A new organic molecule probe has been introduced as a “turn-off” fluorescent sensor to detect trace quantities of UO22+ in the presence of several transition metals with promising results. The procedure is based on quenching the fluorescence intensity of 6-chloro-2H-1,2,4-benzothiadiazine-7-sulfonamide 1,1-dioxide (L) in the presence of various UO22+ concentrations in methanol. The UO22+ and L species interact through electrostatic interaction between negatively charged nitrogen atom of the sulfonamide group of L and positively charged UO22+, thus facilitating the non-radiative recombination of UO22+ and L through the charge transfer or electron transfer processes and leading to the fluorescence quenching of L. The mechanism of quenching was addressed and proved to be static quenching. The impressive quenching of the fluorescence intensity of L by different concentrations of UO22+ has been successfully used as a new sensor to measure UO22+ in methanol at λex = 340 nm, λem = 380 nm with a linear dynamic range of 0.08–5.0 µM and detection limit and quantification limit of 0.0276 and 0.0837 µM, respectively. The L sensor shows interesting advantages compared to other developed sensors with adequate performance, such as broader linear range and lower detection limit, selectivity, and simplicity, which illustrate its useful practical use.
Fluorescence quenching detection of UO22+ in aqueous solution based on an organic molecule probe of 6-chloro-2H-1,2,4-benzothiadiazine-7-sulfonamide 1,1-dioxide.
The fast development of nuclear industry, such as the extraction of uranium minerals, the generation of nuclear electricity, the treatment of spent fuel, and the manufacturing of nuclear weapons, has produced a residue of uranium pollution in the marine environment, posing a clear threat to the ecological system and to human and animal health . In aqueous solution, uranium has many forms, and the most prevalent one is uranyl (UO22+) [2,3,4,5]. The development of a fast and efficient analytical procedure for UO22+ recognition enables it to be measured at a very low concentration to avoid its adverse effects on the atmosphere and on the living organism at an early phase. Analytical techniques such as atomic absorption spectrometry [6,7], atomic emission spectrometry [8,9], or mass spectrometry [10,11] are routinely used in conjunction with a sample pretreatment scheme for high sensitivity UO22+ analysis. However, high cost, advanced and complicated devices, and the needs of skilled staff and well-equipped laboratories limit their use [12,13,14]. This causes problems for an on-site and real-time detection of heavy metal ions. Developing sensors with sensitivities comparable to those advanced instrumental techniques is a major challenge for a long time to come. This is because several metal ions have the same or almost ion radius, charging or other properties, making it hard to assess.
Fluorescence optical sensor is a very promising tool for potential practical applications due to its precision and its inherent sensitivity [15,16]. But fewer reports of UO22+ fluorescent sensors have been published. For instance, Chen et al. introduced an aggregation-induced emission-active sensor, 4-pethoxycarboxyl salicylaldehyde azine (PCSA), which displays high sensitivity towards UO22+ . Also, 2,6-pyridinedicarboxylic acid (PDA) has been used as a fluorescence sensitizing agent for UO22+ measurement by Maji and Viswanathan . However, 2,6-pyridinedicarboxylic acid (PDA) interacts with various metal ions which hinder the detection of UO22+ in the presence of various competing ions. Wu et al. immobilized salophen and fluorescence-labelled oligonucleotide for separation and determination of trace UO22+ concentration . Ganesh et al. used fluorescence enhancing reagent (sodium pyrophosphate) for the determination of UO22+ concentration during spent fuel reprocessing . However, by using these sensors, the presence of some transition metals and thorium ions interfered with the UO22+ detection. In addition, additional chemicals (for instance, oligonucleotide, sodium pyrophosphate, and calcein) or tools are needed to achieve the required sensitivity and selectivity. Therefore, in the presence of transitional metals and less chemical uses, the development of highly selective and sensitive fluorescent sensors for UO22+ recognition remains a challenge.
In the literature, some thiol-based ligands have been found, which have been used for the detection of transition metal ions [21,22,23]. The present study used thiazide-based ligand as a new organic molecule probe [6-chloro-2H-1,2,4-benzothiadiazine-7-sulfonamide-1,1-dioxide (L)] for the assessment of UO22+. The literature review and preliminary study indicated the following: first, L sensor belongs to the thiazide class and has strong fluorescence spectra due to high mobility of its π-electron and high quantum yield. Second, L sensor has high molar absorptivity, which is preferred because the absorption light is increasing at a given wavelength. Third, the preliminary studies of the absorbance and fluorescence emission spectra of L have shown variations that exist when UO22+ is gradually added to L and the fluorescence emission intensity of L is decreased. Therefore, the concentration of UO22+ could thus be detected quantitatively. In this regard, L was studied for the application of a new organic molecule probe for spectrofluorometric assessment of UO22+ in aqueous solution.
The chemicals used in the tests were of analytical grade with no further purification. 6-Chloro-2H-1,2,4-benzothiadiazine-7-sulfonamide 1,1-dioxide (L) was purchased from Sigma-Aldrich. Uranyl nitrate hexahydrate, UO2(NO3)2·6H2O, was manufactured by Mallinckrodt Company. All the other chemicals used in this study were purchased from Alpha Company.
Fluorescence emission spectra were measured using Meslo-PN (Lumina fluorescence Spectrometer; Thermo Scientific, USA). Both emission and excitation slits were at 5.0 nm. The absorption spectra were measured using UV-Vis Evolution 300 (Thermo Fisher Scientific Company, UK). As a reference, UO22+ was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (iCAP 6500 ICP-OES; Thermo Fisher Scientific, UK). Experiments were carried out in safeguards destructive analysis lab (ETZ-, KMP-I), Nuclear and Radiological Regulatory Authority.
2.3 Analytical procedures
In order to assess the UO22+ concentration by L sensor, a stock solution of UO22+ (100 µM) was prepared by dissolving an accurately weighed amount of uranyl nitrate hexhydrate (UO2(NO3)2·6H2O) in deionized water containing few drops of concentrated sulphuric acid and standardized by a known method . All working solutions were prepared by diluting the stock solution suitably with deionized water to give various concentrations of UO22+ from 0.08 to 5.0 µM. These various concentrations of UO22+ were added to L (1.0 mL, 100 µM). The solutions have been mixed with 10 mL of methanol. Measurements of the fluorescence emission intensities were carried out at λex = 340 nm, λem = 380 nm, and 1 min (to reach the equilibrium). The linear range of the fluorescence emission intensity of L sensor was observed between 0.08 and 5.0 µM of UO22+ concentrations.
3 Results and discussion
3.1 Preliminary studies
6-Chloro-2H-1,2,4-benzothiadiazine-7-sulfonamide 1,1-dioxide (L) belongs to the thiazide class. Clearly, it emitted fluorescence as it possessed conjugated functional groups. In addition, its molar absorption is a factor deciding the selection of L to be studied as a fluorescence sensor. High molar absorption of L is favoured because at a specified wavelength it enhances the absorption of light, so as it interacts with the target analyte, the sensitivity of L enhances. The molar absorptivity of L is 2.0 × 104 L/mol cm according to Jothieswari et al. .
The variations in the absorption and fluorescence emission spectra that appear after the addition of UO22+ to L (10 μM) have been examined in order to determine that L can be used as a sensitive fluorescent sensor for UO22+.
The absorbance spectra of L and UO2/L are shown in Figure 1. The L shows three absorption bands, at 230, 270, and 318 nm. This finding was similar to the findings of the literature. When adding UO22+ to L, the absorption band intensity increased at 230 nm and the absorption band intensity decreased at 318 nm, which is a possible indicator of the formation of UO2/L species.
Fluorescence emission spectra of L in the presence of various concentrations of UO22+ (8.0 × 10−8, 3.0 × 10−7, 6.0 × 10−7, 9.5 × 10−7, 1.5 × 10−6, 2.3 × 10−6, 2.8 × 10−6, 3.5 × 10−6, and 5.0 × 10−6 M, respectively) are shown in Figure 2. At an excitation wavelength 340 nm, the L sensor shows a characteristic fluorescent emission band at 380 nm.
The experimental results show that with the increase in the UO22+ concentration, the fluorescence emission intensity decreases and is fully quenched at a UO22+ concentration of 60.0 μM. This indicates that the fluorescence quenching ability of L sensor is good in the presence of UO22+. In addition, in the UO22+ concentration range of 0.08–5.0 µM, the intensity of the fluorescence emission is linearly quenched. The UO22+ and L species interact through electrostatic interaction between positively charged UO22+ and negatively charged nitrogen atom of the sulfonamide group of L. It reveals that it is possible to use the chelating reaction between UO22+ and L to develop a fluorescence sensor for UO22+ determination. Since the absorbance intensity of L at λ = 318 nm decreased, and fluorescence emission intensity of L at λex/λem = 340/380 nm decreased as a function of UO22+ concentration, both indicated the prevalent formation of a UO2/L species.
Scheme 1 shows the mechanism of UO22+ assessment using L as a fluorescent probe. Initially, when L was free in aqueous solution, they showed strong fluorescence intensity at λex/λem = 340/380 nm. In the presence of UO22+, the fluorescence of L was quenched significantly. The results indicate that UO22+ induced fluorescence quenching of L by the chelation of positively charged UO22+ with negatively charged nitrogen atom of the sulfonamide group of L, thus facilitating the non-radiative recombination of UO22+ and L through the charge transfer or electron transfer processes and leading to the fluorescence quenching of L. Job’s method of continuous variation was used to study stoichiometry. The plot of absorbance against the UO22+ mole fraction has shown that the complex formation between UO22+ and L is with a molar ratio of 1:2. Also, the reaction between UO22+ and L was studied by spectrofluorometry at different temperatures (293 and 303 K) (Figure 3) with the Stern–Völmer equation to understand the fluorescence quenching mechanism [26,27]:
3.3 Inner filter effect (INF)
In fluorescence spectroscopy, the INF is a significant issue that especially affects spectral measurements. UO22+ is a fluorescent species and has absorption and fluorescence spectra. The steps that are being carried out in the experimental work to avoid INF are as follows: (1) choose sample with very low concentration to avoid primary INF and (2) select a particular excitation wavelength to reduce the absorption of the sample. The excitation is decreased to 80 nm (λex = 340) below the absorption maximum of UO22+ (420 nm) to avoid secondary INFs.
3.4 Parameter study
3.4.1 Solvent effect
The L sensor showed the fluorescence emission spectra at λex = 340 nm in different solvents. The influences of the solvent on the fluorescence emission intensity of L sensor are shown in Figure 4. The results showed that the fluorescence emission intensity of the L sensor is much higher in methanol than in other solvents. These could be due to various influence factors of solvent such as dielectric constant, polarity, etc. The dielectric constant and polarity of methanol (ε = 33, polarity = 0.762), acetonitrile (ε = 37.5, polarity = 0.46), DMF (ε = 36.71, polarity = 0.386), and DMSO (ε = 47.24, polarity = 0.444), respectively, indicate that the fluorescence quantum yield of L is increasing with the decrease in the solvent dielectric constant and with the increase in solvent polarity. In addition, L is more soluble in methanol. Thus, the high fluorescence quantum yields in methanol could be anticipated, and methanol was selected as the best solvent.
3.4.2 Concentration effect
In order to determine the influence of the concentration of L sensor, three concentrations of L at 20.0, 10.0, and 6.0 µM were studied. The linear range is 0.05–10.0 µM (RSD 1.04%) at the concentration of 20.0 µM, 0.08–5.0 µM (RSD 1.13%) at the concentration of 10.0 µM, and 0.06–3.0 µM (RSD 1.64%) at the concentration of 6.0 µM. By taking into consideration, in terms of linear range, detection limit, and response time, a good linear relationship is found between fluorescence emission intensity and L concentration. For further examination, the concentration of 10.0 μM for the L sensor was selected.
3.4.3 Selectivity study
An important property of the sensor is its selectivity to the analyte compared with other competing metal ions. Interference experiments for the determination of UO22+ in spiked methanol are conducted prior to the development of the L sensor.
The effects of various potential metal ions (aluminium, barium, calcium, cadmium, cobalt, cupper, chromium, iron, lanthanum, magnesium, manganese, nickel, and zinc) likely present in actual samples are investigated by injecting them into solutions containing 5.0 μM of UO22+ and by handling them as mentioned in the earlier procedure. The tolerance limit is the maximum amount of an ion that causes an error not greater than 5% in the fluorescence emission intensity of the consequent solutions. The findings indicate that the concentrations of aluminium (3.0 mg/L), barium (24.0 mg/L), calcium (22.0 mg/L), cadmium (15.0 mg/L), cobalt (11.0 mg/L), cupper (12.0 mg/L), chromium (4.0 mg/L), iron (3.0 mg/L), lanthanum (1.0 mg/L), magnesium (17.0 mg/L), manganese (17.0 mg/L), nickel (13.0 mg/L), and zinc (12.0 mg/L) have little effect on the L sensor fluorescence emission intensity, indicating that the L sensor has sufficient selectivity for UO22+ assessment.
3.5 Analytical figures of merit
The analytical parameters are measured in order to apply the fluorescent L sensor for UO22+ assessment. As shown in Figure 3, a good linear relation was obtained between F0/F versus UO22+ concentration at 293 K. The regression equation was F0/F = 1.0326 + 4.8 × 105 [UO22+] (R2 = 0.9992) within a concentration range of 0.08–5.0 µM. The limit of detection (LOD) is defined as the lowest amount of analyte in a sample, which can be detected but not necessarily quantitated as an exact value. The limit of quantification (LOQ) is defined as the lowest amount of analyte in a sample, which can be quantitatively determined with suitable precision and accuracy. In order to show the sensor constraint, LOD and LOQ were also calculated. According to the ICH guidance, LOD = 3.3S/b and LOQ = 10S/b (where S is the standard deviation and b is the slope) are tabulated in Table 1. These results suggested that the L sensor could be used as a tool for the assessment of UO22+ at very low concentration levels.
|Linear range (M)||8.0 × 10−8–5.0 × 10−6|
|Limit of detection (LOD) (M)||2.76 × 10−8|
|Limit of quantification (LOQ) (M)||8.37 × 10−8|
|Standard deviation (SD)||0.004|
|Variance (SD2)||1.6 × 10−5|
|Correlation coefficient (R2)||0.9992|
In order to investigate the validity of the L sensor to various real aqueous samples, the various R&D samples are assessed by the L sensor and compared with the ICP-OES analysis. The various R&D samples spiked with UO22+ at different concentrations (0.10, 0.30, and 0.50 µM). The L sensor was added to the sample and mixed with 10 mL of methanol and then left for 1 min before measurement of the intensity of fluorescence. Table 2 shows the resulting data obtained. It was indicated that the recovery values are in the range of 95.6–105%. Also, the various R&D samples and their spiked samples are measured by ICP-OES analysis.
|Sample||UO22+ spikeda||Proposed measured (n = 5)||Reference measured (n = 5)|
|UO22+ average measureda ± SDb||Recovery%||RSD%c||UO22+ average measureda ± SDb||Recovery%||RSD%c|
|S1||10||10.5 ± 0.23||105.0||2.19||10.2 ± 0.09||102.0||0.88|
|30||31.1 ± 0.34||103.7||1.09||30.5 ± 0.23||101.7||0.75|
|50||52.4 ± 0.67||104.8||1.27||50.6 ± 0.41||101.2||0.81|
|S2||10||10.4 ± 0.19||104.0||1.83||10.2 ± 0.08||102.0||0.78|
|30||31.0 ± 0.34||103.3||1.09||30.3 ± 0.22||101.0||0.73|
|50||51.8 ± 0.49||103.6||0.95||50.8 ± 0.39||101.6||0.76|
|S3||10||10.4 ± 0.18||104.0||1.73||10.2 ± 0.08||102.0||0.78|
|30||31.2 ± 0.34||104.0||1.09||30.4 ± 0.20||101.3||0.66|
|50||52.3 ± 0.50||104.6||0.96||50.7 ± 0.38||101.4||0.75|
|Tap water||10||9.57 ± 0.12||95.7||1.24||9.87 ± 0.09||98.7||0.93|
|30||28.7 ± 0.31||95.6||1.09||28.9 ± 0.23||96.5||0.81|
|50||47.9 ± 0.87||95.8||1.83||48.8 ± 0.38||97.6||0.78|
|Seawater||10||10.3 ± 0.13||103.4||1.26||10.2 ± 0.11||102.3||1.08|
|30||31.4 ± 0.36||104.7||1.16||30.9 ± 0.31||103.1||1.01|
|50||52.4 ± 0.66||104.8||1.26||51.4 ± 0.48||102.8||0.93|
The values are multiplied by 10−8 M.
SD, standard deviation, the values are multiplied by 10−8 M.
%RSD, relative standard deviation.
The results reveal that the UO22+ concentration detected from the L sensor was in a good agreement with the concentration results using the ICP-OES analysis. To check the accuracy of L sensor, the recovery results and the results from ICP-OES are used. The precision of the L sensor was measured and the three measurements were recorded as relative standard deviation (RSD%) values. The results of RSD% were lesser than 2.5%. These findings indicate that the procedure is precise and can be used for UO22+ assessment.
Further study is carried out by comparing the analytical parameters of the L sensor with some of other sensors developed, as shown in Table 3 [17,18,28,29,30,31,32,33]. The table reveals that, with adequate performance, the L sensor has impressive features, including a broader linear range and a lower detection limit, good selectivity, and remarkable simplicity.
|Active material||Linear range (M)||Interfering ions||References|
|PCSA||3.7 × 10−9–9.2 × 10−8||Cu2+|||
|2,6-Pyridinedicarboxylic acid||2.6 × 10−7–8.8 × 10−6||Tb3+|||
|Tetraphenylethene (TPE) modified with 2-(4,5-dihydrothiazol-2-yl) phenol||1.0 × 10−6–2.0 × 10−5||—|||
|Porphyrin-terminated polymeric||10−7–10−3||Cu2+, Fe3+, Ni2+, Zn2+|||
|Cyclic peptide||3.6 × 10−7–4.2 × 10−5||—|||
|Trimetazidine||4.9 × 10−8–1.7 × 10−6||Th4+, Al3+, Fe3+|||
|Clopidogrel||1.0 × 10−9–4.0 × 10−6||Th4+, some transition metals|||
|Furosemide||7.0 × 10−7–4.0 × 10−6||Al3+, Fe3+, some transition metals|||
|6-Chloro-2H-1,2,4-benzothiadiazine-7-sulfonamide-1,1-dioxide||8.0 × 10−8–5.0 × 10−6||Al3+, Fe3+, some transition metals||This work|
A new turn-off L sensor is studied with regard to the potential application as a fluorescence quenching sensor for UO22+. In particular, L sensor presents sensitivity and selectivity to UO22+ (the detection limit is 0.0276 µM), which is among the best reported results. From this, it was believed that the fluorescence L would be providing a promising and practical UO22+ sensing material.
Funding information: The authors state no funding involved.
Author contributions: Amira A. Elabd: conceptualization, methodology, investigation, writing – original draft, formal analysis, and writing – review and editing; Olivea A. Elhefnawy: conceptualization, methodology, investigation, writing – original draft, formal analysis, and writing – review and editing.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: All data generated or analysed during this study are included in this published article.
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© 2021 Amira A. Elabd and Olivea A. Elhefnawy, published by De Gruyter
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