Sensitive and selective colorimetric nitrite ion assay using silver nanoparticles easily synthesized and stabilized by AHNDMS and functionalized with PABA

Abstract

Nitrite ions (NO₂ ^-), as one of the important inorganic anions, exhibit considerable effects towards the environment and human health. Moreover, over intake of this anion may cause dangerous diseases. Herein, we successfully fabricated silver nanoparticles (AgNPs) using 4-amino-5-hydroxynaphthalene-2, 7-disulphonic acid monosodium salt (AHNDMS) and functionalized them with p-aminobenzoic acid (PABA), and used the functionalised AgNPs as a sensitive and selective colorimetric sensor for nitrite ions. The structure of the as-prepared pure AgNPs was experimentally characterized by different characterizations methods, namely, UV-vis, FT-IR, CV, DPVs, SEM, TEM, and XRD. Additionally, the nitrite ion sensitively and selectively changes the brownish yellow color of the dispersed AgNPs to pinkish red, indicating aggregation of AgNPs, with a detection limit of 0.016 ppm (0.348 μM) and 0.0069 ppm (0.149 μM) by the naked-eye and by UV-vis spectroscopy, respectively. The color change suggested that the aggregation of AgNPs was induced by nitrite-selective diazo-coupling. UV-vis spectra show the disappearance of the absorbance at 474 nm and appearance of a new peak at 532 nm, presumably due to the conversion of AgNPs to silver ions. Moreover, the studies of interference in the proposed sensor confirm its selectivity in the presence of anions as well as cations. Furthermore, linearity was observed between the absorption and the concentration of nitrite ions. More importantly, the proposed sensor was practicably applied for the determination of nitrite in different water samples, such as distilled water, river water, and tap water.

Scheme 1. Schematic for colorimetric sensing of NO₂⁻ using AgNPs synthesized by AHNDMS and functionalized with PABA.

Fig. 1. Absorption spectra of the AgNPs (a), functionalized AgNPs (b), 0.5 mL (b) +0.5 mL pH = 9.4 buffer solution +2 mL DW (c), and (c) +1 ppm NO₂⁻ (d). Inset is the photograph of corresponding solutions.

Fig. 2. FT-IR spectra of AHNDMS (a), PABA (b), synthesized AgNPs (c), and functionalized AgNPs (d).

Fig. 3. XRD pattern of the AgNPs synthesized and stabilized by AHNDMS.

Fig. 4. SEM images of the as-synthesized AgNPs (A), functionalized AgNPs (B), and aggregated AgNPs (C). TEM images of the synthesized AgNPs (D), functionalized AgNPs (E), and aggregated AgNPs (F).

Fig. 5. (A) The corresponding color changes of (B) UV-vis absorption spectra of the sensing of different anions and nitrite (NO₂⁻) ions by the recommended procedure. (C) Absorption enhancement of the anions and nitrite at 532 nm.

Fig. 6. (A) The corresponding color change of (B) UV-vis absorption spectra of the sensing of different cations and nitrite (NO₂⁻) by the recommended procedure. (C) Absorption enhancement of the cations and nitrite at 532 nm.

Fig. 7. (A) The corresponding solution colors of (B) UV-vis absorption spectra for the proposed sensing system with different concentrations of nitrite, and (C) the calibration response for the determination of nitrite. Concentrations range from 0.016 to 1 ppm.