1. EMPA PhD Student’s Symposium 2008 Optical properties of single gold nanoparticles
Michael Ruosch, Dominik Marti, Patrick Stoller,
Jaro Rička and Martin Frenz
Institute of Applied Physics
University of Bern
17. September 2018

2. History Back lightening Front or side lightening
Already the Romans took advantage of the optical properties of gold nanoparticles:
Lycurgus Cup (250 a.Chr.)
Back lightening
Front or side lightening
Applications:
Biological markers for cells, proteins, DNA, etc.
Can act as local, precise and powerful heaters
Tracking
Spectral imaging
Advantages (compared to fluorescent dyes):
Non-toxic (no carcinogen), inert
Easy and cheap to produce
Stable in solution
Gold nanoparticles are embedded in glass
17. September 2018

3. Why gold nanoparticles?
Applications:
Biological markers for cells, proteins, DNA, etc.
Can act as local, precise and powerful heaters (high absorption)
Tracking
Spectral imaging
Advantages (compared to fluorescent dyes):
Non-toxic (no carcinogen), inert
Easy and cheap to produce
Stable in solution
17. September 2018

4. Gold nanoparticles as biological markers
Medical application
Conjugate gold nanoparticles (spheres, rods, shells) to cancer cell antibodies and inject them into the human body
Visualization of the binding of antibodies and cancer cells via gold particles
Destruction of the cancer cells via absorption of light in the gold particles
Multiphoton luminescence microscopy: scan of cancer cell antibodies labeled with gold nanoparticles
17. September 2018

5. Properties of gold nanoparticles: Plasmon resonance
Noble metal nanoparticles are well known for their plasmon resonance
Electromagnetic wave incident on nanoparticle
Build-up of charge on the surfaces
Coulomb restoring force
Plasmon Resonance
dielectric
medium + - E k
signal intensity (a.u.)
wavelength (nm)
60 nm gold spheres
metal
particle
G. Mie, Annalen der Physik 4, 25(3) (1908)
17. September 2018

6. Properties of gold nanoparticles: Plasmon resonance
Noble metal nanoparticles are well known for their plasmon resonance
Electromagnetic wave incident on nanoparticle
Build-up of charge on the surfaces
Coulomb restoring force
Plasmon Resonance
dielectric
medium + + + + + + + + + + E k
signal intensity (a.u.)
wavelength (nm)
60 nm gold spheres - - - - - - - - - -
Explains the cup!
metal
particle
G. Mie, Annalen der Physik 4, 25(3) (1908)
17. September 2018

7. Less well known: Luminescence of gold
A. Mooradian, Phys Rev. Letter (1969)
Gold nanoparticles also luminesce. The signal of the luminescence is weak and has not been studied as extensively as plasmon resonance.
explained by band structure in a 3-step process
17. September 2018

8. 3-step process of gold luminescence*
d-sp transition excitation via photon
Hole scattered to L- or X- point in the Brillouin zone
Recombination of the d- band hole with an arbitrary electron below the Fermi level
t~50fs**
Photon
Erkläre zwar die bandstruktur, aber die lumineszenz von nanopartikel ist nicht durch die bandstruktur sondern durch die plasmon resonance ausgezeichnet!!!!
Figure modified from E. Dulkeith et al., Phys Rev. B, (2004)
*G. T. Boyd et al., Phys. Rev. B (1986)
**M.A. El-Sayed et al., Phys Rev. B 72, (2005)
17. September 2018

9. Multiphoton excited luminescence
t~50fs**
1.65eV
Photon
1.65eV
Erkläre zwar die bandstruktur, aber die lumineszenz von nanopartikel ist nicht durch die bandstruktur sondern durch die plasmon resonance ausgezeichnet!!!!
Incident light (800nm) has higher penetration depth into human tissue
Multiphoton excited luminescence only occurs in the focal spot of a tightly focused laser. Therefore multiphoton microscopy is inherently 3-dimensional
Excitation light can be effectively filtered out.
17. September 2018

10. Multiphoton Microscopy
What do we want to know?
Characterize the multiphoton luminescence spectrum
Only investigate single particles
Determine cross section (efficiency) of the multiphoton luminescence process
Determine best shape & size of the particles for our application
17. September 2018

11. Confocal & multiphoton microscopy scans, spectrum acquisition: setup for single particle luminescence spectroscopy
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12. Results: Not all gold nanoparticles do exhibit multiphoton luminescence
Comparison Confocal scan – Multiphoton scan
100nm gold spheres, confocal scan
multiphoton scan 40 GW/cm²
17. September 2018

13. Results: Particles can be “switched on”
100nm gold spheres, confocal scan
multiphoton scans
48 GW/cm²
136 GW/cm²
163 GW/cm²
41 GW/cm²
17. September 2018

14. Results: Particles exhibit blinking
Observing a 100nm gold sphere at 204 GW/cm²
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15. Measurement of multiphoton luminescence spectrum of single particles
Single gold spheres with diameter of 40nm or 60nm
Irradiate with laser intensity optimized for maximum signal strength without blinking
Investigate only particles which do not show the blinking or “switching on” behavior
Acquire spectrum, integrating over 60s
Determine the dependence of the spectrum on the particle diameter and the refractive index of the surrounding medium
Use a different single particle for every spectrum acquisition
17. September 2018

16. Results: Measured multiphoton luminescence spectrum of one single gold nanosphere (60nm, n=1.518)
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17. Results: Measured multiphoton luminescence spectrum of one single gold nanosphere (60nm, n=1.518)+ -
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18. Using Lorentz-Mie theory for absorption spectrum
Shifted peak position
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19. Results: Multiphoton luminescence peak wavelength dependence on particle diameter and refractive index of surrounding medium
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20. Conclusion
Multiphoton luminescence (as well as single photon luminescence) in single gold nanoparticles with diameters of a few tens of nanometers is strongly dominated by the absorption peak in the plasmon resonance.
Blinking and a hysteretic behavior („switching on“) is observed. These effects are not yet understood and further investigations are necessary!
17. September 2018

21. Thank you! Michael Ruosch Michael.ruosch@iap.unibe.ch
This work was supported by
EU project: PROMET (prostate cancer molecular-oriented detection & treatment of minimal residual disease)
SNF project: „Selective cell diagnosis and killing by nanoparticle absorption of pulsed laser radiation“ No
SNF project: „Particle tracking in three-dimensions using multiphoton luminescence in gold nanoparticles“ No
Michael Ruosch
17. September 2018