Positively Charged Gold Quantum Dots: An Nanozymatic "Off-On" Sensor for Thiocyanate Detection

Abstract

The concentration of thiocyanate (SCN^-) in bodily fluids is a good indicator of potential and severe health issues such as nasal bleeding, goiters, vertigo, unconsciousness, several inflammatory diseases, and cystic fibrosis. Herein, a visual SCN^- sensing method has been developed using the enzyme-like nature of positively charged gold quantum dots (Au QDs) mixed with 3,3',5,5'-tetramethylbenzidine (TMB) and hydrogen peroxide (H₂O₂₎. This research also reports a new method of synthesizing positively charged Au QDs directly from gold nanoparticles through a hydrothermal process. Microscopic imaging has showed that the Au QDs were 3-5 nm in size, and the emission wavelength was at 438 nm. Au QDs did not display any enzyme-like nature while mixed up with TMB and H₂O₂. However, the nanozymatic activity of Au QDs appeared when SCN^- was included, leading to a very low detection limit (LOD) of 8 nM and 99-105% recovery in complex media. The steady-state kinetic reaction of Au QDs showed that Au QDs had a lower Michaelis-Menten constant (Km) toward H₂O₂ and TMB, which indicates that the Au QDs had a higher affinity for H₂O₂ and TMB than horseradish peroxidase (HRP). A mechanism study has revealed that the scavenging ability of hydroxyl (•OH) radicals by the SCN^- group plays an important role in enhancing the sensitivity in this study. The proposed nanozymatic "Off-On" SCN^- sensor was also successfully validated in commercial milk samples.

Keywords: colorimetric detection; gold quantum dots; nanozyme; thiocyanate detection.

Figure 1

Schematic illustration of the present SCN⁻ detection.

Figure 2

Identification of Au NPs and Au QDs: (A) absorbance of Au NPs and Au QDs; (B) fluorescence spectra of Au NPs and Au QDs; (C) TEM image of Au NPs: (D) HR-TEM image of Au QDs (inset: size distribution); (E) close view of Au QDs (inset: lattice fringes of Au QDs).

Figure 3

Nanozymatic activity of Au NPs and Au QDs (A) (inset: the color of the solution) and (B) calibration plot of differently concentrated SCN⁻ (absorbance at 655 nm).

Figure 4

Kinetic study of Au QDs: (A,C) 5 mM TMB with various concentrations of H₂O₂, (B,D) 10 mM H₂O₂ with various concentrations of TMB.

Figure 5

Mechanism study of Au QDs’ enzyme-like nature: (i) formation and adsorption of •OH radicals on the surface of Au QDs; (ii) oxidation reaction of TMB initiated by •OH radicals and (iii) formation of a dimer structure of oxidized TMB molecules that turns the solution color to deep blue.

Figure 6

(A) Fluorescence emission of 2-Hydroxy-1,4-benzenedicarboxylic acid. (B) The absorbance intensity of SCN⁻-Au QDs in the presence of H₂O₂ and TMB (a); after adding citric acid (b) and ascorbic acid on sample a, separately (c).

Figure 7

The specificity and SCN⁻ detection in spiked milk samples: (A) specificity of the proposed assay (a: SCN⁻, b: sucrose, c: citrate, d: BSA, e: fructose, f: SO₄²⁻, g: Mg²⁺, h: Ca²⁺, i: EDTA, j: l-lysine, k: CO₃²⁻, l: NO₃⁻, m: CH₃CO⁻, n: Cl⁻); (B) SCN⁻ concentration vs. absorbance intensity curve measured in the spiked milk samples.