The rational design of artificial solid-state nanopores is of great importance in the discovery of intriguing ion transport phenomena. 2D metal-organic framework (2D MOF) nanosheets with single crystallinity, aligned nanochannels, ultrathin thickness, and diverse functionalities are highly potential solid-state nanopores. An electrophoretic method is developed to successfully fabricate MOF nanopores supported by SiN x substrate, which is confirmed by high-resolution transmission electron microscopy. A giant gap around 4 V together with ionic current rectification is discovered in nonlinear voltage-activated current-voltage curves, revealing the synergy of the hydrophobic effect and charge effect in MOF nanopores. The charge effect embodies the different contribution current which results from the enrichment and depletion of ions in MOF nanopores by COMSOL simulation. Moreover, 2D MOF nanosheets with different surface charges, hydrophobicity, and pore sizes demonstrate the universality of nanopore fabrication and further confirm the synergistic mechanism. The nonlinear ion transport in the ultrathin MOF nanosheets will provide an opportunity to explore further applications in solid-state nanopores.

Two-dimensional (2D) metal-organic framework (MOF) nanosheets, as an emerging type of 2D materials, attract numerous attention due to their unique properties. First, the ultrathin thickness and nanoscale of the materials results in homogeneous dispersion in aqueous solution, giving the materials more opportunities to be utilized in solution chemistry, especially beneficial to the biomimetic catalysis and bio-related analytical applications. Second, the large surface area and accessible active sites of the MOF nanosheets are favorable to the binding between materials and the substrate, leading to their superior performance in catalysis, sensing and enzyme inhibition. Third, the suitable sizes of nanopores on the 2D MOF nanosheets give them the abilities to act as membranes for highly selective and energy-saving gas separation. This minireview covers the synthesis, characterization as well as bio-related and separation applications of 2D MOF nanosheets.

Capturing phosphopeptides from complicated biological samples is essential for the discovery of new posttranslational modification sites and disease diagnostics. Although several two-dimensional (2-D) materials have been used for phosphopeptides capturing, metal−organic framework (MOF) nanosheets have not been reported. The Ti-based MOF nanosheets have well-defined 2-D morphology, high density of active sites, large surface area, and an ultrathin structure. Phosphopeptides can be efficiently extracted and superior detection limits of 0.1 fmol μL −1 can be achieved even for an extremely low molar ratio of phosphoprotein/nonphosphoprotein (1:10000) mixtures. The selectivity over nonphosphopeptides can be enhanced further by pretreatment with a 10 mM salt solution (βglycerophosphate disodium, NaCl, or KCl). The performance of 2-D Ti-based MOF nanosheets is much better than Zr-based MOF (Zr-BTB) nanosheets or any other Ti-based 3-D MOF counterpart, such as MIL-125 and NH 2 -MIL-125. The nanosheets were used for in situ isotope labeling for abnormally regulated phosphopeptides analysis from serum samples of type 2 diabetes patients. The relative quantitative results showed that three of the phosphorylated fibrinogen peptides A (FPA, DpSGEGDFLAEGGGV, DpSGEGDFLAEGGGVR, and ADpSGEGDFLAEGGGVR) were down-regulated, while the other isoform (ADpSGEGDFLAEGGGV) was up-regulated in the serum samples of type 2 diabetes patients compared with those of healthy volunteers. Finally, proteomics analysis showed selective enrichment of phosphopeptides with 2-D Ti-based MOF nanosheets from real samples, including tryptic digests of mouse brain neocortex lysate, mouse spinal cord lysate, and mouse testis lysate, followed by LC-MS/MS analysis. Total numbers of 2601, 3208, and 2866 phosphopeptides were successfully identified from the three samples, respectively. The 2-D Ti-based MOF nanosheets significantly improved sample preparation for mass spectrometric analysis in phosphopeptides and phosphoproteomics research.

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Solid-state nanopore as a versatile alternative to biological nanopore has grown tremendously over the last two decades. It exhibits unique characteristics including mechanical robustness, thermal and chemical stability, easy modifications...

Separation of monophosphopeptides from multiphosphopeptides in complex biological samples is significant in the study of protein kinase signal transduction pathways. To the best of our knowledge, very few materials have been reported that could selectively enrich monophosphopeptides because of the chemical difficulty in retaining the intermediate monophosphopeptides and excluding both non-phosphopeptides and multiphosphopeptides in acidic conditions, which requires unique interactions to balance the metallic affinity and the hydrophobicity. With the large surface area, abundant accessible active sites, and ultrathin structures, two-dimensional (2-D) metal− organic framework (MOF) Hf-1,3,5-tris(4-carboxyphenyl)benzene (BTB) nanosheets were rationally selected. Due to the elongated organic ligands and the balance between metallic affinity of clusters and hydrophobicity from ligands, the 2-D Hf-BTB nanosheets exhibited unique enrichment selectivity toward monophosphopeptides. The 2-D MOF nanosheets demonstrated excellent sensitivity (detection limit of 0.4 fmol μL −1 ) and selectivity [1:1000 molar ratios of β-casein/BSA (bovine serum albumin)] in model phosphopeptides enrichment. The nanosheets were implemented for the analysis of nonfat milk and human saliva samples as well as in situ isotope labeling for dysregulated phosphopeptides from patients' serum with anal canal inflammation, exhibiting 6.6-fold upregulation of serum phosphopeptide HS4 (ADpSGEGDFLAEGGGVR) compared to the control healthy serum. The proteomics analysis of mouse brain cortical samples associated with Alzheimer's disease, which were from Akt (protein kinase B) conditional knockout mouse and littermate control mouse, was further established with 2-D Hf-BTB nanosheets. With high capture efficiency for monophosphopeptides, this method was capable of distinguishing the difference of monophosphopeptides from microtubuleassociated protein τ (MAPT/τ) between the Akt knockout sample and control sample.

In order to investigate the effect of the allelopathic compound pyrogallic acid on Cylindrospermopsis raciborskii F2, the impact on growth, oxidative stress and expression of the psbA, grpE, fabZ, recA, cmpA, ftsZ and cyrJ genes were studied. The results indicated significant decreases in Chl a and cell number following a 24-h incubation with 4 mg L(-1) pyrogallic acid. Additionally, malodialdehyde content, superoxide dismutase activity and catalase activity were enhanced following treatment with 2 and 4 mg L(-1) pyrogallic acid. Expressions of the genes psbA, grpE, fabZ, recA and cyrJ were significantly up-regulated following exposure to 4 mg L(-1) pyrogallic acid, while no changes were observed with concentrations of 1 or 2 mg L(-1). Expression of cmpA was significantly down-regulated following treatment with the lowest tested concentration of pyrogallic acid (1 mg L(-1)), while ftsZ was only significantly down-regulated with concentrations of 2 and 4 mg L(-1). These results suggest that photosynthesis inhibition and oxidative damage are important modes of action for the allelopathic effect of pyrogallic acid on C. raciborskii.

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