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2007, 111, 1549-1552 Published on Web 01/10/2007
Single Particle Spectroscopic Investigation on the Interaction between Exciton Transition of Cyanine Dye J-Aggregates and Localized Surface Plasmon Polarization of Gold Nanoparticles Takayuki Uwada, Ryo Toyota, Hiroshi Masuhara, and Tsuyoshi Asahi* Department of Applied Physics, Osaka UniVersity, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan ReceiVed: NoVember 15, 2006; In Final Form: December 18, 2006
The interaction between molecular exciton and localized surface plasmon (LSP) polarization has been investigated by measuring light scattering spectra of single gold nanoparticles coated with cyanine dye J-aggregate. The spectral dip reflecting absorption spectral band of the J-aggregate varies with the particle size and the distance between the J-aggregates and the surface of gold nanoparticles. By comparing the experimental results with theoretical calculations based on the Mie theory, we demonstrate that the LSP polarization affects the peak and shape of the J-aggregate absorption band when the exciton resonance peak locates near the LSP resonance peak and when the separation of the J-aggregate from the metal surafce is short. The modification of the J-aggregate exciton band induced by LSP is considered to be due to strong coupling between the molecular exciton transition and the LSP resonance of metallic nanoparticles.
Nobel metallic nanoparticles exhibit strong UV-visible absorption and light scattering corresponding to the LSP polariton, which is a resonance between the collective oscillation of conduction electrons and the incident light.1 At the resonance frequency, which depends on the particles size and shape, the electromagnetic field that is confined in a nanometer scale on the particles’s surface is enhanced. This local enhancement of the electromagnetic field is of great importance for optical and spectroscopic applications such as chemical and biological sensors2-10 and surface enhancement of Raman scattering (SERS) spectroscopy,11-13 because the optical transition of molecules near the nanoparticle surface can be affected much by the LSP modes. Now, electromagnetic enhancement effects have been investigated thoroughly and accepted widely in SERS spectroscopy as the enhanced Raman signal from molecules adsorbed on metallic nanoparticles. On the other hand, the enhancement effects on absorption and luminescence spectral properties of molecules have not been elucidated in detail and not been clear yet. Recently, the interaction between molecular electronic transition and LSP modes of metallic nanoparticles is becoming a topic of contemporary spectroscopic research. Considerable attention has been paid to metallic nanoparticles coated with dye molecules, in particular, J-aggregates of cyanine dyes.14-20 Coherent polarization coupling of a narrow exciton band of a J-aggregate to LSP modes has been reported for Au and Ag colloidal nanoparticles coated with a monolayer J-aggregate of 5,5′-dichloro-3,3′-disulfopropylthiacyanine.15 The extinction spectrum is well analyzed by numerical calculation on the basis of Mie theory using a core-shell structure, which indicates that the J-aggregate exciton transition weakly couples to LSP polarization. In this J-aggregate-coated metal nanoparticle * To whom correspondence should be addressed. E-mail: asahi@ ap.eng.osaka-u.ac.jp.
10.1021/jp067565n CCC: $37.00
SCHEME 1: Chemical Structure of PIC
system, however, the exciton resonance frequency is far from the LSP resonance band peak. In general, the coupling strength becomes large when their resonance frequency is close to each other. In such a case, molecular electronic transition properties should be modified by the LSP polarization of metallic nanoparticle. The LSP resonance peak changes with the particle diameter, so that a systematic investigation of the size dependence is important and indispensable for elucidating the electronic interaction between J-aggregate exciton and LSP polarization. However, conventional ensemble measurements of a colloidal dispersion are not useful as the obtained optical spectra show an inhomogeneous broadening due to the size distribution of the nanoparticles. In this letter, we present single particle spectroscopy, using a far-field optical microscope under a dark-field condition, of gold nanoparticles coated with J-aggregate of 1,1′-diethyl-2,2′-cyanine (PIC; Scheme 1). By measuring and comparing light scattering spectra of single gold nanospheres (from 70 to 130 nm diameter) coated with and without PIC J-aggregates, we have demonstrated that the exciton transition of PIC J-aggregates is modified by the LSP of gold nanoparticles when the exciton transition peak locates near the LSP resonance peak. Gold nanoparticles (mean diameter of 80, 100, and 150 nm) were dispersed and fixed on the 3-aminopropyltrimethoxysilane modified glass substrate by adsorption from a diluted solution of gold nanoparticles (EMGC 80, 100, and 150; British Biocell). We carefully prepared a sample where a mean spacing between © 2007 American Chemical Society
1550 J. Phys. Chem. C, Vol. 111, No. 4, 2007 nanoparticles was larger than 10 µm, which was confirmed from dark-field optical microscope images and from SEM images. The PIC J-aggregate was fabricated on the gold nanoparticles by the reported procedure.21 The surface of the gold nanoparticles was modified with a self-assembled monolayer (SAM) of 11-mercaptoundecanoic acid (MUA, Aldrich) or 3-mercaptopropionic acid (MPA, Aldrich) by immersing the substrate into a 1 mM ethanol solution. After rinsing the substrate with ethanol and distilled water, the SAM coated gold nanoparticles were pretreated with AgNO3 by immersing in a 1 mM aqueous solution of AgNO3 (Nakalai Tesque) at room temperature for 1 h and then used for spectral measurement. The prepared sample substrate was covered with a CoverWell perfusion chamber (Depth: 1.0 mm, Diameter: 20 mm, Grace Bio-Labs), and the chamber was filled with either an ethanol/water (1:1) mixture or a PIC bromide (NK-1046, Hayashibara Biochemical Labs) solution (0.1 mM) in ethanol/water. The absorption spectrum of the solution agrees with monomer PIC, indicating that there is no aggregation of PIC in the solution. Scattering spectra of single gold nanoparticles were measured by dark-field light scattering microspectroscopy.10,22,23 The sample on an inverted microscope (IX70, Olympus) was illuminated with white light from a 100-W halogen lamp through a dark-field condenser (U-DCD, Olympus). The scattered light from a single gold nanoparticle was collected with an objective lens (× 60, N.A. 0.7, Olympus) and selected by using a pinhole (300 µm radius), and the spectrum was recorded with CCD (iStar DH720-18, Andor) polychromator (77480, ORIEL). In order to examine the same single gold nanoparticle coated with and without PIC J-aggregate, first we measured the light scattering spectra of many gold particles in ethanol/water mixture and marked the position of each nanoparticle in its darkfield microscope image. After replacing the pure solvent to the PIC solution and waiting about 10 min, we measured LSP resonance spectra of the marked nanoparticles, and then we compared the spectra with and without PIC J-aggregate. We obtained the scattering efficiency spectrum by dividing the detected scattered light intensity of the single gold nanoparticle by that of a frost plate (DFQ1-30C02-240, SIGMA), which showed the uniform scattering efficiency in a spectral range of our experimental setup. In the calculation of the efficiency spectrum spectra in PIC solution, the effect of absorption loss was corrected by its absorption spectrum. Figure 1 illustrates the results of three individual MUA surface-modified gold nanoparticles with different diameters, as representative examples. The spectra in the ethanol/water mixture show a broad peak characteristic of the LSP resonance band of gold nanoparticles. In order to estimate the particle size, we calculated the scattering efficiency spectra of gold nanospheres in the mixture as functions of the diameter on the basis of Mie theory,24 using the dielectric constant of bulk gold26 and setting the refractive index of surrounding medium to be 1.4.10,22 As shown in Figure 1, the calculated spectra can reproduce the observed scattering spectra in the ethanol/water mixture by varying the diameter as an adjustable parameter. The diameters of each particle in Figure 1a-c are estimated to be 80, 100, and 130 nm, respectively. The scattering spectra in the PIC solutions exhibit a red-shift of the LSP resonance peak and a pronounced decreasing of the light scattering from the nanoparticle around the absorption peak of the J-aggregate. Figure 2 shows the temporal change of the light scattering spectra after replacing the methanol/water mixture with a 0.1 mM PIC solution. A broad scattering spectrum similar to that in the solvent mixture was observed just after the replacement. The
Letters
Figure 1. Light scattering spectra of single gold nanoparticles of different diameters [(a) 80, (b) 100, and (c) 130 nm] modified with MUA in ethanol/water mixture (blue line) and in ethanol/water solution (red line) of PIC (0.1 mM), and calculated spectra of spherical gold nanospheres in a medium with refractive index of 1.4. Insertions show the ratio of the spectrum in the pure solvent to that in the PIC solution. The particle diameters are estimated by comparing the spectra in the pure solvent with LSP resonance spectra calculated on the basis of Mie theory.
Figure 2. Temporal change of light scattering spectra of MUA surface modified single gold nanoparticles after replacing ethanol/water mixture with a PIC solution (0.1 mM).
scattering intensity in the wavelength region from 500 to 600 nm decreased with increasing time, whereas the peak intensity was almost constant. The spectral change finished in 10 min. The result indicates clearly that adsorbed PIC formed the J-aggregate on the MUA-SAM on gold nanoparticle, resulting in the formation of a spectral dip in a broad scattering spectrum of the nanoaprticle. We calculated the spectral ratio by dividing the scattering spectrum in methanol/water mixture with that in the PIC solution and determine the dip position from the peak of the spectral ratio. The spectral ratio is shown in insertions of Figure 1a-c. The dip position around 570 nm agrees well with the absorption peak position of the PIC J-aggregate in solution25 and the J-aggregate monolayer on a flat gold surface.15 Therefore, we can conclude that the dip is attributed to the formation of J-aggregate on the surface-modified gold nanoparticle.
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J. Phys. Chem. C, Vol. 111, No. 4, 2007 1551
To explain the scattering spectral modification of the PIC J-aggregate coated nanoparticles, we made theoretical calculations using a core-shell structure as shown in Figure 3a by taking into account the electromagnetic coupling between the gold core polarization and transition dipole of the J-aggregate exciton. The scattering spectra of the core-shell spheres were calculated on the basis of Mie theory, using the algorithm described in ref 24. The dielectric constant of the metal core is assumed to be the same to that of bulk gold,26 and the refractive index of the environment is set to 1.4.10,22 On the other hand, the exciton resonance was modeled with an assumption of a Lorentzian absorption line shape with the transition frequency ω0, oscillator strength S, and the line width γ. Therefore, the dielectric constant of the J-aggregate shell is given by the following equation:
) 1 +
S ω0 - ω2 - iγω 2
Assuming the same absorption peak (581 nm) and line width (10 nm) to those of the J-aggregate monolayer on a flat gold surface,21 and taking 1 ) 2.9, S ) 1.044 × 1030 s-2, ω0 ) 3.232 × 1015 s-1, and γ ) 0.5 × 1014 s-1, we were able to calculate the far-field scattering spectra of the core-shell spheres as functions of metal core’s diameter and shell thickness. As an example, the calculated spectrum of a 100 nm gold particle coated with a 1.0-nm J-aggregate shell is shown in Figure 3c. A broad spectrum having a dip structure is well reproduced in the calculated spectrum. It is clearly confirmed that the formation of a J-aggregate layer on a gold nanoparticle causes the spectral dip in its scattering spectrum. Accordingly, we also measured scattering spectra of many MUA or MPA surface modified gold nanoparticles with different diameters (70-130 nm). Many of them exhibited the similar spectral modification due to the J-aggregate layer formation as it was observed for 80, 100, and 130 nm diameter sized nanoparticles in Figure 1. The relation between the dip position and the LSP resonance peak wavelength is shown in Figure 4 for MUA and MPA modified particles along with the calculated result. In the case of MUA modified particles, the dip position remains constant around 575 nm when the LSP peak is longer than 600 nm and slightly blue-shifts with decreasing of the peak wavelength in the region from 600 to 560 nm. A similar tendency is observed for MPA-modified nanoparticles; however, the shifting value for decreasing of particle size is larger compared to MUA modified ones. This suggests that the dip position depends also on the distance between the J-aggregate shell and metal core, because the distance provided by the SAM of MPA and MUA can be estimated to be 0.8 and 1.7 nm, respectively. It should be noted here that the calculated results resemble the experimental observation in tendency but show a different correlation factor. The calculation relation for the core-shell spheres with different shell thickness (0.5-5 nm) indicated that the dip position did not depend on the thickness. We made also theoretical calculations using a double-layer model in order to take into account the effect of the spacer layer of MPA and MUA. The scattering spectra were calculated for spacer layer (1.7-nm thickness and the refractive index of 1.45) coated gold spheres with the J-aggregate shell. However, the effect of the spacer layer was negligibly small in the theoretical prediction based on the Mie theory. The scattering spectra and the relation between the dip position and the LSP resonance peak are almost the same to the calculation for a simple core-shell model in Figures 3 and 4. Consequently, the experimental result of the size and
Figure 3. (a) Metal core-J-aggregate shell model used for calculating the scattering spectrum of gold nanoparticle coated PIC J-aggregate, (b) calculated scattering spectra of a bare gold nanopartcile with 100 nm diameter (black line) and the nanoparticle coated with J-aggregate layer of 1.0-nm thickness (red line) in a medium of the refractive index of 1.4, and their spectral ratio (blue line), and (c) real (black line) and imaginary (red line) parts of complex refractive index of the J-aggregate layer used for calculation.
Figure 4. Dependence of the dip position on the LSP resonance spectral peak of the gold nanoparticles in ethanol/water mixture for MUA-modified (red open circles) and MPA-modified (blue filled squares) particles, and the calculated result (black dot with line) based on Mie theory using the core-shell model in Figure 3. Dotted lines are guide for eyes.
separation effect on the dip position is significant compared to the theoretical prediction, indicating that the J-aggregate absorption peak depends on the particle size and the distance from the nanoparticle’s surface. It is well recognized that the absorption peak of PIC J-aggregates is sensitive to molecular stacking manner in the aggregate. One may consider the structural deformation of the J-aggregates depending on the particle size, because the curvature of the particle’s surface becomes larger for smaller particles. However, this idea would not explain well the experimental observation that the shift of the dip position for the decreasing of the particle size is larger for the MPA modified particles compared to the MUA ones as shown in Figure 4. The
1552 J. Phys. Chem. C, Vol. 111, No. 4, 2007 difference in thickness of MPA and MUA is much smaller when compared with the gold particle’s size examined here. Therefore, the surface curvature is considered to be the same for both MPA and MUA modified nanoparticles. Thus, we can exclude the structural deformation of J-aggregates due to the changes in the particle size. In the theoretical calculation based on Mie theory, the interaction between the J-aggregate exciton and the LSP polarization is considered as an electromagnetic coupling between them, and the interaction is so weak it does not change the intrinsic properties of optical transition dipole of the J-aggregate and the LSP resonance of the gold nanopaticle. This is deemed to be a weak polarization coupling. On the other hand, we consider that the present results on the size dependent dip position need an additional mechanism inducing modification in the electronic structure of the exciton states, which should be a strong interaction between the J-aggregate exciton transition and the LSP resonance. Indeed, the experimental results were observed as a blue shift of the J-aggregate exciton band when it closes to the LSP resonance peak and when the distance between the J-aggregate and the metal core is short. This strong interaction may be similar to that between a molecular Jaggregate and a propagating surface plasmon polariton generated at a metal surface by total reflection excitation27 and by using a sub-wavelength hole-array structure.28 A strong coupling between molecular exciton and plasmon polarization modes was clearly demonstrated with a Rabi-splitting in the dispersion curve of the transmittance peak for an Ag thin film coated with a J-aggregate/PVA film. At the present state of investigation, it is not clear whether the mechanism discussed in their papers is consistent with our interpretation on the interactions between the molecular electronic transition and the LSP resonance or not. Although the detail mechanism of the interaction is not clear yet, we can point out here a possibility that when the peaks of the J-aggregate exciton and the LSP resonance close to each other a very rapid energy transfer between the molecular exciton state and LSP polarization should take place, resulting in a spectral broadening and a peak shift in the transition line shape of the J-aggregate. In summary, the interaction between the molecular exciton and the LSP polarization of gold nanoaprticles was studied by measuring the light scattering spectra of single gold nanoparticles coated with PIC J-aggregates. The dependence of the spectral dip on the particle size and the separation between the particle and the J-aggregate is not well explained as an effect of enhanced electromagnetic field due to the LSP resonance. The spectral modification of the J-aggregate absorption will occur in the enhanced electromagnetic fields when the exciton peak locates near the LSP resonance peak, indicating a strong coupling between the LSP polarization and the exciton transition. The present results demonstrate clearly that optical responses of hybrid heterostructures consisting of noble metal nanoparticles and dye molecules are modified significantly from
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