Surface functionalization effects on structural, conformational, and optical properties of polyaniline nanofibers

•PAni nanofibers have been synthesized by dilute polymerization technique.•PAni nanofibers are surface functionalized by glutaraldehyde.•Surface functionalization increases the doping level of PAni nanofibers.•The fluorescent signal is enhanced due to surface functionalization.•Surface functionalization induces variation in the redox state of polyaniline. Polyaniline nanofibers have been synthesized by dilute polymerization technique and functionalized with glutaraldehyde under acidic condition. The effects of surface functionalization of polyaniline nanofibers on their structural, conformational, and optical properties have been investigated using transmission electron microscopy (TEM), X-ray diffraction, UV–visible, infrared, and fluorescence spectroscopy. TEM and UV–vis spectroscopy results depict that fiber diameter is decreased from 35.66 to 31.04nm on functionalization. XRD analysis suggests increased doping of surface functionalized polyaniline nanofibers (SF-PNFs) than that of the pristine polyaniline nanofibers (PNFs). Evolution of fluorescence peak at 452nm in the fluorescence spectra and disappearance of absorption peak at 630nm in the absorption spectra of surface functionalized polyaniline nanofibers (SF-PNFs) can be explained on the basis of the dynamic block co-polymeric structure of polyaniline (PAni). Study of optical properties shows that surface functionalization of polyaniline nanofibers (PNFs) brings about a partial transition of emeraldine base (EB) into emeraldine salt (ES) form of polyaniline. This is confirmed from the FTIR analysis, which depicts rearrangement in the polyaniline chain due to oxidation at the amine sites of benzenoid rings and reduction at the imine sites of quinoid rings after functionalization with glutaraldehyde due to its higher reactivity towards amine group. The possible mechanisms of functionalization of polyaniline nanofibers depending on different chemical structures of glutaraldehyde under acidic condition have been discussed in detail.