For the evaluation of the photophysical properties of sensor C and its chromium complex UV-Visible analysis was initially used. For considerable solubility and prospective applicability of sensor C, acetonitrile was selected as a solvent for sensor C in the current study. Ultraviolet-Visible spectrum of sensor C was taken in the presence and absence of different concentration of chromium ions. The Uv/vis spectrum for sensor C showed characteristic main absorption band (λmax) at360 nm .Upon addition of chromium solution bathochromic shift occurred and maximum absorption band appeared at 420nm,confirmingtheformation of chromium complex with sensor C (Fig. 1).Each time baseline was established by putting respective blanks in the reference slot.
Cation sensing studies using fluorescence spectroscopy
Fluorescence-on studies by Cr3+ ion
In our ongoing detection for a better understanding and a feasible intonation of the sensing ability of curcumin based fluorochrome, we have tagged curcumin based sensor with metal ion. In order to realize the photo physical properties of sensor C prior to metal ions recognition, the fluorescence response of free sensor C was taken. According to results obtained, sensor C displayed a very weak emission peak at 560 nm (λem). To investigate metal ion sensing by sensor C, different heavy metal ions Ni2+, Ce3+, Cu2+, Zn2+, Cd2+, Mn2+, Cr3+, Pb2+, Co2+(200 µM) were added to sensor C solution separately. The respective mixtures were allowed for 3 minutes to equilibrated, and were transferred into quartz cuvette separately by monitoring fluorescence intensity in the range of 400–700 nm. The spectra obtained for all the studied metal was compared with the spectrum of sensor C. Accidentally, in our case, a prominent enhancement in fluorescence emission intensity was observedatλem560 nm upon the addition of aqueous chromium ion (200 µM) to sensor C solution. While no apparent enhancement in intensity was spotted for the rest of metal ions (Fig. 2). So among the studied metal ions only chromium form complex with sensor C. This specific fluorescence-on response could be accredited to restriction of ICT phenomena. In the absence of chromium ions, due to ICT phenomena, a weak fluorescence was observed for sensor C. while in the presence of chromium ions ICT process is negligible and fluorescence intensity was extensively enhanced as the result of formation fa stable complex [24].
As discussed, we postulated that the apparent fluorescence enhancement of sensor C could be ascribed to a distinctive chemical resemblance for chromium ion. It is presented in already reported articles, fluorescent molecules like sensor C retain certain preference for chromium ion, thus allow a superficial prejudice from its close neighbor Fe3+ along with other similar co-existing ions through fluorescence enhancement [25].
pH study
In the interests of avoiding interference in chromium ion recognition by protons, we focused on pH analysis on the fluorescence retaliation. The pH value of the test system was set to acquired value by HCl and NaOH solution. The sensor C (8 µM) did not change its fluorescent intensity from pH = 1to 14, indicating its consistency. The weak fluorescent response of sensor C may be owing to intra-molecular photo-induced electron transfer. However, sensor C complex with (16µM) chromium showed strong fluorescence at pH 8, on account of its binding with Cr3+ leading to intra-molecular photo-induced electron transfer process inhibition. The chromium complex was unstable at lower pH thus displaying quenching in intensity as a result of protonation carbonyl oxygen that acts as binding site. By gradually increasing the pH, the fluorescence intensity strappingly enhanced (Fig. 3). At pH 8, high stability was displayed by the chromium complex and maximum enhancement in intensity was observed at λem 560 nm. While at pH > 8, a gradual decrease in intensity occurred due to formation of hydroxyl complexes that had poor solubility in water solution. From the results obtained, it can be assume that the synthesized sensor C can successfully be used for chromium ion determination at pH 8 in aqueous samples [26].
Sensor C concentration effect
The determination of concentration effect of sensor Con fluorescence response of the sensor C-Cr3+ complex is one of the important parameters in fluorimetric analyses. Working solution for analysis were prepared by keeping the chromium concentration constant (16µM), and varying the sensor C concentration in the range of 5–40 µM. The fluorescence emission intensity of each sample was monitored in 440–700 nm range (Fig. 4). With increasing concentration of sensor C, a linear enhancement in the emission intensity was observed. At higher concentration, the fluorescence intensity remains constant.
Quantitative study for Cr3+
For the confirmation of sensitivity of sensor C toward chromium ion, fluorescence titration experiments were carried out. During these titrations, the sensor C concentration was kept constant while changing the chromium ions concentration from 1–15 µM. A linear enhancement in the fluorescence intensity of chromium complex was noted at 560 nm (λem) upon increasing chromium concentration (Fig. 5). Thus showing that sensor C can be used for quantitative determination of Cr3+.
Binding mode study
The binding stoichiometry between sensor C and chromium was investigated using a Job’s plot, A Job’s plot between the different mole fraction of sensor C-Cr3+ complex versus respective emission intensities showed a maxima at 0.7 mole fraction indicating a 2:1 stoichiometric ration between sensor C and Cr3+ (Fig. 6).
This 2:1 binding ratio was further confirmed with the help of Benesi-Helderbrand plot. For this purpose, for determination of association constant the given equation was used
Where K (M− 2) represent association constant. Fo, Fmax and F denote the emission intensity of free sensor C, at [Cr3+] in excess concentration and at different [Cr3+] (λex = 425 nm and λem = 560 nm) respectively. The value of K was determined by plotting Fmax-Fº/F-Fº versus 1/[Cr3+]2, (Fig. 7). According to the Equation, data fitted linearly, showing good liner relationship with slope 6.515× 10− 11, and intercept 1.539. The association constant calculated from the slope is found to be 1.53 x 1010thus confirming the 2:1 binding ratio between sensor C and Cr3+.
The detecting mechanism of fluorescent sensor C forCr3+
The detecting mechanism of sensor C for Cr3+ionwas suggested using the fluorescence spectra. The fluorescent response of the sensor C towards Cr3+ionmay be allocated to intra molecular charge transfer (ICT). Before being coordinated with Cr3+ ions, sensor C displays a weak fluorescence emission spectrum due to lone electron pair of oxygen atom, which result in intra molecular charge transfer. Moreover, the lone electron pair of the oxygen atom give rise to a non-radiative process by the n-π* transition, as a result quenching in fluorescence intensity takes place. On the other hand, after sensor C was coordinated to Cr3+ion, radiation process was primarily viaπ-π*transitions and the coordination complex was more rigid thus the ICT process was restricted upon addition of Cr3+ionat the receptor site.
On the other hand, sensor C act as monodentate ligand as there is one potential coordination cite, the carbonyl oxygen. The coordination sphere of d-block elements like Cr is either 6 or 2 and complexes of chromium with both these coordination spheres have been reported [27–30]. Since the bulk of our sensor C is high so the bonding with chromium is expected to be strong and possible structures of sensor C-Cr3+ complex have been proposed in the (Scheme 2).
Table 1
Photophysical properties of sensor C
Parameter
|
C:Cr3+
|
λem(nm)
|
λex(nm)
|
LOD (Cr3+)
|
Quantum yield (%)
|
Sensor C
|
2: 1
|
560
|
425
|
2.3×10− 9 M
|
72
|
Time study
Rapid response time is highly preferred in flurorimetric studies, therefore the time effect on emission response of sensor C and its complex with chromium was studied in time range of 0–7 min for different concentrations of chromium. During the assay, the fluorescence emission response of sensor C remained unaffected with increasing time, whereas the complex showed a linear response in fluorescence intensity with time and maximum fluorescence response was observed at 2 min and then remained constant (Fig. 8). These results indicate effectivity of sensor C for real time assay of chromium detection in environmental samples.
Effect of competitive metal cations
Competitive binding experiments were taken for evaluation of efficacy of the synthesized sensor C as a selective chromium ion sensor as sensor C should have the potential to detect chromium ions in the presence of co-existing metal cations. For this purpose, different commonly existing metal ions in water were selected like Cu2+,Hg2+,Zn2+,Cd2+,Pb2+,Mn2+,Co2+,Ni2+including the closely related ions likeCe3+. The concentration of each interfering metal ion was taken 8 folds (128µM) as compared to chromium metal ion (16 µM) while keeping the sensor C concentration constant (8 µM). The fluorescence emission profiling was completed between 440–700 nm with 425 nm λex.
No interference in fluorescence results in terms of intensity occurred even in the presence of high concentrations of interfering ions (Fig. 9). Thus, the obtained results may indisputably provide the reported sensor C is particularly valuable in efficiently selective chromium ion determination from its chemically similar cations like cerium and other commonly found interfering ions. Therefore, it can be realistically incidental that the synthesized sensor C exhibits an ultra-sensitive and highly selective fluorescence-on response toward chromium ion in aqueous samples.
Reversibility of sensor C
To determine whether the metal complex formation process is reversible or not, fluorescence emission titration experiment was performed using the sensor-Cr3+ complex with EDTA.
The reversibility of the designed fluorescence–based sensor is curricula for its economy and many times usage. In this regard, EDTA is added into sensor C combined with Cr3+ solution to confirm the effect of Cr3+ ions on the fluorescence response. When EDTA is added to the aqueous solution, the fluorescence spectrum is returned to its original state. The interaction between sensor C and Cr3+ ions is prevented by the formation of Cr3+–EDTA chelate. After the addition of EDTA, the fluorescence of sensor C is recovered. The fluorescence-on sensor C is proved to be reversible using turn off–on mechanism based on the fluorescence emission intensity measurement (Fig. 10). The experiments were repeated 4 times by getting the same results. Furthermore, since the fluorescence emission intensity was restored upon addition of EDTA, the sensing process was considered to be reversible rather than an ion-catalyzed reaction.
Quantitative detection of Cr ion in aqueous samples
To study in deep the suitability and on-field applicability of prepared sensor C for ultra-selective detection of chromium ion, the different water samples were spiked by adding chromium to have sample solutions in (1–10) µM conc. range. Accordingly, the results attained displayed a significant recovery of chromium ions from different environmental water samples (Fig. 11). Thus, we presented a cost efficient method for highly selective determination of Cr3+ ion in aqueous samples.
Significance of Current Work
As compared to other chromium ion detection methods (Table 2), the present methodology has the compensation in terms of linearity and ultra-low sensitivity. In addition, it display higher potential for chromium detection.
Table 2
Comparison of some sensors for chromium ions determination
Sensors
|
Limit of detection
|
Fluores-cence response
|
Analyte
|
Optimal pH
|
Testing media
|
Ref
|
5,5’-bipyridy-
ldicar-
boxaldehyde
|
NA
|
turn-on
|
Cr3+
|
6
|
Aqueous
|
[31]
|
carbon quantum dots
|
0.64 × 10− 6 M
|
turn-off
|
Cr3+
|
8
|
Aqueous
|
[32]
|
5−((2-cyano-
[1,1-
biphenyl]-4-yl)-
methoxy)isophthalic acid
|
1.4× 10− 6 M
|
turn off
|
Cr3+
|
8
|
Aqueous
|
[33]
|
rhodamine B
lactams
|
1× 10− 7 M
|
turn on
|
Cr3+
|
7
|
NA
|
[34]
|
asparagine
derivatives
|
1.6 × 10− 8 M
|
turn on
|
Cr3+
|
7
|
NA
|
[35]
|
curcumin
derivative
|
2.3×10− 9 M
|
turn-on
|
Cr3+
|
8
|
Aqueous
|
Current work
|