We propose a method to measure the variation of the molecular length of self-assembled monolayers (SAMs) when it is exposed to solutions at different pH conditions. The surface immobilized gold nanospheres (SIGNs) shows strong absorption peak at the wavelengths of 600-800 nm when p-polarized light is illuminated. The peak wavelength depends on the length of the gap distance between the SIGNs and the substrate. The gap is supported by the SAM molecules. According to the analytical calculation based on multiple expansion, the relation between the peak wavelength of the SIGN structures and the gap distance is calculated, to evaluate the molecular length of the SAM through the optical absorption spectroscopy for the SIGN structures. The molecular length of the SIGN structure was measured in air, water, acidic, and basic solutions. It was found that the molecular lengths are longer in acidic solutions.
Recently plasmonic biosensors consisting of gold nanoparticles have been developed. In order to understand the response of the biosensors, we have investigated how are gold nanospheres immobilized on a surface covered by a self-assembled monolayer (SAM) which is formed by immersion of the substrate in a solution, by use of surface second-harmonic generation (SHG). The surface immobilized gold nanospheres (SIGNs) are supported by a self-assembled monolayer (SAM) of aminoundecanthiol on a gold thin film. The SIGN substrate was immersed in an ethanol solution of hemicyanine-terminated alkanethiol. The capping angles of the hemicyanine SAM with respect to the top of the SIGN were evaluated from polarization dependence of SHG intensity. The SIGNs are not fully covered with the SAM, and the capping angle is found to be approximately 120 degrees.
We have fabricated gold nanospheres composite multilayer films using the layer-by-layer (LbL) self-assembly
technique, and have investigated the aggregate states of the gold nanosphere. The gold nanospheres composite
multilayer films were fabricated by controlling the gold nanosphere layers with polyelectrolyte layers, and
were characterized with linear and nonlinear optical spectroscopy. The transmission absorption spectra and
scanning electron microscopy (SEM) images show modifications of the optical properties arising from the aggregate
states of the gold nanospheres. The strong longitudinal resonance mode was observed when the gold
nanospheres form aggreagates. Intense optical second-harmonic generation (SHG) was observed from the gold
nanosphere aggregates of which surface was covered with a hemicyanine self-assembled monolayer. This high
SHG response originates from the strong interaction via localized surface plasmon enhancement of the gold
nanosphere aggregates. The gold nanosphere aggregates are promising for applications to optoelectronic devices
and surface-enhanced spectroscopy.
Surface plasmon resonance (SPR) biosensors based on attenuated total reflection (ATR) have been widely used in
biochemistry and genetic engineering, because it is a sensitive and label-free method. The dimension of the sensing
probe is millimeters or more, so that the required amount of a sample solution is more than 100 μL even if a micro
chamber is used. For multifunctional biosensing applications, therefore, a small biosensing platform is needed. We
employed localized surface plasmons (LSPs) in gold nanostructures, instead of the conventional ATR-based SPR, to
realize such small sensing probes. A few works on biosensing developed in our research group will be shown in this
paper. One is a fabrication method of gold nanoparticles by annealing of thin gold film less than 10 nm thick. The
optimized condition for producing nanoparticles for biosensing applications is discussed. The other is a sensitive optical
fiber biosensor based on LSPs in gold nanoparticles. This optical fiber biosensor has advantages: easy handling and
remote sensing. These merits come from the fact that the sensor probe is formed at the endface of a standard multimode
optical fiber whose core diameter is 50 μm. Instead of such a small probe area, it has similar sensitivity to that of the
ATR-based SPR sensors. This optical fiber biosensor enables us to perform biosensing with a sample solution of less
than 100 nL. Finally we show biosensing based on nonlinear optics. Second-harmonic generation is one of the secondorder
nonlinear optical phenomena and is a surface sensitive phenomenon. Here we show that it provides us a highly
sensitive way for biosensing.
We developed an optical fiber biosensor based on localized surface plasmon resonance in gold nanoparticles. The sensor
system enables us to analyze biological molecules in ultra small amount of analytes. In spite of a simple optical setup,
the limit of detection of avidin was about 0.09 μg/mL, which was similar to the value previously reported in a standard
absorption experiment. Since the sensor probe is made at the endface of the optical fiber, it has following advantages:
(1) it is easy to handle, and (2) it is possible to detect a small amounts of samples (<1μL) such as DNA and proteins.
Because of the feature (2), it is possible to carry out DNA and protein detection with a sample solution of 100 nL.
We have found that spherical gold nanoparticles immobilized on a gold substrate with a gap of a few nanometers, which is supported by self-assembled monolayers, show large activity of second-harmonic generation (SHG). Spectroscopic SHG measurements were performed with a Ti:Sa laser in order to investigate the origin of the intense SHG. It was found that the SHG intensity increases with shorter wavelength region, indicating that the enhancement originates from localized surface plasmon resonance in the system. We also fabricated microarrays of the surface immobilized gold nanoparticles through photoregistration of the self-assembled monolayers used to support the nanogap. The nanoparticle microarrays were characterized by SHG microscopy. The microarrays can be applied to multichannel biological sensors using linear optical spectroscopy.
We developed a label-free, highly sensitive, real time, and micrometer-sized optical fiber biosensor without using any attenuated total reflection (ATR) optics. The sensor is based on the localized surface plasmon resonance (LPR) in gold nanoparticles. The gold nanoparitlces show a large absorption band at around 520 nm due to LPR, which is quite sensitive to the environment around the nanoparticles. Thus change of the refractive index of the ambient medium around the nanoparticles or overcoating a thin dielectric layer on the particles results in a red-shift of the absorption band and change of the absorption intensity. This paper reports a novel optical fiber biosensor that is constructed at the endface of the optical fiber. The fiber probe is consisting of gold nanoparticles that plays as a transducer, where ligand is coated. The return light intensity from the fiber probe is changed by the adsorption of the biomolecules that has affinity to the ligand. The sensitivity was evaluated to be 10 pg/mm2 (2x10-5 RIU) with an LED light, which is compatible with the conventional surface plasmon resonance biosensors that use the ATR optics.