108
K. Imaeda and K. Imura
shown in Fig. 6.11b. The black curve in the figure represents the absorption spectrum
of the sample before laser irradiation. The absorption band centered at 740 nm is
assignable to the longitudinal plasmon mode in the gold nanorod. On the other hand,
the three peaks observed near 310, 370, and 540 nm are attributed to the resonance
bands of poly(DE). As depicted by the arrows in Fig. 6.11b, the absorption intensities
at 370 and 540 nm decrease as the laser irradiation time increases. We also found that
a new absorption band appears near 290 nm and becomes stronger with increasing
laser irradiation time. These spectral changes imply that the cycloreversion reaction
of poly(DE) proceeds by near-infrared laser irradiation. To examine the influence
of the gold nanorod on the reaction, we performed the same measurements on the
neat poly(DE) solution. The red and blue plots in Fig. 6.11c are time-traces of the
absorption intensities at 540 nm measured for the assembled and neat poly(DE),
respectively. In this figure, the absorption intensity decays exponentially in both the
assembled and neat samples, indicating that the cycloreversion reaction proceeds
even in the neat poly(DE) solution. We evaluated the reaction rates by fitting the
time-traces with single exponential functions shown as solid curves in Fig. 6.11c.
From the analysis, the reaction rate constants of the assembled and neat poly(DE)
samples are determined to be k GR-DE = 0.020 min
−1 and k DE = 0.012 min
−1 . We
also found that a reaction ratio between the assembled and neat poly(DE) samples is
constant regardless of the incident laser power. This fact indicates that thermal reaction does not occur in the present study. These results demonstrate that the plasmonic
fields induced in the gold nanorods accelerate the one-photon induced cycloreversion
reaction of poly (DE).
Strong interactions between plasmonic fields on mesostructures and molecules
in the vicinity of the structures can amplify various optical signals, including fluorescence and Raman scattering [34–36, 72–74]. This striking property has potential
applications in a wide range of fields, such as high-sensitive sensors and biological imaging. For the practical implementation of plasmon-enhanced light-molecule
interactions, rational design, and construction of wide-scale plasmonic substrates
are highly desirable. The metallic nanohole array is a representative example of
a plasmonic substrate and has attracted enormous attention for its unique optical
characteristics, such as extraordinary light transmission and amplification of local
fields [75–79]. To elucidate the interaction between molecules and plasmonic fields
on a metallic nanohole array, we investigate the fluorescence characteristics of dye
molecules dispersed on a gold nanohole array using the SNOM [80]. Figure 6.12a
shows an SEM image of the gold nanohole array. From this image, the diameter and
periodicity of the nanohole arrays were estimated to be 240 and 480 nm, respectively.
An atomic force microscope (AFM) was used to measure the topography, and the
depth of the nanohole was determined to be 40 nm. To avoid quenching by the gold,
a thin SiO 2 film (20 nm) was deposited on the sample surface. A drop of methanol
solution containing R6G molecules (10
−4 mol/dm
3 ) was spin-coated onto the sample
for the fluorescence measurements. We used a continuous wave laser (532 nm) to
excite the fluorescence of R6G molecules. Figure 6.12b shows near-field fluorescence spectra taken at the red and blue points shown in Fig. 6.12a. We found that
spectral shapes in Fig. 6.12b are different from that of the original R6G molecules.
K. Imaeda and K. Imura
shown in Fig. 6.11b. The black curve in the figure represents the absorption spectrum
of the sample before laser irradiation. The absorption band centered at 740 nm is
assignable to the longitudinal plasmon mode in the gold nanorod. On the other hand,
the three peaks observed near 310, 370, and 540 nm are attributed to the resonance
bands of poly(DE). As depicted by the arrows in Fig. 6.11b, the absorption intensities
at 370 and 540 nm decrease as the laser irradiation time increases. We also found that
a new absorption band appears near 290 nm and becomes stronger with increasing
laser irradiation time. These spectral changes imply that the cycloreversion reaction
of poly(DE) proceeds by near-infrared laser irradiation. To examine the influence
of the gold nanorod on the reaction, we performed the same measurements on the
neat poly(DE) solution. The red and blue plots in Fig. 6.11c are time-traces of the
absorption intensities at 540 nm measured for the assembled and neat poly(DE),
respectively. In this figure, the absorption intensity decays exponentially in both the
assembled and neat samples, indicating that the cycloreversion reaction proceeds
even in the neat poly(DE) solution. We evaluated the reaction rates by fitting the
time-traces with single exponential functions shown as solid curves in Fig. 6.11c.
From the analysis, the reaction rate constants of the assembled and neat poly(DE)
samples are determined to be k GR-DE = 0.020 min
−1 and k DE = 0.012 min
−1 . We
also found that a reaction ratio between the assembled and neat poly(DE) samples is
constant regardless of the incident laser power. This fact indicates that thermal reaction does not occur in the present study. These results demonstrate that the plasmonic
fields induced in the gold nanorods accelerate the one-photon induced cycloreversion
reaction of poly (DE).
Strong interactions between plasmonic fields on mesostructures and molecules
in the vicinity of the structures can amplify various optical signals, including fluorescence and Raman scattering [34–36, 72–74]. This striking property has potential
applications in a wide range of fields, such as high-sensitive sensors and biological imaging. For the practical implementation of plasmon-enhanced light-molecule
interactions, rational design, and construction of wide-scale plasmonic substrates
are highly desirable. The metallic nanohole array is a representative example of
a plasmonic substrate and has attracted enormous attention for its unique optical
characteristics, such as extraordinary light transmission and amplification of local
fields [75–79]. To elucidate the interaction between molecules and plasmonic fields
on a metallic nanohole array, we investigate the fluorescence characteristics of dye
molecules dispersed on a gold nanohole array using the SNOM [80]. Figure 6.12a
shows an SEM image of the gold nanohole array. From this image, the diameter and
periodicity of the nanohole arrays were estimated to be 240 and 480 nm, respectively.
An atomic force microscope (AFM) was used to measure the topography, and the
depth of the nanohole was determined to be 40 nm. To avoid quenching by the gold,
a thin SiO 2 film (20 nm) was deposited on the sample surface. A drop of methanol
solution containing R6G molecules (10
−4 mol/dm
3 ) was spin-coated onto the sample
for the fluorescence measurements. We used a continuous wave laser (532 nm) to
excite the fluorescence of R6G molecules. Figure 6.12b shows near-field fluorescence spectra taken at the red and blue points shown in Fig. 6.12a. We found that
spectral shapes in Fig. 6.12b are different from that of the original R6G molecules.