Temperature and size effects on the behavior of nanoscale water molecule clusters are investigated by molecular dynamics simulations. The flexible three-centered (F3C) water potential is used to model the inter-and intramolecular interactions of the water molecule. The differences between the structural properties for the surface region and those for the interior region of the cluster are also investigated. It is found that as the temperature rises, the average number of hydrogen bonds per water molecule decreases, but the ratio of surface water molecules increases. After comparing the water densities in interior regions and the average number of hydrogen bonds in those regions, we find there is no apparent size effect on water molecules in the interior region, whereas the size of the water cluster has a significant influence on the behavior of water molecules at the surface region.

Water-soluble upconversion nanoparticles (UCNPs) that exhibit significant ultraviolet, blue, and red emissions under 980-nm laser excitation were successfully synthesized for performing near infrared (NIR)-triggered photodynamic therapy (PDT). The lanthanide-doped UCNPs bearing oleate ligands were first exchanged by citrates to generate polyanionic surfaces and then sequentially encapsulated with NH2-terminated poly(amido amine) (PAMAM) dendrimers (G4) and chlorine6 (Ce6) using a layer-by-layer (LBL) absorption strategy. Transmission electron microscopy and X-ray diffraction analysis confirm that the hybrid UCNPs possess a polygonal morphology with an average dimension of 16.0 ± 2.1 nm and α-phase crystallinity. A simple calculation derived through thermogravimetric analysis revealed that one polycationic G4 dendrimer could be firmly accommodated by approximately 150 polyanionic citrates through multivalent interactions. Moreover, zeta potential measurements indicated that the LBL fabrication results in the hybrid nanoparticles with positively charged surfaces originated from these dendrimers, which assist the cellular uptake in biological specimens. The cytotoxic singlet oxygen based on the photosensitization of the adsorbed Ce6 through the upconversion emissions can be readily accumulated by increasing the irradiation time of the incident lasers. Compared with that of 660-nm lasers, NIR-laser excitation exhibits optimized in vitro PDT effects toward human breast cancer MCF-7 cells cultured in the tumorspheres, and less than 40% of cells survived under a low Ce6 dosage of 2.5 × 10−7 M. Fluorescence microscopy analysis indicated that the NIR-driven PDT causes more effective destruction of the cells located inside spheres that exhibit significant cancer stem cell or progenitor cell properties. Moreover, an in vivo assessment based on immunohistochemical analysis for a 4T1 tumor-bearing mouse model confirmed the effective inhibition of cancer cell proliferation through cellular DNA damage by the expression of Ki67 and γH2AXser139 protein markers. Thus, the hybrid UCNPs are a promising NIR-triggered PDT module for cancer treatment.

Deformation behaviors of an (8,8) boron-nitride nanotube (BNNT) under axial tensile strains were investigated via molecular dynamics (MD) simulations. The Tersoff potential was employed in the simulations with potential parameters determined by fitting the MD simulations results to those obtained from density functional theory calculations for BNNTs with the aid of the force-matching method. Variations in the axial stress, bond lengths, bond angles, radial buckling, and slip vectors with tensile strain were all examined. The axial, the radial, and tangential components of the slip vector were employed to monitor the local elongation, the local necking, and the local twisting deformations, respectively, near the tensile failure of the BNNT. From this study, it was noted that the BNNT started to fail at the failure strain of 26.7%. The components of the slip vector grew abruptly and rapidly after the failure strain, especially for the axial component. This implies that the local elongation dominates the tensile failure of the BNNT. With further axial tensile strains, subsequent bond breaking was found in the BNNT and finally resulted in a chain-like failure mode before complete breaking of the BNNT. No apparent yielding was noticed before the tensile failure of the BNNT.

Temperature and size effects on coalescence behavior of water nanoclusters are investigated in this paper by means of molecular dynamics simulations. The flexible three-centered (F3C) water model is employed in the molecular dynamics simulations. To discuss the coalescence behavior, the whole coalescence process is separated into three stages, including the approaching stage, the coalescing stage, and the coalesced stage, according to the shrinkage evolution of the coalescence process. It is observed from the present study that, with a higher initial temperature and a smaller cluster size, the coalescence rate will amplify. The temperature and temperature fluctuation are highest at the coalescing stage, and the fluctuation is more apparent for a higher initial temperature and a smaller cluster size. Moreover, the average number of hydrogen bonds per water molecule will grow slightly after the coalescence of the water nanoclusters. Variations in the average number of hydrogen bonds during the coalescence increase for a higher initial temperature and a smaller cluster size.

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