Interactions between CdTe quantum dots and DNA revealed by capillary electrophoresis with laser-induced fluorescence detection

Activity of CdTe Quantum-Dot-Tagged Superoxide Dismutase and Its Analysis in Capillary Electrophoresis

Quantum dots (QDs) have a broad range of applications in cell biolabeling, cancer treatment, metastasis imaging, and therapeutic drug monitoring. Despite their wide use, relatively little is known about their influence on other molecules. Interactions between QDs and proteins can influence the properties of both nanoparticles and proteins. The effect of mercaptosuccinic acid-capped CdTe QDs on intercellular copper–zinc superoxide dismutase (SOD1)—one of the main enzymatic antioxidants—was investigated. Incubation of SOD1 with QDs caused an increase in SOD1 activity, unlike in the case of CdCl₂, which inhibited SOD1. Moreover, this effect on SOD1 increased with the size and potential of QDs, although the effect became clearly visible in higher concentrations of QDs. The intensity of QD-SOD1 fluorescence, analyzed with the use of capillary electrophoresis with laser-induced fluorescence detection, was dependent on SOD1 concentration. In the case of green QDs, the fluorescence signal decreased with increasing SOD1 concentration. In contrast, the signal strength for Y-QD complexes was not dependent on SOD1 dilutions. The migration time of QDs and their complexes with SOD1 varied depending on the type of QD used. The migration time of G-QD complexes with SOD1 differed slightly. However, in the case of Y-QD complexes with SOD1, the differences in the migration time were not dependent on SOD concentration. This research shows that QDs interact with SOD1 and the influence of QDs on SOD activity is size-dependent. With this knowledge, one might be able to control the activation/inhibition of specific enzymes, such as SOD1.

Polyacrylamide gel electrophoresis of semiconductor quantum dots and their bioconjugates: materials characterization and physical insights from spectrofluorimetric detection

Colloidal semiconductor quantum dot (QD) nanocrystals have ideal fluorescence properties for bioanalysis and bioimaging, but these materials must be functionalized with an inorganic shell, organic ligand or polymer coating, and conjugated with biomolecules to be useful in such applications. Several different analytical techniques are used to characterize QDs and their multiple layers of functionalization. Here, we revisit poly(acrylamide) gel electrophoresis (PAGE), which has been scarcely used for the characterization of QDs and their bioconjugates in deference to the routine use of agarose gel electrophoresis. We implemented PAGE in a novel "stubby" capillary format with spectrofluorimetric detection, the combination of which enabled more rapid and more detailed characterization of QDs than was possible with both poly(acrylamide) and agarose slab gels. Correlations between the peak photoluminescence (PL) emission wavelength and electropherogram peaks, especially when combined with Ferguson analysis, provided new and significant insight into the key factors that determine the electrophoretic mobility of QDs, and helped to resolve heterogeneity and sub-populations in ensembles of QDs. The method was useful for characterization of the inorganic core/shell nanocrystals, their organic ligand and polymer coatings, and their final bioconjugates, the latter of which were in the form of peptide and protein conjugates. With further development and optimization, we anticipate that capillary PAGE with spectrofluorimetric detection will become a valuable addition to the toolbox of characterization techniques suitable for QDs, their bioconjugates, and other nanoparticle materials as well.

Short-term assessment of cadmium toxicity and uptake from different types of Cd-based Quantum Dots in the model plant Allium cepa L

We report on the toxicity and bioaccumulation of three different types of Cd-based quantum dots (QDs), dispersed in aqueous medium, for a model plant Allium cepa L. It is believed that encapsulation of nanoparticles should reduce their toxicity and increase their stability in different environments; in this work we studied how QD encapsulation affects their phytotoxicity. Core, core/shell, and core/shell/shell QDs (CdTe, CdTe/ZnS, and CdTe/CdS/ZnS QDs capped by 2-mercaptopropionic acid) were tested and CdCl₂ was used as a positive control. After 24-h and 72-h exposure, total Cd content (M_Cd) and bioaccumulation factors (BAFs) were determined in all parts of A. cepa plants (roots, bulb, shoot), and the total length of the root system was monitored as a toxicity end-point. Measurements of total Cd content versus free Cd²⁺ content (with Differential Pulse Voltammetry, DPV) in exposure media showed differences in chemical stability of the three QD types. Correspondingly, selected QDs showed different toxicity for A. cepa and different Cd bioaccumulation patterns. CdTe QDs were the most toxic; their effect was similar to CdCl₂ due to the release of free Cd²⁺, which was confirmed by the DPV measurements. Plants exposed to CdTe QDs also bioaccumulated the most Cd among all QD exposure groups. CdTe/ZnS QDs showed no toxicity and very low bioaccumulation of Cd in A. cepa; the main source of measured Cd in the plants were QDs adsorbed on their roots, which was confirmed by fluorescence microscopy. On the contrary, CdTe/CdS/ZnS QD toxicity and bioaccumulation patterns were similar to those of CdTe QDs and pointed to unstable CdS/ZnS shells.

Study on the Interaction of the CpG Alternating DNA with CdTe Quantum Dots

A novel sensitive method for detection of DNA methylation was developed with thioglycollic acid (TGA)-capped CdTe quantum dots (QDs) as fluorescence probes. Recognition of methylated DNA sites would be useful strategy due to the important roles of methylation in disease occurrence and developmental processes. DNA methylation occurs most often at cytosine-guanine sites (CpG dinucleotides) of gene promoters. The QDs significantly interacted with hybridized unmethylated and methylated DNA. The interaction of CpG rich methylated and unmethylated DNA hybrid with quantum dots as an optical probe has been investigated by fluorescence spectroscopy and electrophoresis assay. The fluorescence intensity of QDs was highly dependent to unmethylated and methylated DNA. Specific site of CpG islands of Adenomatous polyposis coli (APC), a well-studied tumor suppressor gene, was used as the detection target. Under optimum conditions, upon the addition of unmethylated dsDNA, the fluorescence intensity increased in linear range from 1.0 × 10^- 10 to 1.0 × 10^- 6M with detection limit of 6.2 × 10^- 11 M and on the other hand, the intensity of QDs showed no changes with addition of methylated dsDNA. We also demonstrated that the unmethylated and methylated DNA and QDs complexes showed different mobility in electrophoresis assay. This easy and reliable method could distinguish between methylated and unmethylated DNA sequences.

In-capillary self-assembly study of quantum dots and protein using fluorescence coupled capillary electrophoresis

As a vast number of novel materials in particular inorganic nanoparticles have been invented and introduced to all aspects of life, public concerns about how they might affect our ecosystem and human life continue to arise. Such incertitude roots at a fundamental question of how inorganic nanoparticles self-assemble with biomolecules in solution. Various techniques have been developed to probe the interaction between particles and biomolecules, but very few if any can provide advantages of both rapid and convenient. Herein, we report a systematic investigation on quantum dots (QDs) and protein self-assembly inside a capillary. QDs and protein were injected to a capillary one after another. They were mixed inside the capillary when a high voltage was applied. Online separation and detection were then achieved. This new method can also be used to study the self-assembly kinetics of QDs and protein using the Hill equation, the KD value for the self-assembly of QDs and protein was calculated to be 8.8 μM. The obtained results were compared with the previous out of-capillary method and confirmed the effectiveness of the present method.