| Nowadays, pollutants detection, emission reduction and waste recycling are advanced strategies to control various pollutants, which are hot topics in the field of environment worldwide. The development of nanomaterials-based techniques has been significantly considered as an important technology to deal with the challenges of organic pollutants conversion and the detection of heavy metal ions, which provide new solutions for controlling environmental pollutants.As a unique nanostructure of gold, gold nanostars (AuNSs) have received great attention from the field of catalysis. Owing to their fascinating plasmon resonance, AuNSs were explored as excellent and useful tools for surface enhanced Raman scattering (SERS)-based detection, imaging and sensing, especially in biomedicine. Although AuNSs are versatile tools for catalytic reduction and detection, its application in environmental field is still limited, even the synthesis mechanism and influence of its complex structure are still unclear. To clarify the influence of AuNSs structure and extend its environmental application, we developed a new method to grow AuNSs by combining benzyldimethylammonium chloride (BDAC) and cetyltrimethylammonium bromide (CTAB) at low mass ratio. Aiming at obtaining an alternative pathway of the organics conversion, we evaluated the effects of branch numbers, illumination pretreatment and the importance of silica shell protection on the catalytic 4-nitrophenol (4-NP) reduction. We also extended the application of AuNSs up to SERS-based detection of Cu2+ in aqueous media by constructing the cysteine-coated AuNSs as a sensor.(1) AuNSs with uniform and sharply-pointed branches were synthesized by combining benzyldimethylammonium chloride (BDAC) and cetyltrimethylammonium bromide (CTAB) as surfactants, at low BDAC/CTAB mass ratio (0.48). Gold seeds were first prepared as reported in the literature. A gold seed-mediated growth method was then employed. The chemicals of 2.5 ×10-4 M HAuCl4 and 0.20 M BDAC mixed with 0.1 M CTAB, were employed as a growth solution by mixing with 0.1 M AgNO3 and then 0.1 M ascorbic acid as a reducing agent. The AuNSs synthesis was performed at 25?. The newly developed method gives the possibility to grow AuNSs with the UV-vis absorbance of 850-1100 nm. Once mixed with CTAB, BDAC lowers the critical micelle concentration for quick formation of the micelles, which provides favorable growth templates for AuNSs formation. Use of less amounts of seeds as the center of nucleation benefited sharper and longer growth of the branches. AuNSs exhibited excellent enhancement of surface-enhanced Raman scattering (SERS) intensities as the result of high electron density localized at the tips. In addition, AuNSs showed high catalytic performance toward the reduction of 4-NP to 4-aminophenol. Efficient catalysis over AuNSs originates from their corners, stepped surfaces and high electron density at the tips.(2) The effectiveness comparison of several tipped AuNSs for the 4-NP reduction by NaBH4 led to the conclusion that among different types of AuNSs (AuNSs with 4-6,6-8 and 8-10 branches), AuNSs with 8-10 branches show the highest catalytic activity evidencing the critical role of AuNSs tips in the catalytic activity. The high catalytic activity obviously benefited from the high number of branches with the tip radius of 2.6-3.6 nm. Therefore, the catalytic activity could be promoted by raising the number of tipped-branches of AuNSs, which reveals that the tips play an important role as active sites during the proton transfer from BH4-anions to 4-NP. By examining the influence of temperature on the catalytic performance of AuNSs catalysts, the induction period was observed at the temperature of 15-30?, and took up to several minutes before the reduction reaction started. With high temperatures (35,40?) the induction time was shortened, hence the higher reaction rate observed.(3) AuNSs were coated with mesoporous silica (AuNSs@mSiO2) using tetraethyl orthosilicate as a precursor, and AuNSs@mSiO2 were irradiated with Xe lamp before use. It was found that pre-irradiating and coating AuNSs with mesoporous silica further enhanced the reduction rate and recyclability, and also contributed to the decrease of the induction period. The value of k as catalyst-dependent rate constant was higher for the irradiated AuNSs@mSiCO2 compared with other catalysts. The activation energy (Ea) was calculated from the slope of the straight lines. ? was found to be 20.2 kJ/mol for pre-irradiated AuNSs@mSiO2,29.8 kJ/mol for AuNSs@mSiO2 and 32.4 kJ/mol for AuNSs catalyzed reaction.(4) The probing system of AuNSs coated with Cysteine (Cys-AuNSs) was successfully constructed and used in SERS-based copper ions (Cu2+) detection in aqueous media.10 metal ions (Hg2+, Cu2+, Cd2+, K+, Mg2+, Pb2+, Zn2+, Ni2+, Cr3+, and Co2+) were used to measure the selectivity of Cys-AuNSs to Cu2+ions in comparison with other metals. SERS signal intensities showed dramatic increase when Cys-AuNSs forms complex with Cu2+. FTIR technique provided some evidences on the coordination of Cys with Cu2+forms Cys-Cu-Cys complex through acidic (COOH) and basic (NH2) functional groups of cysteine, leading to change of electrostatic repulsion between stable AuNSs and inducing the aggregation phenomenon on AuNSs, which significantly enhanced SERS signals. Using this SERS-based sensing method, we have achieved a practical detection limit of 10 ?M. |
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