This paper reports the chemosensing properties of a commercially available fluorescent probe, 1,8-diamino naphthalene for copper ion detection in the aqueous medium and its application in bioimaging. The proposed mechanism involves fluorescence quenching of the ligand by the paramagnetic spin–orbit coupling of the coordinated Cu2+ ions. The article reports a thorough investigation on chemosensing properties of the probe. I have some observations which would add to the research that the authors have done.
The Reactions of Interest:
The sole reacting metal ion species in this experiment for a detection limit of 0.05 M was Cu2+ by the mechanism described by the authors in section 3.7 as “The proposed mechanisms of detection and the structures of the probes, both with and without the addition of Cu2+, are shown in Fig. 5. Probe containing a naphthalene ring and two amidogens has strong fluorescence. After coordinated with Cu2+, the paramagnetic effect from spin-orbit coupling of the Cu2+ induces fluorescence quenching”.
Practical observations and Comments:
- The authors have included Cu+ as one of the metal ions which did not show any change in fluorescence intensity of the probe. So they have suggested that it does not bind to the ligand. But, it does show quenching which can be observed in the absorption spectrum. The observation is obvious as the chelating ligand would favor the transformation of Cu(I) to Cu(II) as the latter is more stable especially for a chelating ligand in the aqueous medium and forms a structure similar to the one shown in Fig. 5. The detection of Cu(II) over Cu(I) by this method is overemphasized, and not suitable for any practical purposes. Although the authors have claimed to use deionized water, the stability of Cu(I) even in deionized water is very much doubtful.
- In Fig. 1, the authors have not included the data for Hg2+. This is a big mistake. As I found, Hg2+ does have very minor quenching effect on the probe, but competitively, Cu2+ is clearly dominant.
- The fluorescence titration, UV-vis spectra and pH dependence measurements are very well reproducible.
- The stoichiometry for Cu(II) and ligand binding is 1:1 as mentioned in section 3.6 and this can also be verified by Job’s method.
- The results of the bioimaging studies indicate the effectiveness of the system, although I did not try these myself. Overall, the study is good and replicable except for the Cu(I) system.