Electrochemical Synthesis of Silver Nanoparticles for Applications in Nitrate Detection, Catalysis and Antibacterial Activity

In this thesis results are presented and discussed on the formation and characterisation of silver nanoparticles. The potential applications of the silver nanoparticles are investigated using the reduction of 4-nitrophenol and the detection of nitrates. In addition, data are presented on the antibacterial activity of the silver nanoparticles. The silver nanoparticles were synthesized as colloidal particles protected with polyvinylpyrrolidone (PVP) with a diameter of 5.73 nm. The colloidal silver nanoparticles were generated in solutions containing 0.05 mol dm-3 AgNO3, 0.1 mol dm-3 KNO3 and 423.75 g dm-3 PVP with an electrochemical pulse of -6.0 V vs Ag+/Ag. The PVP-protected silver nanoparticles showed excellent stability over a 30 day period and instability was only observed on the addition of AgNO3 or KNO3. The PVP-protected nanoparticles were successfully immobilised in a polyacrylamide hydrogel and employed in the reduction of 4-nitrophenol in the presence of NaBH4. The 4-nitrophenol was completely reduced after a 60 min period with a first order rate constant of 7.4 x 10-4 s-1. The activity of the hydrogel composite was maintained on dehydrating and rehydrating and following repeated reactions. The hydrogel composite showed antibacterial activity for Staphyloccoccus aureus, MRSA, Escherichia coli and Pseudomonas aeruginosa and very good antibacterial activity was observed on loading the composite with mobile Ag+ ions. However, the composite showed poor detection of nitrates, due to diffusion limitations. The silver nanoparticles were successfully deposited at a glassy carbon electrode using a double pulse or a single pulse technique. For the double pulse technique, a pulse at a potential of E1 was applied to instantaneously nucleate nanoparticle seeds. Then, a longer pulse at E2 was applied to grow the seeds into established particles. In the single pulse technique a potential sufficient to nucleate and grow the particles was applied for the entire period, giving a much higher density of silver particles consistent with a progressive nucleation and 3D growth model with a narrow size distribution. The deposited silver nanoparticles were successfully used in the electrochemical detection of nitrates. The limit of detection was obtained as 2.0 x 10-5 mol dm-3 using cyclic voltammetry and the performance was increased using constant potential amperometry, giving a limit of detection of 9.8 x 10-6 mol dm-3, which falls well below the maximum concentration of 8.0 x 10-4 mol dm-3 as set out by the EPA under the Nitrate Directive (91.676/EEC). However, significant interference was observed on the addition of nitrites and chloride anions. The rate constant for the reduction of nitrate was calculated as 9.79 x 10-3 s-1, while the reaction order was computed as 0.736, consistent with the adsorption of nitrates. Higher peak currents were observed at lower pH values between 2.0 and 4.0, indicating that the adsorption of H+ ions facilitates the reduction of the nitrate anion