Biopolymer capped silver nanoparticles as fluorophore for ultrasensitive and selective determination of malathion

This paper describes a novel luminescent sensor for malathion using chitosan capped silver nanoparticles (Chi-AgNPs) as fluorophore. The Chi-AgNPs were synthesized by the wet-chemical method and were characterized by absorption, fluorescence, HR-TEM, XRD and DLS techniques. The Chi-AgNPs show the absorption maximum at 394 nm and emission maximum at 536 nm. While adding 10 mu M malathion, yellow color Chi-AgNPs was changed to brown and the absorbance was decreased along with a redshift. The observed spectral and color changes were mainly due to the aggregation of Chi-AgNPs. This was confirmed by zeta potential, DLS and HR-TEM studies. No significant absorption spectral change was observed for Chi-AgNPs in the presence of less than micromolar concentrations of malathion. However, the emission intensity of Chi-AgNPs was decreased and the emission maximum was shifted toward higher wavelength in the presence of picomolar concentration of malathion. Based on the decrease in emission intensity, the concentration of malathion was determined. The Stern-Volmer constant, Gibbs free energy change, association constant, quantum yield and binding constant were calculated and the quenching mechanism was proposed. The Chi-AgNPs show good selectivity toward the determination of 10 nM malathion in the presence of 1000-fold higher concentrations of common interferents. A good linearity was observed for the emission intensity against 1 x 10(-9)-10 x 10(-12) M malathion and the detection limit was found to be 94 fM L-1 (S/N=3). The proposed method was successfully applied to determine malathion in fruits and water samples and the obtained results were validated with HPLC. (c) 2013 Elsevier B.V. All rights reserved.