Absorption-based colorimetric detection of nickel(II) ion by phase separation of thermoresponsive magnetic nanoparticles under microflow.

Therma-Max™ LSA Streptavidin is a thermoresponsive magnetic nanoparticle (TMNP). It can be introduced conveniently to molecular recognition groups by avidin-biotin interaction. In this study, we demonstrated the detection of nickel(II) ions by the magnetic separation of TMNP induced by their phase transition under microflow. The NTA-tagged TMNP solution mixed with a Ni2+ sample was introduced into a microchannel with a well structure. Moreover, the sample was heated to induce the thermally induced aggregation of TMNP. The Ni-capturing TMNP were trapped in the well by magnetic fields. The supernatant was removed from the outlet, and a dimethylglyoxime (DMG) solution was introduced into the device for colorimetric detection in the well. Because DMG has a higher stability constant with Ni2+, sensitive colorimetric detection of Ni2+ can be achieved in devices where the sample volume, e.g., optical pathlength, is short. To demonstrate the feasibility of the proposed method, a recovery test was conducted using a commercially available cosmetic sample. Therein, complete collection was achieved.

Elucidating the Quenching Mechanism of Tris(2,2'-bipyridyl)ruthenium(II) Complex in the Water-Glycerol Binary System Based on the Microscopic Structure of the Media.

Kinetic studies on the photochemical quenching reaction of the tris(2,2'-bipyridyl) ruthenium(II) complex ([Ru(bpy)3]2+) in water-glycerol binary media were conducted based on the Einstein-Smoluchowski (E-S) theory. Dynamic and static quenching behaviors were analyzed by comparing results from time-resolved spectroscopy and emission spectroscopy. While the dynamic quenching reaction aligns well with the E-S theory, static quenching was observed, leading to a notable increase in the overall photoquenching reaction rate constant. Employing chromatography and infrared spectroscopy, we correlated the microscopic molecular structure of the binary solvent system and the solvation environment around the emitters with the reaction mechanism. This correlation was found to correspond to ion pair formation and the confinement effect of the emitter, respectively.

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.

Incorporation of a Morphologically Controlled Ice Grain Boundary into a Microfluidic Device for Size-Selective Separation of Micro/Nanospheres.

A frozen aqueous solution was integrated into a microfluidic device as a size-tunable separation field for the size-selective separation of micro/nanospheres. The width of the ice grain boundaries formed in frozen aqueous solutions could be altered by controlling the operating temperature. A freezing chamber was placed adjacent to the microfluidic channel. A sample-dispersing aqueous sucrose solution was injected into the chamber and frozen, allowing the freeze-concentrated solution (FCS) to run vertically to the microfluidic channel, where the eluting solution flows. The operating temperature can be used to control the physical interaction between the ice wall and micro/nanospheres, enabling size-selective migration. The eluted micro/nanospheres in the microchannel were passed through the eluting solution collected from the outlet. We achieved size-selective separation and collection of microspheres and nanospheres. We separated the exosomes and yeast cells to demonstrate their applicability in bioseparation. The present method is suitable not only for size-selective separation but also for evaluating the biological expression of extracellular vesicles under cryogenic conditions.

Rapid acquisition of absorption spectra to monitor proton migration in nanoliter space using the RGB-spectrum-conversion method with a 10 ms interval.

Acquisition protocol of absorption spectra at nanoliter spaces from the RGB values preserved in video data at 10 ms intervals was established using the principal-component-analysis-based RGB-conversion method. Proton behavior was monitored using a camera to acquire the video footage to monitor colorimetric change in the nanoliter space. The RGB values observed in the video were converted into a score vector using a conversion matrix. A linear combination of the predetermined loading vectors with the score values was calculated to reproduce the absorption spectra. The reproduced absorption spectra correlated well with those acquired using a conventional spectrophotometer during a short period. This method was applied to monitor the proton diffusion from a single cationic ion-exchange resin into hydrogels at low concentrations. The rapid acquisition and quick response of this method may enable the monitoring of the initial diffusion of protons, which is challenging with conventional spectrophotometry and electrochemical approaches.

Fabrication of paper-based analytical devices by a laminating method with thermal ink ribbons, sticky notes, and office appliances.

A stencil printing method utilizing sticky notes, a thermal transfer ink ribbon, and office appliances for paper-based analytical device (PAD) fabrication was proposed. A sticky note was attached to a filter paper, and a mask pattern was cut using a cutting machine. A commercially available thermal ink ribbon was then placed over the mask and laminated. We have characterized the fabricated devices. This approach could be used for the fast and mass prototyping of PADs using simple office appliances with no need for a wax printer.

Charge-Selective Aggregation Behavior of Thermoresponsive Polyelectrolytes Having Low Charge Density in Aqueous Solutions of Organic Counterions.

The aggregation behavior of thermoresponsive polyelectrolytes with low charge density in aqueous solutions of organic counterions was investigated. We synthesized two thermoresponsive polyelectrolytes: anionic poly(N-isopropylacrylamide-co-(3-sulfopropyl)acrylamide potassium) (P-NIP-SPAK) and cationic poly(N-isopropylacrylamide-co-(3-acrylamidepropyl)trimethylammonium chloride) (P-NIP-AAPTAC). The polyelectrolytes remained soluble in their aqueous solutions even above the lower critical soluble temperature of P-NIP owing to the strong hydration property of the ionic groups. The aggregation occurred when organic counterions were added to the solution. In these solution systems, the concentration of counterions exceeds those of ionic groups introduced into the polyelectrolytes. The aggregation behavior is attributed to the salting-out effect of counterions accommodated near the polyelectrolyte surface by electrostatic interaction. This aggregation behavior was utilized for the charge-selective recognition of amino acids. P-NIP-SPAK aggregated only when basic amino acids were added under acidic conditions, whereas P-NIP-AAPTAC aggregated only when acidic amino acids were added under basic conditions. The results herein demonstrate that P-NIP-SPAK and P-NIP-AAPTAC have the potential to be used as charge-selective polymer sensors for amino acids without having to strictly control the experimental conditions.

Total protein assay by PCA-based RGB-spectrum conversion methods with smartphone-acquired digital images.

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.

Geometrical pH mapping of Microfluids by principal-component-analysis-based xyz-spectrum conversion method.

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.

Dataset for reproducing absorption spectra of methyl orange from the RGB values of microscopic images.

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.

Reproducing absorption spectra of pH indicators from RGB values of microscopic images.

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.

Interaction between antifreeze protein and ice crystal facet evaluated by ice-channel electrophoretic measurements of threshold electric field strength.

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.

Size-Tunable Micro-/Nanofluidic Channels Fabricated by Freezing Aqueous Sucrose.

Upon freezing aqueous sucrose at temperatures higher than the eutectic point (-14 °C in this case), two phases, that is, ice and freeze concentrated solution (FCS), are spontaneously separated. FCS forms through-pore fluidic channels when thin ice septum is prepared from aqueous sucrose. Total FCS volume depends on temperature but is independent of the initial sucrose concentration. This allows us to control the size of the FCS channels simply by changing the initial sucrose concentration as long as temperature is kept constant. In this paper, we show that the size of the channel, which has a layered structure, can be controlled in a range from 50 nm to 3 µm. Thus, the FCS channel is suitable for size-sorting of micro- and nanoparticles. We discuss the size-sorting efficiency of the channel and demonstrate the separation of particles with different sizes.

Zeta potential determination with a microchannel fabricated in solidified solvents.

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.

Electrophoresis in ice surface grooves for probing protein affinity to a specific plane of ice crystal.

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.

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.