Molecule counting with alkanethiol and DNA immobilized on gold microplates for extended gate FET

Several molecule counting methods based on electrochemical characterization of alkanethiol and thiolated single-stranded oligonucleotide (HS-ssDNA) immobilized on gold microplates, which were used as extended gates of field effect transistors (FETs), have been investigated in this paper. The surface density of alkanethiol and DNA monolayers on gold microplates were quantitatively evaluated from the reductive desorption charge by using cyclic voltammetry (CV) and fast CV (FCV) methods in strong alkali solution. Typically, the surface density of 6-hydroxy-1-hexanethiol (6-HHT) was evaluated to be 4.639 molecules/nm2, and the 28 base-pair dsDNA about 1.226–4.849 molecules/100 nm2 on Au microplates after post-treatment with 6-HHT. The behaviors on surface potential and capacitance of different aminoalkanethiols on Au microplates were measured in 0.1 mol/L Na2SO4 and 10 mmol/L Tris–HCl (pH = 7.4) solutions, indicating that the surface potential increases and the double-layer capacitance decreases with the length of carbon chain increased for the thiol monolayers, which obey a physics relationship for a capacitor. Comparably, a simple sensing method based on the electronic signals of biochemical reaction events on DNA immobilization and hybridization at the Au surface of the extended gate FET (EGFET) was developed, with which the surface density of the hybridized dsDNA on the gold surface of the EGFET was evaluated to be 1.36 molecules per 100 nm2, showing that the EGFET is a promising sensing biochip for DNA molecule counting. - Graphical abstract: Several molecule counting methods based on electrochemical characterization of alkanethiol and thiolated single-stranded oligonucleotide (HS-ssDNA) immobilized on gold microplates, which used as extended gates of field effect transistors (FETs), have been investigated in this paper. The surface density of the hybridized dsDNA on the gold surface of the extended gate FET was evaluated to be 1.36 molecules/100 nm2, showing that the extended gate FET is a promising sensing biochip for DNA molecule counting. Highlights: ► Molecule counting is very important for the biosensing chip which deserves to develop. ► FCV was employed for counting DNA molecules on Au microplates quantitatively. ► It was examined how the physical parameters of SAM changed with the carbon number. ► The molecule counting method with EGFET electronic detection was developed. ► The surface density of dsDNA was 1.36 molecules/100 nm2 on Au of EGFET