An eco‑friendly ultrasound‑assisted deep eutectic solvent‑based liquid–phase microextraction method for enrichment and quantification of nickel in environmental samples

An eco-friendly and easy ultrasound-assisted liquid phase microextraction approach using deep eutectic solvent (UA-DES-LPME) was established to preconcentrate and separate trace amount of nickel (Ni(II)) in various environmental samples before flame atomic absorption spectrometric estimation. In this method, Ni(II) was complexed with 2-(benzothiazolyl azo) orcinol reagent. The impacts various parameters on the microextarction of Ni(II) was investigated. The calibration graph is linear in the range of 1–500 µg L −1 and limits of detection and quantification were determined as 0.27 and 0.90 μg L −1 , respectively. The RSD% and preconcentration factor were 2.30% and 100, respectively. The analysis of certified reference materials demonstrated the validity of the established procedure. The microextraction method provided here simple, rapid, cheap, green and was effectively used to determine nickel levels in a variety of environmental samples with recoveries ranged of 95.0–98.54%.


Introduction
Nickel (Ni(II)) is harmful to living things at certain concentrations. Nickel enters the body via the air, polluted food and drink, and the smoke of cigarettes. Ni(II) compounds have been classified as carcinogens, and excessive exposure to Ni(II) compounds has been linked to a variety of chronic respiratory issues, lung cancer, and skin dermatitis [1]. As a result, estimating trace Ni(II) in diverse samples using unique and sensitive approaches is a critical goal [2,3]. Because the Ni(II) concentration is lower than the detection limit of certain devices, such as GFAAS or FAAS, and because matrix ions are inert, a preconcentration and separation procedure is required before measurements to overcome these constraints by increasing responsiveness and improving accuracy.
To alleviate the growing environmental issues, nontoxic solvents are necessary. Researchers have been looking for a better series of minimal, healthy, and environmentally acceptable solvents to offset the high cost and toxic effects of ionic liquids (ILs). They developed creative, ecologically friendly, and cost-effective deep eutectic solvents (DESs) as a result of their research [36].
DESs are essentially prepared by mixing two or more inexpensive constituents that can join through hydrogen bonding [37]. DESs are often achieved by using cheap, safe, and biodegradable choline chloride (Vitamin B4, ChCl) (through hydrogen bond donors (HBDs)). DESs produced from ChCl have a number of advantages, including being cheap, having simplicity in the synthesis, requiring no further pretreatment, being biodegradable, and being environmentally friendly [38]. When it comes to measuring hazardous metal ions in diverse environmental materials, many instrumental approaches such as ETAAS, FAAS, electro-analytical, and ICP-OES were applied [39][40][41][42][43]. Therefore, the use of the UA-DES-LPME method in conjunction with FAAS offers several advantages including ease of use, cost savings, a lower detection limit, a better preconcentration factor, and environmental friendliness.
At room temperature, some DESs, including the DES employed in the study being presented, are in a liquid state. This provides an advantage for usage of DESs in microextraction studies. Our goal was to develop a green UA-DES-LPME method for preconcentrating and precisely assessing Ni(II) in a variety of environmental samples using FAAS. The effect of numerous variables on the developed method's performance was optimized. The validity of the procedure was tested using certified reference materials.

Apparatus
The Ni concentration was determined using an Agilent 55B AA spectrometer (Agilent Technologies Inc., Santa Clara, USA) with an air-acetylene flame burner and Ni-hollow cathode lamp (231.1 nm). The sample introduced a FAAS nebulizer utilising micro-injection method. The pH of the buffer solutions was determined using an AD1000 pHmeter (Adwa Instruments Kft., Szeged, Hungary). To speed up phase separation, a centrifuge (Isolab, GmbH, Germany) was used. A Grant ultrasonic water bath (LabGear, Australia) was used to facilitate analyte separation from
Daily dilutions of the stock standard solution resulted in a diluted Ni(II) working solution.

DESs preparation
In a glass bottom flask on a water bath at 80 °C for 5.0 min with continuous agitation, four different types of DES were created by combining choline chloride (ChCl) as a hydrogen bond acceptor and hydrogen bonding donors (urea (U), oxalic acid (Ox), lactic acid (LA), and ethylene glycol (EG) at a 1:2 molar ratio.  60 (0.5 mL) and 400 μL DES were added, then vortexes were run for 30 s to make a homogeneous solution, and then 300 μL THF was injected rapidly and the mixture was transferred into an ultrasonication bath for 3.0 min. The solution was centrifuged for 5.0 min at 4000 rpm to speed up phase separation. The aqueous phase was removed with a syringe. Finally, the residual DES-rich phase was diluted with acidic ethanol to 500 μL, and the Ni(II) concentration was determined using FAAS.

Application to real samples
Tap, mineral, sea, well, and waste water samples from Saudi Arabia were passed via a cellulose membrane filter (0.45-μm pore size) and diluted HNO 3 was used for Various fresh food samples and cigarette tobacco samples collected from the markets in Saudi Arabia. The samples were dried at 90 °C overnight in an oven before being homogenised in an agate porcelain mortar. Firstly, SRM 1570a spinach leaves or SRM 1573a tomato leaves (0.2 g) (National Institute of Standard Technology, Gaithersburg, MD, USA), food (1.0 g), and cigarette tobacco (0.5 g) samples were weighted in a glass beaker, then processed with 10 mL of a concentrated HNO 3 -H 2 O 2 (2:1, v/v) combination and placed into Teflon tubes. The microwave digestion procedure has been applied for sample preparation after requiring dilution and pH adjustments [32]. The digested samples were then put through the UA-DES-LPME process.

Effect of pH
pH affects both the extraction recovery and the metalchelate complex creation. As a result, the effect of pH was investigated at pH values ranging from 3 to 9. The recovery of the complex enhanced by rising pH, and highest quantitative recoveries were obtained at pH range of 6.0-8.0 as presented in (Fig. 1). At low pH values, the extraction of Ni(II) ions not effective, due to the hydronium ions disturbance on DES functional groups. Because of the production of the corresponding Ni(II) hydroxides at pH values over 8.0, the extraction recovery also falls. Therefore, pH 7.0 of phosphate buffer solution (3.0 mL) was used in following studies.

Effect of amount of BTAO reagent
In order to acquire quantifiable results, the amount of reagent utilized has a considerable impact on the Ni(II)-BTAO complex recovery. Various quantities of BTAO (0.2%, w/v) were tested in the range of 0.1-1.0 mL, and the curves of the findings are presented in (Fig. 2). By raising the BTAO volume to 0.5 mL, the recovery value was raised, and larger quantities of BTAO had no influence on the recovery value. As a result, in subsequent investigations, 0.5 mL of BTAO (0.2%, w/v) was used as the optimal amount.

Effect of DES type, composition and volume
DES type used is an important factor in the quantitative microextraction of analytes [31][32][33][34][35]. As extraction solvents, four distinct DES with varying compositions were produced (see Table 1). Quantitative recoveries were achieved, as demonstrated in Table 1. The (ChCl with U) DES solution was chosen for future investigation based on the results obtained. The extraction solvent volume is a significant criterion that must be adjusted. In the range of 100-800 µL of ChCl: U (1:3 mol ratio), the influence of DES solution volume on the extractability of Ni(II) was investigated. Figure 3 shows that with raising the DES volume, the recovery of Ni(II) ions increased up to 400 μL. Then larger than 400 μL the recovery decreased due to dilution. Hence, DES volume (400 μL) was used for further experiments.

Effect of the molar ratio of DES
After choosing the optimum DES solvent (ChCl with U), various molar ratios were investigated in the developed UA-DES-LPME method at 1:2, 1:3, and 1:4 ( Table 1). The results in Table 1 revealed that raising the U ratio enhanced the quantitative recovery. Because phase separation is

Effect of volume of THF
Tetrahydrofuran (THF) is employed as an emulsifier solvent in liquid phase microextraction procedures. When THF is introduced to an aqueous sample solution, the DES phase begins to emulsify, the DES clumps becomes water/THF insoluble. Then, extraction of the analyte from the aqueous phase to the DESrich phase. THF was chosen for its high extraction efficiency and ability to separate Ni(II)-DES-rich phases precisely. Furthermore, varied volumes of THF (100-600 μL) at a constant volume of DES (400 μL) were utilized to investigate the influence of THF volume (Fig. 4). Because of the solubility of the ChCl: U molecule, no extraction phase was collected in the absence of THF. However, increasing the THF volume increased the volume of the extraction phase. With the addition of 200-400 μL of THF, quantifiable findings for nickel(II) were achieved. So, 300 μL of THF was selected as the optimal volume.

Effect of sonication time
The duration of ultrasonic radiation exposure has a considerable impact on the developed method's efficiency and performance. The optimal ultrasonication time was found to be between 1.0 and 6.0 min (Fig. 5). The recovery was raised up to 3.0 min, according to the results. There was no notable change in recovery beyond this time until 4.0 min. At time more than 4.0 min, the recovery slightly decreased due to increase the temperature of solution. As a result, the optimum sonication time for future experiments was determined to be 4.0 min.

Study of centrifuging conditions
In the ranges of 1000-5000 rpm and 2-15 min, the influence of centrifugation rate and time on Ni(II) extraction efficiency was investigated. The rate was raised to 4000 rpm, which was determined to be the best rate. Also, to guarantee complete phase separation, the quantitative recovery was resulted at 5.0 min. When the centrifuging period exceeded 5.0 min, the recovery decreased due to generation of heat which may enhance the dissolving the metal complex into the aqueous phase. For additional research, the optimal centrifuge rate and time were determined to be 4000 rpm and 5.0 min, respectively.

Effect of sample volume
The sample volume is a key component for calculation the preconcentration factor (PF) for metal ion preconcentration. The PF is the ratio of the original sample volume to the final dilution volume. Over the range of 10-100 mL model solutions, the effect of sample volume on extraction efficiency was tested (Fig. 6). The results revealed that Ni(II) ion recoveries in volumes greater than 50 mL were not quantifiable. As a result, in all future tests, 50 mL of Ni(II) solution was selected as the largest volume. As a result, PF was set to 100.

Effect of matrix
The effect of certain cations and anions on Ni(II) ions recovery was examined. The highest tolerance limits for Ni(II) ions are shown in Table 2. There was no evidence of matrix ion interference in the estimation of Ni(II) ions under the experimental settings, demonstrating the usability of the developed approach for Ni(II) assessment in a variety of real environmental samples.     Table 3 shows the analytical features and characteristics of the developed method with the optimised parameters. Linearity was obtained in the range 1-500 µg L −1 . LOD and LOQ were computed as 3S b /m and 10S b /m, respectively, where S b is the standard deviation from blank solution measurements (n = 10) and m is the calibration graph slope with preconcentration. The ratio between the slopes of calibration curves with and without preconcentration is known as the sensitivity enrichment factor (EF). The consumptive index (CI) is the ratio of the analyte solution volume to EF. The RSD% of ten determinations of 300 µg L −1 Ni(II) solution in the same day (intraday) and various days (inter-day) was used to assess the repeatability and precision of the proposed approach. The RSD% intra-day and inter-day were determined to be 2.30 and 2.0%, respectively, demonstrating the high method's precision.

Method validation
Detrmination of Ni(II) content in four standard reference materials (TMDA-51.3 fortified water, TMDA-53.3 fortified water, SRM 1570a Spinach leaves, and SRM 1573a Tomato leaves) was used to validate the accuracy of the UA-DES-LPME approach. The results represented in Table 4 revealed that the found values were in a good accordance with the reported certified values. Analysis of 100, 200, and 300 μg L −1 Ni(II) (n = 3) on the same day and on five subsequent days were used to estimate intra-day and inter-day precisions. As indicated in Table 5, the RSD% and recovery values were ≤ 3.50% and ≥ 95.30, respectively, respectively. The satisfactory results revealed that the developed method had good potential to detect Ni(II) ions in various samples.

Analysis of environmental samples
The developed UA-DES-LPME preconcentration method was tested for its ability to recognize and separate Ni(II) ions in a variety of genuine water and food and cigarette tobacco samples. To validate the devised procedure reliability and accuracy, certain amounts of Ni(II) ions were spiked to the sample solutions utilizing addition/recovery test and the recoveries and RSD% were determined. The Ni(II) analyte had excellent quantitative recoveries, ranging from 95.0-98.0% with RSD% ≤ 1.70% for water samples (Table 6) and 95.15-98.54% with RSD% ≤ 1.49% for food and cigarette tobacco samples ( Table 7). As a result of these findings, the method may be used to separate, preconcentrate, and analyse Ni(II) in real environmental samples at trace levels.

Comparison with other approaches
The UA-DES-LPME approach was compared to various extraction strategies described earlier for preconcentration of Ni(II). The comparison provides for a more thorough examination of the suggested method's benefits in comparison to other alternatives. The low limit of detection, extensive working ranges, excellent PF, greater reliability, good precision and the use of green DES, were the method's primary advantages, as shown in Table 8. As a result of these findings, the proposed UA-DES-LPME methodology for analysing Ni(II) in different environmental samples could be successfully executed.

Declarations
Conflict of interest There are no competing interests declared by the author.
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