| Abstract | For this thesis, two sensors and two biosensors were designed based on carbon composite nanomaterials using electrochemical methods and surface enhanced Raman spectroscopy as detection methods for food, environmental, and clinical applications.
In the first section, an electrochemical sensor for food applications was fabricated based on a glassy carbon paste electrode (GCPE) modified with graphene nanoplatelets (GNP) functionalized with ionic liquid (IL). The sensor was applied for the detection of bisphenol A (BPA) and performed by differential pulse voltammetry (DPV). Under optimum conditions, the proposed sensor exhibited a linear range for BPA determination from 0.02-5.0 micro M with a detection limit (LOD) of 6.4 nM. The GCPE/GNP-IL sensor was successfully applied to the determination of BPA in drinking water and plastic drinking water bottles. The results demonstrated a high degree of accuracy and are in agreement with high-performance liquid chromatography (HPLC).
In the second section, an electrochemical biosensor for clinical applications was designed based on CEA antibody (anti-CEA) anchored with core shell Fe3O4@Au nanoparticles which were immobilized on a screen-printed carbon electrode (SPCE) modified with manganese dioxide laid out on graphene nanoplatelets (GNP-MnO2). A biosensor was applied for label-free detection of carcinoembryonic antigens (CEA), which was monitored by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). The difference in signal response owing to redox reactions of Fe(CN)63-/4- before and after a direct binding of CEA to a fixed amount of anti-CEA on the electrode surface was regarded as the biosensor response corresponding directly to the CEA concentration. Under optimized conditions, the biosensor exhibited a linear range of 0.001-100 ng/mL (LSV) and 0.30 pg/mL (EIS). The applicability of the biosensor was verified by determination of CES in human serums compared to electrochemiluminescence immunoassay.
In the third section, an electrochemical biosensor for environmental applications was constructed based on SPCE modified with reduced graphene oxide (rGO) and silver nanoparticles (AgNPs). The biosensor was applied for indirect detection of glyphosate herbicide, which relied on the inhibition of acid phosphatase enzymes (ACP) immobilized on the SPCE/rGO-AgNPs surface. In the presence of glyphosate, the current signal was decreased, owing to the enzymatic reaction of ACP to its substrate. The signal was measured by chronoamperometry and quantitative measurements proportional to the glyphosate concentration. The biosensor exhibited two linear ranges from 0.05 to 0.5 mg/L and 0.5 to 22.0 mg/L, and the LOD of 16 micro g/L were obtained. The proposed biosensor was successfully applied for the determination of glyphosate in water and soil samples, and the results were in full accordance with the HPLC method.
In the last section, the analytical method for detection of glyphosate in environmental samples was also designed based on surface-enhanced Raman spectroscopy (SERS). A vertical heterostructure composed of titanium dioxide nanotube arrays (TiO2 NTs), AgNPs and rGO was constructed and served as a SERS-based sensor. Under optimum conditions, the TiO2 NTs/AgNPs-rGO surface exhibited high SERS activity, which provided analytical enhancement factors (AEF) as high as 7.1x108. The modified SERS sensor was successfully applied to glyphosate detection ranging from 0.1 to 100 mg/L and the LOD as 0.05 mg/L. The practical applications of glyphosate determination in environmental waters and soils were investigated and the results are in great accord with those obtained by the HPLC method.
The research is successful in developing sensors and biosensors. The sensors and biosensors developed are analytical devices employed as low-cost platforms for simple use and rapid detection. The addition, the new knowledge obtained will increase not only in the research field but also for commercial use in the future.
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