PNIPAAm-modified silicon nanowire arrays (SiNWAs) appear to adsorb denatured proteins, according to new experiments by researchers at Soochow University in China. The amount of protein adsorbed on the nanostructures varies with temperature and, more interestingly, cycling around PNIPAAm’s lower critical solution temperature (LCST) appears to further increase the quantities taken up. These results not only broaden our current understanding of the interaction between proteins and PNIPAAms, but may also lead to applications in which PNIPAAm-modified surfaces are used to separate different types of protein from a mix.
PNIPAAm is a shortened form of poly(N-isopropylacrylamide). This polymer is known to be temperature-responsive, because it possesses a LCST of about 32°C in aqueous solution.
Combining wet etching of silicon and a surface-initiated atom transfer radical polymerization (SI-ATRP) technique, we prepared arrays of silicon nanowires, and tethered a PNIPAAm layer about 10 nm thick onto the surface of each wire.
While the surface of PNIPAAm-decorated silicon nanowire arrays only absorbed small amounts of native proteins, the amount of denatured proteins they absorbed was found to be notably higher. The amount of denatured protein absorbed also varied much more strongly with temperature. Furthermore, temperature cycling around the LCST, a protocol that we put in place to prevent native proteins from thermally denaturing, further increased the amount of denatured protein adsorbed.
Native and denatured proteins appear to have different affinities for PNIPAAm-modified SiNWAs, a phenomenon that could be used to remove the denatured ones from a mix of the two. We have already conducted proof-of-concept experiments and have found that, for such a mix, the amount of native protein in solution remains nearly at its original level, whereas that of denatured protein greatly reduces.