Ice Nucleation Activity of Graphene and Graphene Oxides

Microbial ice-binding structures: A review of their applications

Microorganisms' ice-binding structures (IBS) are macromolecules with potential commercial value in agriculture, food technology, material technology, cryobiology, and medicine. Microbial ice-structuring or microbial ice-binding particles, with their multi-applications, are simple to use, effective in low amounts, non-toxic, and environmentally friendly. Due to their source and composition diversities, microbial ice-binding structures are gaining attention because they are useable in various conditions. Some microorganisms also produce structures with dual ice-nucleating and anti-freezing properties. Structures that promote ice formation (ice nucleating particles- INPs) act as ice nuclei, lowering the energy barrier between supercooled liquid and ice, causing ice crystals to form. In contrast, anti-freeze particles (AFPs) prevent ice formation and recrystallization through several mechanisms, including disturbing the formation of string hydrogen bonds amongst water molecules, melting already formed ice crystals, and preventing crystal formation by binding to specific sites. Knowledge of the type and function of microbial ice-binding structures lends fundamental insight for possible scaling the production of cheap, functional, and advanced microbial structure-inspired mimics and by-products. This review focuses on microbial ice-binding structures and their potential uses in the food, medicinal, environmental, and agricultural sectors.

On the Role of Starchy Grains in Ice Nucleation Processes

Little is known about the role of starchy food on climate change processes like ice nucleation. Here, we investigate the ice nucleation efficiency (INE) of eight different starchy food materials, namely, corn (CO), potato (PO), barley (BA), brown rice (BR), white rice (WR), oats (OA), wheat (WH), and sweet potato (SP), in immersion freezing mode under mixed-phase cloud conditions. Notably, among all these food materials, PO and BA exhibit the highest ice nucleation efficiency with ice nucleation temperatures as high as -4.3 °C (T50 ∼ -7.0 ± 0.5 °C) and -6.5 °C (T50 ∼ -7.2 ± 0.2 °C), respectively. We also explore the effect of environmentally relevant physicochemical conditions on ice nucleation efficiency, including different pH, temperature, UV/O3/NOx exposure, and various cocontaminants. The change in shape, size, surface properties, hydrophobicity, and crystallinity of materials accounted for the altered INE. The increase in shape, size, and hydrophobicity of the sample generally reduces the INE, whereas an increase in crystallinity enhances the INE of the sample under our experimental conditions. The results suggest that environmentally relevant concentrations slightly alter INE, indicating their role as catalysts in environmental matrices. The outcome of studies on the ice nucleation properties of these food-containing aerosols might help in the physicochemical understanding of other biomolecule-induced ice nucleation, which is still an underdeveloped research area.

Active Sites and Their Individual Turnover Frequencies for Ethylene Hydrogenation on Reduced Graphene Aerogel

Graphene aerogel (GA) was reduced at various temperatures to prepare a series of reduced graphene aerogels (rGAs) with different surface characteristics. Detailed characterization demonstrated that an increase in the thermal reduction temperature leads to an increase in surface area accompanied by an increase in surface density of defect sites formed by the removal of the oxygen-containing functional groups. rGA samples were then tested for ethylene hydrogenation under identical conditions. A comparison of catalytic performances of each catalyst demonstrated that the rGA sample prepared by reduction in Ar at 900 °C (rGA-900) provides the highest performance compared with others prepared at lower temperatures. Next, we analyzed the per-gram activity of each catalyst as a sum of individual contributions from different defect sites quantified by Raman spectroscopy and CHNS-O analysis to determine the individual turnover frequencies (TOFs) of each active site. This analysis identified polyene-like structures and interstitial defects associated with amorphous sp2 bonded carbon atoms as the dominant active sites responsible for hydrogenation. A comparison of their TOFs further indicated that the polyene-like structures provide approximately ten times higher TOF compared to those associated with the amorphous carbon defects. These results, identifying the dominant active centers and quantifying their corresponding TOFs, provide opportunities toward the rational design of GA-based carbocatalysts.

Cryopreservation of organoids: Strategies, innovation, and future prospects

Organoid technology has demonstrated unique advantages in multidisciplinary fields such as disease research, tumor drug sensitivity, clinical immunity, drug toxicology, and regenerative medicine. It will become the most promising research tool in translational research. However, the long preparation time of organoids and the lack of high-quality cryopreservation methods limit the further application of organoids. Although the high-quality cryopreservation of small-volume biological samples such as cells and embryos has been successfully achieved, the existing cryopreservation methods for organoids still face many bottlenecks. In recent years, with the development of materials science, cryobiology, and interdisciplinary research, many new materials and methods have been applied to cryopreservation. Several new cryopreservation methods have emerged, such as cryoprotectants (CPAs) of natural origin, ice-controlled biomaterials, and rapid rewarming methods. The introduction of these technologies has expanded the research scope of cryopreservation of organoids, provided new approaches and methods for cryopreservation of organoids, and is expected to break through the current technical bottleneck of cryopreservation of organoids. This paper reviews the progress of cryopreservation of organoids in recent years from three aspects: damage factors of cryopreservation of organoids, new protective agents and loading methods, and new technologies of cryopreservation and rewarming.

Toward Elimination of Soot Emissions from Jet Fuel Combustion

Soot from jet fuel combustion in aircraft engines contributes to global warming through the formation of contrail cirrus clouds that make up to 56% of the total radiative forcing from aviation. Here, the elimination of such emissions is explored through N₂ injection (containing 0-25 vol % O₂) at the exhaust of enclosed spray combustion of jet fuel that nicely emulates aircraft soot emissions. It is shown that injecting N₂ containing 5 vol % of O₂ enhances the formation of polyaromatic hydrocarbons (PAHs) that adsorb on the surface of soot. This increases soot number density and volume fraction by 25 and 80%, respectively. However, further increasing the O₂ concentration to 20 or 25 vol % enhances oxidation and nearly eliminates soot emissions from jet fuel spray combustion, reducing the soot number density and volume fraction by 87.3 or 95.4 and 98.3 or 99.6%, respectively. So, a judicious injection of air just after the aircraft engine exhaust can drastically reduce soot emissions and halve the radiative forcing due to aviation, as shown by soot mobility, X-ray diffraction, Raman spectroscopy, nitrogen adsorption, microscopy, and thermogravimetric analysis (for the organic to total carbon ratio) measurements.

The dependence of soot particle ice nucleation ability on its volatile content

Aviation soot can affect contrail and cirrus cloud formation and impact climate. A product of incomplete combustion, soot particles, are fractal and hydrophobic aggregates comprising carbonaceous spheres with complex physicochemical properties. In the cirrus cloud regime, the surface wettability and pore abundance of soot particles are important determinants for their ice nucleation ability via pore condensation and freezing. In the atmosphere, soot particles can undergo various ageing processes which modify their surface chemistry and porosity, thus acting as ice nucleating particles with varying abilities as a function of ageing. In this study, size-selected soot particles were treated by thermal denuding at 573 K in a pure nitrogen (N₂) or synthetic air (N₂ + O₂) flow and then exposed to varying relative humidity conditions at a fixed temperature in the range from 218 to 243 K, to investigate the role of volatile content in the ice nucleation ability. Both organic-lean and organic-rich propane (C₃H₈) flame soot particles, as well as two types of commercially available carbon black soot particles with high and low surface wettability, were tested. The size and mass distribution of soot aerosol were monitored during the ice nucleation experiments. Bulk soot samples also prepared in pure N₂ or synthetic air environments at 573 K were characterised by thermogravimetric analysis, Fourier transform infrared spectroscopy and dynamic vapour sorption measurements, to reveal the relation between denuding volatile content, associated soot particle property modifications and the ice nucleation ability. Our study shows that thermal denuding induces a change in soot particle porosity playing a dominant role in regulating its ice nucleation via the pore condensation and freezing mechanism. The enrichment in mesopore (2-50 nm) availability may enhance soot ice nucleation. The presence of O₂ in the thermal denuding process may introduce new active sites on soot particles for water interaction and increase soot surface wettability. However, these active sites only facilitate soot ice nucleation when mesopore structures are available. We conclude that a change in volatile content modifies both morphological properties and surface chemistry for soot particles, but porosity change plays the dominant role in regulating soot particle ice nucleation ability.

Influences of Oxidation Degree and Size on the Ice Nucleation Efficiency of Graphene Oxide

Figuring out the influences of carbonaceous particle properties on ice nucleation is important to atmospheric science, but it is still a challenge, especially for experimental investigations due to the coupling effect of multiple properties. Here we separately investigate the effects of oxidation degree and size, two typical and debated factors, on ice nucleation efficiency by choosing graphene oxide (GO) as the model. The results show that with the decrease of oxidation degree, ice nucleation efficiency increases through decreasing the ice nucleation free energy barrier (ΔGheter*) on GO surface. Interestingly, although the chosen GO sizes are sufficiently large compared with the sizes of critical ice nuclei, the increase of GO size leads to the increase of ΔGheter* and thus the decrease of ice nucleation efficiency, unlike the general thought that ΔGheter* is not affected by the particle size any more when the size of particle increases to several times that of the critical ice nucleus.

Seasonal ice nucleation activity of water samples from alpine rivers and lakes in Obergurgl, Austria

Heterogeneous ice nucleation plays an important role in many environmental processes such as ice cloud formation, freezing of water bodies or biological freeze protection in the cryosphere. New information is needed about the seasonal availability, nature, and activity of ice nucleating particles (INPs) in alpine environments. These INPs trigger the phase transition from liquid water to solid ice at elevated subzero temperatures. We collected water samples from a series of alpine rivers and lakes (two valleys and their rivers, an artificial pond, and a natural lake system) in Obergurgl, Austria in June 2016, July 2016, November 2016, and May 2017. Each alpine river and lake was sampled multiple times across different seasons, depending on site access during different times of the year. Water samples were filtered through a 0.22 μm membrane filter to separate microbial INPs from the water, and both fractions were analyzed for ice nucleation activity (INA) by an emulsion freezing method. Microorganisms were cultured from the filters, and the cultures then analyzed for INA. Portions of the filtered samples were concentrated by lyophilization to observe potential enhancement of INA. Two sediment samples were taken as reference points for inorganic INPs. Sub-micron INPs were observed in all of the alpine water sources studied, and a seasonal shift to a higher fraction of microbial ice nucleators cultured on selective media was observed during the winter collections. Particles larger than 0.22 μm showed INA, and microbes were cultured from this fraction. Results from 60 samples gave evidence of a seasonal change in INA, presence of submicrometer INPs, and show the abundance of culturable microorganisms, with late spring and early summer showing the most active biological INPs. With additional future research on this topic ski resorts could make use of such knowledge of geographical and seasonal trends of microbial INPs in freshwater habitats in order to improve the production of artificial snow.

Biological mediated synthesis of RGO-ZnO composites with enhanced photocatalytic and antibacterial activity

In this study, we reported the biological approach to synthesis of ZnO nanorod (NR) on the reduced graphene oxide (RGO) for photocatalytic, antibacterial activity and hydrogen production under sunlight. Bacillus subtilis played a vital role in the production of biogenic ammonia from synthetic urine and utilized for the synthesis of ZnONR on the RGO sheet. The morphological study revealed that RGO sheets displayed a tremendous role in anchoring ZnONR. XRD patterns showed the ZnO crystal phase on the RGO sheets. XPS and Raman spectra confirmed that the bio-hydrothermal method as suitable for GO converted into RGO. The transient photocurrent and I/V measurement are exhibited as an increment on the RGO-ZnONR compared to ZnONR. The RGO-ZnONR composites showed excellent performance with decolorization of MB and textile dyes and efficient control of the E. coli and S. aureus. RGO-ZnONR exhibited remarkable noted as a higher photocatalytic hydrogen evolution rate (940 μmol/h/g_cat) than the ZnONR (369.5 μmol/h/g _cat). As a result of photocatalytic performance to correlate with sunlight intensity was extensively studied. RGO plays an essential role in interface electron transfer from sunlight to ZnONR for enhancing •OH radical formation to cleavage of dye color substance and eradicated bacterial cells.

Ice Inhibition for Cryopreservation: Materials, Strategies, and Challenges

Cryopreservation technology has developed into a fundamental and important supporting method for biomedical applications such as cell-based therapeutics, tissue engineering, assisted reproduction, and vaccine storage. The formation, growth, and recrystallization of ice crystals are the major limitations in cell/tissue/organ cryopreservation, and cause fatal cryoinjury to cryopreserved biological samples. Flourishing anti-icing materials and strategies can effectively regulate and suppress ice crystals, thus reducing ice damage and promoting cryopreservation efficiency. This review first describes the basic ice cryodamage mechanisms in the cryopreservation process. The recent development of chemical ice-inhibition molecules, including cryoprotectant, antifreeze protein, synthetic polymer, nanomaterial, and hydrogel, and their applications in cryopreservation are summarized. The advanced engineering strategies, including trehalose delivery, cell encapsulation, and bioinspired structure design for ice inhibition, are further discussed. Furthermore, external physical field technologies used for inhibiting ice crystals in both the cooling and thawing processes are systematically reviewed. Finally, the current challenges and future perspectives in the field of ice inhibition for high-efficiency cryopreservation are proposed.

Atomic Steps Induce the Aligned Growth of Ice Crystals on Graphite Surfaces

Heterogeneous ice nucleation on atmospheric aerosols strongly affects the earth's climate, and at the microscopic level, surface-irregularity-induced ice crystallization behaviors are common but crucial. Because of the lack of visual evidence and effective experimental methods, the mechanism of atomic-structure-dependent ice formation on aerosol surfaces is poorly understood. Here we chose highly oriented pyrolytic graphite (HOPG) to represent soot (a primary aerosol), and environmental scanning electron microscopy (ESEM) was performed for in situ observations of ice formation. We found that hexagonal ice crystals show an aligned growth pattern via a two-stage pathway with one a axis coinciding with the direction of atomic step edges on the HOPG surface. Additionally, the ice crystals grow at a noticeably higher speed along this direction. This study reveals the role of atomic surface defects in heterogeneous ice nucleation and may pave the way to control icing-related processes in practical applications.

Water Freezes at Near-Zero Temperatures Using Carbon Nanotube-Based Electrodes under Static Electric Fields

Although static electric fields have been effective in controlling ice nucleation, the highest freezing temperature (T_f) of water that can be achieved in an electric field (E) is still uncertain. We performed a systematic study of the effect of an electric field on water freezing by varying the thickness of a dielectric layer and the voltage across it in an electrowetting system. Results show that T_f first increases sharply with E and then reaches saturation at -3.5 °C after a critical value E of 6 × 10⁶ V/m. Using classical heterogeneous nucleation theory, it is revealed that this behavior is due to saturation in the contact angle of the ice embryo with the underlying substrate. Finally, we show that it is possible to overcome this freezing saturation by controlling the uniformity of the electric field using carbon nanotubes. We achieve a T_f of -0.6 °C using carbon nanotube-based electrodes with an E of 3 × 10⁷ V/m. This work sheds new light on the control of ice nucleation and has the potential to impact many applications ranging from food freezing to ice production.

A Drone-Based Bioaerosol Sampling System to Monitor Ice Nucleation Particles in the Lower Atmosphere

Terrestrial ecosystems can influence atmospheric processes by contributing a huge variety of biological aerosols (bioaerosols) to the environment. Several types of biological particles, such as pollen grains, fungal spores, and bacteria cells, trigger freezing processes in super-cooled cloud droplets, and as such can contribute to the hydrological cycle. Even though biogenic particles are known as the most active form of ice nucleation particles (INPs), the transport to high tropospheric altitudes, as well as the occurrence in clouds, remains understudied. Thus, transport processes from the land surface into the atmosphere need to be investigated to estimate weather phenomena and climate trends. To help fill this knowledge gap, we developed a drone-based aerosol particles sampling impinger/impactor (DAPSI) system for field studies to investigate sources and near surface transport of biological INPs. DAPSI was designed to attach to commercial rotary-wing drones to collect biological particles within about 100 m of the Earth’s surface. DAPSI provides information on particulate matter concentrations (PM10 & PM2.5), temperature, relative humidity, and air pressure at about 0.5 Hz, by controlling electrical sensors with an onboard computer (Raspberry Pi 3). Two remote-operated sampling systems (impinging and impacting) were integrated into DAPSI. Laboratory tests of the impinging system showed a 96% sampling efficiency for standardized aerosol particles (2 µm polystyrene latex spheres) and 84% for an aerosol containing biological INPs (Betula pendula). A series of sampling missions (12 flights) were performed using two Phantom 4 quadcopters with DAPSI onboard at a remote sampling site near Gosau, Austria. Fluorescence microscopy of impactor foils showed a significant number of auto-fluorescent particles < 0.5 µm at an excitation of 465–495 nm and an emission of 515–555 nm. A slight increase in ice nucleation activity (onset temperature between −27 °C and −31 °C) of sampled aerosol was measured by applying freezing experiments with a microscopic cooling technique. There are a number of unique opportunities for DAPSI to be used to study the transport of bioaerosols, particularly for investigations of biological INP emissions from natural sources such as birch or pine forests.

Hydroxyl Groups on the Graphene Surfaces Facilitate Ice Nucleation

Although it is crucial to the formation of cirrus clouds and consequently the climate on Earth, the unambiguous effect of carbonaceous materials (CM) on ice nucleation remains to be unveiled as the chemical variation on the surface of CM is always complicated by the change in morphology. Here, we separately investigate the effects of the surface chemistry and morphology of CM on ice nucleation by studying ice nucleation on highly oriented pyrolytic graphite surfaces treated with different types of plasmas. We discover unambiguously that increasing the density of hydroxyl groups leads to an increased activity of ice nucleation on the surface of graphene, while no observable effects are found when carboxylic groups are introduced. Analysis based on the classical nucleation theory reveals that the increase in the density of hydroxyl groups on the graphene surface results in an increased binding energy between the ice nucleus and the graphene surface, which consequently facilitates the formation of the critical ice nucleus.