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

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 www.nd.edu/~ghartlan/images/cup.jpeg 17. September 2018

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

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

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

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

Less well known: Luminescence of gold A. Mooradian, Phys Rev. Letter. 22 185 (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

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, 205424 (2004) *G. T. Boyd et al., Phys. Rev. B 33 7923 (1986) **M.A. El-Sayed et al., Phys Rev. B 72, 235405 (2005) 17. September 2018

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

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

Confocal & multiphoton microscopy scans, spectrum acquisition: setup for single particle luminescence spectroscopy 17. September 2018

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

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

Results: Particles exhibit blinking Observing a 100nm gold sphere at 204 GW/cm² 17. September 2018

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

Results: Measured multiphoton luminescence spectrum of one single gold nanosphere (60nm, n=1.518) 17. September 2018

Results: Measured multiphoton luminescence spectrum of one single gold nanosphere (60nm, n=1.518) + - 17. September 2018

Using Lorentz-Mie theory for absorption spectrum Shifted peak position 17. September 2018

Results: Multiphoton luminescence peak wavelength dependence on particle diameter and refractive index of surrounding medium 17. September 2018

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

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) http://www.fp6-promet.net/index.html SNF project: „Selective cell diagnosis and killing by nanoparticle absorption of pulsed laser radiation“ No.205320-116343 SNF project: „Particle tracking in three-dimensions using multiphoton luminescence in gold nanoparticles“ No.200021-113284 Michael Ruosch Michael.ruosch@iap.unibe.ch 17. September 2018