By comparing different samples, the samples’ color shift gives information about
the analyte concentration, and therefore the assay can be quantitative.
An overview of various assays with aptamer-modified gold nanoparticles is given
in Table 1.
2.2 Biosensors Using Aptamer-Modified Nanoparticles
Besides colorimetric applications, aptamer-modified nanoparticles can be used in
various types of biosensors. Depending on the nanoparticle composition, fluorescence signals, UV-Vis analysis, or electrochemical signals can be used for
biosensors.
2.2.1 Fluorescence Biosensors
Fluorescence sensors benefit from the property of metal nanoparticles to alter the
fluorescence of fluorophores. Dependent on the nanoparticles size and the distance
of the fluorophore to the particle, the fluorescence can be enhanced or more likely be
reduced; hence most particles quench the fluorescence signal. Crucial for the fluorescence change is the plasmon field around the particle, which is generated by
incident light and the dipole energy around the particle [27, 52].
Many sensors are FRET (Förster resonance energy transfer)-based sensors
[53]. For FRET sensors the aptamer or cOligo is modified with a fluorophore. In
one setup fluorescence-modified aptamers get adsorbed onto nanoparticles and due
to quenching generate a low fluorescence signal. When the analyte is present,
aptamers detach from the nanoparticles to bind their target; thus the fluorescence
increases. This principle is called “signal-on.” Depending on the nanoparticles and
fluorophore properties, the opposite “signal-off” can be used as the sensor setup. For
the “signal-off” sensor, the aptamer is covalently or semi-covalently bound to the
nanoparticle, with the fluorophore having the furthest distance from the nanoparticle
as possible. Once the target is present, the aptamers’ structure changes, and therefore
the distance of the fluorophore to the nanoparticle decreases, leading to a higher
quenching efficiency and lower signal intensity [52, 54].
Another setup has aptamers immobilized to the nanoparticle. When the cOligo is
fluorescence-modified and is competing with the target to bind the aptamer conjugated to the AuNP, the cOligo fluorophore quenching evaluates the assay. A
sandwich assay would work similarly.
Another fluorescence sensor setup uses the aptamers’ structural changes, operating in a fluorophore distance change [53]. Fluorescent-modified aptamers that bound
to nanoparticles generate different signals depending whether the target is bound or
not bound to the aptamer. Crucial for this signal change is the fluorophore distance to
the particle, which has to change distinctly with the aptamer structural change upon
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the analyte concentration, and therefore the assay can be quantitative.
An overview of various assays with aptamer-modified gold nanoparticles is given
in Table 1.
2.2 Biosensors Using Aptamer-Modified Nanoparticles
Besides colorimetric applications, aptamer-modified nanoparticles can be used in
various types of biosensors. Depending on the nanoparticle composition, fluorescence signals, UV-Vis analysis, or electrochemical signals can be used for
biosensors.
2.2.1 Fluorescence Biosensors
Fluorescence sensors benefit from the property of metal nanoparticles to alter the
fluorescence of fluorophores. Dependent on the nanoparticles size and the distance
of the fluorophore to the particle, the fluorescence can be enhanced or more likely be
reduced; hence most particles quench the fluorescence signal. Crucial for the fluorescence change is the plasmon field around the particle, which is generated by
incident light and the dipole energy around the particle [27, 52].
Many sensors are FRET (Förster resonance energy transfer)-based sensors
[53]. For FRET sensors the aptamer or cOligo is modified with a fluorophore. In
one setup fluorescence-modified aptamers get adsorbed onto nanoparticles and due
to quenching generate a low fluorescence signal. When the analyte is present,
aptamers detach from the nanoparticles to bind their target; thus the fluorescence
increases. This principle is called “signal-on.” Depending on the nanoparticles and
fluorophore properties, the opposite “signal-off” can be used as the sensor setup. For
the “signal-off” sensor, the aptamer is covalently or semi-covalently bound to the
nanoparticle, with the fluorophore having the furthest distance from the nanoparticle
as possible. Once the target is present, the aptamers’ structure changes, and therefore
the distance of the fluorophore to the nanoparticle decreases, leading to a higher
quenching efficiency and lower signal intensity [52, 54].
Another setup has aptamers immobilized to the nanoparticle. When the cOligo is
fluorescence-modified and is competing with the target to bind the aptamer conjugated to the AuNP, the cOligo fluorophore quenching evaluates the assay. A
sandwich assay would work similarly.
Another fluorescence sensor setup uses the aptamers’ structural changes, operating in a fluorophore distance change [53]. Fluorescent-modified aptamers that bound
to nanoparticles generate different signals depending whether the target is bound or
not bound to the aptamer. Crucial for this signal change is the fluorophore distance to
the particle, which has to change distinctly with the aptamer structural change upon
Aptamer-Modified Nanoparticles in Medical Applications
169