Fluorescence quenching detection of UO 2 2 + in aqueous solution based on an organic molecule probe of 6-chloro-2 H-1 , 2 , 4-benzothiadiazine-7-sulfonamide 1 , 1-dioxide

A new organic molecule probe has been introduced as a “turn-off” fluorescent sensor to detect trace quantities of UO2 2+ in the presence of several transition metals with promising results. The procedure is based on quenching the fluorescence intensity of 6-chloro-2H1,2,4-benzothiadiazine-7-sulfonamide 1,1-dioxide (L) in the presence of various UO2 2+ concentrations in methanol. The UO2 2+ and L species interact through electrostatic interaction between negatively charged nitrogen atom of the sulfonamide group of L and positively charged UO2 2+, thus facilitating the non-radiative recombination of UO2 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 UO2 2+ has been successfully used as a new sensor to measure UO2 2+ 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.


Introduction
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 [1]. In aqueous solution, uranium has many forms, and the most prevalent one is uranyl (UO 2 2+ ) [2][3][4][5]. The development of a fast and efficient analytical procedure for UO 2 2+ 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 UO 2 2+ 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] [20]. However, by using these sensors, the presence of some transition metals and thorium ions interfered with the UO 2 2+ 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 UO 2 2+ recognition remains a challenge.

2+
. 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 UO 2 2+ is gradually added to L and the fluorescence emission intensity of L is decreased. Therefore, the concentration of UO 2 2+ could thus be detected quantitatively. In this regard, L was studied for the application of a new organic molecule probe for spectrofluorometric assessment of UO 2 2+ in aqueous solution.

Materials
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, UO 2 (NO 3 ) 2 ·6H 2 O, was manufactured by Mallinckrodt Company. All the other chemicals used in this study were purchased from Alpha Company.

Instruments
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, UO 2 2+ 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.

Analytical procedures
In order to assess the UO 2 2+ concentration by L sensor, a stock solution of UO 2 2+ (100 µM) was prepared by dissolving an accurately weighed amount of uranyl nitrate hexhydrate (UO 2 (NO 3 ) 2 ·6H 2 O) in deionized water containing few drops of concentrated sulphuric acid and standardized by a known method [24]. All working solutions were prepared by diluting the stock solution suitably with deionized water to give various concentrations of UO 2 2+ from 0.08 to 5.0 µM. These various concentrations of UO 2 2+ 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 λ ex = 340 nm, λ em = 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 UO 2 2+ concentrations.

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 × 10 4 L/mol cm according to Jothieswari et al. [25].
The variations in the absorption and fluorescence emission spectra that appear after the addition of UO 2 2+ to L (10 μM) have been examined in order to determine that L can be used as a sensitive fluorescent sensor for UO 2 2+ .
The absorbance spectra of L and UO 2 /L are shown in Since the absorbance intensity of L at λ = 318 nm decreased, and fluorescence emission intensity of L at λ ex /λ em = 340/380 nm decreased as a function of UO 2 2+ concentration, both indicated the prevalent formation of a UO 2 /L species.

Mechanism
Scheme 1 shows the mechanism of UO 2 2+ assessment using L as a fluorescent probe. Initially, when L was free in aqueous solution, they showed strong fluorescence intensity at λ ex /λ em = 340/380 nm. In the presence of UO 2 2+ , the fluorescence of L was quenched significantly. The results indicate that UO 2 2+ induced fluorescence quenching of L by the chelation of positively charged UO 2 2+ with negatively charged nitrogen atom of the sulfonamide group of L, thus facilitating the nonradiative recombination of UO 2 2+ 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 UO 2 2+ mole fraction has shown that the complex formation between UO 2 2+ and L is with a molar ratio of 1:2. Also, the reaction between UO 2 2+ and L was studied by spectrofluorometry at where F 0 and F are the fluorescence emission intensities of L in the absence and presence of UO 2 2+ , respectively, Q is the quencher concentration of UO 2 2+ , and K sv is the constant of Stern-Völmer. Stern-Völmer's plot of F 0 /F versus [Q] at different temperatures (293 and 303 K) is shown in Figure 3. And the calculated K sv are 4.8 × 10 5 and 4.6 × 10 5 mol −1 at 293 and 303 K, respectively. This result indicates that there is a good agreement between quenching process and the Stern-Völmer equation, which is an indicator for static quenching mechanism because K sv decreases with increase in temperature.

Inner filter effect (INF)
In fluorescence spectroscopy, the INF is a significant issue that especially affects spectral measurements. UO 2 2+ 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 (λ ex = 340) below the absorption maximum of UO 2 2+ (420 nm) to avoid secondary INFs.

Parameter study 3.4.1 Solvent effect
The L sensor showed the fluorescence emission spectra at λ ex = 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

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 UO 2 2+ 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 UO 2 2+ 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

Analytical figures of merit
The analytical parameters are measured in order to apply the fluorescent L sensor for UO 2 2+ assessment. As shown in Figure 3, a good linear relation was obtained between 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 UO 2 2+ at very low concentration levels.

Application
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 UO 2 2+ 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.
The results reveal that the UO 2 2+ 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 UO 2 2+ 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.

Conclusion
A new turn-off L sensor is studied with regard to the potential application as a fluorescence quenching sensor for UO 2 2+ . In particular, L sensor presents sensitivity and selectivity to UO 2 2+ (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 UO 2 2+ sensing material.
Funding information: The authors state no funding involved.

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.