Fast detection of melamine using silver nanoparticles capped with l-cysteine functionalized carbon dots

Abstract

Melamine is an additive used fraudulently to enrich foods with nitrogen, particularly in the dairy industry. It is also known as the main metabolite or degradation phytosanitary product of cyromazine. However, the numerous incidents involving living beings in aquatic environments, children and pets fed with products made from melamine in China and certain African countries have led to distrust of melamine in food. In order to ensure strong food safety and security, and good quality of the ecosystem free of melamine, it is important to design a fast, simple, reliable and efficient method for the detection of melamine. For this purpose, silver nanoparticles capped with l-cysteine functionalized carbon dots (cCDs/AgNPs) were designed for the detection of melamine. The results showed that a yellow solution of cCDs/AgNPs turns pink and gradually blue within two minutes of heating at 90 °C in the presence of melamine even at a concentration of 0.1 μg mL^-1. This color change reflects the sensitivity of cCDs/AgNPs towards melamine. The investigation of cCDs/AgNPs-based on ultraviolet-visible spectroscopy exhibits good linearity in the range from 0.5 μg mL^-1 to 4.5 μg mL^-1 for melamine detection, with a detection limit of 0.03 μg mL^-1. This method was successfully applied to determine melamine in a milk matrix, suggesting that this method can be applied for food monitoring with the aim of obtaining melamine-free food in dairy products.

Fig. 1. UV-vis spectrum of (a) cCDs and (b) cCDs/AgNPs. (c) SEM characterization of cCDs/AgNPs. (d) FTIR spectra of (D1) cCDs, (D2) cCDs/AgNPs and (D3) cCDs/AgNPs-melamine. XPS spectrum of (e) cCDs, (f) cCDs/AgNPs, and (g) cCDs/AgNPs-melamine.

Scheme 1. Mechanism of designing of cCDs/AgNPs as sensor and the process of melamine detection.

Fig. 2. (a) Effect of temperature at (A1) 28 °C, (A2) 70 °C, (A3) 80 °C and (A4) 90 °C on the detection of 3 mL of 1.2 μg per mL melamine with 300 μL of cCDs/AgNPs. (b) Associated UV-vis spectrum for the detection of melamine at 28 °C (B1), 70 °C (B2), 80 °C (B3) and 90 °C (B4).

Fig. 3. (a) and (b) Effect of pH on cCDs/AgNPs in the absence and presence of melamine (3 mL, 1.2 μg mL⁻¹) for 2 min of heating at 90 °C and at different pH: (A1 and B1) pH 3, (A2 and B2) pH 5, (A3 and B3) pH 6, (A4 and B4) pH 8, (A5 and B5) pH 10, (A6 and B6) pH 11 et (A7 and B7) pH 12. (c) UV-vis spectra cCDs/AgNPs-melamine mixture at different pH. (d) pH detection trend curve obtained from (e) at wavelength λ = 410 nm.

Fig. 4. (a) Detection of melamine for 2 min of heating at 90 °C in different concentrations (A1) 0 μg mL⁻¹, (A2) 0.5 μg mL⁻¹, (A3) 1.0 μg mL⁻¹, (A4) 1.5 μg mL⁻¹, (A5) 2.0 μg mL⁻¹, (A6) 3.5 μg mL⁻¹, (A7) 4.0 μg mL⁻¹ and (A8) 4.5 μg mL⁻¹ at pH8. (b) UV-vis absorption spectrum of cCDs/AgNPs in each sample from (a). (c) Calibration curve of melamine detection collected from (b) using the difference of absorbance (without and with melamine) versus the concentration. (d) Interference tests on cCDs/AgNPs with 1.0 μg mL⁻¹ of (D1) Na₂SO₄, (D2) CaSO₄, (D3) MgSO4, (D4) FeSO₄, (D5) glycine, (D6) starch and (D7) cystine. (e) Melamine (1.0 μg mL⁻¹) detection in the presence of 10.0 μg mL⁻¹ interferents (Ei is the correspondent of Di with the presence of melamine). (f) Melamine recovery diagram obtained from (e).

Fig. 5. Melamine recovery diagram in different milk powder samples: E1: extract from” Bonnet rouge”, E2: extract from “Nido”, E3: extract from “full cream milk powder Lp”, E4: extract from Laity sample and E5: extract from “Top-Lait”.