Synthesis and Characterization of Tunable, pH-Responsive Nanoparticle–Microgel Composites for Surface-Enhanced Raman Scattering Detection

The synthesis of microgels with pH-tunable swelling leads to adjustable and pH-responsive substrates for surface-enhanced Raman scattering (SERS)-active nanoparticles (NPs). Sterically stabilized and cross-linked latexes were synthesized from random copolymers of styrene (S) and 2-vinylpyridine (2VP). The pH-dependent latex-to-microgel transition and swellability were tuned based on their hydrophobic-to-hydrophilic content established by the S/2VP ratio. The electrostatic loading of polystyrene/poly(2-vinylpyridine) microgels [PSxP2VPy (M)] with anions such as tetrachloroaurate (AuCl4–) and borate-capped Ag NPs was quantified. The PSxP2VPy (M) can load ∼0.3 equiv of AuCl4– and the subsequent photoreduction results in Au NP-loaded PSxP2VPy (M) with NPs located throughout the structure. Loading PSxP2VPy (M) with borate-capped Ag NPs produces PSxP2VPy (M) with NPs located on the surface of the microgels, where the Ag content is set by S/2VP. The pH-responsive SERS activity is also reported for these Ag NP-loaded microgels. Analytical enhancement factors for dissolved crystal violet are high (i.e., 109 to 1010) and are set by S/2VP. The Ag NP-loaded microgels with ∼80 wt % 2VP exhibited the most stable pH dependent response.