2 Surface Plasmons for Chiral Sensing
29
Considering, therefore, the importance of chiral sensing in research, it is vital
to develop alternative schemes that overcome the above-mentioned limitations of
path-length enhancement and/or chiral-field enhancement techniques. In particular,
it is apparent that a general chiral sensing scheme should be able to discriminate
between the contributions of Re(κ) and Im(κ), especially far from the chiral molecular
resonances, where Im(κ) is weak and Re(κ) is dominant. Additionally, it should be
sensitive to both the magnitude of the chirality parameter, |κ|, and its sign sgn(κ), as
|κ| is a function of the molecular properties (i.e. polarizability) and its concentration,
while sgn(κ) depends on the handedness of the medium.
In this chapter we discuss how surface plasmon resonance (SPR) allows for the
complete measurement of chirality (handedness and magnitude) of a chiral system.
SPR has become an important technology in the areas of biochemistry, biology, and
medical sciences because of its real-time, label-free, and noninvasive nature (see,
e.g., [38, 39]). We demonstrate how chiral-sensitive SPR, i.e. CHISPR, is able to
quantitatively detect both the real and imaginary part of the refractive index of a chiral
substance (responsible for the refraction and absorption, respectively), contrary to
most demonstrations that employ nanophotonic structures. We show that CHISPR
is particularly suitable for chiral sensing from thin samples which are not easily
measurable using alternative polarimetric techniques, and that it makes use of the
advantage of being applicable directly on existing SPR instrumentation without the
need for additional elaborate fabrication.
2.2 Surface Plasmon Resonance (SPR)
Surface plasmon resonance (SPR) refers to the resonant excitation of a surface plasmon polariton (SPP) at the interface between a metal and a dielectric (or, in general,
between two materials, one with negative and one with positive permittivity). SPPs,
in particular, are electromagnetic excitations that propagate along the metal-dielectric
interface and are evanescently confined in the perpendicular direction. They involve
the collective oscillation of conduction electrons at the surface of the metal (hence the
term ‘surface plasmon’) and they arise via the coupling of the electromagnetic fields
to the surface plasmon (hence the term ‘polariton’). SPPs are TM (or ‘p’)-polarized
waves, i.e., their magnetic field lies entirely on the metal-dielectric interface.
SPR is the basis for many biosensor applications and different lab-on-a-chip sensors, owing to the sensitive dependence of the SPP characteristics on the permittivity
of the dielectric region extending over the metal. In essence, slight changes in the
permittivity of the dielectric lead to different propagation characteristics in the SPP
and therefore a frequency shift in its resonance. By measuring such frequency shifts,
one can detect the material changes occurring at the metal-dielectric interface.
Typical SPR setups involve a thin metallic layer (usually Ag or Au in the order
of 50 nm) placed directly on a glass substrate, as shown in Fig. 2.1a in the wellknown Kretschmann configuration (see [40, 41] for other customary setups). In this
configuration the metallic layer extends along the x y-plane and SPP propagates along
29
Considering, therefore, the importance of chiral sensing in research, it is vital
to develop alternative schemes that overcome the above-mentioned limitations of
path-length enhancement and/or chiral-field enhancement techniques. In particular,
it is apparent that a general chiral sensing scheme should be able to discriminate
between the contributions of Re(κ) and Im(κ), especially far from the chiral molecular
resonances, where Im(κ) is weak and Re(κ) is dominant. Additionally, it should be
sensitive to both the magnitude of the chirality parameter, |κ|, and its sign sgn(κ), as
|κ| is a function of the molecular properties (i.e. polarizability) and its concentration,
while sgn(κ) depends on the handedness of the medium.
In this chapter we discuss how surface plasmon resonance (SPR) allows for the
complete measurement of chirality (handedness and magnitude) of a chiral system.
SPR has become an important technology in the areas of biochemistry, biology, and
medical sciences because of its real-time, label-free, and noninvasive nature (see,
e.g., [38, 39]). We demonstrate how chiral-sensitive SPR, i.e. CHISPR, is able to
quantitatively detect both the real and imaginary part of the refractive index of a chiral
substance (responsible for the refraction and absorption, respectively), contrary to
most demonstrations that employ nanophotonic structures. We show that CHISPR
is particularly suitable for chiral sensing from thin samples which are not easily
measurable using alternative polarimetric techniques, and that it makes use of the
advantage of being applicable directly on existing SPR instrumentation without the
need for additional elaborate fabrication.
2.2 Surface Plasmon Resonance (SPR)
Surface plasmon resonance (SPR) refers to the resonant excitation of a surface plasmon polariton (SPP) at the interface between a metal and a dielectric (or, in general,
between two materials, one with negative and one with positive permittivity). SPPs,
in particular, are electromagnetic excitations that propagate along the metal-dielectric
interface and are evanescently confined in the perpendicular direction. They involve
the collective oscillation of conduction electrons at the surface of the metal (hence the
term ‘surface plasmon’) and they arise via the coupling of the electromagnetic fields
to the surface plasmon (hence the term ‘polariton’). SPPs are TM (or ‘p’)-polarized
waves, i.e., their magnetic field lies entirely on the metal-dielectric interface.
SPR is the basis for many biosensor applications and different lab-on-a-chip sensors, owing to the sensitive dependence of the SPP characteristics on the permittivity
of the dielectric region extending over the metal. In essence, slight changes in the
permittivity of the dielectric lead to different propagation characteristics in the SPP
and therefore a frequency shift in its resonance. By measuring such frequency shifts,
one can detect the material changes occurring at the metal-dielectric interface.
Typical SPR setups involve a thin metallic layer (usually Ag or Au in the order
of 50 nm) placed directly on a glass substrate, as shown in Fig. 2.1a in the wellknown Kretschmann configuration (see [40, 41] for other customary setups). In this
configuration the metallic layer extends along the x y-plane and SPP propagates along