b2815 Tissue Engineering and Nanotheranostics “9.61x6.69”
184 Tissue Engineering and Nanotheranostics
the fluorescence was bright like a flare.^173 Later, this system was
extended for multiplexed detection of different targets simultaneously
in living cells.174–176 Besides nanoflare, NEST sensor based on molecu
lar beacon, composed of a loop structure and a stem structure,177,178 has
been employed. In the hairpin structure, the nucleotides at each termi
nus, where the fluorophore and quencher are anchored respectively,
are complementary to each other, resulting in the quenched fluoro
phore. Once the target nucleic acid hybridize with molecular beacon,
the stem structure opens and the fluorescence recovers. AuNPs behave
excellently in molecular beacons due to their high quenching effi
ciency. Dubetret et al. used AuNPs to assist molecular beacon to detect
DNA targets, they found that sensitivity had improved 100 times
compared to conventional molecular beacon.^179 Fan et al.180,181
reported a similar probe to detect three different tumorsuppressor
genes (Fig. 7(b)). Additionally, the AuNPsassisted molecular beacon
demonstrate a singlebase mismatch selectivity of 25:1, so they hold
great potential in multiplexed detection of singlebase mismatch.
Generally, small AuNPs (with size less than 20 nm) are more often
used in NEST, as absorption is dominant over scattering. While large
AuNPs are more often used in PRET as scattering is dominant above
600 nm.^182
6.2. Surface-Enhanced Raman Scattering
Raman effect originates from the inelastic scattering when light inter
acts with matter, and it means that an energy exchange occurs
between the incident photons and the scattering substance. Like NIR
spectrum, the Raman spectrum of different molecules is so unique
that it has also been called a “Raman fingerprint”.^183 A major limita
tion of traditional Raman spectroscopy is that its SNR ratio is too low,
especially in in vivo imaging with short acquisition time. SERs is the
most crucial solution to amplify Raman intensity.^184 SERs is a plas
monic effect in which the Raman intensity of a molecule is enhanced
enormously (up to 10^14 fold) when adsorbed on a rough metal (e.g.
Ag or Au) surface with high curvature, such as a nanoparticle.185,186
Both electromagnetic (EM) enhancement and chemical enhancement
