Ecotoxicology impact of silica-coated CdSe/ZnS quantum dots internalized in Chlamydomonas reinhardtii algal cells

Highlights

• Silica coating improve the fluorescence efficiency and life time of QD.

• C. reinhardtii might be good for bioremediation of heavy metals.

• Si-QD has been internalized in C. reinhardtii and accelerating the cell cycle.

• The Si-QD uptake inside C. reinhardtii was confirmed by confocal microscopy.

• Si-QD exhibited a minor toxic effect on C. reinhardtii cells.

Abstract

The use of quantum dots (QD) in various medical and industrial applications may cause these nanoparticles to leak into waterways and subsequently enter the food chain. Therefore, if we intend to use QD, we must first know their potential environmental implications. In this work, cadmium selenide/zinc sulfide core/shell QD were synthesized, and then, biocompatible, water-dispersed QD were coated with silica (Si-QD). The QD were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) combined with energy-dispersive X-ray spectroscopy (EDX), and UV–Vis absorption analysis, which revealed that these surface-engineered QD have a highly crystalline, homogeneous spherical shape measuring approximately 25 nm. The cytotoxicity of the nanoparticles in the green algae Chlamydomonas reinhardtii was studied by incubating the algae cells with Si-QD and determining the optical density of algal cell culture, cell counts, and cells sizes by microflow cytometry. These measurements indicated that Si-QD are biocompatible up to a concentration of 25 ng/ml. Finally, the cellular uptake of Si-QD into C. reinhardtii was monitored by confocal laser scanning microscopy (CLSM). In conclusion, our results reveal that surface-engineered Cd-QD can penetrate the cells of aquatic organisms, which ensures a serious impact on the food chain and consequently the environment. On the other hand, the results also highlight a new potential method for bioremediation of Cd-QD by green algae, especially C. reinhardtii.

Introduction

Quantum dots (QD) are a special type of nanomaterial due to their unique physiochemical properties, such as a broad absorption band, long first exaction radiative lifetime, small Stokes shift, and the ability to tune the semiconductor core particle aspect ratio to obtain linearly polarized PL emission (Gui et al., 2015; Khosroshahy et al., 2017; Steckel et al., 2015), that make QD useful for various potential scientific and industrial applications, including agricultural, environmental (Khalil et al., 2015), biological (Polo and Puertas, 2012), biosensing (Matea et al., 2017; Tan et al., 2012), in vitro assay (Rocha et al., 2014), and imaging applications (Rocha et al., 2015; Tan et al., 2013). While cadmium-based QD (Cd-QD) have the most favourable physiochemical properties (Kovalenko et al., 2015; Oh et al., 2016), their toxicity, instability in aqueous phase and loss of brightness at high temperature are some of the obstacles that prevent the use of Cd-QD (Hanson et al., 2017). In 2002, the Quantum Dot Corporation in Hayward, California, began to sell QD to cell biologists as fluorescent imaging labels for proteins and other biological molecules (Bourzac, 2013). Additionally, due to their high light-harvesting capability in the visible region (Zhang et al., 2018), Cd-QD have been shown to be very attractive tools for electronic industrial applications, such as solar cells, photoresistors, light emitting diodes, and TV screens (Bourzac, 2013).

Unfortunately, despite the progress in methodologies for preparation of Cd-QD with various type of coating, the Cd release from this nanoparticles still represents an environmental risk as Cadmium is a biologically toxic and highly persistent metal that can enter the food chain. Direct health effects of Cd include renal dysfunction, cancer risk, and disturbances in calcium homeostasis (McElroy et al., 2017). For these health and environmental risks, current production has shifted from the Cd-based to Cd-free QD.

Fortunately, the effect of Cd toxicity can be eliminated by coating by encapsulation layers that used to safeguard QD from water and isolate Cd based core from the environment (Dai et al., 2017). Numerous coatings were applied to QD, the silica coating is a very common type of coating technology and as it increases the photoluminescence efficiency (PL) and biocompatibility of the QD (Qu et al., 2015). While, the best yielded cadmium-based QD usually synthesized in an organic phase (Goldman and Mattoussi, 2005). Silica is a good solution to transfer the QD to aqueous media (Vibin et al., 2014).

However, application of Cd- QD represents a potential risk as these nanomaterials can be released into marine environments and enter food chain (Agrawal and Rathore, 2014); in fact, and the environmental and health hazards of these materials are unclear (Xiao et al., 2010). Therefore, evaluating the release, exposure, and uptake behaviours of Cd-QD as model compounds in the environment is an essential step in the ecological risk assessment of these nanomaterials (Kühnel et al., 2014; Navarro et al., 2012). Additionally, the potential for bioremediation of Cd-QD using microalga, such as the green microalga Chlamydomonas reinhardtii (C. reinhardtii), may exist. Green microalga is used in aquaculture, human health food, biofuel, etc. (Acien et al., 2017; Ben Halima, 2017; Bolhuis and Cretoiu, 2016) and is also beneficial for bioremediation. Previous studies have provided evidence of the successful uptake of free Cd by C. reinhardtii (Siripornadulsil et al., 2002). Investigations on the Cd-QD exposure of algal cells did not distinguish how the Cd-QD were internalized by the algal cells (Navarro et al., 2012). The successful uptake of QD into human cells (Jackson et al., 2012; Navarro et al., 2012) and plant cells (Morelli et al., 2012) has been clearly observed, but the toxicity of QD to algae remains unclear (Navarro et al., 2012).

To better understand the biological impact of nanomaterials on algae, this study aims to determine the probability of C. reinhardtii, a model green algae, internalizing silica-coated CdSe/ZnS core/shell quantum dots (Si-QD) and investigate the toxicological effects of these nanocrystals on the life cycle of C. reinhardtii. Green algae, a key part of the food chain, were selected based on the premise that an aquatic environment might provide information on how nanomaterials can affect the environment (Peng and Peng, 2001). Si-QD were selected because of their importance as prospective industrial-use surface-modified nanocrystals (Sampat et al., 2015).