Large-Scale Arrays of Bowtie Nanoaperture Antennas for Nanoscale Dynamics in Living Cell Membranes

Hybrid Plasmonic Nanostructures for Enhanced Single-Molecule Detection Sensitivity

Biosensing applications based on fluorescence detection often require single-molecule sensitivity in the presence of strong background signals. Plasmonic nanoantennas are particularly suitable for these tasks, as they can confine and enhance light in volumes far below the diffraction limit. The recently introduced antenna-in-box (AiB) platforms achieved high single-molecule detection sensitivity at high fluorophore concentrations by placing gold nanoantennas in a gold aperture. However, hybrid AiB platforms with alternative aperture materials such as aluminum promise superior performance by providing better background screening. Here, we report on the fabrication and optical characterization of hybrid AiBs made of gold and aluminum for enhanced single-molecule detection sensitivity. We computationally optimize the optical properties of AiBs by controlling their geometry and materials and find that hybrid nanostructures not only improve signal-to-background ratios but also provide additional excitation intensity and fluorescence enhancements. We further establish a two-step electron beam lithography process to fabricate hybrid material AiB arrays with high reproducibility and experimentally validate the higher excitation and emission enhancements of the hybrid nanostructures as compared to their gold counterpart. We foresee that biosensors based on hybrid AiBs will provide improved sensitivity beyond the capabilities of current nanophotonic sensors for a plethora of biosensing applications ranging from multicolor fluorescence detection to label-free vibrational spectroscopy.

Broadband Plasmonic Nanoantennas for Multi-Color Nanoscale Dynamics in Living Cells

Recently, the implementation of plasmonic nanoantennas has opened new possibilities to investigate the nanoscale dynamics of individual biomolecules in living cells. However, studies so far have been restricted to single molecular species as the narrow wavelength resonance of gold-based nanostructures precludes the simultaneous interrogation of different fluorescently labeled molecules. Here, broadband aluminum-based nanoantennas carved at the apex of near-field probes are exploited to resolve nanoscale-dynamic molecular interactions on living cell membranes. Through multicolor excitation, the authors simultaneously recorded fluorescence fluctuations of dual-color labeled transmembrane receptors known to form nanoclusters. Fluorescence cross-correlation studies revealed transient interactions between individual receptors in regions of ≈60 nm. Moreover, the high signal-to-background ratio provided by the antenna illumination allowed the authors to directly detect fluorescent bursts arising from the passage of individual receptors underneath the antenna. Remarkably, by reducing the illumination volume below the characteristic receptor nanocluster sizes, the molecular diffusion within nanoclusters is resolved and distinguished from nanocluster diffusion. Spatiotemporal characterization of transient interactions between molecules is crucial to understand how they communicate with each other to regulate cell function. This work demonstrates the potential of broadband photonic antennas to study multi-molecular events and interactions in living cell membranes with unprecedented spatiotemporal resolution.

Complex 10-nm resolution nanogap and nanowire geometries for plasmonic metasurface miniaturization

Emerging electromagnetic inverse design methods have pushed nanofabrication methods to their limits to extract maximum performance from plasmonic aperture-based metasurfaces. Using plasmonic metamaterial-lined apertures as an example, we demonstrate the importance of fine nanowire and nanogap features for achieving strong miniaturization of plasmonic nanoapertures. Metamaterial-lined nanoapertures are miniaturized over bowtie nanoapertures with identical minimum feature sizes by a factor of 25% without loss of field enhancement. We show that features as small as 10 nm can be reliably patterned over the wide areas required of metasurfaces using the helium focused ion beam microscope. Under imperfect fabrication conditions, we achieve 11-nm-wide nanogaps and 12-nm-wide nanowires over an area of 13 µm², and successfully validate our results with optical characterization and comparable full-wave simulations.

Photonic metacrystal: design methodology and experimental characterization

We report a design methodology for creating high-performance photonic crystals with arbitrary geometric shapes. This design approach enables the inclusion of subwavelength shapes into the photonic crystal unit cell, synergistically combining metamaterials concepts with on-chip guided-wave photonics. Accordingly, we use the term "photonic metacrystal" to describe this class of photonic structures. Photonic metacrystals exploiting three different design freedoms are demonstrated experimentally. With these additional degrees of freedom in the design space, photonic metacrystals enable added control of light-matter interactions and hold the promise of significantly increasing temporal confinement in all-dielectric metamaterials.

Correlative nanophotonic approaches to enlighten the nanoscale dynamics of living cell membranes

Dynamic compartmentalization is a prevailing principle regulating the spatiotemporal organization of the living cell membrane from the nano- up to the mesoscale. This non-arbitrary organization is intricately linked to cell function. On living cell membranes, dynamic domains or 'membrane rafts' enriched with cholesterol, sphingolipids and other certain proteins exist at the nanoscale serving as signaling and sorting platforms. Moreover, it has been postulated that other local organizers of the cell membrane such as intrinsic protein interactions, the extracellular matrix and/or the actin cytoskeleton synergize with rafts to provide spatiotemporal hierarchy to the membrane. Elucidating the intricate coupling of multiple spatial and temporal scales requires the application of correlative techniques, with a particular need for simultaneous nanometer spatial precision and microsecond temporal resolution. Here, we review novel fluorescence-based techniques that readily allow to decode nanoscale membrane dynamics with unprecedented spatiotemporal resolution and single-molecule sensitivity. We particularly focus on correlative approaches from the field of nanophotonics. Notably, we introduce a versatile planar nanoantenna platform combined with fluorescence correlation spectroscopy to study spatiotemporal heterogeneities on living cell membranes at the nano- up to the mesoscale. Finally, we outline remaining future technological challenges and comment on potential directions to advance our understanding of cell membrane dynamics under the influence of the actin cytoskeleton and extracellular matrix in uttermost detail.

Ultra-sensitive amplitude engineering and sign reversal of circular dichroism in quasi-3D chiral nanostructures

Circular dichroism (CD), as one of the most representative chiroptical effects, provides a simple strategy for the detection and characterization of the molecular chirality. The enhancement and sign reversal of CD are of great importance for its practical applications in chiral bio-sensing, chirality switching and optical filtering, etc. Here, we realize considerable adjustments and the sign reversal of CD in quasi-three-dimensional (quasi-3D) combined Archimedean spiral nanostructures. With special local and lattice configurations, the nanostructures have both right-handed and left-handed geometric chirality, which are designed based on the proximity effect of stencil lithography. We find that the CD response of the nanostructures becomes obvious once its height exceeds 200 nm and can be adjusted by the further increase of the height or the change of the blade spacing of the nanostructures. The CD reversal is achieved by utilizing the competition of two chiral centers when the height or blade spacing exceeds a critical value. Further analysis of the scattering power of multipole moments reveals that the CD modulation is determined by both magnetic dipole moment and electric quadrupole moment. Benefiting from the highly sensitive CD response to the height, the extreme sign reversal of CD is achieved when a sub-10-nm ultrathin medium layer is anchored on the surface of the nanostructures, which provides a promising strategy for ultra-sensitive chiral bio-sensing.

Enhancing Single-Molecule Fluorescence Spectroscopy with Simple and Robust Hybrid Nanoapertures

Plasmonic nanoapertures have found exciting applications in optical sensing, spectroscopy, imaging, and nanomanipulation. The subdiffraction optical field localization, reduced detection volume (~attoliters), and background-free operation make them particularly attractive for single-particle and single-molecule studies. However, in contrast to the high field enhancements by traditional "nanoantenna"-based structures, small field enhancement in conventional nanoapertures results in weak light-matter interactions and thus small enhancement of spectroscopic signals (such as fluorescence and Raman signals) of the analytes interacting with the nanoapertures. In this work, we propose a hybrid nanoaperture design termed "gold-nanoislands-embedded nanoaperture" (AuNIs-e-NA), which provides multiple electromagnetic "hotspots" within the nanoaperture to achieve field enhancements of up to 4000. The AuNIs-e-NA was able to improve the fluorescence signals by more than 2 orders of magnitude with respect to a conventional nanoaperture. With simple design and easy fabrication, along with strong signal enhancements and operability over variable light wavelengths and polarizations, the AuNIs-e-NA will serve as a robust platform for surface-enhanced optical sensing, imaging, and spectroscopy.

Plasmon-enhanced fluorescence correlation spectroscopy for super-localized detection of nanoscale subcellular dynamics

In this report, we investigate plasmon-enhanced imaging fluorescence correlation spectroscopy (p-FCS). p-FCS takes advantage of extreme light confinement by localization at nanogap-based plasmonic nanodimer arrays (PNAs) for enhanced signal-to-noise ratio (SNR) and improved precision by registration with surface plasmon microscopy images. Theoretical results corroborate the enhancement by PNAs in the far-field. Near-field scanning optical microscopy was used to confirm near-field localization experimentally. Experimental confirmation was also conducted with fluorescent nanobeads. The concept was further applied to studying the diffusion dynamics of lysosomes in HEK293T cells stimulated by phorbol 12-myristate 13-acetate treatment. It was found that lysosomes demonstrate stronger super-diffusive behavior with relatively weaker sub-diffusion after stimulation. SNR measured of p-FCS was improved by 9.77 times over conventional FCS. This report is expected to serve as the foundation for an enhanced analytical tool to explore subcellular dynamics.

Impact of Glycans on Lipid Membrane Dynamics at the Nanoscale Unveiled by Planar Plasmonic Nanogap Antennas and Atomic Force Spectroscopy

Lateral compartmentalization of the plasma membrane is a prominent feature present at multiple spatiotemporal scales that regulates key cellular functions. The extracellular glycocalyx matrix has recently emerged as an important player that modulates the organization of specific receptors and patterns the lipid bilayer itself. However, experimental limitations in investigating its impact on the membrane nanoscale dynamics have hampered detailed studies. Here, we used photonic nanoantenna arrays combined with fluorescence correlation spectroscopy to investigate the influence of hyaluronic acid (HA), a prominent glycosaminoglycan, on the nanoscale organization of mimetic lipid bilayers. Using atomic force microscopy and force spectroscopy, we further correlated our dynamic measurements with the morphology and mechanical properties of bilayers at the nanoscale. Overall, we find that HA has a profound effect on the dynamics, nanoscale organization, and mechanical properties of lipid bilayers that are enriched in sphingolipids and/or cholesterol, such as those present in living cells.

Chiral metamaterials via Moiré stacking

Chiral metamaterials have attracted strong interest due to their versatile capabilities in spin-dependent light manipulation. Benefiting from advancements in nanofabrication and mechanistic understanding of chiroptical effects, chiral metamaterials have shown potential in a variety of applications including circular polarizers, chiral sensors, and chiroptical detectors. Recently, chiral metamaterials made by moiré stacking, superimposing two or more periodic patterns with different lattice constants or relative spatial displacement, have shown promise for chiroptical applications. The moiré chiral metamaterials (MCMs) take advantage of lattice-dependent chirality, giving cost-effective fabrication, flexible tunability, and reconfigurability superior to conventional chiral metamaterials. This feature article focuses on recent progress of MCMs. We discuss optical mechanisms, structural design, fabrication, and applications of the MCMs. We conclude with our perspectives on the future opportunities for the MCMs.

Utilization of Resist Stencil Lithography for Multidimensional Fabrication on a Curved Surface

The limited ability to fabricate nanostructures on nonplanar rugged surfaces has severely hampered the applicability of many emerging technologies. Here we report a resist stencil lithography based approach for in situ fabrication of multidimensional nanostructures on both planar and uneven substrates. By using the resist film as a flexible stencil to form a suspending membrane with predesigned patterns, a variety of nanostructures have been fabricated on curved or uneven substrates of diverse morphologies on demand. The ability to realize 4 in. wafer scale fabrication of nanostructures as well as line width resolution of sub-20 nm is also demonstrated. Its extraordinary capacity is highlighted by the fabrication of three-dimensional wavy nanostructures with diversified cell morphologies on substrates of different curvatures. A robust general scheme is also developed to construct various complex 3D nanostructures. The use of conventional resists and processing ensures the versatility of the method. Such an in situ lithography technique has offered exciting possibilities to construct nanostructures with high dimensionalities that can otherwise not be achieved with existing nanofabrication methods.

Nano-spectroscopic imaging of proteins with near-field scanning optical microscopy (NSOM)

Understanding the hierarchical structure of proteins at their fundamental length scales is essential to get insights into their functions and roles in fundamental biological processes. Near-field scanning optical microscopy (NSOM), which overcomes the diffraction limits of conventional optics, provides a powerful analytical tool to image target proteins at nanoscale resolution. Especially, by combining NSOM with infrared (IR) or Raman spectroscopy, near-field nanospectroscopic imaging of a single protein is achieved. In this review, we present the recent technical progress of NSOM setup for nanospectroscopic imaging of proteins, and its application to nanospectroscopic analysis of protein structures is highlighted and critically reviewed. Finally, current challenges and perspectives on application of NSOM in emerging areas of industrial, environmental and medical biotechnology are discussed.

Optical Antenna-Based Fluorescence Correlation Spectroscopy to Probe the Nanoscale Dynamics of Biological Membranes

The plasma membrane of living cells is compartmentalized at multiple spatial scales ranging from the nano- to the mesoscale. This nonrandom organization is crucial for a large number of cellular functions. At the nanoscale, cell membranes organize into dynamic nanoassemblies enriched by cholesterol, sphingolipids, and certain types of proteins. Investigating these nanoassemblies known as lipid rafts is of paramount interest in fundamental cell biology. However, this goal requires simultaneous nanometer spatial precision and microsecond temporal resolution, which is beyond the reach of common microscopes. Optical antennas based on metallic nanostructures efficiently enhance and confine light into nanometer dimensions, breaching the diffraction limit of light. In this Perspective, we discuss recent progress combining optical antennas with fluorescence correlation spectroscopy (FCS) to monitor microsecond dynamics at nanoscale spatial dimensions. These new developments offer numerous opportunities to investigate lipid and protein dynamics in both mimetic and native biological membranes.

Planar Optical Nanoantennas Resolve Cholesterol-Dependent Nanoscale Heterogeneities in the Plasma Membrane of Living Cells

Optical nanoantennas can efficiently confine light into nanoscopic hotspots, enabling single-molecule detection sensitivity at biological relevant conditions. This innovative approach to breach the diffraction limit offers a versatile platform to investigate the dynamics of individual biomolecules in living cell membranes and their partitioning into cholesterol-dependent lipid nanodomains. Here, we present optical nanoantenna arrays with accessible surface hotspots to study the characteristic diffusion dynamics of phosphoethanolamine (PE) and sphingomyelin (SM) in the plasma membrane of living cells at the nanoscale. Fluorescence burst analysis and fluorescence correlation spectroscopy performed on nanoantennas of different gap sizes show that, unlike PE, SM is transiently trapped in cholesterol-enriched nanodomains of 10 nm diameter with short characteristic times around 100 μs. The removal of cholesterol led to the free diffusion of SM, consistent with the dispersion of nanodomains. Our results are consistent with the existence of highly transient and fluctuating nanoscale assemblies enriched by cholesterol and sphingolipids in living cell membranes, also known as lipid rafts. Quantitative data on sphingolipids partitioning into lipid rafts is crucial to understand the spatiotemporal heterogeneous organization of transient molecular complexes on the membrane of living cells at the nanoscale. The proposed technique is fully biocompatible and thus provides various opportunities for biophysics and live cell research to reveal details that remain hidden in confocal diffraction-limited measurements.

Transient Nanoscopic Phase Separation in Biological Lipid Membranes Resolved by Planar Plasmonic Antennas

Nanoscale membrane assemblies of sphingolipids, cholesterol, and certain proteins, also known as lipid rafts, play a crucial role in facilitating a broad range of important cell functions. Whereas on living cell membranes lipid rafts have been postulated to have nanoscopic dimensions and to be highly transient, the existence of a similar type of dynamic nanodomains in multicomponent lipid bilayers has been questioned. Here, we perform fluorescence correlation spectroscopy on planar plasmonic antenna arrays with different nanogap sizes to assess the dynamic nanoscale organization of mimetic biological membranes. Our approach takes advantage of the highly enhanced and confined excitation light provided by the nanoantennas together with their outstanding planarity to investigate membrane regions as small as 10 nm in size with microsecond time resolution. Our diffusion data are consistent with the coexistence of transient nanoscopic domains in both the liquid-ordered and the liquid-disordered microscopic phases of multicomponent lipid bilayers. These nanodomains have characteristic residence times between 30 and 150 μs and sizes around 10 nm, as inferred from the diffusion data. Thus, although microscale phase separation occurs on mimetic membranes, nanoscopic domains also coexist, suggesting that these transient assemblies might be similar to those occurring in living cells, which in the absence of raft-stabilizing proteins are poised to be short-lived. Importantly, our work underscores the high potential of photonic nanoantennas to interrogate the nanoscale heterogeneity of native biological membranes with ultrahigh spatiotemporal resolution.

Growth Of Organic Semiconductor Thin Films with Multi-Micron Domain Size and Fabrication of Organic Transistors Using a Stencil Nanosieve

To grow small molecule semiconductor thin films with domain size larger than modern-day device sizes, we evaporate the material through a dense array of small apertures, called a stencil nanosieve. The aperture size of 0.5 μm results in low nucleation density, whereas the aperture-to-aperture distance of 0.5 μm provides sufficient crosstalk between neighboring apertures through the diffusion of adsorbed molecules. By integrating the nanosieve in the channel area of a thin-film transistor mask, we show a route for patterning both the organic semiconductor and the metal contacts of thin-film transistors using one mask only and without mask realignment.

In-Plane Plasmonic Antenna Arrays with Surface Nanogaps for Giant Fluorescence Enhancement

Optical nanoantennas have a great potential for enhancing light-matter interactions at the nanometer scale, yet fabrication accuracy and lack of scalability currently limit ultimate antenna performance and applications. In most designs, the region of maximum field localization and enhancement (i.e., hotspot) is not readily accessible to the sample because it is buried into the nanostructure. Moreover, current large-scale fabrication techniques lack reproducible geometrical control below 20 nm. Here, we describe a new nanofabrication technique that applies planarization, etch back, and template stripping to expose the excitation hotspot at the surface, providing a major improvement over conventional electron beam lithography methods. We present large flat surface arrays of in-plane nanoantennas, featuring gaps as small as 10 nm with sharp edges, excellent reproducibility and full surface accessibility of the hotspot confined region. The novel fabrication approach drastically improves the optical performance of plasmonic nanoantennas to yield giant fluorescence enhancement factors up to 10⁴-10⁵ times, together with nanoscale detection volumes in the 20 zL range. The method is fully scalable and adaptable to a wide range of antenna designs. We foresee broad applications by the use of these in-plane antenna geometries ranging from large-scale ultrasensitive sensor chips to microfluidics and live cell membrane investigations.

Dual-band moiré metasurface patches for multifunctional biomedical applications

There has been strong interest in developing multi-band plasmonic metasurfaces for multiple optical functions on single platforms. Herein, we developed Au moiré metasurface patches (AMMP), which leverage the tunable multi-band responses of Au moiré metasurfaces and the additional field enhancements of the metal-insulator-metal configuration to achieve dual-band plasmon resonance modes in near-infrared and mid-infrared regimes with high field enhancement. Furthermore, we demonstrate the multifunctional applications of AMMP, including surface-enhanced infrared spectroscopy, optical capture and patterning of bacteria, and photothermal denaturation of proteins. With their multiple functions of high performance, in combination with cost-effective fabrication using moiré nanosphere lithography, the AMMP will enable the development of highly integrated biophotonic platforms for a wide range of applications in disease theranostics, sterilization, and the study of microbiomes.

Nanoscopic Cellular Imaging: Confinement Broadens Understanding

In recent years, single-molecule fluorescence imaging has been reconciling a fundamental mismatch between optical microscopy and subcellular biophysics. However, the next step in nanoscale imaging in living cells can be accessed only by optical excitation confinement geometries. Here, we review three methods of confinement that can enable nanoscale imaging in living cells: excitation confinement by laser illumination with beam shaping; physical confinement by micron-scale geometries in bacterial cells; and nanoscale confinement by nanophotonics.

3D nanostructures fabricated by advanced stencil lithography

This letter reports on a novel fabrication method for 3D metal nanostructures using high-throughput nanostencil lithography. Aperture clogging, which occurs on the stencil membranes during physical vapor deposition, is leveraged to create complex topographies on the nanoscale. The precision of the 3D nanofabrication method is studied in terms of geometric parameters and material types. The versatility of the technique is demonstrated by various symmetric and chiral patterns made of Al and Au.

Tunable multiband metasurfaces by moiré nanosphere lithography

Moiré nanosphere lithography (MNSL), which features the relative in-plane rotation between two layers of self-assembled monodisperse nanospheres as masks, provides a cost-effective approach for creating moiré patterns on generic substrates. In this work, we experimentally and numerically investigate a series of moiré metasurfaces by MNSL. Due to the variety of gradient plasmonic nanostructures in arrays, single moiré metasurfaces can support multiple localized surface plasmon (LSP) modes with a wide range of resonant wavelengths from ∼600 nm to ∼4200 nm. We analyze the origin of the LSP modes based on the optical spectra and near-field electromagnetic distributions. In addition, we fabricate and analyze the metasurfaces with high-density nanogap structures. These nanogap structures support plasmonic gap modes with significant field enhancements. With their tunable multiband optical responses from visible to near-infrared to mid-infrared regimes, these moiré metasurfaces are applicable for ultrabroadband absorbers, multiband surface-enhanced infrared and Raman spectroscopy, and broadband single-molecule spectroscopy.

Current approaches to studying membrane organization

The local structure and composition of the outer membrane of an animal cell are important factors in the control of many membrane processes and mechanisms. These include signaling, sorting, and exo- and endocytic processes that are occurring all the time in a living cell. Paradoxically, not only are the local structure and composition of the membrane matters of much debate and discussion, the mechanisms that govern its genesis remain highly controversial. Here, we discuss a swathe of new technological advances that may be applied to understand the local structure and composition of the membrane of a living cell from the molecular scale to the scale of the whole membrane.

Nanoscale volume confinement and fluorescence enhancement with double nanohole aperture

Diffraction ultimately limits the fluorescence collected from a single molecule, and sets an upper limit to the maximum concentration to isolate a single molecule in the detection volume. To overcome these limitations, we introduce here the use of a double nanohole structure with 25 nm gap, and report enhanced detection of single fluorescent molecules in concentrated solutions exceeding 20 micromolar. The nanometer gap concentrates the light into an apex volume down to 70 zeptoliter (10(-21) L), 7000-fold below the diffraction-limited confocal volume. Using fluorescence correlation spectroscopy and time-correlated photon counting, we measure fluorescence enhancement up to 100-fold, together with local density of optical states (LDOS) enhancement of 30-fold. The distinctive features of double nanoholes combining high local field enhancement, efficient background screening and relative nanofabrication simplicity offer new strategies for real time investigation of biochemical events with single molecule resolution at high concentrations.