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

Abstract

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 Cu 2+ , and its sensitivity was evaluated. Coordination with Cu 2+ was accompanied by a binding constant of 5.95 × 10 3 M −1 whilst its detection limit is remarkably low (0.48 μM; WHO acceptable limit = 31.5 μM). Moreover, detection of Cu 2+ 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 Cu 2+ was verified. Additionally, THB can be reversibly used for up to three times with administration of EDTA and the coordination of THB with Cu 2+ was corroborated by the shifting of carbonyl amide peak in the FT-IR spectra. Then, the prototype of THB as an on-site test kit for detection of Cu 2+ 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 non-cancerous colon cell line CCD-18Co (IC 50 via MTT: HT-29 = 39.24 ± 1.59 μM; CCD-18Co > 50.62 μM). Nonetheless, these findings proposed the potential of THB as a simple and feasible sensor in multiple practical applications. • Excellent selectivity of THB towards copper ions. • Wide pH detection range for Cu 2+ . • Versatile application of THB in both acidic and alkaline conditions. • Detection limit of 0.48 μM, well below WHO acceptable levels (31.5 μM). • Application of THB as an on-site testing kit.