Localized Surface Plasmon Resonance for Optical Fiber-Sensing Applications

It is well known that optical fiber sensors have attracted the attention of scientific community due to its intrinsic advantages, such as lightweight, small size, portability, remote sensing, immunity to electromagnetic interferences and the possibility of multiplexing several signals. This field has shown a dramatic growth thanks to the creation of sensitive thin films onto diverse optical fiber configurations. In this sense, a wide range of optical fiber devices have been successfully fabricated for monitoring biological, chemical, medical or physical parameters. In addition, the use of nanoparticles into the sensitive thin films has resulted in an enhancement in the response time, robustness or sensitivity in the optical devices, which is associated to the inherent properties of nanoparticles (high surface area ratio or porosity). Among all of them, the metallic nanoparticles are of great interest for sensing applications due to the presence of strong absorption bands in the visible and near-infrared regions, due to their localized surface plasmon resonances (LSPR). These optical resonances are due to the coupling of certain modes of the incident light to the collective oscillation of the conduction electrons of the metallic nanoparticles. The LSPR extinction bands are very useful for sensing applications as far as they can be affected by refractive index variations of the surrounding medium of the nanoparticles, and therefore, it is possible to create optical sensors with outstanding properties such as high sensitivity and optical self-reference. In this chapter, the attractive optical properties of metal nanostructures and their implementation into different optical fiber configuration for sensing or biosensing applications will be studied

Creation of novel gold-nanorod-based localized surface plasmon resonance biosensors

Starting with a comprehensive review of both surface plasmon resonance (SPR) based and localized surface plasmon resonance (LSPR) based sensors, this thesis reports the studies on the development of a novel sensitive gold nanorod (GNR) based label-free LSPR optical fibre biosensor, and the development of a novel robust method for effectively modifying the surface of cetyl-trimethyl ammonium bromide (CTAB) capped GNRs and their LSPR biosensing applications. A novel GNR-based LSPR optical fibre sensor was fabricated and evaluated in this work. The sensor probe was prepared by covalently immobilizing GNRs, synthesized using a seed-mediated growth method, on the decladed surface of a piece of multimode optical fibre. In order to operate the LSPR sensor as a reflective sensor, a silver mirror was also coated at one distal end of the sensor probe by a dip coating method. In the refractive index sensitivity test, it was found that the longitudinal plasmon band (LPB) of GNRs is highly sensitive to the refractive index change close to the GNRs surface, and the sensitivity of the LSPR optical fibre sensor increases with the increase of the aspect ratio of GNRs. The results showed that the GNR-based LSPR optical fibre sensors prepared in this work have linear and high refract index sensitivities. For sensors based on GNRs with aspect ratios of 2.6, 3.1, 3.7 and 4.3, their refractive index sensitivities were found to be 269, 401, 506 and 766 nm/RIU (RIU = refractive index unit), respectively, in the refractive index range from 1.34 to 1.41. In order to evaluate the biosensing performance, the GNR-based LSPR optical fibre sensor with aspect ratio of 4.1 and a 2 cm sensing length was further functionalized with human IgG to detect the specific target — anti-human IgG, and a detection limit of 1.6 nM was observed using a wavelength-based interrogation approach. In another study, in order to overcome the drawbacks of the CTAB-capped GNRs found in biosensing and biomedical applications, a simple yet robust pH-mediated method for effectively modifying the surface of CTAB-capped GNRs synthesized by the seed-mediated growth method was developed. This method allows the complete replacement of the CTAB molecules attached on the GNRs surface with the 11-mercaptoundecaonic acid (MUA) molecules to take place in a total aqueous environment by controlling the pH of the MUA aqueous solution, thus avoiding the irreversible aggregation of GNRs during the complex surface modification process observed in the previous reported methods. The success of the complete replacement of CTAB with MUA was confirmed by the surface elemental analysis using an X-ray photoelectron spectroscopy (XPS), and the MUA-modified GNRs created in this work demonstrated a high stability up to 4 months at least when stored in a buffer solution at pH 9 at 4°C. The MUA-modified GNRs with an aspect ratio of 3.9 were furthered developed as a solution-phase-based label-free LSPR biosensor by functionalizing the GNRs with human IgG. A detection limit as low as 0.4 nM for detecting anti-human IgG was achieved by this sensor. The achievements of this work are concluded and the directions of future work are also pointed out

Plasmonic Nanostructures for Biosensor Applications

Improving the sensitivity of existing biosensors is an active research topic that cuts across several disciplines, including engineering and biology. Optical biosensors are the one of the most diverse class of biosensors which can be broadly categorized into two types based on the detection scheme: label-based and label-free detection. In label-based detection, the target bio-molecules are labeled with dyes or tags that fluoresce upon excitation, indicating the presence of target molecules. Label-based detection is highly-sensitive, capable of single molecule detection depending on the detector type used. One method of improving the sensitivity of label-based fluorescence detection is by enhancement of the emission of the labels by coupling them with metal nanostructures. This approach is referred as plasmon-enhanced fluorescence (PEF). PEF is achieved by increasing the electric field around the nano metal structures through plasmonics. This increased electric field improves the enhancement from the fluorophores which in turn improves the photon emission from the fluorophores which, in turn, improves the limit of detection. Biosensors taking advantage of the plasmonic properties of metal films and nanostructures have emerged an alternative, low-cost, high sensitivity method for detecting labeled DNA. Localized surface plasmon resonance (LSPR) sensors employing noble metal nanostructures have recently attracted considerable attention as a new class of plasmonic nanosensors.;In this work, the design, fabrication and characterization of plasmonic nanostructures is carried out. Finite difference time domain (FDTD) simulations were performed using software from Lumerical Inc. to design a novel LSPR structure that exhibit resonance overlapping with the absorption and emission wavelengths of quantum dots (QD). Simulations of a composite Au/SiO2 nanopillars on silicon substrate were performed using FDTD software to show peak plasmonic enhancement at QD emission wavelength (560nm). A multi-step fabrication process was developed to create plasmonic nanostructures, and the optical characterization of emission enhancement was performed

Lab on fiber technology: a nanospectroscopic approach for biochemical sensing

169 p.Hoy en día, gracias al uso de fibras ópticas, el desarrollo de sensores bioquímicos económicos y de altas prestaciones capaces de realizar medidas en tiempo real es posible, modificando esta tecnología por la tradicional basada en equipamientos caros, grandes y complejos. Por esta razón, en esta tesis hemos desarrollado un sensor mediante nanopartículas de oro inmovilizadas en la cara de una fibra óptica. El sensor que proponemos combina las ventajas de las fibras ópticas con el efecto plasmón de las nanopartículas, que proporcionan gran sensibilidad a cambios en el medio externo. Sin embargo, la mayor novedad que esta tesis proporciona es el uso de la nano-espectroscopia. Esta técnica se basa en hacer coincidir las frecuencias de resonancia de las nanopartículas con el elemento bioquímico que se quiera detectar, consiguiendo altos niveles de selectividad y sensibilidad, en contraposición con los métodos convencionales que se basan en medir cambios en longitud de onda de la frecuencia de resonancia de las nanopartículas. Para demostrar la validez de la nano-espectroscopia en la punta de una fibra óptica, se han realizado medidas para detectar iones de cobre (II) y Citocromo c, consiguiendo unos límites de detección varios órdenes de magnitud por debajo de los sensores basados en nano-espectroscopia mediante microscopios. Además, esta tesis contribuye también a un mayor entendimientodel proceso de inmovilización de las nanopartículas en la fibra óptica gracias a la amplia caracterizaciónque se ha realizado

Lab on fiber technology: a nanospectroscopic approach for biochemical sensing

169 p.Hoy en día, gracias al uso de fibras ópticas, el desarrollo de sensores bioquímicos económicos y de altas prestaciones capaces de realizar medidas en tiempo real es posible, modificando esta tecnología por la tradicional basada en equipamientos caros, grandes y complejos. Por esta razón, en esta tesis hemos desarrollado un sensor mediante nanopartículas de oro inmovilizadas en la cara de una fibra óptica. El sensor que proponemos combina las ventajas de las fibras ópticas con el efecto plasmón de las nanopartículas, que proporcionan gran sensibilidad a cambios en el medio externo. Sin embargo, la mayor novedad que esta tesis proporciona es el uso de la nano-espectroscopia. Esta técnica se basa en hacer coincidir las frecuencias de resonancia de las nanopartículas con el elemento bioquímico que se quiera detectar, consiguiendo altos niveles de selectividad y sensibilidad, en contraposición con los métodos convencionales que se basan en medir cambios en longitud de onda de la frecuencia de resonancia de las nanopartículas. Para demostrar la validez de la nano-espectroscopia en la punta de una fibra óptica, se han realizado medidas para detectar iones de cobre (II) y Citocromo c, consiguiendo unos límites de detección varios órdenes de magnitud por debajo de los sensores basados en nano-espectroscopia mediante microscopios. Además, esta tesis contribuye también a un mayor entendimientodel proceso de inmovilización de las nanopartículas en la fibra óptica gracias a la amplia caracterizaciónque se ha realizado

Optical Microfibre Based Photonic Components and Their Applications in Label-Free Biosensing

Optical microfibre photonic components offer a variety of enabling properties, including large evanescent fields, flexibility, configurability, high confinement, robustness and compactness. These unique features have been exploited in a range of applications such as telecommunication, sensing, optical manipulation and high Q resonators. Optical microfibre biosensors, as a class of fibre optic biosensors which rely on small geometries to expose the evanescent field to interact with samples, have been widely investigated. Due to their unique properties, such as fast response, functionalization, strong confinement, configurability, flexibility, compact size, low cost, robustness, ease of miniaturization, large evanescent field and label-free operation, optical microfibres based biosensors seem a promising alternative to traditional immunological methods for biomolecule measurements. Unlabeled DNA and protein targets can be detected by monitoring the changes of various optical transduction mechanisms, such as refractive index, absorption and surface plasmon resonance, since a target molecule is capable of binding to an immobilized optical microfibre. In this review, we critically summarize accomplishments of past optical microfibre label-free biosensors, identify areas for future research and provide a detailed account of the studies conducted to date for biomolecules detection using optical microfibres

Dynamic detection of the bio-molecular interaction at the surface of plasmonic nanoarrays

Nanophysics and plasmonics have recently become fields of relevant interest in the world of research and, in particular, in biosensing and biochemistry. Nanoparticles of noble metals interact with incident light giving rise to the Localized Surface Plasmon Resonance (LSPR), a sharp peak of the extinction spectra of the nanoparticles as a result of the collective oscillation at a resonant frequency of the conduction electrons. The shape of the peak and its position strongly depend on both nano system properties, as composition, size, shape, orientation, and on the local dielectric environment. A change in the medium in which the nanoparticle is embedded is indeed detected and transduced as a distortion and shift of the peak. This mechanism is at the basis of the biosensing application of plasmonic structures, revealing binding events of molecules to the surface or extremely small variation in concentration of substances in the proximity. For this reason, LSPR plasmonic biosensors gained great popularity in a broad range of applications, in particular as diagnostic devices able to quantitatively detect biomarker molecules. MicroRNA, among the others, are biomolecules of prominent interest associated to thumoral or other kind of diseases. The aim of this project is to realize and test a sensitive, specific and label-free plasmonic nanobiosensor able to detect microRNA target molecules and to investigate the dynamics of the binding of the biomolecules on the surface of the optical transducers. To accomplish this task, Au nanoprisms arrays (NPA) are chosen as reference structure, with a LSPR wavelength around 800 nm and nanofabricated via NanoSphere Lithography (NSL) and thermal evaporation deposition. All the samples are morphologically characterized with AFM or SEM microscopy. Post-treating procedure and functionalization protocols are employed to allow the binding of the analyte molecule to be detected to the sensor, and all the functionalization signals are detected by linear optical spectroscopy in the visible or near-infrared spectral range. Static measurements are performed to control the peak shift of the sample after each functionalization step, and dynamic measurements in a microfluidic setup allow to monitor the temporal evolution of the optical signal and to reconstruct in real-time the hybridization kinetics at the surface of the plasmonic sensor. A 217nm/RIU bulk sensitivity and 50fMoles limit of detection is reached with the employed structures, indicating that both the nanofabrication and functionalization strategy are successful in the detection of analyte molecules down to low concentration limits. Of course, optimization is desirable, to push even further the sensitivity and solve challenges as for example the aspecific target binding on the sensor surface. Another purpose of the work is to extract interesting information about the dynamics of the hybridization reaction that takes place when the analyte microRNA is bound to the surface of the nanoarray. Hybridization kinetics is studied, determining the time and affinity constants characterizing the reaction. The results obtained will prove the non- ideal behaviour of the association, laying the basis for future and advanced outlook about the building of a non-Langmuir association model able to analytically describe the bi-molecular binding system

Spectroscopy-Based Biosensors

Biosensors are analytical devices capable of providing quantitative or semi-quantitative information by using a biological recognition element and a transducer. Depending upon the nature of the recognition element, different surface sensitive techniques can be applied to monitor these molecular interactions. In order to increase sensitivities and to lower detection limits down to even individual molecules, nanomaterials are promising candidates. This is possible due to the potential to immobilize more bioreceptor units at reduced volumes and their ability to act as transduction elements by themselves. Among such nanomaterials, gold nanoparticles, quantum dots, polymer nanoparticles, carbon nanotubes, nanodiamonds, and graphene are intensively studied. Biosensors provide rapid, real-time, accurate, and reliable information about the analyte under investigation and have been envisioned in a wide range of analytical applications, including medicine, food safety, bioprocessing, environmental/industrial monitoring, and electronics. A variety of biosensors, such as optical, spectroscopic, molecular, thermal, and piezoelectric, have been studied and applied in countless fields. In this book, examples of spectroscopic and optical biosensors and immunoassays are presented. Furthermore, two comprehensive reviews on optical biosensors are include

Recent advances in plasmonic sensor-based fiber optic probes for biological applications

Funding: This research was funded by National Natural Science Foundation of China (NSFC), grant number [61675008]. Acknowledgments: KN wishes to thank The Royal Society Kan Tong Po International Fellowship 2018 for the travel fund to visit Hong Kong Polytechnic University and Shenzhen Science and Technology Innovation Commission (Project GJHZ20180411185015272).Peer reviewedPublisher PD