Figure 3: Structures of some Metal Organic Frameworks. BPDC [Biphenyl Dicarboxylate], BDC [Benzene-1,4-dioic acid], M-TCPP [meso-Tetra(4-carboxyphenyl)porphine], TBAPy [1,3,6,8-tetrakis(p-benzoic acid) purene], MeIM [2-methylimidazole], LH6 [triphenylene-based hexamine].
As an example, Yue Q et al [6] have developed two MOFs from the same metal cluster (Zn4 SBU) just by changing the organic dicarboxylate linker (Figure 4).
Figure 4: Views showing the evolution of two different MOFs 3-D Structure, 3D kag net (e) and the default pcu net (f) from the 2D kag subnet (c) and the 44 subnet (d) constructed by the same Zn4 SBU (a, b) [8]
MOFs are now widely explored as advanced materials for various important applications in many areas of interest, such as, hydrogen storage [9,10], gas separation [11,12,13], catalysis [14,15], sensing [16], imaging [17,18], and drug delivery and release [19,20], but in this review we will focus on MOFs applications in analytical chemistry.
Recently, Metal Organic Frameworks have drawn researchers' attention for its distinct properties and characteristics, and the possibility of using it in many analytical chemistry applications, which include, sampling, sample pre-concentration processes, solid phase extraction (SPE) and solid phase micro-extraction (SPME), liquid and gas chromatography. The results reveal that MOFs are attractive as high-efficient adsorbents and advanced separation media for the targets of environmental, biological, or industrial importance [6,21,22,23].
MOFs for sample preparation
MOFs for pre-concentration processes
In high sensitive analytical methods, a pre-concentration process is usually occurred using pre-concentrator with an appropriate adsorbent, then thermal desorption step is applied for recovering substance of interest. Many classical adsorbent agents are commercially available, which have various structures and physiochemical properties. Such as, Tenax® polymers (like Tenax TA) and Carboxen® polymers (like Carboxen 1000) (Figure 5).
Figure 5: Classical Carboxen pre-concentrator equipped with GC system [24]
Recently, Metal Organic Frameworks have been successfully used as novel adsorbents in the sampling and pre-concentration processes in many applications. In particular, Iso-Reticular MOFs, like IRMOF-1 which is showed in (Figure 4-f) was applied as a sorbent for trapping and pre-concentration of standard dimethyl methylphosphonate (DMMP) with high selectivity, adsorption capacity, and fast adsorption kinetics. These results can be explained as DMMP can have strong dipole-dipole interactions with the framework and there is a single binding site for DMMP in IRMOF-1 with a binding energy of about 19 kcal/mol. Moreover, the adsorbed DMMP is released upon heating to 250 °C [25].
Ni Z et al. observed pre-concentration gains of over 5000 for DMMP in IRMOF-1 with sampling times of 4 sec. By comparison, the gain was only 2 by using Tenax TA trap instead.
In another study [26], quartz tubes filled with MOF-5 have been used for in-field sampling and pre-concentration of atmospheric formaldehyde. The sampling was performed by pumping air directly through the sorbent tube. Then analytes were desorbed on a Markes UNITY/ULTRA desorption system, which is a multistage desorption system includes (1) thermal desorption from the sorbent tube for 4 min, (2) re-concentration on a focusing cryogenic quartz tube filled with graphitized carbon at -10 °C for 4 min, and then (3) flash heating to 300 °C (in less than 8 sec) to release the concentrated analytes into the GC/MS system with a narrow band to enhance the resolution. All these stages are showed in Figure 6.
In this study, MOF-5 gave a 53 and 73 times better concentration effect than Tenax TA (organic polymers) and Carbograph 1TD (graphitized carbon black), respectively. These results can be explained as MOF-5 has a relatively larger surface area which is more favorable to the adsorption of formaldehyde than Tenax TA and Carbograph 1TD. In addition, formaldehyde has specific adsorption sites on zinc-corners of MOF-5, these binding sites are not existed in Tenax TA, neither Carbograph 1TD. Thereby, this developed MOF-5-based TD-GC/MS technique offered high sensitivity, and good reproducibility which are important parameters for analytical techniques to be applied in environmental applications [26].
Figure 6: Schematic figure of atmospheric formaldehyde sampling, desorption, and analysis processes using quartz tube filled with MOF-5 sorbent
MOFs for Solid Phase Extraction (SPE)
Solid phase extraction is a type of sampling processes in which the dissolved or dispersed substance of interest is separated from other sample components, and then concentrated to be ready for analysis.
The extraction process occurs due to the difference in physiochemical properties, and then the affinity of sample components to the solid phase. In this method, either the other sample components or the desired substance are retained on the solid phase, and thus in the last case, we may need an additional Elution step to retrieve the substance to be analysed.
Many researchers have been equipped MOFs materials as potential sorbents in solid phase extraction [27,28,29,30]. The first application of MOFs in Solid Phase Extraction (SPE) was in 2006, which was to identify polycyclic aromatic hydrocarbons (PAHs) traces in water samples and coal fly ash. PAHs are a group of persistent organic pollutants (POPs) emitted from industrial processes, and known to be strongly mutagenic and/or carcinogenic [27].
Due to their low solubility in water, these pollutants exist in low concentrations which are difficult to analyse directly, and require initial concentration step using SPE method [31].
Metal organic frameworks constructed by Copper(II) isonicotinate [Cu(4-C5H4NCOO)2(H2O)4] were used for packing SPE columns, which are then on-line coupled with high-performance liquid chromatography in a flow injection system for determination of PAHs in environmental samples. This developed method is considered to be simple, sensitive, selective and cost-effective [27].
Recently, zinc benzotriazole coordination networks have been used as sorbent agents for solid phase extraction of Benzo[α]pyrene (BaP) traces in edible oils, to be then analyzed using gas chromatography coupled with mass spectrophotometric detector (GC/MS).
BaP is a common contaminant in edible oils, which has carcinogenic effects on humans. As BaP has strong lipophilic properties and existed in low concentrations, it is difficult to direct analyse this pollutant in oils, and an initial SPE step is required.
The high selective retention of BaP on zinc benzotriazole coordination polymers is explained due to the existence of five aromatic rings in BaP, which are considered П-electron substrates, that strongly bind with coordination polymers by aromatic ligands, whereas the neutral triglyceride and tocopherol in edible oil are non П-electron substrates and thus leave the SPE column without retention (Figure 7) [30].
Figure 7: Structural formulas of Benzo[α]pyrene (A), and Benzotriazole (B).
MOFs for solid Phase Micro-Extraction (SPME)
Solid phase micro-extraction technique depends on the use of fibers coated by sorbent agent in a nanoscale, and is used primarily for volatile samples extraction. The quantity of analyte extracted by the fiber is proportional to its concentration in the sample as long as equilibrium is reached or, in case of fixed short time pre-equilibrium. Then, thermal desorption step is applied and gas chromatography system is usually used to analyse the sample (Figure 8). SPME is considered to be fast, high efficient, solvent free, and high sensitive technique (detection limits can reach parts per trillion p.p.t for certain applications).
Although a number of SPME coatings are commercially available, such as polyacrylate (PA) and poly dimethylsiloxane (PDMS), they cannot be reliable in some applications due to their thermal or solvent instability, limited selectivity, and some technical limitations. Therefore, and due to the high stability of MOFs, they were thought to be used in SPME fibers coating [32,33]. Such as MOF-199, which was successfully used to coat stainless steel fibers prepared for the extraction of volatile benzene homologues with large enhancement factors, wide linear range, and good reproducibility [34].
Figure 8: schematic figure of solid phase micro-extraction (SPME) technique coupled with GC system [32]
MOFs for gas chromatography
MOFs for packed column gas chromatography
The first application of MOFs in packed column gas chromatography has been shown by Chen BL et al. [32], and was for the separation of linear and branched alkanes mixtures. MOF-508 was used due to its distinct characteristics and properties. This coordination network is constructed by regular 1-D channels of approximately
(4 X 4 0A) in cross section, which are slightly larger than a methane molecule (3.8 X 3.8 0A). These properties make MOF-508 a suitable stationary phase for the separation of linear alkanes from branched form, wherein the last are less retained on the stationary phase.
This application is beneficial particularly in petroleum refining. It can also be used to identify the impurities in natural gas, and to monitor the amounts of mono- and multi-branched alkanes formed in cracking reactions.
Another study performed in Cambridge University on CUK-1 coordination polymer [Co3(2,4-pyridine dicarboxylic)2(µ3-OH)2]·9H2O, showed that this MOF has unusual and selective sorption properties of light gases (O2 – H2 – N2 – CH4 – CO2), which make it possible to efficiently separate these gases mixtures using one packed column filled with CUK-1 instead of using two different GC column conventionally [36].
MOFs for capillary gas chromatography:
It is well known that packed columns usage contribute to wide peaks on the chromatogram and low separation quality even by using MOFs as a stationary phase. Moreover, MOFs packed columns consume a large quantity of stationary phase, and then high cost to prepare this type of chromatographic columns. Therefore, MOFs have been also used as a thin film on the inner wall of the chromatographic column, which is referred to as capillary gas chromatography.
The first example of using MOFs in capillary columns gas chromatography was reported in 2010 [37], by using MIL-101 coordination networks as a suspension to coat capillary columns by a dynamic coating method under fixed gas pressure. These prepared capillary columns were used to separate xylene isomers and ethyl benzene, which are mono aromatic substances constructed by eight carbon atoms (Figure 9). These substances are important raw materials in industry. In particular, p-xylene is used in the manufacture of terephthalic acid for the polyester industry [38]. Thus, the separation and detection of xylene isomers and ethyl benzene are of great practical interest in environmental and industrial applications [39,40].
For these reasons, numerous conventional stationary phases have been developed for GC separation of xylene isomers and ethyl benzene, such as, 7,8-benzoquinoline [41], tetrachlorophthalate [42], and β-cyclodextrin derivatives [39]. However, long analysis time (27–90 min) or temperature programming is often needed.
In contrast, MIL-101 is a chromium terephthalate MOF with a high surface area, large pores (2.9–3.4 nm), coordinatively unsaturated sites (CUS), and high chemical and thermal stability [43]. These features illustrate the ability of MIL-101 capillary columns to perform the separation of xylene isomers and ethyl benzene in short time (within 1.6 min) with high resolution, high selectivity, and without the need of temperature programming.
To demonstrate the role of CUS on the separation process, they grafted these CUS sites of MIL-101 with pyridine by post-synthetic modification. The obtained pyridine-grafted MIL-101 coated capillary column was less selective, because it was not able to separate p-xylene from m-xylene in the mixture [37].
Figure 9: Structural formulas of xylene isomers and ethyl benzene [34]
New advance in the utilizing of MOFs in capillary gas chromatography has recently been reported [44,45]. Münch AS and Mertens FORL merge the Controlled Secondary Building Unit Approach (CSA) with the process of Self-Assembled Monolayer (SAM) of HKUST-1 coordination networks on the inner wall of capillary column. This column was used in the separation of low molecular weight aromatic hydrocarbons mixtures (benzene – toluene – xylene – ethyl benzene) in short time, and with high selectivity.
MOFs for chiral gas chromatography
The chiral mixtures analysis is considered one of the main challenges in analytical chemistry. Thus, many researchers are trying to find and explore new chiral-recognition sites containing stationary phases, which have the ability to recognize the stereo chemical differences, and therefore separate enantiomers [46].
A variety of chiral-recognition sites are found in some MOFs (called chiral MOFs), thus make them appropriate stationary phases for chiral gas chromatography. For example, three-dimensional metal organic frameworks with chiral channels composed of [Cu(sala)n] have been successfully used to separate racemic mixtures of aldehydes, ketones, organic acids, amino acids, and alcohols [47].
The explanation of chiral recognition properties in chiral MOFs is usually complex. The previously mentioned coordination network [Cu(sala)n] for example, possesses a left-handed helical channels with a 25 Å screw pitch (Figure 10). This distinct structure is considered to play the main role in chiral recognition process. To demonstrate that, they prepare capillary column coated by non-condensed [Cu(sala)]2.2H2O metal-complex for comparison. As a result, they noticed that this modification leads to significant decrease in the chiral recognition ability in comparison with condensed [Cu(sala)n] structure.
Figure 10: The crystal structure of MOF [Cu(sala)n] (right side), and a view of 1-D left handed helix channels in MOF [Cu(sala)n] (left side).
MOFs for high performance liquid chromatography
MOFs for Normal Phase HPLC (NP-HPLC)
The first application of metal organic frameworks in normal phase HPLC was showed in 2007 by Alaerts L et al. [48], by using vanadium terephthalate coordination networks called MIL-47 packed in 5 cm HPLC column utilized for the separation of xylene isomers and ethyl benzene mixtures (Figure 9), using hexane as a mobile phase.
In this study, they suggest that there are interactions occur between the protons of the CH3 groups of sample components and the terephthalate ligands, thus causing a change in the lattice constants of MIL-47 frameworks. The interactions mentioned above in addition to the high uptake capacity and moderately hydrophobic nature of studied stationary phase, are considered to play the main role in the separation process.
In another study, MOFs also showed great potential in fullerene separation. Yang CX et al. [49] reported the utilization of Cr-MIL-101 as a novel HPLC stationary phase for the separation of C60 and C70 fullerenes mixtures (Figure 11). This MOF is selected due to its high surface area, large pores (29-34 Å), and excellent chemical and solvents stability.
Figure 11: Topological structure of fullerenes C60 (A), and C70 (B) [50]
The high selectivity of Cr-MIL-101 for HPLC separation of fullerenes C60 and C70 can be resulted from the difference of fullerenes affinity to both stationary and mobile phases. For clarification, the solubility of C70 in the organic mobile phase is lower than that of C60. In addition, C70 fullerenes have more П electrons, and thus more П - П interactions with the terephthalate ligands of Cr-MIL-101 stationary phase. Moreover, the larger C70 fullerene molecules tend to have stronger van der Waals interactions with Cr-MIL-101 than C60. All these reasons make C70 more retained on the stationary phase than C60. Figure 12 is showing fast, high selective, and reproducible separation of C60 and C70 on the mentioned column.
Figure 12: Chromatograms for thirteen replicate HPLC separations of C60 and C70 on Cr-MIL-101 packed column, using CH2Cl2/CH3CN (98:2) as a mobile phase, with UV detector at 340 nm.
MOFs for Reverse Phase HPLC (RP-HPLC)
The above-mentioned studies were about the usage of MOFs in normal phase HPLC, where the polarity of stationary phase is higher than that of mobile phase. However, the HPLC technique in which the polarity of stationary phase is lower than that of mobile phase (reverse phase HPLC) is now the most widely used technique in pharmaceutical, biomedical and environmental research. Therefore, many researchers have illustrated the possibility of MOFs utilization in RP-HPLC applications.
As an example, Liu SS et al. [47] have showed the ability of Al-MIL-53 coordination networks to separate a wide range of analytes with different polarity and characteristics.
Al-MIL-53 is a three-dimensional framework containing one-dimensional diamond-shaped pore channels constructed by aromatic rings, and offers high surface area, suitable crystal size, and excellent chemical and solvent stability. Moreover, Al-MIL-53 network has been shown to exhibit a breathing phenomenon due to hydration–dehydration equilibrium (Figure 13), which probably plays an important role in its retention properties as a stationary phase.
Figure 13: Structure of Al-MIL-53 hydrated (A), and dehydrated (B).
Researchers showed the ability of Al-MIL-53 HPLC columns to separate toluene and ethyl benzene mixtures with a longer retention time compared with C18 stationary phase (Figure 14-a), this result confirm that Al-MIL-53 is more hydrophobic than C18. Apart from the difference in hydrophobic interactions between Al-MIL-53 and conventional C18 stationary phase, the first is considered to have nanoscale size-exclusion mechanisms which are responsible for some substances to leave the column without any retention.
In the same mentioned study, Al-MIL-53 columns also equipped successfully for the separation of Poly-cyclic aromatic hydrocarbons (PAHs) (Figure 14-b), benzenediol isomers (Figure 14-c), and xanthine derivatives mixtures (Figure 14-d).
Figure 14: HPLC chromatograms on the Al-MIL-53 packed column for the separation of (a) ethyl benzene and toluene mixture using CH3CN/H2O (7:3) as a mobile phase, (b) poly-cyclic aromatic hydrocarbons (PAHs) using pure CH3CN as a mobile phase, (c) benzenediol isomers using CH3CN/H2O (1:9) as a mobile phase, and (d) xanthine derivatives using CH3CN/H2O (5:5) as a mobile phase. All samples detected by Diode Array Detector (DAD) at 210 nm, 256 nm, 280 nm, 270 nm, respectively.
Another example of utilizing MOFs in reverse phase HPLC was shown by Centrone A et al. [50], in which ZIF-8 coordination networks were equipped for the successful separation of some substances produced in pharmaceutical industry (Figure 15). These substances containing di-methylamine group (the desired main product), which are produced in combination with mono-methylamine substances (secondary by-products) as a chemical synthesis products. This mixture is difficult to separate on conventional C18 stationary phase, thus resulting in a significant decrease in the synthesis reaction yield, and more secondary reactions to be occur due to mono-methyl substances by-products.
The results of this study can contribute to design a continuous separation and manufacturing systems, which can increase the synthesis yield obtained, and decrease the formation of mono-methyl undesired by-products by further direct substitution step to result the desired di-methyl amine derivative again.
Figure 15: structural formulas of mono- (by-product) and di-methyl amine (product of interest) produced in pharmaceutical industry.
CONCLUSION:
Although there are many challenges block the application of some Metal organic frameworks (MOFs) in analytical chemistry techniques, such as, thermal and solvent instability, but some of these coordination networks have a good thermo-stability and distinct mechanical and structural characteristics (like ZIF and MIL classes), which make it possible to utilize these MOFs as a potential sorbent agents in several analytical applications, including pre-concentration processes, Solid Phase Extraction (SPE), Solid Phase Micro-Extraction (SPME), Gas Chromatography (GC), and High Performance Liquid Chromatography (HPLC).
The use of MOF networks in such analytical techniques has led to a significant progress, particularly improving sensitivity and selectivity parameters, and meeting many analytical challenges. However, more studies should be performed to reveal all potential features and advantages of these micro-porous materials to analytical chemistry.
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Received on 11.10.2017 Modified on 17.11.2017
Accepted on 24.12.2017 © RJPT All right reserved
Research J. Pharm. and Tech 2018; 11(8): 3514-3522.
DOI: 10.5958/0974-360X.2018.00650.9