Quantum dots (QDs) and magnetic nanoparticles (MPs) are of interest for biological imaging, drug targeting, and bioconjugation because of their unique optoelectronic and magnetic properties, respectively. To provide for water solubility and biocompatibility, QDs and MPs were encapsulated within a silica shell using a reverse microemulsion synthesis. The resulting SiO2/MP-QD nanocomposite particles present a unique combination of magnetic and optical properties. Their nonporous silica shell allows them to be surface modified for bioconjugation in various biomedical applications.

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...

Bifunctional nanocomposites comprising semiconductor and magnetic nanoparticles are of interest in biological applications for biolabeling, bioimaging, and cell separation. To this end, a nanocomposite system that consists of Fe2O3 magnetic nanoparticles and CdSe quantum dots and which exhibits superparamagnetism and tunable emission properties was prepared and used to label different live cell membranes (see picture).

Herein, we describe the synthesis of functional and multifunctional nanoparticles (NPs), derived from our recent work, for bioimaging and biosensing applications. The functionalized NPs involve quantum dots (QDs), magnetic particles (MPs) and noble metal NPs for the aforementioned applications. A diverse silica coating approaches (reverse microemulsion and thin silanization) are delineated for the design of water-soluble NPs. We also review the synthesis of silica-coated bifunctional NPs consisting of MPs and QDs for live cell imaging of human liver cancer cells (HepG2) and mouse fibroblast cells (NIH-3T3). Using silica coated NPs, various NPs that are functionalized with antibody, oligonucleotide, biotin and dextran are efficiently used for protein detection.

We have investigated the effects of nanometer-sized silver particles on the optical properties of Eu3+ ions in SiO2 glass. Glass samples were prepared by the sol-gel method. The mean particle size and volume fraction of silver particles, which were estimated from UV-VIS absorption spectra using Mie-Drude theory modified by electron mean free path model, rapidly increased with the reduction time of Ag+ up to ∼3 min at 900 °C. The fluorescence from Eu3+ ions for the excitation by N2 laser was greatly enhanced in the presence of silver particles of 4.3 nm size. Our experimental results suggest that the origin of enhanced fluorescence is from local field enhancement around Eu3+ ions, owing to the surface plasmon resonance of small silver particles.