4 research outputs found
Thermal Structural Transitions and Carbon Dioxide Adsorption Properties of Zeolitic Imidazolate Framework‑7 (ZIF-7)
As a subset of the
metal–organic frameworks, zeolitic imidazolate
frameworks (ZIFs) have potential use in practical separations as a
result of flexible yet reliable control over their pore sizes along
with their chemical and thermal stabilities. Among many ZIF materials,
we explored the effect of thermal treatments on the ZIF-7 structure,
known for its promising characteristics toward H<sub>2</sub> separations;
the pore sizes of ZIF-7 (0.29 nm) are desirable for molecular sieving,
favoring H<sub>2</sub> (0.289 nm) over CO<sub>2</sub> (0.33 nm). Although
thermogravimetric analysis indicated that ZIF-7 is thermally stabile
up to ∼400 °C, the structural transition of ZIF-7 to an
intermediate phase (as indicated by X-ray analysis) was observed under
air as guest molecules were removed. The transition was further continued
at higher temperatures, eventually leading toward the zinc oxide phase.
Three types of ZIF-7 with differing shapes and sizes (∼100
nm spherical, ∼400 nm rhombic-dodecahedral, and ∼1300
nm rod-shaped) were employed to elucidate (1) thermal structural transitions
while considering kinetically relevant processes and (2) discrepancies
in the N<sub>2</sub> physisorption and CO<sub>2</sub> adsorption isotherms.
The largest rod-shaped ZIF-7 particles showed a delayed thermal structural
transition toward the stable zinc oxide phase. The CO<sub>2</sub> adsorption
behaviors of the three ZIF-7s, despite their identical crystal structures,
suggested minute differences in the pore structures; in particular,
the smaller spherical ZIF-7 particles provided reversible CO<sub>2</sub> adsorption isotherms at ∼30–75 °C, a typical
temperature range of flue gases from coal-fired power plants, in contrast
to the larger rhombic-dodecahedral and rod-shaped ZIF-7 particles,
which exhibited hysteretic CO<sub>2</sub> adsorption/desorption behavior
Healing of Microdefects in SSZ-13 Membranes via Filling with Dye Molecules and Its Effect on Dry and Wet CO<sub>2</sub> Separations
It
is quite challenging to avoid microdefect formation during hydrothermal
growths and/or calcination processes, while manufacturing high-quality
zeolite membranes in a reproducible manner. Even less than 1% of defects,
which generally provide nonselective pathways, will considerably worsen
the intrinsic, high molecular sieving-based separation performance
of a continuous zeolite membrane. Herein, we propose a simple and
reliable method for blocking defects using water-soluble dye molecules,
which were originally used for the visualization of nonzeolitic, defective
structures in a zeolite membrane. Because the dye molecules are ∼1
nm in size, they cannot diffuse into the zeolitic pores and selectively
access the defects. For the demonstration of dye-based defect healing,
we chose a siliceous chabazite type SSZ-13 zeolite membrane (pore
size = 0.37 × 0.42 nm<sup>2</sup>) with some degree of defects
and investigated the effect of defect healing on the final CO<sub>2</sub> separation performance. Because the defects were gradually
filled by the dye molecules, both CO<sub>2</sub>/N<sub>2</sub> and
CO<sub>2</sub>/CH<sub>4</sub> separation performances were concomitantly
increased. Intriguingly, the CO<sub>2</sub> perm-selectivity test
with ternary mixtures including H<sub>2</sub>O vapor (the third largest
component in the flue and natural/shale/bio gas streams) in the feed
diminished CO<sub>2</sub> separation performance. This could be ascribed
to inhibited transport of the fast permeating species, here CO<sub>2</sub>, from the adsorbed H<sub>2</sub>O molecules on the dye-treated
and water-friendly (relatively hydrophilic) membrane surface. On the
contrary, the intact, siliceous (water-repelling or hydrophobic) SSZ-13
membranes showed improved CO<sub>2</sub> perm-selectivities in the
presence of H<sub>2</sub>O vapor, seemingly due to defect blocking
by the physisorbed H<sub>2</sub>O molecules
Thermosensitive Structural Changes and Adsorption Properties of Zeolitic Imidazolate Framework‑8 (ZIF-8)
We compared four types of ZIF-8 with
varying sizes and shapes to
determine their thermal-structural stability and derive appropriate
thermal activation conditions and correlation between structural characteristics
and adsorption properties. Under air, the ZIF-8 phase for all the
samples was converted completely into the zinc oxide phase above ∼300
°C, though thermalgravimetric analysis (TGA) indicated that the
original structure was stable to ∼300–350 °C. Longer
exposures (∼30 d) suggested that thermal activation at ∼200
°C was appropriate for the removal of guest and/or solvent molecules
under air without structural damage. Despite no noticeable change
in X-ray diffraction (XRD) patterns after activation at 250 °C
under air, the resulting BET surface areas and CO<sub>2</sub> adsorption
amounts (at 1 bar and 30 °C) of ZIF-8s were reduced to ∼44–54
and ∼72–87%, respectively, as compared to those of appropriately
activated ZIF-8s. It appears that after the activation at 250 °C
under air, some Zn and N atoms were dissociated and converted to ZnOH
and NOH, respectively, causing the partial structural damage of ZIF-8s
Multiplexed Detection of Epigenetic Markers Using Quantum Dot (QD)-Encoded Hydrogel Microparticles
Epigenetic
alterations in gene expression are influenced by experiences
and environment, resulting in significant variation of epigenetic
markers from individual to individual. Therefore, it is imperative
to measure various epigenetic markers simultaneously from samples
of individual subjects to accurately analyze the epigenetic markers
in biological samples. Moreover, the individualized genome-wide analysis
has become a critical technology for recent trends in clinical applications
such as early diagnosis and personalized medicine screening of numerous
diseases. The array-based detection of modified histones, conventionally
used for multiplexed analysis of epigenetic changes, requires pooling
of samples from many subjects to analyze population-wise differences
in the expression of histone markers and does not permit individualized
analysis. Here, we report multiplexed detection of genome-wide changes
in various histone modifications at a single-residue resolution using
quantum dot (QD)-encoded polyethylene glycol diacrylate (PEGDA) hydrogel
microparticles. To demonstrate the potential of our methodology, we
present the simultaneous detection of (1) acetylation of lysine 9
of histone 3 (Ac–H3K9), (2) dimethylation of H3K9 (2Me-H3K9),
and (3) trimethylation of H3K9 (3Me-H3K9) from three distinct regions
in the brain [nucleus accumbens (NAc), dorsal striatum (DSt), and
cerebellum (Cbl)] of cocaine-exposed mice. Our hydrogel-based epigenetic
assay enabled relative quantification of the three histone variants
from only 10 μL of each brain lysate (protein content = ∼
1 μg/μL) per mouse. We verified that the exposure to cocaine
induced a significant increase of acetylation while a notable decrease
in methylation in NAc