Both of temperature（in water） and composition（in the water/methanol mixed solvent） can induce the coil-toglobule transition of poly（N-isopropylacrylamide）（PNIPAM）. The atomic force microscope（AFM） based single molecule force spectroscopy（SMFS） has been exploited to investigate the interactions between the polymer chain and solvent at the single-molecule level. It is found that the single-chain mechanics of PNIPAM show a remarkable dependence on the two external stimuli. A confusing experimental result is that all the force-extension（F-E） curves of unfolding an individual PNIPAM globule present a feature of elastic（monotonically increasing force） stretching but not plateau（constant force） stretching predicted by theory. In this article, we clarify that the presence of the interior solvent molecules in the single-chain globule is the origin of the discrepancy between the F-E curves obtained from theory and experiment. Although both of the external stimuli do tend to lower the solvent quality for PNIPAM, water and the water/methanol mixed solvent will never be the strongly poor solvent for PNIPAM, even at the worst condition.
Four double-stranded DNA films with different chain lengths were prepared on 3-aminopropyltriethoxysilane(APS)-modified mica surfaces in the NaCl solution with concentration ranging from 0.001 to 0.1 M.By using an atomic force microscope,the force-distance curves and friction behaviour of each DNA film were studied in the NaCl solution that was used in the sample preparation.When adsorbed on mica as films in salt solution,the conformation of DNA molecules would be a combination of loops and"train-like".As the chain length increased from 50 to 20000 bp,the extension rate of DNA film increased from 7.1to 11.5 in 0.001 mol/L NaCl solution,which suggested that the DNA molecule with long chain likely resulted in more extended conformation.In addition,under low normal load,low NaCl concentration could increase the friction of DNA film and the chain length revealed insignificant effect on the friction force of DNA film.Therefore,long chain DNA with low salt concentration is more conducive to the nanopore sequencing process,since extended conformation can make DNA molecules easier to reach into nanopore and the high friction can reduce the translocation speed.These results may benefit the development of the third-generation sequencing technique based on nanopore.
A novel environment-friendly system is proposed to fabricate polymer brush, which has the advantages including non-toxic and inexpensive initiator （eosin Y）, visible-light exposure （λ = 515 nm）, water medium and ambient environment. The experimental results from UV-Vis spectroscopy, AFM-based single molecule force spectroscopy （SMFS） and other measurements indicate that a polymer brush with a living nature is fabricated via free radical polymerization. This polymer brush may find applications in coatings, bio-interfaces and so forth.
