Directional two-photon induced surface plasmon-coupled emission

Surface plasmon-coupled emission imaging for biological applications

Fluorescence imaging technology has been extensively applied in chemical and biological research profiting from its high sensitivity and specificity. Much attention has been devoted to breaking the light diffraction-limited spatial resolution. However, it remains a great challenge to improve the axial resolution in a way that is accessible in general laboratories. Surface plasmon-coupled emission (SPCE), generated by the interactions between surface plasmons and excited fluorophores in close vicinity of the thin metal film, offers an opportunity for optical imaging with potential application in analysis of molecular and biological systems. Benefiting from the highly directional and distance-dependent properties, SPCE imaging (SPCEi) has displayed excellent performance in bioimaging with improved sensitivity and axial confinement. Herein, we give a brief overview of the development of SPCEi. We describe the unique optical characteristics and constructions of SPCEi systems and highlight recent advances in the use of SPCEi for biological applications. We hope this review provides readers with both the insights and future prospects of SPCEi as a new promising imaging platform for potentially widespread applications in biological research and medical diagnostics. Graphical abstract.

Particle sensing with confined optical field enhanced fluorescence emission (Cofefe)

We describe the development and performance of a new type of optical sensor suitable for registering the binding/dissociation of nanoscopic particles near a gold sensing surface. The method shares similarities with surface plasmon resonance microscopy but uses a completely different optical signature for reading out binding events. This new optical read-out mechanism, which we call confined optical field enhanced fluorescence emission (Cofefe), uses pulsed surface plasmon polariton fields at the gold/liquid interface that give rise to confined optical fields upon binding of the target particle to the gold surface. The confined near-fields are sufficient to induce two-photon absorption in the gold sensor surface near the binding site. Subsequent radiative recombination of the electron-hole pairs in the gold produces fluorescence emission, which can be captured by a camera in the far-field. Bound nanoparticles show up as bright confined spots against a dark background on the camera. We show that the Cofefe sensor is capable of detecting gold and silicon nanoparticles, as well as polymer nanospheres and sub-μm lipid droplets in a label-free manner with average illumination powers of less than 10 μW/μm2.

Surface plasmon-coupled emission: what can directional fluorescence bring to the analytical sciences?

Surface plasmon-coupled emission (SPCE) arose from the integration of fluorescence and plasmonics, two rapidly expanding research fields. SPCE is revealing novel phenomena and has potential applications in bioanalysis, medical diagnostics, drug discovery, and genomics. In SPCE, excited fluorophores couple with surface plasmons on a continuous thin metal film; plasmophores radiate into a higher-refractive index medium with a narrow angular distribution. Because of the directional emission, the sensitivity of this technique can be greatly improved with high collection efficiency. This review describes the unique features of SPCE. In particular, we focus on recent advances in SPCE-based analytical platforms and their applications in DNA sensing and the detection of other biomolecules and chemicals.

Optimizing silver film for surface plasmon-coupled emission induced two-photon excited fluorescence imaging

In this study, the optimal condition of a silver (Ag) film deposited on a cover slip for surface plasmon-coupled emission (SPCE) induced two-photon excited fluorescence (TPEF) based on an objective-based, total internal reflection (TIR) microscope was investigated. According to the theoretical simulations of local electric field enhancement and fluorescence coupled emission efficiency, the thickness of the Ag film should be about 40 nm in order to maximize the TPEF collection efficiency by the objective. The deposited Ag film with a germanium seed layer on a cover slip exhibits additional improvement in surface smoothness by reducing variations in surface roughness to below 1.0 nm, thereby reduces local hot spots which degrade the image uniformity. Moreover, an Ag film with a 20 nm-thick SiO2 spacer not only prevents damage caused through interaction with the aqueous solution under high laser power irradiance, but also reduces the fluorescence quenching effect by the Ag film. By optimizing the Ag film thickness, surface smoothness, and a protective dielectric spacer, efficient TIR TPEF imaging can be achieved through SPCE.

Surface plasmon-enhanced and quenched two-photon excited fluorescence

This study investigated theoretically and experimentally that two-photon excited fluorescence is enhanced and quenched via surface plasmons (SPs) excited by total internal reflection with a silver film. The fluorescence intensity is fundamentally affected by the local electromagnetic field enhancement and the quantum yield change according to the surrounding structure and materials. By utilizing the Fresnel equation and classical dipole radiation modeling, local electric field enhancement, fluorescence quantum yield, and fluorescence emission coupling yield via SPs were theoretically analyzed at different dielectric spacer thicknesses between the fluorescence dye and the metal film. The fluorescence lifetime was also decreased substantially via the quenching effect. A two-photon excited total internal reflection fluorescence (TIRF) microscopy with a time-correlated single photon counting device has been developed to measure the fluorescence lifetimes, photostabilities, and enhancements. The experimental results demonstrate that the fluorescence lifetimes and the trend of the enhancements are consistent with the theoretical analysis. The maximum fluorescence enhancement factor in the surface plasmon-total internal reflection fluorescence (SP-TIRF) configuration can be increased up to 30 fold with a suitable thickness SiO(2) spacer. Also, to compromise for the fluorescence enhancement and the fluorophore photostability, we find that the SP-TIRF configuration with a 10 nm SiO(2) spacer can provide an enhanced and less photobleached fluorescent signal via the assistance of enhanced local electromagnetic field and quenched fluorescence lifetime, respectively.

Direct image of surface-plasmon-coupled emission by leakage radiation microscopy

Leakage radiation microscopy (LRM) is used to directly image surface-plasmon-coupled emission (SPCE). When compared with the prism-based setup commonly used in SPCE research, LRM has the advantages of directly giving out the emitting direction without angle scanning, and the image generated of surface plasmon polariton (SPPs) propagation, which helps to understand the optical process of SPCE. LRM also can give a clearer SPCE image than that obtained by a prism-based setup. Based on the LRM, we find that the SPCE pattern and propagation of SPPs can be modified by the shape of PMMA films containing fluorescence molecules.

Kinetics of a single cross-bridge in familial hypertrophic cardiomyopathy heart muscle measured by reverse Kretschmann fluorescence

Familial hypertrophic cardiomyopathy (FHC) is a serious heart disease that often leads to a sudden cardiac death of young athletes. It is believed that the alteration of the kinetics of interaction between actin and myosin causes FHC by making the heart to pump blood inefficiently. We set out to check this hypothesis ex vivo. During contraction of heart muscle, a myosin cross-bridge imparts periodic force impulses to actin. The impulses are analyzed by fluorescence correlation spectroscopy (FCS) of fluorescently labeled actin. To minimize observation volume and background fluorescence, we carry out FCS measurements in surface plasmon coupled emission mode in a reverse Kretschmann configuration. Fluorescence is a result of near-field coupling of fluorophores excited in the vicinity of the metal-coated surface of a coverslip with the surface plasmons propagating in the metal. Surface plasmons decouple on opposite sides of the metal film and emit in a directional manner as far-field p-polarized radiation. We show that the rate of changes of orientation is significantly faster in contracting cardiac myofibrils of transgenic mice than wild type. These results are consistent with the fact that mutated heart muscle myosin translates actin faster in in vitro motility assays.

Surface plasmon-enhanced two-photon fluorescence microscopy for live cell membrane imaging

A surface plasmon-enhanced two-photon total-internal-reflection fluorescence (TIRF) microscope has been developed to provide fluorescent images of living cell membranes. The proposed microscope with the help of surface plasmons (SPs) not only provides brighter fluorescent images based on the mechanism of local electromagnetic field enhancement, but also reduces photobleaching due to having a shorter fluorophore lifetime. In comparison with a one-photon TIRF, the two-photon TIRF can achieve higher signal-to-noise ratio cell membrane imaging due its smaller excitation volume and lower scattering. By combining the SP enhancement and two-photon excitation TIRF, the microscope has demonstrated it's capability for brighter and more contrasted fluorescence membrane images of living monkey kidney COS-7 fibroblasts transfected with an EYFP-MEM or EGFP-WOX1 construct.

Plasmon-controlled fluorescence towards high-sensitivity optical sensing

Fluorescence spectroscopy is widely used in chemical and biological research. Until recently most of the fluorescence experiments have been performed in the far-field regime. By far-field we imply at least several wavelengths from the fluorescent probe molecule. In recent years there has been growing interest in the interactions of fluorophores with metallic surfaces or particles. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. The spectral properties of fluorophores can dramatically be altered by near-field interactions with the electron clouds present in metals. These interactions modify the emission in ways not seen in classical fluorescence experiments. Fluorophores in the excited state can create plasmons that radiate into the far-field and fluorophores in the ground state can interact with and be excited by surface plasmons. These reciprocal interactions suggest that the novel optical absorption and scattering properties of metallic nanostructures can be used to control the decay rates, location, and direction of fluorophore emission. We refer to these phenomena as plasmon-controlled fluorescence (PCF). An overview of the recent work on metal-fluorophore interactions is presented. Recent research combining plasmonics and fluorescence suggest that PCF could lead to new classes of experimental procedures, novel probes, bioassays, and devices.

Plasmon-controlled fluorescence: a new paradigm in fluorescence spectroscopy

Fluorescence spectroscopy is widely used in biological research. Until recently, essentially all fluorescence experiments were performed using optical energy which has radiated to the far-field. By far-field we mean at least several wavelengths from the fluorophore, but propagating far-field radiation is usually detected at larger macroscopic distances from the sample. In recent years there has been a growing interest in the interactions of fluorophores with metallic surfaces or particles. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. The spectral properties of fluorophores can be dramatically altered by near-field interactions with the electron clouds present in metals. These interactions modify the emission in ways not seen in classical fluorescence experiments. In this review we provide an intuitive description of the complex physics of plasmons and near-field interactions. Additionally, we summarize the recent work on metal-fluorophore interactions and suggest how these effects will result in new classes of experimental procedures, novel probes, bioassays and devices.

Fluorescence correlation spectroscopy in a reverse Kretchmann surface plasmon assisted microscope

Fluorescence Correlation Spectroscopy (FCS) demands a high rate of photon detection per molecule, low background, and large fluctuations of fluorescence associated with translational motion. The new approach presented here, Surface Plasmon Assisted Microscope (SPAM), meets these requirements by drastically limiting the observation volume. In this method, the observational layer is made so thin that fluctuations are mostly due to the axial motion of molecules. This is conveniently realized by placing a sample on a thin metal film and illuminating it with a laser beam through an aqueous medium. The excited fluorophores close to the surface couple (via near-field interactions) to surface plasmons in the metal. Propagated surface plasmons decouple on opposite side of the metal film as a far-field radiation and emit in directional manner. Fluorescence is collected with a high Numerical Aperture objective. A confocal aperture inserted in its conjugate image plane reduces lateral dimensions of the detection volume to a diffraction limit. The thickness of the detection layer is reduced further by metal quenching of excited fluorophores at a close proximity (about 30 nm) to the surface. We used a suspension of fluorescent microspheres to show that FCS-SPAM is an efficient method to measure molecular diffusion.

Application of surface plasmon coupled emission to study of muscle

Muscle contraction results from interactions between actin and myosin cross-bridges. Dynamics of this interaction may be quite different in contracting muscle than in vitro because of the molecular crowding. In addition, each cross-bridge of contracting muscle is in a different stage of its mechanochemical cycle, and so temporal measurements are time averages. To avoid complications related to crowding and averaging, it is necessary to follow time behavior of a single cross-bridge in muscle. To be able to do so, it is necessary to collect data from an extremely small volume (an attoliter, 10(-18) liter). We report here on a novel microscopic application of surface plasmon-coupled emission (SPCE), which provides such a volume in a live sample. Muscle is fluorescently labeled and placed on a coverslip coated with a thin layer of noble metal. The laser beam is incident at a surface plasmon resonance (SPR) angle, at which it penetrates the metal layer and illuminates muscle by evanescent wave. The volume from which fluorescence emanates is a product of two near-field factors: the depth of evanescent wave excitation and a distance-dependent coupling of excited fluorophores to the surface plasmons. The fluorescence is quenched at the metal interface (up to approximately 10 nm), which further limits the thickness of the fluorescent volume to approximately 50 nm. The fluorescence is detected through a confocal aperture, which limits the lateral dimensions of the detection volume to approximately 200 nm. The resulting volume is approximately 2 x 10(-18) liter. The method is particularly sensitive to rotational motions because of the strong dependence of the plasmon coupling on the orientation of excited transition dipole. We show that by using a high-numerical-aperture objective (1.65) and high-refractive-index coverslips coated with gold, it is possible to follow rotational motion of 12 actin molecules in muscle with millisecond time resolution.