Q. What is the composition of NOM Quantum Dots (QDs)?
A. NOM QDs consist of a semiconductor core, an outer shell of a second, different semiconductor, and a monolayer of surfactant covering the outer surface. The diameter of the core and its composition determines the wavelength of the fluorescent emission of the QD. The cores of NOM QDs consist of either CdSe, CdTe, CdSexTe1-x, or PbS depending on the range of fluorescent wavelengths required (blue to near infrared). All Cd containing QDs have an outer shell of ZnS that enhances the quantum confinement effect and increases the quantum yield (brightness) of the QD emission. PbS QDs have an outer shell of CdS that also enhances fluorescence intensity as well increases wavelength stability upon storage.
The outermost surfactant coating determines the solubility of NOM QDs and often is used to provide reactive functional groups at the terminal end of the surfactant molecule to allow attachment of QDs to other molecules. NOM QDs that are soluble in organic solvents (e.g. toluene, chloroform, hexane) normally have a surfactant coating of trioctylphosphine oxide (TOPO). Other surfactant coatings providing solubility in organic solvents are available upon customer request.
NOM QDs that are hydrophilic and soluble in water as well as other polar solvents (e.g. methanol, ethanol, glycerol) are coated with hydrophilic surfactants that can have reactive terminal groups for attachment to other molecules. Standard NOM hydrophilic surfactant coatings have terminal functional groups that are either, carboxylic acids, primary amines, or diols (two hydroxyl groups on adjacent carbons). The carboxylic acid and primary amine terminated coatings are suitable for allowing attachment of the QDs to other molecules. Other special surfactant coatings are available on a custom synthesis basis.
Q. What are the sizes of NOM QDs?
A. The total diameter (and mass) of NOM QDs is the diameter (mass) of the core nanocrystal plus the thicknesses (and masses) of the shell and surfactant coating. For CdSe QDs the core diameters range from 2.6 nm for a QD emitting at 520 nm to 7.8 nm for a QD emitting at 650 nm. For CdTe QD cores, diameters range from 4.3 nm for a QD emitting at 650 nm to 6.0 nm for a QD emitting at 700 nm. For CdSexTe1-x cores, diameters range from 2.7 nm for QDs emitting at 700 nm to 9.3 nm for QDs emitting at 900 nm. For PbS cores, diameters range from 2.9 nm for QDs emitting at 900 nm to 4.2 nm for QDs emitting at 1200 nm.
For the Cd containing NOM QDs, the ZnS shell adds an additional 1.8 nm to the diameters of the nanoparticles. The surfactant TOPO adds another 4.5 nm to the diameter of NOM QDs while the hydrophilic surfactants add 1 to 5 nm to the diameter. Thus, for example, a CdSe/ZnS TOPO coated NOM QD emitting at 520 nm will have a diameter of 2.6 + 1.8 + 4.5 = 8.9 nm. NOM QDs with thinner or thicker shells can be provided on a custom synthesis basis.
Q. Are other special coatings for QDs available?
A. Yes we work with our customers to apply special coatings to our
QDs for their applications. In many cases thiols are used to attach monolayer coatings to the QDs. Thiol molecules of many different types and structures can be used to meet many needs.
Additionally we can custom synthesize QDs with silica outer shells and attach many other types of molecules to the outside of these shells. Please contact us for special needs.
Q. Can you supply QDs for lighting applications?
A. Yes, NOM QDs can be used to produce colored light, white light and near infrared light when excited by a blue or ultraviolet light source such as a LED array. NOM can blend a mixture of QDs to produce the type of light desired. The QDs can be dispersed in different polymer mixtures including polyacrylates. Please contact us for special needs.
Q. How can one covalently bond QDs to other molecules or nanoparticles?
A. NOM water soluble QDs are available with outer surface coatings that are terminated in either a carboxylic acid, primary amine or a diol. Established chemistries using commercial crosslinking agents are available to achieve covalent attachment of the QDs to other molecules having functional groups including primary amines, carboxylic acids sulfhydryls, and diols. Please contact us for more information on crosslinking agents and chemical procedures.