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2.3.4 Biosensing with Luminescent Semiconductor
Quantum Dots
Recently, luminescent semiconductor nanocrystals or quantum dots (QDs) have
become a successful novel nanomaterial possessing unique photophysical
fluorescent properties, which have helped create a new generation of robust
fluorescent biosensors [47–53]. It should be stated that the fluorescent properties of
QDs have overcome most of the liabilities of conventional organic and proteinbased fluorophores. The biosensing QD properties of interest include high quantum
yields, broad absorption spectra coupled to narrow size-tunable photo-luminescent
emissions, and exceptional resistance to both photo-bleaching and chemical
degradation. In this section, we will investigate the advancement in using QDs for
many invitro biosensing applications, including their use in immunoassays, as
generalized probes [47–53].
2.3.4.1 Immunoassays Using Quantum Dots
The unique advantages of using QDs are owed to their inherent photostability, their
improved sensitivity, and size-tunable photoluminescence coupled to their broad
absorption spectra. These unique advantages have allowed them to serve as multicolor or multiplexed immunoassays. It should be mentioned that in terms of coupling
QDs to antibodies, the most common method reported in the literature utilizes
biotin-avidin interactions [48]. The avidin/streptavidin-coated QDs are commercially
available, while biotin-labeling of antibodies are usually prepared in-house. The
QDs containing free carboxylic acid groups from their capping agents may also be
covalently attached to the epsilon amine of an antibody’s lysine residues by using
EDC/NHS coupling chemistry [47–53]. Instead, simple electrostatic interactions
can be used depending on the overall protein charge at the pH of conjugation [48].
For example, sandwich immunoassays were used by Goldman et al. for the
simultaneous recognition of four toxins in a single microtiter well, as shown in
Fig. 2.9 [47, 48, 53].
In this assay, antibodies immobilized in a microtiter well plate were first subjected to the mixed toxin sample. Antibodies particular for each of the toxins linked
to a different color QD were then put in the microtiter well plate. The subsequent
signal from the mixed toxin samples was then deconvoluted using a simple algorithm. Similarly, QD-antibody bioconjugates were used to detect and distinguish
between diphtheria toxin and tetanus toxin proteins, which were generally immobilized onto poly-L-lysine coated coverslips and for the simultaneous detection of
Escherichia coli O157:H7 and Salmonella typhimurium bacteria using various colored QDs as immunoassay labels [54].
J. H. Banoub and A. Mikhael
2.3.4 Biosensing with Luminescent Semiconductor
Quantum Dots
Recently, luminescent semiconductor nanocrystals or quantum dots (QDs) have
become a successful novel nanomaterial possessing unique photophysical
fluorescent properties, which have helped create a new generation of robust
fluorescent biosensors [47–53]. It should be stated that the fluorescent properties of
QDs have overcome most of the liabilities of conventional organic and proteinbased fluorophores. The biosensing QD properties of interest include high quantum
yields, broad absorption spectra coupled to narrow size-tunable photo-luminescent
emissions, and exceptional resistance to both photo-bleaching and chemical
degradation. In this section, we will investigate the advancement in using QDs for
many invitro biosensing applications, including their use in immunoassays, as
generalized probes [47–53].
2.3.4.1 Immunoassays Using Quantum Dots
The unique advantages of using QDs are owed to their inherent photostability, their
improved sensitivity, and size-tunable photoluminescence coupled to their broad
absorption spectra. These unique advantages have allowed them to serve as multicolor or multiplexed immunoassays. It should be mentioned that in terms of coupling
QDs to antibodies, the most common method reported in the literature utilizes
biotin-avidin interactions [48]. The avidin/streptavidin-coated QDs are commercially
available, while biotin-labeling of antibodies are usually prepared in-house. The
QDs containing free carboxylic acid groups from their capping agents may also be
covalently attached to the epsilon amine of an antibody’s lysine residues by using
EDC/NHS coupling chemistry [47–53]. Instead, simple electrostatic interactions
can be used depending on the overall protein charge at the pH of conjugation [48].
For example, sandwich immunoassays were used by Goldman et al. for the
simultaneous recognition of four toxins in a single microtiter well, as shown in
Fig. 2.9 [47, 48, 53].
In this assay, antibodies immobilized in a microtiter well plate were first subjected to the mixed toxin sample. Antibodies particular for each of the toxins linked
to a different color QD were then put in the microtiter well plate. The subsequent
signal from the mixed toxin samples was then deconvoluted using a simple algorithm. Similarly, QD-antibody bioconjugates were used to detect and distinguish
between diphtheria toxin and tetanus toxin proteins, which were generally immobilized onto poly-L-lysine coated coverslips and for the simultaneous detection of
Escherichia coli O157:H7 and Salmonella typhimurium bacteria using various colored QDs as immunoassay labels [54].
J. H. Banoub and A. Mikhael