Ultrathin Layer-by-Layer Hydrogels with Incorporated Gold Nanorods as pH-Sensitive Optical Materials

We report ultrathin pH-responsive plasmonic membranes of [poly(methacrylic acid)-gold nanorods]₂₀ (PMAA-Au NRs)₂₀ with gold nanorods embedded into swollen cross-linked LbL hydrogels. In contrast to the most of known pH responsive materials which rely on pH-triggered change in the intensity of photoluminescence or plasmon bands, the responsive structures suggested here exhibit a significant pH-triggered shift in easily detectable, strong plasmon resonance band. We show that a pH-induced deswelling of the (PMAA-Au NRs)₂₀ hydrogel film in the pH change from 8 to 5 causes a dramatic blue-shift of the longitudinal plasmon peak by 21 nm due to the increased side-by-side interactions of adjacent gold nanorods. These composite hydrogel multilayer films can be released from the substrates yielding free-floating and optically pH-responsive ultrathin hydrogel films which can be transferred to the appropriate solid substrates.