Editorial pubs.acs.org/JPCL
Semiconductor Nanostructures for Energy and Biomedical Applications
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room temperature, and are less likely to photobleach than typical dye molecules, resulting in high sensitivity and stability ideally suited for biosensing and bioimaging applications. Application examples of single QD imaging discussed by Chang and Rosenthal include mapping receptor membrane diffusion, monitoring endosomal trafficking and endocytosis, measuring dynamic processes of intracellular targets, and biosensing based on fluorescence resonance energy transfer (FRET). In addition, 3D single QD tracking in live cells has been demonstrated and showed clearly an advantage over 2D tracking by providing additional information about diffusion and transport characteristics. It is worth mentioning that besides sensing and imaging applications, QDs have also been investigated for therapeutical applications, for example, in treatment of cancers. In summary, while semiconductor QDs have shown encouraging success in various applications including solar energy conversion and biomedical imaging, further work is still needed to address issues of long-term stability, reproducibility, synthesis on a large scale, and rational control of the structure on the atomic level. There is also a need to better understand the fundamental interaction and intricate interface between QDs and other systems such as organic or biological systems. Advances in sophisticated characterization techniques with high spatial, temporal, and energy resolution should help to address these issues. High-level theoretical and computational work needs to be developed to corroborate and guide experimental research efforts. The growing need for sustainable solutions in the energy and biomedical fronts will demand more advanced and complex materials including QDs and their composites or heteostructures with improved properties and functionalities.
emiconductor nanostructures or quantum dots (QDs) possess unique and useful chemical and physical properties that are promising for technological applications ranging from energy conversion to chemical sensing and biomedical imaging and therapy. The quantum confinement effect allows the optical properties of QDs to be varied with size and, to a lesser degree, shape. The extremely large surface-to-volume ratio leads to the opportunity of surface functionalization. Recent advances in synthesis and fabrication techniques make it possible to create complex, multicomponent or multicomposition nanostructures with diverse and enhanced functionalities for various applications of interest. Two of the most exciting emerging applications are solar energy conversion and biomedical detection/imaging. In solar energy conversion, QDs can be used as active light absorbers with tunable absorption for converting light into electricity in solar cells or into chemical fuel such as hydrogen in photoelectrochemical cells. In one Perspective of this issue, Teranishi and Sakamoto provide an overview of recent activities in the study of semiconductor nanostructures, particularly heteojunctions with epitaxial interfaces and proper band alignment, that have desired electronic and optical properties for charge generation and separation in solar energy conversion applications. Examples include type-II semiconductor heterodimers composed of chalcogenide−chalcogenide blends, such as CdS−Cu2−xS (0 ≤ x ≤ 0.0625) and CdS−CdTe, in which CdS is used as an n-type semiconductor while both Cu2−xS and CdTe are p-type semiconductors for light absorption in solar cells. Such heterodimers with a staggered alignment of band edges at the heterointerface can be synthesized by seeded growth or ion exchange to promote the spatial charge separation between electrons and holes in different parts of the heterostructures. The type-II band-edge alignment between the two constituting components allows for the realization of spatial charge carrier separation in the heterodimer in which photogenerated electrons and holes are separately confined within each component, respectively. This is highly desirable for subsequent charge carrier collection in solar cells or for driving redox reactions in photoelectrochemical devices. In biomedical applications, QDs can be used for detection, imaging, and therapeutic purposes. In a second Perspective by Chang and Rosenthal, applications of QDs in exploring membrane dynamics and intracellular trafficking are reviewed, with emphasis on single QD techniques that can reveal single protein/vehicle dynamics in real time. One of the principal advantages of using single QDs is the high signal-to-noise (S/ N) ratio originating from their extraordinarily high molar extinction coefficients and large effective Stokes shifts. The basic principle of single QD imaging techniques is based on detection of photoluminescence (PL) from the QD conjugated or linked to biological systems of interest. By monitoring the PL in space and time, one can determine the structural and dynamic properties of the biological systems. QDs with proper surface capping can have very high PL yield, as high as 50% at © 2013 American Chemical Society
Jin Z. Zhang, Senior Editor
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University of California, Santa Cruz, California
AUTHOR INFORMATION
Notes
Views expressed in this Editorial are those of the author and not necessarily the views of the ACS.
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RELATED READINGS
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