Since the RhB-Im was not fully soluble in 100% aqueous media, an appropriate amount of DMF (40%, v/v) was used as a solubilizing medium in whole experi- ments. The UV–vis and fluorescence titrations were carried out by syringing aliquots of each metal stock solution into 100 mmol L À 1
of RhB-Im in 3 mL acetate-buffer/DMF (3/2, v/v, pH 3.6). After each addition, the solution was fully mixed and equilibrated for ca. 2 min. The absorption spectra were taken by scanning the solution within 400–650 nm.