Long-range excitation transfer via DNA nanowire by fs laser pulse
excitation of silver nanoparticles
J. Wirth, F. Garwe, A. Csáki, N. Jahr, O. Stranik, W. Paa, W. Fritzsche*
1Institute
of Photonic Technology, Albert-Einstein-Straße 9, 07745 Jena, Germany
*fritzsche@ipht-jena.de
Metal nanoparticles show not only exceptional electronic and optical properties
but also a compatibility with the biomolecular world. Thus, they have attracted
increasing attention with growing interest in several nanobiotechnological
disciplines, such as the manipulation of biomolecules at the nanometer length
scale. Here, we present the effects of excited silver nanoparticles attached to DNA
molecules by fs laser pulses.
Introduction
Because of their unique optical properties based on localized surface plasmon resonance
(LSPR), which can be tuned to specific wavelengths, e.g. by modifying the size, shape or
material, noble metal nanoparticles can act as laser light antennas and allow a very local
energy conversion of laser pulses from the nanoparticle into the surrounding medium.
Thereby, the pulse width of the laser light plays an important role. While the excitation
of metallic nanoparticles with nanosecond- or picosecond laser pulses results already
during the pulse duration in an electron-phonon relaxation process and in an increased
temperature in the proximal dielectric surrounding, resonant femtosecond laser pulses
are too short and do not result in such a increase of the temperature [1]. However,
highly excited electrons in the particles can easily overcome the metal’s work function
by absorption of three to four photons. These electrons and the amplified electric near
field interact directly with the dielectric surrounding [2].
Results and Discussion
To study the interaction of femtosecond laser pulses with silver nanoparticle labelled
DNA molecules, we used the UV- and electron sensitivity of an underlying
Polymethylmethacrylate (PMMA) layer as an indirect detection method. First of all, we
ensured that the DNA itself and the PMMA layer were not affected by femtosecond laser
pulses. Afterwards, nanoparticle labelled DNA bundles were stretched onto the PMMA
layer and irradiated with the same laser parameters, resulting in a quite impressive
change of these DNA structures [3]. Thereby the plasmonic excitation caused by the
irradiation of the nanoparticles is guided along the attached DNA-molecules, which
results in a structuring of the PMMA along the former DNA position and leads to a thin
groove reaching even micrometers away from the excited nanoparticle. These grooves
are visible as dark structures in the AFM images with a depth of 3-4 nm. To explain this
apparent energy transfer from the particle to the DNA, two mechanisms are eligible. On
the one hand, the electrons of the nanoparticle could overcome their affinity and
transfer their kinetic energy to the electrons of the PMMA and the DNA or on a very high
electric field (108 - 109 V/m) in the environment of the irradiated nanoparticle
accelerate electrons in the PMMA or DNA. In DNA, momentum or electron transfer over
π stacked DNA base pairs mediated by molecular vibration of the backbone could also
take place and result in breaking of DNA bonds. In PMMA the accelerate electrons could
result in a removal of PMMA side groups (radical formation) at a random position
nearby the nanoparticle and the DNA, leading to local PMMA degradation. These small
molecule fragments can evaporate and grooves occur in PMMA. On the other hand, the
nonlinear excited electrons in the metallic nanoparticles can be seen as light on the
nanoscale far below the diffraction limit. Regarding the DNA bundles as a
subwavelength optical fibre, a light coupling of the localized surface plasmons in the
nanoparticle onto the nearest DNA bundles could occur. The light could propagate
through the DNA bundle as quasi-particles, called phonon polaritons caused by electron
polarization.
This combination of the optically controlled nanoantenna effect with the plethora of
geometries provided by DNA could have applications in fields such as lithography,
nanoplasmonics or molecular electronics.
[1] F. Garwe et.al., Nanotechnoloy 19, p.055207 (2008)
[2] A. Csaki et. al. Nano Lett. 7, p.247 (2007)
[3] J. Wirth et.al., Nano Lett., dx.doi.org:10.1021/nl104269x