Study Of The Aggregation And Catalytic Mechanisms Of Metal Nanoparticles And Their Analytical Applications

Metal nanoparticles possess some unique chemical and physical properties due to the quantum size effect, surface effect and quantum tunneling effect, such as special optics, electronics and catalytic activity. Based on the special distance and size-dependent surface plasmon resonance (SPR) and catalytic activity of metal nanoparticles, they have been largely applied to colorimetric and catalysis.The quaternary ammonium surfactants are widely used in industrial applications and pharmaceutical/cosmetic preparations.When surfactants discharge into the environment, they are deposited on land or into water systems rather than being degraded, so they will become environmental pollutants. In this paper, we studied the reaction between citrate-capped silver nanoparticles (Ag NPs) and quaternary ammonium surfactants. We used citrate-capped silver nanoparticles for the rapid colorimentric detection of quaternary ammonium surfactants. We also make a comparison between citrate-capped Ag NPs and gold nanoparticles(Au NPs) in the response to quaternary ammonium surfactants. On the other hand, we studied the difference of catalytic activities between dispersed and aggregated Ag NPs towards the reaction of methyl orange (MO) and NaBH4. We found that Ag NPs can reversibly catalyze the reaction of MO and NaBH4 We also further studied the reversible catalytic mechanism of Ag NPs for the reduction of MO and used for the colorimetric detection of cationic surfactants.Details are as follows:1、A rapid, sensitive and low-cost colorimetric method for detection of quaternary ammonium surfactants using citrate-capped Ag NPs was developed. The quaternary ammonium surfactants can induce the aggregation of Ag NPs through the hydrophobic effect. The addition of cationic surfactant results in color change of Ag NPs solution from yellow to red and finally to colorless, which is due to the broadening of the surface plasmon band. The color change was monitored using a UV-vis spectrophotometer. The LOD of different cationic surfactants was in the range of 0.5-5 μM. More importantly, this detection method was successfully utilized to the disinfectant residual sample.2、A rapid and low-cost colorimetric method for detection of quaternary ammonium surfactants using citrate-stabilized AuNPs based on the hydrophobic effect was developed. According to our previous research, quaternary ammonium surfactants can induce the aggregation of Ag NPs and used for the colorimetric detection of cationic surfactants, so we also make a comparison between citrate-capped Ag NPs and Au NPs in the response to quaternary ammonium surfactants. The LODs of different surfactants ranged from 0.5 to5 μM. We find that, compared to the Ag NPs, the red-to-blue color change resulting from the aggregation of the Au NPs is more remarkable than the yellow-to-colorless change of Ag NP solution. Therefore, the Au NPs should be more sensitive to the cationic surfactants. What is more, the UV/Vis spectra of the Au NP solution response to the surface plasma resonance changes are completely different from Ag NP solution.3、Ag NPs can catalyze the degradation of MO in the presence of NaBH4, but the catalytic mechanism of this reaction was not deeply understood. We have deeply studied the reduction of MO catalyzed by Ag NPs in the presence of NaBEH4. We surprised to discover the reversible catalysis for the reaction between MO and NaBH4 by Ag NPs. When MO molecules adsorbed onto the surface of the dispersed Ag NPs, they were polarized and activated. The electron transfer between MO and NaBH4 was accelerated and then the activation energy of the reaction was reduced. The products of the reaction between MO and NaBH4 were generated on the surface of the Ag NPs and then diffused into solution. If the diffusion of these products was restricted, these products could be catalyzed to generate MO again. This discovery could contribute to the deep comprehension of the catalytic mechanism of nanoparticles in the future.the interesting reversible catalysis by Ag NPs was applied to the rapid detection of CTAB.