Lanthanide-doped upconversion nanocrystals (UCN) converting low energy photons to high energy photons have emerged as an efficient and versatile bioimaging and therapeutic tool. However, the upconversion efficiency of the UCNs is low, which limits their applications. Plasmonic modulation makes it possible to enhance the luminescence of these nanocrystals. We hypothesize that the enhancement of the upconversion luminescence for all the emission peaks simultaneously could be achieved if the UCNs are coated with a gold nanoshell and the surface plasmon resonance (SPR) peak is tuned to the near-infrared (NIR) region and made resonant with the absorption of the UCNs, thereby substantially increasing the excitation flux via local field enhancement (LFE) effect. Furthermore, the nanoparticles could be used for darkfield imaging due to the light scattering caused by the gold nanoshell. Herein, we report a poly-(amino acid)-templated gold-shell encapsulation of the silica coated NaYF 4 :Yb,Er UCNs and show how a deft tuning of the SPR peak from visible to NIR region dramatically transforms the luminescence quenching into an enhancement effect and how the nanoparticles are used for combined upconversion fluorescence and darkfield light scattering imaging.

Hollow silver nanoshells with tunable plasmon bands have been synthesized using Ag2O nanoparticles at an optimized temperature of 20 °C. The plasmon peak has been tuned in a wide range from 460 nm to 605 nm employing a combination of mild reductant and a mild stabilizer, hydrazine hydrate and sodium citrate, respectively. In contrast, the combination of strong reductant and strong stabilizer, NaBH4 and hydrophilic thiols, resulted in limited plasmon tunability (455-510 nm). The differential behaviour is attributed to the change in dynamics of the diffusion-reaction process. For thiols, the effect of free end-groups was quite evident as plasmon peak shifted from 449 nm to 470 nm on replacing thioglycolic acid (HS-CH2-COOH) with mercaptoethanol (HS-CH2-CH2-OH). Transmission electron microscopy (TEM) revealed that the aspect ratio [outer diameter (d)/shell thickness (t)] was 2.8 (d: 40.0 ± 1.6 nm, t: 14.0 ± 1.3 nm) and 5 (d: 84 ± 2.3 nm, t: 16.8 ± 1.9 nm) for the nanoshells exhibiting a plasmon peak at 460 nm and 605 nm, respectively. The crystal phase of nanoshells was found to be face centered cubic (fcc) as deduced from HR-TEM and electron diffraction data. Using the same Ag2O template, morphological transformation from non-porous to mesoporous has also been achieved by simply reversing the order of addition.

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Selective quenching of luminescence of quantum dots (QDs) by Cu2+ ions vis-à-vis other physiologically relevant cations has been reexamined. In view of the contradiction regarding the mechanism, we have attempted to show why Cu2+ ions quench QD-luminescence by taking CdS and CdTe QDs with varying surface groups. A detailed study of the solvent effect and also size dependence on the observed luminescence has been carried out. For a 13% decrease in particle diameter (4.3 nm -->3.7 nm), the quenching constant increased by a factor of 20. It is established that instead of surface ligands of QDs, conduction band potential of the core facilitates the photo-induced reduction of Cu (II) to Cu (I) thereby quenching the photoluminescence. Taking the advantage of biocompatibility of dendrimer and its high affinity towards Cu2+ ions, we have followed interaction of Cu2+-PAMAM and also dendrimer with the CdTe QDs. Nanomolar concentration of PAMAM dendrimer was found to quench the luminescence of CdTe QDs. In contrast, Cu2+-PAMAM enhanced the fluorescence of CdTe QDs and the effect has been attributed to the binding of Cu2+-PAMAM complex to the CdTe particle surface. The linear portion of the enhancement plot due to Cu2+-PAMAM can be used for determination of Cu2+ ions with detection limit of 70 nM.

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