Preparation of nanomaterials for sensitive electrochemical As (III) detection and H2O2 reduction
Creators: name: Ren, Baiyu
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340103
subject: Electroanalytical chemistry

description: The two major research areas in this thesis are electrochemical As (III) determination and H2O2 reduction in aqueous solution. Although a variety of nanomaterials have been developed, there is still room for new materials and greater understanding of current material classes that may lead to higher sensitivity, lower limit of detection (LoD) and acceptable selectivity. These two research areas are linked by the fact that that they both rely on the careful preparation of well characterised nanomaterials for electrochemical applications, which is the central theme of this thesis. Each of the research areas and their main results are presented below.  <br><br>The first result presented is the electrodeposition of gold nanoparticles (AuNPs) on a gold substrate for Arsenic (III) detection. To obtain the best As (III) analysis sensitivity, a series of conditions were optimized. Also, two most commonly used electroanalysis techniques, square wave anodic stripping voltammetry (SWASV) and amperometric i-t plots were compared, indicating that SWASV leads to a better sensitivity of 0.124 µA ppb-1 and the lower limit of detection (LoD) of 0.244 ppb. Cu (II) interference studies revealed acceptable anti-interference ability at lower concentrations of Cu (II) (below 50 ppb).    <br><br>Next, a series of Au nanospikes and dendrites were electrodeposited with either an inorganic (Pb2+) or organic (cysteine) growth directing agent for different times to obtain varied morphology. These structures were compared with gold nanoparticles of three different shapes (Octahedral, Cubic and Rhombic Dodecahedral) for detection of As (III) by SWASV. The sensitivity and LoD was dependent on the surface crystallographic orientations and the morphology, with superior sensitivity confirmed with a maximum amount of Au (111) facets on the surface for the nanoparticles. The study confirms that electrodeposition parameters or nanoparticle synthesis methods for Au surfaces in arsenic sensing needs to be carefully controlled, to either maximise the (111) facets, or minimise the steps on a polycrystalline Au surface, and that inorganic and organic shape directing agents used for electrodeposition will have differing effects.<br><br>A new material for As (III) detection was prepared based on ceria cubes decorated with manganese oxide nanoparticles (Mn2O3/CeO2 nanocubes), and used to modify a Au electrode for analysis of As (III) in aqueous solution. This modified electrode displayed improved sensitivity than either oxide on their own, indicating a synergistic effect. The improved sensitivity could be ascribed to the enhanced As (III) adsorption ability of the Mn2O3/CeO2 nanocube during electrochemical pre-concentration, combined with the well-known As (0) deposition and stripping qualities of the gold substrate. Also, a graphene oxide framework (GOF) material was constructed and used to modify a gold substrate. The obtained GOF/Au electrode was shown as a highly sensitive electrochemical arsenic (III) sensor. The improved electrochemical activity towards arsenic was ascribed to the larger surface area, strong adsorption ability of GOF and enhanced electron transfer between modified GOF and the gold electrode, in conjunction with the excellent electrocatalytic capabilities of gold substrate.<br> <br>Prussian blue (PB) and platinum have long been used for arsenite detection separately due to their individual characteristics. This thesis investigated the synergistic effect of these two materials for As (III) detection. Two classes of materials, PBPt composite and PB@Pt core-shell cubes were prepared. PBPt and PB@Pt modified glassy carbon electrodes (GCE) were used for electrochemical detection of As (III) via its oxidation to As (V). Compared with the pure PtNPs and PBPt composite decorated GCE, PB@Pt modified GCE obtained the highest sensitivity and lowest limit of detection (LoD). Cu (II) ions at low concentration (less than 377.21 ppb) at PB@Pt modified GCE did not interfere with detection of As (III). It was found that that much higher concentrations of Cu (II) (up to 377.21 ppb) can be used without interference on PB@Pt, compared with the lower concentrations of Cu (II) on Au-based electrodes (below 50 ppb).  <br><br>Similarly, rationally designed nanomaterials are of great importance in improving the electrocatalytic activity of H2O2 sensors including sensitivity, selectivity and stability. Firstly, a stable AuCN/Prussian Blue (PB) Nanocube composite was prepared by galvanic replacement of PB cubes with HAuCl4 solution. When the electrochemical detection of H2O2 was studied, the AuCN/PB composite showed considerably improved stability. This stability was attributed to the stabilisation of the surface of PB with AuCN, which prevents decomposition of the reduced form of PB during electrocatalysis. The new material shows that redox active Metal Organic Frameworks (MOFs) used for electrochemical sensing or catalysis can be tuned for surface porosity and stabilised via galvanic replacement with a noble metal salt, leading to improved electroanalytical performance.<br><br>Furthermore, two kinds of nanomaterials supported on MoS2 flakes, i.e. Pt@Au core-shell nanostructures and AuCN/prussian blue (PB) nanocomposites, were prepared. Both nanostructures were used for electrochemical H2O2 reduction, showing good catalytic performance. Both nanostructures show high anti-interference ability towards common interfering species, with good stability, and are promising for practical applications in H2O2 sensing.  <br><br>Finally, combining several of the synthetic strategies and materials common to the thesis, a strategy was presented for the direct preparation of Au nanoparticles (AuNPs) on a Fe-based support, encapsulated with porous carbon (PC), via pyrolysis of AuCN functionalised Prussian Blue (PB) metal organic frameworks (MOF). The resulting structures were shown to be active Au-based nanomaterials for model applications including catalysis (4-nitrophenol reduction) and electroanalysis (arsenic (III) detection), confirming a general strategy for preparation of supported noble metal nanoparticles evenly distributed on a magnetic support, allowing separation of catalysts from products for heterogeneous applications. The general performance of the materials was confirmed with a rate constant of 0.725 min-1 for 4-nitrophenol reduction, and a sensitivity of 0.04 µA ppb<sup>-</sup>1 and LoD of 0.15 ppb for electrochemical analysis of As (III).<br><br>In summary, several kinds of nanomaterials with interesting structures were designed and prepared, aiming at obtaining a deeper understanding towards some classical nanomaterials such as gold or improving the electrochemical performance via creation of new materials for electrochemical As (III) determination and H2O2 reduction in aqueous solution.<br>