Influence of the Electrolyte Salt Concentration on DNA Detection with Graphene Transistors

Abstract

Liquid-gated Graphene Field-Effect Transistors (GFET) are ultrasensitive bio-detection platforms carrying out the graphene's exceptional intrinsic functionalities. Buffer and dilution factor are prevalent strategies towards the optimum performance of the GFETs. However, beyond the Debye length (λD), the role of the graphene-electrolytes' ionic species interactions on the DNA behavior at the nanoscale interface is complicated. We studied the characteristics of the GFETs under different ionic strength, pH, and electrolyte type, e.g., phosphate buffer (PB), and phosphate buffer saline (PBS), in an automatic portable built-in system. The electrostatic gating and charge transfer phenomena were inferred from the field-effect measurements of the Dirac point position in single-layer graphene (SLG) transistors transfer curves. Results denote that λ_D is not the main factor governing the effective nanoscale screening environment. We observed that the longer λ_D was not the determining characteristic for sensitivity increment and limit of detection (LoD) as demonstrated by different types and ionic strengths of measuring buffers. In the DNA hybridization study, our findings show the role of the additional salts present in PBS, as compared to PB, in increasing graphene electron mobility, electrostatic shielding, intermolecular forces and DNA adsorption kinetics leading to an improved sensitivity.

Keywords: DNA; Debye length; graphene; liquid gate; phosphate buffer (PB); phosphate buffer saline (PBS); salts.

Figure 1

(a) The integrated portable system with automatic micropump and electronic reader of the graphene channel on the chip, (b) surface functionalization and DNA hybridization strategy.

Figure 2

(a) Raman spectra of bare and modified graphene channel, (b) the corresponding 2D/G Raman intensity ratio maps of the single-layer graphene (SLG).

Figure 3

(a) I_DSV_GS characteristic, and (b) extracted Dirac voltage point as the function of different phosphate buffer (PB) concentrations, (c) I_DSV_GS characteristic, and (d) extracted Dirac voltage point as the function of varying pH.

Figure 4

I_DSV_GS curves of DNA hybridization detection using background solution of PB of (a) 10 mM, (b) 1 mM and (c) 0.1 mM and PBS (d) 1×, (e) 0.1×, and (f) 0.01×.

Figure 5

Calibration curves of DNA hybridization detection using a background solution of (a) PB, and (b) phosphate buffer saline (PBS) with R² value corresponds to the linear fitting. Illustration of the ionic gradient on the interface with (c) PB, and (d) PBS backgrounds and (e) The schematic comparison of the electric potential at the interface from the measurement with PB and PBS. The large and small red symbols are positive ions with bigger sizes and smaller sizes, respectively. The large and small cyan symbols are negative ions with bigger and smaller sizes, respectively.

Figure 6

Adsorption kinetics curves (solid squares and circles) of the probe DNA immobilization on functionalized Graphene Field-Effect Transistors (GFET) sensor surface measured in two different electrolyte backgrounds. The solid lines are fittings to the Langmuir isotherm.