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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 17 — Aug. 26, 2013
  • pp: 20260–20267
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Dynamic light scattering microscope: Accessing opaque samples with high spatial resolution

Takashi Hiroi and Mitsuhiro Shibayama  »View Author Affiliations


Optics Express, Vol. 21, Issue 17, pp. 20260-20267 (2013)
http://dx.doi.org/10.1364/OE.21.020260


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Abstract

We developed a new technique that conducts dynamic light scattering (DLS) under a microscope with high spatial resolution. This technique dramatically extends the range of DLS application from transparent to opaque samples. The total scattered electric field contains both electric field generated from the samples and time-independent reflected electric field. These two components are decomposed by applying a partial heterodyne method. By using this technique, we successfully calculate the characteristic size distribution of both multiple-scattering samples and strong light-absorbing samples. This is the first study to observe the collective motion of particles in a highly concentrated solution by using DLS.

© 2013 OSA

1. Introduction

Dynamic light scattering (DLS) is a technique for obtaining the size distribution of particles in solution. This technique is especially powerful for the characterization of polymer solutions, colloids, emulsions, etc. However, it cannot be applied to opaque samples. There are two types of opaque samples [1

1. H. C. van de Hulst, Light Scattering by Small Particles (Dover Publications, Inc., 1981).

, 2

2. C. F. Bohren, Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (Dover Publications, Inc., 2001).

]. One is a black sample, which is a strong light-absorbing material such as ink. In this case, scattered light is completely absorbed by the sample itself and one cannot obtain the signal. The other type is milky samples, which are strong light-scattering materials such as dense suspension of polystyrene latex. In this case, multiple scattering is inevitable although we want to obtain is single scattering. In practice, opaque samples need to be diluted for DLS measurement. However, there is no evidence that highly diluted samples have the same size distribution as that of a neat solution. This disadvantage has been a long-standing issue for the application of the DLS technique to opaque samples such as paint and food. In addition, poor spatial resolution sometimes also becomes a problem. In typical DLS, the irradiated volume is of the order 100 μm on a side. However, there is a growing demand to investigate the dynamic behavior of samples with a higher spatial resolution. One such example is the tracking of material transportation in biological cells. Another example is the investigation of spatial inhomogeneity of soft materials such as gels [3

3. M. Shibayama, “Spatial inhomogeneity and dynamic fluctuations of polymer gels,” Macromol. Chem. Phys. 199(1), 1–30 (1998). [CrossRef]

]. To realize such applications, the spatial resolution needs to be near the diffraction limit (~1 μm).

Many studies have been conducted on how to overcome these disadvantages. A cross correlation spectroscopy is a technique to extract singly scattered light by using two-beam, two-detector light scattering spectrometer [4

4. G. D. J. Phillies, “Suppression of multiple scattering effects in quasielastic light scattering by homodyne crosscorrelation techniques,” J. Chem. Phys. 74(1), 260–262 (1981). [CrossRef]

, 5

5. G. D. J. Phillies, “Experimental demonstration of ruultiple-scattering suppression in quasielastic-light-scattering spectroscopy by homodyne coincidence techniques,” Phys. Rev. A 24(4), 1939–1943 (1981). [CrossRef]

]. In this experiment, a cross correlation was used to remove the effect from the multiply scattered light. Another example is a low-coherence DLS [6

6. K. Ishii, R. Yoshida, and T. Iwai, “Single-scattering spectroscopy for extremely dense colloidal suspensions by use of a low-coherence interferometer,” Opt. Lett. 30(5), 555–557 (2005). [CrossRef] [PubMed]

]. The unique thing about this technique is the use of a superluminescent diode as the light source. Its short coherence length was efficiently utilized to remove multiple scattering. The different approach for the measurement of dense media is the evaluation of the intensity correlation function in consideration of multiple scattering explicitly, called diffusing wave spectroscopy [7

7. G. Maret and P. E. Wolf, “Multiple light scattering from disordered media. The effect of brownian motion of scatterers,” Z. Phys. B 65(4), 409–413 (1987). [CrossRef]

, 8

8. D. J. Pine, D. A. Weitz, P. M. Chaikin, and E. Herbolzheimer, “Diffusing wave spectroscopy,” Phys. Rev. Lett. 60(12), 1134–1137 (1988). [CrossRef] [PubMed]

]. From a viewpoint of the application of DLS to biological cells, novel technique was proposed by Dzakpasu and Axelrod [9

9. R. Dzakpasu and D. Axelrod, “Dynamic light scattering microscopy,” in Nanotechnologies for the Life Sciences, C. Kumar, ed. (Wiley-VCH, 2006).

]. They successfully created spatial maps of fluctuation decay rates with a high spatial resolution (μm order) by a combination of confocal microscopy and a streak camera. However, no studies have been reported on the application of DLS to opaque media with high spatial resolution.

In this paper, we propose a new DLS technique, a DLS microscope that mitigates both the above-mentioned disadvantages. Although there are many reports on how to perform DLS under a microscope [10

10. Y. Takagi and K. Kurihara, “Application of a microscope to Brillouin scattering spectroscope,” Rev. Sci. Instrum. 63(12), 5552–5555 (1992). [CrossRef]

12

12. D. C. Liptak, J. C. Reber, J. F. Maguire, and M. S. Amer, “On the development of a confocal Rayleigh-Brillouin microscope,” Rev. Sci. Instrum. 78(1), 016106 (2007). [CrossRef] [PubMed]

], all of them utilize forward scattering. This restricts the scope of its application to transparent materials. The proposed DLS microscope obtains a signal with a backscattering geometry similar to that of a Rayleigh light scattering microscope [13

13. C. Casiraghi, A. Hartschuh, E. Lidorikis, H. Qian, H. Harutyunyan, T. Gokus, K. S. Novoselov, and A. C. Ferrari, “Rayleigh imaging of graphene and graphene layers,” Nano Lett. 7(9), 2711–2717 (2007). [CrossRef] [PubMed]

, 14

14. G. Louit, T. Asahi, G. Tanaka, T. Uwada, and H. Masuhara, “Spectral and 3-dimensional tracking of single gold nanoparticles in living cells studied by Rayleigh light scattering microscopy,” J. Phys. Chem. C 113(27), 11766–11772 (2009). [CrossRef]

]. This enables us to measure opaque samples with high spatial resolution. However, this technique leads to contamination of reflected light from the interface of the sample. We regard this reflected light as a local oscillator and actively utilize it for data analysis.

2. Principle and experiment

The schematic of the DLS microscope is shown in Fig. 1
Fig. 1 Schematic of the proposed DLS microscope: VND, variable neutral density filter; PH1, pinhole, ϕ = 25 μm; HM, half mirror; BD, beam diffuser; PH2, pinhole, ϕ = 50 μm; APD, avalanche photodiode.
. An Ar-Kr ion laser (λ = 514.5 nm, Stabilite 2018, Spectra-Physics) is used as the light source. The beam is reflected by a half mirror and focused onto the sample by an objective (Olympus: UPlan FL 40x / NA 0.75). In order to minimize the effect of the laser on the sample, such as that of absorption, the laser power at the sample is kept at less than 1 mW. The sample is shielded in a hole-slide glass, covered by a plate of cover glass. Note that typical DLS needs approximately 1 mL of sample. However, because of the small irradiated volume of our DLS compared with that of typical DLS, we need only 50 μL of sample. Scattered light from the sample is collected with a backscattering geometry and guided to an avalanche photodiode, and then to the autocorrelator (ALV, Langen, Germany). To improve the performance, we adopt a confocal optical system. Concretely speaking, there are following two reasons. Firstly, the multiple scattered lights can be eliminated because these lights have changed their traveling direction and cannot pass the pinhole (PH2 shown in Fig. 1). Secondly, the reflection from the boundary of the sample and cover glass can be decreased when the focal point is sufficiently far from the boundary. Although signal is strongly affected by the reflection when the focal point is near the boundary, this can be precisely evaluated by using a partial heterodyne analysis, shown in the latter of this section. The irradiated volume achieved by this confocal optical system is approximately 1 μm in a lateral direction and 10 μm in a vertical direction with the loose setting of PH2 in mind. High spatial resolution originated from this small irradiated volume is also experimentally shown in the latter of this section.

What we obtain thorough the DLS experiment is a time-averaged normalized intensity correlation function, g(2)(τ)1. This function shows an exponential decay whose initial amplitude is 1. Practically, the initial amplitude is a little smaller than 1, mainly due to the finite size of the irradiated volume, which is called the smearing effect. This reduction is represented by a coherence factor. However, the smearing effect is considered to be a minor factor in case of the DLS microscope because the irradiated volume is very small. In addition to the smearing effect, the initial amplitude can be reduced when some time-independent electric field is mixed to the signal from the sample. In the case of the DLS microscope, this time-independent electric field is the reflection from the boundary. Quantitative relationship between the initial amplitude and the proportion of time-independent component was formulated as follows:
11A=IsTItotT,
(1)
where A is the initial amplitude of the time-correlation function of the scattered intensity, , IsT is the intensity from the sample, and ItotT is the total intensity. This analysis is based on a partial heterodyne method, which is originally developed for the analysis of the nonergodic media such as gels [15

15. P. N. Pusey and W. van Megen, “Dynamic light scattering by non-ergodic media,” Physica A 157(2), 705–741 (1989). [CrossRef]

17

17. M. Shibayama, T. Norisuye, and S. Nomura, “Cross-link density dependence of spatial inhomogeneities and dynamic fluctuations of poly(N-isopropylacrylamide) gels,” Macromolecules 29(27), 8746–8750 (1996). [CrossRef]

]. Since we obtain A and ItotT from one experiment, we can calculate IsT from each experiment. In response to the variation of the initial amplitude, the apparent diffusion constant, DA, is also changed as follows:
DA=11AAD,
(2)
where D is the genuine diffusion coefficient.

The sample is polystyrene latex suspensions (3000 Series Nanospheres, Duke Scientific Cooperation) and Chinese ink. The nominal diameter of the polystyrene latex particles used is 50 nm and 100 nm. These samples were used as purchased or after the appropriately dilution.

3. Results and discussion

3.1 Polystyrene beads: multiple scattering media

To see the performance of the DLS microscope, we measure the polystyrene latex suspension whose diameter of the particles is 50 nm. To compare the DLS microscope to a typical DLS system, we measure the sample without dilution by the DLS microscope and 103 times-diluted sample by the typical DLS system. Figure 2
Fig. 2 Intensity correlation functions for a polystyrene latex suspension, 1 wt%. The nominal diameter of the polystyrene latex particles is 50 nm. Solid lines: Several data sets obtained from the DLS microscope (λ = 514.5 nm, θ = 180°) at different points within the suspension. Dashed line: Data obtained from a typical DLS system (DLS/SLS 5000 compact goniometer, ALV, λ = 632.8 nm, θ = 90°). The inset is a plot of DA vs. (11A)/A. The solid line is the best fit result based on Eq. (2).
shows some intensity correlation functions obtained by the proposed DLS microscope and the typical DLS system. The decay times obtained from the DLS microscope and typical DLS are different in principle because they have different λ (wavelength of the incident light) and θ (scattering angle). We find the initial amplitude obtained from the DLS microscope to be significantly below 1. In this experiment, we obtain values of A in the range 0.19 - 0.86. In addition to the variation in A, the values of the diffusion constants obtained from the decay time also vary.

The inset of Fig. 2 indicates a plot of DA vs. (11A)/A. Each point lies on a straight line from the origin, as expected from Eq. (2). This result strongly supports the validity of the assumption that the total intensity is decomposed into the components from the samples and that from the boundary. This linearity also verifies that we can evaluate the genuine diffusion constant, D, from a single measurement. From the slope of the straight line in the inset of Fig. 2, D is obtained as 8.22 x 10−12 m2 s−1. From this value, the hydrodynamic diameter, dh, is obtained using the Stokes-Einstein equation. In this experiment, the calculated diameter is 58 nm, which is in good agreement with the value determined by the typical DLS system (57 nm).

By using the partial heterodyne method, we measure a 1 wt% polystyrene latex suspension having a nominal diameter of 100 nm. This suspension is so dense that it has a milky liquid appearance because of multiple scattering (Fig. 3(a)
Fig. 3 (a) The appearance of the sample. The cover glass is fixed with manicure. (b) The definition of z. (c) Position dependence of ItotT (thin solid line), IR (dashed line), and IsT (thick solid line) for polystyrene latex suspension, 1 wt%. The nominal diameter of the polystyrene latex particles is 100 nm. The intensity is expressed in s−1, the photon counts in a second. (d) Position dependence of the diameter of the polystyrene latex suspension. The diameter calculated from the typical DLS is 116 nm, which is indicated in the figure. Solid lines: The diameter corrected using Eq. (2). Dashed line: The apparent diameter calculated by using apparent diffusion constant, DA.
). Taking advantage of the high spatial resolution, we measure the intensity correlation functions by scanning the focus along the vertical (z) axis to show the effect of reflection explicitly. The definition of z is shown in Fig. 3(b). Figure 3(c) shows the position dependence of the scattered intensity of the polystyrene latex suspension. Near the boundary of the sample and glass (0 - 40 μm, 170 - 180 μm), the scattered intensity is strongly enhanced. The intensity decrease in the region 40 < z < 170 μm is mainly due to multiple scattering; the greater the amount of medium the light passes through, the more the light is scattered and the intensity attenuated. We experimentally determined the coherence factor as 0.97, which is the maximum value of the initial amplitude in the data set for this experiment. This large value corresponds to the small irradiated volume of this DLS microscope. The observed intensity, ItotT, is decomposed into its two components; IsT, the time-averaged intensity from the sample and IR, the intensity of reflection from the boundary of the sample and glass. Note that we do not have to take the time-average for IR because this quantity is time-independent. We calculate the proportion of these components from the initial amplitude of each intensity correlation function by using Eq. (1). The result is also shown in Fig. 3(c). This figure shows that these two components are successfully decomposed into two components. It is experimentally demonstrated that the source of the irregular intensity enhancement near the boundary region is IR enhancement, while the decrease in intensity between the boundaries is due to IsT. In addition, the position dependence of the diameter is calculated at each point by using Eq. (2). The result is shown in Fig. 3(d). The apparent diameter (dashed line) shows that the hydrodynamic diameter increases near the edge. However, by using partial heterodyne analysis, the hydrodynamic diameter shows almost the same value regardless of the focus point. In other words, we do not observe the effect of the interface in this case, as previously reported [18

18. H. Xia, K. Ishi, and T. Iwai, “Hydrodynamic radius sizing of nanoparticles in dense polydisperse media by low-coherence dynamic light scattering,” Jpn. J. Appl. Phys. 44(8), 6261–6264 (2005). [CrossRef]

].

3.2 Ink: strong light-absorbing media

3.3 Concentration dependence of the size distribution

Although we cannot see the difference between the neat ink and the diluted ink from the viewpoint of the absorption coefficient, this does not mean that the microscopic morphology is invariant with dilution. To see the concentration dependence of the microscopic morphology, we measure the intensity correlation function with various concentrations. Highly concentrated solutions are measured with the DLS microscope and low-concentration solutions are measured with the typical DLS system. At intermediate concentrations, we measure the sample with both DLS instruments. From these data, the characteristic decay time distribution functions are obtained with an inverse Laplace transform program (a constrained regularization program, CONTIN provided by ALV). These functions are converted into the characteristic size distribution, G(dh), by using the Stokes-Einstein equation.

Figure 5(a)
Fig. 5 (a) Concentration dependence of the size distribution of a polystyrene latex suspension. The nominal diameter of the polystyrene latex particles is 100 nm. The 1 - 0.01 wt%, as measured by the DLS microscope, is represented by the solid lines. The 0.01 - 0.0001 wt%, as measured by the typical DLS system, is represented by the dashed lines. (b) Concentration dependence of the size distribution of Chinese ink. The 10 - 0.05 wt%, as measured by the DLS microscope, is represented by the solid lines. The 0.05 - 0.001 wt%, as measured by the typical DLS system, is represented by the dashed lines.
shows the results for the polystyrene latex suspension, and Fig. 5(b) shows that for Chinese ink. Since Fig. 5(a) shows peaks at almost the same position in all concentrations, we can conclude that the polystyrene latex suspension shows no morphology change through varying the concentration. This is consistent with the following molecular interpretation: each particle has a negative charge on the surface, and the particles repel each other keeping their form unchanged and showing Brownian motion. This result corresponds to the result obtained by using diffusing wave spectroscopy [19

19. P. Navabpour, C. Rega, C. J. Lloyd, D. Attwood, P. A. Lovell, P. Geraghty, and D. Clarke, “Influence of concentration on the particle size analysis of polymer latexes using diffusing-wave spectroscopy,” Colloid Polym. Sci. 283(9), 1025–1032 (2005). [CrossRef]

].

In contrast, initial observations seem to show that the average particle size increases and the size distribution broadens at higher concentrations of Chinese ink. However, as Chinese ink is a protective colloidal solution, the particles are considered not to cluster. One of the possible explanations for our result is the existence of attractive interactions between colloidal particles. In other words, colloidal particles show collective motion rather than Brownian motion at higher concentrations. This consideration is supported by the fact that the viscosity of neat ink is approximately five times higher than that of water. To the best of our knowledge, this is the first observation of such collective motion within a dense opaque media by using DLS.

4. Conclusion

We have developed a new DLS technique that allows us to obtain intensity correlation functions from opaque samples under a microscope. The intensity correlation functions show the initial amplitude as much less than 1 because of the mixing of the time-independent reflected electric field with the electric field from the sample. The data are analyzed by the partial heterodyne method, and the size of the polystyrene latex suspension particles is determined precisely without being affected by multiple scattering. In addition, the intensity for the sample is extracted from the total intensity by using the value of the initial amplitude. We also apply the DLS microscope to Chinese ink and observe the collective motion in highly concentrated samples. In addition, we find that this instrument can also measure the absorption coefficient of the ink without dilution. Taking advantage of its high spatial resolution, this technique can also be readily applied to other media such as biological cells and gels.

Acknowledgment

This study is supported by “Advanced Leading Graduate Course for Photon Science, Program for Leading Graduate Schools, Japan Society for the Promotion of Science granted to TH.

References and links

1.

H. C. van de Hulst, Light Scattering by Small Particles (Dover Publications, Inc., 1981).

2.

C. F. Bohren, Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (Dover Publications, Inc., 2001).

3.

M. Shibayama, “Spatial inhomogeneity and dynamic fluctuations of polymer gels,” Macromol. Chem. Phys. 199(1), 1–30 (1998). [CrossRef]

4.

G. D. J. Phillies, “Suppression of multiple scattering effects in quasielastic light scattering by homodyne crosscorrelation techniques,” J. Chem. Phys. 74(1), 260–262 (1981). [CrossRef]

5.

G. D. J. Phillies, “Experimental demonstration of ruultiple-scattering suppression in quasielastic-light-scattering spectroscopy by homodyne coincidence techniques,” Phys. Rev. A 24(4), 1939–1943 (1981). [CrossRef]

6.

K. Ishii, R. Yoshida, and T. Iwai, “Single-scattering spectroscopy for extremely dense colloidal suspensions by use of a low-coherence interferometer,” Opt. Lett. 30(5), 555–557 (2005). [CrossRef] [PubMed]

7.

G. Maret and P. E. Wolf, “Multiple light scattering from disordered media. The effect of brownian motion of scatterers,” Z. Phys. B 65(4), 409–413 (1987). [CrossRef]

8.

D. J. Pine, D. A. Weitz, P. M. Chaikin, and E. Herbolzheimer, “Diffusing wave spectroscopy,” Phys. Rev. Lett. 60(12), 1134–1137 (1988). [CrossRef] [PubMed]

9.

R. Dzakpasu and D. Axelrod, “Dynamic light scattering microscopy,” in Nanotechnologies for the Life Sciences, C. Kumar, ed. (Wiley-VCH, 2006).

10.

Y. Takagi and K. Kurihara, “Application of a microscope to Brillouin scattering spectroscope,” Rev. Sci. Instrum. 63(12), 5552–5555 (1992). [CrossRef]

11.

P. D. Kaplan, V. Trappe, and D. A. Weitz, “Light-scattering microscope,” Appl. Opt. 38(19), 4151–4157 (1999). [CrossRef] [PubMed]

12.

D. C. Liptak, J. C. Reber, J. F. Maguire, and M. S. Amer, “On the development of a confocal Rayleigh-Brillouin microscope,” Rev. Sci. Instrum. 78(1), 016106 (2007). [CrossRef] [PubMed]

13.

C. Casiraghi, A. Hartschuh, E. Lidorikis, H. Qian, H. Harutyunyan, T. Gokus, K. S. Novoselov, and A. C. Ferrari, “Rayleigh imaging of graphene and graphene layers,” Nano Lett. 7(9), 2711–2717 (2007). [CrossRef] [PubMed]

14.

G. Louit, T. Asahi, G. Tanaka, T. Uwada, and H. Masuhara, “Spectral and 3-dimensional tracking of single gold nanoparticles in living cells studied by Rayleigh light scattering microscopy,” J. Phys. Chem. C 113(27), 11766–11772 (2009). [CrossRef]

15.

P. N. Pusey and W. van Megen, “Dynamic light scattering by non-ergodic media,” Physica A 157(2), 705–741 (1989). [CrossRef]

16.

J. G. H. Joosten, J. L. McCarthy, and P. N. Pusey, “Dynamic and static light scattering by aqueous polyacrylamide gels,” Macromolecules 24(25), 6690–6699 (1991). [CrossRef]

17.

M. Shibayama, T. Norisuye, and S. Nomura, “Cross-link density dependence of spatial inhomogeneities and dynamic fluctuations of poly(N-isopropylacrylamide) gels,” Macromolecules 29(27), 8746–8750 (1996). [CrossRef]

18.

H. Xia, K. Ishi, and T. Iwai, “Hydrodynamic radius sizing of nanoparticles in dense polydisperse media by low-coherence dynamic light scattering,” Jpn. J. Appl. Phys. 44(8), 6261–6264 (2005). [CrossRef]

19.

P. Navabpour, C. Rega, C. J. Lloyd, D. Attwood, P. A. Lovell, P. Geraghty, and D. Clarke, “Influence of concentration on the particle size analysis of polymer latexes using diffusing-wave spectroscopy,” Colloid Polym. Sci. 283(9), 1025–1032 (2005). [CrossRef]

OCIS Codes
(180.1790) Microscopy : Confocal microscopy
(290.1350) Scattering : Backscattering
(290.4210) Scattering : Multiple scattering
(300.6310) Spectroscopy : Spectroscopy, heterodyne

ToC Category:
Microscopy

History
Original Manuscript: April 22, 2013
Revised Manuscript: July 21, 2013
Manuscript Accepted: August 7, 2013
Published: August 22, 2013

Virtual Issues
Vol. 8, Iss. 9 Virtual Journal for Biomedical Optics

Citation
Takashi Hiroi and Mitsuhiro Shibayama, "Dynamic light scattering microscope: Accessing opaque samples with high spatial resolution," Opt. Express 21, 20260-20267 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-17-20260


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References

  1. H. C. van de Hulst, Light Scattering by Small Particles (Dover Publications, Inc., 1981).
  2. C. F. Bohren, Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics (Dover Publications, Inc., 2001).
  3. M. Shibayama, “Spatial inhomogeneity and dynamic fluctuations of polymer gels,” Macromol. Chem. Phys.199(1), 1–30 (1998). [CrossRef]
  4. G. D. J. Phillies, “Suppression of multiple scattering effects in quasielastic light scattering by homodyne crosscorrelation techniques,” J. Chem. Phys.74(1), 260–262 (1981). [CrossRef]
  5. G. D. J. Phillies, “Experimental demonstration of ruultiple-scattering suppression in quasielastic-light-scattering spectroscopy by homodyne coincidence techniques,” Phys. Rev. A24(4), 1939–1943 (1981). [CrossRef]
  6. K. Ishii, R. Yoshida, and T. Iwai, “Single-scattering spectroscopy for extremely dense colloidal suspensions by use of a low-coherence interferometer,” Opt. Lett.30(5), 555–557 (2005). [CrossRef] [PubMed]
  7. G. Maret and P. E. Wolf, “Multiple light scattering from disordered media. The effect of brownian motion of scatterers,” Z. Phys. B65(4), 409–413 (1987). [CrossRef]
  8. D. J. Pine, D. A. Weitz, P. M. Chaikin, and E. Herbolzheimer, “Diffusing wave spectroscopy,” Phys. Rev. Lett.60(12), 1134–1137 (1988). [CrossRef] [PubMed]
  9. R. Dzakpasu and D. Axelrod, “Dynamic light scattering microscopy,” in Nanotechnologies for the Life Sciences, C. Kumar, ed. (Wiley-VCH, 2006).
  10. Y. Takagi and K. Kurihara, “Application of a microscope to Brillouin scattering spectroscope,” Rev. Sci. Instrum.63(12), 5552–5555 (1992). [CrossRef]
  11. P. D. Kaplan, V. Trappe, and D. A. Weitz, “Light-scattering microscope,” Appl. Opt.38(19), 4151–4157 (1999). [CrossRef] [PubMed]
  12. D. C. Liptak, J. C. Reber, J. F. Maguire, and M. S. Amer, “On the development of a confocal Rayleigh-Brillouin microscope,” Rev. Sci. Instrum.78(1), 016106 (2007). [CrossRef] [PubMed]
  13. C. Casiraghi, A. Hartschuh, E. Lidorikis, H. Qian, H. Harutyunyan, T. Gokus, K. S. Novoselov, and A. C. Ferrari, “Rayleigh imaging of graphene and graphene layers,” Nano Lett.7(9), 2711–2717 (2007). [CrossRef] [PubMed]
  14. G. Louit, T. Asahi, G. Tanaka, T. Uwada, and H. Masuhara, “Spectral and 3-dimensional tracking of single gold nanoparticles in living cells studied by Rayleigh light scattering microscopy,” J. Phys. Chem. C113(27), 11766–11772 (2009). [CrossRef]
  15. P. N. Pusey and W. van Megen, “Dynamic light scattering by non-ergodic media,” Physica A157(2), 705–741 (1989). [CrossRef]
  16. J. G. H. Joosten, J. L. McCarthy, and P. N. Pusey, “Dynamic and static light scattering by aqueous polyacrylamide gels,” Macromolecules24(25), 6690–6699 (1991). [CrossRef]
  17. M. Shibayama, T. Norisuye, and S. Nomura, “Cross-link density dependence of spatial inhomogeneities and dynamic fluctuations of poly(N-isopropylacrylamide) gels,” Macromolecules29(27), 8746–8750 (1996). [CrossRef]
  18. H. Xia, K. Ishi, and T. Iwai, “Hydrodynamic radius sizing of nanoparticles in dense polydisperse media by low-coherence dynamic light scattering,” Jpn. J. Appl. Phys.44(8), 6261–6264 (2005). [CrossRef]
  19. P. Navabpour, C. Rega, C. J. Lloyd, D. Attwood, P. A. Lovell, P. Geraghty, and D. Clarke, “Influence of concentration on the particle size analysis of polymer latexes using diffusing-wave spectroscopy,” Colloid Polym. Sci.283(9), 1025–1032 (2005). [CrossRef]

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