Surface-Enhanced Fluorescence from Fluorophore-Assembled Monolayers by Using Ag@SiO2 Nanoparticles

Silicon-assisted surface enhanced fluorescence toward improved assay performances

A novel scheme of silicon-assisted surface enhanced fluorescence (SEF) is presented for SEF-based assays, where the blank signal suppression and the fluorescence signal enhancement is combined. The P-doped, (100) oriented silicon substrate is used to quench the fluorescence of Rose Bengal (RB) molecules attached to it, resulting in an effectively suppressed background signal, which is useful for a lower limit of detection (LOD). When a proper quantity of silver nanoparticles (AgNPs) is deposited on the RB-attached silicon substrate, a significant fluorescence enhancement of up to around 290 fold is obtained, which helps to improve the sensitivity in fluorescence-based assays. Besides, conventional gold nanoparticles (AuNPs) have also been demonstrated to exhibit excellent SEF effect using the presented scheme, providing improved stability and biocompatibility. The mechanism of the observed SEF effect has been investigated, and both the decreased apparent quantum yield and the silicon-induced electric field redistribution are considered to play important roles. The experimental results suggest that the presented scheme holds great potential in the SEF-based assays aiming at higher sensitivity and lower LOD.

A straightforward and sensitive “ON–OFF” fluorescence immunoassay based on silicon-assisted surface enhanced fluorescence

A straightforward immunoassay based on silicon-assisted surface enhanced fluorescence (SEF) has been demonstrated using a silicon-based fluorescent immune substrate and silver-antibody nanoconjugate (SANC). The P-doped, (100) oriented silicon wafers are used for both fluorophore attachment and antigen immobilization. The silicon substrate offers a very low blank signal in the “OFF” state, due to its fluorescence quenching effect. In the detection process, the capture of the SANCs by the surface-immobilized antigen leads to an effectively enhanced fluorescence to produce an “ON” state. The analytical performance of the presented scheme has been investigated and a limit of detection of 31.4 pg mL⁻¹ has been obtained. Besides the broadened application range compared with the conventional immunoassays, the presented scheme is straightforward, cost effective and sensitive, and is hence expected to find widespread applications in immunoassays as well as other fluorescence-based assays.

A straightforward immunoassay based on silicon-assisted surface enhanced fluorescence (SEF) has been demonstrated using a silicon-based fluorescent immune substrate and silver-antibody nanoconjugate (SANC).

Fluorescence enhancement of europium complexes by core-shell Ag@SiO₂ nanoparticles

Three kinds of core-shell Ag@SiO2 nanoparticles with shell thickness of around 10, 15, and 25 nm, respectively, have been prepared by modified Stöber method and used for fluorescence enhancement. Six kinds of europium complexes with halobenzoic acid have been synthesized. Elemental analysis and lanthanide coordination titration show that the complexes have the compositions of Eu(p-XBA)3·H2O and Eu(o-XBA)3·2H2O (X=F, Cl, Br). The fluorescence spectra investigation indicates that the introduction of Ag@SiO2 nanoparticles into the europium complexes' solution can significantly enhance the fluorescence intensities of the complexes. The sequence of enhancement factors for halobenzoic acid complexes with different halogen atoms is F<Cl<Br, and the fluorescence enhancement factors increase as the excitation wavelength of complexes increase. When the thickness of the SiO2 shell is 25 nm, the fluorescence intensity of the europium complexes can reach a maximum enhancement factor of 5.1. The fluorescence enhancement mechanism may be the metal-enhanced fluorescence resulting from surface plasmon resonance of nanoparticles. And the nanoparticles near the complexes can effectively prevent complexes from the interaction with the solvent molecules, leading to a decrease of nonradiative energy transfer and the suppression of luminescence quench.

Plasmon enhanced fluorescence of a bisphthalonitrile-based dye via a dopamine mediated interfacial crosslinking reaction on silver nanoparticles

Dopamine modified silver nanoparticles have been employed to enhance the fluorescence of a bisphthalonitrile end-capping phenolphthalein dye in solution via plasmon enhanced fluorescence methodology.

Dual labeled Ag@SiO₂ core-shell nanoparticle based optical immunosensor for sensitive detection of E. coli

An optical nanobiosensor is presented using a fluorescent dye and anti-E. coli McAb anchored Ag@Silica core shell nanoparticles, for rapid and sensitive Escherichia coli detection in environmental samples. The synthesized dual labeled core shell (DLCS) nanoparticle shows intense fluorescence at 620 nm in solution, having a narrow emission with full width at half maxima (FWHM) of 10 nm, as a prerequisite to develop a sensitive detection platform for various biosensing applications. The specific E. coli was captured using an anti-E. coli antibody functionalized quartz glass, followed by a treatment with DLCS, where the photoluminescence spectroscopy was used to detect the target pathogen. The fabrication of the quartz glass based optical-immunosensor was monitored, and the results show changes in the photoluminescent patterns, which substantiate that varied species were immobilized on the surface of the antibody modified quartz glass. Consequently, the optical immunosensor demonstrated specificity and improved sensitivity, as compared to the customary methods, and was able to detect as low as 5CFU/mL. The developed DLCS based optical immunosensor was evaluated with environmental water samples, which showed acceptable precision, reproducibility and stability, and could be readily applied to the routine monitoring of pathogenic microorganisms in the environmental samples, and most importantly, demonstrate the potential of a prototype development of a simple and inexpensive diagnostic technique.

Ag-SiO₂-Er₂O₃ nanocomposites: highly effective upconversion luminescence at high power excitation and high temperature

Rare Earth (RE) activated upconversion phosphors (UCPs), have demonstrated significant application potentials in some front fields, including solar energy conversion and bio-application. However, some bottleneck problems should be overcame, such as the lower upconversion efficiency, narrower excitation band, concentration-quenching and temperature-quenching. To solve these problems, the Ag-SiO₂-Er₂O₃ nanocomposites were fabricated, in which the upconversion luminescence (UCL) of Er₂O₃ was white broadband. Through the interaction of Er₂O₃ with surface plasmon (SP) of silver nanoparticles (SNPs), the threshold power for generating broadbands was suppressed largely in contrast to the Er₂O₃ nanoparticles (NPs), while the UCL brightness was enhanced remarkably, ranging from several to 10⁴ times, which strongly depended on the power density of excitation light. At excitation power density of 1.50 W/mm² of 980 nm light, the UCL intensity of Ag-SiO₂-Er₂O₃ is 40-folds than the well-known NaYF₄:Yb³⁺,Er³⁺ commercial powders. And more, it is also interesting to observe that the composites demonstrate two excitation bands extending of 780–980 nm, highly improved UCL with elevated temperature and excitation power density. The UCL mechanism related to UCL enhancement was carefully studied.

A straightforward immunoassay applicable to a wide range of antibodies based on surface enhanced fluorescence

A straightforward immunoassay based on surface enhanced fluorescence (SEF) has been demonstrated using a fluorescent immune substrate and antibody functionalized-silver nanoparticles. Unlike the conventional SEF-based immunoassay, which usually uses the dye-labeled antibodies and the metallic nanostructured-substrates, the presented immune system does not need the antibodies to be labeled with dye molecules. Thus, this immunoassay can be easily applied to the detection of a wide range of target antigens, which is of great importance for its practical application. The experimental results show that this immunoassay has a good specificity as well as the capacity of quantitative detection. Basically, the surface density of the immuno-adsorbed silver nanoparticles increases with the increased amount of target antigens, resulting in a fluorescence enhancement up to around 7 fold. The dose-responsive performance of the immunoassay has been investigated and the limit of detection (LOD) is 1 ng/mL. Due to its simple preparation method and the wide range of detectable antigens, this presented immunoassay is expected to be helpful for extending the SEF-based application.