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Near-infrared (NIR) light can penetrate biological samples and even tissues without causing sample damage and avoid autofluorescence from biological samples in fluorescence detection. Thus, a luminescence resonance energy transfer (LRET)-based immunoassay that can be excited by NIR irradiation is a promising approach to the analysis of biological samples. Here we demonstrate the use of NIR-to-visible upconversion nanoparticles (UCNPs) as an energy donor, which can emit a visible light upon the NIR irradiation, and gold nanoparticles (An NPs) as an energy acceptor, which can absorb the visible light emitted from the donor, to develop a sandwich-type LRET-based immunoassay for the detection of goat antihuman immunoglobulin G (IgG). Amino-functionalized [NaYF.sub.4]:Yb, Er UCNPs and Au NPs were first prepared and then conjugated with the human IgG and rabbit antigoat IgG, respectively. The NIR-excited fluorescence emission band of human IgG-conjugated [NaYF.sub.4]: Yb, Er UCNPs ([[lambda].sub.max] = 542 nm) partially overlaps with the visible absorption band of the rabbit antigoat IgG-conjugated colloidal Au NPs ([[lambda].sub.max] = 530 nm), satisfying the requirement of spectral overlap between donors and acceptors for LRET. A LRET system was then formed when goat antihuman IgG was added to a mixture of human IgG-modified [NaYF.sub.4]:Yb, Er UCNPs (donor) and rabbit antigoat IgG-modified Au NPs (acceptor). The sandwich-type immunoreactions between the added goat antihuman IgG (primary antibody) and the two different proteins (antigen and secondary antibody on the surface of the donors and acceptors, respectively) cross-bridge the donors and acceptors and thus shorten their spacing, leading to the occurrence of LRET from UCNPs to Au NPs upon NIR irradiation. As a result, the quenching of the NIR-excited fluorescence of the UCNPs is linearly correlated to the concentration of the goat antihuman IgG (in the range of 3-67 [micro]g x [mL.sup.-1]) present in the system, enabling the detection and quantification of the antibody. Such sandwich-type LRET-based approach can reach a very low detection limit of goat antihuman IgG (0.88 [micro]g x [mL.sup.-1]), indicating that this method is applicable for the trace protein detection. This approach is expected to be extended to the detection of other biological molecules once the donor and acceptor nanoparticles are modified by proper molecules that can recognize the target biomolecules. 10.1021/ac901808q

In the present work, CdS quantum dots (QDs) were successfully biosynthesized at room temperature by using saccharomyces cerevisiae yeast as a carrier. Fluorescence emission spectra, ultraviolet-visible (UV/Vis) absorption spectra, and inverted fluorescence microscope images confirmed that saccharomyces cerevisiae can be used to biosynthesize CdS QDs. The as-prepared CdS QDs show the fluorescence emission peak at 443 nm and emit blue-green fluorescence under UV light (with excitation at 365 nm). Transmission electron microscopy (TEM) was applied to characterize the as-prepared CdS QDs and the TEM results showed that the as-prepared CdS QDs had the structure of hexagonal wurtzite. Fluorescence emission spectrum and UV/Vis absorption spectrum were used as the performance indicatiors to study the effects of saccharomyces cerevisiae yeast incubation times, reactant Cd2+ concentrations and reaction times on CdS QDs synthesis. Saccharomyces cerevisiae yeast grown in early stable phase can get the highest fluorescence intensity of CdS QDs when they were co-cultured with 0.5 mmol x L(-1) of Cd2+ with 24 h incubation time. Furthermore, much more CdS QDs can be obtained by changing the culture medium during the synthesis process.

Monodisperse NaYF4 : Yb, Er upconversion fluorescent nanoparticles were firstly synthesized via a co-precipitation method in the presence of diethylenetriamine pentoacetic acid (DTPA). The nanoparticles were characterized by using of X-ray diffraction (XRD), transmission electron microscope (TEM), fluorescence (FL) spectrum, and thermogravimetry-differential scanning calorimetry (TG-DSC) analysis. The as-prepared nanoparticles were uniform, and their size could be controlled in a range of 20 to 120 nm by varying the amount of DTPA. During the precipitation reaction, DTPA molecules could form a complex with the rare earth ions, and then the rare earth ions were released slowly to react with F- ions, which slowed down the speed of the reaction. In addition, DTPA molecules could also be capped on surface of the growing nanoparticles, which prevented the nanoparticles from aggregation. After annealing, the nanoparticles were transformed from cubic phase to hexagonal phase, and their upconversion fluorescence intensity was enhanced remarkably. The synthesis conditions including the amount of chelating agents and temperature for annealing, which showed great influence on the size, phase and upconversion fluorescence intensity of the nanoparticles, were also discussed. It was confirmed by XRD and TG-DSC analysis that the presence of DTPA suppressed the cubic-to-hexagonal phase transition of the nanoparticles. However, while the small nanoparticles were obtained with well control, the annealed crystals synthesized by adding DTPA could still emit strong fluorescence under a low exciting power, which could well fulfill the demand for bio-labeling. These nanoparticles are envisioned to find potential applications in biological detections.