Study of the AlS4Pc-Polylysine Ion Pair, a Red-Fluorescent Probe: Its High Specificity, Large Concentration Range...

Tetrasulfonated aluminum phthalocyanine (AlS₄Pc), a strongly red-emitting compound, shows high detection sensitivity, little effect of photobleaching, and photochemical stability, making it an excellent red-fluorescent probe. We have observed that in acid media, a low concentration of poly-L-lysine (PLL) has a strong fluorescence-quenching effect on AlS₄Pc, forming the ion-pair complex as AlS₄Pc-PLL with almost no fluorescence. However, in the presence of Bi³⁺, the fluorescence of AlS₄Pc-PLL dramatically recovers and the emission is visual because of the remarkable recovery. Screening experiments with other metal ions reveal that only Bi³⁺ can restore the fluorescence of the AlS₄Pc-PLL complex. The presence of other metal ions does not result in the recovery of fluorescence, indicating the high specificity of the response to Bi³⁺ of AlS₄Pc-PLL. This is the key finding of the present study. It was also observed that the response to Bi³⁺ of AlS₄Pc-PLL exhibits a linear relationship over a large concentration range (three orders of magnitude). Based on these findings, we have established a new quantitative analysis method for Bi³⁺ with high specificity and high sensitivity, using the ion-pair AlS₄Pc-PLL complex as a red-fluorescent probe, and we discuss the reaction mechanism. The detection limit of this method is 0.0021 mg L^-1 The linear relationship applies to the range of 0.007-34.9 mg L^-1 with a correlation coefficient of 0.9993. The method established addresses the complex operation and time-consuming problems in traditional methods and is thus suitable for real applications. Satisfactory results have been obtained when the method was applied to the measurement of real samples. This study further expands the scope of new applications of phthalocyanine-based red-fluorescent probes in analytical sciences.