Biophotonics_Concepts_to_Applications

is incident on the metal surface at a specific angle (called theplasmon resonance
angle) is coupled to the surface plasmon modes in the metalfilm and is totally
internally reflected toward a photodetector. The resonance angle is very sensitive to
small changes in the refractive index at the interface between the metal and the
receptor layer. For example, these changes occur when the chemically activated
surface captures biological samples. These events will modify the surface plasmon
mode thereby causing a shift in the resonance peak of the reflected light. This shift
can be viewed by a photodetector, for example, a charge-coupled device (CCD).
This shifting process enables the construction of a biosensor that correlates the
shifts in the resonance angle to quantitative molecular binding data at the sensor
surface.

7.9 Optical Fiber Nanoprobes

The recent joining of SPR, nanotechnology, and opticalfiber technology has led to
thelab-on-fiberconcept [ 72 – 78 ]. In this currently evolving technology, major
efforts include attaching patterned layers of nanoparticles either on the tip of afiber
or as a nanocoating over a Bragg grating written inside a microstructured optical
fiber [ 3 ]. When a patterned nanostructure is deposited on the tip of a standard
opticalfiber,localized surface plasmon resonances(LSPRs) can be excited by an
illuminating optical wave, because of a phase matching condition between the
scattered waves and the modes supported by the nanostructure. Figure7.28shows
an example in blue of the surface plasmon resonance wave associated with a
nanostructure pattern for the condition when no analyte is covering the pattern on
the end of afiber. These LSPRs are very sensitive to the surrounding refractive
index (SRI) at thefiber tip. Thus, inserting thefiber tip into afluid will cause the
analyte liquid to cover the nanoparticle pattern. This action changes the refractive
index (RI) of the nanolayer-fluid interface, thereby causing a wavelength shift in the
LSPR peak due to a change in the phase matching condition, as shown by the red

Reflectance

Wavelength (nm)

0.8

0.6

0.4

0.2

1350 1400 1450

Reflectance

without an

analyte

Reflectance

with an

analyte

Fig. 7.28 Example of the
shift in the surface plasmon
resonant peak when there is a
relative index change at the
tip of afiber covered with a
nanoarray pattern (J. Biomed.
Opt. 19(8), 080902 (Aug 28,
2014). doi:10.1117/1.JBO.19.
8.080902)

226 7 Optical Probes and Biosensors