ABSTRACT
Quantum dots (QDs), tiny light-emitting particles on the nanometer scale, are rapidly emerging as a
new class of fluorescent probes for biomolecular and cellular imaging. In comparison with organic dyes
and fluorescent proteins, quantum dots have unique optical and electronic properties such as size-
tunable light emission, improved signal brightness, resistance against photobleaching, and simultaneous
excitation of multiple fluorescence colors. These properties are most promising for improving the
sensitivity of molecular imaging and quantitative cellular analysis by 1-2 orders of magnitude. Recent
advances have led to multifunctional nanoparticle probes that are highly bright and stable under
complex in-vivo conditions. As illustrated in Figure 10.1, the novel properties of QDs arise from
quantum size confinement, which was first reported by Ekimov and Onushchenko in 1982 when they
observed sharp and discreet absorption peaks in CuCl nanocrystals embedded in a transparent insulat-
ing matrix [1]. A theoretical framework was presented in the same year [2]. About 10 years later,
procedures for synthesis of high-quality CdSe QDs dispersed in organic solvents were developed by
Murray et al. [3]. However, it was not until 1998 that QDs entered their new role as fluorescent probes
when two groups simultaneously reported procedures for making QDs water soluble and conjugating
them to biomolecules [4,5]. Following these initial reports, extensive research has been directed towards
developing QDs for use in biodetection and bioimaging. In particular, high-quality QDs have been made
water-soluble by coating them with amphiphilic polymers [6]. Water-soluble QDs have also been linked
to small proteins [7,8], peptides [9], nucleic acids [10], carbohydrates [11,12], polymers [9], and small
molecules [13]. Using bioconjugated QDs as fluorescent probes, recent research has achieved real-time
imaging of single cell surface receptors [14] and noninvasive detection of small tumors in live animal
models [15]. In this chapter, we present a brief overview of research in QDs and their biological
applications. Beginning with the novel properties of QDs, we discuss their synthesis, functionalization,
and applications in biology and medicine.
