Michael J. R. Previte

Plasmonic engineering of singlet oxygen generation

In this article, we report metal-enhanced singlet oxygen generation (ME1O2). 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.

Metal-enhanced fluorescence from copper substrates

In this letter, the authors report the observation of metal-enhanced fluorescence emission from fluorophores deposited on copper (Cu) substrates. Different thicknesses of Cu particulate films (from 1 to 5nm) were deposited onto glass slides using thermal vapor deposition. Fluorophores positioned in close proximity to the Cu films show fluorescence enhancement as a function of the Cu thickness increases, reaching a maximum (2.5-fold) at ≈3nm. The findings strongly suggest that surface plasmons from Cu can radiate and therefore enhance a fluorophore’s spectral properties, similar to observations reported for both silver and gold nanoparticle deposited substrates.

Metal-enhanced singlet oxygen generation: a consequence of plasmon enhanced triplet yields.

In this Rapid Communication, we report the first observation of Metal-Enhanced singlet oxygen generation (ME1O2). Rose Bengal in close proximity to Silver Island Films (SiFs) can generate more singlet oxygen, a three-fold increase observed, as compared to an identical glass control sample but containing no silver. The enhanced absorption of the photo-sensitizer, due to coupling to silver surface plasmons, facilitates enhanced singlet oxygen generation. The singlet oxygen yield can potentially be adjusted by modifying the choice of MEF (Metal-Enhanced Fluorescence) & MEP (Metal Enhance Phosphorescence) parameters, such as distance dependence for plasmon coupling and wavelength emission of the coupling fluorophore. This is a most helpful observation in understanding the interactions between plasmons and lumophores, and this approach may well be of significance for singlet oxygen based clinical therapy.

Metal-enhanced fluorescence: Surface plasmons can radiate a fluorophore’s structured emission

In this letter, the authors report the observation of metal-enhanced structured fluorescence emission. Perylene in close proximity to silver island films (SiFs) enhances the structured fluorescence emission intensity. In this regard, an approximately two-fold higher perylene fluorescence intensity was observed from SiFs as compared to a glass control sample, containing no silver nanoparticles. The findings strongly suggest that surface plasmons can radiate a fluorophore’s vibrational structure. This observation is helpful in the authors’ understanding, not only for studying the interactions between plasmons and lumophores but also for their laboratories’ continued efforts to develop a unified plasmon-lumophore theory.

Surface Plasmon Coupled Fluorescence in the Ultraviolet and Visible Spectral Regions Using Zinc Thin Films

The use of zinc thin films deposited onto glass supports for surface plasmon coupled fluorescence (SPCF) over a broad 200 nm wavelength range is demonstrated. Fresnel calculations performed in the ultraviolet and visible spectral range are predicted to generate surface plasmon modes in 30 nm zinc thin films. In this spectral range, the extent of coupling of light to zinc thin films was shown to be significant as compared to similar aluminum, gold, and silver thin films. The experimental demonstration of SPCF using 30 nm zinc thin films in the ultraviolet and visible spectral regions was undertaken using three different fluorophores 2-AP, POPOP, and FITC, respectively. Surface plasmon coupled fluorescence from zinc thin films was p-polarized and highly directional with lambda max conferred at an angle of 58, 68, and 60 degrees for FITC, POPOP, and 2-AP, respectively. s-Polarized emission from zinc thin films was negligible for all fluorophores except for a sample spin coated from a 10% PVA solution, which resulted in significant s-polarized emission due to the generation of waveguide modes. The experimental results are consistent with reflectivity curves that are theoretically predicted using Fresnel calculations. Given the growing use and utility of plasmon-enhanced fluorescence in the analytical and biological sciences, our findings will serve as a useful tool for workers in the ultraviolet and visible spectral regions.

Microwave-accelerated surface plasmon-coupled directional luminescence 2: A platform technology for ultra fast and sensitive target DNA detection in whole blood

The application of Microwave-Accelerated Surface Plasmon-Coupled Luminescence (MA-SPCL) to fast and sensitive DNA hybridization assays in buffer and whole blood is presented. In this regard, a model DNA hybridization assay whereby a fluorophore-labeled target ssDNA specific to human immunodeficiency, Hepatitis C (Hep C), is probed by an anchor probe immobilized on thin gold films, is driven to completion within 1 min with microwave heating, as compared to an identical assay completed in approximately 4 h at room temperature. Finite-Difference Time-Domain calculations show that gold disks are preferentially heated around the edges creating a temperature gradient along the disks, which in turn results in the larger influx of complementary DNA towards anchor probe-modified surface. Thermal images of the assay platform during microwave heating also provide additional information on the microwave heating pattern in the microwave cavity. Finally, the effects of low power microwave heating on the ability of DNA to re-hybridize with the complimentary target on the surface gold films, which allows the multiple re-use of the gold films, is demonstrated. The MA-SPCL technique offers an alternative approach to current DNA based detection technologies, especially when speed and sensitivity are required, such as in the identification of DNA or even RNA-based diseases using whole blood samples that affect human health.

Surface plasmon coupled fluorescence from copper substrates

Previously, we reported the observation of metal enhanced emission from copper nanoparticles. In this letter, we report the observation of surface plasmon coupled fluorescence (SPCF) or emission from thin copper (Cu) continuous films. Using Fresnel calculations to theoretically calculate the reflectance curves for polymethyl methacrylate films doped with sulforhodamine 101, we correlated the minimum reflectance angles for both s- and p-polarized lights with maximum fluorescence emission. These observations reveal that we can achieve SPCF with copper substrates and possibly design inexpensive copper substrates for fluorescent sensing applications and potentially, fluorescent based electrical signals and circuits.

Metal-enhanced e-type fluorescence.

In this letter, we report metal-enhanced e-type fluorescence. Eosin in close proximity to silver island films (SiFs) shows enhanced e-type fluorescence with an approximately two-fold higher intensity observed from SiFs, as compared to a control sample. Our findings suggest two complementary mechanisms for the enhancement: surface plasmons can radiate e-type delayed fluorescence efficiently and enhanced absorption also facilitates enhanced emission from both S(1) and T(1) states. This observation is helpful in our understanding not only for studying the interactions between plasmons and fluorophores but also for our laboratories continued efforts to develop a unified plasmon-lumophore description.

Metal-enhanced superoxide generation: A consequence of plasmon-enhanced triplet yields

The authors report significant enhancements in the generation of superoxide for fluorophores in close proximity to silver nanoparticles. A distance dependence study of the fluorophores from the metallic nanostructures, combined with carefully chosen control samples, confirms that the enhancements in superoxide generation are due to plasmon-enhanced triplet yields, a consequence of the distance dependent sensitizer extent of excitation. This observation strongly agrees with current models developed by the authors. Given that the generation of superoxide and other oxygen species is important for many chemical and biological applications, then we believe that our findings are likely to fuel a wealth of oxygen-based plasmon-enhanced triplet assays.

Microwave-triggered metal-enhanced chemiluminescence (MT-MEC): application to ultra-fast and ultra-sensitive clinical assays.

In this rapid communication we describe a new approach to protein detection with chemiluminescence. By combining common practices in protein detection with chemiluminescence, microwave technology, and metal-enhanced chemiluminescence, we show that we can use low power microwaves to substantially increase enzymatic chemiluminescent reaction rates on metal substrates. As a result, we have found that we can in essence trigger chemiluminescence with low power microwave (Mw) pulses and ultimately, perform on-demand protein detection assays. Using microwave triggered metal-enhanced chemiluminescence (MT-MEC), we not only improve the sensitivity of immunoassays with enhanced signal-to-noise ratios, but we also show that we can accurately quantify protein concentrations by integrating the photon flux for discrete time intervals.