Abstract: Introduction Gold nanoparticles (Au NP) exhibit interesting optical, physical and chemical features that can be tuned by modifying their morphology or by functionalizing them [1]. Therefore, a wide variety of sensing applications based on optical fiber have been developed using them, such as the colorimetric detection of toxic metal ions, cysteine or hydrogen peroxide. For those purposes, Au NP are typically embedded into polyelectrolyte multilayers or polymeric matrices, in order to be deposited onto optical fibers. Recent studies showed the possibility of depositing Au NP onto substrates seeded with block copolymers forming nanoparticle clusters [2] . The resulting array gave rise to a localized surface plasmon resonance, whose wavelength can be tailored, for instance, by modifying the amount of nanoparticles of each cluster or the distribution of the clusters. In order to check out the feasibility of the array as sensing structure, citrate-stabilized Au NPs have been self-assembled onto a polymeric template deposited along an optical fiber. The utilization of this device as a pH sensor opens the doors for the fabrication of sensors for different applications by controlling the seeding process and the morphology of Au NP clusters onto the surface of the optical fiber. Self-assembly of patterned gold nanoparticle cluster array along the optical fiber Au NP clusters were self-assembled by electrostatic interactions with a polymeric template [2] along the core of a 200 μm-core optical fiber. The optical fiber was first cleaned and activated by immersing it for 20 minutes in each one of the following solutions: soap, ultrapure water, potassium hydroxide (KOH) and ultrapure water. Subsequently, in order to build the template and so seeding the substrate, the fiber was dipped for 30 seconds in a 5 mg/mL solution of the cationic copolymer poly(styrene-b-2-vinylpyridine) in o-xylene. Finally, gold nanoparticles were self-assembled onto the template by immersing the fiber for 60 minutes into the anionic citrate-stabilized gold nanoparticles solution. The resulting coating was analyzed with images obtained by a Scanning Electronic Microscope (SEM). The deposition of the Au NP cluster array along the fiber was controlled by analyzing the absorption spectra during the different steps of the process explained above. The experimental set up used to study the processes consisted of the connection of the optical fiber between a white light source in the Visible Spectra (VIS) and a spectrometer (see Figure 1). Results The averaged diameter of the clusters is 120nm whereas the distance between the centers is 140nm (see Figure 2). Due to their formation on the seeded surface of the optical fiber, an absorption peak was observed in the transmitted spectra: as the formation of the cluster array moved forward, the absorption peak broadened and red-shifted from 630 nm up to 700 nm, while the absorbance increased, as it is shown in Figure 3. These changes in the absorption spectra are characteristic of the formation of the cluster array along the surface of the optical fiber. In order to evaluate the utilization of this Au NP cluster array as a possible sensing structure, the as-patterned optical fiber was exposed to different pH values ranging from 2 to 6. For each distinct pH value, the absorption peak was located at a different wavelength: as it can be observed in Figure 4, the absorption peak was red-shifted as the pH value decreased, showing a repetitive and reversible response. Conclusions The results exposed previously showed how the formation of a patterned gold nanoparticle cluster array yield to a localized surface plasmon resonance that could be used for measuring pH variations. The utilization of this array as sensing structure opens the possibility of utilizing this pattern for other sensing purposes by employing modified of functionalized Au NP. These parameters are currently been studied, together with different modifications of the patterned array, such as the distribution of the clusters and the amount of gold nanoparticles adsorbed in each cluster. Acknowledgements This research was supported by the Spanish State Research Agency (AEI) through the TEC2016-79367-C2-2-R project. References [1] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment, J. Phys. Chem. B. 107 (2003) 668–677. doi:10.1021/jp026731y. [2] F.L. Yap, P. Thoniyot, S. Krishnan, S. Krishnamoorthy, Nanoparticle cluster arrays for high-performance SERS through directed self-assembly on flat substrates and on optical fibers, ACS Nano. 6 (2012) 2056–2070. doi:10.1021/nn203661n. Figure 1

Abstract: Introduction Nanofilms surfaces with a high roughness are relevant to develop optical fiber sensors (OFS). It has been reported, in previous publications [1], that this type of structures has been used in several applications to detect parameters as for example, gases and VOCs. One of the most important features of these sensors, based on this type of nanofilms, is the high surface/volume ratio; thanks to this fact, the molecules of the parameter to be measured can interact in a better way, with the rough surface improving the final sensitivity. Moreover, in most of the cases, these structures can be functionalized to make the final sensor more selective. This is the case of the structure proposed in this work formed by PAH (Poly(Allylamine Hydrochloride)) / PSP (Poly(Sodium Phosphate)) when it is deposited along the optical fiber by means of the technique Layer-by-Layer (LbL) nano assembly. The main idea of LbL technique consists of the nanoassembly of oppositely electrically charged polyelectrolytes: PAH is the material used as polycation and PSP is the inorganic compound employed as polyanion. It has been studied and exposed in [2] that the final nanofilm obtained with both polymers yields into an increasing roughness as the thickness does so making it use very interesting for humidity sensing. Nano Film Construction and experimental set up As it can be mentioned above, LbL nano assembly technique was employed with the goal of obtaining a specific roughness for the PSP/PAH nanofilm (Sensor A). The roughness surface developed enables the interaction between the PSP/PAH thin film and the evanescent field of the light guided and, due to the surface/volume ratio obtained, the molecules of water can penetrate through the nanofilm deposited improving in this manner the sensitivity. Different constructions with a distinct number of pair of layers (15 for sensor A and 100 for sensor B) and were carried out with polyelectrolyte concentrations of 10 -3 M along the core of a 200 μm-core optical fiber using a robotic arm (Nadetech S.L.). Another construction with a polyelectrolyte concentration of 10 -2 M was made (Sensor C: 15 pair of layers). Firstly, the optical fibers were cleaned by immersing them for 20 minutes in each one of the following solutions: soap, ultrapure water, potassium hydroxide (KOH) and ultrapure water. After that, in order to facilitate the deposition of the first layer of PSP, an anchoring layer of Poly(ethyleneimine) (PEI) was deposited during another 20 minutes. Every sensor was exposed to different concentrations of relative humidity (RH) and their responses were recorded and studied. For sensor characterization, a transmission architecture was employed. For humidity sensing, the interrogation was carried out with a halogen broadband source (HL-2000-FHSA) connected to one end of the optical fiber and a spectrometer (USB2000+XR1) connected to the other one (see Figure 1). The sensors were placed in a climatic chamber where changes in the RH, from 20% to 90%, were performed. This cycle of RH variations was repeated several times and the responses of the sensors were recorded every minute. Results and Conclusions The responses of the sensors towards RH were carried out by monitoring the signal power level at 600 nm. As Figures 2 and 3 show, the optical power variations of Sensor A (0,6 dB), when the RH changes from 20% to 90%, are higher than Sensor B variations (0,18 dB); this is because with a low number of pair of layers, the final nanofilm thickness enables a better interaction between the evanescent field of the optical fiber and Sensor A nanofilm. Both factors lead to increased sensitivity. On the other hand, the value of the nanofilm roughness, as with the thickness, of Sensor B is higher than Sensor A; thanks to this fact, the molecules of H 2 O can penetrate and spread more in its structure. As it can be appreciated in Figure 3, the consequence of this fact is the reduction of the noise component of the sensor B response. Finally, Figure 4 shows the response of the Sensor C towards HR. In this case, due to the polyelectrolyte concentration used (10 -2 M), the roughness obtained was the lowest and consequently, the response of the sensor C is the worst. Therefore, this fact proves that roughness is a very important factor in the sensitivity optimization process of this type of nanofilms. Acknowledgements This work was carried out with the financial support of MINECO (Spain) through TEC2016-79367-C2-2-R (AEI/FEDER, UE) as well as Public University of Navarre PhD grants program. References [1] N. Cini, T. Tulun, G. Decher, and V. Ball, “Step-by-step assembly of self-patterning polyelectrolyte films violating (Almost) all rules of layer-by-layer deposition,” J. Am. Chem. Soc. , vol. 132, no. 24, pp. 8264–8265, 2010. [2] C. Elosua, D. Lopez-Torres, M. Hernaez, I. R. Matias, and F. J. Arregui, “Comparative study of layer-by-layer deposition techniques for poly(sodium phosphate) and poly(allylamine hydrochloride).,” Nanoscale Res. Lett. , vol. 8, p. 539, 2013. Figure 1

Abstract: The present work is an electrochemical evaluation of hydrotalcite Mg/Al impregnated with caffeine for the protection of the AZ31 alloy, where different concentrations of the inhibitor were evaluated to determine the best result in order to be applied as a coating in orthopedic uses. The efficiency of the hydrotalcite coating as a function of concentration was determined using electrochemical impedance spectroscopy and it was shown that HT MgAl with 5 ppm perezone impregnated achieved 99% inhibition efficiency.