Suitable labels are at the core of Luminescence and fluorescence imaging and sensing. One of the most exciting, yet also controversial, advances in label technology is the emerging development of quantum dots (QDs)--inorganic nanocrystals with unique optical and chemical properties but complicated surface chemistry--as in vitro and in vivo fluorophores. Here we compare and evaluate the differences in physicochemical properties of common fluorescent labels, focusing on traditional organic dyes and QDs. Our aim is to provide a better understanding of the advantages and limitations of both classes of chromophores, to facilitate label choice and to address future challenges in the rational design and manipulation of QD labels.

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Quantum dots versus organic dyes as fluorescent labels

Colloidal nanocrystals are solution-grown, nanometre-sized, inorganic particles that are stabilized by a layer of surfactants attached to their surface. The inorganic cores possess useful properties that are controlled by their composition, size and shape, and the surfactant coating ensures that these structures are easy to fabricate and process further into more complex structures. This combination of features makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. Chemists are achieving ever more exquisite control over the composition, size, shape, crystal structure and surface properties of nanocrystals, thus setting the stage for fully exploiting the potential of these remarkable materials.

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Colloidal nanocrystal synthesis and the organic–inorganic interface

Colloidal nanocrystals are nanometer-sized, solution-grown inorganic particles stabilized by a layer of surfactants attached to their surface. The inorganic cores exhibit useful properties controlled by composition as well as size and shape, while the surfactant coating ensures that these structures are easy to fabricate and process. It is this combination of features that makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. But their full potential can only be exploited if we achieve exquisite control over their composition, size, shape, crystal structure and surface properties. Here we review what is known about nanocrystal growth and outline strategies for controlling it.

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Colloidal nanocrystal synthesis and the organic-inorganic interface - eScholarship

Electrically driven semiconductor lasers are used in technologies ranging from telecommunications and information storage to medical diagnostics and therapeutics. The success of this class of lasers is due in part to well-developed planar semiconductor growth and processing, which enables reproducible fabrication of integrated, electrically driven devices. Yet this approach to device fabrication is also costly and difficult to integrate directly with other technologies such as silicon microelectronics. To overcome these issues for future applications, there has been considerable interest in using organic molecules, polymers, and inorganic nanostructures for lasers, because these materials can be fashioned into devices by chemical processing. Indeed, amplified stimulated emission and lasing have been reported for optically pumped organic systems and, more recently, inorganic nanocrystals and nanowires. However, electrically driven lasing, which is required in most applications, has met with several difficulties in organic systems, and has not been addressed for assembled nanocrystals or nanowires. Here we investigate the feasibility of achieving electrically driven lasing from individual nanowires. Optical and electrical measurements made on single-crystal cadmium sulphide nanowires show that these structures can function as Fabry-Perot optical cavities with mode spacing inversely related to the nanowire length. Investigations of optical and electrical pumping further indicate a threshold for lasing as characterized by optical modes with instrument-limited linewidths. Electrically driven nanowire lasers, which might be assembled in arrays capable of emitting a wide range of colours, could improve existing applications and suggest new opportunities.

Single-nanowire electrically driven lasers

The use of semiconductor nanocrystals (quantum dots) as fluorescent labels for multiphoton microscopy enables multicolor imaging in demanding biological environments such as living tissue. We characterized water-soluble cadmium selenide-zinc sulfide quantum dots for multiphoton imaging in live animals. These fluorescent probes have two-photon action cross sections as high as 47,000 Goeppert-Mayer units, by far the largest of any label used in multiphoton microscopy. We visualized quantum dots dynamically through the skin of living mice, in capillaries hundreds of micrometers deep. We found no evidence of blinking (fluorescence intermittency) in solution on nanosecond to millisecond time scales.

https://science.sciencemag.org/content/sci/300/5624/1434.full.pdf

Water-Soluble Quantum Dots for Multiphoton Fluorescence Imaging in Vivo

For transient ellipsometry, the sample is placed between crossed polarizers at an angle of 54 between the sample normal and the laser beam. A laser diode at a wavelength k = 905 nm is employed and the residual birefringence of the sam- ple is compensated by a crystal compensator. The transmission through the el- lipsometry setup is measured in response to a step function of the electric field applied to the sample with a rise time of ~100 ls. The two-wave mixing is measured in a standard geometry (29) with the two beams having external angles of 30 and 60 to the sample normal and beam ra- tio 1:1. The laser source is a Kr ion laser at a wavelength of 647 nm. The time transients are measured by opening a shutter for both beams with a switching time of about 150 ls. After the measurements, the gratings are erased by a larg- er, non-Bragg matched erasing beam. In the beam fanning experiments, the same setup as in the two-wave mixing is used except only one beam is present. The angle of the beam to the sample normal is 60 and the applied electric field is inverted compared to two-wave mixing to enhance the fanning. Additionally, an aperture is placed in the beam path 50 cm behind the sample, which clips only 5 % of the unperturbed beam.

Lasing from Semiconductor Quantum Rods in a Cylindrical Microcavity

This study evaluates the influence of particle size, PEGylation, and surface coating on the quantitative biodistribution of near-infrared-emitting quantum dots (QDs) in mice. Polymer- or peptide-coated 64Cu-labeled QDs 2 or 12 nm in diameter, with or without polyethylene glycol (PEG) of molecular weight 2000, are studied by serial micropositron emission tomography imaging and region-of-interest analysis, as well as transmission electron microscopy and inductively coupled plasma mass spectrometry. PEGylation and peptide coating slow QD uptake into the organs of the reticuloendothelial system (RES), liver and spleen, by a factor of 6-9 and 2-3, respectively. Small particles are in part renally excreted. Peptide-coated particles are cleared from liver faster than physical decay alone would suggest. Renal excretion of small QDs and slowing of RES clearance by PEGylation or peptide surface coating are encouraging steps toward the use of modified QDs for imaging living subjects.

Particle Size, Surface Coating, and PEGylation Influence the Biodistribution of Quantum Dots in Living Mice

Correlated atomic-force and fluorescence microscopy are used to study single-particle versus ensemble fluorescence quantum yields (QY) of semiconductor nanocrystals by measuring a simultaneous map of the topography and the single-particle fluorescence. CdSe/ZnS nanocrystal quantum dots and quantum rods with high QY were investigated. A significant portion of dark particles is detected. Comparison with the ensemble solution QY shows that samples with higher QY have a larger fraction of bright particles accompanied by an increased single-particle QY. Saturated emission from single nanocrystals could not be detected because of particle darkening under high-power excitation.

Fluorescence quantum yield of CdSe/ZnS nanocrystals investigated by correlated atomic-force and single-particle fluorescence microscopy

The cloning of long DNA segments, especially those containing large gene clusters, is of particular importance to synthetic and chemical biology efforts for engineering organisms While cloning has been a defining tool in molecular biology, the cloning of long genome segments has been challenging Here we describe a technique that allows the targeted cloning of near-arbitrary, long bacterial genomic sequences of up to 100 kb to be accomplished in a single step The target genome segment is excised from bacterial chromosomes in vitro by the RNA-guided Cas9 nuclease at two designated loci, and ligated to the cloning vector by Gibson assembly This technique can be an effective molecular tool for the targeted cloning of large gene clusters that are often expensive to synthesize by gene synthesis or difficult to obtain directly by traditional PCR and restriction-enzyme-based methods

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Cas9-Assisted Targeting of CHromosome segments CATCH enables one-step targeted cloning of large gene clusters

Nanotechnology is poised to transform research, prevention, and treatment of cancer through the development of novel diagnostic imaging methods and targeted therapies. In particular, the use of nanoparticles for imaging has gained considerable momentum in recent years. This review focuses on the growing contribution of quantum dots (QDs) for in vivo imaging in small-animal models. Fluorescent QDs, which are small nanocrystals (1-10 nm) made of inorganic semiconductor materials, possess several unique optical properties best suited for in vivo imaging. Because of quantum confinement effects, the emission color of QDs can be precisely tuned by size from the ultraviolet to the near-infrared. QDs are extremely bright and photostable. They are also characterized by a wide absorption band and a narrow emission band, which makes them ideal for multiplexing. Finally, the large surface area of QDs permits the assembly of various contrast agents to design multimodality imaging probes. To date, biocompatible QD conjugates have been used successfully for sentinel lymph node mapping, tumor targeting, tumor angiogenesis imaging, and metastatic cell tracking. Here we consider these novel breakthroughs in light of their potential clinical applications and discuss how QDs might offer a suitable platform to unite disparate imaging modalities and provide information along a continuum of length scales.

https://jnm.snmjournals.org/content/jnumed/50/4/493.full.pdf

Yuval Ebenstein

Papers

Lasing from Semiconductor Quantum Rods in a Cylindrical Microcavity

Particle Size, Surface Coating, and PEGylation Influence the Biodistribution of Quantum Dots in Living Mice

Fluorescence quantum yield of CdSe/ZnS nanocrystals investigated by correlated atomic-force and single-particle fluorescence microscopy

Cas9-Assisted Targeting of CHromosome segments CATCH enables one-step targeted cloning of large gene clusters

Quantum Dots for In Vivo Small-Animal Imaging