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Chemical Physics Letters 465 (2008) 92–95
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Chemical Physics Letters
journal homepage: www.elsevier .com/locate /cplet t
Observation of surface plasmon-coupled emission using thin platinum films
Krishanu Ray *, Mustafa H. Chowdhury, Joseph R. LakowiczCenter for Fluorescence Spectroscopy, Department of Biochemistry and Molecular Biology, University Maryland School of Medicine, 725 W. Lombard Street,Baltimore, MD 21201, United States
a r t i c l e i n f o
Article history:Received 22 July 2008In final form 22 September 2008Available online 26 September 2008
0009-2614/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.cplett.2008.09.057
* Corresponding author. Fax: +1 410 706 8408.E-mail address: [email protected] (K. Ra
a b s t r a c t
Surface plasmon-coupled emission (SPCE) is the directional radiation of light into a glass substrate due toexcited fluorophores above a thin metal film. The sharp angular distribution of SPCE is a striking phenom-enon and is in stark contrast with the isotropic fluorescence emission. In this Letter, we show that SPCEcan occur with thin platinum films at green and red wavelengths and was found to be mostly p-polarized.This SPCE emission is the result of near-field interactions of the excited fluorophores with the thin plat-inum film, and is not simply a reflective or transmissive phenomenon. Our preliminary observation sug-gests that platinum nanostructures can be part of several novel bio-analysis surfaces.
� 2008 Elsevier B.V. All rights reserved.
1. Introduction
During the past several years there has been a growing interestin the interactions of fluorophores with metallic surfaces and par-ticles [1–3]. Several laboratories have reported on favorable spec-tral changes such as increase intensity, photostability [4–8] andincreased distances for fluorescence resonance energy transfer[9,10]. Excited state fluorophores can also resonantly interact withsmooth thin metal attached to glass prisms that can lead to thegeneration of surface plasmons and the highly directional emissionof the spectral properties of the excited fluorophores on the glassside [11–14]. This phenomenon is called surface plasmon-coupledemission (SPCE). Most of the previous reports showed SPCE andmetal-enhanced fluorescence (MEF) with metallic silver surfacesand particles. It was also found that both SPCE and MEF could occurwith both gold and aluminum, thereby, increasing the versatility ofSPCE and MEF. Gold was found to be more useful at longer wave-lengths [15–18] and aluminum found to be useful in the near-UV[19,20].
Because of our interest in SPCE and MEF we tested differentmetals such as silver, gold and aluminum. During the past severalyears of our research on metal-fluorophore interactions we foundthat SPCE generally occurs over longer distances than MEF [21–24]. For fluorophores near a smooth metal surface there appearsto be three possible processes. The fluorophore can be quenched,can couple to surface plasmons or can emit into free-space. If thefluorophore is close to the surface (<10 nm) there is a high ratefor radiationless deactivation and the emission appears to bequenched. At distances between 10 and 100 nm the dominant de-cay rate is expected to be due to coupling to the surface plasmons.This is important for the applications of SPCE because coupling will
ll rights reserved.
y).
occur over a significant volume in the sample allowing detection oflower overall analyte concentrations. At longer distances(>100 nm) the reflected field interferes constructively or destruc-tively, resulting in the oscillatory behavior reported for fluoro-phores in front of mirrors [25].
In the present report we show that SPCE can occur with thinplatinum films at green and red wavelengths. The most convincingdemonstration of SPCE can be obtained with reverse Kretchmann(RK) excitation where the incident light cannot induce surfaceplasmons. In RK configuration, the fluorophores are excited fromthe air side of the sample and the emission is observed on the distalglass/quartz side of the substrate. In this case, the incident lightwould not create surface plasmons and there is no evanescent fielddue to the incident light. An excited fluorophore near the metalfilm would not know how it was excited; that is, the emissionshould be the same whether the fluorophore is excited by evanes-cent field (Kretchmann configuration) or from a light source notcoupled to the surface plasmon (RK configuration). In the RK geom-etry, the fluorophores are excited uniformly across the sample.However, only those fluorophores within a fractional wavelengthdistant from the metal will couple to the surface and result in SPCE.This allows the use of SPCE without the surface plasmon excitation,which can simplify the devices based on this phenomenon. Sincefluorophores distant from the metal will not couple, autofluores-cence from molecules not localized by the surface chemistryshould be suppressed in the SPCE signal. We believe there will soonbe a variety of metallic nanostructures for use in genomics, proteo-mics and medical testing. Our observation suggests that platinumnanostructures can be part of these novel bioanalysis surfaces.
2. Experimental
Forty nanometer of platinum were deposited on quartz slidesusing an AJA sputtering system under high vacuum. The metal
Notch Filter
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Fig. 1. Schematic representation for measurement of angle-dependent emissionand sample configuration.
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Fig. 2. Angle-dependent intensities of (a) fluorescein and (b) rhodamine Bcontaining PVA films on 40 nm thick platinum coated quartz slides.
K. Ray et al. / Chemical Physics Letters 465 (2008) 92–95 93
deposition step was followed by the deposition of 5 nm of silica viaevaporation without breaking vacuum. This step served to protectthe metal surface and also it adds a distance of 5 nm from the me-tal surface. The Pt-coated slides were spin coated with a solution of�50 lM fluorescein or rhodamine B in 0.5% PVA. The dye/PVA filmis estimated to be approximately 15 nm in thickness. Fig. 1 pre-sents a schematic representation of the sample configuration.
The metal coated slides containing the samples were attachedto a hemicylindrical prism made from quartz and the refractive in-dex was matched using spectrophotometric grade glycerol be-tween the back of the quartz slide (uncoated side) and the prism.This unit was then placed on a precise 360� rotatory stage. Therotatory stage allowed the collection of light at all angles aroundthe sample chamber (shown in Fig. 1). The excitation source usedis a diode pumped solid state continuous-wave laser lasing at470 or 532 nm. An Ocean Optics low OH 600 lm diameter opticalfiber with NA of 0.22 (Dunedin, FL) used for collecting the SPCE andfree-space signals was mounted on a holder that was screwed on to
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Fig. 3. Top Panel: free-space and surface plasmon-coupled emission spectra of (a) fluorescein and (b) rhodamine-B in PVA films. Bottom panel: polarized and unpolarizedsurface plasmon-coupled emission spectra of (c) fluorescein and (d) rhodamine-B in PVA films.
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94 K. Ray et al. / Chemical Physics Letters 465 (2008) 92–95
the base of the rotatory stage. SPCE and free-space spectra werecollected using a model SD 2000 Ocean Optics spectrometer con-nected to the optical fiber. The spectra were collected with an inte-gration time of 1 s. Unpolarized, p- and s-polarized signalinformation were collected for the SPCE signal (from 0–90� and270–360� with respect to the front of the prism) and the free-space(FS) signal (from 90� to 270� with respect to the front of the prism).Fig. 1 presents a geometrical scheme for the measurement of an-gle-dependent emission. All data processing was performed usingORIGINPRO 7. Time-resolved intensity decays were recorded usinga PicoQuant Fluotime 100 time-correlated single-photon countingfluorescence lifetime spectrometer. The excitation source was apulsed laser diode (PicoQuant PDL800-B) with a 20 MHz repetitionrate. The instrument response function (IRF) is about 60 ps.
The fluorescence intensity decays were analyzed in terms of themulti-exponential model as the sum of individual single exponen-tial decays:
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Fig. 4. Intensity–time decays of (a) fluorescein and (b) rhodamine containing PVAfilms in the free-space (blue, square) and SPCE (black, circle) configuration. IRF isalso included in the plots. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)
3. Results and discussion
We measured the emission intensities for all accessible anglesaround the z-axis in the plane of the Pt films. We used RK excita-tion since the incident light from this direction cannot excite sur-face plasmons. Fig. 2 shows polar intensity plots showing theSPCE and free-space emission of PVA films containing fluorescein(top panel) and rhodamine B (bottom panel) respectively on 40-nm platinum films. The emission is sharply directed back intothe hemicylinder and is distributed equally on both sides of thenormal axis as shown in Fig. 2. The emission was sharply distrib-uted at 50� and 48� for fluorescein and rhodamine, respectively.Fig. 2 also shows the highly directional and predominantly p-polar-ized SPCE emission on Pt films, suggesting that the observed signalis due to surface plasmons. This is in stark contrast to the free-space emission which does not show any polarization or direc-tional preference. It can be seen that the p-polarized signal inten-sity at the SPCE peak angle is lower in magnitude than theunpolarized signal. This occurs because the sheet polarizers usedin the experiment have less than 100% peak transmission efficiencyfor p- and s-polarizations. It is important to note that the angulardistribution shown in Fig. 2 is much sharper than those found forfluorophores on uncoated glass, and sharper than those due toreflections above a mirror.
We measured emission spectra of thin PVA films containingeither fluorescein or rhodamine B on Pt films in both free-spaceand SPCE configuration to confirm the absence of scattered light.The emission spectra were recorded at the angle of maximumemission as shown in Fig. 3. The emission spectra on the SPCE side
are spectrally equivalent to the free-space emission spectra forboth fluorescein and rhodamine B samples (Fig. 3a and b). We havealso recorded the SPCE spectra through a polarizer as shown inFig. 3c and d. The SPCE was almost completely polarized in the hor-izontal direction, which is p-polarized. The high polarization valuedemonstrates that SPCE signal is due to plasmons which, becauseof the wavevector matching conditions are created by the ex-cited-state fluorophores. Furthermore, the SPCE signal is p-polar-ized irrespective of the polarization of the incident light. Thesharp angular distribution and p-polarization are characteristic ofSPCE [11–14]. Our SPCE observations with reverse-Kretschmann(RK) configuration support that the surface plasmon coupling oc-curs in the excited state and that the SPCE is not directly createdby the incident light. This suggests that the SPCE emission ob-served with thin Pt films is not due to direct transmission of fluo-rescence, but rather due to increased electromagnetic couplingefficiency between the excited state fluorophores and smooth Ptfilms.
Fig. 4 shows intensity decays for the free-space emission andthe SPCE emission for thin PVA films of fluorescein and rhodamineB on thin platinum films. We carefully considered possible artifacts
K. Ray et al. / Chemical Physics Letters 465 (2008) 92–95 95
and the effects of sample geometry, while recording the intensitydecays. The most important conclusion from these intensity decaymeasurements is that the lifetimes of SPCE and the free-spaceemission are essentially the same.
In summary, we have examined fluorescein and rhodamine B inthin polymer films on 40 nm thick Pt films on quartz substrates.We observed efficient SPCE through thin Pt films. This suggeststhat the energy effectively coupled through the Pt film into thequartz substrate at a sharply defined angle. A significant portionof the total emission appeared as SPCE. The SPCE emission wasp-polarized with different wavelengths appearing at different an-gles. The high polarization indicates that the emission is probablydue to surface plasmons which radiate into the prism. The emis-sion lifetimes of fluorophores were mostly unchanged in thefree-space and SPCE modes.
The phenomenon of SPCE has now been reported for four met-als Al, Ag, Au and Pt. These metals are biologically benign and ifneeded can be coated with silica for which there is a great dealof surface chemistry available for linking biomolecules. Addition-ally, these metals are easily used in nanofabrication methods suchas focused ion beam (FIB) and e-beam nanolithography (EBL). Weexpect SPCE to become widely used in devices for DNA and proteinassays, and in diagnostic systems.
Acknowledgments
This work was supported by National Institutes of Health (GrantNos. HG-002655 and EB-006521).
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