Biomedical Engineering Reference
In-Depth Information
narrow and symmetric spectral line profi le (the full width half maximum is typically 25-35 nm),
making them ideal for simultaneous detection of multiple fl uorophores by excitation of a single light
source [307,308]. The photoluminescence has a long lifetime (
20-50 ns), which allows imaging of
living cells without interference from background autofl uorescence. QDs overcome several disad-
vantages of fl uorescence dyes, which have intriguing stability against photobleaching [309], large
molar extinction coeffi cients, high quantum yield [310], and large surface-to-volume ratios.
An application of QDs as sensors exploits the fl uorescence resonance energy transfer (FRET).
QDs are promising FRET donors or acceptors. Because of their continuously tunable adsorption
and emission, and high FRET effi ciency which has been well-documented with QDs connected to
various acceptors [311,312], complexes are made by grafting complementary bioconjugates, such as
antibody-antigen pairs.
Goldman et al. [313] used CdSe/ZnS core or shell QDs functionalized with antibodies to per-
form multiplexed fl uoroimmunoassays for simultaneously detecting four toxins (Figure 14.8). This
type of sensor could be used for environmental purposes for simultaneously identifying pathogens
(like cholera toxin or ricin) in water.
Besides immunoassays, optical biosensors based on QDs can also be used to detect proteins
(e.g., NADH or GOD) and organic molecules (e.g., AA and cholesterol) that are related to funda-
mental biological processes. As an example, specifi c binding of different proteins was observed
through measurements of FRET between a CdSe-ZnS QD donor, attached to one of the proteins,
and some organic acceptor dyes attached to the other protein under study. In the presence of
specifi c interactions between both proteins, strong enhancement of the acceptor-dye fl uorescence
was observed [312]. In a more fundamental study, conjugation of BSA with luminescent CdTe
NPs (capped with l-cysteine) resulted in a signifi cant increase in the CdTe fl uorescent emission,
attributed to an effi cient resonance energy transfer from the tryptophan moieties of the protein units
to the CdTe NPs acting as acceptors [314].
Magnetic NPs have also been used in sensor applications. They can be prepared in the form of
superparamagnetic magnetite (Fe
3
O
4
), greigite (Fe
3
S
4
), Maghemite (γ-Fe
2
O
3
), and various types
of ferrites (MeO·Fe
2
O
3
, where Me
∼
Ni, Co, Mg, Zn, Mn, etc.) [315]. Since the late 1990s, magne-
toelectronics [316] has emerged as one of the several new platform technologies for biosensor and
=
QD
MBP
PG-zb
IgG
FIGURE 14.8
Cartoon of a mixed-surface QD conjugate. Both PG-zb adapter proteins binding antibodies
through their Fc domain and MBPzb purifi cation tool proteins are shown conjugated to the QD. (From Gold-
man, E.R., Clapp, A.R., Anderson, G.P., Uyeda, H.T., Mauro, J.M., Medintz, I.L., and Mattoussi, H.,
Anal.
Chem
., 76, 684, 2004. With permission.)
