Fig 2: BNP detection results based on AlGaN/GaN sensor with magnetic beads (Device B).aRDS-4probe-t response to BNP with concentrations ranging from 0.1 ppt to 1 ppb, using Device B as the transducer and anti-BNP immobilized on the magnetic beads. The inset is the optical microscope (OM) top view of the AlGaN/GaN device immobilized with magnetic beads. b ΔRDS-4probe results of BNP detection with the Kelvin connection technique and VG = −3.8 V. c Transconductance of Device B and its SNR versus different VG biases of the reference electrode. d Comparison of the LOD values for the Au-electrode, Device A using the two-probe method, Device A using the Kelvin connection technique, and Device B biased at the optimal SNR quiescent working point (VG = − 3.8 V), using the Kelvin connection technique.

Fig 3: BNP detection results based on Au-electrode.a Change of RCT in the EIS with the surface functionalization process. The inset is the circuit model used to simulate the RCT value. Z′Re is the real part, and Z′Im is the imaginary part of the impedance in the EIS of the Au electrode. b Change in RCT at different concentrations of BNP.

Fig 4: Schematic diagram and structure model diagrams of the Kelvin connection devices.a Diagram of the Kelvin connection device; high force (H. F) and high sense (H. S) are connected to the drain terminal, low force (L. F) and low sense (L. S) are connected to the source terminal. VDS voltage is applied between the H. F and L. F to form the current loop. The voltage between the H. S and L. S is measured as VDS−S. The diagram on the left is the schematic diagram and that on the right is the OM (Optical microscope (OM) top view of the real device. b Device A. The open gate area of device A is deposited by the Au layer. AuNPs (Au nanoparticles) are electroplated on the Au gate, and the anti-BNP is immobilized on the AuNPs to selectively detect BNP. c Device B. The open gate area of Device B is bare AlGaN. Magnetic beads immobilized with anti-BNP are absorbed on the AlGaN surface with a permanent magnet on the back side of the chip.

Fig 5: Schematics of bioprotonic devices with biotin-Streptavidin and aptamer-peptide.a i) 2-cholesterol handled DNA nanopores with biotin in the absence of streptavidin (6HB-2C-2B). Created pathway facilitates H+ transfer without inhibition of binding due to absence of streptavidin; ii) 2-cholesterol-handled DNA nanopores with binding of biotin-streptavidin (6HB-2C-2B/S-avidin). H+ transfer is inhibited by blocked pore channels; iii) 2-cholesterol-handled DNA nanopores without biotin in the presence of streptavidin (6HB-2C/S-avidin). The pores are not blocked by binding due to lacking biotin. b IH+ versus time plot for V = −400 mV. Orange trace 6HB-2C-2B (3a-i), green trace 6HB-2C-2B/S-avidin (3a-ii) and blue trace 6HB-2C/S-avidin (3a-iii). We measured IH+ = −96 ± 21 nA, −12 ± 6 nA and −92 ± 9 nA with 6HB-2C-2B, 6HB-2C-2B/S-avidin and 6HB-2C/S-avidin, respectively. Error bars are 1 s.d. (n = 3). c i) 2-cholesterol handled DNA nanopores with SELEX based DNA aptamer in the absence of B-type natriuretic peptide (6HB-2C-2AP). Created pathway facilitates H+ transfer without inhibition of binding due to absence of peptide; ii) 2-cholesterol-handled DNA nanopores with binding of aptamer-peptide (6HB-2C-2AP/BNP). H+ transfer is slightly inhibited by blocked pore channels; iii) 2-cholesterol-handled DNA nanopores without aptamer in the presence of peptide (6HB-2C/BNP). d IH+ versus time plot for V = −400 mV. Orange trace 6HB-2C-2AP (3c-i), green trace 6HB-2C-2AP/BNP (3c-ii) and blue trace 6HB-2C/BNP (3c-iii). We measured IH+ = −90 ± 3 nA, −51 ± 1 nA and −96 ± 9 nA with 6HB-2C-2AP, 6HB-2C-2AP/BNP and 6HB-2C/BNP, respectively. Error bars are 1 s.d. (n = 3).