The sensor PMB3 was synthesized through a two-step condensation reaction, as outlined in scheme 1, commencing with 2-hydroxy-1-naphthaldehyde. PMB3 demonstrated pronounced solubility in organic solvents like dimethyl sulfoxide (DMSO) and dimethylformamide (DMF). Its comprehensive characterization encompassed elemental analysis, IR spectrum (Fig.S.1),1H NMR spectrum (Fig.S.2), 13C NMR spectrum (Fig.S.3), and HR-MS(ESI) analysis (Fig.S.4).
3.1 Colorimetric analysis
The sensing efficacy of PMB3 was systematically evaluated using a visual assessment in the presence and absence of diverse metal ions. Upon the addition of Cu2+ and Ni2+ ions, an immediate colour change of the PMB3 solution in DMF was observed, transitioning from colourless to yellow (Fig. 1). On another hand the addition of other metal ions (Al3+, Hg2+, Zn2+, Co2+, Fe3+, Cd2+, Ag+, Mn2+, Mg2+, Ba2+, Ca2+, Cr3+, Pb2+, and Na+) yielded no discernible effect on the colour. This remarkable finding underscores the ability of PMB3 for visual detection, specifically demonstrating its selectivity for divalent copper and nickel ions.
3.2. UV-Visible Absorption studies
Figure 2 depicts the UV-visible absorption spectrum of PMB3 in DMF in the presence of diverse metal ions. The DMF solution of PMB3 exhibited two broad bands at 332nm and 387nm, corresponding to the π-π* and n-π* transitions of the 2-hydroxy-1-naphthaldehyde moiety respectively. The addition of Cu2+ and Ni2+ induced substantial alterations in the absorption spectral profile. Specifically, the introduction of bivalent copper led to a reduction in the peak intensity at 387nm and the emergence of a new peak at 454nm. Similarly, the addition of bivalent nickel resulted in a diminished intensity of the peak at 387nm, accompanied by the appearance of a new peak at 472nm. Notably, the position or intensity of the absorption peaks remained relatively unaffected with the introduction of all other metal ions under investigation.
3.3 UV -Visible absorption titration on Cu2+ and Ni2+ ions:
The sensitivity of the PMB3 sensor to Cu2+ and Ni2+ is delineated in Fig. 3a and b. Regarding Cu2+ ions, with increasing concentration, the absorption band at 387nm gradually attenuated, concomitant with the emergence of a new absorption band at 454nm, and a distinct isosbestic point was observed at 416nm, indicating the formation of the PMB3-Cu2+ complex in the solution state. Furthermore, the initially colourless solution underwent a transition to a pale-yellow hue, intensifying with higher concentrations of Cu2+, and the absorption peak exhibited a redshift.
In assessing the specificity of PMB3 towards Ni2+, the absorption peak of PMB3 at 387nm systematically shifted downwards with the gradual addition of Ni2+ ions and subsequently, an additional intense peak is registered at 472nm. This spectral alteration, coupled with a distinct isosbestic point at 421nm, confirms the formation of the PMB3-Ni2+ complex. The observed changes in the absorption spectrum facilitated a colorimetric transition from colourless to pale-yellow. The molar absorptivity value for each concentration of Cu2+ and Ni2+ was also calculated and tabulated in Table S1 and Table S2.
3.4 Stoichiometry of metal complexes
The Job's plot, given in Fig. 4, illustrates the variation in the maximum absorption at 454nm and 472nm concerning the mole fraction. This analysis reveals that in PMB3-Cu2+ and PMB3-Ni2+ complexes, PMB3 engages with the respective metal ions in each complex at a consistent 1:1 binding ratio. Substantiating these findings, the ESI-MS spectrum analysis in Fig. 5a and b reveals molecular peaks at m/z = 561 and m/z = 680, signifying PMB3-Cu2+ and PMB3-Ni2+ complexes respectively.
3.5 Limit of Detection (LOD)
The determination of detection limits for these metal ions was conducted by employing standard deviation methods, as provided in Fig. 6. The limit of detection was calculated using the equation, Limit of detection (LOD) = 3× SD/k, where ‘SD’ is the standard deviation obtained for blank measurements and ‘k’ is the slope obtained from calibration plot. The detection limits were calculated to be 4.56µM for PMB3-Cu2+ and 2.68µM for PMB3-Ni2+, emphasizing the efficacy of PMB3 as a sensor for the selective detection of divalent copper and nickel in drinking water. A comparison was carried out between the obtained LOD value PMB3 with that of some of the colourimetric probes that have been reported (Table 1). Even at lower concentration levels, PMB3 was found to have comparatively better Cu2+ and Ni2+ ion sensitivity than the other reported probes, which is a desired quality of an effective colourimetric probe. The assessment binding interactions between the metal ions and PMB3 utilized the Benesi-Hildebrand equation, disclosing association constants of 3.47 × 104M−1 and 8.02 × 104M−1 for the PMB3-Cu2+ and PMB3-Ni2+ complex respectively.
Moreover, to validate the enhanced selectivity of PMB3 toward Cu2+ and Ni2+, the sensing performance in the presence of other competitive metal ions was assessed. Various metal ions, including Al3+, Zn2+, Co2+, Fe3+, Cr3+, Cd2+, Hg2+, Ag+, Mn2+, Mg2+, Pb2+, Ba2+, Ca2+, and Na+, which could potentially interact with PMB3, were examined under identical conditions to evaluate selectivity of PMB3 as a colorimetric sensor for Cu2+ and Ni2+. In Fig. 7a, the proportional changes in PMB3 absorbance induced by the addition of various metal ions are depicted, and the figure clearly illustrates that the sensing characteristics of PMB3 for Cu2+ and Ni2+ remained largely unaffected by the addition of different metal ions.
Additionally, the interaction between PMB3 and Cu2+ or Ni2+ in the presence of diverse anions was examined, and the resultant shifts in absorption are illustrated in Fig. 7b. PMB3 manifested a pink colour upon the introduction of anions such as fluoride, cyanide, and acetate, followed by a transition to yellow upon the subsequent addition of Cu2+ and Ni2+. This observation distinctly underscores the selectivity of PMB3 in the presence of competing anions.
3.6 Selectivity and reversibility of the complexation
The reversible sensing behaviour stands as a pivotal characteristic of the sensor, and in enhancing the practical utility of novel sensors, the ability for reversibility in the detection process is of utmost significance. To probe the regeneration and reversibility of complexation of PMB3 with Cu2+ and Ni2+, we investigated the interactions with a potent chelator, the disodium salt of EDTA. The absorbance bands at 454nm and 472nm corresponding to the PMB3-Cu2+ and PMB3-Ni2+ complex, respectively, disappeared upon the addition of Na2EDTA to the mixtures, confirming restoration and regeneration of free PMB3. Furthermore, reintroducing Cu2+ and Ni2+ ions into the solution mixture reinstated the absorption bands (Fig. 8). Consequently, the PMB3 sensor proves to be effective and reusable for real-time applications owing to its regeneration capability.
3.6 Effect of pH on sensing behaviour of PMB3
The effect of pH on the sensing behaviour of PMB3 towards Cu2+ and Ni2+ was investigated using buffer solutions. The absorption spectra of PMB3, PMB3 with the addition of Cu2+, and PMB3 with the addition of Ni2+ were recorded at different pH solutions and depicted in Fig. 9. The absorbance of PMB3 at 387nm has almost similar absorbance value from pH 3 to 10 and after it goes decreasing. With the addition of Cu2+ ion the absorbance value at 454nm and with the addition of Ni2+ the absorbance value at 472nm shows similar trends as PMB3 and hence it indicated the applicability of PMB3 for sensing these ions at a biological pH range.
3.7 Discrimination between PMB3-Cu2+ and PMB3-Ni2+
It is essential to figure out how to differentiate between PMB3-Cu2+ and PMB3-Ni2+ because they both displayed the same yellow colour with a slight difference in wavelength of absorption. It is commonly known that amino acids and peptides containing thiols bind strongly to Cu2+ion[28–31]. Thus, to address the issue, we employed glutathione. Upon addition of glutathione to PMB3-Cu2+ and PMB3-Ni2+ complex, only PMB3-Cu2+ complex demonstrated spectral and discernible colour changes from yellow to colourless as displayed in Fig. 10 and PMB3-Ni2+ complex did not show any spectral and noticeable colour change from yellow to colourless. This finding showed that the chelation of Cu2+ with glutathione caused PMB3-Cu2+ to return to free PMB3.
3.8 Analysis of Real Sample
To demonstrate the practical applicability of PMB3 in quantifying Cu2+ and Ni2+, these metal ions were scrutinized in real samples sourced from the natural environment. The assessment of metal ion recovery involved the examination of real samples spiked with varying concentrations of the metal ions, facilitating an evaluation of the accuracy of the procedure. The presented results in Tables 2 and 3 indicated successful recovery for both analytes, affirming the practical viability of PMB3 for the precise detection of Cu2+ and Ni2+ in real environmental samples.