Finding Photonic Ways to Monitor Blood Sugar
Spectroscopic and fluorescence techniques might someday take the sting out of self-checking by
with diabetes mellitus do not naturally produce enough insulin to process the sugars
that they ingest into useful energy for their cells. Instead, they must regulate
their sugar intake, increase their level of exercise and — if they are among
the 5 to 10 percent of diabetics who have Type 1 of the disease — inject insulin
every day. Furthermore, they should regularly monitor their blood sugar —
the amount of glucose in their blood serum.
Unfortunately, current methods for home-based
glucose monitoring require using sharp lancets to draw blood — typically from
a fingertip. Testing blood sugar in this manner is often uncomfortable (even painful)
and inconvenient, which discourages diabetics from regular monitoring. To address
this disadvantage, investigators are looking into new methods that can continuously
measure blood glucose levels with minimum invasiveness.
A group of chemists and biomedical
engineers at Northwestern University in Evanston, Ill., is using surface-enhanced
Raman spectroscopy (SERS) to develop a method for glucose monitoring. In this technique,
proximity of a tested material to a metallic surface (usually silver or gold) enhances
the Raman scattering of laser light directed onto the material, thus improving the
However, according to team members
Olga Lyandres and Nilam Shah, the researchers had to overcome the fact that glucose
does not bind to a bare metal surface. To do so, they used a self-assembled monolayer
composed of decanethiol and mercaptohexanol. The monolayers, they said, form well-ordered
layers with uniform thickness and surface properties that can be tailored to particular
The investigators chose decanethiol
and mercaptohexanol because of their hydrophobic and hydrophilic natures, respectively,
which help to pull glucose out of an aqueous solution and localize it within a few
nanometers of the enhancing electromagnetic field generated near the metallic surface.
The monolayer also aids the sensitivity of the technique by helping exclude nonglucose
The researchers report in the Oct.
15 issue of Analytical Chemistry that they employed the Raman technique to
monitor the blood glucose levels in live rats. They created SERS-active surfaces
by depositing a 200-nm-thick silver film over an 18-mm-diameter copper disk that
was coated with a solution containing 390-nm-diameter spheres. They then incubated
the coated plate in decanethiol and mercaptohexanol, producing a self-assembled
monolayer on each surface.
Researchers at Northwestern
University have developed a glucose sensor based on surface-enhanced Raman spectroscopy
using silver nanoparticles to boost the Raman signal. They tested the technique
by surgically embedding a sensor into a rat and measuring the Raman spectra through
an implanted optical window. Reproduced with permission of Analytical Chemistry.
© 2006 American Chemical Society.
They implanted the plate into a rat
by removing the skin and placing the plate beneath a transparent window through
which they could measure the Raman spectra. To vary the glucose concentration in
the rat, they intermittently injected the animal with a glucose solution over several
hours. They acquired SERS spectra using a 785-nm Ti:sapphire laser from Spectra-Physics
of Mountain View, Calif., at 50 mW. For comparison with current home-monitoring
methods, they also used a consumer glucometer to measure blood glucose levels. According
to Lyandres and Shah, they chose the wavelength because it is compatible with in
vivo studies, and it reduces the fluorescence of other biological substances that
could create an interfering signal.
The graph shows the time course of in vivo glucose measurement. Glucose
infusion began at t = 60 min (arrow). Triangles represent measurements made using
a consumer-brand blood glucometer; squares, measurements made with the SERS-based
sensor. The inset shows a typical in vivo spectrum compared with a typical ex vivo
spectrum of the same surface. Reproduced with permission of Analytical Chemistry.
© 2006 American Chemical Society.
They found that both the glucometer
and the SERS method effectively tracked changes in glucose concentration. Moreover,
they noted that changes in concentration as fast as ∋30 s were detectable
with the Raman technique because the binding of glucose to the monolayer was rapidly
The investigators are looking to refine the accuracy
of their approach as well as to explore other substrates, such as inorganic capture
layers and hydrogels, to aid glucose trapping and localization.
Comprising primarily water, hydrogels
increasingly are becoming important to biological studies because they are nontoxic,
resist degradation and readily immobilize molecules that can act as biosensors.
Now researchers at the University of California, Santa Cruz, are using hydrogels
as a thin-film substrate to carry a fluorescent dye along with quencher molecules
that change the dye’s fluorescence when in the presence of glucose.
The investigators, led by Bakthan Singaram,
report in the Oct. 10 issue of Langmuir that they used a hydrogel that incorporated
poly(2-hydroxyethyl methacrylate) as their substrate material. Team member Soya
Gamsey, who is now at GluMetrics Inc. in Irvine, Calif., said that they used a hydrogel
because it constrains the dye and quencher molecules so that they do not leach into
the glucose solution, but it does permit the glucose itself to pass through.
Besides the dye, which had an excitation
wavelength at 490 nm and an emission at 540 nm, the scientists used viologen, a
permanently charged salt of 4,4’-bipyridine that quenches a dye’s
fluorescence. They combined the viologen with boronic acid, which acts as a glucose
receptor but does not affect the salt’s quenching ability, then put the compound
and dye into the hydrogel.
In the presence
of viologen, the fluorescence of a dye molecule is quenched (left). When glucose
is exposed to the boronic acid attached to the viologen, quenching stops and the
fluorescence intensity increases. This principle enabled investigators at the University
of California, Santa Cruz, to create a hydrogel-based glucose sensor.
The key to the technique, according
to Gamsey, is that glucose infused into the hydrogel binds to the receptor, which
decreases the viologen’s quenching ability, thereby restoring the dye’s
fluorescence. The investigators then measured the fluorescence and calculated the
glucose concentration from that data.
They tested the method using two setups.
In one, they excited the infused hydrogel via front-facing illumination created
with a 490-nm xenon discharge lamp with 7.3 W of average power at 50 Hz. They gradually
added various amounts of glucose to the gel and measured the resulting increase
in fluorescence with a spectrometer from PerkinElmer of Wellesley, Mass.
To test the hydrogel technique’s ability to work efficiently
in a compact system, the researchers constructed a platform comprising an optical
fiber that is encased in a glass tube and capped with the embedded hydrogel. Reproduced
with permission of Langmuir. © 2006 American Chemical Society.
They found that the hydrogel displayed
a high dynamic response to glucose concentrations within 45 to 360 mg/dl, a range
that is especially relevant to diabetics. By decreasing the glucose concentration
with the hydrogel and observing the resulting reduction in fluorescence, they also
demonstrated the reversibility of the system.
“It is important for [the system]
to be reversible,” Gamsey said, “because physiological glucose levels
in diabetics and in critically ill hospitalized patients rise and fall. The sensor
must be efficient at tracking peaks as well as valleys in glucose concentrations.”
The investigators then devised and
tested a miniature hydrogel-based sensing system composed of fiber optics. They
threaded one end of an optical fiber through an epoxy-filled glass tube, attached
a disk comprising the hydrogel to the end of the tube assembly and immersed the
hydrogel-capped end in a cell through which glucose solution flowed.
They connected the other end of the
fiber to an Ocean Optics Inc. spectrophotometer equipped with a blue LED light source
that provided an excitation wavelength centered at 470 nm. As with the front-facing
illumination setup, the scientists found that changes in the fluorescence intensity
corresponded directly with glucose concentration. Better still, they saw that the
fiber optic system provided faster response times.
In addition, they tested the fiber
optic setup’s ability to provide a stable signal throughout extended exposure
to glucose, which could indicate the potential for continuous monitoring in diabetics.
They exposed the hydrogel sensor to a high concentration of glucose for 36 hours,
noting that the fluorescence intensity did not significantly change during this
The researchers are hoping to improve
the system’s selectivity by finding a way to prevent the boronic acid from
binding to molecules that are structurally similar to glucose — such as fructose
— which affects the fluorescence signal. They also are modifying the porosity
of the hydrogel to attempt to further improve the response time.
“These are the two main hurdles
which must be overcome before the system becomes clinically viable,” Gamsey
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