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A
new quantum mechanical-based biosensor designed by a team at University
of California, Santa Barbara offers tremendous potential for detecting
biomolecules at ultra-low concentrations, from instant point-of-care
disease diagnostics, to detection of trace substances for forensics and
security.
Kaustav
Banerjee, director of the Nanoelectronics Research Lab and professor of
Electrical and Computer Engineering at UCSB, and PhD student Deblina
Sarkar have proposed a methodology for beating the fundamental limits of
a conventional field-effect transistor (FET) by designing a Tunnel-FET
(T-FET) sensor that is faster and four orders of magnitude more
sensitive. The details of their study appeared in the April 2, 2012
issue of the journal Applied Physics Letters.
“This
study establishes the foundation for a new generation of
ultra-sensitive biosensors that expand opportunities for detection of
biomolecules at extremely low concentrations,” said Samir Mitragotri,
professor of Chemical Engineering and director of the Center for
Bioengineering at UCSB. “Detection and diagnostics are a key area of
bioengineering research at UCSB and this study represents an excellent
example of UCSB’s multi-faceted competencies in this exciting field.”
Biosensors
based on conventional FETs have been gaining momentum as a viable
technology for the medical, forensic, and security industries since they
are cost-effective compared to optical detection procedures. Such
biosensors allow for scalability and label-free detection of
biomolecules—removing the step and expense of labeling target molecules
with fluorescent dye.
The
principle behind any FET-based biosensor is similar to the FETs used in
digital circuit applications, except that the physical gate is removed
and the work of the gate is carried out by charged versions of the
biomolecules it intends to detect. For immobilizing these biomolecules,
the dielectric surface enclosing the semiconductor is coated with
specific receptors, which can bind to the target biomolecules—a process
called conjugation.
“The
thermionic emission current injection mechanism of conventional FET
based biosensors puts fundamental limitations on their maximum
sensitivity and minimum detection time,” said Banerjee, who conceived
the idea in 2009 while studying the design of tunnel-FETs for ultra
energy-efficient integrated electronics.
“We
overcome these fundamental limitations by making Quantum Physics join
hands with Biology” explained Sarkar, the lead author of the paper. “The
key concept behind our device is a current injection mechanism that
leverages biomolecule conjugation to bend the energy bands in the
channel region, leading to the quantum-mechanical phenomenon of
band-to-band tunneling. The result is an abrupt increase in current
which is instrumental in increasing the sensitivity and reducing the
response time of the proposed sensor.”
“The
abruptness of current increase in an electrical switch is quantified by
a parameter called subthreshold swing and the sensitivity of any FET
based biosensor increases exponentially as the subthreshold swing
decreases. Thus, similar devices such as Impact-ionization- or
Nano-electromechanical-FETs are promising for biosensing applications,”
explained Banerjee. “But since theT-FETs can be easily integrated in
the widely available silicon-based semiconductor technology, they can be
mass produced in a cost effective manner.”
According
to the researchers, their T-FET biosensor is expected to have
tremendous impact on research in genomics and proteomics, as well as
pharmaceutical, clinical and forensic applications—including the growing
market of in-vitro and in-vivo diagnostics. Banerjee and Sarkar have
filed a patent disclosure for their technology, which the researchers
anticipate can be ready for the marketplace in as few as two years.
Proposal for tunnel-field-effect-transistor as ultra-sensitive and label-free biosensors
