Photonics: Sensing on the way

Schematic illustration of a hollow fiber. The chemiluminescent liquid in the core (yellow) is guided through the fiber, also with help of further hole structures (dark blue). © A*STAR

Processing biological samples on a small substrate the size of a
computer chip is becoming a common task for biotechnology
applications. Given the small working area, however, probing
samples on the substrate with light can be difficult. To address
this issue, Xia Yu and co-workers at the A*STAR Singapore Institute
of Manufacturing Technology have now developed an optical fiber
system that is able to deliver light to microfluidic chips with
high efficiency.

“Our compact optical fibers are designed for use with
high-throughput detection systems,” says Yu. “They are
ideal for use in space-restrictive locations.”

A common way of probing biological samples is by light. In this
method, the sample is excited by an external light source and the
light emitted in response is detected, which provides a unique
fingerprint of the substance. Conventional techniques are able to
deliver light to samples and probe the response, but they are not
very efficient at probing a small sample volume. A solution to this
is to use optical fibers that are able to guide light to small
spaces. The drawback with this technique, however, has been that it
can be difficult to insert the external probe light into the
optical fiber with sufficient efficiencies.

Yu and her co-workers have now circumvented this problem by using
optical fibers with a hollow core (see image). The empty hollow
core can be filled with liquids — in this case, with
chemiluminescent solutions. The liquid is important to promote the
transport of light through the core. In addition, these solutions
consist of two liquids that when brought together initiate a
chemical reaction that emits light. If such a solution is placed
directly within the hollow core the problem of coupling light into
the fiber is circumvented. This not only avoids external light
sources but also promotes an established technology.

“The use of chemical luminescence is a common technique for a
variety of detection assays in biology,” says Yu. “By
incorporating the emission mechanism into optical fibers, we can
use it as a light source for sensing applications in microfluidics
systems.”

First tests for such sensing applications are already underway,
although some challenges remain. For example, there might be losses
in the light emitted by the fluid if the emitted light is not
perfectly confined within the fiber. Such problems can be solved
through improved fiber designs and an appropriate choice of
materials, and applications of these fibers for microfluidic
systems are promising.

The A*STAR-affiliated researchers contributing to this research are
from the Singapore Institute of Manufacturing Technology

Reference:

Yu, X. et al. Chemiluminescence detection in liquid-core
microstructured optical fibers. Sensors and Actuators B: Chemical
160, 800–803 (2011).