The extensive research of nanoparticles in connection to their various biological and medical applications has been the preamble for the development of quantum dots (QDs). These represent a heterogenous class of nanoparticles composed of a semiconductor core including group II-VI or group III-V elements encased within a shell comprised of a second semiconductor material . Due to their unique optical and chemical properties, i.e., their broad absorption spectra, narrow fluorescence emission, intense fluorescence, and photo bleaching resistance [2, 3], QDs were proposed as nanoprobes which were able to replace the conventional organic dyes and fluorescent proteins . The use of different core material combinations and appropriate nanocrystal sizes has rendered QDs useful in biosensing , energy transfer , in vivo imaging , drug delivery , and diagnostic and cancer therapy applications .
Despite their special properties, most types of QDs have limited use in biology and medicine due to their toxicity . Numerous concerns regarding the cytotoxicity of different types of QDs were presented in a recent review , which detailed that QD toxicity depends on a number of factors including the experimental model, concentration, exposure duration, and mode of administration.
Interestingly, efforts to reduce QD toxicity include the encapsulation in a SiO2 shell [7, 12], with silicon-based QDs being expected to be less toxic than heavy metal-containing ones. Due to previously known benefits of silicon, like reduced elemental toxicity, its potential biodegradability to silicic acid and its abundance and low costs are adding to the promising results of recent investigations that indicate silicon use in in vivo imaging to be a good alternative to cadmium QDs [13, 14]. Nanoporous and microparticulate forms of silicon have shown great promise in terms of compatibility and cytotoxicity . Nonetheless, studies concerned with the biological and medical applications of silicon-based QDs are less numerous and still at preliminary stages [16–18].
A step towards overcoming the toxicity issue is to elucidate the in vivo distribution and biological effects of QDs that due to their variable characteristics must be addressed individually. It is now accepted that nude nanoparticles, including QDs, become entrapped in the cells of the reticuloendothelial system and are preferentially transported and accumulated into the liver, spleen, and also in the kidney [4, 19–24]. Once localized at this levels, nanoparticles interact with the surrounding tissue and cells .
In vitro and in vivo studies suggest that intracellular reactive oxygen species (ROS) production is a possible mechanism for silicon-based QDs toxicity [16, 26–28]. ROS are formed continuously in all living aerobic cells as a consequence of both oxidative biochemical reactions and external factors, with them being involved in the regulation of many physiological processes . When the production of ROS exceeds the ability of the antioxidant system to balance them, oxidative stress occurs . Because ROS are highly reactive, most cellular components are prone to oxidative damage. Consequently, lipid peroxidation, protein oxidation, reduced glutathione (GSH) depletion, and DNA single strand breaks could be initiated by ROS excess. Taken together, all these changes can ultimately lead to cellular and tissue injury and dysfunction .
Aquatic organisms are known for their sensitivity to oxidative stress . Fish possess systems for generating as well as for protection against the adverse effects of free radicals [32, 33]. Due to their dependence on oxygen availability in their environment, fish metabolism has adapted to diminish oxygen requirements. More interestingly, carp and gibel carp are capable to tolerate anoxia for periods that extend to months, depending on temperature . Similarly to other aestivating animals, these fish have developed remarkable antioxidant defense mechanisms to cope with the return to normal environmental conditions . The most potent antioxidant mechanisms are found particularly in the organs with high metabolic activity such as the liver, kidney, and brain . Thus, the freshwater fish Carassius gibelio is a suitable model system to evaluate the changes induced by QDs and their putative oxidative stress related effects.
In this study, we highlighted the in vivo accumulation of silicon-based QDs and described the histological changes that occurred in the hepatic tissue of the gibel carp. We also focused on revealing the biochemical alterations that appeared. We evaluated the GSH concentration and the levels of oxidative stress markers such as: malondialdehyde (MDA), carbonyl derivates of proteins (CP), protein sulfhydryl groups (PSH), and advanced oxidation protein products (AOPP). Additionally, we concentrated on the activity of the antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), and glutathione-S-transferase (GST), as well as glutathione reductase (GR) and glucose 6-phosphate dehydrogenase (G6PDH) due to their key roles in antioxidant defense.