used a layer-by-layer deposition technique to line nanotubes with ZnCdSe QDs, resulting in a graded bandgap structure with three-layers of QDs in a ring structure (Determine 14) [123]

used a layer-by-layer deposition technique to line nanotubes with ZnCdSe QDs, resulting in a graded bandgap structure with three-layers of QDs in a ring structure (Determine 14) [123]. in the application at hand [15]. This caution applies doubly in the case of QD-QD FRET, where both the donor and acceptor moieties are emissive semiconductor nanoparticles, as it is necessary that the BRL-50481 advantages of this fluorophore choice overcome some of the inherent limitations in QD-QD FRET devices. A primary advantage of QDs in QD-QD FRET sensing is usually their extreme photostability compared to organic dyes or fluorescent proteins, making QDs uniquely suited for longitudinal studies, where measurements or images are taken repeatedly over extended periods of time. In addition, the extraordinary brightness of QDs, due primarily to their large absorption cross-section, yields considerable fluorescence output with relatively fewer emitters, potentially lowering the limit of detection in sensing applications. However, QD-QD FRET is usually challenging due to the broad, overlapping excitation spectra from the two nanocrystals, precluding selective excitation of the donor. This introduces crosstalk and artificially creates a large background signala major challenge in QD-QD FRET sensor design. This review provides an overview of the foundational work on QD-QD FRET and discusses published applications. The challenges of achieving efficient QD-QD FRET and how they might be overcome are briefly discussed as well. 2. Background 2.1. Semiconductor Nanocrystal Quantum Dots Semiconductor quantum dots (QDs) are crystalline nanoparticles often composed of group IICVI or IIICV elements from the periodic table with diameters smaller than their exciton Bohr radius [35], typically just a few nanometers. Quantum confinement effects present at this size range give rise to unique optical and electronic properties that are not present in the bulk materials. As the size of the single-crystalline nanoparticle decreases below the exciton Bohr radius of that particular semiconductor, the bandgapthe energy difference between the highest energy valence band and lowest energy conduction bandincreases in energy. Since the size of the bandgap dictates the emission energy, the quantum confinement effect ultimately leads to the size-tunable emission of QDs (Physique 1). In addition to size, the bandgap of nanocrystals also depends on its chemical composition. For example, the bulk bandgap of CdSe is usually 1.74 eV (712 nm) while that of PbS is 0.37 eV (3350 nm). Nanoparticles of CdSe and PbS with decreasing size exhibit increasing bandgaps, which can reach ~3.6 eV (350 nm) [36] and 1.3 eV (950 nm) [15], respectively. CdSe particles with diameters ranging from 2 to 6 nm thus emit photons with energies spanning the visible wavelength range, Rabbit Polyclonal to GPR18 while PbS QDs emit in BRL-50481 the near infrared (NIR). QDs most commonly used in the visible wavelength range are CdSe/ZnS core/shell nanoparticles; the CdSe core confers the particle its unique optical properties, while the ZnS shell serves as a passivation layer, protecting the core from oxidation and enhancing the quantum yield (QY) [19,37]. Open in a separate window Physique 1 (a) CdSe/ZnS core/shell QDs with CdSe core diameters ranging from 6.9 nm to 1 1.8 nm in diameter emitting with peaks from 1.9C2.8 eV (655C443 nm) from left to right under UV illumination. Adapted with permission from [20]; (b) Bandgap energy increases as the nanocrystal size decreases. Reprinted with permission from [38]; (c) Absorption (top) and emission (bottom) spectra of CdSe quantum dots. Reprinted with permission from [39]. Copyright (2010) American Chemical Society. Colloidal core/shell QDs are manufactured in answer through step-wise injections of organometallic precursors at high temperature. Nucleation and growth are thermally activated and growth continues until arrested BRL-50481 by cooling [19]..

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