Elsevier

Thin Solid Films

Volume 491, Issues 1–2, 22 November 2005, Pages 173-176
Thin Solid Films

Directional two-photon induced surface plasmon-coupled emission

https://doi.org/10.1016/j.tsf.2005.06.010Get rights and content

Abstract

We measured a directional surface plasmon-coupled emission (SPCE) induced by a two-photon absorption. A 60 nm thick layer of poly(vinyl alcohol) film doped with rhodamine 123 was deposited on a silvered (50 nm Ag) glass slide, which was attached to a hemicylindrical glass prism. The 820 nm excitation from a femtosecond Ti:Sapphire laser was used either in reverse Kretschmann or Kretschmann configuration. The angular distribution of two-photon induced SPCE does not depend on the used configuration. The two-photon induced SPCE can be applied to improve immunoassays and deoxyribonucleic acid detection.

Introduction

Surface plasmon-coupled emission (SPCE) is related to surface plasmon resonance (SPR) analysis, a technique widely used to measure bioaffinity reactions [1], [2], [3]. In SPR measurements absorption of light is observed at the specific angles of incidence, which depend on the optical properties of system near a metal–dielectric interface. In SPCE experiments emission is being detected rather than absorption. Surface plasmons in a thin noble metal layer, induced by oscillating dipoles of exciting molecules, can radiate into a glass-coupling prism. This hollow cone of surface plasmon-coupled radiation occurs within a sharply defined angle. The SPCE angle depends on the optical properties of the dielectric layers, coupling glass, a metal layer and the emission wavelength. It should be noted that the interaction of excited molecules with surface plasmons is a near-field effect occurring without emission of photons, similar to Forster fluorescence resonance energy transfer. The coupling of the excitation energy to the surface plasmons can be an effective deactivation channel for excited molecules. The interaction of excited dipoles with metallic surfaces has been already described by Chance et al. [4] in 1978 and more recently by Enderlein [5], [6].

Although the SPCE angle can be roughly estimated from the SPR theory and thin film calculations [7], [8], [9], [10], [11], the SPCE theory has been recently described [12], [13], [14].

The strong excitation of fluorophores near a metal layer by an evanescent field of SPR has been used in bioassays [15], [16]. The near-surface localized excitation, directional and polarization properties of SPCE provided effective background suppression in DNA hybridization [17] and immunoassays studies [18], [19].

Two-photon excitation (TPE) is now commonly used in microscopy and spectroscopy [20], [21], [22], [23]. This is a result of the development of a robust Ti:Sapphire femtosecond laser, which provides superior TPE. The long-wavelength and localized excitation enables the use of minimal volumes and provides less photodamage to the bulk sample. Two-photon induced fluorescence depends on the squared intensity of the incident light.

Since local evanescent field intensity due to SPR can be up to 100-fold larger than it for the incident light field [7], [8], [9], the TPE in SPR/SPCE experiments has a great potential creating the possibilities of 104-fold enhancements. Strong enhancements with TPE have been already observed by intensified fluorescence and near-field probing methods [24], [25].

We believe that SPCE with TPE will be effectively employed in the near future with enhanced immunoassays and array techniques for improved DNA detection. In this manuscript we described the directional SPCE induced by TPE.

Section snippets

Experimental details

Glass slides 45 × 12.5 × 0.7 mm were coated by vapor deposition by EMF Corp. (Ithaca, NY). A 50 nm thick layer of Ag was deposited on the slide, which was prior coated with 2 nm chromium for improved adhesion. Rhodamine 123 (R123; Aldrich) was deposited on the silvered surface by spin coating at 3000 rpm a 1.5% by weight solution of low-molecular-weight poly(vinyl alcohol) (PVA, Aldrich) in water. The concentration of R123 in the solution was about 2 × 10 3 M. The thickness of the PVA film was

Results and discussion

First, we calculated the reflectance curves for the 820 nm excitation wavelength and 550 nm emission (Fig. 1). For these calculations we used TFCalc. software provided by Spectra, Inc. Portland, OR. The four-phase system of layers adequate to our experiment is shown in the insert of Fig. 1. In this case the minima of reflectance are predicted at about 48° and 64° for 820 and 550 nm, respectively. In these calculations the refractive indices for glass (BK7) and PVA film were 1.52 and 1.50 for

Conclusions

In this manuscript we demonstrated the use of TPE in SPCE measurements. The directional surface plasmon-coupled emission provides complementary data to the SPR analysis. The most promising future of SPCE is exceptional background rejection [18], [19]. TPE offers a localized excitation and a possibility to use minimal volumes of the sample, which is important in assays and measurements involving microplates and microarrays. The large separation of angles θI and θF with TPE–SPCE measurements,

Acknowledgements

This work was supported by NIH, National Center for Research Resources, RR-08119, and Biomolecular Interaction Technologies Center (University of New Hampshire).

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