Fe3O4/hydroxyapatite/graphene quantum dots as a novel nano-sorbent for preconcentration of copper residue in Thai food ingredients: Optimization of ultrasound-assisted magnetic solid phase extraction
Graphical abstract
Introduction
The well known “Tom Yum Kung” is a traditional Thai soup that is generally cooked with giant shrimp (Fig. S1). It is one of the most famous and popular dishes not only in Thailand but also worldwide in Thai restaurants, which have made it well known among foreigners who have experimented with Thai foods. This soup is characterized by its distinct hot and sour flavors with fragrant herbs generously used. To enhance its flavoring appeal, savory complements of various Thai herbs such as chili pepper, tomato, shallot, lemon grass, kaffir lime leaves, galangal, coriander, leek, stinkweed, and lime are combined. The soup is not only delicious but also nutritious. The different herbs and spices used have several health benefits, as they possess antioxidant, antimicrobial, antibacterial antidiabetic, and anticancer properties [1], [2].
At a global level, environmental pollution due to heavy metals has become a major concern. Metals are natural occurring chemical compounds. They can be present at various levels in the environment, e.g. soil, water and atmosphere. Metals can also occur as residues in food because of their presence in the environment, as a result of human activities such as farming, industry or car exhausts or from contamination during food processing and storage. People can be exposed to these metals from the environment or by ingesting contaminated food or water. Their accumulation in the body can lead to harmful effects over time, depending on the dose and toxicity. Among the heavy metals, copper is an essential micronutrient that is indispensable for life as it is involved in various biological processes [3]. It plays a central role in human metabolism. Most of the copper in the body is tightly bound with different metalloproteins. Furthermore, many copper enzymes have been characterized to have a variety of cellular and extracellular activities, including those in immune reactions, neural function, bone metabolism, hematopoietic system, antioxidant function, and energy production [4]. Moreover, copper deficiency can possibly increase the incident risk of coronary heart diseases [5]. However, copper is one of the most harmful pollutants in the environment as it is highly toxic and non-degradable; it also has a high bioaccumulation rate [6]. High intake of copper in the blood system may generate reactive oxygen species and damage the DNA, lipids and protein as well as affect the kidney, pancreas, and liver [7]. In addition, it causes liver damage in infants [8]. Furthermore, patients suffering from Wilson’s disease must carefully control their copper intake through water and food [9]. With the increasing use of new food production technologies, the possibility of food contamination with various environmental pollutants, particularly heavy metals, has also increased [10]. Therefore, the internationally accepted maximum level of copper is set at 30 mg kg−1 to safeguard public health [7]. It is thus extremely essential to analyze and measure copper in food samples. Modern instrumental techniques, such as flame [11], [12], [13] and/or graphite furnace atomic absorption spectrometry [14], inductively coupled plasma-optical emission spectrometry [15], [16], inductively coupled plasma-mass spectrometry [17], electrochemical method [18], and X-ray absorption spectrometry [19], have been used to accurately and precisely determine heavy metal ions at trace levels in food, water, and environmental samples.
Ultrasound-assisted extraction (UAE) is another popular technique that has several advantages (e.g., expeditious, inexpensive, and environmental friendly) over other conventional techniques [20], [21], [22]. It has been increasingly used for the primary extraction of target compounds from complex matrices. UAE is widely used for analytical sample preparations because it requires much lower extraction solvent volume, reduces extraction time, enhances the extraction efficiency through disruption of cell walls, reduces particle size, and enhances mass transfer of the cell contents as a result of cavitation [23], [24], [25], [26]. This technique has been proven to be a very useful tool in intensifying the mass transfer process and breaking the affinity between adsorbate and adsorbent [27].
To remove the interference or sample matrix and increase the analyte concentration, pre-concentration techniques such as ionic liquid extraction [28], ion exchange [29], [30], liquid–liquid extraction [31], [32], cloud point extraction [33], [34], co-precipitation [35], and solid-phase extraction (SPE) [36], [37], [38], [39], [40], [41] have been used. SPE has several major advantages including operation simplicity, minimum eluent volume, shorter extraction time, high pre-concentration factor and reduction of disposal cost [42], [43]. In addition, the availability of a wide variety of sorbent materials primarily affects the extraction efficiency. Therefore, SPE has become a well-established sample preparation method to extract and pre-concentrate the desired components. Currently, the application of iron oxide (such as Fe3O4)-based nanomaterials or magnetic nanoparticles (NPs) in SPE (MSPE) simplifies sample pre-treatment and overcomes some limitations of conventional SPE [44]. The sorbent does not need to be packed into the cartridge and the phase separation can be easily realized by applying an external magnetic field. NPs possess large surface area, high adsorption capacity, and rapid adsorption rate. Therefore, to extract analytes from the large volumes of samples, low amounts of sorbent and short equilibrium time are required [45]. In recent years, this novel solid material has become increasingly important owing to its special properties. One of its special properties is that most of the atoms are on the surface of the NPs. The surface atoms are unsaturated, and therefore, they can bind with other atoms that feature high chemical reactivity. Subsequently, the NPs can adsorb metal ions with a great adsorption speed [46]. Among various NPs, hydroxyapatite (HAP) and graphene quantum dots (GQDs) are of particular interest due to their wide applications. HAP (Ca10(PO4)6(OH)2), the main inorganic calcium phosphate mineral component of human bones and teeth, has been used extensively for medical and dental applications [47]. It shows excellent potential in heavy metals treatment because of their high specific surface area, abundant hydroxyl groups, low water solubility, high stability under reducing and oxidizing conditions, good dispersibility, low cost, environmental friendly and simple synthesis method with calcium hydroxide or nitrate as precursors. The reports on the use of hydroxyapatite for the capture of several heavy metals, such as Cr, Cu, Pb, Cd, Zn, Co, V, Ni and Sb [48]. GQDs are graphene sheets with lateral size smaller than 100 nm in single, double and multiple layers, and diameters spanning the range 3–20 nm mainly. These materials possess special properties including low toxicity, high biocompatibility, high fluorescent activity, robust chemical inertness and excellent photostability. The presence of oxygen-containing (carbonyl, epoxy, hydroxyl, carboxyl) functional groups at the edge of GQDs. GODs had great interaction ability toward metal ion such as Hg2+, Fe2+, Cu2+, Ni2+, Mn2+ and Co2+ [49]. It can be assumed that using these types of adsorbent decorated magnetite Fe3O4 NPs in MSPE with ultrasound as agitator can improve the adsorption performance of copper and reduce the analysis time through the rapid isolation of MNPs with a strong magnet from large volumes of the sample solution. Ultrasonic technique was also applied for ions determination because of improving the mass transfer in the adsorption and desorption processes [13], [40].
In this regard, the preparation, characterization, and application of Fe3O4/HAP/GQDs magnetic nanocomposite as a novel adsorbent for an ultrasound-assisted magnetic solid-phase extraction (UA-MSPE) have been described. Fe3O4/HAP/GQDs was synthesized with a simple co-precipitation method. The resultant nanocomposite was characterized by Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDX), and vibrating sample magnetometry (VSM) techniques. Subsequently, the as-prepared nanocomposite was used for the pre-concentration and determination of copper in Thai food ingredients using inductively coupled plasma-atomic emission spectrometry (ICP-AES). The green approach, i.e., UAE, was used as an efficient sample pre-treatment method, followed by UA-MSPE and ICP-AES.
Section snippets
Chemicals and reagents
All the chemicals and reagents were of analytical grade. Copper(II) chloride, calcium nitrate tetrahydrate, sodium phosphate monobasic monohydrate, sodium phosphate dibasic anhydrous, and sodium acetate were purchased from Sigma-Aldrich (USA). Citric acid was purchased from Carlo Erba (Italy). Sodium hydroxide, paraffin oil, ammonium hydroxide, ferric chloride hexahydrate and iron(II) sulphate heptahydrate were obtained from QRec™ (New Zealand). Acetic acid, phosphoric acid, hydrochloric acid,
Characterization of Fe3O4/HAP/GQDs
Fig. 1 shows the TEM and SEM images of Fe3O4, Fe3O4/HAP and Fe3O4/HAP/GQDs. The particle size of the materials was analyzed using ImageJ software, and expressed as mean ± standard deviation (SD). Uniform and consistent spherical shapes were obtained with a mean diameter of approximately 8.4 ± 3.2 nm (Fe3O4), 10.6 ± 3.9 nm (Fe3O4/HAP), and 15.8 ± 4.4 nm (Fe3O4/HAP/GQDs). They were aggregated with many NPs, resulting in a rough surface.
Furthermore, to evaluate the relative quantity of the main elements of
Conclusion
A new reusable robust Fe3O4/HAP/GQDs NPs can be prepared by rapid, simple, and inexpensive synthetic method. The Fe3O4/HAP/GQDs NPs has a number of advantages, such as high surface area, easy elution of analytes and complete regeneration of adsorbent using deionized water treatment. The magnetic separation greatly improved the separation rate while avoiding the time-consuming column passing or filtration operation. Both UAE and UA-MSPE operation are convenient, effective, and rapid for the
Conflicts of interest
The authors have declared no conflict of interest.
Acknowledgements
The authors thank the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission, through the Food and Functional Food Research Cluster of Khon Kaen University, Materials Chemistry Research Center, Department of Chemistry, Center of Excellence for Innovation in Chemistry (PERCH-CIC), Thailand for financial support of this study.
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