1 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.

Multiple approaches to preconcentrate and separate Ni(II) have lately been published in the literature, including cloud point extraction [4,5,6,7,8,9,10,11,12], solid-phase extraction [13,14,15,16], membrane filteration [17], and co-precipitation [18,19,20].

Using dispersive liquid-liquid microextraction (DLLME), the toxicity of extraction solvents has been reduced or eliminated. The DLLME technique has many advantages, including simplicity, speed, cheap cost, ease of use, green solvent use, and high enrichment factors. Many improved liquid-phase microextraction (LPME) methods for Ni(II) separation and microextraction from diverse sources have recently been developed and described [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35].

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.

2 Experimental

2.1 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 pH-meter (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 sample matrices and the formation of a hazy solution. To produce bidistilled water, Milli-Q (Millipore, USA) was utilised. Before the experiment, glass wares were immersed in HNO3 (5.0% v/v) nightly and cleaned numerous times with bidistilled water. The samples were digested utilising microwave digestion systems from Milestone Ethos UP (Milestone, Sorisole, Italy) up to a maximum temperature of 300 °C and 1450 psi maximum pressure.

2.2 Reagents and solutions

All chemicals and reagents were acquired from the companies (Merck, Darmstadt, Germany) and (Sigma Aldrich, St. Louis, USA).

A Ni(NO3)2·6H2O (Fluka Chemie AG, Basel, Switzerland) with purity 98% was used to make the Ni(II) stock standard solution (1000 µg mL−1) and standardisation by EDTA [44]. Daily dilutions of the stock standard solution resulted in a diluted Ni(II) working solution.

Acetate (CH3COONa–CH3COOH) pH (3.0–5.5), phosphate (Na2HPO4–NaH2PO4) pH (6.0–7.0), ammoniacal (NH3-NH4Cl) solution pH 8.0, and borate (sodium tetraborate and boric acid) pH (9.0–10) are some of the specific buffer series that have been utilised. According to the literature [45], HCl and NaOH are employed in particular to modify pH values.

The 2-(benzothiazolyl)-azo orcinol reagent (BTAO) was prepared according to the protocol described [6]. In a 100 mL measuring flask, a suitable amount of reagent was dissolved in ethanol (purity, 95%) to make a BTAO stock solution (0.2%, w/v).

2.3 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.

2.4 Recommended procedure

An aliquot of Ni(II) solution (50 mL) containing 1–500 µg L−1 was combined with phosphate buffer (pH 7.0) (3.0 mL) in a centrifuge tube. Posteriorly, BTAO (0.2% w/v) solution (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.

2.5 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 HNO3 was used for acidification, then stored in a polyethylen container in darkness at 4.0 °C. Then, the proposed approach was successfully implemented on water samples. Also, the proposed approach has been used with the reference materials (TMDA-51.3 and TMDA-53.3 fortified water) developed by (National Water Research Institute of Environment Burlington, Canada). The calibration graph was used to calculate the concentration of Ni(II).

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 HNO3–H2O2 (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.

3 Results and discussion

3.1 Effect of pH

pH affects both the extraction recovery and the metal-chelate 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.

Fig. 1
figure 1

Effect of pH on the recovery of Ni(II) via UA-DES-LPME approach (N = 3)

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3.2 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.

Fig. 2
figure 2

Effect of the BTAO amount on the recovery of Ni(II) using UA-DES-LPME approach (N = 3)

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3.3 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.

Table 1 Effect of type DES and molar ratio on the recovery of Ni(II) (N = 3)
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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.

Fig. 3
figure 3

The influences of DES volume on the recovery value of Ni(II) using through UA-DES-LPME approach (N = 3)

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3.4 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 apparent at a 1:3 molar ratio and the findings obtained from this location are quantitative, 1:3 was chosen as the best ChCl:U ratio and utilized for all tests.

3.5 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 DES-rich 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.

Fig. 4
figure 4

Effect of THF volume on the recovery value of Ni(II) using UA-DES-LPME approach (N = 3)

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3.6 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.

Fig. 5
figure 5

Effect of sonication time on the Ni(II) extarction using UA-DES-LPME approach (N = 3)

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3.7 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.

3.8 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.

Fig. 6
figure 6

Influence of sample volume on the Ni(II) preconcentration using UA-DES-LPME approach (N = 3).

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3.9 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 2 Effect of matrix ions on the recovery of Ni(II) (N = 3)
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3.10 Analytical features and validation of the UA-DES-LPME method

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 3Sb/m and 10Sb/m, respectively, where Sb 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.

Table 3 Analytical features of the proposed UA-DES-LPME approach for the determination of Ni(II)
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3.11 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.

Table 4 The validation of the proposed UA-DES-LPME approach for Ni(II) determination in CRMs (N = 3)
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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.

Table 5 Intra-day and inter-day accuracy and precision of the proposed approach
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3.12 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.

Table 6 Addition/recovery test for extraction of Ni(II) in water samples using the developed UA-DES-LPME approach (N = 3)
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Table 7 Addition/recovery test for the extraction of Ni(II) in food and cigarrate tobacco samples using the developed UA-DES-LPME approach (N = 3.0)
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3.13 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.

Table 8 Comparison of the proposed UA-DES-LPME approach with some other reported preconcentration techniques
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4 Conclusions

This study used the green and efficient UA-DES-LPME approach for preconcentration Ni(II) ions in environmental samples prior to FAAS assessment. The suggested approach has several advantages, including small LOD (0.27 µg L−1), a wide linear range, a large PF (100), simplicity, less operational costs, and small reagent and sample consumption. Reproducibility and repeatability are satisfactory (RSD% < 2.5). The suggested approach has good analytical performance, indicating that it is dependable, and used successfully for trace Ni(II) ions measurement in environmental samples.