###### Abstract  
A comparative, fully parallel study of nanoparticles (NPs) toxicity by in vitro cell viability is shown looking for reliable comparability of nanotoxicological results, a well-recognized bottleneck in the context. This procedure is suitable to compare toxicity of similar NPs, as well as the influence on toxicity of the size, surface, and other characteristics. As a case of study, semiconductor (SQDs) and graphitic-carbon quantum dots (CQDs) with identical surface groups and size were evaluated. All experiments were conducted at same conditions, involving two types of cells (mouse fibroblasts (3T3-LI) and carcinoma human hepatocellular cells (HepG2)) and different extracellular components (in the absence or presence of fetal bovine serum (FBS)). Cell viability demonstrated the excellent biocompatibility of CQDs compared to SQDs, which caused higher percentage of cell death at lower concentrations, as predicted but never clearly demonstrated. However, our comparative studies established that the toxicity of SQDs and CQDs are cellular type-dependent, and the absence or presence of serum proteins reduces the minimal concentration necessary of NPs to produce toxicity. c 20  
Accordingly, the number of published applications with SQDs has experienced a considerable increase, especially in biology and clinics [2]. In particular, this has demonstrated their potential in fluorescence resonance energy transfer (FRET) analysis, drug delivery, in vivo animal imaging, therapeutics, and cell tracking [6; 7]. However, their use is limited by their potential toxicity associated with the release of heavy metals; thus, further analysis focused on identifying new fluorescent NPs with enhanced properties in biological and medical applications as alternatives of SQDs will be necessary. In this line, the next QD generation consists of green nanomaterials that afford these mentioned multipurpose applications in nanomedicine. Thus, graphitic CQDs, discovered in 2004 during the purification of carbon nanotubes [8], have emerged as excellent candidates for both sensing and imaging biological systems. These QD generations show low toxicity to overcome the toxic effects faced by organic dyes and SQDs, and require an easy and low-cost production. CQDs exhibit stable photoluminescence properties, which can be tuned from the synthetic route chosen to the passivation/functionalization, following steps. Since their discovery, applications related to (bio)chemical sensing, photocatalysis, bioimaging, or drug delivery have been reported [2; 9; 10; 11; 12; 13; 14; 15; 16]. In recent years, CQDs have been explored in cell viability studies. [PERSON] et al. [17] proposed the use of fluorescent carbon NPs as an ideal cell-imaging probe able to get into cells without any specific surface modification, whereas [PERSON] et al. [18] used polyethylenimine functionalized carbon-based nanodots to both reduce toxicity and increment gene expression of plasmid DNA in carcinoma cells. However, others reported a comparison of the same fluorescent nanodots in fibroblasts and demonstrated their compatibility and use as gene delivery carriers for cancer treatment [19]. On the one hand, a systematic safety evaluation, via acute and subacute toxicities of carbon-based nanodots, reported by [PERSON] et al. [20] demonstrated the lack of toxic effects. In fact, CQDs were reported to be biocompatible NPs in a variety of cell lines upon different experimental conditions. In this sense, a recent report on the effects of CQDs with different surface charges on the viability, proliferation, differentiation, cellular uptake, and stability of CQDs in human umbilical cord-derived mesenchymal stem cells showed that lower positively-charged CQDs maintain a good balance between biocompatibility and cellular uptake in these cells [21]. Likewise, [PERSON] et al. [22] reported an interesting in vitro toxicological study that demonstrated the correlation of toxicity with nanoparticle concentration at different exposition times (24, 48, and 72 h).