(Invited) Rapid C-reactive Protein Detection with AlGaN/GaN High Electron Mobility Transistors in an Integrated Microfluidic System

As living style and eating patterns of people have been changed a lot in recent years, many severe diseases come with those habits and risk human health. Cardiovascular diseases are one of the most serious diseases worldwide and cause 25 million people every year. In order to control the cardiovascular diseases in the early stage or even to prevent the diseases from happing, monitoring c-reactive protein (CRP) in human blood is a very efficient and promising solution. CRP is a well known biomarker related to cardiovascular diseases. It is synthesized by the liver and is usually under a very low concentration in blood. The definitions of risks for cardiovascular diseases are low for a CRP concentration below 1.00 mg/L (9.00 nM), moderate for a CRP from 1.00 to 3.00 mg/L (9.00 nM to 27.00 nM), and high for concentrations over 3.00 mg/L (27.00 nM). An aptamer with high sensitivity and specificity to CRP has been developed. It can be used as a receptor to catch CRP by immobilizing the aptamer on high electron mobility transistors (HEMTs). HEMTs have already been demonstrated in gas, ion and biomolecule sensing purpose. With the high sensitivity of the electrical properties of the transistors and the extremely small size, the CRP detections complete in very short response time and consume a little amount of CRP sample. To control a stable detection environment and the amount of CRP sample, a multifunctional microfluidic system was used in the detection and also in the immobilization of the aptamer.

Figure 1 (a) and (b) show the schematic of HEMT and the top-view of the multifunctional microfluidic system. To create a high quality HEMT device, a 3 μm-thick undoped GaN buffer layer and a 150 Å-thick undoped Al_0.25Ga_0.75N layer were deposited by metal-organic chemical vapor deposition (MOCVD) on the substrate. Mesa isolation was formed by Inductively Coupled Plasma (ICP) etching system for for 28 seconds. Ti, Al, Ni and Au were deposited by Electron Beam Evaporation and were annealed at 850 °C for 45 seconds under N₂flowing to form Ohmic contacts. A 100 Å-thick Au layer was deposited on the gate region as gate metal. All the area of HEMT was encapsulated by photoresist with only gate region open for aptamer immobilization and CRP sample detection. The multifunctional microfluidic system was covered on top of the HEMT and sealed by double- site tape under hot press.

Figure 2 shows the procedure of aptamer immobilization and CRP detection. First, the aptamer was injected into the channel and mixed for 24 hours at 25 °C  to allow the aptamer immobilized on the gate surface. Unbound aptamer was washed away by aptamer washing buffer. Second, CRP sample was injected into the channel and mixed for a few minutes for CRP binding with aptamer. Unbound CRP was washed away by CRP washing buffer. Third, the current change induced from the binding of CRP and aptamer was extracted by a constant bias of 0.5V applied on HEMT.

Figure 3 shows the current changed induced by different concentrations of CRP after normalized. The current changes shows a high linear relation to the CRP concentrations with R²= 0.98455. The detection limit is 0.026 nM and the detection only completed in 3000 seconds. Comparing to the conventional method, the detection by using HEMT integrated microfluidic system comes with high sensitivity and low response time. It will be a potential method in the further CRP research.