Visible Illumination Enhanced Nonenzymatic Glucose Photobiosensor based on TiO2 Nanorods Decorated with Au Nanoparticles
Abstract—A nonenzymatic glucose photobiosensor was developed based on Au-nanoparticle-decorated TiO2 nanorods (NRs) under visible illumination. Au nanoparticles (NPs) absorbed the visible illumination, resulting in surface plasmon resonance (SPR). The SPR of the Au NPs indicated there was a strong electric field around them, which promoted the transport of more electrons to the TiO2 NRs and enhanced the glucose sensing properties. The sensing current under visible illumination was 5 times higher than in the dark when in 0.1 M NaOH solution at a potential of 0.17 V. Moreover, the Michaelis-Menten constant (Km) of the Au NPs/TiO2 NRs/FTO under visible illumination was
0.52 mM, which is much smaller than that reported previously. Moreover, these results indicate that the Au NPs/TiO2 NRs/FTO under visible illumination feature outstanding properties as a nonenzymatic glucose photobiosensor.
I.INTRODUCTION
ITANIUM dioxide (TiO2), zinc oxide (ZnO) [1], and tin oxide (SnO2) are important transparent conducting oxide (TCO) materials, which are commonly applied to fabricates transparent electrode contacts due to their outstanding physical and optical characteristics [2]. With a large band gap of 3.0 eV (for rutile), TiO2 has limited optical absorption within the ultraviolet (UV) light region [3]. TiO2 nanorods (NRs) have good biocompatibility, a large surface area, and a large number of active sites for chemical reactions. Accordingly, TiO2 has been extensively investigated for use in solar cells [4-6], field emission devices [7], photodetectors [8], gas sensors [9], and biosensors [10, 11]. Diabetes mellitus is a worldwide public health problem, with diagnosis and treatment requiring a quick and reliable blood glucose sensor. In response, electrochemical sensors and biosensors have received great interest. Many studies have employed the use of glucose oxidase (GOD) to detect glucose [12-13]. However, using an enzyme has certain drawbacks due to the intrinsic properties of proteins, namely chemical instability and temperature sensitivity, both of which lead to poor long-term stability. As such, much effort has been directed toward producing nonenzymatic glucose sensors, which is the fourth generation of analytical glucose oxidation, based on the direct oxidation of glucose on a modified electrode surface [14-15].
Noble metals such as Pd, Ag, and Au have been explored as catalysts for nonenzymatic glucose sensors [16-18]. In addition, noble metals are also attractive because of their nanoscale size for absorbing visible light and generating the surface plasmon resonance (SPR) effect, which is the free electrons of surface NPs that have collective oscillation [19-21]. Then, the SPR effect creates a strong electromagnetic field to excite the electrons, which gain greater energy for glucose sensing enhancement. With the two features of SPR, Kochuveedu et al. used SPR to induce the visible-light photocatalytic activity (absorption wavelength of 500-600 nm) of TiO2 via Au NPs [22]. Also the SPR-based sensors have been used for a variety of applications, including chemical sensors [23], biosensors [24], photovoltaics [25], and solar cells [26]. Based on the nonenzymatic glucose sensor and the SPR of noble metal, in this study, we investigated the performance of TiO2 NRs and Au-NPs-decorated TiO2 NRs, respectively, on a fluorine-doped tin oxide (FTO) substrate as a nonenzymatic glucose sensor. Results show that the sensing properties of the Au NPs/TiO2 NRs glucose biosensor were enhanced by visible illumination due to the SPR of the Au NPs. Moreover, this configuration not only solved the enzyme storage problem, but also improved the sensing characteristics via the visible illumination.
II.EXPERIMENTS
A fluorine-doped tin oxide (FTO) thin film was deposited on 2 cm × 2 cm glass substrate (Corning 1737) by RF sputtering. The growth area (1.5 cm × 2 cm) of the TiO2 NRs was defined, and a photoresist (PR) mask with the photolithography process was used. In the hydrothermal synthesis of the TiO2 NRs, 0.5 ml of titanium tetrachloride (TiCl4, Sigma-Aldrich corp.) was added dropwise to a solution of 6 ml of concentrated HCl and 4 ml of deionized (DI) water. After sufficient mixing, the solution was added to a small Teflon beaker, in which the FTO/glass substrate had been placed at an angle and immersed in the solution. Then, the Teflon beaker was placed in a sealed stainless steel autoclave, and maintained at 190 °C in an electric oven for 1 hr. Subsequently, the beaker was allowed to cool to room temperature. After the TiO2 NRs growth, a 5-nm-thick Au film was deposited onto the as-prepared TiO2 NRs/FTO/glass substrate using thermal evaporation. The device was then subjected to annealing in a furnace at 300 °C in air for 30 min to form the Au NPs. After the Au NPs/TiO2 NRs had grown on the FTO/glass substrate, the PR was removed, leaving the FTO to act as the electrode.Attributes of the as-prepared TiO2 NRs/FTO substrate were evaluated as follows. The crystal quality of the as-grown TiO2 NRs and Au NPs on the TiO2 NRs was characterized using X-ray diffraction (XRD, MAC MXP18). Surface morphology of the samples and the size distribution of the NRs were characterized by field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7001F) and high-resolution transmission electron microscopy (HR-TEM, JEOL-JEM2100F). while cyclic voltammetry (CV) and amperometetric response measurements were performed using a potentiostat (Autolab PGSTAT101). A conventional three-electrode system was employed, in which TiO2 NRs/FTO and Au NPs/TiO2 NRs/FTO served as the working electrodes, a Pt electrode acted as the counter electrode, and an Ag/AgCl electrode as the reference electrode. The visible illumination power was 7 W.
III.RESULTS AND DISCUSSION
SEM images of the TiO2 NRs and Au NPs/TiO2 NRs synthesized on the FTO/glass substrate are shown in Figures 1
(a) and (b), with the insets providing high-magnification SEM images. The average length and diameter of the TiO2 NRs were approximately 830 nm and 130 nm, respectively. As can be seen in the figures, the Au NPs, with diameters of about 5~30nm, could be distinctly observed on the surface of TiO2 NRs. The TiO2 NRs surface area was calculated to be around 4.73×10-3 m2, as observed from the SEM images.According to the energy-dispersive X-ray spectroscopy analysis of Au NPs/TiO2 NRs/FTO (Figure 1 (c)), the atomic ratio of Au was 2.87%. Further, the XRD patterns in Figure 2 indicate that the NRs were in the TiO2 rutile phase (JCPDS card no. 87-0920). In general, the XRD pattern of rutile TiO2 NPs show a notable (110) diffraction peak at 27.5°, but the (110) peak of the TiO2 NRs/FTO samples was slight. The XRD pattern reveals a strong (101) diffraction peak at 36.1°. The weak (110) and strong (101) peaks are similar to the XRD results of previous reports on TiO2 NRs/FTO [5, 6]. Figure 3 displays the room-temperature absorbance spectra, recorded in a wavelength range of 300-800 nm, of TiO2 NRs and the Au NPs/TiO2 NRs. It should be noted that absorption at wavelengths below 400 nm can be attributed to the characteristic absorption of TiO2 NRs, and that the Au NPs/TiO2 NRs had an extra absorption peak at around 400-700 nm due to the surface plasmon absorption of Au NPs [27]. The electroluminescence (EL) spectrum of visible illumination (Figure 3 inset) was measured at 445 nm and 556 nm, thereby matching the absorbed visible wavelength of the Au NPs.
The hydrophilic properties of the Au NPs have been investigated with temperature-programmed desorption and X-ray photoelectron spectroscopy [28]. The monolayer desorption peak was observed and confirmed from enhanced H2O–Au adsorption energy, which increased the hydrophilic
properties of the Au NPs/TiO2 NRs sample. A previous study reported that TiO2 thin film could be enhanced to feature super hydrophilic properties by UV light illumination [29]. The UV photo-generated electrons and hole pairs dissociate and adsorb water to create hydrophilic OH groups on the TiO2 surface. The blue energy of visible light is larger than the deep level of TiO2, which means that visible light can excite and generate some electrons and hole pairs to increase the hydrophilic properties.Figure 6 presents the CV curves of the FTO, TiO2 NRs/FTO, and Au NPs/TiO2 NRs/FTO in a 0.1 M NaOH and 10 mM glucose solution obtained at a scan rate of 50 mV/s under visible illumination. The peaks for the FTO substrate did not increase with glucose concentration, indicating that FTO had no glucose-sensing capability. TiO2 NRs/FTO had a sharp oxidation peak in the 0.1 M NaOH/glucose solution at around 0 V, indicating that TiO2 NRs have excellent and accurate glucose-sensing ability. The CV curves of the Au NPs/TiO2 NRs/FTO indicate superior performance compared to those of TiO2 NRs/FTO. The Au NPs, acting as a catalyst, oxidized the glucose.
In the negative direction scan of Au NPs/TiO2 NRs/FTO, the reduction of Au oxide caused the Au NPs to behave as action sites exposed for the direct oxidation of glucose, which resulted in the sharp current peaks at 0.09 and
-0.22 V. According to chemisorption of the hydroxide anions, the surface of the Au NPs can be oxidized with OHads to form an initial Au[OH]ads layer; then, the Au[OH]ads was oxidized with glucose to generate gluconolactone. The detailed mechanism of Au NPs sensing glucose has been discussed in previous reports [30-31] sensitivity was at 0.17 V. Further, for the 0.1 M NaOH and 10 mM glucose solution shown in Figure 7 (c), the current under visible illumination (11.6 μA at 0.17 V) at a scan rate of 50 mV/s almost five times to that in the dark (2.79 μA at 0.17 V). This was attributed to the Au NPs absorbing the visible illumination due to SPR, and thereby generating a strong electric field that facilitated the transport of electrons from glucose oxidation to the TiO2 NRs [32]. The CV peaks were decreased when the concentration of glucose exceeded 20 mM. These measured high glucose concentration samples were observed with an optical microscope. The surface of these samples absorbed and reacted to form an organic film, which caused that reaction surface area to decrease.
It should be noted that the smaller the Km value, the greater the affinity for glucose [34-36]. Compared to other glucose biosensors (Table 1) [37-41], the Au NPs/TiO2 NRs/FTO under visible illumination had relatively good characteristics. The affinity toward glucose of the illuminated electrode is higher than natural glucose oxidase, due to these Au NPs absorbing visible illumination to produce the SPR effect, which causes strong electron motion and an associated electromagnetic field that enhances the glucose affinity.The mechanisms of the glucose sensing activity of TiO2 the TiO2 interface, thereby trapping the electrons generated from glucose oxidation at the surface of the TiO2 NRs and reducing the current response. Moreover, the activated chemisorption model suggests that the electrocatalysis process, involving the d-electrons and d-orbitals of a metal substrate, allows it to form suitable bonds for the absorption of glucose to the electrode surface [41]. Au NPs were chosen to modify the TiO2 NRs (Figure 9 (b)); however, the difference in the work functions of Au (5.47 eV) and TiO2 (4.13 eV) created a Schottky barrier, which in turn made the collection of electrons by TiO2 more difficult. Consequently, the glucose sensing current did not increase, as expected. Visible illumination was therefore employed not only to generate electrons via visible light absorption, but also to generate a strong electric field near the Au NPs via SPR [42]. In this manner, the glucose-sensing current could be enhanced via electrons crossing the Au NPs/TiO2 NRs Schottky barrier and being collected by the TiO2 NRs, as shown in Figure 9 (c). The Schottky barrier height depends on the work functions of TiO2 (4.9 eV) and Au (5.1 eV). The Schottky barrier height was calculated to be ~0.2 eV by the following equation, (qψSB =qψTiO2－qψAu).
IV.CONCLUSION
TiO2 NRs grown on an FTO substrate and decorated with Au NPs were employed for glucose sensing. Although the TiO2 NRs exhibited intrinsic sensitivity toward glucose, the Au NPs acted as a catalyst and enhanced the sensitivity. The currents of the Au NPs/TiO2 NRs/FTO in the dark and under visible illumination were 2.79 and 11.6 μA, respectively, in 0.1 M NaOH and 10 mM glucose at a potential of 0.17 V. Moreover, the Km values were correspondingly 4.47 and 0.52 mM in the dark and under visible illumination. These values are smaller than those previously reported due to the Au NPs absorbing visible illumination to produce the SPR effect, and thereby creating a strong electric field that promotes the transport of electrons to the TiO2 NRs. Accordingly, the Au NPs/TiO2 NRs/FTO under visible illumination exhibited great affinity for Dac51 glucose.