Direct Detection of NT- Pro BNP As a Cardiac Biomarker Using High Electron Mobility Transistors in Physiological Salt Environment
Authors: Regmi Abiral , Sarangadharan Indu , Chen Yan Wen , Hsu Chen-Pin , Wang Yu-Lin 
Published: February 2020

Cardiovascular disease (CVD) is one of the major causes to claim millions of lives all across the globe every year. While the chances of CVD increase with age, it can also occur in any age group of people, both male and female. Despite all efforts and tremendous accomplishments in the precaution and treatment of CVD, its detection at an early stage is still a big challenge for scientists in this present modern world. Here, NT-pro BNP plays a crucial role in acute congestive heart failure since it is biologically inactive, binds better with its receptors and has a longer half-life than any other, making it an ideal biomarker. The detection method is based upon the ligand-receptor interaction, also called as immunoassay, which can be successfully detected with the use of High Electron Mobility Transistors (HEMTs). With its unique characteristic features  like good electrical property, chemical stability, biocompatibility, thermal stability and low power consumption HEMT sensors also have been used in several other applications like ion-sensing, pH sensing, gas sensing, etc. Therefore, the detection method becomes simple, fast, convenient and cost effective.

 AlGaN/ GaN are the most promising candidates for HEMT sensor materials. A thin film of undoped Al0.25Ga0.75N layer (150 Å) is deposited onto the surface of 3 μm-thick undoped GaN buffer layer, which in turn is deposited on the top of Silicon substrate by metal-organic chemical vapor deposition (MOCVD) process. Using mesa ICP etching, active device area is formed. Then four different metals of various thicknesses, i.e. Ti, Al, Ni and Au, are deposited to make an ohmic contact and is annealed at an elevated temperature of 850ºC. Later, the third mask helps in making the final metal layer and the gate electrode used for immobilizing the receptor. Finally, the fourth mask lets us open some portion of the gate and the transistor region to allow liquid to cross through.

The next step to sensor fabrication and device gate opening is making the surface ready for immobilization of receptor, or so called surface functionalization. The antibody disulfide bridges between the heavy chains are cleaved in the presence of mercaptoethylamine (MEA) and EDTA whereas the disulfide bridges between the heavy chain and the light chain remains unaffected. This leaves behind a number of disulfide bonds which can be bound onto gold surface easily because of the strong interactions between the native thiol group and the gold surface. Finally, the sensor measurement is performed with Agilent B1530 parameter analyzer.

The figure 1 (c) represents the detection of NT- pro BNP at various concentrations. The graph shows the total charge accumulated against various concentrations of target proteins prepared in physiological environment (1X PBS). From the experiment performed, it can be observed that there is an increase in the change of charge with increase in the concentration of target proteins, which confirms the immobilization of antibody and ultimately the detection of target proteins. Moreover, the calibration curve for the sensor can be drawn by plotting change in the total charge against the concentrations with repeated testing of the device. Therefore, this method of detection is cheap, robust and time efficient making it also ideal for multi-sensor array and point-of-care diagnostics.

Figure 1