Graphene-Oxide-Derived Nanomaterials for the Extraction of Metals

Graphene-Oxide-Derived Nanomaterials for the Extraction of Metals: History

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Graphene oxide is a compound with a form similar to graphene, composed of carbon atoms in a sp2 single-atom layer of a hybrid connection. Due to its significant surface area and its good mechanical and thermal stability, graphene oxide has a plethora of applications in various scientific fields including heterogenous catalysis, gas storage, environmental remediation, etc. In analytical chemistry, graphene oxide has been successfully employed for the extraction and preconcentration of organic compounds, metal ions, and proteins.

graphene oxide
sample preparation
metal ions
food samples
environmental samples
biological samples
agricultural samples

1. Introduction

Among environmental pollutants, metals are of high importance due to their potential toxic effects and tendency to bioaccumulate in aquatic ecosystems. In small quantities, certain metals like iron, copper, cobalt, manganese, and zinc are nutritionally essential for a healthy life, while other metals such as mercury, arsenic, chromium, cadmium, and lead have no known role in biological systems and they are considered toxic and extremely dangerous even at trace levels ^[1]^[2]^[3]. The instrumental spectroscopic techniques commonly used for the determination of metals are flame atomic absorption spectroscopy (FAAS) ^[4], cold vapor atomic absorption spectroscopy (CVAAS) ^[5], electrothermal atomic absorption spectroscopy (ETAAS) ^[6], inductively coupled plasma optical emission spectrometry (ICP-OES) ^[7], and inductively coupled plasma mass spectrometry (ICP-MS) ^[8]. Metal ions are present in environmental samples at ultra-trace concentrations and, due to the existence of potential interferences and the complexity of matrices, the implementation of an extraction and preconcentration technique is required for their efficient determination ^[9]^[10].

A wide variety of novel sorbents including graphene, graphene oxide, carbon nanotubes, metal–organic frameworks, covalent organic frameworks, and zeolitic imidazole frameworks have been employed for the extraction of metal ions ^[11]^[12]^[13]^[14]^[15]^[16]. Those materials have been utilized in a wide range of analytical sample preparation techniques including dispersive solid phase extraction (d-SPE) ^[17], magnetic solid-phase extraction (MSPE) ^[18], pipette tip solid-phase extraction (PT-SPE) ^[19], solid phase microextraction (SPME) ^[20], stir bar sorptive extraction (SBSE) ^[21], etc.

Graphene is a non-polar hydrophobic carbon-based nanomaterial that was discovered by Geim et al. in 2004 ^[22]. Since then, graphene has attracted a lot of attention due to its extraordinary mechanical, thermal, structural, and electronic properties, as well as its high specific surface area. It consists of a single layer of carbon atoms densely packed in a honeycomb crystal lattice that forms graphite sheets. Applications of graphene include preparation of nanocomposites, heterogenous catalysis, drug delivery, gas storage, molecular probing, and electrochemical sensors ^[23]^[24]^[25]^[26]^[27]^[28]^[29]^[30]. However, graphene is insoluble and hard to disperse in most solvents because of strong intermolecular van der Waals interactions ^[31].

Graphene oxide (GO) is the oxidized form of graphene, which can be obtained from natural graphite powder through oxidation with an anhydrous mixture of sulfuric acid, sodium nitrate, and potassium permanganate ^[32]. Graphene and graphene oxide show a similar structure, which is composed of carbon atoms in sp² hybridization linked within a single-atom layer ^[33]^[34]^[35]. GO is of more polar and hydrophilic character than graphene, since it contains a large number of oxygen-containing groups including hydroxyl, carboxyl, and epoxy groups. Graphene oxide sheets are negatively charged in aqueous solutions due to the ionization of carboxylic groups and since they contain oxygen atoms with a lone pair of electrons, they are ideal sorbents to bind metal ions both through ionic and coordinative interaction. The adsorbed metal ions can be subsequently eluted with the addition of acid with the H⁺ competing for the binding site ^[10]^[35]^[36].

Due to the two-dimensional plane structure of GO, the material has a high sorption capacity and is an excellent sorbent for solid-phase extraction. A limitation of graphene oxide is the significant π–π stacking interactions between the GO nanosheets, which lead to aggregation and restacking of the nanosheets. As a result, some active adsorption sites of the adsorbent are blocked and thus its specific surface area is reduced. Functionalization of graphene oxide can take place in order to prevent aggregation, improve its behavior in aqueous solutions, and enhance its selectivity towards the target metal ion and/or the extraction efficiency of the sorbent. Moreover, GO can form magnetic nanocomposites with Fe₃O₄ nanoparticles through electrostatic interactions between the negatively charged GO nanosheets and the positively charged surface of magnetite. The magnetic nanocomposites combine the high adsorption efficiency of GO and the convenience of magnetic separation of Fe₃O₄ nanoparticles ^[37]^[38]^[39]^[40]^[41]^[42]^[43]^[44].

2. Synthesis of Graphene-Oxide-Derived Materials

Hummers′ method with or without modification is the most common synthetic route for the preparation of graphene oxide. Typically, graphite is dispersed in sulfuric acid and the dispersion is stirred. Subsequently, potassium permanganate is added dropwise to prevent a temperature rise. The resulting brownish slurry is diluted in water and final addition of hydrogen peroxide takes place. The mixture is centrifuged and washed with hydrochloric acid and water. Finally, filtration and freeze-drying of the obtained GO material take place ^[32].

Magnetic graphene oxide (GO/Fe₃O₄) can be produced by various synthetic routes. The one-step chemical co-precipitation approach is the most common approach for the preparation of magnetic graphene oxide. In this case, graphene oxide is dispersed in water, salts of Fe²⁺ and Fe³⁺ are added in appropriate concentrations, and the mixture is heated under reflux. Subsequently, ammonia is added dropwise to precipitate the ferric and ferrous ions. Graphene oxide can be also prepared with the solvothermal approach or by subjecting a mixture of GO and Fe₃O₄ to stirring, mechanical shaking, or ultrasonic treatment ^[45]^[46]^[47]^[48].

Reduced graphene oxide (RGO) is a nanomaterial obtained by chemical reduction of graphene oxide. RGO contains fewer oxygen groups than GO and reduction leads to an increase in the porosity as a result of exfoliation and rearrangement of layers ^[49]. RGO can be easily synthesized by dispersing GO in water and using hydrazine hydrate as a reductant. The reduction is normally carried out under stirring and heating. Magnetic RGO can be also prepared by approaches similar to those for the preparation of GO/Fe₃O₄, such as chemical co-precipitation, solvothermal, or hydrothermal approaches. In this way, the benefits of RGO and magnetic nanoparticles are combined to prepare highly efficient sorbents. Other reduction methods including electrochemical reduction, thermal reduction, microwave and photo reduction, photo-catalyst reduction, and reduction with green chemicals (e.g., ascorbic acid) can also be implemented for the preparation of RGO from GO ^[50]^[51]^[52].

Functionalization of graphene oxide can be employed in order to increase its potential applications. The functional modification of graphene oxide can not only maintain its excellent properties, but it also introduces new functional groups able to provide new characteristic to the sorbent. For this purpose, multifunctional organic materials such as polymers, nanoparticles, organic compounds, and multidentate chelating ligands have been examined. Various synthetic routes and various functional groups have been employed for the functionalization of GO and GO/Fe₃O₄ nanoparticles.

The selection of the functional group is based on the scope of the application (i.e., extraction of a metal ion, extraction of a complex compound of a metal ion, etc.), since different functional groups result in different characteristics. The modification of GO (e.g., with organic functional groups, such as amino, carboxyl, and mercapto groups) aims to enhance the selectivity of the sorbent towards the target analyte, the sensitivity of the determination, and the overall performance of the extraction procedure. Functionalization methods for graphene oxide mainly include covalent functionalization, non-covalent functionalization, and elemental doping.

Covalent functionalization involves combining graphene oxide with functional groups by forming covalent bonds in order to improve processability and introduce new functions to the sorbent. Covalent functionalization can be mainly divided into carbon-skeleton functionalization, hydroxyl functionalization, and carboxyl functionalization. On the other hand, non-covalent functionalization is based on π–π bond interaction, hydrogen bond interaction, ion interaction, and electrostatic interaction. In this approach, the structure and excellent properties of graphene oxide are maintained, while its dispersibility and stability are improved. Finally, element doping modifications are performed to incorporate different elements into the sorbent and thus enhance the overall performance of the material ^[53]^[54].

3. Extraction of Metal Ions with Graphene-Oxide-Derived Materials

The applications of graphene-oxide-derived materials for the extraction of metal ions are summarized in Table 1.

Table 1. Application of graphene-oxide-derived materials for the extraction of metal ions.

Analyte	Sample Matrix	Sorbent	Functional Groups	Analytical Technique ¹	LODs (µg L⁻¹)	Adsorption Time (min)/ Desorption Time (min)	Recovery (%)	Adsorption Capacity (mg g⁻¹)	Reusability	Ref.
Hg(II)	Seafood	GO/Fe₃O₄	Polythiophene	FI-CVAAS	0.03	21/2	85	1		^[55]
	Fish, rice, tea, milk	GO/Fe₃O₄	2-Pyridinecarboxaldehyde	ICP-OES	0.008	3/4	97	NA		^[56]
	Water	GO/Fe₃O₄	Chitosan, Mercaptopropyltrimethoxysilane	CVAAS	0.06	10/10	>95	>400		^[57]
Cr(VI) & Cr(III) species	Water	GO/Fe₃O₄		FAAS	0.1	>5 min/3	97–103	60	At least 10 times	^[58]
	Water	GO/Fe₃O₄	Triethylenetetramine	FAAS	1.4–1.6	30/-	>96	9.6–16.4		^[59]
	Water	GO/Fe₃O₄	Imidazolium, thioamine	GFAAS	1.2 × 10⁻³	9/16.5	>95	304 (total)		^[60]
Cr(VI)	Water	GO/Fe₃O₄	Polyaniline	GFAAS	5 × 10⁻³	20/4	68	14.8		^[61]
Cd(II)	Water, rice	GO/Fe₃O₄		FAAS	0.21 × 10⁻³	2/1	>95	11.1		^[62]
Au(II)	Water	GO/Fe₃O₄		FI-FAAS	4 × 10⁻³	Rapid/40 s	98–102	9.8	At least 10 times	^[63]
Au(II)	Water	GO/Fe₃O₄		MP-AES	5 × 10⁻³	10/5	97–101	192.1	Up to 20 times	^[64]
Co(II)	Water, food, biological samples	GO/Fe₃O₄		ETAAS	0.02	½	70–106	60		^[65]
Zn(II)	Water, food	GO/Fe₃O₄	Polythionine	FAAS	0.08	7/-	>87		At least 5 times	^[66]
Zn(II)	Water, food	GO/Fe₃O₄	Chitosan, Zn-imprinted polymer	FAAS	0.09	10/5	>96	71.4	At least 9 times	^[67]
Cu(II)	Eggplant, red lentil and mushroom	GO/Fe₃O₄	1,6-Hexadiamine	FAAS	0.9	10/2	>97		Up to 5 times	^[68]
Pb(II)	Water, food	GO/Fe₃O₄	4-(2-pyridylazo)resorcinol	ETAAS	0.18 × 10⁻³	-/3	>98	133		^[69]
Pb(II)	Water, food	GO	Polystyrene	FAAS	2.5	Not applicable	>99	227.9	Up to 50 times	^[70]
Tl(III)	Water	GO/Fe₃O₄	4-methyl-2(2-pyrazinyl)-1,3-thiazole-5-carboxy acid	GFAAS	12 × 10⁻³	8/3	65	20.0		^[71]
Ce(III)	Water	RGO/Fe₃O₄	Thioglycolic-acid-capped Cadmium–tellurium quantum dots	ICP-OES	0.1	10/6	>96	56.8	At least 12 times	^[72]
Sa(III)	Water	GO/Fe₃O₄	10-phenanthroline-2,9-dicarboxilic acid	ICP-OES	1.4	20/12	>97			^[73]

¹ FI-CVAAS: Flow-injection cold vapor atomic absorption spectrometry, ICP-OES: Inductively coupled plasma optical emission spectrometry, CVAAS: Cold vapor atomic absorption spectrometry, FAAS: Flame atomic absorption spectroscopy, GFAAS: graphite furnace atomic absorption spectrometry, FI-FAAS: Flow-injection flame atomic absorption spectroscopy, MP-AES: Microwave plasma-atomic emission spectrometry, ETAAS: Electrothermal atomic absorption spectroscopy.

4. Multielement Extraction with Graphene-Oxide-Derived Materials

The applications of graphene-oxide-derived materials for multielement extraction are summarized in Table 2.

Table 2. Applications of graphene-oxide-derived materials for multielement extraction.

Analytes	Sample Matrix	Sorbent	Modification	Analytical Technique ¹	LODs (µg L⁻¹)	Adsorption Time (min)/ Desorption Time (min)	Recovery (%)	Adsorption Capacity (mg g⁻¹)	Reusability	Ref.
Co(II), Ni(II), Cu(II), Zn(II), Pb(II)	Water	GO		ICP-OES	0.5–1.8	5/-	94–106	294–1119		^[74]
Cr(III), Co(II), Ni(II), Cu(II), Zn(II), Pb(II)	Water	GO		EDXRF	0.07–0.25	15/-	94–104			^[75]
Cr(III), Cd(II), Pb(II)	Water, saliva, urine	GO		ETAAS	5-12 × 10⁻³	Few seconds/-	94–103			^[76]
Co(II), Ni(II)	Water, black tea, tomato	GO		FAAS	0.18–0.25	Not applicable	>95	6.8–7		^[77]
Cu(II), Pb(II)	Water	GO	SiO₂	FAAS	0.08–0.27	Not applicable	>95	6.0–13.6	At least 50 times	^[78]
Mn(II), Co(II), Ni(II), Cu(II), Cd(II), Pb(II)	Water	GO	Silica	ICP-MS	0.39–22 × 10⁻³	5/1	85–119	4.6–25	At least 50 times	^[79]
Co(II), Ni(II), Cu(II), Cd(II), Pb(II)	Plasma, Urine	GO/Fe₃O₄		ICP-MS	0.02–0.40	7/7	81–113	1.3–9.7	At least 20 times	^[80]
Cr(III), Pb(II)	Rice, milk, wine, water	GO/Fe₃O₄	Polyaniline–polypyrrole, SiO₂	ICP-MS	3.4–4.8 × 10⁻³	6.3/3.7	96–103	188.9–213.3	At least 6 times	^[81]
Cu(II), Pb(II), Zn(II), Cr(III), Cd(II)	Water, agricultural samples	GO/Fe₃O₄	Polypyrrole–polythiophene, SiO₂	FAAS	0.15–0.65	6.5/12	90–106	80–230	At least 5 times	^[82]
Pb(II), Cd(II), Cu(II), Ni(II), Co(II)	Water, food samples	GO/Fe₃O₄	Poly(vinylacetate-co-divinylbenzene)	FAAS	0.37–2.39	-/-	>95			^[83]
Fe(III), Co(II), Ni(II), Cu(II), Zn(II),Pb(II)	Water	GO	Ethylene diamine	EDXRF	0.06–0.1	5/-	>90			^[84]
Cd(II), Pb(II)	Water, vegetables	GO/Fe₃O₄	Diethylenetriamine (DETA)	FAAS	0.38–0.40	10/2	>99	59.9–172.4		^[85]
Cr(III), Cu(II), Zn(II), Pb(II)	Water	GO/Fe₃O₄	Mercapto-groups	EDXRF	0.06–0.10	10/-	>95	191.5–487.3		^[86]
Au(III), Pd(II), Ag(I)	Water, ore and automobile catalyst	Magnetic GO	2-mercaptobenzothiazole	ICP-OES	0.05–0.08	10/3	90–103	28–45	At least 5 times	^[87]
Cd(II), Cu(II), Pb(II)	Water, vegetables	GO/Fe₃O₄	2-mercaptobenzothiazole	FAAS	0.19–0.35	4/5	>99	156–179		^[88]
Co(II), Ni(II), Cu(II), As(III), Cd(II), Pb(II)	Water	GO	(3-mercaptopropyl)-trimethoxysilane	TXRF	0.05–9.11	10/2	>94	18.1–108.3		^[89]
Pb(II), Cu(II)	Water	GO/Fe₃O₄	Trithiocyanuric acid	FAAS	0.13–0.32	Not applicable	>90	0.46–0.75		^[90]
Mn(II), Fe(III)	Water, food and biological samples	GO	3-(1-methyl-1H-pyrrol-2-yl)-1H-pyrazole-5-carboxylic acid	FAAS	0.31–355	Not applicable	>95	21.6–24.0		^[91]
Cr(III), Fe(III), Pb(II), Mn(II)	Wastewater	GO	Multi-walled carbon nanotubes -DETA	ICP-OES	0.16–0.50	Not applicable	>95	5.4–13.8		^[92]
Cd(II), Pb(II)	Vegetables, fish, lipstick	GO/Fe₃O₄	8-Hydroxyquinoline	FAAS	0.09–0.27	5/5	>96	133–150		^[93]
Cr(III), Zn(II), Cu(II)	Water	GO/Fe₃O₄	Glycine	EDXRF	0.07–0.15	10/-	>97			^[94]
Noble metals, Sb(III), Hg(II)	Seawater	GO/Fe₃O₄	1,5-bis(di-2-pyridyl)methylene thiocarbohydrazide	ICP-OES	0.05–2.60	Not applicable	90–106	4.5–9.7		^[95]
Cu (II), Pb(II), La(III), Ce(III), Eu(III), Dy(III), Yb(III)	Water	GO	TiO₂	ICP-OES	0.13–2.64	Not applicable	>90	0.8–13.5	At least 90 times	^[96]
REEs	Water	GO/Fe₃O₄	Polyaniline, SiO₂	ICP-MS	0.04–1.49 × 10⁻³	2/5	80–121	7.7–16.3	At least 30 times	^[97]
REEs	Nuts, water	Oxidized GO		ICP-MS	0.03–1.8	15/1	60–90	6.1–12.2	At least 12 times	^[8]

¹ ICP-OES: Inductively coupled plasma optical emission spectrometry, EDXRF: Energy-dispersive X-ray fluorescence spectrometry, ETAAS: Electrothermal atomic absorption spectroscopy, FAAS: Flame atomic absorption spectroscopy, ICP-MS: Inductively coupled plasma mass spectrometry, TXRF: Total-reflection X-ray fluorescence spectrometry.

Non-functionalized graphene oxide was employed for the dispersive solid-phase extraction of Co(II), Ni(II), Cu(II), Zn(II), and Pb(II) prior to their determination by energy-dispersive X-ray fluorescence spectrometry (EDXRF) ^[74]. Good recovery, reproducibility, and extraction recovery were obtained. Graphene oxide has been also employed for the d-SPE of Cr(III), Co(II), Ni(II), Cu(II), Zn(II), and Pb(II) as their complexes with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) using graphene oxide nanoparticles ^[75]. The chelation reagent does not form complexes with the alkali and alkaline earth metals, and it was therefore employed to enhance the selectivity of the extraction. In order to enhance the convenience of the d-SPE method, Ghazaghi et al. developed a coagulating homogenous extraction procedure based on coagulation of homogeneous GO solution with the aid of polyethyleneimine (PEI) ^[76]. In this work, PEI assisted the separation of the dispersed GO from the sample solution and provided satisfactory extraction recovery.

Solid-phase extraction of Co(II) and Ni(II) was performed by Pourjavid et al. using graphene oxide as adsorbent. In order to enhance the method’s selectivity, N-(5-methyl-2- hydroxyacetophenone)-N′-(2-hydroxyacetophenone) ethylene diamine (MHE) was used as a chelating agent and the complex of MHE and metal ions was extracted by graphene oxide in a SPE column ^[77].

Sitko et al. synthesized GO functionalized with spherical silica (GO@SiO₂), coupling the amino groups of spherical aminosilica and the carboxyl groups of GO. The GO@SiO₂ sorbent was packed into a SPE column and used for the extraction of Cu(II) and Pb(II) from water prior to their determination by FAAS ^[78]. Since small particles of GO can cause serious problems in SPE, such as high pressure in SPE system and the loss of adsorbent material, silica was covalently bonded with GO nanosheets to overcome these problems. Graphene-oxide–silica-composite-coated hollow fibers were synthesized and used for the online SPME of Mn(II), Co(II), Ni(II), Cu(II), Cd(II), and Pb(II) in environmental water samples prior to their determination by ICP-OES ^[79]. The novel fibers exhibited high adsorption capacity, reproducibility, and stability as well as long lifespan (more than 50 SPME cycles). Compared to the silica-coated hollow fiber, the GO–silica composite showed a different adsorption behavior, resulting in higher extraction efficiencies.

In order to combine the properties of GO with the ease in separation of magnetic nanoparticles, Sun et al. synthesized a magnetic graphene oxide nanocomposite with the one-step co-precipitation approach and used it for the MSPE of heavy metals from biological samples ^[80]. The GO/Fe₃O₄ sorbent was successfully employed for the extraction of Co(II), Ni(II), Cu(II), Cd(II), and Pb(II) from plasma and urine samples prior to their determination with ICP-MS.

Various polymers have been employed for the functionalization of GO, including polyaniline–polypyrrole (PANI-PPy) ^[81], polypyrrole–polythiophene (PPy-PTh) ^[82], and poly(vinylacetate-co-divinylbenzene) (DVB-VA) ^[83]. A polyaniline–polypyrrole-functionalized SiO₂-coated magnetic graphene oxide composite was prepared and used for the MSPE of chromium and lead ions at trace levels in food and environmental water samples. Both polyaniline and polypyrrole contain amine and imine groups that can serve as good sorption sites for metals from complex matrices. Adsorption of metal ions to those polymers is based on hydrogen bonding π–π interactions, as well as ion-exchange interactions and chemical sorption. As a result, the adsorption capacity and the selectivity towards the target analytes was enhanced after functionalization ^[81].

In 2017, Molaei et al. synthesized a SiO_2-coated magnetic GO modified with polypyrrole–polythiophene. For this purpose, magnetic GO was coated with SiO₂ and the composite was modified by a copolymer with an in situ simultaneous oxidative polymerization of pyrrole and thiophene monomers, using iron chloride as an oxidant and dopant. The presence of N- and S-containing moieties enhanced the extraction efficiency of the nanocomposite and the selectivity of the MSPE procedure. The novel sorbent was successfully employed for the extraction of trace amounts of heavy metals (copper, lead, chromium, zinc, and cadmium) from water and agricultural samples prior to FAAS determination ^[82].

5. Application of Ionic Liquids and Deep Eutectic Solvents for the Modification of GO

Ionic liquids are an alternative to environmentally harmful ordinary organic solvents that are gaining more and more popularity lately ^[98]. These materials are generally composed of bulky, nonsymmetrical organic cations including imidazolium, pyrrolidinium, pyridinium, ammonium, or phosphonium and different inorganic or organic anions such as tetrafluoroborate or bromide anions ^[99]. In the field of analytical chemistry, ionic liquids have been used as ionic-liquid-supported membranes, as additives in mobile phases, as surface-bonded stationary phases, and as extraction solvents for sample preparation ^[99]^[100]^[101]^[102]. ILs have a tunable nature and their properties can be optimized through the choice of their cationic and anionic constituents. Among their extraordinary chemical and physical properties are negligible vapor pressure, excellent thermal stability, tunable viscosity, good miscibility with organic solvents and water, and their good extraction efficiency of metal ions and organic compounds ^[98]^[103]. The combination of ionic liquids and graphene-oxide-derived materials makes it possible to design and develop new extraction adsorbent phases with outstanding properties ^[98]. The applications of ILs as modifiers of GO-based materials for the extraction of metal ions are summarized in Table 3.

Table 3. Applications of ILs for the modification of GO-based materials for the extraction of metal ions.

Analyte	Sample Matrix	Sorbent	Ionic Liquids	Analytical Technique ¹	LODs (µgL⁻¹)	Adsorption Time (min)/ Desorption Time (min)	Recovery (%)	Adsorption Capacity (mg g⁻¹)	Ref.
Cd(II)	River and seawater, carrot, lettuce and tobacco	GO/Fe₃O₄	1-ethyl-3-methylimidazolium tetrafluoroborate	FAAS	0.12	Few seconds/1 min	98–102	33.7	^[104]
Pb(II), Cd(II), Ni(II), Cu(II) and Cr(III)	Medicine capsules	GO/Fe₃O₄ modified with (3-mercaptopropyl)trimethoxysilane	1,4-diazabicyclo [2.2.2]octane	FAAS	0.2–1.8	4 min/1 min	95–102	18.1–47.6	^[105]
Ni(II)	Sea and river water, tea, spinach, cacao powder, cigarette	GO/Fe₃O₄	1-hexadecyl-3-methylimidazolium chloride	FAAS	0.16	15 min/2 min	97–99	129.9	^[106]
Al(III), Cr(III), Cu(II), Pb(II)	Environmental water	Fe₃O₄-SiO₂−GO	N-(3- Dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride	ICP-OES	0.5–30 × 10⁻³	5 min/3 min	89–118	5.0–11.7	^[107]
Cu(II), Zn(II), Cd(II), Cr(III), Pb(II) and Co(II)	Environmental water	GO/Fe₃O₄	1-butyl-3-methylimidazolium hexafluorophosphate	ICP-OES	0.1–1	10 min/6 min	34–94	312.5	^[108]

¹ FAAS: Flame atomic absorption spectroscopy, ICP-OES: Inductively coupled plasma optical emission spectrometry.

This entry is adapted from the peer-reviewed paper 10.3390/molecules25102411

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