Metal-enhanced Fluorescence Substrate Research Articles

Conversion of just-continuous metallic films to large particulate substrates for metal-enhanced fluorescence.

We report the effects of thermally annealing, non-, just-, and thick continuous silver films for their potential applications in metal-enhanced fluorescence, a near-field concept which can alter the free-space absorption and emissive properties of close-proximity fluorophores (excited states). We have chosen to anneal a noncontinuous particulate film 5 nm thick and two thicker continuous films, 15 and 25 nm thick, respectively. Our results show that the annealing of the 25 nm film has little effect on close-proximity fluorescence when coated with a monolayer of fluorophore-labeled protein. However, the 15 nm continuous film cracks upon annealing, producing large nanoparticles which are ideal for enhancing the fluorescence of close-proximity fluorophores that are indeed difficult to prepare by other wet-chemical deposition processes. The annealing of 5 nm noncontinuous particulate films (a control sample) has little influence on metal-enhanced fluorescence, as expected.

Aluminum nanostructured films as substrates for enhanced fluorescence in the ultraviolet-blue spectral region.

Particulate aluminum films of varied thicknesses were deposited on quartz substrates by thermal evaporation. These nanostructured substrates were characterized by scanning electron microscopy (SEM). With the increase of aluminum thickness, the films progress from articulate toward smooth surfaces as observed by SEM images. To date, metal-enhanced fluorescence (MEF) has primarily been observed in the visible - NIR wavelength region using silver or gold island films or roughened surfaces. We now show that fluorescence could also be enhanced in the ultraviolet-blue region of the spectrum using nanostructured aluminum films. Two probes, one in the ultraviolet and another one in the blue spectral region, have been chosen for the present study. We observed increased emission, decrease in fluorescence lifetime, and increase in photostability of a DNA base analogue 2-aminopurine and a coumarin derivative (7-HC) in 10-nm spin-casted poly(vinyl alcohol) film on Al nanostructured surfaces. The fluorescence enhancement factor depends on the thickness of the Al films as the size of the nanostructures formed varies with Al thickness. Both probes showed increased photostability near aluminum nanostructured substrates, which is consistent with the shorter lifetime. Our preliminary studies indicate that Al nanostructured substrates can potentially find widespread use in MEF applications particularly in the UV-blue spectral regime. Furthermore, these Al nanostructured substrates are very stable in buffers that contain chloride salts compared to usual silver colloid-based substrates for MEF, thus furthering the usefulness of these Al-based substrates in many biological assays where high concentration of salts are required. Finite-Difference Time-Domain calculations were also employed to study the enhanced near-fields induced around aluminum nanoparticles by a radiating fluorophore, and the effect of such enhanced fields on the fluorescence enhancement observed was discussed.

Metal-enhanced fluorescence from plastic substrates.

We report the first findings of Metal-Enhanced Fluorescence (MEF) from modified plastic substrates. In the past several years our laboratories have reported the favorable effects of fluorophores in close proximity to silver nanoparticles. These effects include, enhanced fluorescence intensities, (increased detectability), and reduced lifetimes, (enhanced fluorophore photostability). All of these reports have featured silver nanostructures and fluorophores which have been immobilized onto clean glass or quartz surfaces. In this report we show how plastic surfaces can be modified to obtain surface functionality, which in turn allows for silver deposition and therefore metal-enhanced fluorescence of fluorophores positioned above the silver using a protein spacer. Our findings show that plastic substrates are ideal surfaces for metal-enhanced phenomena, producing similar enhancements as compared to clean glass surfaces. Subsequently, we speculate that plastic substrates for MEF will find common place, as compared to the more expensive and less versatile traditional silica based supports.

Fluorescence enhancements on silver colloid coated surfaces.

We observed a strong, more than 16-fold, enhancement of Texas Red-labeled BSA fluorescence emission when deposited on silver colloid coated surfaces (SCCS). The same labeled protein deposited on silver island films (SIFs) showed an approximate 8-fold fluorescence enhancement. The lifetimes of Texas Red-BSA fluorescence are significantly shorter on silvered surfaces than on uncoated quartz substrate indicating a strong change in radiative decay rate of the dyes. We also observed a 36-fold increased brightness of overlabeled fluorescein-HSA deposited on silver colloid coated surface. Stronger enhancement observed for overlabeled Fl-HSA protein indicates that presence of silver particles partially decreased self-quenching. Our results indicate that surfaces coated with silver colloids are valuable substrates for metal-enhanced fluorescence.