Biomedical Engineering Reference
In-Depth Information
of multiple different functions, such as glucose detection and self-regulated
insulin delivery in a single nano-enabled system that can work at physiological
conditions.
The size-tunable optical properties allow QDs to act as optical acceptors as
well as donors for biosensing based on fluorescence resonance energy trans-
fer (FRET).
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FRET is a process that involves nonradiative energy transfer
from a donor in its electronic excited state to an acceptor through dipole-dipole
interactions. The acceptor will be relaxed to its ground state through emitting
photons or releasing heat energy where the FRET efficiency strongly depends
on the separation distance between the donors and acceptors. Generally, FRET
sensors require separation of absorption profile of acceptors from the emission
of donors to avoid direct excitation.
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QD-based detection of hybridization and cleavage of DNA was reported by
Willner and coworkers.
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DNA/QD conjugates were initially hybridized with
the complementary Texas Red-labeled DNA. Addition of DNase I resulted
in the cleavage of DNA and the partial recovery of fluorescence emission
of QDs. The previous studies used organic dyes as quenchers, which have
promoted the development of biosensing. AuNPs have been demonstrated to
be more efficient quenchers than conventional organic dyes. The nonradiative
quenching of QD's emission by AuNPs is due to long-distance dipole-metal
interactions that extend significantly beyond the classical Förster distance
(~6nm). The quenching efficiency strongly depends on the particle size and
the quenching constant increases with increase in the AuNP size because the
absorption cross-section of AuNPs is the main parameter affecting the energy
transfer efficiency. The assembly with QDs as the donor and the AuNP as
acceptor is extremely attractive for bioassays.
Nanotube and nanowire-based fluorescent biosensors are also attractive for
optical bioassays because of their confined electron transportation along the 1D
direction. Chen et al.
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have assembled a DNA aptamer probe through attach-
ment of thiolated thrombin-binding aptamer on gold nanowires (AuNWs). In
this study, exposure of the modified AuNWs probes to the biotinylated throm-
bin led to specific recognition of targets that were labeled with a fluorescent
reporter. Their biosensor was used to detect thrombin at a single-molecule level
with a limit of detection of 100 fM.
Novel composite NMs, such as QD-carbon nanotubes and QD-graphene
oxides, are among the emerging NMs that are efficient candidates for fluores-
cent biosensors. Applications of graphene oxide have been reported for biosensors
because of its facile surface modification, high mechanical strength, good water
dispersibility, and photoluminescence.
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Aside from the advantageous planar
structure that facilitates electron and energy transportation, graphene oxide is also
ideal support for NP loading. Jung and coworkers reported a graphene oxide-based
immunobiosensor for rotavirus pathogen detection with high sensitivity and selec-
tivity. This sensor was based on the photoluminescence quenching of graphene
oxides through the FRET process induced by the AuNPs. In this study, the anti-
body-modified graphene oxides could recognize the pathogen due to the specific
