A novel ultrasensitive surface plasmon resonance-based nanosensor for nitrite detection

Abstract

Nitrite is a common food additive, however, its reduction product, nitrosamine, is a strong carcinogen, and hence the ultra-sensitive detection of nitrite is an effective means to prevent related cancers. In this study, different sized gold nanoparticles (AuNPs) were modified with P-aminothiophenol (ATP) and naphthylethylenediamine (NED). In the presence of nitrite, satellite-like AuNPs aggregates formed via the diazotization coupling reaction and the color of the system was changed by the functionalized AuNPs aggregates. The carcinogenic nitrite content could be detected by colorimetry according to the change in the system color. The linear concentration range of sodium nitrite was 0-1.0 μg mL^-1 and the detection limit was determined to be 3.0 ng mL^-1. Compared with the traditional method, this method has the advantages of high sensitivity, low detection limit, good selectivity and can significantly lower the naked-eye detection limit to 3.0 ng mL^-1. In addition, this method is suitable for the determination of nitrite in various foods. We think this novel designed highly sensitive nitrate nanosensor holds great market potential.

Fig. 1. The schematic of NO₂⁻ detection by Au satellite-like aggregate colorimetry.

Fig. 2. Characterization of ATP-AuNPs. (A) The absorption was monitored at 567 nm for citrate-protected AuNPs etched by ATP at different concentrations. The inset cartoons represent the changing of ATP ligand numbers capped on AuNPs. (B) The zeta potential (ζ) change before and after ATP conjugation to AuNPs. (C) The hydrodynamic size change before and after ATP conjugation to AuNPs. (D) UV absorption and colour change (inset) before and after ATP conjugation to AuNPs.

Fig. 3. Characterization, covalent conjugation and nitrite detection of NG-AuNPs. (A) UV-vis absorption spectrum of G-AuNP. (B) Size distribution of G-AuNPs measured by DLS. (C) Agarose gel electrophoresis of G-AuNPs and citrate-protected AuNPs. (D) Brief description of the detection principle of the system. (E) Absorption spectra of NO₂⁻ with gradient concentrations in the system. (F) The raw 567 nm-absorbance plot in a larger concentration range. (G) Plot of 567 nm-absorbance with a linear fit (red line) function (y = 0.7204x + 0.0037, where y is the 567 nm-absorbance and x is the concentration of NO₂⁻).

Fig. 4. The sensitivity and selectivity of the sensing system. (A) Colorimetric changes of the sensing system in the presence of NO₂⁻ (1.0 μg mL⁻¹) under different pH conditions. (B) The ionic selectivity test of the sensing system; inset is the absorbance at 567 nm monitored with the addition of equimolar ions.

Fig. 5. Application of the nanosensor to nitrite detection for samples of bread (A), milk (B), pork luncheon meat (C), ham (D) and spicy cabbage (E). The photos are of the real tested samples and the plots below are 567 nm-absorbance against the food exposure time in a −4 °C refrigerator. The comparisons of chromogenic effects between the new method and traditional method are shown in both photos and plots. (F) The standard detection plot derived from the traditional detection method showed a detection limit of 0.06 g mL⁻¹.