Differential-mode HEMT-based biosensor for real-time and label-free detection of C-reactive protein

In this paper, we have investigated the characteristics of the differential-mode high electron mobility transistor (HEMT)-based biosensor. An Au-gate HEMT is the sensing device that reacts with biomolecules, whereas the Pt-gate HEMT is the dummy device used in the differential-mode detection circuit. Both the biosensing HEMT (Bio-HEMT) and reference HEMT (Ref-HEMT) were biased by a Pt quasi-reference electrode (QRE). The Pt QRE was used to replace the bulky conventional reference electrode. The binding of 11-mercaptoundecanoic acid, CRP-antibody, and CRP caused a variation in Bio-HEMT gate surface potential, which was confirmed by X-ray photoelectron spectroscopy. Moreover, we measured the variation of output voltage with light intensity and temperature, which was eliminated as a common-mode signal by the differential-mode measurement technique. Therefore, this technique can improve the detection limit in low CRP concentration. The output voltage of the sensor system is nonlinearly proportional to the CRP concentration from 0.01 ng/mL to 1000 ng/mL. These results indicated that the Bio-HEMT/Ref-HEMT/QRE could be applied to a portable biomolecule detection system.