Reductant-free Synthesis of Gold Nanoparticles on Citric Acid/Carbon Nanotube Matrix for the Simultaneous Detection of Ascorbic Acid, Dopamine, and Uric Acid

Ascorbic acid (AA), dopamine (DA), and uric acid (UA) are biologically important molecules whose concentrations are closely related to several diseases such as Parkinson's disease, Alzheimer's disease, and gout.Therefore, real-time and accurate monitoring of these analytes is crucial for early diagnosis and treatment.Although various analytical methods such as Raman scattering, chromatography, fluorescence, and electrochemical techniques have been developed, electrochemical sensing is particularly attractive due to its high sensitivity, fast response, and costeffectiveness, making it suitable for portable biosensing platforms.However, electrochemical detection of AA, DA, and UA faces challenges due to their overlapping oxidation potential, which hinders selective analysis.To overcome this, electrode surface modification using nanomaterialssuch as metal nanoparticles, graphene, and carbon nanotubes-has been proposed to enhance sensitivity and enable peak separation.In this study, we fabricated a modified glassy carbon electrode (GCE) by drop-casting citric acid-functionalized multi-walled carbon nanotubes (CA-MWCNTs) and reducing gold ions (Au) in situ using citric acid to form gold nanoparticles (AuNPs).This reductant-free synthesis enables facile control over nanoparticle formation simply by adjusting the gold ion concentration and incubation time, without the need for toxic chemical reductants or high-temperature conditions required in conventional methods.The resulting AuNPs/CA-MWCNT/GCE was characterized by SEM and EDS, and its electrochemical properties were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV).The modified electrode exhibited distinct oxidation peaks for AA, DA, and UA at -0.088 V, 0.148 V, and 0.276 V, respectively, with detection limits of 300 M, 1.5 M, and 6.0 M.The electrode also showed pH-dependent oxidation behavior, high selectivity against common interferents, and excellent long-term electrochemical stability.