Facile sensor based on thiol capped quantum dots for sensing of human epidermal growth factor receptor (HER-2)

Abstract

Quantum dots (QDs) are a novel class of materials that have attracted much interest in the fields of biology, material science, and physical optics. Herein, we report the facile synthesis of thiol-protected SnS QDs for HER-2 sensor fabrication. The QDs were synthesized using two synthetic methods with different sulfur sources (sodium sulfide (Na2S) vs. thiourea). The synthesis was performed by heating in the presence of simple precursors in a diol-stabilized chemical medium. To improve material stabilization and bio-compatibility, three thiols, namely, Glutathione (GSH), L-Cysteine (L-cyst) and Mercapto-propanoic acid (MPA), were investigated as potential capping agents in each method. The effect of synthetic parameters such as reaction time, Sn: S ratio, reaction solvent (H20 vs. C3H8O2), reaction method (heat vs. reflux), ZnS shell and pH were also investigated to obtain optimum reaction conditions. An optical property comparison between the two methods was conducted. SnS QDs synthesized using method 2 exhibited high PL peak intensities at 1:1 Sn:S and pH 6 when L-Cysteine is used as a thiol capping agent. Adding the ZnS shell on the SnS QDs enhanced the PL intensity in both methods. Subsequently, L-Cysteine capped SnS QDs were synthesized using heat-up method at Sn:S ratio (1:1), and pH 6 with 30 min reaction time. These QDs were used as labels to create a sensor for HER2 biomarker detection.The optical properties of the synthesized QDs were characterized using ultraviolet-visible (UV–Vis), photoluminescence (PL) spectroscopy, and energy dispersive spectroscopy (EDS). The size and the shape of the QDs were confirmed using dynamic light scattering (DLS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The electroactivity of the as-synthesized L-cyst-SnS/ZnS QDs were successfully investigated using non-film (L-cyst-SnS/ZnS (solution) and film (L-cyst-SnS/ZnS/GCE) modes. Electroactivity properties were studied using Cyclic voltammetry (CV) and differential-pulse voltammetry (DPV). A choice was made between non-film and film methods based on depicted electroactivity and the film preparation problems encountered. The effect of varying supporting electrolytes was made using aqueous hydrochloric acid and Phosphate-buffered saline (PBS). PBS showed better electroactivity; hence, used in chemical reactivity and electron transfer kinetics of L-cyst-SnS/ZnS QDs studies. Both Randles-Sevcik and Lavron's method results for the core-shell L-Cyst-SnS/ZnS (non-film) QDs showed enhanced electrochemical properties, such as heterogeneous electron-rate constants at 6.72x10-5 cm.s-1 and diffusion coefficients (D) of 9.43x10-3 cm2s-1, in comparison to the reported values. These results suggest the formation of smaller L-cyst-SnS/ZnS QDs which might be the cause of aggregation. The L-cyst-SnS/ZnS (non-film) was successfully employed for sensing of HER-2. Different operating conditions were evaluated for the successful detection of the HER-2 biomarker. The electrochemical interaction between the biomarker and the as-synthesized L-Cyst-SnS/ZnS QDs were carried out using CV, DPV and SWV respectively at different time intervals. The findings showed time-depent interactions between the biomarker and the QDs. Amongst all the methods assessed, SWV technique demonstrated superior sensitivity in HER-2 detection, hence it was selected as the best method. The addition of Glutaraldehyde as a cross-linker led to an enhanced peak of the HER-2. The developed L-Cyst-SnS/ZnS (non-film) QDs sensor demonstrated an optimal limit of detection of 0.23 ng/mL. Additionally, the limits of quantification (LOQ) were determined to be 0.77 ng/mL, and the linear dynamic range of the method was obtained between 0.99 ng/mL and 3.3 ng/mL. The proposed sensor showed great potential for detecting HER-2 in real samples.