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In this study, the growth of Escherichia coli was monitored using a complementary metal-oxide-semiconductor (CMOS) near-field sensor array. Each of the 1488 integrated elements, arranged in a 3 mm square, has a resonator that oscillates at 65 GHz. The effective capacitance of the resonator is altered by changes in the dielectric properties of the sensor surface, which shifts the resonance frequency. Growth curves of E. coli at different initial concentrations (OD600 = 0.01, 0.03, and 0.05) were monitored. A suspension with initial turbidity of OD600 = 0.05 was cultured in a medium, and the sensor successfully distinguished between viable E. coli and heat-treated dead E. coli in 20 min. Moreover, the apparent suppression of growth was observed in the presence of 500 μg/mL streptomycin. As the sensor is composed of arrayed elements, and the area of sensitivity distribution of the element is larger than the size of one bacteria, the variation in the output value of each element may reflect the number and movement of bacteria. This study revealed that the presence of viable E. coli could be rapidly confirmed by using the change in permittivity caused by the displacement of media by E. coli near the sensor surface.

The biological functions of proteins depend on harmonization with hydration water surrounding them. Indeed, the dynamical transition of proteins, such as thermal denaturation, is dependent on the changes in the mobility of hydration water. However, the role of hydration water during dynamical transition is yet to be fully understood due to technical limitations in precisely characterizing the amount of hydration water. A state-of-the-art CMOS dielectric sensor consisting of 65 GHz LC resonators addressed this issue by utilizing the feature that oscillation frequency sensitively shifts in response to the complex dielectric constant at 65 GHz with extremely high precision. This study aimed to establish an analytical algorithm to derive the hydration number from the measured frequency shift and to demonstrate the transition of hydration number upon the thermal denaturation of human serum albumin. The determined hydration number in the native state drew a “global” hydration picture beyond the first solvation shell, with substantially reduced uncertainty of the hydration number (about ±1%). This allowed the detection of a rapid increase in the hydration number at about 55 °C during the heating process, which was in excellent phase with the irreversible rupture of the α-helical structure into solvent-exposed extended chains, whereas the hydration number did not trace the forward path in the subsequent cooling process. Our result indicates that the weakening of water hydrogen bonds trigger the unfolding of the protein structure first, followed by the changes in the number of hydration water as a consequence of thermal denaturation.

The bioprotective properties of disaccharides have been linked to destructuring effect on the hydrogen-bond structure of the interfacial water around the disaccharide solute, but its detailed mechanisms are yet to be provided. In this study, we characterized the destructuring effect based on the complex dielectric constants of interfacial water around sucrose and trehalose in the H-O-H bending region. Our analysis showed that the destructuring effect around disaccharides involves substantial disordering of the hydrogen-bond structure and formation of strong disaccharide-water hydrogen-bond. Such a destructuring effect caused by disaccharides is totally distinct from what happens with temperature increases of neat water.

The dynamical and structural properties of water at protein interfaces were characterized on the basis of the broadband complex dielectric constant (0.25 to 400 THz) of albumin aqueous solutions. Our analysis of the dielectric responses between 0.25 and 12 THz first revealed hydration water with retarded reorientational dynamics extending ∼8.5 A (corresponding to three to four layers) out from the albumin surface. Second, the number of nonhydrogen-bonded water was decreased in the presence of the albumin solute, indicating protein inhibits the fragmentation of the water hydrogen-bond network. Finally, water molecules at the albumin interface were found to form a distorted hydrogen-bond structure due to topological and energetic disorder of the protein surface. In addition, the intramolecular O-H stretching vibration of water (∼100 THz), which is sensitive to hydrogen-bond environment, pointed to a trend that hydration water has a larger population of strongly hydrogen-bonded water molecules compared with that of bulk water. From these experimental results, we concluded that the "strengthened" water hydrogen bonds at the protein interface dynamically slow down the reorientational motion of water and form the less-defective hydrogen-bond network by inhibiting the fragmentation of water-water hydrogen bonds. Nevertheless, such a strengthened water hydrogen-bond network is composed of heterogeneous hydrogen-bond distances and angles, and thus characterized as structurally "distorted."

Aqueous solutions of poly(N-isopropylacrylamide), P-NIPAAm, exhibit a noticeable temperature responsive change in molecular conformation at a cloud point temperature (Tcp). As the temperature rises above Tcp, the extended coil-like P-NIPAAm structure changes into a swollen globule-like conformation as hydration levels decrease and hydrophobic interactions increase. Though water plays an important role in this coil-to-globule transition of P-NIPAAm, the behavior of water molecules and the associated hydrogen-bond (HB) network of the surrounding bulk water are still veiled in uncertainty. In this study, we elucidate changes in the hydration state and the dynamical structure of the water HB network of P-NIPAAm aqueous solutions during the coil-to-globule transition by analyzing the complex dielectric constant in the terahertz region (0.25-12 THz), where bulk water reorientations and intermolecular vibrations of water can be selectively probed. The structural properties of the water HB network were examined in terms of the population of the non-HB water molecules (not directly engaged in the HB network or hydrated to P-NIPAAm) and the tetrahedral coordination of the water molecules engaged in the HB network. We found the hydration number below Tcp (≈10) was decreased to approximately 6.5 as temperature increased, in line with previous studies. The HB network of bulk water becomes more structured as the coil-to-globule phase transition takes place, via decreases in non-HB water and reduction in the orderliness of the tetrahedral HB architecture. Together these results indicate that the coil-to-globule transition is associated with a shift to hydrophobic-dominated interactions that drive thermoresponsive structural changes in the surrounding water molecules.

We present a double-beam dielectric spectroscopic system using an integrated photonic phase modulator for glucose hydration sensing from 0.25 to 1.0 THz. Compact phase modulators with a photonic circuit and a double-beam scheme using a broadband beam splitter enable us to improve signal stability in sub-THz generation and detection. Using the system, we carried out fast vector measurement on glucose solutions with effective signal stabilization. By incorporating a simple calibration procedure for the extraction of dielectric properties, the Fabry-Perot effect in liquid cell window was mitigated. The system demonstrated accurate detection of glucose dielectric spectra for hydration analysis of glucose-water interaction in sub-THz region.

Recent studies of saccharides' peculiar anti-freezing and anti-dehydration properties point to a close association with their strong hydration capability and destructuring effect on the hydrogen bond (HB) network of bulk water. The underlying mechanisms are, however, not well understood. In this respect, examination of the complex dielectric constants of saccharide aqueous solutions, especially over a broadband frequency region, should provide interesting insights into these properties, since the dielectric responses reflect corresponding dynamics over the time scales measured. In order to do this, the complex dielectric constants of glucose solutions between 0.5 GHz and 12 THz (from the microwave to the far-infrared region) were measured. We then performed analysis procedures on this broadband spectrum by decomposing it into four Debye and two Lorentz functions, with particular attention being paid to the β relaxation (glucose tumbling), δ relaxation (rotational polarization of the hydrated water), slow relaxation (reorientation of the HB network water), fast relaxation (rotation of the non-HB water), and intermolecular stretching vibration (hindered translation of water). On the basis of this analysis, we revealed that the hydrated water surrounding the glucose molecules exhibits a mono-modal relaxational dispersion with 2-3 times slower relaxation times than unperturbed bulk water and with a hydration number of around 20. Furthermore, other species of water with distorted tetrahedral HB water structures, as well as increases in the relative proportion of non-HB water molecules which have a faster relaxation time and are not a part of the surrounding bulk water HB network, was found in the vicinity of the glucose molecules. These clearly point to the HB destructuring effect of saccharide solutes in aqueous solution. The results, as a whole, provide a detailed picture of glucose-water and water-water interactions in the vicinity of the glucose molecules at various time scales from sub-picosecond to hundreds of picoseconds.

In spite of intensive efforts to elucidate the cellular functions and activities of biological macromolecules, such as DNA and proteins, the intermediary role of intracellular water in these activities is still cloaked in mystery. This is because bulk water dynamics, which occur on sub-picosecond and picosecond timescales, cannot be directly probed by conventional experimental techniques. With a periodic electric field on a sub-picosecond or picosecond timescale, terahertz waves are selectively sensitive to the dynamics of the hydrogen bond network associated with the bulk water. We use THz TD-ATR measurements, combined with a two-interface model, to determine the complex dielectric constant of a living cell monolayer, and thus characterize their intracellular water dynamics. Further interrogation was undertaken by decomposing the complex dielectric constant into its slow relaxation, fast relaxation and intermolecular stretching vibration components. The results indicated that intracellular water has a fragile hydrogen bond network, releasing water molecules from the network. Additionally, tetrahedrally coordinated water in the cell interior is structurally disordered compared to the extracellular water.

Studies with THz split ring resonators (THz-SRRs) have revealed the possibility of using them as sensors to monitor changes in the complex dielectric constant of samples. In this regard, THz reflection spectroscopy in combination with a SRR could be used to measure even target samples in aqueous solution since the interference of water's excessive absorption is minimized with this technique. In this letter, we demonstrate that reflectance intensity at the resonant frequency is more sensitive than frequency shift to changes in the imaginary part of the dielectric constant associated with the absorbance of the sample. Moreover, we distinguished significant changes in reflectance between a cell monolayer in a medium cultured directly onto the SRR and the SRR with medium alone. We confirmed that THz-SRR can be a highly sensitive biosensor to monitor changes in the complex dielectric constant of dielectric samples, such as living cells where abundant extracellular water is present.

The complex dielectric constant of human cultured cell, HeLa, is determined in the terahertz region. Since they are sensitive to picosecond motions, intracellular water dynamics mediated by hydrogen bonds are discussed. As a result, it is suggested that about 25 % of water molecules are slightly perturbed and hydrogen bonds are more “unstructured” in the cellular milieu, indicating 6 % of total intracellular water molecules are isolated from hydrogen bond network of water.

We demonstrated reflection spectroscopy of human cancer cell, DLD-1, cultured in the liquid medium on the metamaterial. Based on the complex dielectric constant of distilled water and DLD-1 monolayer determined by terahertz time-domain attenuated total reflection spectroscopy, we found weakened resonance inherent in the metamaterial in the terahertz frequencies reflects dielectric properties within the field of depth “sampled” (several micrometers from the metamaterial surface in the above case), which is defined by the architecture of the metamaterial. This finding suggests that optimization of a frequency-tunable metamaterial can be used to develop a novel, sensitive, cellular sensor that measures whole cell changes; a result that cannot be realized by conventional approaches.

Water conformation around hydrophobic side chains of four amino acids (glycine, L-alanine, L-aminobutyric acid, and L-norvaline) was investigated via changes in complex dielectric constant in the terahertz (THz) region. Each of these amino acids has the same hydrophilic backbone, with successive additions of hydrophobic straight methylene groups (-CH2-) to the side chain. Changes in the degree of hydration (number of dynamically retarded water molecules relative to bulk water) and the structural conformation of the water hydrogen bond (HB) network related to the number of methylene groups were quantitatively measured. Since dielectric responses in the THz region represent water relaxations and water HB vibrations at a sub-picosecond and picosecond timescale, these measurements characterized the water relaxations and HB vibrations perturbed by the methylene apolar groups. We found each successive straight -CH2- group on the side chain restrained approximately two hydrophobic hydration water molecules. Additionally, the number of non-hydrogen-bonded (NHB) water molecules increased slightly around these hydrophobic side chains. The latter result seems to contradict the iceberg model proposed by Frank and Evans, where water molecules are said to be more ordered around apolar surfaces. Furthermore, we compared the water-hydrophilic interactions of the hydrophilic amino acid backbone with those with the water-hydrophobic interactions around the side chains. As the hydrophobicity of the side chain increased, the ordering of the surrounding water HB network was altered from that surrounding the hydrophilic amino acid backbone, thereby diminishing the fraction of NHB water and ordering the surrounding tetrahedral water HB network.

We present a method to determine the complex dielectric constant of a cell monolayer using terahertz time-domain attenuated total reflection spectroscopy combined with a two-interface model. The imaginary part of the dielectric constant of the cell monolayer shows a lower absorption of slow relaxation mode than that of the liquid medium. This result allows us to estimate the intracellular water dynamics on a picosecond time scale, and the existence of weakly hydrated water molecules inside the cell monolayer was indicated. This method will provide a perspective to investigate the intracellular water dynamics in detail.