Electrochemistry of silver nanoparticles

This thesis presents findings realising two main objectives. The first aim is to investigate the electrochemical detection of nanomaterials with an emphasis on silver nanoparticles. The second goal is to employ silver nanoparticles in electroanalysis to aid in the detection of other analytes. First, the detection of silver nanoparticles was demostrated through two different electrochemical methods, stripping voltammetry and 'nano-impacts'. For stripping voltammetry, the potential of metallic nanoparticles oxidation was quantified by various new analytical expressions for peak potential. For the novel method of 'nano-impacts', individual silver nanoparticles were successfully detected in an optically opaque suspension. Then, a comparison between the two techniques was achieved via the oxidation of silver nanoparticles with different capping agents. Strong capping agent effects was found for stripping voltammetry and one may markedly underestimate the amount of silver nanoparticle present on the electrode surface. The electrochemical sizing of nanoparticles via 'nano-impacts' remained unaffected by the capping agent effect. Amidst the study on the various types of capping agent, it was discovered that cetyltrimethylammonium bromide (CTAB) is electroactive due to the oxidation of its bromide content. This inspired the use of 'nano-impacts' to detect the presence of large CTAB micelles which self-assembled at concentrations above the critical micelle concentration. Next, various types of silver nanoparticles were applied to different electroanalytical systems to aid in the measurement of other analytes. (a) Small silver nuclei, remaining after the oxidative stripping of an electrode modified by silver nanoparticle suspension drop casting, allowed subsequent signal enhancement (at least a factor of three) in anodic stripping voltammetry of silver ions. (b) The thermodynamic favourable formation of silver halide complexes allowed the silver nanoparticle modified electrode to analyse the halide content of a solution. Hence, a proof-of-concept for an electrochemical sensor based on silver nanoparticle modified electrode for chloride ions was established. This might be applied to the pre-screening of cystic fibrosis, a genetic disease detrimental to many infants' lives. (c) Another key halide in human body, iodide ions, was also measured using a related concept. The level of iodide ions in synthetic human urine was determined. Last, the strong affinity of silver to thiol groups also warranted a study devoted to their interaction through electrochemical and spectroscopic measurements. It was found that there is no general mechanism for silver-thiol interaction and each thiol must be treated as a separate entity. </p

Electrocatalytic effect of ZnO nanoparticles on reduction of nitroaromatic compounds

Nitroaromatic compounds have garnered a lot of attention due to their wide applications in military and industrial fields. Electrochemical techniques can be applied for sensing these compounds due to their electrochemical activity. Electrocatalytic reduction of nitroaromatic explosives was shown previously on silver nanoparticles and graphene based surfaces. Here, we demonstrate electrocatalytic reduction of explosives on zinc oxide nanoparticles. Nitroaromatic explosives were tested using glassy carbon electrodes modified with zinc or zinc oxide nanoparticles to study the catalytic effects on the electrochemical reduction of the nitroaromatic compounds with the aim of improving the sensitivity of their detection. A zinc oxide nanoparticle modified electrode has shown favourable improvement in peak height and hence greater sensitivity compared to bare glassy carbon electrodes and zinc nanoparticle modified electrodes.Accepted versio

A proof-of-concept – Using pre-created nucleation centres to improve the limit of detection in anodic stripping voltammetry

Anodic stripping voltammetry is a much-utilised method for trace metal analysis. We provide a simple proof-of-concept technique to improve the sensitivity of the method, which is illustrated by the detection of silver cations. This approach requires an electrode pre-treatment, which involves drop casting a metal nanoparticle suspension and oxidising the nanoparticles, leaving small metal nuclei on the electrode surface. In turn, the small metal nuclei act as nucleation sites for subsequent metal deposition when used to interrogate target solutions. In particular, the pre-treatment increases the amount of deposited metal in a given amount of time. Silver nitrate concentrations from 30 nM to 1 μM were tested and at silver ion concentration of 300 nM, the pre-treated electrode gave a signal, which was 40 times larger than the untreated electrode. The larger signal leads to the enhancement of sensitivity and a lowering of the detection limit of anodic stripping voltammetry without introducing other organic molecules, metals or impurities

Nanoelectrocatalytic oxidation of lactic acid using nickel nanoparticles

We report the electrocatalytic effect of oxidized nickel nanoparticles on lactic acid oxidation in alkaline solution. A boron-doped diamond electrode was modified with nickel nanoparticles by electrodeposition. Various electrodeposition charges were used to deposit nanoparticles of different sizes onto the electrode. Afterward, the nickel nanoparticle modified boron-doped diamond electrodes were initially oxidized in NaOH solution to form beta-Ni(OH)(2) on the surface. Further oxidation forms the Ni(2+/3+) redox couple, beta-Ni(OH)(2)/beta-NiOOH, which catalyzes lactic acid oxidation. Studies in the absence and in the presence of lactic acid were performed and compared to the behavior of the Ni(2+/3+) redox couple on a bulk nickel electrode. The anodic peak was greater than the cathodic peak current in both situations and attributed to the catalytic effect of NiOOH on the oxidation of lactic acid. The ratio between the anodic and cathodic peaks was normalized to the amount of the Ni((2+/3+)) couple on the electrode, and it was found that for suitably small nickel nanoparticles (diameter = 31.8 +/- 7.0 nm) authentic catalytic behavior was seen when compared to a bulk nickel electrode.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP