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Local Extraction and Condensation under a Microscope Using the Optically Controlled Phase Separation of a Thermoresponsive Polymer



In-situ extraction and condensation of various dyes were carried out in a phase-separation region of a thermoresponsive polymer aqueous solution generated by near infrared (NIR) laser heating under a microscope. The NIR laser irradiation was directed at a chromium line deposited on a glass substrate, thereby causing local heating of the solution due to the photothermal effect. A phase-separation region was formed by dehydration of the thermoresponsive polymer followed by ejection of water outside of the phase-separation region. When various dyes were included in the solution, some dye molecules were extracted into the phase-separation region, where they condensed. In the case of poly(N-isopropylacrylamide) (PNIPAM, 10 wt % in an aqueous solution) as the thermoresponsive polymer and crystal violet (CV) as the dye (0.1 mM), CV condensed by about 25 times. It was found that one of the necessary conditions for the extraction/condensation is the hydrophobicity of the dye molecule; however, the dominant cause for accumulating inside the PNIPAM chain is the molecular interaction between the amide group in the side chain of PNIPAM and the functional groups such as carbonyl or amino groups in the dye molecules.