Surface plasmon-coupled emission (SPCE) is the directional radiation of light into a substrate due to excited fluorophores above a thin metal film. To date, SPCE has only been observed with visible wavelengths using silver or gold films. We now show that SPCE can be observed in the ultraviolet region of the spectrum using thin (20 nm) aluminum films. We observed directional emission in a quartz substrate from the DNA base analogue 2-aminopurine (2-AP). The SPCE radiation occurs within a narrow angle at 59° from the normal to the hemicylindrical prism. The excitation conditions precluded the creation of surface plasmons by the incident light. The directional emission at 59° is almost completely p-polarized, consistent with its origin from surface plasmons due to coupling of excited 2-AP with the aluminum. The emission spectra and lifetimes of the SPCE are those characteristic of 2-AP. Different emission wavelengths radiate at slightly different angles on the prism providing intrinsic spectral resolution from the aluminum film. These results indicate that SPCE can be used with numerous UV-absorbing fluorophores, suggesting biochemical applications with simultaneous surface plasmon resonance and SPCE binding assays.Surface plasmon resonance (SPR) is widely used for detection of bioaffinity reactions on surfaces. [1][2][3] Surface plasmons are oscillating electrical charges on a metallic surface. When a thin metal film is illuminated through a glass prism, and the angle of incidence is appropriate, the surface plasmons in resonance with the incident light occur at the metal-air or water interface. This results in strong absorption, which is measured as a decrease of reflectivity. The SPR angle is sensitive to the refractive index of the sample above the metal, distal from the glass prism. The origin of this sensitivity is the evanescent field from the plasmons, which penetrates approximately λ/3 into the sample.In several recent reports we described a related phenomenon, surface plasmon-coupled emission (SPCE). 4,5 We found that excited fluorophores near a thin metal surface can couple with the surface plasmons, resulting in directional radiation into the glass substrate. The spectral properties of the radiation were found to be essentially identical to those of the fluorophore, except for a highly p-polarized emission, too large to be due to optical photoselection. The angular dependence of the radiation, as well as the p-polarization, are consistent with radiating surface plasmons, the reverse process of surface plasmon absorption.Our previous studies of SPCE used thin silver 5-7 or gold 8 surfaces. This choice of metals restricts the selection of fluorophores to those absorbing and emitting at visible or longer wavelengths. However, many widely used fluorophores absorb or emit at ultraviolet wavelengths. We now report that SPCE at UV wavelengths can be observed using thin *To whom correspondence should be addressed: lakowicz@cfs.umbi.umd.edu.
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MATERIALS AND METHODS
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