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Chapter 1
Surface Plasmons for Biodetection
Pavel Adam, Marek Piliarik, Hana Å ĆpovĆ”, TomĆ”Å” Å pringer, Milan Vala and Ji
Ć Homola
Institute of Photonics and Electronics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
1.1 Introduction
The diffusion of inorganic and biological worlds represents an important paradigm of modern science and technology [1]. Biophotonics stands as an emerging field of research at the crossroads of physical, chemical, and life sciences. The integration of photonics, biology, and nanotechnology is leading to a new generation of devices that makes it possible to characterize chemical and other molecular properties and to discover novel phenomena and biological processes occurring at the molecular level. Biophotonics is widely regarded as the key science on which the next generation of clinical tools and biomedical research instruments will be based.
The last two decades have witnessed an increasing effort devoted to the research and development of optical biosensors and biochips worldwide. Recent scientific and technological advances have demonstrated that such devices hold tremendous potential for applications in areas such as genomics, proteomics, medical diagnostics, environmental monitoring, food analysis, agriculture, and security [2ā4]. Label-free optical biosensors present a unique technology that enables the direct observation of molecular interaction in real-time and allows for the study of molecular systems, which cannot be labeled and studied by fluorescence spectroscopy [2]. Optical label-free biosensors measure binding-induced refractive index changes and are typically based on interferometric transducers, such as the integrated optical MachāZehnder interferometer [5], the integrated Young interferometer [6], and the white light interferometer [7], and transducers based on spectroscopy of guided modes of dielectric waveguides, such as the resonant mirror sensor [8] and the grating coupler sensor [9], or metal-dielectric waveguides, such as the surface plasmon resonance (SPR) sensor.
Since the first demonstration of the SPR method for the study of processes at the surfaces of metals [10] and sensing [11] in the early 1980s, SPR sensors have received a great deal of attention and allowed for great advances both in terms of technology and applications [12]. Thousands of research papers on SPR biosensors have been published and SPR biosensors have been extensively featured in books [1, 2, 4, 13] and reviews [3, 12, 14ā18]. SPR biosensors have become a crucial tool for characterizing and quantifying biomolecular interactions. SPR biosensors have also been increasingly developed for the detection of chemical and biological species and numerous SPR biosensors for the detection of analytes related to medical diagnostics, environmental monitoring, food safety, and security have been reported as well.
This chapter describes the principles of SPR biosensors and discusses the advances that SPR biosensors have made both in terms of technology and applications over the last decade. The first part (Section 1.2) describes the fundamentals of SPR biosensors. Sections 1.3 and 1.4 are concerned with the optical configurations and immobilization methods used in current SPR sensors. The last part (Section 1.5) presents examples of applications of SPR biosensors for the detection of chemical and biological species with an emphasis on food safety and security applications.
1.2 Principles of SPR Biosensors
1.2.1 Surface Plasmons
Surface plasmons (SPs) are electromagnetic modes guided by metallic waveguides. The simplest geometry supporting SPs comprises a planar boundary between a semi-infinite metal and a semi-infinite dielectric. The optical properties of the metal are characterized by a complex permittivity
, where
and
are the real and imaginary parts of Īµ
m) and the dielectric is characterized by the refractive index
nd. Analysis of Maxwell's equations with appropriate boundary conditions suggests that this structure can only support a single guided mode of electromagnetic fieldāan SP [19]. The vector of intensity of the magnetic field of SP lies in the plane of the metalādielectric interface and is perpendicular to the direction of propagation. Such a mode of the electromagnetic field is referred to as the transversally magnetic (TM) mode. A typical profile of the magnetic field of an SP is shown in
Figure 1.1(a). The intensity of the magnetic field reaches its maximum at the metalādielectric interface and decays into both the metal and the dielectric. The field decay in the direction perpendicular to the metalādielectric interface is characterized by the penetration depth. The penetration depth depends on the wavelength and permittivities of the materials involved. The penetration depth into the dielectric for an SP propagating along the interface of gold and a dielectric with
nd = 1.32 increases with a wavelength and ranges from 100 to 600 nm in the wavelength region 600ā1000 nm [19].
Propagation constant of SP Ī²SP at the metalādielectric interface can be expressed as
where
c is the speed of light in a vacuum, Ļ is the angular frequency, and
nef is the effective index of the SP [20, 21]. If the structure is lossless
,
Equation 1.1 represents a guided mode only if the metal permittivity
is negative and
. Metals such as gold, silver, and aluminum exhibit a negative real part of permittivity in the visible and near-infrared region of the spectrum.
Figure 1.1b depicts the wavelength dependence of the effective index of SP
nef for the gold waveguide. The imaginary part of the propagation constant is associated with the imaginary part of the metal permittivity
and determines attenuation of the SP in the direction of propagation [20].
A special example of the metallic waveguide is a symmetric dielectricāmetalā dielectric planar structure. When the metal film thickness is much larger than the SP penetration depth into the metal, an independent SP may propagate at each metalādielectric boundary. If the thickness of the metal film is decreased, coupling between the SPs at opposite sides of the metal film can occur, giving rise to mixed modes of electromagnetic fieldāsymmetric and antisymmetric SPs [22, 23]. The profiles of magnetic intensity of symmetric and antisymmetric SPs are symmetric or antisymmetric with respect to the plane of symmetry of the structure. The field of the symmetric SP penetrates much deeper into the dielectric medium than the field of the antisymmetric SP or the field of a conventional SP at a single metalādielectric interface. Moreover, the symmetric SP exhibits a lower attenuation than its antisymmetric counterpart and therefore it is referred to as a long-range surface plasmon (LRSP) while the antisymmetric mode is referred to as a short-range surface plasmon [22].
1.2.2 Excitation of Surface Plasmons
1.2.2.1 Prism Coupling
The most common approach to the excitation of SPs is by means of a prism coupler and the attenuated total reflection method (ATR). In the Kretschmann geometry of the ATR method [24], a high refractive index prism with refractive index np is interfaced with a metalādielectric waveguide consisting of a metal film with permittivity Īµ m and a semi-infinite dielectric with a refractive index nd (nd < np), Figure 1.2.
When a light wave propagating in the prism totally reflects on the prism base, an evanescent electromagnetic wave decays exponentially in the direction perpendicular to the prismāmetal interface [25]. If the metal film is sufficiently thin (less than 100 nm for light in the visible and near-infrared part of spectrum), the evanescent wave penetrates through the metal film and couples with an SP at the outer boundary of the metal film. In terms of the effective index, this coupling condition can be written as follows:
where
nef is the effective index of the SP, and the perturbation in effective index
, and the respective propagation constant of SP
describe the effect of the presence of the prism.
Figure 1.3 shows the angular and wavelength spectra calculated using a rigorous Fresnel model of light reflection on a multilayer structure calculated at two different wavelengths and for two angles of incidence, respectively. The reflectivity spectra exhibit distinct dips in TM polarization, which are associated with the transfer of energy from the incident light wave into an SP and its subsequent dissipation in the metal film.
The reflectivity spectra can be rigorously calculated using Maxwell equations and the boundary condition of the planar multilayer structure. Assuming that the permittivity of metal Īµ
m obeys
and
, a Lorentzian (with respect to
nef) approximation of the reflectivity can be used as follows [20]:
where Ī³i = Im{Ī²SP}Ī»/2Ļ and Ī³rad = Im{Ī²(1)}Ī»/2Ļ denote the attenuation coefficients of SPs owing to absorption and radiation, respectively. As follows from Equation 1.3, the minimum of the dip in the reflectivity spectrum occurs when the coupling condition (Eq. 1.2) is matched and the shape of the reflectivity dip depends strongly on the strength of the coupling between the excitation wave and SP represented by Ī³rad. This approximation has been shown to provide a good estimate of the position of the reflectivity dip and to predict the shape of the reflectivity curve in the neighborhood of the minimum [19]. In addition, the Lorentzian curve exhibits the same width as the dips calculated using the rigorous approach [26].
1.2.2.2 Grating Coupling
Another approach to optical excitation of SPs is based on the diffraction of light on a diffraction grating. In this method, a light wave is incident at an angle of incidence Īø from a diele...
