of one aptamer or different aptamers directed against the same target can be
immobilized on a nanoparticle. This can dramatically increase the affinity of the
system due to avidity effects [14]. While the term affinity describes the interaction
between one ligand and its target structure, avidity describes the overall affinity of
multiple binding events between the multimeric ligand construct and the target
[15]. Avidity effects are possible when either different aptamers binding to distinct
sites of a target molecule are available or in case of multivalent target structures, such
as multimeric proteins or targets expressed in high density on a cell surface [15].
Multivalent aptamer-modified nanoparticles have already been proposed for
therapeutic applications. They were used to develop theragnostic agents for cancer
cell therapy [16] and drug delivery [17]. Moreover aptamer-modified nanoparticles
were used to develop a system to control thrombin activity [18, 19]; one of these
systems is described in Sect. 3.3 of this chapter in detail.
2 Assays with Aptamer-Modified Nanoparticles
in Diagnostics
Common types of diagnostic applications are immunoassays using antibodies or
analytical methods like liquid chromatography (LC) [20–23]. Because of their high
costs and some other disadvantages, efforts are made to replace antibodies by
aptamers [4, 6, 12, 24]. Due to their low cost production and the possibility to target
even small and toxic analytes, aptamers provide the possibility for new diagnostic
assays and applications. For generation of a signal, the aptamers often get conjugated
to nanoparticles [12, 23]. The nanoparticles can produce a visible signal, either based
on their color like gold nanoparticles or, e.g., an electrochemical signal [25]. Different assays exploiting aptamer-modified nanoparticles for diagnostic applications are
discussed in the following sections.
2.1 Assays Using Aptamer-Modified Gold Nanoparticles
In assays with gold nanoparticles (AuNPs), the optical properties of AuNPs are
utilized. Color shifts due to the size and distance-dependent light absorption can be
analyzed by the naked eye. Colloidal gold nanoparticles are red; agglomerated
particles show a color shift over purple to blue and black eventually. The AuNPs
are mostly either produced by reduction of tetrachloroauric acid (HAuCl 4 ) by
sodium citrate or by laser ablation of gold [26].
AuNPs can easily be modified with ligands, such as aptamers but also proteins
like antibodies or other molecules [27, 28].
Often the high affinity of thiol toward gold is exploited, e.g., to immobilize thiolmodified aptamers on the AuNP surface, resulting in a strong semi-covalent
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immobilized on a nanoparticle. This can dramatically increase the affinity of the
system due to avidity effects [14]. While the term affinity describes the interaction
between one ligand and its target structure, avidity describes the overall affinity of
multiple binding events between the multimeric ligand construct and the target
[15]. Avidity effects are possible when either different aptamers binding to distinct
sites of a target molecule are available or in case of multivalent target structures, such
as multimeric proteins or targets expressed in high density on a cell surface [15].
Multivalent aptamer-modified nanoparticles have already been proposed for
therapeutic applications. They were used to develop theragnostic agents for cancer
cell therapy [16] and drug delivery [17]. Moreover aptamer-modified nanoparticles
were used to develop a system to control thrombin activity [18, 19]; one of these
systems is described in Sect. 3.3 of this chapter in detail.
2 Assays with Aptamer-Modified Nanoparticles
in Diagnostics
Common types of diagnostic applications are immunoassays using antibodies or
analytical methods like liquid chromatography (LC) [20–23]. Because of their high
costs and some other disadvantages, efforts are made to replace antibodies by
aptamers [4, 6, 12, 24]. Due to their low cost production and the possibility to target
even small and toxic analytes, aptamers provide the possibility for new diagnostic
assays and applications. For generation of a signal, the aptamers often get conjugated
to nanoparticles [12, 23]. The nanoparticles can produce a visible signal, either based
on their color like gold nanoparticles or, e.g., an electrochemical signal [25]. Different assays exploiting aptamer-modified nanoparticles for diagnostic applications are
discussed in the following sections.
2.1 Assays Using Aptamer-Modified Gold Nanoparticles
In assays with gold nanoparticles (AuNPs), the optical properties of AuNPs are
utilized. Color shifts due to the size and distance-dependent light absorption can be
analyzed by the naked eye. Colloidal gold nanoparticles are red; agglomerated
particles show a color shift over purple to blue and black eventually. The AuNPs
are mostly either produced by reduction of tetrachloroauric acid (HAuCl 4 ) by
sodium citrate or by laser ablation of gold [26].
AuNPs can easily be modified with ligands, such as aptamers but also proteins
like antibodies or other molecules [27, 28].
Often the high affinity of thiol toward gold is exploited, e.g., to immobilize thiolmodified aptamers on the AuNP surface, resulting in a strong semi-covalent
164
A. Eilers et al.