Fluorescent tagged probing agent and structure-directing amphiphilic molecular design for polyaniline nanomaterials via self-assembly process

Abstract

We have designed and developed a unique amphiphilic dopant molecule, 4-[4-hydroxy-2((Z)-pentadec-8-enyl)phenylazo]-benzenesulfonicacid, from a renewable resource, cardanol, which acts as a fluorescent probe and structure-directing agent for polyaniline nanomaterials. The amphiphilic dopant is fluorescing in water and forms stable emulsion for a wider composition of dopant:aniline ratio from 1:1 to 1:1500 (in moles), which is rarely noticed in the polyaniline synthesis. The azobenzene dopant exists in two supramolecular aggregates, such as bi-layer or micelle, depending upon its concentration in water, and the critical micelle concentration (CMC) is directly obtained from the emission properties. Above the CMC, the dopant-aniline complex exists as either aggregated or isolated micelles, and subsequent oxidation produces polyaniline nanomaterials such as hollow spheres (1-2 mu M), dendritic nanofibers, and linear nanofibers of 8-10 mu M length with a diameter of 130-180 nm. Below the CMC, the dopant aggregated in the form of bi-layers which produce mixtures of nanotubes plus nanofibers or nanotubes of 60 nm pore and 80 nm wall thicknesses. The WXRD patterns of nanofibers showed a sharp peak at 2 theta = 6.4 (d-spacing = 13.6), which corresponds to the highly ordered polyaniline chain followed by the effective interdigitations of dopant molecules. The intensity and percent (%) crystallinity of the ordered peak increase and reach a maximum up to CMC and then decrease gradually. It reveals that above CMC, the dopant effectively penetrates into the polymer interlayer and produces highly three dimensionally ordered solid-state nanofibers. Below the CMC, the dopant molecule loses the collective penetration ability to form highly ordered fibers. The polyaniline nanofibers showed enhanced emission in water, and the amount of the dopant in the nanomaterial plays a crucial role in luminescent intensity and quantum yield of the nanofibers. In a nutshell, by understanding the mechanistic aspects of renewable resource amphiphilic dopant-aniline complex by fluorescent spectroscopy, the properties of polyaniline nanomaterials were precisely controlled in a single system.