Despite the increasing use of semiconductor nanocrystals (quantum dots, QDs) with unique size-controlled optical and chemical properties in (bio)analytical detection, biosensing and fluorescence imaging and the obvious relevance of reliable values of fluorescence quantum yields for these applications, evaluated procedures for the determination of the fluorescence quantum yields (Φf) of these materials are still missing. This limits the value of literature data of QDs in comparison to common organic dyes and hampers the comparability of the performance of QDs from different sources or manufacturers. This encouraged us to investigate achievable uncertainties for the determination of Φf values of these chromophores and to illustrate common pitfalls exemplarily for differently sized water-soluble CdTe QDs. Special attention is dedicated to the colloidal nature and complicated surface chemistry of QDs thereby deriving procedures to minimize uncertainties related to these features.

Determination of the Fluorescence Quantum Yield of Quantum Dots: Suitable Procedures and Achievable Uncertainties

We review the syntheses, optical properties, and biological applications of cadmium selenide (CdSe) and cadmium selenide-zinc sulfide (CdSe-ZnS) quantum dots (QDs) and gold (Au) and silver (Ag) nanoparticles (NPs). Specifically, we selected the syntheses of QDs and Au and Ag NPs in aqueous and organic phases, size- and shape-dependent photoluminescence (PL) of QDs and plasmon of metal NPs, and their bioimaging applications. The PL properties of QDs are discussed with reference to their band gap structure and various electronic transitions, relations of PL and photoactivated PL with surface defects, and blinking of single QDs. Optical properties of Ag and Au NPs are discussed with reference to their size- and shape-dependent surface plasmon bands, electron dynamics and relaxation, and surface-enhanced Raman scattering (SERS). The bioimaging applications are discussed with reference to in vitro and in vivo imaging of live cells, and in vivo imaging of cancers, tumor vasculature, and lymph nodes. Other aspects of the review are in vivo deep tissue imaging, multiphoton excitation, NIR fluorescence and SERS imaging, and toxic effects of NPs and their clearance from the body.

Semiconductor quantum dots and metal nanoparticles: syntheses, optical properties, and biological applications

A strong photoluminescence enhancement effect of illumination on CdTe nanocrystals stabilized by thioglycolic acid (TGA) was observed. Under optimized conditions, the fluorescence quantum yield of CdTe nanocrystals in aqueous solution reached 85% at room temperature. Optical measurements indicated that the photodegradation of TGA rather than NCs was the main reason for the PL enhancement effect since sulfide ions released from TGA during illumination coordinated with cadmium ions on the surface of CdTe and formed a shell structure of CdS there. X-ray photoelectron spectroscopic results further proved the formation of CdS shell on the CdTe core during illumination. The colloidal solution of CdTe/CdS core−shell nanocrystals with photoluminescence quantum yield higher than 80% turned out to be very stable. Therefore, the current investigations not only reveal the PL enhancement effect of illumination, but also provide a useful synthetic route for producing water-soluble and highly fluorescent CdTe/CdS core−shell nanocrystals

Enhancement Effect of Illumination on the Photoluminescence of Water-Soluble CdTe Nanocrystals: Toward Highly Fluorescent CdTe/CdS Core−Shell Structure

We report a protocol for manipulating the surface composition of CdSe nanocrystals. By combining the successive ion layer adhesion and reaction (SILAR) method developed by Li et al. J. Am. Chem. Soc. 2003, 125, 12567 with a phosphine-free selenium precursor, the surface stoichiometry of CdSe can be tunably altered from Cd- to Se-rich. By changing the overall surface stoichiometry, we demonstrate ligand binding to specific surface sites. Tertiary phosphines produce a dramatic enhancement in photoluminescence quantum yield of CdSe particles with Se-rich surfaces but have little effect on Cd-rich surfaces. Unpassivated selenium surface sites are also shown to be a cause of the photobrightening behavior of CdSe nanocrystals.

From Cd-rich to se-rich--the manipulation of CdSe nanocrystal surface stoichiometry.

The synthesis of quantum dot semiconductors is now an established science and a large amount of work has been carried out into the chemistry and physics of the interface between nanoparticle surface and ligand. Here, we review the basic ligands and cover some of their properties.

The nature of quantum dot capping ligands

We report the changes in surface state of tri-n-octylphosphine oxide (TOPO) capped CdSe nacocrystals by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). In the photobrightening state, where photoluminescence (PL) intensity increased during the illumination, surface ligand rearrangement such as the TOPO−Se compelx and TOPO dimers are formed, which might be deeply correlated with the PL enhancement. It was also observed that surface oxidation and decomposition of TOPO, which is followed by the desorption, occurred in photodarkening state.

Yoshimi Abe

Papers

Surface State Analysis of Photobrightening in CdSe Nanocrystal Thin Films

Angle-resolved photoemission study of the surface state on NbC(111)