Figure 1: A schematic illustration
describing the encapsulation of 5-FU into the UiO-66 MOF thin film optical
fiber and its release upon switching on the light at the other end of the
optical fiber
Trace
contaminants in water are often measured by taking samples from the environment
to a lab for analysis, which can lead to inaccurate results due to delayed and
irregular sample collection or long-transportation and handling times. Thus,
techniques enabling in-situ or real-time measurements of water contaminants are
no doubt one of the major steps towards effective control of water quality.
Optical
fiber chemical sensors based on optical absorption feature high specificity,
fast response, and a much longer lifetime compared to other chemical sensors,
qualities that offer significant potential for application in pollution
monitoring, environmental protection, and hazardous-material detection. Now by
integrating metal organic framework (MOF) materials—a new class of highly porous crystalline
material—with optical fibers, researchers from Victoria University and Monash
University, Australia, have co-developed a novel, highly sensitive chemical sensor based on
an optical fiber coated
with a thin film of a specific MOF (namely, UiO-66), which could be potentially
used for real-time detection of heavy organic contaminants such as herbicides
or pesticides in water. In a paper published this week in the journal Optics
Letters, from The Optical Society (OSA), the researchers described their
work.
"Metal
organic frameworks (MOFs) are networks of metal atoms linked and separated by
carbon-based (organic) compounds. The UiO-66 MOF we used in the experiment is
made from Zirconium and is well known for the stability in water," said
Stephen Collins, professor of engineering, Victoria University, Australia.
"We have demonstrated for the first time that the advanced porous material
MOFs can be coated onto the end-face of optical fibers to create a novel,
faster and more sensitive chemical sensor potentially used for measuring heavy
organic contaminants on site and in real-time."
Collins
said various porous adsorbents such as pyrene-labeled monomer, silica sol-gel
and zeolites have been studied recently by scientists for detecting hazardous
compounds. However, the low porosity and small pores of the above adsorbents
limit their use in the sensing area to small molecules. That is, they cannot
detect larger or heavy organic molecules (e.g. herbicides or pesticides) in
water.
Metal
organic frameworks are about 10 times more porous than any material previously
known, so they can absorb larger molecules. MOFs form as crystals and careful
selection of MOF constituents can yield crystals of ultrahigh porosity and high
thermal and chemical stability.
To
fabricate the MOF-fiber sensor, the researchers removed the polymer coating of
a conventional single mode fiber several centimeters from the end and activated
the fiber surface using plasma. Then, the fiber was placed in MOF liquid
solution and heated at 120 degrees Celsius for 24 hours, which allowed the
activated fiber surface to attract the MOF to grow on the end-face of the fiber,
resulting in a MOF thin film of 17- to 22-micrometer thickness.
Collins
explained that the MOF-fiber sensor can be used as an in-fiber Fabry-Perot
interferometer, which is a well-established method for detecting the
"optical thickness" of a thin film by studying the interference
signals generated by the film interfaces. As the MOF-fiber sensor absorbs more
and more contaminants, the optical thickness of the MOF thin film increases
accordingly, leading to a change in the interference spectra. By using the
established optical model and mathematical procedure, the researchers can
calculate the optical thickness of the MOF thin film from the experimentally
measured interference spectra, and hence infer the concentration of
contaminants in water.
In
the experiment, Collin's team used the MOF-fiber sensor to detect a specific
contaminant in water called Rhodamine-B (RhB) dye, a bright pink dye known as
Opera Rose, which is used in the textile industry and is known to be
potentially carcinogenic if ingested.
"Our
experimental results showed a positive detection response of the MOF-fiber
sensor to RhB in water down to 48 parts per million or 0.1 millimolar, which is
a very promising result, demonstrating the sensor's ability to detect
pollutants at a low concentration before the pollution goes worse," said
Collins.
He
explained the high sensitivity and fast response of the MOF-fiber sensor are
attributed to the MOF's ability to pre-concentrate molecules, which can be
imaged as a sponge "soaking" up molecules into its pores.
Additionally, the MOF sponge selectively absorbs molecules to fit into its
pores and rejects unfit ones, which enhance the sensor's sensitivity and
reliability.
The
researchers also found the sensor's absorption process of RhB dye is non-reversible,
which is ideal for long-term monitoring where RhB concentrations are minimal
and a marked increase in the dye's concentration would be recognized easily,
said Collins.
"While
the non-reversible mode suits many applications, we have also developed methods
of releasing absorbed molecules by shining light down the fiber, which would
make the sensor re-usable," Collin said.
The
researchers' next step is to further explore the MOF-fiber sensor's responses
to other heavy organic contaminants such as pesticides and herbicides in water.
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