The investigation on the free motion of nanoparticles and their interaction with other media has become an attrac-tive field for extending the practical application.However,the real-time monitoring of dynamic behaviors still exists signifi-cant challenge.In this paper,based on surface enhanced Raman spectroscopy(SERS)and the formation of"hot spots"during collision between Au nanoparticles and Au single crystal microplate under Brownian motion,the real-time monitoring of free motion behaviors of Au nanoparticles during collision and the dynamic SERS study were realized accordingly by using thio-phenol(TP)as the probe molecule.The nature and influencing factors of microscopic motion of nanoparticles were investi-gated by statistical analysis of the"spikes"in the SERS trajectories,including"single spikes"and"cluster spikes".The re-sults reveal that the"spike"is mainly attributed to the"hot spots"formed by reversible collision of nanoparticle and plane."Single spikes"correspond to the rapid departure of Au nanoparticles from the surface of the Au microplate after collision with the microplate,and"cluster spikes"correspond to the process of Au nanoparticles staying on the surface of the Au mi-croplate for a short time after colliding with the plane and then leaving or possibly multiple nanoparticles colliding continu-ously.Increasing the concentration of nanoparticles is beneficial to the formation of"cluster spike".The intensity distribution of the corresponding SERS characteristic peaks is concentrated in 5.2 cps and 8.7 cps,respectively.The relative intensities of SERS peaks of TP in the"spikes"are critically depended on the vibrational modes.It demonstrates that the probability of stretching vibrational modes is higher,and it is mainly due to the different orientations of molecules in the localized area during the dynamic collision processes.The realization of dynamic collision is beneficial to deeply understand the nature of microscopic motion of nanoparticles.It provides the basis for the investigation of dynamic interfacial chemical reactions in localized area.

Fenton technology is one of advanced oxidation process (AOP) methods to treat wastewater through chemical oxidation. Due to the limitations of classical iron-based catalysts, it is still challenging to find suitable catalysts for Fenton-like reactions. Here, MoS2/Au heterojunctions were successfully synthesized by reduction of chloroauric acid in the solution of layered MoS2 prepared by hydrothermal method. As a model molecule, methylene blue (MB) was used as the species to be degraded to evaluate the performance of the catalyst. It was determined by UV–visible spectra that the optimal catalyst can be obtained when MoS2 (mg): HAuCl4 (wt. % mL) is 2:2. The Fenton-like reaction process was monitored by introducing highly sensitive surface enhanced Raman spectroscopy (SERS). The results show that MB can be degraded by 83% in the first 10 min of the reaction, indicating that MoS2/Au has good catalytic performance. In addition, as a fingerprint spectrum, SERS was used to preliminarily analyze the molecular structure changes during the degradation process. The result showed that C-N-C bond was easier to break than the C-S-C bond. NH2 group and the fused ring were destroyed at the comparable speed at the first 30 min. In terms of application applicability, it was showed that MB degradation had exceeded 95% at all the three pH values of 1.4, 5.0, and 11.1 after the reaction was carried out for 20 min. The test and analysis of the light environment showed that the catalytic efficiency was significantly improved in the natural light of the laboratory compared to dark conditions. The possible mechanism based on ·OH and ·O2− from ESR data was proposed. In addition, it was demonstrated to be a first-order reaction from the perspective of kinetics. This study made a positive contribution to broaden of the applicable conditions and scope of Fenton-like reaction catalysts. It is expected to be used as a non-iron catalyst in practical industrial applications. From the perspective of detection method, we expect to develop SERS as a powerful tool for the in situ monitoring of Fenton-like reactions, and to further deepen our understanding of the mechanism.

Hypothesis: The dynamic behaviors of colloidal particles have already been considered as one of the key issues in their practical application, such as aggregation and dispersion. However, it is still remained sig-nificant challenge in developing the real time techniques to capture their dynamic tracks. The nano/sub-nanometer scale gap generated during the colloidal collisions served as the critical location for amplifying the Raman signal, so called as gap ("hot spots") based surface enhanced Raman spectroscopy (SERS). The alternating reversible "spike" of SERS intensity and irreversible step in baseline intensity are contributed to the preferred stability and the aggregation of colloid respectively.Experiments: A facile approach is developed to track colloidal stability in real-time based on collisions and SERS. The effects of particle concentration, the dispersion medium, and solution pH on colloidal sta-bility are systematically investigated, and the SERS intensity of a simulated single-like "hot spot" was cal-culated by combining a SEM position with SERS mapping technology to estimate the intensity of single -particle collision.Findings: The colloidal particles exhibited higher stability in the solution with lower particle concentra-tion, higher viscosity and neutral medium. The SERS intensity of single-particle collision was estimated to be about 2.06 x 10-7 counts, and the average number of collisions for the 0.13 mmol/dm3 SiO2@Ag solu-tion was about 1.11 x 108 times/spike in the "spikes" with SERS intensity of 23.0 cps. It is believed that the SERS based strategy would be developed as a promising tool for obtaining the deeper insight into the nature of collisions in the colloidal science.(c) 2022 Elsevier Inc. All rights reserved.