Detection of chromium and iron in water sample

Sensing and bioimaging

2.3. Results and discussion

2.3.3. Detection of chromium and iron in water sample

Figure 2.5b shows that the PL intensity is not significantly affected by a substantial increase in the ionic strength of the solution. The indication of the non-interference of Cl⁻ and Na⁺ ions is a favourable and desired situation in the present study because the CDs are meant for the detection of iron and chromium in a real water sample, in which the generation of the actual results is important among the many factors that can interfere with the accuracy of the sensing system. The stability of the CDs is shown in figure 2.5c.

The PL of the as-synthesized CDs retained their intensity of ~ 89% up to a period of 4 months of storage.

can be inferred that these two metal ions (Cr⁶⁺ and Fe³⁺) are capable of effectively quenching the fluorescence of the CDs. This quenching is attained by electron transfer or energy transfer [30, 34]. The sensitivity of the CDs towards these metal ions have also been studied in this work. For chromium, the fluorescence of the CDs with different known concentrations of Cr⁶⁺ ion ranging from 0 μM−100 μM was investigated.

Figure 2.7a shows the PL spectra of the CD solution with different Cr⁶⁺ concentrations.

This figure shows that the PL intensity is decreased with the increase in Cr⁶⁺ ions at 427 nm. This trend solidifies the fact that the sensing system is sensitive to Cr⁶⁺ ion concentration. The PL quenching data trails the Stern−Volmer equation. The mechanism

Figure 2.7. (a) PL spectra of the CD solution in the presence of different Cr⁶⁺

concentrations (from top to bottom: 0, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 50, and 100 μM). (b) Linearly fitted Stern−Volmer plot for the quenching of PL of CD with Cr⁶⁺ (excitation wavelength: 345 nm; F and F0 are the PL intensities of the CD solution at 427 nm in the presence and absence of Cr⁶⁺, respectively; the error bars represent the SD value considering three different measurements).

is either static or dynamic, and the equation can be written is as follows:

𝐹0

𝐹 − 1 = 𝐾_𝑆𝑉(𝑀) (2.3)

where (M) denotes the metal ion concentration (here, Cr⁶⁺); F0 and F denote the PL intensities of CDs at 427 nm in the absence and presence of chromium in the form of Cr⁶⁺ ions; and K SV is the Stern−Volmer quenching coefficient. Figure 2.7b shows the Stern−Volmer plot for the Cr⁶⁺ ions, and it is observed that it shows an adequate linear fit with a correlation coefficient (R²) of 0.982. In addition, this sensing system detected chromium up to a concentration of 0.012 μM. According to WHO, the acceptable chromium ion (Cr⁶⁺) concentration in drinking water should be below 900 nM (0.9 μM) [34], which is easily achievable with the present detection technique. In fact, the present detection limit is quite lower than the acceptance limit specified by WHO. Moreover, the Table 2.1. Comparison of works for Cr⁶⁺ detection.

Methods Linear range (μM) LOD (μM) Ref.

g-C3N4

fluorescence probe 0.6−300 0.15 [41]

CDs fluorescence

sensor 2−180 2.10 [42]

GQD-modified

membranes 1−500 0.19 [43]

N,S-CDs

fluorescence sensor 0.5−125 0.02 [44]

N-CDs

fluorescence sensor 0.01−250 0.005 [45]

P,N-CDs

fluorescence sensor 1.5−30 0.023 [46]

wsCQDs - 0.073 [34]

GQDs fluorescence

sensor 0.05−500 0.0037 [47]

CDs sensor from

potato 0.5−100 0.012 Current work

comparison of LOD for Cr⁶⁺ reported in the literature, as shown in table 2.1, highlights the superior detection efficiency of the present detection system, besides the work done by Zhang et al. [45] and Huang et al. [47] Most importantly, the CDs have been used without further purification and stabilization in the proposed technique of the present study.

Different studies have been carried out regarding the quenching mechanism [48]. The quenching of PL due to the presence of Cr⁶⁺ can be explained as a non-radiative recombination of the electron-hole pair by energy or electron transfer process. This is due

Figure 2.8. (a) PL spectra of the CD solution in the presence of different Fe³⁺

concentrations (from top to bottom: 0, 0.5, 1, 2, 3, 4, 5, 10, 20, 50, and 100 μM). (b) Stern−Volmer plot for the quenching of PL of CD with Fe³⁺. The top inset shows the fitted curve for data from 10 μM to 100 μM and the inset below shows the fitted curve for data from 0 to 5 μM (excitation wavelength: 345 nm; F and F0 are the PL intensities of the CD solution at 427 nm in the presence and absence of Fe³⁺, respectively; the error bars represent the SD value considering three different measurements).

to the presence of vacant d orbital and low-lying d-d transition states [49], which ultimately aids the fluorescence quenching of the CDs. A similar procedure was followed to verify the sensitivity of the Fe³⁺ ion. Figure 2.8a shows the PL intensity change of the CD solution at different Fe³⁺ ion concentrations (from 0 μM-100 μM). Figure 2.8b shows

the Stern−Volmer plot for Fe³⁺ to show the quenching of this metal ion, referring to equation 2.3. In this case, (M) represents the concentration of Fe³⁺, and F0 and F denote the PL intensities of CDs at 427 nm in the absence and presence of Fe³⁺ ions. This figure shows that the Stern−Volmer plot does not fit linearly over the whole concentration range. Two linear fits are there for two different concentration ranges and these two fits are shown in the insets of figure 2.8b (lower inset: 0 μM−5 μM of Fe³⁺; upper inset: 10 μM−100 μM of Fe³⁺). The correlation coefficients (R²) are 0.95 and 0.96 for concentration ranges 0 μM−5 µM and 10 μM−100 μM, respectively. This indicates that in this sensing system, both dynamic and static quenching may occur. Fe³⁺ has an outer electronic configuration of 4s²3d⁵, which means that it has five half-filled d orbitals in the outer shell. Therefore, when they are added to the CD solution, the excited electrons from the conduction band of CD are transferred to the half-filled 3d orbital of Fe³⁺ and hence a PL quenching takes place [25]. However, when KH2PO4 is added to the CD- Fe³⁺ solution, then the PL property of the CD particles returns. This is due to the strong

Figure 2.9. Normalized PL spectra of solutions containing only CDs (curve a), CDs- Fe³⁺mixture (curve b), CDs-Cr⁶⁺mixture (curve c), CDs-Fe³⁺-KH2PO4 mixture (curve d), and CDs-Cr⁶⁺-KH2PO4 mixture (curve e).

affinity of Fe³⁺ towards the phosphate group, which helps the Fe³⁺ to form a complex with the phosphate group by Fe─O─P bonds [50]. The regaining of the PL property of the CD is shown in figure 2.9. The lower limit for Fe³⁺ detection by our sensing system is 0.000549 μM, which is significantly below the limit provided by WHO for Fe³⁺ which is 5.36 μM [11]. A comparative study of LOD of Fe³⁺ with the available methods is shown in table 2.2. The table signals the significant contribution of the present work since it possesses the lowest detection limit among the existing methods. Moreover, we calculated the recovery percentage of the two metal ions by using our sensing system. We used a known concentration (5 μM) of the metal ions in the experiments and then measured the concentrations using our sensing system. The calculated recovery percentages for Cr⁶⁺ and Fe³⁺ were therefore determined as 104.53% and 89.52%, respectively.

Table 2.2. Comparison of works for Fe³⁺ detection.

Methods Linear range

(μM) LOD (μM) Ref.

Chemiluminescent

CDs 5−80 0.7 [7]

CDs from sweet

potato 1−100 0.32 [11]

CQDs from

graphite electrode 10−200 1.8 [17]

CDs fluorescence

sensor 1−40 0.87 [42]

C-dots from

banana peel 2−16 0.211 [29]

N-doped C-Dots

sensor 0.01−500 0.0025 [51]

CDs sensor from

potato 0.5−5 0.000 549 Current work