Surface Plasmon Resonances in Oriented Silver Nanowire Coatings

Article pubs.acs.org/JPCC

Surface Plasmon Resonances in Oriented Silver Nanowire Coatings on Optical Fibers Jean-Michel Renoirt,†,‡ Marc Debliquy,‡ Jacques Albert,§ Anatoli Ianoul,∥ and Christophe Caucheteur*,† †

Electromagnetism and Telecommunication Department, Faculté Polytechnique, University of Mons, Place du Parc, 20, 7000 Mons, Belgium ‡ Materials Science Department, Faculté Polytechnique, University of Mons, Place du Parc, 20, 7000 Mons, Belgium § Department of Electronics, Carleton University, 1125 Colonel By Drive, K1S 5B6 Ottawa, ON, Canada ∥ Department of Chemistry, Carleton University, 1125 Colonel By Drive, K1S 5B6 Ottawa, ON, Canada ABSTRACT: Silver nanowires 1−3 μm in length and diameters of 0.04−0.05 μm were synthesized by a polyol process and deposited on a single mode optical fiber with the Langmuir−Blodgett technique. For nanowire surface coverage of ∼40% and partial orientation of their long axis obtained by controlling the deposition parameters, the optical properties of the nanowire coating become identical to those of a uniform metal coating obtained by sputtering or evaporation. Excitation of the nanowires by the polarized evanescent field of fiber cladding modes at near-infrared wavelengths near 1.5 μm results in surface plasmon-like resonances in the transmission spectrum of the optical fiber. The polarizationdependent loss (PDL) spectrum of the tilted fiber Bragg grating used to excite the cladding modes shows a pronounced characteristic dip indicative of a plasmon resonance for radially polarized light waves and complete shielding of light for azimuthally polarized light. The PDL dip shifts at a rate of 650 nm/(refractive index unit) when the surrounding refractive index is changed, a 10-fold increase compared to uncoated fiber gratings and similar to that of uniform metal coated gratings. The advantage of the nanowire approach is to provide a much increased contact surface area for biomolecular recognition-based immunosensing.

1. INTRODUCTION Surface plasmon resonance (SPR) assisted optical fiber sensors overcome the practical limitations of their bulky prism counterparts, ensuring remote and real-time operation in volumes of analyte as small as microliters.1−3 An optical fiber is a cylindrical waveguide made of two concentric layers (socalled core and cladding) that guide light by a slight difference of refractive index between the layers. As light is intrinsically confined in the fiber core, an optical fiber is not naturally sensitive to surrounding refractive index (SRI) changes. SRI sensing is achieved by exposing the evanescent field of the light to the surrounding medium, either via complete or partial removal of the cladding (by etching, polishing, or tapering).4−7 Alternatively, the cladding can be left intact but the light extracted out of the core through the use of core-cladding coupling gratings photoinscribed in the fiber.8 The first solution is the most straightforward as it exposes the evanescent waves associated with the propagating core mode(s) (single mode or numerous modes depending on the fiber core dimensions) to the surrounding medium. However, it considerably weakens the optical fiber and may prevent its use in practical applications, especially outside of laboratory settings. Gratings maintain the fiber integrity while providing a strong coupling between the core-guided light and the cladding at specific wavelengths © 2014 American Chemical Society

corresponding to grating resonances. Once the light is guided by the cladding, its evanescent field extends into the surrounding medium. Two grating configurations compete with similar figures of merit for the ratio of the width of the resonances to their refractometric sensitivity: long period fiber gratings (LPFGs) and tilted fiber Bragg gratings (TFBGs). LPFGs consist in a periodic refractive index modulation of the fiber core with a period of a few hundred micrometers, coupling the forward-going core mode into forward-going cladding modes.9 Their transmitted power spectrum is composed of resonances that have high sensitivities to SRI (of the order of 104 nm/RIU10) but relatively large spectral widths (full width at half-maximum (fwhm) ∼20 nm) and spectral separation, with only a few resonances occurring in a wavelength range over 200 nm. TFBGs are short period (∼500 nm) gratings characterized by a refractive index modulation slightly angled with respect to the perpendicular to the optical fiber axis. In addition to the self-backward coupling of the core mode, they couple light into backward-propagating cladding modes. Their transmitted power spectrum is composed of several tens of “narrow” Received: March 12, 2014 Revised: April 28, 2014 Published: April 28, 2014 11035

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were oriented parallel to the evanescent electric fields of the cladding modes and thus broadly resonant with the nearinfrared light used (in the 1550 nm wavelength band). The effect reported here is completely different: packing and orienting the nanowires results in semicontinuous coating that is very uniform azimuthally but highly structured in the fiber axis direction, on a length scale that is deeply subwavelength. This coating behaves much in the same way as a uniform metal film but retains the higher surface area of nanoparticles. The nanowires were synthesized with a polyol process and deposited on optical fibers using the Langmuir− Blodgett technique.33−36 Under certain conditions of density and surface tension, it was possible to orient the nanowires in the azimuthal direction on the fiber cladding surface. Interrogation of the TFBG by measuring its polarizationdependent loss (PDL) spectrum then revealed the presence of a characteristic SPR signature. We show that this signature is absent from samples with randomly oriented nanowires of similar surface densities. Furthermore, the refractometric sensitivity of the SPR signature in oriented nanowire-coated TFGBs was found to be 650 nm/RIU, which is a 10-fold increase compared to bare gratings, but most importantly essentially identical to that of TFBGs with uniform metal coatings. It is concluded that such oriented nanowires coatings may well be an ideal solution for biosensors with an improved limit of detection resulting from the high surface area while maintaining the narrow line widths and amplitudes of the resonances of conventional SPR sensors.

cladding mode resonances (fwhm ∼ 100 pm) located on the short wavelength side of the Bragg resonance (the resonance corresponding to self-backward core mode coupling). Those resonances have refractometric sensitivities not exceeding 60 nm/RIU.11 However, the presence of the Bragg resonance provides a convenient reference for the total optical power in the fiber, and also to determine the local temperature at the sensor, as it is not sensitive to SRI changes.12,13 The high spectral density of the cladding mode resonances of a TFBG provides a wealth of information about the fiber and its immediate environment and is thus privileged in our work.14 Optical fiber sensors that utilize SPR enhancements of the probing electromagnetic fields can be obtained from fiber gratings with a thin noble metal (most often gold or silver) deposited on the fiber cladding. As in the widely used Kretschmann-Raether prism configurations, metal thicknesses ranging between 30 and 70 nm provide the best SPR responses. SPR generation is achieved under TM-polarized light, which for cylindrical structures corresponds to the electric field of the guided modes polarized radially at the surrounding medium interface. The orthogonal polarization state (TE, or azimuthal in this case) is not able to excite the SPR, as the electric field of the light modes is tangential to the metal surface and thus cannot couple energy to the surface plasmon waves. In this respect, another great feature of TFBGs is that cladding guided modes with predominantly radial and azimuthal polarizations can be excited individually by controlling the wavelength and the orientation of the input polarization of the light launched in the fiber core relative to the tilt direction of the grating planes.15,16 Input light polarized linearly in the tilt plane (ppolarized) excites radially polarized cladding modes while spolarized light (perpendicular to the tilt plane) excites azimuthally polarized modes.17 This antagonist behavior between p- and s-polarized signals has been advantageously exploited to process the refractometric response of gold-coated TFBGs and to identify clear SPR signatures.18−20 The large sensitivity of plasmon wave properties to the permittivity of the medium immediately adjacent to the metal surface has led to the development of many biosensing platforms and configurations in which bioreceptors (antibodies, aptamers, etc.) are attached to the metal surface.21−30 It was also determined, especially in prism and planar waveguides configurations, that nanostructuring of the metal coatings increases the effective surface area for attachment of bioreceptors and leads to enhanced detection limits for molecules, proteins, and other particle-like objects. However, such nanostructuring also prevents the propagation of surface plasmon waves with a well-defined resonance wavelength that is highly sensitive to the SRI.31 An ideal SPR-based sensor should have both high surface area and well-defined plasmon resonances. In this paper, we experimentally demonstrate that SPR generation with the same properties as those seen in uniform metal layers is possible in TFBGs coated with high surface area silver nanowires, provided that the density and orientation of the silver nanowires are suitably controlled. In a preliminary experiment reported earlier, it had already been demonstrated that sparse coatings of randomly oriented silver nanowires did lead to a plasmon-related enhancement of the refractometric sensitivity of the coated TFBGs (relative to uncoated ones), by a factor of 3−5 for a surface density of only 14%.32 In that case the improvement was due to a small enhancement of the localized electric field in the vicinity of those nanowires that

2. EXPERIMENTS 2.1. Photoinscription of TFBGs. TBFGs were manufactured into hydrogen-loaded standard telecommunication-grade single-mode optical fiber (CORNING SMF-28). The gratings were photoinscribed by means of an ultraviolet light interference pattern produced by a pulsed KrF excimer laser (248 nm) and a uniform phase mask, following the same experimental parameters as those used by Bialiayeu et al.32 The tilt angle was chosen equal to 10° to ensure a strong coupling to high-order cladding modes with phase velocities matched to those of surface plasmons for gold−water interfaces typical of biosensing experiments. 2.2. Synthesis of Silver Nanowires. The growth of anisotropic structures starting from isotropic solutions is a delicate task, especially for almost all the metals that crystallize into highly symmetric cubic lattices. One way to obtain this kind of structure is the polyol process.37−42 Silver nitrate (AgNO3) is reduced by ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP). More details on the growth mechanism can be found in the work of Sun et al.43 All reagents used in this work were purchased from SigmaAldrich. Glassware was cleaned by means of aqua regia, rinsed in deionized water (18.2 MΩ·cm), and dried into an oven prior to all experiments. In a 50 mL round-bottom flask, 10 mL of 99.8% ethylene glycol, 400 mg of poly(vinylpyrrolidone) (50 000 units of monomers), and 4 mg of sodium chloride (99.99%) were heated at 160 °C for 30 min under moderate stirring. Then, 5 mL of 0.12 M AgNO3 (99%+) dissolved in EG was added in the flask, using a plastic syringe. The injection rate was about 0.125 mL min−1. A few minutes after injection, the solution turned to redbrown and slowly became a turbid light greenish-gray at the end of the reaction. Reaction monitoring was performed by 11036

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taking small amounts of the solution and measuring their UV− vis spectra after dilution in ethanol. When the reaction was complete, the flask was put into an ice bath in order to stop any further growth. Nanowires were cleaned by adding ethanol to the solution and centrifuging at 12 000 rpm for 20 min in order to remove any excess PVP and other reaction byproducts. Supernatant was thrown away, and the wires were dispersed in ethanol. This process was repeated 3 times at 10 000 rpm and 3 times at 5000 rpm in order to separate nanowires from other nanoparticles synthesized during the reaction. TEM (transmission electron microscopy) and AFM (atomic force microscopy) images of the nanowires were taken together with UV−vis−NIR spectra on flat glass substrates. The size of the nanowires can be estimated through TEM images (Figure 1) which indicate diameters in the range of 50−100 nm and

the wide dispersion in the length and orientation of the nanoparticles. 2.3. Nanowire Coatings on TBFGs. In order to coat the gratings with oriented nanowires, we have used the Langmuir− Blodgett technique. Previous studies have shown the potential of this technique to assemble large-sized nanowire monolayers with some level of spatial organization (alignment).33,34 2.3.1. Langmuir−Blodgett Technique. The Langmuir− Blodgett (LB) technique is widely used in order to form dense monolayers on substrates. Molecules or particles are spread out on the surface of a subphase (here, water) contained in a device made from Teflon due to its suitable properties such as hydrophobicity and chemical neutrality, which are required for this kind of apparatus. The LB trough comprises a Wilhelmy plate to measure surface pressure and a movable Teflon barrier to control the compression of the floating monolayer (Figure 3). A dipping mechanism is provided to deposit coatings one

Figure 1. TEM image of silver nanowires before purification.

Figure 3. Schematic of the Langmuir−Blodgett trough used to orient and dip-coat the silver nanowires on the optical fiber surface (not in scale).

lengths between 1 and 3 μm. Nanocubes and other nanocrystals can also be seen in Figure 1. They are byproducts of the nanowire synthesis. While centrifugation was used to get rid of those, it was impossible to remove them all. In the UV−vis−IR spectrum (Figure 2), a strong absorption peak can be seen at 480 nm, which corresponds to the transverse plasmon resonance of the nanowire, while the short wavelength tail of an important absorption band is measured in the near-IR range, corresponding to the longitudinal plasmon resonance. The near-IR peak is considerably broadened due to

monolayer at a time. The barrier, Wilhelmy plate, and dip coater are driven by a computer, which allows the surface pressure to remain constant as the layer gets transferred to substrates, depending on the dipping speed and cross section of the substrate. 2.3.2. Sample Preparation. Prior to deposition, the nanowire samples must be carefully conditioned. The first step is a 5 times cleaning in ethanol, following the procedure used after the synthesis, in order to eliminate any PVP excess and others residues. Indeed, the LB process is very sensitive to any contaminant, which can dramatically modify the interfacial tension and lead to imperfect monolayer formation. The LB technique requires the use of hydrophobic molecules in order to form a monolayer on top of the water surface. For this purpose, the silver nanowires were functionalized with 1dodecanethiol as follows: nanowires were first centrifuged at 5000 rpm for 10 min. Ethanol supernatant was removed, and a 25 mM 1-dodecanethiol in chloroform was added. Nanowires were dispersed using an ultrasonic bath (power = 160 W for 5 min). Then, the solution containing the nanoparticles was left reacting for several hours and then centrifuged and sonicated under the same conditions as above. Finally, the supernatant was removed, and pure chloroform was added to eliminate the excess dodecanethiol. The process was repeated several times. At this stage, nanowire solutions may be stored for up to 2 weeks at ambient temperature, without degradation. Monolayers of nanowires were dispersed on the water surface from a manually operated glass syringe. This must be done slowly enough to allow chloroform to evaporate. Surface

Figure 2. UV−vis−NIR spectrum obtained by drop-casting of the nanowires solution. 11037

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2.3.3. Fiber Preparation and Nanowires Deposition. Because of the need to dip the TFBGs into the LB trough, the grating spectra were obtained in pseudoreflection mode by cleaving the fiber just beyond the TFBG and depositing a gold mirror fiber tip. The gold mirror was deposited by putting the fiber tip into a vial containing gold nanoparticles (diameter about 8 nm) in water for 1 h. Following that, a reduction of gold was done using 1% chloroauric acid and hydroxylamine hydrochloride. This process was repeated three times in order to generate a gold layer sufficiently thick to form a close to 100% broad-band mirror at the end of the fiber. Before their introduction in the LB apparatus, fibers containing TFBGs were first washed in a piranha solution (hydrogen peroxide and sulfuric acid in volume ratio 1:4), then washed with water, and dipped in a 1% APMES (aminopropyltrimethoxysilane) in methanol for 1 h to allow adhesion of silver nanowires on the glass fiber and washed again with ethanol. Coated fibers were imaged using SEM (scanning electron microscopy). Figure 5 displays the results of two widely different nanowire depositions with similar densities. The nanowire densities were estimated by contrast enhancement of the images and comparing the number of bright pixels (corresponding to the nanowires) to the number of the dark background pixels. Nanowires are clearly randomly oriented on the left-hand side SEM picture. Their density has been computed to be 42%. Conversely, the nanowires are aligned predominantly in the direction perpendicular to the fiber axis (horizontal in the picture) for the right-hand-side picture. The density of the oriented nanowires in the right-hand-side image is equal to 41%. The successful orientation of the nanowires was obtained by following the operating principles described in section 2.3.2, using six successive compression cycles. The dipcoating process took place under a surface tension of ∼25 mN/ m. In contrast, the randomly oriented deposition was obtained under the same surface tension, but after a single compression cycle. As already mentioned, some residual nanoparticles were not filtered out even by repeated centrifugations due to their relatively equal weight compared to nanowires, and they appear in the SEM images. It is also clear from Figure 5 that the orientation of the nanowires in the multicycle deposition is not perfect but retains some randomness. In spite of this, it will be shown in the next section that there is a significant difference in

pressure is monitored during the process and kept constant by the movable barrier. A monolayer area of about 250 cm2 was prepared, and the nanowires were randomly oriented at this stage. The alignment of the nanowires in the direction parallel to the moveable barrier is obtained by carrying out several compression−release cycles of the layer. When the fiber is dipped in the solution containing the oriented nanowires, they maintain their privileged orientation when sticking on the optical fiber surface. Hence, as the fiber is immersed vertically, the nanowires appear aligned with their longitudinal axis perpendicular to the optical fiber propagation axis. Figure 4

Figure 4. Compression isotherm of nanowires measured by the Wilhelmy plate.

depicts the final compression isotherm obtained before deposition on the fiber, as measured by the Wilhelmy plate. The slope of the isotherm evolves during the compression and indicates pressure changes in the monolayer. This evolution can be understood by the fact that the pressure does not increase as long as wires are far from each other but changes as they get closer, at a rate that depends on the distribution of their orientations. When the nanowires become very close to each other, the pressure increases drastically and excess pressure can even lead to overlapping of the nanowires. As will be discussed later, this isotherm is the key parameter to ensure a correct oriented deposition of the nanowires.

Figure 5. SEM image of random nanowires (left) and aligned nanowires perpendicular to the fiber axis (right). The fiber axis is horizontal in the picture. 11038

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Figure 6. Sketch of the experimental setup used to record the TFBG response in pseudoreflection mode.

Figure 7. PDL spectrum of a TFBG coated with randomly oriented nanowires (left) and with orthogonally aligned nanowires (right).

of the cladding mode resonances shows a Gaussian-like shape, similar to that of bare TFBGs. In this spectrum a discontinuity in the upper envelope amplitude appears around 1538 nm, which reflects the position of the mode cutoff, i.e., the point at which radially polarized cladding modes cease to be guided by the glass−water boundary. It is well-known that in such a configuration the cladding mode most sensitive to SRI change is the last mode guided before cutoff.11 For the TFBG coated with nanowires oriented perpendicularly to the optical fiber axis, a different PDL spectral feature appears: a sudden decrease of the PDL amplitude near 1544 nm. This is the unambiguous SPR signature, already reported for continuous nanoscale gold coatings on TFBGs.18 These results provide clear experimental evidence that perpendicularly oriented nanowires in sufficient density behave as a continuous metal layer in terms of SPR generation. If this hypothesis is correct, the minimum point in the PDL spectrum should shift at a rate of ∼500−600 nm/RIU. Figure 8 depicts the evolution of the PDL spectrum measured in different refractive index solutions and the position of PDL minimum is

the optical properties of the two kinds of deposition shown in Figure 5 and in particular that even the partial orientation that was achieved is sufficient to allow surface plasmon resonances that are similar to those of a uniform metal layer of similar average thickness. 2.4. Experimental Setup for Measuring the Optical Properties of the Nanowire Coatings. The transmission of TFBGs depends on the properties of the medium in which the fiber is immersed and on any coating deposited on the fiber. The refractometric response of the TFBGs coated with nanowires was tested by immersing them in different refractive index solutions. Figure 6 depicts the sketch of the experimental setup that simply consists in connecting the coated TFBGs to a dedicated instrument for measurement with polarized light, socalled optical vector analyzer. In our case, each measurement consisted of four successive polarized power spectra obtained at different input states of polarization by an optical vector analyzer JDSU Swept Wavelength System OMNI 2, from which the PDL spectrum of the TFBG can be computed.44 In the case of TFBGs, the PDL spectrum corresponds to the absolute value of the difference between the p- and s-polarized mode inputs, and it provides a clear spectral signature when an SPR is present.15 The SRI range studied here goes from 1.330 to 1.347, obtained using solutions of ethylene glycol in water at different concentrations. The temperature was kept constant during all experiments and any remaining small variations removed by referencing all wavelengths to the Bragg wavelength (which is totally isolated from the SRI). The TFBG is also totally strainfree because it is held on one side only, upstream from the grating. Therefore, all the wavelength shift results reported in the following section must be due to SRI changes.

3. EXPERIMENTAL RESULTS AND DISCUSSION The two nanowires configurations (oriented and random) were compared in terms of their PDL spectra. Figure 7 displays the PDL measured for TFBGs immersed in water in the wavelength range between 1520 and 1600 nm. For the TFBG with randomly dispersed silver nanowires, the envelope

Figure 8. PDL spectrum of a 10° TFBG with aligned nanowires (density ∼40%) measured in different refractive index solutions. 11039

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extracted and shown in Figure 9. A linear fit of the raw data yields a mean sensitivity of 651 nm/RIU for the orthogonally

Finally, it is worth mentioning that silver nanowires may degrade over time due to oxidation effects. So, the ultimate solution would be to use gold nanowires or alternatively use silver nanowires with thin silica, alumina, or titania shells to prevent degradation. According to our expertise, we are confident that similar effects as the ones reported in this work can be achieved with gold nanowires or with coated silver nanowires.

4. CONCLUSION In this paper, highly sensitive refractometry measurements in water solutions by a TFBG coated with an oriented monolayer of silver nanowires occupying 40−50% of the fiber surface were demonstrated. Comparative experiments with randomly oriented nanowires or layers that were denser or sparser did not result in similar sensitivity enhancements. The sensitivity of 650 nm/RIU that was achieved is equal to or slightly higher than that obtained using TFBGs coated with uniform metal layers. However, the results shown here are significant for biochemical analysis applications considering the possibility of functionalizing the nanowires with biomolecules over a much increased surface area. In fact, the surface area improvement factor (over a uniform flat surface over the same length of fiber) is easily found from basic geometry considerations to be equal to π (3.14). In general, however, the improvements in the limit of detection of biochemical sensors achieved by incorporating metal nanoparticles go well beyond geometric factors and involve electromagnetic field trapping and localization effects.45

Figure 9. SPR mode wavelength shift as a function of SRI changes.

aligned nanowire-coated TFBG. Compared to a bare grating, the refractometric sensitivity is ∼10 times higher and essentially identical to that obtained with 50 nm thick uniform gold coatings obtained by evaporation or sputtering. It is worth to mention that experiments were also conducted with nanowires oriented parallel to the fiber axis. No SPR response was obtained in such samples. It was also determined that apart from their relative orientation with respect to the optical fiber axis, the density of nanowires is another critical parameter to ensure a correct SPR generation. Through repeated experiments, it was empirically found that SPR generation only occurs when the surface tension is comprised between ∼20 and ∼30 mN/m, yielding a silver nanowire surface coverage fraction ranging from 40 to 50% of the total surface. Figure 10 shows two SEM pictures corresponding to counterexamples obtained for coatings with nanowire densities that were too sparse and too dense. The sparse coating was deposited by several compression cycles but at a surface tension of only ∼5 mN/m. The resulting surface coverage fraction was determined to be equal to 11%. The dense coating was deposited with a surface tension exceeding 35 mN/m, and the resulting surface coverage was 58%, without taking into account the superimposition between nanowires that is clearly visible on the picture. In both cases, no SPR signature was observed, and the maximum SRI sensitivity (of the cutoff point) did not exceed 100 nm/RIU.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was performed in the framework of the Opti2 Mat project financially supported by the Walloon region in Belgium. C. Caucheteur is supported by the Fonds National de la Recherche Scientifique (F.R.S.-FNRS) and by the ERC (European Research Council) Starting Independent Researcher Grant PROSPER (grant agreement no. 280161). Jean-Michel Renoirt was supported by the Wallonia−Brussels Federation (travel grant 2013). A. Ianoul and J. Albert are supported by NSERC.

Figure 10. Silver nanowires deposited on the optical fiber surface using the LB technique with a surface tension of ∼5 mN/m (left) and ∼35 mN/m (right). 11040

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(23) Voisin, V.; Pilate, J.; Damman, P.; Mégret, P.; Caucheteur, C. Highly Sensitive Detection of Molecular Interactions with Plasmonic Optical Fiber Grating Sensors. Biosens. Bioelectron. 2014, 51, 249−254. (24) Pollet, J.; Janssen, K. P. F.; Knez, K.; Lammertyn, J. Real-Time Monitoring of Solid-Phase PCR Using Fiber-Optic SPR. Small 2011, 7, 1003−1006. (25) Peng, W.; Banerji, S.; Kim, Y.-C.; Booksh, K. S. Investigation of Dual-Channel Fiber-Optic Surface Plasmon Resonance Sensing for Biological Applications. Opt. Lett. 2005, 30, 2988−2990. (26) Sharma, A.; Gupta, B. Absorption-Based Fiber Optic Surface Plasmon Resonance Sensor: A Theoretical Evaluation. Sens. Actuators, B 2004, 100, 423−431. (27) Pollet, J.; Delport, F.; Janssen, K. P. F.; Jans, K.; Maes, G.; Pfeiffer, H.; Wevers, M.; Lammertyn, J. Fiber Optic SPR Biosensing of DNA Hybridization and DNA-Protein Interactions. Biosens. Bioelectron. 2009, 25, 864−869. (28) Chen, Y.; Dong, B.; Zhou, W. Surface Plasmon Resonance Biosensor Modified with Multilayer Silver Nanoparticles Films. Appl. Surf. Sci. 2010, 257, 1021−1026. (29) Lu, H. B.; Homola, J.; Campbell, C. T.; Nenninger, G. G.; Yee, S. S.; Ratner, B. D. Protein Contact Printing for a Surface Plasmon Resonance Biosensor with on-Chip Referencing. Sens. Actuators, B 2001, 74, 91−99. (30) Green, R. J.; Frazier, R. A.; Shakesheff, K. M.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. Surface Plasmon Resonance Analysis of Dynamic Biological Interactions with Biomaterials. Biomaterials 2000, 21, 1823−1835. (31) Zhou, W.; Mandia, D. J.; Griffiths, M. B. E.; Barry, S. T.; Albert, J. Effective Permittivity of Ultrathin Chemical Vapor Deposited Gold Films on Optical Fibers at Infrared Wavelengths. J. Phys. Chem. C 2014, 118, 670−678. (32) Bialiayeu, A.; Bottomley, A.; Prezgot, D.; Ianoul, A.; Albert, J. Plasmon-Enhanced Refractometry Using Silver Nanowire Coatings on Tilted Fibre Bragg Gratings. Nanotechnology 2012, 23, 444012. (33) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Langmuir−Blodgett Silver Nanowire Monolayers for Molecular Sensing Using Surface-Enhanced Raman Spectroscopy. Nano Lett. 2003, 3, 1229−1233. (34) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Large-Scale Hierarchical Organization of Nanowire Arrays for Integrated Nanosystems. Nano Lett. 2003, 3, 1255−1259. (35) Langley, D.; Giusti, G.; Mayousse, C.; Celle, C.; Bellet, D.; Simonato, J.-P. Flexible Transparent Conductive Materials Based on Silver Nanowire Networks: A Review. Nanotechnology 2013, 24, 452001. (36) Acharya, S.; Hill, J. P.; Ariga, K. Soft Langmuir-Blodgett Technique for Hard Nanomaterials. Adv. Mater. 2009, 21, 2959−2981. (37) Zhang, Y.; Wang, J.; Yang, P. Convenient Synthesis of Ag Nanowires with Tunable Length and Morphology. Mater. Res. Bull. 2013, 48, 461−468. (38) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Uniform Silver Nanowires Synthesis by Reducing AgNO3 with Ethylene Glycol in the Presence of Seeds and Poly(Vinyl Pyrrolidone). Chem. Mater. 2002, 14, 4736−4745. (39) Sun, Y.; Xia, Y. Large-Scale Synthesis of Uniform Silver Nanowires through a Soft, Self-Seeding, Polyol Process. Adv. Funct. Mater. 2002, 14, 833−837. (40) Korte, K. E.; Skrabalak, S. E.; Xia, Y. Rapid Synthesis of Silver Nanowires through a CuCl- or CuCl2-Mediated Polyol Process. J. Mater. Chem. 2008, 18, 437. (41) Wiley, B.; Sun, Y.; Xia, Y. Polyol Synthesis of Silver Nanostructures: Control of Product Morphology with Fe(II) or Fe(III) Species. Langmuir 2005, 21, 8077−8080. (42) Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Shape-Controlled Synthesis of Metal Nanostructures: The Case of Silver. Chemistry 2005, 11, 454−463. (43) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Polyol Synthesis of Uniform Silver Nanowires: A Plausible Growth Mechanism and the Supporting Evidence. Nano Lett. 2003, 3, 955−960.

REFERENCES

(1) Homola, J. Surface Plasmon Resonance Based Sensors; Springer: Berlin, 2006; Vol. 4. (2) Sharma, A. K.; Jha, R.; Gupta, B. D. Fiber-Optic Sensors Based on Surface Plasmon Resonance: A Comprehensive Review. IEEE Sens. J. 2007, 7, 1118−1129. (3) Cheng, S.-F.; Chau, L.-K. Colloidal Gold-Modified Optical Fiber for Chemical and Biochemical Sensing. Anal. Chem. 2003, 75, 16−21. (4) Schroeder, K.; Ecke, W.; Mueller, R.; Willsch, R.; Andreev, A. A Fibre Bragg Grating Refractometer. Meas. Sci. Technol. 2001, 12, 757− 764. (5) Iadicicco, A.; Cusano, A.; Campopiano, S.; Cutolo, A.; Giordano, M. Thinned Fiber Bragg Gratings as Refractive Index Sensors. IEEE Sens. J. 2005, 5, 1288−1295. (6) Cusano, A.; Paladino, D.; Cutolo, A.; Iadicicco, A.; Campopiano, S. Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation; Bentham Science Publishers: Sharjah, 2012; pp 238−269. (7) Jiang, B.; Zhao, J.; Rauf, A.; Qin, C.; Jiang, W. Applications of Tilted Fiber Bragg Grating in Liquid Parameters Measurement. Proc. SPIE 8788, Optical Measurement Systems for Industrial Inspection VIII 2013, 8788, 878812. (8) Baldini, F.; Brenci, M.; Chiavaioli, F.; Giannetti, A.; Trono, C. Optical Fibre Gratings as Tools for Chemical and Biochemical Sensing. Anal. Bioanal. Chem. 2012, 402, 109−116. (9) Bhatia, V.; Vengsarkar, A. M. Optical Fiber Long-Period Grating Sensors. Opt. Lett. 1996, 21, 692−694. (10) Erdogan, T.; Sipe, J. Tilted Fiber Phase Gratings. J. Opt. Soc. Am. A 1996, 13, 296−313. (11) Chan, C.-F.; Chen, C.; Jafari, A.; Laronche, A.; Thomson, D. J.; Albert, J. Optical Fiber Refractometer Using Narrowband CladdingMode Resonance Shifts. Appl. Opt. 2007, 46, 1142−1149. (12) Laffont, G.; Ferdinand, P. Tilted Short-Period Fibre-BraggGrating-Induced Coupling to Cladding Modes for Accurate Refractometry. Meas. Sci. Technol. 2001, 765. (13) Caucheteur, C.; Megret, P. Demodulation Technique for Weakly Tilted Fiber Bragg Grating Refractometer. IEEE Photonics Technol. Lett. 2005, 17, 2703−2705. (14) Albert, J.; Shao, L.-Y.; Caucheteur, C. Tilted Fiber Bragg Grating Sensors. Laser Photon. Rev. 2013, 7, 83−108. (15) Shevchenko, Y. Y.; Albert, J. Plasmon Resonances in GoldCoated Tilted Fiber Bragg Gratings. Opt. Lett. 2007, 32, 211−213. (16) Caucheteur, C.; Chen, C.; Voisin, V.; Berini, P.; Albert, J. A Thin Metal Sheath Lifts the EH to HE Degeneracy in the Cladding Mode Refractometric Sensitivity of Optical Fiber Sensors. Appl. Phys. Lett. 2011, 99, 041118−041118−3. (17) Caucheteur, C.; Voisin, V.; Albert, J. Polarized Spectral Combs Probe Optical Fiber Surface Plasmons. Opt. Express 2013, 21, 3055− 3066. (18) Caucheteur, C.; Shevchenko, Y.; Shao, L.-Y.; Wuilpart, M.; Albert, J. High Resolution Interrogation of Tilted Fiber Grating SPR Sensors from Polarization Properties Measurement. Opt. Express 2011, 19, 1656−1664. (19) Voisin, V.; Caucheteur, C.; Mégret, P.; Albert, J. Interrogation Technique for TFBG-SPR Refractometers Based on Differential Orthogonal Light States. Appl. Opt. 2011, 50, 4257−4261. (20) Renoirt, J.-M.; Zhang, C.; Debliquy, M.; Olivier, M.-G.; Mégret, P.; Caucheteur, C. High-Refractive-Index Transparent Coatings Enhance the Optical Fiber Cladding Modes Refractometric Sensitivity. Opt. Express 2013, 21, 29073−29082. (21) Shevchenko, Y.; Francis, T. J.; Blair, D. A. D.; Walsh, R.; DeRosa, M. C.; Albert, J. In Situ Biosensing with a Surface Plasmon Resonance Fiber Grating Aptasensor. Anal. Chem. 2011, 83, 7027− 7034. (22) Knez, K.; Janssen, K. P. F.; Pollet, J.; Spasic, D.; Lammertyn, J. Fiber-Optic High-Resolution Genetic Screening Using Gold-Labeled Gene Probes. Small 2012, 8, 868−872. 11041

dx.doi.org/10.1021/jp5025069 | J. Phys. Chem. C 2014, 118, 11035−11042

The Journal of Physical Chemistry C

Article

(44) Bialiayeu, A.; Caucheteur, C.; Ahamad, N.; Ianoul, A.; Albert, J. Self-Optimized Metal Coatings for Fiber Plasmonics by Electroless Deposition. Opt. Express 2011, 19, 18742−18753. (45) Lyon, L. A.; Musick, M. D.; Natan, M. J. Colloidal Au-Enhanced Surface Plasmon Resonance Immunosensing. Anal. Chem. 1998, 70, 5177−5183.

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