An Aminosalicylic Acid-based Schiff Base As A Fluorescent Switch And Its Analytical Application

A fluorescent molecular switch was a molecule that whose fluorescent intensity could be reversibly shifted between two states just like enhancement or quenching. The fluorescent intensity might be shifted between the states in response to changes in pH, light, temperature, metal ions and so on. Fluorescent molecular switche had be widely interested as chemical analysis method due to its convenient in determination, high sensitivity and short response time. An salicylic acid-based schiff base was synthesized and the fluorescent switch properties were studied in this paper. The work in this paper mainly includes two parts as follow:First, 2-hydroxy-4-(pyridin-2-ylmethyleneamino) benzoic acid (PA) was synthesized with 2-pyridylaldehyde and 4-aminosalicylicacid. The product was identified by IR spectra and LC-MS. It was demonstrated that Fe³⁺ could selectively quench the fluorescence of PA in acidic aqueous solution, whereas other metal ions such as Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, Mn²⁺, Cr³⁺ and Fe²⁺ can't. In addition, the fluorescent intensity of PA could be reversibly tuned by ligand (just like EDTA) or reductant (just like hydroxylamine hydrochloride). Thus PA could perform the "OFF-ON" function of fluoresce emission intensity and could be used as a double control fluorescent molecule switch. In the addition, they could executed one excellent logic operation, which integrated one OR logic gates.Second, PA, which showed high specificity for Fe³⁺ recognition, could be used as a fluorescent molecular probe. The results indicated that the fluorescence intensity of PA at 392 nm could be significantly quenched by Fe³⁺, and the fluorescence intensity was in proportion to the concentration of Fe³⁺. Under the optimum conditions, the linear range for the determination was 4.0×10^-7 to 6.5×10^-6 mol L^-1, and the detect limit was 4.8×10^-8 mol L^-1. The proposed method was successfully applied to assay Fe³⁺ in milk powders with good recovery rate (93.0¹05%). While citrate added to the PA-Fe³⁺ system, the fluorescence of PA recovered. These might because that citrate had stronger coordinating ability to Fe³⁺, it would prohibit the formation of PA-Fe³⁺ complex. And the fluorescence intensity of PA-Fe³⁺-citrate system was in proportion to the concentration of citrate. Based on that, a novel fluorescence enhance method was established for citrate detection, and the detect limit and linear range are 2.2×10^-7 mol L^-1 and 2.0×10^-6 to 2.0×10^-5 mol L^-1, respectively. This method was simple, practical, sensitive, and it was selective and successfully applied to determination of citrate in soft drink. The fluorescence enhancement and quenching mechanisms were also discussed in the text.