Comparison of Toxicity and Cellular Uptake of CdSe/ZnS and Carbon Quantum Dots for Molecular Tracking Using Saccharomyces cerevisiae as a Fungal Model

Abstract

Plant resource sharing mediated by mycorrhizal fungi has been a subject of recent debate, largely owing to the limitations of previously used isotopic tracking methods. Although CdSe/ZnS quantum dots (QDs) have been successfully used for in situ tracking of essential nutrients in plant-fungal systems, the Cd-containing QDs, due to the intrinsic toxic nature of Cd, are not a viable system for larger-scale in situ studies. We synthesized amino acid-based carbon quantum dots (CQDs; average hydrodynamic size 6 ± 3 nm, zeta potential -19 ± 12 mV) and compared their toxicity and uptake with commercial CdSe/ZnS QDs that we conjugated with the amino acid cysteine (Cys) (average hydrodynamic size 308 ± 150 nm, zeta potential -65 ± 4 mV) using yeast Saccharomyces cerevisiae as a proxy for mycorrhizal fungi. We showed that the CQDs readily entered yeast cells and were non-toxic up to 100 mg/L. While the Cys-conjugated CdSe/ZnS QDs were also not toxic to yeast cells up to 100 mg/L, they were not taken up into the cells but remained on the cell surfaces. These findings suggest that CQDs may be a suitable tool for molecular tracking in fungi (incl. mychorrhizal fungi) due to their ability to enter fungal cells.

Keywords: Saccharomyces cerevisiae; carbon dots; molecular tracking; mycorrhizal fungi; nanoparticle toxicity; nanoparticle uptake; nutrients; quantum dots; viability.

Figure 1

Absorbance spectrum (black dashed line) and normalized fluorescence spectra (solid colored lines) of a 0.29 g/L aqueous solution of carbon quantum dots (PL stands for photoluminescence).

Figure 2

FTIR spectrum of the carbon quantum dots (CQDs) made from L-cysteine and citric acid by microwave technology.

Figure 3

Electronmicroscopic characterization of carbon quantum dots (CQDs). High-resolution TEM (A,B), STEM BF (C), and HAADF (D) images of CQDs.

Figure 4

High-resolution TEM images of CdSe/ZnS-Cys QDs with different magnifications (A,B).

Figure 5

(A) The survey XPS spectra of quantum dots. The origins of the main peaks are indicated. The Si signal originates from the substrate and is particularly visible for thin samples. (B) XPS spectra of different quantum dots in the Cd3d and N1s regions.

Figure 6

Size distributions of CdSe/ZnS QDs (A), CdSe/ZnS-Cys QDs (B), and CQDs (C), based on the dynamic light scattering intensity (upper graph in each panel) and particle volume (bottom graph in each panel). The hydrodynamic sizes were measured in MilliQ water using ZetaSizer Nano ZS (Malvern). The average hydrodynamic diameters for each type of QDs (Table 1) were calculated by the instrument software based on the dynamic light scattering intensity distributions.

Figure 7

A viability assay (spot test) showing the colony-forming ability of yeast S. cerevisiae BY4741 after 24 h of exposure to CdSe/ZnS quantum dots (QDs), CdSe/ZnS QDs connected to cysteine (Cys), and carbon quantum dots (CQDs) and AgNO₃ (as a positive control) in MilliQ water at 30 °C. Two replicates per tested compound were presented.

Figure 8

Confocal laser scanning microscopy (CLSM) images of the cellular interactions of quantum dots (QDs) in S. cerevisiae BY4741 cells stained with CellBrite Fix 555 membrane stain (orange pseudocolour) were incubated with QDs (blue pseudocolour) for 24 h. After the incubation, cells were either not washed (A–D) or washed (E–H) with MilliQ water to assess the strength of interactions. Scale bars correspond to 5 µm.