Optical Guided-wave Chemical and Biosensors I

Sensing platforms based on the principle of total internal reflection (TIR) represent a fairly mature yet still expanding and exciting field of research. Sensor development has mainly been driven by the need for rapid, stand-alone, automated devices for application in the fields of clinical diagnosis and screening, food and water safety, environmental monitoring, and chemical and biological warfare agent detection. The technologies highlighted in this chapter are continually evolving, taking advantage of emerging advances in microfabrication, lab-on-a-chip, excitation, and detection techniques. This chapter describes many of the underlying principles of TIR-based sensing platforms and additionally focusses on planar TIR fluorescence (TIRF)-based chemical and biological sensors.

The field of chemical and biosensors based on waveguide technology is rapidly growing, with new developments focusing on higher sensitivity and stability. This key demand is prompting researchers and developers to explore new materials for waveguide sensor systems, with especially high-refractive-index materials as promising components. This chapter gives an overview of different sensor platforms implementing high-refractive-index waveguide materials, with applications in both research and commercial sensor systems. This is accompanied by a theoretical background of waveguide-sensing principles, especially focusing on the key steps to high sensor sensitivities.

Interferometry is an optical technique that compares the differences experienced by two light beams traveling along similar paths. Planar waveguides have evanescent fields sensitive to changes in the index of refraction in the volume immediately above the waveguide surface. Placing a chemically sensitive film within this region provides the basis for chemical sensing. Film–analyte interactions change the index of refraction, causing the propagating light speed or phase to change in a direction of opposite sign to that of the index change. To measure this change, a reference propagating beam, which is adjacent to the sensing beam, is combined optically with the sensing beam, thus creating an interference pattern of alternating dark and light fringes. When chemical or physical changes occur in the sensing arm, the interference pattern shifts, producing a sinusoidal output. Waveguides and interferometers come in a variety of designs, but all rely on the evanescent field interacting with a chemically selective film to produce a measured response. The sensing mechanism can be passive (a physical change) or active (reactive sites in the film). Through a judicious choice of sensing films, interferometers can be designed to detect a wide variety of chemical and biological materials. Multi-interferometer devices with several different sensing films can be used to detect and identify a variety of different chemical or biological analytes either through specific sensing chemistry or through analysis of patterned response from an array of different films.

Electron transfer processes to/from monolayers or submonolayers of surface-confined molecules are at the core of several established or emerging sensor technologies. Spectroelectrochemical techniques to monitor these redox processes combine spectroscopic information with the normally monitored electrochemical parameters, such as changes in current or voltage, and can be much more sensitive to changes in optical properties coupled with electron transfer than electrochemical techniques alone. Spectroelectrochemical techniques based on absorbance measurements typically suffer from low sensitivity owing to the low concentrations of redox active species on the surface, and their low absorptivities. Electro-active, single-mode waveguide technologies, developed over the last decade, have provided more than adequate sensitivity to characterize electron transfer to surface-confined molecules where the coverage can be as low as a few percent of a monolayer. In this chapter, we review the major developments in combining electrochemical analysis with optical platforms that maximize optical sensitivity, through the development of electro-active integrated planar waveguides operating in the single-mode optical regime. We provide here a general overview of the theoretical formalisms associated with light propagation and absorbance measurements in integrated optical waveguides, and their electro-active counterparts. We also describe the major implementations of the technology, including the extension of the single-mode configuration into a broadband spectroscopic tool to facilitate the interrogation of the entire visible wavelength region during the redox event, and review some specific applications of these techniques, which demonstrate its sensitivity and broad utility.

Surface plasmon resonance (SPR) is a surface optical technique that measures minute changes in refractive index at a metal-coated surface. It has become increasingly popular in the study of biological and chemical analytes because of its label-free measurement feature. In addition, SPR allows for both quantitative and qualitative assessment of binding interactions in real time, making it ideally suited for probing weak interactions that are often difficult to study with other methods. This chapter presents the biosensor development in the last 3 years or so utilizing SPR as the principal analytical technique, along with a concise background of the technique itself. While SPR has demonstrated many advantages, it is a nonselective method and so, building reproducible and functional interfaces is vital to sensing applications. This chapter, therefore, focuses mainly on unique surface chemistries and assay approaches to examine biological interactions with SPR. In addition, SPR imaging for high-throughput screening based on microarrays and novel hyphenated techniques involving the coupling of SPR to other analytical methods is discussed. The chapter concludes with a commentary on the current state of SPR biosensing technology and the general direction of future biosensor research.

Ordered arrays of subwavelength holes in optically thick metal films exhibit optical properties that may be exploited to achieve chemical and biological sensing. The fundamental phenomena governing these interactions, the sensing methodologies they enable, and the on-chip integration of nanohole array sensors are described in this chapter. The fundamental phenomena of confinement, or guiding of electromagnetic waves at a metal surface that are central to the sensing capabilities offered by nanohole arrays in metal films are described first. The fundamental basis for surface plasmon resonance on smooth planar metal-dielectric interfaces as well as the extension and localization of these phenomena to nanostructures is described. Nanohole-array-based sensing methodologies are discussed next. The extraordinary optical transmission through nanohole arrays is described with the application of that phenomenon to surface plasmon resonance-based sensing. Field localization, related to the surface plasmon excitation, enables surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence spectroscopy (SEFS). The application of nanohole arrays in these sensing methodologies are described, as are recent efforts to further localize the electromagnetic field via overlapped double-hole structures. A selection of recently presented experimental results are highlighted throughout the chapter to demonstrate the relevant phenomena and sensing capabilities. In addition to the variety of sensing opportunities offered, both the small footprint of nanohole arrays and the simplified transmission mode operation at normal incidence are highly advantageous with respect to device-level miniaturization. Finally, the micro- and nanofluidic integration of nanohole-array-based sensors is discussed. Integration efforts to date, as well as future prospects for nanohole arrays in a lab-on-chip format and potential to exploit transport phenomena in these structures to the benefit of chemical and biological sensing applications, are described.

This chapter reviews the characteristics of localized surface plasmon resonance (LSPR), the excitation of which is mediated by nanostructures, and its applications to biosensing. The LSPR is explored in three regimes in terms of creation and coupling of localized surface plasmons (LSPs): LSPs created in surface-relief patterns coupled to propagating surface plasmons (SPs), LSPs in surface-relief patterns coupled to particle plasmons, and LSPs created in particles. The results, in general, suggest that localized field enhancement in the near-field be correlated with enhanced detection sensitivity for LSPR over conventional thin film-based SP resonance while LSPR-based biosensors can potentially maintain flexibility by using nanoparticles.

First, a short overview of the sensor activities on surface plasmon waveguide mode coupling via wave vector match in metal-coated channel waveguides as well as in slab waveguides and optical fibers is given. Both monomode and multimode approaches were demonstrated as well as the implementation of Bragg gratings and hollow fibers are described. Then, the use of gold nanoparticles for sensor application in combination with these optical device systems is discussed. At the beginning, a channel waveguide approach with gold nanoparticles is described without taking the typical optical features of gold nanoparticles into account. Then, waveguide devices, which use the localized surface plasmon resonance, an absorption band, intrinsic to gold nanoparticles, and the color changes of gold colloids upon clustering for sensor operation are described. This is achieved on quasi-waveguides, channel waveguides, and on optical fibers. Transmission and reflectance experiments have been performed, either with spectral information or with monochromatic light and pure intensity information. An electro-optical approach is discussed. The activities in photonic crystal sensor are described for planar-waveguide systems and hollow photonic crystal fiber bundles.