Control of Luminescence and Interfacial Properties as Perspective for Upconversion Nanoparticles.

Near-infrared (NIR) light is highly suitable for studying biological systems due to its minimal scattering and lack of background fluorescence excitation, resulting in high signal-to-noise ratios. By combining NIR light with lanthanide-based upconversion nanoparticles (UCNPs), upconversion is used to generate UV or visible light within tissue. This remarkable property has gained significant research interest over the past two decades.

Bilayer-Coating Strategy for Hydrophobic Nanoparticles Providing Colloidal Stability, Functionality, and Surface Protection in Biological Media.

The surface chemistry of nanoparticles is a key step on the pathway from particle design towards applications in biologically relevant environments. Here, a bilayer-based strategy for the surface modification of hydrophobic nanoparticles is introduced that leads to excellent colloidal stability in aqueous environments and good protection against disintegration, while permitting surface functionalization via simple carbodiimide chemistry. We have demonstrated the excellent potential of this strategy using upconversion nanoparticles (UCNPs), initially coated with oleate and therefore dispersible only in organic solvents.

Small and Bright Water-Protected Upconversion Nanoparticles with Long-Time Stability in Complex, Aqueous Media by Phospholipid Membrane Coating.

Chemical and colloidal stability in complex aqueous media are among the main challenges preventing nanoparticles from successfully entering into the biomedical field. Small core-shell upconversion nanoparticles (UCNPs) NaYF:Yb,Er@NaYF of 12 nm in diameter with a high surface-to-volume ratio are utilized to demonstrate that self-assembling phospholipid bilayers (PLMs) have several benefits compared to common ligand-exchange and ligand-addition particle coatings such as poly(acrylic acid) and amphiphilic polymers. An efficient hydrophobic barrier against water quenching and toward particle disintegration is formed by PLM.

Plasmonic Enhancement of NIR to UV Upconversion by a Nanoengineered Interface Consisting of NaYF:Yb,Tm Nanoparticles and a Gold Nanotriangle Array for Optical Detection of Vitamin B12 in Serum.

A nanoengineered interface fabricated by self-assembly enables the online determination of vitamin B12 via a simple luminescence readout in serum without any pretreatment. The interplay of Tm-doped NaYF nanoparticles (UCNPs) and a gold nanotriangle array prepared by nanosphere lithography on a glass slide is responsible for an efficient NIR to UV upconversion. Hot spots of the gold assembly generate local electromagnetic-field enhancement, favoring the four-photon upconversion process at the low-power excitation of approximately 13 W·cm.