Biomedical Engineering Reference
(IV-VI) (Alivisatos et al. 2005 ; Vanmaekelbergh and Liljeroth 2005 ) . Initially, QDs
made from CdSe are of particular interest because when excited optically, by vary-
ing the QD size and chemical composition, the wavelength of the fl uorescence light
emitted can be tuned to a broad emission spectrum that includes visible light
(400-2,000 nm) (Murray et al. 2000 ; Fu et al. 2005 ). CdSe QDs were produced in
1993 using high-temperature organometallic chemical techniques (Goldstein et al.
1992 ; Peng and Peng 2001 ). This allowed for monodispersed QDs with narrow size
variations (SD < 5%) (Murray et al. 2000 ). Further development in depositing a sur-
face capping layer of ZnS or CdS increased quantum yields from ~10% to ~40-50%
(Chan et al. 2002 ) .
Quantum Dot Optical Properties Expand
Optical detection is perhaps the most widely used modality for detecting and imag-
ing biological systems. Over the past decade, rapidly increasing number of fl uores-
cent dyes and proteins have been developed, resulting in improved ability to image
subcellular processes with higher resolution and enabled major advances in neuro-
science (Miyawaki et al. 2003 ). Fluorescent QDs exhibit unique photophysical
properties that are dramatically different from organic dyes. These properties have
been demonstrated in an increasing number of diverse cellular systems and prepara-
tions, thus expanding the biologist's toolbox of capabilities that have not been pos-
sible with existing techniques.
QD fl uorescence is exceedingly bright and photostable compared to conventional
organic dyes. Brightness can be approximated by the product of quantum yield and
extinction coeffi cient at the excitation wavelength. Although QDs possess quantum
yields that are comparable to that of organic dyes, their molar extinction coeffi cients
(10 5 -10 6 M −1 cm −1 ) far exceed that of organic dyes by ×10-100 ( http://www.qdots.
com/live/render/content.asp?id=84 ; Bruchez et al. 1998 ; Chan et al. 2002 ) . QD
brightness has been calculated to be about ~20 times brighter than that of single
organic dye molecules such as rhodamine (Chan et al. 2002 ). Due to their inorganic
composition, QD fl uorescence is also exceptionally photostable compared to exist-
ing organic fl uorophores (Alivisatos et al. 2005 ). For example, Jaiswal et al. showed
that under continuous 50-mW illumination QDs did not photobleach even after
14 h. In contrast, most organic fl uorophores are subject to fast photobleaching, com-
pletely losing their ability to emit in less than 20 min (Jaiswal and Simon 2004 ) . In
in vitro and in vivo studies, QDs have proven to not undergo any signifi cant bleach-
ing and have enabled monitoring of molecules in live cells for hours, days, weeks,
and months (Jaiswal and Simon 2004 ). Such unprecedented QD brightness and pho-
tostability means that cellular structures can be detected at high sensitivity over
extended periods of time without signifi cant reduction in intensity. Indeed, it has
been reported that a very small number of individual QDs are needed to immunolabel
subcellular receptors and may in fact approach that of one QD per target molecule.