MTT Detection of the Toxicity on CdTe Quantum Dots

Objective: The quantum dots are the useful materials in microelectronics and biomedical research. However its toxicity has to be considered. We studied the effect of cell inhibition with ZnS core quantum dots and CdTe quantum dots modified with Thioglycolic acid or Cysteine functional group (TGA-CdTe, TGA-CdTe/ZnS, Cys-CdTe, Cys-CdTe/ZnS) on Caco-2 cell proliferation. Methods: We studied the effect of cell inhibition with ZnS core QDs and CdTe QDs modified with functional group on Caco-2 cell proliferation by MTT assay at 0, 12.5, 25, 50, 100 μg/ml and 6, 24, 48 h. Result: Our results showed that all QDs have inhibited cell proliferation and reached maximum 79.21%. The inhibition rate of Cys-modified QDs increased with the increase of concentration and reached maximum 66.72%. The inhibition rate of TGA-modified QDs increased with the increase of time. The ratios of Cys-modified to TGA-modified were less than 1 at all concentrations and three exposure times (P ≤ 0.01). The average ratios of Cys-CdTe/ZnS to Cys-CdTe reached 1.11 only for 48 h (P ≤ 0.05). The ratios of TGA-CdTe/ZnS to TGA-CdTe were closed to 1 at all concentrations and exposure times. Conclusion: The regularity of QDs modified with functional group is that inhibition of TGA-modified higher than Cys-modified. Inhibition exhibited dose-dependent for Cys-modified while exhibited time-dependent for TGA-modified. The regularity of CdTe-QDs with ZnS or not is that the inhibition of Cys-CdTe/ZnS was higher than Cys-CdTe while TGA-CdTe/ZnS and TGA-CdTe were consistent. concentrations and three exposure times. The facilitates the further understanding of the inhibition effect on cell proliferation of CdTe QDs with core or and provides useful information for the


Introduction
QDs possess unique optical and electronic properties including tunable emission wavelength, broadband absorption spectrum, and photostability that make them useful materials in microelectronics and biomedical research [1]. QDs toxicity and surface coatings to render them biologically compatible have been intensively researched (Rizvi et al., 2012). QDs were appealing alternatives to conventional fluorophores due to their superior optical properties and have the potential to meet some of these outstanding challenges in biotechnology [2].
Cadmium QDs (Cd-QDs) including CdTe-QDs and CdSe-QDs have shown great potential for use as fluorescent tags in therapeutic targeting and in medical and molecular imaging but its toxicity has to be considered [1]. QDs toxicity limits their biomedical applicatio, although they can now be aqueously synthesized. QDs toxicity was multifactorial and was determined by their physiochemical properties, including composition of the core, size, surface charge, concentration, surface chemistry, bioactivity, oxidative, photolytic and mechanical stability, as well as their environmental interactions [3] [4]. Cd-QDs toxicity has been intensively researched. The toxicity of Cd-based QDs has been proposed to be associated with the oxidation reaction of the metal core. The reaction generates reactive oxygen species (ROS) and Cd 2+ , which were toxic to cells of animals, plants and microbes [5]. Another research found that the physicochemical characteristics of CdTe core QDs influenced subcellular localization and cytotoxicity; quantified as generation of ROS [6]. Cadmium (Cd), which was capable of inducing known toxicities in humans including hepatic, renal, neurologic, and/or genetic toxicities, was the most abundant component of QDs [7]. A research found that CdTe-QDs with a smaller size showed greater hematopoiesis toxicity and CdTe-QDs effects on immune system [5] [8]. CdTe-QDs can induce cytotoxicity, autophagy, oxidative stress, ER stress, chromatin condensation, reducing cell viability and apoptosis in cells [9]. Based on the above research, we intend to study the effect of cell inhibition with CdTe core QDs and CdTe QDs modified with functional group in vitro experiments. Our results provide new insights into Cd-QDs toxicology and provide evidence for the future application.

Preparation and Characteristics of QDs
The CdTe-QDs used in the study were synthesized by Janus New-Materials The emission wavelength of the resulting solution was 450 nm. Emission wavelength was 537 ± 5 nm. Maximum absorption peaks was 500 nm.

Cell Treatment
Confluence of about 80% of the logarithmic cells was collected. Cells were seeded in 96-well plates at 106 cells/well in 100 μl of medium and incubated at 5%

Statistical Analysis
The experiments were performed in triplicate. Statistical analyses were carried out using the statistical software SPSS 20.0. Pearson's correlation coefficient (r) was calculated. The difference between the experimental groups and the control group was analyzed by one-way analysis of variance (ANOVA) followed by Dunnett's t-test. The statistically significant difference was considered to * P ≤ 0.05 compared to control cells and ** P ≤ 0.01 compared to control cells.         Figure 4(b)). With the concentration increases, the ratio increased slowly for 24 h. The ratios reached lowest (0.025 for Figure 4(a), 0.18 for Figure   4(b)) at concentration of 12.5 μg/ml for 6 h. The ratios were closed to 0.85 at maximum concentration (100 μg/ml) at three exposure times. Figure 4(b) exhibited time-dependent at concentration of 12.5 μg/ml (r = 0.951, P ≤ 0.05). Table 1

Discussion
Our result showed that four QDs (CdTe-QDs, TGA and Cys-modified CdTe/ZnS and CdTe QDs) have inhibited cell proliferation. A study reported CdTe QDs have inhibited to HepG2 and HELF cells proliferation and have dose dependent  [10], which supports our observations.
It has reported that cytotoxicity of QDs depend on their surface modification [11]. Our result showed that the inhibition rate of TGA-modified QDs was higher than Cys-modified QDs. For TGA-modified QDs, the inhibition on cell proliferation exhibited obviously time-dependent and no effected by design-concentration range. The inhibition was almost consistent in all concentrations and had already reached this level at the lowest concentration. At a low concentration, TGA-CdTe QDs also showed the cytotoxic effects to HepG2 cells [12]. Contrary to TGA, Cys-modified QDs of the inhibition exhibited obviously dose-dependent and no obviously time-dependent. The inhibition was similar at 24 h and 48 h. The inhibition was almost no effect at the designed-lowest concentration and shortest time. At a low concentration and short time, Cys-CdTe QDs also had no apparent effect in cell metabolic activity and apoptosis [13].
The chemical composition of the QDs core is important factors affecting QDs toxicity [14]. For Cys-modified QDs, the inhibition of CdTe/ZnS QDs was higher than CdTe QDs at the higher concentration in the maximum time despite they both inhibited to cell proliferation. For TGA-modified QDs, the inhibition of CdTe/ZnS QDs and CdTe QDs were almost consistent at all concentrations and three exposure times.

Conclusion
We found that four CdTe-QDs, modified with TGA or Cys, with ZnS core or not, all have inhibited cell proliferation. The inhibition of TGA-modified QDs was higher than Cys-modified QDs. Inhibition exhibited obviously time-dependent for TGA-modified QDs while exhibited obviously dose-dependent for Cys-modified QDs. The inhibition of CdTe/ZnS QDs was higher than CdTe QDs for Cys-modified QDs. The inhibition of CdTe/ZnS QDs was almost consistent at all concentrations and three exposure times. The study facilitates the further understanding of the inhibition effect on cell proliferation of CdTe QDs with core or functional group and provides useful information for the use of CdTe QDs.