Optical properties and electric conductivity of gold nanoparticle-containing, hydrogel-based thin layer composite films obtained by photopolymerization

Poly(acrylamide) [poly(AAm)] and poly(N-isopropyl-acrylamide) [poly(NIPAAm)] based gel films containing Au nanoparticles (d = 14 +/- 2.5 nm) were synthesized. Monomers and cross-linker were added to a gold nanodispersion, and after the addition of the initiator, polymer films were prepared on the surface of an interdigital microelectrode by photopolymerization. In the course of the syntheses the gold concentration of the films was constant (10.8 mu g/cm(2)) and the volume fraction of Au nanoparticles (phi(Au)) in the polymer gel films varied in the range of 0.58-85.3%. Poly(AAm)-based films swell when the temperature increases: due to a temperature shift of 15 degrees C the Au plasmon absorption maximum at lambda = similar to 532 nm was shifted towards shorter wavelengths by 16.6 nm (blue shift) through the swelling of the polymer gel film. In the case of poly(NIPAAm) the temperature-induced shrinking resulted in a red shift, namely the maximum was shifted by 18.07 nm by a temperature shift of 15 degrees C. In the case of both composites, the electric conductivity of the samples was shown to increase with increasing Au particle concentration. In the case of the poly(AAm)-based composite containing phi(Au) = 0.85 gold the resistance of the film spread on the surface of the electrode was 0.16 M Omega at 25 degrees C and 0.66 M Omega at 50 degrees C, i. e. the conductivity of the sample decreased with increasing temperature. The opposite effect is observed in the case of the poly(NIPAAm)-based composite: as temperature is raised, the resistance of the composite abruptly drops at the point of collapse of the NIPAAm gel (it is 0.28 M Omega at 32 degrees C and only 0.021 M Omega at 35 degrees C). This thermosensitive effect was detectable only at sufficiently high Au contents (phi(Au) = 0.85) in both gels. (C) 2009 Elsevier B.V. All rights reserved.