Fig. 1. (a) Schematic diagram of a-Si:H ISFET structure fabricated on a glass substrate and (b) picture of packaged chip. The external Ag/AgCl electrode is immersed in a drop of 50–100 μL phosphate buffer 100 mM, pH 7.0. The source (S), drain (D) and gate (G) electrodes are identified.

Fig. 2. Schematic diagram of the DNA probe immobilization on the ISFET surface. Chemical functionalization steps: (1) cleaning and hydroxylation with cholic acid; (2) silanization with APTES; (3) functionalization with sulfo-EMCS spacer arm molecule; (4) DNA probe immobilization; (5) blocking of the unreacted immobilization sites with BSA (pre-hybridization). The BSA protein is not to scale.

Fig. 3. a-Si:H ISFET transfer curves obtained after each chemical reaction step: CA—after cholic acid washing (initial state); FUNCT—functionalization, involving silanization and reaction with spacer arm; IMMOB—19-nt DNA probe covalent immobilization; PREHYB—surface blocking and pre-hybridization with BSA; HYB—DNA complementary target hybridization. The inset shows an enlargement of the transfer curve.

Fig. 4. (a) Gate voltage shift, ΔV_G, measured at 30 pA, relative to the initial state (cholic acid) after chemical reaction steps. DNA complementary hybridization detection is shown. The inset in (a) shows the comparison of the shifts upon complementary and 50%-mismatch hybridization, as well as the corresponding fluorescence microscopy micrographs. (b) Point-of-zero charge (pH_pzc) of the chemically treated surfaces after each surface modification step.

Fig. 5. Threshold voltage shift of the a-Si:H ISFET relative to the initial state due to the unspecific adsorption of biomolecules. The shift is plotted as a function of the concentration of biomolecules in solution: (a) 19-nt oligonucleotides; (b) horseradish peroxidase, HRP; (c) green fluorescent protein, GFP. The symbols correspond to the average of two independent measurements and the error bars to the respective standard deviation. The lines are Langmuir isotherm fits to the data points and the corresponding calculated parameters are shown.

Fig. 6. Schematic illustration of biomolecules adsorption at the surface of ISFETs and expected V_T shift: (a) negatively charged biomolecules and distribution of positive counter-ions; (b) positively charged biomolecules and distribution of negative counter-ions. The Debye length is indicated by the dotted line (λ_D = 0.66 nm).