Low-Cost Plastic Plasmonic Substrates for Operationin Aqueous Environments

RAMAMURTHY SAI SATHISH, YORDAN KOSTOV,* and GOVIND RAOCenter for Advanced Sensor Technology and Department of Chemical and Biochemical Engineering, University of Maryland Baltimore County,

1000 Hilltop Circle, Baltimore, Maryland 21250

We report a novel design of a multilayer stack to attain surface-plasmon-

coupled emission (SPCE) enhancements in liquid medium. Variation in

thickness of the multilayer affects the position and depth of resonance

plasmon dips. Numerical investigation resulted in an optimal stack

configuration that supports long-range surface plasmons. SPCE sub-

strates were prepared on plain BK7 glass and Teflon-AF coated

polycarbonate (PC-T) substrates by modifying their surface functional-

ities using plasma etching. The changes in refractive indices due to the

presence of the fluoropolymer layer help reduce the SPCE exit angle from

a ¼ 758 (plain BK7) to a ¼ 608 (PC-T) in water without requiring

specialized optics.

Index Headings: Surface-plasmon-coupled emission; Solution deposition;

Plastics; Plasmonics; Long-range surface plasmons.

INTRODUCTION

Surface-plasmon-coupled emission (SPCE) is a well-known1–3 phenomenon that has recently enjoyed a lot ofattention4–8 due to a number of properties that are beneficial forfluorescence sensing.9 SPCE is based on the coupling of SPswith more than 50% of the emission from fluorophores within200 nm from the metal film. This results in a highly polarizedand directional emission with significant enhancements influorescence intensity.3–9 It presents a unique approach fordetection of fluorescence emission from monolayers10 andsingle molecules.11 The application of SPCE to studies ofcomplex sample matrices such as whole blood8,12 and for thedesign of ultrasensitive immunoassays13–15 is highly promis-ing. However, a specific problem with any SPCE measurementis the fact that the SPCE angle a (see Fig. 1) strongly dependson the refractive index of the medium in close proximity to thefluorescence layer. The higher the refractive index, the longerthe angle. For example, when the medium is air, a ¼ 46.58;however, in water a exceeds 758. As a result, collection of theSPCE fluorescence is difficult and requires specialized andexpensive optics, such as large-diameter sapphire lenses.16 Thisproblem limits the use of SPCE for solution-based measure-ments.

As a large number of fluorescence-based analytical assaysare performed in aqueous environments, it is desirable to havea simple, low-cost SPCE platform that would afford lower exitangles, which in turn would permit the use of standard, mass-produced optics. Such an opportunity is presented byintroducing a dielectric layer below the thin metal film with arefractive index similar to the refractive index above the metalfilm.17–19 The coupling of SPs propagating along the oppositeinterfaces results in a transverse standing wave known as long-range surface plasmons (LRSPs). We report here the design of

a novel multilayer stack suitable for LRSP that allowsobtaining smaller SPCE angles amenable to lower-costinstrumentation. Importantly, our experimental results suggestthat the fluorescence enhancements and collection efficienciesin water are similar to those obtained with regular plasmonicsubstrates in air. In addition, the structures are fabricated usinglow-cost solution-based techniques, which makes them usefulfor measurements in low-resource settings.

EXPERIMENTAL DETAILS

The multilayer structure that supports LRSPs for SPCEapplications consists of a polycarbonate (PC) substrate, afluoropolymer buffer layer (Teflon AF 1600, DuPont, USA), athin silver film, and a thin fluoroprobe/polymer layer. The PCslides were initially subjected to a CHF3 plasma etch (TechnicsSeries 85-RIE) to both clean the surface and simultaneouslygenerate surface fluoro-alkyl functionalities such as CF3, CF2,and CF having hydrophobic properties20 that improve adhesionof Teflon AF to the PC substrate. For the experiments theplasma parameters were as follows: pressure, 250 mTorr; rfpower, 150 W; and treatment time, 3 min. To preserve thesesurface modifications, each of the substrates was immersed inTeflon AF solution until further use. Commercially availablefluoropolymer: Teflon AF with a refractive index (n ¼ 1.30)marginally below that of water (n ¼ 1.33) was spin-coated onCHF3 plasma-etched PC substrate (n ¼ 1.58) to yield atransparent fluorine-containing film that is highly waterrepellant. The rotation speed was maintained at 800 rpm usinga Chemat Technology Spin coater to attain the desired layerthickness of 550 nm measured using a Tencor Alpha StepProfilometer. The Teflon AF coated PC (PC-T) substrates weredried at room temperature for 1 h. The Teflon AF coveredsurface was exposed to oxygen plasma to subsequentlygenerate oxygen-containing surface hydrophilic functional-ities,21,22 which enable rapid silvering. A short burst of oxygenplasma with the following parameters was used: pressure, 150mTorr; rf power, 100 W; and treatment time, 6 s. The finalthickness of the fluoropolymer layer, measured using thesurface profiler, was found to be 500 nm after the oxygenplasma etch. To preserve these surface modifications, each ofthe substrates was immersed in distilled water until silverdeposition.

A detailed low-cost procedure for the solution-baseddeposition of thin silver films onto plastic substrates using acommercially available silvering kit from Peacock LaboratoriesInc. has been recently described by us.23,24 The wet-chemistryapproach presents silver films of 47 6 3 nm with uniformthickness on PC-T slides by optimizing the deposition time andtemperature. The silvered substrates were subsequently spin-coated with a 20-nm-thick film of R-6G (1 mM solution) in0.2% PMMA (polymethylmethacrylate, in acetonitrile) at 800

Received 13 April 2010; accepted 17 August 2010.* Author to whom correspondence should be sent. E-mail: kostov@umbc.edu.

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� 2010 Society for Applied Spectroscopy

rpm for 1 min. The dye-doped PMMA films were thoroughlyrinsed with deionized water. The PC-T slide described abovewas attached to a BK7 hemicylindrical prism (n ¼ 1.51) withglycerol (n¼ 1.47) as the index matching fluid and the samplesystem was placed on the 3608 rotary SPCE stage. A hand-made three-sided demountable cuvette was attached to thesample side for holding a 3-mL volume of water. A schematicof the SPCE phenomenon with excitation of the sampleachieved in reverse Kretschmann (RK) configuration25 with a405-nm, 3-mW laser diode (TE cooled module, PhotonicsProducts) is shown in Fig. 1. A uniform excitation of the

fluorophores across the plastic substrate, from the water side,was achieved and the SPCE peak was observed on the BK7prism side of the substrate in RK geometry. Directional SPCEwas collected at specific angles between 08 and 908 and 2708and 3608 and the free-space signal was collected from 908 to1808, with respect to the front of the prism (Fig. 1). Emissionfrom one point of the SPCE cone (;1 arc degree) wascollected. The emission light was filtered with a 450-nm long-pass filter placed in front of the photomultiplier tube andmonochromator of an ISS K2 fluorometer mounted on therotation stage. A polarizer was placed in front of the filter toperform the polarization measurements. The theoretical SPCEangles for the system shown in Fig. 1 were determined fromequations that govern the theory of surface plasmon reso-nance.26 The position and coupling of plasmon resonance dipsassociated with the radiated light from SPCE were calculatedwith commercially available TFCalc. 3.5 software (SoftwareSpectra, Inc., Portland, OR).

RESULTS AND DISCUSSION

The focus of this study was to develop low-cost plasmonicsubstrates that can be employed in a water environment withobservable fluorescence enhancements accompanied by asignificant bend in the SPCE exit angles. To this end, we firstinvestigated the influence of the thicknesses of Teflon AF,silver, and dye-doped PMMA on the position and depth of thesurface plasmon resonance (SPR) dip. Simulation curves for airand water environments in the case of BK7 and PC substrateswere calculated for the four-phase system shown in Fig. 1. The

FIG. 1. Schematic representation of multilayer structures supporting angle-dependent SPCE.

FIG. 2. (Top) Resonance plasmon dips calculated for a four-layer system at an emission wavelength of 583 nm in air and water media, consisting of a 47-nm thinsilver film and 20 nm PMMA in the presence (500 nm) and absence (0 nm) of Teflon AF: (a) for a PC substrate; (b) for a BK7 glass substrate. (Bottom) SPR angle-dependent reflectivity curves for a variation in Teflon AF thickness from 300 to 1500 nm: (c) in air as exit medium; (d) in water as exit medium.

APPLIED SPECTROSCOPY 1235

simulation of the angular reflectivity spectrum in air and watermedia, in the presence (500 nm) and absence (0 nm) of TeflonAF for PC and BK7 glass substrates are presented in Figs. 2aand 2b. As expected, in the absence of Teflon AF only short-range surface plasmons are present. The exit angles in air forBK7 and PC are approximately 46.58 and 448, respectively.However, in water medium they increase to approximately 758

and 688, respectively. With the presence of a 500-nm TeflonAF layer, these angles change dramatically. Loss of degeneracythat results from coupling of SPs propagating along theopposite interfaces of the sandwich sample appears as twocoupled modes: LRSPs and short-range surface plasmons(SRSPs).18,19 Now, in air two dips—sharp and broad—arepresent. The smaller angle (;448 for the PC and ;46.58 forBK7) corresponds to the LRSPs. The second dip, which occursat ;648 for PC and ;708 for BK7, corresponds to SRSP. Inwater, the LRSP peak shifts to ;608 (from ;448 in air) for PCand ;658 (from ;46.58 in air) for BK7. More importantly, theSRSP peak completely disappears, which forces all the coupledfluorescence to be emitted only at that angle. It was concludedthat the most promising system for operation in water includesPC substrate, Teflon sub-layer, 50-nm silver layer, and up to20-nm-thick fluorescent layer. Earlier work suggests that 50 6

5 nm is the ideal metal thickness that provides the bestcompromise between the enhancements and the angularity ofthe plasmon emission.27 Our models suggest (data not shown)that further increase of the fluorescent polymer thickness whilemaintaining the rest of the system constant results in wider

SPCE dips in water, which would complicate fluorescenceemission collection.

Next, the influence of the Teflon AF thickness on the SPCEangular distribution (both in air and in water) was investigated(Figs. 2c and 2d). Maintaining the thickness of silver at 47 nmand that of PMMA at 20 nm, a variation in Teflon AF thicknessfrom 300 to 1500 nm in air (Fig. 2c) and water (Fig. 2d)environments showed that 500 nm of Teflon AF offers the bestconditions for attaining the minimum SPR reflectance (andmaximum fluorescence intensity) in water at approximately608. At this thickness, there is a single reflectivity minimumand at this angle the wave vectors match ideally: the reflectanceis almost zero. At smaller thicknesses, SRSP starts appearing.At higher thicknesses, there is a mismatch, resulting in pooroutput coupling and losses of fluorescence. Remarkably, theLRSP peak does not change its depth with the variation of theTeflon thickness when probed in air. On the other hand, theposition and especially the depth of the SRSP peak is thicknessdependent (Fig. 2c). The plasmon resonance dip at 43.858 has anear constant reflectance value of 17.63, unlike the dip at63.648, which varies from 7.92 to 98.23 for all Teflon AFthicknesses in the range 300–1500 nm. In this way, it ispossible to quickly evaluate the thickness of the fluoropolymerbuffer layer. The ratio of the reflectance values (fluorescenceintensities) at these two SPR angles (SPCE exit angles) isunique for a particular thickness of the fluoropolymer and canbe used as a quality control as well as a predictor of intensity ofthe exiting fluorescence.

The preparation of multilayer structures supporting LRSPs

FIG. 3. (Top) Free-space (FS) and SPCE spectra obtained for a system consisting of a plain BK7 glass substrate vs. a PC substrate coated with 500 nm Teflon AF. A47-nm thin silver film and 20 nm PMMA doped with 1 mM Rhodamine-B are coated on both the substrates and studied: (a) in air as exit medium; (c) in water as exitmedium. Insets in (a) and (c) display photographs taken at the SPCE angles (448, 648 in air and 608 in water) for the PC substrate, with a 550-nm long-pass filter.(Bottom) Polarized SPCE from the plain BK7 glass substrate and the Teflon AF coated PC substrate: (b) in air as exit medium; (d) in water as exit medium.

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has so far been demonstrated only on glass substrates involvingvapor deposition of thin metal films on the fluoropolymer layer.The adhesion of Teflon AF to the BK7 substrate in thesestudies has been achieved by generation of a monolayer offluoro-alkyl silane (perfluorodecyltrichlorosilane) under inertatmosphere conditions that renders the glass surface hydro-phobic.28 In this work, we report an interfacing procedure forthe PC substrate. A treatment with CHF3 plasma not onlyremoved any surface residues, but simultaneously generatedCFx surface layer on the substrate, which resulted in strongadhesion to Teflon AF. CHF3 plasma creates perfluorinatedislands due to grafting of the fluorocarbon radicals.29 Thesubsequent treatment of the Teflon AF layer with oxygenplasma generated strongly oxidized islets with hydrophilicsurface functional groups (–COOH, –OH, .C¼O) available forrapid solution-based deposition of Ag.21,22 The simple changeof the plasma gasses (CHF3 or oxygen) rendered the desiredstrong adhesion between PC, Teflon, and silver. Plasmatreatments were critical for spin-coating the fluoropolymerand the solution deposition of the thin silver film, as we werenot able to obtain continuous layers on untreated plastics. Toour knowledge, this is the first study that uses plasma treatmenttogether with wet deposition techniques for preparation ofmultilayer SPCE structure on plastic substrates.

Using the described approach, SPCE substrates wereprepared on plain BK7 and Teflon AF coated PC. Fluorescenceemission intensities at the specified angles were initiallymeasured in air. Figure 3a compares the isotropic free space(FS) spectra of a 1 mM Rhodamine-B-doped 20-nm PMMAlayer on plain BK7 and PC-T substrates with the directionalemission on the SPCE side. At 448 on PC-T and 468 on BK7, a12- to 14-fold enhancement of the fluorescence was observed,as expected. Additionally, at 648 the second SPCE peak wasobserved. It exhibited 4- to 6-fold intensity enhancements ascompared with the free-space emission. This observationconfirmed that we can observe SPCE both due to LRSPs (at448) and SRSP (at 648). The emissions on PC-T were highly p-polarized, 89.70% at 448 and 87.28% at 648 (Fig. 3b). Thisconfirms the plasmonic origin of the emissions, as the SPCE isa reverse process from SPR. Fluorescence was excited inreverse Kretschmann configuration,25 which allows the use ofSPCE with any, including non-polarized, light source. Thissimplifies the devices that need to be used to study thisphenomenon in aqueous environments and is the significantimprovement in the current study. In water, only one SPCEpeak from PC-T was observed, as expected (Fig. 3c). Itappeared at 608, a significantly more manageable angle ascompared with 758 for a plain BK7 substrate. Remarkably,fluorescence intensity enhancements remained nearly the same:10 to 12 fold. Again, the SPCE peak in water was highly p-polarized (88.84%) for PC-T substrate as shown in Fig. 3d.The insets in Figs. 3a and 3c present photographs of SPCEpeaks from PC-T substrate at the respective angles, takenthrough a 550-nm long-pass filter. Theoretical angles for themultilayer system on the PC substrate in air (43.858, 63.648)and water (60.328) media were in excellent agreement with theexperimental SPCE angularity measurements in air (448, 648)and water (608), respectively. The presence of the Teflon AFlayer on the PC substrate thus helps bend the SPCE exit anglefrom a ¼ 758 (plain BK7 substrate) and a ¼ 688 (plain PCsubstrate) to a¼ 608 with improved intensity enhancements inwater.

CONCLUSIONS

The ability to obtain comparable SPCE intensities in waterand air environments from solution-deposited thin silver filmson plain BK7 glass and PC-T substrates with excitation in theRK configuration offers a technically simple route forinstrument design based on this phenomenon. The lowerSPCE observation angle is favorable for easier light extractionand simplifies the requirements for the observation optics. Theuse of plastic substrates presents avenues for mass productionof SPCE devices for rapid assays and low-cost diagnostics. Thesensitivity of the system coupled with the directionality andvisibility of the emission can be exploited for lab-on-a-chipapplications used to monitor chemical species and biomolec-ular detection in medicine, biotechnology, and the environ-ment.

ACKNOWLEDGMENT

The authors acknowledge financial support from NSFBES Grant No.0517785.

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