João Pedro Mendes

Abstract
The lower refractive index sensitivity (RIS) of plasmonic nanoparticles (NP) in comparison to their plasmonic thin films counterparts hindered their wide adoption for wavelength-based sensor designs, wasting the NP characteristic field locality. In this context, high aspect-ratio colloidal core-shell Ag@Au nanorods (NRs) are demonstrated to operate effectively at telecommunication wavelengths, showing RIS of 1720 nm/RIU at 1350 nm (O-band) and 2325 nm/RIU at 1550 nm (L-band), representing a five-fold improvement compared to similar Au NRs operating at equivalent wavelengths. Also, these NRs combine the superior optical performance of Ag with the Au chemical stability and biocompatibility. Next, using a side-polished optical fiber, we detected glyphosate, achieving a detection limit improvement from 724 to 85 mg/L by shifting from the O to the C/L optical bands. This work combines the significant scalability and cost-effective advantages of colloidal NPs with enhanced RIS, showing a promising approach suitable for both point-of-care and long-range sensing applications at superior performance than comparable thin film-based sensors in either environmental monitoring and other fields.

Abstract
The phase-matching conditions for exciting surface plasmon resonances (SPR) in plasmonic films are typically satisfied via prism, optical fibers or grating-assisted coupling. We recently showed that plasmonic nanospheres can act as local emitters, exciting SPR waves on thin films-termed nanoparticle-induced SPR (NPI-SPR). This structure holds promise for sensing, but the effects of optical fiber geometry and nanoparticle anisotropy on the response were unexplored. This study examines these factors, showing that an etched multimode fiber with a 200 mu m core diameter, taper ratio of 4, and etching angle of 20 degrees optimizes interaction with plasmonic nanoparticles. Tuning the nanoparticle aspect ratio from 1 to 3 shifts the NPI-SPR band from 780 to 1580 nm, with excitation highly dependent on the incident light angle. Notably, for light incident parallel to the film plane, a refractive index sensitivity exceeding 1000 nm/RIU is achieved. This efficient light emission combines the field locality enhancements of plasmonic nanoparticle-on-film structures with the emission efficiency of plasmonic nanoantennas, advancing plasmonic optical fiber chemical and biosensors.

Abstract
Reinforced concrete structures form the backbone of civil infrastructure due to their durability, longevity, affordability, and availability. However, aging concrete poses challenges, with decay often beginning internally and becoming visible only at advanced stages, leading to costly repairs, restricted functionality, and safety risks. To address these challenges, sensors are crucial for enhancing infrastructure resilience and optimizing repairs. This study employs multimode optical fibers to monitor concrete curing, water ingress, relative humidity (RH), cement paste carbonation, and rebar corrosion. Four sensors monitor changes in reflection at the fiber tip of a 600 mu m multimode fiber (MMF) using LEDs and photodiodes, connected via a fiber bundle containing two 200 mu m MMF. Variations in the refractive index around the fiber tip are used to monitor water throughout the concrete lifecycle, including curing, RH changes and water intrusion. Colorimetric changes in a cement paste layer and an iron-thin film are used to monitor carbonation and corrosion. The curing sensor is temperature-independent and correlates strongly with cumulative heat release from cement hydration (R=0.95). The RH sensor monitors up to and beyond 100% RH, detecting water intrusion. The corrosion sensor detects early corrosion stages and distinguishes between reflection losses from corrosion and mechanical changes. The layer of cement paste for carbonation monitoring increases reflected intensity by 17% due to carbonation, with 63% of the increase occurring in 80 minutes in a 3% CO2 atmosphere. The broad monitoring scope and low implementation cost make this sensor a unique option among commercially available solutions for structural health monitoring of reinforced concrete.

Abstract
This study investigates the fabrication of plasmonic optical fiber sensors for glyphosate detection, employing silver thin film coatings deposited via the Tollens' reaction and further enhanced with protective gold plating. Silver films were produced through electroless deposition, forming rough plasmonic surfaces with localized hotspots that amplify the electromagnetic field. Surface roughness effects on the creation of hotspots were first evaluated numerically using the finite element method (FEM) and later experimentally assessed the impact on optical response. Furthermore, to address the inherent susceptibility of silver to oxidation and corrosion, a gold plating was applied using the Kirkendall effect, selectively replacing surface silver atoms with gold. This approach significantly improved the chemical stability of the sensors while preserving their plasmonic properties. This configuration was applied in developing a biosensor, using aptamers, for detecting glyphosate in concentrations ranging from 10(-1) to 10(4) mu g/L. The results demonstrated a sensitivity of 25.08 +/- 0.22 nm/(mu g/L) and a limit of detection (LOD) of 0.04 mu g/L, nearly ten times lower than the European Union's safety limit for glyphosate. Experimental results highlight the potential of this fabrication approach for developing sensitive, stable, and scalable plasmonic sensors tailored for environmental and agricultural monitoring applications.

Abstract
Concrete structures require precise temperature and humidity monitoring during curing to ensure optimal strength and prevent defects like cracking. A compact optical sensing system was developed using a single fiber that can be embedded directly within the concrete. The system functions as both a temperature and humidity sensor when paired with a spectral interrogation unit operating in the 1500-1600 nm range. Temperature monitoring is achieved through a Fiber Bragg Grating, while humidity sensing is facilitated by a Fabry-Perot interferometer at the fiber tip. The interferometer cavity is formed with a layer of polyvinylpyrrolidone (PVP). Initial air humidity sensor tests showed a significant change in the interference period with RH, demonstrating low hysteresis and high reproducibility. Calibration of one sensor revealed an approximately 3 nm period decrease when RH increased from 55% to 95%, with results suggesting a quadratic relationship between the interference period and RH values.