Thermally responsive wettability of self-assembled methylcellulose nanolayers

Thermo-responsive cellulosic nanolayers were prepared from methylcellulose (MC), which is known to have a unique lower critical solution temperature. Thiosemicarbazide (TSC) was selectively introduced into the MC reducing end groups, and the corresponding MC-TSC derivative was spontaneously chemisorbed on an Au substrate at 4 °C to give MC self-assembled monolayers (SAMs). Linear MC chains were stably fixed onto the Au substrate, yielding an MC-SAM of thickness ca. 15 nm with a root mean square value less than 1 nm. The MC-SAM surface exhibited thermally responsive wetting characteristics; the water contact angle was found to rise and fall around 70 °C, possibly due to the solid-state phase transition of the MC nanolayers resulting from the inherent gelation of MC molecules in water. Such wetting behavior was shown to be reversible following repeated heating and cooling. The MC-SAM immersed in salt solution revealed lower phase transition temperatures, and an increase in sodium chloride concentration ranging from 0.0 to 1.0 M brought about a dramatic decrease in the apparent phase transition temperature from ca. 70 to 30 °C. For the purposely designed MC nanolayers, such controllable wetting properties are expected to prompt growing interest in the applications of cellulosic biopolymer interfaces.