Semiconducting Quantum Dots for Bioimaging

INTRODUCTION There are several noninvasive imaging techniques available for molecular imaging purposes, such as fluorescence imaging, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), ultrasonography, and many more (1). Across the electromagnetic spectrum, these techniques span from ultrasound to X-rays to gamma rays. Currently, MRI, optical imaging, and nuclear imaging are emerging as the key molecular imaging techniques (1). They differ in terms of sensitivity, resolution, complexity, acquisition time, and operational cost. However, these techniques are complementary to each other most of the time. There are several reviews on the physical basis of these techniques (1,2), instrumentation (3,4), and issues that affect their performance (5,6). Currently, a significant amount of research is aimed at using the unique optical properties of quantum dots (Qdots) in biological imaging. Much of optical bioimaging is based on traditional dyes (7,8), but there are several drawbacks associated with their use. It is well known that cell autofluorescence in the visible spectrum (9) leads to the following five effects: (i) The autofluorescence can mask signals from labeled organic dye molecules. (ii) Instability of organic dye under photoirradiation is well known in bioimaging, which results in only short observation times. (iii) In general, conventional dye molecules have a narrow excitation window, which makes simultaneous excitation of multiple dyes difficult. (iv) Dyes are sensitive to the environmental conditions, such as variation in pH. (v) Most of the organic dyes have a broad emission spectrum with a long tail at red wavelengths, which creates spectral crosstalk between different detection channels and makes it difficult to quantitate the amounts of different probes. Qdots, on the other hand, are of interest in biology for several reasons, including (i) higher extinction coefficients, (ii) higher quantum yields (QYs), (iii) less photobleaching, (iv) absorbance and emissions can be tuned with size, (v) generally broad excitation windows but narrow emission peaks, (vi) multiple Qdots can be used in the same assay with minimal interference with each other, (vii) toxicity may be less than conventional organic dyes, and (viii) the Qdots may be functionalized with different bioactive agents. In addition, near infrared (NIR) emitting Qdots can be used to avoid interference from the autofluorescence, because cell, hemoglobin, and water have lower absorption coefficient and scattering effects in the NIR region (650-900) (Fig. 1). Light is routinely used for intravital microscopy, but imaging of deeper tissue (500 m-1 cm) requires the use of NIR light (10). Inorganic Qdots are more photostable under ultraviolet excitation than organic molecules, and their fluorescence is more saturated. In general, as-synthesized Qdots are very hydrophobic. Qdots have been synthesized by different bottom-up chemical methods, such as0.10.01A bsor ptio nco effic ient (cm1 )500 600 700 Wavelength (nm)800 900Near IR windowH2OHbO2Hbsol-gel (11,12), microemulsion (13,14), competitative reaction chemistry (15,16), hot solution decomposition method (17,18), microwave irradiation process (19,20), and hydrothermal synthesis procedure (21,22). For the production of highly crystalline, monodispersed Qdots, the hot solution decomposition method is the best method known to date. To convert Qdots from hydrophobic to hydrophilic, a silica shell is generally grown on the Qdots. Growth of silica shell can be achieved by microemulsion and/or sol-gel methods. Several review articles and book chapters (23-27) can be found with elaborate discussions on Qdots. Hence, the properties of Qdots are briefly overviewed in the following section.