[Homogeneous phase fluorescence assay based on NaYF4 : Er3+, Yb3+ UCNPs as donors].

Upconversion nanoparticles (UCNPs) of small size have great potential in homogeneous assay based on fluorescence resonance energy transfer (FRET). A novel approach of surface ligand-exchange for preparing water-soluble and amido-functionalized NaYF4 : Er3+, Yb3+ UCNPs with 12 nm was developed. The ligand exchange process was confirmed by Fourier transform infrared spectra. Investigations by scan electron microscopy showed no obvious variations in the size and shape of the UCNPs. The circular dichroism characterization demonstrates that the secondary structure of the avidin remains largely intact after the conjugation. Avidin-biotin served as a bridge to make the short enough distance for FRET between the acceptor biotinylated R-phycoerythrin and the donors avidin-conjugated UCNPs. When the free biotins were added into this system, they competitively combined with avidin on the UCNPs surface and impede the FRET to triggered fluorescence changes. According to the relationship between this change and the addition of the amount of biotin, such FRET-based approach can reach a limit of detection in the nanomolar concentration range.

[A new optical switch using upconversion nanoparticles conjugated aptamer].

As new class of fluorescent biolabels, upconversion nanoparticles (UCNPs) were very attractive for use as labels, compared with these traditional downconversion materials. It can eliminate problems associated with autofluorescence and scattered excitation light under near infrared light (NIR) excitation. In the present work, water-soluble NaYF4:Yb3+, Er+ UCNPs were successfully prepared by solvothermal synthesis, using branched polyethylenimine as the surfactants, and a generic design strategy for UCNPs conjugated aptamer based optical switch was presented. In the absence of the target thrombin protein, such a duplex structure brings the UCNPs and TAMRA close proximity of each other, leading to luminescence resonance energy transfer from UCNPs to TAMRA upon near-infrared light irradiation. When target is introduced, the quadruplex conformation of the aptamer should be preferentially stabilized, resulting in the displacement of the TAMRA labelled oligonucleotide conjugate and triggering UCNPs fluorescence intensity to increase and the corresponding TAMRA fluorescence intensity to decrease. Since the aptamer responds to its target molecule by switching on its fluorescent properties, the authors named this an "optical switch".

[Homogeneous immunoassay technology based on near infrared upconversion fluorescence resonance energy transfer].

A FRET based assay utilizing NaYF4 : Yb3+, Tm3+ UCNPs as an energy donor, which can emit intense near infrared (NIR) upconversion emission around 800 nm ranges under illumination with a 980 nm laser, and GNPs as an energy acceptor, which has an surface plasmon absorption maximum at 784 nm, was demonstrated. Their optical properties satisfy the requirement of spectral overlap between donors and acceptors for FRET. A model assay for human IgG was then constructed, in which amino-functionalized NaYF4 : Yb3+, Tm3+ UCNPs and GNPs were first prepared and then conjugated with the antibody (goat antihuman IgG) and antigen (human IgG), respectively. The mutual affinity of the antigen and antibody brought the nanocrystals close enough together to allow the FRET to occur, resulting in a significant quenching of UCNPs upconversion emission at 800 nm. When free human IgG was added to the immunocomplex, it competitively binds to UCNPs-goat antihuman IgG, thereby replacing human IgG-GNPs from the immunocomplex and inhibits the FRET process. As a result, the gradually increasing the NIR emission was observed. The authors associate the fluorescence enhancement effect with the concentration of human IgG. Under our experimental conditions, the detection limit is 5 microg x mL(-1). This approach is expected to be extended to the detection of other biological fields, enabling measurements without background fluorescence interference.

[CdSe/ZnSe quantum dots via a two-phase solution system process solubilization and spectroscopic characterization].

In the present work, the CdSe/ZnSe core/shell quantum dots (QDs) were successfully transferred from organic phase to water phase via a two-phase solution system process by surface coating with amphiphilic polymer. Surface coating with amphiphilic polymer is an effective method, which can form stable soluble QDs in water. However, the conventional polymer coating method is performed in homogeneous phase, and it easily induces the aggregation of the QDs attributing to the long chain of enlace of the polymer. It is thus necessary and meaningful to develop surface coating technique for getting monodisperse coating QDs with amphiphilic polymer. In comparison with previously reported coating method, the authors' experiment process is performed in two-phase solution system, and can effectively reduces the possibility of aggregation of the QDs. The resulting hydrophilic CdSe/ZnSe core/shell QDs have long-term stability in water, and high quantum yield. The polymer coating process was affirmed by various characterizations. Fourier transform infrared spectra suggest that the octylamine modified polymer was successfully coated on the surface of the CdSe/ZnSe QDs. The transmission electron microscopy suggests that the size and shape of the QDs showed no obvious change before and after the coating process. Dynamic light scattering results indicate that the hydrophilic QDs exhibit narrow hydrodynamic size distribution with the mean hydrodynamic diameters of about 19.7 nm. The luminescence properties of the QDs were investigated with photoluminescence spectra and ultraviolet-visible absorption spectra. This polymer coating process has less effect on luminescence capability. The quantum yield decreased from 43% to 30%. Further, in order to confirm that the polymer capped QDs is biocompatible, the QDs were used for specific detection of the human IgG with fluorescence mapping. The specific molecular recognition capacity of goat anti-human IgG-modified QDs confirms that the polymer coated QDs have compatible functional chemical groups for bioconjugation and are suitable for biological applications.

[Preparation, characterization and specific biological labeling of silica coated upconversion fluorescent nanocrystals].

The authors synthesized a kind of upconversion nanocrystals NaYF4:Yb3+, Er3+ via the hydrothermal assisted homogeneous precipitation method, and then the nanocrystal was coated with silica. The SEM image demonstrated that the as-prepared samples were uniform in size distribution with ca. 25 nm before and ca. 250 nm after silica coating, respectively. The upconversion spectra and photoluminescence lifetime measurement showed that the silica shell had hardly effect on the properties of fluorescence of the NaYF4:Yb3+, Er3+ nanocrystals. At the same time, the naked eye-visible green upconversion fluorescence pattern was acquired from the as-prepared upconversion nanoparticles in the PBS buffer (2 wt%) excited by 980 nm laser at room temperature. These water-soluble nanoparticles were linked to the antibodies using the coupling reagents glutaraldehyde. The circular dichroism (CD) spectra of antibody and upconversion nanoparticles-antibody conjugates were very similar to each other, indicating that the secondary structure of antibody remained largely intact after the conjugation. Finally, antigen-antibody recognition reaction was performed on the surface of a silicon slide. The immunofluorescence in vitro indicated that the upconversion nanoparticles-antibody bioconjugates had excellent species-specific detection ability with hardly non-specific binding. Based on the present results, it is anticipated that the silica-coated upconversion nanoparticles are suitable for use as biolabeling materials.

[Effect of hole transporting materials on photoluminescence of CdSe core/shell quantum dots].

Photoluminescence quenching of colloidal CdSe core/shell quantum dots in the presence of hole transporting materials was studied by means of steady state and time resolved photoluminescence spectroscopy. With increasing hole transporting materials concentration in the CdSe core/shell quantum dot solution, the photoluminescence intensity and lifetime decreased gradually. The photoluminescence quenching of CdSe/ZnSe quantum dots with adding hole transporting material N,N'-bis(1-naphthyl)-N, N'-diphenyl-1,1 '-biphenyl-4, 4'-diamine (NPB) is more efficient than N,N'-diphenyl-N, N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD). And compared with CdSe core/shell quantum dots with ZnSe shell, the ZnS shell is an effective one on the surface of CdSe quantum dots for reducing photoluminescence quenching efficiency when interacting with hole transporting material TPD. Based on the analysis, there are two pathways in the photoluminescence quenching process: static quenching and dynamic quenching. The static quenching results from the decrease in the number of the emitting centers, and the dynamic quenching is caused by the hole transfer from quantum dots to hole transporting materials molecules. The efficiency of the photoluminescence quenching in CdSe core/shell quantum dots is strongly dependent on the structure of the shells and the HOMO levels of the hole transporting materials. The results are important for understanding the nature of quantum dots surface and the interaction of quantum dots and hole transporting materials.

[Study of applying fluorescence spectrum imaging to quantitative assay of proteins in bio-chip].

In the present work, the amount and the activity of the goat anti-human IgG, to bind the human IgG labelled with fluorescein isothiocyanate (FITC), immobilized on silicon surfaces modified with APTES and APTES-Glu, respectively, were studied using the fluorescence spectrum imaging (FSI), the results of which were compared with that of ellipsometry. It is shown that the amount of the human IgG labeled with FITC on APTES-Glu measured using FSI is 2.8 times higher than that on APTES, which is nearly coincident with the 2.2 times obtained using ellipsometry, showing that the activity of the goat anti-human IgG on APTES-Glu is higher than that on APTES. It is reasoned that the FSI is used in the fluorescence immunoassay the for measurement of quasi-quantification or quantification.

[Surface enhanced Raman spectroscopic study on the gold-labeled protein self-assembled surface].

The human IgG molecules were labeled with 13 nm gold nanoparticles and the complex of the gold-labeled human IgG molecules was immobilized on a silicon surface modified by 3-aminopropyltriethoxysilane and glutaraldehyde. The method increases not only the tightness but also the surface coverage for immobilization of the complex and retains protein configuration well on the silicon surface. The self-assembled complex surface was observed by AFM. The complex aggregated on the silicon surface and the "island" monolayer of the complex was obtained. Meanwhile the SERS spectrum of the complex self-assembled "island" monolayer on silicon surface was presented. In the present paper, the gold labeled human IgG molecules were self-assembled on the silicon surface, SERS spectra of protein were obtained and as SERS active substrates were provided for the study of the protein molecules.