Fluorescence Quenching of the Probes L-Tryptophan and Indole by Anions in Aqueous System

Abstract

A fluorescence quenching study of the fluorescent probes L-tryptophan and indole by anions (NO₃⁻, Cl⁻, SO₄²⁻) was carried out in an aqueous system. The ions NO₃⁻, Cl⁻ and SO₄²⁻ showed very good quenching of both the probes. Quenching of L-tryptophan by all the anions studied was higher as compare to the quenching of indole. The data was fitted with the Stern–Volmer equation. Stern–Volmer constants were observed in the order NO₃⁻ > Cl⁻ > SO₄²⁻. Stern–Volmer constants reflect the sensitivity of the method for the studied anions. Limit of detection (LOD) was calculated as three times the standard deviation of the blank for n = 10 (3 × SD) while the limit of quantification (LOQ) was calculated as ten times the standard deviation of the blank for n = 10 (10 × SD). In the case of L-tryptophan LOD and LOQ varied from 4.08 × 10⁻⁵ – 4.56 × 10⁻⁴ mol L⁻¹, while in the case of indole the values ranged from 3.87 × 10⁻⁵ – 6.59 × 10⁻⁴ respectively. Fluorescence quenching of L-tryptophan and indole by the studied anions showed good reproducibility and the method could be very effective for the determination of anions.