Surface plasmon coupled fluorescence in the ultraviolet and visible spectral regions using zinc thin films.
ABSTRACT 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.
Article: Polypyrrole-coated electrodes for the delivery of charge and neurotrophins to cochlear neurons.[show abstract] [hide abstract]
ABSTRACT: Sensorineural hearing loss is associated with gradual degeneration of spiral ganglion neurons (SGNs), compromising hearing outcomes with cochlear implant use. Combination of neurotrophin delivery to the cochlea and electrical stimulation from a cochlear implant protects SGNs, prompting research into neurotrophin-eluting polymer electrode coatings. The electrically conducting polypyrrole/para-toluene sulfonate containing neurotrophin-3 (Ppy/pTS/NT3) was applied to 1.7 mm2 cochlear implant electrodes. Ppy/pTS/NT3-coated electrode arrays stored 2 ng NT3 and released 0.1 ng/day with electrical stimulation. Guinea pigs were implanted with Ppy/pTS or Ppy/pTS/NT3 electrode arrays two weeks after deafening via aminoglycosides. The electrodes of a subgroup of these guinea pigs were electrically stimulated for 8 h/day for 2 weeks. There was a loss of SGNs in the implanted cochleae of guinea pigs with Ppy/pTS-coated electrodes indicative of electrode insertion damage. However, guinea pigs implanted with electrically stimulated Ppy/pTS/NT3-coated electrodes had lower electrically-evoked auditory brainstem response thresholds and greater SGN densities in implanted cochleae compared to non-implanted cochleae and compared to animals implanted with Ppy/pTS-coated electrodes (p<0.05). Ppy/pTS/NT3 did not exacerbate fibrous tissue formation and did not affect electrode impedance. Drug-eluting conducting polymer coatings on cochlear implant electrodes present a clinically viable method to promote preservation of SGNs without adversely affecting the function of the cochlear implant.Biomaterials 01/2009; 30(13):2614-24. · 7.40 Impact Factor
[show abstract] [hide abstract]
ABSTRACT: Polypyrrole (PPy) is a biocompatible, electrically conductive polymer that has great potential for battery, sensor, and neural implant applications. Its amorphous structure and insolubility, however, limit the experimental techniques available to study its structure and properties at the atomic level. Previous theoretical studies of PPy in bulk are also scarce. Using ab initio calculations, we have constructed a molecular mechanics force field of chloride-doped PPy (PPyCl) and undoped PPy. This model has been designed to integrate into the OPLS force field, and parameters are available for the Gromacs and TINKER software packages. Molecular dynamics (MD) simulations of bulk PPy and PPyCl have been performed using this force field, and the effects of chain packing and electrostatic scaling on the bulk polymer density have been investigated. The density of flotation of PPyCl films has been measured experimentally. Amorphous X-ray diffraction of PPyCl was obtained and correlated with atomic structures sampled from MD simulations. The force field reported here is foundational for bridging the gap between experimental measurements and theoretical calculations for PPy based materials.Polymer 10/2010; 51(21):4985-4993. · 3.44 Impact Factor
Article: Impact of Scaffold Micro and Macro Architecture on Schwann Cell Proliferation under Dynamic Conditions in a Rotating Wall Vessel Bioreactor.[show abstract] [hide abstract]
ABSTRACT: Over the last decade tissue engineering has emerged as a powerful alternative to regenerate lost tissues owing to trauma or tumor. Evidence shows that Schwann cell containing scaffolds have improved performance in vivo as compared to scaffolds that depend on cellularization post implantation. However, owing to limited supply of cells from the patients themselves, several approaches have been taken to enhance cell proliferation rates to produce complete and uniform cellularization of scaffolds. The most common approach is the application of a bioreactor to enhance cell proliferation rate and therefore reduce the time needed to obtain sufficiently significant number of glial cells, prior to implantation.In this study, we show the application of a rotating wall bioreactor system for studying Schwann cell proliferation on nanofibrous spiral shaped scaffolds, prepared by solvent casting and salt leaching techniques. The scaffolds were fabricated from polycaprolactone (PCL), which has ideal mechanical properties and upon degradation does not produce acidic byproducts. The spiral scaffolds were coated with aligned or random nanofibers, produced by electrospinning, to provide a substrate that mimics the native extracellular matrix and the essential contact guidance cues.At the 4 day time point, an enhanced rate of cell proliferation was observed on the open structured nanofibrous spiral scaffolds in a rotating wall bioreactor, as compared to static culture conditions. However, the cell proliferation rate on the other contemporary scaffolds architectures such as the tubular and cylindrical scaffolds show reduced cell proliferation in the bioreactor as compared to static conditions, at the same time point. Moreover, the rotating wall bioreactor does not alter the orientation or the phenotype of the Schwann cells on the aligned nanofiber containing scaffolds, wherein, the cells remain aligned along the length of the scaffolds. Therefore, these open structured spiral scaffolds pre-cultured with Schwann cells, in bioreactors could potentially shorten the time needed for grafts for peripheral nerve regeneration.Materials Science and Engineering C 01/2011; 31(1):22-29. · 2.69 Impact Factor
Surface Plasmon Coupled Fluorescence in the
Ultraviolet and Visible Spectral Regions Using Zinc
Kadir Aslan, Michael J. R. Previte, Yongxia Zhang, and Chris D. Geddes*
Institute of Fluorescence, Laboratory for Advanced Medical Plasmonics and Laboratory for Advanced Fluorescence
Spectroscopy, Medical Biotechnology Center, University of Maryland Biotechnology Institute, 725 West Lombard
Street, Baltimore, Maryland 21201
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 dif-
ferent fluorophores 2-AP, POPOP, and FITC, respec-
tively. Surface plasmon coupled fluorescence from zinc
thin films was p-polarized and highly directional with λmax
conferred at an angle of 58, 68, and 60° 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 fluores-
cence in the analytical and biological sciences, our find-
ings will serve as a useful tool for workers in the ultraviolet
and visible spectral regions.
Since the early observations that the spontaneous emission
rate of fluorescent species can be modified with close-proximity
planar metal surfaces,1-5numerous studies have focused on
metal-fluorophore interactions.6-8The spontaneous emission of
fluorescent species near-to metals follows a distance dependent
radiative and/or nonradiative decay channel, which is affected by
the dipole orientation of the fluorescent species.9The emission
rate oscillates as the distance between the metal and the dipole
is steadily increased, and the strength of these oscillations
decreases since the dipole is a point source. A parallel dipole (s-
polarized) is canceled out by its own image on the metal surface
while a perpendicular dipole (p-polarized) is coupled to the surface
plasmon modes and is subsequently enhanced. Several authors
have calculated the distance dependent emission rate with the
assumption that the metal surface is perfect and the rotation of
the dipole moment is within the excited-state lifetime.10
species is larger than 300 nm, decay processes occur through
traditional far-field radiation (i.e., fluorescence), while excited states
can couple/induce surface plasmons at ≈10-300 nm. Below 10 nm,
fluorescence typically decays through nonradiative channels, often
referred to as fluorescence damping or quenching.11The coupling
of fluorescence from a population of randomly oriented fluorescent
species and the subsequent polarized and directional emission from
the metal surface is well described by a phenomenon called surface
plasmon coupled fluorescence.8,12,13As a result, it became possible
to achieve better detectability of fluorophores at metal interfaces,
which facilitates the detection of surface bound biomolecules with a
technique often referred to as surface plasmon fluorescence spec-
In SPFS, the excitation of the fluorescent species can be
achieved either directly from the air side (reverse Kretschmann
configuration) or through a prism (Kretschmann configuration).13
In the reverse Kretschmann configuration, the fluorescent species
are excited from the air side (at any incidence angle) and the
coupled fluorescence emission is then detected from the prism
side. This configuration also allows the detection of isotropic (free-
space) emission from the air side. In the Kretschmann configu-
ration, the p-polarized light emerges through the prism at the
surface plasmon resonance angle and generates surface plasmon
modes at the metal-dielectric interface. The generated evanescent
wave penetrates the subsequent dielectric layers, and the excita-
tion of the fluorescent species is achieved. The fluorescence
emission is then detected from the air side as free-space emission
or from the prism side, where the surface plasmon coupled
(9) Barnes, W. L. J. Mod. Opt. 1998, 45, 661–699.
(10) Vasilev, K.; Knoll, W.; Kreiter, M. J. Chem. Phys. 2004, 120, 3439–3445.
(11) Ford, G. W.; Weber, W. H. Phys. Rep. 1984, 113, 195–287.
(12) Morawitz, H.; Philpott, M. R. Phys. Rev. B 1974, 10, 4863–4868.
(13) Liebermann, T.; Knoll, W. Colloids Surf., A 2000, 171, 115–130.
* Corresponding author. E-mail: email@example.com.
(1) Drexhage, K. H. Ber. Bunsen-Ges. 1968, 72, 329.
(2) Kuhn, H. J. Chem. Phys. 1970, 70, 101.
(3) Philpott, M. R. Chem. Phys. Lett. 1973, 19, 435–439.
(4) Tews, K. H. Ann. Phys. (Weinheim, Ger.) 1973, 29, 97–120.
(5) Chance, R. R.; Prock, A.; Silbey, R. J. Chem. Phys. 1974, 60, 2744–2748.
(6) Persson, B. N. J. J. Phys. C: Solid State Phys. 1978, 11, 4251–4269.
(7) Benner, R. E.; Dornhaus, R.; Chang, R. K. Opt. Commun. 1979, 30, 145–
(8) Weber, W. H.; Eagen, C. F. Bull. Am. Phys. Soc. 1979, 24, 441–441.
Anal. Chem. 2008, 80, 7304–7312
10.1021/ac800923n CCC: $40.75 2008 American Chemical Society
Analytical Chemistry, Vol. 80, No. 19, October 1, 2008
Published on Web 09/03/2008
fluorescence is coupled at a specific angle (cone). Both configura-
tions are highly attractive in analytical sensing applications.14,15
In previous studies on SPCF, aluminum thin films were
demonstrated to have use in the UV spectral range16and silver
thin films were mostly used in the visible spectral range.17Gold17
and copper18thin films, which have similar optical and electronic
properties, have found use in the red spectral region. The use of
these noble metals in SPFS is due to their nearly free-electron
behavior in these spectral regions, giving rise to a low εi/εrratio
(εi, imaginary component of the dielectric function; εr, real
component of the dielectric function), which in turn yields sharp,
narrow, and deep reflectivity minima. Planar gold films are usually
the substrate of choice13,17due to their versatility, i.e., they are
inert and can undergo further chemical modification without losing
physical and electronic properties. The optimum thickness of the
metals for SPCF was previously determined to be ≈20 nm for
aluminum thin films,1635 nm for copper thin films18to ≈50 nm
for gold thin films17and silver thin films.19
One can find numerous reports on the utilization of SPCF in
bioassays.20-25These include studies for DNA hybridization,20,21,24
immunoassays,25-28and protein detection.14In all of these reports,
one of the biological assay components is covalently linked to the
metal surface and their interactions with the biomolecule of
interest in the presence of a third fluorophore-labeled biomolecule
is monitored via the change in SPCF intensity from the back of
the prism. Subsequently, the concentration of the biomolecule of
interest is directly calculated from the SPCF intensity.14,15
Zinc has been rarely used in combination with fluorescence;29,30
however, considerable attention has been given to zinc due to
other settings to its wide bandgap (3.37 eV) and large exciton
binding energy (60 meV) at room temperature.31Zinc has a
hexagonal crystal structure32,33and readily forms an oxide. In
addition, the imaginary component (εi) of the dielectric function
for zinc is very large in the visible region of the spectrum with
peaks at 1.6 and 0.8 eV readily arising from the band transitions.33
This results in a high εi/εr(as compared to the noble metals)
and broad and shallow surface plasmon resonances, which can
readily be calculated.
In this work, the applicability of zinc thin films in combination
with fluorescence to surface plasmon fluorescence spectroscopy
is demonstrated both theoretically and experimentally. Theoretical
Fresnel calculations were employed to determine the optimum
thickness for the generation of surface plasmon modes in zinc
thin films. Fresnel calculations were also used in the investigation
of the spectral range where the surface plasmons in zinc can
effectively be generated as well as in direct comparison with
aluminum, gold, and silver films. In this regard, to demonstrate
the applicability of zinc thin films for use in SPFS, 30 nm zinc
thin films with a 10 nm SiOxoverlayer were thermally evaporated
onto glass supports that have optical transmission above 365 nm.
Surface plasmon coupled and free-space fluorescence from fluo-
rophores with emission in the ultraviolet and visible spectral
regions were measured using the reverse Kretschmann config-
uration. The theoretical calculations and experimental data are
in very good agreement. Finally, we place our findings in context
with immunoassays and analytical based sensing, where the
penetration depth into optically dense media can be both modeled
and tuned using different metallic thin film sensing supports.
Materials. All fluorophores, 2-aminopurine (2-AP), 1,4-bis(5-
phenyl-2-oxazolyl)benzene (POPOP), fluorescein isothiocyanate
(FITC), poly(vinyl)alcohol (PVA, 98% hydrolyzed 13 000-23 000
MW), poly(methyl methacrylate) (PMMA, 100 000 MW), chlo-
roform (99.8% ACS Reagent) and silane-prep glass microscope
slides were purchased from Sigma-Aldrich chemical company
(Milwaukee, WI). Thirty nanometers thick zinc thin films with
10 nm thick SiOxoverlayer were deposited onto silane-prep glass
microscope slides by Thin Films, Inc., Hillsborough, NJ.
Sample Preparation. Fluorophores were deposited onto zinc
thin films by spin coating a solution of polymers (PVA in water
or PMMA in chloroform) containing the fluorophores. Stock
solutions of FITC (1 mM) and 2-AP (3.5 mM) were prepared in
deionized water and then diluted with various solutions of PVA
that were prepared in water. The final concentrations of fluoro-
phore/polymer solutions used to spin coat zinc thin films were
as follows: 0.1 mM FITC in 0.1, 1, and 10% PVA and 1 mM 2-AP
in 5% PVA. A stock solution of POPOP (1 mM) was prepared in
chloroform and was mixed with a 10% PMMA solution to make a
POPOP solution with the following final concentrations: 0.1 mM
POPOP in 5% PMMA. A volume of 40 µL of fluorophore/polymer
solutions were spin-coated onto zinc thin films (1 cm × 1 cm) using
a Chemat Technology spin coater (model KW-4A) with the
following speeds: setting 1, 9 s; setting 2, 20 s. The thickness of
the polymer films was measured using a Molecular Imaging
Picoplus atomic force microscope at a scan rate of 1 Hz with 512
× 512 pixel resolution in the tapping mode, and the average film
thickness was determined to be 25, 70, and 400 nm for the 0.1, 5,
and 10% PVA films and 750 nm for the 5% PMMA film (Supporting
Information, Figure S2).
(14) Aslan, K.; Malyn, S. N.; Geddes, C. D. J. Immunol. Methods 2007, 323,
(15) Aslan, K.; Previte, M. J.; Zhang, Y.; Geddes, C. D. J. Immunol. Methods
2008, 331, 103–113.
(16) Gryczynski, I.; Malicka, J.; Gryczynski, Z.; Nowaczyk, K.; Lakowicz, J. R.
Anal. Chem. 2004, 76, 4076–4081.
(17) Liebermann, T.; Knoll, W.; Sluka, P.; Herrmann, R. Colloids Surf., A 2000,
(18) Previte, M. J. R.; Zhang, Y. X.; Aslan, K.; Geddes, C. D. Appl. Phys. Lett.
(19) Gryczynski, I.; Malicka, J.; Nowaczyk, K.; Gryczynski, Z.; Lakowicz, J. R. J.
Phys. Chem. B 2004, 108, 12073–12083.
(20) Kwon, S. H.; Hong, B. J.; Park, H. Y.; Knoll, W.; Park, J. W. J. Colloid Interface
Sci. 2007, 308, 325–331.
(21) Liu, J.; Tiefenauer, L.; Tian, S.; Nielsen, P. E.; Knoll, W. Anal. Chem. 2006,
(22) Lossner, D.; Kessler, H.; Thumshirn, G.; Dahmen, C.; Wiltschi, B.; Tanaka,
M.; Knoll, W.; Sinner, E. K.; Reuning, U. Anal. Chem. 2006, 78, 4524–
(23) Stengel, G.; Knoll, W. Nucleic Acids Res. 2005, 33, e69.
(24) Tawa, K.; Yao, D.; Knoll, W. Biosens. Bioelectron. 2005, 21, 322–329.
(25) Yu, F.; Yao, D.; Knoll, W. Anal. Chem. 2003, 75, 2610–7.
(26) Matveeva, E.; Gryczynski, Z.; Gryczynski, I.; Lakowicz, J. R. J. Immunol.
Methods 2004, 286, 133–140.
(27) Matveeva, E.; Gryczynski, Z.; Gryczynski, I.; Malicka, J.; Lakowicz, J. R.
Anal. Chem. 2004, 76, 6287–6292.
(28) Matveeva, E.; Malicka, J.; Gryczynski, I.; Gryczynski, Z.; Lakowicz, J. R.
Biochem. Biophys. Res. Commun. 2004, 313, 721–726.
(29) Dorfman, A.; Kumar, N.; Hahm, J. Adv. Mater. 2006, 18, 2685-+.
(30) Dorfman, A.; Kumar, N.; Hahm, J. I. Langmuir 2006, 22, 4890–4895.
(31) Kumar, N.; Dorfman, A.; Hahm, J. I. J. Nanosci. Nanotechnol. 2005, 5,
(32) Mel’nichuk, A. V.; Mel’nichuk, L. Y.; Pasechnik, Y. A. Technical Phys. 1998,
(33) Nash, D. J.; Sambles, J. R. J. Mod. Opt. 1998, 45, 2585–2596.
Analytical Chemistry, Vol. 80, No. 19, October 1, 2008
It is important to note that since the thickness of the polymer
film spin coated onto the metal films is dependent on the size of
the support, the type, and the settings of the spin coater itself,
similar solution preparation conditions and settings were used to
reproduce the results presented in this study.
Surface Plasmon Fluorescence Spectroscopy (SPFS).
coated zinc thin films were attached to a right-angle prism made of
BK7 glass with index matching fluid. This combined sample was
positioned on a precise rotary stage (x-z) that allows excitation and
observation at any desired angle relative to the vertical axis (z-axis)
along the prism. The sample was excited using the reverse
Kretschmann configuration from the air or sample side, which has
a refractive index lower than the prism. The excitation of 2-AP and
FITC was carried out with a laser (335 and 473 nm, respectively) at
(Mikropack D-2000 deuterium) that was collimated to a 5 mm spot
on the sample geometry at an angle of 90°.
Observation of the surface plasmon coupled and free-space
emission were performed with a 600 µm diameter fiber bundle,
covered with a 200 µm vertical slit, and positioned about 15 cm
from the sample. This corresponds to an acceptance angle below
0.1°. The output of the fiber was connected to an Ocean Optics
HD2000 spectrofluorometer to measure the florescence emission
spectra through a 400 nm long-pass filter for 2-AP and POPOP
and a 488 nm super notch filter (Semrock) for FITC.
Real-color photographs of the surface plasmon coupled emis-
sion were taken through an emission filter used for the excitation
of the samples placed on a hemispherical prism.
Theory and Fresnel Calculations. It is well-known that
surface plasmon modes in metallic thin films can be generated
by fluorescent species in close proximity, i.e, in the near-field.9
The optical properties of metals can be predicted using the Drude
dispersion model.34The coupling of light emission from fluores-
cent species to metal surfaces depends on the matching of the
wavevector of the incident light (k0) with the wavevector of the
surface plasmons (ksp) according to the following equation13,35
where εmand εsare the real parts of the dielectric constants of
the metal (εm) εr+ εi) and the sample (εs) εr+ εi) above the
metal film, respectively. The conditions for SPR excitation are met
when the following condition is satisfied
where npis the refractive index of the prism, θspis the surface
plasmon angle, kx and k0 are the wavevectors in the metal
(x-component) and free space (incident light), respectively. From
eq 2, it follows that the reflectivity and transmissivity of incident
light at the metal-dielectric interface are determined by θsp. The
reflectivity at the surface below the surface plasmon coupling
angle, θsp, is very high due to the presence of an evaporated metal
layer that acts as a mirror and reflects most of the transmitted
light.13At just above θsp, the metal surface acts as a resonator for
Figure 1. Determination of the thickness of zinc substrates for surface plasmon coupled fluorescence. Four-phase Fresnel reflectivity curves
of p- (top) and s- (bottom) polarized light at (A) 345 and (B) 507 nm for various thicknesses of zinc substrates with a 10 nm SiOxoverlayer.
Analytical Chemistry, Vol. 80, No. 19, October 1, 2008
incident light and gives rise to the resonant excitation of a surface
plasmon, which reradiates according to the dispersion curve for
surface plasmons.13Fresnel calculations (using a macro procedure
written for Igor Pro software) were performed to account for each
the different optical properties of each dielectric layer and their
respective thicknesses, surface plasmon resonance conditions.
Penetration depth calculations were performed for metals using
three-phase (glass/metal/water) Fresnel calculations. In this
regard, the maximum value for the z-component of the electric
field (Ez2) that occurs at the angle of reflectivity minimum is
normalized with respect to the highest value and plotted against
the thickness (depth) above the metal.36
RESULTS AND DISCUSSION
Since zinc has a band gap energy of 3.37 eV (368 nm) at room
temperature and finite-difference time-domain (FDTD) calculations
predict the occurrence of a surface plasmon resonance (SPR) peak
and increased electric fields around zinc nanostructures at 380
nm, one can expect that surface plasmon modes of zinc can be
generated by p-polarized light or near-field fluorescent species
emitting at these wavelengths. In this regard, Fresnel calculations
for two wavelengths of light (1) overlapping (345 nm) and (2)
red-shifted (507 nm) with respect to the SPR peak of zinc were
performed to determine the ideal thickness for the zinc thin
films for SPCF. Figure 1 shows the four-phase Fresnel reflectivity
curves of 345 and 507 nm p- and s-polarized light for various
thicknesses of zinc thin films and with a 10 nm SiOxoverlayer.
SiOxis commonly used as a protective layer and affords for silane
chemistry to be carried out on metallic films. Fresnel calculations
show, in both cases, that a reflectivity curve minimum occurs for
30 nm thick zinc films, and thus the optimum thickness with zinc
thin films is 30 nm. The calculated optimum thickness for zinc
thin films is in the same range of thicknesses reported for other
metallic thin films employed in SPFS.16-18The angle of the
reflectivity minimum, which varies with wavelength, occurs at 56
and 47° for 345 and 507 nm p-polarized light, respectively. As
shown before,18s-polarized light incident on metallic thin films
does not couple and induce surface plasmons, which is found to
hold for zinc thin films (Figure 1A,B,bottom).
To determine the range of fluorophores that will most efficiently
couple to zinc thin films, four-phase Fresnel reflectivity curves for
multiple wavelengths were calculated and are shown in Figure 2.
The wavelength range, 305-545 nm, was chosen based on the
predicted occurrence of an SPR peak at and around 380 nm. Figure
2A shows that p-polarized light over a 200 nm wavelength range
(305-545 nm) generates surface plasmon modes of 30 nm zinc thin
films with a 10 nm SiOxoverlayer, while s-polarized light does not
couple to surface plasmons. It is important to note that Fresnel
nm) does not efficiently generate surface plasmon modes of zinc
(34) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370–4379.
(35) Burstein, E.; Chen, W. P.; Chen, Y. J.; Hartstein, A. J. Vac. Sci. Technol.
1974, 11, 1004–1019.
(36) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569–638.
Figure 2. Determination of the wavelength range for surface plasmon coupled fluorescence from zinc substrates. Four-phase Fresnel reflectivity
curves of (A) p- (top) and s- (bottom) polarized light at 305, 345, 385, 425, 465, 507, and 545 nm for 30 nm thick zinc substrates with a10 nm
SiOxoverlayer and (B) a plot of reflectivity minimum angles versus wavelength of incident light for 30 nm zinc substrates with a 10 nm SiOx
Analytical Chemistry, Vol. 80, No. 19, October 1, 2008
(data not shown). Figure 2B shows the plot of normalized reflectivity
of light. The angle of the reflectivity minimum occurs at 64° for
p-polarized light at 305 nm and decreases as the wavelength is
increased. Since the normalized reflectivity is less than 5% in the
wavelength range of 305-507 nm (Figure 2B), fluorophore emission
in this wavelength range is predicted to most effectively couple to
zinc thin films.
In addition to the thickness of the metal film, surface plasmon
coupled emission is also influenced by the thickness of the sample
containing the fluorophores.19In a typical fluorescence-based
bioassay (a sandwich immunoassay) constructed on a surface
support, the fluorophores can be located as far as 30 nm from
the surface. To determine the effect of sample thickness on the
optical response from zinc thin films, five-phase Fresnel reflectivity
curves for light at 345 and 507 nm, for 30 nm thick zinc thin films,
10 nm SiOx overlayer, and various thicknesses of PVA were
calculated, Figure 3. These wavelengths are chosen to emulate
the typical emission wavelengths of commonly used fluorophores
used in bioassays, namely, 2-AP and fluorescein. Figure 3A shows
that surface plasmons can efficiently be generated only by
p-polarized emission at 345 nm from a sample up to 25 nm in
thickness (10 nm SiOx+ 15 nm PVA). The degree of coupling of
p-polarized light (or fluorescence emission) decreases and be-
comes inefficient for >40 nm thick samples. The Fresnel calcula-
tions also predict a 5° shift in angle of the reflectivity minimum
for every 5 nm increase in sample thickness. Figure 3B shows
that the generation of surface plasmons by p-polarized light at
507 nm is efficient up to 90 nm (10 nm SiOx+ 80 nm PVA). A
shift and broadening in the reflectivity minimum, as a result of
an increase in sample thickness, for the coupling of p-polarized
light at 507 nm is also predicted. Figure 3 also shows that
s-polarized light both at 345 and 507 nm is not predicted to
generate or couple to surface plasmons of zinc thin films with a
SiOxoverlayer and the various PVA thicknesses.
The optimum thickness of metals other than zinc used in SPFS
was previously determined to be 20 nm for aluminum thin films,16
35 nm for copper thin films,18to 50 nm for gold thin films17and
silver thin films.19To demonstrate the advantages of using zinc
thin films over aluminum, gold, and silver thin films in SPFS, five-
phase Fresnel reflectivity curves for light at 305-517 nm were
calculated and compared for these metals, cf. Figures 4 and 5.
Figure 4 shows the comparison of Fresnel reflectivity curves for
zinc (30 nm) and aluminum (20 nm)16thin films with 10 nm SiOx
and 12 nm PVA overlayers. These overlayer thicknesses (22 nm)
were used in a previous study on SPFS16and also encompasses
the range of distances of fluorophores from the metal surface used
in SPCF-based bioassays. These reflectivity curves show that only
p-polarized light efficiently generates surface plasmon modes of
zinc and aluminum thin films in the 345-517 and 305-517 nm
spectral range, respectively. Two major differences in the reflec-
tivity curves for zinc and aluminum thin films can be observed:
Figure 3. Optimization of 30 nm zinc substrates with polymer overlayer for surface plasmon coupled fluorescence. Five-phase Fresnel reflectivity
curves of p- (top) and s- (bottom) polarized light at (A) 345 and (B) 507 nm for 30 nm thick zinc substrates with various thicknesses of a PVA
Analytical Chemistry, Vol. 80, No. 19, October 1, 2008