There is a continuous demand for imaging probes offering excellent performance in various microscopy techniques for comprehensive investigations of cellular processes by more than one technique. Fluorescent nanodiamond-gold nanoparticles (FND-Au) constitute a new class of "all-in-one" hybrid particles providing unique features for multimodal cellular imaging including optical imaging, electron microscopy, and, and potentially even quantum sensing. Confocal and optical coherence microscopy of the FND-Au allow fast investigations inside living cells via emission, scattering, and photothermal imaging techniques because the FND emission is not quenched by AuNPs. In electron microscopy, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) analysis of FND-Au reveals greatly enhanced contrast due to the gold particles as well as an extraordinary flickering behavior in three-dimensional cellular environments originating from the nanodiamonds. The unique multimodal imaging characteristics of FND-Au enable detailed studies inside cells ranging from statistical distributions at the entire cellular level (micrometers) down to the tracking of individual particles in subcellular organelles (nanometers). Herein, the processes of endosomal membrane uptake and release of FNDs were elucidated for the first time by the imaging of individual FND-Au hybrid nanoparticles with single-particle resolution. Their convenient preparation, the availability of various surface groups, their flexible detection modalities, and their single-particle contrast in combination with the capability for endosomal penetration and low cytotoxicity make FND-Au unique candidates for multimodal optical-electronic imaging applications with great potential for emerging techniques, such as quantum sensing inside living cells.

Abstract:We present a novel extended-focus optical coherence microscope (OCM) attaining 0.7 µm axial and 0.4 µm lateral resolution maintained over a depth of 40 µm, while preserving the advantages of Fourier domain OCM. Our system uses an ultra-broad spectrum from a supercontinuum laser source. As the spectrum spans from near-infrared to visible wavelengths (240 nm in bandwidth), we call the system visOCM. The combination of such a broad spectrum with a high-NA objective creates an almost isotropic 3D submicron resolution. We analyze the imaging performance of visOCM on microbead samples and demonstrate its image quality on cell cultures and ex-vivo brain tissue of both healthy and alzheimeric mice. In addition to neuronal cell bodies, fibers and plaques, visOCM imaging of brain tissue reveals fine vascular structures and sub-cellular features through its high spatial resolution. Sub-cellular structures were also observed in live cells and were further revealed through a protocol traditionally used for OCT angiography.

An ideal nanofabrication method should allow the organization of nanoparticles and molecules with nanometric positional precision, stoichiometric control, and well-defined orientation. The DNA origami technique has evolved into a highly versatile bottom-up nanofabrication methodology that fulfils almost all of these features. It enables the nanometric positioning of molecules and nanoparticles with stoichiometric control, and even the orientation of asymmetrical nanoparticles along predefined directions. However, orienting individual molecules has been a standing challenge. Here, we show how single molecules, namely, Cy5 and Cy3 fluorophores, can be incorporated in a DNA origami with controlled orientation by doubly linking them to oligonucleotide strands that are hybridized while leaving unpaired bases in the scaffold. Increasing the number of bases unpaired induces a stretching of the fluorophore linkers, reducing its mobility freedom, and leaves more space for the fluorophore to accommodate and find different sites for interaction with the DNA. Particularly, we explore the effects of leaving 0, 2, 4, 6, and 8 bases unpaired and find extreme orientations for 0 and 8 unpaired bases, corresponding to the molecules being perpendicular and parallel to the DNA double-helix, respectively. We foresee that these results will expand the application field of DNA origami toward the fabrication of nanodevices involving a wide range of orientation-dependent molecular interactions, such as energy transfer, intermolecular electron transport, catalysis, exciton delocalization, or the electromagnetic coupling of a molecule to specific resonant nanoantenna modes.

We study experimentally the excitation of the radially polarized conical surface plasmon polariton (SPP) in a fully metalcoated conically tapered M-profile fiber which works as a "plasmonic tip" for the scanning near-field optical microscope (SNOM). This structure extends the Kretschmann configuration to the conical geometry. In this plasmonic tip, the radially polarized waveguide mode, propagating inside the fiber, resonantly excites the radially polarized SPP on the metal surface, which consequently gets confined at the apex where the field oscillates longitudinally along the tip axis. We also demonstrate the reverse process, where a longitudinal field excites the radially polarized SPP mode which then resonantly excites the radially polarized waveguide mode. This plasmonic tip combines the advantageous properties of near-field optical probes. Though, it has the shape of an apertureless SNOM tip, it can simplify the detection/excitation procedure and suppresses the background signal by its fiber-based design. Unlike the sharp apertureless SNOM tips that detects only the longitudinal field component or aperture SNOM tips that detect mostly the transversal component, the plasmonic tip detects both longitudinal and transversal field in collection mode and backward-scattering mode, respectively. The plasmonic tip, with further improvements, can become an advanced tool in SNOM due to its ability for background-free near-field detection, ease of operation, and higher conversion efficiency from far-field to near-field than conventional tips. KEYWORDS: plasmonic tip, radially polarized conical SPP, radially polarized fiber mode, longitudinal and transversal field, SNOM, M-profile fiber T he biggest challenge for studying the optical properties of nano-objects arises in efficient delivery and detection of light to and from nanoscale regions. Surface plasmon polaritons (SPPs) help to overcome this limit. 1,2 It enables strong confinement and enhancement of electromagnetic energy below the diffraction limit of light in a variety of structures, particularly in tapered metallic structures with sharp edges or tips. 3,4 Being hybrid electromagnetic waves, SPPs comprise properties of transverse (photon) and longitudinal (plasmon) waves. When propagating in tapered metallic structures toward the sharp edges or apexes, longitudinal component of SPPs field becomes more pronounced. 5−14 This leads to a decrease of the wavelength, thus, allowing it to get localized at the tip apex and resulting in a highly confined and enhanced field at the apex with a strong longitudinal component. 5,7−9 This phenomenon is called SPP superfocusing, and in a metallic cone, it takes place only for the radially polarized SPP mode. 7,8 Tip-enhanced microscopy technique, which provides the highest spatial optical resolution among the optical detection methods, takes advantage of SPP localization at a nanoscale apex of a conical metallic tip. 15−23 The localization can be achieved either by exciting SPPs on a tip shaft 21−23 or simply by placing a tip...