Preparation, Characterization and Toxicological Impacts of Monodisperse Quantum Dot Nanocolloids in Aqueous Solution

Abstract

Recently, mass production and extensive use of novel engineered nanomaterials (e.g., carbon nanotubes, fullerene, nano-TiO2, nano-Au and quantum dots) raise increasing concerns about their potential harmful effects on our environment and human health. Therefore, for better understanding of their environmental and biological fate upon accidental release during their manufacturing and transport processes, increasing number of studies on the toxicities of engineered nanomaterials were conducted during last few years. However, regarding the toxicity of engineered nanomaterials, contradictory results were frequently found in the current literature, probably due to the widely varying physicochemical properties of engineered nanomaterials and inconsistency of the dosage/exposure conditions. Among various engineered nanoparticles, semiconductor quantum dots (QDs) have received significant attention as a new type of fluorophore for the biological and medical imaging. However, they also induced increasing concern as a potential pollutant, due to its unknown toxicity as a nanometer-scale material as well as the toxic components (i.e., Cd, Zn, Se and etc). But, their colloidal stability, photochemical property, dissolution behavior and overall cytotoxic impact of these QD colloids have not been systematically studied and completely understood yet. In this study, using various analytical tools, we have prepared and characterized water-soluble monodisperse QD nanocolloids with well-defined core and hydrodynamic size, functional groups at the interface, impurities, nanoparticle concentration and impurities in aqueous solution. Then, toxicological impacts of QD nanocolloids collected at different stage of preparation (i.e., different degree of impurity) were tested on gram negative bacterial cells (i.e., Escherichia coli). Preparation, surface modification and characterization of QD nanoparticles. From XRD analysis of QD nanoparticles, we confirmed that the QDs synthesized using SiPOP mothod predominantly have the wurtzite CdSe crystal structure (see supporting information, Figure S1). Additionally, absences of CdO or ZnO peaks in the XRD results indicated that negligible degree of oxidations occurred during the preparation of this QD nanoparticle. Furthermore, application of schrrer equation to the width of CdSe (110) peak resulted in the estimated value of CdSe core size, which is 3.25 nm (see supporting information, Figure S2), while estimation of CdSe core size from the position (559 nm) of the first excitonic absorption peak resulted in very similar value (3.24 nm). To make this QD soluble in aqueous solution, ligand exchange reaction of TOPO capped QD (i.e., QD) was performed using MAA (mercaptoacetic acid) and aqueous colloid of carboxylic acid group terminated QD was prepared. Then, several cycles of wash-out procedure were conducted to remove residual MAA from aqueous solution. To assure the monodispersity of this QDs in aqueous solution, hydrodynamic sizes of QDs were also measured by dynamic light scattering methods (QD = 4.56 nm, see Fig. 1) and confirmed that the individual QD particles are well dispersed in solution with least degree of aggregation. Concentrations of QDs in aqueous solutions were determined from the intensities of the first excitonic absorption peak and extinction coefficients based on the method reported by Yu et al. and presented in the units of moles of QD per liter (i.e., [QD]). Fourier Transform Infrared (FT-IR) spectroscopy was used to monitor changes in surface modifying groups of QDs as well as impurities included during ligand exchange process. During surface modification process, the replacement of TOPO and changes in adsorbed MAA molecules were monitored by FT-IR and corresponding spectra were presented in Fig. S3 (In supporting information). The IR spectrum shown in Fig. S3(e) shows spectrum collected from QD solutions after several cycles of wash-out process and showed quite distinct spectral features with that of Fig. S3(d). First of all, absence of thiol peaks around 2570 cm−1 indicate that there is no free MAA with thiol groups and most of the MAA are already lost their hydrogen and