Fluorophore-Induced Plasmonic Current Generation from Aluminum Nanoparticle Films.

In this paper we demonstrate fluorophore induced plasmonic current (FIPC) from aluminum nanoparticle films. It has been previously shown that near-field excited fluorophores are able to plasmonically couple with metal nanoparticle films (MNF's) and induce surface plasmons, which in turn leads to a direct measurable electrical current through the MNF. These currents have been detected and quantified in noble metal MNF's, however due to future envisioned cost considerations there has been a push to adapt FIPC for use with less expensive metals. Subsequently, we observe that plasmonic aluminum films are able to produce these current changes when in close proximity to excited fluorophores, and the magnitude of the current changes are respective to the magnitude of the extinction coefficients of the fluorophores themselves. These findings also further support recent literature reports showing the inverse relationship between metal-enhanced fluorescence (MEF) and FIPC.

Plasmonic enhancement of nitric oxide generation.

While the utility of reactive oxygen species in photodynamic therapies for both cancer treatments and antimicrobial applications has received much attention, the inherent potential of reactive nitrogen species (RNS) including nitric oxide (NO˙) for these applications should not be overlooked. In recent years, NO˙ donor species with numerous-including photodynamic-mechanisms have been classified with efficacy in antimicrobial and therapeutic applications. While properties of NO˙ delivery may be tuned structurally, herein we describe for the first time a method by which photodynamic NO˙ release is amplified simply by utilizing a plasmonic metal substrate. This is a process we term "metal-enhanced nitric oxide release", or ME-NO˙. Using donor agents known as brominated carbon nanodots (BrCND), also the first carbon nanodot variation classified to release NO˙ photodynamically, and the fluorescence-on probe DAF-FM, we report metal-enhanced release of NO˙ 2- to 6-fold higher than what is achieved under classical conditions. Factors affecting the plasmon-amplified photodynamic system are subsequently studied, including exposure times, excitation powers, and surface area, and consistent ME-NO˙ factors are reported from BrCND across these tunable conditions. Only probe concentration is determined to impact the detected ME-NO˙ factor, with higher concentrations resulting in improved detectability of "actual" NO˙ release enhancement. Further, principles of metal-enhanced fluorescence (MEF) are applied to achieve a faster, high-throughput experimental method with improved data resolution in ME-NO˙ detection. The results have significant implications for the improvement of not just carbon nanodot NO˙ donor agents, but a wide spectrum of photoactivated NO˙ donor systems as well.

Low-concentration trypsin detection from a metal-enhanced fluorescence (MEF) platform: Towards the development of ultra-sensitive and rapid detection of proteolytic enzymes.

Proteolytic enzymes, which serve to degrade proteins to their amino acid building blocks, provide a distinct challenge for both diagnostics and biological research fields. Due to their ubiquitous presence in a wide variety of organisms and their involvement in disease, proteases have been identified as biomarkers for various conditions. Additionally, low-levels of proteases may interfere with biological investigation, as contamination with these enzymes can physically alter the protein of interest to researchers, resulting in protein concentration loss or subtler polypeptide clipping that leads to a loss of functionality. Low levels of proteolytic degradation also reduce the shelf-life of commercially important proteins. Many detection platforms have been developed to achieve low-concentration or low-activity detection of proteases, yet many suffer from limitations in analysis time, label stability, and ultimately sensitivity. Herein we demonstrate the potential utility of fluorescein derivatives as fluorescent labels in a new, turn-off enzymatic assay based on the principles of metal-enhanced fluorescence (MEF). For fluorescein sodium salt alone on nano-slivered 96-well plates, or Quanta Plates™, we report up to 11,000x enhancement for fluorophores within the effective coupling or enhancement volume region, defined as ~100 nm from the silver surface. We also report a 9% coefficient of variation, and detection on the picomolar concentration scale. Further, we demonstrate the use of fluorescein isothiocyanate-labeled YebF protein as a coating layer for a MEF-based, Quanta Plate™ enzymatic activity assay using trypsin as the model enzyme. From this MEF assay we achieve a detection limit of ~1.89 ng of enzyme (2.8 mBAEE activity units) which corresponds to a minimum fluorescence signal decrease of 10%. The relative success of this MEF assay sets the foundation for further development and the tuning of MEF platforms for proteolytic enzyme sensing not just for trypsin, but other proteases as well. In addition, we discuss the future development of ultra-fast detection of proteases via microwave-accelerated MEF (MAMEF) detection technologies.

Phosphorus sequestration in the form of polyphosphate by microbial symbionts in marine sponges.

Marine sponges are major habitat-forming organisms in coastal benthic communities and have an ancient origin in evolution history. Here, we report significant accumulation of polyphosphate (polyP) granules in three common sponge species of the Caribbean coral reef. The identity of the polyP granules was confirmed by energy-dispersive spectroscopy (EDS) and by the fluorescence properties of the granules. Microscopy images revealed that a large proportion of microbial cells associated with sponge hosts contained intracellular polyP granules. Cyanobacterial symbionts cultured from sponges were shown to accumulate polyP. We also amplified polyphosphate kinase (ppk) genes from sponge DNA and confirmed that the gene was expressed. Based on these findings, we propose here a potentially important phosphorus (P) sequestration pathway through symbiotic microorganisms of marine sponges. Considering the widespread sponge population and abundant microbial cells associated with them, this pathway is likely to have a significant impact on the P cycle in benthic ecosystems.

Ultra-fast and sensitive detection of non-typhoidal Salmonella using microwave-accelerated metal-enhanced fluorescence ("MAMEF").

Certain serovars of Salmonella enterica subsp. enterica cause invasive disease (e.g., enteric fever, bacteremia, septicemia, meningitis, etc.) in humans and constitute a global public health problem. A rapid, sensitive diagnostic test is needed to allow prompt initiation of therapy in individual patients and for measuring disease burden at the population level. An innovative and promising new rapid diagnostic technique is microwave-accelerated metal-enhanced fluorescence (MAMEF). We have adapted this assay platform to detect the chromosomal oriC locus common to all Salmonella enterica subsp. enterica serovars. We have shown efficient lysis of biologically relevant concentrations of Salmonella spp. suspended in bacteriological media using microwave-induced lysis. Following lysis and DNA release, as little as 1 CFU of Salmonella in 1 ml of medium can be detected in <30 seconds. Furthermore the assay is sensitive and specific: it can detect oriC from Salmonella serovars Typhi, Paratyphi A, Paratyphi B, Paratyphi C, Typhimurium, Enteritidis and Choleraesuis but does not detect Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Streptococcus pneumoniae, Haemophilus influenzae or Acinetobacter baumanii. We have also performed preliminary experiments using a synthetic Salmonella oriC oligonucleotide suspended in whole human blood and observed rapid detection when the sample was diluted 1:1 with PBS. These pre-clinical data encourage progress to the next step to detect Salmonella in blood (and other ordinarily sterile, clinically relevant body fluids).

Surface plasmon coupled fluorescence in the visible to near-infrared spectral regions using thin nickel films: application to whole blood assays.

Nickel thin films thermally evaporated onto glass supports are used to demonstrate surface plasmon coupled fluorescence (SPCF) over a broad 400 nm wavelength range (400-800 nm) for potential assays that can be run in buffer and/or whole blood. In contrast to traditional fluorescence-based assays, SPCF converts otherwise isotropic emission into highly directional and polarized emission, an attractive concept for surface assays. Theoretical Fresnel calculations performed in the ultraviolet to near-infrared spectral range (344-1240 nm) predict the near-field generation of surface plasmons in 15 and 20 nm nickel thin films. The angles of minimum reflectivity for nickel thin films with 10 nm SiO(x) and 30 nm polymer overcoats over the 428-827 nm wavelength range occur over a 10 degree range. To experimentally corroborate the theoretical calculations, a polymeric solution of five different fluorophores, POPOP (lambda(max, emission) = 428 nm), FITC (517 nm), S101 (600 nm), Zn PhCy (710 nm), and IR 780 (814 nm), were spin coated separately onto 15 and 20 nm nickel thin films. SPCF intensity (s- and p-polarized) from fluorophores at the corresponding emission lambda(max) was measured at angles between 0-90 degrees. In addition, the free-space emission and SPCF intensity of FITC on 20 nm nickel thin film were also measured to demonstrate the angular-dependent nature of SPCF. SPCF from nickel thin films was p-polarized and highly directional with lambda(max) confirmed at an angle of 65 degrees for all the fluorophores as predicted by Fresnel calculations. The utility of nickel thin films for whole blood bioassays is demonstrated with a long-wavelength fluorophore, where the SPCF intensity of Zn PhCy (50 pM-50 microM) in whole blood at 710 nm was measured at 65 degrees. Using Fresnel calculations it is also predicted that the evanescent field above the nickel films penetrates deeper into solution than for other metals used to date for SPCF, an attractive notion in SPCF-based biosensing applications.

Directional, Broad, and Fixed Angle Surface Plasmon Coupled Fluorescence from Iron Thin Films.

Fixed angle surface plasmon coupled fluorescence (SPCF) from iron thin films is demonstrated for the first time. The optimum thickness of the iron thin films was determined to be 15 nm using Fresnel calculations. The angles of minimum reflectivity for light at 496-814 nm were predicted to occur at a fixed and broad range of angles of 60-70°. Experimental corroboration of these predictions were undertaken by employing fluorescein isothiocyanate (emission peak at 517 nm), rhodamine B (580 nm), zinc phthalocyanine (710 nm), and IR 780 dye (814 nm). SPCF emission from all four fluorophores was directional, p-polarized, and observed at a fixed angle of ∼65°.

Plasmonic engineering of singlet oxygen generation.

In this article, we report metal-enhanced singlet oxygen generation (ME(1)O2). We demonstrate a direct relationship between the singlet oxygen yield of a common photosensitizer (Rose Bengal) and the theoretical electric field enhancement or enhanced absorption of the photosensitizer in proximity to metallic nanoparticles. Using a series of photosensitizers, sandwiched between silver island films (SiFs), we report that the extent of singlet oxygen enhancement is inversely proportional to the free space singlet oxygen quantum yield. By modifying plasmon coupling parameters, such as nanoparticle size and shape, fluorophore/particle distance, and the excitation wavelength of the coupling photosensitizer, we can readily tune singlet oxygen yields for applications in singlet oxygen-based clinical therapy.

Microwave-triggered chemiluminescence with planar geometrical aluminum substrates: theory, simulation and experiment.

Previously we combined common practices in protein detection with chemiluminescence, microwave technology, and metal-enhanced chemiluminescence to demonstrate that we can use low power microwaves to substantially increase enzymatic chemiluminescent reaction rates on particulate silvered substrates. We now describe the applicability of continuous aluminum metal substrates to potentially further enhance or "trigger" enzymatic chemiluminescence reactions. Furthermore, our results suggest that the extent of chemiluminescence enhancement for surface and solution based enzyme reactions critically depends on the surface geometry of the aluminum film. In addition, we also use FDTD simulations to model the interactions of the incident microwave radiation with the aluminum geometries used. We demonstrate that the extent of microwave field enhancement for solution and surface based chemiluminescent reactions can be ascribed to "lightning rod" effects that give rise to different electric field distributions for microwaves incident on planar aluminum geometries. With these results, we believe that we can spatially and temporally control the extent of triggered chemiluminescence with low power microwave (Mw) pulses and maximize localized microwave triggered metal-enhanced chemiluminescence (MT-MEC) with optimized planar aluminum geometries. Thus we can potentially further improve the sensitivity of immunoassays with significantly enhanced signal-to-noise ratios.