Dual-band chirality-selective absorbing by plasmonic metasurfaces with breaking mirror and rotational symmetry.

In this work, we proposed a state-of-the-art metasurface model that breaks the mirror symmetry and rotation symmetry of the structure. It consists of two-layer rotating gold split rings, and has the capability of chirality-selective absorption for circularly polarized light (CPL) in two bands. The absorption peaks for left- and right- circularly polarized (LCP&RCP) light appeared at 989 nm and 1404 nm, respectively, with the maximum absorptivity of 98.5% and 96.3%, respectively. By changing the rotation angle of the two-layer gold split rings, it could also be designed as a single-band chiral metasurface absorber, which only absorbed RCP light but not LCP light, and the absorptivity of RCP light could be up to 97.4%. Furthermore, we found our designed absorbers had the characteristics of great circular dichroism (CD) and symmetric absorption. The physical mechanism of the selective absorption of CPL by the absorbers may be explained by the current vector analysis. In addition, the absorption peak could be tuned with the changing of the geometrical parameters of the structure. The proposed chirality-selective metasurface absorbers could be used in CD spectral detection, optical communication, optical filtering, and other fields.

High-Sensitivity Sensing of Divalent Copper Ions at the Single Upconversion Nanoparticle Level.

Single-nanoparticle-level sensing allows us to measure individual molecular interactions and probe environmental stimuli at nanometer-scale resolution. Despite these premises, limited success has been met hitherto due to the demanding challenge to distinguish a dimmed signal from a noisy background. Here, we describe an approach for high-sensitivity single-nanoparticle-level sensing of divalent copper (Cu2+) ions through near-infrared-to-visible upconversion luminescence against a near-null background. This nanosensor utilizes ytterbium- (Yb3+) and erbium (Er3+)-doped sodium yttrium fluoride (NaYF4) upconversion nanoparticles (UCNPs) (maximal emission at 540 nm when excited at 980 nm) as an energy donor, of which the surface attaches Cu2+-dependent DNAzymes labeled with BHQ1 dye (Black Hole Quencher 1, maximal absorption at 548 nm) as energy acceptors. Adding a hint amount of Cu2+ ions resulted in the cleavage of a BHQ1-containing moiety in DNAzymes, thus turning on upconversion luminescence for sensitive detection. Indeed, this approach allows us to perform single-nanoparticle-level detection of Cu2+ ions with extraordinary signal-to-noise ratios (SNRs, >277) for all measured concentrations that cover 3 orders of magnitude (from sub-nM to µM). Importantly, a limit of detection of 220 pM was achieved, about sevenfold lower than the one at the ensemble level. Moreover, a stochastic particle-to-particle sensing behavior was also identified, featuring single-nanoparticle-level detection. This work untaps the usage of UCNPs for high-sensitivity single-nanoparticle-level biosensing.

Recent advances of lanthanide-doped upconversion nanoparticles for biological applications.

Near infrared (NIR) excited lanthanide-doped upconversion nanoparticles (UCNPs) are emerging as a new type of fluorescent tag for biological applications, which can emit multi-photon ultraviolet, visible or NIR luminescence for imaging or activation of photosensitive molecules. Here, we present a comprehensive review on recent advances of UCNPs for a manifold of biological applications, including upconversion mechanisms, building bright multicolor upconversion nanocrystals, single nanoparticle and super resolution imaging, in vivo optical and multimodal imaging, photodynamic therapy, light-controlled drug release, biosensing, and toxicities. Our perspectives on the future development of UCNPs are also described.

Efficient sub-15 nm cubic-phase core/shell upconversion nanoparticles as reporters for ensemble and single particle studies.

Single particle imaging of upconversion nanoparticles (UCNPs) has typically been realized using hexagonal (ß) phase lanthanide-doped sodium yttrium fluoride (NaYF4) materials, the upconversion luminescence (UCL) of which saturates at power densities (P) of several hundred W cm-2 under 980 nm near-infrared (NIR) excitation. Cubic (α) phase UCNPs have been mostly neglected because of their commonly observed lower UCL efficiency at comparable P in ensemble level studies. Here, we describe a set of sub-15 nm ytterbium-enriched α-NaYbF4:Er3+@CaF2 core/shell UCNPs doped with varying Er3+ concentrations (5-25%), studied over a wide P range of ∼8-105 W cm-2, which emit intense UCL even at a low P of 10 W cm-2 and also saturate at relatively low P. The highest upconversion quantum yield (ΦUC) and the highest particle brightness were obtained for an Er3+ dopant concentration of 12%, reaching the highest ΦUC of 0.77% at a saturation power density (Psat) of 110 W cm-2. These 12%Er3+-doped core/shell UCNPs were also the brightest UCNPs among this series under microscopic conditions at high P of ∼102-105 W cm-2 as demonstrated by imaging studies at the single particle level. Our results underline the potential applicability of the described sub-15 nm cubic-phase core/shell UCNPs for ensemble- and single particle-level bioimaging.