Decreasing photobleaching by silver island films: application to muscle

Imaging of Cell Morphology Changes via Metamaterial-Assisted Photobleaching Microscopy

Determining the axial position of an emitter with nanoscale precision is critical to a fundamental imaging methodology. While there are many advanced optical techniques being applied to high-resolution imaging, high-axial-resolution topography imaging of living cells is particularly challenging. Here, we present an application of metamaterial-assisted photobleaching microscopy (MAPM) with high-axial resolution to characterize morphological properties of living cells. Quantitative imaging of changes in the morphology of live cells is obtained by topographic and statistical analysis. The time-lapse topography image using the metamaterial-induced photostability provides information about growth factor induced changes in the cell morphology with high-axial resolution.

Metamaterial-Assisted Photobleaching Microscopy with Nanometer Scale Axial Resolution

The past two decades have witnessed a dramatic progress in the development of novel super-resolution fluorescence microscopy technologies. Here, we report a new fluorescence imaging method, called metamaterial-assisted photobleaching microscopy (MAPM), which possesses a nanometer-scale axial resolution and is suitable for broadband operation across the entire visible spectrum. The photobleaching kinetics of fluorophores can be greatly modified via a separation-dependent energy transfer process to a nearby metamaterial. The corresponding photobleaching rate is thus linked to the distance between the fluorophores and the metamaterial layer, leading to a reconstructed image with exceptionally high axial resolution. We apply the MAPM technology to image the HeLa cell membranes tagged with fluorescent proteins and demonstrate an axial resolution of ∼2.4 nm with multiple colors. MAPM utilizes a metamaterial-coated substrate to achieve super-resolution without altering anything else in a conventional microscope, representing a simple solution for fluorescence imaging at nanometer axial resolution.

Giant suppression of photobleaching for single molecule detection via the Purcell effect

We report giant suppression of photobleaching and a prolonged lifespan of single fluorescent molecules via the Purcell effect in plasmonic nanostructures. The plasmonic structures enhance the spontaneous emission of excited fluorescent molecules, reduce the probability of activating photochemical reactions that destroy the molecules, and hence suppress the bleaching. Experimentally, we observe up to a 1000-fold increase in the total number of photons that we can harvest from a single fluorescent molecule before it bleaches. This approach demonstrates the potential of using the Purcell effect to manipulate photochemical reactions at the subwavelength scale.

Europium chelate (BHHCT-Eu3+) and its metal nanostructure enhanced luminescence applied to bioassays and time-gated bioimaging

We report the use of europium chelate, 4,4'-bis(1'',1'',1'',2'',2'',3'',3''-heptafluoro-4'',6''-hexanedion-6''-yl)chlorosulfo-o-terphenyl-Eu(3+) (BHHCT-Eu(3+)), in silver nanostructure-enhanced luminescence and its application to bioassays and bioimaging. The highest luminescence intensity enhancement factor of BHHCT-Eu(3+) achieved in this study was about 11 times, while the simultaneously measured luminescence lifetime was reduced 2-fold. The luminophore photostability was also improved by a factor of 3. On the basis of these experimental results, we estimated the impact of silver nanostructures on the excitation and emission enhancement factors. Luminescence enhancement was demonstrated in two geometries: on planar glass substrates and on silica beads. In the biotin-modified IgG antibody assay the bead geometry provided slightly higher enhancement factor and greater sensitivity. Subsequently, we applied such bead substrates to time-gated luminescence imaging of Giardia lamblia cells stained by BHHCT-Eu(3+) where we observed improved brightness by a factor of 2. Such improved photostability and brightness of BHHCT-Eu(3+) in the presence of metal nanostructures are highly desirable for ultrasensitive bioassays and bioimaging, especially with time gating.

Total internal reflection with fluorescence correlation spectroscopy: Applications to substrate-supported planar membranes

In this paper, the conceptual basis and experimental design of total internal reflection with fluorescence correlation spectroscopy (TIR-FCS) is described. The few applications to date of TIR-FCS to supported membranes are discussed, in addition to a variety of applications not directly involving supported membranes. Methods related, but not technically equivalent, to TIR-FCS are also summarized. Future directions for TIR-FCS are outlined.

Monolayers of silver nanoparticles decrease photobleaching: application to muscle myofibrils

Studying single molecules in a cell has the essential advantage that kinetic information is not averaged out. However, since fluorescence is faint, such studies require that the sample be illuminated with the intense light beam. This causes photodamage of labeled proteins and rapid photobleaching of the fluorophores. Here, we show that a substantial reduction of these types of photodamage can be achieved by imaging samples on coverslips coated with monolayers of silver nanoparticles. The mechanism responsible for this effect is the interaction of localized surface plasmon polaritons excited in the metallic nanoparticles with the transition dipoles of fluorophores of a sample. This leads to a significant enhancement of fluorescence and a decrease of fluorescence lifetime of a fluorophore. Enhancement of fluorescence leads to the reduction of photodamage, because the sample can be illuminated with a dim light, and decrease of fluorescence lifetime leads to reduction of photobleaching because the fluorophore spends less time in the excited state, where it is susceptible to oxygen attack. Fluorescence enhancement and reduction of photobleaching on rough metallic surfaces are usually accompanied by a loss of optical resolution due to refraction of light by particles. In the case of monolayers of silver nanoparticles, however, the surface is smooth and glossy. The fluorescence enhancement and the reduction of photobleaching are achieved without sacrificing the optical resolution of a microscope. Skeletal muscle myofibrils were used as an example, because they contain submicron structures conveniently used to define optical resolution. Small nanoparticles (diameter approximately 60 nm) did not cause loss of optical resolution, and they enhanced fluorescence approximately 500-fold and caused the appearance of a major picosecond component of lifetime decay. As a result, the sample photobleached approximately 20-fold more slowly than the sample on glass coverslips.

Metal Enhanced Fluorescence on Silicon Wafer Substrates

We report on the fluorescence enhancement induced by silver island film (SIF) deposited on a silicon wafer. The model immunoassay was studied on silvered and unsilvered wafers. The fluorescence brightness of Rhodamine Red X increased about 300% on the SIF, while the lifetime was reduced by several fold and the photostability increased substantially. We discuss potential uses of silicon wafer substrates in multiplex assays in which the fluorescence is enhanced due to the SIF, and the multiplexing is achieved by using micro transponders.

Reduction of photobleaching and photodamage in single molecule detection: observing single actin monomer in skeletal myofibrils

Recent advances in detector technology make it possible to achieve single molecule detection (SMD) in a cell. SMD avoids complications associated with averaging signals from large assemblies and with diluting and disorganizing proteins. However, it requires that cells be illuminated with an intense laser beam, which causes photobleaching and cell damage. To reduce these effects, we study cells on coverslips coated with silver nanoparticle monolayers (NML). Muscle is used as an example. Actin is labeled with a low concentration of fluorescent phalloidin to assure that less than a single molecule in a sarcomere is fluorescent. On a glass substrate, the fluorescence of actin decays in a step-wise fashion, establishing a single molecule detection regime. Single molecules of actin in living muscle are visualized for the first time. NML coating decreases the fluorescence lifetime 17 times and enhances intensity ten times. As a result, fluorescence of muscle bleaches four to five times slower than on glass. Monolayers decrease photobleaching because they shorten the fluorescence lifetime, thus decreasing the time that a fluorophore spends in the excited state when it is vulnerable to oxygen attack. They decrease damage to cells because they enhance the electric field near the fluorophore, making it possible to illuminate samples with weaker light.

Fluorescence Quenching/Enhancement Surface Assays: Signal Manipulation Using Silver-coated Gold Nanoparticles

Gold nanoparticles covalently attached to the indium tin oxide coated glass slide drastically quench fluorescence of a surface immunoassay (approximately 5-fold). Silver electrochemically deposited over the gold particles leads to fluorescence amplification: signal increases approximately 7-8 times if compared to the signal on gold particles not covered with silver. This phenomenon allows enhancing of the surface immunoassays utilizing both types of nanoparticles.

Fluorescence blinking dynamics of silver nanoparticle and silver nanorod films

Fluorescence blinking of silver nanoparticle films is observed when illuminated with red light (635 nm). The observed power-law off-time distribution is attributed to random surface diffusion and subsequent agglomeration of atomic Ag leading to the formation of photo-active Ag nanoclusters. These nanoclusters can in turn diffuse randomly to form non-emitting Ag clusters after aggregation with another Ag species. This is revealed in the power-law on-time distribution. Silver oxides found on the surfaces of Ag nanostructures are important for photoblinking to take place since nanostructures with a protective layer of polymeric citrate or cetyltrimethylammonium bromide (CTAB) against atmospheric O(2) do not display obvious emission intermittency.

Decreasing photobleaching by silver nanoparticles on metal surfaces: application to muscle myofibrils

Recently it has become possible to study single protein molecules in a cell. However, such experiments are plagued by rapid photobleaching. We recently showed that the interaction of fluorophores with localized surface plasmon polaritons (LSPs) induced in the metallic nanoparticles led to a substantial reduction of photobleaching. We now investigate whether the photobleaching could be further reduced when the excited fluorophore interacts with the LSP excited in the metallic nanoparticles resident on mirrored surface. As an example we use myofibrils, subcellular structures within skeletal muscle. We compare nanoparticle-enhanced fluorescence of myofibrils in the presence and in the absence of a mirrored surface. The proximity of the mirrored surface led to enhancement of fluorescence and to a decrease in fluorescent lifetime, much greater than that observed in the presence of nanoparticles alone. We think that the effect is caused by the near-field interactions between fluorophores and LSP, and between fluorophores and propagating surface plasmons (PSPs) produced in the metallic surface by the nanoparticles. Photobleaching is decreased because fluorescence enhancement enables illumination with a weaker laser beam and because the decrease in fluorescence lifetime minimizes the probability of oxygen attack during the time a molecule is in the exited state.