Hg2+ Optical Fiber Sensor Based on LSPR Generated by Gold Nanoparticles Embedded in LBL Nano-Assembled Coatings

Recent advances in colorimetric and photoluminescent fibrillar devices, photonic crystals and carbon dot-based sensors for mercury (II) ion detection

The escalating environmental contamination with mercury has become a pressing issue, significantly impacting human beings and nature. For instance, small-scale gold mining has led to severe contamination in the Brazilian Yanomami village, highlighting the urgent need for action. The development of fibrillar-based sensors for mercury (II) ions represents an important issue to be considered in the point-of-care and simple detection of contaminants in water. Herein, this review discussed different colorimetric/photoluminescent-based prototypes for Hg2+ ions sensors and corresponding strategies to improve selectivity and sensitivity associated with the regeneration and reuse of the devices. Given these aspects, the electrospinning technique is promising for developing advanced mercury (II) ion sensors.

A disposable fiber-optic plasmonic sensor for chemical sensing

The integration of fiber optics and plasmonic sensors is promising to improve the practical usability over conventional bulky sensors and systems. To achieve high sensitivity, it typically requires fabrication of well-defined plasmonic nanostructures on optical fibers, which greatly increases the cost and complexity of the sensors. Here, we present a fiber-optic sensor system by using chemical absorption of gold nanoparticles and a replaceable configuration. By functioning gold nanoparticles with aptamers or antibodies, we demonstrate the applications in chemical sensing using two different modes. Measuring shift in resonance wavelength enables the Pb2+ detection with a high linearity and a limit of detection of 0.097 nM, and measuring absorption peak amplitude enables the detection of E. coli in urinary tract infection with a dynamic range between 103 to 108 CFU/mL. The high sensitivity, simple fabrication and disposability of this sensing approach could pave the way for point-of-care testing with fiber-optic plasmonic sensors.

Ion-Imprinted Chitosan-Based Localized Surface Plasmon Resonance Sensor for Ni2+ Detection

Heavy metals are important sources of environmental pollution and cause disease in organisms throughout the food chain. A localized surface plasmon resonance sensor was proposed and demonstrated to realize Ni2+ detection by using ion-imprinted chitosan. Au nanoparticles were coated on the multimode fiber to excite the local surface plasmon resonance, and Ni2+-imprinted chitosan was then functionalized by using the dip coating technique. Ethylene diamine tetra-acetic acid was used to release the Ni2+ ions and hence form countless voids. Ni2+ was refilled into the voids to increase the refractive index of the sensing material, thus realizing the measurement of Ni2+ by monitoring the wavelength shift in the localized surface plasmon resonant peak. The coating thickness of the Ni2+-chitosan gel was optimized to obtain greater sensitivity. Experimental results show that the proposed Ni2+ sensor has a sensitivity of 185 pm/μM, and the limit of detection is 0.512 μM. The comparison experiments indicated that the ion-imprinted chitosan has better selectivity than pure chitosan.

In Situ Synthesis of Gold Nanoparticles in Layer-by-Layer Polymeric Coatings for the Fabrication of Optical Fiber Sensors

A new method is proposed to tune the interferometric response of wavelength-based optical fiber sensors. Using the nanoparticle in situ synthesis (ISS) technique, it is possible to synthesize gold nanoparticles (AuNPs) within a pre-existing polymeric thin film deposited at the end-face of an optical fiber. This post-process technique allows us to adjust the optical response of the device. The effect of the progressive synthesis of AuNPs upon polymeric film contributed to a remarkable optical contrast enhancement and a very high tuning capability of the reflection spectra in the visible and near-infrared region. The spectral response of the sensor to relative humidity (RH) variations was studied as a proof of concept. These results suggest that the ISS technique can be a useful tool for fiber optic sensor manufacturing.

Self-Referenced Optical Fiber Sensor Based on LSPR Generated by Gold and Silver Nanoparticles Embedded in Layer-by-Layer Nanostructured Coatings

In this work, an optical fiber sensor based on the localized surface plasmon resonance (LSPR) phenomenon has been designed for the detection of two different chemical species (mercury and hydrogen peroxide) by using Layer-by-Layer Embedding (LbL-E) as a nanofabrication technique. In the first step, silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) have been synthesized by using a chemical protocol as a function of the strict control of three main parameters, which were polyelectrolyte concentration, a loading agent, and a reducing agent. In the second step, their incorporation into nanometric thin films have been demonstrated as a function of the number of bilayers, which shows two well-located absorption peaks associated to their LSPR in the visible region at 420 nm (AgNPs) and 530 nm (AuNPs). Finally, both plasmonic peaks provide a stable real-time reference measurement, which can be extracted from the spectral response of the optical fiber sensor, which shows a specific sensing mechanism as a function of the analyte of study.

Optical Biomedical Diagnostics Using Lab-on-Fiber Technology: A Review

Point-of-care and in-vivo bio-diagnostic tools are the current need for the present critical scenarios in the healthcare industry. The past few decades have seen a surge in research activities related to solving the challenges associated with precise on-site bio-sensing. Cutting-edge fiber optic technology enables the interaction of light with functionalized fiber surfaces at remote locations to develop a novel, miniaturized and cost-effective lab on fiber technology for bio-sensing applications. The recent remarkable developments in the field of nanotechnology provide innumerable functionalization methodologies to develop selective bio-recognition elements for label free biosensors. These exceptional methods may be easily integrated with fiber surfaces to provide highly selective light-matter interaction depending on various transduction mechanisms. In the present review, an overview of optical fiber-based biosensors has been provided with focus on physical principles used, along with the functionalization protocols for the detection of various biological analytes to diagnose the disease. The design and performance of these biosensors in terms of operating range, selectivity, response time and limit of detection have been discussed. In the concluding remarks, the challenges associated with these biosensors and the improvement required to develop handheld devices to enable direct target detection have been highlighted.

Multiarchitecture-Based Plasmonic-Coupled Emission Employing Gold Nanoparticles: An Efficient Fluorescence Modulation and Biosensing Platform

Surface plasmon-coupled emission (SPCE) is an efficient surface-enhanced fluorescence method based on the near-field coupling process of surface plasmons and fluorophores. Based on this, we developed multiple coupling structures for an SPCE system by introducing gold nanoparticles (AuNPs) with different architectures by adjusting different modification methods and configurations. By assembling AuNPs on a gold substrate through electrostatic adsorption and spin-coating, 40- and 55-fold enhancements were obtained compared to free space (FS) emission, respectively. After theoretical simulations and the optimization of experimental conditions, a novel "hot-spot" plasmonic structure, an intense electromagnetic field within the system, plasmonic properties, and the coupled process were found to be mainly responsible for the diverse enhancement effects observed. For the spin-coating deposition method, new enhancing systems with high efficiency can be easily built without complex modification. Additionally, the subsequent detection system based on the uniform modification of AuNPs through electrostatic adsorption is convenient to establish with high sensitivity and stability, which can broaden the application of SPCE in both fluorescence-based sensing and imaging. This AuNP-enhanced SPCE using an electrostatic adsorption method was designed as an immunosensor to prove feasibility.

Trends in the Implementation of Advanced Plasmonic Materials in Optical Fiber Sensors (2010–2020)

In recent years, the interaction between light and metallic films have been proven to be a highly powerful tool for optical sensing applications. We have witnessed the development of highly sensitive commercial devices based on Surface Plasmon Resonances. There has been continuous effort to integrate this plasmonic sensing technology using micro and nanofabrication techniques with the optical fiber sensor world, trying to get better, smaller and cost-effective high performance sensing solutions. In this work, we present a review of the latest and more relevant scientific contributions to the optical fiber sensors field using plasmonic materials over the last decade. The combination of optical fiber technology with metallic micro and nanostructures that allow plasmonic interactions have opened a complete new and promising field of study. We review the main advances in the integration of such metallic micro/nanostructures onto the optical fibers, discuss the most promising fabrication techniques and show the new trends in physical, chemical and biological sensing applications.

Giant Displacement Sensitivity Using Push-Pull Method in Interferometry

We present a giant sensitivity displacement sensor combining the push-pull method and enhanced Vernier effect. The displacement sensor consists in two interferometers that are composed by two cleaved standard optical fibers coupled by a 3 dB coupler and combined with a double-sided mirror. The push pull-method is applied to the mirror creating a symmetrical change to the length of each interferometer. Furthermore, we demonstrate that the Vernier effect has a maximum sensitivity of two-fold that obtained with a single interferometer. The combination of the push-pull method and the Vernier effect in the displacement sensors allows a sensitivity of 60 ± 1 nm/μm when compared with a single interferometer working in the same free spectral range. In addition, exploring the maximum performance of the displacement sensors, a sensitivity of 254 ± 6 nm/μm is achieved, presenting a M-factor of 1071 and MVernier of 1.9 corresponding to a resolution of 79 pm. This new solution allows the implementation of giant-sensitive displacement measurement for a wide range of applications.

Preparation and Application of Metal Nanoparticals Elaborated Fiber Sensors

In recent years, surface plasmon resonance devices (SPR, or named plamonics) have attracted much more attention because of their great prospects in breaking through the optical diffraction limit and developing new photons and sensing devices. At the same time, the combination of SPR and optical fiber promotes the development of the compact micro-probes with high-performance and the integration of fiber and planar waveguide. Different from the long-range SPR of planar metal nano-films, the local-SPR (LSPR) effect can be excited by incident light on the surface of nano-scaled metal particles, resulting in local enhanced light field, i.e., optical hot spot. Metal nano-particles-modified optical fiber LSPR sensor has high sensitivity and compact structure, which can realize the real-time monitoring of physical parameters, environmental parameters (temperature, humidity), and biochemical molecules (pH value, gas-liquid concentration, protein molecules, viruses). In this paper, both fabrication and application of the metal nano-particles modified optical fiber LSPR sensor probe are reviewed, and its future development is predicted.

Hg2+ Optical Fiber Sensor Based on LSPR with PDDA-Templated AuNPs and CS/PAA Bilayers

An optical fiber localized surface plasmon resonance (LSPR) sensor was proposed and experimentally demonstrated to detect Hg2+ ions by functionalizing the optical fiber surface with gold nanoparticles (AuNPs) and chitosan (CS)/poly acrylic acid (PAA) bilayers. A flame-brushing technology was proposed to post-process the polydimethyl diallyl ammonium chloride(PDDA)-templated nanoparticles, avoiding the aggregation of AuNPs and achieving well-dispersed AuNPs arrays. LSPR stimulated by the AuNPs is sensitive to changes in the refractive index induced by Hg2+ ions absorption on the CS/PAA bilayers. Experimental results demonstrated that the LSPR peak wavelength linearly shifts with the concentrations of Hg2+ ions from 1 to 30 μM with a sensitivity of around 0.51 nm/ppm. The sensor also exhibits good specificity and longtime stability.

Design and Optimization of Plasmon Resonance Sensor Based on Micro–Nano Symmetrical Localized Surface

Surface Plasma resonance (SPR) sensors combined with biological receptors are widely used in biosensors. Due to limitations of measurement techniques, small-scale, low accuracy, and sensitivity to the refractive index of solution in traditional SPR prism sensor arise. As a consequence, it is difficult to launch commercial production of SPR sensors. The theory of localized surface plasmon resonance (LSPR) developed based on SPR theory has stronger coupling ability to near-field photons. Based on the LSPR sensing theory, we propose a submicron-sized golden-disk and graphene composite structure. By varying the thickness and diameter of the array disk, the performance of the LSPR sensor can be optimized. A graphene layer sandwiched between the golden-disk and the silver film can prevent the latter from oxidizing. Symmetrical design enables high-low concentration of dual-channel distributed sensing. As the fixed light source, we use a 632.8-nm laser. A golden nano-disk with 45 nm thickness and 70 nm radius is designed, using a finite difference time domain (FDTD) simulation system. When the incident angle is 42°, the figure of merit (FOM) reaches 8826, and the measurable refractive index range reaches 0.2317.