Colorimetric detection of mercury (Hg2+) using UV-vis spectroscopy and digital image analysis based on gold nanoparticles functionalized with bromelain enzyme

Abstract

A very sensitive and selective colorimetric biosensor for the measurement of mercury ion (Hg²⁺) in environmental samples has been developed using functionalized gold nanoparticles with bromelain enzyme (brn-AuNPs). This work has shown that Hg²⁺ measurement based on spectrophotometer and digital image analysis is a very innovative and successful method for providing an effective preliminary system and has promise for the future of water quality biomonitoring. Response Surface Methodology (RSM), a Box-Behnken design-based technique, was used to identify the optimum levels of functionalization of bromelain to AuNPs. The created model's validity was confirmed, and statistical analysis revealed that the ideal functionalize conditions were 1 mM of AuNPs, functionalize with 0.59 mM bromelain concentration on 14 ℃ temperature and 72 h incubation time. The lowest colorimetric detection concentration (LOD) of brn-AuNPs of Hg²⁺ was 0.0092 ppm and 0.011 ppm for spectrophotometer and digital image analysis. As shown, digital image analysis had advantages based on the LOD result comparable to UV-VIS spectrophotometer. The practical application of the brn-AuNPs sensing was proven with mercury determination in water samples. The present study developed a robust sensor, which successfully implemented in a compact portable sensor kit, turning this sensor into a very potent tool for the development water quality biomonitoring system of Hg²⁺ application.

Keywords: Bromelain; Colorimetric detection; Gold nanoparticles; Mercury; Response surface methodology.

Fig. 1

Schematic illustration for the interaction between brn-AuNPs and Hg²⁺

Fig. 2

Multimodal size distribution graph

Fig. 3

Diagnostic plots showing a the externally studentized residuals plotted against the normal probability, b the predicted versus the externally studentized residuals, c the run number versus the externally studentized residuals, and d the actual responses versus the predicted response

Fig. 4

Response Surface Method plots showing interactive effect of a (B) bromelain concentration; (A) AuNPs concentration; b. (C)incubation time and (A)AuNPs concentration; c (D) temperature and (A) AuNPs concentration; d (C) incubation time and (B) bromelain concentration; e (D) Temperature and (B) Bromelain concentration; f (D) Temperature and (C) incubation time

Fig. 5

Re-dispersed functionalized brn-AuNPs

Fig. 6

UV–vis absorption spectra of AuNPs and brn-AuNPs at 524 nm

Fig. 7

a Transmission Electron Microscopy image of AuNPs at 50 nm magnification. AuNPs were spherical in shape, uniform and independent from each other. b Transmission Electron Microscopy image of brn-AuNPs at 50 nm magnification shows that the outer layer of the bromelain-encapsulated around the AuNPs lighter than the core area. Bromelain protease hydrophobic nature enables it to be contained in the hydrophobic part of the AuNPs

Fig. 8

The cropped digital image of brn-AuNPs with different heavy metals after colorimetry reaction

Fig. 9

UV–vis absorption spectra of brn-AuNPs on different heavy metals

Fig. 10

The cropped digital image of brn-AuNPs contained different concentration of mercury (ppm) after colorimetry reaction

Fig. 11

UV–vis absorption spectra of brn-AuNPs on different concentration of mercury

Fig. 12

A₆₂₆/A₅₂₄ of Brn-AuNPs versus the different concentration of Hg²⁺

Fig. 13

Plotted graph mercury different concentration against effective intensity of color R

Fig. 14

Representative TEM images with scale bar at 50 nm of a brn-AuNPs in the presence of 1 ppm of Hg²⁺; b brn-AuNPs in the presence of 10 ppm of Hg²⁺

Fig. 15

The cropped digital image of brn-AuNPs after reaction with different water samples