Single-UCNP correlative imaging.

<p>(A) TEM and (B) epi-luminescence microscopy images corresponding to the same areas of the sample TEM grid. The distances between the individual (encircled) nanoparticles/clusters, given in (A), were precisely matched to those in (B) to identify the same UCNP constellation. (C) Close-up TEM image of the same area as in (A), where UCNP sites designated ‘cluster 1’, ‘cluster 2’, and ‘single’ correspond to the three sites in (B). The individual UCNPs within ‘cluster 1’ and ‘cluster 2’ were optically unresolvable. “Single” designates a single UCNP particle clearly observable, as a diffraction-limited spot in (B). The excitation wavelength, intensity and exposure time were 978 nm, ∼250 W/cm² and 0.7 s, respectively. The pixel values were converted to photons/second (ph/s) and color-coded according to the look-up color bar in (B).</p

Spectral imaging of UCNPs.

<p>Emission spectra of UCNPs in (A) single, (B) small cluster (designated ‘cluster 2’ in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0063292#pone-0063292-g002" target="_blank">Figure 2</a>) and (C) powder form (data points joined by solid lines) captured using hyperspectral epi-luminescence microscopy, overlaid with the ensemble-averaged spectra of UCNP powder captured by a calibrated spectrometer (dashed lines). The corresponding exposure times and EMCCD camera electron-multiplication (EM) gains were (A) 4 sec and 255; (B) 1.5 sec and 44; and (C) 0.014 sec and 9. Since the samples (A) and (B) contained considerably less emitters than the powder sample (C), the excitation intensities at λ_ex = 978 nm were varied, respectively, from 250 W/cm² to 8 W/cm² to accommodate for the large disparity in the emission signals that would otherwise exceed the dynamic range of the EMCCD. The decreased I_ex resulted in an increased green-to-red emission ratio in (C) due to the varied upconversion energy redistribution between the green and red multiplets. Top panel, schematic diagram of NaYF₄:Yb,Er UCNP.</p

Diagram of the custom-modified epi-luminescence imaging system employed for single-UCNP and spectral imaging.

<p>A wide-field inverted epi-luminescence microscope was modified to allow external fiber-coupled laser illumination. The optical fiber was dithered to average out speckles. The excitation light was configured to uniformly illuminate the field-of-view at the sample plane via a modified Köhler illumination scheme. The sample plane was imaged using an EMCCD camera, optionally mounted with an AOTF for hyperspectral imaging. An adjustable iris diaphragm allowed reduction of the field-of-view to restrict imaging to several single UCNP particles and small clusters.</p

Epi-luminescence imaging of a single UCNP using a “blood-immersion” objective.

<p>(A) A photograph of the hemolyzed blood layer between the objective and cover slip. (B) Absorption spectrum of the hemolyzed blood (red solid curve) and UCNP emission spectrum (green dashed curve). (C) Low-magnification images of the UCNP sample recorded through the eyepiece port using the water- (top) and blood- (bottom) immersion objective. The dried UCNP colloid rims appeared green (top) and red (bottom) due to the green light absorption by blood. (D) Epi-luminescence microscopy image of the UCNP constellation identified in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0063292#pone-0063292-g002" target="_blank">Figure 2C</a>, imaged using the blood-immersion objective. The single UCNP is clearly observable, although blurred. The EMCCD camera settings and excitation parameters were equivalent to these of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0063292#pone-0063292-g002" target="_blank">Figure 2</a>. The pixel values were converted to photons/second (ph/s) and color-coded using the look-up bar in (D).</p

Conversion efficiency, size and morphology of UCNPs synthesized in-house.

<p>(A) Plot of the absolute conversion efficiency (<i>η</i>_uc) [W/W] of the reported upconversion nanoparticle sample versus the excitation intensity at <i>λ</i>_ex = 978 nm measured using a calibrated integrating sphere set-up. <i>η</i>_uc is the ratio of the emitted power integrated over the entire emission spectral range (500–700 nm) to the absorbed power. (B) Size histogram obtained by analyzing the transmission electron microscopy (TEM) images of NaYF₄:Yb,Er UCNPs (330 particles). A typical TEM-image is shown in (C).</p

Theoretical estimation of single-emitter detection sensitivity in skin.

<p>A plot of the optical (confocal) detection contrasts of a single upconversion nanoparticle (UCNP, brown) and organic fluorescence dye (fluorescein, FC, green) versus their depth in skin, as modeled theoretically. The inset shows more detailed quantitative plots of the imaging signal (dashed) and background (dotted) of UCNP and FL versus depth in skin expressed in electrons per second (e⁻/s). The black dotted line demarcates the contrast value of 1. See text for details.</p

Facile Assembly of Functional Upconversion Nanoparticles for Targeted Cancer Imaging and Photodynamic Therapy

The treatment depth of existing photodynamic therapy (PDT) is limited because of the absorption of visible excitation light in biological tissue. It can be augmented by means of upconversion nanoparticles (UCNPs) transforming deep-penetrating near-infrared (NIR) light to visible light, exciting PDT drugs. We report here a facile strategy to assemble such PDT nanocomposites functionalized for cancer targeting, based on coating of the UCNPs with a silica layer encapsulating the Rose Bengal photosensitizer and bioconjugation to antibodies through a bifunctional fusion protein consisting of a solid-binding peptide linker genetically fused to <i>Streptococcus</i> Protein G′. The fusion protein (Linker-Protein G) mediates the functionalization of silica-coated UCNPs with cancer cell antibodies, allowing for specific target recognition and delivery. The resulting nanocomposites were shown to target cancer cells specifically, generate intracellular reactive oxygen species under 980 nm excitation, and induce NIR-triggered phototoxicity to suppress cancer cell growth in vitro

Development of Bright and Biocompatible Nanoruby and Its Application to Background-Free Time-Gated Imaging of G‑Protein-Coupled Receptors

At the forefront of developing fluorescent probes for biological imaging applications are enhancements aimed at increasing their brightness, contrast, and photostability, especially toward demanding applications of single-molecule detection. In comparison with existing probes, nanorubies exhibit unlimited photostability and a long emission lifetime (∼4 ms), which enable continuous imaging at single-particle sensitivity in highly scattering and fluorescent biological specimens. However, their wide application as fluorescence probes has so far been hindered by the absence of facile methods for scaled-up high-volume production and molecularly specific targeting. The present work encompasses the large-scale production of colloidally stable nanoruby particles, the demonstration of their biofunctionality and negligible cytotoxicity, as well as the validation of its use for targeted biomolecular imaging. In addition, optical characteristics of nanorubies are found to be comparable or superior to those of state-of-the-art quantum dots. Protocols of reproducible and robust coupling of functional proteins to the nanoruby surface are also presented. As an example, NeutrAvidin-coupled nanoruby show excellent affinity and specificity to μ-opioid receptors in fixed and live cells, allowing wide-field imaging of G-protein coupled receptors with single-particle sensitivity