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

Plasmonic Coupling on an Optical Microfiber Surface: Enabling Single-Molecule and Noninvasive Dopamine Detection

Optical fibers can be effective biosensors when employed in early-stage diagnostic point-of-care devices as they can avoid interference from molecules with similar redox potentials. Nevertheless, their sensitivity needs to be improved for real-world applications, especially for small-molecule detection. This work demonstrates an optical microfiber biosensor for dopamine (DA) detection based on the DA-binding-induced aptamer conformational transitions that occur at plasmonic coupling sites on a double-amplified nanointerface. The sensor exhibits ultrahigh sensitivity when detecting DA molecules at the single-molecule level; additionally, this work provides an approach for overcoming optical device sensitivity limits, further extending optical fiber single-molecule detection to a small molecule range (e.g., DA and metal ions). The selective energy enhancement and signal amplification at the binding sites effectively avoid nonspecific amplification of the whole fiber surface which may lead to false-positive results. The sensor can detect single-molecule DA signals in body-fluids. It can detect the released extracellular DA levels and monitor the DA oxidation process. An appropriate aptamer replacement allows the sensor to be used for the detection of other target small molecules and ions at the single-molecule level. This technology offers alternative opportunities for developing noninvasive early-stage diagnostic point-of-care devices and flexible single-molecule detection techniques in theoretical research.

Review of Optical Fiber Sensors for Temperature, Salinity, and Pressure Sensing and Measurement in Seawater

Temperature, salinity, and pressure (TSP) are essential parameters for the ocean. Optical fiber sensors (OFSs) have rapidly come into focus as an ocean detection technology in recent years due to their advantages of electromagnetic interference, light weight, low cost, and no waterproof requirement. In this paper, the most recently developed TSP sensors for single parameter and multi-parameter TSP sensing and measurement based on different OFSs are reviewed. In addition, from the practical point of view, encapsulation methods that protect fibers and maintain the normal operation of OFSs in seawater, and the response time of the OFS, are addressed. Finally, we discuss the prospects and challenges of OFSs used in marine environments and provide some clues for future work.

Prefab Hollow Glass Microsphere-Based Immunosensor with Liquid Crystal Sensitization for Acute Myocardial Infarction Biomarker Detection

Quantitative detection of cardiac troponin biomarkers in blood is an important method for clinical diagnosis of acute myocardial infarction (AMI). In this work, a whispering gallery mode (WGM) microcavity immunosensor based on a prefab hollow glass microsphere (HGMS) with liquid crystal (LC) sensitization was proposed and experimentally demonstrated for label-free cardiac troponin I-C (cTnI-C) complex detection. The proposed fiber-optic immunosensor has a simple structure; the tiny modified HGMS serves as the key sensing element and the microsample reservoir simultaneously. A sensitive LC layer with cTnI-C recognition ability was deposited on the inner wall of the HGMS microcavity. The arrangement of LC molecules is affected by the cTnI-C antigen—antibody binding in the HGMS, and the small change of the surface refractive index caused by the binding can be amplified owing to the birefringence property of LC. Using the annular waveguide of the HGMS, the WGMs were easily excited by the coupling scanning laser with a microfiber, and an all-fiber cTnI-C immunosensor can be achieved by measuring the resonant wavelength shift of the WGM spectrum. Moreover, the dynamic processes of the cTnI-C antigen—antibody binding and unbinding was revealed by monitoring the wavelength shift continuously. The proposed immunosensor with a spherical microcavity can be a cost-effective tool for AMI diagnosis.

Optical Microfiber with a Gold Nanorods-Black Phosphorous Nanointerface: An Ultrasensitive Biosensor and Nanotherapy Platform

The detection and therapy of cancers in the early stage significantly alleviate the associated dangers. Optical devices offer new opportunities for these early measures. However, the clinical translation of the existing methods is severely hindered by their relatively low sensitivity or unclear physiological metabolism. Here, an optical microfiber sensor with a drug loading gold nanorod-black phosphorous nanointerface, as an ultrasensitive biosensor and nanotherapy platform, is developed to meet the early-stage requirement. With interface sensitization and functionalization of the hybrid nanointerface, the microfiber sensor presents an ultrahigh sensing performance, achieving the selective detection of the HER2 biomarker with limits of detection of 0.66 aM in buffer solution and 0.77 aM in 10% serum. It can also distinguish breast cancer cells from other cells in the early stage. Additionally, enabled by the interface, the optical microfiber is able to realize cellular nanotherapy, including photothermal/chemotherapy with pump laser coupling after diagnosis, and evaluate therapy results in real time. The immobilization of the interface on the optical microfiber surface prevents the damage to normal cells induced by nanomaterial enrichment, making the device more efficient and intelligent. This study opens up a new avenue for the development of smart optical platforms for sensitive biosensing and precision therapy.

Label-Free Physical Techniques and Methodologies for Proteins Detection in Microfluidic Biosensor Structures

Proteins in biological fluids (blood, urine, cerebrospinal fluid) are important biomarkers of various pathological conditions. Protein biomarkers detection and quantification have been proven to be an indispensable diagnostic tool in clinical practice. There is a growing tendency towards using portable diagnostic biosensor devices for point-of-care (POC) analysis based on microfluidic technology as an alternative to conventional laboratory protein assays. In contrast to universally accepted analytical methods involving protein labeling, label-free approaches often allow the development of biosensors with minimal requirements for sample preparation by omitting expensive labelling reagents. The aim of the present work is to review the variety of physical label-free techniques of protein detection and characterization which are suitable for application in micro-fluidic structures and analyze the technological and material aspects of label-free biosensors that implement these methods. The most widely used optical and impedance spectroscopy techniques: absorption, fluorescence, surface plasmon resonance, Raman scattering, and interferometry, as well as new trends in photonics are reviewed. The challenges of materials selection, surfaces tailoring in microfluidic structures, and enhancement of the sensitivity and miniaturization of biosensor systems are discussed. The review provides an overview for current advances and future trends in microfluidics integrated technologies for label-free protein biomarkers detection and discusses existing challenges and a way towards novel solutions.

Ultrasensitive nanoplasmonic biosensor based on interferometric excitation of multipolar plasmonic modes

We propose a nanoplasmonic interferometric biosensor, which exploits the selective excitation of multipolar plasmonic modes in a nanoslit to provide a novel scheme for highly-sensitive biosensing. In this design, two counter-propagating surface plasmon polaritons interfere at the location of the nanoslit, selectively exciting the dipolar and quadrupolar modes of the structure depending on the phase relationship induced by the analyte. The contrasting radiation patterns produced by these modes result in large changes in the angular distribution of the transmitted light that depends on the analyte concentration. The resultant far-field is numerically modeled and the sensing performance of the structure is assessed, resulting in maximum bulk and surface sensitivities of S_B = 1.12 × 10⁵ deg/RIU and S_S = 302 deg/RIU, respectively, and a bulk-sensing resolution of the order of 10^-8 RIU. The design allows ample control over the trade-off between operating range and resolution through the slit's width, making this platform suitable for a broad range of sensing requirements.

3D nanointerface enhanced optical microfiber for real-time detection and sizing of single nanoparticles

Portable devices, which can detect and characterize the individual nanoparticles in real time, are of insignificant interest for early diagnosis, homeland security, semiconductor manufacturing and environmental monitoring. Optical microfibers present a good potential in this field, however, are restricted by the sensitivity limit. This study reports the development of a 3D plasmonic nanointerface, which is made of a Cu-BTC framework supporting Cu_3-xP nanocrystals, enhancing the optical microfiber for real-time detection and sizing of single nanoparticles. The Cu_3-xP nanocrystals are successfully embedded in the 3D Cu-BTC framework. The localized-surface plasmon resonance is tuned to coincide with the evanescent field of the optical microfiber. The 3D Cu-BTC framework, as the scaffold of nanocrystals, confines the local resonance field on the microfiber with three dimensions, at which the binding of target nanoparticles occurs. Based on the evanescent field confinement and surface enhancement by the nanointerface, the optical microfiber sensor overcomes its sensitivity limit, and enables the detection and sizing of the individual nanoparticles. The compact size and low optical power supply of the sensor confirm its suitability as a portable device for the real-time single-nanoparticle characterization, especially for the convenient evaluation of the ultrafine particles in the environment. This work opens up an approach to overcome the sensitivity limit of the optical microfibers, as long with stimulating the portable real-time single-nanoparticle detection and sizing.

State-of-the-Art Optical Microfiber Coupler Sensors for Physical and Biochemical Sensing Applications

An optical fiber coupler is a simple and fundamental component for fiber optic technologies that works by reducing the fiber diameter to hundred nanometers or several micrometers. The microfiber coupler (MFC) has regained interest in optical fiber sensing in recent years. The subwavelength diameter rationales vast refractive index (RI) contrast between microfiber “core” and surrounding “cladding”, a large portion of energy transmits in the form of an evanescent wave over the fiber surface that determines the MFC ultrasensitive to local environmental changes. Consequently, MFC has the potential to develop as a sensor. With the merits of easy fabrication, low cost and compact size, numerous researches have been carried out on different microfiber coupler configurations for various sensing applications, such as refractive index (RI), temperature, humidity, magnetic field, gas, biomolecule, and so on. In this manuscript, the fabrication and operation principle of an MFC are elaborated and recent advances of MFC-based sensors for scientific and technological applications are comprehensively reviewed.

An Optical Microfiber Biosensor for CEACAM5 Detection in Serum: Sensitization by a Nanosphere Interface

The detection of carcinoembryonic antigen (CEA)-related cell adhesion molecules 5 (CEACAM5) is significant in cancer prewarning. Early diagnosis can effectively alleviate the danger of cancer. Point-of-care testing (POCT) has become a competitive technology for early detection. Fiber optic biosensors have great potential as POCT tools. However, their limits of detection (LODs) are not sufficient to afford ultralow concentration detection at the early stage. Herein, this work presents an optical microfiber sensor functionalized by a polystyrene@gold nanosphere (PS@Au nanosphere) interface for a synergistic sensitization effect to detect the ultralow CEACAM5 concentrations in serum at the early stage. The sensor's LOD achieves 3.54 × 10^-17 M in pure solution and 5.27 × 10^-16 M in serum, with the sensitization effect coupled with surface area enlargement and electromagnetic enhancement of interface. This LOD is about 6 orders of magnitude lower than that of current methods. It can be employed to detect the biomarkers at ultralow concentrations present in serum in the early stages of cancer. As the interfacial synergistic sensitization strategy is suitable for refractive index (RI)-based optical transducers, this work provides new opportunities to employ fiber optic biosensors as effective POCT tools.

Micro-/Nanofiber Optics: Merging Photonics and Material Science on Nanoscale for Advanced Sensing Technology

Micro-/nanofibers (MNFs) are optical fibers with diameters close to or below the wavelength of the guided light. These tiny fibers can offer engineerable waveguiding properties including optical confinement, fractional evanescent fields, and surface intensity, which is very attractive to optical sensing on the micro-/nano scale. In this review, we first introduce the basics of MNF optics and MNF optical sensors from physical and chemical to biological applications and review the progress and current status of this field. Then, we review and discuss hybrid MNF structures for advanced optical sensing by merging MNFs with functional structures including chemical indicators, quantum dots, dye molecules, plasmonic nanoparticles, 2-D materials, and optofluidic chips. Thirdly, we introduce the emerging trends in developing MNF-based advanced sensing technology for ultrasensitive, active, and wearable sensors and discuss the future prospects and challenges in this exciting research field. Finally, we end the review with a brief conclusion.

Single-molecule detection of biomarker and localized cellular photothermal therapy using an optical microfiber with nanointerface

For early-stage diagnostics, there is a strong demand for sensors that can rapidly detect biomarkers at ultralow concentration or even at the single-molecule level. Compared with other types of sensors, optical microfibers are more convenient for use as point-of-care devices in early-stage diagnostics. However, the relatively low sensitivity strongly hinders their use. To this end, an optical microfiber is functionalized with a plasmonic nanointerface consisting of black phosphorus-supported Au nanohybrids. The microfiber is able to detect epidermal growth factor receptor (ErbB2) at concentrations ranging from 10 zM to 100 nM, with a detection limit of 6.72 zM, enabling detection at the single-molecule level. The nanointerface-sensitized microfiber is capable of differentiating cancer cells from normal cells and treating cancer cells through cellular photothermal therapy. This work opens up a possible approach for the integration of cellular diagnosis and treatment.

Micro/Nanofibre Optical Sensors: Challenges and Prospects

Micro/nanofibres (MNFs) are optical fibres with diameters close to or below the vacuum wavelength of visible or near-infrared light. Due to its wavelength- or sub-wavelength scale diameter and relatively large index contrast between the core and cladding, an MNF can offer engineerable waveguiding properties including optical confinement, fractional evanescent fields and surface intensity, which is very attractive to optical sensing on the micro and nanometer scale. In particular, the waveguided low-loss tightly confined large fractional evanescent fields, enabled by atomic level surface roughness and extraordinary geometric and material uniformity in a glass MNF, is one of its most prominent merits in realizing optical sensing with high sensitivity and great versatility. Meanwhile, the mesoporous matrix and small diameter of a polymer MNF, make it an excellent host fibre for functional materials for fast-response optical sensing. In this tutorial, we first introduce the basics of MNF optics and MNF optical sensors, and review the progress and current status of this field. Then, we discuss challenges and prospects of MNF sensors to some extent, with several clues for future studies. Finally, we conclude with a brief outlook for MNF optical sensors.

A Review of Multimode Interference in Tapered Optical Fibers and Related Applications

In recent years, tapered optical fibers (TOFs) have attracted increasing interest and developed into a range of devices used in many practical applications ranging from optical communication, sensing to optical manipulation and high-Q resonators. Compared with conventional optical fibers, TOFs possess a range of unique features, such as large evanescent field, strong optical confinement, mechanical flexibility and compactness. In this review, we critically summarize the multimode interference in TOFs and some of its applications with a focus on our research project undertaken at the Optoelectronics Research Centre of the University of Southampton in the United Kingdom.

Nano-materials for use in sensing of salmonella infections: Recent advances

Salmonella infectious diseases spreading every day through food have become a life-threatening problem for millions of people and growing menace to society. Health expert's estimate that the yearly cost of all the food borne diseases is approximately $5-6 billion. Traditional methodologies for salmonella analysis provide high reliability and very low limits of detection. Among them immunoassays and Nucleic acid-based assays provide results within 24h, but they are expensive, tedious and time consuming. So, there is an urgent need for development of rapid, robust and cost-effective alternative technologies for real-time monitoring of salmonella. Several biosensors have been designed and commercialized for detection of this pathogen in food and water. In this overview, we have updated the literature concerning novel biosensing methods such as various optical and electrochemical biosensors and newly developed nano- and micro-scaled and aptamers based biosensors for detection of salmonella pathogen. Furthermore, attention has been focused on the principal concepts, applications, and examples that have been achieved up to diagnose salmonella. In addition, commercial biosensors and foreseeable future trends for onsite detecting salmonella have been summarized.

Aryl Diazonium Chemistry for the Surface Functionalization of Glassy Biosensors

Nanostring resonator and fiber-optics-based biosensors are of interest as they offer high sensitivity, real-time measurements and the ability to integrate with electronics. However, these devices are somewhat impaired by issues related to surface modification. Both nanostring resonators and photonic sensors employ glassy materials, which are incompatible with electrochemistry. A surface chemistry approach providing strong and stable adhesion to glassy surfaces is thus required. In this work, a diazonium salt induced aryl film grafting process is employed to modify a novel SiCN glassy material. Sandwich rabbit IgG binding assays are performed on the diazonium treated SiCN surfaces. Fluorescently labelled anti-rabbit IgG and anti-rabbit IgG conjugated gold nanoparticles were used as markers to demonstrate the absorption of anti-rabbit IgG and therefore verify the successful grafting of the aryl film. The results of the experiments support the effectiveness of diazonium chemistry for the surface functionalization of SiCN surfaces. This method is applicable to other types of glassy materials and potentially can be expanded to various nanomechanical and optical biosensors.