生物论文代写:纳米金催化抗坏血酸的电催化氧化

Figure 3

The DPVs of AA oxidation on bare TiO2, PPy/TiO2 and AuNP/PPy/TiO2 electrodes are presented in Fig.3. No characteristic peak on TiO2 and PPy/TiO2 (curve a and b) was obtained indicating no oxidation of AA. But on the PPy/TiO2 electrode, an increase in charging current was observed due to the coating of conducting polymer. On the AuNPs modified electrode a sharp peak at 0.15 V appears corresponding to the oxidation of AA to dehydroascorbic acid. This observed potential is about 0.34 V less positive than that of the oxidation on bare gold electrode [64-65], illustrating the electrocatalytic effect of the AuNPs on the electrode surface. Since the oxidation takes place at a lower potential, the commonly interfering molecules which got oxidized at a relatively positive potential will not interfere in the detection of AA. Further deposition of AuNPs on the electrode increases the peak current depicting the enhancement of electrocatalytic activity towards the oxidation of AA. A similar trend in peak current for AA oxidation was observed upto 12 CVs of AuNP immobilization. The peak current increases linearly with concentration of AA with a very slight shift in peak potential (Fig. 4). Each addition corresponds to an increment of 100 µM and a linear response was observed with a linear regression equation ip(µA) = 0.0083C (µM) + 1.3041 (r = 0.9989) throughout the concentration.

Figure 4

After 12 potential cycles of AuNP deposition, further depositions did not cause any increase in the peak current for AA oxidation, instead the peak current was found to decrease considerably. Again, a new peak was observed at 0.30 V (Fig. 5). The peak current intensity increases with concentration of AA accompanied with a positive shift in peak potential. The reduction in peak current and appearance of new peak can be attributed to the conversion of AuNPs into the bigger clusters of gold at very high gold loading [66] since the size of the nanocluster and their aggregation state could influence the catalytic efficiency.

Figure 5

CV obtained in 100 µM AA in 0.1 M PBS (Fig. S3 in the supporting information) showed an anodic peak but no peak was observed in the cathodic scan indicating irreversibility of electrocatalytic oxidation of AA on AuNP/PPy/TiO2 electrode. LSVs obtained at various scan rates showed that the peak current varies linearly with square root of the scan rate, which establishes the diffusion controlled nature of the oxidation process (Fig. S4 in the supporting information).

3.4 Amperometric Detection of AA

Figure 6A, B

The steady state current response observed at 0.2 V with successive additions of AA is shown in Fig. 6A. Time required to obtain a stable response after the injection of AA was less than one second. Three different ranges of concentrations were tested by successive additions of AA in 0.1 M PBS of pH 7. The first set of 4 additions was introduced in increments of 1 μM, the next set of 4 additions in increments of 10 μM and the last set of 5 additions in increments of 100 μM. Inset in Fig. 6A shows the calibration curve. The sensor exhibits linearity in the range of 1 μM to 5 mM with a linear regression equation ip(µA) = 0.02686 + 0.00416 C(µM) (r = 0.9996) and excellent sensitivity of 63.912 μA mM−1 cm−2. The detection limit was found to be 0.1 μM. It is worth mentioning that the observed sensitivity in this study was remarkably higher than that of similar AA sensors reported [67-68].

The stability of the response at higher concentration and fouling due to the oxidized products during the repeated measurements were studied amperometrically by adding higher concentration (100 µM) of AA solution to the constantly stirred PBS solution at 0.2V. The current response obtained for all additions remains the same and is given by the calibration curve (Fig. 6B). This clearly eliminates the possibility of fouling caused by the oxidized products of AA at the AuNP/PPy/TiO2.

3.5 Reproducibility and Storage Stability

The reproducibility of the sensor was evaluated by making five numbers of AuNP/PPy/TiO2 electrodes and testing with 100 µM AA solution in 0.1 M PBS at 0.2 V. The variation was observed to be less than 1.5% (Fig. S5A in the supporting information) which suggests that the electrode modification method adopted in this work was highly reproducible. The sensor was preserved in 0.1 M PBS at room temperature (25 ± 2 -C) when not in use. The long term storage stability of the sensor was examined by measuring the amperometric response to 100 µM AA once in every three days over a period of one month (Fig. S5B in the supporting information). The decrease in sensitivity was less than 3% of its original value. This study has convincingly proved that the sensor has good reproducibility and storage stability.

3.6 Effect of Interfering Species

Table 1

The interference of species such as UA, DA, AP and glucose in the determination of AA was studied by amperometric method. Interfering species were injected along with AA into a constantly stirred solution of PBS and their response was noted and presented in Table 1. AA was found to respond in the same way irrespective of the presence or absence of interferents, thus illustrating that the quantitative determination of AA was in no way affected by interfering species. Further, the response current obtained for these interfering species was less than 2% of that observed for AA.

3.7 Analytical Applications

Figure 7

The sensor developed was directly tested for determination of AA in lemon. Fresh juice was injected to 10 mL of constantly stirred solution of 0.1 M PBS and the amperometric response was recorded at 0.2 V and the results are presented in Fig.7. First three additions are 10 µL and the fourth addition is of 100 µL of lemon juice. The concentration of AA was calculated and found to be 339 µg mL-1, which is very close to the value determined by capillary zone electrophoresis [69].

Conclusion

Highly sensitive and selective determination of the AA has been achieved by developing a sensor based on AuNP and PPy on TiO2 nanotube arrays. The oxidation potential of AA at this electrode was very low (0.15 V). The sensor showed linearity over a wide range of concentration. In this study AA solutions of concentrations from 1 µM to 5 mM were injected and a linear steady state current response was obtained. This linearity even at very high concentration establishes that no fouling has taken place due to the oxidized products of AA. The interference of other biomolecules was tested and found that the commonly interfering species did not affect the detection of AA at this electrode. The applicability of this sensor was extended for the testing of AA in lemon.