Introduction

Metal–organic frameworks (MOFs) comprise a new class of functional materials that have received a great deal of attention due to their permanent porosities, chemically tunable structures, and diverse functionalities [1,2,3,4,5,6,7]. Many pioneering works on the applications of MOFs, including separation, energy conversion, gas storage, chemical sensing, detoxification, catalysis, and biomedicine [8,9,10,11,12,13,14], have been reported. In this review, we comprehensively summarize and discuss another field of application for MOFs: as novel materials and matrices for sample preparation in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS).

MALDI, a soft ionization technique, was invented and developed for use in conjunction with time-of-flight mass spectrometry (TOF MS) in the late 1980s [15, 16]. Although significant advances have recently been made in liquid chromatography–mass spectrometry (LC-MS), particularly in relation to biomolecular analysis using electrospray orbitrap MS [17,18,19], MALDI-MS is still a valuable tool for high-throughput analysis as well as MS imaging due to its advantageous features, such as its high sensitivity, ability to provide fast analysis, simple sample preparation, low sample consumption, and high salt tolerance [20,21,22,23,24,25].

Peptides are fundamental structural units that are found extensively in biomolecules, and are closely associated with the developmet of life itself [26, 27]. Moreover, recent studies have confirmed that specific peptides play vital roles in biological processes such as immunoregulation [28], growth regulation [29], antimicrobial activity [30], and antioxidant activity [31]. LC-MS is a state-of-the-art analytical technique that can be employed for the highly selective and quantitative detection of peptides in biological matrices, providing high mass accuracy and resolution. Antibody-based and/or conventional titanium dioxide (TiO2)-based enrichment of phosphopeptides are well-established methods for identifying and quantifying peptides such as the phosphopeptides investigated in LC-MS proteomics analysis. While MALDI has excellent sensitivity, it is not currently used as a standard method to analyze peptides because it is more difficult to couple MALDI to liquid separation techniques than in LC-MS [32, 33]. However, MALDI has been extensively applied as a high-throughput, sensitive, and fast MS technique for screening and evaluating materials in peptide analysis.

Various enrichment strategies utilizing different materials, including metals, metal oxides, carbon, silicon, polymers, covalent organic frameworks (COFs), and composite materials, have been reported to give good enrichment performance for the analysis of peptides such as endogenous peptides, phosphopeptides, and glycopeptides [34,35,36,37,38,39,40,41]. Some of the above sorbents have been employed in LC-MS-based proteomics research. As they are emerging materials, MOFs have only rarely been used in LC-MS, although they have shown good performance in MALDI and great potential in LC-MS due to their advantageous porous characteristics, tunable structures, and abundant surface functional groups.

Although MALDI-TOF MS has shown itself to be a very useful technique for analyzing large molecules (> 1000 Da) such as proteins [42, 43], peptides [44, 45], oligosaccharides [46], nucleic acids [47], and synthetic polymers [48], it is still a big challenge to analyze smaller molecules (< 700 Da) using this technique due to the interference from traditional organic matrices (e.g., 2,5-dihydroxybenzoic acid (DHB), α-cyano-4-hydroxycinnamic acid (CHCA), and sinapic acid (SA)) in the low-mass region [49,50,51]. These limitations can be efficiently addressed by using nanoparticles and nanostructured surfaces as matrices. This alternative method is called surface-enhanced laser desorption ionization mass spectrometry (SELDI-MS) [52]. Recently, many studies performed in this field have trialed various new materials as matrices, such as porous silicon [53], organic materials [54], and graphitic materials [55]. When MOFs are used as matrices, due to their high absorption capabilities in the UV-visible range, proton/electron transfer to analytes is relatively easy. Thus MOFs meet the requirements of an efficient matrix for application in SELDI-MS. By modifying the framework of the MOF and exfoliating the MOF into a nanosheet, the porosity of the MOF can be tuned and the surface properties of the MOF nanosheet can be varied. Thus, MOFs and MOF nanosheets play important roles in the MS analysis of endogenous peptides, phosphopeptides, glycopeptides, and matrices in multiple biological processes. Although this topic has already been partially reviewed by other researchers [56, 57], we provide our perspectives here, and we summarize representative works that have been published in this field since our first report in 2011 (Scheme 1). Furthermore, very recent progress made in the application of 2D MOF nanosheets to peptide enrichment and matrices is also reviewed here for the first time.

Scheme 1
scheme 1

Schematic illustration of the applications of MOFs in MALDI MS

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Endogenous peptide analysis

Endogenous peptides and proteins include well-characterized families of hormones, neuropeptide modulators, neuropeptide transmitters, and fragments of functional proteins. These play crucial roles in regulating many pathological and physiological processes, such as cytokines, growth factors, and hormones [58,59,60,61]. However, owing to the huge complexity of biological samples as well as the wide dynamic range of endogenous peptides, it is still a big challenge to selectively extract and enrich these peptides. Recently, MOFs have shown excellent potential for use in peptidome profiling (Table 1).

Table 1 Recent applications of MOF-based affinity materials for the analysis of endogenous peptides
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The first attempt to enrich peptides with MOFs was made by Gu et al. [62] (Fig. 1). In order to analyze a mixture of standard peptides as well as human plasma and human urine samples, three MOFs were prepared: MIL-53(Al), MIL-100(Cr), and MIL-101(Cr). They facilitated the efficient enrichment of low-abundance peptides while simultaneously effectively excluding high-abundance proteins.

Fig. 1
figure 1

Schematic of MOFs employed by Gu et al. for the efficient enrichment of peptides and the simultaneous exclusion of proteins from complex biological samples. Reprinted with permission from [62]. Copyright 2011 by the Royal Society of Chemistry

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Among MIL-53(Al), MIL-100(Cr), and MIL-101(Cr), MIL-100(Cr) provided the best enrichment performance; using this MOF, the S/N ratios of peptides from the tryptic digest of BSA were enhanced from 0 to 64, with a limit of detection (LOD) of 2 fmol μL−1. The number of matched peptides as well as the sequence coverage were markedly increased from 3 to 20 and from 0 to 39%, respectively. Meanwhile, the efficient exclusion of proteins by these MOF materials was also confirmed. MIL-101(Cr) successfully excluded all high-abundance proteins > 10 kDa. This result can be attributed to the molecular sieving effect of MIL-101(Cr), with pore windows of 1.2 and 1.6 nm. Instead of requiring a tedious centrifugation process, the various magnetic MOF composites with trapped peptides were collected using an external magnet.

As glucose can serve as a carbon source, Wei et al. [63] coated it onto magnetic nanospheres and combined these with MIL-100. The resulting Fe3O4/C@MIL-100 composites were used to capture peptides from tryptic digests of BSA and human serum prior to MALDI-TOF MS analysis. Over 170 peptide peaks with S/N ratios of > 3 were obtained from the tryptic digest of human serum. Moreover, it was found to be possible to distinguish between persons with and without type 2 diabetes mellitus (T2DM) by digesting sera from them and then examining their peptide mass fingerprints.

In contrast to MIL-100, ZIFs combine the unique properties of both zeolites and MOFs. Zhao et al. [64] reported a facile polymerization reaction to obtain magnetic microspheres which were subsequently modified with polydopamine (PDA) and then coated with ZIF-8. The resulting magnetic Fe3O4@PDA@ZIF-8 showed high selectivity for a mixture of BSA tryptic digest and BSA proteins (molar ratio 1:200). Moreover, due to the low coordination number of the Zn2+ ion, the Fe3O4@PDA@ZIF-8 composite exhibited strong affinities for low-abundance peptides, especially those with histidine residues.

In order to gain a core–shell structure for Fe3O4 nanoparticles and MOFs, mercaptoacetic acid (MAA) was employed to modify the Fe3O4 nanoparticles. Xiong et al. [65] assembled the MIL-100(Fe) on the surfaces of the Fe3O4 magnetic nanoparticles using a layer-by-layer assembly method, and core–shell magnetic nanospheres (Fe3O4@MIL-100(Fe)) were prepared. Unlike for Fe3O4@MIL-100(Fe), directly coating a layer of MOFs onto the magnetic sphere without any modification was also tried, but the core–shell structure was not obtained. Following the enrichment of peptides from a tryptic digest of human serum, 563 unique peptides were identified from 5 μL of primary human serum by LC-MS/MS.

Other modifications that were performed to prepare magnetic MOF composite materials have been reported. Wei et al. [66] directly assembled MIL-101(Cr) on carboxylic-functionalized magnetite (Fe3O4-COOH) particles through a one-step hydrothermal method. Application of the as-prepared composites (Fe3O4-COOH@MIL-101) led to high enrichment of BSA tryptic digest peptides, with a LOD of 0.25 fmol μL−1. Thereafter, the composites were employed to discriminate different Escherichia coli strains and to capture protein biomarkers from bacterial cell lysates. Aside from the above Fe3O4-COOH particles, the MAA modification strategy was also used to modify carboxyl groups on the surfaces of microspheres. The resulting magnetic particles, Fe3O4@[Cu3(btc)2], were used for peptide enrichment, and they showed excellent affinity for the peptides, with a LOD of 10 pM [67].

Graphene-based composites have been completely and homogeneously coated with well-defined MOF layers in order to prepare magnetic MOF composites. Using a controllable self-assembly strategy, Cheng et al. [68] prepared MOF-coated magnetic graphene (MGMOF) composites. Briefly, magnetic graphene nanosheets were prepared through a solvothermal method and then densely and uniformly coated with the highly porous MOF MIL-100(Fe). The resulting MGMOF was then successfully employed to extract 27 peptides from human urine.

Phosphopeptide analysis

Protein phosphorylation plays a vital role in the control mechanisms for multiple biological processes such as cell division, growth, and signal transduction. Therefore, it is crucial to study protein phosphorylation if we are to gain a deep understanding of the phosphorylation pathways in biological processes at a molecular level. However, it is difficult to analyze phosphopeptides using MS detection without enriching the analytes because they are present at low levels and their signals are strongly suppressed in the presence of nonphosphorylated peptides [32, 69,70,71,72]. Thus, given their abundant metal sites and high specific surface areas, MOFs and their complexes had been used as novel affinity materials for the selective capture of phosphopeptides prior to MS analysis (Table 2).

Table 2 Recent applications of MOF-based affinity materials for the analysis of phosphopeptides
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In 2013, the MOF [Er2(PDA)3(H2O)]·2H2O, consisting of two Er(III) ions linked together by 1,4-phenylenediacetate, was successfully synthesized in a one-pot reaction [73]. This MOF was subsequently applied as an affinity material in the selective enrichment of 14 phosphopeptides from a standard protein digest mixture (α-casein, β-casein, and ovalbumin). Quantum mechanical calculations indicated that the binding energy of (CH3CH2PO4H)− (242.1 kcal mol−1) with Er(III) was much higher than those of CH3CH2COOH (88.3 kcal mol−1) and (H2O)2 (57.2 kcal mol−1) with Er(III), which explains why this MOF has such a high affinity for phosphopeptides. Similarly, the magnetic Er-containing MOF Fe3O4@PDA@Er(btc) was prepared by Xie et al. [74] and used as an affinity material to capture phosphopeptides.

Recently, two easily synthesized Zr-based MOFs (UiO-66 and UiO-67) were used by Zhu et al. [75] to capture phosphopeptides. The MOFs were applied to a tryptic digest of human serum to test their efficiencies and selectivities for phosphopeptides. Due to the pore sizes and surface areas of these MOFs, as well as the abundance of ZrO clusters in them, these MOFs facilitated the detection of four endogenous phosphopeptides with m/z values of 1389.6, 1460.8, 1545.6, and 1616.6 in human serum using MALDI-TOF MS. In order to improve upon the enrichment efficiency of UiO-66, a new approach to designing and synthesizing MOFs for use as adsorption materials was proposed by Chen et al. [76], who used a simple solvothermal approach to create another Zr-based MOF, UiO-66-(OH)2. Due to the presence of Zr–O–P bonds, this MOF showed a high affinity for phosphate groups and facilitated highly selective enrichment of phosphopeptides in a tryptic digest mixture of β-casein/BSA 1:100. In order to improve the enrichment efficiency of MOFs that are used to detect phosphopeptides, hydrophilic groups have been added to the surfaces of MOFs. Yang et al. [77] synthesized urea-modified MIL-101(Cr) (MIL-101(Cr)-UR2) using a facile two-step method and applied this MOF to enrich phosphopeptides. Nine phosphopeptides were detected after enrichment from a tryptic digest of non-fat milk.

Various magnetic MOFs that can enrich phosphopeptides more conveniently have also been prepared. Chen et al. [78] synthesized MAA-modified Fe3O4 nanoparticles that were covered with MIL-100(Fe) (denoted as Fe3O4@MIL-100(Fe)) using a layer-by-layer method. Owing to the rapid magnetic responsiveness, very large surface area, and uniform core–shell structure of this composite, it yielded a low detection limit (0.5 fmol) and high selectivity (β-casein:BSA 1:500) for phosphopeptides. Another example of a magnetic MOF composite, Fe3O4/MIL-101(Fe), was synthesized via electrostatic self-assembly involving positively charged MIL-101(Fe) and negatively charged Fe3O4 magnetic nanoparticles [79]. Due to the excellent magnetic properties and highly specific surface area of Fe3O4/MIL-101(Fe), it permitted 51 phosphorylation sites to be identified in a tryptic digest of tilapia egg proteins. Zhao et al. [80] prepared a magnetic Fe3O4@PDA@Zr-MOF with a core–shell–shell structure that was more hydrophilic than the two materials mentioned above using a simple PDA coating strategy, and then used this MOF as a novel IMAC material to enrich phosphopeptides. Due to strong chelation between Zr4+ and PDA, this magnetic MOF showed a good reusability (it could be used at least five times) with phosphopeptides. Moreover, the Fe3O4@PDA@Zr-MOF allowed greater enrichment of phosphopeptides than its precursor Fe3O4@PDA@Zr4+, implying that MOF shells enhance the interactions between phosphopeptides and Zr4+. Moreover, Peng et al. [81] prepared a dual Zr-MOF (DZMOF) with two Zr coordination environments for the highly selective capture of phosphopeptides (Fig. 2). The resulting DZMOF was zirconium-rich, as it included not only the immobilized Zr(IV) but also the inherent ZrO cluster. When applied in combination with IMAC and MOAC, DZMOF allowed both multi- and monophosphopeptides to be captured. As a result, 17 phosphopeptides, including 12 multiphosphopeptides and 5 monophosphopeptides, were detected in 5 μL of human saliva by MALDI-TOF MS/MS.

Fig. 2
figure 2

Schematic representation of a the preparation of DZMOF and b the capture of phosphopeptides in a biological sample. Reprinted with permission from [81]. Copyright 2016 by the American Chemical Society

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Magnetic graphene has been applied as a substrate material for immobilizing MOFs that are used to enrich phosphopeptides. Zhao et al. [82] synthesized a sandwich-structured composite of Zr-bdc MOF-modified, PDA-coated magnetic graphene (denoted magG@PDA@Zr-MOFs). This magnetic MOF was able to enrich 180 phosphopeptides with 261 phosphorylation sites from a tryptic digest of mouse liver.

The hard acid cations Fe(III), Zr(IV), and Er(III) have typically been used in phosphopeptide enrichment schemes owing to the hard basicity (according to the hard and soft acids and bases (HSAB) theory) of phosphopeptides [83]. However, alkaline earth metal cations such as Ca(II), Sr(II), and Ba(II), which are also regarded as hard Lewis acids, have only rarely been used to capture phosphopeptides. Recently, our group prepared 2D nanosheets of Egyptian blue (CaCuSi4O10) [84] and its analogues (SrCuSi4O10 and BaCuSi4O10), which were subsequently employed to enrich phosphopeptides for the first time [85]. Surprisingly, the 2D CaCuSi4O10 nanosheets were seen to be highly selective towards multiphosphopeptides but not monophosphopeptides across a wide range of buffer compositions and acidic conditions. Moreover, these nanosheets were employed in the phosphoproteome profiling of tau proteins, which are associated with Alzheimer’s disease and play a significant role in biomedicine.

In addition, MOFs can be utilized as precursors to create affinity materials with specific structures. For instance, Zhao et al. [86] prepared a hierarchical porous anatase TiO2 (HPT) through hydrolysis and thermal decomposition from MIL-125(Ti). Due to its large total pore volume, hierarchical porous structure, and substantial surface area, the obtained HPT was used as a multifunctional nanoreactor as well as for the in situ enrichment of phosphopeptides.

Two-dimensional (2D) MOF nanosheets are an emerging class of MOFs that have attracted considerable attention. They have abundant accessible active sites on their surfaces and large surface areas, in contrast to bulk MOFs. Recently, our group used exfoliated Ti-based MOF nanosheets with a well-defined 2D morphology, a high density of active sites, and an ultrathin structure to enrich phosphopeptides for the first time. These nanosheets permitted a superior detection limit of 0.1 fmol μL−1 in a phosphoprotein/nonphosphoprotein mixture containing an extremely low molar ratio of phosphoprotein (1:10000). Moreover, the 2D Ti-based MOF nanosheets were further used for the selective enrichment of phosphopeptides from complicated samples, including tryptic digests of mouse brain neocortex lysate, mouse spinal cord lysate, and mouse testis lysate [87].

Glycopeptide analysis

Protein glycosylation, one of the most frequent and significant post-translational modifications, is of great importance in protein function (e.g., in protein localization, protein–protein interactions, and enzymatic activity). Aberrant glycosylation of N- and O-linked glycoproteins is closely associated with a variety of diseases, such as neurodegenerative disease and cancer [88, 89]. In order to identify these disease biomarkers more effectively, it is necessary to determine glycoproteins and identify their glycosylation sites. The recent rapid development of MS has made it a highly effective technique for characterizing glycoproteins/glycopeptides [90]. However, due to the extremely low abundances and the relatively low ionization efficiencies of glycopeptides, it is still a challenge to directly analyze glycopeptides with MS. Thus, methods of enriching glycopeptides before MS analysis have been proposed that mainly involve introducing hydrophilic groups and specific affinity sites onto affinity materials. In this context, due to their readily modifiable surfaces and tunable frameworks, MOFs have been applied in the selective enrichment of glycopeptides (Table 3).

Table 3 Recent applications of MOF-based affinity materials for the analysis of glycopeptides
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Zhang et al. [91] synthesized an amino-functionalized MOF, MIL-101(Cr)-NH2, and then applied it to the highly specific enrichment of glycopeptides for the first time. Due to the special characteristics of this MOF, including its strong hydrophilicity, porous structure, and large surface area, it permitted a total of 116 glycopeptides and 42 different glycoproteins to be identified in a 10-μL human serum sample. The specificity and sensitivity of MIL-101(Cr)-NH2 for glycopeptides derives from the extra electrostatic interaction between the amino group on the functionalized MOF and the negatively charged carboxyl group on the sialic acid. Moreover, a maltose-functionalized MOF, MIL-101(Cr)-maltose, was prepared via a facile two-step postsynthesis of MIL-101(Cr)-NH2 by Ma et al. [92]. The parent MIL-101(Cr)-NH2 was treated with tert-butylnitrate (tBuONO) and azidotrimethylsilane (TMSN3) in tetrahydrofuran (THF) to form azide-functionalized MIL-101(Cr)-N3. After that, MIL-101(Cr)-maltose was obtained via a click reaction using 1-propargyl-O-maltose. Because of the presence of the maltose groups and the characteristics of the MOF, 15 and 33 glycopeptides were enriched by MIL-101(Cr)-maltose from tryptic digests of HRP and human immunoglobulin G (IgG) proteins, respectively. However, no peptides were detected after enrichment using MIL-101(Cr)-N3, illustrating that the azide groups do not contribute to glycopeptide enrichment. Moreover, 111 glycopeptides and 115 N-glycosylation sites corresponding to 65 glycoproteins were enriched by MIL-101(Cr)-maltose from only 5 μL of serum.

Other MOFs that facilitate the selective enrichment of glycopeptides via hydrophilic interactions have also been prepared. A crosslinked CD-MOF (LCD-MOF) was obtained that had γ-cyclodextrin as the ligand and possessed a nanosized cubic structure, superior hydrophilicity, and biocompatibility, allowing it to selectively enrich glycopeptides [93], in particular 20 glycopeptides from 67 fmol of a human IgG digest. Moreover, four glycopeptides with S/N > 3 were still observed when as little as 3.3 fmol of the tryptic digest of human IgG was examined, with satisfactory recoveries of 84–103%. Finally, when 100 μg of a tryptic digest of mouse liver were enriched by the LCD-MOF, 290 glycopeptides and 334 unique N-glycosylation sites were identified in a single LC-MS/MS experiment.

In addition to amino-functionalized MOFs, Ma et al. [94] prepared a cysteine-functionalized MOF via a straightforward two-step approach. Firstly, Au nanoparticles were loaded in situ onto an amino-derived MOF, MIL-101(NH2), and then L-cysteine (Cys) was immobilized. The resulting MIL-101(NH2)@Au-Cys was applied in the enrichment of N-linked glycopeptides, where it showed high glycopeptide enrichment efficiencies (16 and 31 glycopeptides were captured from digests of HRP and human IgG), a short incubation time (5 min), and a low detection limit (1 fmol) due to its ultrahigh hydrophilicity and large specific surface area (Fig. 3). Moreover, 1069 N-glycopeptides and 614 N-glycoproteins containing 1123 N-glycosylation sites were identified from a HeLa cell lysate. Thereafter, Wang et al. [95] synthesized a functional dual hydrophilic dendrimer-modified MOF, MIL-101(Cr)-NH2@PAMAM, in which poly(amidoamine) (PAMAM) was grafted onto MIL-101(Cr)-NH2, as a means to capture glycopeptides based on hydrophilic interactions. The obtained functional MOF exhibited a low detection limit (1 fmol μL−1) and good selectivity (glycopeptides:nonglycopeptides 1:100). Moreover, 92 N-glycopeptides and 46 unique glycoproteins were identified from the human serum digest.

Fig. 3
figure 3

Schematics of a the synthesis of MIL-101(NH2)@Au-Cys and b the glycopeptide enrichment process. Reprinted with permission from [94]. Copyright 2017 by the American Chemical Society

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Along with the postsynthesis modification of MOFs mentioned above, functional MOFs can also be prepared by introducing functional groups during the synthesis. Xie et al. [96] synthesized an amino-functionalized magnetic MOF (denoted Fe3O4@PDA@UiO-66-NH2) for glycopeptide enrichment. This MOF showed excellent hydrophilicity, an ultrahigh surface area, and strong magnetic responsiveness. Briefly, a magnetic Fe3O4 core was first prepared, and then the self-polymerization of dopamine onto the Fe3O4 was realized. Next, Zr3+ was fixed to the surface through a one-pot reaction of the MOF with an amino ligand. The resulting Fe3O4@PDA@UiO-66-NH2 exhibited excellent selectivity (HRP:BSA mass ratio 1:100) and good sensitivity (0.2 fmol μL−1) for glycopeptides. After enrichment by Fe3O4@PDA@UiO-66-NH2, a total of 301 N-glycopeptides and 121 glycoproteins were identified in a human serum digest by nanoLC-MS/MS. By varying the MOF shell, other magnetic materials were also obtained. Similarly, Fe3O4@PDA@Zr-SO3H was synthesized for use in the enrichment of glycopeptides [97]. Furthermore, a carboxylic-functionalized MOF (UiO-66-COOH) was prepared using ZrCl4 and binary ligands (isophthalic acid (IPA) and terephthalic acid (TPA)) by Liu et al. [98]. Compared to pure UiO-66 (with a single ligand), the glycopeptide enrichment efficiency of this MOF was much higher due to the presence of binary ligands with greater hydrophilicity. The selectivity of UiO-66-COOH was demonstrated by applying it to a tryptic digest mixture of HRP and nonglycosylated BSA with a HRP:BSA mass ratio of 1:20. After enrichment by the UiO-66-COOH, the glycopeptides were clearly observed with no interference from nonglycopeptides. Moreover, a total of 255 glycopeptides from 93 unique glycoproteins were identified in a human serum digest by nano-LC/MS/MS.

Aside from the above methods of obtaining MOF composites, MOFs can also be grafted onto the surface of magnetic graphene. Wang et al. [99] synthesized a glycopeptide-capturing composite of magnetic graphene and Zn-MOF (denoted MG@Zn-MOFs) by coating Zn-MOF (ZIF-8) crystals onto a magnetic graphene surface. Five glycopeptides were clearly detected in a tryptic digest of HRP by MALDI-TOF MS after enrichment by the MOF composite, even when the concentration of the digest was as low as 0.8 fmol μL−1. Moreover, 517 unique N-glycopeptides and 151 glycoproteins were identified in 1 μL of human serum digest after incubation with the MG@Zn-MOFs. Furthermore, a nanospherical magnetic composite, Fe3O4@Mg-MOF-74, was readily synthesized using an epitaxial growth strategy [100]. Due to its 1D-porous (channel-containing) structure, the enrichment efficiency of Fe3O4@Mg-MOF-74 was found to be unusually high. The results showed that 441 N-glycosylation sites corresponding to 418 N-glycopeptides from 125 glycoproteins were detected in 1 μL of human serum.

MOFs as matrices

In particular, due to their high absorption in the UV-visible range, MOFs can be used as matrices in mass spectrometry [101, 102] (Table 4). The first time that MOFs were used as matrices in the analysis of small molecules by MALDI-TOF MS was by Shih et al. in 2013 [103]. They synthesized a series of MOFs, including MIL-100(Fe), MIL-100(Cr), MIL-100(Al), MIL-101(Cr), DUT-4(Al), DUT-5(Al), and CYCU-3(Al). The efficiencies of these MOFs as matrices were investigated using five polycyclic aromatic hydrocarbons (PAHs) as test analytes: anthracene (Ant), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), and benzo[a]pyrene (BaP) (Fig. 4). Among these MOFs, the cage type MIL-100(Fe) yielded the best signal shot-to-shot reproducibility of 2%. Moreover, MIL-100(Fe), MIL-100(Cr), and MIL-100(Al) were used to assess the effects of different metal ions on the performance when analyzing PAHs. The results indicated that PAH intensity and signal reproducibility varied depending on the metal ions present in the MOF matrices.

Table 4 Recent applications of MOFs as matrices for the determination of analytes
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Fig. 4
figure 4

Schematic of the MALDI-MS process using porous MIL-100(Fe) as a matrix. Reprinted with permission from [103]. Copyright 2013 by the Royal Society of Chemistry

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Composites of MOFs have also been synthesized and investigated as potential matrices for MALDI-MS analysis. For example, Lin et al. [104] recently prepared a ZIF-8-coated magnetic nanocomposite (denoted Fe3O4@ZIF-8 MNCs) by a straightforward method and then used it as a matrix for the sensitive analysis of small molecules. Due to the combination of the unique properties of ZIF-8 and the Fe3O4 magnetic nanoparticles, the as-prepared Fe3O4@ZIF-8 MNCs exhibited strong UV-visible absorption, a large specific surface, superparamagnetic behavior, and excellent thermal stability. The results indicated that this composite MOF provided an interference-free background, good salt tolerance, and good reproducibility, and it allowed the quantitative analysis of melatonin at different concentrations in human urine and serum samples (R2 = 0.996).

Recently, Yang et al. [105] prepared a series of ZIFs, including ZIF-7, ZIF-8, and ZIF-90. Compared with ZIF-7 and ZIF-90, ZIF-8 exhibited better MS performance—such as low background interference, good reproducibility, and high signal intensity—due to its large surface area and small particle size. Five bisphenol environmental pollutants—BPA, bisphenol B (BPB), bisphenol F (BPF), bisphenol S (BPS), and bisphenol AF (BPAF)—were detected using the ZIFs as matrices in negative ion mode, with LODs in the range 1.17–2.98 ng mL−1.

Chen et al. [76] introduced a new approach to designing and synthesizing MOFs for use as matrices that was based on the structural similarity of ligands and common matrices. Since 2,5-pyridinedicarboxylic acid (PDC) and 2,5-dihydroxyterephthalic acid (DHT) have similar chemical structures to the traditional matrix materials 2-picolinic acid (PA) and 2,5-dihydroxybenzoic acid (DHB), they were selected as ligands to synthesize MOFs. In this work, two Zr(IV)-based MOFs (UiO-66-PDC and UiO-66-(OH)2) were obtained and used as matrices for the analysis of low molecular weight compounds by MALDI-MS. UiO-66-(OH)2 demonstrated excellent performance when applied to the quantitative determination of glucose (R2 = 0.997).

Recently, Shih et al. [106] prepared two carbonized MOFs (cMIL-53 and cCYCU-3) with large pore volumes and high surface areas. In contrast to conventional carbon-based materials, these carbonized MOFs are hydrophilic without requiring surface modification, and were therefore used as matrices. Due to the high UV absorptivities and low heat capacities of the carbonized MOFs, they permitted much higher desorption/ionization efficiencies for nonpolar and polar small-molecule analytes and produced no matrix interference in the low-mass region, unlike the pristine MOFs.

As well as the MOFs used as matrices that are mentioned above, carbon-based nanomaterials such as graphene and mesoporous carbon can also be employed as matrices for MALDI MS because of their inherent advantages. Liu et al. [107] presented a graphene-based matrix for use in negative ion MALDI MS analysis of flavonoids and derivatives of coumarin. The results illustrated that the signal intensity was higher for flavonoids when GO was used as the matrix rather than rGO, even at a low concentration (1 pmol μL−1). They also showed that the lateral size of the GO affected the desorption/ionization efficiency.

Moreover, ordered mesoporous carbons have been investigated as possible matrices, and they have been applied for the high-throughput screening and identification of toxic chemicals in a single drop of human whole blood. Huang et al. [108] reported matrices based on several commercial mesoporous materials (CMK-3, CMK-8, SBA-15, and MCM-41), and the results demonstrated that CMK-8 provided the best performance in negative-ion MALDI MS. CMK-8 allowed high sensitivity (detection limits at ppt levels) and good reproducibility in the analysis of typical toxic compounds. Moreover, six perfluorinated compounds (PFCs) in individual drops of whole blood collected from workers in a perfluorochemical plant were successfully identified and screened by the established method. This research highlighted how MOFs could be used in real-world applications.

Recently, our group used water-stable 2D Zn2(bim)4 nanosheets with high surface area and ionization efficiency as well as efficient photon absorption as a candidate matrix for MALDI MS [109]. These homogeneous and stable 2D Zn2(bim)4 nanosheets offer a large accessible surface and a stable layered structure (Fig. 5a) that exhibits a strong Tyndall effect (Fig. 5b and c). They also provide clean background MS spectra in dual-ion modes (Fig. 5d and e) and good reproducibility in the analysis of amino acids, neurotransmitters, nucleobases, hormones (Fig. 5f and g), and pollutant molecules. However, the 2D Zn2(bim)4 nanosheet matrix was found to degrade in the presence of a high salt concentration; for instance, 1000 mM NH4HCO3 or NaCl. Thereafter, 2D NTU-9 nanosheets exfoliated from a bulk Ti-MOF (NTU-9) were also investigated as a promising matrix by our group [110]. Compared to the other six water-stable MOFs, the obtained NTU-9 nanosheets produced a much lower matrix background, showed ultrahigh ionization efficiency, and exhibited high acid resistance in the analysis of saccharides in both negative-ion and positive-ion reflector modes. Given their advantages, including speed and high throughput, the NTU-9 nanosheets were successfully applied to the quantitative determination of glucose in human serum (R2 = 0.999), which could open up a new research avenue in the field of matrices based on 2D MOF nanosheets.

Fig. 5
figure 5

a TEM image of 2D Zn2(bim)4 nanosheets. Photographs of b a pristine aqueous suspension of Zn2(bim)4 and c a colloidal suspension of 2D Zn2(bim)4 nanosheets. d Mass spectra of the three amino acids Glu, His, and Trp obtained in positive-ion and e negative-ion reflector modes. Ions that produce background interference are shown as red stars. MALDI-TOF mass spectra of serotonin obtained with a matrix of 2D Zn2(bim)4 MOF nanosheets in f positive-ion and g negative-ion reflector modes. Reprinted with permission from [109]. Copyright 2016 by the Royal Society of Chemistry

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Conclusion

In summary, we have reviewed the recent applications of MOFs in MALDI MS analysis. Due to their specific porosities, affinities, and photon absorption capacities, MOFs can play multiple roles in MALDI MS. First, they can be applied as highly efficient adsorbents for enriching peptides, which makes use of the inherent interactions between peptides and MOFs. MALDI is a method that allows rapid evaluation of peptides following enrichment. Although MOFs have not been widely used in the field of LC-MS, the characteristics mentioned above make MOFs an appealing class of materials for potential use in detailed proteome/peptidome analysis. Second, MOFs can be used as a new type of matrix in MALDI MS, where they exhibit excellent sensitivities and high ionization efficiencies and produce a clear MS background across a wide mass range. This feature can expand the range of analytes that can be analyzed using MALDI MS to include both large and small molecules. Thus, the application of MOFs to MALDI MS has pushed the boundaries of this MS technology.

Nevertheless, it is important to recognize that there are still numerous challenges to overcome in the application of MOFs to MALDI MS. For instance, more studies of the mechanisms for MOF-assisted MALDI processes are needed, so that general guidelines on the selection of MOFs or the modification strategies needed for different analytes can be identified and reported. Also, research into the application of MOFs for quantitative analysis using LC-MS must be improved. Because nano-MOFs and 2D MOF nanosheets provide more stable and abundant active sites than other materials can, quantitative applications of MOFs in LC-MS are in great demand. Finally, most of the newly developed applications of MOFs in MALDI MS have only been investigated at a fundamental research level, so their feasibility when used for complex real samples requires further evaluation.