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 (c gly ini). However, the viscosity of the FCS measured at a given temperature increases with decreasing c gly ini, 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 depends on the size of the space in which the FCS is confined and is enhanced in smaller spaces. This result agrees well with several reports of anomalous phenomena in a microspace confined in ice. These phenomena should 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 has ice-like physicochemical properties and structures, giving higher viscosity than the corresponding bulk solution.
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 pK a 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 interaction of ions other than H + with the ice surface is mainly driven by coordination of the dangling bonds to the ions. Therefore, smaller ions are adsorbed more readily on the ice surface than their larger counterparts. Additionally, uptake of F − in the ice crystal lattice is suggested. Elucidation of the ice/water interface properties will allow us not only to understand the unique properties found in nano-or micro-sized liquid phases confined in ice but also to develop novel separations and reactions using frozen aqueous media as platforms.
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.
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