High-angle deflection of metagrating-integrated laser emission for high-contrast microscopy.

Flat metaoptics components are looking to replace classical optics elements and could lead to extremely compact biophotonics devices if integrated with on-chip light sources and detectors. However, using metasurfaces to shape light into wide angular range wavefronts with high efficiency, as is typically required in high-contrast microscopy applications, remains a challenge. Here we demonstrate curved GaAs metagratings integrated on vertical-cavity surface-emitting lasers (VCSELs) that enable on-chip illumination in total internal reflection and dark field microscopy. Based on an unconventional design that circumvents the aspect ratio dependent etching problems in monolithic integration, we demonstrate off-axis emission centred at 60° in air and 63° in glass with > 90% and > 70% relative deflection efficiency, respectively. The resulting laser beam is collimated out-of-plane but maintains Gaussian divergence in-plane, resulting in a long and narrow illumination area. We show that metagrating-integrated VCSELs of different kinds can be combined to enable rapid switching between dark-field and total internal reflection illumination. Our approach provides a versatile illumination solution for high-contrast imaging that is compatible with conventional microscopy setups and can be integrated with biophotonics devices, such as portable microscopy, NIR-II range bioimaging, and lab-on-a-chip devices.

Label-free nanofluidic scattering microscopy of size and mass of single diffusing molecules and nanoparticles.

Label-free characterization of single biomolecules aims to complement fluorescence microscopy in situations where labeling compromises data interpretation, is technically challenging or even impossible. However, existing methods require the investigated species to bind to a surface to be visible, thereby leaving a large fraction of analytes undetected. Here, we present nanofluidic scattering microscopy (NSM), which overcomes these limitations by enabling label-free, real-time imaging of single biomolecules diffusing inside a nanofluidic channel. NSM facilitates accurate determination of molecular weight from the measured optical contrast and of the hydrodynamic radius from the measured diffusivity, from which information about the conformational state can be inferred. Furthermore, we demonstrate its applicability to the analysis of a complex biofluid, using conditioned cell culture medium containing extracellular vesicles as an example. We foresee the application of NSM to monitor conformational changes, aggregation and interactions of single biomolecules, and to analyze single-cell secretomes.

Non-equilibrium properties of an active nanoparticle in a harmonic potential.

Active particles break out of thermodynamic equilibrium thanks to their directed motion, which leads to complex and interesting behaviors in the presence of confining potentials. When dealing with active nanoparticles, however, the overwhelming presence of rotational diffusion hinders directed motion, leading to an increase of their effective temperature, but otherwise masking the effects of self-propulsion. Here, we demonstrate an experimental system where an active nanoparticle immersed in a critical solution and held in an optical harmonic potential features far-from-equilibrium behavior beyond an increase of its effective temperature. When increasing the laser power, we observe a cross-over from a Boltzmann distribution to a non-equilibrium state, where the particle performs fast orbital rotations about the beam axis. These findings are rationalized by solving the Fokker-Planck equation for the particle's position and orientation in terms of a moment expansion. The proposed self-propulsion mechanism results from the particle's non-sphericity and the lower critical point of the solution.

Nanoplasmonic-Nanofluidic Single-Molecule Biosensors for Ultrasmall Sample Volumes.

Detection of small amounts of biological compounds is of ever-increasing importance but also remains an experimental challenge. In this context, plasmonic nanoparticles have emerged as strong contenders enabling label-free optical sensing with single-molecule resolution. However, the performance of a plasmonic single-molecule biosensor is not only dependent on its ability to detect a molecule but equally importantly on its efficiency to transport it to the binding site. Here, we present a theoretical study of the impact of downscaling fluidic structures decorated with plasmonic nanoparticles from conventional microfluidics to nanofluidics. We find that for ultrasmall picolitre sample volumes, nanofluidics enables unprecedented binding characteristics inaccessible with conventional microfluidic devices, and that both detection times and number of detected binding events can be improved by several orders of magnitude. Therefore, we propose nanoplasmonic-nanofluidic biosensing platforms as an efficient tool that paves the way for label-free single-molecule detection from ultrasmall volumes, such as single cells.

Optical Rotation and Thermometry of Laser Tweezed Silicon Nanorods.

Optical rotation of laser tweezed nanoparticles offers a convenient means for optical to mechanical force transduction and sensing at the nanoscale. Plasmonic nanoparticles are the benchmark system for such studies, but their rapid rotation comes at the price of high photoinduced heating due to Ohmic losses. We show that Mie resonant silicon nanorods with characteristic dimensions of ∼220 × 120 nm2 can be optically trapped and rotated at frequencies up to 2 kHz in water using circularly polarized laser light. The temperature excess due to heating from the trapping laser was estimated by phonon Raman scattering and particle rotation analysis. We find that the silicon nanorods exhibit slightly improved thermal characteristics compared to Au nanorods with similar rotation performance and optical resonance anisotropy. Altogether, the results indicate that silicon nanoparticles have the potential to become the system of choice for a wide range of optomechanical applications at the nanoscale.

Nanoscale Inorganic Motors Driven by Light: Principles, Realizations, and Opportunities.

The prospect of self-propelled artificial machines small enough to navigate within biological matter has fascinated and inspired researchers and the public alike since the dawn of nanotechnology. Despite many obstacles toward the realization of such devices, impressive progress on the development of its basic building block, the nanomotor, has been made over the past decade. Here, we review this emerging area with a focus on inorganic nanomotors driven or activated by light. We outline the distinct challenges and opportunities that differentiate nanomotors from micromotors based on a discussion of how stochastic forces influence the active motion of small particles. We introduce the relevant light-matter interactions and discuss how these can be utilized to classify nanomotors into three broad classes: nanomotors driven by optical momentum transfer, photothermal heating, and photocatalysis, respectively. On the basis of this classification, we then summarize and discuss the diverse body of nanomotor literature. We finally give a brief outlook on future challenges and possibilities in this rapidly evolving research area.

Biomolecular charges influence the response of surface plasmon resonance biosensors through electronic and ionic mechanisms.

Surface plasmon resonance (SPR) biosensors have become an important label-free optical biomolecular sensing technology and a "gold standard" for retrieving information on the kinetics of biomolecular interactions. Even though biomolecules typically contain an abundance of easily ionizable chemical groups, there is a gap in understanding of whether (and how) the electrostatic charge of a biomolecular system influences the SPR biosensor response. In this work we show that negative static charge present in a biomolecular layer on the surface of an SPR sensor results in significant SPR spectral shifts, and we identify two major mechanisms responsible for such shifts: 1) the formation of an electrical double layer (ionic mechanism), and 2) changes in the electron density at the surface of a metal (electronic mechanism). We show that under low ionic strength conditions, the electronic mechanism is dominant and the SPR wavelength shift is linearly proportional to the surface concentration of biomolecular charges. At high ionic strength conditions, both electric and ionic mechanisms contribute to the SPR wavelength shift. Using the electronic mechanism, we estimated the pKa of surface-bound carboxylic groups and the relative concentration of the carboxyl-terminated alkanethiols in a binary self-assembled monolayer of alkanethiols. The reported sensitivity of SPR to surface charge is especially important in the context of biomolecular sensing. Moreover, it provides an avenue for the application of SPR sensors for fast, label-free determination of the net charge of a biomolecular coating, which is of interest in material science, surface chemistry, electrochemistry, and other fields.

Antibody-Antigen Interaction Dynamics Revealed by Analysis of Single-Molecule Equilibrium Fluctuations on Individual Plasmonic Nanoparticle Biosensors.

Antibody-antigen interactions are complex events central to immune response, in vivo and in vitro diagnostics, and development of therapeutic substances. We developed an ultrastable single-molecule localized surface plasmon resonance (LSPR) sensing platform optimized for studying antibody-antigen interaction kinetics over very long time scales. The setup allowed us to perform equilibrium fluctuations analysis of the PEG/anti-PEG interaction. By time and frequency domain analysis, we demonstrate that reversible adsorption of monovalently bound anti-PEG antibodies is the dominant factor affecting the LSPR fluctuations. The results suggest that equilibrium fluctuation analysis can be an alternative to established methods for determination of interaction rates. In particular, the methodology is suited to analyze molecular systems whose properties change during the initial interaction phases, for example, due to mass transport limitations or, as demonstrated here, because the effective association rate constant varies with surface concentration of adsorbed molecules.