Histidine-Functionalized Diblock Copolymer Nanoparticles Exhibit Enhanced Adsorption onto Planar Stainless Steel

RAFT aqueous emulsion polymerization of isopropylideneglycerol monomethacrylate (IPGMA) is used to prepare a series of PGEO5MA46-PIPGMAy nanoparticles, where PGEO5MA is a hydrophilic methacrylic steric stabilizer block bearing pendent cis-diol groups. TEM studies confirmed that well-defined spherical nanoparticles were obtained while DLS analysis indicated that the z-average particle diameter could be adjusted from 68 nm to 188 nm by systematically varying the target degree of polymerization for the core-forming PIPGMA block. Sodium periodate was employed as a selective oxidant to convert the cis-diol groups on PGEO5MA46-PIPGMA500 and PGEO5MA46-PIPGMA1000 nanoparticles into the analogous aldehyde-functionalized nanoparticles, which were then reacted with histidine via reductive amination. In each case, the extent of functionalization was more than 99% as judged by 1H NMR spectroscopy. Moreover, aqueous electrophoresis studies indicated that such derivatization converted initially neutral nanoparticles into nanoparticles that exhibit an isoelectric point (IEP) at around pH 7. Interestingly, DLS studies confirmed that such histidine-derivatized nanoparticles remained colloidally stable over a wide pH range, with no sign of any aggregation at around the IEP. A quartz crystal microbalance (QCM) was employed at 25 °C to assess the extent of adsorption of both the cis-diol- and histidine-functionalized nanoparticles onto a planar stainless steel substrate at pH 6. The histidine-bearing nanoparticles adsorbed much more strongly than their cis-diol counterparts. For the highest adsorbed amount of 70.5 mg m-2, the adsorbed layer of nanoparticles was examined by SEM, which enabled a fractional surface coverage of 0.23 to be estimated via digital image analysis.