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on its surface was used to selectively recognize Cu2+ down to 1 nM
over K+, Na+, Ca2+, Mg2+, Ni2+, Co2+, in the aqueous solutions.^161
Another example, AuNPs transfer their energy to adsorbed cytochrome
c, and “spectral dips” are displayed in the single nanoparticle scatter
ing spectrum, where the positions of spectral dips match the
cytochrome c absorption peak positions of about 550 nm.^167
Meanwhile, since nanoplasmonic optical sensor do not photobleach
or blink, real time production of cytochrome c in living HepG2 cells
has been dynamically imaged for long term using PRET spectros
copy.^162 Highly sensitive and selective metal ion sensing has also been
enabled by PRET spectroscopy.163,168 Besides gold nanosphere,
nanorods are also applied as nanoplasmonic sensors to monitor the
opening process in molecular beacon.^165
In addition to offering high spatial resolution owing to the small
nanometerscale size of the biosensor, this method is 100–1,000
times more sensitive and much more faster than organic reporter
based methods.^166
6.1.2. Nanometal surface energy transfer
In order to describe the rate of energy transfer in NEST from a dipole
to a metallic surface interband transition, Chance et al. used a for
mula, which is similar to FRET, however, Persson and Lang extend
this formula and create a surface energy transfer rate.169–172 Thus,
energy transfer to a surface follows a very different distance trend and
magnitude of interaction. In brief, in NEST, energy transfer distances
are much more longer than typical Förster distance (typically double),
and quenching of dyes of different emission frequencies forms the
visible range to the NIR and could be realized by the same nanopar
ticles rather than different dyes.^164
To bring the NEST sensor into the application, metal nanoparti
cles quench the fluorescence of the fluorophores adsorbing on the
surface by chemical binding or electrostatic interaction at first. Then,
the target molecules bind selectively to the probe and, in turn, the flu
orescence restores. The restored intensity is proportional to the con
centration of the target molecular structure. For instance, Ray et al.
