Food samples such as milk, beverages, meat and chicken products, fish, etc. are complex and demanding matrices. Various novel materials such as molecular imprinted polymers (MIPs), carbon-based nanomaterials carbon nanotubes, graphene oxide and metal-organic frameworks (MOFs) have been recently introduced in sample preparation to improve clean up as well as to achieve better recoveries, all complying with green analytical chemistry demands. Metal-organic frameworks are hybrid organic inorganic materials, which have been used for gas storage, separation, catalysis and drug delivery.
1. Introduction
Sample preparation is the most challenging step of the analytical procedure for the analysis of most samples. An appropriate sample preparation technique should not only be simple, fast and economical, but it should also be in regard with the main principles of green chemistry
[1][2]. Solid-phase extraction (SPE) is a well-established sample preparation technique; which however shows some fundamental disadvantages, such as including complicated and time-consuming steps, as well as requiring large amounts of sample and organic solvents. As a result, many novel techniques have been developed
[1][2][3][4][5]. Nowadays, a trend in analytical chemistry is to develop new sorbents either for the well-established SPE procedure or for the novel microextraction procedures, which have been gaining more and more attention
[1]. New sorbents such as molecular imprinted polymers (MIPs), carbon-based nanomaterials, carbon tubes, graphene based materials, or metal-organic frameworks (MOFs) are becoming more and more popular
[6][7][8]. Metal-organic frameworks are a new class of hybrid organic inorganic supramolecular materials, which are based on the coordination of metal ions or clusters with bi- or multidentate organic linkers
[9][10]. Metal-organic frameworks became popular in 1995, when Yangi and Li reported the synthesis of a metal-organic framework containing large rectangular channels
[11] What makes the use of MOF materials so promising is the fact that they bare great physical and chemical properties, such as their high surface areas (up to 10,000 m
2/g), in addition with their tunable pore size and functionality, and can act as hosts for a variety of guest molecules. Some of MOFs great properties are luminosity, flexibility of their structure, charge transfer ability from the ligand to the metal or from the metal to the ligand, thermal stability, properties that include electronic and conducting effects and pH-sensitive stability
[11][12].
For the synthesis of MOF materials many alternative ways have been proposed. The most famous method is the solvothermal method, which is normally performed in an autoclave with high temperature and pressure and with the use of an organic solvent at its boing point (typically dialkyl formamides, alcohols and pyridine)
[13]. Other synthetic methods that have been applied for MOF materials include microwave, electrochemical, mechanochemical, ultrasonic, high-throughput syntheses and more novel techniques include post-synthetic deprotection
[12].
Therefore, MOFs have been applied in many different scientific fields and their most famous application is for storage of gas fuels such as hydrogen and methane
[14]. Other applications of MOFs include gas separation proton, electron, and ion conduction, capture of carbon dioxide and organic reaction catalysis applications
[15] Biomedical applications of MOFs include biomedical imaging, disease diagnosing, drug delivery, biosensing and magnetic resonance imaging
[15][16][17][18][19]. In the field of analytical chemistry many different applications of MOF materials have been reported. In 2006, Chen et al. used for the first time MOF-508 material as stationary phase in a packed column in gas chromatography (GC)
[20]. After that, some other MOFs have been used in packed GC columns
[21][22][23]. Moreover, MOFs have been used as stationary phases in HPLC columns both for normal-phase and for reversed-phase high performance liquid chromatography (HPLC) applications
[24][25][26]. However, the most popular applications of the use of MOFs in analytical chemistry are in the field of sample preparation as absorbents for the extraction of a wide range of analytes in different matrices
[27].
In the last few years the very promising properties of MOF materials, such as the high surface area, made MOF ideal materials to be used as absorbents for sample preparation to meet various separation needs for many different compounds including either organic compounds or metal compounds from a wide range of matrices, such as environmental samples, food samples, drinking water etc. Typical examples of MOF materials that have been used as absorbents for sample preparation are MOF-199, MOF-5(Zn), ZIF-8, and MIL-53(Al). Most of the times, the mechanism of absorption may be due to the π–π stacking interaction between the MOF material and the analytes because of the presence of sp
2 hybridized carbons
[15].
Another interesting category of materials are metal organic frameworks derived nanoporous carbons, which are also useful materials for sample preparation. These materials have properties similar to MOFs and therefore they can form π-interactions between them and benzene rings of the target analytes. Direct carbonization or carbonization/polymerization after impregnation of MOF carbon precursors with furfuryl alcohol can lead to the formation of those materials. As a result, MOF derived nanoporous carbons are also considered as useful adsorbents for sample preparation
[28].
2. Food Matrices
Metal-organic frameworks have been used for many different food matrices (Figure 1).
Figure 1. Food matrices treated with MOFs for analytical purposes.
Table 1 summarizes the use of different MOFs in various food matrices as well as some analytical characteristics of the novel developed methods.
Table 1. Applications of MOF use for food sample preparation.
Matrix
|
Analytes
|
Analytical Technique
|
MOF Material
|
Sample Preparation Technique
|
Recovery
|
LODs
|
Reference
|
Milk
|
Sulfonamides
|
UHPLC-MS/MS
|
MIL-101(Cr)@GO
|
d-μSPE
|
79.83–103.8%
|
0.012–0.145 μg/L
|
[29]
|
Milk
|
Penicillins
|
UHPLC-TUV
|
MIL-53
|
In tube SPME
|
80.8–90.9%
|
0.06–0.26 μg/L
|
[30]
|
Milk
|
Tetracyclines
|
HPLC-PDA
|
ZIF-8
|
on-line SPE
|
70.3–107.4%
|
1.5–8.0 μg/L
|
[31]
|
Milk
|
Estrogens
|
HPLC-UV
|
MOF-5
|
SPME
|
73.1–96.7%
|
0.17–0.56 ng/mL
|
[32]
|
Fruit tea
|
Polycyclic aromatic hydrocarbons
|
UHPLC-FLD
|
Fe3O4@HKUST-1
|
D-μSPE
|
On average 75%
|
0.8 ng/L
|
[33]
|
Tea samples
|
Pyrethroids
|
GC-ECD
|
MIL-101(Cr)
|
MSPE-DLLME-SFO
|
>0.015 ng/mL
|
78.3–103.6%
|
[34]
|
Chrysanthemum tea
|
Luteolin
|
Square wave anodic stripping voltammetry
|
Cu3(BTC)2/GO
|
SPE
|
7.9 × 10−10 mol/L
|
99.4–101.0%
|
[35]
|
In tea and mushroom
|
Hg(II)
|
AFS
|
JUC-62
|
SPE
|
>0.58 mg/kg
|
On average 93.3%
|
[36]
|
Fish
|
Polychlorinated biphenyls
|
GC-MS
|
Fe3O4-MOF-5(Fe)
|
SBSE
|
0.061–0.096 ng/g
|
>80%
|
[37]
|
Fish
|
Polychlorinated biphenyls
|
GC-MS
|
MOF-5
|
SBSE
|
0.003–0.004 ng/mL
|
>80%
|
[38]
|
Fish
|
Aromatic hydrocarbons and gibberellic acids
|
GC-MS LC-MS/MS
|
MOF-5
|
MSPE
|
0.91–1.96 ng/L for PAHs and 0.006–0.08 μg/L for GAs
|
66.4–120.0% for PAHs and 90.5–127.4% for GAs
|
[39]
|
Fish
|
Triphenylmethane dyes
|
HPLC-MS/MS
|
MOF-5
|
MSPE
|
0.30–0.80 ng/mL
|
83.15–96.53
|
[40]
|
Fish
|
Cd(II) and Pb(II)
|
FAAS
|
MOF-199
|
MSPE
|
0.2–1.1 μg/L
|
92.8–117%
|
[41]
|
Fish
|
Cd(II), Zn(II), Ni(II), and Pb(II)
|
FAAS
|
MOF-199
|
MSPE
|
0.12–1.2 ng/mL
|
>90%
|
[42]
|
Fish
|
Hg(II)
|
Cold Vapor AAS
|
MOF-199
|
MSPE
|
10 ng/L
|
95–102%
|
[43]
|
Fish and shrimps
|
Cd(II), Pb(II), and Ni(II)
|
FAAS
|
Fe3O4@TAR
|
MSPE
|
0.15–0.8 ng/mL
|
NA
|
[44]
|
Shrimp samples, chicken and pork meat
|
Sulfonamides
|
HPLC-DAD
|
Fe3O4@JUC-48
|
MSPE
|
1.73–5.23 ng/g,
|
76.1–102.6%
|
[45]
|
Chicken breast
|
Drug traces
|
LC-MS/MS
|
MIL-101(Cr)@GO
|
d-μSPE
|
0.08 and 1.02 ng/kg
|
88.9–102.3%
|
[46]
|
Lettuce
|
Pesticides
|
GC-MS
|
∞[(La0.9Eu0.1)2(DPA)3(H2O)3]
|
MSPD
|
0.02–0.05 mg/kg
|
78–107%
|
[47]
|
Fruits and vegetables
|
Phytohormones
|
HPLC-FLD
|
UiO-66
|
Pipette Tip SPE
|
0.01–0.02ng/mL
|
88.3–105.2%
|
[48]
|
Fruits
|
Plant growth regulator
|
HPLC-FLD
|
UIO-67
|
d-SPE
|
89.3–102.3%
|
0.21–0.57 ng/mL
|
[49]
|
Fruits
|
Phytohormones
|
HPLC-UV
|
Zeolitic imidazolate framework-8
|
SBSE
|
82.7–111%
|
0.11–0.51μg/L
|
[50]
|
Fruits and vegetables
|
of insecticides
|
HPLC-UV
|
Fe3O4@SiO2-GO MOF
|
MSPE
|
81.2–105.8%
|
0.30–1.58 μg/L
|
[51]
|
Shellfish
|
Shellfish poisoning toxin
|
LC-MS/MS
|
Fe3O4@SiO2@UiO-66
|
MSPE
|
93.1% and 107.3%
|
1.45 pg/mL
|
[52]
|
Rice
|
Herbicides
|
HPLC-UV
|
MIL-101(Cr)
|
MSPE
|
83.9–103.5%
|
0.010–0.080 μg/kg
|
[53]
|
Tomato sauce
|
Sudan dyes
|
HPLC-DAD
|
Fe3O4-NH2@MIL-101
|
MSPE
|
69.6–92.9%
|
0.5–2.5 μg/kg
|
[54]
|
Peanuts
|
Herbicides
|
HPLC-DAD
|
MIL-101(Cr)
|
d-SPE
|
89.5–102.7%
|
0.98–1.9 μg/kg
|
[55]
|
In cereal, beverages and water samples
|
Lead
|
FAAS
|
MOF-545
|
Vortex Assisted SPE
|
91–96%
|
1.78 μg/L
|
[56]
|
3. MOF-Derived Carbon Materials
Recently, the use of magnetic nanoporous carbons derived from metal-organic framework as adsorbent for sample preparation is gaining more and more attention. Since MOFs are known for their high surface area and in combination with their mesoporous properties and the high carbon content, these materials consist a useful template to synthesize porous carbons with many potential uses, such as hydrogen storage, toxic aromatic compounds sensing, electrocatalysis, etc.
[28]. As Lim et al. have found, even non-porous MOFs could result in highly nanoporous carbons
[57]. In general, there are two different techniques to construct a MOF-derived nanoporous carbon. The first attempt includes impregnation of MOF carbon precursors with furfuryl alcohol as carbon source and then polymerization/carbonization as a second step. Another simpler attempt is a single-step direct carbonization
[28]. MOF-5 is the most common MOF material that has been used for the synthesis of MOF derived nanoporous carbons.
In 2008, Liu et al. first used A MOF as a template to make porous carbon and subsequently many applications have been reported on the literature. Those materials as well as MOFs show high surface area, large porous volume and due to their thermal stability and great electrochemical performance can be used for the same purposes as metal-organic frameworks
[58][59]. Moreover, due to the presence of sp
2 hybridized carbons they are able to form π-stacking interaction with benzene ring and aromatic compounds. As a result, those materials can be used for the adsorption of these kind of chemical compounds. When combined with magnetic precursors, MOFs can form magnetic nanoporous carbons that combine the great adsorption ability of porous carbons and handling convenience of magnetic materials
[28].
Table 2 summarizes the applications of MF derived carbons for food sample preparation and some analytical details about the novel developed method.
Table 2. Applications of MOF-derived carbons for food sample preparation.
In 2015, Liu et al., used the well-known material MOF-5 as template to form a magnetic porous material. For this purpose, MOF-5 was loaded in quartz boat and transferred in tube furnace at 80 °C for one hour in order to be carbonized under argon atmosphere at 900 °C for 6 h. Afterwards, iron(III) chloride hexahydrate and iron(II) sulfate heptahydrate was added to the porous carbon under nitrogen atmosphere at 50 °C for one hour in combination with mechanical stirring. The material was used for the determination of carbamates in apple samples with HPLC. For the sample preparation a quantity of 25.0 g of homogenized apple samples was placed in a 50 mL centrifugal tube and was centrifuged at 4000 rpm for 10 min. The supernatant was collected and filtered, and the procedure was repeated after the addition of 10 mL of water to the sediment and vortex mixing. Then, the whole extract was transferred in a conical flask for the MSPE process, where 60 mg of the MOF derived magnetic porous carbon was added and mechanical shaking was implemented.
After 25 min, the material was collected to the bottom of the flask with the use of external field (magnet) and the liquid was discarded. Elution was achieved with 200 μL of methanol and the procedure was repeated three times prior to HPLC analysis. During extraction method optimization it was found that pH value should not be higher than 6 and no salt addition is needed. The developed method showed good repeatability, linearity, precision, recovery values (89.3–109.7%) and low LODs (0.1–0.2 ng/g). Moreover, the material can be used 13 times without any loss in functionality
[59].
The same research group published in 2015 an analytical method for simultaneous determination of phenylurea herbicides in grapes and bitter gourd samples, using magnetic carbon as adsorbent material for the sample preparation. The magnetic nanoporous carbon was synthesized by direct carbonization of Co-based metal-organic framework, ZIF-67. For the fabrication of ZIF-67, cobalt(II) nitrate hexahydrate and 2-methylimidazole were used. For the carbonization, ZIF-67 was heated at 150 °C for 1 h, and after that it was heated at 700 °C for 6 h under nitrogen to pyrolyze the organic species. For the sample preparation of grapes and gourd sample homogenization, centrifugation and collection of the supernatant and filtration was carried out with the same procedure as in apple sample preparation.
[58][66]. For the MSPE process, a quantity of 10 mg of the magnetic porous carbon material was placed in a 100 mL flask that contained the sample solution. Shaking of the mixture took place for 25 min for the extraction and then the magnetic material was gathered to the bottom of the flask with the use of a magnet. After discarding the supernatant 0.1 mL of acetone was added to desorb the analytes for the HPLC analysis. It was found that no pH adjusting, or salt addition was required for the optimum extraction procedure. No significant loss of adsorption capacity was observed when the material was used 15 times. The developed method showed good repeatability, linearity and precision, LODs were 0.17–0.4 ng/g for the grape samples and 0.23–0.46 ng/g for the bitter gourd samples, while recoveries were 88.9–105.1% for grape sample and 89.6–104.0% for bitter gourd sample
[60].
The same MOF derived magnetic nanoporous carbon was used for the determination of neonicotinoid insecticides from water and fat-melon samples by high-performance liquid chromatography ultraviolet detection (HPLC-UV). The samples were cut, homogenized and centrifuged and the MSPE procedure was similar to the above- mentioned procedure for grapes and gourd samples. Same amount of material and volume of extraction solvent was used, however shaking for the extraction took place for 20 min. Separation of the phase was carried out with the use of a magnet and after discarding the liquid phase, a volume of 0.2 mL acetone was added into the isolated MOF and vortexed for 1 min to desorb the chemical compounds. Desorption procedure was repeated one more time before HPLC analysis. During method optimization it was found that pH 6 was the ideal value for extraction and no salt addition was necessary. The MOF derived material can be used at least 15 times without functionality loss. Linearity, repeatability and method precision were good. Moreover, and LOD for the analytes in fat melon samples were 0.2–0.5 ng/g and recoveries ranged from 93.0% to 99.3%
[61].
In 2016, Li et al. synthesized a Zn/Co bimetallic metal–organic framework by introducing cobalt into ZIF-8 and by direct carbonization of the resulting Zn/Co-ZIF-8 and used it as an adsorbent for the extraction of chlorophenols from water and honey tea samples prior to their determination by HPLC-UV. The MOF material was prepared by mixing cobalt(II) nitrate hexahydrate, zinc nitrate hexahydrate and 2-methylimidazole. Different molar ratios of Zn and Co complex compounds were examined and finally the ratio of Zn:Co 7:1 was chosen. Carbonization of the material took place at 900 °C for 6 h under nitrogen. For the honey tea sample preparation, the samples were diluted in a volume of 1:1 with distilled water and filtered and 15 mg of the material was added in 100 mL of the solution and the mixture was shaken for 20 min for the MSPE procedure. The material was separated from the mixture with the use of a magnet and desorption took place with 2 × 0.2 mL alkaline methanol solution and pH was neutralized with HCl solution prior to the injection to the HPLC system. As a result, a rapid, convenient, and efficient MSPE method was developed with low LOD values (0.1–0.2 ng/mL) and good recoveries (83.0–114.0%)
[62].
The same year Liu et al. developed a nanoporous carbon/iron composite material MIL-53-C by one-step carbonization of the MOF material MIL-53. The novel material was used as an adsorbent for MSPE for the determination of endocrine disrupting compounds (EDCs) in fruit juices and milk by HPLC. Firstly, MIL-53 (Fe) was fabricated by mixing terephthalic acid and iron(III) chloride hexahydrate at high temperature and pressure and then carbonization was achieved by heating the material at 700 °C for 6 h under nitrogen atmosphere to pyrolyze the organic species. After juice samples were filtrated and milk samples were deproteinized and extracted with acetone, a portion of 12 mg of the material was added to the solutions for the MSPE procedure. Under optimum conditions extraction lasted for 20 min with mixing, adsorption was achieved with 0.2 mL alkaline acetone thrice and no pH adjusting, or salt addition was needed. LODs were 0.05–0.10 ng/mL for fruit juice and 0.10–0.20 ng/mL, while recovery values ranged from 92.2% to 108.3%. The method showed high adsorption capability for trace levels of EDCs and could be a promising extraction method for preconcentration of other organic compounds
[63].
In 2016, Hao et al. used a metal-organic framework-derived nanoporous carbon (MOF-5-C) modified with Fe
3O
4 magnetic nanoparticles for the extraction of chlorophenols from mushroom samples prior to HPLC-UV determination. Excellent adsorption capacity was achieved. The carbonization of the MOF-5 nanoparticles was performed at 900 °C for 6 h under Ar. For the MSPE, 8.0 mg of Fe
3O
4@MOF-5-C was added to 50 mL sample solution obtained from homogenization and centrifugation of mushroom samples. The mixture was shaken on a slow-moving platform shaker for 10 min. Subsequently, the material was separated from the sample solution by putting an external magnet and 0.4 mL (0.2 mL × 2) of alkaline methanol was used for elution. The developed method was characterized as simple, fast and sensitive. Limit of detection ranged between of 0.25–0.30 ng/g, while recovery values were 85.4–97.5%
[64].
In 2017, Wang et al. synthesized three-dimensional porous Cu@graphitic octahedron carbon cages that were constructed by rapid room-temperature synthesis of a Cu-based metal–organic framework (MOF) followed by further pyrolysis at 700 °C under nitrogen for the dispersive solid phase extraction of four fluoroquinolones (FQs) from chicken muscle and fish tissue prior to their determination with HPLC. The material was synthesized by the reaction of 1,3,5-benzenetricarboxylic acid and copper(II) nitrate trihydrate. Chicken and fish samples were homogenized and treated with methanol with sonication for 10 min to extract the analytes. The resulting solution was filtered, and 36.0 mg of the porous Cu@graphitic carbon cages was added into it. The mixture was vibrated for 30 min followed by centrifugation to separate the material. Elution was performed with ethanol (EtOH)/NaOH 1 mol L
−1) (7/1,
v/
v) and the liquid was evaporated under nitrogen. Finally, acidic methanol was added for HPLC analysis. Low detection limits (0.18–0.58 ng/g) were obtained in combination with satisfying recoveries (81.3–104.3%). Good method performance was obtained showing great potential to further increase the applications of this novel material
[65].
This entry is adapted from the peer-reviewed paper 10.3390/molecules23112896