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A stencil printing method for paper-based analytical device (PAD) fabrication utilizing sticky notes, a thermal transfer ink ribbon, and office appliances is proposed.

Total protein concentrations in the aqueous solutions were determined from the absorption spectra reproduced from smartphone-captured digital color images. We employed two different procedures for protein determination: the pyrogallol red molybdate method and Bradford's method. The principal-component-analysis-based reproduction process, which was previously reported by our research group, enabled the conversion of RGB values to score values for a linear combination of loading vectors to generate reproduced absorption spectra. The reproduced spectra were identical to those measured using a commercially available spectrophotometer. The total protein assays of commercial soymilk and human serum samples were carried out with both coloration reagents, and the obtained results were in good agreement with those attained using a conventional spectrophotometer. These results show that the proposed method enables smartphone-based ratiometric analysis of real samples without requiring any monochromating equipment.

The anisotropic properties of one-dimensional (1D) supramolecules have generally been the sole way to input molecular information along a structure of high density. Although the chain reaction of a...

The phase separation of an aqueous ionic liquid in migration by electrophoresis was utilized for online sample enrichment in microchip electrophoresis. The enrichment of rhodamine 6G in a sample so...

The chemical interaction between antifreeze proteins (AFPs) and ice crystals is evaluated via electrophoresis of AFP-anchored microparticles in fluidic channels formed in frozen aqueous sucrose. Straight fluidic channels are created in a flat glass chamber connecting two Ag/AgCl electrodes. This configuration allows us to estimate an electric field strength exerted on probe particles migrating along the channel. When the channel width is comparable to the particle size, the particle is immobile because of the resistance force induced by the interaction with the ice wall. However, when the overall electrophoretic force surpasses the resistance force, the microsphere starts to migrate. From the threshold electric field strengths determined for unmodified and AFP-modified particles, the resistance forces for the chemical interaction between AFPs and ice wall are estimated.

The present dataset is related to the research paper entitled “Reproducing Absorption Spectra of pH Indicators from RGB Values of Microscopic Images” (Inagawa et al., 2020). The dataset contains microscopic images of aqueous methyl orange (MO), absorption spectra acquired with a spectrophotometer, loading spectra and calculation sheets for reproducing absorption spectra of aqueous MO from their RGB values of the microscopic images. The microscopic transmission images of the standard MO solutions at various pH conditions were acquired with a CMOS camera equipped with an invert microscope. Meanwhile, the loading spectra were obtained by principle component analysis of a series of absorption spectra of the standard solutions. The conversion matrix from RGB values in a region of interests (ROI) to score values were linear-algebraically determined from the RGB values and score values of the standard solutions. The absorption spectra of the sample solutions of which pH conditions are unknown were then reproduced by calculating the linear combination of the loading spectra with the score values obtained from the conversion process. Herein, the absorption spectra of MO are reproduced at various pH and ROI conditions.

We reveal the charging mechanism and behavior of ions at the ice/solution interface through measurements of the zeta potential of ice. The zeta potential of ice, which is calculated from the migration of a probe in an ice channel under various conditions, is interpreted using the Stern double layer model. The zeta potential of ice is generated by the deprotonation of dangling OH bonds, the adsorption of ions on the ice surface, and ion uptake in the ice crystal lattice. The deprotonation of the dangling OH bonds on the surface of ice is enhanced compared to that in bulk liquid water; the pKa of the former is estimated to be ∼3. Interestingly, only 1.41% of the total dangling OH bonds on the ice surface are deprotonated, even at pH > 6, suggesting that the deprotonation of a dangling bond suppresses further dissociation of the nearby OH sites. This is caused by the facilitated reorientation of the water molecules in ice in the presence of L-defects. The ion adsorption constants indicate that the interactio...

This paper proposes a simple and versatile method for the determination of the zeta potential of a channel wall and discusses the values measured for the surface of frozen solvents, which are not only of scientific interest but also of potential use for microfluidic platforms. The zeta potential of the solid surface is an important parameter for discussing its electrokinetic properties, the distribution and reaction of ions in an electric double layer, and the fluidic behavior in the space surrounded by the surface. While the zeta potential of colloidal matters can be determined from their electrophoretic mobility, it is often difficult to determine that of a bulk material. In this paper, the zeta potential of a microchannel fabricated in a frozen solvent is determined by measuring the apparent mobility of microparticles as the probe. The electrophoretic mobility of the microparticles has been measured in advance using microchip electrophoresis under various conditions. This approach allows us to determine the zeta potential of water-ice and frozen cyclohexane. We discuss the pH dependence of the zeta potential of ice and also effects of the NaCl concentration on that of ice and frozen cyclohexane.

The absorption spectra of bromothymol blue (BTB) solution introduced in microfluidic devices were reproduced by principal component analysis (PCA)-based xyz-spectrum conversion methods for geometric mapping of the pH values of fluids. We fabricated PDMS-made microfluidic devices with a channel depth of 1 mm to overcome the lower detection limits of transmittance image acquisition. Aqueous solutions of pH indicators under various pH conditions were hydrodynamically introduced into the channel, and RGB values of the region of interest (ROI) were obtained via image analysis. The xyz values were then converted into absorption spectral data of the pH indicator using the PCA-based spectral reproduction previously proposed by the authors. The high reproducibility of the spectra was confirmed to be comparable to that of the conventional method using a spectrophotometer. We applied the present method to elucidate the pH gradient at an aqueous biphasic interface in the microfluidic channels generated by contacting multiple laminar flows of two or three buffered solutions. We confirmed that the pH gradient ranged from approximately 70 to 140 μm, which is consistent with the results reported using other approaches. The results demonstrate the applicability of the present method to the fluctuation field in micro/nanospaces to acquire spectrophotometric information in the order of milliseconds without monochromating equipment.

Aggregation-induced emission (AIE) is a new technique for enhancing fluorescence. To demonstrate manipulation of near-infrared AIE by thermal stimuli for analytical use, we synthesized a thermoresponsive fluorescent polymer containing Au(I)–S bonds as near infrared fluorophores. We also examined the polymer for use as a fluorescent chemosensor based on the manipulation of AIE. The thermoresponsive fluorescent polymer was prepared from poly(N-isopropylacrylamide-co-methacrylic acid) by introducing thiol groups which were further modified to form fluorescent Au(I)–S bonds. The thermoresponsive fluorescent polymer aggregated and then fluoresced in the near infrared region due to AIE when heated above the phase transition temperature of 32 °C. The change in fluorescence intensity of the polymer was reversible by changing the solution temperature. Because the residual carboxyl groups make the thermoresponsive fluorescent polymer negatively ionizable, neutralization by the addition of positively charged analytes facilitates its aggregation to enhance fluorescence above the phase transition temperature. When hemoglobin, which has an isoelectric point of 6.8, was added to a pH 6.0 polymer solution at 40 °C, the polymer aggregated to enhance its fluorescence as a result of neutralization. A linear relationship between the hemoglobin concentration and the fluorescence intensity of the polymer was obtained for 0.5–2.0 g/L hemoglobin with a detection limit (3 s) of 0.014 g/L.

Absorption spectra of pH indicators in aqueous solutions were reproduced from RGB values of microscopic images utilizing principal component analysis (PCA) and linear algebraic treatments. The reproduction of absorption spectra comprises the following three steps: (1) determining the loading spectra by PCA, (2) determining the conversion matrix from the RGB values to the score vectors, and (3) reproducing the absorption spectra by linear combination of the loading spectra and the score vectors. The reproducibility of the absorption spectra was demonstrated by employing bromothymol blue and methyl red solutions as pH indicators. The reproduced spectra of both indicators were in good agreement with the spectra measured with a conventional spectrophotometer. The pKa values of both indicators calculated from the reproduced spectra are in good agreement with those obtained from the spectrophotometric spectra and the literature values, confirming validity of the reproduction. This approach was applied to measure pH of freeze concentrated solutions in micro drains formed in ice. A change in pH was successfully observed on freezing and was compared with that reported in previous literature. Since this method does not necessitate the use of grating systems, spectral changes can be traced in milliseconds; this elucidates the phenomena occurring in fluctuating fields.

In this chapter, the migration behavior of bio-materials in ice grain boundaries is explored to demonstrate the applicability of the concept proposed in Chap. 2 to bio-separation. Yeast cells and T4 GT7 giant DNA samples were employed. The size-tunability of the ice grain boundary channel (Inagawa et al. in Sci Rep 5:17308 [1], Inagawa et al. in Talanta 183:345–351 [2]) allows the migration control of yeast cells. On the other hand, for DNA, the difference in the migration behavior of the random-coil and globule states was investigated. This difference in state affects the contour flexibility of the DNA, resulting in different threshold temperatures at which the DNA molecules begin to migrate. Ice grain boundary electrophoresis was also applied to the detection of the chemical interactions between the biomolecules and ice crystals. AFPs were anchored on 1 μm PS particles, and their migration behavior in the grooves on the surface ice was studied. The threshold temperature at which the particles begin to migrate is an effective criterion for the evaluation of the chemical interactions between the particles and ice walls. The threshold temperature was lowered by 2.5 °C when the AFPs were bound onto the particles, indicating the presence of interactions between the bound AFPs and the ice wall. Because the AFPs studied in this work exhibit selectivity towards the prism plane of the ice crystal, it is critical that the prism plane of the ice crystal should be in contact with the FCS in the surface grooves.

This chapter discusses a new principle, namely “ice grain boundary electrophoresis,” for the fabrication of size-tunable fluidic nano- and microchannels by freezing an aqueous solution (Inagawa and Okada in Ice grain boundary electrophoresis. In: Proceeding of MicroTAS2013, pp 560–562, 2013, [1]; Inagawa et al. in Sci Rep 5:17308, 2015, [2]). Grooves filled with FCSs are spontaneously formed on the surface of ice when an aqueous solution of the appropriate dopant is frozen. Sucrose-doping allows the development of grooves with lengths of 300 μm along the boundaries of the ice crystal grains. This chapter focuses on the control of the size of the liquid-filled groove and reveals its applicability to the size-selective differentiation of nano- and micromaterials. The width of this groove can be varied across the range 200 nm–4 μm by adjusting the working temperature of the frozen platform. The channel dimension is reproducible when the same frozen conditions are employed. The size-selective entrapment of particles by controlling the physical interference of the ice wall is demonstrated.

This chapter proposes a simple and versatile method for the determination of the zeta potentials of frozen solvents (Inagawa et al. in J Colloid Interface Sci 532:231–235, [1]; J Phys Chem C 123:6062–6069, [2]). A microchannel was fabricated in a solidified solvent. The zeta potential of the frozen solvent was determined by measuring the apparent mobility of the probe microspheres. Thus, the ice zeta potentials under various conditions, including the dependences of temperature, salt concentration, and pH, were determined. Based on the above results, the charging mechanism at the ice/solution interface as well as the ionic behavior was revealed using the Stern double-layer model. The zeta potential of ice is generated by the deprotonation of dangling OH bonds, ion adsorption on the ice surface, and ion uptake in the ice crystal lattice. The deprotonation of the dangling OH bonds on the ice surface (pKa, ∼3) is enhanced compared to that in bulk liquid water. Interestingly, only 1.41% of the total dangling OH bonds on the ice surface are deprotonated, even at pH > 6, suggesting that the deprotonation of a dangling bond suppresses further dissociation of the neighboring OH sites. This is caused by the facilitated reorientation of the water molecules in ice in the presence of L-defects. The ion adsorption constants indicate that the interaction of ions, other than H+ ions, with the ice surface is mainly driven by the coordination of the dangling bonds with the ions. Therefore, smaller ions are adsorbed more readily on the ice surface than their larger counterparts. Additionally, the uptake of F− ions in the ice crystal lattice is suggested.

In this chapter, the viscosities of the FCS in frozen glycerol/water solutions are evaluated by two spectrometric methods of different principles: (1) the reaction rate of the diffusion-controlled emission quenching and (2) fluorescence correlation (FCor) spectroscopy (Inagawa et al. in J Phys Chem C 121:12321–12328, 2017 [1]). Thermodynamics indicates that the concentration of glycerol in the FCS is constant at a constant temperature, regardless of the glycerol concentration in the original solution before freezing (\(c_{\text{gly}}^{\text{ini}}\)). However, the viscosity of the FCS measured at a given temperature increases with decreasing \(c_{\text{gly}}^{\text{ini}}\) and becomes more pronounced with decreasing measurement temperature. Further, the viscosity of the FCS in a rapidly frozen solution is higher than that observed in a slowly frozen solution. These results suggest that the viscosity of the FCS depends on the size of the space in which the FCS is confined and increases in smaller spaces. This result agrees well with several reports of anomalous phenomena in a microspace confined in ice. These phenomena originate from the fluctuation of the ice/FCS interface, which is macroscopically stable but microscopically dynamic and undergoes continuous freezing and thawing. Thus, the FCS near the interface displays ice-like physicochemical properties and structures, thereby affording higher viscosity than the corresponding bulk solution.

Channel-like grooves are formed on the surface of frozen aqueous sucrose. They are filled with a freeze concentrated solution (FCS) and act as an efficient size-tunable separation field for micro and nanoparticles. The width of the channel can be easily varied by changing the temperature. Because the channel width decreases with decreasing temperature, particles become immobilized due to physical interference from the ice wall when the temperature reaches a threshold point specific to the particle size. Surface modification of particles can add a factor of chemical interaction between the particles and ice walls. In this study, anti-freeze proteins (AFPs) are anchored on 1µm-polystyrene (PS) particles, and their behavior in the surface grooves on the ice is studied. The threshold temperature is an effective criterion for evaluating chemical interactions between particles and ice walls. The AFP binding on 1µm PS particles lowers the threshold temperature by 2.5°C, indicating interactions between AFPs on the PS particles and the ice wall. Because the AFPs studied here show selectivity towards the prism plane, it is critical that the prism plane of the ice crystal is in contact with the FCS in the surface grooves.

Simple and convenient preparation of metal nanoparticles has been a challenging issue to overcome in nano-technologies. To address this, spontaneous growth of gold nanoclusters to form gold nanoparticles in the presence of high molecular weight poly(ethylene glycol) (PEG) was studied under ambient conditions. Gold nanoclusters with a particle size of ∼2 nm spontaneously grew to form larger gold nanoparticles with a plasmonic absorption band at 520 nm only when in the presence of structurally organized PEG with a molecular weight of at least 2 × 106 g/mol, producing a pinkish colored solution. UV–vis absorption spectra indicate the existence of two steps responsible for gold nanocluster growth to form gold nanoparticles. The first step involves an increase in the absorbance of the spectral band at 520 nm during the first 10 min due to aggregation of gold nanoclusters. The second step is the broadening of this band at the same wavelength as a result of fusion followed by growth of the gold nanoclusters. Redox potentials and transmission electron microscopy (TEM) images reveal that structurally organized high molecular weight PEG induces aggregation of the gold nanoclusters and their subsequent growth to form larger nanoparticles, which exhibit plate-like triangular structures with sharp edges. Our study unveils the role that PEG plays in the growth process from gold nanoclusters to gold nanoparticles.

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.

An aqueous solution separates into ice and a freeze-concentrated solution (FCS) when frozen at temperatures above the eutectic point. The FCS acts as important reaction media in natural environment and industrial processes. The viscosities of the FCS in frozen glycerol/water solutions are evaluated by two spectrometric methods with different principles: (1) the reaction rate of the diffusion-controlled emission quenching and (2) fluorescence correlation spectroscopy. Thermodynamics indicates that the concentration of glycerol in the FCS is constant at a constant temperature regardless of the glycerol concentration in the original solution before freezing (cglyini). However, the viscosity of the FCS measured at a given temperature increases with decreasing cglyini, and this trend becomes more pronounced with decreasing measurement temperature. Further, the viscosity of the FCS in a rapidly frozen solution is higher than that in a slowly frozen solution. These results suggest that the viscosity of the FCS d...