Spinel CuMn₂O₄ nanoflakes as an effective adsorbent for dispersive solid phase microextraction of silver and lead in spices and water†

Şerife Tokalıoğlu*, Samet Pekşin, Yakup Yılmaz and Şaban Patat
Erciyes University, Faculty of Sciences, Chemistry Department, Kayseri, 38039, Türkiye. E-mail: serifet@erciyes.edu.tr

First published on 25th July 2025

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1 Introduction

Food safety and environmental pollution are the most important issues of today. Among all pollutants, heavy metals pose the greatest risk to food safety and environmental pollution. Their main sources include industry, the application of fertilizers and pesticides, the atmospheric deposition related to coal combustion and vehicle emissions, mining and sewage irrigation.¹ Among them, lead is the most serious one. It is an enzyme inhibitor and metabolic poison. Lead exposure results in neurological system changes, leading to neurological function loss. It may cause brain damage, high blood pressure, kidney, liver, and heart injuries, seizures, and anemia in adults and a slight deficit in children's learning abilities.^2–4 Silver is a precious metal. It has been widely used in pharmaceuticals, jewelry, photography, and electroplating. This led to the discharge of a large amount of wastewater containing silver.⁵ Silver has caused risks to human and aquatic lives because of its toxicity.⁶ Soluble silver ions in water systems are classified as hazardous substances by the WHO and the EPA and the silver concentration in drinking water is limited to 100 μg L⁻¹.⁷ Therefore, selective and sensitive analytical methods for Ag(I) and Pb(II) ions are needed for the preconcentration and separation of these ions in various samples.

Sample preparation eliminates most matrix interferences and decreases the sample complexity while providing preconcentration.⁸ Traditional solid phase extraction (SPE) is one of the most common extraction methods. However, it has certain shortcomings such as producing large secondary wastes, solvent loss, a need for complex equipment, and being a long procedure.^9,10 In this sense, analytical chemistry studies have endeavored to improve greenness, miniaturization, simplification, and time reduction methods. Dispersive solid phase microextraction (D-SPμE) is classified as a miniaturized SPE. D-SPμE reveals a lot of advantages over traditional SPE, like recovery efficiency, reduced solvent consumption, and short time requirement. Moreover, it is economic, easy, and simple.¹¹ Besides the rapid extraction, D-SPμE does not require column sorbent packing and large sample volume passing. D-μ-SPE is carried out by dispersing the sorbent in the solution, followed by sonication, vortexing, or shaking. This dispersion provides fast and uniform interaction which leads to the extraction and enrichment of analytes.¹² The analyte–sorbent contact surface and thus the analyte–sorbent interaction increase upon sorbent dispersion in the solution. It reduces the extraction time and increases extraction efficiency.¹³ Low organic solvent consumption, high analyte recoveries, speed, and high operation simplicity are advantages of D-SPμE.¹²

The sorbents having a micro or nano-size in D-SPμE have a high surface area that decreases the extraction time. Moreover, in D-SPμE, the consumption of eluent is reduced.¹⁴ Compared with microscale materials, nanostructured sorbents provide higher adsorption capacity.¹⁵ Generally, adsorbents with high capacity and good dispersion ability are required to obtain fast sorption, dispersion, and analyte elution. Therefore, sorbent preparation is a main factor in achieving high sorption capacity and selectivity. The sorbent selectivity in the adsorption depends on the strength and type of the analyte–sorbent interactions, or the shape and size of the sorbent and analyte pore in the absorption.¹³ Therefore, a continuous effort for many researchers is the exploration and design of novel adsorbents.

Nano-metal oxide materials contain metal oxides. Their sizes change from 1 to 100 nm. They are relatively cheap and indicate enhanced adsorption and redox capabilities.¹⁶ Recently, new spinel oxides have drawn a lot of attention as researchers explore their structural stability as well as their optical, electrical, and magnetic characteristics.¹⁷ In comparison with single metal oxides, the spinel AB₂O₄ oxides (A = Mg, Ba, Sr, Ca, Fe, Mn, Cu, Co, Zn, Ga, Ni, and Cd; B = Fe, Al, Mn, Cr, Co, etc.) include two different valence transition metals. The synergism can support the internal electron transfer rate and decrease the charge transfer activation energy (Gao et al., 2021).¹⁸ They have high adsorption capacity and photocatalytic activity. Among AB₂O₄ spinel oxides, CuB₂O₄ (B = Fe, Cr, Mn, and Al) is expected to have high adsorption capacity and photocatalytic activity.¹⁹

In this work, a spinel CuMn₂O₄ nanoflake adsorbent was used for the extraction of silver and lead in spice and water samples. It was synthesized and characterized by different methods. Up to now, there is no report in the literature on D-SPμE of silver and lead using a CuMn₂O₄ nanoflake sorbent. Especially, the number of works reported for D-SPμE of silver with nanomaterial adsorbents from various samples is very small. The spinel CuMn₂O₄ nanoflakes have a multistage porous structure. In its structure, tetrahedral and octahedral positions are occupied by a certain amount of Cu²⁺ and Mn³⁺ ions, respectively. Therefore, the synergism of Cu and Mn ions with different valences in the CuMn₂O₄ results in a greater number of active centers. Its abundant oxygen vacancies, and redox-active Cu²/Mn³⁺ centers provide specific binding sites and enhanced affinity¹⁸ for Ag(I) and Pb(II). The pH, eluent type/volume/concentration, volume of sample, adsorption/elution time, adsorption capacity of the CuMn₂O₄ and competing ion effects were examined. The linear range, detection limit (LOD), and precision as inter-day/intra-day of the method were tested. After the optimized D-SPμE method, it was applied in water (tap water, river water, wastewater, and well water), spices purchased from both a herbalist and supermarket (red pepper, black pepper, cinnamon, and cumin), and CRMs (BCR-482 Lichen, NW-TMDA-54.6 Lake water and SPS-WW1 Batch 121 Wastewater).

2 Experimental

2.1 Instrumentation

Silver and lead determination was performed by a- FAAS (AAnalyst 800 model, PerkinElmer, Waltham, MA, USA) with a 100 mm burner head. The instrumental conditions for Ag and Pb were as follows: resonance lines, 328.1 nm and 283.3 nm; currents, 10 mA and 10 mA; slit widths, 0.7 nm and 0.7 nm, respectively. The flame was produced with an air (17 L min⁻¹) and acetylene (2.0 L min⁻¹) mixture. To control the purity of the CuMn₂O₄, a- X-ray powder diffraction pattern (XRD) was measured at 2θ angles from 5° to 90° on a- X-ray diffractometer (PANalytical Empyrean with Cu-K_α tube, Malvern, UK). The surface characteristics and particle sizes of the CuMn₂O₄ were investigated using a field emission scanning electron microscope (FESEM, Zeiss, Gemini-500 Oberkochen, Germany) with an energy dispersive X-ray spectrometer (EDS). The Fourier transform infrared spectrum (FT-IR) of the CuMn₂O₄ was measured on a Perkin Elmer-400 FTIR spectrometer (ATR, Waltham, MA, USA) in the wavelength range of 450–4000 cm⁻¹ to explore the functional groups of CuMn₂O₄. Zeta potential of the CuMn₂O₄ was measured using a Malvern Zetasizer Nano system (England). N₂ adsorption/desorption isotherms were recorded using a Micromeritics Gemini VII instrument (Norcross, USA) to determine the pore size distribution and specific surface area of the CuMn₂O₄ powder. Before the measurement, the CuMn₂O₄ powder was degassed for 8 h at 300 °C. A vortex (Wiggen Hauser, Malaysia) and a Nüve NF 400 model centrifuge (Ankara, Türkiye) were used. A WTW pH 3110 meter (Weilheim, Germany) was used for pH adjustments. An analytical balance (Precisa 125 A SCS) was used for all weighings, and a hot plate (Elektromag M 4060) was used for acid digestions.

2.2 Reagents and solutions

All chemicals were obtained from Merck, Darmstadt, Germany. For synthesis of CuMn₂O₄, analytical grade Mn(CH₃COO)₂·4H₂O, Cu(CH₃COO)₂·H₂O, KOH, and ethylene glycol (EG) were used. Ultra-high purity water (U-HPW, 18.2 MΩ cm) was used in all experiments. U-HPW was prepared using a Millipore water purification system (Millipore Corp., USA). The Pb(II) stock solution (1000 mg L⁻¹) was prepared from Pb(NO₃)₂ (Merck) in 1% nitric acid. An Ag(I) stock solution of 1000 mg L⁻¹ was obtained from Merck, Darmstadt, Germany. Working and calibration solutions of Ag(I) and Pb(II) were prepared by diluting the stock solutions before use. As buffer solutions, H₃PO₄/NaH₂PO₄ buffer for pH 2.0 and 3.0, CH₃COOH/CH₃COONa buffer for pH 4.0, 5.0, and 6.0 and NH₃/CH₃COONH₄ for pH 7.0 were used. For pH 1.0, only diluted HCl was used.

2.3 Synthesis of CuMn₂O₄

The porous CuMn₂O₄ spinel nanoflakes were synthesized according to the literature.¹⁸ In a nutshell, 0.10 mol L⁻¹ Cu(CH₃COO)₂·H₂O and 0.16 mol L⁻¹ Mn(CH₃COO)₂·4H₂O were dissolved in mixed ethylene glycol-U-HPW (V_EG:V_H2O = 1:9). Afterwards, 1 mol L⁻¹ KOH solution was added dropwise to the mixture until the pH value was 10. After mixing with a magnetic stirrer for 12 hours at room temperature, the precipitate was separated by centrifugation. It was washed with U-HPW and ethanol a few times, and dried at 60 °C in an oven. Then, the powder was calcined at 650 °C for 4 h to obtain the CuMn₂O₄ nanoflake material.

2.4 Dispersive solid phase microextraction procedure

The D-SPμE procedure is shown in Fig. 1. It is described as follows: 100 mg of CuMn₂O₄ nanoflakes was added to 20 mL of an aqueous solution including 2 μg Ag(I) and 8 μg Pb(II). Then, the mixture's pH was adjusted to 2.0 by adding one milliliter of buffer solution (pH 2.0). Adsorption of analytes was carried out without vortexing. After extraction, the sorbent was separated from the adsorbent by centrifugation for 3 min. at 4100 rpm and the upper liquid was removed. Then 2 mL HCl (2 mol L⁻¹) was added to the CuMn₂O₄ nanoflakes, and it was vortexed for 30 s. After centrifugation for 3 min. at 4100 rpm, the eluent was collected, and the concentrations of Ag and Pb were quantified by FAAS.

2.5 Sample preparation

The CuMn₂O₄ nanoflakes as a sorbent were used for D-SPμE of analytes in various spices and waters. Tap water, river water, well water, and wastewater were collected from the research laboratory, Pınarbaşı, Bünyan, and Industrial Region in Kayseri, respectively. The samples were immediately filtered by a membrane filter (pore size 0.45 μm). Then the D-SPμE was applied for water samples of 20 mL, 10 mL of NW-TMDA-54.6, and 40 mL of SPS-WW1 Batch 121. The same type of spice was purchased from both a herbalist (n = 4) and supermarkets (n = 4) in Kayseri, Türkiye, and then dried for 2 hours at 40 °C and ground. For dissolving spices, 0.50 g of red pepper, black pepper, cinnamon, and cumin was weighed into glass beakers. After adding 10 mL (65%, w/w) of conc. HNO₃ to the beaker, it was evaporated to dryness on a hot plate. After cooling, 3 mL (30%, w/w) of conc. H₂O₂ was added to the beaker, and the heating process to dryness was again applied.²⁰ Then, 20 mL of U-HPW was added to each beaker, and the D-SPμE method was applied. 0.20 g of BCR-482 Lichen was weighed into a beaker. A concentrated HNO₃ and HClO₄ mixture (V_HNO3:V_HClO4 = 9:1) was added to the beaker.²¹ Then it was evaporated. After adding U-HPW of 20 mL to the beaker, the solution pH was adjusted to 2, and then the D-SPμE was done.

3 Results and discussion

3.1 Characterization of CuMn₂O₄

The XRD powder pattern of the porous CuMn₂O₄ spinel nanoflakes is given in Fig. 2a. As shown in Fig. 2a, the Fdm space group and face-centered cubic structure of porous CuMn₂O₄ spinel nanoflakes are identified in the XRD pattern, matching well with standard JCPDS card no. 01-076-2296.¹⁸

FESEM images of the CuMn₂O₄ are given in Fig. 2b. It is evident from the FESEM images that the CuMn₂O₄ nanoflake material has high porosity. Moreover, it can be described as disordered nanoparticles with a small size and multistage porous structure. The EDS spectrum and the elemental composition of the CuMn₂O₄ are given in Fig. 2c. As shown in Fig. 2c, the EDS spectrum indicates that the powder sample consists of Cu, Mn, and O elements, and their stoichiometries are close to the targeted values.

The FTIR spectrum of the CuMn₂O₄ nanoflake material is given in Fig. 2d. The absorption peaks in the range from 800 to 400 cm⁻¹ are assigned to M–O lattice vibration. Especially, the two strong peaks at 582 cm⁻¹ and 477 cm⁻¹ correspond to Cu–O and Mn–O stretching vibration modes, respectively.¹⁸

The N₂ adsorption/desorption isotherms and pore size distribution of the CuMn₂O₄ are given in Fig. 2e. N₂ adsorption/desorption of the CuMn₂O₄ indicates type IV isotherms indicating the presence of meso- and macropores. The BET specific surface area and pore size of the material were calculated to be 6 m² g⁻¹ and 5–67 nm, respectively, representing its porous structure.

The zeta potential versus pH for CuMn₂O₄ is given in Fig. 2f. As can be seen from Fig. 2f, the adsorbent surface charge is negative for pH > 2.5 whereas it is positive for pH < 2.5.

3.2 Effect of pH

The sample pH affects the form of analytes and the surface charge of the sorbent.⁹ Therefore, the effect of sample pH on CuMn₂O₄ nanoflake adsorption of Ag(I) and Pb(II) was tested at different pH ranging from 1.0 to 7.0. The results are illustrated in Fig. 3a. The percentage recoveries (R%) for Pb(II) were quantitative at all the pH (R% = 92–100) whereas the adsorption of Ag(I) ions significantly increased with the increase from pH 1.0 to 3.0 and then was decreased with the increase from pH 4.0 to 7.0. At pH 1, the active groups on the CuMn₂O₄ nanoflake surface were protonated and therefore, repulsion between the active sites of the sorbent and silver ions occurred. Upon increasing the pH, the CuMn₂O₄ nanoflake adsorbent is deprotonated and in this case, a coordination interaction between the active sites of the sorbent and Ag(I) and Pb(II) ions may be possible. After pH 4.0, the silver ions' extractability was decreased owing to the competition of hydroxide ions with silver ions and hydrolysis of Ag(I).²² Besides, Ag(I) is more stable at acidic pH.²³ According to these results, pH = 2.0 was used as the optimal pH. Another interaction between the analyte ions and adsorbent is electrostatic interaction owing to the negative surface charge of the CuMn₂O₄ nanoflakes. The zeta potential results (Fig. 2f) show that the surface charge of the CuMn₂O₄ nanoflakes is negative at pH > 2.5. With increasing solution pH, the negative charges on the CuMn₂O₄ nanoflakes increase and electrostatic attraction between the adsorbent and analyte increases, except for pH > 4.0 for the silver ions owing to Ag(I) hydrolysis. Despite the positive zeta potential of the adsorbent at pH 2.0, the high recoveries of Ag(I) and Pb(II) at pH 2.0 can be explained by surface complex formation through covalent coordinate binding between the analyte ions and CuMn₂O₄ nanoflake adsorbent. A similar comment was given by Tokalioglu et al., 2023.²¹ The lead mainly exists as Pb(II) and PbOH⁺ cations in the pH range from 2.0 to 6.0. The concentration of cationic species, i.e., Pb(II), starts to decrease at pH > 6.0 owing to its precipitation. The surface functional group available on the CuMn₂O₄ adsorbent surface resulted in surface complexation between Pb²⁺ and PbOH⁺.²⁴

3.3 Effect of adsorption and elution contact times

To ensure sufficient contact between the Ag(I) and Pb(II) ions and CuMn₂O₄ nanoflake sorbent, selection of appropriate contact times is necessary for adsorption/elution.²⁰ In this work, the adsorption contact times of 0, 30 s, 1 min, 2 min, and 3 min and elution contact times of 0, 15 s, 30 s, 1 min, 2 min, and 3 min on the D-SPμE of Ag(I) and Pb(II) were examined. As shown in Fig. 3b, for all the adsorption contact times, the recoveries changed from 93 to 100% for Ag(I) and from 99 to 102% for Pb(II), which indicates that the adsorption equilibrium of analytes on CuMn₂O₄ nanoflakes could be reached without vortexing. Hence, in further tests based on the principle of saving energy and time, vortexing for the extraction of analytes was not done. The effect of elution contact time is given in Fig. 3c. The results revealed that the Ag(I) recoveries increased with increasing elution contact times from 0 to 3 min. Ag(I) elution was quantitative for contact times ≥30 s, while the elution of Pb(II) could be realized without vortexing. In the subsequent experiments, an elution contact time of 30 s for Ag(I) and Pb(II) was chosen. The results without vortexing adsorption and elution of 30 s show that a rapid diffusion in the nanosorbent would provide a high adsorption rate.^25,26

3.4 Effect of eluent volume/type/concentration

Elution conditions were investigated to elute the adsorbed analytes from the CuMn₂O₄ nanoflakes completely.²⁷ Different volumes and concentrations of HNO₃ and HCl were examined. In this work, 3 mL of 1 mol L⁻¹ HCl, 2 mL, 3 mL and 5 mL of 2 mol L⁻¹ HCl, and 2 mL of 2 mol L⁻¹ HNO₃ solutions were investigated as eluents (Fig. 3d). The results revealed that 2–5 mL of 2 mol L⁻¹ HCl solutions was sufficient to elute quantitatively the analyte ions, whereas the recoveries were low for other eluents. The Ag(I) and Pb(II) recoveries were 95 and 97%, respectively, when 2 mL of 2 mol L⁻¹ HCl eluent was used. Consequently, it was chosen to desorb the metal ions on CuMn₂O₄ nanoflakes.

3.5 Effect of sample volume

The volume of the sample affects the preconcentration factor (PF). For this, 2 μg Ag(I) and 8 μg Pb(II) were added to the volumes of 20, 30, 40, 50, 60, 80, and 100 mL containing 100 mg adsorbent at pH 2.0. The results are given in Fig. 3e. All solutions were extracted using adsorption without vortexing/30 s elution time. The D-SPμE quantitative recoveries could be obtained when the volumes of 20 and 30 mL for Ag(I) and 20–45 mL for Pb(II) ions were used. The analyte recoveries were decreased for changing volumes from 50 to 100 mL. Therefore, PF for analytes was calculated as 15 for Ag(I) and 22.5 for Pb(II), which was the highest sample volume/eluent volume. A 30 mL volume was chosen as the highest volume to determine Ag(I) and Pb(II) simultaneously and to obtain a lower method LOD. Based on the above results, the optimized D-SPμE conditions were pH 2.0; extraction time, no vortexing and elution time, 30 s; eluent volume, 2 mL; eluent concentration, 2 mol L⁻¹ HCl; and sample volume, 30 mL.

3.6 Effect of competing ions

In order to interact with the sorbent, there is a competition between the analyte and the interfering species.^13,21 Therefore the effect of interfering ions like K⁺, Ca²⁺, Na⁺, Mg²⁺, Zn²⁺, Fe³⁺, Al³⁺, and SO₄²⁻ was examined to evaluate the D-SPμE selectivity. Solutions of 20 mL containing 0.1 mg L⁻¹ Ag(I), 0.4 mg L⁻¹ Pb(II), and interfering ions at increasing concentrations were treated with the optimized D-SPμE (pH 2.0, no vortexing extraction/30 s elution, 2 mL of 2 mol L⁻¹ HCl elution). The results of the interfering ions are listed in Table S1.† The tolerance limits are defined as the highest interfering ion concentration making a variation of ±10% (R% = 90–110) in the recoveries of analytes (Table 1). The tolerable concentrations (in mg L⁻¹) for the D-SPμE of Ag(I) and Pb(II) were 500 and 750 for Na⁺, 100 and 10000 for K⁺, 75 and 25000 for Ca²⁺, 75 and 25000 for Mg²⁺, 50 and 50 for Fe³⁺, 50 and 50 for Al³⁺, 10 and 25 for Zn²⁺, and 25 and 100 for SO₄²⁻, respectively. As can be seen from Table 1, the tolerance limits for Pb(II) are higher than those of the Ag(I). Besides, the tolerance limits for Ag(I) have a negligible effect on its D-SPμE in real samples. Hence, it can be concluded that the CuMn₂O₄ nanoflakes have a satisfactory selectivity for the Ag(I) and especially Pb(II) ion adsorption, which is used for the D-SPμE of analyte ions in water and spice samples.

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3.7 Adsorption capacity of CuMn₂O₄ nanoflakes

CuMn₂O₄ nanoflakes' adsorption capacity was examined by using the solutions of 20 mL, including the 250 mg L⁻¹ Ag(I) or 250 mg L⁻¹ Pb(II) ions. The optimized D-SPμE, pH 2.0, extraction without vortexing and elution time of 30 s, 2 mL of 2 mol L⁻¹ HCl elution was applied to these solutions. After centrifuging, the upper liquid was diluted 10-fold for Ag(I) and 10-fold for Pb(II). The adsorption capacity (q_e) equation is as follows:

qe = V(Co − Ce)/W	(1)

where q_e is the adsorption capacity in mg g⁻¹, C_o is the initial concentration of Ag(I) and Pb(II) in mg L⁻¹, C_e is the equilibrium concentration in mg L⁻¹, V is the volume in L, and W the adsorbent weight (g). The adsorption capacities of CuMn₂O₄ nanoflakes were found to be 42.4 ± 0.3 mg g⁻¹ for Ag(I) and 42.7 ± 0.3 mg g⁻¹ Pb(II).

3.8 Reusability of the adsorbent

The regeneration and stability of CuMn₂O₄ nanoflakes are the key factors in terms of adsorbent performance.⁴ In the current work, 100 mg CuMn₂O₄ nanoflakes used in the D-SPμE procedure were rinsed with 5 mL U-HPW after each use. To evaluate the re-usability of CuMn₂O₄ nanoflakes, the Ag(I) and Pb(II) recoveries were used. The stability of the adsorbent was investigated by testing the decrease in the Ag(I) and Pb(II) recoveries by performing multiple adsorption–elution cycles at pH 2.0, no vortexing extraction/30 s elution, and 2 mL of 2 mol L⁻¹ HCl elution conditions. The results revealed that the CuMn₂O₄ nanoflakes can be reused 20 times with 90% ± 3 for Ag(I) and 93% ± 3 for Pb(II) (average recovery ± SD). In subsequent experiments, a decrement in the Ag(I) and Pb(II) recoveries was observed because of the decreased adsorbent amount owing to the dissolution after multiple elutions with 2 mol L⁻¹ HCl.

3.9 Analytical figures of merit

The limit of detection (LOD) for analytes of the D-SPμE method was described as 3SD/b. The D-SPμE was applied for twenty blank solutions of 30 mL for Ag(I) and 45 mL for Pb(II) (optimized conditions: pH 2.0, no vortexing extraction/30 s elution, eluent: 2 mL of 2 mol L⁻¹ HCl). It was 1.7 μg L⁻¹ for Ag(I) and 2.1 μg L⁻¹ for Pb(II) with PFs of 15 and 22.5, respectively. The calibration curves with eight standards for Ag(I) and seven standards for Pb(II) revealed good linearity. The linear ranges for analytes with the D-SPμE were achieved in the concentration range of 6.67–333 μg L⁻¹ for Ag(I) and 22–444 μg L⁻¹ for Pb(II) with determination coefficients (R²) of 0.9989 for Ag(I) and Pb(II). The linear ranges for analytes without the D-SPμE were obtained in the range of 0.10–5.0 mg L⁻¹ with R² = 0.9995 for Ag(I) and 0.50–10 mg L⁻¹ with R² = 0.9998 for Pb(II). The intra- and inter-day precisions (as RSD%) were determined by analysis of 20 mL solutions including 0.1 mg L⁻¹ Ag(I) and 0.4 mg L⁻¹ Pb(II) with seven experiments on the same day for intra-day and three different days for inter-day RSD%. The intra- and inter-day RSDs were 2.0% and 2.7% for Ag(I) and 2.2% and 3.1% for Pb(II), respectively.

3.10 CRM, water, and spice analyses

The reliability of D-SPμE results was assessed by analysis of BCR-482, SPS-WW1 Batch 121 and NW-TMDA-54.6 and by recovery studies in water and spices. The Ag and Pb concentrations achieved using the D-SPμE agreed with the CRM results (Table 2). Different sample matrices, tap water, wastewater, well water, and river water, and also red pepper, black pepper, cinnamon, and cumin purchased both from a herbalist and supermarkets, were investigated to evaluate the D-SPμE's accuracy and applicability. The analysis results with recovery values for the spiked water and spices are given in Tables 3 and 4, respectively. The percentage recoveries for the analytes changed between 88 and 102% for water and 89 and 102% for spices. The Student's t-test was applied to compare data for the same type of spice purchased from a supermarket and herbalist. The t_experimental for the three replicates of the spices is lower than their t_critical (t_0.05,₄ = 2.78) values. There is no significant difference between the spice data for p = 0.05, except for silver in red pepper. Silver concentrations in the black pepper were found to be lower than its limit of quantification. The concentrations of spices changed from 0.02 μg g⁻¹ to 0.37 μg g⁻¹ for Ag and from 1.17 μg g⁻¹ to 2.17 μg g⁻¹ for Pb. The red pepper for Ag and cumin for Pb had the highest concentrations. The results reveal that the D-SPμE is selective (especially for lead), accurate, reliable and sensitive to determine silver and lead in water and spice matrices.

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3.11 Comparison with other D-SPμE methods

A comparison table for the D-SPμE of Ag(I) and Pb(II) is given in Table 5. The D-SPμE work done with Ag(I) is very few. The main superiority of the described method is to reach equilibrium very quickly without vortexing adsorption and 30 s elution. Besides the D-SPμE has higher re-usability (20 times), comparable or higher adsorption capacity (except for ref. 6 and 27), better precision (except for ref. 28, 33 and 34) and more acidic working pH. For Pb(II), it has higher tolerable Na⁺, K⁺, Ca²⁺, and Mg²⁺ concentrations. Hence, the method's applicability for various water and spice matrices is rather high. Moreover, the CuMn₂O₄ nanoflakes are prepared easily and have low toxicity. It revealed lower LOD and wider or comparable linear ranges to those of works done using FAAS (except for ref. 29 and 30). Because of these reasons, the CuMn₂O₄ nanoflakes are preferred for the D-SPμE of silver and lead ions.

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4 Conclusions

In this work, we reported CuMn₂O₄ nanoflakes as an adsorbent for the D-SPμE of Ag(I) and Pb(II) ions in various water, black pepper, red pepper, cinnamon and cumin samples purchased from both a herbalist and supermarkets. The Student's t-test shows that there is no significant difference between the results for p = 0.05, except for silver in red pepper. The D-SPμE using 100 mg adsorbent was optimized at pH 2.0, with short contact times (without vortexing adsorption and 30 s for desorption) and 2 mL of HCl (2.0 mol L⁻¹) eluent. Selectivity of the adsorbent (especially for lead), re-usability (20), low cost, low toxicity, and simple synthesis are important advantages of this work. Also, the chelating reagent was not used. The developed D-SPμE method can be used as a cheap, simple, fast, selective, sensitive, and accurate method. Finally, CuMn₂O₄ nanoflakes can be preferred as a good adsorbent for D-SPμE of silver and lead ions in spices and water.

Data availability

The data supporting this article have been included in the article.

Author contributions

Şerife Tokalıoğlu: writing – review & editing, writing – original draft, visualization, supervision, validation, conceptualization, project administration, investigation, methodology, funding acquisition. Samet Pekşin: validation, investigation, methodology. Yakup Yılmaz: writing – review & editing, investigation, methodology, visualization. Şaban Patat: writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by a grant from the Scientific Research Project Unit of Erciyes University, Türkiye (Grant No. FYL-2024-13747). We would like to thank the Proofreading & Editing Office of the Dean for Research at Erciyes University for the copyediting and proofreading service for this manuscript.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ay00883b

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