Nanoparticle Layer Deposition for Plasmonic Tuning of Microstructured Optical Fibers

Plasmonic nanoparticles with spectral properties in the UV-to-near-IR range have a large potential for the development of innovative optical devices. Similarly, microstructured optical fibers (MOFs) represent a promising platform technology for fully integrated, next-generation plasmonic devices; therefore, the combination of MOFs and plasmonic nanoparticles would open the way for novel applications, especially in sensing applications. In this Full Paper, a cost-effective, innovative nanoparticle layer deposition (NLD) technique is demonstrated for the preparation of well-defined plasmonic layers of selected particles inside the channels of MOFs. This dynamic chemical deposition method utilizes a combination of microfluidics and self-assembled monolayer (SAM) techniques, leading to a longitudinal homogeneous particle density as long as several meters. By using particles with predefined plasmonic properties, such as the resonance wavelength, fibers with particle-adequate spectral characteristics can be prepared. The application of such fibers for refractive-index sensing yields a sensitivity of about 78 nm per refractive index unit (RIU). These novel, plasmonically tuned optical fibers with freely selected, application-tailored optical properties present extensive possibilities for applications in localized surface plasmon resonance (LSPR) sensing.


Introduction
Noble metal nanoparticles show distinguished optical properties due to resonant behavior based on the density oscillations of their conductive electrons. These oscillations excite the so-called particle plasmon polaritons with defi ned localized surface plasmon resonance (LSPR). [ 1 , 2 ] The position of the LSPR strongly depends on the material properties, [ 3 , 4 ] composition (e.g., alloy [ 5 ] or core-shell [ 6 , 7 ] ), dimension, and shape of the particles. These factors can be adjusted by chemical synthesis. Using colloidal synthesis, gold, silver, copper,

Nanoparticle Layer Deposition for Plasmonic Tuning of Microstructured Optical Fibers
platinum, and palladium nanoparticles can be prepared in the shape of spheres, [8][9][10] triangles, [ 11 , 12 ] nanorods, [ 8 , 9 ] and other geometries. [ 9 -11 ] However, since the LSPR effect is an interface phenomenon, not only the particle properties but also the immediate surroundings determine the optical behavior. Plasmon particles show a large spectral response to changes in the surrounding media, for example, by binding analyte molecules onto the particle surface using (bio)affi nity interactions between capture and probe, [ 12 , 13 ] which indicates their potential for LSPR sensing. [ 12 ] Different kinds of plasmon particles have varying levels of sensitivity. Particles with anisotropic geometries and core-shell particles offer higher sensitivity compared to spheres and homometallic particles, respectively. [ 14 ] Plasmon particles can act as transducer structures in the form of single particles, [ 15 , 16 ] (sub-) mono layers, [ 17 ] solutions, [ 18 ] or complex nanostructures. [ 19 ] Additionally, the particles can induce local fi eld enhancement for other sensoric principles, like surface-enhanced Raman spectroscopy (SERS), [ 20 , 21 ] as well as for enhanced luminescence or fl uorescence. [ 22 , 23 ] In this Full Paper, we present the use of NLD to prepare defi ned plasmon layers from selected particles with tailored plasmon resonances inside optical fi ber structures for the development of novel LSPR sensing devices.
Microstructured optical fi bers (MOFs) hold such promise for the creation of new optical devices that they are the subject of intensive study in the scientifi c community. [ 29 , 30 ] Such fi bers exhibit various arrangements of air holes and, by choosing an appropriate structure, the spectral and spatial characteristics of the guided light can be engineered. One type of MOF is the suspended core fi ber (SCF). SCFs consist of a solid central core surrounded by an arrangement of 2-6 air holes, which run longitudinally along the length of the fi ber, and with the core suspended on thin bridges (see Scheme 1 ). [ 24 ] The light is guided by the effective index contrast between the massive core and the surrounding air holes. It has been proposed to harness the evanescent fi eld of the guided light for the sensing of gases, liquids, and analytes surrounding the core (within the holes of the fi ber). [ 25 ] In general, such fi bers offer a high sensitivity due to their potential use for long interaction lengths.
Recently, there has been great interest in fi bers with incorporated metallic thin fi lms or nanoparticles in order to bring about the next generation of photonic or plasmonic devices. The incorporation of metals into optical-fi ber geometry allows the guiding of the photon transport within the active plasmonic region, yielding highly integrated devices with unique excitation and detection geometries. [26][27][28][29] In recent years, high-pressure chemical deposition techniques, for example, chemical vapor deposition (CVD), have been developed to include a wide range of optically important materials within the MOF capillaries. [ 29 , 30 ] One such integration was the high-pressure deposition of silver nanoparticles, which allowed the development of a fi ber-optic SERS sensor. [ 31 , 32 ] Besides the high pressure, 10-100 MPa, an additional heat treatment with a temperature of 200 ° C is required, which can have adverse effects on the mechanical stability of the fi ber by damaging the standard outer polymer coating. The metal layer can be realized only for lengths of 15 cm with an approximately 50-μ m channel diameter and only after a 2-h incubation period. In addition, such a particle coating is only homogeneous for 5-6 cm in the middle section of the MOF since covering gradients were induced by depletion of particles. Therefore, when capillary fi lling is utilized for SERS applications, only the front end is suffi ciently coated. [ 33 ] This method can be performed at 60 ° C. A mixture of analyte and nanoparticles were used for SERS in MOFs. [ 34 ] The capillaries of 40-cm-long pieces of MOF were coated by in situ synthesis of silver particles from dextrose and silver nitrate in static deposition procedures. [ 35 ] Such coating shows a relatively rough and granular surface and the layer thickness cannot be adjusted exactly. Homogeneous silver layers can be produced by vigorous shaking during the deposition; [ 36 ] however, such shaking is only possible for short fi ber pieces.
In general, deposition methods for long MOFs must necessarily be performed at room temperature with homogeneous particle coverage and adjustable covering density. We introduce here a dynamic low-pressure chemical deposition of metal nanoparticles, which are attached to a self-assembled adhesive monolayer on the inner surfaces of the MOF. This so-called NLD technique is based on the self-assembled monolayer (SAM) techniques [ 37 , 38 ] for oxide surfaces using silane chemistry and controlled microfl uidic management, with a microstructured fl uid chip for the covering procedure, and can be employed with various types of metal nanoparticles as layer components.

Nanoparticle Layer Deposition in Holes of MOFs
The preparation of nanoparticle-based plasmonic structures on the internal capillary walls of MOFs was realized using NLD. This technique combines SAM techniques, microfl uidic control of the surface chemistry, and guided particle deposition. Microfl uidic chips were designed for optimally interfacing the MOFs. The preferred MOFs, such as the SCFs, were coupled into the microfl uidic chip and the capillary walls were chemically modifi ed by the perfusion of the silanes. The resulting functional layer was a chemical adhesive for metal nanoparticles due to its amino modifi cation, [ 39 ] as displayed in Scheme 1 . Metal nanoparticles were prepared in different shapes, dimensions, and with different materials and the selected particle solutions were incubated by continuous fl ow. The incubation time was < 1 h for 40-cm-long fi ber pieces and ∼60 h for 6-m-long pieces; for the latter, only 4 mL of particle solution was used. A successive inner saturation of the MOF with nanoparticles can be observed over the incubation time: the color front migrated along the fi ber with a speed of ∼2 cm min − 1 . The coating uniformity that resulted, that is, the nanoparticle density and the thickness of our layers, was constant over tens of centimetres up to 6 m. A homogeneous coating density was observed independently on the local curvature of the capillary-channel cross section ( Figure 1 a,b). Scanning electron microscopy (SEM) images clearly show particle (sub) monolayers at saturation coverage (Figure 1 c,d). A density of ∼450 particles μ m − 2 for 30-nm gold nanoparticles was determined in both the start and end region. Compared to other coating techniques with gelatine precursor layers for 35-nm silver particles (density of 1 particle μ m − 2 ), [ 40 ] the presented dynamic deposition technique offers a signifi cantly higher particle density.
The method presented for the modifi cation of MOFs/ SCFs was shown as a defi ned coating technique of the capillaries using a fl uidic chip, the respective fl uidic periphery, and The resulting layer thickness is adjustable by changing the selected particle dimension. The next section focuses on the optical characterization of plasmonically tuned SCF fi bers prepared by the method discussed.

Optical Properties of the Plasmonically Tuned SCF Fibers
The optical properties of the resulting, internally coated MOFs (SCFs) are identical to the properties of the employed colloidal suspensions of plasmon particles, as shown in Figure 2 . The optical behavior was adjusted by selecting plasmon particles that absorb in the UV range (Pt and Ag spheres), visible range (Au spheres and Ag triangles), or near-infrared range (Ag triangles and Au nanorods).
As shown in Figure 3 , a successful inner coating with plasmon particles in the visible spectral range can be easily confi rmed by microscopic inspection or even by the naked eye, either from the end face (Figure 3 a,c) or from the side (Figure 3 b,d). The plasmonically tailored fi bers showed measurable transmission only for short fi ber pieces of ∼3-mm long. The high losses are clearly explainable by the high particle coverage realized, which causes a very strong interaction of the relatively small core with the extremely high number of nanoparticles. By comparison, the 1 particle μ m − 2 coverage described in Oo et al. [ 40 ] resulted in a loss of 0.57 dB m − 1 and the particle density of the presented dynamic deposition technique is ∼450 times higher. Therefore, the expected attenuation should be approximately three orders of magnitude higher. Changing the particle surface density directly infl uences the effi ciency and the length needed to make such a sensor useful. High particle coverage allows for dissection of the plasmonically tuned long fi ber on a short (mm) length scale and therefore sensoric applicable segments. So, the costeffective preparation of novel miniaturized sensor devices is possible. Otherwise, for the utilization of long interaction length in MOFs, the density of gold particles on the inner surfaces has to be decreased. The use of a mixed monolayer in the NLD process enables the control of the particle density and thereby the adjustment of the attenuation. Investigations for such a control of the particle coverage are in progress.
Transmission measurements of the MOFs are needed in order to utilize the plasmonic particle layer as a transducer for sensing changes in the refractive index. The transmission spectrum of a fi ber will usually be measured longitudinally, that is, in the fi ber axis along the fi ber core. However, in the case of plasmonically tuned SCFs with saturated particle coverage, a longitudinal transmission measurement was not possible due to the strong attenuation already mentioned. Therefore, a transversal measurement setup was preferred, by   which illumination as well as collection of light transversally to the fi ber axis occurred. The resulting effective interaction length was about four times the thickness of the nanoparticle layer, which proved to be suffi cient for transmission measurements on a SCF coated with particles at high surface density. The fi ber piece to be measured was positioned vertically in the collimated beam of the white-light source. Directly behind the SCF, a large core fi ber selected only that part of the light that was transmitted through the SCF. A spectrum of a SCF without inner coating was used as the reference to calculate an extinction spectrum from the transmissions. In Figure 4 , an extinction spectrum of plasmonic particle solutions and SCFs coated with correlated particles are compared. The extinction peaks of certain nanoparticles (30-nm gold spheres, silver triangles with ∼120-nm, ∼50-nm, and ∼26-nm edge lengths) are well separated spectrally. The extinction spectrum measured from nanoparticles in solution (Figure 4 a) can also be reproduced with the inner-coated SCF (Figure 4 b).
The graphs for the 30-nm gold spheres (dashed lines in Figure 4 ) are nearly identical. For the silver triangles with ∼50-m edge length (dotted lines in Figure 4 ) and the silver triangles with ∼120-nm edge length (solid lines in Figure 4 ), only the peak positions in both systems fi t well, although the peak widened when the particles were deposited inside the glass fi ber. This is the result of a substrate effect, as the particles in colloidal solutions are surrounded only by water. In SCFs, the particles are partially surrounded with the fi ber material, silica glass. This induces a different proportional refractive index in the medium around the particles and effects small shifts in spectra and broader peaks of the larger and nonspherical particles. [ 1 , 41 ] In addition, dipole-dipole interactions between the particles in the plasmonic layer are possible. [ 42 ] Transmission measurements have shown that layers of plasmonic particles from gold seem to be stable in the fi ber over several months. SCFs with a layer of 30-nm gold spheres turned out to be very stable in spite of the fi ll and refi ll processes. The measured transmission curves were reproducible for more than fi ve refi ll cycles. In addition, the testing of the same plasmonically tuned SCF in different positions shows that not only are their SEM images very similar but so are their spectral properties.
In order to characterize the optical properties, SCFs coated with 30-nm gold particles were tested as the sensor. The sensitivity was determined by transversal measurement with solutions of different refractive index, which were injected into fi ber channels with the same fl uidic setup as for the particle-layer preparation. For the 30-nm gold spheres, the theoretical calculations show a strong dependence on the refractive index (   theoretical simulations. The difference in peak width and the subsidiary peak was induced by the substrate effect for plasmonic particle layers on planar glass surfaces and by the nonspherical geometry of the real particle samples. [ 1 , 41 , 43 , 44 ] The sensitivity of 30-nm-gold-particle-modifi ed SCF was determined to be ∼78 nm per refractive index unit (RIU). This value compares well to the sensitivity for LSPR sensing with gold nanospheres, ∼72 nm RIU − 1 , in so-called nanoSPR . [ 17 ] By proper selection of plasmon particles for plasmonic tuning of SCFs, an increased sensitivity of ∼600 nm RIU − 1 can be achieved. [ 14 ]

Conclusion
We have demonstrated a novel method for the generation of plasmonic nanoparticle layers inside the channels of microstructured optical fi bers, especially for SCFs, based on SAM techniques. By using microchips for the fl uidic coupling, a reproducible, cost-effective, and contamination-free nanoparticle deposition is possible. With the presented method, nanoparticles in a great variety of materials, shapes, and sizes can be used for the deposition. Optical characterization, as well as electron microscopic evaluation, confi rmed the even deposition of the holes and the constant population density over fi ber lengths of several meters. The possibility of preparing fi bers with plasmonic properties in the UV-to-near-infrared spectral range then exists. A transversal measurement setup can be utilized because of the high population density, allowing the ports for optical and fl uidic coupling to be separated. This enables the principle testing of the plasmonic tuned fi ber system for sensing applications. For detection and measurements of a liquid analyte, the fi lling setup tested in the coating procedure described above can be applied. This technique allows the complete fi lling of a piece of a SCF without bubbles remaining and the capillaries of the same piece of SCF are still easy to repeatedly clean, refi ll, and dry. In a proof-of-principle experiment, the refractive-index-dependent shift of the LSPR peak was demonstrated. For particle layers with 30-nm gold spheres in MOFs, a sensitivity of ∼78 nm RIU − 1 was measured. The presented system offers a vast potential for the development of innovative sensors based on LSPR and local fi eld enhancement, like SERS or enhanced fl uorescence. The fi ber channels can be used both for the transport of analyte molecules as well as for the generation of the sensor signal on the particlebased plasmonic transducer layer. The actual transmission losses, and with this the usable fi ber length, could be tuned with an adapted particle density. Experiments concerning adjustable particle density are in progress. This method provides a miniaturized, cost-effective sensor for bioanalytical and diagnostic applications.

Experimental Section
Preparation of Microfl uidic Chips and Fluid-Coupling : Microfl uidic chips for interfacing the MOFs/SCFs were prepared by wet etching and anodic bonding of two glass substrates using a silicon-bond support layer. [ 45 ] In brief, fl uid and fi ber port channels were etched into two glass substrates, with an etch depth of 65 μ m. After the bonding of two half-channels, a total height of 130 μ m was realized, which was optimally suited for the interfacing of optical fi bers with an outer diameter of 125 μ m. The SCFs were prepared using high silica glass capillaries by "stack-anddraw" technology. [ 46 ] Their outer diameter was 125 μ m, the core diameter was 3.2 μ m, and the dimension of the holes was 30 × 40 μ m with 0.9-μ m-thick bridges. [ 47 ] SCFs with the protective plastic coating (acrylate) were inserted into the microfl uidic chip and glued into the fl uidic output. The chip was then connected to a syringe pump system (neMESYS, cetoni GmbH, Korbussen, Germany) by Tefl on tubes (Jasco, Gross-Umstadt, Germany).
Characterization of the Plasmonic Particle Layer in MOFs : The coating uniformity was characterized for cleaved fi ber pieces from different positions along the inner-coated SCFs by SEM measurements (Zeiss DSM 960, Jena, Germany). For the spectral characterization of the plasmonically tuned fi bers, a white-light source (Mikropack DH2000, OceanOptics, Duiven, Netherlands) and fi ber spectrometer (Spectro 320D, Instrument Systems GmbH, Munich, Germany) were employed. Microscopic images were taken with an AxioImager , equipped with a color camera (AxioCam mrc5, Carl Zeiss, Jena, Germany) in transmission and refl ection mode.