000, ACS Paragon Plus Environment


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1:10000) while exposed to UV irradiation to synthesize the Au NPs. Accordingly, 9 synthesized samples were fabricated and applied as the working electrode for electrochemical measurements. The non-enzymatic glucose sensors were investigated with cyclic voltammetry (CV) between −1.0V and 1.0V in the dark. Figures 2(a)-2(c), 2(d)-2(f), and 2(g)-2(i) present the CV curves of the 15, 20, and 40 nm thick Au/ZnO core-shell NSs decorated with Au NPs via the diluted 1:1000, 1:5000, 1:10000, respectively. These Au NPs/Au/ZnO NS-based working electrodes operated at a 50 mV/s scan rate in various concentrations of glucose (0~20mM) and 0.1 M NaOH electrolyte solution. Although the measured CV results of these various samples all had similar shapes of their curves, the current intensities differed according to the synthesis conditions. Figure 2(j) shows the glucose sensitivity of the various Au NPs/Au/ZnO NSs sensors. As can be seen, the sample comprising the 20nm thick Au/ZnO core-shell NSs decorated with Au NPs via the 1:5000 dilution ratio featured the maximum glucose sensitivity of 3371.9 µA/cm2mM-1. According to the incipient hydrous oxide/adatom mediator (IHOAM) model, the Au surface monolayer will be oxidized with OHads to form an initial Au[OH]ads layer by chemisorption of hydroxide anions.14-15 In the positive-scan at the applied potential starting from −1.0V to 1.0V, the measured current featured two peaks at around −0.45 V and 0.20 V. The −0.45 V peak is attributed to the oxidation of glucose, the reaction of which follows Glucose + AuOH → Gluconolactone + Au. This indicates that glucose oxidation is determined by the amount of AuOH.14 The ~0.20 V peak was caused by the oxidation of gluconolactone. When the scan potential is larger than ~0.20 V, AuOH will be reduced and


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the oxidation of glucose repressed due to the AuOH being transferred to Au2O3. Therefore, the current decreases with decreasing AuOH. In the negative-scan at the applied potential from 1.0V to −1.0V, the rising current starts at ~0.15V due to Au2O3 being transferred to AuOH. The obviously strong oxidation peak at ~0.05 V was associated with the recovery of AuOH. These CV peaks demonstrate that the sensor features excellent catalytic activity toward glucose oxidation, which accords with the IHOAM model. Moreover, by increasing the glucose concentration, the catalytic activity of Au NPs/Au/ZnO NSs toward glucose was further characterized. The sensitivity is related to the working electrode surface area and sensor morphology, which was examined by SEM. As mentioned, the maximum glucose sensitivity of 3371.9 µA/cm2mM-1 was found for the sample comprising 20nm thickness Au/ZnO core-shell NSs decorated with Au NPs via the 1:5000 dilution ratio, the top view and cross-sectional FE-SEM images of which are shown in Figures 3(a) and (b). The FE-SEM image shows that high density ZnO nanowires and nanosheets were hydrothermally synthesized on the ITO/glass substrate. The average length and diameter of the ZnO nanowires were ~2.3 µm and 200~600 nm, respectively. The thickness of the ZnO nanosheet is around 20~70 nm. The inset image of Fig. 3 in the upper -right corner shows that the energy dispersive X-ray (EDX) spectrum of the sample indicates a composition of Zn (36.22 atomic %), O (39.81 atomic %) and Au (23.97 atomic %). Additionally, the inset image in the upper-left corner



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shows a zoomed-in FE-SEM image revealing that many Au NPs were evenly distributed on the ZnO nanowires’ surface. The Au NPs diameter was measured to be around 10nm. Figure S1. (Supporting Information) reveals the top view SEM images of sputtered 40 nm-thick Au/ZnO core-shell NSs sample. Figure S1(a) shows the many small pieces of Au thin films cover on NWs and the Figure S1(b) is zoom-in image of Fig. S1(a). The Au layer thickness of NWs is increasing and the spacing between NWs is decreasing with increase sputtering time. The DC sputtering process of Au layer is poor step coverage for ZnO NW morphology. The Au deposition ratio of NWs top portion is higher than NWs bottom portion. In the partial area of 40 nm-thick Au sample, the spacing between Au/ZnO NWs top portion is disappear and the morphology become a thin film. It is speculated that surface area to volume ratio of 40 nm-thick Au/ZnO core-shell NSs is lower compared with 20 nm-thick Au sample and its lower surface area causes the glucose sensitivity worse. Figures 4(a)-(c) show the transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images of the sample composed of 20nm Au/ZnO core-shell NSs decorated Au NPs via the 1:5000 dilution ratio. Figure 4(a) TEM image revealed that the density of the Au NPs was lower than that from the SEM images. In general, 30 minutes sonication has been used to finely disperse the ZnO NSs for the preparation of TEM samples. However, the disparity was attributed to the Au NPs in TEM samples being structurally damaged and shed by the ultrasonic agitation. The HR-TEM images show that the diameter of the Au NPs is around 3~10 nm. Figure 4(d) presents the EDX mapping


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images of the Au NPs/Au/ZnO NSs, which indicates that the O, Zn and Au atoms were evenly distributed throughout the NW. The few and disperse of Au NPs of preparation TEM sample caused the Au low contrast on the EDX map. Figures 5(a)-5(f) present the 20nm Au/ZnO core-shell NSs decorated with Au NPs via the 1:5000 dilution ratio as the working electrode at a 50 mV/s scan rate under glucose concentrations of 0~20mM and 0.1 M NaOH electrolyte solution. The CVs for the samples were measured in the dark, and under blue and green LED illumination. In the dark, the glucose sensing current of the Au NPs/Au/ZnO NSs increases with increasing glucose concentration. As the positive-scan oxidation peaks shifted from 0.18V to 0.33V with increasing glucose concentrations, the measured current gradually increased to 41.1, 2.20×103, 2.39×103, 3.34×103, 4.38×103, 5.06×103, 5.74×103, 6.63×103, and 7.25×103 µA at the glucose concentrations of 0, 1, 2, 4, 6, 8, 10, 15 and 20mM, respectively. The area of the glucose sensor immersed in solution was around 0.652 cm2. The working currents of the blue and green LEDs were the same, namely 0.7A at an applied bias 3.6V and 4.0V, respectively. The lumens of the blue (wavelength 420nm) and green (wavelength 540nm) LEDs were measured via spectrometer to be 6.05×103 and 3.27×104 lux, respectively. The photosynthetic photon flux (PPF) values of the blue and green LEDs were respectively calculated to be ~112.0 and ~605.6 µmol·m-2·s-1, which was transferred to ~6.72×1019 and ~3.63×1020 photons m-2·s-1. The blue and green exposure enhanced the glucose sensing current of the Au NPs/Au/ZnO NSs sample at all glucose



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concentrations. Under blue exposure, the sensor’s oxidation peaks shifted from 0.27V to 0.59 V and the sensing currents increased to 82.3, 2.89×103, 4.27×103, 5.47×103, 5.72×103, 6.85×103, 7.25×103, 8.11×103, and 9.25×103 µA for the glucose concentrations of 0, 1, 2, 4, 6, 8, 10, 15 and 20mM, respectively. The position of the oxidation peaks under green illumination shifted from 0.231 V to 0.516 V and the sensing currents were 75.9, 2.73×103, 2.91×103, 5.05×103, 5.43×103, 6.39×103, 7.25×103, 86.8×103, and 9.84×103 µA for the glucose concentrations of 0, 1, 2, 4, 6, 8, 10, 15 and 20mM, respectively. It was noted that the green illumination sample featured the maximum glucose sensing current due to the high PPF values of ~605.6 µmol·m-2·s-1, which is higher than that of the blue illumination at ~112.0µmol·m-2·s-1. The glucose sensitivities of the Au NPs/Au/ZnO NSs sensor in the dark, and under blue and green illumination were 3371.9, 4410.9, and 4157.8 µA/cm2mM-1, respectively. The blue light had the highest sensitivity enhancement due to its photo energy (420nm, 2.95eV) being higher than the green light (540nm, 2.30eV). This demonstrates that visible light enhanced the glucose sensing current and sensitivity. I and I0 represent the glucose sensing current and the initial background current, respectively. The I/I0 ratios of the sensor under blue (~112) and green (~130) illumination were smaller than that in the dark (~176) at glucose concentration 20mM, primarily because the I0 of the blue (82.3µA) and green (75.9µA) illuminated samples were higher than in the dark (41.1µA) by around 2-fold. Base on the previously reported,16 the oxidation peaks shifted toward the positive direction. The shift ratio of oxidation peaks is dependent with Au NPs size. Their


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theoretical model has been built for free energy changes with the Au NPs surface area. The positive-scan oxidation peaks shifted to larger voltage side with increasing glucose concentrations in the dark. Because 50 mV/s scan rate and electrode size are constant, the oxidation peaks shift is presume that increasing glucose concentration needs the more reaction time and higher scan potential for oxidation of gluconolactone. The oxidation peaks shifts of under blue and green LED illumination sample are higher than measured in the dark. The blue LED illumination presents the maximum oxidation peaks shifts. It is speculate that ratio of oxidation shift is strongly related to photon energy. According the past reported,1, 13 the green light could excite the stronger LSPR of Au NPs than blue light excited. By the way, the LED light cannot irradiate a monochromatic light, which indicate that blue LED (420nm) may irradiate a small amount of shorter wavelength light, such as UV light. It is mean that blue LED excites the interband transition of the Au NPs/Au film and Au NPs/ZnO NSs for increase sensor conductivity. Therefore, the glucose sensor was performed higher overpotential under green and blue LED illumination. Figs. 5(a)-5(f) have been used for the plots in Figures 6(a) and 6(b), which display the current peak of the glucose sensing and I/I0 ratio of the glucose sensors at various glucose concentrations in the dark, and under blue and green illumination. The working electrode was the synthesized Au NPs/Au/ZnO NSs on ITO/glass substrate. The sensing currents of the glucose sensor under blue and green illumination were around 1.27- and 1.35-fold higher than the dark measurements, which mean that the glucose sensor was enhanced 27% and



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35%, respectively. The oxidation peak position of the glucose sensor under blue and green exposure shifted toward a larger voltage side in comparison with measured results in the dark. The blue illumination sample had the largest oxidation peak shift due to the photo energy of blue light (420 nm, 2.95 eV) being larger than green light (540 nm, 2.30 eV), which enhanced the electrochemical reaction. I/I0 ratio curves of the glucose sensors presents similarly line characteristic in the dark, under blue and green illumination. The I/I0 ratio of dark is higher than blue (~1.57 fold) and green (~1.35 fold) illumination due to the smaller dark sensing current. Figures 7 (a)-(c) show the chronoamperometry curves of the Au NPs/Au/ZnO NSs sample measured in the dark, and under blue and green illumination in 0.1 M NaOH at a bias of 0.2V with increasing glucose concentrations. The glucose concentration increment consisted of 1.5mM every 50sec, accumulating to a high of 15 mM at a bias of 0.2 V. Prior to adding the glucose, the sensing current backgrounds of the Au NPs/Au/ZnO NSs sample in the dark, and under blue and green illumination were ~1.1, ~3.6 and ~2.1 µA, respectively. The sensing current increased linearly with increased glucose concentration. However, it was noted that because each glucose injection increased the volume of the solution, the immersed area of the working electrode also increased. In order to maintain a fixed immersed area of the working electrode, the injection volume was manually drawn out, which led to current drift. Although manual operation cannot prevent current drift, an automated measurement system would be able to achieve significant improvements. The


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stable glucose concentration measurement should be from manually drawn out to next glucose injection. The mean value and error bar of glucose sensing current was calculated and plotted in inset of the Figure 7 during every stable glucose concentration measurement. The linear fit of the dark, and blue and green illumination samples were R2 = 0.9706, R2 = 0.9687 and R2 = 0.999, respectively. The green illumination sample provided the highest linear R2 and a high sensing current, which indicates that the green light enhanced both the glucose-sensing ability and stability of the Au NPs/Au/ZnO NSs sample. Figure S2 displays that optical transmittance, reflectance, absorptivity measurement for ZnO NSs/ITO/glass and Au NPs/Au/ZnO NSs/ITO/glass samples by UV/VIS/IR Spectrophotometer (JASCO, V600). The optical transmittance of ZnO NWs/ITO/glass and Au NPs/Au/ZnO NWs/ITO/glass samples are lower than 0.58% and 0.30%. The optical reflectance of Au NPs/Au/ZnO NSs/ITO/glass is higher than ZnO NSs/ITO/glass around, which indicate the Au NPs/Au layer increase the reflectance by their metal scattering effect. However, the absorptivity of Au NPs/Au/ZnO NSs/ITO/glass is wider in vision region compared with ZnO NSs/ITO/glass. Speculate these Au NPs/Au was increased the visible light absorption region due to noble metal nanostructure LSPR effect. Because of the Au NPs is not high density and their diameter only 3~10 nm, which cause that Au NPs only absorb a part of the incident light and the remaining incident light is transmitted to 20nm Au film. In addition, the 3~10nm Au NPs and 20nm Au film are nanoscale dimensional size of



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noble metal materials, which have ability to absorb the visible light for generate the LSPR effect. Figure 8 (a) shows the schematic illustration of Au NPs/Au/ZnO NSs structures. In general, the sputter deposition is exhibit poor step coverage on sidewall, which causes that Au thin film of ZnO NSs sidewall is ultra-thin and discontinuous distribution. The photochemical synthesized Au NPs is randomly distributed on Au/ZnO NSs surface. Most of the Au NPs will be deposited on the Au film, and a few Au NPs will be deposited and directly contacted to ZnO NSs sidewall. Because of the ZnO NSs is high density and the space of between NSs is tiny, which cause that very small amount Au pass through ZnO NSs and deposit on ITO thin film. It is indicating the Au NPs/Au film/ZnO NSs and Au NPs/ZnO NSs are dominate the glucose sensing due to their larger surface area. In this investigate; the upper and bottom portion of ZnO NSs should be the Au NPs/Au film/ZnO NSs and Au NPs/ZnO NSs interface, respectively. Figure 8 (b) and 8(c) display that band diagrams of the Au/ZnO core-shell NSs decorated with Au NPs and Au NPs/ZnO NSs nanocomposites non-enzymatic glucose sensor in the dark. The Au NPs/Au film act as electrocatalyst reaction sites and the glucose oxide is controlled by the combination of Au-OH species.14-15 The glucose-detecting reaction may be attributed to the Au NPs/Au film oxidation as follows:17-18 Au + OH－ + h+ → Au(OH) Au(OH)ads + 2OH－ → AuO(OH) + H2O + 2e－


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AuO(OH) + Glucose → Au(OH) + Gluconolactone Based on the above reaction equation, the surface of the Au NPs/Au film will accumulate a high density of electrons during the oxidation process. Up to now, noble metal have been used and alloyed with metal oxide NSs to improve the glucose sensing sensitivity; however, the semiconductor metal oxide NSs in contact with the noble metal produces a Schottky barrier at the semiconductor–metal heterojunction interface. In general, a Schottky barrier is a high resistance electron path. The sensing current will also be limited by the heterojunction interface barriers. Nevertheless, these accumulated electrons on the Au NPs/Au film diffuse through the Schottky barrier of the heterojunction to the working electrode with an applied CV bias. In this study, the Au shell layer was sputter-coated onto the ZnO NSs template which was then decorated with Au NPs. The grain boundary of the Au shell layer and Au NPs have been reduced or eliminated by thermal annealing to increase the sensing current and sensitivity. The schematics in Figures 9(a) and 9(b) display the band diagrams of the Au NPs/Au/ZnO NSs and the Au NPs/ZnO nanocomposites non-enzymatic glucose sensors under visible light illumination, respectively. Plasmonic photocatalysis has been applied with Au NPs/Au film with visible light illumination to produce localized surface plasmonic resonance (LSPR) on the NPs’ surface.19 According to Figure S2(f), blue (2.95 eV) and green (2.30 eV) light energy were absorbed by the Au NPs/Au film, the surface of which produced a strong electric field with the LSPR effect.18 The strong LSPR electric field



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caused by the electron–electron relaxation roots of the Au NPs/Au film originated from high energy electrons colliding with other electrons. The higher Fermi level E’F was created by collisions redistributing electron energy to form a Fermi–Dirac distribution.19 The LSPR excited hot electrons have overcome the Schottky barrier and transferred to ZnO NSs. Accordingly, the increased sensing current of the glucose sensor under visible light illumination, as compared with in the dark, is attributed to the LSPR effect. Figure 10 reveals the amperometric response of the interfering effect of the compounds and the detection limit of the Au NPs/Au/ZnO NSs sensor as measured in the dark, and under blue and green LED illumination. One of important analytical abilities of biosensors is discriminate the target analyte and interfering species with similar electroactivity. In physiological fluids, ascorbic acid (AA) and uric acid (UA) are the most significant interferences, which maybe interfere the glucose sensing. However, the glucose of normal human blood was approximately 2~7 mM, which concentration are much higher than interfering species of human blood around several decade folds. The measurement solution consisted of the sequential additions of 2 mM glucose, 0.02 mM UA, 0.1 mM AA, and 2 mM glucose in a 0.1 M NaOH solution in the dark, and under blue and green LED illumination. The applied bias of the amperometric measurements was 0.2 V at room temperature. In the dark and under visible light illumination, the interfering UA and AA species response was so small as to be negligible compared with the 2mM glucose response. The limit of detection (LOD) for the Au NPs/Au/ZnO NSs glucose sensor is 0.5µM, and the


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green illumination enhanced the highest sensing current to approximately ~2.2µA. This means that the Au NPs/Au/ZnO NSs sensor has high sensitivity and features good selectivity toward sensing glucose. It is important to evaluate the long-term performance stability of the Au NPs/Au/ZnO NSs based sensor in the dark, and under blue and green illumination, as shown in Figure 11. These sensor samples were stored in a sealed box under atmospheric and room temperature conditions. The sensing response of the sensors were measured in the dark, and under blue and green LED illumination at various glucose concentrations (4.0~20.0 mM) every 5 days. The proposed sensor displayed good long-term stability and reproducibility in the dark and green illumination during 20 days; however, the performance of blue illumination sample had poor stability and decayed to a normalized sensitivity of ~70.0%. Although the PPF (~605.6 µmol·m-2·s-1) of green light is around 5.4-fold greater than blue light (~112.0 µmol·m-2·s-1), their glucose sensing currents and sensitivities are similar, which suggests that the high photo energy (2.95eV) of the blue light not only increases the LSPR effect, but may also excite the electrochemical reaction and cause the stability to decay. Table 1 presents the key parameters of the noble metal NSs based sensors acting as the working electrodes in non-enzymatic and enzymatic glucose sensors. The Au NPs/Au/ZnO NSs based working electrode sensor features the maximum sensitivity of 3371.9 µA/cm2mM-1, which is much higher compared with previous enzymatic glucose sensor reports.20-28 Further, the sensitivity of the Au NPs/Au/ZnO NSs sensor can be enhanced by



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approximately ~30.8% (4410.9 µA/cm2mM-1) and ~23.3% (4157.8 µA/cm2mM-1) by blue and green illumination, respectively. It is believed that the high sensitivity of this sensor is due to the very large surface-area-to-volume ratio attributed to the 20 nm thick Au thin film sputtered on ZnO nanowires and nanosheets to increase the surface area of the Au thin film by the ZnO NSs template. The large amount of Au NPs were synthesised on Au/ZnO NSs for increasing the working electrode surface area, and produced electrochemical catalytic quantum dots. This sensor contains 2-D, 1-D, and 0-D NSs and has no interface barrier due to the same metal, namely Au, being used as well as the annealing process. The Au NPs/Au/ZnO NSs also produces the LSPR effect to enhance the sensitivity by blue and green LED illumination.

Conclusion

High density ZnO nanowires and nanosheets were synthesized via the hydrothermal process on ITO/glass substrate. The 20nm Au thin film was sputtered onto ZnO nanowires and nanosheets to form core-shell NSs, after which they were decorated with Au NPs via a solution with a dilution ratio of 1:5000. SEM images showed that the average length and diameter of the ZnO nanowires were ~2.3 µm and 200~600 nm, respectively. The thickness of the ZnO nanosheet was around 20~70 nm, while the diameter of Au NPs was around 3~10 nm according to TEM observation. The 2-D, 1-D, and 0-D NSs of the sensor have a large surface-area-to-volume ratio, the electrochemical catalytic quantum effect and avoid issues association with the heterojunction interface barrier. The ACS Paragon Plus Environment

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glucose sensitivities of the Au NPs/Au/ZnO NSs sensor in the dark, and under blue and green illumination were 3371.9, 4410.9, and 4157.8 µA/cm2mM-1, respectively. The blue and green illumination enhanced the sensitivity of the Au NPs/Au/ZnO NSs sensor by approximately ~30.8% and ~23.3%, respectively. The illumination peaks of the blue and green LEDs and the corresponding increase in CV sensing currents were respectively ~420 and ~540 nm, and ~27% and ~35% compared with measurements in the dark. Moreover, the glucose detection limit of the Au NPs/Au/ZnO NSs glucose sensor was 0.5µM, and the visible light illumination enhancements were attributed to the LSPR effect.

ASSOCIATED CONTENT

Supporting Information.

Top view FE-SEM images of the 40nm thickness Au film/ZnO NSs synthesized on ITO/glass substrate (Figure S1). UV-Visible transmittance, reflectance, absorptivity spectra for the ZnO NWs/ITO/glass and Au NPs/Au/ZnO NWs/ITO/glass samples (Figure S2) (PDF)

AUTHOR INFORMATION Corresponding Author * Tel: +886-6-260-6123 ext. 7785; Fax: +886-6- 2602305; E-mail: [email protected]



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Present Addresses Department of Electrical Engineering, National University of Tainan, Tainan 700, Taiwan (Republic of China).

ACKNOWLEDGMENT

The authors would like to thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under Contract MOST 105-2221-E-024-015-.

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