Separation of peptides with polyionic nanosponges for MALDI-MS analysis.

A polymer brush consisting of 70% poly(N-isopropylacrylamide) (PNIPAAM) and 30% polymethacrylic acid (PMAA) was synthesized from gold substrates with a "grafting from" AIBN-type free-radical initiator. Fractionation of two peptides, bradykinin and buccalin, was accomplished in less than 120 s by placing a 30 pM (pH approximately 6.2) droplet onto the polymer brush substrate. The eluant containing the anionic buccalin is pipetted away for MALDI analysis while the cationic bradykinin adsorbed to the swollen anionic brush and was subsequently released by adding a droplet of formic acid to the substrate. This caused the brush to collapse and release the bradykinin, much like squeezing a sponge; these nanosponge substrates exhibited very high loading capacity (>2.0 mg/mL) compared to plasma-polymer-modified MALDI substrates. Ellipsometric measurements showed that complementary peptides adsorb rapidly while those of the same charge do not, and MALDI-MS analysis of the two fractions showed separation of both peptides. The adsorption of bradykinin was monitored over time, and 85% of the peptide had been adsorbed to the nanosponge in 1 min from a 0.5 mg/mL aqueous solution.