28.3 CMOS biosensor IC focusing on dielectric relaxations of biological water with 120GHz and 60GHz oscillator arrays

The water present in the close vicinity of biomolecules is particularly relevant to the function of the biomolecules and is termed “biological water” in contrast to ordinary “bulk water” that fills the region beyond this vicinity (Fig. 28.3.1). Many studies have revealed that these two types of water exhibit clear differences in their dielectric relaxations. First, bulk water has two major relaxation modes: (i) the first mode represents the dielectric relaxation of the cooperative organization of hydrogen-bonded water molecules with a time constant τ
_d
(≈8ps), and (ii) the second mode exhibits a faster relaxation time constant τ
_f
(≈0.27ps) and is usually attributed to the rotation of the water molecules that are not involved in hydrogen bonding. On the other hand, a slower relaxation (τ
_s
) appears in biological water because of “bound water” that is strongly bonded to biomolecules and less movable compared with bulk water. Owing to the presence of τ
_s
-mode water, the fractions of τ
_d
- and τ
_f
-mode waters in biological water are smaller than those in bulk water. In this manner, the existence and status of biological water is reflected in the complex dielectric constant at a frequency around 100GHz. Because the ionic polarization of electrolytes (e.g., Na, K, Ca) strongly affects the dielectric loss in the frequency range lower than 50GHz, a difficulty arises in differentiating the dielectric relaxations of water from that of the ionic polarization [2]. By limiting the observation frequency to higher than 50GHz, we can eliminate this difficulty. By focusing on the dielectric relaxations of biological water, we present a CMOS biosensor IC that can detect and visualize the target biomolecules by using biological water as a label.