Multi-stimuli responsive tetra-PPO60-PEO20 ethylene diamine block copolymer enables pH, temperature, and solvent regulation of Au nanoparticle composite plasmonic response
A gold nanoparticle (NP) hydrogel composite prepared by multi-step cascade synthesis undergoes temperature, pH, and solvent dependent nanoscale structural transformations promoting modulation of the plasmon resonance. The composite is spontaneously prepared from a non-covalent lamellar structured lyotropic mesophase (complex fluid) composed of amphiphiles (DMPC and Triton X-100) that support the reactive constituents, a mixture of hydroxyl and acrylate end-derivatized Tetronic (Poloxamine) polymer and [AuCl4]-. The reaction sequence begins with the reduction of [AuCl4]- by the ethylene diamine linkage and PEO blocks of the Tetronic, yielding Au NP. The redox reaction generates free radicals initiating crosslinking of the acrylate end-derivatized polymer producing a chemical hydrogel. Optical spectroscopy and STEM confirm Au NP formation. ATR/FT-IR spectroscopy and thermal analysis establish acrylate crosslinking and network formation. X-ray scattering reveals the composite reversibly converts between multilamellar and bicontinuous cubic structures as temperature is increased above 35°C for composites containing protonated ethylene diamine linkages (un-buffered water or acidic buffer). The reversible changes in nanoarchitecture red shifts and broadens the collective plasmon (from 523 nm to 575 nm) of the composite, an observation attributed to re-organization of confined NPs from 2D sheets (lamellae) to 1D columns (bicontinuous cubic channels). Composites prepared in alkaline buffer do not undergo a structural conversion with temperature, retaining a multilamellar architecture. In addition to pH and thermo-responsivity, the composite swells reversibly in polar solvents, which acts to de-aggregate and aggregate the encapsulated nanoparticles and varies the plasmon resonance by Δ52 nm. This work reports the first combined use of a polymer to achieve both the in-situ synthesis of spatially confined plasmonic NPs and their reversible stimuli-triggered re-organization mediated by interconvertible changes in scaffold nanoarchitecture.