Biomedical Engineering Reference
In-Depth Information
dissociated neural cells and cytoarchitecturally intact neural retinal tissue sections
(Pathak et al. 2006, 2009 ), illustrates the use of such protocols and methods, and
discusses the direct quantitative estimation of the number of functionally available
antibodies conjugated to quantum dot nanocrystals for applications to biological
labeling (Pathak et al. 2007 ). The interested reader is referred to other review arti-
cles by the author and colleagues that discuss applications of nanotechnologies to
neuroscience (Provenzale and Silva 2009 ; Silva 2004, 2005, 2006, 2007, 2008a, b ;
Yu and Silva 2008 ) .
Semiconductor Quantum Dots and Their Utility
for Neurobiological Imaging
Semiconductor fl uorescent quantum dots are nanometer-sized, functionalized par-
ticles that display unique physical properties that make them particularly well-suited
for visualizing and tracking molecular processes in cells using standard fl uores-
cence (Biju et al. 2008 ; Gao 2003 ; Jaiswal et al. 2004 ; Wu et al. 2003 ) . They are
readily excitable and have broad absorption spectra with very narrow emission
spectra, allowing multiplexing of many different colored quantum dots; they display
minimal photobleaching, thereby allowing molecular tracking over prolonged peri-
ods; they also display a blinking property that allows the identifi cation of individual
quantum dots. As a result, single-molecule binding events can be identifi ed and
tracked using optical fl uorescence microscopy, allowing the pursuit of experiments
that are diffi cult or not possible given other experimental approaches.
Quantum dots are nanometer-sized particles composed of a heavy metal core,
such as cadmium selenium or cadmium telluride, with an intermediate unreactive
zinc sulfi de shell and a customized outer coating of different bioactive molecules
tailored to a specifi c application (Fig. 1 ). The composition and very small size of
quantum dots (5-8 nm) gives them unique and very stable fl uorescent optical prop-
erties that are readily tunable by changing their physical composition or size. The
photochemical properties of quantum dots allow selective fl uorescent tagging of
proteins similar to classical immunocytochemistry (ICC). However, the use of quan-
tum dots is associated with minimal photobleaching and a much higher signal-to-
noise ratio. Their broad absorption spectra but very narrow emission spectra allows
multiplexing of many quantum dots of different colors in the same sample, some-
thing which cannot be achieved with traditional fl uorophores. The physics respon-
sible for these effects are beyond the scope of this brief introduction, but the small
size of quantum dot particles results in large but specifi c energy jumps between the
energy band gaps of excited electron-hole pairs in the semiconductor core. This
effect results in scaled changes of absorption and emission wavelengths as a func-
tion of particle size so that small changes in the radius of quantum dots translate into
very distinct changes in color. This physical property represents another major
advantage over traditional organic fl uorophores that, in general, require distinct
chemistries to produce different colors. For biological applications, quantum dots
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