生物论文代写:纳米金催化抗坏血酸的电催化氧化

生物论文代写:纳米金催化抗坏血酸的电催化氧化

Experimental

2.1 Chemicals and Reagents

Titanium foil of (99.7%) 0.25 mm thickness, pyrrole (98%) and glucose (99.9%) were purchased from Sigma-Aldrich and used as received. L-ascorbic acid (99%) and uric acid (99%) were procured from Lobachemie, India. Ammonium fluoride, glycerol, chloroauric acid and other chemicals were of analytical grade and used without further purification. 5 mM potassium ferricyanide in 1 M KCl was used for electrochemical characterisation and 0.1 M PBS of pH 7 for amperometric measurements. Solutions used in this study were prepared with double distilled water.

Instrumentation

Anodization of titanium foil was carried out using a conventional electrolytic cell with titanium foil as the anode and platinum foil as the cathode. An indigenously fabricated DC power supply (5 A, continuous variable 60 V) was employed for anodisation and a magnetic stirrer for agitation of the electrolyte during anodization. The morphology of the anodized titanium foil was studied by a variable pressure FESEM (Hittachi SU 6600) and AFM (NT-MDT NTEGRA Prima). All electrochemical experiments were carried out using CHI 660C electrochemical workstation (CH Instruments, Austin, TX, USA), with a conventional three-electrode cell consisting of TiO2 or modified TiO2 as working, platinum foil as counter and Ag/AgCl as reference electrode. All potentials measured in this work were referenced to Ag/AgCl (3 M KCl) electrode.

Fabrication of TiO2

Prior to anodization, the titanium strips were washed with double distilled water, followed by ethyl alcohol and acetone and then dried in nitrogen atmosphere. Titanium was potentiostatically anodized from a mixture of glycerol and water (10%v/v) containing ammonium fluoride (0.15 M) at 20 V for 5 hours [60]. The anodized specimen was washed in double distilled water and preserved in deaerated double distilled water.

2.4 Fabrication of Sensor Electrodes

The electrodeposition of PPy on TiO2 nanotube electrode was carried out by immersing in a solution of 0.1 M pyrrole in 1 M KCl and cycled 5 times between 0 and 1.2 V at a scan rate of 100 mV s−1 and rinsed with water. It was denoted as PPy/TiO2. Electrodeposition of AuNPs on this was carried out by CV in a potential window of 0.5 to -1.0 V at a scan rate of 100 mV s-1 from 5 mM solution of chloroauric acid. Then the modified electrode was taken out and rinsed with water. It is denoted as AuNP/PPy/TiO2.

2.5 Electrochemical Characterisation

All electrochemical characterization was carried out in 5 mM potassium ferricyanide in 1 M potassium chloride. CVs were carried out in a potential range of 0 of 0.6 V at a scan rate of 100 mV s−1. EIS of the bare and modified electrodes was carried out at open circuit potentials, in the frequency range of 0.01 Hz to 100 KHz with potential amplitude of 10 mV. The impedance spectra were plotted in the form of complex plane diagrams.

2.6 Electrochemical Oxidation of AA

The CV and LSV were carried out in 0.1 M PBS in a potential window of 0 to 0.6 V at a scan rate of 100 mV s−1. DPV experiments were conducted at a scan rate of 20 mV s−1 under 50 mV pulse amplitude, pulse width of 50 ms and pulse time of 200 ms. The steady state response of the sensor to AA was carried out at 0.2 V in a constantly stirred solution of 0.1 M PBS. 10 μL of AA solution was injected at regular intervals so that the resultant concentration varied from 1μM to 5 mM. The interference of other biomolecules was studied by injecting 10 μL of the solution into the test solution. All experiments were conducted at room temperature (25 ± 2 oC) and were repeated at least three times to check reproducibility.

3. Results and Discussion

3.1 Surface Characterization

Fig. 1 shows a comparison of the morphology of the bare TiO2, PPy/TiO2 and AuNP/PPy/TiO2 electrodes by FESEM and AFM.

Figure 1A-F

Highly ordered titanium dioxide arrays can be seen in Fig. 1A. The diameter of the nanotube is about 40 nm. Fig. 1B is polypyrrole coated (for five potential cycles) on TiO2 electrodes. It is seen from the image that the coating of the PPy is not uniform. This arises because the anodized surface is not planar in the nanoscopic regime and hence the electric field on the surface is not uniformly distributed resulting in formation of polymer at more projected areas. It was found that AuNP deposited on the sites where PPy has formed (Fig. 1C), again due to the differential electric field. Here, the deposition of AuNPs was carried out for four potential cycles. On increasing the number of potential cycles, the size of the AuNPs increased and formed nanoclusters. Fig. 1D shows the FESEM image for 10 potential cycles of AuNPs deposition. The AFM study carried out with AuNP/PPy/TiO2 supports these observations. The particle size of the AuNP deposited by four potential cycles (Fig. 1E) was much smaller than the one obtained after 8 potential cycles of deposition (Fig. 1F).

Since the PPy has inherent ability to accommodate metal nanoparticles and the nanotubular structures of the TiO2 makes the modified electrode a suitable matrix for AuNPs and prevent aggregation. After deposition of AuNPs on the PPy/TiO2 the colour turns to reddish violet which is the typical colour of gold nanoparticle. This was not in the case of direct electrodeposition of gold on unmodified titanium electrode from the same chloroauric acid solution under similar conditions and a golden yellow coloured deposit was obtained. From this observation it is reasonable to assume that the deposition of Au on the modified electrode takes place as gold nanoparticles and on the unmodified electrode as bulk gold.

3.2 Electrochemical Characterisation of Sensor Electrodes

Figure 2A, B

Fig. 2A shows the CVs of the bare and modified electrodes in 1 mM potassium ferricyanide solution in 0.1 M KCl. The curve ‘a’ shows no characteristic peak in ferricyanide solution, indicating that the TiO2 nanotube electrode surface does not catalyse the redox of ferricyanide. The curve ‘b’ depicts an increase in charging current because of the deposition of PPy on the TiO2 nanotube electrode which is a characteristic feature of the conducting polymer coated electrodes. The curve ‘c’ obtained on the AuNP/PPy/TiO2 electrode shows typical redox peaks of [Fe(CN)6]3-/4-. The peak observed was sharp and peak separation was only 80 mV indicating good electronic communication between the AuNPs and titanium through the PPy/TiO2 matrix.

The effect of AuNPs on the electrocatalyic activity was studied by increasing the deposit of AuNPs on the TiO2 electrode. The CVs carried out after 2, 4, 6, 8, 10 and 12 cycles of AuNP deposition is presented in Fig. 2B and it shows that the peak current increases with increase in AuNPs on the electrode surface. As reported, the electron transfer reaction of [Fe(CN)6]3-/4- depends on the surface coverage of AuNPs [30, 61-62], and hence the observed increase in peak current with the increase in AuNPs. Further, the CVs observed in ferricyanide with repeated immobilization of AuNPs showed same peak separation which indicates that the particles did not aggregate but dispersed, as the size of the immobilized AuNPs increases the peak separation also increases [63]. The low value of peak separation suggests that the electron transfer on the modified electrode is relatively fast [63].

Again, the peak current increases linearly with square root of scan rate (Fig. S1 in the supporting information) with a linear regression equation, ipa (µA) = 2.7232 + 1.1921ν1/2 (mV s-1)1/2 and (r = 0.9989). This illustrates that the redox of ferricyanide is a typical diffusion controlled process on the modified electrode.

The AC impedance studies on the bare and modified electrodes show that the electron transfer resistance (Ret) of the TiO2 electrode decreases due to the deposition of PPy on the electrode (Fig. S2 in the supporting information). Further, repeated immobilization of AuNPs on the PPy/TiO2 electrode resulted in a gradual decrease of impedance. This again confirms that during each CV in chloroauric acid the amount of AuNPs deposited on the electrode surface increases.

3.3 Electrocatalytic oxidation of AA