Characterization of Silver Nanoparticle Plasmon Effects on Systems of Free and

Bound Chromophores

19TH ANNUAL ARGONNE SYMPOSIUM FOR UNDERGRADUATES

IN SCIENCE, ENGINEERING AND MATHEMATICS

November 7th, 2008

Undergraduate Researcher: Will Boyd

Faculty Mentor: Dr. Joseph Perry

Graduate Student Mentor: Wojciech Haske

I. Project Background

Why Silver Nanoparticles?

The Plasmon Effect: A Strongly Localized Electric Field

Incident Electromagnetic Radiation at Plasmon Resonance Frequency

Oscillations of Nanoparticle Conduction Band Electrons

[1]

The Chromophore: TPD

Incident electromagnetic radiation at chromophore resonance frequency

Oscillating molecular dipole

TPD-thiolN

N

O

(CH2)12SH

Δ+Δ-

Theoretical Predictions

Modeling of Excited State Chromophore – AgNP Interactions a. Theoretical expression for molecules near spheroids derived by Gersten and Nitzan [2]

b. Enhanced and suppressed fluorescence radiative and non-radiative constants, kr and knr, depending on chromophore orientation

c. kr is inversely related to fluorescence lifetime:

d. Already reports of shorter lifetimes for chromophores on silver nanoparticles [3], [4], [5]

II. Physical Characterization of Chromophore-Coated AgNPs

Synthesis of AgNPs

Silver Acetate (AgAc)

NH2Oleylamine (OLA)

Ave. Diam.: 4.5 nm Std. Dev.: 18.8%

AgAc + OLAo-Xylene

145°C

AgOLA NP

Ag

Chromophore Exchange Reaction

Exchange Reactiona. Thiolated chromophores replace OLA

on surface of AgNPs

b. Thiol “footprint” (~20 Å2) used to estimate relative concentrations of AgNPs to chromophores

Purificationb. Aggregation in hexane

c. Precipitation following centrifugation

c. Extraction of supernatant

d. Dissolution of chromophore-coated AgNPs in toluene (TPD)

Est. Surface Coverage TPD((CH2)12SH): ~70%

III. Optical Characterization of Chromophore-Coated AgNPs

Stability of AgNP-TPD((CH2)12SH)

Time-Based Fluorimetry

a. Silver strongly quenches chromophore fluorescence

b. Increasing fluorescence intensity: desorption of chromophores

c. Stability was found to be highly solvent-dependent

Time-Based Fluorimetry of AgNPs-TPD((CH2)12SH) in Toluene/DMF (5:1)

UV-vis of AgNPs-TPD((CH2)12SH) in Toluene/DMF (5:1)

UV-vis Spectroscopy

a. Taken before and after time-based

fluorimetry

b. AgNPs stained cuvette in toluene

c. DMF and other organic solvents

strongly quenched fluorescence

Measuring Fluorescence Lifetimes

Time Correlated Single Photon Counting (TC/SPC)

TC/SPC generates ahistogram of the decay times for single photons from fluorescence emission

An analysis of the histogram finds the best fit exponential decay lifetimes

Mea

sure

d

Ph

oto

ns

Time

TC/SPC Results

TC/SPC Results: Exponential Decay Lifetimes

System TPD((CH2)12SH)AgNPs-

TPD((CH2)12SH)

Solvent Toluene/DMF (5:1) Toluene/DMF (5:1)

τ1 [ns] 1.599 < 0.070

A1 [%] 100.00 2.70

τ2 [ns] - 0.919

A2 [%] - 33.16

τ3 [ns] - 2.86

A3 [%] - 64.13

Mono-exponential decay of free TPD demonstrated

Tri-exponential decay of bound TPD

a. Ultra-short component too fast to resolve with TCSPC

b. Long component could represent TPD bound in parallel orientation

IV. Conclusions and Future Work

Physical Characterization of AgNP-Chromophore Systemsa. OLA-coated AgNPs can be synthesized with relatively small size distribution

b. Thiolated chromophores can be attached via a ligand exchange reaction

c. Relatively high chromophore coverage can be achieved

Optical Characterization of AgNP-Chromophore Systemsa. More work needed to fully characterize fluorescence lifetimes

b. Difficulty resolving short fluorescence lifetimes with small error

c. Utilization of femtosecond transient absorption to measure ultra-fast component of the excited state lifetimes

V. References

[1] Kelly, K. Lance; Coronado, Eduardo; Zhao, Lin Lin; Schatz, George C. J. Phys. Chem. B 107(3): 668-677 (2003)

[2] J. Gersten, A. Nitzan, J. Chem. Phys. 75(3): 1139-1152 (1981)

[3] Tovmachenko, Oleg G.; Graf, Christina; van den Heuvel, Dave J.; van Blaaderen, Alfons; Gerritsen, Hans C., Advanced Materials 18(1): 91-95 (2006)

[4] Fu, Yi; Zhang, J.; Lakowicz, J. R., J. Fluorescence 17: 811-816 (2007)

[5] Viger, M. L.; Live, L. S.; Therrien, O. D.; Boudreau, D., Plasmonics 3: 33-40 (2008)

VI. Acknowledgements

Special thanks to the following individuals and organizations for making this research possible:

National Science Foundation

MDITR REU Grant # 0120967

Georgia Tech President’s Undergraduate Research Award

Dr. Joseph Perry

Wojciech Haske

Michal Malicki

Zac Heacker

Matteo Cuzzuol

Sarah Chi

Dr. Matthew Sartin

Dr. Joel Hales

Questions?