Fabrication of a Label-Free Immunosensor Using Surface-Engineered AuPt@GQD Core–Shell Nanocomposite for the Selective Detection of Trace Levels of Escherichia coli from Contaminated Food Samples

Fabrication of label-free immunosensors is highly necessitated due to their simplicity, cost-effectiveness, and robustness. Herein, we report the facile development of a label-free, direct, rapid, capacitive immunosensor for ultrasensitive and rapid recognition of trace levels of Escherichia coli from contaminated food samples. This was achieved using gold platinum core–shell nanoparticles loaded with graphene quantum dots (AuPt@GQDs) that were utilized as electrode modifiers. The incorporation of GQDs to the surface of AuPt core–shell nanoparticles was performed using the “ greener ” probe-sonication method. The electrochemical properties of AuPt@GQDs, determined using cyclic voltammetry and electrochemical impedance spectroscopy, suggested the optimized loading concentration of AuPt to be 0.05% in the core–shell nanocomposite to exhibit the highest current response. Furthermore, immobilization of anti- E. coli monoclonal antibodies (anti- E. coli mAb) onto the surface of modified electrodes was performed using amine coupling. The high specific binding of E. coli cells onto the surface of the immuno-electrode was measured as a direct function of change in transient capacitance with time that was measured at low and high frequencies. The resultant immunosensor (bovine serum albumin/anti- E. coli mAb/AuPt 0.05 @GQDs/FTO) demonstrated a detection range (5 to 4.5 × 10 3 cells/mL), with the detection limit as low as 1.5 × 10 2 cells/mL, and an excellent sensitivity ∼171,281.40 μF –1 mL cells –1 cm –2 without the use of any labels ( R 2 -0.99). These findings were further verified using real sample analysis wherein the immuno-electrode demonstrated outstanding sensitivity, the highest noticed so far. More interestingly, the high resuability ∼48 weeks (RSD-5.92%) and excellent reproducibility in detection results (RSD ∼ 9.5%) testify its potential use in a clinical setting. The results reveal the usefulness of the surface-engineered AuPt@GQDs core–shell nanocomposite as an electrode modifier that can be used for the development of newer on-site monitoring devices to estimate trace levels of pathogens present as contaminants in food samples.