Yong-Qiang Li

Distance-Dependent Metal-Enhanced Quantum Dots Fluorescence Analysis in Solution by Capillary Electrophoresis and Its Application to DNA Detection

Here the distance dependence of metal-enhanced quantum dots (QDs) fluorescence in solution is studied systematically by capillary electrophoresis (CE). Complementary DNA oligonucleotides-modified CdSe/ZnS QDs and gold nanoparticles (Au NPs) were connected together in solution by the hybridization of complementary oligonucleotides, and a model system (QD-Au) for the study of metal-enhanced QDs fluorescence was constructed, in which the distance between the QDs and Au NPs was controlled by adjusting the base number of the oligonucleotide. In our CE experiments, the metal-enhanced fluorescence of the QDs solution was only observed when the distance between the QDs and Au NPs ranged from 6.8 to 18.7 nm, and the maximum enhancement by a factor of 2.3 was achieved at 11.9 nm. Furthermore, a minimum of 19.6 pg of target DNA was identified in CE based on its specific competition with the QD-DNA in the QD-Au system. This work provides an important reference for future study of metal-enhanced QDs fluorescence in solution and exhibits potential capability in nucleic acid hybridization analysis and high-sensitivity DNA detection.

High-sensitivity quantum dot-based fluorescence resonance energy transfer bioanalysis by capillary electrophoresis

Here a new method for high-sensitivity quantum dot (QD)-based fluorescence resonance energy transfer (FRET) bioanalysis was developed. In this method, capillary electrophoresis (CE) with fluorescence detection was applied. The FRET system consisted of water-soluble 532-nm emitting CdTe QDs donor and 632-nm emitting CdSe/ZnS QDs acceptor which were covalently conjugated with mouse IgG and goat anti-mouse IgG, respectively. The bio-affinity between antigen and antibody brought two kinds of QDs close enough to make the FRET happen between them. In the CE experiments, highly efficient separation of donor-acceptor immunocomplexes was obtained, and the process of FRET was monitored. Results showed that FRET efficiency obtained by CE (38.56-69.58%) improved substantially in comparison with that obtained by ensemble measurement (12.77-52.37%). The high efficient separation of donor-acceptor immunocomplexes and the possible conformation change of antigen and antibody, contributes to the lower analysis uncertainty (variance) and higher FRET efficiency obtained in CE and consequentially, this makes the analysis of FRET more sensitive. This novel CE-based technique can be easily extended to other FRET system based on QDs and may have potential application in the study of biomolecule conformation change.

Simultaneous detection of dual single-base mutations by capillary electrophoresis using quantum dot-molecular beacon probe

Here a novel capillary electrophoresis (CE) for simultaneous detection of dual single-base mutations using quantum dot-molecular beacon (QD-MB) probe is described. Two QD-MB probes were designed using 585 and 650-nm emitting CdTe QDs which were covalently conjugated to MBs with different DNA oligonucleotide sequences by amide linkage and streptavidin-biotin binding, respectively. The hybridizations of QD-MB probes with different DNA targets were then monitored by CE, and results indicated that the two QD-MB probes specifically hybridized with their complementary DNA sequences, respectively. Target DNA identification was observed to have a high sensitivity of 16.2 pg in CE. Furthermore, the simultaneous detection of dual single-base mutations in a given DNA oligonucleotide was successfully achieved in CE using above two QD-MB probes. This novel CE-assisted QD-MB biosensor offers a promising approach for simultaneous detection of multiple single-base mutations, and exhibits potential capability in the single nucleotide polymorphism (SNP) analysis and high-sensitivity DNA detection.

Influence of quantum dot's quantum yield to chemiluminescent resonance energy transfer

The resonance energy transfer between chemiluminescence donor (luminol-H2O2 system) and quantum dots (QDs, emission at 593 nm) acceptors (CRET) was investigated. The resonance energy transfer efficiencies were compared while the oil soluble QDs, water soluble QDs (modified with thioglycolate) and QD-HRP conjugates were used as acceptor. The fluorescence of QD can be observed in the three cases, indicating that the CRET occurs while QD acceptor in different status was used. The highest CRET efficiency (10.7%) was obtained in the case of oil soluble QDs, and the lowest CRET efficiency (2.7%) was observed in the QD-HRP conjugates case. This result is coincident with the quantum yields of the acceptors (18.3% and 0.4%). The same result was observed in another similar set of experiment, in which the amphiphilic polymer modified QDs (emission at 675 nm) were used. It suggests that the quantum yield of the QD in different status is the crucial factor to the CRET efficiency. Furthermore, the multiplexed CRET between luminol donor and three different sizes QD acceptors was observed simultaneously. This work will offer useful support for improving the CRET studies based on quantum dots.

A highly efficient capillary electrophoresis-based method for size determination of water-soluble CdSe/ZnS core–shell quantum dots

This paper describes a highly efficient method for size determination of water-soluble CdSe/ZnS core-shell quantum dots (QDs) by capillary electrophoresis (CE) using polymer additive as sieving medium. The influence of some factors, such as kinds and concentrations of the sieving medium, pH, concentrations of the background electrolyte (BGE) and applied voltage, on the separation of QDs was investigated. Under the optimal separation conditions, four different sized QDs were successfully separated, and the relative standard deviation (RSD) of the migration times for these QDs was below 1.013%. In addition, an equation was fit by taking into account the correlation existing between the electrophoretic mobilities and the sizes of a set of QDs. The feasibility of this equation to measure the sizes of other QDs was confirmed by comparison with the sizes obtained by transmission electron microscopy (TEM) experiment. This work offers a novel method for size determination of QDs, and provides an important reference on the study of QDs based on CE.

Modification of CdTe quantum dots as temperature-insensitive bioprobes

CdTe quantum dots (QDs) were synthesized in aqueous solution with 3-mercaptopropionic acid (MPA) as the stabilizer. The photoluminescence (PL) of CdTe QDs (3.5nm) is found to be temperature-dependent: as the temperature arising from 278K to 323K, the PL intensity declines to 50.2% of its original and PL emission peak shows obvious red-shift ( approximately 7nm). After modification of the QDs surface with denatured ovalbumin, the PL is more temperature-insensitive than before. The PL intensity retains more than 70% of its original and the emission peak shows less red-shift ( approximately 2nm). Moreover, it is found that the PL intensity and wavelength of denatured ovalbumin coated CdTe QDs are reversible during heating (323K)-cooling (278K) cycles. All the studies provide an important theoretical basis for searching temperature-insensitive bioprobes.

Special method to prepare quantum dot probes with reduced cytotoxicity and increased optical property.

Quantum dots (QDs) are widely used in the life sciences because of their novel physicochemical properties. However, the cytotoxity of these nonoparticles have attracted great attention recently because this has not been well resolved. Four probes were synthesized by chemical coupling and protein denaturation with CdSeZnS, CdTe QDs, and transferrin. Sodium dodecyl sulfate polyacrylamide gel electrophoresis and capillary electrophoresis were used to verify the conjugation of these luminescent probes. The cytotoxicity of these four luminescent probes and the original QDs were evaluated in HeLa cells. The results showed that over 92% of HeLa cells were still alive after being exposed to 3.2-microM CdSeZnS QDs capped with denatured transferrin for 72 h. Furthermore, while the probe preparation was very simple, the photoluminescence quantum yield of this probe was 7% higher than the original CdSeZnS QDs. This provides a new way for exploiting QD probes with low cytotoxicity, which will expand applications of nanocomposite assembly in biolabeling and imaging.

Characterization of CdTe/CdSe quantum dots-transferrin fluorescent probes for cellular labeling

In this paper, we prepared three types of transferrin-quantum dots conjugates (QDs-Tf) using three different methods (electrostatic interaction, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) coupling, denatured transferrin (dTf) coating). Fluorescence emission spectra, surface characteristics, zeta potentials of quantum dots (QDs) and QDs-Tf fluorescent probes were characterized by spectrophotometer, capillary electrophoresis, and dynamic light scattering. Fluorescent imaging of HeLa cells was also performed by QDs and QDs-Tf fluorescent probes. It was found that the fluorescence imaging performances of QDs-Tf probes prepared by electrostatic interaction and EDC coupling were better compared with the one prepared by dTf coating. Then a real-time single cell detection system was established to quantitatively evaluate cell labeling effects of QDs-Tf fluorescent probes. It was found that for cell labeling efficiency, the proportion of cells labeled by quantum dot probes to a group of cells, QDs-Tf probe prepared by EDC coupling showed the highest labeling efficiency (85.55±3.88%), followed by electrostatic interaction (78.86±9.57%), and dTf coating showed the lowest (40.09±10.2%). This efficiency order was confirmed by flow cytometry results. This study demonstrated the relationship between conjugation methods and the resultant QDs-Tf probes and provided a foundation for choosing appropriate QDs-Tf probes in cell labeling.

Real-time observation of the effect of iron on receptor-mediated endocytosis of transferrin conjugated with quantum dots

The optical properties of antiphotobleaching and the advantage of long-term fluorescence observation of quantum dots are fully adopted to study the effects of iron on the endocytosis of transferrin. Quantum dots are labeled for transferrin and endocytosis of transferrin in HeLa cells is observed under the normal state, iron overloading, and an iron-deficient state. In these three states, the fluorescence undergoes a gradual process of first dark, then light, and finally dark, indicating the endocytosis of transferrin. The fluorescence intensity analysis shows that a platform emerges when fluorescence changes to a certain degree in the three states. Experienced a same period of time after platform, the fluorescence strength of cells in the normal state is 1.2 times the first value, and the iron-deficiency state is 1.4 times, but the iron overloading state was 0.85 times. We also find that the average fluorescence intensity in cells detected by the spectrophotometer in the iron-deficiency state is almost 7 times than that in a high iron state. All this proves that iron overloading would slow the process, but iron deficiency would accelerate endocytosis. We advance a direct observational method that may contribute to further study of the relationship of iron and transferrin.