Ferrocene Functionalized Gold Nanoparticles on Carbon Nanotube Electrodes for Portable Dopamine Sensor

Abstract

Introduction Abnormal dopamine levels lead to severe diseases and disorders, including Parkinson's disease, Alzheimer's disease, and schizophrenia [1]. There is a need for sensitive and cost-effective dopamine sensors to provide rapid diagnosis at the point of care. Several approaches, such as high-performance liquid chromatography-mass spectrometry, fluorescence, and chemiluminescence based sensors, have been reported for dopamine detection with good sensitivity. However, these techniques generally are tedious, time-consuming, require sophisticated instruments, and thus not suitable for point of care (POC) testing. On the contrary, electrochemical sensors are promising due to their numerous advantages, including simple operation, faster response, low-cost, and accessibility for onsite monitoring. In addition, tracing dopamine, an electroactive molecule, with an electrochemical technique is also favorable. However, dopamine's electrochemical oxidation is often disrupted by the interference from other electroactive molecules (e.g., ascorbic acid, uric acid) due to their overlapping oxidation peak potentials and the higher physiological concentrations comparing to dopamine in biological fluids.A wide range of chemically modified electrodes is explored to improve the electrocatalytic capability towards dopamine oxidation. In this regard, the modification of electrodes with carbon-based nanomaterials and redox mediators has gained keen interest. For instance, ferrocene (Fc) and its derivatives can catalyze the oxidation or reduction of several biomolecules with advantages of fast response, stable redox states, electrochemical reversibility, and regeneration at low potential [2]. However, immobilization of Fc on electrodes remains challenging due to their weak adsorption and low stability. In this work, we create a new sensor by decorating ferrocene (Fc) functionalized gold nanoparticles (AuNPs) on carbon nanotube (CNT) to form an electroactive composite (Fc-AuNPs-CNT) for electrochemical detection of dopamine. The Fc functionalized AuNPs provide excellent catalytic activity towards dopamine with improved electron conductivity. Furthermore, the larger active surface area provided by CNT also enhances the dopamine reaction on the electrode. Method The synthesis of AuNPs decorated multiwall CNT (MWCNT) was obtained through citrate reduction. During the pre-mixture of MWCNT and chloroauric acid (HAuCl4), HAuCl4 molecules are absorbed on the MWCNT and initiate the growth of AuNPs upon the addition of sodium citrate at room temperature. The AuNP surfaces were then functionalized with cysteamine (Cys) for the conjugation of ferrocene carboxaldehyde (Fc-CHO), leading to a Fc-AuNPs-CNT nanocomposite. The resulting nanocomposite was drop-casted with chitosan (CS) as a binder onto a screen-printed carbon electrode (SPE). The film morphology and elemental composition were characterized by scanning electron microscopy (SEM) and energy dispersive X-Ray analysis (EDX). Electrochemical responses were obtained with cyclic voltammetry (CV). Results and discussion The Fc-AuNPs-CNT composite exhibits a pair of stable and well-defined redox peaks in CV contributed by the ferrocene and ferrocenium (Fc/Fc+) redox couples. The Fc element peak in EDX further confirms the successful conjugation of Fc towards AuNPs. The Fc-AuNPs-CNT composite was demonstrated for dopamine sensing within 10 s incubation. Dopamine oxidation peak appears around +0.3V in anodic sweep, and its two reduction peaks appear around 0.15V and -0.25V. Increasing dopamine concentrations raises both oxidation and reduction currents. Compared with the AuNPs-CNT composite, Fc-AuNPs-CNT composite exhibits ~3 times higher response for the oxidation of 1 mM dopamine. The synergistic effects of Fc-AuNPs-CNT improve electron transfer rate between dopamine and electrode and convey superior electrocatalytic activity toward dopamine oxidation and reduction. The proposed sensor has a dynamic range of detection ranging from 10 µM to 1 mM dopamine and a low detection limit of 3 µM. The sensor also provides selective detection to discriminate ascorbic acid, glucose, hydrogen peroxide. The electrochemical property of Fc remains stable under repetitive potential sweep. No significant change in peak intensity was observed. Conclusion The Fc-AuNPs-CNT nanocomposite-based sensor presented here provides simple, cost-effective, portable, and rapid detection of dopamine with good sensitivity and selectivity, making it suitable for POC testing.