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私たちはプラズモニクス研究を応用する可能性・実用性を光化学と光物理学分野の見地から探求しています。
 
  1. Plasmonic Waveguiding
  2. Single Molecule Studies　
  3. Plasmon Associated Energy Harvesting　
  4. Drug Delivery System based on Plasmonics　
  • プラズモン導波
  • 単一分子研究
  • 環境発電に関わるプラズモン
  • プラズモニクスを基とする薬輸送システム

 

1. Plasmonic Waveguiding - プラズモン導波
Remote-excitation surface enhanced Raman spectroscopy
Surface Plasmons are promising to be the next medium for energy and information transport applications. Surface Plasmon Polaritons (SPPs) can be guided along the metal-dielectric interface, called the plasmonic waveguide effect in metal nanowires. By coupling of free-space photons to the electron gas of the metal nanowire to form propagating SPPs, allows this needed spatial confinement and the transport of light through structures with subdiffraction limited diameters. If the SPPs are confined to the surface of a single crystalline metal nanowire, they can propagate over tens of microns before the energy is lost due to Ohmic damping. In our lab, the properties of waveguiding nanowires are tested at full extent. Uji-i et al. demonstrated that focused laser excitation at the end of silver nanowires can excite Surface Enhanced Raman Spectroscopy hot-spots at points micrometers along the nanowire due to this remarkable property of plasmon waveguiding (figure 1).

Uji-i, H. et al., Nano Lett. 9, 995 (2009).

A nanowire/nanoparticle aggregate system is proposed as a potential probe for SERS applications. The total SERS intensity detected at the hot-spots following wire-end excitation correlates with the known wavelength, polarization, and distance dependences of surface plasmon polariton (SPP) propagation in nanowires. The SERS spectra obtained at the hot-spots following wire-end excitation show very little background compared to when excitation occurs directly at the hot-spot, suggesting that a much smaller SERS excitation volume is achieved by remote, waveguide excitation (figure 2).

 

Silver nanowires as plasmonic antennas
Kenens, B. et  al., J. Phys. Chem. C 117, 2547 (2012).

Uji-i et al demonstrated that by using propagating SPPs along a silver nanowire, Remote Excitation-Surface Enhanced Raman Scattering (RE-SERS) is possible. The coupling position of the incident light has to be precisely controlled; since the diffraction limit of the focused laser is ~300nm and the diameter of the nanowire is ~100nm, it is safe to assume the precision of localization of the laser light has to be 100nm. Kenens et al. showed this by combining scanning optical microscopy (SOCM) with AFM to investigate in- and out-coupling of the light of a focused 632.8 nm laser. To improve the coupling of light into the SPPs on the nanowire, a nanoparticle is attached on the end of the wire. We have showed that a nanoparticles positioned at the end of a nanowire enhances the in-and out-coupling efficiency with a factor 4 (figure 3). 

 

Tip-enhanced Raman microscopy

Raman spectroscopy is a powerful analytical technique providing a detailed vibrational fingerprint from the molecules under investigation. However, Raman scattering is relatively weak compared to, e.g., fluorescence.

Single wall carbon nanotubes (SWCNTs) on gold surface mapped with STM mode TERS microscope

Surface enhanced Raman scattering (SERS) has helped to overcome this problem, offering signal enhancements of several orders of magnitude over conventional Raman scattering. The (often random) heterogeneity of SERS substrates leads to high variations in electromagnetic field enhancement across the sample surface, which limits the potential range of application. Reducing the enhancing substrate to a single local 'hot spot' at the end of a very sharp tip, which can be accurately positioned on the sample surface (e.g. by using a scanning probe microscope) circumvents this issue and is called Tip-Enhanced Raman Scattering (TERS) microscopy.

TERS microscopy enables us to obtain topographical and Raman maps simultaneously within less than 10 nanometers in an ambient condition. Such an extremely high spatial resolution in TERS can be obtained by irradiating laser light to a sharp noble metal probe, exciting localized surface plasmon on the probe apex. 
 
 

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2. Single Molecule Studies - 単一分子研究
A major challenge for present research lies in the unraveling and understanding of the nanometer-scale structuring and ordering of complex molecules and materials. One technique that has shown strong promise in this regard is single-molecule fluorescence spectroscopy (SMFS). With this technique, an individual molecule can be used as a nanometer-sized probe, where its emission dynamics report on its nanoscale surroundings.

 

Super resolution microscopy on metal nanostructures
Lin, H. et al. ChemPhysChem 13, 973 (2012).
Metal nanostructure exhibit great plasmonics enhanced phenomenon which has great potential in various researches and applications. Normally the metal nanostructure and the probe are small then the diffraction limitation of the conventional optical microscopy, thus the features of the interaction cannot be resolved. By applying super resolution microscopy, we can discriminate effects of labeling density when estimating the enhancement factor and resolved the nanostructures with sizes below the diffraction limitation.

 

Defocused wide-field imaging 

Developing molecular systems with functions analogous to those of macroscopic machine components, such as rotors, gyroscopes and valves, is a long-standing goal of nanotechnology. However, macroscopic analogies go only so far in predicting function in nanoscale environments, where friction dominates over inertia. Here, we visualize the motions of surface-bound molecular rotors using defocused fluorescence imaging, and observe the transition from hindered to free Brownian rotation by tuning medium viscosity.

Hutchison, J.A. et al. Nat. Nanotechnol. 9, 131 (2014). Dedecker, P. et al. Adv. Mater. 21, 1079 (2009).

 

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3. Plasmon Associated Energy Harvesting - 環境発電に関わるプラズモン
Light induced synthesis
The successful application of nanoparticles (NPs) in optoelectronic and other nanoscale devices imposes stringent conditions on the NPs, for example, monodispersity both in size and shape, stability in ambient  conditions, a crystalline morphology with well-defined edges and smooth surface, and so on.
Paul A. et al., Langmuir 28, 8920 (2012).
In general, the synthesis of silver NPs through the so called bottom-up approach or wet-chemical route yields particles with good crystalline morphology and well-controlled shapes, whereas the top-down lithographic techniques allow for the successful engineering of ordered arrays of prototype nanostructures on solid substrates. However, the top-down approach suffers from a poor crystalline morphology and surface roughness as well as from reduced stability under ambient conditions of the obtained metal nanostructures. This compromises the efficiency of devices due to large losses of surface plasmons generated on these structures. In our lab, an effective route to grow triangular flat-top silver NPs (designated as silver nanotriangles or NTs) directly on a solid substrate from small NPs using a light induced synthesis has been developed. The silver NTs show very distinctive size-dependent extinction spectra in the UV−vis range and it has been shown that the progressive red shift of the applied photoexcitation wavelength yields increasingly larger NTs with more red-shifted extinction spectra. Apparently, the surface plasmon resonances of the nanoparticles play a crucial role in the growth process, and the photoexcitation of the dipolar plasmon mode by visible light of appropriate wavelength has been invoked to explain the growth of small NTs into the larger ones.

 

Plasmon-enhanced photocatalysis
A transition from the classical thermally driven processes to photo-stimulated reactions can contribute to the development of sustainable chemistry. In this transition, photocatalytic materials could play a major role. So far, UV responsive materials like titania- and zincoxide have shown catalytic activity in, mostly, oxidation reactions. However, to further extend the uses of these materials in sustainable chemistry, an optical response in the visible light and therefore the possible use of solar light-energy, is necessary.
 
In our group, to achieve a visible light-response for the photocatalytic materials, we aim to use the electric field enhancement caused by the interaction of plasmonic nanostructures and the electromagnetic field. Catalyzed oxidation reactions by e.g. TiO2 occur via an excitation of an electron from the valence band to the conduction band in the TiO2. The electron can be excited by absorption of a photon in the UV. The local field enhancement by localized surface plasmons (LSPs) is considered to decrease this bandgap and therefore shift the absorption of the catalyst to the visible light region.  

For the plasmon-enhanced electric field to have a significant effect on the catalysts performance, the catalysts have to be located inside that field. To achieve the localization of the catalyst, we use two approaches. 

In the first approach, a plasmonic nanostructure is placed parallel to a catalytic surface. In this case, it is important that the nanostructure is flat so the enhanced electric field can penetrate into the catalyst surface.
In the second approach, the catalyst is coated onto the building blocks of the plasmonic nanostructures, the noble metal nanoparticles (NPs), followed by an assembly into the desired structure. In this way, the enhanced electric field created between two (or more) NPs will always penetrate a part of the catalyst.

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4. Drug Delivery System Based on Plasmonics - プラズモニクスを基とする薬輸送システム
Plasmon-enhanced singlet oxygen generation 
Photodynamic therapy (PDT) is a promising technique for the treatment of cancer and Singlet oxygen is known to be the responsible cytotoxic agent of this therapy. Singlet oxygen is an excited state of molecular oxygen and can react with surrounding biomolecules in a cell, leading to cell death by apoptosis or necrosis(1). 

 
Upon irradiation at the appropriate wavelength, an organic dye (called Photosensitizer) becomes excited and can transfer its energy to oxygen in the surrounding media, producing singlet oxygen (Fig.1)(2). However, efficiency of single oxygen generation and local concentration of sensitizer greatly affect on efficiency and specificity of photodynamic therapy. In order to cover these deficiencies, organic photosensitizers, such as methylene blue and porphirine derivatives, are encapsulated in meso-porous silica coated metal nanoparticle (3) (Fig.2). 

Due to plasmonic interaction, photophysical properties of the photo-sensitizer are modified, resulted in change in efficiency of single-oxygen generation (4) (5) (Fig.3).

 

Plasmon-heating triggered drug delivery
The leakage problem in drug delivery system (DDS) is vital since it may introduce serious side effect, thus it is important and urgent to develop a “zero leakage" DDS. In our research, by introducing plasmonic structure, the drug release from DDS can be triggered by plasmon-heating, making "zero leaking" practically possible in DDSs.
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北海道大学電子科学研究所 光科学研究部門 ナノ材料光計測研究分野
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