(395b) Detection of Ions and DNA Hybridization Using Diamond Solution Gate Fets

1. Diamond Solution Gate FETs. Ion sensitive field effect transistors (ISFETs) have been investigated for a long time for particular ionsensing application because the transistor sensors have a capability of not only sensing the ion concentration in the biochemical reactions, but transducing them to electrical signals and amplifying them. They can be scaled down in the device size without sacrificing their performance. Several type of ISFETs have been developed based on the metal oxide semiconductor FET (MOSFET) of Si technology or on the modulation doped FETs (MODFETs) of III-V compound semiconductors. In these cases, the sensing layer and the transducing and amplifying layer are separated by an interlayer which prevents ions from invading into the transducing layer. However, the performance of sensors such as sensitivity, time response has been affected by the interlayer which attenuate the surface potential change by the ion adsorption or causes a slow time response when the sensing molecules diffuse into the interlayer. We have demonstrated a new type of FET sensor without such an interlayer [1-3]. Since the diamond surface is nearly inert to the chemical solutions at room temperature and has a wide potential window within which the redox reaction does not occur. The reproducible FET performance can be obtained in a wide pH region on the condition that the diamond surface channel is exposed to the electrolyte solution directly. This new type of FET is called as electrolyte solution gate FET (SGFET). Recently, pH sensing [4,5], urea and glucose sensing [4], and detection of antigen ? antibody reaction [6] have been reported using the diamond SGFETs.

2. Variation of Ion Sensitivity by Surface Modification and Biomolecule Immobilization. The surface chemical modification can be realized from the H-terminated surface which is a starting diamond surface after the chemical vapour deposition of diamond in the hydrogen atom rich condition. The surface has highly p-type semiconductivity after the adsorption of negatively charged ions in air. The H-terminated surface can attract negative charges because the surface dipoles caused by the electronegativity difference between hydrogen (2.1 in Pauling unit) and carbon (2.5). The charge density of H -C dipoles is about 1E14 e/cm2. This effect is valid in air or in electrolyte solution where negative charged particles are effectively physisorbed on the H-terminated diamond. Using the properties halogen ions such as Cl-, Br-, and I- are detected by the SGFETs down to 1E-7 M [2,3]. Negatively charged biomolecules such as DNA might be also detected if they present in the Helmholtz layer.

3. Sensing of DNA molecules in Helmholtz Layer. The surface carrier density has been affected by the change in electric charge in the Helmholtz layer. When probe DNAs are in the Helmholtz layer hybridise with target DNAs with the equivalent length of charge, the surface holes can be induced by the increase of surface negative charge in the hybridised DNAs and can be detected as the drain current increase or the positive shift of the threshold voltage in the p channel SGFET. It is clearly and reproducibly observed in the diamond SGFET where the surface has been partially aminated (NH2 terminated) and the probe DNAs have been immobilized [7]. The difference between complementary and non complementary target DNA has been detected by the real time shift of gate voltage to keep the constant drain current. The surface density of probe (immobilized) DNA and effective charge of DNA are discussed in our presentation considering the thickness of Debye length.

[1] H.Kawarada et al., Phys. Status Solidi A 185 (2001) 79. [2] H.Kanazawa et al., Diamond Relat. Mater., 12 (2003) 618. [3] K.S. Song, et al., Biosens. Bioelectron. 19(2003) 137. [4] K.S. Song, et al., Jpn. J. Appl. Phys., 43 (2004) L814. [5] J.Garrido, et al., Appl. Phys. Lett. 86 (2005) 073504. [6] W.Yang, et al. Appl. Phys. Lett. 85 (2004) 3626. [7] K.S. Song , H.Kawarada et al. (submitted).