Core-shell polymeric micelles based on the 2-(hydroxyimino)aldehyde group as self-contained nanoreactors to obtain single and clustered ultra-small silver nanoparticles

Colloidal Silver NanoParticles (AgNPs) are routinely obtained from Ag+ salts with chemical reductants in the presence of polymers or small molecules as capping agents to control their size and shape and enhance their stability. The use of amine-containing polymers, polyethers and polyols for AgNPs fabrication circumvents the need for added reductants, thus avoiding purification steps and enhancing process sustainability. However, high temperatures or irradiation may still be needed, and characterization of the resulting chemical species may be difficult. In this work, we incorporate Ag+ into pre-formed micelles obtained through a pH-gradient method from two block copolymers pTEGMA-b-pHIABMA (TEGMA: tetra(ethylene glycol) methyl ether methacrylate, 75 units; HIABMA: 4-[(hydroxyimino)aldehyde]butyl methacrylate), 38 or 73 units) and a 37:37 random copolymer pOEGMA-r-pHIABMA (OEGMA having nine EG units). The micelles act as self-contained nanoreactors through the oxidation of aldehyde groups to carboxylates (1H and 13C NMR), to yield AgNPs with polymer-dependent characteristics. The core–shell micelles obtained with the two block copolymers afford ultra-small (2 nm diameter) spherical AgNPs clustered within the polymer nanoaggregates (DLS, TEM, SAXS). More numerous AgNPs are embedded within the polymer having more HIABMA units. The loose micelles obtained with the random copolymer, instead, affords single 10 nm AgNPs. The polymer-coated AgNPs exhibit Surface Plasmon Resonance (λ = 418 nm). No significant change in the UV–Vis spectra and DLS measurements was observed over a six-month period at room temperature. TEM micrographs show that the 2 nm clusters of AgNPs are stable over 20 months at 4 ◦C, with no evidence of coalescence, although partial solubilization of Ag+ ions occurs over time. Given the different arrangement of the ultra-small AgNPs within the polymeric matrices of different architecture, the proposed nanoreactors may provide new tools in the fields of metamaterials and nanoarchitectonics. As for antimicrobial effects, a preliminary evaluation of the microbiological activity of the 2 nm clustered AgNPs shows bacteriostatic activity against Staphylococcus aureus (5 μg mL 1 Ag), and bactericidal activity (11 μg mL 1 Ag) against Pseudomonas aeruginosa, in concentration ranges that are comparable to previous literature on ultra-small AgNPs.