Topics in Current Chemistry (2020) 378:35
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although QDs coated with PEG spacers have reduced nonspecific protein binding,
which may often limit the applicability of these NPs in bioanalytical methodologies. For such uses, PEG molecules should be previously activated with appropriate
functional groups (e.g. amine, thiols or carboxyls) to provide hydrophilic bridges
between the QD surface and the PEG chains [52].
A variant of this third stabilization approach consists of the encapsulation of the
hydrophobic QDs in appropriate hydrophilic vehicles, such as liposomes [53]. The
hollow spherical structure of liposomes and the high loading capacity makes them
attractive carriers for hydrophobic QDs. Moreover, the surface of liposomes can
be easily modified with the appropriate functional groups so as to allow a simple
further bioconjugation with proteins, thereby minimizing nonspecific interactions
of water-soluble and water-insoluble QDs with surface material and amplifying the
analytical signal due to the possibility to incorporate several QDs in a single nanoliposome. In this context, a signal amplification platform based on the measurement
of fluorescence from QDs encapsulated in liposomes has been recently proposed for
the highly sensitive detection of human telomerase activity [54]. In the approach
described, similar to a typical hybridization bioassay, biotinylated liposomes containing the QDs were recognized by a capture probe conjugated with streptavidin.
In a final step, the QD-encapsulated liposomes were disrupted by the controlled
addition of Triton X-100, and the fluorescence intensity of the released QDs was
measured to detect telomerase activity [54]. Liposomes containing hydrophobic
QDs have also been employed for tumor imaging applications through the specific
recognition of aptamers conjugated to the surface of the liposomes (see Fig. 5) [55].
However, a significant drawback of liposomes is their low stability when entering
in vivo media. Additionally, QDs stabilized by this approach have a substantially
increased hydrodynamic diameter, which could limit their application in bioimaging
and targeting procedures.
It must also be taken into account that the procedure selected for hydrophilization
of the QDs likely affects their suitability in subsequent bioconjugation and future
bioanalytical applications. For example, some approaches can significantly increase
the hydrodynamic ratios of the NPs, which in turn can lead to non-specific binding
or reduced accessibility to some targets. In this context, ligand exchange provides
QDs with a small hydrodynamic diameter but also with lower photoluminescence
quantum yields, while encapsulation results in larger nanoprobes sizes with higher
quantum yields [56].
4 QD Bioconjugation Strategies
One of the main challenge to the use of QDs in biomedical applications can be considered to be the generation of robust bonding between the appropriate target recognition molecule and the surface of the NP, as this bonding will be a key parameter affecting the direct application of the QDs in biological media. In this section,
we summarize some of the most relevant physicochemical processes used to attach
BMs to the QD surface, a process referred to as bioconjugation.
142
Reprinted from the journal
1 3
although QDs coated with PEG spacers have reduced nonspecific protein binding,
which may often limit the applicability of these NPs in bioanalytical methodologies. For such uses, PEG molecules should be previously activated with appropriate
functional groups (e.g. amine, thiols or carboxyls) to provide hydrophilic bridges
between the QD surface and the PEG chains [52].
A variant of this third stabilization approach consists of the encapsulation of the
hydrophobic QDs in appropriate hydrophilic vehicles, such as liposomes [53]. The
hollow spherical structure of liposomes and the high loading capacity makes them
attractive carriers for hydrophobic QDs. Moreover, the surface of liposomes can
be easily modified with the appropriate functional groups so as to allow a simple
further bioconjugation with proteins, thereby minimizing nonspecific interactions
of water-soluble and water-insoluble QDs with surface material and amplifying the
analytical signal due to the possibility to incorporate several QDs in a single nanoliposome. In this context, a signal amplification platform based on the measurement
of fluorescence from QDs encapsulated in liposomes has been recently proposed for
the highly sensitive detection of human telomerase activity [54]. In the approach
described, similar to a typical hybridization bioassay, biotinylated liposomes containing the QDs were recognized by a capture probe conjugated with streptavidin.
In a final step, the QD-encapsulated liposomes were disrupted by the controlled
addition of Triton X-100, and the fluorescence intensity of the released QDs was
measured to detect telomerase activity [54]. Liposomes containing hydrophobic
QDs have also been employed for tumor imaging applications through the specific
recognition of aptamers conjugated to the surface of the liposomes (see Fig. 5) [55].
However, a significant drawback of liposomes is their low stability when entering
in vivo media. Additionally, QDs stabilized by this approach have a substantially
increased hydrodynamic diameter, which could limit their application in bioimaging
and targeting procedures.
It must also be taken into account that the procedure selected for hydrophilization
of the QDs likely affects their suitability in subsequent bioconjugation and future
bioanalytical applications. For example, some approaches can significantly increase
the hydrodynamic ratios of the NPs, which in turn can lead to non-specific binding
or reduced accessibility to some targets. In this context, ligand exchange provides
QDs with a small hydrodynamic diameter but also with lower photoluminescence
quantum yields, while encapsulation results in larger nanoprobes sizes with higher
quantum yields [56].
4 QD Bioconjugation Strategies
One of the main challenge to the use of QDs in biomedical applications can be considered to be the generation of robust bonding between the appropriate target recognition molecule and the surface of the NP, as this bonding will be a key parameter affecting the direct application of the QDs in biological media. In this section,
we summarize some of the most relevant physicochemical processes used to attach
BMs to the QD surface, a process referred to as bioconjugation.
142
Reprinted from the journal