Nano-friction behavior and deformation study of hydroxyapatite in ultra-precision polishing process.

CONTEXT: In order to study the effect of ultra-precision machining on the surface quality of hydroxyapatite semiconductor materials as well as the material removal mechanism of hydroxyapatite, the mechanical polishing behaviors of hydroxyapatite at different polishing depths were studied by molecular dynamics method. The results show that the subsurface damage of hydroxyapatite increases with increasing polishing depth. The polishing temperature and the polishing force showed a positive correlation with the polishing depth, and the variation of the polishing force was related to the accumulation-release effect of the potential energy of hydroxyapatite material. In addition, the variation of stresses in hydroxyapatite during polishing is mainly influenced by the thermal softening effect. With a smaller polishing depth, the hydroxyapatite semiconductor material has fewer structural defects, fewer atoms undergoing phase transitions, lower surface roughness, and better surface quality. Therefore, to ensure the long-lasting service life of hydroxyapatite semiconductor materials, a small polishing depth should be used in ultra-precision machining. Additionally, this study also provides a theoretical reference for future research on the mechanical properties of hydroxyapatite-based composites. METHODS: A Large-Scale Atomic/Molecular Parallel Simulator (LAMMPS) was utilized to perform molecular dynamics simulations. The output was visualized and analyzed by the Open Visualization Tools (OVITO) software. The intermolecular interactions were described by the polymer consistent force-field and the 12/6 Lennard-Jones potential functions. The workpiece was polished under a micro-canonical ensemble with the temperature settled at 300 K. Periodic boundary conditions were adopted and the velocity-Verlet algorithm was used to integrate the atomic motion with a timestep of 0.1 femtoseconds (fs).

MoS2/graphene van der Waals heterojunctions combined with two-layered Au NP for SERS and catalysis analyse.

MoS2-plasmonic hybrid platforms have attracted significant interest in surface-enhanced Raman scattering (SERS) and plasmon-driven photocatalysis. However, direct contact between the metal and MoS2 creates strain that deteriorates the electron transport across the metal/ MoS2 interfaces, which would affect the SERS effect and the catalytic performance. Here, the MoS2/graphene van der Waals heterojunctions (vdWHs) were fabricated and combined with two-layered gold nanoparticles (Au NP) for SERS and plasmon-driven photocatalysis analyse. The graphene film is introduced to provide an effective buffer layer between Au NP and MoS2, which not only eliminates the inhomogeneous contact on MoS2 but also benefits the electron transfer. The substrate exhibits excellent SERS capability realizing ultra-sensitive detection for 4-pyridinethiol molecules. Also, the surface catalytic reaction of p-nitrothiophenol (PNTP) to p,p-dimercaptobenzene (DMAB) conversion was in situ monitored, demonstrating that the vdWHs-plasmonic hybrid could effectively accelerate reaction process. The mechanism of the SERS and catalytic behaviors are investigated via experiments combined with theoretical simulations (finite element method and quantum chemical calculations).

Noble metal modified ReS2 nanocavity for surface-enhanced Raman spectroscopy (SERS) analysis.

The rhenium disulphide (ReS2) nanocavity-based surface enhanced Raman scattering (SERS) substrates ware fabricated on the gold-modified silicon pyramid (PSi) by thermal evaporation technology and hydrothermal method. In this work, the ReS2 nanocavity was firstly combined with metal nanostructures in order to improve the SERS properties of ReS2 materials, and the SERS response of the composite structure exhibits excellent performance in sensitivity, uniformity and repeatability. Numerical simulation reveals the synergistic effect of the ReS2 nanocavity and the plasmon resonance generated by the metal nanostructures. And the charge transfer between the metal, ReS2 and the analytes was also verified and plays an non-ignorable role. Besides, the plasmon-driven reaction for p-nitrothiophenol (PNTP) to p,p'-dimercaptobenzene (DMAB) conversion was successfully in-situ monitored. Most importantly, it is found for the first time that the SERS properties of ReS2 nanocavity-based substrates are strongly temperature dependent, and the SERS effect achieves the best performance at 45 °C. In addition, the low concentration detection of malachite green (MG) and crystal violet (CV) molecules in lake water shows its development potential in practical application.

Film wrap nanoparticle system with the graphene nano-spacer for SERS detection.

Film wrap nanoparticle system (FWPS) is proposed and fabricated to perform SERS effect, where the Ag nanoparticle was completely wrapped by Au film and the double-layered graphene was selected as the sub-nano spacer. In this system, the designed nanostructure can be fully rather than partly used to generate hotspots and absorb probe molecules, compared to the nanoparticle to nanoparticle system (PTPS) or nanoparticle to film system (PTFS). The optimal fabricating condition and performance of this system were studied by the COMSOL Multiphysics. The simulation results show that the strongly large-scale localized electromagnetic field appears in the whole space between the Ag nanoparticle and Au film. The experimental results show that the FWPS presents excellent sensitivity (crystal violet (CV): 10-11 M), uniformity, stability and high enhancement factor (EF: 2.23×108). Malachite green (MG; 10-10 M) on the surface of fish and DNA strands with different base sequence (A, T, C) were successfully detected. These advanced results indicate that FWPS is highly promising to be applied for the detection of environmental pollution and biomolecules.

[Discrimination of common oral streptococcus with metabonomics method].

OBJECTIVE: To evaluate the feasibility of identifying oral streptococcus by comparing their metabolic profiling, and to find a convenient and rapid way to discriminate oral microorganisms. METHODS: The pure cultivation of Streptococcus sanguis ATCC 10556 and Streptococcus sobrinus 6715 (reference strain) from solid culture were respectively inoculated in TPY liquid medium. Then the growth quantity was measured periodically by turbidimetry and the growth curves of the inoculated bacteria were completed. The culture solutions in the stationary phase of the two bacteria were centrifuged, and then tested with the 1H-nuclear magnetic resonance (1H-NMR) spectrometer respectively. The gained free induction decay (FID) data were all inputted into MestReC Soft and finally transformed into metabolic profiling. The metabolic profiles were integrated segmentingly and the results were inputted into SIMCA-P Soft for principal components analysis (PCA). RESULTS: The PCA results showed the obvious clustering phenomena and the points of two group data differentially centralized in two clusters. Therefore, the NMR-based metabonomics profiles can discriminate the two different kinds of bacteria. CONCLUSION: The metabonomics can be expected to be a kind of promising useful method in quick discrimination of oral streptococcus.

[Preliminary study on the discrimination of putative periodontal pathogens with a metabonomics method].

OBJECTIVE: To evaluate the feasibility of identifying oral pathogenic bacteria by comparing the metabolic profiling of putative periodontal pathogens and try to find a convenient and rapid way to discriminate oral microorganisms. METHODS: Suspensions of Porphyromonas gingivalis, Prevotella intermedia and Fusobacterium nucleatum with same density were prepared and cultured respectively at liquid BHI medium. Then the growth quantity was measured periodically through turbidimetry and the growth curves of the inoculated bacteria were completed. The culture solutions of stable growth phase were sampled and characterized by 1H-nuclear magnetic resonance 1H-NMR). The data of 1H-NMR spectroscope results were analyzed by principal components analysis (PCA). RESULTS: The PCA showed the obvious clustering phenomena and the points of three groups differentially centralized to three clusters. Therefore, the NMR-based metabonomics profiles could discriminate the three different kinds of bacteria. CONCLUSION: The metabonomics is a potential classable method to identify the oral pathogenic bacteria.