Optically active pH-dependent colloids of silver nanoparticles capped by polygalacturonic acid

Optically active nanobiocolloids (NBC) containing plasmonic nanoparticles (NP) covered by helical biomacromolecules are interesting for various theranostic applications. We report the study of NBC colloids containing from 4.4 to 9.3 w/w % of small (ca. 10 nm) spherical AgNPs covered by polygalacturonic acid (PGA). The AgNPs were obtained by reduction of silver ions with the functional groups of PGA. The NBC structure was formed by nanoprecipitation of the final solution into an anti-solvent (ethanol) bath. The NBC powders were characterized by transmission electron and atomic force (AFM) microscopies, FT-IR, and X-ray diffraction. The FT-IR analysis showed that the carboxylate groups of PGA were attached to the AgNPs’ surface by bidentate binding. The PGA (the major constituent of the main chain of pectins) is a weak anionic polysaccharide which helical conformations and their self-assembly depend on the deprotonation of carboxyl groups. We studied how pH influences the form, size, depolarization of scattered light, and chiroplasmonic properties of NBC. The optical rotatory dispersion of NBC exhibited plasmonically enhanced Cotton effect related to helical PGA macromolecules capped to AgNPs. The Cotton effect changed its sign from negative (at pH 4.01) to positive (at pH 6.86) implying the inversion of handedness of the PGA helixes. By using dynamic and depolarized dynamic light scattering, the effective hydrodynamic radii were calculated for translational (R_h) and rotational (R_rot) diffusion. Their characteristic ratio R_rot/R_h expressed in terms of the Perrin frictional coefficients showed that the form of NBC colloids changed from spherical to elongated with decreasing pH. The elongated form in the acidic buffer was attributed to the side-by-side stacking of the helical segments of PGA. Low fraction of coupled plasmons and plasmonic enhancement of elliptical polarization of light by the helical conformers of PGA were both responsible for high depolarization of light scattered by NBC colloids. The highest depolarization degree of 40% observed in the acidic buffer was ascribed to the non-uniform elliptical shell of PGA around AgNPs. Contrary to the pristine particles of sodium polygalacturonate (PGA@Na), the NBC retained their size upon drying, as shown by AFM. In accord, comparison of the FT-IR spectra of NBC and PGA@Na showed that the NBC were stabilized by additional hydrogen bonds.