## 3 Results and Discussion
### Discussion  
Fluorescent nanodots provide a solid basis for the creation of sensors and imaging nanoagents. Amongst the two big families of fluorescent nanodots, which are CQDs and SQDs, there is plenty of room for designing and engineering their optical properties as a function of the wide variety of functional surfaces and sizes, but also of their core/shell composition for SQDs. There are many variables to be considered when assessing NP stability, cytotoxicity, and biocompatibility as a basis for judging the ideal bioscaffold. Thus, it is essential to compare the toxicity of both big families of fluorescent nanodot-based probes in situ by selecting CQDs and SQDs with the same size, superficial groups, and charges as standard selection criteria. As a proof of concept, 3-nm sized CQDs containing carboxyl groups at surface attracted our attention for biological applications, owing to their excellent water solubility, stability, and controlled fluorescent properties with specific pH [25; 29]. Accordingly, hydrophilic SQDs composed of a CdTe core/shell with carboxyl groups at surface and the same size than CQDs, yet with different emission wavelengths, were selected. Fluorescence characteristics of CQDs could be attributed to the entire surface shell of the dots rather than only to the carboxylic groups, as reported elsewhere [2]. However, emission mechanism of CQDs is still under debate [2; 29], and the use of these CQDs as bioprobes has been less extensively evaluated. In this study, the characteristics and nanotoxicity of CQDs and SQDs with identical surface groups and size were evaluated in two distinct cell types, fibroblasts and hepatocytes. We report herein that, although both types of NPs may be suitable for biological and/or biomedical applications, they markedly differ in their interaction with cells, in terms of their internalization capacity and toxicity. Furthermore, the response is cell type-specific and highly dependent on microenvironmental clues. We previously demonstrated that the toxicity of NPs, in particular, SWCNTs-PEG, increased in 3T3-L1 cells cultured in serum-free media [26]. This contrasts with the results obtained herein for CQDs, even though CQDs and SWCNTs-PEG are both carbon NPs [2; 26]. To be more specific, higher CQDs doses (400 mg\(\cdot\)L\({}^{-1}\), 800 mg\(\cdot\)L\({}^{-1}\), or 1600 mg\(\cdot\)L\({}^{-1}\)) were required to decrease 3T3-L1 cell viability in the absence of FBS, compared to SWCNTs-PEG (2.6 mg\(\cdot\)L\({}^{-1}\) and 13 mg\(\cdot\)L\({}^{-1}\)) [26]. Interestingly, studies in several cell types, such as lung epithelial A549 cells, have shown that silica NPs are internalized under serum-free conditions, even with higher efficiency than in medium containing serum, via adhesion of the bare NPs on the cell membrane [30]. In this line, our previous microscopy studies on SWCNTs-PEG [26] and present data on CQDs (Figure 3) demonstrate that both NP types can be internalized into 3T3-L1 cells in the absence of serum, further supporting the notion that NP uptake is not dependent on the presence of serum proteins, irrespective of the type of cell and/or NP. As a matter of fact, both CQDs and SWCNTs-PEG induced cell toxicity in the absence of serum yet with different efficiency, with the latter being more toxic when administered to 3T3-L1 cells at concentrations ranging from 2-10 mg\(\cdot\)L\({}^{-1}\)[26]. This could be related to physicochemical properties of NPs, as it is clearly established that the shape, size, surface charge, and core/shell composition affect the interaction of nanodots with the cells [31]. It is noteworthy that CQDs and SWCNTs-PEG behaved differentially when administered in serum-containing media. Thus, in the presence of serum, CQDs reduced cell viability at lower doses than those observed for serum-free media, while the opposite occurred for SWCNTs-PEG [26]. These results support the notion that nanoparticle properties are also important to define their association with serum proteins, which, in turn, have an impact on cell biological responses.