Specific detection of Cu2+ by a pH-independent colorimetric rhodamine based chemosensor

A new colorimetric "off-on" rhodamine based chemosensor for metal ions (2,4,6-trihydroxybenzaldehyde rhodamine B hydrazone, THB) was synthesized via Schiff base chemistry between rhodamine B hydrazide and 2,4,6-trihydroxybenzaldehyde. Of all the metal ions tested, it selectively turned pink in the presence of Cu2+, and its sensitivity was evaluated. Coordination with Cu2+ was accompanied by a binding constant of 5.95 x 10(3) M-1 whilst its detection limit is remarkably low (0.48 mu M; WHO acceptable limit = 31.5 mu M). Moreover, detection of Cu2+ with THB was independent of pH as pinkish mixture with high absorbance was observed in a wide range from pH 4-12. Consequently, a 1:1 binding ratio of THB to Cu2+ was verified. Additionally, THB can be reversibly used for up to three times with administration of EDTA and the coordination of THB with Cu2+ was corroborated by the shifting of carbonyl amide peak in the FT-IR spectra. Then, the prototype of THB as an onsite test kit for detection of Cu2+ in 100% aqueous condition was also successfully demonstrated. However, THB is weakly cytotoxic against the colorectal adenocarcinoma cell line HT-29 albeit non-toxic towards the noncancerous colon cell line CCD-18Co (IC50 via MTT: HT-29 = 39.24 +/- 1.59 mu M; CCD-18Co > 50.62 mu M). Nonetheless, these findings proposed the potential of THB as a simple and feasible sensor in multiple practical applications.