Real-time label-free quantitative monitoring of biomolecules by floating-gate complementary metal-oxide semiconductor sensor

We report a label-free field-effect sensing array integrated with complementary metal-oxide semiconductor (CMOS) readout circuitry to detect the surface potential determined by the negative charge in DNA molecules. For real-time DNA quantification, we have demonstrated the measurements of DNA molecules without immobilizing them on the sensing surface which is composed of an array of floating-gate CMOS transistors. This nonimmobilizing technique allows the continuous monitoring of the amount of charged molecules by injecting DNA solutions sequentially. We have carried out the real-time quantitative measurement of 19 bp oligonucleotides and analyzed its sensitivity as a function of pH in buffer solutions.

Recently, various electrochemical techniques have been investigated for detecting deoxyribonucleic acid (DNA) molecules based on sensing their intrinsic properties. Among these methods, the detection of the intrinsic charges in DNA molecules is one of the most attractive approaches because they provide many advantages. Typically, ion-sensitive field-effect devices fabricated in complementary metal-oxide semiconductor (CMOS) process are used for label-free detection with a high sensitivity in a small form factor. These sensors can be easily miniaturized at low cost using standard integrated circuit fabrication processes and provide the capability of monolithic integration with readout circuitry to enhance the signal-to-noise ratio. However, in order to detect DNA molecules, it is required to immobilize single-stranded probes on the surface of the ion-sensitive devices for the hybridization of single-stranded complementary target molecules in the electrolyte solution or to deposit a positively charged layer such as poly-L-lysine on the sensing electrodes to attract the negatively charged nucleic acid molecules. These additional processes increase reaction time and the complexity in detection protocols. Furthermore, it is difficult to incorporate the continuous monitoring for repetitive assays (e.g., real-time quantitative PCR) for these protocols because the regeneration of the sensing surface requires a complicated rinsing sequence. We have explored a nonsurface binding detection technique for the real-time sequential monitoring of DNA molecule concentrations. The charges of DNA molecules in a buffer solution can directly affect the potential of the field-effect transistors (FETs) where the gates of FETs are floated. The potential at the interface between electrolyte and electrodes depends on the amount of charges in a buffer solution within Debye length which is determined by the ionic strength of the electrolyte solution.

Figure 1: (a) Schematics of a floating-gate field-effect transistor sensing device and label-free detection. The surface potential change is determined by the amount of DNA molecules in the Debye length of the solution. (b) Microphotographs of the fabricated sensor chip and the magnified view of the sensing element.

Related Publication
  1. S.-J. Kim, K. Yoo, J. Shim, W. Chung, C. Ko, M. Im, L.-S. Kim, and E. Yoon, "Real-time label-free quantitative monitoring of biomolecules without surface binding by floating-gate complementary metal-oxide semiconductor sensor array integrated with readout circuitry," Applied Physics Letters, Vol. 91, No. 20, pp. 203903-1-203903-3, Nov. 2007.