Colorimetric Method for Sensitive Detection of Microcystin-LR Using Surface Copper Nanoparticles of Polydopamine Nanosphere as Turn-On Probe

Abstract

A novel, facile sensor was further developed for microcystin-LR (MC-LR) determination by visible spectroscopy. Antibody-functionalized SiO₂-coated magnetic nanoparticles (Fe₃O₄@SiO₂) and aptamer-functionalized polydopamine nanospheres decorated with Cu nanoparticles (PDA/CuNPs) recognized specific sites in MC-LR and then the sandwich-type composites were separated magnetically. The Cu in the separated composites was converted to Cu²⁺ ions in solution and turn-on visible absorption was achieved after reaction with bis(cyclohexanone)oxaldihydrazone (BCO) (λ_max = 600 nm). There was a quantitative relationship between the spectral intensity and MC-LR concentration. In addition, under the optimum conditions, the sensor turns out to be a linear relationship from 0.05 to 25 nM, with a limit of detection of 0.05 nM (0.05 μg/L) (S/N = 3) for MC-LR. The sensitivity was dependent on the low background absorption from the off-to-on spectrum and label amplification by the polydopamine (PDA) surface. The sensor had high selectivity, which shows the importance of dual-site recognition by the aptamer and antibody and the highly specific color formed by BCO with Cu²⁺. The bioassay was complete within 150 min, which enabled quick determination. The sensor was successfully used with real spiked samples. These results suggest it has potential applications in visible detection and could be used to detect other microcystin analogs.

Keywords: colorimetric sensor; dual-site recognition; magnetic separation; microcystin-LR; turn-on visible absorption.

Scheme 1

Schematic diagram of microcystin-LR (MC-LR) detection with sensor.

Figure 1

Transmission electron microscopy (TEM) images of Fe₃O₄ (A), Fe₃O₄@SiO₂ (B), PDA (C), CuNPs (D) and PDA@CuNPs (E); (F) vibrating sample magnetometry curves for (a) Fe₃O₄ and (b) Fe₃O₄@SiO₂; (G) Fourier transform infrared (FTIR) spectra of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂ and (c) Fe₃O₄@SiO₂@PAA; (H) FTIR spectra of (a) dopamine hydrochloride, (b) PDA and (c) PDA@CuNPs; thermogravimetric analysis (TGA) (I) and differential scanning calorimetry (DSC) (J) curves for (a) PDA and (b) PDA@CuNPs.

Figure 2

(A) Absorbance spectra of (a) Cu²⁺ and (b) bis(cyclohexanone)oxaldihydrazone (BCO) in solution of sodium citrate and triethanolamine and (c) mixture of Cu²⁺ and BCO in the solution; (B) absorbance spectra and photographs of BCO mixed with various metal ions, namely Cu²⁺, K⁺, Na⁺, Ag⁺, Mg²⁺, Ca²⁺, Cd²⁺, Co²⁺, Al³⁺ and Fe³⁺ (20 μM); and (C) absorbance spectra and photographs of sensing solution in absence (blue curve) and presence of MC-LR (15 nM, red curve).

Figure 3

Effects of various concentration of Fe₃O₄@SiO₂-antibody (A) and PDA/CuNPs-aptamer (B), pH (C), incubation time (D), incubation temperature (E) and coloration time (F) on signal intensity. MC-LR concentration was 25 nM. Error bars are standard deviations across three repeated experiments.

Figure 4

UV-Vis responses of sensor to various concentrations of MC-LR. (A) photographs of MC-LR solutions of various concentrations (a–k) and colorimetric spectra of sensor for various MC-LR concentrations (nM): (a) 0, (b) 0.05, (c) 1.0, (d) 2.0, (e) 3.0, (f) 4.0, (g) 5.0, (h) 10.0, (i) 15.0, (j) 20.0, (k) 25.0, (l) 30.0 and (m) 35.0; (B) corresponding calibration curve of absorbance values at 600 nm against MC-LR concentration.

Figure 5

Effects of coexisting substances on colorimetric response of MC-LR. Solution composition: (A) (1) 10.0 nM MC-LR in Tris-HCl buffer solution (0.2 M, pH 7.4); mixtures of 10.0 nM MC-LR with (2) MC-LF, (3) MC-RR, (4) MC-YR, (5) OA, (6) MC-LW, (7) K⁺, (8) Cu²⁺, (9) Ag⁺, (10) Ca²⁺, (11) Na⁺, (12) Co²⁺, (13) Mg²⁺, (14) Al³⁺, (15) Fe³⁺, (16) Cd²⁺, (17) thiacloprid, (18) thiamethoxam, (19) dinotefuran and (20) phenol; (21) mixture of 1 with the other 20 types of interferent. (B) Molecular structures of interferents.