Colorimetric detection of Pd2+ and Ni2+ ions using a probe having benzo[d]imidazo[2,1-b]thiazole and pyrene rings: Application in detection of Pd2+ ion in anti-hypertensive drugs

Abstract

Pd-complexes are widely used in the pharmaceutical industry to synthesize APIs and drug molecules. It is imperative to detect even trace amounts of Pd impurities in them. A probe (E/Z)-N'-(pyren-1-ylmethylene)benzo[d]imidazo[2,1-b]thiazole-2-carbohydrazide (LH) having benzo[d]imidazo[2,1-b]thiazole and pyrene rings has been synthesized. LH was found to exhibit a change from colourless to yellowish green in presence of Pd2+ and Ni2+ ions. This colorimetric “turn on” response has also been observed with Pd2+ ions in the presence of “sartan” drugs, namely telmisartan, valsartan, irbesartan, olmesartan medoxomil and losartan potassium. The limit of detection ranged from 46.1 to 93.9 nM (4.9 to 6.0 ppb) for PdCl2 and 10.6 to 19.6 nM (1.1 to 2.1 ppb) for Pd(PPh3)2Cl2 and these are lower than the limit suggested by the EMA. The LOD values of Ni2+ ion was 9.301 nM which is also lower than the WHO guideline value. This probe showed a remarkable ability to recover Pd2+ ions from the spiked solution of drug samples. The probe formed stable 1:1 complexes with both the metal ions with binding constant values 3.72 × 103 L mol−1 (for Pd2+) and 8.26 × 104 L mol−1 (for Ni2+). Binding constant values of Pd-L complex increased in the presence of drug molecules, thereby indicating the absence of interference. The pH tolerance of the probe was found to be in the range of 3–11 (for Ni2+) and 2–11 (for Pd2+). DFT/TDDFT studies performed on LH, [Pd(L)(CH3CN)Cl] and [Ni(LH)(H2O)(CH3CN)Cl2] in CPCM continuum solvation model in CH3CN, showed good correlation between calculated UV–Vis and experimental spectra.