Gradient alloy quantum dots (GA-Qdots) are a class of novel fluorescent Qdots with superior luminescence properties over “traditional” cadmium based Qdots. “Traditional” Qdots products based on chalcogen sulfide on the market have size-dependent fluorescence properties, with larger size Qdots having longer wavelength emissions. Thus, Qdots with smaller sizes have a lower extinction coefficient, resulting in smaller fluorescence light output for smaller Qdots. For example, the most commonly used CdSe Qdots emit colors from green to red when the particles’ diameters range from ~2 nm to ~6 nm. A species labeled with 6 nm CdSe Qdots will emit red light that is ~30 times stronger than the green light from a species that is labeled with 2 nm Qdots, given that the 2 nm and 6 nm Qdots have the same photoluminescence quantum efficiencies. Secondly, the quantum efficiency of these Qdots is lower in the range of light from green to deep red. Photoluminescence quantum efficiency of CdSe Qdots with the emission below ~530 nm or above ~630 nm is significantly lower than that of Qdots emitting in other visible wavelength range. In addition, the photoluminescence from individual “traditional” Qdot exhibits intermittent “blinking” behavior. The existence of photoluminescence ‘off’ periods greatly limits the number of photons that can be detected in a given time period and also makes the photon arrival times from a single QD highly unpredictable.
Figure 1. of the gradient core/shell/shell structure of GA-Qdots developed in Mesolight
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Figure 2.GA-Qdots with same overall size but distinct photoluminescence (PL) emissions covering the range from ultraviolet to near-infrared.
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Addressing the limitation of “traditional” Qdots, Mesolight dedicates to develop GA-Qdots (Figure 1) as next generation Qdots. As shown in Figure 2, GA-Qdots have the same overall size of 7 nm. Instead of tuning emitting wavelength through the “traditional” method of controlling the Qdot size, GA-Qdots emit different colors by tuning their composition in the core and in the gradient layer instead. This brings a number of unique advantages for GA-Qdots as fluorescent labels:
This feature is not available to binary Qdots. This allows for the tuning of optical/electronic properties of Qdots by composition instead of solely relying on particle size. This makes preparation of alloyed Qdots with the approximately the same particle sizes (hence the same absorption and emission intensity) but different emission wavelength possible. Subsequently, this will enable the measurement of each of the species with the equal sensitivity.
Narrow emission. Due to the quantum confinement effect (QCE), semiconductor nanocrystals show strong size-dependent properties when their sizes are significantly smaller than the exciton Bohr radius (RB). Consequently, the finite size distribution of semiconductor nanocrystals results the inhomogeneous emission broadening. When the particle size is close to or somewhat larger than the Bohr radius, the QCE is relatively week, and accordingly the size distribution has less effect on the emission broadening. For a similar size range, QCE is more pronounced in materials with smaller band gap energy and larger Bohr radius. It’s been revealed out that alloyed CdSe1-xTex nanocrystals process a very strong nonlinear relation between their band gap energy and composition. This nonlinear relation would allow the synthesis of alloyed Qdots with the same emission wavelength as their binary counterpart but with smaller Bohr radius, and consequently narrower emission.
High stability. Due to their size-dependent absorption and PL emission that are tunable across the visible range, CdSe nanocrystals have been extensively investigated in the past ten years. The best PL efficiency reported for CdSe core-shell nanocrystals can reach over 50% in the wavelength window of around 570 nm, but the efficiency for the blue and deep red spectral range is still low (>20%). In comparison, GA-Qdots possess higher (PL efficiency up to 70-85% after transferred into water) and more stable PL emission due to larger particle size, higher crystalline, hardened lattice structure, lower inter-diffusion, and spatial compositional fluctuation.
Figure 3.Time dependent photoluminescence intensity traces from a single GA-Qdot synthesized in Mesolight showing the particle is at “on” state for above 90% of the scanned time period.
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No-blinking. Qdots with an alloyed composition gradient from the core to the surface do not blink but rather remain continuously ‘on’. As shown in Figure 3, GA-Qdots show greatly reduced blinking phenomenon, with 90% of time in “on” state. This is especially important for their applications in single molecular fluorescence spectroscopy studies as well as for applications for which a fast fluorescence detection timing window is required.
Figure 4.Photoluminescence emission spectra of GA-Qdots products in Mesolight
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Figure 5. GA-Qdots products in Mesolight (left) with emissions in 490-655 nm dispersed in 50 mM borate buffer (pH=9.0) and –COOH functionalized; (right) Gel electrophoresis (real image) of Qdots products from Mesolight and a reference sample.
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Figure 6. Chromaticity coordinate values of GA-Qdots products in Mesolight.
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Current GA-products in Mesolight have emissions in 490-655 nm range (Figure 4), with narrow emission bandwidth (full-width-at-half-maximum less than 30 nm), deep emission colors (Figure 6), high brightness, high quantum yield (50%-110% with reference to organic dyes), high dispersity, and high purity. Products are dispersed in either organic solvent, or dispersed in aqueous buffer solution (Figure 5) after surface ligands modification with –COOH or –NH2 and other functional groups for future conjugations.
