This paper reports the binding effect of some novel benzimidazole derivatives with Cu2+ metal ion by solution spectral studies. Benzimidazole complexes (1 – 6) were studied for fluorescence quenching by Cu2+ in comparison to Fe3+, Mg2+, Ag+, K+ and Na+ metal ions. The crystal structure of 5 was determined by X-ray crystallography and DFT studies on it were done. Absorption spectra for all the complexes formed by reaction of ligands 1 – 6 with Cu2+ metal ion were also studied comparatively. Although the paper is aimed at presenting the selectivity of sensing Cu2+ metal ions with fluorescence quenching, there are some big loopholes in the methodology which need to be looked at.
The Reaction of Interest:
- The authors do not mention the names of ligands anywhere in the main article. Another glaring mistake that I noticed was no mention of solvent for the absorbance and fluorescent studies. Moreover, they have not mentioned the instrument and conditions for FT-IR analysis. Generally, such associations are reversible in nature and for FT-IR spectral analysis during the reaction; it is a must to mention the solvent. There is no mention of any such data to be available in supplementary files as well.
- The procedure for synthesis of ligands was found to be lengthy and low yielding (58%). I found it much better to use Fe(III) or Cu(II) salts as catalysts Like FeCl3 or Cu(NO3)2 in methanolic medium for quick reaction time (30 mins) and very good yields (>90%).
- I used methanol as the solvent for studying the effect of binding on absorbance and fluorescence spectra of 5. I noticed that the authors have omitted the results of absorbance and instead preferred fluorescence for studying selectivity. Also, the number of metal ions selected for such a study is too low.
- I used Al3+, Co2+, Ni2+, Cd2+ and Hg2+ salts for studying the effect on absorption spectra. Al3+, Ni2+, Hg2+ gave red, green and white solutions respectively. When used for competitive fluorescence analysis, Al3+ was found to interfere.
- In general, the paper lacks the minute details necessary for replicating the experiments. The preference for fluorescence detection is not explained when the colour changes recognized with naked eye indicate towards bonding by several competing metal ions.
- The procedure for ligand synthesis should be modified as explained by me.
- The detection of ligand-metal binding is to be ascertained by a Job’s plot for definite understanding.
- Fluorescence quenching was monitored around 350 to 400 nm. The absorption spectra were also monitored up to 350 nm. Clearly, a more comprehensive study is needed to ascertain the bonding of copper to metal as this study excludes the d-d transition.
- Overall, the methodology described in this paper is not up to the mark and needs sufficient modification for it to be applicable for similar investigations.