Highly Selective Colorimetric Sensor of Mercury(II) Ions by Andrographolide-Stabilized Silver Nanoparticles in Water and Antibacterial Evaluation

Abstract

Silver nanoparticles (AgNPs) are well known for their exceptional properties and versatility in various applications. This study used andrographolide as a biochemical stabilizer to synthesize AgNPs (andro-AgNPs). The andro-AgNPs were characterized by using UV-vis spectroscopy, revealing a surface plasmon resonance peak at 440 nm. Fourier transform infrared spectroscopy was also used to confirm the presence of AgNPs. Transmission electron microscopy was used to investigate the morphology of andro-AgNPs, which showed a spherical shape with an average diameter of 18.30 ± 5.57 nm (n = 205). Andro-AgNPs were utilized as a colorimetric sensor to detect mercury ions (Hg²⁺) in water, and the optimized detection conditions were evaluated using UV-vis spectroscopy with a linear range of 15-120 μM. The limit of detection and the limit of quantification for Hg²⁺ detection were found to be 11.15 and 37.15 μM, respectively. Furthermore, andro-AgNPs exhibited antibacterial properties against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. The results imply that andro-AgNPs hold promising potential for future biomedical applications.

Figure 1

Synthesis of AgNPs using andrographolide as a reducer and a stabilizer.

Figure 2

(a) UV–vis absorption spectra, (b) TEM image, and (c) size distribution plot of andro-AgNPs from TEM (n = 205).

Figure 3

UV–vis absorption spectra of andro-AgNPs and andro-AgNPs + Hg²⁺.

Figure 4

Schematic representation of the sensing mechanism of Hg²⁺ and andro-AgNPs.

Figure 5

(a) UV–vis absorption spectra for various andro-AgNPs concentrations interacting with Hg²⁺ and (b) the corresponding relative change of the absorbance of andro-AgNPs at 430 nm with respect to the andro-AgNPs concentration in the presence of 20 μM Hg²⁺, (c) the corresponding UV–vis absorption spectra of andro-AgNPs at 430 nm with respect to time in the presence of 0.08 nM Hg²⁺, (d) relative change in the absorbance of andro-AgNPs for the data in (c),(e) UV–vis absorption spectra of andro-AgNPs at 430 nm in the presence of 20 μM Hg²⁺ and various metal ions, and (f) relative change of the absorbance of andro-AgNPs for the data in (e).

Figure 6

(a) Comparison of colorimetric responses for 0.08 nM andro-AgNPs in the presence of 20 μM of various metal ions. (b) Colorimetric response to different concentrations of Hg²⁺ (from left to right: 0, 0.1 0.15, 0.20, 0.25, 0.30, and 0.50 mM).

Figure 7

(a) UV–vis absorption spectra of 0.08 nM andro-AgNPs treated with various concentrations of Hg²⁺ ranging from 15 to 120 μM, (b) absorbance ratio (A₀ – A) at 430 nm plotted against Hg²⁺ concentrations, where “A₀” represents the absorbance of andro-AgNPs (blank) and “A” represents the absorbance of andro-AgNPs in the presence of Hg²⁺ concentrations ranging from 15 to 120 μM.

Figure 8

Andro-AgNPs and their antimicrobial activity. Antimicrobial activity of position (i) andro-AgNPs, position (ii) andrographolide, position (iii) DI water, and position (iv) gentamicin 33.5 μM against (a) S. aureus and (b) E. coli.