Abstract: In HCl medium, AuCl₄^- and GaCl₄^- could react with methylene blue(MB) to form two ionic associates, which could be easily extracted by 1,2-dichloromethane solvent and resulted in the fluorescence enhanced significantly in organic phase. Their maximum excitation wavelengths(λ_ex) located at 656 and 666 nm, respectively; and their maximum emission wavelengths(λ_em) were 674 and 685 nm, respectively. The synchronized fluorescence spectra(Δλ＝0) could be recorded with synchronous scanning at λ_ex=λ_em, and it could be proved by the fluorescence polarization experiment that they were resonance fluorescence. The resonance fluorescence peaks of two ionic associates located at 660 and 671 nm, respectively, which was found in the intersection of excitation spectra and the emission spectra. Both intensities(ΔF) of fluorescence and resonance fluorescence were directly proportional to the concentration of MB in a certain ranges, so the high sensitivity methods of determination of trace amount MB were established, and their detection limits(3σ) were 0.2 and 0.6 ng/mL for fluoresence method, as well as 1.1 and 2.8 ng/mL for resonance fluoresence method, respectively. The fluorescence method has a higher sensitivity and is more suitable in determination of trace amount MB. In the paper, the optimum conditions for organic solvent extration, the spectra characteristics of fluorescence and resonance fluorescence were discussed. Besides, the composition and structure of [MeCl₄]^--MB ionic associates were studied. The fluorescence method could be applied to the determination of MB in urine samples and human serum samples with satisfactory results.

Key words: Methylene blue, Solvent extraction, Ionic associate, Resonance fluorescence