Photoluminescent Quantum Dots
“Semiconductor nanocrystalline quantum dots (QDs) with bioconjugates on the surface have been studied extensively because of their unique optical properties. QDs are inorganic nanoparticles that emit light at a specific wavelength when excited by light. When light shines on QDs, electrons in the valence band are excited to the conduction band, forming short-lived (nanoseconds) electron-hole pairs called excitons, which emit photons of a specific wavelength when the electron-hole pairs eventually recombine. The excitonic emission is independent of the wavelength of the excitation light. This makes it easier to excite QDs to luminescence than the traditional fluorescent molecules that require a specific excitation wavelength. The wavelength of the emitted photons of QDs, on the other hand, is specific and can be controlled by the QDs' particle size and composition. The synthesis of QDs was developed mostly in the 1990's. In the last few years, the interest in using QDs in biomedical imaging has exploded due to advances in surface modification of QDs that have made them accessible for antibody immobilization and detection of antibody-antigen binding.” (Shih et al, US Patent 7,824,653; 11/2/2010)
Recent US Patents
12/28/201
7,858,386
Method of controlling quantum dot photoluminescence and other intrinsic properties through biological specificity
Medinitz of the US Navy is able to control quantum dot photoluminescence by a protein, such as a polyhistidine metal complex or horse radish peroxidase. (RDC 7/7/20110
11/2/2010
7,824,653
Water soluble quantum dots
Shih et al of Drexel University, Pennsylvania produced water soluble quantum dots that are ready for bioconjugation. The method can produce aqueous QDs with emission wavelengths varying from 400 nm to 700 nm. Highly luminescent metal sulfide (MS) QDs are produced via an aqueous synthesis route. MS QDs are capped with thiol-containing charged molecules in a single step. The resultant MS QDs exhibit the distinctive excitonic photoluminescence desired of QDs and can be fabricated to avoid undesirable broadband emissions at higher wavelengths. The aqueous QDs are stable in biological fluids over a long period of time. In addition, nontoxic ZnS QDs have been produced with good photoluminescence properties by refluxing the ZnS QD suspensions over a period of time. (RDC 2/23/2011)
Recent Journal Articles
Two-Photon 3D FIONA of Individual Quantum Dots in an Aqueous Environment
(4074–4078) Nano Letters 11 #10 (2011)
Zhang et al , Illinois, United Kingdom, Texas and Massachusetts, report the first two-photon (2P) microscopy of individual quantum dots (QDs) in an aqueous environment with both widefield and point-scan excitations at nanometer accuracy. Thiol-containing reductants suppress QD blinking and enable measurement of the 36 nm step size of individual Myosin V motors in vitro. We localize QDs with an accuracy of 2–3 nm in all three dimensions by using a 9 × 9 matrix excitation hologram and an array detector, which also increases the 3D scan imaging rate by 80-fold. With this 3D microscopy we validate the LamB receptor distribution on E. coli and the endocytosis of EGF-receptors in breast cancer cells. (RDC 10/13/2011)
Spectral Properties of Multiply Charged Semiconductor Quantum Dots
(4425–4430) Nano Letters 11 #10 (2011)
Yalcin et al of the University of Massachusetts, Massachusetts, used electrostatic force microscopy (EFM) on the electrospray QD samples to show a subpopulation of charged QDs with 4.7 ± 0.7 excess electrons, as well as a significant fraction of uncharged QDs as evidenced by the distinct cantilever response under bias. The blue-shifted peak recombination energy can be understood as a first-order electronic perturbation that affects the band-edge electron- and hole-states differently. Spectrally resolved fluorescence imaging of single CdSe/ZnS quantum dots (QDs), charged by electrospray deposition under negative bias has revealed a surprising net blue shift (60 meV peak-to-peak) in the distribution of center frequencies in QD band-edge luminescence. (RDC 10/13/2011)
Photo-induced suppression of plasmonic emission enhancement of CdSe/ZnS quantum dots
(# 405202 ) Nanotechnology 22 #40 (2011)
Sadeghi and West and Nejat showed that when thin films of colloidal quantum dots are placed in the vicinity of Au nano-islands, the plasmonic enhancement of the radiative decay rates of quantum dots and Forster energy transfer can hinder the photo-induced fluorescence enhancement of these films. This in turn leads to significant suppression of their plasmonic emission enhancement when they are irradiated with a laser beam. (RDC 9/7/2011)
Dynamic analysis of the photoenhancement process of colloidal quantum dots with different surface modifications
(#385703) 385703
Llopis et al of the Universidad de Oviedo, Spain, critically compared the behavior of CdSe QDs (widely used in bioanalytical applications) with different surface modifications (ligand exchange and polymer coating), in different controlled experimental conditions, in the presence–absence of the ZnS layer and in different media when exposed for long times to intense UV irradiation. Thus six different types of colloidal QDs were finally studied. The results showed a different behavior of the studied colloidal QDs after UV irradiation in terms of their photoluminescence characteristics, potential toxicity due to metal release to the environment, nanoparticle stability and surface coating degradation. (RDC 9/2/2011)
Quantum Dot/Plasmonic Nanoparticle Metachromophores with Quantum Yields That Vary with Excitation Wavelength
(2725–2730) Nano Letters 11 #7 (2011)
Munechika et al of the University of Washington, Washington, showed that CdSe quantum dots (QDs) near single silver nanoprisms can exhibit photoluminescence lifetimes and quantum yields that depend on the excitation wavelength, in apparent violation of the Kasha–Vavilov rule. We attribute the variation in QD lifetime with excitation wavelength to the wavelength-dependent coupling of higher-order plasmon modes to different spatial subpopulations of nearby QDs. At the QD emission wavelength, these subpopulations are coupled to far-field radiation with varying efficiency by the nanoprism dipolar resonance. These results offer an easily accessible new route to design metachromophores with tailored optical properties. (RDC 7/20/2011)