Characterization of Silver Nanoparticle Plasmon Effects on Systems of Free and Bound Chromophores

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?