The effect of hydrophilic quantum dots on autofluorescence of unicell freshwater algae

Abstract

This study explored the effect of hydrophilic quantum dots (QD) CdSe/ZnS-COOH on the autofluorescence of unicellular freshwater algae Scenedesmus sp. and Chlorella sp. in the enviroment of modified Wilkins-Chalgren medium (MWC) and deep well water media. These media are important for the study of algal autofluorescence as they allow a comparison between the MWC medium, which is adapted for the growth of algae in a way that is not nutrient deficient, and the deep well water medium, which is quite close to the natural conditions for the growth of freshwater algae. Several spectroscopic techniques were used to analyse the autofluorescence of unicellular algae in both media including Kautsky curves. These methods can also be used to compare how the photoluminescence of quantum dots is affected by the presence of algae in these media. Finally, we can assess how quantum dots introduced into these media can affect the autofluorescence of these algae. Where we find that the autofluorescence intensity and excitation wavelength ratio of 483nm/435nm in deep well water of unicellular freshwater algae Scenescesmus sp. and Chlorella sp. under "day/night" illumination conditions after 7 days are lower compared to the autofluorescence of algae in MWC medium. Also during the first week we see that irradiated CdSe/ZnS-COOH quantum dots in MWC medium kept under "day/night" illumination conditions have a decrease in scattering of the optical density spectra, a slight increase in the photoluminescence intensity and a narrowing of the spectrum compared to the spectroscopic QD parameters measured just after irradiation, which is attributed to the degradation of the unstable QD aggregates formed during the irradiation. As an addition in the same medium partial recovery of the photoluminescence intensity of the quantum dots induced by the algal cells and/or the ectracellular material released from them, which, although lower than in the sample without algae, was accompanied by a narrowing of the spectrum in the red part. What we see in deep well water is in fact similar. Here we see that the degradation of the quantum dot structure results in a decrease in the absorption and photoluminescence intensity of the irradiated CdSe/ZnS-COOH quantum dots and a shift of the spectrum towards the short-wavelength side, compared to the spectra of the quantum dots measured immediately after irradiation, In the medium with algae, this decay is partially stabilised and a partial recovery of quantum dot photoluminescence is recorded during the first week, which is due to the formation of an additional layer when the quantum dots interact with the algae and/or their extracellular materials. And finaly CdSe/ZnS-COOH quantum dots induced a medium-independent response in the unicellular freshwater species Scenescesmus sp. and Chlorella sp. algae, but the photochemical attenuation and the electron transfer coefficient were lower in deep well water than in MWC medium, so quantum dots in deep well water have a greater effect on the photosystem II of algae