## 3 Results and Discussion
### Results
#### 3.1.2 Evaluation of Toxicity of CQDs and SQDs  
Next, it is important to test the cell type; each cell has different morphological and metabolic characteristics. It is also essential to test the different external conditions to which the cells are exposed, since they can also be of interest in the medium-quantum dots interaction. In this study, the use of two different cell lines, grown with different culture media, was proposed to elucidate the effect of quantum dots on the cell type and on the interactions with different extracellular components. Toxicity of CQDs and SQDs in 3T3-L1 Cells  
* Defining the optimal cellular conditions prior to cytotoxicity assay. Previous in vitro toxicity assays involving exposure of 3T3-L1 cells to polyethylene glycol coated single-walled carbon nanotubes (SWCNTs-PEG) demonstrated that changes in cell viability could be already detected after 24 h of treatment [26]. Moreover, in our previous study, we showed that the presence of serum was essential to maintain 3T3-L1 cell viability after exposure to NPs [26]. Figure 1: Absorbance and emission curves at their maximal excitation wavelengths of (**A**) graphitic-carbon quantum dots (CQDs) and (**B**) semiconductor quantum dots (SQDs). [Nanodots] = 200 mg\(\cdot\)L\({}^{-1}\); slit widths at emission and excitation were set at 3 and 8 nm, respectively. _Condition 1_: Absence of serum in cell culture:  
* MTT cell viability assay. Herein, we demonstrate that 3T3-L1 cells are fully viable after a 24 h-treatment with CQDs at concentrations up to 100 mg\(\cdot\)L\({}^{-1}\) in the absence of serum (Figure 2A). However, from 400 mg\(\cdot\)L\({}^{-1}\) of CQDs, cell viability decreased significantly (18.6%, 21.1%, and 28.3% for 400 mg\(\cdot\)L\({}^{-1}\), 800 mg\(\cdot\)L\({}^{-1}\), and 1600 mg\(\cdot\)L\({}^{-1}\), respectively) (Figure 2A). In contrast, a 24 h-treatment with SQDs induced significant cell death in serum-deprived 3T3-L1 cells at concentrations between 2 and 100 mg\(\cdot\)L\({}^{-1}\), which prevented further testing of higher SQDs doses on cell viability under these conditions (Figure 2B). Figure 2: Cell viability data in 3T3-L1 cells cultured in the absence of serum and exposed to CQDs (**A**) and SQDs (**B**) at concentrations ranging from 0\(-\)1600 mg\(\cdot\)L\({}^{-1}\) for 24 h. Control cells were cultured in Dulbecco’s Modified Eagle’s Medium-low glucose (DMEM) alone. Cell viability was measured by the MTT assay. Data (mean \(\pm\) SEM of at least three independent experiments with three replicate wells per treatment) were analyzed by one-way ANOVA multiple comparison test followed by [PERSON]’s least significant difference (LSD) test. Significant differences are indicated as \({}^{\star}\)\(p\)\(<\) 0.05, \({}^{\star\star}\)\(p\)\(<\) 0.01, \({}^{\star\star\star}\)\(p\)\(<\) 0.001 vs. control; \(\#\)\(p\)\(<\) 0.01, \(\#\)\(\#\)\(p\)\(<\) 0  
Imaging assay. Confocal microscopy images from 3T3-L1 cells exposed to either CQDs or SQDs at 25 mg-L\({}^{-1}\) in the absence of serum showed that both types of nanodots were incorporated by the cells while evoking different cell responses (Figure 3). In accordance with the MTT data, SQDs induced morphological changes indicative of cell death (i.e., DNA decondensation and cell membrane damage), while no apparent modifications were observed in cells exposed to CQDs (Figure 3). When SQDs were administered at a low, non-toxic dose (0.2 mg-L\({}^{-1}\)), only a few numbers of cells, with normal morphology, exhibited fluorescent puncta indicative of nanoparticle uptake (Figure S3). Cell uptake assay. Along with the microscopy studies, particle incorporation into the cells was also assessed by spectrophotometry.