Topics in Current Chemistry (2020) 378:35
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exciting class of fluorescent probes for use in photoluminescence imaging due to
their tunable optical properties, high stability and ability to be used in guided-targeting based on NP surface functionalization with appropriate recognition elements
(e.g. antibodies, aptamers, peptides, etc.). Researchers have explored various methods based on QD functionalization to enhance fluorescence imaging. In this section,
we include a brief overview of the emerging applications for bioconjugated QDs in
bioimaging for clinical diagnosis.
6.1 Bioconjugated QDs for Biomedical Labeling and Imaging
Researchers are increasingly focused on the luminescence-based imaging of biological specimens for use in biomedical studies and, in particular, in medical diagnostics as developing technological advances point to enhanced biomedical capabilities
(e.g. innovative fluorescence-based imaging systems). Typical fluorescent imaging reagents can be endogenous, which often require an enzyme-mediated process
inside the organism to stimulate the production of measurable visible light, or exogenous; the latter are currently the more versatile and popular fluorescent imaging
agents. NP-based optical contrast agents (including QDs) fall within the group of
exogenous reagents that are still dominated by sensitive organic fluorescent probes.
However, important advances in the bioconjugation of QDs to selected recognition
elements (e.g. antibodies, peptides, genetic material, etc.) constitute a significant
trust in the development of novel and improved bioimaging methodologies [140].
The QD–BM bioconjugates thus produced constitute outstanding nanoplatforms for
the fluorescence labeling of target BMs. Such fluorescent labels have been successfully employed in fluorescence imaging in studies on the single-molecule dynamics of living cells, monitoring of intracellular protein–protein interactions, disease
detection in deeper tissues, detection of tumor cells based on selective binding of
the tailored, bioconjugated QDs to known cancer biomarkers, and many more [141].
Pioneer studies on the use of QDs as luminescence tags for imaging exploited the
high sensitivity of the luminescence from these NPs to the surface state. Eventual
chemical or physical interactions between chemical species present in the media and
the surface of the QDs would result in detectable changes in the fluorescence emission. Based on this basic approach, Liu et al. evaluated mercaptoacetic acid (MAA)capped CdSe/ZnSe/ZnS QDs for the detection of changes in pH within SKOV-3
human ovarian cancer cells [142]. These authors made use of the changes in the
intrinsic fluorescence emission of QDs with pH, with more intense fluorescence
emission obtained at higher pHs. Therefore, after the QDs were uptaken by endocytosis within the lysosomes in both fixed and living cells, where the pH is rather low,
the addition of chloroquine produced an increase on the pH and an enhancement of
the photoluminescence intensity of the QDs [142].
However, it must be taken into account that such methods based on the
direct interaction of the analytes with the QDs suffer from poor selectivity and
so have a rather limited applicability in real-life settings. However, advances in
the controlled bioconjugation of QDs to active recognition elements explain the
recent emergence of a myriad of applications of QDs for use in target-guided
160
Reprinted from the journal
1 3
exciting class of fluorescent probes for use in photoluminescence imaging due to
their tunable optical properties, high stability and ability to be used in guided-targeting based on NP surface functionalization with appropriate recognition elements
(e.g. antibodies, aptamers, peptides, etc.). Researchers have explored various methods based on QD functionalization to enhance fluorescence imaging. In this section,
we include a brief overview of the emerging applications for bioconjugated QDs in
bioimaging for clinical diagnosis.
6.1 Bioconjugated QDs for Biomedical Labeling and Imaging
Researchers are increasingly focused on the luminescence-based imaging of biological specimens for use in biomedical studies and, in particular, in medical diagnostics as developing technological advances point to enhanced biomedical capabilities
(e.g. innovative fluorescence-based imaging systems). Typical fluorescent imaging reagents can be endogenous, which often require an enzyme-mediated process
inside the organism to stimulate the production of measurable visible light, or exogenous; the latter are currently the more versatile and popular fluorescent imaging
agents. NP-based optical contrast agents (including QDs) fall within the group of
exogenous reagents that are still dominated by sensitive organic fluorescent probes.
However, important advances in the bioconjugation of QDs to selected recognition
elements (e.g. antibodies, peptides, genetic material, etc.) constitute a significant
trust in the development of novel and improved bioimaging methodologies [140].
The QD–BM bioconjugates thus produced constitute outstanding nanoplatforms for
the fluorescence labeling of target BMs. Such fluorescent labels have been successfully employed in fluorescence imaging in studies on the single-molecule dynamics of living cells, monitoring of intracellular protein–protein interactions, disease
detection in deeper tissues, detection of tumor cells based on selective binding of
the tailored, bioconjugated QDs to known cancer biomarkers, and many more [141].
Pioneer studies on the use of QDs as luminescence tags for imaging exploited the
high sensitivity of the luminescence from these NPs to the surface state. Eventual
chemical or physical interactions between chemical species present in the media and
the surface of the QDs would result in detectable changes in the fluorescence emission. Based on this basic approach, Liu et al. evaluated mercaptoacetic acid (MAA)capped CdSe/ZnSe/ZnS QDs for the detection of changes in pH within SKOV-3
human ovarian cancer cells [142]. These authors made use of the changes in the
intrinsic fluorescence emission of QDs with pH, with more intense fluorescence
emission obtained at higher pHs. Therefore, after the QDs were uptaken by endocytosis within the lysosomes in both fixed and living cells, where the pH is rather low,
the addition of chloroquine produced an increase on the pH and an enhancement of
the photoluminescence intensity of the QDs [142].
However, it must be taken into account that such methods based on the
direct interaction of the analytes with the QDs suffer from poor selectivity and
so have a rather limited applicability in real-life settings. However, advances in
the controlled bioconjugation of QDs to active recognition elements explain the
recent emergence of a myriad of applications of QDs for use in target-guided
160
Reprinted from the journal