Preparation Of N-acetyl-L-cysteine Modified Gold Nanoparticle And Its Application In Sensor

Thiolate-protected AuNPs, also called monolayer-protected clusters, with the size of 1-5 nm have attracted particular exploration because of their unique physical and chemical properties. Moreover, thiolate-protected AuNPs have many advantages, for example, easy of preparation and functionalization, excellent biocompatibility and water-solubility. In this work, N-acetyl-L-cysteine-protected gold nanoparticles (NAC-AuNPs) were synthesized. The as-synthesized NAC-AuNPs were characterized by several methods. Finally, the NAC-AuNPs were applied to develop H2O2 sensor and tyrosinase-based biosensor.Chapter 1:The characteristics, classification and application of nanomaterials were discussed briefly. The preparation methods, characterization techniques and analytical application of gold nanoparticles were reviewed in detail.Chapter 2:The N-acetyl-L-cysteine-protected gold nanoparticles were synthesized by the NaBH4 reduction of HAuCl4·3H2O. The morphology, particle size and structure of NAC-AuNPs were studied by transmission electron microscopy (TEM), infrared (IR) spectroscopy and thermogravimetric analysis (TGA). The TEM images show that small spherical AuNPs were evenly dispersed on the carbon-coated copper grids and the size of AuNPs was determined as 1.89±0.04 nm. The absorption spectrum of NAC-AuNPs displays that the typical 520 nm surface plasmon absorbance band was not observed, indicating that the particle size was smaller than 2.0 nm. The photoluminescence (PL) properties of NAC-AuNPs have been investigated. The NAC-AuNPs show broad PL excitation wavelength range from 420 to 540 nm with an emission spectrum ranging from visible to NIR region. The spectral characteristics are very similar under various excitation wavelengths and the emission peak maximum is at ca.730 nm. The fluorescence lifetime of NAC-AuNPs was measured and the photoluminescence mechanism was also discussed.Chapter 3:A rapid and simple near-infrared (NIR) luminescence quenching method for the detection of phenolic compounds based on the combination the unique property of gold nanoparticles (AuNPs) and tyrosinase (Tyr) enzymatic reactions is described. This method relies on the luminescence quenching of gold nanoparticles-tyrosinase (AuNPs-Tyr) by phenolic compounds. Quinone intermediates produced from the enzymatic catalytic oxidation of phenolic compounds were believed to play the major role in the luminescence quenching. Dynamic quenching mechanism was confirmed by using time-resolved luminescence spectroscopy. Optimization of the experimental parameters including the concentration of AuNPs-Tyr (20μg/mL), excitation wavelength (450 nm), pH 6.0, and temperature (20℃) was determined. A linear range 0.50μM-1.0 mM and a detection limit 0.10μM of catechol were obtained under the optimal conditions. The sensitivity of different phenolic compounds was compared and follows the trend:catechol >p-cresol> phenol. The proposed NIR luminescence quenching method exhibits high sensitivity, good repeatability, and long-term stability. It has the potential for further development of NIR luminescence phenol biosensors.Chapter 4:The NAC-AuNPs and tyrosinase were immobilized on fumed silica which was activated by different silane agents. N-acetyl-L-cysteine-protected AuNPs were synthesized according to a previous report. The size and character of NAC-AuNPs were studied using transmission electron microscopy, UV-visible absorption spectroscopy and fluorescence spectroscopy. Experimental parameters influencing the immobilization of NAC-AuNPs and tyrosinase on fumed silica have been determined. The fluorescence of solid powder NAC-AuNPs and tyrosinase immobilized on fumed silica was employed to detect catechol. A linear range 0.50μM to 0.40 mM and a detection limit of 0.10μM catechol were obtained.Chapter 5:A new method was developed for the detection of hydrogen peroxide (H2O2) with NAC-AuNPs as the fluorescence probe because the fluorescence of NAC-AuNPs can be quenched by H2O2. Different experimental parameters were studied including the concentration of NAC-AuNPs, excitation wavelength, pH 6.0 and temperature. Under the optimal conditions, a linear range 10μM-30 mM, r2=0.9964 and a detection limit of 0.10μM H2O2 were obtained. Dynamic quenching mechanism was confirmed by using time-resolved luminescence spectroscopy. The proposed NIR luminescence quenching method exhibits high sensitivity, good repeatability and long-term stability.Chapter 6:A novel tyrosinase-based biosensor using NAC-AuNPs-chitosan nanocomposites has been developed for detection of phenolic compounds. The large surface area of N-acetyl-L-cysteine-protected gold nanoparticles, remarkable biocompatibility and the porous morphology of chitosan led to high enzyme loading. The entrapped enzyme could retain its bioactivity. The prepared tyrosinase-based biosensor was used to detect phenolic compounds by amperometric detection of the biocatalytically liberated quinone at-0.20 V (vs. saturated calomel electrode). Experimental parameters including pH of supporting electrolyte, working potential and temperature have been studied and optimized. The biosensor was successfully applied to detect catechol with a linear range of 0.10-60μM (r2=0.999) and the detection limit of 50 nM. The tyrosinase-based biosensor exhibits good repeatability and stability.