Tunable resistive pulse sensing and nanoindentation of pH-responsive expansile nanoparticles

Experimental studies of pH-responsive expansile nanoparticles (eNPs) have been carried out using tunable resistive pulse sensing (TRPS) and nanoindentation. The eNPs are initially ~200 nm in diameter, but increase in size owing to a compositional change resulting from cleavage of pH-labile protecting groups on the polymer backbone at low pH. TRPS detected expected changes in the size of eNPs, most clearly through an increase in the size distribution close to ~1 μm diameter with time after exposure to pH 5.0 electrolyte. Heavily cross-linked nanoparticles and eNPs kept at neutral pH did not produce such pronounced changes. Nanoindentation results demonstrate that for accurate measurement of eNP mechanical properties, particles must be precisely located and securely fixed to the substrate. Moreover, the expanded eNPs may be too weak to respond to conventional nanoindentation. Nanoscale physical characterisation of soft matter is of burgeoning importance in medical and biotechnological research and applications.