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.

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.

Single-Step DNA Detection Assay Monitoring Dual-Color Light Scattering from Individual Metal Nanoparticle Aggregates

Efficiently detecting DNA sequences within a limited time is vital for disease screening and public health monitoring. This calls for a new method that combines high sensitivity, fast read-out time, and easy manipulation of the sample, avoiding the extensive steps of DNA amplification, purification, or grafting to a surface. Here, we introduce photon cross-correlation spectroscopy as a new method for specific DNA sensing with high sensitivity in a single-step homogeneous solution phase. Our approach is based on confocal dual-color illumination and detection of the scattering intensities from individual silver nanoparticles and gold nanorods. In the absence of the target DNA, the nanoparticles move independently and their respective scattering signals are uncorrelated. In the presence of the target DNA, the probe-functionalized gold and silver nanoparticles assemble via DNA hybridization with the target, giving rise to temporal coincidence between the signals scattered by each nanoparticle. The degree of coincidence accurately quantifies the amount of target DNA. To demonstrate the efficiency of our technique, we detect a specific DNA sequence of sesame, an allergenic food ingredient, for a range of concentration from 5 pM to 1.5 nM with a limit of detection of 1 pM. Our method is sensitive and specific enough to detect single nucleotide deletion and mismatch. With the dual-color scattering signals being much brighter than fluorescence-based analogs, the analysis is fast, quantitative, and simple to operate, making it valuable for biosensing applications.

Single-step homogeneous immunoassay for detecting prostate-specific antigen using dual-color light scattering of metal nanoparticles

Conventional sandwich-type immunoassays are widely used for protein biomarker detection, yet their workflows are challenged by the need for multiple incubation steps separated by washing cycles.

Plasmonic Nanoantennas Enable Forbidden Förster Dipole-Dipole Energy Transfer and Enhance the FRET Efficiency

Förster resonance energy transfer (FRET) plays a key role in biochemistry, organic photovoltaics, and lighting sources. FRET is commonly used as a nanoruler for the short (nanometer) distance between donor and acceptor dyes, yet FRET is equally sensitive to the mutual dipole orientation. The orientation dependence complicates the FRET analysis in biological samples and may even lead to the absence of FRET for perpendicularly oriented donor and acceptor dipoles. Here, we exploit the strongly inhomogeneous and localized fields in plasmonic nanoantennas to open new energy transfer routes, overcoming the limitations from the mutual dipole orientation to ultimately enhance the FRET efficiency. We demonstrate that the simultaneous presence of perpendicular near-field components in the nanoantenna sets favorable energy transfer routes that increase the FRET efficiency up to 50% for nearly perpendicular donor and acceptor dipoles. This new facet of plasmonic nanoantennas enables dipole-dipole energy transfer that would otherwise be forbidden in a homogeneous environment. As such, our approach further increases the applicability of single-molecule FRET over diffraction-limited approaches, with the additional benefits of higher sensitivities and higher concentration ranges toward physiological levels.

All-Dielectric Silicon Nanogap Antennas To Enhance the Fluorescence of Single Molecules

Plasmonic antennas have a profound impact on nanophotonics as they provide efficient means to manipulate light and enhance light-matter interactions at the nanoscale. However, the large absorption losses found in metals can severely limit the plasmonic applications in the visible spectral range. Here, we demonstrate the effectiveness of an alternative approach using all-dielectric nanoantennas based on silicon dimers to enhance the fluorescence detection of single molecules. The silicon antenna design is optimized to confine the near-field intensity in the 20 nm nanogap and reach a 270-fold fluorescence enhancement in a nanoscale volume of λ(3)/1800 with dielectric materials only. Our conclusions are assessed by combining polarization resolved optical spectroscopy of individual antennas, scanning electron microscopy, numerical simulations, fluorescence lifetime measurements, fluorescence burst analysis, and fluorescence correlation spectroscopy. This work demonstrates that all-silicon nanoantennas are a valid alternative to plasmonic devices for enhanced single molecule fluorescence sensing, with the additional key advantages of reduced nonradiative quenching, negligible heat generation, cost-efficiency, and complementary metal-oxide-semiconductor (CMOS) compatibility.

Picosecond Lifetimes with High Quantum Yields from Single-Photon-Emitting Colloidal Nanostructures at Room Temperature

Minimizing the luminescence lifetime while maintaining a high emission quantum yield is paramount in optimizing the excitation cross-section, radiative decay rate, and brightness of quantum solid-state light sources, particularly at room temperature, where nonradiative processes can dominate. We demonstrate here that DNA-templated 60 and 80 nm diameter gold nanoparticle dimers, featuring one fluorescent molecule, provide single-photon emission with lifetimes that can fall below 10 ps and typical quantum yields in a 45-70% range. Since these colloidal nanostructures are obtained as a purified aqueous suspension, fluorescence spectroscopy can be performed on both fixed and freely diffusing nanostructures to quantitatively estimate the distributions of decay rate and fluorescence intensity enhancements. These data are in excellent agreement with theoretical calculations and demonstrate that millions of bright fluorescent nanostructures, with radiative lifetimes below 100 ps, can be produced in parallel.

Coupling Emitters and Silver Nanowires to Achieve Long-Range Plasmon-Mediated Fluorescence Energy Transfer

The development of quantum plasmonic circuitry requires efficient coupling between quantum emitters and plasmonic waveguides. A major experimental challenge is to simultaneously maximize the surface plasmon propagation length, the coupling efficiency into the plasmonic mode, and the Purcell factor. Addressing this challenge is also the key to enabling long-range energy transfer between quantum nanoemitters. Here, we use a dual-beam scanning confocal microscope to carefully investigate the interactions between fluorescent nanoparticles and surface plasmons on single-crystalline silver nanowires. By exciting the fluorescent nanoparticles via nanowire surface plasmons, we maximize the light-matter interactions and reach coupling efficiencies up to 44% together with 24× lifetime reduction and 4.1 μm propagation lengths. This improved optical performance enables the demonstration of long-range plasmon-mediated fluorescence energy transfer between two nanoparticles separated by micrometer distance. Our results provide guidelines toward practical realizations of efficient long-range fluorescence energy transfer for integrated plasmonics and quantum nano-optics.

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.

Matching Nanoantenna Field Confinement to FRET Distances Enhances Förster Energy Transfer Rates

Förster resonance energy transfer (FRET) is widely applied in chemistry, biology, and nanosciences to assess distances on sub-10 nm scale. Extending the range and applicability of FRET requires enhancement of the fluorescence energy transfer at a spatial scale comparable to the donor-acceptor distances. Plasmonic nanoantennas are ideal to concentrate optical fields at a nanoscale fully matching the FRET distance range. Here, we present a resonant aluminum nanogap antenna tailored to enhance single molecule FRET. A 20 nm gap confines light into a nanoscale volume, providing a field gradient on the scale of the donor-acceptor distance, a large 10-fold increase in the local density of optical states, and strong intensity enhancement. With our dedicated design, we obtain 20-fold enhancement on the fluorescence emission of donor and acceptor dyes, and most importantly up to 5-fold enhancement of the FRET rate for donor-acceptor separations of 10 nm. We also provide a thorough framework of the fluorescence photophysics occurring in the nanoscale gap volume. The presented enhancement of energy transfer flow at the nanoscale opens a yet unexplored facet of the various advantages of optical nanoantennas and provides a new strategy toward biological applications of single molecule FRET at micromolar concentrations.

Three-dimensional nanometre localization of nanoparticles to enhance super-resolution microscopy

Meeting the nanometre resolution promised by super-resolution microscopy techniques (pointillist: PALM, STORM, scanning: STED) requires stabilizing the sample drifts in real time during the whole acquisition process. Metal nanoparticles are excellent probes to track the lateral drifts as they provide crisp and photostable information. However, achieving nanometre axial super-localization is still a major challenge, as diffraction imposes large depths-of-fields. Here we demonstrate fast full three-dimensional nanometre super-localization of gold nanoparticles through simultaneous intensity and phase imaging with a wavefront-sensing camera based on quadriwave lateral shearing interferometry. We show how to combine the intensity and phase information to provide the key to the third axial dimension. Presently, we demonstrate even in the occurrence of large three-dimensional fluctuations of several microns, unprecedented sub-nanometre localization accuracies down to 0.7 nm in lateral and 2.7 nm in axial directions at 50 frames per second. We demonstrate that nanoscale stabilization greatly enhances the image quality and resolution in direct stochastic optical reconstruction microscopy imaging.

FRET analysis of CP12 structural interplay by GAPDH and PRK

CP12 is an intrinsically disordered protein playing a key role in the regulation of the Benson-Calvin cycle. Due to the high intrinsic flexibility of CP12, it is essential to consider its structural modulation induced upon binding to the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK) enzymes. Here, we report for the first time detailed structural modulation about the wild-type CP12 and its site-specific N-terminal and C-terminal disulfide bridge mutants upon interaction with GAPDH and PRK by Förster resonance energy transfer (FRET). Our results indicate an increase in CP12 compactness when the complex is formed with GAPDH or PRK. In addition, the distributions in FRET histograms show the elasticity and conformational flexibility of CP12 in all supra molecular complexes. Contrarily to previous beliefs, our FRET results importantly reveal that both N-terminal and C-terminal site-specific CP12 mutants are able to form the monomeric (GAPDH-CP12-PRK) complex.

FRET enhancement in aluminum zero-mode waveguides

Zero-mode waveguides (ZMWs) can confine light into attoliter volumes, which enables single molecule fluorescence experiments at physiological micromolar concentrations. Of the fluorescence spectroscopy techniques that can be enhanced by ZMWs, Förster resonance energy transfer (FRET) is one of the most widely used in life sciences. Combining zero-mode waveguides with FRET provides new opportunities to investigate biochemical structures or follow interaction dynamics at micromolar concentrations with single-molecule resolution. However, prior to any quantitative FRET analysis on biological samples, it is crucial to establish first the influence of the ZMW on the FRET process. Here, we quantify the FRET rates and efficiencies between individual donor-acceptor fluorophore pairs that diffuse into aluminum zero-mode waveguides. Aluminum ZMWs are important structures thanks to their commercial availability and the large amount of literature that describe their use for single-molecule fluorescence spectroscopy. We also compared the results between ZMWs milled in gold and aluminum, and found that although gold has a stronger influence on the decay rates, the lower losses of aluminum in the green spectral region provide larger fluorescence brightness enhancement factors. For both aluminum and gold ZMWs, we observed that the FRET rate scales linearly with the isolated donor decay rate and the local density of optical states. Detailed information about FRET in ZMWs unlocks their application as new devices for enhanced single-molecule FRET at physiological concentrations.

Nanophotonic enhancement of the Förster resonance energy-transfer rate with single nanoapertures

Tailoring the light-matter interaction and the local density of optical states (LDOS) with nanophotonics provides accurate control over the luminescence properties of a single quantum emitter. This paradigm is also highly attractive to enhance the near-field Förster resonance energy transfer (FRET) between two fluorescent emitters. Despite the wide applications of FRET in nanosciences, using nanophotonics to enhance FRET has remained a debated and complex challenge. Here we demonstrate enhanced energy transfer within single donor-acceptor fluorophore pairs confined in single gold nanoapertures. Experiments monitoring both the donor and the acceptor emission photodynamics clearly establish a linear dependence of the FRET rate on the LDOS in nanoapertures, demonstrating that nanophotonics can be used to intensify the near-field energy transfer. Strikingly, we observe a significant six-fold increase in the FRET rate for large donor-acceptor separations exceeding 13 nm. Exciting opportunities are opened to investigate biochemical structures with donor-acceptor distances much beyond the classical Förster radius. Importantly, our approach is fully compatible with the detection of single biomolecules freely diffusing in water solution under physiological conditions.

Gold nanoparticles for enhanced single molecule fluorescence analysis at micromolar concentration

Individual metal nanoparticles represent an inexpensive and versatile platform to enhance the detection of fluorescent species at biologically relevant concentrations. Here we use fluorescence correlation spectroscopy to quantify the near-field detection volume and average fluorescence enhancement factors set by a single gold nanoparticle. We demonstrate detection volumes down to 270 zeptoliters (three orders of magnitude beyond the diffraction barrier) together with 60-fold enhancement of the fluorescence brightness per molecule.

Plasmonic band structure controls single-molecule fluorescence

Plasmonics and photonic crystals are two complementary approaches to tailor single-emitter fluorescence, using strong local field enhancements near metals on one hand and spatially extended photonic band structure effects on the other hand. Here, we explore the emergence of spontaneous emission control by finite-sized hexagonal arrays of nanoapertures milled in gold film. We demonstrate that already small lattices enable highly directional and enhanced emission from single fluorescent molecules in the central aperture. Even for clusters just four unit cells across, the directionality is set by the plasmonic crystal band structure, as confirmed by full-wave numerical simulations. This realization of plasmonic phase array antennas driven by single quantum emitters opens a flexible toolbox to engineer fluorescence and its detection.

A plasmonic 'antenna-in-box' platform for enhanced single-molecule analysis at micromolar concentrations

Single-molecule fluorescence techniques are key for a number of applications, including DNA sequencing, molecular and cell biology and early diagnosis. Unfortunately, observation of single molecules by diffraction-limited optics is restricted to detection volumes in the femtolitre range and requires pico- or nanomolar concentrations, far below the micromolar range where most biological reactions occur. This limitation can be overcome using plasmonic nanostructures, which enable the confinement of light down to nanoscale volumes. Although these nanoantennas enhance fluorescence brightness, large background signals and/or unspecific binding to the metallic surface have hampered the detection of individual fluorescent molecules in solution at high concentrations. Here we introduce a novel 'antenna-in-box' platform that is based on a gap-antenna inside a nanoaperture. This design combines fluorescent signal enhancement and background screening, offering high single-molecule sensitivity (fluorescence enhancement up to 1,100-fold and microsecond transit times) at micromolar sample concentrations and zeptolitre-range detection volumes. The antenna-in-box device can be optimized for single-molecule fluorescence studies at physiologically relevant concentrations, as we demonstrate using various biomolecules.

Cost and resource implications with serum angiogenic factor estimation in the triage of pre-eclampsia

OBJECTIVES

To analyse the economic and resource implications of using plasma soluble fms-like tyrosine kinase-1 s(Flt1) and placenta growth factor (PlGF) measurements in pre-eclampsia evaluation and management.

DESIGN

Retrospective cost analysis of our prospective cohort study.

SETTING

Boston, Massachusetts (USA).

POPULATION

Women (n = 176) presenting to the hospital at <34 weeks of gestation for evaluation of possible pre-eclampsia during 2009-10. Cases without complete cost or outcome data (n = 9) and re-enrolments (n = 18) were excluded.

METHODS

Modelled comparisons between the standard approach (combination of blood pressure, urinary protein excretion, alanine aminotransferase and platelet counts) and a novel approach (ratio of plasma sFlt1 and PlGF) using actual hospital data converted to 2012 US dollars in accordance with the Centers for Medicare and Medicaid Services.

MAIN OUTCOME MEASURES

Direct 2-week costs and resource use by groups having true or false positive and negative test results for adverse outcomes according to approach.

RESULTS

The improved specificity of the novel approach decreased the proportion of women falsely labelled as test-positive from 42.3% (34.4-50.2%) to 4.0% (0.85-7.15%) and increased the proportion correctly labelled as test-negative from 23.5% (16.7-30.3%) to 61.7% (53.9-69.5%). This could potentially reduce average per-patient costs by $1215. Substantial quantities of resources [47.2% (35.7-58.7%) of antenatal admissions and 72.5% (68.0-77.0%) of tests for fetal wellbeing] were unnecessarily used for women who were truly negative. A proportion of iatrogenic preterm deliveries among women with negative results was potentially avoidable representing further cost and resource savings.

CONCLUSIONS

Clinical use of the plasma sFlt1 and PlGF ratio improves risk stratification among women presenting for pre-eclampsia evaluation and has the potential to reduce costs and resource use.