Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> Abstract The metallisation of glass for decorative and/or functional purposes is now a well-established technique. The most popular methods are electroplating or sputter deposition. In order to obtain suitable adhesion, substrate pretreatment is a substantial part of a vapour-deposited coating that generally cannot be dispensed with. The pretreatment costs can reach the same order of magnitude as those associated with the actual coating process, or can even exceed them. In this work we studied the adhesion of Au thin films on glass. The substrates were pretreated by an ion-beam-mixing step, consisting of the deposition of Au/C bilayers or C/Au/C multilayers followed by Xe + implantation. After such preparation, the specimens were further coated (using a sputtering machine) with 150-nm-thick Au or Au-alloy films. Adhesion properties of the films were examined using a scratch tester in conjunction with scanning electron microscopy. It was observed that, without the ion-beam-mixing pretreatment, the coatings were poorly adherent. Strong adhesion enhancement was observed in the pretreated samples. The key mechanism envisaged to explain this is related to the formation of mixed SiC–Au phases at the interface region. Moreover, the mechanical properties of the pure and alloyed Au films were quantified by nano-indentation, and hardness results are in good agreement with a simple rigid-sphere model of substitutional hardening. <s> BIB001 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> The effects of annealing at 300 °C on gold nanostructures sputtered onto glass substrate were studied using XRD, SAXSees, the Van der Pauw method and ellipsometry. As-sputtered and annealed samples exhibit a different dependence of the gold lattice parameter on the sputtering time. With increasing sputtering time the average thickness of the layer and the size of gold crystallites increased. Another rapid enlargement of the crystallites is observed after annealing. The volume resistivity decreases rapidly with the increasing sputtering time for both, as-deposited and annealed structures. With increasing sputtering time initially discontinuous gold coverage changes gradually in a continuous one. Electrically continuous gold coverage on the as-sputtered and annealed samples exhibits the same concentration of free charge carriers and Hall mobility. Optical constants of as-deposited and annealed gold films determined by ellipsometry support resistivity measurements and clearly manifest the presence of plasmons in discontinuous films. <s> BIB002 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> As alternatives to conventional metals, new plasmonic materials offer many advantages in the rapidly growing fields of plasmonics and metamaterials. These advantages include low intrinsic loss, semiconductor-based design, compatibility with standard nanofabrication processes, tunability, and others. Transparent conducting oxides such as Al:ZnO, Ga:ZnO and indium-tin-oxide (ITO) enable many high-performance metamaterial devices operating in the near-IR. Transition-metal nitrides such as TiN or ZrN can be substitutes for conventional metals in the visible frequencies. In this paper we provide the details of fabrication and characterization of these new materials and discuss their suitability for a number of metamaterial and plasmonic applications. <s> BIB003 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> The study presented herein investigates a novel arrangement of fiber-optic biosensors based on a tilted fiber Bragg grating (TFBG) coated with noble metal nanoparticles, either gold nanocages (AuNC) or gold nanospheres (AuNS). The biosensors constructed for this study demonstrated increased specificity and lowered detection limits for the target protein than a reference sensor without gold nanoparticles. The sensing film was fabricated by a series of thin-film and monolayer depositions to attach the gold nanoparticles to the surface of the TFBG using only covalent bonds. Though the gold nanoparticle integration had not yet been optimized for the most efficient coverage with minimum number of nanoparticles, binding AuNS and AuNC to the TFBG biosensor decreased the minimum detected target concentrations from 90 nM for the reference sensor, to 11 pM and 8 pM respectively. This improvement of minimum detection is the result of a reduced non-specific absorption onto the gold nanoparticles (by functionalization of the external surface of the gold nanoparticles), and of an optical field enhancement due to coupling between the photonic modes of the optical fiber and the localized surface plasmon resonances (LSPR) of the gold nanoparticles. This coupling also increased the sensitivity of the TFBG biosensor to changes in its local environment. The dissociation constant (Kd) of the target protein was also characterized with our sensing platform and found to be in good agreement with that of previous studies. <s> BIB004 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> A miniaturized, localized surface plasmon resonance (LSPR)-coupled fiber-optic (FO) nanoprobe is reported as a biosensor that is capable of label-free, sensitive detection of a cancer protein biomarker, free prostate specific antigen (f-PSA). The biosensor is based on the LSPR at the reusable dielectric-metallic hybrid interface with a robust, gold nano-disk array at the fiber end facet that is directly fabricated using EBL and metal lift-off process. The f-PSA has been detected with a mouse anti-human PSA monoclonal antibody (mAb) as a specific receptor linked with a self-assembled monolayer at the LSPR-FO facet surfaces. Experimental investigation and data analysis found near field refractive index (RI) sensitivity at ~226 nm/RIU with current LSPR-FO nanoprobe, and demonstrated the lowest limit of detection (LOD) at 100 fg/mL (~3 fM) of f-PSA in PBS solutions. The control experimentation using 5mg/mL bovine serum albumin in PBS and nonspecific surface test shows the excellent specificity and selectivity in the detection of f-PSA in PBS. These results present important progress towards a miniaturized, multifunctional fiber-optic technology that integrates informational communication and sensing function for developing a high performance, label-free, point-of-care (POC) device. <s> BIB005 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> Mercaptopropylsilatrane (MPS) was investigated as a novel self-assembled film on silica surfaces and also as a novel adhesive layer for the construction of a gold colloid monolayer on silica surfaces. We compare the preparation procedure and film quality of the MPS films to those of (3-mercaptopropyl)trimethoxysilane (MPTMS), which is commonly used for anchoring of gold nanoparticles on silica surfaces. The films were characterized by Fourier transform infrared spectroscopy, contact angle measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and Ellman's reagent to determine surface mercaptan concentration. The process in preparing the MPS films involves more environmentally friendly aqueous or polar organic solvents, takes significantly shorter time (<30 min), and results in more uniform and reproducible films due to its insensitivity to moisture. The MPS films also have higher mercaptan surface density than that of the MPTMS films, resulting in higher saturation coverage of the gold colloid monolayers on the MPS-coated substrates. The higher ambient stability of the MPS films as compared to the MPTMS films is important for applications where sufficient durability of the self-assembled films under ambient conditions is required. Thus, mercaptosilatrane may have the potential to replace mercaptosilane for surface modification and as an adhesive layer for the construction of a noble metal colloid monolayer on oxide surfaces. <s> BIB006 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> This study presents a facile, rapid and effective method for the fabrication of optical fiber surface plasmon resonance (SPR) sensors via polydopamine (PDA)-accelerated electroless plating (ELP). The bioinspired PDA coating formed through the facile self-polymerization of dopamine (DA) was utilized as a versatile material for optic-fiber functionalization. Gold seeds were then rapidly and firmly adsorbed onto the PDA functional layer by amino and imino intermediates generated during the polymerization, and a gold film sensor was fabricated after metal deposition. The fabrication time of the sensor was decreased by 6–12 times, and the fabricated sensor exhibited higher sensitivity, better reproducibility and adhesion stability compared with those fabricated by the traditional ELP. Some key experimental parameters, including DA polymerization temperature, DA polymerization time, and plating time, were investigated in detail. The optimized sample exhibited high sensitivity ranging from 1391 nm per RIU to 5346 nm per RIU in the refractive index range of 1.328 to 1.386. Scanning electron microscopy images indicated that the sensor surface consisted of gold nanoparticles with a uniform particle size and an orderly arrangement, and the film thickness was approximately 60 nm. Another PDA layer was formed on the gold film for facile immobilization of antibodies. The sensor exhibited effective antibody immobilization ability and high sensitivity for human IgG detection over a wide range of concentrations from 0.5 to 40 μg mL−1, which indicate the potential applications of the fabricated sensor in immunoassays. <s> BIB007 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> The surface plasmon resonance (SPR) performance of gold coated tilted fiber Bragg gratings (TFBG) at near infrared wavelengths is evaluated as a function of the angle between the tilt plane orientation and the direction of single- and double-sided, nominally 50 nm-thick gold metal depositions. Scanning electron microscope images show that the coating are highly non-uniform around the fiber circumference, varying between near zero and 50 nm. In spite of these variations, the experimental results show that the spectral signature of the TFBG-SPR sensors is similar to that of simulations based on perfectly uniform coatings, provided that the depositions are suitably oriented along the tilt plane direction. Furthermore, it is shown that even a (properly oriented) single-sided coating (over only half of the fiber circumference) is sufficient to provide a theoretically perfect SPR response with a bandwidth under 5 nm, and 90% attenuation. Finally, using a pair of adjacent TFBG resonances within the SPR response envelope, a power detection scheme is used to demonstrate a limit of detection of 3 × 10−6 refractive index units. <s> BIB008 </s> Plasmonic Optical Fiber-Grating Immunosensing: A Review <s> Metal Layer Deposition <s> Abstract The morphological properties of thin gold (Au) films sputtered onto fiber optic (FO) substrates play an essential role in the overall performance of the sensing devices relying on surface plasmon resonance (SPR) effects. In this work, the influence of thermal treatments on the structural changes of the Au layer coated on the FO-SPR sensors, and consequently on their plasmonic biosensing performance, was systematically investigated. First, the sensors were exposed to different annealing temperatures for different durations and their sensitivity was evaluated by refractometric measurements in sucrose dilutions. The attained results suggested the optimal annealing conditions that were further validated using a split-plot experimental design statistical model. Room-temperature scanning tunneling microscopy (STM) imaging of the FO substrates revealed changes in the granular surface texture of the thermally treated Au films that could be linked to the observed increase in sensitivity of the treated sensors. The FO sensors, annealed under optimal conditions, were finally tested as label-free aptamer-based biosensors for the detection of Ara h 1 peanut allergen. Remarkably, the results demonstrated a superior biosensing performance of the thermally treated FO sensors, as the limit of detection (LOD) was improved with up to two orders of magnitude compared to a similar non-annealed FO-SPR sensor. This significant increase in sensitivity represents a major step forward in the facile and cost-effective preparation of FO-SPR sensors capable of label-free detection of various biomolecular targets. <s> BIB009
The shape of deposited metal particles and the coating homogeneity directly impact the sensitivity and the reproducibility of plasmonic sensors. Metal deposits can be successfully applied to optical fibers using well-established technologies, such as nanoparticles immobilization BIB004 , nanostructures elaboration BIB005 , electroless deposition BIB007 , evaporation or sputtering BIB008 . The latter is the most spread technique for planar covering, because it provides very high quality depositions. However, in case of cylindrical surfaces, as for optical fibers, two consecutive depositions are usually performed in the same conditions, with a 180° rotation between both processes to ensure an entire surface covering. More sophisticated devices may allow a continuous rotation during the sputtering process, providing a more uniform coverage, as sketched in Figure 8 . To enhance the adherence of gold on silica substrates, different methods have been investigated, such as the deposition of 2-3 nm of chromium or titanium under the Au layer  , ion beam bombardment BIB001 and finally, the use of polymers and adhesive molecules BIB006 . Another option consists in thermally annealing the gold film, modifying its morphology BIB002 and consequently, the sensor's potential of detection BIB009 . Regardless of the chosen deposition technique, the main challenge is to obtain a very uniform layer where thickness, rugosity and morphology are finely managed for an optimum SPR excitation. A gold thickness of about 50 nm is needed to get the narrowest and the deepest SPR attenuation, corresponding to the highest surface sensitivity of the sensor. Recent papers also mention the use of non-metal layers for SPR analysis, such as semiconductors and oxides BIB003 but these kinds of materials have to be explored for optical fiber sensors and represent interesting perspectives for the future.  The shape of deposited metal particles and the coating homogeneity directly impact the sensitivity and the reproducibility of plasmonic sensors. Metal deposits can be successfully applied to optical fibers using well-established technologies, such as nanoparticles immobilization BIB004 , nanostructures elaboration BIB005 , electroless deposition BIB007 , evaporation or sputtering BIB008 . The latter is the most spread technique for planar covering, because it provides very high quality depositions. However, in case of cylindrical surfaces, as for optical fibers, two consecutive depositions are usually performed in the same conditions, with a 180 • rotation between both processes to ensure an entire surface covering. More sophisticated devices may allow a continuous rotation during the sputtering process, providing a more uniform coverage, as sketched in Figure 8 . To enhance the adherence of gold on silica substrates, different methods have been investigated, such as the deposition of 2-3 nm of chromium or titanium under the Au layer  , ion beam bombardment BIB001 and finally, the use of polymers and adhesive molecules BIB006 . Another option consists in thermally annealing the gold film, modifying its morphology BIB002 and consequently, the sensor's potential of detection BIB009 . Regardless of the chosen deposition technique, the main challenge is to obtain a very uniform layer where thickness, rugosity and morphology are finely managed for an optimum SPR excitation. A gold thickness of about 50 nm is needed to get the narrowest and the deepest SPR attenuation, corresponding to the highest surface sensitivity of the sensor. Recent papers also mention the use of non-metal layers for SPR analysis, such as semiconductors and oxides BIB003 but these kinds of materials have to be explored for optical fiber sensors and represent interesting perspectives for the future. 