Systematic Computational Study of the Effect of Silver Nanoparticle Dimers on the Coupled Emission from Nearby Fluorophores

Center for Fluorescence Spectroscopy, Medical Biotechnology Center, University of Maryland School of Medicine, 725 West Lombard Street, Baltimore, Maryland 21201.
The Journal of Physical Chemistry C (Impact Factor: 4.77). 07/2008; 112(30):11236-11249. DOI: 10.1021/jp802414k
Source: PubMed


We use the finite-difference time-domain method to predict how fluorescence is modified if the fluorophore is located between two silver nanoparticles of a dimer system. The fluorophore is modeled as a radiating point dipole with orientation defined by its polarization. When a fluorophore is oriented perpendicular to the metal surface, there is a large increase in total power radiated through a closed surface containing the dimer system, in comparison to the isolated fluorophore and the case of a fluorophore near a single nanoparticle. The increase in radiated power indicates increases in the relative radiative decay rates of the emission near the nanoparticles. The angle-resolved far-field distributions of the emission in a single plane are also computed. This is informative as many experimental conditions involve collection optics and detectors that collect the emission along a single plane. For fluorophores oriented perpendicular to the metal surfaces, the dimer systems lead to significant enhancements in the fluorescence emission intensity in the plane. In contrast, significant emission quenching occurs if the fluorophores are oriented parallel to the metal surfaces. We also examine the effect of the fluorophore on the near-field around the nanoparticles and correlate our results with surface plasmon excitations.

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    • "All of these effects are based on the same phenomenon, which is the interaction of fluorophores (dipoles) with resonant or nonresonant plasmons on the metal nanoparticles or surfaces [28] [29] [30] [31]. These interactions can occur both during excitation, via the high light-induced fields, and during emission, by changes in the radiative decay rate [32] [33] [34] [35]. Fluorophore–plasmon coupling offers the opportunity to combine fluorescence with the rapidly developing field of plasmonics [36] [37] [38] [39]. "
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    ABSTRACT: There is a continuing need to increase the brightness and photostability of fluorophores for use in biotechnology, medical diagnostics and cell imaging. One approach developed during the past decade is to use metallic surfaces and nanostructures. It is now known that excited state fluorophores display interactions with surface plasmons, which can increase the radiative decay rates, modify the spatial distribution of emission and result in directional emission. One important example is Surface Plasmon-Coupled Emission (SPCE). In this phenomenon the fluorophores at close distances from a thin metal film, typically silver, display emission over a small range of angles into the substrate. A disadvantage of SPCE is that the emission occur at large angles relative to the surface normal, and at angles which are larger than the critical angle for the glass substrate. The large angles make it difficult to collect all the coupled emission and have prevented use of SPCE with high-throughput and/or array applications. In the present report we describe a simple multi-layer metal-dielectric structure which allows excitation with light that is perpendicular (normal) to the plane and provides emission within a narrow angular distribution that is normal to the plane. This structure consist of a thin silver film on top of a multi-layer dielectric Bragg grating, with no nanoscale features except for the metal or dielectric layer thicknesses. Our structure is designed to support optical Tamm states, which are trapped electromagnetic modes between the metal film and the underlying Bragg grating. We used simulations with the transfer matrix method to understand the optical properties of Tamm states and localization of the modes or electric fields in the structure. Tamm states can exist with zero in-plane wavevector components and can be created without the use of a coupling prism. We show that fluorophores on top of the metal film can interact with the Tamm state under the metal film and display Tamm state-coupled emission (TSCE). In contrast to SPCE, the Tamm states can display either S- or P-polarization. The TSCE angle is highly sensitive to wavelength which suggests the use of Tamm structures to provide both directional emission and wavelength dispersion. Metallic structures can modify fluorophore decay rates but also have high losses. Photonic crystals have low losses, but may lack the enhanced light-induced fields near metals. The combination of plasmonic and photonic structures offers the opportunity for radiative decay engineering to design new formats for clinical testing and other fluorescence-based applications.
    Analytical Biochemistry 10/2013; 445(1). DOI:10.1016/j.ab.2013.10.009 · 2.22 Impact Factor
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    ABSTRACT: In recent years metal-enhanced fluorescence (MEF) using silver particles has been reported for a number of fluorophores emitting at visible wavelengths. However it was generally thought that silver particles would always quench fluorescence at shorter wavelengths. We now report the observation of metal-enhanced fluorescence of the tryptophan analogue N-acetyl-L-tryptophanamide (NATA) on silver nano-structured surfaces. NATA is a model for the intrinsic tryptophan emission from proteins. We have also studied the effects of silver nanostructures on the emission of N-acetyl-L-tyrosinamide (NATA-tyr). In the case of NATA we observed increased emission, decrease in fluorescence lifetimes, and increase in photostability when NATA was embedded in 15 nm thick spin-casted poly(vinyl alcohol) film on silver nanostructured surfaces. We have also investigated the effects of silver nanostructures on the emission from thin poly(vilnyl alcohol) films containing NATA-tyr. However, we have observed no increase in fluorescence signal for NATA-tyr on silver nanostructures. To understand these results we performed numerical calculations using the Finite-Difference Time-Domain (FDTD) technique to model a tryptophan-wavelength dipole near a spherical silver particle. Our calculations reveal an enhancement of the power of the radiated emission by the excited-state fluorophore in proximity to a 100 nm silver nanoparticle covering the emission spectra of NATA and NATA-tyr. These calculations show a clear wavelength dependence with the specific spectral region displaying low-enhancement at the shorter NATA-tyr wavelength and higher enhancement at NATA emission wavelength. Our FDTD calculations also reveal that excited fluorophores in the near-field of a 100 nm silver nanoparticle can induce enhancement fields of varying degrees of the intensity of the near-fields around the particle that is dependent on the wavelength of the emission. We believe this enhanced near-fields play a role in our observation of MEF from metal surfaces. The enhanced emission of NATA on silver nanostructures suggests that the extension of MEF to the UV region opens new possibilities to study tryptophan-containing proteins without labeling with longer wavelength fluorophores towards label free detection of biomolecules.
    The Journal of Physical Chemistry C 02/2008; 112(46):17957-17963. DOI:10.1021/jp807025n · 4.77 Impact Factor
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    ABSTRACT: We use finite-difference time-domain calculations to show that aluminum nanoparticles are efficient substrates for metal-enhanced fluorescence (MEF) in the ultraviolet (UV) for the label-free detection of biomolecules. The radiated power enhancement of the fluorophores in proximity to aluminum nanoparticles is strongly dependent on the nanoparticle size, fluorophore-nanoparticle spacing, and fluorophore orientation. Additionally, the enhancement is dramatically increased when the fluorophore is between two aluminum nanoparticles of a dimer. Finally, we present experimental evidence that functionalized forms of amino acids tryptophan and tyrosine exhibit MEF when spin-coated onto aluminum nanostructures.
    Analytical Chemistry 02/2009; 81(4):1397-403. DOI:10.1021/ac802118s · 5.64 Impact Factor
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