In this work, silver nanoparticles (AgNPs) obtained from photographic waste were synthesized and electrodeposited via cyclic voltammetry using epoxy-graphite composite as an electrochemical substrate. Both electrodes, unmodified (Epoxy/C) and modified (Epoxy/C/AgNPs), were characterized electrochemically by electrochemical impedance spectroscopy (EIS), charge transfer constant (K0), and electroactive area. The modified electrode provided lower charge transfer resistance (275 Ω), more kinetically favored electron transfer (K0 = 1.15 × 10-3 cm s-1), and a 1.7-fold increase in the active area compared to the unmodified electrode. Additional characterizations by scanning electron microscopy (SEM) and Raman spectroscopy confirmed the presence of AgNPs structures on the carbonaceous surface. As a proof of concept, Pb2+ was used as a model analyte, and a square wave anodic stripping voltammetry (SWASV) method was developed to evaluate the analytical performance of both electrodes. A wider linear range (4.0 to 40.0 µg L-1), the appropriate limit of detection (1.2 µg L-1), and a 6-fold increase in sensitivity were found using the modified electrode, suggesting that the AgNPs significantly contributed to the performance of the electrode. The proposed method was applied to three real water samples, where the Pb2+ levels varied from 11.3 to 19.5 µg L-1. The proposed protocol (reuse of silver waste) has proven to be a powerful tool for improving the detection of Pb2+, which can be helpful for other electrochemical sensing applications in locations with minimal infrastructure.

This work presents the development and characterization of a new chemically modified electrode exploiting recordable Au-rewritable compact discs (AuCDs) as substrate for drop casting of a chemically-reduced graphene-oxide slurry focusing on simple and low-cost electrochemical sensors. Increase in electrochemical responses in cyclic voltammetric and amperometric measurements were achieved using the proposed sensor. Both scanning electron microscopy and atomic force microscopy data showed increase in rugosity (about 46% higher) for the chemically reduced graphene oxide (CRGO) AuCD surface while Raman spectroscopy confirmed the presence of structural defects of graphene. The calculated charge transfer resistances (Rct) and heterogeneous electron transfer rate constants (k0) were 1638 Ω and 0.0022 cm s-1 for the bare substrate, and 91 Ω and 0.0027 cm s-1 for the modified electrode, evidencing the facilitated electron transfer of the CRGO-AuCD surface. The dopamine (DP) amperometric detection using CRGO-AuCD provided low detection limit (0.12 µg L-1) compared with other modified electrodes already reported, high precision (relative standard deviation (RSD) < 5%) and analytical frequency (370 h-1). As a proof-of-concept, DP determination in synthetic saliva samples was performed and a satisfactory recovery value (98.6 ± 0.9%) was obtained.

This work demonstrates the application of batch-injection analysis (BIA) with amperometric detection on a multi-walled carbon nanotube (MWCNT) modified electrode for the sensitive determination of the antioxidant pyrogallol in biodiesel. Samples were diluted in electrolyte (0.1 mol L-1 HClO4) before injection using an electronic micropipette on the BIA cell. The size effect of MWCNTs on electrochemical oxidation of pyrogallol was investigated and 3.5-fold increase in the amperometric sensitivity using the modified electrode with MWCNTs of shorter dimensions was obtained. This result was in agreement with cyclic voltammetry and electrochemical impedance spectroscopy measurements. After optimization of BIA parameters for the amperometric detection on the MWCNT-modified electrode, the proposed method presented a linear range from 0.3 to 1000 µmol L-1, detection limit of 0.013 µmol L-1, precision of 3%, and sample throughput of 240 h-1. Spiked biodiesel samples were analyzed and recovery values between 95-99% were obtained.