Excitation of luminescing biolabels via two-photon absorption processes allows the use of near-infrared (NIR) light, which is only weakly absorbed by biological tissue.[1] Excitation in the NIR induces only a very weak autofluorescence background and avoids photodegradation in biotagging applications, thus simplifying the detection of the labeled target molecules and increasing the sensitivity of the method. Organic dyes as well as semiconductor nanoparticles can be employed as emitters. [1±3] Due to the nature of the two-photon absorption (TPA) process which involves a non-stationary (ªvirtualº) quantum mechanical state, however, its efficiency is very low, requiring high excitation densities. To avoid thermal decomposition processes in the sample, expensive pulsed lasers are frequently employed as light sources with pulse durations in the pico-or even femtosecond range. Photon upconversion is an alternative process for the generation of visible radiation by NIR excitation. It is based on sequential absorption and energy transfer steps involving real metastable excited states of the chromophore.[4] Therefore its efficiency can be much higher than for TPA processes, and continuous wave (CW) laser or lamp excitation is possible. Typical excitation densities fall in the range of 1±10 3 W cm ±2for upconversion, compared to 10 6 ±10 9 W cm ±2 for twophoton absorption. [2,4,5] Applications range from display devices [6] and lasers [7] to commercially used reporters for nucleic acid microarrays. [8,9] [10] Crystalline materials doped with these ion couples, however, normally consist of sub-micrometer [8,9] to micrometer-sized grains [10] which do not form transparent colloids and are much too large to substitute for molecular dyes in biological tagging applications. Therefore it was a challenging task to synthesize nanocrystals of these materials which can be transparently dispersed in solution. Recently we have been able to dope lanthanide phosphate nanocrystals with these ion couples and have demonstrated upconversion emission in transparent colloidal solution for the first time.[11] Due to competing radiationless processes, however, the efficiency of the upconversion luminescence was still rather poor. In the present paper we report on the successful synthesis and very intense multicolor upcon- nanocrystals transparently dispersed in solution. The upconversion efficiency of such solutions is about eight orders of magnitude higher than for the previously reported colloids of lanthanide-doped phosphate nanocrystals. We believe that this enormous improvement of the upconversion efficiency of these materials opens the door for interesting future applications in the field of biolabeling. The characterization of NaYF 4 :20 %Yb,2 %Er is summarized in Figure 1. The transmission electron microscopy (TEM) images (Fig. 1A) show crystalline particles of roughly spherical shape. A histogram of the particle size distribution, deduced from several overview TEM images, is given in Figure 1B and shows a relatively broad distributi...

Quantum leap: Very high quantum yields, thought to be impossible for nanocrystalline phosphor materials, were achieved with CePO4:Tb/LaPO4 core–shell nanoparticles (see picture). Because the LaPO4 shell reduces energy losses at the particle surface, transparent colloidal solutions of these 8‐nm particles are highly photoluminescent.

The synthesis of bifunctional Au/Pt/Au nanoraspberries for use in quantitative in situ monitoring of platinum-catalyzed reactions by surface-enhanced Raman scattering (SERS) is presented. Highly convolved SERS spectra of reaction mixtures can be decomposed into the contributions of distinct molecular species by multivariate data analysis.

A variety of redispersible Eu(3+)-doped LaPO(4) nanoparticles have been prepared in a high-boiling coordinating solvent mixture, and the Eu(3+) lattice sites of these materials have been investigated by luminescence line-narrowing measurements. In this spectroscopic method, Eu(3+) ions occupying different lattice sites are selectively excited with a tunable narrow-bandwidth laser system and distinguished by their luminescence spectra ("site-selective spectroscopy"). Depending on the concentration of the dopant, up to three different lattice sites could be identified in the interior of the LaPO(4) nanoparticles. These sites correspond to those known from bulk LaPO(4). In addition, a variety of surface sites is observed, which could be converted completely into bulk sites by overgrowing the nanoparticles with a shell of pure LaPO(4). The surface sites are identical to those obtained by reacting Eu(3+) with the surface of pure LaPO(4) nanoparticles. The spectroscopic properties of Eu(3+)-doped LaPO(4) nanoparticles differ from those of pure EuPO(4) nanoparticles, which were also investigated. Remarkably, the core/shell synthesis investigated in this paper allows one to prepare doped nanoparticles that contain no other dopant sites than those known from the corresponding bulk material.

A biocompatible, seed-mediated synthesis of monodisperse ~60 nm gold nanostars, followed by hydrophilic stabilization with ethylene glycol-modified Raman reporter molecules, is presented. Their application as SERS labels for imaging of the tumor suppressor p63 in prostate biopsies by immuno-SERS microscopy is demonstrated.