Observation of a brine layer on an ice surface with an environmental scanning electron microscope at higher pressures and temperatures

Impact of secondary ice in a frozen NaCl freeze-concentrated solution on the extent of methylene blue aggregation

Freezing and lyophilization have been utilized for decades to stabilize pharmaceutical and food products. Freezing a solution that contains dissolved salt and/or organic matter produces pure primary ice crystal grains separated by freeze-concentrated solutions (FCS). The microscopic size of the primary ice crystals depends on the cooling conditions and the concentration of the solutes. It is generally accepted that primary ice crystals size influences the rate of sublimation and also can impact physico-chemical behaviour of the species in the FCS. This article, however, presents a case where the secondary ice formed inside the FCS plays a critical role. We microscoped the structures of ice-cast FCS with an environmental scanning electron microscope and applied the aggregation-sensitive spectroscopic probe methylene blue to determine how the microstructure affects the molecular arrangement. We show that slow cooling at -50 °C produces large salt crystals with a small specific surface, resulting in a high degree of molecular aggregation within the FCS. In contrast, fast liquid nitrogen cooling yields an ultrafine structure of salt crystals having a large specific surface area and, therefore, inducing smaller aggregation. The study highlights a critical role of secondary ice in solute aggregation and introduces methylene blue as a molecular probe to investigate freezing behaviour of aqueous systems with crystalline solute.

Investigating freezing-induced acidity changes in citrate buffers

Citrate buffers are commonly utilized in the field of biomolecule stabilization. We investigate their applicability in the frozen state within a range of initial pHs (2.5 to 8.0) and concentrations (0.02 to 0.60 M). Citrate buffer solutions subjected to various cooling and heating temperatures are examined in terms of the freezing-induced acidity changes, revealing that citrate buffers acidify upon cooling. The acidity is assessed with sulfonephthalein molecular probes frozen in the samples. Optical cryomicroscopy combined with differential scanning calorimetry was employed to investigate the causes of the observed acidity changes. The buffers partly crystallize and partly vitrify in the ice matrix; these processes influence the resulting pH and allow designing the optimal storage temperatures in the frozen state. The freezing-induced acidification apparently depends on the buffer concentration; at each pH, we suggest pertinent concentration, at which freezing causes minimal acidification.

ESEM Methodology for the Study of Ice Samples at Environmentally Relevant Subzero Temperatures: "Subzero ESEM"

Frozen aqueous solutions are an important subject of study in numerous scientific branches including the pharmaceutical and food industry, atmospheric chemistry, biology, and medicine. Here, we present an advanced environmental scanning electron microscope methodology for research of ice samples at environmentally relevant subzero temperatures, thus under conditions in which it is extremely challenging to maintain the thermodynamic equilibrium of the specimen. The methodology opens possibilities to observe intact ice samples at close to natural conditions. Based on the results of ANSYS software simulations of the surface temperature of a frozen sample, and knowledge of the partial pressure of water vapor in the gas mixture near the sample, we monitored static ice samples over several minutes. We also discuss possible artifacts that can arise from unwanted surface ice formation on, or ice sublimation from, the sample, as a consequence of shifting conditions away from thermodynamic equilibrium in the specimen chamber. To demonstrate the applicability of the methodology, we characterized how the true morphology of ice spheres containing salt changed upon aging and the morphology of ice spheres containing bovine serum albumin. After combining static observations with the dynamic process of ice sublimation from the sample, we can attain images with nanometer resolution.

Studying Ice with Environmental Scanning Electron Microscopy

Scanning electron microscopy (SEM) is a powerful imaging technique able to obtain astonishing images of the micro- and the nano-world. Unfortunately, the technique has been limited to vacuum conditions for many years. In the last decades, the ability to introduce water vapor into the SEM chamber and still collect the electrons by the detector, combined with the temperature control of the sample, has enabled the study of ice at nanoscale. Astounding images of hexagonal ice crystals suddenly became real. Since these first images were produced, several studies have been focusing their interest on using SEM to study ice nucleation, morphology, thaw, etc. In this paper, we want to review the different investigations devoted to this goal that have been conducted in recent years in the literature and the kind of information, beyond images, that was obtained. We focus our attention on studies trying to clarify the mechanisms of ice nucleation and those devoted to the study of ice dynamics. We also discuss these findings to elucidate the present and future of SEM applied to this field.

Structures of ions accommodated in salty ice Ih crystals

Frozen aqueous electrolytes are ubiquitous and involved in various phenomena occurring in the natural environment. Although salts are expelled from ice during freezing of aqueous solutions, minor amounts of the constituent ions are accommodated in the crystal lattice of ice. This phenomenon was associated with the generation of the Workman-Reynolds freezing potential. Molecular simulations also confirmed the ion incorporation in the crystal lattice of ice Ih upon freezing of aqueous electrolytes and identified possible local structures of the ions. However, no experimental information is available on the structure of ions accommodated in the crystal lattice of ice Ih. In this work, we use X-ray absorption fine structure (XAFS) to study the local structures of K⁺ and Cl^- accommodated in ice Ih single crystals. Previous molecular simulations predicted that ions are trapped in the hexagonal cavities of the ice structure or replace two water molecules in the crystal lattice. Four possible configurations are considered and optimized by the calculations using ONIOM (QM/QM/QM). The results are evaluated in terms of the agreement between the experimental XAFS spectra and those simulated from the optimized structures. The spectra are most reasonably interpreted by assuming that K⁺ replaces one water molecule in the ice crystal lattice and is accommodated in a tetrahedral coordination cage. Similarly, Cl^- probably adopts the same configuration, because it explains the coordination number better than other structures, such as that assuming the replacement of two water molecules belonging to the same hexagonal planes.

Making good's buffers good for freezing: The acidity changes and their elimination via mixing with sodium phosphate

Solutions of three Good's buffers (HEPES, MOPS, and MES), both pure and mixed with sodium phosphate buffers (Na-P), are investigated in terms of the freezing-induced acidity changes in their operational pH ranges. The Good's buffers have the tendency to basify upon freezing and, more intensively, at lower pHs. The acidity varies most prominently in MES, where the change may reach the value of two. Importantly, the Good's buffers are shown to mitigate the strong acidification in the Na-P buffer. Diverse concentrations of the Good's buffers are added to cancel out the strong, freezing-induced acidity drop in 50 mM Na-P that markedly contributes to the solution's acidity; the relevant values are 3 mM HEPES, 10 mM MOPS, and 80 mM MES. These buffer blends are therefore proposed to be applied in maintaining approximately the acidity of solutions even after the freezing process and, as such, should limit the stresses for frozen chemicals and biochemicals.

Using Excimeric Fluorescence to Study How the Cooling Rate Determines the Behavior of Naphthalenes in Freeze-Concentrated Solutions: Vitrification and Crystallization

We utilized fluorescence spectroscopy to learn about the molecular arrangement of naphthalene (Np) and 1-methylnaphthalene (MeNp) in frozen aqueous solutions. The freezing induces pronounced compound aggregation in the freeze-concentrated solution (FCS) in between the ice grains. The fluorescence spectroscopy revealed prevalent formation of a vitrified solution and minor crystallization of aromatic compounds. The FCS is shown as a specific environment, differing significantly from not only the pure compounds but also the ice surfaces. The results indicate marked disparity between the behavior of the Np and the MeNp; the cooling rate has a major impact on the former but not on the latter. The spectrum of the Np solution frozen at a faster cooling rate (ca 20 K/min) exhibited a temperature-dependent spectral behavior, whereas the spectrum of the solution frozen at a slower rate (ca 2 K/min) did not alter before melting. We interpret the observation through considering the varied composition of the FCS: Fast freezing leads to a higher water content expressed by the plasticizing effect, allowing molecular rearrangement, while slow cooling produces a more concentrated and drier environment. The experiments were conceived as generalizable for environmentally relevant pollutants and human-made freezing.

Dynamical in-situ observation of the lyophilization and vacuum-drying processes of a model biopharmaceutical system by an environmental scanning electron microscope

The paper discusses the real-time monitoring of the changing sample morphology during the entire lyophilization (freeze-drying) and vacuum-drying processes of model biopharmaceutical solutions by using an environmental scanning electron microscope (ESEM); the device's micromanipulators were used to study the interior of the samples in-situ without exposing the samples to atmospheric water vapor. The individual collapse temperatures (T_c) of the formulations, pure bovine serum albumin (BSA) and BSA/sucrose mixtures, ranged from -5 to -29 °C. We evaluated the impact of the freezing method (spontaneous freezing, controlled ice nucleation, and spray freezing) on the morphologies of the lyophiles at the constant drying temperature of -20 °C. The formulations with T_c above -20 °C resulted in the lyophiles' morphologies significantly dependent on the freezing method. We interpret the observations as an interplay of the freezing rates and directionalities, both of which markedly influence the morphologies of the frozen formulations, and, subsequently, the drying process and the mechanical stability of the freeze-dried cake. The formulation with T_c below -20 °C yielded a collapsed cake with features independent of the freezing method. The vacuum-drying produced a material with a smooth and pore-free surface, where deep cracks developed at the end of the process.

A Technological Understanding of Biofilm Detection Techniques: A Review

Biofouling is a persistent problem in almost any water-based application in several industries. To eradicate biofouling-related problems in bioreactors, the detection of biofilms is necessary. The current literature does not provide clear supportive information on selecting biofilm detection techniques that can be applied to detect biofouling within bioreactors. Therefore, this research aims to review all available biofilm detection techniques and analyze their characteristic properties to provide a comparative assessment that researchers can use to find a suitable biofilm detection technique to investigate their biofilms. In addition, it discusses the confluence of common bioreactor fabrication materials in biofilm formation.

Simulation-based optimisation of thermodynamic conditions in the ESEM for dynamical in-situ study of spherical polyelectrolyte complex particles in their native state

We present a complex analysis and optimisation of dynamic conditions in the environmental scanning electron microscope (ESEM) to allow in-situ observation of extremely delicate wet bio-polymeric spherical particles in their native state. According to the results of gas flow and heat transfer simulations, we were able to develop an improved procedure leading to thermodynamic equilibrium between the sample and chamber environment. To quantify and hence minimise the extent of any sample deformation during specimen chamber pumping, a strength-stress analysis is used. Monte Carlo simulations of beam-gas, -water, and -sample interactions describe beam scattering, absorbed energy, interaction volume and the emission of signal electrons in the ESEM. Finally, we discuss sample damage as a result of drying and the production of beam-induced free radicals. Based on all experimental and simulation results we introduce a Delicate Sample Observation Strategy for the ESEM. We show how this strategy can be applied to the characterization of polyelectrolyte complex spherical particles containing immobilized recombinant cells E. coli overexpressing cyclohexanone monooxygenase, used as a model biocatalyst. We present the first native-state electron microscopy images of the viscous core of a halved polyelectrolyte complex capsule containing living cells.

Emerging investigator series: spatial distribution of dissolved organic matter in ice and at air-ice interfaces

Dissolved organic matter (DOM) is a common solute in snow and ice at Earth's surface. Its effects on reaction kinetics in ice and at air-ice interfaces can be large, but are currently difficult to quantify. We used Raman microscopy to characterize the surface and bulk of frozen aqueous solutions containing humic acid, sodium dodecyl sulfate (SDS), and citric acid at a range of concentrations and temperatures. The surface-active species (humic acid and SDS) were distributed differently than citric acid. Humic acid and SDS are almost completely excluded to the air-ice interface during freezing, where they form a film that coats the surface nearly completely. A liquid layer that coats the majority of the surface was observed at all humic acid and SDS concentrations. Citric acid, which is smaller and less surface active, is excluded to liquid channels at the air-ice interface and within the ice bulk, as has previously been reported for ionic solutes such as sodium chloride. Incomplete surface wetting was observed at all citric acid concentrations and at all temperatures (up to -5 °C). Citric acid appears to be solvated in frozen samples, but SDS and humic acid do not. These results will improve our understanding of the effects of organic solutes on environmental and atmospheric chemistry within ice and at air-ice interfaces.

Growth and Morphology of Liquid Phase in Frozen Aqueous NaCl Probed by Voltammetry and Simulations

Cyclic voltammograms (CVs) of Fe(CN)₆ ^4- are measured using a microelectrode in frozen aqueous NaCl solutions to obtain morphological information on the liquid phase developed on the electrode surface. CVs in frozen solutions feature the radial diffusion similar to that measured in bulk solution in some cases but the linear diffusion in other cases. The former suggests the sufficient growth of the liquid phase, whereas the latter implies the diffusion paths in particular directions are hindered. Two parameters, i. e. a ratio of the maximum current to the steady-state current (R) and current amplification (r_amp ), are extracted from CVs and compared with those of simulated ones. CV simulations are carried out for four geometrical models. From the relationship between r_amp and R, the FCS developed on the electrode surface can be regarded as a thin layer developed in the direction parallel to the electrode surface or a cylinder running in the direction away from the electrode. Since solutes are concentrated in this liquid phase, highly sensitive voltammetric analysis would be possible if the growth of the FCS were successfully managed. The liquid phase morphology on the electrode, which cannot be probed by other methods, is useful information for designing such highly sensitive voltammetric analyses.

Size Controllable, Transparent, and Flexible 2D Silver Meshes Using Recrystallized Ice Crystals as Templates

Ice templates have been widely utilized for the preparation of porous materials due to the obvious advantages, such as environmentally benign and applicable to a wide range of materials. However, it remains a challenge to have controlled pore size as well as dimension of the prepared porous materials with the conventional ice template, since it often employs the kinetically not-stable growing ice crystals as the template. For example, there is no report so far for the preparation of 2D metal meshes with tunable pore size based on the ice template, although facile and eco-friendly prepared metal meshes are highly desirable for wearable electronics. Here, we report the preparation of 2D silver meshes with tunable mesh size employing recrystallized ice crystals as templates. Ice recrystallization is a kinetically stable process; therefore, the grain size of recrystallized ice crystals can be easily tuned, e.g., by adding different salts and changing the annealing temperature. Consequently, the size and line width of silver meshes obtained after freeze-drying can be easily adjusted, which in turn varied the conductivity of the obtained 2D silver film. Moreover, the silver meshes are transparent and display stable conductivity after the repeated stretching and bending. It can be envisioned that this approach for the preparation of 2D conducting films is of practical importance for wearable electronics. Moreover, this study provides a generic approach for the fabrication of 2D meshes with a controllable pore size.

Abnormal gas-liquid-solid phase transition behaviour of water observed with in situ environmental SEM

Gas-liquid-solid phase transition behaviour of water is studied with environmental scanning electron microscopy for the first time. Abnormal phenomena are observed. At a fixed pressure of 450 Pa, with the temperature set to −7 °C, direct desublimation happens, and ice grows continuously along the substrate surface. At 550 Pa, although ice is the stable phase according to the phase diagram, metastable liquid droplets first nucleate and grow to ~100–200 μm sizes. Ice crystals nucleate within the large sized droplets, grow up and fill up the droplets. Later, the ice crystals grow continuously through desublimation. At 600 Pa, the metastable liquid grows quickly, with some ice nuclei floating in it, and the liquid-solid coexistence state exists for a long time. By lowering the vapour pressure and/or increasing the substrate temperature, ice sublimates into vapour phase, and especially, the remaining ice forms a porous structure due to preferential sublimation in the concave regions, which can be explained with surface tension effect. Interestingly, although it should be forbidden for ice to transform into liquid phase when the temperature is well below 0 °C, liquid like droplets form during the ice sublimation process, which is attributed to the surface tension effect and the quasiliquid layers.

Application of Scanning Electron Microscopy With Energy-Dispersive X-Ray Spectroscopy for Analyzing Ocular Surface Particles on Schirmer Strips

PURPOSE

To demonstrate the application of scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM/EDS) for analyzing Schirmer strips for particle concentration, size, morphology, and type distribution.

METHODS

A cross-sectional design was used. Patients were prospectively recruited from the Miami Veterans Affairs (VA) Healthcare System eye clinic, and they underwent a complete ocular surface examination. The size, type, and chemical composition of particulate matter on Schirmer strips (from the left eye) were analyzed using SEM/EDS.

RESULTS

Schirmer strips from all 6 patients showed particle loading, ranging from 1 to 33 particles, whereas the blank Schirmer strip that served as a control showed no particle loading. Most particles were coarse, with an average size of 19.7 μm (95% confidence interval 15-24.4 μm). All samples contained organic particles (eg, pollen and mold), and 5 of the 6 samples contained nonorganic particles. The nonorganic particles were composed of silicon, minerals, and metals, including gold and titanium. The size of aluminum and iron particles was ≥62 μm, whereas the size of 2 other metals, zinc and gold, was smaller, that is, <20 μm. Most metal particles were elongated compared with the organic particles, which were round.

CONCLUSIONS

Although SEM/EDS has been extensively used in biomedical research, its novel application to assess the size, morphology, and chemical composition of the ocular surface particles offers an unprecedented opportunity to tease out the role of particulate matter exposure in ocular surface disease and disorders.

Spectroscopic Properties of Anisole at the Air-Ice Interface: A Combined Experimental-Computational Approach

A combined experimental and computational approach was used to investigate the spectroscopic properties of anisole in aqueous solutions and at the ice-air interface in the temperature range of 77-298 K. The absorption, diffuse reflectance, and emission spectra of ice samples containing anisole prepared by different techniques, such as slow freezing (frozen aqueous solutions), shock freezing (ice grains), or anisole vapor deposition on ice grains, were measured to evaluate changes in the contaminated ice matrix that occur at different temperatures. It was found that the position of the lowest absorption band of anisole and its tail shift bathochromically by ∼4 nm in frozen samples compared to liquid aqueous solutions. On the other hand, the emission spectra of aqueous anisole solutions were found to fundamentally change upon freezing. While one emission band (∼290 nm) was observed under all circumstances, the second band at ∼350 nm, assigned to an anisole excimer, appeared only at certain temperatures (150-250 K). Its disappearance at lower temperatures is attributed to the formation of crystalline anisole on the ice surface. DFT and ADC(2) calculations were used to interpret the absorption and emission spectra of anisole monomer and dimer associates. Various stable arrangements of the anisole associates were found at the disordered water-air interface in the ground and excited states, but only those with a substantial overlap of the aromatic rings are manifested by the emission band at ∼350 nm.

Suppression of protein inactivation during freezing by minimizing pH changes using ionic cryoprotectants

Freezing and lyophilization are often used for stabilization of biomolecules; however, this sometimes results in partial degradation and loss of biological function in these molecules. In this study we examined the effect of freezing-induced acidity changes on denaturation of the model enzyme haloalkane dehalogenase under various experimental conditions. The effective local pH of frozen solutions is shown to be the key causal factor in protein stability. To preserve the activity of frozen-thawed enzymes, acidity changes were prevented by the addition of an ionic cryoprotectant, a compound which counteracts pH changes during freezing due to selective incorporation of its ions into the ice. This approach resulted in complete recovery of enzyme activity after multiple freeze-thaw cycles. We propose the utilization of ionic cryoprotectants as a new and effective cryopreservation method in research laboratories as well as in industrial processes.

X-ray Fluorescence Imaging of Frozen Aqueous NaCl Solutions

In frozen aqueous NaCl, the liquid phase (LP) should coexist with ice at temperatures between the melting point and eutectic point of the system. The LP forms grooves on the surface of ice. In the present study, the morphology of the surface LP is examined by synchrotron radiation X-ray fluorescence (XRF). These measurements afford 2D distribution maps for Cl(-) as a major component, in addition to coexistent metal ions (Mn(2+), Co(2+), Cu(2+), and Zn(2+)). The 2D images obtained for Cl(-) indicate that a Y-shaped surface groove is formed in an observation area. Mn(2+) and Co(2+) are simply enriched in the LP together with Cl(-). In contrast, Cu(2+) and Zn(2+) exhibit behavior that is different from that of Mn(2+) and Co(2+), and they are concentrated at particular locations that are obviously off of the LP; this tendency is more apparent for Zn(2+). The 2D images are converted into 3D images by taking into consideration the freeze concentration in the LP and the attenuation of X-rays. The depth is largest in the middle of the groove, particularly at the intersection of surface grooves.

Fluidic Grooves on Doped-Ice Surface as Size-Tunable Channels

We propose a new principle for fabrication of size-tunable fluidic nano- and microchannels with a ubiquitous green material, water. Grooves filled with a solution are spontaneously formed on the surface of ice when an appropriate dopant is incorporated. Sucrose doping allows the development of grooves with lengths of 300 μm along the boundaries of ice crystal grains. This paper focuses on controlling the size of the liquid-filled groove and reveals its applicability to size-selective differentiation of nano- and micromaterials. The width of this groove can be varied in a range of 200 nm to 4 μm by adjusting the working temperature of the frozen platform. The channel dimension is reproducible as long as the same frozen condition is employed. We demonstrate the size-selective entrapment of particles as well as the state evaluation of DNA by controlling the physical interference of the ice wall with the electrophoretic migration of particles.