Ultraviolet Surface Plasmon-Coupled Emission Using Thin Aluminum Films

Abstract

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. NIH Public Access MATERIALS AND METHODS Sample PreparationQuar...

First observation of surface plasmon-coupled chemiluminescence (SPCC)

“…It is however worth noting that the signal at the SPCC peak angle is not entirely ppolarized. This is in contrast with our past experiences with optically pumped SPCE experiments where the SPCE signal was almost entirely p-polarized [8][9][15][16][17][18][19][20]. Figure 2 (Bottom) is the normalized SPCC and free-space emission spectra showing a high degree of overlap between the spectra.…”

“…This is unlike our previous experiences with optically pumped SPCE studies for a thin layer of fluorophores, where the SPCE peak angle was approximately 1-2 degrees [8][9][15][16][17][18][19][20]. In order to investigate if the broadness of the SPCC peak is dependent on the surface area of the sample, we repeated the experiment with a sample chamber built inhouse that had approximately half the surface area when compared to the samples made with commercially available imaging chambers that had been used thus far.…”

“…However, the hydrophobic nature of the surface globulated the chemiluminescence liquid, preventing films less < 250 nm thick to be produced. Given that clinical/biochemical assays are typically substantially smaller than 250 nm, indeed as we have shown in numerous SPCE publications [15][16][17][18][19][20], then similarly we expect surface-bound chemiluminescence assays to be also highly useful. Figure 2 (Top) is an enlarged figure showing the highly directional and p-polarized SPCC emission, suggesting that the observed signal is due to surface plasmons.…”

“…We subsequently termed this phenomenon as surface plasmon-coupled emission (SPCE) [8][9][15][16][17][18][19][20]. We have also shown that SPCE can be generated on gold films by electrochemical excitation of [Ru(bpy) 3 ] 2+ near a gold electrode [21].…”

Aluminum Nanostructured Films as Substrates for Enhanced Fluorescence in the Ultraviolet-Blue Spectral Region

“…18 However, it has been shown that excited fluorophores over smooth aluminum film could create surface plasmons, which eventually radiate into the substrate. 19 The spectral properties of the radiation were found to be essentially identical to those of the fluorophore, except for a highly p-polarized emission. The angular dependence of the radiation, as well as the p-polarization, are consistent with radiating surface plasmon.…”