Functionalization of Microstructured Optical Fibers by Internal Nanoparticle Mono-Layers for Plasmonic Biosensor Applications

218 IEEE SENSORS JOURNAL, VOL. 12, NO. 1, JANUARY 2012

Functionalization of Microstructured Optical Fibersby Internal Nanoparticle Mono-Layers for

Plasmonic Biosensor ApplicationsKerstin Schröder, Andrea Csáki, Anka Schwuchow, Franka Jahn, Katharina Strelau, Ines Latka, Thomas Henkel,

Daniell Malsch, Kay Schuster, Karina Weber, Thomas Schneider, Robert Möller, and Wolfgang Fritzsche

Abstract—For fully integrated next-generation plasmonic de-vices, microstructured optical fibers (MOFs) represent a promisingplatform technology. This paper describes the use of a dynamictechnique to demonstrate the wet chemical deposition of gold andsilver nanoparticles (NPs) within MOFs. The plasmonic struc-tures were realized on the internal capillary walls of a three-holesuspended core fiber. Electron micrographs, taken of the insideof the fiber holes, confirm the even distribution of the NP in theMOF over a length of up to 6 m. Accordingly, this procedure ishighly productive and makes the resulting MOF-based sensorspotentially (very) cost efficient. In proof-of-principle experimentswith liquids of different refractive indices, the dependence of thelocalized surface plasmon resonance (LSPR) on the surroundingswas confirmed. Comparing Raman spectra of MOFs with andwithout NP layers, each one filled with crystal violet, a significantsignal enhancement demonstrates the usability of such function-alized MOFs for surface-enhanced Raman spectroscopy (SERS)experiments.

Index Terms—Localized surface plasmon resonance (LSPR),metal nanoparticle (NP), microstructured optical fiber (MOF),surface-enhanced raman spectroscopy (SERS).

I. INTRODUCTION

T HE DEVELOPMENT of the next generation of photonic-plasmonic devices has aroused interest in fibers which in-

corporate metallic thin films or nanoparticles (NPs). Localizedsurface plasmons (LSP) are generated using NPs. With LSP afixed and determined state of polarization is not needed (when

Manuscript received November 12, 2010; revised April 11, 2011; acceptedApril 11, 2011. Date of publication May 10, 2011; date of current version De-cember 01, 2011. This work was supported in part by the Executive Committeeof the IPHT Jena, and is a result of the cooperation of different groups in theInstitute of Photonic Technology (IPHT), Jena, Germany. An earlier version ofthis paper was presented at EWOFS 2010 in Porto, Portugal, and was publishedin its proceedings. The associate editor coordinating the review of this paper andapproving it for publication was Prof. Jose Santos.

K. Schröder is with the Department of Fiber Sensor Systems, Insti-tute of Photonic Technology (IPHT) Jena, 07745 Jena, Germany (e-mail:[email protected]).

A. Csáki, A. Schwuchow, F. Jahn, K. Strelau, I. Latka, T. Henkel,D. Malsch, K. Schuster, K. Weber, T. Schneider, R. Möller, and W. Fritzscheare with the Institute of Photonic Technology (IPHT) Jena, 07702 Jena,Germany (e-mail: [email protected]; [email protected];[email protected];; [email protected]; [email protected]; [email protected]; [email protected];[email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2011.2144580

NPs are circular or noncircular NPs are arranged in a nonregularorder), in contrast to surface plasmon resonance [2]. The sensi-tivity of the plasmon excitation to changes in the refractive index(RI) of the surrounding dielectric renders the photonic-plas-monic devices excellent candidates for optical sensing. All-fiberdevices offer a number of advantages over conventional planarplasmonic structures in that they are cheap, compact, robust,flexible, and compatible with existing fiber infrastructures.

In this paper, we present the experimental realization ofNP-based plasmonic structures on the internal capillary wallsof MOFs. It has been proposed that the evanescent field couldbe used for the sensing of gases and liquids within the holes ofthe fiber [3]. The deposition of NP inside the fiber voids hasthe potential to explore new directions in micro/nanomaterialstechnology.

Recently, high-pressure chemical deposition techniques orstatic procedures, respectively, have been developed for the in-clusion of a wide range of technologically important materials,such as silicon and germanium, within MOF capillaries. We pro-pose a dynamic low pressure deposition of metal NP, in whichNP are chemically attached in a self-assembled monolayer(SAM) to the inner surfaces of the MOF. With this nanoparticlelayer deposition (NLD) method, an even deposition of NP ispossible without the threat of damaging the thin struts of thefiber. Possible fields of applications include surface-enhancedraman spectroscopy (SERS) [4] and surface-enhanced fluores-cence, which use the field enhancement near the particle surface[5], refractive index measurement, biomolecule detection [6],or THz waveguiding [7], respectively.

Metal NP are under investigation in LSPR sensors as trans-ducers for signal transfers. The effect is based on the spectralshift of the localized plasmon resonance which occurs when theanalyte binds to the particle surface. In [8], the performancesof LSPR sensing and surface plasmon resonance (SPR) sensingwere compared for a special sensing example.

MOFs which are modified with gold NPs could also be in-teresting for a fiber-based analysis of samples via SERS. Thisanalytical method enables the enhancement of the intrinsicallyweak Raman signal since the analyte molecules interact with ananostructured metal surface. Commonly used metals for theseso-called SERS substrates are the coin metals, gold, and silver.When using appropriate SERS substrates an increase of sensi-tivity by several orders of magnitude and even single moleculedetection can be achieved [9]. Besides the enhancement effect,the application of a vibrational spectroscopic method enables

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SCHRÖDER et al.: FUNCTIONALIZATION OF MOFS BY INTERNAL NP MONO-LAYERS FOR PLASMONIC BIOSENSOR APPLICATIONS 219

Fig. 1. (a) Micrograph of the end face of the MOF with NP layer. (b) NP mod-ified fibers appear colored (upper fiber with gold NP) in comparison to unmod-ified fibers (lower fiber).

the detection and identification of nearly every analyte due to thehighly specific Raman spectra of each molecule, the so-calledmolecular fingerprint. In a proof-of-principle experiment, thesuitability of the fabricated MOF for SERS measurements wasdemonstrated by detecting the standard analyte molecule crystalviolet.

II. SAMPLE PREPARATION

The MOFs we employed were prepared in-house by theso-called “stack-and-draw” technology from preforms of highpurity silica glass. For the first filling experiments, we useda three-holed suspended core fiber [Fig. 1(a)] with a corediameter of 3.2 , hole diameters of 30 (radial), and anouter diameter of 125 . This fiber type was chosen for ourproject because it was readily available and easy to handle.

Other fiber concepts predict better signal-to-noise ratio for thesensor evaluation. Big hole diameters (no smaller than 20 )allow easy handling during fiber filling, so that long fiber lengths(about 6 meters) can be filled with moderate pressure (approxi-mately 8 bar).

Originally, the cleaved MOF was fixed to the feeding tubes(PVC) via a conventional adapter. A peristaltic pump, adjustedto a rate of 1 , was used for the delivery of the liquids.With this delivery rate a complete liquid exchange in a 40 cmlong piece of the described fiber type requires about 15 min.

Distilled water was fed through the fiber for 20 min to re-move possibly present residuals originating from fiber fabrica-tion. After cleaning and surface activation steps with a mixtureof different acids, a silane solution (APTES) was fed through forat least 1 h. The silane chemically binds to the surface ofthe fiber and provides the coupling sites for the NP (for detailssee [10]–[12]). By using a well chosen mixture of bonding andnonbonding silanes for the NLD process, we should be able toadjust the particle density on the capillary surfaces. In the exper-iments described here, we used a very high population density.After a final washing step, the fiber was prepared to be filledwith NP.

Different NP, prepared via the Turkevich/Frens methodmethod [13], [14], were tried due to their deposition behavior,even though, only one particle type in one fiber sample. Trian-gles, spheres or nanorods, respectively, were used with sizesranging from 12 to 120 nm. Usually, the NP solution, whichmust be pumped through the MOF, has a specific color. Forinstance, light red for gold NP and blue for silver triangles

Fig. 2. Photograph of a microfludic chip (16 mm� 12.5 mm) with fiberchannel with a glued fiber and five fluidic channels. The adapters for fluidiccoupling are not attached yet.

with edge length. A “spent” solution is impoverishedof NP and loses its color. With the help of this effect an easycontrol of the process is possible: as long as some binding sitesare unoccupied, the solution which leaves the fiber at its end isclear. By the time all available binding sites are occupied, thesolution becomes colored because of the remaining NP in it.This filling process takes about 30–60 min for fiber lengths ofapproximately 40 cm. After a cleaning and drying process thefiber appears colored, with the brightness depending on the NPdensity [see Fig. 1(b)].

A major improvement in the preparation efficiency was theutilization of a microfluidic chip [15] for fiber–fluidic coupling.This chip was specifically designed for this application (Fig. 2).Within the chip we created a section for fiber fixation (viagluing) and five different fluid ports which can be connectedwith pressure stable adapters. The chip device was preparedwith wet etching technology and anodic bonding of two glasssubstrates using a bond support layer (details are given in[16]). In brief, channels were etched in two glass substratesusing Hydrofluoric acid. Closed channels were constructedby bonding the two glass substrates together using an anodicbonding process mediated by a silicon bond support layer.The channel diameter is adapted to the fiber’s outer diameter.A V-slot was incorporated to ease the mounting of the MOFinto the fiber channel from the chips edge. The five differentports connected to the fiber channel improve the handlingduring the filling process because they make tube cleaning andtube exchanging unnecessary. In addition, as a result of thesmaller and shorter delivery tubes, the necessary amount of NPsolution decreases significantly. With the coupling chip, thefluidic–fiber connection can be made leakproof for pressuresof up to 10 bar, allowing the NLD of much longer fibers inone procedure. A NP modified fiber approximately 6 meters inlength was the longest tested in our experimentation. This canbe cut into desired lengths for sensors afterwards. Accordingly,this procedure is highly productive and makes the resultingMOF-based sensors potentially very cheap.

As shown in Fig. 1, the easiest way to confirm a successful NPattachment on the inner surfaces is achieved by optical inspec-tion, e.g., with a light-optical microscope, either from the endface or from the side. To check the NP density and layer uni-formity, scanning electron micrographs (SEM Zeiss DSM 960)


Fig. 3. SEM-pictures (45 angled view) from the two fiber ends. (a) Filling side. (b) Exit side of a MOF modified with 30 nm Ø gold spheres layer.

were taken of different sections of the modified fibers (Fig. 3).In preparation for the scans these fibers were cut into pieces witha fiber optic cleaver. No images of the NP layer could be takenof the top view as the only access to it was from the fiber endface. Fig. 3(a) and (b) show images of a suspended core fiberwith a 30 nm gold sphere layer. The images are taken from op-posite ends of a 30 cm fiber, from an angled position (45 ), anddemonstrate that the population density is nearly constant overthe entire fiber length, namely 443 and 459 forthese two pictures. In addition, good homogeneity of the NPlayer is documented; there were no large areas without NP andonly a few clusters were detected. In comparison to other testedNP layer deposition techniques, with the NLD the NP densityis also homogeneous along the hole’s diameter, independent ofthe local curvature of the capillary channel cross section.

III. MEASUREMENTS AND DISCUSSION

To check the usability of the NP modified MOF, we carriedout proof-of-principle measurements with our fiber samples.We checked that an extinction peak was achieved in the LSPRmeasurements and also that as the refractive index of the sur-rounding material was changed, this peak moved correspond-ingly. In order to determine if NP modified fibers could be usedas the substrate in a SERS method, the fibers were filled withcrystal violet and the Raman spectrum was analyzed.

A. LSPR Characterization

To use the NP layer as a transducer for refractive indexchange detections, like DNA analytics, a transmission spectrumor transmissions at some specially defined wavelengths have tobe measured.

From this transmission, we calculate the extinction spectrum:

This extinction spectrum included the wavelength dependentabsorbing and scattering behavior of the NP solution or NPlayer.

Fig. 4. Experimental setup for transversal transmission measurements.

Because of the expected high signal-to-noise ratio, layerswith high NP density ( ) were prepared.

1) Measurement Setup: Accessing both light and measurand(e.g., fluids) via the same end of the MOF is a difficult problem.A possible method for characterizing the NP layer is to illumi-nate and collect the light transversally to the fiber axis (Fig. 4).With this approach both fiber ends are available for filling withthe fluids to be tested. The fiber–fluidic coupling can be per-formed in the same way as for NLD. As a result of the highNP density there was enough NP–light interaction in the illumi-nated area to measure clearly visible extinction spectra (Fig. 5).An illumination and measurement along the fiber axis wouldsuffer from the difficulties of strong attenuation with regard tothe metallic NP, which is not limited to the resonance region,and to the demanding fiber alignment.

A white light source or a Xenon lamp were used fortransversal fiber illumination. The plasmonically modifiedMOF was positioned into the collimated beam of that source.Using a well matched collecting fiber the transmission spectraare lead into the spectrometer for analysis. This fiber wasdirectly connected to a spectrometer (Instrument SystemsSpectro 320–164 or homemade compact spectrometer MINOS,respectively [17]).

A MOF without NP was used as the reference.


Fig. 5. Extinction spectra of MOFs with internal NP layers of: Ag triangle withbase length of about 50 and 100 nm and Au spheres with diameter 30 nm.

First, measurements with this setup were made on MOFsplasmonically modified with different types of NPs. In Fig. 5,typical extinction spectra for different Ag triangles and Auspheres are diagrammed. These spectra agree very well withthose taken with the same NP in solution [18]. In some cases,a subsidiary peak appeared when the NP were bound onto asurface. This could be caused by surface effects [19], nonspher-ical size distribution of the NP, and/or dipole-dipole interactionbetween the particles in the plasmonic layers.

All measurements presented in the following were made with30 nm Ø Au spheres.

2) Results: To determine the sensitivity of the LSP resonanceto RI changes, liquids with defined RI were injected into theMOF sample arranged in a similar setup as for the particle layerpreparation. All measurements were taken on exactly the sameplace of the MOF and the collecting fiber, avoiding uncertain-ties from differing alignments. This seems to be the only wayto guarantee reproducible spectra. Transmission spectra weretaken [Fig. 6(a)] and after each liquid measurement the fiber wascleaned with water, dried, and a control spectrum was recorded.This procedure tested the complete removal of the liquids, aswell as the stability of the bond of the NP to the silica surface.If needed, the cleaning procedure can be repeated but we foundthat no more than one additional cleaning was ever necessary.

To determine the sensitivity, i.e., the wavelength shift of theLSP resonance versus refractive index shift, not only the mainpeak – which relates to the NP resonance itself, but also thesecond peak – which was observed only with surface bound NP,were investigated. At first, a baseline subtraction was achieved.Following the fitting of both peaks, their position is given over

, the RI of the surrounding medium, in Fig. 6(b). For the firstpeak, we got a sensitivity , roughly approximatedas a linear fit of the measured curve from to 1.515,of ( ). Thesecond peak’s sensitivity was determined with the same approx-imation as . To assess these results, it isnecessary to take into account that the second peak is smallerand therefore, the peak position cannot be determined withouthigher inaccuracy than that of the first peak.

To evaluate the sensitivity of the first peak, extinction crosssections for a single spherical NP in different surrounding ma-

Fig. 6. (a) Baseline subtracted extinction spectra for a MOF with internal NPlayers of 30 nm Au spheres and filled with liquids of different RI. (b) Twomaxima of these spectra over the prevailing RI.

terials were calculated. Details are discussed in a former publi-cation [1]. The calculated RI sensitivity ofis reproduced by our measurement.

The sensitivity of the LSP resonance to the surroundingrefractive index can be increased as it is dependent on thesize, the shape, and the material of the NP. For example,when using core-shell-particles the sensitivity should reachvalues of [20]. These findings will be furtherinvestigated in upcoming experiments.

The aforementioned measurements were performed afterremoval of the protective acrylate coating. As the removal ofthe coating is disadvantageous for the mechanical stability ofthe fiber, its influence on the transmission measurements waslooked into. The coating material Acrylat DeSolite® 3471–3-14has no absorption bands in the wavelength region of interest.Comparison of transmission spectra of coated and uncoatedfibers (Fig. 7) revealed that the spectral position or the width ofthe resonance peak, respectively, were not affected.

The measurement through the fiber coating opens the oppor-tunity for a quality control of the whole fiber directly after NPdeposition process.


Fig. 7. Comparison of extinction spectra measured through the coating andthrough an uncoated MOF, both filled with air.

Fig. 8. Measured spectra of a 100�� crystal violet solution. While the Ramanspectrum, measured in an unmodified fiber, shows only noise and no specificbands, the SERS spectrum, detected in a NP modified fiber, shows a spectrumwith many specific bands.

B. Application as Fiber Optic SERS Sensor

1) Measurement Setup: The coating was removed from thefiber in preparation for the measurements. Subsequently, onefiber with NP coating and one fiber without NP coating were in-cubated with a 100 solution of the standard analyte crystalviolet. The SERS measurements were performed using a con-focal Raman microscope system alpha300 R (WITec, Ulm, Ger-many) and a 514 nm argon ion laser served as the excitationsource. The laser light was focused through a 20 microscopeobjective onto the prepared fiber, resulting in laser strength ofapproximately 1 mW on the sample. The 180 back-scatteredlight was detected with a CCD camera (1024 127 pixels) op-erating at 208 K. The integration time for each spectrum was 1 s.

2) Results: In the first test, the possibility of using the pre-pared MOF to detect an analyte via SERS was demonstrated.Therefore, a solution of crystal violet was pumped through a NPcoated fiber followed by a SERS measurement. The results areshown in Fig. 8. To demonstrate the enhancement effect of theNP layers, a MOF without NPs was prepared and measured aswell. In comparison to the modified fiber, no Raman spectrumof the analyte was detectable.

Fig. 9. (a) Microscopic and (b) Raman images of a part of the with 30 nm Auspheres coated MOF. (a) Microscopic image with the scanned area. (b) Falsecolor image of the scanned region. The bright areas display regions with a highSERS intensity of the 1368 cm-1 peak of the analyte molecule crystal violet.

For the application of SERS as an analytical tool an even dis-tribution of SERS active areas across a SERS substrate is nec-essary. The uniform distribution of the NPs within the fiber wasshown by SEM measurements. Due to this uniform distributiona homogeneous SERS signal was expected within the MOF. Toinvestigate the distribution of the SERS signal, an image scanwith a size of 136 118 was analyzed. Within this area16048 spectra of the analyte molecule crystal violet were mea-sured. To visualize the distribution of the SERS signal, the char-acteristic Raman mode at 1368 , a N-phenyl stretch vibra-tion was taken and their integrated SERS intensity was plotted.The results are shown in Fig. 9. The false color image clearlyshows the SERS active areas within the fiber where the NPcoating is present. Within the silica glass material of the fiber,where no NPs were immobilized, no noteworthy SERS signalscould be detected. These observations indicate that the Ramansignal of the analyte molecule crystal violet is enhanced dueto the proximity to the metal NPs within the fiber. Thus, theMOF with NP layers fiber can be used as substrate for SERSexperiments. An exact enhancement factor was not calculatedyet because improvements in the measurement setup and sensordesign are still under investigation.

IV. SUMMARY

Self assembled monolayer (SAM) techniques were used forthe deposition of metal NPs into the channels of the microstruc-tured optical fibers. In our first experiments, a suspended corefiber with three channels was used. SAM techniques are veryflexible concerning the material, shape and the size of the em-ployed NPs, which offers the possibility of tailoring the de-posited layer for the individual sensor applications. An adaptedmultichannel microfluidic chip for the consecutive steps of thecoating procedure ensures a reproducible, cost-effective, andcontamination-free NP deposition. Optical inspection as well aselectron microscopic evaluation confirmed the even depositionon the inner walls and constant population density over the fiberlength.


The NLD technology offer the opportunity to design a sensorspecifically for an intended application.

In an easy to use transversal measurement setup which sep-arates the light from the analyte “path,” the sensitivity of theLSPR peak and the subsidiary peak to the refractive index ofthe surrounding medium was measured and the correlation withsimulations confirmed. The promising experiments with regardto a possible application as fiber-optic SERS sensor were per-formed, using crystal violet as model substance.

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[18] A. Csaki, F. Jahn, I. Latka, T. Henkel, D. Malsch, T. Schneider, K.Schröder, K. Schuster, A. Schwuchow, R. Spittel, D. Zopf, and W.Fritzsche, “Nanoparticle layer deposition for plasmonic tuning of mi-crostructured optical fibers,” Small, vol. 6, no. 22, pp. 2584–2589, Doi:10.1002/smll.201001071.

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Kerstin Schröder graduated as an Engineer inelectro-technique/optical communication techniquefrom Technische Hochschule Karlsruhe, Karlsruhe,Germany, in 1996 and a Dr. rer. nat. from the PhysicsFaculty, Friedrich-Schiller-Universität Jena, Jena,Germany, in 2001.

She works at the Institute of Photonic Technology(IPHT) Jena, Germany, since 1996, in the devel-opment of fiber-optic sensing systems and theirapplications.

Andrea Csáki was born on October 26, 1967in Nagykörös. She received the Ph.D. de-gree in nanotechnology/biology at the IPHTJena/Friedrich-Schiller-Universität (biological phar-maceutical faculty) Jena, Jena, Germany, in 2003.

Since 2003, she has been a Research Scientistat IPHT. Her first research aim was focused on theDNA-based molecular nanotechnology. Currentworks are focused on the molecular plasmonics andplasmonic bioanalytics.

Anka Schwuchow was born on April 18, 1970, in Jena, Germany. She re-ceived her diploma in communications engineering at the Technische Univer-sität Dresden, in 1994.

She works at the Institute of Photonic Technology (IPHT) Jena, Germany, asa Research Scientist, since 1994, active on characterization of rare earth dopedand other special fibers and glass samples.

Franka Jahn was born on November 15, 1063 in Jena, Germany. She receivedthe Diploma in mathematics and physics lectureship at the Friedrich-Schiller-Universität (mathematical faculty) Jena, Jena, Germany, in 1985.

Since 1990, she has been a Technical Scientist at the Institute of PhotonicTechnology (IPHT), Jena, Germany. She is a specialist for TEM and SEM mea-surements and characterization of plasmonic structures.

Katharina Strelau, photograph and biography not available at the time ofpublication.

Ines Latka studied physics at the Technical Univer-sity Ilmenau, Ilmenau, Germany, and the Friedrich-Schiller University Jena, Jena, Germany.

Currently, she is with the Institute of PhotonicTechnology (IPHT), Jena, Germany. After her degreeshe worked several years in the field of fiber-opticsensors for industrial applications, particularly withfiber Bragg gratings. In 2009, she joined the molec-ular imaging group at IPHT. Her new interests arefocused on CARS applications as well as fiber-opticendoscopes, e.g., employing Raman scattering.

Thomas Henkel studied bio-organic chemistry at the Friedrich-Schiller-Uni-versität Jena, Jena, Germany, and received the Dr. rer. nat. degree in 1994.

Since 1997, he has been with the Institute of Photonic Technology (IPHT),Jena, active in microfluidics and lab-on-a-chip technology.


Daniell Malsch, photograph and biography not available at the time ofpublication.

Kay Schuster, photograph and biography not available at the time ofpublication.

Karina Weber was born in 1979. She studied biotechnology at the University ofApplied Sciences Jena, Jena, Germany (1997-2002). After her studies in chem-ical and process engineering at the Technical University, Clausthal-Zellerfeld(2002-2004), she received the Ph.D. degree from the University of Applied Sci-ences Jena in cooperation with the Technical University in Clausthal-Zellerfeldin 2006.

Currently she is working at the Friedrich-Schiller-Univisität Jena. Her re-search is focused on the development and optimization of novel chip-based de-tection technologies for multiplex analysis of biomolecules and low molecularweight substances. Further she works on modification and functionalization ofsensor surfaces.

Thomas Schneider received the Degree in biology from the Friedrich-Schiller-University (FSU) Jena, Jena, Germany, in 2007. Currently, he is working to-wards the Ph.D. degree in nano-biophotonics at the Institute of Photonic Tech-nology (IPHT), Jena, Germany.

His research activities focus on the spectroscopic characterization of singlemetal nanoparticles and their application as optical biosensors.

Robert Möller, photograph and biography not available at the time ofpublication.

Wolfgang Fritzsche, photograph and biography not available at the time ofpublication.

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Functionalization of Microstructured Optical Fibers by Internal Nanoparticle Mono-Layers for Plasmonic Biosensor Applications