Frank Garwe

Molecular plasmonics: light meets molecules at the nanoscale.

Certain metal nanoparticles exhibit the effect of localized surface plasmon resonance when interacting with light, based on collective oscillations of their conduction electrons. The interaction of this effect with molecules is of great interest for a variety of research disciplines, both in optics and in the life sciences. This paper attempts to describe and structure this emerging field of molecular plasmonics, situated between the molecular world and plasmonic effects in metal nanostructures, and demonstrates the potential of these developments for a variety of applications.

Plasmonic Nanofabrication by Long-Range Excitation Transfer via DNA Nanowire

Driven by the demand for ongoing integration and increased complexity of todays microelectronic circuits, smaller and smaller structures need to be fabricated with a high throughput. In contrast to serial nanofabrication techniques, based, e.g., on electron beam or scanning probe methods, optical methods allow a parallel approach and thus a high throughput. However, they rarely reach the desired resolution. One example is plasmon lithography, which is limited by the utilized plasmonic metal structures. Here we show a new approach extending plasmonic lithography with the potential for a highly parallel nanofabrication with a higher level of complexity based on nanoantenna effects combined with molecular nanowires. Thereby femtosecond laser pulse light is converted by Ag nanoparticles into a high plasmonic excitation guided along attached DNA structures. An underlying poly(methyl methacrylate) (PMMA) layer acting as an electron-sensitive resist is so structured along the former DNA position. This apparently DNA-guided effect leads to nanometer grooves reaching even micrometers away from the excited nanoparticle, representing a novel effect of long-range excitation transfer along DNA nanowires.

Tuning of Spectral and Angular Distribution of Scattering from Single Gold Nanoparticles by Subwavelength Interference Layers

Localized surface plasmon resonance (LSPR) as the resonant oscillation of conduction electrons in metal nanostructures upon light irradiation is widely used for sensing as well as nanoscale manipulation. The spectral resonance band position can be controlled mainly by nanoparticle composition, size, and geometry and is slightly influenced by the local refractive index of the near-field environment. Here we introduce another approach for tuning, based on interference modulation of the light scattered by the nanostructure. Thereby, the incoming electric field is wavelength-dependent modulated in strength and direction by interference due to a subwavelength spacer layer between nanoparticle and a gold film. Hence, the wavelength of the scattering maximum is tuned with respect to the original nanoparticle LSPR. The scattering wavelength can be adjusted by a metallic mirror layer located 100-200 nm away from the nanoparticle, in contrast to near-field gap mode techniques that work at distances up to 50 nm in the nanoparticle environment. Thereby we demonstrate, for the first time at the single nanoparticle level, that dependent on the interference spacer layer thickness, different distributions of the scattered signal can be observed, such as bell-shaped or doughnut-shaped point spread functions (PSF). The tuning effect by interference is furthermore applied to anisotropic particles (dimers), which exhibit more than one resonance peak, and to particles which are moved from air into the polymeric spacer layer to study the influence of the distance to the gold film in combination with a change of the surrounding refractive index.

Plasmonic Coupling and Long-Range Transfer of an Excitation along a DNA Nanowire

We demonstrate an excitation transfer along a fluorescently labeled dsDNA nanowire over a length of several micrometers. Launching of the excitation is done by exciting a localized surface plasmon mode of a 40 nm silver nanoparticle by 800 nm femtosecond laser pulses via two-photon absorption. The plasmonic mode is subsequently coupled or transformed to excitation in the nanowire in contact with the particle and propagated along it, inducing bleaching of the dyes on its way. In situ as well as ex situ fluorescence microscopy is utilized to observe the phenomenon. In addition, transfer of the excitation along the nanowire to another nanoparticle over a separation of 5.7 μm was clearly observed. The nature of the excitation coupling and transfer could not be fully resolved here, but injection of an electron into the DNA from the excited nanoparticle and subsequent coupled transfer of charge (Dexter) and delocalized exciton (Frenkel) is the most probable mechanism. However, a direct plasmonic or optical coupling and energy transfer along the nanowire cannot be totally ruled out either. By further studies the observed phenomenon could be utilized in novel molecular systems, providing a long-needed communication method between molecular devices.

Plasmonically Enhanced Electron Escape from Gold Nanoparticles and Their Polarization-Dependent Excitation Transfer along DNA Nanowires

Here we show plasmon mediated excitation transfer along DNA nanowires over up to one micrometer. Apparently, an electron excitation is initiated by a femtosecond laser pulse that illuminates gold nanoparticles (AuNP) on double stranded DNA (dsDNA). The dependency of this excitation on laser wavelength and polarization are investigated. Excitation of the plasmon resonance of the AuNPs via one- and two-photon absorption at 520 and 1030 nm, respectively, was explored. We demonstrate an excitation transfer along dsDNA molecules at plasmon supported four-photon excitation of AuNP cluster or at laser field driven nanoparticle electron tunneling for an alignment of the attached dsDNA to the polarization of the electric field of the laser light. These results extend the previously observed plasmonically induced three-photon excitation transfer along DNA nanowires to another nanoparticle material (gold) and the adapted irradiation wavelengths.

Bioanalytics using single plasmonic nanostructures

Plasmonic nanostructures promise to provide sensing capabilities with the potential for sensitive and robust assays in a high parallelization. We present here the use of individual nanostructures for the detection and manipulation of biomolecules such as DNA based on optical approaches [1]. The change in localized surface plasmon resonance of individual metal nanoparticles is utilized to monitor the binding of DNA directly or via DNA-DNA interaction. The influence of different size (length) as well as position (distance to the particle surface) is thereby studied [2]. Holes in a Cr layer present another interesting approach for bioanalytics. They are used to detect plasmonic nanoparticles as labels or to sense the binding of DNA on these particles. This hybrid system of hole and particle allows for simple (just using RGB-signals of a CCD [3]) but a highly sensitive (one nanoparticle sensitivity) detection. On the other hand, the binding of molecular layers around the particles can be detected using spectroscopic features of just an individual one of these systems. Besides sensing, individual plasmonic nanostructures can be also used to manipulate single biomolecular structures such as DNA. Attached particles can be used for local destruction [4] or cutting as well as coupling of energy into (and guiding along) the molecular structure [5].

Nanoprocessing of DNA with femtosecond laser

Sequence specific cutting of DNA is a standard method in molecular biology. This cutting is realized with enzymes which have a defined recognition sequence and cutting sequence. Therefore one can manipulate only sequences for which an enzyme is available. With current physical methods (AFM) any sequences can be cut, but the precise sequence specific and highly parallel cutting is not possible. Near infrared (NIR) femtosecond laser systems have been used to optically knock out genomic regions of highly condensed DNA in human chromosomes as well as of single expanded (stretched) DNA molecules. Working with 80 MHz laser pulses at 800 nm of low 2 nJ pulse energy but at high TW/cm2 light intensities, multiphoton ionization and optical breakdown (OB) resulted in highly precise material ablation with sub-100 nm cut sizes. This is far below the diffraction-limited spot size. A minimum FWHM cut size of 65 nm was achieved in the case of the nanodissection of a laser-treated stretched λ-DNA (48kb) molecule which corresponded to 200 optically knocked out bases. By the use of metal nanoparticles as energy coupling objects for fs laser radiation we expect a specific highly local destruction effect within the DNA molecule (cut). Thereby, a sequence-specific binding of DNA nanoparticle complexes along the target DNA is a fundamental condition. The effect of laser exposure on DNA and DNA-nanoparticle complexes are presented.

Design of a scalable AuNP catalyst system for plasmon-driven photocatalysis

In this work we present a simple, fast and cost-efficient synthesis of a metal nanoparticle catalyst on a glass support for plasmon driven heterogeneous photocatalysis. It is based on efficient mixing of metal salts as particle precursors with porous glass as the supporting material in a mixer ball mill, and the subsequent realization of a complete catalyst system by laser sintering the obtained powder on a glass plate as the support. By this, we could obtain catalyst systems with a high particle proportion and an even spatial particle distribution in a rapid process, which could be applied to various kinds of metal salt resulting in plasmon active metal nanoparticles. Furthermore, the catalyst production process presented here is easily scalable to any size of area that is to be coated. Finally, we demonstrate the catalytic performance of our catalysts by a model reaction of ethanol degradation in a self-designed lab-scale reactor.

Plasmonic nanoparticles sensors utilizing hybrid modes, electrical excitation, and anisotropic particles

Surface Plasmon Resonance (SPR) in metallic nanostructures is an optical effect that can be exploited for the detection of small molecules. There is a broad range of metallic nanostructures supporting different SPR modes, and nanostructures can be even geometrically combined leading to the creation of new hybridised SPR modes. In our study, we investigated the properties of a hybridised SPR mode (gap modes GM) created by the placement of metallic nanoparticles onto metallic layers and its use as a sensitive sensor. A tunneling current passing through a metal-insulator-semiconductor structure can generate supported SPR modes that can be scattered through GM, which was experimentally confirmed. Moreover, we were able to experimentally follow the degradation of anisotropic (silver nanoprism) nanoparticles under ambient conditions in real time. Using atomic force microscopy and optical spectroscopy we observed an anisotropic corrosion that is starting from the tips of the nanoparticles.

Localization of laser energy conversion by metal nanoparticles: basic effects and applications

Manipulation of material by optical means represents an emerging field with numerous applications. Especially in biology and medicine, the flexible and powerful potential of laser utilization holds great promises. For many applications, the resolution of the induced effects is essential. Besides focusing of the beam by various means, the use of sub-wavelengths nanoantenna could overcome this problem. The optical absorption of certain nanostructures is based on plasmon effects. We present studies of the use of metal (homogeneous gold or gold/silver core/shell systems) nanoparticles as antennas that convert the incident laser light into irreversible destructive effects. Based on the established field of DNA-conjugated nanoparticles, we investigated the sequence-specific attachment of DNA-nanoparticle complexes onto DNA with complementary sequences, in the state of double-stranded either isolated or metaphase chromosomal DNA. Important points were the adjustment of the absorption properties of the nanoparticles by control of their material composition (e.g., by addition of a silver layer to a gold core) and diameter. Another group of experiments studied chromosome-conjugated particles before and after laser treatment, in order to reveal the lateral extension of damages as well as the underlying mechanism.