Rheological and fluorescent properties of riboflavin-poly(N-isopropylacrylamide) hybrid hydrogel with a potentiality of forming Ag nanoparticle

The riboflavin (R) and poly(N-isopropylacrylamide) (PNIPAAM) hybrid hydrogels have been prepared using the free radical polymerization of N-isopropylacrylamide in the presence of N, N'-methylene bisacrylamide as a cross linker for 1, 2 and 3 mM concentrations of R. The invariance of storage (G') and loss (G '') moduli over a wide range of angular frequency, where G' > G '' for R-PNIPAAM system characterize it to be behaving as a gel in the hybrid state. Both G' and G '' decrease with the increase of R concentration, but the decrease is four times higher in the former than in the latter. R-PNIPAAM gel has a higher critical strain value than PNIPAAM gels and it gradually increases with the increase in R concentration indicating that R is acting as a supramolecular cross-linker. The fluorescent intensity of R-PNIPAAM gels increases with the increase in R concentration and its variation with temperature at different pH shows an increase in the intensity value with temperature, showing the maximum at similar to 30 degrees C because of the coil-to-globule transition of PNIPAAM chains, suggesting that the R-PNIPAAM gel can be used to be a probe for temperature detection. The sensitivity index of the fluorescent temperature sensing of R-PNIPAAM gel is moderate and it is highest at pH 7. R becomes gradually released from the R-PNIPAAM gel when dipped into water (pH 7) at 20 degrees C, initially at a slower rate, then at a higher rate, and finally it shows a leveling for the release of 19% of embedded R at 24 h of aging. Silver nanoparticles that are grown in the R-PNIPAAM gels by immersing the slashed gel pieces in AgNO3 solution stay at both the surface and the interior of the R-PNIPAAM fibres, maintaining the gel structure intact with a decrease in critical strain and causing a quenching of the fluorescence intensity of R-PNIPAAM gels. The average size and size distribution of AgNPs linearly increase with R concentration at a constant AgNO3 concentration.