Enhancing selectivity in spectrofluorimetric determination of tryptophan by using graphene oxide nanosheets

Reaction of formaldehyde with amino acids followed by oxidation with hydrogen peroxide to produce a fluorophore Norharman product is well known and was used for the spectrofluorimetric determination of l-tryptophan (Trp). This study aimed to use graphene oxide (GO) to enhance the selectivity and sensitivity of Trp in presence of other amino acids and possible interfering compounds. Different parameters such as pH, temperature, incubation time, and concentrations of formaldehyde, H₂O₂ and GO were studied to optimize the condition of determination. Experimental data showed that the maximum fluorescence intensity was achieved in pH 7.0–9.0 phosphate buffer mixed with 7–10% (v/v) formaldehyde and 1–2% (v/v) H₂O₂ as oxidizing agent at 60 ?C for 1 h. On the basis of calibration curve of various concentrations of Trp in the presence of 20 μg mL⁻¹ GO, the lower limit of detection (LOD) of Trp was determined as 0.092 nmol mL⁻¹ and the lower limit of quantification (LOQ) was 0.3 nmol mL⁻¹. The selectivity of Trp in presence of other amino acids and possible interfering compounds were studied with and without GO. The data obtained after inner filter effect corrections revealed that the selectivity of Trp in presence of amino acids and other possible interfering agents was improved in the range of 76–96%, compared with that in absence of GO. The enhancement of selectivity in the presence of GO indicates that the Trp and other amino acid and possible interfering compounds were adsorbed by GO, and the selective uptaking of Trp-by the reaction with formaldehyde followed by oxidation with H₂O₂ at 60 ?C with high selectivity and sensitivity was achieved successfully.