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Surface Enhanced Fluorescence

Ellane J. Park

Turro Group MeetingJuly 15, 2008

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Surface Enhanced Fluorescence (SEF)

• Motivation: biological systems– Enhancement of Fluorescence

Intensity– Adjust fluorescence lifetime

• Techniques to couple SPP and photons– High index materials and specific

optical geometry --> Increase momentum of photon

• Example: metal on glass (e.g. Kretschmann configuation)

– Break up translational motion with corrugated or roughened metallic surfaces.

Aluminum Silver

Gold Copper

Nanoholes Nanoparticles Thin Films

Metals

SEFS Applications

J. R. Lackowicz et. al. Proc. Of SPIE, 6099, 609909-1 (2006)

J. Phys. D: Appl. Phys. (2008) 41, 013001

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SEF Distance and Orientation Dependence

• What is the “enhancement factor”? – Magnitude of increase in fluorescence intensity

of fluorophore in presence of metal, compared to that of fluorophore in the absence of metal

• Distance– Less than 10 nm: Non-radiative energy transfer

can occur (i.e. quenching)

– Greater than 10 nm: Fluorescence Intensity can be enhanced by up to 100-fold

• Dipole Orientation– Dipoles that oscillate perpendicular to surface

couple efficiently with surface plasmon modes

J. Phys. D: Appl. Phys. (2008) 41, 013001

Calculated Fluorescence intensity of free fluorophore as a function of distance to Au NP

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Fluorescence Intensity of Au Nanorods

CSR. 35, 209 (2006)

y

xAR=

x

y

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Fluorescence Enhancement in Nanoholes

• Lifetime reduction from 3.87 ns in open solution to 0.9 ns above nanohole

• Effects of nanohole diameters and periodicity

J. Phys. D: Appl. Phys. (2008) 41, 013001

(a) Fluorescence decay curves of Rhodamine 6G in open solution and into a d=150 nm hole

(b) 3-D finite-element time-domain simulation of intensity distribution for a d=50 nm nanohole

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Theory: SEF in Ag Nanoshells

J. R. Lackowicz et. al. Proc. Of SPIE, 6099, 609909-1 (2006)

• Quantum Dots are highly photostable.

• Light absorbed by metal --> fluorophore

• Potential Effects of fluorophore in Nanoshell: – Photostability (Protection from

oxygen)– Higher radiative decay rates

and higher quantum yields – Emission is 2-fold narrower– Shorter lifetime --> less time for

photochemistry in excited state

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Thank You

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Surface Plasmons

J. Phys. D: Appl. Phys. (2008) 41, 013001

: angular frequencyc: Speed of light: Incident angle with the normal to surface in dielectric mediumkspp: In-plane wavevector of SPP modesKspp (greater momentum) is greater than k0 (free space photon of same pulsation)

+: lower e- density-: higher e- density

• Note: Energy flow from fluorophore to SPP modes can be detrimental to SEF process

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