UV-Vis spectroscopic studies of L1 in presence of metal ions

Scheme 1 L1 is used as ligand in this Chapter

2.3. Results and discussion

2.3.1. UV-Vis spectroscopic studies of L1 in presence of metal ions

The UV–Vis spectrum of the sensor L1 recorded in MeOH:HEPES buffer solution showed an absorption band having a maximum at 359 nm. The intensity of this absorption maximum remains unchanged or slightly decreased with the addition of up to two equivalents of metal chlorides mentioned above. However, upon the gradual addition of PdCl2 solution to L1 solution, 359 nm peak gradually decreased in intensity (Fig. 1). Three new overlapping absorption bands having their maxima at 395, 420 and 440 nm grew in intensity along with an isosbestic point at 388 nm (Fig. 2). The presence of isosbestic point indicates that only two species viz., free L1 and L1-bound Pd(II), are involved during the course of the titration. A significant change in colour from light blue to yellow was observed, which is the consequence of red shift of the absorption band. This color change can easily be observed by the naked eye and hence L1 enables visual detection of Pd²⁺ ion. Since with other metal ions noted above such a red shift was not observed, it is imperative that L1 is very selective towards Pd(II) salts.

Fig. 1 The emission spectra of L1 with solutions of various metal ions along with Pd²⁺ ion.

Inset compares colors of L1 and L1 + Pd²⁺ ion.

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Fig. 2 The change in absorption spectra of solution of L1 upon gradual addition of PdCl2. 2.3.2. Fluorescence spectroscopic studies of L1 in presence of metal ions

Upon irradiation of solution of L1 with λex = 359 nm using an emission slit of 3 nm, emission at λem = 454 nm was observed. In order to evaluate the possible selective fluorescence behaviour of L1 towards palladium(II) in presence of various other metal chloride salts, a study of fluorescence emission behaviour was performed. Upon titrating L1 with chloride salts of Na⁺, K⁺, Li⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Fe³⁺, Cr³⁺, Al³⁺ and Pd²⁺ ions, the pattern of emission at λem = 454 nm was largely invariant, but Fe(III) exhibited a small reduction (~10%) in the fluorescence intensity (Fig. 3). The exception is Pd(II) salts, whichcaused an outstanding decrease in the fluorescence intensity (Fig. 4) of L1 (~90%) and significant colour change from deep sky blue to yellow. This colour change is readily observable by the naked eye (Fig. 5). It is evident that L1 has high selectivity towards Pd²⁺ cations with respect to other metal salts. The decrease in fluorescence intensity of L1 in presence of Pd²⁺ ion is due to chelation enhanced quenching (CHEQ) mechanism. Again, to check the practical utility of L1, competition experiments were performed by first treating L1 (Fig. 6) with equimolar amount of competing metal salts and then adding one equimolar quantity of Pd²⁺ cations. From this experiment, it was seen that the fluorescence of the compound L1 was greatly quenched by Pd(II) in presence of other metal salts. These results clearly indicate that L1 is highly selective towards the Pd²⁺

ion, even in presence of other metal ions.

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Fig. 3 The emission spectra of L1 with solutions of various metal ions along with Pd²⁺ ion.

Change in fluorescence of the solution in presence of Pd²⁺ ion while illuminated in UV- chamber is shown in the inset.

Fig. 4 The change in fluorescence intensity of solution of L1 upon adding of Pd²⁺ solution.

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Fig. 5 Photograph of L1 solutions in presence of various metal ions observed under visible light (top), short UV light (middle) and long UV light (bottom).

Fig. 6 Bar diagram of fluorescence intensity of solution of L1 in the presence of other metal chloride and after addition of equimolar solution of PdCl2 into them.

In order to ascertain the pH window for selective detection of Pd²⁺ ion by L1, fluorescence of free L1 and in presence of Pd²⁺ cation was recorded using MeOH-HEPES buffer solution (pH was adjusted using HCl or NaOH solutions). In the case of free L1, the fluorescence intensity increased on increasing the pH from 2 to 7 and then did not deviate much in the range 8–14.

Upon carrying out the same experiment in presence of one equivalent of PdCl2, initially, the solution was non-fluorescent in the range of pH = 2–8. Then fluorescence intensity gradually enhanced with further increase in pH (Fig. 7). This indicates that in alkaline conditions free

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L1 is released into the solution. Hence it could be concluded that the fluorescence intensity of L1 was effectively quenched by PdCl2 in the pH range of 2–8, which is the favourable range.

Fig. 7 Plot of fluorescence intensities at λem = 454 nm vs pH.

Further, the reversibility of L1 binding to Pd²⁺ ion has also been established using Na2EDTA method. The fluorescence emission at λem = 454 nm of solution of L1 bound with Pd²⁺ ion, was restored upon addition of one equivalent Na2EDTA solution (Fig. 8). This indicates that Na2EDTA effectively sequester Pd(II) ion thereby releasing free L1 into the solution thus re-establishing the fluorescence intensity (Scheme 2). After that, the fluorescence intensity at 454 nm was quenched again after the addition of another equivalent of PdCl2. From this experiment, it can be inferred that L1 binding can be reversed and be utilised multiple times.

Fig. 8 Fluorescence response for reversibility of L1 binding to PdCl2.

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Scheme 2 Possible mode of sensing Pd²⁺ ion.