Photo-controllable coil-to-globule transition of single polymer molecules

•The coil-to-globule conformation transition of PNIPAM is studied at single molecular level.•Isomerization of the azobenzene moiety at the chain end of PNIPAM changes the polarity of the end.•The temperature for conformation transition of PNIPAM is shifted up to 12 °C with UV irradiation.

This study demonstrates that the conformation transition of single polymer molecules can be adjustable by applying external irradiation. By choosing poly N-isopropylacrylamide as the model system, its coil-to-globule conformation transition temperature is shifted with application of external UV irradiation, due to the photo-isomerization of the azobenzene moiety attached to the chain end. The investigation was conducted at single molecular level by fluorescence correlation spectroscopy, using hydrodynamic radius as the measure of chain dimension. It is discovered that the coil-to-globule transition temperature of the PNIPAM-azo can be controlled up to a difference of 12 °C upon external irradiation, due to the difference in polarity of the azobenzene end group.

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