Kinetic description of water transport during spontaneous emulsification induced by Span 80.

Spontaneous emulsification is a phenomenon that forms nanometer-sized droplets (nanodroplets) without the application of any external force, and the mechanism has been actively studied for application to various technologies. In this study, we analyzed the kinetics of spontaneous emulsification induced by Span 80. The measurement of water concentration in Span 80 hexadecane solution indicated that the chemical potential of water in the nanodroplets decreased as the amount of water in the nanodroplets decreased. Based on this result, water transport between the aqueous phase and nanodroplets in which the chemical potential of water was controlled was quantitatively investigated by using a microfluidic device. The results demonstrate that the kinetics of water transport during spontaneous emulsification induced by Span 80 was described by a model of osmotic transport through an organic liquid film between the aqueous phase and nanodroplets.

Rapid determination of domoic acid in seafood by fluorescence polarization immunoassay using a portable analyzer.

Monitoring phycotoxin accumulation in marine products such as edible shellfish is a regulatory requirement in many countries. Therefore, a simple and rapid onsite quantification method is sought. Herein, we present a fluorescence polarization immunoassay (FPIA), a well-known one-step immunoassay, using a portable fluorescence polarization analyzer for domoic acid (DA), widely referred to as the primary toxin of amnesic shellfish poisoning (ASP). To establish FPIA for DA, the matrix effect of methanol, which is widely used to extract DA from shellfish, on FPIA was investigated. To validate this method, we performed a spike recovery test using oysters containing DA at a concentration equivalent to the regulatory limits of North America and the European Union (20 mg/kg). The recovery rate was found to be 79.4-114.7%, which is equivalent to that of the commercially available enzyme-linked immunosorbent assay (ELISA). We expect that this FPIA system will enable the quantitative onsite analysis of DA and significantly contribute to the safety of marine products.

Sensitive and simple multi-ion detection using organic nanocrystal enrichment in paper analytical devices.

Microfluidic paper analytical devices (µPADs) are among the most promising platforms for heavy metal ion analysis. On the other hand, achieving simple and highly sensitive analysis of µPADs is challenging. In this study, we developed a simple enrichment method for sensitive multi-ion detection utilizing water-insoluble organic nanocrystals accumulated on µPAD. By combining the enrichment method with multivariate data analysis, three metal ion concentrations in the ion mixtures were simultaneously quantified with high sensitivity owing to the sensitive responses of the organic nanocrystals. In this work, we successfully quantified Zn2+, Cu2+, and Ni2+ at 20 ng L-1 in the mixed ion solution using only two dye indicators with a larger sensitivity improvement than those reported in previous studies. Interference studies revealed possibilities for a practical application in real sample analysis. This developed approach also can be used for other analytes.

Detection of Fibril Nucleation in Micrometer-Sized Protein Condensates and Suppression of Sup35NM Fibril Nucleation by Liquid-Liquid Phase Separation.

Elucidating the link between amyloid fibril formation and liquid-liquid phase separation (LLPS) is crucial in understanding the pathologies of various intractable human diseases. However, the effect of condensed protein droplets generated by LLPS on nucleation (the initial step of amyloid formation) remains unclear because of the lack of available quantitative analysis techniques. This study aimed to develop a measurement method for the amyloid droplet nucleation rate based on image analysis. We developed a method to fix micrometer-sized droplets in gel for long-term observation of protein droplets with known droplet volumes. By combining this method with image analysis, we determined the nucleation dynamics in droplets of a prion disease model protein, Sup35NM, at the single-event level. We found that the nucleation was unexpectedly suppressed by LLPS above the critical concentration (C*) and enhanced below C*. We also revealed that the lag time in the Thioflavin T assay, a semi-quantitative parameter of amyloid nucleation rate, does not necessarily reflect nucleation tendencies in droplets. Our results suggest that LLPS can suppress amyloid nucleation, contrary to the conventional hypothesis that LLPS enhances it. We believe that the proposed quantitative analytical method will provide insights into the role of LLPS from a pathological perspective.

Nano-confined electrochemical reaction studied by electrochemical surface forces apparatus.

Electrochemical reactions in a nano-space are different from those in bulk solutions due to structuring of the liquid molecules and peculiar ion behavior at the electric double layer and are important for applications involving sensors and energy devices. The electrochemical surface forces apparatus (EC-SFA) we developed enabled us to study the electrochemical reactions in a solution nano-confined between the electrodes with varying distance (D) at nm resolution. We recorded measurements of the current-distance profiles due to the electrochemical reaction of the redox couples in the electrolyte nano-confined between Pt electrodes using our EC-SFA. We observed a long-range feedback current due to redox cycling and the sudden current increase at a short distance, the latter for the first time. This sudden current increase was two orders greater than the conventional feedback current and was observed at D < 5 nm when the electrodes were approaching and D < 200 nm on separation. We simultaneously measured the electric double layer force and the current between the electrodes in the solution to study the mechanisms of this sudden current increase in the short distance range. The results revealed a molecular insight as to how the redox species affect the current between two electrodes under nano-confinement. This study demonstrated that EC-SFA is a powerful tool for obtaining fundamental knowledge about the nano-confined electrochemical reactions for nanoelectrodes which can be applied to sensors and energy devices.

Facile and rapid detection of SARS-CoV-2 antibody based on a noncompetitive fluorescence polarization immunoassay in human serum samples.

Antibody detection methods for viral infections have received broad attention due to the COVID-19 pandemic. In addition, there remains an ever-increasing need to quantitatively evaluate the immune response to develop vaccines and treatments for COVID-19. Here, we report an analytical method for the rapid and quantitative detection of SARS-CoV-2 antibody in human serum by fluorescence polarization immunoassay (FPIA). A recombinant SARS-CoV-2 receptor binding domain (RBD) protein labeled with HiLyte Fluor 647 (F-RBD) was prepared and used for FPIA. When the anti-RBD antibody in human serum binds to F-RBD, the degree of polarization (P) increases by suppressing the rotational diffusion of F-RBD. The measurement procedure required only mixing a reagent containing F-RBD with serum sample and measuring the P value with a portable fluorescence polarization analyzer after 15 min incubation. We evaluated analytical performance of the developed FPIA system using 30 samples: 20 COVID-19 positive sera and 10 negative sera. The receiver operating characteristic curve drawn with the obtained results showed that this FPIA system had high accuracy for discriminating COVID-19 positive or negative serum (AUC = 0.965). The total measurement time was about 20 min, and the serum volume required for measurement was 0.25 µL. Therefore, we successfully developed the FPIA system that enables rapid and easy quantification of SARS-CoV-2 antibody. It is believed that our FPIA system will facilitate rapid on-site identification of infected persons and deepen understanding of the immune response to COVID-19.

Evaluation of Interfacial pH Using Surface Forces Apparatus Fluorescence Spectroscopy.

The fluorescence spectrum measurement of a fluorescence pH probe, C. SNARF-4F, was performed for monitoring the interfacial pH of aqueous electrolytes between mica or silica surfaces while varying the surface separation (D) using surface force apparatus (SFA) fluorescence spectroscopy. The pH of the aqueous CsCl between mica exponentially decreased with decreasing D. The order of the decay lengths of the interfacial pH obtained from the exponential fitting (L) at various electrolyte concentrations was L1mM > L0.1mM ≈ L0.4mM > L10mM. For studying the mechanisms of these changes, we performed the electric double layer (EDL) model calculation of the interfacial pH based on the surface potentials, which were evaluated from the EDL forces between the substrates in aqueous electrolytes using the same SFA. The calculated pH value for the 0.1 mM aqueous electrolyte corresponded to the values obtained from fluorescence spectroscopy, indicating that the interfacial pH was attributed to only the general EDL effect. On the other hand, the measured pH value for the higher concentrations of aqueous electrolytes (0.4-10 mM) decreased in the longer D ranges than the values calculated from the model, indicating that there was an additional factor affecting the interfacial pH for those concentrations. We also studied the effects of the cationic species of the electrolytes (Cs+, Na+, and Li+) and of the silica substrate on the interfacial pH. The systematic studies of the interfacial pH revealed that it depended on all three factors studied here, that is, the electrolyte concentration, electrolyte species, and the substrates. The results also suggested that the interfacial pH was not only due to the simple EDL theory but could also be affected by an additional factor due to the ion adsorption at the interface and chemical states of the substrates.

Ice premelting layer of ice-rubber friction studied using resonance shear measurement.

We performed a resonance shear measurement (RSM) based on a low-temperature surface force apparatus to evaluate the frictional properties of the interface between butadiene rubber and ice at various temperatures below 0 °C. Friction between the rubber and ice was high and constant at temperatures below -5 °C, but sharply decreased when the temperature rose above -5 °C. We performed the same measurement by replacing the rubber with polystyrene and silica films which were rigid and exhibited practically minimal elastic deformation in comparison to the rubber. The friction decreased gradually with the increase in temperature from -20 °C to 0 °C at both the polystyrene-ice and the silica-ice interfaces. These results indicated that the elasticity of rubber was responsible for the differences in the rubber-ice interface and the other two samples. To understand the detailed mechanism of friction between the rubber and the ice, we analyzed the obtained RSM data using a physical model. The result indicated that the friction between ice and rubber was determined by the elastic deformation of the rubber film at temperatures below -5 °C, and by the viscosity of the ice premelting layer above -5 °C.

Ice Premelting Layer Studied by Resonance Shear Measurement (RSM).

The viscosity of the ice premelting layer in contact with silica in the temperature range of -18 to -1 °C was studied by resonance shear measurement (RSM). The viscosity of the ice premelting layer was determined to be ∼5 orders of magnitude greater than that of the bulk liquid water and continuously decreased with the increasing sliding speed between the two surfaces over the temperature range employed in this study, which was the same behavior as for the typical confined liquids including water. On the other hand, the normal load and the contact pressure did not influence the viscosity, indicating that the premelting layer behaved differently from the typical confined liquids.

Viscosity measurement of Xanthan-Poly(vinyl alcohol) mixture and its effect on the mechanical properties of the hydrogel for 3D modeling.

Biomodels made of poly(vinyl alcohol) (PVA) are demanded because they can represent the geometries and mechanical properties of human tissues realistically. Injecting and molding, commonly used in three-dimensional (3D) modeling, help to represent the blood vessels accurately. However, these techniques sometimes require higher pressures than the upper pressure limit of the dispensers for pouring in high viscosity materials; the material viscosity should therefore be lower. Moreover, the mechanical properties of the biomodels should be reproduced. This study proposes a PVA solution through the addition of xanthan gum (XG) for 3D modeling, which lowers liquid viscosity while maintaining the mechanical properties of biomodels. XG is known to facilitate the achievement of non-Newtonian fluidity; however, the effects of XG on a PVA solution and PVA hydrogel (PVA-H) are not confirmed. The viscosity measurement using 15 wt% PVA with XG solution (PVA/XG) shows that it will provide easier pouring than 17 wt% PVA solution. The tensile test using the PVA-H of PVA(15 wt%)/XG(0.2 wt%) reveals that the gel is comparable in Young's modulus to 17 wt% PVA-H. X-ray diffraction shows the crystalline structures of the PVA/XG gel and PVA-H are identical. Thus, this PVA/XG would be useful for fabricating biomodels using injection molding techniques.

Low-Temperature Surface Forces Apparatus to Determine the Interactions between Ice and Silica Surfaces.

We have developed a low-temperature surface forces apparatus (SFA) using a thermoelectric Peltier module inserted below the bottom surface of the lower sample holder, giving easy access to the samples and allowing quick temperature changes. In air, the temperature can be decreased to ca. -20 °C. To demonstrate the performance of the apparatus, we measured the interactions between ice and a silica surface at -11.5 ± 0.5 °C. An exponentially decaying repulsion of the decay length, 11.2 ± 1.0 nm, was observed, and attributed to the electric double layer (EDL) repulsion. The surface potential of the ice was calculated to be -35 mV by fitting the data to the EDL model.

Selection of Diacrylate Monomers for Sub-15 nm Ultraviolet Nanoimprinting by Resonance Shear Measurement.

In UV nanoimprinting, the selection of monomers suitable for sub-15 nm patterning is difficult because the filling behavior of resin at this scale still remains scientifically unclear. We demonstrate sub-15 nm patterning by UV nanoimprinting using silica molds with 20, 15, and 7 nm diameter holes; however, the 7 nm diameter pillar patterns were not fabricated using hydroxy-containing monomers. The filling behavior into silica holes of around 10 nm depended on the chemical structure of the monomers. Resonance shear measurements revealed the following: (1) The viscosities of hydroxy-containing monomers confined between chlorodimethyl(3,3,3-trifluoropropyl)silane (FAS3-Cl)-modified surfaces began to increase at distances shorter than those of the monomers between unmodified surfaces. (2) The monomers confined between tridecafluoro-1,1,2,2-tetrahydrooctyltrimethoxysilane-modified surfaces were squeezed out when the surface-surface distance decreased at less than 7 nm. The measured viscosities between the FAS3-Cl-modified silica surfaces were correlated with the insufficient filling behavior into the silica holes of around 10 nm in UV nanoimprinting. Contact angle measurements provided an additional insight that a higher wettability of the monomers onto the antisticking chemisorbed monolayers resulted in imprinted patterns with higher aspect ratios. Considering the increase in the monomer viscosity in the nanospace and the wettability of monomers onto chemisorbed monolayers, we concluded that the monomer showing low viscosity under confinement and high wettability onto the mold surface was suitable for single-digit nanometer UV nanoimprinting.

Nanotribological Characterization of Lubricants between Smooth Iron Surfaces.

We performed the resonance shear measurement (RSM) for evaluating the nanorheological and tribological properties of model lubricants, hexadecane and poly(α-olefin) (PAO), confined between iron surfaces. The twin-path surface forces apparatus (SFA) was used for determining the distance between the surfaces. The obtained resonance curves for the confined lubricants showed that the viscosity of the confined hexadecane and PAO increased due to liquid structuring when the surface separation (D) decreased to a value less than 24 and 20 nm, respectively. It was also determined that the iron surfaces were lubricated by the hexadecane when normal load (L) was less than 1.1 mN, while the confined hexadecane behaved almost solid-like and showed poor lubricity when L was greater than 1.1 mN. In contrast, PAO between the iron surfaces showed high lubricity even under the high load (L > 2 mN). The surface separation of hexadecane and PAO at a hard wall contact between the iron surfaces was determined to be 4.6 ± 0.5 and 5.0 ± 0.4 nm by applying the fringes of equal chromatic order (FECO) for half-transparent iron films deposited on mica surfaces as substrates. We also characterized hexadecane and PAO confined between mica surfaces for studying the effect of substrates on the confined lubricants.

Nanometer-Resolved Fluidity of an Oleophilic Monomer between Silica Surfaces Modified with Fluorinated Monolayers for Nanoimprinting.

Ultraviolet (UV) nanoimprinting has the potential to fabricate sub-15 nm resin patterns, but the interfacial fluidity of organic monomers near monomer liquid/mold solid interfaces related to filling nanoscale mold recesses with UV-curable resins still remains unclear. In this study, we demonstrated that surface forces and resonance shear measurements were helpful to select a surface modifier appropriate for silica mold surfaces for UV nanoimprinting with the low-viscosity monomer 1,10-decanediol diacrylate. Surface forces between silica surfaces mediated with the diacrylate monomer and fluidities of the monomer were investigated with nanometer resolution. Chemical vapor surface modification of silica surfaces with chlorodimethyl(3,3,3-trifluoropropyl)silane (FAS3-Cl) and tridecafluoro-1,1,2,2-tetrahydrooctyltrimethoxysilane (FAS13) gave fluorinated silica surfaces with root-mean-square roughness of less than 0.24 nm suitable for the measurements. When the distance D between two silica surfaces was decreased stepwise in the range of 0-30 nm, monomer viscosity between cleaned silica surfaces increased markedly at D < 6 nm. Surface modification with FAS3-Cl suppressed this increase of interfacial monomer viscosity. In contrast, FAS13-modified silica surfaces caused a jump-in phenomenon at approximately D = 7-9 nm, suddenly decreasing to D = 1 nm as the monomer fluid layer was squeezed out. We concluded that FAS3-Cl was appropriate as a fluorinated surface modifier for silica molds used in UV nanoimprinting with an oleophilic low-viscosity monomer, because the chemisorbed monolayer maintained low monomer viscosity near the surface/monomer interface, in addition to its low surface free energy and short CF3CH2CH2- group.

Characterization of ferrocene-modified electrode using electrochemical surface forces apparatus.

A electrochemical surface forces apparatus (EC-SFA) was employed to measure the interactions between gold electrodes modified with self-assembled monolayers of ferrocene alkyl thiol (Fc-SAM) and oxidized ferrocene (ferrocenium cation, Fc(+)-SAM) in a 1 mM aqueous electrolyte. The double-layer repulsion in both cases of the Fc-SAM and Fc(+)-SAM electrodes was observed. The surface charge density (σ) evaluated from the double-layer repulsions between the Fc(+)-SAM electrodes in 1 mM aqueous KClO4 was 0.0027 C/m(2), which was 2.5 times greater than that of the Fc-SAM, at 0.0011 C/m(2). The σ values of the Fc(+)-SAM were evaluated for various counteranions using the same method, which were 0.0048, 0.0040, and 0.0104 C/m(2) for NO3(-), SO4(2-), and CF3SO3(-), respectively. The degrees of dissociation (αd) between the ferrocenium cation and these counteranions were obtained from σ and the density of the ferrocenium on the electrode. The αd value of CF3SO3(-), 4.1%, was the highest, followed in the order, SO4(2-) > NO3(-) > ClO4(-), indicating that most of the positive charges of the ferrocenium cation were compensated by formation of an ion pair with counteranions.

Two types of two-component gels formed from pseudoenantiomeric ethynylhelicene oligomers.

Two-component gels formed from pseudoenantiomeric ethynylhelicene oligomers in toluene exhibited two different properties depending on difference in numbers of helicenes in the two components. The combinations (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/(P)-4, which contained oligomers with comparable numbers of helicenes, formed transparent gels (Type I gels). The combinations (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/(P)-3, which contained oligomers with considerably different numbers of helicenes, formed turbid gels (Type II gels). Negative Cotton effects were observed for the Type I gels in the region between 350 and 450 nm, and were positive for the Type II gels, despite the use of (M)-oligomers for the longer components. UV/vis exhibited absorption maxima at 350 nm for the Type I gels and at 338 nm for the Type II gels. Different behaviors in gel formation processes were observed by fluorescence studies. Atomic force microscopy analysis showed fiber structures of 25-50 nm diameter for Type I gels and bundles of 100-150 nm diameter for Type II gels. The stoichiometry in gel formation also differed: The Type I gels showed 1:1 stoichiometry of the two components; the Type II gels showed no 1:1 stoichiometry, likely 1:2 stoichiometry. Using the Type I and II gels, two-layer gel systems were constructed.

Resonance shear measurement of nanoconfined ionic liquids.

Two types of imidazolium-based ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(4)mim][NTF(2)]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF(4)]), confined between silica surfaces were investigated by surface force apparatus (SFA)-based resonance shear measurements together with surface force measurements. The surface force profiles in the ILs showed oscillatory solvation forces below the characteristic surface separations: 10.0 nm for [C(4)mim][NTf(2)] and 6.9 nm for [C(4)mim][BF(4)]. The more pronounced solvation force found in [C(4)mim][NTf(2)] suggests that the crystal-forming ability of the IL contributes to the stronger layering of the ILs adjacent to the surface. The resonance shear measurement and the physical model analysis revealed that the viscosities of the confined ILs were 1-3 orders of magnitude higher than that of the bulk IL. This paper also focused on the correlation between the resonance shear behaviour and the lubrication property of the ILs, and the suspension rheology in the ILs. An understanding of the solid-IL interface and of ILs confined in nanospace will facilitate the further development of novel applications employing ILs.

Water expansion dynamics after pulsed IR laser heating.

A nanosecond pulsed IR (1.9 microm) laser rapidly heated water, in an open vessel, to temperatures well below the boiling point. The subsequent dynamics of volume expansion were monitored using time-resolved interferometry in order to measure the increase in the water level in the heated area. The water expanded at less than the speed of sound, taking just less than 100 ns to increase its height by approximately 500 nm at surface temperature jumps of 20 K. The initial expansion was followed by an apparent contraction and then a re-expansion. The first expansion phase occurred more slowly than the timescale for bulk H-bond re-structuring of the water, as determined from vibrational bands in the Raman spectra, and represents the limit to the rate at which the overpressure caused by sudden heating can be released. The second phase of the expansion was caused by hydrodynamic effects and is accompanied by morphological changes resulting in light scattering as well as droplet spallation.

Density changes accompanying wave propagation in the cerium-catalyzed Belousov-Zhabotinsky reaction.

Refractive index measurement using an interferometric imaging system and observation of chemical wave shapes were carried out during chemical wave propagation of a cerium-catalyzed Belousov-Zhabotinsky (BZ) reaction. Densities increased as chemical waves propagated in samples without NaBr, and decreased in samples with NaBr. Concentration changes of malonic acid, bromomalonic acid, and BrO3- were estimated from Raman spectral measurements in a stirred batch BZ reaction, and these also exhibited differences between samples with and without NaBr. It is proposed that a reaction subset yielding low molecular weight carboxylic acids is predominant in samples with NaBr, whereas a pathway leading to dibromoacetic acid or tribromoacetic acid production is the major process in samples without NaBr.