Metallic nanoparticles containing valuable metals such as silver and gold exhibit unique optical, physical, chemical, and biological properties that are exploited in a wide range of applications (Hagarová, 2020). Their strong affinity for inorganic and organic mercury species can be utilized in extraction procedures for mercury binding. However, metallic nanoparticles containing noble metals can also be applied for the separation of other heavy metals (Li et al., 2020), organic species (pyrene, olefins, methylene blue) and gaseous species (NO, SO₂) (Dastafkan et al., 2015).

Commercially available silver nanoparticles (AgNPs) have been lately used as solid sorbent in an ultrasound-assisted DMSPE (UA-DMSPE) procedure for separation and pre-concentration of Hg²⁺ ions in real spring water, tap water and ground water samples (Krawczyk and Stanisz, 2015). In this procedure, the adsorption of mercury on AgNPs is followed by separation of the liquid phase from the solid phase. Liquid phase is then removed and solid phase is immediately dissolved in 7 M HNO₃. The mercury content is quantified by high-resolution continuum source electrothermal atomic absorption spectrometry (HR-CS ETAAS). The limit of detection (LOD) of this method has been calculated as 5 ng/L which demonstrates a great potential of the method in an ultratrace analysis of mercury ions in natural waters.

In a multistep process, Yordanova et al. (2014) prepared AgNPs supported on the surface of NH₂-functionalized silica submicrospheres (SiO₂/AgNPs) to be used as a sorbent for separation and pre-concentration of inorganic mercury in surface water samples during extraction procedure. This novel nanocomposite adsorbent was synthesized by a completely green procedure and was rigorously characterized by transmission electron microscopy (TEM), UV-Vis spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM).

Besides silver, utilization of gold in a form of nano-sized material in analytical chemistry has become a topic of interest during the last decades. The continued popularity of AuNPs can be attributed to their size- and shape- dependent optical and electrical properties and high electrocatalytic activity. Moreover, the surface chemistry of AuNPs is versatile, allowing the linking of various biofunctional groups (e.g., amphiphilic polymers, silanols, sugars, nucleic acids, proteins) through strong Au–S or Au–N bonding, or through physical adsorption (Lin et al., 2011). Such types of biomolecules can be adsorbed spontaneously onto gold surfaces generating well-organized, self-assembled monolayers (Khajeh et al., 2013), which favors these nanoparticles for application in procedures designed particularly for organic compounds separation.

Following the earlier work of Turkevich and Frens, Kiran (2015) synthesized AuNPs capped with 2-mercapto succinic acid in order to separate mercury from environmental water samples, vegetable, and crop samples collected near industrial areas. Nanoparticle structure and morphology was characterized by high-resolution transmission electron microscopy (HR-TEM) and scanning electron microscopy (SEM). The method was successfully applied to detect mercury up to ppb levels.

Palladium is considered the most valuable of the four major precious metals with extraordinary catalytic, powerful mechanical, and electroanalytical properties. Palladium nanoparticles (PdNPs) have been developed as self-therapeutics with proven anti-bacterial and cytotoxic pharmacological activity (Yaqoob et al., 2020). PdNPs have been applied as coprecipitation carriers for pre-concentration of Cu, Pb, and Cd in seawater and synthetic water samples prior to their measurement by atomic absorption spectrometry (AAS) (Zhuang et al., 1996). PdNPs have been found very useful in speciation studies conducted to determine Cr(VI) and Cr(III) in soil and aqueous media (Omole et al., 2007), and As(III) and As(V) in environmental water samples (Sounderajan et al., 2009).

The adsorption capacity of PdNPs as a DSPE sorbent for the separation and pre-concentration of selenium species in ground water has been evaluated by Kumar et al. (2015). The authors used sodium borohydrate for the reduction of Pd(II) to Pd(0) to obtain PdNPs. The same reducing agent was employed for the reduction of Se(IV) to Se(0). The reduced elemental selenium was then adsorbed on PdNPs surface. The PdNPs with adsorbed selenium were collected and dissolved in a minimum amount of nitric acid and the extracted selenium was quantified by electrothermal atomic absorption spectrometry (ETAAS). The adsorption capacity of PdNPs for Se(0) was 28 mg/g and the recoveries for Se(IV) and Se(VI) were in the range of 97–102%.

Zirconium is a transition metal with extreme resistance to oxidation and corrosion which makes it ideal as an alloying agent choice for materials that are frequently exposed to corrosive environments (such as high temperatures, water, steam, acids, organic solvents, salts, etc.). Besides application of this metal in processes encountered in chemical engineering, zirconium and its compounds have found diverse uses in modern analytical chemistry as summarized by Pechishcheva et al. (2018). Very recently, the first work on application of zirconium nanoparticles (ZrNPs) in extraction procedures has been conducted.

Tekin et al. (2020) established a novel, green, highly accurate, and precise ZrNPs-based solid phase extraction strategy for separation and pre-concentration of trace cadmium prior to its quantification by slotted quartz tube-flame atomic absorption (SQT-FAAS). ZrNPs proved to be an effective adsorbent for the removal of Cd from water, the recovery rates of 96–100% were achieved.

ZrNPs have also been a choice of Karlidag and his co-workers who used it as sorbent material for the adsorption of antimony (Karlidag et al., 2020a) and selenium (Karlidag et al., 2020b) from tea samples by vortex assisted ligandless dispersive solid phase extraction (VA-LDSPE) and dispersive solid phase extraction (DSPE) method. Analyte quantification was performed by SQT-FAAS. Using Zr-NPs-VA-LDSPE-SQT-FAAS method, the analyte was detectable at concentrations 180-fold lower than those detected by conventional method. The obtained recovery values ranged between 93 and 102% for Sb and 92 and 101% for Se.

Zirconium dioxide (ZrO₂), also known as zirconia, is a widely used inorganic material with exceptional biocompatibility, high mechanical and thermal stability, wear and corrosion resistance, chemical inertness and low systemic toxicity. However, in comparison with other metallic oxides used in extraction procedures for trace elements, it is considered less frequently used one. Yet, it has been successfully applied for the pre-concentration of Ti(IV) in water samples by Zheng et al. (2009) in a batch extraction procedure. Zirconium dioxide was in the form of ultrafine amorphous spherical powder prepared by precipitation in micro-droplets which act as nanoreactors in the microemulsion system composed of cyclohexane/water/TritonX-100/hexyl alcohol. The powder so obtained had particles substantially uniform in diameter and with low-aggregation behavior (Ma et al., 2004). The concentration of Ti(IV) was determined using a UV-1200 PC spectrophotometer. After optimization of experimental conditions, the authors achieved a limit of detection of 0.1 μg/L.

Titanium dioxide nanoparticles (TiO₂NPs) have already shown outstanding performances in photoelectrochemical and photocatalytic processes. They have also found their application in extraction procedures as solid sorbents, owing to their optimal physicochemical properties (e.g., high thermal and chemical stability), high specific surface area, high adsorption capacity, low cost, and low toxicity (Krawczyk and Stanisz, 2016). TiO₂NPs have been used for separation and pre-concentration of various trace elements in extraction procedures (Xu et al., 2018).

The method combining ultrasound-assisted DMSPE (UA-DMSPE) with cold vapor atomic absorption spectrometry (CVAAS), where TiO₂NPs were successfully used for separation and pre-concentration of total mercury and mercury species (Hg²⁺ and CH₃Hg⁺) in biological, geological, and water samples, has been presented by Krawczyk and Stanisz (2016). After extraction, a sorbent with entrapped analyte was separated from the bulk solution and mixed with 1 M HNO₃ to prepare a slurry that was then measured for mercury. However, it is known that detection limits that can be achieved using CVAAS after pre-concentration on TiO₂NPs are better than those obtained with conventional CVAAS.

In a study by Chen et al. (2020a), the fibrous TiO₂@g-C₃N₄ nanocomposites (FTGCNCs) were tested as a new adsorbent for separation of various antimony species [Sb(III), Sb(V), residual, digestible, and total Sb] in a cow milk by DMSPE prior to inductively coupled plasma mass spectrometry (ICP-MS) analysis. The results showed that Sb(III) was quantitatively adsorbed on FTGCNCs in the pH range of 2–4, while Sb(V) remained in an aqueous phase. Compared to other separation and detection methods, particularly solid-phase microextraction coupled to inductively coupled plasma mass spectrometry (SPME-ICP-MS), ion chromatography coupled to inductively coupled plasma mass spectrometry (IC-ICP-MS), magnetic solid phase extraction coupled to inductively coupled plasma mass spectrometry (MSPE-ICP-MS), high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (HPLC-ICP-MS), cloud point extraction combined with electrothermal vaporization inductively coupled plasma mass spectrometry (CPE-ETV-ICP-MS) and hydride generation inductively coupled plasma mass spectrometry (HG-ICP-MS), this approach allows for more accurate determination of sample composition and results in a lower LOD [0.37 pg/mL for Sb(III)] and higher enrichment factor (100).

The efficiency of FTGCNCs as sorbent material was supported in their following study (Chen et al., 2020b), where FTGCNCs were used for DMSPE of Co and Ni prior to their determination by ICP-MS. The apparent selectivity of this material for some metal ions can be attributed to a large number of –NH₂, –NH– and =N functional groups distributed on FTGCNCs' surface. For Co and Ni retention on FTGCNCs, the optimal pH ranges between 5 and 8.

Although graphitic carbon nitride (g-C₃N₄) shows great potential as a new sorbent in future sample pretreatment techniques, it has some issues from the DMSPE point of view. Pure g-C₃N₄ easily aggregates which results in a decrease of its specific surface area (Sun et al., 2016). The weak polarity of g-C₃N₄ causes its poor dispersion in water. Fortunately, these problems can be solved by the in-situ growth of g-C₃N₄ on the surface of TiO₂ nanofibers (TDNFs) to form the fibrous TiO₂@g-C₃N₄ nanocomposites (FTGCNCs).

The features such as large specific surface area, high porosity, low toxicity, and easy preparation have ensured great success of zinc oxide nanoparticles (ZnONPs) throughout the field of chemistry. In SPE procedures, ZnONPs have found application as effective nanosorbents. This material has been successfully used for efficient separation and pre-concentration of trace germanium in various food samples including tea leaves and mixed herbs by an ultrasound-assisted dispersive micro-solid phase extraction (UA-DMSPE) prior to its determination by high-resolution continuum source electrothermal atomic absorption spectrometry (HR-CS ETAAS) (Stanisz and Krawczyk-Coda, 2017).

Zinc oxide is a unique material with versatile industrial and commercial applications. However, there is a constant race toward the performance enhancement. Doping is one of the methods commonly used for nanoparticle modification in order to enhance their electrical, optical, physical, biological, and chemical properties. Doping can increase the surface area, reduce the mass, and alter nanoparticle morphology resulting in optimized functional properties of nanomaterials (Khan et al., 2016). Although the methodology of the following work is based on thermodynamic and kinetic principles, the general steps followed in dispersive extraction (e.g., addition of 25 mg of Ca-doped ZnONPs to the metal ion solution) classify this procedure as DMSPE. Khan et al. (2016) developed simple, sensitive and innovative Ca-doped ZnONPs extraction system for the recognition and elimination of Pb(II) ions in real water samples (drinking water, tap water, lake water, and seawater) based on a high selectivity of Ca-doped ZnONPs for Pb(II). Although this sorbent material was also applied for the extraction of Cd(II), Cu(II), Mn(II), Hg(II), Pd(II), La(III), and Y(III), it was found to be highly selective for Pb(II). For this metal ion, the extraction efficiency of 94–99% was achieved. The adsorption process for Pb(II) was characterized as monolayer adsorption with the adsorption capacity of 84.66 mg/g. Nanoparticle structure was studied by field emission scanning electron microscope (FESEM), X-ray microanalysis (EDS), X-ray diffraction (XRD), X-ray Photoelectron (XPS), Fourier transform infrared spectroscopy (FTIR), and UV-Vis spectrophotometry.

Aluminum oxide has become a typical sorbent material in SPE procedures. The high-performance of Al₂O₃ nanoparticles (Al₂O₃NPs) is attributed to a large specific surface area, high adsorption capacity, mechanical strength, high chemical activity, enhanced sorption kinetics, and low-temperature modification (Mohammadifar et al., 2015).

Hassanpoor et al. (2015) proposed a simple and efficient method for the extraction and speciation of trace quantities of arsenic in spiked real water, food and biological samples. For this purpose, Al₂O₃NPs were functionalized by a ligand containing donor atoms such as oxygen, nitrogen, and sulfur [3,3′-bis-(3-triethoxysilylpropyl)-2,2′-dithioxo [5,5′] bithiazolidinylidene-4,4′-dione]. Desorption of the analyte was done by 1 M HCl and final measurements were performed by ETAAS. The adsorption capacity of the sorbent was tested in multiple cycles of adsorption and desorption. The obtained results showed that functionalized Al₂O₃NPs could be reused up to 70 times without any considerable loss in adsorption efficiency.

The different modification of Al₂O₃NPs used for selenium speciation in surface water samples by DMSPE procedure has been presented in a work published by Nyaba et al. (2016). In this study, Al₂O₃NPs were functionalized with quaternary ammonium salt (Aliquat336) and characterized by FTIR. Desorption of the analyte was done by diluted HNO₃ and its quantification was performed by inductively coupled optical emission spectrometry (ICP-OES). The introduction of DMSPE prior to ICP-OES detection resulted in a significantly low LOD and LOQ (limit of quantification). The optimum pH for successful separation of inorganic Se species in sample solution was between 2 and 7. The Se(IV) recovery was >90%, however, the recovery of Se(VI) was only near 5% over most of the pH range.

Baranik et al. (2018a,b) obtained novel aluminum oxide/graphene oxide and aluminum oxide/nano-graphite sorbents for As(V), Cr(III), and Cr(VI) pre-concentration in natural waters by means of (ultrasound-assisted-) DMSPE. Graphene oxide (GO) was synthesized by improved Hummers' method (Marcano et al., 2010) out of high purity graphite, sodium nitrate, sulphuric acid, potassium permanganate, and hydrogen peroxide. Alumina supported on graphene oxide (Al₂O₃/GO) nanocomposite was prepared by a simple sono-thermo-chemical method involving dispersion of GO in a high purity water followed by addition of Al(NO₃)₃·9H₂O. After an ultrasonic bath treatment followed by drying at 100°C, the solid was heated at 500°C in a furnace to initiate the growth of Al₂O₃NPs on the GO surface. The aluminum oxide coated nano-graphite (Al₂O₃/nano-G) was synthesized out of Al(NO₃)₃·9H₂O, Triton X-100 and high purity graphene nanosheets in a sono-thermo-chemical process. Thus, prepared adsorbents were characterized by SEM, TEM, powder XRD, and Raman spectroscopy.

Al₂O₃/nano-G has exhibited high selectivity for Cr(III) in the presence of Cr(VI). Al₂O₃/GO has demonstrated selectivity toward arsenates in the presence of arsenites at pH 5 and chromium(III) ions in the presence of chromate anions at pH 6. Under optimized conditions, As(V) and Cr(III) ions could be determined with very good precision (RSD 2.7–4%), excellent LOD (0.02 ng/mL As and 0.11 ng/mL Cr) and recovery rates (92–108%). Compared to Al₂O₃/GO nanocomposite, Al₂O₃/nano-G has been characterized by better LOD values for Cr(III). Figure 2 illustrates the attraction/repulsion between positively charged metal ions and positively- and negatively-charged surface of a nanoparticle in a different pH environment.

Pytlakowska et al. (2020a,b) ran experiments on two variants of graphene, graphene oxide (GO) and reduced graphene oxide (rGO), coupled with molybdenum disulfide (MoS₂) in order to investigate the potential of the hybrid material for the adsorption of metal ions. MoS₂ was supported on GO by hydrothermal method. The resulting nanocomposite (MoS₂-rGO) was characterized by XPS, SEM and TEM. The material was successfully applied for chromium speciation in real lake, spring and river water samples and artificial sea water by UA-DMSPE method. At pH 2, MoS₂-rGO showed a strong affinity for Cr(VI) leaving Cr(III) ions in an aqueous solution. The strong affinity of a target analyte toward MoS₂-rGO NPs results from both electrostatic interaction and outsphere surface complexation. The adsorption capacity of 583.5 mg/g was nearly 6.5 times higher than that reported for untreated MoS₂. The adsorption process was predominantly ruled by chemisorption.

Reduced graphene oxide (rGO) decorated with molybdenum disulfide (MoS₂) was synthesized by hydrothermal method and studied by different spectroscopic techniques, such as FTIR, Raman spectroscopy and XPS. The detailed high resolution images of the hybrid material were provided by SEM and TEM. MoS₂-rGO has been evaluated as a solid nanoadsorbent for the pre-concentration of heavy metal ions in different water samples by DMSPE prior to energy dispersive X-ray fluorescence spectrometry (EDXRF) analysis. The maximum adsorption capacities were ~1.5–5-folds higher than those of unmodified GO; q_maxwere 235.1, 130.8, 279.3, 414.8, 483.7, 381.9 mg/g for Cr(III), Co(II), Ni(II), Cu(II), Zn(II), and Pb(II) ions, respectively. The introduction of ultrasound in adsorption process has resulted in excellent recovery rates (close to 100% after 10 min of sonication).