Large Fluorescence Enhancements of Fluorophore Ensembles with Multilayer Plasmonic Substrates: Comparison of Theory and Experimental Results

Emerging Strategies in Enhancing Singlet Oxygen Generation of Nano-Photosensitizers Toward Advanced Phototherapy

The great promise of photodynamic therapy (PDT) has thrusted the rapid progress of developing highly effective photosensitizers (PS) in killing cancerous cells and bacteria. To mitigate the intrinsic limitations of the classical molecular photosensitizers, researchers have been looking into designing new generation of nanomaterial-based photosensitizers (nano-photosensitizers) with better photostability and higher singlet oxygen generation (SOG) efficiency, and ways of enhancing the performance of existing photosensitizers. In this paper, we review the recent development of nano-photosensitizers and nanoplasmonic strategies to enhance the SOG efficiency for better PDT performance. Firstly, we explain the mechanism of reactive oxygen species generation by classical photosensitizers, followed by a brief discussion on the commercially available photosensitizers and their limitations in PDT. We then introduce three types of new generation nano-photosensitizers that can effectively produce singlet oxygen molecules under visible light illumination, i.e., aggregation-induced emission nanodots, metal nanoclusters (< 2 nm), and carbon dots. Different design approaches to synthesize these nano-photosensitizers were also discussed. To further enhance the SOG rate of nano-photosensitizers, plasmonic strategies on using different types of metal nanoparticles in both colloidal and planar metal-PS systems are reviewed. The key parameters that determine the metal-enhanced SOG (ME-SOG) efficiency and their underlined enhancement mechanism are discussed. Lastly, we highlight the future prospects of these nanoengineering strategies, and discuss how the future development in nanobiotechnology and theoretical simulation could accelerate the design of new photosensitizers and ME-SOG systems for highly effective image-guided photodynamic therapy.

Surface-enhanced fluorescence imaging on linear arrays of plasmonic half-shells

Here, we perform a Surface-Enhanced Fluorescence (SEF) intensity and lifetime imaging study on linear arrays of silver half-shells (LASHSs), a class of polarization-sensitive hybrid colloidal photonic-plasmonic crystal unexplored previously in SEF. By combining fluorescence lifetime imaging microscopy, scanning confocal fluorescence imaging, Rayleigh scattering imaging, optical microscopy, and finite difference time domain simulations, we identify with high accuracy the spatial locations where SEF effects (intensity increase and lifetime decrease) take place. These locations are the junctions/crevices between adjacent half-shells in the LASHS and locations of high electromagnetic field enhancement and strong emitter-plasmon interactions, as confirmed also by simulated field maps. Such detailed knowledge of the distributed SEF enhancements and lifetime modification distribution, with respect to topography, should prove useful for improved future evaluations of SEF enhancement factors and a more rational design of efficiency-optimized SEF substrates. These linear arrays of metal-coated microspheres expand the family of hybrid colloidal photonic-plasmonic crystals, platforms with potential for applications in optoelectronic devices, fluorescence-based (bio)chemical sensing, or medical assays. In particular, due to the polarized optical response of these LASHSs, specific applications such as hidden tags for anti-counterfeiting or plasmon-enhanced photodetection can be foreseen.

Polarization-sensitive anisotropic plasmonic properties of quantum dots and Au nanorod composites

Polarization-sensitive anisotropic plasmonic interaction between gold nanorods (AuNRs) and quantum dots (QDs) encapsulated in an epoxy resin polymer has been experimentally investigated. The anisotropic plasmonic interaction utilized the polarization-dependent plasmonic properties of aligned AuNR in AuNR-QD composite. AuNRs were aligned by an external AC electric field of 3.5 ×10⁵ Vm^-1. The plasmonic interaction modified QD absorption and emission dependent on excitation light polarization and maximum enchantment of 10% and 59%, respectively. Moreover, anisotropic plasmonic interaction induced directional emission of QDs has improved emission decay rate by 20% and modulated emission polarization ratio of out-of-plane (vertical) and in-plane (horizontal) from 1 to 0.84.

Fluorescence quenching by lipid encased nanoparticles shows that amyloid-β has a preferred orientation in the membrane

Short range plasmonic fields around a nanoparticle can modulate fluorescence or Raman processes.

Imaging optical fields below metal films and metal-dielectric waveguides by a scanning microscope

Laser scanning confocal fluorescence microscopy (LSCM) is now an important method for tissue and cell imaging when the samples are located on the surfaces of glass slides. In the past decade, there has been extensive development of nano-optical structures that display unique effects on incident and transmitted light, which will be used with novel configurations for medical and consumer products. For these applications, it is necessary to characterize the light distribution within short distances from the structures for efficient detection and elimination of bulky optical components. These devices will minimize or possibly eliminate the need for free-space light propagation outside of the device itself. We describe the use of the scanning function of a LSCM to obtain 3D images of the light intensities below the surface of nano-optical structures. More specifically, we image the spatial distributions inside the substrate of fluorescence emission coupled to waveguide modes after it leaks through thin metal films or dielectric-coated metal films. The observed spatial distribution were in general agreement with far-field calculations, but the scanning images also revealed light intensities at angles not observed with classical back focal plane imaging. Knowledge of the subsurface optical intensities will be crucial in the combination of nano-optical structures with rapidly evolving imaging detectors.

Dye-Labeled Polyacryloyl Hydrazide-Ag Nanoparticle Fluorescent Probe for Ultrasensitive and Selective Detection of Au Ion

The efficiency of a fluorescence sensing device based on metal-enhanced fluorescence (MEF) is dependent on the optimization of interaction between the fluorophore and the metal nanoparticle (NP). Herewith, ultrasensitive and selective turn-on sensing of Au³⁺ is achieved by using a suitable combination of fluorophore and metal NP system through sequential MEF effect. Dansyl hydrazide-tagged Ag NPs in the polyacryloyl hydrazide cavity are utilized to sense the picomolar concentration of Au³⁺ in aqueous media. We demonstrated that the selective Au³⁺ sensing is due to the selective deposition of Au on the Ag NP surface over the 16 other metal ions studied. The sensitivity is assigned to the strong overlapping of the emission band of the fluorophore with the surface plasmon band of the Au and improvement of fluorescence signal through successive MEF by Ag and Au colloids. The sensing is associated with a fivefold increase in fluorescence intensity and appearance of violet color of the solution. These luminescent Ag-Au bimetallic NPs may be utilized to trace cancer cells in biological systems and for cell imaging applications.

Several hundred-fold enhanced fluorescence from single fluorophores assembled on silver nanoparticle-dielectric-metal substrate

We observed over 400-fold enhanced fluorescence from single Cy5 molecules assembled on multilayer silver nanoparticle-dielectric-metal (PDM) substrate. This substantial enhancement is due to the near-field enhanced excitation, emission, and interaction of Cy5 with plasmonic nanostructures. Experimental observation is supported by finite-element method calculations.

Selectable Surface and Bulk Fluorescence Imaging with Plasmon-Coupled Waveguides

In this letter, we propose a new method for selective imaging of surface bound probes or simultaneous imaging of surface bound plus fluorescence from dye molecules in bulk water solution. The principle of this method relies on use of two optical modes with different mode distributions, filed decay lengths and polarization states that are sustaining in a plasmon waveguide. The two modes with different decay lengths couple with dye molecules of different regions, at different distances from the PCW-water interface. The emission from two different regions occur as two coupled emission rings with different polarizations and emitting angles in the back focal plane (BFP) images. By using an electric-driven liquid crystal in BFP imaging, we selectively imaged surface or surface plus bulk fluorescence. Accordingly two coupled emissions can be switched ON or OFF independently, that are for either surface or bulk fluorescence imaging. Our work provides a new method for fluorescence imaging or sensing just by using a planar multilayer film, which may be a useful for fluorescence-based techniques in chemistry, materials science, molecular biology, and medicine.

Fluorescence Spectroscopy with Metal-Dielectric Waveguides

We describe a hybrid metal-dielectric waveguide structures (MDWs) with numerous potential applications in the biosciences. These structures consist of a thin metal film coated with a dielectric layer. Depending on the thickness of the dielectric layer, the modes can be localized near the metal, within the dielectric, or at the top surface of the dielectric. The optical modes in a metal-dielectric waveguide can have either S (TE) or P (TM) polarization. The dielectric spacer avoids the quenching, which usually occurs for fluorophores within about 5 nm from the metal. Additionally, the resonances display a sharp angular dependence and can exhibit several hundred-fold increases in intensity (E²) at the silica-air interface relative to the incident intensity. Fluorophores placed on top of the silica layer couple efficiently with the metal, resulting in a sharp angular distribution of emission through the metal and down from the bottom of the structure. This coupling occurs over large distances to several hundred nm away from the metal and was found to be consistent with simulations of the reflectivity of the metal-dielectric waveguides. Remarkably, for some silica thicknesses, the emission is almost completely coupled through the structure with little free-space emission away from the metal-dielectric waveguide. The efficiency of fluorophore coupling is related to the quality of the resonant modes sustained by the metal-dielectric waveguide, resulting in coupling of most of the emission through the metal into the underlying glass substrates. Metal-dielectric waveguides also provide a method to resolve the emission from surface-bound fluorophores from the bulk-phase fluorophores. Metal-dielectric waveguides are simple to fabricate for large surface areas, the resonance wavelength can be adjusted by the dielectric thickness, and the silica surface is suitable for coupling to biomolecules. Metal-dielectric waveguides can have numerous applications in diagnostics and high-throughput proteomics or DNA analysis.

Binding of fusion protein FLSC IgG1 to CCR5 is enhanced by CCR5 antagonist Maraviroc

The CCR5 chemokine receptor is crucial for human immunodeficiency virus type 1 (HIV-1) infection, acting as the principal coreceptor for HIV-1 entry and transmission and is thus an attractive target for antiviral therapy. Studies have suggested that CCR5 surface density and its conformational changes subsequent to virion engagement are rate limiting for entry, and consequently, infection. Not all CCR5 antibodies inhibit HIV-1 infection, suggesting a need for more potent reagents. Here we evaluated full length single chain (FLSC) IgG1, a novel IgG-CD4-gp120(BAL) fusion protein with several characteristics that make it an attractive candidate for treatment of HIV-1 infections, including bivalency and a potentially increased serum half-life over FLSC, the parental molecule. FLSC IgG1 binds two domains on CCR5, the N-terminus and the second extracellular loop, lowering the levels of available CCR5 viral attachment sites. Furthermore, FLSC IgG1 synergizes with Maraviroc (MVC), the only licensed CCR5 antagonist. In this study, we used both microscopy and functional assays to address the mechanistic aspects of the interactions of FLSC IgG1 and MVC in the context of CCR5 conformational changes and viral infection. We used a novel stochastic optical reconstruction microscopy (STORM), based on high resolution localization of photoswitchable dyes to visualize direct contacts between FLSC IgG1 and CCR5. We compared viral entry inhibition by FLSC IgG1 with that of other CCR5 blockers and showed FLSC IgG1 to be the most potent. We also showed that lower CCR5 surface densities in HIV-1 infected primary cells result in lower FLSC IgG1 EC50 values. In addition, CCR5 binding by FLSC IgG1, but not CCR5 Ab 2D7, was significantly increased when cells were treated with MVC, suggesting MVC allosterically increases exposure of the FLSC IgG1 binding site. These data have implications for future antiviral therapy development.

Tamm State-Coupled Emission: Effect of Probe Location and Emission Wavelength

We report the effect of the probe location and wavelength on the emission spatial distribution and spectral properties of fluorophores located on structures which display Tamm states. Our structure consists of a one-dimensional photonic crystal (1DPC)-that is, a multilayer structure of alternate high and low refractive index dielectrics-and a thin top silver film. Simulations show the presence of Tamm and surface plasmon modes in the structure. The electric field intensities for the Tamm modes are located mostly in the dielectric layer below the metal film. The corresponding field intensities for the surface plamon modes are located above the metal film in the distal side. Tamm states can be in resonance with the incident light normal or near normal to the surface, within the light line, and can be accessed without the use of a coupling prism or gratings. We investigated the emission spectra and angular distribution of the emission for probes located above and below the metal film to explore the interaction of fluorophores with Tamm plasmons and surface plasmons modes. Three probes were chosen with different overlap of the emission spectra with the Tamm modes. The fluorophores below the metal film coupled predominantly with the Tamm state and displayed more intense and only Tamm state-coupled emission (TSCE). Probes above the metal film display both surface plasmon-coupled emission (SPCE) and Tamm state-coupled emission. In contrast to SPCE, which shows only KR, P-polarized emission, the Tamm states can display both S- and P-polarized emission and can be populated using both RK and KR illuminations. The TSCE angle is highly sensitive to wavelength, which suggests the use of Tamm structures to provide both directional emission and wavelength dispersion. The combination of plasmonic and photonic structures with directional emission close to surface normal offers the opportunities for new design formats for clinical testing, portable devices, and other fluorescence-based applications.

Control of radiative processes using tunable plasmonic nanopatch antennas

The radiative processes associated with fluorophores and other radiating systems can be profoundly modified by their interaction with nanoplasmonic structures. Extreme electromagnetic environments can be created in plasmonic nanostructures or nanocavities, such as within the nanoscale gap region between two plasmonic nanoparticles, where the illuminating optical fields and the density of radiating modes are dramatically enhanced relative to vacuum. Unraveling the various mechanisms present in such coupled systems, and their impact on spontaneous emission and other radiative phenomena, however, requires a suitably reliable and precise means of tuning the plasmon resonance of the nanostructure while simultaneously preserving the electromagnetic characteristics of the enhancement region. Here, we achieve this control using a plasmonic platform consisting of colloidally synthesized nanocubes electromagnetically coupled to a metallic film. Each nanocube resembles a nanoscale patch antenna (or nanopatch) whose plasmon resonance can be changed independent of its local field enhancement. By varying the size of the nanopatch, we tune the plasmonic resonance by ∼ 200 nm, encompassing the excitation, absorption, and emission spectra corresponding to Cy5 fluorophores embedded within the gap region between nanopatch and film. By sweeping the plasmon resonance but keeping the field enhancements roughly fixed, we demonstrate fluorescence enhancements exceeding a factor of 30,000 with detector-limited enhancements of the spontaneous emission rate by a factor of 74. The experiments are supported by finite-element simulations that reveal design rules for optimized fluorescence enhancement or large Purcell factors.

Controllable metal-enhanced fluorescence in organized films and colloidal system

In recent years, considerable efforts have been devoted to better understand the unique emission properties of fluorophores enhanced by the localized surface plasmon resonance of metal nanoparticles (NPs), due to the widespread applications of fluorescence techniques. It is demonstrated by experiment and theoretical calculation that the enhancement efficiency strongly depends on the morphology of the metal NPs, the spectral overlap between metal and fluorophores, the separation distance between them, and other factors. Among these aspects to be considered are suitable spacer material and assembling methods to control the spatial arrangement of plasmonic NPs and fluorophore with proper optical properties and interactions. In this contribution, we provide a brief overview on recent progress of metal-enhanced fluorescence in organized films and colloidal systems.

Imaging of Protein Secretion from a Single Cell Using Plasmonic Substrates

Detecting, imaging, and monitoring cell function on a single cell basis is very important in the field of immunology research where many molecules are secreted from cells in response to external stimuli including immunization. Here we introduce substrates with plasmonic nanoparticles and fluorescence microscopy as promising imaging methods for studies on molecular processes controlling cell behavior, particularly secretion of cytokines. We developed unique composition of silver and silica layers of plasmonic nanostructures which resulted in fluorescence enhancement of more than 200-fold for ensemble of molecules in the immunoassay. For the proof of concept demonstration, we used primary mouse macrophages and imaged tumor necrosis alpha (TNF-α) secretion after stimulation of the cells with lipopolysaccharide (LPS). We demonstrate that metal-enhanced fluorescence assay provides imaging capability detection of cytokine secretion from a single cell without extensive biochemical procedures as required with standard methods. In addition it is demonstrated that cell viability can be controlled during secretion.