FABRICATION AND SURFACE MODIFICATION OF PT NANOWIRES FOR GLUCOSE DETECTION

In this paper we present a new fabrication technique that only uses conventional techniques of microtechnology such as microlithography, thin-film deposition and directional ion beam etching to makevery narrow, wafer-scale length platinum (Pt) nanowires, named deposition and etching under angles (DEA). Then fabricated Pt nanowires electrodes were modified by using several chemicals to immobilize glucose oxidase (GOD) enzyme for application in glucose detection. A cyclic voltammetry (CV) technique was used to determine glucose concentrations. The detection results showed that GOD was immobilized on all of the tested surfaces and the highest glucose detection sensitivity of 60μM was obtained when the Pt nanowires were modified by PVA. Moreover, the sensors also showed very high current response when the Pt nanowires were modified with the cysteamine SAM.


INTRODUCTION
Nanoscale devices based on nanowires have been realized for applications in electronics, optics, gas, and especially biomedical sensing [1][2][3].One-dimensional structures such as nanowires are particularly compelling for electronic interconnects and biosensing applications due to their suitability for large-scale high-density integration and high sensitivity to surface interactions.
Although nanowires have been fabricated by various methods [4][5][6], simple fabrication techniques which are not only easily addressed electrically, but also maintain reasonable costs for practical application, are also highly desirable.
Surface properties are especially of concern because the interaction of any metal electrode with its environment mainly occurs at the surface, and also because of the dependence of the response on the surface state of the electrode.Many analytical applications, such as electron transfer reaction, preferential accumulation, or selective membrane permeation, can benefit from chemically modified electrodes [7][8][9].Other important applications including electrochromic display devices, controlled release of drugs, electrosynthesis, corrosion protection, etc [10][11][12][13][14] can also benefit from the rational design of

Fabrication of Pt nanowires by the DEA technique
The new fabrication process that has been   Figure 3 shows a diced chip that contains an array of Pt nanowires, while the inset image shows that each nanowire from the array is individually electrically addressed.This allows the fabricated nanowires to easily be further connected to an outer electronics for detailed device measurement and applications.

Cyclic voltammograms of enzyme electrodes
The response current of glucose on three types of biosensors was recorded and is shown in figure 6 with a potential scan rate of 100 mVs.The results show that all enzyme electrodes have high electron transfer efficiencies.We observed that with an increase in glucose concentration the redox current increased monotonously at a potential higher than 0.4 V and it just became stable only when the applied voltage was higher than 0.6 V.In contrast, the CV curve of a gel-SiO2 modified electrode had an unstable current, and the applied voltage was higher than 0.7 V because of the influence of the oxygen concentration in Moreover, we also found that the oxidation current or reduction current increased linearly with the concentration of glucose, and this important result is reported in detail in the next section.

Amperometric response of glucose sensor
three-electrode conventional cell including the gold nanowires chip as working electrode, a platinum rod 0.5 mm diameter was used as a counter electrode, and a Ag/AgCl electrode as reference.All measurements were carried out under room temperature.
developed and allows the fabrication of long and narrow Pt nanowires is shown schematically in figure 1.Briefly, a layer of 1000 nm silicon dioxide (SiO2 ) is grown on a 4 inch, (100) silicon wafer by means of wet oxidation.Conventional microlithography is then carried out to define patterns on the wafer, followed by isotropic etching of SiO 2 for 1 min in a buffered oxide etching (BHF) solution.This isotropic etching creates an under-etching or nano-spacer with width about 65-70 nm below the photoresist layer.Layers of 40 nm platinum/5 nm chromium are then deposited by an E-beam evaporator with an inclined angle of 30 o on the surface of the patterned wafer.The typical evaporation rate is 1 Å s−1 for both Cr and Pt.As the result of inclined deposition, a small part of the Pt/Cr is deposited into the nano-spacer or hidden below the photoresist film.In our work, Cr is used as an adhesive material for deposition of Pt film, and the width of the hidden metallic part depends on several parameters, such as the dimensions of the nano-spacer and the inclined evaporation angle.

Figure 1 .Figure 2 .
Figure 1.DEA fabrication process to make waferscale Pt nanowire using only conventional microfabrication techniques.

Figure 3 .Figure 2
Figure 3.A diced chip contains an array of Pt nanowires.The inset image shows individually electrically addressed Pt nanowires, thus making the nanowires ready for measurement.2.3.Preparation of enzyme electrode on different modified surface of Pt nanowire Pt nanowires chips were immersed in dicholoromethane, propanol, acetone and deionized water (DI) for 5 min, respectively.Then the samples were dried with blown nitrogen and cleaned by using oxygen plasma (power of 250 W for 6-7 min).Then it was electrochemically scanned repeatedly until the voltammogram characteristic was obtained.In the first generation of glucose sensor, the cleaned electrode was immersed into the compound of 1 ml gelatin-SiO2 (3:1 v/v mixture of concentrated gelatin, SiO2 stirred in 2 h) and 0.5 ml GOD (5 mg/ml of acetate buffer, pH 5.5) solution.Afterwards, the electrode was dried at 4 0 C and washed with DI water before being used for glucose detection.In the next experiment, the electrode was reduced by scanning it in 0.001 M H2 SO4.Then it was

Figure 4 Figure 4 . 3 . 3 .
Figure 4 shows an I-V characterization of the 20 µm length Pt nanowires.It can be seen that the wires have good electrical characteristics with linear IV behavior of the bulk metal Pt.Moreover, the measurement results show a resistance of about 1540 ± 40 Kfor the fabricated Pt nanowire.This value is only about 30% higher than the value calculated using the bulk material.

3. 4 .
Effect of pH on enzyme electrode The influence of pH buffer solution on glucose detection has been studied by several authors [7-10].Investigation of the effect of pH value on the performance of the glucose sensor is very important because the activity of immobilized GOD is pH dependent [8].In our work, the pH dependence of a modified electrode by PVA compound and PB mediator was evaluated over the pH range from 5.6 to 8.4.When the pH of the buffer was very low or very high, the GOD electrode exhibited low current response to glucose.An optimum response current was observed at a pH value of 7.2.

Figure 7
Figure 7 shows the dependence on glucose concentration (0-16 mM) of the CV curves of the electrodes modified by the three immobilizing methods.Obviously, the gelatin/SiO2 modified Pt had the lowest response current and corresponding coefficient ( R2 = 0.8335).This indicated that this

Figure 8 .
Figure 8. CV of enzyme electrode in 3 mM glucose solution at different times.From down to up 30, 20, 20 and 0 days, respectively.