Observation of Defocus Images of a Single Metal Nanorod - The

Article pubs.acs.org/JPCC

Observation of Defocus Images of a Single Metal Nanorod Toshinori Motegi,† Hideki Nabika,†,§ Yasuro Niidome,‡ and Kei Murakoshi*,† †

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan Department of Applied Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

‡

S Supporting Information *

ABSTRACT: We examined an emission of light from a single metallic nanoparticle based on surface plasmon (SP) resonance for determination of three-dimensional orientation of nanoparticles as well as their optical properties. The defocused image of individual Au nanorods (Au NRs) is recorded by changing the focus distance under total internal reflection microscopy (TIRM) observations. Numerical and statistical analysis revealed that the observed light distribution patterns of Au NRs defocused images were classified into two groups. One is explained by considering that a single dipole dominates its light emission property. The other is explained by assuming the presence of multidipoles. This result leads us to a consideration that the emission of light coupled with the transverse and the longitudinal SP modes was observed reflecting the optical characteristics of NRs. Additionally, unique multiple ring patterns were also observed by placing Au NRs at the vicinity of nanoscopic structure, reflecting the distance between NRs and the wall of the structure in the scale less than a few tens of nanometers. The inclusive SP measurement for both the transverse and longitudinal axes of these anisotropic metal NRs using a defocused imaging system brings us reliable optical and conformational information.

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INTRODUCTION Optical probes that yield large photon counts with little background noise, have high photostability, and show good biocompatibility are at the peak of demand for nanoscale imaging. High-accuracy polarization anisotropy is also desirable because it allows observation of the probe orientation, which can consequently provide important information on the nanoscale system dynamics. Meanwhile, single molecule spectroscopy has become a major tool for determining local nanoscale orientation of biological probes because it avoids ensemble averaging of the observables, even in heterogeneous systems. Polarization-sensitive single molecule optical imaging has thus led to a wealth of new information on many complex processes in ordered systems. Examples include the rotational mechanism of the F1-ATPase, which is strongly correlated to its biological function,1 as well as the structural changes of myosin V2 and the dynamical changes in polymers near the glass transition temperature.3 These studies used fluorescent dye molecules, polymers bearing dye groups, and inorganic semiconductor nanoparticles with high photostability and bright emission as probes.4,5 However, with using the fluorescence molecules, the relatively weak stability against strong light irradiation sometimes becomes a problem which creates a barrier for biological imaging that requires high accuracy and long-term observation. Recently, metallic nanoparticles which display surface plasmon (SP) resonance have attracted attention as a new class of probes for biological applications.6 The collective oscillation of free electrons in noble metallic nanoparticles gives rise to large scattering and absorption cross sections.7 In © 2012 American Chemical Society

addition to high photostability of the anisotropic nanoparticles, the lack of photoblinking and high polarization-dependent light absorption and scattering allows orientation imaging.8 One of the best studied metallic nanoparticles is gold nanorods (Au NRs) that have longitudinal and transverse SP modes polarized parallel to the long and short axes of the NR, respectively.9 Precise optical information on the longitudinal SP, the main SP mode of the NR, can be gained by scattering based techniques because its scattering cross-section is much larger than the transverse SP. We have described that certain emissive modes of metal nanoparticles are also selectively coupled with the SP mode.10 However, full analysis of the optical properties of the NRs which includes quantitative investigation of its transverse SP is difficult using the average scattering based observation for a large number of NRs. Such a limitation comes from the fact that the absorption effect dominates over the scattering for the transverse SP of the small NRs with a diameter less than 100 nm.11 Consequently, the weak scattering light from the transverse SP should be buried in the strong scattering light from the longitudinal SP. In addition, only a few methods have been reported that are capable of determining NR orientation by the observation of anisotropic SP in situ. One method is polarization imaging, which uses dark-field microscopy combined with a birefringent Special Issue: Nanostructured-Enhanced Photoenergy Conversion Received: July 1, 2012 Revised: October 9, 2012 Published: October 31, 2012 2535

dx.doi.org/10.1021/jp3065008 | J. Phys. Chem. C 2013, 117, 2535−2540

The Journal of Physical Chemistry C

Article

experiment, 5 μL of water-diluted NRs solution were dropped onto a precleaned 24 mm × 24 mm cover glass (thickness 0.12−0.17 mm, Matsunami, Japan) and immediately dried in a vacuum desiccator. Imaging of single Au NRs was carried out in air using objective-type TIRM (IX-71; Olympus). The excitation laser (532 nm, 10 mW) was focused onto the sample through an objective lens (×100, N.A. = 1.45) and immersion oil (see also the Supporting Information Figure S1). Emission of light with wavelengths longer than 590 nm was collected, and the defocused image from Au NRs was acquired with a digital CCD camera (Ixon; Andor). Images were recorded at 300−500 ms/frame. The computer simulations were performed using Matlab according to previously reported single dipole model methods which was developed by Enderlein et al.16,21

crystal to split the perpendicular and parallel polarized scattering light from anisotropic SP.12 In that study, only relative two-dimensional (2-D) orientation information was obtained because out-of-plane angles were completely obscured, but out-of-plane angular information is vital for accurate 3-D orientation determination. Another approach is to attach a single Au nanoparticle to the end of a sharp glass fiber tip and then illuminate it with white light.13 By collecting the emission of light as a function of the incident light polarization angle, ellipticity and angular information of the Au nanoparticle was resolved. However, the time resolution of this method is limited such that large numbers of randomly orientated single nanoparticles cannot be resolved simultaneously. Another effective method is imaging the SP with confocal microscopy through higher-order-mode laser excitation.14 This method was initially developed to determine the dipole orientation of fluorescent dye molecules and requires sophisticated excitation sources and complex laser mode modulation techniques.15 Image acquisition is also time-consuming, which makes it difficult to apply the method for observation of dynamic behaviors such as biological phenomena. Therefore, it is important to develop a convenient, in situ, and high-throughput method to extract 3-D orientation information from large numbers of single nanoparticles simultaneously. Recently, a novel technique to observe the Au NR 3-D orientation using standard optical darkfield microscopes was developed. In this method, the spatial angles of Au NRs were conveniently derived by deciphering the light distribution pattern in the defocused darkfield image16 which are derived within the defined image defocusing framework first developed by Sepioł17 and later expanded by Enderlein and coworkers.18,19 This method showed improvements over previously reported methods, such as fast full 3-D angleresolving capability, high stability against photobleaching, highly parallel data acquisition, and a simple and inexpensive experimental setup. However, the statistical analysis for emission of light from single NRs is needed for the full understanding of light emission properties along the both transverse and longitudinal axis of single NRs. In this study, we have described new insights into the defocused images of metallic NRs from distinct image observations using total internal reflection microscopy (TIRM) and statistical analysis of defocused images from various single metallic NRs. By tuning the excitation wavelength to the SP resonance and exciting only the probes embedded very near the surface with TIRM, one may improve selectivity against nonmetallic objects and access sizes down to a few tens of nanometers. By exciting with using a monochromatic light under TIRF configuration, the origin of the emission of light from metallic NRs through the SP resonance can be discussed. We used Au NRs and silvershell/gold-core nanorods (Au@Ag NRs) in this study to expand the versatility of the present system. The spatial angles of these NRs were derived by deciphering the light distribution pattern in the defocused image.

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RESULTS AND DISCUSSION As shown in Figure 1a, the extinction spectrum of Au NRs in aqueous solution has longitudinal and transverse SP bands at

Figure 1. Extinction spectrum of (a) Au NRs and (b) Au@Ag NRs in water.

870 and 510 nm, respectively. In addition to Au NRs, we also used Au@Ag NRs to investigate how the detailed optical properties vary. The extinction spectrum of Au@Ag NRs in aqueous solution, shown in Figure 1b, also shows an SP band at 646 nm which should be associated with a peak shift of the longitudinal SP band of the Au core rod during the silver shell formation. Multiple bands appeared in the shorter wavelength region that have recently been assigned to dipolar and higher order plasmon resonances.20,22 The two bands at 444 and 394 nm, which were known to be shifted toward longer wavelength during the shell formation, would relate to the transverse SP of the Au core rod. Because we used a 532-nm laser excitation source, the transverse SP would be principally excited in both Au NRs and Au@Ag NRs. However, the longitudinal SP occasionally shows large peak deviations, e.g., 100 nm from the ensemble peak.23 In these cases, a portion of transverse SP energy is relaxed to the longitudinal SP, which brings the longitudinal SP contribution to the emission of light from NR. Additionally, when the transverse SP is excited, luminescence should arise through electron−hole pair recombination in the particles.24 Although the nature of this luminescence has not been fully assigned yet,25 emissive sites that are resonant with localized surface plasmon resonance can selectively emit photons.10 Thus, the present system yields not only the direct emission of light from NRs dominated by the transverse and longitudinal SPs but also the luminescence from the certain emissive sites, providing us accurate orientation characterization as well as information on the emitted light via the excitation of the SP of metallic NRs.

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EXPERIMENTAL PROCEDURES Au@Ag NRs and Au NRs were fabricated as previously reported.20 According to the transmission electron microscope observation, the Au@Ag NRs have a size with longitudinal length of 52 ± 14 nm and the transverse length 19 ± 1 nm. The thickness of the silver shell along the transverse axis is about 4− 5 nm. The Au NRs has the size with longitudinal length 51 ± 7 nm and the transverse length 10 ± 1 nm. In a typical 2536



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Figure 2. (top panel) Defocused image of (a) Au NR and (b) Au@Ag NR on glass substrate. Exposure time is 500 ms. (bottom panel) Polar plots of experimental profile (solid line) and theoretical fitting (dot line). (c) Image produced by the calculation with Θ = 80° and Φ = 60°.

light distribution patterns of these quantum dots were wellfitted with the multidipole equation:

Figure 2 shows the defocused images for two different NRs, along with corresponding integrated polar intensity plots. In the integrated intensity analyses, we set the circular acquisition region which includes the bright optical image. Then, we integrated the intensity in segments of circumference angle, Δφ (12° in the present system), and calculated the contour intensity, 0 (φ). Finally, we plotted 0 (φ) and fit it using eqs 1 and 2.26 0(φ) =

P [1 + Α cos(φ − Φ) + Β cos(φ − Φ)] 4π

[Amulti , Bmulti ] = (3f − 1)/2[Asingle , Bsingle ]

where 1 − f is the fraction of total radiated power for the twodimensional dipole and f is the fraction for the other single dipole. The integrated intensity plots are fitted by eq 1 which the Amulti and Bmulti in eq 3 were assigned to the coefficients A and B, respectively. Here, the actual fitting parameters are P, Φ, Θ, and f. If the value of f falls within anywhere from 0 to 1, the defocused image should be associated with the multidipoles. According to this, one can obtain the multidipoles with oscillations polarized perpendicular to each other with coordinates (Θ + π/2, Φ), (π/2, Φ + π/2), and (Θ, Φ). For both single and multidipole systems, we obtained the quantitative error sum of squares, χ2. In Figure 3, the relationship between the χsingle2 and χmulti2 values from calculations based on single and multidipole

(1)

where P is the total radiated power. For a single dipole, the coefficients A and B can be written as follows: ⎡ ⎤ 1 [Asingle , Bsingle] = ⎢ −sin Θ cos Θ, − sin 2 Θ⎥ ⎣ ⎦ 2

(3)

(2)

This analysis yields the individual dipole orientation defined by its spherical coordinate parameter, out-of-plane angle Θ and in-plane angle Φ. In this case, the in-plane reference x- and yaxes are set at the transverse and vertical axes of Figure 2, respectively. The z-axis corresponds to the optical axis. As shown in Figure 2a, the defocused image of Au NR generally consists of a bright pair of inner lobes and an ambiguous outer ring. Compared with Au NR, both features were brighter for Au@Ag NR (Figure 2b). The brightness of the defocused image indicates that the spectrum overlap between the excitation wavelength and the transverse SP resonance of Au@Ag NR is larger than that of the Au NR. From the polar plots of these NRs, the main SP axis of the NRs was estimated as Θ = 85° and Φ = 54° for Au NR and Θ = 78° and Φ = 56° for Au@Ag NR. In Figure 2c, we show the simulated defocused image with angles of Θ = 80° and Φ = 60°. The simulated and experimental images show good agreement. From these images, we conclude that each specific angle is correlated with a unique defocused image. As a result, the Au NR orientations on a substrate can be easily determined by referring to their corresponding fitting results. We have also found that some NRs results in images that could not be fitted by eqs 1 and 2. To investigate the origin of this light emission behavior, further analysis considering multidipoles were carried out for 36 Au and 53 Au@Ag NRs. Some kinds of fluorescent quantum dots have been found to have multidipoles that are composed of the two-dimensional dipole in a plane perpendicular to the c-axis of their hexagonal structure and the single dipole located along the c-axis.26 The

Figure 3. χ2 value plot of Au NR (red) and Au@Ag NR (blue) Dashed line indicates the region of χsingle2 = χmulti2.

analyses, respectively, for each individual NR is plotted. Two groupings were found in the χsingle2−χmulti2 plot. One is the group with χsingle2 = χmulti2, which are indicated by the black dashed line in Figure 3. These NRs can be well-fit by the single dipole treatment, indicating that for these NRs, the single dipole dominated the light emission properties. According to 2537



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extinction spectra for different individual NRs displayed large dispersions in the SP resonance wavelengths as much as 100 nm.20 Thus, it is possible in the present system that the Au@Ag NR longitudinal SP shows large peak dispersion from the ensemble peak at 646 nm as shown in Figure 1b. Hence, the longitudinal SP of some Au@Ag NRs can be indirectly excited due to the energy relaxation of the transverse SP. For these NRs, the two transverse and one longitudinal dipole may contribute to the light emission properties at the same time, which would entirely account for the defocused images. From these results, it becomes clear that these various dipoles should be taken into account for interpretation of single NR defocused imaging experiments. While the out-of-plane polarization component could not be estimated in previous experiments, TIRM and the defocused imaging technique enabled the accurate estimation of the cross-sectional angle and the shape of Au NRs. Therefore, applying the defocused imaging technique to anisotropically shaped materials can provide reliable optical and conformational information. Finally, we observed unique defocused images on gold nanostructures. The gold nanostructure (thickness 30 nm) was made on a glass surface by conventional electron beam lithography. Au@Ag NRs were dispersed on the substrate, and defocused images were acquired (Figures 5a and b). The defocused images showed bright concentric rings, quite different from the NR defocused images on the glass substrate, such as in Figures 2 and 4. The gold structures are also seen as faint blurs in the defocused images in Figure 5a and b. Atomic force microscopy (AFM) images of the gold nanostructures are shown in Figure 5c and d. The position of the marker (+) in each AFM image, c and d, corresponds to the center of the NR defocused images shown in Figure 5a and b, respectively. In the case of the NR that was positioned near the structures (Figure 5a and c), the maximum ring number was found to be four. According to simulation results that considered phase shifting for emission of light,16,28 the maximum ring number reaches four at the same defocus distance as the present experimental conditions. Thus, this observed fringe should be associated with phase shifting for emission of light from the NR itself. On the other hand, when the NR was positioned on the nanostructure (Figure 5b and d), the maximum ring number increased to greater than six (Figure 5e, red line) irrespective of the same defocus distance in the case of Figure 5a. In this case, the higher fringe numbers cannot be ascribed to only the emission of light from the NR which positioned on the thin nanostructures (thickness; ∼30 nm). In order to explain these higher number fringes, it is necessary to consider that the NR emission of light is scattered back by the underlying nanostructure, resulting in coherent superposition between the emission and backscattering light.31 A similar interference effect that altered the defocused images was recently reported only for a pair of second harmonic fluorescence emission particles.32 In that study, the appearance of interference patterns was dependent on the distance between the particles. In our case, the high number fringes observed in the defocused images would be sensitive to the relative position to the underlying nanostructure. As such, in the combination with substrate patterning, the defocused imaging will provide a unique opportunity to investigate not only the particles optical properties and orientation but also the relative position through the direct interference pattern for emission of light coupling between the particles and nanostructures.

the previous quantitative luminescence observation of single NRs, this single dipole should be associated from the indirectly excited longitudinal SP.27,28 On the other hand, nearly all of the points that deviated from this line showed χmulti2 < χsingle2, which strongly suggests that more than two dipoles contributed to the light emission properties of these NRs. To confirm the difference between these two groups, we have compared the typical defocused images from both groups (Figure 4).

Figure 4. Two kinds of defocused images and intensity profiles of Au@Ag NRs. The fitting curves were written with dotted and solid lines for single and multidipole fitting, respectively.

Significant differences were observed in the inner lobe shapes. The single dipole image displayed two lobes, whereas a bright ring was seen instead of lobes for the multidipole image. This light distribution pattern shows similarities with those of fluorescent quantum dots that have multidipoles.29 The differences in the light distribution patterns can also be seen in the integrated intensity plots. The profiles were able to be fit only when assuming a multidipole in Figure 4b (also see Figure S2c and d in the Supporting Information), whereas both single and multidipole analyses could fit the profile in Figure 4a (also see Figure S2a and b in the Supporting Information). These analysis results lead us to the conclusion that the defocused image shown in Figure 4b definitely originated from the multidipoles of NRs. The appearance of the multidipole defocused image can be explained by taking into account the present optical configuration. The transverse SP of the cylindrically shaped metallic NRs can be approximated as two dipoles oscillations which are perpendicular to each other. The observation of two transverse SPs at the same time has been previously reported in polarization-sensitive photothermal imaging experiments.30 The determination of the in-plane transverse SP angle of Au NRs was achieved using low numerical aperture objective lenses (N.A. = 0.7) to exclude the contributions of the out-of-plane polarization components. In the present TIRM setup, both of these oscillations can be excited and the emissions of light from each dipole were collected with a high numerical aperture objective lens (N.A. = 1.45). Thus, the observed multidipole generation partially arose from the combination of these transverse SPs. The other dipole should be caused by the longitudinal SP. Previous studies have shown that the 2538



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Present Address §

Department of Chemistry, Faculty of Material and Biological Science, Yamagata University, Yamagata 990−8560, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Grant-in-Aid for scientific research 18750001, 16205026, and 17034002, from the Ministry of Education, Science and Culture, Japan. Especially, that on Priority Area “Strong Photon-Molecule Coupling Fields (No. 470)” is acknowledged. T.M. also thanks the Japan Society for the Promotion of Science (JSPS) Research Fellowships for Young Scientists.

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Figure 5. (a and b) Defocused images of Au@Ag NRs deposited on the substrate with metallic structures. The exposure time is 300 ms and averaged for 20 frames. The NR is positioned (a) near and (b) on the structure. Two defocused images were taken in same defocused distance. (c and d) AFM images of the metallic structures. The position of markers (+) in each AFM image (c and d) are corresponding to the center of the defocused images which are shown in a and b, respectively. (e) Emission intensity profiles of images shown in a (black) and b (red).

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CONCLUSION Defocused imaging and dipole analyses yielded optical properties and orientation information of single metallic NRs. With careful observation, the defocused images associated with multidipole generation were characterized. In addition, the metallic NRs show unique interference patterns in combination with gold nanostructures. In the future, one can use this technique as a tool to investigate plasmon-mediated interactions in highly localized regions. Thus, the present study should allow informative optical properties of metallic NRs to be obtained for biomedical applications, for example, by detecting Au NRs as nonbleaching sensors.

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ASSOCIATED CONTENT

S Supporting Information *

Details of experimental setup and defocused images for other NRs. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 81-11-706-4684. 2539



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Observation of Defocus Images of a Single Metal Nanorod - The

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