Detection of HIV-1 RT Protein Using AlGaN/GaN High Electron Mobility Transistors

As AIDS caused by (human immunodeficiency virus type 1) has now spread to every country in the world. Statistics show that approximately 40 million people are currently living with HIV infection, and an estimated 35 million have died from this disease since the beginning of the epidemic. In this study, we use DNA-immobilized AlGaN/GaN high electron mobility transistors (HEMTs) for detecting low concentration of HIV-1 RT protein (1 fM) binding with DNA electronically, showing that this technique is promising for biosensor applications.

Figure 1 (a) and (b) show the schematic of RT immobilized HEMT and the top-view of the device respectively. The HEMT structure consisted of a 3 μm-thick GaN buffer, 150 Å-thick undoped Al0.25Ga0.75N and 10 Å-thick undoped GaN cap layer. Mesa isolation was formed by Inductively Coupled Plasma (ICP) etching system for 28 seconds. Ti, Al, Ni and Au were deposited by Electron Beam Evaporation and were annealed at 850 °C for 45 seconds under N₂ flowing to form ohmic contacts. The source and drain of HEMT were isolated by photoresist except the gate metal region. The DNA was immobilized on the gate region for 24 hours followed by surface blocking with bovine serum albumin (BSA). The HIV-1 RT protein solutions were then added on the sensing region of the sensor, and then waited for 30 minute for binding. After HIV-1 RT protein binding with DNA, the sensor was washed with 1X PBS to remove the unbound HIV-1 RT protein. 1X PBS was added on the sensor for electrical measurements. The sensor was measured by applying a dc bias of drain-source voltage (Vds) at 0.5V first, and then the gate voltage (Vg) was applied 0.5 V on the gate electrode. The drain current (Id) of the transistor was recoded in real-time when the drain-source voltage and the gate voltage were applied. The total charge was calculated by integrating the drain currents with the increasing time. Due to calculation of the total charges, any tiny difference in drain currents resulted from different HIV-1 RT protein concentrations may cause a significantly huge difference in the total charges.

The total charge counted at a certain time for various concentrations of HIV-1 RT protein is shown in figure 2. It is obviously seen that the difference in total charges among different surface modification. Higher HIV-1 RT protein concentrations, including 1fM, 10fM, 100fM and 1pM were tested and caused more accumulated charges. Therefore, the change in total charge with varying concentration of HIV-1 RT protein can represent sensor calibration curve. The detection limit was then determined to be 1fM for HIV-1 RT protein measurement. The high sensitivity and the low detection limit of the sensor resulted from the charge accumulation and the less screening effect during the dynamic change of the drain current. In the detection, only a small amount of sensing materials is required and a short detection time is taken. The developed system has a great potential to measure an extremely low concentration of different proteins for biomedical applications.