Takumi Sannomiya

In situ sensing of single binding events by localized surface plasmon resonance.

Single binding events of nanoparticle-labeled DNA strands were detected as stepwise peak shifts in localized surface plasmon resonance by single particle measurement. We confirmed the number of binding events by observing label particles by scanning electron microscopy. Our simulation based on a multiple multipole program showed that the peak shift is dependent on interparticle gap size and binding position. The experimental peak shift distribution was also reproduced by simulation.

Investigation of Plasmon Resonances in Metal Films with Nanohole Arrays for Biosensing Applications

Biosensing with nanoholes is one of the most promising applications of nanoplasmonic devices. The sensor properties, however, are complex due to coupled resonances through propagating and localized surface plasmons. This Full Paper demonstrates experimental and simulation studies on different plasmonic hole systems, namely various patterns of circular holes in gold films. In contrast to most previous work, here, the challenging situation of optically thin films is considered. The refractive-index-sensing properties, such as sensitive locations in the nanostructure and sensitive spectral features, are investigated. The multiple multipole program provides the complete field distribution in the nanostructure for different wavelengths. It is shown that the spectral feature most sensitive to refractive-index changes is the extinction minimum, rather than the maximum. The results are consistent with theory for perfect electrical conductors. The spectral response is investigated for molecular adsorption at different positions inside or outside a hole. Furthermore, the optical properties of nanohole arrays with long-range and short-range order are compared and found to demonstrate remarkable similarities. Our results help to predict the resonance wavelengths of nanoholes with arbitrary patterns, including short-range order. The results presented here are highly important since they extend and challenge several aspects of the current understanding of plasmon resonances in nanohole arrays. These theoretical models, simulation results, and experimental data together help provide the understanding necessary for the development of efficient biomolecular analysis tools based on metallic nanoholes.

Single plasmonic nanoparticles for biosensing

Along with remarkable progress of nanoplasmonics over the past 10 years, single plasmonic nanoparticle sensors have introduced a completely new dimension to the sensing scale, considering that nanoparticles are comparable in size to biomolecules such as nucleic acids or antibodies. Single particle sensing methods have recently shown the possibility of detecting the adsorption of single biomolecules, and have already provided information about conformational changes of single molecules. For practical application, arrays of such compact sensor units are expected to realize massive multiplexing and high throughput in diagnostics and drug discovery in the near future. In this review, recent achievements and perspectives of this emerging biosensing technique are discussed.

Metallic Nanodot Arrays by Stencil Lithography for Plasmonic Biosensing Applications

The fabrication of gold nanodots by stencil lithography and its application for optical biosensing based on localized surface plasmon resonance are presented. Arrays of 50-200 nm wide nanodots with different spacing of 50-300 nm are fabricated without any resist, etching, or lift-off process. The dimensions and morphology of the nanodots were characterized by scanning electron and atomic force microscopy. The fabricated nanodots showed localized surface plasmon resonance in their extinction spectra in the visible range. The resonance wavelength depends on the periodicity and dimensions of the nanodots. Bulk refractive index measurements and model biosensing of streptavidin were successfully performed based on the plasmon resonance shift induced by local refractive index change when biomolecules are adsorbed on the nanodots. These results demonstrate the potential of stencil lithography for the realization of plasmon-based biosensing devices.

Electrochemistry on a Localized Surface Plasmon Resonance Sensor

The optical signal of a localized surface plasmon resonance (LSPR)-based sensor combined with electrochemistry was investigated. Gold nanoparticles were immobilized on an indium tin oxide (ITO) substrate, which functioned as working electrode. Using cyclic voltammetry synchronized with LSPR sensing, surface reactions on gold were detected both electrically and optically. In the capacitive charging regime, optical signals linear to the applied potential were detected. Gold was found to be dissolved above the oxidation potential and partially redeposited during the reduction, which changed size and conformation of the gold nanoparticles. In kinetic measurements, slower potential establishment was observed at lower salt concentrations. Simulations by multiple multipole program (MMP) suggested the formation of a lossy layer by combination of charge depletion of gold and negative ion adsorption even below the reaction potential. We consider the results presented here of importance for any future sensors based on combined plasmonics and electrochemistry.

Measurement method of aberration from Ronchigram by autocorrelation function

Aberrations up to the fifth-order were successfully measured using an autocorrelation function of the segmental areas of a Ronchigram. The method applied to aberration measurement in a newly developed 300kV microscope that is equipped with a spherical aberration corrector for probe-forming systems. The experimental Ronchigram agreed well with the simulated Ronchigram that was calculated by using the measured aberrations. The Ronchigram had an infinite magnification area with a half-angle of 50mrad, corresponding to the convergence angle of a uniform phase.

Achieving 63 pm Resolution in Scanning Transmission Electron Microscope with Spherical Aberration Corrector

The performance of a newly developed high-resolution 300 kV microscope equipped with a spherical aberration corrector for probe-forming systems is reported. This microscope gave the highest resolution for the distance between atomic columns, as determined by a high-angle annular dark field imaging method using a GaN[211] crystalline specimen, where the distance between the neighboring columns of Ga was 63 pm.

Electrochemical plasmonic sensors

The enormous progress of nanotechnology during the last decade has made it possible to fabricate a great variety of nanostructures. On the nanoscale, metals exhibit special electrical and optical properties, which can be utilized for novel applications. In particular, plasmonic sensors including both the established technique of surface plasmon resonance and more recent nanoplasmonic sensors, have recently attracted much attention. However, some of the simplest and most successful sensors, such as the glucose biosensor, are based on electrical readout. In this review we describe the implementation of electrochemistry with plasmonic nanostructures for combined electrical and optical signal transduction. We highlight results from different types of metallic nanostructures such as nanoparticles, nanowires, nanoholes or simply films of nanoscale thickness. We briefly give an overview of their optical properties and discuss implementation of electrochemical methods. In particular, we review studies on how electrochemical potentials influence the plasmon resonances in different nanostructures, as this type of fundamental understanding is necessary for successful combination of the methods. Although several combined platforms exist, many are not yet in use as sensors partly because of the complicated effects from electrochemical potentials on plasmon resonances. Yet, there are clearly promising aspects of these sensor combinations and we conclude this review by discussing the advantages of synchronized electrical and optical readout, illustrating the versatility of these technologies.

Biosensing by densely packed and optically coupled plasmonic particle arrays.

Densely packed plasmonic particle arrays are investigated for biosensing applications. Such particle arrays exhibit interparticle optical coupling creating a strong field between the particles, which is useful for sensing purposes. The sensor properties, such as bulk sensitivity, layer sensitivity, and the depth of sensitivity are investigated with the aid of a multiple multipole program. Sensitivity to the analyte with low concentration is also examined by a dynamic adsorption processes. The detectable concentration limit of streptavidin within 3000 s in the detection system is expected from the signal-to-noise to be less than 150 pM.

High-Resolution Resistless Nanopatterning on Polymer and Flexible Substrates for Plasmonic Biosensing Using Stencil Masks

The development of nanoscale lithographic methods on polymer materials is a key requirement to improve the spatial resolution and performance of flexible devices. Here, we report the fabrication of metallic nanostructures down to 20 and 50 nm in size on polymer materials such as polyimide, parylene, SU-8, and PDMS substrates without any resist processing using stencil lithography. Metallic nanodot array analysis of their localized surface plasmon spectra is included. We demonstrate plasmon resonance detection of biotin and streptavidin using a PDMS flexible film with gold nanodots. We also demonstrate the fabrication of metallic nanowires on polyimide substrates with their electrical characteristics showing an ohmic behavior. These results demonstrate high-resolution nanopatterning and device nanofabrication capability of stencil lithography on polymer and flexible substrates.