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Towards Full-Length Accumulative Surface-EnhancedRaman Scattering-Active Photonic Crystal Fibers

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By Yun Han, Siliu Tan, Maung Kyaw Khaing Oo, Denis Pristinski,

Svetlana Sukhishvili,* and Henry Du*

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[*] Prof. H. Du, Y. Han, Dr. S. Tan, M. K. K. OoDepartment of Chemical Engineering and Materials ScienceStevens Institute of TechnologyHoboken, NJ 07030 (USA)E-mail: hdu@stevens.edu

Dr. D. PristinskiPolymers Division, NISTGaithersburg, MD 20899 (USA)

Prof. S. SukhishviliDepartment of Chemistry, Chemical Biology, and BiomedicalEngineeringStevens Institute of TechnologyHoboken, NJ 07030 (USA)E-mail: ssukhish@stevens.edu

DOI: 10.1002/adma.200904192

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The integration of microfluidics with photonics on a singleplatform using well-established planar device technology has ledto the emergence of the exciting field of optofluidics.[1] As both alight guide and a liquid/gas transmission cell, photonic crystalfiber (PCF, also termed microstructured or holey fiber),[2–7]

synergistically combines microfluidics and optics in a single fiberwith unprecedented light path length not readily achievable usingplanar optofluidics. PCF, an inherent optofluics platform, offersexcellent prospects for a multitude of scientific and technologicalapplications.[8–14] The accessibility to the air channels of PCF hasalso opened up the possibility for functionalization of the channelsurfaces (silica in nature) at the molecular and the nanometerscales, in particular to impart the functionality of surfa-ce-enhanced Raman scattering (SERS) in PCF for sensing anddetection.[15–18]

SERS, an ever advancing research field since its discovery inthe 1970s,[19–21] has tremendous potential for label-freemolecularidentification at trace or even single-molecule levels due to up to1014 increase in the Raman scattering cross-section of a moleculein the presence of Ag or Au nanostructures.[22–24] Seminal workhas been reported on the development of 1D[25] and 2D[26,27]

SERS substrates for a variety of sensing applications.[28] The useof 3D[29–32] geometry, i.e., substrates obtained by the deposition ofnoble nanoparticles onto porous silicon[29] or porous aluminummembrane[30–32] offered additional advantages of increased SERSintensity due to increased SERS probing area,[29] as well as themembrane waveguiding properties.[30–32] Specifically, severalorders of magnitude higher SERS intensity, affording pico- orzeptogram-level detection of explosives, has been demonstratedwith porous alumina membranes containing �60-mm-longnanochannels, as compared to a solid planar substrate.[30–32]

SERS-active PCF optofluidics, as a special fiber optic SERS

platform, offers easy system integration for in situ flow-throughdetection, and, more importantly, much longer light interactionlength with analyte, thus promising to open a new vista inchemical/biological sensing, medical diagnosis, and processmonitoring, especially in geometrically confined or samplingvolume-limited systems.

Various attempts have been made over the last several years tointegrate SERS with the PCF platform by incorporating Ag or Aunanostructures albeit inside a very limited segment (typically afew centimeters) of the fiber air channels. Examples includedeposition of Ag particulates and thin films by chemical vapordeposition at high pressure[15] or coating of Ag and Aunanoparticles using colloidal solutions driven into the micro-scopic air channels via capillary force with backscattering as thetypical data acquisition mode.[16,17] Building uniform SERSfunctionality throughout the length of the PCF while preservingits light guide characteristics has remained elusive as measuredRaman intensity is a combination of the accumulative gain fromRaman scattering and the continuous loss from nanoparti-cle-induced light attenuation over the path length. As a result, wehave recommended and recently described in a brief studyforward scattering as a more suitable detection mode tounambiguously assess the SERS-active nature of PCF.[18] Tothe best of our knowledge, this article is the first report of netaccumulative SERS from the full-length Ag-nanoparticle-functionalized PCFs. The finding has been enabled by a finecontrol of the coverage density of Ag nanoparticles and studies ofa competitive interplay between SERS gain and light attenuationin the Raman intensity with PCFs of varying length. Using twodifferent types of PCF, i.e., solid-core PCF and hollow-core PCF,we show that Raman gain in PCF prevails at relatively lownanoparticle coverage density (below 0.5 particlemm�2), allowingfull benefit of accumulation of Raman intensity along the fiberlength for robust SERS sensing and enhanced measurementsensitivity. Light attenuation dominates at higher nanoparticlecoverage density, however, diminishing the path-length benefit.

Shown in Figure 1 are cross-sectional scanning electronmicroscopy (SEM) images of solid-core PCFand hollow-core PCFused in this work. Also depicted in the figure is the light-guideprocess in the corresponding PCFs that contain immobilized Agnanoparticles and are filled with aqueous solution throughout thecladding air channels for solid-core PCFand in the center air coreonly for hollow-core PCF. Note that light is guided via totalinternal reflectance in both cases. The presence of the aqueoussolution in the cladding air channels does not fundamentallychange the contrast of the higher index silica core and the lowerindex liquid-silica cladding in the solid-core PCF. The selective

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Figure 1. Cross-sectional SEM images of solid-core PCF and hollow-corePCF and schematic illustrations of light guiding in the correspondingliquid-filled structures with immobilized Ag nanoparticles. Scale bar inthe SEM image: 10mm.

Figure 2. A) Molecular- and nanometer-scale modification leading to controlof Ag nanoparticles in the air channels of the PCFs. B) Effect of the deposition p(2mgmL�1, deposited for 20min) on nanoparticle coverage density. Ag nattached to the PAH-modified air channels using 1012 particlemL�1 solution foindicate standard deviation from six measurements taken at various sectiolength. C,D,E,F) SEMs of immobilized Ag nanoparticles in the cladding air chaPCF and hollow-core PCF, respectively.

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filling of the center air channel with the aqueous solution hasturned an otherwise bandgap hollow-core PCF to an index-guidedliquid-core PCF. SERS is generated by evanescent field of thepropagating light in the silica core for solid-core PCFand by directexcitation of the propagating light in the liquid core for thesolution-filled hollow-core PCF.

To realize SERS-active PCF while preserving the light-guideproperty of the fiber, an important step is uniform incorporationof Ag nanostructures inside the air channels along the entire fiberlength with controlled surface coverage density. Similar to workby others,[30,31] we are using polyelectrolyte-mediated surfaceimmobilization of noble nanoparticles. However, in contrast toprevious work where Au nanoparticles were attached to�60-mm-long nanocanal walls of porous alumina,[30,31] we focuson controlling nanoparticle density in a wide range, and achievinglow and uniform coverage density of Ag nanoparticlesinside much longer air channels of PCFs. To mediatenanoparticle attachment, polyallylamine hydrochloride (PAH)

led immobilizationH of PAH solutionanoparticles werer 30min. Error barsns along the fibernnels of solid-core

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was first allowed to adsorb at the surface of theair channels, served as anchoring site for Agnanoparticles. Ag nanoparticles 35� 5 nm insize with zeta potential of approximately�35mV were synthesized using a modifiedLee and Meisel method.[33]

Surface functionalization of the fiber airchannels and immobilization of Ag nanopar-ticles (Fig. 2A) were carried out in acustom-built pressure cell maintaining a200 psi pressure differential between theentrance end (submerged in a solution) andthe exit end (exposed to the laboratoryambient) of the PCF. In hollow-core PCF,only the hollow core underwent the varioustreatment steps by selectively sealing thecladding air channels at both distal ends of thefiber using a fusion splicer. The PCFs typically25–30 cm in length were first filled with PAHsolutions at pH 4–7 for polymer self-assemblyon the air channels, then with Ag colloidalsolution at pH 5.6. Intense rinsing withMilli-Q water was applied after each deposi-tion step to remove unbound PAH or Agnanoparticles from the channels. The nega-tive surface charge of silica, whose chargedensity increases with pH >3, affords controlover the amount of adsorbed polycation andthus the availability of binding sites for Agnanoparticles.[34] Ag nanoparticle coveragedensity strongly correlates with thesolution pH used during PAH adsorptionstep, with more nanoparticles immobilized inthe air channels at higher pH values of thePAH assembly (Fig. 2B). We were specificallyinterested in exploring the lower limit of Agnanoparticles (0.1–5 particlemm�2) achievedat PAH deposition pH <6.2 as we revealedsolid evidence that higher nanoparticlecoverage density effectively terminates thewaveguide property of PCF. SEM microscopy

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Figure 3. A,B) SERS spectra of various R6G solutions using 20 cm long PCFs coated with Agnanoparticles: A) solid-core PCF, 0.1 particlemm�2; and B) hollow-core PCF, 0.2 particlemm�2.C,D) SERS intensity of 10�6

M R6G as a function of fiber length for solid-core PCF (C) andhollow-core PCF (D). Raman excitation wavelength was 632.8 nm; the laser power 5mW; andacquisition time 60 s.

images of various segments of the PCF showed similar coverageof discrete Ag nanoparticles (Fig. 2C–F). These results indicatethat our experimental approach allows a high degree of control inimmobilization of Ag nanoparticles inside the microscopic airchannels along the entire PCF length.

The PCFs with immobilized Ag nanoparticles were filled withRhodamine 6G (R6G) solution in Milli-Q water at 200 psipressure differential. Raman measurements were performedusing a forward propagating transmission geometry at anexcitation wavelength of 632.8 nm with a custom-built Ramansetup.[18] Note that the use of 532-nm excitation wavelength,generally preferable for SERSmeasurements using individual Agnanoparticles, was not pursued because of severe interferencefrom the fluorescence of R6G. Figure 3A and 3B compare theSERS spectra of R6G solutions using a 20-cm-long solid-core PCFand hollow-core PCF containing immobilized Ag nanoparticles ofparticle coverage densities of �0.1–0.2 particlemm�2. In the caseof solid-core PCF, dominant peaks at 400–1200 cm�1 (with themain peak at 485 cm�1 assigned to the Si-O bending vibration)originate from direct laser excitation of the silica core.[14] In thecase of hollow-core PCF, however, Raman spectra weredominated by intense water peak around 3381 cm�1 caused bylaser excitation of the liquid core. The silica bands present areweak as expected.With both PCF types, peaks at 1307, 1351, 1509,1570, and 1645 cm�1 are associated with characteristic aromaticC�C stretching vibrations of R6G molecule.[35,36] The R6Gvibrational features were evident even at 10�7M and becamemore distinct and intense at higher R6G concentrations.

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Importantly, for planar substrates[37] with thesame Ag nanoparticle coverage density, noSERS could be observed for a much higherR6G concentration of 10�5M.

The effect of PCF length on SERS intensitywas evaluated in fiber cut-back experiments.PCFs with Ag nanoparticles immobilized tovarious particle coverage densities were filledwith 10�6M R6G. The dependence of themeasured SERS R6G peak intensity at1351 cm�1 on the fiber length of solid-corePCF and hollow-core PCF is shown in Fig. 3Cand Fig. 3D. In both cases, SERS intensityincreased with the fiber length at the lower endof the Ag nanoparticle coverage (below0.5 particlemm�2), indicating a net signalaccumulation along the optical path. Thisdemonstrates a new feature enabled by thefull-length functionalized PCF, i.e., anincrease in sensitivity in SERS sensing via afacile increase in the PCF length. Note that theRaman signal declined with the fiber length athigher nanoparticle densities. The pattern is aresult of the interplay between accumulativeRaman signal gain and the progressivescattering and absorption loss of boththe excitation light intensity and the Ramansignal intensity as the path length increases.The Raman gain prevails at lower nanoparticlecoverage densities where light attenuation islow, whereas at higher nanoparticle coverage

densities the loss overwhelms the Raman gain over the fiberlength. Importantly, substantial gain in Raman sensitivity can beachieved using longer PCF with very low nanoparticle densities(such as 0.1–0.2 particlemm�2).

Our SERS spectral measurements in the forward propagatinggeometry have conclusively shown both the SERS-active andwaveguiding features of the PCF when Ag nanoparticles arecontrollably immobilized inside the air channels. They do notallow, however, the assessment as to how the waveguide core(fundamental core mode) and the cladding structure (claddingmode) contribute to the overall Raman intensity. We demonstratehere that hyperspectral imaging at the distal end of the PCF offersan excellent means of mapping the intensity distribution of aspecific Raman line when overlaying with the cross-sectionalmicrostructure of the PCF. The hyperspectral Raman imagingwas performed on 20-cm-long SERS-active solid-core PCF andhollow-core PCF, both coated with 0.5 particlemm�2 Agnanoparticles, also using forward-propagating Raman signal atthe distal end of the PCF. A relatively high 10�5M R6Gconcentration was used to allow hyperspectral Raman imaging asthe employment of a line filter substantially suppresses theRaman signal intensity. Shown in the far left of Fig. 4A andFig. 4B are Raman spectra obtained respectively from solid-corePCF and hollow-core PCF with the cladding air channels (forsolid-core PCF) or the hollow core (for hollow-core PCF) filledwith 10�5M R6G solution. The right three images in Figure 4Aand 4B show Raman intensity distribution of silica (485 cm�1),R6G (1351 cm�1), and water (3381 cm�1) in solid-core PCF and

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Figure 4. SERS spectra of 10�5M R6G and hyperspectral Raman images of silica, R6G, and water from solid-core PCF (A) and hollow-core PCF (B) with

immobilized Ag nanoparticles of �0.5 particlemm�2 in coverage density. The measurements were done using forward-propagating geometry at anexcitation wavelength of 632.8 nm, a power of 5mW, and an acquisition time varying from 0.2 s for strong signal intensity to 20 s for weaker one.

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hollow-core PCF, respectively. For solid-core PCF (Fig. 4A), thepropagating core mode contributed primarily to the measuredRaman intensity of both silica (by direct excitation) and R6G (viaevanescent field interaction), with effective confinement of theSERS intensity of R6G to the light-guide core. Weak contributionsby the triple-air-channel junctions (each junction is like a silicalight guide surrounded by three air channels) of both silica andR6G are also seen in the hyperspectral images in the claddingregion (middle two images in Fig. 4A). No water Raman imagecould be acquired from solid-core PCF.

In contrast, Raman imaging of hollow-core PCF yieldedunusual dashed ring-like Raman intensity distribution of silicasurrounding the liquid core (left image in Fig. 4B), a clearevidence that the triple air channel junctions between the coreand two adjacent air channels allowed light guiding, i.e., forwardpropagating of silica signal. The Raman distributions of R6G andwater (right two images in Fig. 4B) were also confined in theliquid core, suggesting dominant core mode contribution to themeasured Raman intensities. These results provide strongevidence that the liquid core is a robust waveguide, and thatthe SERS signal of R6G can be effectively coupled andforward-propagated along the liquid core. The Raman image ofthe water line appears to be larger in diameter than that of R6Gdue to the dispersion property of light propagation.[38,39]

In summary, we have shown that the full-length SERS-activePCF optofluidic platform can be endowed the capacity ofaccumulation of Raman signal along the fiber length, demon-strating a novel way of enhancing SERS measurement sensitivityby a simple increase in the fiber length. We have shown thecompetitive interplay between SERS gain and light attenuation asthe optical path length increases for a PCF containingimmobilized Ag nanoparticles, with low particle coverage densitybeing essential for a net accumulative Raman gain along the fiber

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length. Key to achieving the SERS-active PCF optofluidic platformlies in the high degree of control of nanoparticle coverage densityvia polyelectrolyte-based surface modification technique whichcan be applied to PCF far longer than the 20 plus cm fiber lengthwe explored in this study.

Of particular interest and as a future direction is theintroduction of sparsely distributed nanoparticle clusters (e.g.,dimers and trimers) as SERS hot spots inside the air channels ofPCFs for further increase of detection sensitivity down to thesingle-molecule level. SERS-active PCF optofluidic platform isinherently easy for system integration, robust in light couplingand harvesting, and unparalleled in optical path length forlabel-free and sensitive identification. Its potential applicationsinclude fundamental studies of chemical, biological, and catalyticinteractions in geometrically confined systems; chemical andbiological sensing and detection; and in situ process monitoring.Specific examples include detection of contaminants in waterfor environmental protection, diagnosis of body fluids forhealthcare, and in-line measurements of trace chemicals forquality control.

Experimental

PCFs: Solid-core PCF (model RB087) and hollow-core PCF (modelHC19-1550-01) used in our investigation were obtained respectively fromOFS Laboratories, Somerset, NJ, USA and Crystal-Fibers A/S, Birkerød,Denmark. Specifically, solid-core PCF contains a �2.5-mm silica coresurrounded by an array of 126 cladding air channels �3.5mm in diameter.Hollow-core PCF has a large hollow core of �20mm in diametersurrounded by 282 cladding air channels with an average diameter of�3.5mm.

Synthesis of Ag Nanoparticles: Ag nanoparticles were produced by aUV-assisted citrated reduction based upon modified Lee and Meisel recipe[34]. sodium citrate (8mg) in water (0.8mL) was dropwise added into

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AgNO3 solution (1mM, 40mL) in a glass beaker (50mL) under continuousstirring and then the mixture transferred under UV lamp (UV Flood CuringSystem, Cure Zone 2 (CON-TROL-CURE, Chicago, IL). A water bath wasused to keep the AgNO3 and sodium citrate mixture solution below waterbath level to avoid locally increased temperature induced by strong lightabsorption of glass beaker. The mixture solution was kept under UV lightwith continuous stirring for �4 hr. The water bath was changed every halfan hour during the nanoparticle synthesis with a temperature range of25–40 8C. The z-potential of produced Ag colloids measured by means ofdynamic light scattering using Zetasizer Nano Series (MalvernInstruments) was �35mV. Particle size of the Ag colloids characterizedboth by transmission electron microscopy (TEM, Philips CM20) and byZetasizer was 35 nm� 5 nm. UV–visible absorption spectrum of Agnanoparticle solution obtained from the Synergy HT Multi-DetectionMicroplate Reader (BioTek Instruments) showed an absorbance peak at408 nm.

Immobilization of Ag Nanoparticles in PCFs and on PlanarSubstrates: The PCF with both distal ends freshly cut (or sealed in thecase of hollow-core PCF) was filled with PAH (weight-average molecularweight of 70000 gmol�1, 2mgmL�1) in water at certain solution pH undera pressure differential of �200 psi. The applied pressure was then quicklyreleased to leave the PAH solution inside the fiber for a deposition time of20min. After thorough rinsing the PAH-modified fiber by continuousflowingMilli-Q water at the same pH as used in PAH solution, the unboundand/or weakly absorbed PAH molecules were completely removed fromthe fiber channels. In a similar way, the Ag collodial solution(1012 particlesmL) was filled into the fiber and sit for 30min to allowfor Ag nanoparticles immobilize. Finally, the fiber was copiously rinsed bycontinuous flowing of Milli-Q water to remove free Ag nanoparticles.Custom-made glass cells were prepared using a previously developedmethod [37] for immobilization of Ag nanoparticles on planar substrates.The glass cells were filled with PAH (2mgmL�1) in water, pH adjusted to4.5, for 20min, and thoroughly rinsed with Milli-Q water at pH 4.5. Finally,they were filled with the Ag colloidal solution for 30min, then rinsed withMilli-Q water, and kept full with Milli-Q water for future SERSmeasurements. SEM measurements revealed a coverage density of0.2 particlemm�2 by immobilized Ag nanoparticles on the bottom of thecell.

SERS Measurements and Raman Imaging: All SERS and Raman imagemeasurements were conducted in a forward propagating geometry usingthe PCFs with immobilized Ag nanoparticles. A home-built Ramanspectrometer in conjunction with high-resolution hyperspectral Ramanimaging system was used in the study.[18] For comparison, SERSmeasurements on planar substrates were carried out using the samesystem as that for fiber measurements but reconfigured for plannergeometry.

Acknowledgements

We thank Dr. Dennis J. Trevor of OFS Laboratories for supplying solid-corePCF, Dr. Yinian Zhu and Dr. Rainer Martini of Stevens for their valuablediscussions, and Mr. Vassili Belov for his help with the preparation ofthe manuscript. This work was supported by NSF under ECS-0404002 andby the US Army ARDEC under W15QKN-05-D-0011.

Received: December 7, 2009

Published online:

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