A field-portable electrochemical immunosensor based on a multifunctional Ag2O/g-C3N4@MA-DBB covalent organic framework receptor interface for single-step detection of aflatoxin M1 in raw milk samples

Abstract

Aflatoxin M₁ (AFM₁), a hydroxylated metabolite of aflatoxin B₁ (AFB₁), contaminates milk and dairy products from livestock via ingestion of feed contaminated with a species of Aspergillus. As AFM₁ is a Group-II B carcinogen, it is indispensable to develop novel, easy-to-handle, sensitive, portable and cost-effective strategies for its detection. Herein, a covalent organic framework (COF)-based electroactive nanocomposite, Ag₂O/g-C₃N₄-COOH@MA-DBB-COF (silver oxide/carboxy-functionalized graphitic carbon nitride@melamine-dibromo butane COF), is designed to serve as a multifunctional receptor surface. The Ag₂O/g-C₃N₄-COOH@MA-DBB-COF formation was characterized through X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, micro-Raman spectroscopy (RAMAN), dynamic light scattering (DLS) and thermogravimetric analysis (TGA), and each step of the sensor fabrication was monitored using field emission scanning electron microscopy (FE-SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Under the optimal conditions, the designed immunosensor permitted the detection of AFM₁ in the linear range of 0.03-1000 fg mL^-1, with a 0.01 fg mL^-1 limit of detection (LOD). The selectivity of the designed immunosensor was validated via an anti-interference study. The practical applicability of the immunosensor was demonstrated by the detection of AFM₁ in real milk samples, and good recovery values (97.28-102.62%) were obtained. Furthermore, the developed immunosensor and high-performance liquid chromatography (HPLC) were employed in parallel to detect AFM₁ in local market milk samples from twenty different sites to validate the performance of the newly designed immunosensor. Additionally, the designed immunosensor was stable over an extended period of time. This work reports a single-step field-portable multifunctional innovative electrochemical immunoreceptor design for on-site and label-free detection of AFM₁ in milk samples. Hence, the present study is the first report on the fabrication of a multifunctional innovative electrochemical immunoreceptor based on PGE/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF for the detection of AFM₁ in milk samples.

Fig. 1. (A) FTIR spectra, (B) Raman spectra, (C) XRD patterns, and (D) thermogravimetric analysis of (a) g-C₃N₄, (b) Ag₂O/g-C₃N₄-COOH, (c) MA-DBB-COF and (d) Ag₂O/g-C₃N₄-COOH@MA-DBB-COF.

Scheme 1. (A) Preparation of acidified Ag₂O/g-C₃N₄-COOH@MA-DBB-COF; (B) immunosensor fabrication.

Fig. 2. (A) Cyclic voltammograms and (B) electrochemical impedance spectroscopy (Nyquist plot) of 0.1 M [Fe(CN)₆]^3−/4− at a scan rate of 50 mV s⁻¹ for the (a) bare electrode, (b) EDC-NHS/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF/PGE, (c) anti-AFM₁ antibody/EDC-NHS/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF/PGE, (d) BSA/anti-AFM₁ antibody/EDC-NHS/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF/PGE and (e) AFM₁/BSA/anti-AFM₁ antibody/EDC-NHS/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF/PGE. FE-SEM images of the step-wise modification of the pencil graphite electrode for the (C) bare electrode, (D) EDC-NHS/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF/PGE, (E) anti-AFM₁ antibody/EDC-NHS/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF/PGE, (F) BSA/anti-AFM₁ antibody/EDC-NHS/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF/PG and (G) AFM₁/BSA/anti-AFM₁ antibody/EDC-NHS/Ag₂O/g-C₃N₄-COOH@MA-DBB-COF/PGE.

Fig. 3. (A) Differential voltammograms for different concentrations of the analyte: (a) 0.03, (b) 16, (c) 90, (d) 200, (e) 328 (f) 505, (g) 650, (h) 820, and (i) 1000 fg mL⁻¹. (B) Calibration curve for AFM₁ concentration. (C) Specificity of the immunosensor and (D) stability of the immunosensor.