IEEE Spectrum April, 2014 - 51

DiagnOsis
by Light
4

Gold
coating

A lab on fiber uses
near-infrared frequencies to detect
precise concentrations of chemicals
or biological molecules in a solution.
Here's how it works.
1

A laser transmits light
into the fiber's core.

A tilted grAting reflects
certain wavelengths known
as resonances out of the core.
these resonances depend on the
properties of the grating and the
fiber's surface.
2

Aptamers
(binding
agents)

When tArget molecules
bind to receptors on the surface
of the fiber, they shift the wavelength
and intensity of the resonances.
3

A gold mirror on the tip
of the fiber reflects the light
remaining in the core back to a
spectrometer at the opposite end.
4

Before
After

the spectrometer detects changes
in the returning light, revealing the
concentration of target molecules.

Cladding
1

with a standard telecom fiber, which costs
next to nothing.
This hair-thin glass fiber consists of an
inner core and outer cladding encased
in a protective polymer jacket. While the
cladding is made of pure silica, the core is
laced with tiny amounts of germanium oxide,
which raises its refractive index, a measure
of the speed of light in a material. If light
traveling through the core strikes the cladding within a certain critical angle, the slight
difference in refractive indices between the
two layers causes the beam to bend back
into the core-a property called total internal reflection. In this way, light can pingpong down a fiber's core for up to several
kilometers. The ability to carry light over
long distances with very little loss could
allow fiber labs to be inserted into water
supply networks, lowered into the ocean,
or strung throughout hospitals.
To transform our inert fiber into a chemical sensor, we must first choose where to
put the detection site. Some groups are investigating designs that place this probe
on a fiber's tip or inside tiny air tunnels in
the cladding of an experimental type of
fiber called a microstructured optical fiber
[see sidebar, "Where to Put the Probe?"].
We have found, though, that the simplest-
and therefore cheapest-approach is to use
a short segment of a fiber's outer surface,
around 1 to 10 millimeters long.
We coat this probe with a chemical compound, called a reagent, that will interact
with whatever target molecules we want to
measure, such as blood enzymes or food
additives. There are many different compounds that can serve as reagents and many
ways to attach them to a fiber. One of the
most promising processes that we are developing takes two steps. First we cover the surface of the probe with a thin layer of metal
using standard deposition techniques such
as sputtering, evaporation, or electroless
plating. Then we dip the probe into a salt
bath filled with synthetic molecules called
aptamers, which bind to the surface coating, forming the reagent.
Chemical supply companies can make a
huge variety of aptamers that each bind to
a different target molecule, including vari|

nORTh aMERICan

aPR 2014

Table of Contents for the Digital Edition of IEEE Spectrum April, 2014