Simultaneous Electrodeposition of Gold Nanoparticles and Electropolymerization of L-Cysteine through Paper-Based Microfluidic Sensor
Authors: Ghanbari Mohammad Hossein , Etzold Bastian J.M. 
Published: August 2024

Introduction

Using catalytic active modifiers and additives e.g. to improve electron or ion transport in electrochemical sensors usually offers significant benefits by improving sensitivity and selectivity. An elegant way to introduce both is electrodeposition and electropolymerization. Examples are known, where through these methods enhanced catalytic activity, stability, reproducibility, and active surface area could be achieved [1,2]. Paper-based microfluidic electrochemical sensors (μCS) have gained significant attention due to their simplicity, portability, cost-effectiveness, and potential for point-of-care applications. Although electrodeposition of AuNPs and electropolymerization of (L-cysteine) have been widely studied in static voltammetry using glassy carbon electrodes or screen-printed carbon-based electrodes (SPCE), their application by the microfluidic systems has been limited, and based on our knowledge, no report has been done on electrodeposition and electropolymerization through a μCS.
In this work, we show the electrodeposition of AuNPs and electropolymerization of (L-cysteine) onto the electrodes of paper-based microfluidic electrochemical sensor, while using the microfluidic channel also for transport of the electrodeposition solution.

Experimental

The alignment of the component for the μCS device is sketched in Scheme 1. After providing the HAuCl4 and L-cysteine solution through the sponge the electrochemical fabrication and detection are carried out through cyclic voltammetry (CV). AuNPs were electrodeposited and L-cysteine were electropolymerized simultaneously through the μCS by handling a potential window of −1.5 to +2.2 V at a scan rate of 100 mV/s for 10 cycles in a solution of 0.1 M PBS at pH = 5.5 consists of 1.0 mM HAuCl4 and 1.0 mM L-cysteine. Scanning electron microscope (SEM), CV, and electrochemical impedance spectroscopy (EIS) were employed to certify that the AuNPs and poly (L-cysteine) were attached successfully to the working electrode surface.

Results and discussion

CVs of the platforms were recorded in the redox probe solution consisting of 5.0 mM of [Fe(CN)6]3−/4−. By modifying the platforms by simultaneous electrodeposition of AuNPs and electropolymerization of (L-cysteine) not only did peak currents increase dramatically but also ΔEp were decreased to 17 mV and 13 mV for SPCE/S (ED & EP) and μCS/S (ED & EP), respectively. This decrement behavior indicating the presence of AuNPs, and poly (L-cysteine) enhances the reversible nature of the electrochemical reaction.
By comparing the Nyquist plots related to the bare SPCE and μCS it is clear that the Rct value for μCS (46 Ω) is much lower than SPCE (690 Ω) It may be attributed to the 3D and sandwich-like structure of μCS devices. SPCE/S (ED & EP) and μCS/S (ED & EP) present very smaller semicircles with Rct values of 225 Ω and 13 Ω, respectively. This remarkable increase in the current and substantial decrease in resistance of the modified platforms at first confirms which AuNPs and poly (L-cysteine) were successfully achieved onto the WE surface. Interestingly, the simultaneous deposition of AuNPs and poly (L-cysteine) by using μCS shows the lowest Rct and could be a highly interesting sensing electrode.
To ensure the performance of both simultaneous electrodeoposition of AuNPs and electropolymerization of (L-cysteine) and μCS in the enhancement of the surface area, accordingly, the calculated A of the SPCE/S (ED & EP) and μCS/S (ED & EP) were estimated to be 0.09 cm2 and 1.14 cm2, respectively. In comparison with the A value of the bare SPCE (0.06 cm2) and μCS, (0.31 cm2) these increases are 1.5 times for the SPCE and 3.6 times for the μCS.

Conclusion

The μCS/S (ED & EP) offers several core advantages over SPCE/S (ED & EP) in terms of cost, simplicity, and sensitivity. These unique advantages are due to the combined microfluidic configuration, 3D electrode layout, and a unique electrochemical modifier. According to the SEM results, the images corroborated that the simultaneous synthesis of nanocomposite on the working electrode surface is carried out successfully. This strategy can assist as the proper way to boost the surface area of the sensor. We believe our findings would have significant implications in developing other portable, fast, and cost-effective electrochemical detection platforms, such as clinical diagnosis and security inspection.

References

Mei, Xue, et al. "A novel electrochemical sensor based on gold nanobipyramids and poly-l-cysteine for the sensitive determination of trilobatin." Analyst 148.10 (2023): 2335-2342.

Ghanbari, Mohammad Hossein, et al. "Simultaneous electrochemical detection of uric acid and xanthine based on electrodeposited B, N co-doped reduced graphene oxide, gold nanoparticles and electropolymerized poly (L-cysteine) gradually modified electrode platform." Microchemical Journal 175 (2022): 107213.

Bard, Allen J., Larry R. Faulkner, and Henry S. White. Electrochemical methods: fundamentals and applications. John Wiley & Sons, 2022.

Figure 1