A Bifunctional Nanosilver-Reduced Graphene Oxide Nanocomposite for Label-Free Electrochemical Immunosensing

Abstract

A bi-functional material based on silver nanoparticles (AgNPs)-reduced graphene oxide (rGO) composite for both electrode modification and signal generation is successfully synthesized for use in the construction of a label-free electrochemical immunosensor. An AgNPs/rGO nanocomposite is prepared by a one-pot wet chemical process. The AgNPs/rGO composite dispersion is simply cast on a screen-printed carbon electrode (SPCE) to fabricate the electrochemical immunosensor. It possesses a sufficient conductivity/electroreactivity and improves the electrode reactivity of SPCE. Moreover, the material can generate an analytical response due to the formation of immunocomplexes for detection of human immunoglobulin G (IgG), a model biomarker. Based on electrochemical stripping of AgNPs, the material reveals signal amplification without external redox molecules/probes. Under optimized conditions, the square wave voltammetric peak current is responded to the logarithm of IgG concentration in two wide linear ranges from 1 to 50 pg.ml^-1 and 0.05 to 50 ng.ml^-1, and the limit of detection (LOD) is estimated to be 0.86 pg.ml^-1. The proposed immunosensor displays satisfactory sensitivity and selectivity. Importantly, detection of IgG in human serum using the immunosensor shows satisfactory accuracy, suggesting that the immunosensor possesses a huge potential for further development in clinical diagnosis.

Keywords: Immunoglobulin G; electrochemistry; immunosensor; reduced graphene oxide; screen-printed carbon electrode; silver nanoparticles.

Scheme 1

Fabrication of AgNP/rGO-based electrochemical immunosensor.

Figure 1

SEM images of GO (A) and AgNPs/rGO (B) coated on SPCEs. A TEM image (C) of AgNPs/rGO and a particle size distribution profile (D) of AgNPs on rGO sheets.

Figure 2

CV results of bare SPCE, and GO and AgNPs/rGO-modified SPCEs in contact with PB (pH 7.4).

Figure 3

(A) CV curves at a scan rate of 100 mV s⁻¹ and (B) Nyquist plots of bare SPCE, and GO- and AgNP/rGO-modified SPCEs in contact with 0.10 M PB (pH 7.4) containing 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆].

Figure 4

CVs of the AgNPs/GO-modified SPCE in contact with 0.010 M PB (pH 7.4) containing 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] at different scan rates: 10, 20, 40, 50, 80, 100, 125, 200, 250, and 400 mVs⁻¹. Inset: plots of the anodic peak current (I_pa) and the cathodic peak current (I_pc) vs. the square root of the scan rate.

Figure 5

Influence of incubation time of anti-IgG (A), incubation time of IgG (B), pH of phosphate buffer (C) on current responses of the developed immunosensor.

Figure 6

SWVs of the immunosensor after incubation with different concentrations of IgG. Inset: a calibration curve based on the change in the SWV peak current vs. the logarithm of the concentration (n = 3).

Figure 7

Selectivity study of the immunosensors incubated with different solutions: blank (blue bar), IgG solution (1 ng ml⁻¹, red bar), interference solutions (100 ng ml⁻¹) with presence of 1 ng ml⁻¹ IgG (red bars), and individual interference solutions (100 ng ml⁻¹) with no IgG (blue bars).