Sunlight Induced Preparation of Functionalized Gold Nanoparticles as Recyclable Colorimetric Dual Sensor for Aluminum and Fluoride in Water.

A sunlight induced simple green route has been developed for the synthesis of polyacrylate functionalized gold nanoparticles (PAA-AuNPs), in which poly(acrylic acid) functions as a reducing as well as stabilizing agent. This material has been characterized on the basis of spectroscopic and microscopic studies; it exhibited selective colorimetric detection of Al3+ in aqueous media, and the Al3+ induced aggregated PAA-AuNPs exhibited detection of F- with sharp color change and high selectivity and sensitivity out of a large number of metal ions and anions tested. The mechanistic study revealed that, for Al3+, the color change is due to a shift of the SPR band because of the Al3+ induced aggregation of PAA-AuNPs, whereas for F-, the reverse color change (blue to red) with return of the SPR band to its original position is due to dispersion of aggregated PAA-AuNPs, as F- removes Al3+ from the aggregated species by complex formation. Only concentration-dependent fluoride ion can prevent Al3+ from aggregating PAA-AuNPs. The method is successfully used for the detection of F- in water collected from various sources by the spiking method, in toothpastes of different brands by the direct method. The solid Al3+-PAA-AuNPs were isolated, adsorbed on ZIF@8 (zeolitic imidazolate framework) and on a cotton strip, and applied as solid sensing material for detection of F- in aqueous media.