Robust, Non‐Cytotoxic, Silica‐Coated CdSe Quantum Dots with Efficient Photoluminescence

Abstract

Near-monodisperse semiconductor quantum dots (QDs) have been synthesized by wet-chemical methods for fluorescent biological labels [1±5] and light-emitting devices.[6±8] Organic capping of QDs with surfactants can provide electron passivation and form a barrier against aggregation of crystallites. Typically, CdSe QDs capped with trioctylphosphine oxide (TOPO) have a quantum yield (QY) of~10 % at room temperature. [9] Coating of CdSe QDs with semiconductors of larger bandgaps (such as ZnS) has been shown to improve the photoluminescence QY to over 60 % by passivating the surface non-radiative recombination sites.[10] While the assynthesized QDs are stable in non-aqueous solution, their photophysical behavior is affected by the use of other solvents, ligands, and environments. Photobrightening and photodarkening may also be caused by the photoionization of QDs. [11] In order to improve the photostability of QDs, they need to be encapsulated within a rigid matrix. Silica is an ideal choice, and it can be applied as a coating using a versatile sol± gel process.[12]Recent advances in synthetic routes with less-toxic precursors (e.g., CdO) have made it possible to produce highly photoluminescent CdSe nanocrystals.[13±16] However, it is a real challenge to make the plain CdSe dots water-soluble while also achieving colloidal stability, photostability, efficient fluorescence, and low non-specific adsorption under aqueous biological conditions. Two main approaches exist in the literature for the design of water-soluble QDs. The first method involves an organic coating, using either polymers, [17,18] micelles, [5] or thiol groups such as mercaptoacetic acid (MAA) [2] and mercaptoundecanoic acid [19] as the linker molecules. The second method is based on the well-known silica chemistry developed for coating metal nanoparticles. [20±23] This strategy has a number of advantages over organic coating of nanoparticles. Silica acts as a robust, inert layer against the degradation of optical properties and imparts water solubility. Silica-coated (and silanized) QDs are very useful for biological applications since they allow for surface conjugation with amines, thiols, and carboxyl groups, which in turn would facilitate the linking of biomolecules such as biotin and avidin. Alivisatos and co-workers [1] first utilized the silanization approach to coat ZnS±CdSe QDs. Although this was carried out in a more-polar methanol solution using 3-mercaptopropyl trimethoxysilane (MPS), the procedure involved numerous steps and appeared to be complex to control. [24] Conversely, Rogach et al. encapsulated water-soluble CdTe QDs in 40±80 nm silica spheres through the Stöber method, but the emission spectrum was broadened with reduced intensity.[25] Using a reverse microemulsion, monodisperse silica particles can be synthesized.[26] Dyes encapsulated within silica shells showed enhanced luminescence and lifetimes due to improved chemical stability and photostability. [27] This communication describes a simple strategy for making plain CdSe QD...

Applications of Nanoparticles in Biology

“…[139,140] The problem of acute toxicity and photo-oxidation can be overcome by capping with a protective shell of insulating material or semiconductor, for example, ZnS-coated CdSe core/shell QDs. [141] As water solubility is key to their applications in imaging, there are a range of methods reported to make the QDs water soluble and biocompatible for biological imaging, such as fabricating the surface with suitable thiolated ligand, [142] over-coating with silica, [143,144] and encapsulating with amine-modified polymer. [145] Likewise, there are a number of strategies for their functionalization.…”

Bioconjugation of quantum dots: Review & impact on future application

“…Cd). In addition, silica is a relatively biocompatible material and its surface can be easy functionalized, which makes QDs@SiO 2 suitable for bioconjugation [15,16]. The third approach is based on encapsulation of the hydrophobic nanoparticles in different carrier vehicles e.g.…”

“…Liposomes, which are spherical lipid vesicles, can also be used for encapsulation. Their hollow spherical structure and high loading capacity makes them attractive carriers for hydrophobic QDs [15,19,25]. It was shown that encapsulation of QDs into liposomes provides transfer of waterinsoluble QD into aqueous media, facilitates their bioconjugation with proteins, minimizes nonspecific interaction of water-soluble and water-insoluble QD with surface material, and amplifies the analytical signal [2].…”

Effects of nanoscale quantum dots in male Chinese loaches (Misgurnus anguillicaudatus): Estrogenic interference action, toxicokinetics and oxidative stress

“…Many researchers demonstrated that QDs' toxicity depends on multiple factors derived from individual QDs' physicochemical properties such as QDs size [7,8] , charge [9] , stability [10] , coating [11,12] , and ingredients [13] . Derfus et al [14] concluded that QDs' toxicity was directly correlated to the release of the heavy metal ion (Cd 2+ ) derived from the oxidation of capping materials.…”