plasmons are generated when polarized light of a particular wavelength is
shone on it at the SPR angle. This gold sensor surface serves as the bottom
of a flow cell through which solutions of various analytes can be passed. If
the analytes interact with the sensor surface (or with molecules that have
been pre-adsorbed to the surface), then a change in the SPR angle is
observed by the detector and recorded by the computer software. The
flow cell itself is typically very small (20–60 nL) and common flow rates
are on the order of 1–100 µL/min.
While SPR sensors can be employed for a variety of purposes that involve
the characterization of ultrathin films, they are primarily used as
biosensors—to detect interactions between proteins and substrates,
between strands of DNA, and between drug molecules and protein
targets. SPR can also be used to monitor the formation of self-assembled
monolayers on gold surfaces, particularly the formation of alkanethiol
monolayers.
The SPR effect is not limited to planar surfaces, but can also be observed
using gold or silver nanoparticles in solution. In this case, the nanoparticles have a wavelength-dependent absorbance that is a result of the
excitation of surface plasmons within the nanoparticles, and this wavelength is measured rather than an angle-dependent response as with
planar SPR (which would be difficult to measure for a nanoparticle). The
wavelength at which the maximum absorbance is observed is a function
of the particle size and particle shape. For example, spherical gold
nanoparticles with a diameter of ∼13 nm exhibit a maximum absorbance
at 520 nm.
The wavelength at which the SPR effect is maximized also depends on the
dielectric environment (i.e., the refractive index) near the nanoparticle
surface and on the distance between neighboring nanoparticles. These
two properties have allowed gold nanoparticles to be used as sensitive
biosensors. For example, short pieces of DNA called oligonucleotides
have been covalently attached to gold nanoparticles through thiol chemistry, and when complementary DNA is introduced into the solution, the
oligonucleotide-coated nanoparticles aggregate as they hybridize to the
strands in solution. This aggregation causes the distance between neighboring nanoparticles to decrease and a blueshift in the wavelength of
maximum absorbance is observed. In this manner, gold nanoparticles
can be used to determine the complementarity of two DNA sequences.
Similar methods have also been developed to examine the changes in
refractive index that occur on the surface of a gold nanoparticle when
OTHER TECHNIQUES FOR MEASURING THICKNESS AND REFRACTIVE INDEX 283
shone on it at the SPR angle. This gold sensor surface serves as the bottom
of a flow cell through which solutions of various analytes can be passed. If
the analytes interact with the sensor surface (or with molecules that have
been pre-adsorbed to the surface), then a change in the SPR angle is
observed by the detector and recorded by the computer software. The
flow cell itself is typically very small (20–60 nL) and common flow rates
are on the order of 1–100 µL/min.
While SPR sensors can be employed for a variety of purposes that involve
the characterization of ultrathin films, they are primarily used as
biosensors—to detect interactions between proteins and substrates,
between strands of DNA, and between drug molecules and protein
targets. SPR can also be used to monitor the formation of self-assembled
monolayers on gold surfaces, particularly the formation of alkanethiol
monolayers.
The SPR effect is not limited to planar surfaces, but can also be observed
using gold or silver nanoparticles in solution. In this case, the nanoparticles have a wavelength-dependent absorbance that is a result of the
excitation of surface plasmons within the nanoparticles, and this wavelength is measured rather than an angle-dependent response as with
planar SPR (which would be difficult to measure for a nanoparticle). The
wavelength at which the maximum absorbance is observed is a function
of the particle size and particle shape. For example, spherical gold
nanoparticles with a diameter of ∼13 nm exhibit a maximum absorbance
at 520 nm.
The wavelength at which the SPR effect is maximized also depends on the
dielectric environment (i.e., the refractive index) near the nanoparticle
surface and on the distance between neighboring nanoparticles. These
two properties have allowed gold nanoparticles to be used as sensitive
biosensors. For example, short pieces of DNA called oligonucleotides
have been covalently attached to gold nanoparticles through thiol chemistry, and when complementary DNA is introduced into the solution, the
oligonucleotide-coated nanoparticles aggregate as they hybridize to the
strands in solution. This aggregation causes the distance between neighboring nanoparticles to decrease and a blueshift in the wavelength of
maximum absorbance is observed. In this manner, gold nanoparticles
can be used to determine the complementarity of two DNA sequences.
Similar methods have also been developed to examine the changes in
refractive index that occur on the surface of a gold nanoparticle when
OTHER TECHNIQUES FOR MEASURING THICKNESS AND REFRACTIVE INDEX 283