Recent Advances in DNA Biosensor - International Frequency ...

3.1. Molecular Beacons (MBs)
Sensors & Transducers Journal, Vol. 92, Issue 5, May 2008, pp. 122-133
MBs are oligonucleotides with a stem-and-loop structure, labeled with a fluorophore at one end and a
quencher on the other end of the stem that become fluorescent upon hybridization. In addition to their
direct monitoring capability, MB probes offer high sensitivity and specificity. A biotinylated molecular
beacon probe was developed to prepare a DNA biosensor using a bridge structure. MB was
biotinylated at quencher site of the stem and linked on a biotinylated glass through strepatavidin,
which acted as bridge between MB and glass matrix. The fluorescence change was measured by
confirmation change of MB in the presence of complementary target DNA [34, 35].
3.2. Surface Plasmon Resonance (SPR)
Surface plasmon resonance (SPR) is an quantum optical-electrical phenomenon arising from the
interaction of light with metal surface. Under specific conditions the energy carried by photons of light
is transferred to packets of electrons (photons) on a metal surface. Energy transfer occurs only at
specific resonance wavelength of light [33].
These biosensors are based on monitoring changes in surface optical properties (change in resonance
angle due to change in the interfacial refractive index) resulting from the surface binding reaction.
Such devices thus combine the simplicity of SPR with the sensitivity of wave guiding devices. The
resonance conditions are influenced by the material adsorbed onto the thin metal film. A linear
relationship is found between resonance energy and mass concentration of molecules such as proteins,
sugars and DNA. The SPR signal which is expressed in resonance units is therefore a measure of mass
concentration at the sensor chip surface [36-38].It has been reported that alkane thiol modified
oligonucleotide can be immobilized onto gold surface to detect DNA hybridization using SPR based
detection in agro food industry [39].
3.3. Quantum–Dot
An ultrasensitive nanosensor based on fluorescence resonance energy transfer (FREET) can detect
very low concentration of DNA and do not require separation of unhybridized DNA. Such type of
technique is based on quantum-dots (QDs) which are linked to specific DNA probes to capture target
DNA. The target DNA strand binds to a fluorescent-dye (fluorophore) labeled reporter strand and thus
forming FREET donor-acceptor assembly. Quantum dot also functions as to concentrate the signal by
confining several targets at nanoscale domain. Unbound DNA strand produce no fluorescence but on
biding of even small amount of target DNA (≤ 50 copies) may produce very strong FREET signal.
Several FREET based DNA probes (molecular beacons and TaqMan probes) whose fluorescence
signals change as a result of hybridization or enzymatic reactions have been developed for separation
free (unhybridized DNA strand) detection of target DNA [40-43].
DNA nanosensor consists of two target specific DNA probes i.e. reporter and capture probe. The
reporter probe is labeled with fluorophore whereas capture probe is labeled with biotin which binds
with streptavidin conjugated with QD. The QD functions as target concentrator as well as FREET
energy donor. When target DNA is present in solution, it becomes sandwiched by reporter and capture
probes. Several sandwiched hybrids are then captured by a single QD through biotin-streptavidin
binding and concentrate at nanoscale domain [44]. The fluorophore acceptor and QD donor close
proximity causing fluorescence from acceptor by means of FREET on illumination of the donor. The
detection of acceptor emission indicates the presence of target DNA. The unhybridized probe do not
participate in FREET and do not give fluorescence, therefore, it is not necessary to remove. The CdSe-
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