Diamond biosensors are forever
Lynn M. Savage
To make effective biosensors, the detection molecules must be attached
to a microelectronics-compatible surface in such a way that it is highly stable
for long periods. Combinations of biomolecules and commonly used substrate materials
such as glass, silicon or gold, however, are not known to be very stable over time.
In the April 11 issue of Langmuir, researchers
at Waseda University in Tokyo and at the University of Tokyo have described a photolithographic
technique in which a highly stable diamond surface is coated with select amine groups.
These groups then can be functionalized with various biomolecules, which stay attached
to the diamond substrate even after multiple washings.
The investigators chose diamond not
only for its chemical stability, but also because it has high sensitivity in electrochemical
reactions and can be deposited as a thin film on various substrates. They deposited
an ∋8-μm-thick film of polycrystalline diamond onto a silicon substrate,
then produced amine groups directly on the diamond’s surface by irradiating
the material with 253.7-nm light from a halogen lamp in the presence of ammonia
They used a 100-nm-thick gold photomask
and a coating of photoresist film to create an array of dots composed of the amine
deposits. Where the gold and resist layers were removed during the photolithographic
technique, they exposed the areas to C3F8, which created fluorine-terminated surfaces
intended to prevent subsequent hybridization of the biomolecule.
The researchers created arrays with
dots that were 5, 10, 15 or 20 μm in diameter and used optical microscopy to
confirm that the dots were regular in size and shape. They also used spatially resolved
x-ray photoelectron spectroscopy to verify the differentiation of the surface
areas with the amine groups and those terminated with fluorine.
They functionalized the exposed amine
groups to either complementary or noncomplementary sequences of DNA and hybridized
them with DNA that was tagged with the fluorophore Cy5. Fluorescence imaging showed
that the shape of the hybridized amine groups corresponded with those seen in the
optical microscopy, although the noncomplementary DNA did not hybridize at all.
Furthermore, fluorescence intensity stayed intact after up to 20 repeated cycles
of hybridization and denaturization with sodium hydroxide.
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