Yongxuan Su

Postsynthetic ligand exchange as a route to functionalization of ‘inert’ metal–organic frameworks

Herein, we report that the exchange of ligands from an intact metal–organic framework (MOF) can be exploited as a means to introduce functionalized ligands into MOFs under mild conditions. It is shown that ligand exchange can occur with ‘inert’ Zr(IV)-based UiO-66 MOFs in a solvent dependent manner. We call this process postsynthetic exchange (PSE) and show that it provides access to MOFs that are not readily prepared in high quality by solvothermal methods. It was found that ligand exchange can occur between UiO-66 MOFs as monitored by aerosol time-of-flight mass spectrometry (ATOFMS). ATOFMS was used to analyze the chemical composition of microcrystalline MOFs on the single particle level, providing information not available through bulk analysis. PSE is an important postsynthetic approach to the modification of MOFs, and the ligand exchange revealed by ATOFMS requires a re-evaluation of the assumed ‘stability’ of even the most robust MOFs.

Measurements of Isoprene-Derived Organosulfates in Ambient Aerosols by Aerosol Time-of-Flight Mass Spectrometry - Part 1: Single Particle Atmospheric Observations in Atlanta

Organosulfate species have recently been identified as a potentially significant class of secondary organic aerosol (SOA) species, yet little is known about their behavior in the atmosphere. In this work, organosulfates were observed in individual ambient aerosols using single particle mass spectrometry in Atlanta, GA during the 2002 Aerosol Nucleation and Characterization Experiment (ANARChE) and the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS). Organosulfates derived from biogenically produced isoprene were detected as deprotonated molecular ions in negative-ion spectra measured by aerosol time-of-flight mass spectrometry; comparison to high-resolution mass spectrometry data obtained from filter samples corroborated the peak assignments. The size-resolved chemical composition measurements revealed that organosulfate species were mostly detected in submicrometer aerosols and across a range of aerosols from different sources, consistent with secondary reaction products. Detection of organosulfates in a large fraction of negative-ion ambient spectra - ca. 90-95% during ANARChE and ~65% of submicrometer particles in AMIGAS - highlights the ubiquity of organosulfate species in the ambient aerosols of biogenically influenced urban environments.

Development and Characterization of an Aircraft Aerosol Time-of-Flight Mass Spectrometer

Vertical and horizontal profiles of atmospheric aerosols are necessary for understanding the impact of air pollution on regional and global climate. To gain further insight into the size-resolved chemistry of individual atmospheric particles, a smaller aerosol time-of-flight mass spectrometer (ATOFMS) with increased data acquisition capabilities was developed for aircraft-based studies. Compared to previous ATOFMS systems, the new instrument has a faster data acquisition rate with improved ion transmission and mass resolution, as well as reduced physical size and power consumption, all required advances for use in aircraft studies. In addition, real-time source apportionment software allows the immediate identification and classification of individual particles to guide sampling decisions while in the field. The aircraft (A)-ATOFMS was field-tested on the ground during the Study of Organic Aerosols in Riverside, CA (SOAR) and aboard an aircraft during the Ice in Clouds Experiment-Layer Clouds (ICE-L). Initial results from ICE-L represent the first reported aircraft-based single-particle dual-polarity mass spectrometry measurements and provide an increased understanding of particle mixing state as a function of altitude. Improved ion transmission allows for the first single-particle detection of species out to approximately m/z 2000, an important mass range for the detection of biological aerosols and oligomeric species. In addition, high time resolution measurements of single-particle mixing state are demonstrated and shown to be important for airborne studies where particle concentrations and chemistry vary rapidly.

Measurements of isoprene-derived organosulfates in ambient aerosols by aerosol time-of-flight mass spectrometry - Part 2: Temporal variability and formation mechanisms

Organosulfate species have recently gained attention for their potentially significant contribution to secondary organic aerosol (SOA); however, their temporal behavior in the ambient atmosphere has not been probed in detail. In this work, organosulfates derived from isoprene were observed in single particle mass spectra in Atlanta, GA during the 2002 Aerosol Nucleation and Characterization Experiment (ANARChE) and the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS). Real-time measurements revealed that the highest organosulfate concentrations occurred at night under a stable boundary layer, suggesting gas-to-particle partitioning and subsequent aqueous-phase processing of the organic precursors played key roles in their formation. Further analysis of the diurnal profile suggests possible contributions from multiple production mechanisms, including acid-catalysis and radical-initiation. This work highlights the potential for additional SOA formation pathways in biogenically influenced urban regions to enhance the organic aerosol burden.

Exploiting adaptive laboratory evolution of Streptomyces clavuligerus for antibiotic discovery and overproduction.

Adaptation is normally viewed as the enemy of the antibiotic discovery and development process because adaptation among pathogens to antibiotic exposure leads to resistance. We present a method here that, in contrast, exploits the power of adaptation among antibiotic producers to accelerate the discovery of antibiotics. A competition-based adaptive laboratory evolution scheme is presented whereby an antibiotic-producing microorganism is competed against a target pathogen and serially passed over time until the producer evolves the ability to synthesize a chemical entity that inhibits growth of the pathogen. When multiple Streptomyces clavuligerus replicates were adaptively evolved against methicillin-resistant Staphylococcus aureus N315 in this manner, a strain emerged that acquired the ability to constitutively produce holomycin. In contrast, no holomycin could be detected from the unevolved wild-type strain. Moreover, genome re-sequencing revealed that the evolved strain had lost pSCL4, a large 1.8 Mbp plasmid, and acquired several single nucleotide polymorphisms in genes that have been shown to affect secondary metabolite biosynthesis. These results demonstrate that competition-based adaptive laboratory evolution can constitute a platform to create mutants that overproduce known antibiotics and possibly to discover new compounds as well.

Steroids initiate a signaling cascade that triggers rapid sporulation in Dictyostelium.

Encapsulation of prespore cells of Dictyostelium discoideum is controlled by several intercellular signals to ensure appropriate timing during fruiting body formation. Acyl-CoA-binding protein, AcbA, is secreted by prespore cells and processed by the prestalk protease TagC to form the 34 amino acid peptide SDF-2 that triggers rapid encapsulation. AcbA is secreted when γ-aminobutyric acid (GABA) is released from prespore cells and binds to GrlE, a G protein-coupled receptor (GPCR). Analysis of SDF-2 production in mutant strains lacking Gα subunits and GPCRs, either as pure populations or when mixed with other mutant strains, uncovered the non-cell-autonomous roles of GrlA, Gα4 and Gα7. We found that Gα7 is essential for the response to GABA and is likely to be coupled to GrlE. GrlA-null and Gα4-null cells respond normally to GABA but fail to secrete it. We found that they are necessary for the response to a small hydrophobic molecule, SDF-3, which is released late in culmination. Pharmacological inhibition of steroidogenesis during development blocked the production of SDF-3. Moreover, the response to SDF-3 could be blocked by the steroid antagonist mifepristone, whereas hydrocortisone and other steroids mimicked the effects of SDF-3 when added in the nanomolar range. It appears that SDF-3 is a steroid that elicits rapid release of GABA by acting through the GPCR GrlA, coupled to G protein containing the Gα4 subunit. SDF-3 is at the head of the cascade that amplifies the signal for encapsulation to ensure the rapid, synchronous formation of spores.

Supramolecular Tetrahedra of Phosphines and Coinage Metals

Supramolecular metal–ligand clusters, sometimes referred to as metal–organic polyhedra (MOPs), have been a major area of chemical research for at least 20 years. The groups of Stang, Raymond, Fujita, and many others have discovered an incredibly wide array of such molecules. Many of the assemblies have demonstrated spectacular host–guest chemistry, including the ability to catalyze chemical reactions. These clusters have also been linked together or assembled in such a way as to generate solid-state materials, such as metal–organic frameworks (MOFs, or “crystalline molecular flasks”). Despite the vast literature on these types of self-assembled structures, such constructs have only been rarely assembled from soft Lewis base ligands derived from second-row heteroatoms (for example, sulfur and phosphorus). Such ligands would be expected to form a distinct class of assemblies in conjunction with lower oxidation-state metal ions or metalloids. Indeed, Johnson and co-workers have successfully used thiol-based ligands to obtain self-assembled clusters based on P, As, Sb, and Bi. Herein, we present a unique series of tetrahedral clusters derived from a rigid tris(diphenylphosphine) ligand and the soft Lewis acid coinage-metal ions of Group 11. To best of our knowledge, this is the first time isostructural supramolecular structures of a platonic solid (tetrahedron) have been prepared with all members of a transition metal group of the periodic table. Some supramolecular assemblies have been described with multidentate phosphine ligands. Of particular note is work from the groups of Yip and James, both of whom have prepared self-assembled structures from M-phosphine interactions (M = Ag, Au). From a variety of rigid, multidentate phosphine ligands these groups have constructed rings, helicates, and a small, adamantoid-shaped cluster. The use of Ag and Au has also engendered some of these supramolecular species with luminescent properties. Furthermore, Hahn and co-workers have also prepared cylindertyped supramolecular structures combining polydentate Nheterocyclic carbene (NHC) ligands with coinage-metal ions by metal-controlled self-assembly. Despite the impressive progress made by these groups and others, no large, threedimensional structures, reminiscent of the platonic solids described with many other metal–ligand systems, have been reported. The supramolecular tetrahedra prepared herein (Scheme 1) with Cu, Ag, and Au are the first to realize this challenging target in metal–ligand self-assembly.

Real-Time Characterization of the Composition of Individual Particles Emitted From Ultrafine Particle Concentrators

Particle concentrators are commonly used for controlling exposure levels to ambient ultrafine, fine, and coarse aerosols over a broad range of concentrations. For ultrafine aerosols, these concentrators require water condensation technology to grow and enrich these smaller sized particles (D a < 100 nm). Because the chemistry of the particles is directly related to their toxicity, any changes induced by ultrafine concentrators on ambient particles need to be better characterized in order to fully understand the results obtained in health exposure studies. Using aerosol time-of-flight mass spectrometry (ATOFMS), the size-resolved chemistry was measured of concentrated ultrafine and accumulation mode (50–300 nm) particles from several particle concentrators with different designs. This is the first report detailing the size-resolved distributions of elemental carbon (EC) and organic carbon (OC) particles sampled from concentrators. Experimental measurements of the single particle mixing state of particles in concentrated versus non-concentrated ambient air show transformations of ultrafine EC particles occur as they become coated with organic carbon (OC) species during the concentration process. Based on relative ion intensities, concentrated ultrafine particles showed a 30% increase in the amount of OC on the EC particles for the same aerodynamic size. An increase in the number fraction of aromatic- and polycyclic aromatic hydrocarbon-containing particles was also observed in both the ultrafine and fine size modes. The most likely explanation for such changes is gas-to-particle partitioning of organic components (e.g., water-soluble organic compounds) from the high volume of air used in the concentrator into aqueous phase ultrafine and fine aqueous particles created during the particle enrichment process.

Capillary-discharge-based portable detector for chemical vapor monitoring

Conventional portable instruments for sensing chemical vapors have certain limitations for on-site use. In this article, we develop a genuinely portable detector that is sensitive, powerful, rugged, of simple design, and with very low power needs. Such a detector is based on a dry-cell battery-powered, capillary-discharge-based, microplasma source with optical emission detection. The microscale plasma source has very special features such as low thermal temperature and very low power needs. These features make it possible for the plasma source to be powered with a small dry-cell battery. A specially designed discharge chamber with minielectrodes can be configured to enhance the plasma stability and the system performance. A very small amount of inert gas can be used as sample carrier and plasma supporting gas. Inert gases possess high excitation potentials and produce high-energy metastable particles in the plasma. These particles provide sufficient energy to excite chemical species through Penning ionization and/or energy transfer from metastable species. A molecular emission spectrum can be collected with a palm-sized spectrometer through a collimated optical fiber. The spectrum can be displayed on a notebook computer. With this design and arrangement, the new detector provides high sensitivity for organic chemical species. The advantages and features of the newly developed detector include high sensitivity, simple structure, low cost, universal response, very low power consumption, compact volume with field portable capability, and ease of operation.

Design and development of a highly sensitive, field portable plasma source instrument for on-line liquid stream monitoring and real-time sample analysis

The development of a highly sensitive, field portable, low-powered instrument for on-site, real-time liquid waste stream monitoring is described in this article. A series of factors such as system sensitivity and portability, plasma source, sample introduction, desolvation system, power supply, and the instrument configuration, were carefully considered in the design of the portable instrument. A newly designed, miniature, modified microwave plasma source was selected as the emission source for spectroscopy measurement, and an integrated small spectrometer with a charge-coupled device detector was installed for signal processing and detection. An innovative beam collection system with optical fibers was designed and used for emission signal collection. Microwave plasma can be sustained with various gases at relatively low power, and it possesses high detection capabilities for both metal and nonmetal pollutants, making it desirable to use for on-site, real-time, liquid waste stream monitoring. An effectiv...