Xiu-Ping Yan

Metal-organic frameworks for analytical chemistry: from sample collection to chromatographic separation.

In modern analytical chemistry researchers pursue novel materials to meet analytical challenges such as improvements in sensitivity, selectivity, and detection limit. Metal-organic frameworks (MOFs) are an emerging class of microporous materials, and their unusual properties such as high surface area, good thermal stability, uniform structured nanoscale cavities, and the availability of in-pore functionality and outer-surface modification are attractive for diverse analytical applications. This Account summarizes our research on the analytical applications of MOFs ranging from sampling to chromatographic separation. MOFs have been either directly used or engineered to meet the demands of various analytical applications. Bulk MOFs with microsized crystals are convenient sorbents for direct application to in-field sampling and solid-phase extraction. Quartz tubes packed with MOF-5 have shown excellent stability, adsorption efficiency, and reproducibility for in-field sampling and trapping of atmospheric formaldehyde. The 2D copper(II) isonicotinate packed microcolumn has demonstrated large enhancement factors and good shape- and size-selectivity when applied to on-line solid-phase extraction of polycyclic aromatic hydrocarbons in water samples. We have explored the molecular sieving effect of MOFs for the efficient enrichment of peptides with simultaneous exclusion of proteins from biological fluids. These results show promise for the future of MOFs in peptidomics research. Moreover, nanosized MOFs and engineered thin films of MOFs are promising materials as novel coatings for solid-phase microextraction. We have developed an in situ hydrothermal growth approach to fabricate thin films of MOF-199 on etched stainless steel wire for solid-phase microextraction of volatile benzene homologues with large enhancement factors and wide linearity. Their high thermal stability and easy-to-engineer nanocrystals make MOFs attractive as new stationary phases to fabricate MOF-coated capillaries for high-resolution gas chromatography (GC). We have explored a dynamic coating approach to fabricate a MOF-coated capillary for the GC separation of important raw chemicals and persistent organic pollutants with high resolution and excellent selectivity. We have combined a MOF-coated fiber for solid-phase microextraction with a MOF-coated capillary for GC separation, which provides an effective MOF-based tandem molecular sieve platform for selective microextraction and high-resolution GC separation of target analytes in complex samples. Microsized MOFs with good solvent stability are attractive stationary phases for high-performance liquid chromatography (HPLC). These materials have shown high resolution and good selectivity and reproducibility in both the normal-phase HPLC separation of fullerenes and substituted aromatics on MIL-101 packed columns and position isomers on a MIL-53(Al) packed column and the reversed-phase HPLC separation of a wide range of analytes from nonpolar to polar and acidic to basic solutes. Despite the above achievements, further exploration of MOFs in analytical chemistry is needed. Especially, analytical application-oriented engineering of MOFs is imperative for specific applications.

Doped quantum dots for chemo/biosensing and bioimaging

Quantum dots (QDs) have received great interest for diverse applications due to their distinct advantages, such as narrow and symmetric emission with tunable colors, broad and strong absorption, reasonable stability, and solution processibility. Doped QDs not only potentially retain almost all of the above advantages, but also avoid the self-quenching problem due to their substantial ensemble Stokes shift. Two obvious advantages of doped QDs, especially doped ZnS QDs, over typical CdSe@ZnS and CdTe QDs are longer dopant emission lifetime and potentially lower cytotoxicity. The lifetime of dopant emission from transition-metal ion or lanthanide ion-doped QDs is generally longer than that of the bandgap or defect-related emission of host, and that of biological background fluorescence, providing great opportunities to eliminate background fluorescence for biosensing and bioimaging. For bioimaging applications, fluorescent dopants may mitigate toxicity problems by producing visible or infrared emission in nanocrystals made from less-harmful elements than those currently used. In this review, recent advances in utilizing doped QDs for chemo/biosensing and bioimaging are discussed, and the synthetic routes and optical properties of doped QDs that make them excellent probes for various strategies in chemo/biosensing and bioimaging are highlighted. Moreover, perspectives on future exploration of doped QDs for chemo/biosensing and bioimaging are also given.

Metal–Organic Framework MIL-101 for High-Resolution Gas-Chromatographic Separation of Xylene Isomers and Ethylbenzene†

Metal–organic frameworks (MOFs) have received great attention because of their fascinating structures and intriguing applications in hydrogen storage, gas separation, catalysis, chiral separation, sensing, and imaging. Recently, MOFs such as MOF-508, MIL-47, and MIL53 have been shown to be promising as stationary phases for gas chromatography (GC) and liquid chromatography. 11] All these pioneering works on the utilization of MOFs as stationary phases in chromatography were performed on packed columns. However, packed columns usually result in low resolution as a result of peak broadening, which impairs the separation efficiency of MOFs. Moreover, gram-scale MOFs are needed for packed columns, leading to high-cost applications of MOFs as stationary phases in chromatographic separation. In contrast, capillary columns, either wall-coated open tubular (WCOT) columns or porous layer open tube (PLOT) columns, involve a thin film of MOFs coated on their inner walls, and thus improve the resolving power of MOFs and reduce the amount of MOFs required for GC applications. However, to the best of our knowledge, no work on the utilization of MOFs as stationary phases for high-resolution capillary GC separation has been reported so far. Herein we show the first fabrication of the MOF-coated capillary column for high-resolution GC separation. For a proof-of-concept demonstration, we choose MIL-101 as the stationary phase and xylene isomers and ethylbenzene (EB) as the targets for separation. Xylene isomers and EB are important raw chemicals in industry; in particular, p-xylene is used in the manufacture of terephthalic acid for the polyester industry. The separation and detection of individual xylene isomers and EB are also of environmental concern, and of great practical interest in air monitoring and blood analysis. For these reasons, numerous stationary phases have been developed for GC separation of xylene isomers and EB, for example, 7,8benzoquinoline, tetrachlorophthalate, 1,8-diaminonaphthalene, modified organo-clays Bentone-34, liquid-crystalline compounds, b-cyclodextrin derivatives, poly(ethylene glycol), and MIL-47. However, long analysis time (27–90 min) or temperature programming is often needed. MIL-101 is a chromium terephthalate MOF with coordinatively unsaturated sites (CUS). We utilized MIL-101 as the stationary phase because of its high surface area, large pores (2.9–3.4 nm), accessible CUS, and excellent chemical and thermal stability, which make it an attractive candidate for isomer separation. However, MIL-101 has never been explored as the stationary phase for chromatographic separation before, even though the tiny crystal size characteristic of MIL-101 is beneficial to the fabrication of MIL-101 coated capillary columns by a dynamic coating method. In this work, we prepared the MIL-101 coated capillary column and achieved a baseline separation of p-xylene, oxylene, m-xylene, and EB on the fabricated MOF coated capillary column by GC within 1.6 min without the need for temperature programming (Figure 1).

Surface molecular imprinting on Mn-doped ZnS quantum dots for room-temperature phosphorescence optosensing of pentachlorophenol in water.

A new type of molecularly imprinted polymer (MIP)-based room-temperature phosphorescence (RTP) optosensor was developed by anchoring the MIP layer on the surface of Mn-doped ZnS quantum dots (QDs) via a surface molecular imprinting process. The synergetic combination of the RTP property of the Mn-doped ZnS QDs and the merits of the surface imprinting polymers not only improves the RTP selectivity of the Mn-doped ZnS QDs but also makes the MIP-based RTP optosensor also applicable to selective detecting of those nonphosphorescent analytes without the need for any inducers and derivatization. The new MIP-based RTP sensing protocol was applied to detect trace pentachlorophenol (PCP) in water samples without the interference of autofluorescence and scattering light of matrixes. The detection limit for PCP was 86 nM, and the precision for five replicate detections of 0.4 microM PCP was 2.8% (relative standard deviation). The recovery of spiked PCP in river water samples ranged from 93% to 106%.

Zeolitic imidazolate framework-8 nanocrystal coated capillary for molecular sieving of branched alkanes from linear alkanes along with high-resolution chromatographic separation of linear alkanes.

A zeolitic imidazolate framework-8 (ZIF-8) nanocrystal coated capillary is shown not only to have a strong ability to sieve branched alkanes from linear alkane isomers owing to the narrow pore windows but also to offer excellent features for high-resolution gas chromatographic separation of linear alkanes due to van der Waals interaction between linear alkanes and the hydrophobic inner surfaces of the micropores. This makes the ZIF-8 coated capillary very promising for the specific adsorption and separation of alkanes in complicated matrices.

In Situ Hydrothermal Growth of Metal−Organic Framework 199 Films on Stainless Steel Fibers for Solid-Phase Microextraction of Gaseous Benzene Homologues

Metal-organic frameworks (MOFs) have received great attention due to their fascinating structures and intriguing potential applications in various fields. Herein, we report the first example of the utilization of MOFs for solid-phase microextraction (SPME). MOF-199 with unique pores and open metal sites (Lewis acid sites) was employed as the coating for SPME fiber to extract volatile and harmful benzene homologues. The SPME fiber was fabricated by in situ hydrothermal growth of thin MOF-199 films on etched stainless steel wire. The MOF-199-coated fiber not only offered large enhancement factors from 19,613 (benzene) to 110,860 (p-xylene), but also exhibited wide linearity with 3 orders of magnitude for the tested benzene homologues. The limits of detection for the benzene homologues were 8.3-23.3 ng L(-1). The relative standard deviation (RSD) for six replicate extractions using one SPME fiber ranged from 2.0% to 7.7%. The fiber-to-fiber reproducibility for three parallel prepared fibers was 3.5%-9.4% (RSD). Indoor air samples were analyzed for the benzene homologues using the SPME with the MOF-199-coated fiber in combination with gas chromatography-flame ionization detection. The recoveries for the spiked benzene homologues in the collected indoor air samples were in the range of 87%-106%. The high affinity of the MOF-199-coated fiber to benzene homologues resulted from the combined effects of the large surface area and the unique porous structure of the MOF-199, the pi-pi interactions of the aromatic rings of the analytes with the framework 1,3,5-benzenetricarboxylic acid molecules, and the pi-complexation of the electron-rich analytes to the Lewis acid sites in the pores of MOF-199.

Conjugation of glucose oxidase onto Mn-doped ZnS quantum dots for phosphorescent sensing of glucose in biological fluids.

Integrating various enzymes with nanomaterials provides various nanohybrids with new possibilities in biosensor applications. Furthermore, the enzymatic activity and stability are also improved due to the large surface area of nanomaterials. Here we report the conjugation of glucose oxidase (GOD) onto phosphorescent Mn-doped ZnS quantum dots (QDs) using 1-ethyl-3-(3-dimethylaminopropy)carbodiimide (EDC)/N-hydroxysuccinimide (NHS) as coupling reagents for glucose biosensing based on the effective quenching of the room temperature phosphorescence (RTP) of Mn-doped ZnS QDs by the H(2)O(2) generated from GOD-catalyzed oxidation of glucose. The obtained bioconjugate not only provided improved enzymatic performance with Michaelis-Menten constant of 0.70 mM but also favored biological applications because the phosphorescent detection mode avoided the interference from autofluorescence and scattering light from the biological matrix. In addition, the GOD-conjugated Mn-doped ZnS QDs showed better thermal stability in the temperature range of 20-80 degrees C. The GOD-Mn-doped ZnS QDs based RTP sensor for glucose gave a detection limit of 3 microM and two linear ranges from 10 microM to 0.1 mM and from 0.1 to 1 mM. The developed biosensor was successfully applied to the determination of glucose in real serum samples without the need for any complicated sample pretreatments.

Facile magnetization of metal–organic framework MIL-101 for magnetic solid-phase extraction of polycyclic aromatic hydrocarbons in environmental water samples

The unusual properties such as high surface area, good thermal stability, uniform structured nanoscale cavities and the availability of in-pore functionality and outer-surface modification make metal-organic frameworks (MOFs) attractive for diverse analytical applications. However, integration of MOFs with magnets for magnetic solid-phase extraction for analytical application has not been attempted so far. Here we show a facile magnetization of MOF MIL-101(Cr) for rapid magnetic solid-phase extraction of polycyclic aromatic hydrocarbons (PAHs) from environmental water samples. MIL-101 is attractive as a sorbent for solid-phase extraction of pollutants in aqueous solution due to its high surface area, large pores, accessible coordinative unsaturated sites, and excellent chemical and solvent stability. In situ magnetization of MIL-101 microcrystals as well as magnetic solid-phase extraction of PAHs was achieved simultaneously by simply mixing MIL-101 and silica-coated Fe(3)O(4) microparticles in a sample solution under sonication. Such MOF-based magnetic solid-phase extraction in combination with high-performance liquid chromatography gave the detection limits of 2.8-27.2 ng L(-1) and quantitation limits of 6.3-87.7 ng L(-1) for the PAHs. The relative standard deviations for intra- and inter-day analyses were in the range of 3.1-8.7% and 6.1-8.5%, respectively. The results showed that hydrophobic and π-π interactions between the PAHs and the framework terephthalic acid molecules, and the π-complexation between PAHs and the Lewis acid sites in the pores of MIL-101 play a significant role in the adsorption of PAHs.

Functional Near Infrared-Emitting Cr3+/Pr3+ Co-Doped Zinc Gallogermanate Persistent Luminescent Nanoparticles with Superlong Afterglow for in Vivo Targeted Bioimaging

Near infrared (NIR)-emitting persistent luminescent nanoparticles (PLNPs) have great potential for in vivo bioimaging with the advantages of no need for in situ excitation, high signal-to-noise ratio, and deep tissue penetration. However, functional NIR-emitting PLNPs with long afterglow for long-term in vivo imaging are lacking. Here, we show the synthesis of NIR-emitting long-persistent luminescent nanoparticles (LPLNPs) Zn2.94Ga1.96Ge2O10:Cr(3+),Pr(3+) by a citrate sol-gel method in combination with a subsequent reducing atmosphere-free calcination. The persistent luminescence of the LPLNPs is significantly improved via codoping Pr(3+)/Cr(3+) and creating suitable Zn deficiency in zinc gallogermanate. The LPLNP powder exhibits bright NIR luminescence in the biological transparency window with a superlong afterglow time of over 15 days. A persistent energy transfer between host and Cr(3+) ion in the LPLNPs is observed and its mechanism is discussed. PEGylation greatly improves the biocompatibility and water solubility of the LPLNPs. Further bioconjugation with c(RGDyK) peptide makes the LPLNPs promising for long-term in vivo targeted tumor imaging with low toxicity.

Metal–Organic-Framework-Based Tandem Molecular Sieves as a Dual Platform for Selective Microextraction and High-Resolution Gas Chromatographic Separation of n-Alkanes in Complex Matrixes

Metal-organic frameworks (MOFs) were employed to design tandem molecular sieves as a dual platform for selective solid-phase microextraction (SPME) and high-resolution gas chromatographic (GC) separation of target analytes in complex matrixes. An elegant combination of a ZIF-8-coated fiber for SPME with a ZIF-8-coated capillary for GC allows selective extraction and separation of n-alkanes from complex matrixes such as petroleum-based fuel and biological fluids. The proposed tandem ZIF-8 molecular sieves not only offered good enhancement factors from 235 (hexane) to 1212 (nonane), but also exhibited wide linearity with 3 orders of magnitude for the tested linear alkanes. The limits of detection for the linear alkanes ranged from 0.46 ng L(-1) (nonane) to 1.06 ng L(-1)(hexane). The relative standard deviations of retention time, peak area, peak height, and half peak width for five replicate determinations of the tested n-alkanes at 30 ng L(-1) were 0.02-0.26%, 1.9-8.6%, 1.4-6.0%, and 1.3-7.2%, respectively. The developed tandem ZIF-8 molecular sieves were further used for the determination of linear alkanes in petroleum-based fuel and human serum. The large diversity in structure and pore size allows various combinations of MOFs for designing an MOF-based tandem molecular sieve platform to achieve different selectivities in extraction and chromatographic separation and to solve headache problems in complex real sample analysis.