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Spherical and Flower-Shaped Gold Nanoparticles Characterization by Scattering Correlation Spectroscopy Nadia Djaker,*,† Sadequa Sultana,† Dahia Issaad,‡ Sanda Boca,§ Hanane Moustaoui,† Jolanda Spadavecchia,† Aicha Medjahed,‡ Mohamed Bouafia,‡ Simion Astilean,§ and Marc Lamy de la Chapelle† †

Laboratoire CSPBAT, CNRS (UMR 7244), Université Paris 13, Sorbonne Paris Cité, UFR SMBH, 74 rue Marcel Cachin, F-93017 Bobigny, France ‡ Laboratoire d’optique appliquée, Université Ferhat Abbas Sétif 1, El- Bez, DZ-19000 SETIF, Algeria § Institute for Interdisciplinary Research in Bio-Nanosciences and Faculty of Physics, Nanobiophotonics and Laser Microspectroscopy Center, Babes-Bolyai University, 1 Str. M Kogalniceanu, RO-400084 Cluj-Napoca, Romania S Supporting Information *

ABSTRACT: The aim of this study is to compare the optical scattering properties of different gold nanoparticles (GNPs), with different shapes (spherical, GNSs, and flower-shaped, GNFs), sizes (20, 30, and 50 nm), and surface chemistries (with and without PEG). These scattering properties give geometrical characterization of hydrodynamic sizes of GNPs by using the scattering correlation spectroscopy. Afterward, a multiparametric comparative study of the scattering efficiency is presented depending on various parameters such as GNPs geometry, excitation wavelength (532 and 633 nm) and powers (from 5 to 100 μW). As predicted by Mie theory, we demonstrate that the increase in GNSs size leads to an increase of the scattered intensity, proportional to the excitation power. The scattered signal is the highest when the excitation wavelength is closer to the localized surface plasmon resonance. In the case of GNFs, the measured scattered signal is around 1000 times stronger than that for GNSs of the same size and concentration. For GNFs, a scattering coefficient at the plasmon resonance of around 2 × 10−13 m2 was calculated, which is comparable to the scattering coefficient of a GNS with a diameter of 300 nm. Due to their strong scattering properties, GNFs appear as a good alternative to GNSs of the same size for cell imaging.

1. INTRODUCTION

The characterization and detection of GNPs are very important steps before their bio-application. One of the recent powerful techniques used to characterize GNPs is the scattering correlation spectroscopy (SCS).25−27 As fluorescence correlation spectroscopy (FCS), the SCS technique is based on the analysis of intensity fluctuations within a well-defined confocal volume (∼1 fL). The correlation curve is directly related to the hydrodynamic radius of molecules or nanoparticles,26−33 to their diffusion coefficient, concentration and shape.27,34−36 The SCS is very sensitive to GNPs morphology and brightness since the scattering intensity depends on the GNPs volume.37−40 The analysis of the size effect on the scattered intensity in case of GNSs using SCS technique was reported by Sabanayagam et al.5 They found that for GNSs sized from 38 to 100 nm, the measured scattering intensities are proportional to the scattering cross sections predicted by Mie theory. The

Thanks to their strongly enhanced surface plasmon resonance, gold nanoparticles (GNPs) at optical frequencies are excellent scatterers and absorbers of the visible light1 and, as a consequence, make them visible at low concentrations using optical microscopy.2−5 In addition to their optical properties, GNPs are considered as biocompatible6−10 and can be conjugated to a variety of biological molecules for different medical applications.11−15 Thus, due to all these properties, gold nanospheres (GNSs) have been already applied to cellular imaging and medical applications.16−18 While, the synthesis of branched and flowershaped gold nanoparticles (GNFs) is a fairly recent development,19,20 they have additional valuable properties for bioapplications.21,22 In fact, their localized surface plasmon resonance (LSPR) can be tuned from the visible to the nearinfrared (NIR) region and their branched shape dramatically enhances their optical properties.23,24 However, a recent study demonstrated that they are of higher toxicity than the GNSs.10 © 2016 American Chemical Society

Received: March 8, 2016 Revised: May 9, 2016 Published: May 11, 2016 11700

DOI: 10.1021/acs.jpcc.6b02436 J. Phys. Chem. C 2016, 120, 11700−11708

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The Journal of Physical Chemistry C

450 to 850 nm. The wavelength of the extinction maxima was used to calculate the stock concentration of the colloids by using the extinction coefficients corresponding to each nanoparticle type. While the molar extinction coefficient for GNSs is known,37 for GNFs, we extrapolated the theoretical molar extinction coefficient from their extinction spectra and as previously reported in the case of other rough nanoparticles46 (see Figure S1 in the Supporting Information). 2.2. Optical Setup for Scattering Correlation Spectroscopy. SCS was performed using a home-built inverted confocal microscope (Nikon, Japan). Briefly, light from a 532 nm (Optoelectronics company, UK) or 633 nm (Thorlabs, France) laser was collimated and expanded to overfill the back aperture of a microscope objective (Olympus, 40×, NA = 1.2). The beam was circularly polarized using a λ/4 waveplate (Thorlabs, France). The focal plane of the objective was set to approximately ∼10 μm inside the solution to avoid excessive scattered light from the glass−water interface. Different fluorescent solutions of Alexa Fluor 532 (AF532) and Alexa Fluor 647 (AF647) (see Table 1), and Orange/red fluorescent

sensitivity of SCS in the determination of GNPs size was demonstrated by Dominguez-Medina et al.35 The surface chemistry modification during the GNPs-proteins interactions was also monitored by SCS, which showed the dependence of the sensitivity of the scattered signal to a small variation of GNSs hydrodynamic size (∼4 nm) due to the variations on the surface chemistry.35 The SCS characterization of GNSs (diameter ∼20 and 100 nm) and gold nanorods size by the determination of their hydrodynamic radii was reported by Tcherniak et al.27 and Zhang et al.41 Both experimental results showed a good agreement with Mie theory. They highlight the fact that the excitation of GNPs at low power (below 100 μW) avoids the effect of optical trapping on the measurement of time diffusion, hence the determination of the hydrodynamic size. In this paper, we use SCS to characterize the hydrodynamic sizes of two different shapes of GNPs (GNSs and GNFs), with two different surface chemistries (with and without PEG) and different sizes (20, 30, and 50 nm). We explore the scattering properties of these GNPs at two wavelengths, close and far from their plasmon resonances. For that we integrate the scattered signal of GNPs during 300 s at different excitation powers. As predicted by Mie theory, we demonstrate that the increase in GNSs size leads to the increase of the scattered intensity with the excitation power. In the case of GNFs, we observe a large increase of the scattered signal due to their specific surface morphology. Such results make this type of nanoparticles a better candidate for both cell imaging and photothermal therapy.21,22

Table 1. Calibration Parameters of the Focal Volume for Both Excitation Wavelengths: 532 and 633 nm Iexc (nm) 532 633

sample

diffusion coefficient D (10−6 cm2/s)

time diffusion τD (μs)

lateral waist wxy (μm)

axial waist wz (μm)

Veff (fL)

AF532 AF647

3.96 3.3

102 170

0.40 0.47

1.3 1.5

1.1 1.8

polystyrene beads (Molecular Probes) (see figures S2 and S3, in the Supporting Information) were used for alignment and calibration of the focal volume. The confocal volume was determined in order to calculate the GNPs hydrodynamic radii from the measured diffusion times.28,47−49 GNPs diffusing across the focal volume scatter light due to their inherent surface plasmon resonance. For the GNPs size characterization, an excitation power of 10 μW was used to avoid the trapping effect (Figure S4, Supporting Information).41 The scattered light was collected in the backward direction and redirected to a 30 μm pinhole for 532 nm excitation and 50 μm pinhole for 633 nm excitation wavelength (Thorlabs). The signal was then recorded by two avalanche photodetectors ((PerkinElmer SPCM-AQR-14, Canada) and the signal cross-correlation was performed by a hardware correlator (ALV7004, ALV-Laser Vertriebsgesellschaft m.b.H. Germany). Each individual SCS measurement was obtained by averaging 5 runs of 60 s duration. For the system calibration using the fluorescence signal, emission filters were added in front of the detectors. To switch from fluorescence to scattering correlation spectroscopy, the dicroı̈que mirror was switched to a 50/50 beam splitter and the emission filter was removed. The signal of the pure solvent sample (water) was measured using similar excitation powers to make sure that the signal originates only from the GNPs. Each sample was measured at least five independent times to yield mean values and corresponding errors. All data were analyzed using the standard correlation function (eq 1) to model nanoparticles diffusing in a threedimensional Gaussian volume. Such analysis was performed with the Igor software (WaveMetrics, Portland, U.S.A.) as previously described.36 2.3. Analysis of Scattering Correlation Spectroscopy. Correlation spectroscopy yields the characteristic diffusion time τD of an analyte of interest as it diffuses through a diffraction

2. MATERIAL AND METHODS 2.1. GNPs Synthesis and Surface Modification. The synthesis of nanoparticles was already reported by the authors.10 Briefly, colloidal GNSs of 20 and 30 nm were synthesized by the aqueous reduction of HAuCl4 with trisodium citrate according to the Turkevich−Frens method.42,43 For the synthesis of 20 nm gold nanoparticles, an amount of 100 mL of 10−3 M HAuCl4:3H2O was boiled. A solution of 38.8 × 10−3 M sodium citrate (10 mL) was quickly added under vigorous stirring until a red burgundy colloidal solution was formed.44 30 nm gold nanoparticles were obtained by decreasing the ratio between trisodium citrate and the gold salt. GNSs of 50 nm were produced by stirring 10 mL of a solution of 0.5 × 10−3 M HAuCl4 at room temperature for several minutes, followed by the addition of a proper volume of a freshly prepared ascorbic acid (7.5 × 10−3 M) solution. GNFs were prepared by the rapid mixture of a 20 mL solution of 20 × 10−3 M ascorbic acid with 200 μL of 10−2 M HAuCl4 at ice temperature.45 One batch of each type of GNPs was modified using mPEG-SH polymer of 5 kDa molecular weight, that provided more stability to the particles. Depending on the nanoparticle type and hence on its surface area, various amounts of 10−3 M polymeric solution were added to the colloidal solution by dripping. The polymer−nanoparticle mixtures were subjected to vigorous stirring after which were left to sit for 24 h at 4 °C to afford the complete binding of the polymer. Both as-prepared and polymer stabilized GNPs (aGNPs and PEG-GNPs, respectively) were purified by centrifugation at high speed and resuspended in ultrapure water. To determine GNPs concentration in colloidal solution, optical extinction spectra were measured using a UV−vis spectrometer (Kontron Instr. France) on a spectral range from 11701


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Figure 1. Schematic view of the SCS experimental setup (a). Typical fluorescence cross-correlation of free (b) AF532 and (c) AF647 molecules. The fitting curve using eq 1 gives a typical diffusion time of τD = 102 μs for AF532 and τD = 170 μs for AF647 molecules in the observation volume.

where wxy is the radius of the observation volume considered generally as a three-dimensional Gaussian intensity profile. For our system the values of wxy and wz are given in Table 1 for each wavelength 532 and 633 nm (several techniques to measure wxy and wz are presented in the first part of the Supporting Information). η is the water viscosity (10−3 kg.m−1 s−1), k the Boltzmann constant and T is the temperature (all measurements were done at room temperature 25 °C). Figure 1 presents typical fluorescence cross-correlations G(τ) recorded with the AF532 (Figure 1b) and AF647 (Figure 1c) diluted in water (at 10 nM concentration) and excited with a 100 μW excitation power. The nanoparticle sizes used in our experiment were much smaller than the wavelength of illumination lasers (532 and 633 nm), which obeys to the Mie theory.37,56−58

limited focal volume. Due to the low concentrations used (typically < nM), it is assumed that only one analyte crossed the focal volume (∼1 fL) at the same time. Fluctuations in the scattering intensity I(τ) were observed when GNPs were optically excited while diffusing across this focal volume. Temporal cross-correlation analysis of the scattering signal was performed over a range of lag times τ. The analysis of this SCS data relies on a numerical fit based on a three-dimensional Brownian diffusion model:50−53 ⎡ 1⎢ 1 G (τ ) = 1 + ⎢ N ⎢1 + ⎣

1 τ τD

τ

1 + s2 τ

D

⎤ ⎥ ⎥ ⎥⎦

(1)

where N is the average number of particles crossing the focal volume and s is the ratio of transversal (wxy) to axial (wz) dimensions of the analysis volume (s = 0.3, see Figure S3 in the Supporting Information). Numerical fit of the SCS data following eq 1 provides the diffusion time τD, and this time is directly related to both translational diffusion coefficient (D) and hydrodynamic radius (Rh) of the GNPs by the Stokes− Einstein equation:28,31,54,55

τD =

D=

3. RESULTS AND DISCUSSION 3.2. GNPs Characterization by UV−vis Spectroscopy and Zetasizer. The primary shape and size for all the nanoparticles were determined by transmission electron microscopy (TEM; JEOL 100 U, 100 kV accelerating voltage). The TEM images show that the GNSs have a round shape, a smooth surface, and a narrow size distribution whereas the GNFs have a large number of tips at the surface. By image analysis using the software ImageJ, we also measured the sizes of GNFs with respect to their core and branches and included these values: a mean size of (42 ± 6) nm, a core size of (36 ± 5) nm, and a branch size of (7 ± 2) nm (Figure 2a−d). GNPs sizes are confirmed by the Zetasizer measurements that are

ωxy2 4D

(2)

kT 6πηR h

(3) 11702


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This characterization of the colloidal nanoparticles is relevant with the literature on the nanoparticles fabricated using such methods.44,45 The PEG-GNPs have a larger size due to the PEG thickness around nanoparticles. For all GNPs, we observe a red-shift when the PEG is added. This confirms the exchange of the as prepared surface chemistry by PEG. The PEG layer is approximately around 5 nm (SD ± 3 nm), which is consistent with the size difference measured with the Zetasizer (Table 2). The surface ligand exchange was further confirmed by a shift of the ζ potential of colloidal nanoparticles measured with the Zetasizer (Table 2). We observed a ζ potential shift from −14 to −5 mV (64% decrease) for 50 GNSs and from −10 to −5 mV (50% decrease) in case of GNFs. For both types of PEG-GNPs, the surface charge was decreased near to neutral. Moreover, all GNPs were found to be highly stable, as no aggregates were observed even after three months of storage at room temperature in ultra pure water. The as prepared GNPs, are stabilized by their negatively charged surface via electrostatic repulsion, even for the GNFs that have a lower zeta potential than the GNSs. Additionally, PEG-GNPs are stabilized by steric stabilization and the polymer coating can prevent the nanoparticle aggregation even at neutral zeta potential. 3.3. Determination of the Hydrodynamic Radius of GNPs by SCS. First, SCS was used to characterize GNPs hydrodynamic size. The characterization of the hydrodynamic radii of GNPs was achieved close to their resonance wavelength, 532 nm for GNSs and 633 nm for GNFs to reduce the measurement uncertainties as reported before27 due to the weakness of the scattering signal of GNPs out of resonance. Figure 3 shows the average of 5 cross-correlation curves (the acquisition time for each curve is fixed to 60s) of GNSs and GNFs. Each curve has been fitted using eq 1 and the τD has been deduced for each GNPs (see insert to each curve on Figure 3). By combining eqs 2 and 3, the hydrodynamic radius (Rh) of the GNPs was calculated as it depends directly on the τD. In the case of 20-a-GNS, a Rh of 10.8 ± 0.2 nm was found (see the Supporting Information for the calculation of Rh and the experimental error, Table S1). Note that, the total uncertainties on the hydrodynamic size are about ±0.4 nm and this is due to the sample heterogeneity. The correlation analysis is very sensitive to GNPs homogeneity as reported before.5,27,47,49 By increasing GNPs size, the time diffusion is increased leading to a hydrodynamic radius of Rh = 16.3 ± 0.2 nm for 30a-GNS, Rh = 23.5 ± 0.2 nm for 50-a-GNS, and Rh = 23.0 ± 0.2 nm for a-GNF. After adding PEG, the hydrodynamic radius is increased to Rh = 25.5 ± 0.2 nm for 50- PEG-GNS and Rh = 26.8 ± 0.2 nm for PEG-GNF, and the increase in Rh corresponds to a PEG layer of 2 nm for GNSs and 3.8 nm

Figure 2. TEM images of GNPs: (a) 20-a-GNS, (b) 30-a-GNS, (c) 50a-GNS, and (d) a-GNF. (e) Extinction spectra of GNPs with (dotted lines) and without (solid lines) PEG.

summarized in Table 2. All the prepared a-GNPs have the expected size with a size distribution around 10%. These results are consistent with the extinction spectra that give the localized surface plasmon resonance (Table 2). Indeed, for the 20-aGNS, 30-a-GNS, and the 50-a-GNS, the LSPR positions measured at 522, 526, and 535 nm, respectively, are in agreement with the GNPs diameter (the normalized extinction spectra of the gold colloidal solutions are illustrated in Figure 2e). In the case of the GNFs, the LSPR is red-shifted at 605 nm and is very broad compared to GNSs. This is the result of the particle anisotropy and of the multiple tips at the particle surface.45

Table 2. Characterization of GNPs by UV−vis, Zetasizer, and SCS Spectroscopya 20-a-GNS 30-a-GNS 50-a-GNS 50-PEG-GNS a-GNF PEG-GNF a

shape

surface chemistry

LSPR (nm)

ζ potential mV (±SD)

spherical spherical spherical spherical flower flower

citrate citrate ascorbate PEG ascorbate PEG

522 526 535 537 605 620

−40.3 ± 1.7 −25.1 ± 1.9 −14 ± 0 −5 ± 0 −9.08 ± 0.8 −4.65 ± 0

size by Zetasizer nm (±SD) 21.1 32.1 47.9 57.8 46.7 58.1

± ± ± ± ± ±

2.1 3.3 5.2 5.2 6.5 6.5

size by SCS nm (±SD) 21.6 32.6 47.0 51.0 46.0 53.6

± ± ± ± ± ±

0.4 0.4 0.4 0.4 0.4 0.4

Note: SD: standard deviation. 11703


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Figure 3. Normalized cross-correlation curves (dashed lines) and their fits following eq 1 (solid lines) of the scattered light by GNSs excited at 532 nm excitation wavelength (a−d) and by GNFs excited at 633 nm excitation wavelength (e and f).

for GNFs. GNPs characteristics are summarized in Table 2. The hydrodynamic radii Rh obtained by scattering correlation spectroscopy are in excellent agreement with those measured by Zetasizer but with a largely higher precision (the uncertainties is reduced by 1 order of magnitude compared to the zetasizer measurements). This is also remarkable to notice that the GNFs size measured by SCS is in perfect agreement with the ones determined by zetasizer. This means that the specific surface morphology of the GNFs do not influence the SCS measurements and the dynamic properties of the GNFs compared to round shaped GNSs. Note that, from the Stokes−Einstein equation, the measured GNFs hydrodynamic radius is the radius of a hypothetical hard sphere that

diffuses with the same speed as the GNF under examination. The translational diffusion coefficient (D) will depend not only on the size of the GNP, but also on any surface structure (PEG), as well as the concentration and the surrounding medium. Factors that affect the diffusion speed of GNPs. 3.4. Effect of GNPs Properties and Excitation Intensity on Scattering Properties of GNPs. The optical properties of GNSs37,38,59 have been well documented using other optical methods as UV−visible spectroscopy. The scattering light intensity of GNSs linearly depends on the intensity of illumination laser, as already published before.41 While in the case of GNFs, few experimental works were reported on their scattering properties.20,60 Nehl et al. reported from dark-field 11704


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Figure 4. Scattering signal recorded in water, solvent noise, (a) and for 20-a-GNS at 75pM (b) and 750 pM (c) concentration. Integrated scattered signal by removing noise during 300 s of GNPs (d,e) with different excitation powers. Integrated scattered signal without noise during 300 s of different concentrations of 50- a-GNS (f).

Figure 5. Comparison between the scattered signal of GNSs (a) and GNFs (b) with different excitation powers for both excitation wavelengths 532 and 633 nm. Experimental data (markers) and linear fits (solid lines).

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The Journal of Physical Chemistry C microscopy combined to transmission electron microscopy (TEM) that individual star-shaped GNPs support multiple plasmon resonances, which result in polarization-dependent multipeak scattering that is sensitive to the local dielectric environment.24 Single particle techniques which work best for diffusion on surfaces or in thin films because NPs drifting out of focus makes the measurement and analysis difficult or requires complex setups. Dark-field imaging was a sensitive method and can be used to investigate the diffusion behaviors, but it cannot provide the concentration information on NPs in solution. In our work, we propose another solution which is to follow not a single NP but to monitor changes in signal due to NP diffusion from a small focal volume. In this work, we make a multiparametric study to compare the effects of GNPs geometrical properties and excitation wavelength and intensity on GNPs scattering properties. In the case of the excitation wavelength at 532 nm, the photodetected signal was first recorded at different excitation powers (from 5 to 100 μW), for the water, solvent, alone (Figure 4a) and for a-20-GNS and a-50-GNS at several concentrations (Figure 4b,c). To study the effect of GNPs properties and excitation power on the scattering signal, the recorded intensity from GNPs was integrated during 300 s and water (noise) signal was removed. The scattered signal is linearly dependent on the excitation power (Figure 4d, e) and the GNPs concentration (Figure 4f). The GNPs scattering intensity signal versus excitation power is fitted by a linear curve passing through the origin. One can first notice that the scattered signal is stronger for 50-GNSs than for 20-GNS. As already demonstrated by Mie theory, the scattering coefficient increases as GNSs size is increased. This trend has been already observed in experiments37,61 by other techniques and reflects the dependence of the GNSs extinction crosssection on the sphere volume.1,59 In the case of GNFs, the scattered signal is 10 times stronger than by 50-GNS (GNFs concentration is 10 time less than 50-GNSs whereas close scattering signal is measured for both GNPs.). Moreover, even if the GNFs and GNSs do not have the same shape, the slope of the scattered intensity versus power is approximately the same for both particles, meaning that this effect is only related to the GNPs size. The effect of the excitation wavelength on the scattering intensity is shown in Figure 5. Higher concentrations of GNSs are taken to avoid a very weak signal-to-noise ratio at 633 nm excitation wavelength. To compare the results in the same conditions, the final power value at 633 nm excitation wavelength is 40 μW, to avoid the saturation of the detectors in the case of GNFs. Figure 5a) shows the scattered signal of GNSs at 532 and 633 nm. This signal is weaker at 633 nm since we are far from the plasmon resonance wavelength as predicted by the Mie theory (see in the Supporting Information, the calculation for both GNSs, in Figure S6). Whereas, for GNFs (Figure 5b) the scattered intensity is stronger at 633 nm and the ratio of the scattering intensity of GNFs between 633 and 532 nm is around 50 times, as the plasmon resonance of GNFs is very close to 633 nm. In Table 3, the integrated signal for each sample at 20 μW excitation power is extracted from Figure 5. To compare the scattering properties of all GNPs, the scattered intensity was normalized by the concentration parameter and the excitation power. Hence, the scattering factor per mole and per power unit (Csca) is calculated by the following equation:

Table 3. Summary of the Integrated Scattering Signal (Isca) for Each Sample with 20 μW Excitation Power 50-a-GNS (5 pM) 50-a-GNS (50 pM) a-GNF (0.5 pM)

Csca =

532 nm

633 nm

38 kHz 425 kHz 43,4 kHz

211 kHz 2000 kHz

Isca IexcC

where Isca is the integrated scattered signal, Iexc is the excitation power (in our case we have made our calculation for a power of 20 μW) and C the GNPs concentration. Table 4, shows the calculated scattering factors (Csca) for all GNPs. We can demonstrate that at 532 nm excitation Table 4. Calculation of the Scattering Factor per Mole for Each Sample Csca [Hz μW1− pM−1] 20-a-GNS 50-a-GNS a-GNF

532 nm

633 nm 1.5 400 4500

1 200 200 000

wavelength, Csca (50-GNS)/Csca (20-GNS) ≈ 267, which is in agreement with the theoretical ratio predicted by Mie theory as 250 (see table S2). The theoretical calculations based on Mie theory give a scattering factor of 10−15 m2 for 50-GNSs at 532 nm and 2.10−16 m2 at 633 nm (table S2). Comparing GNSs and GNFs, Csca(GNF)/Csca(50-GNS) ≈ 10 at 532 nm, we can estimate in this case, that the scattering factor of GNFs is around 10−14 m2 at 532 nm. At 633 nm excitation wavelength, the scattering factor increases by a factor 50 in the case of a-GNF and Csca(GNF)/ Csca(50-GNS) ≈ 1000 at 633 nm. This increase of 2 orders of magnitude between 532 and 633 nm can be explained by the LSPR position for both nanoparticles (535 nm for 50-GNSs and 605 for GNFs). By using the theoretical values obtained by Mie theory, we can calculate the scattering factor of GNFs at 633 nm, which is around 2.10−13 m2. To take into account this resonance effect and to compare actually the scattering efficiency of the GNFs and the 50-GNSs, we have to calculate the ratio of the scattering factors for both nanoparticles in resonance. Such ratio Csca(GNF at 633 nm)/ Csca(50-GNS at 532 nm) is equal to 500, and we can conclude that the GNFs are 500 times more efficient than GNSs at the same size. The higher scattering efficiency of the GNFs can be explained by their surface roughness. Indeed, each tip can be considered as a hot spot during the interaction with the excitation light and acts as a single scatterer. The GNFs exhibit larger number of scatterers than GNSs and as a consequence a larger scattering signal. At 633 nm excitation wavelength, one can estimate that the GNFs (close to their plasmon resonance wavelength) have the same scattering behavior than a GNS with a diameter of around 300 nm (see Supporting Information, Figure S6).

4. CONCLUSION By recording the scattering signal of different GNPs, we have demonstrated that the scattering correlation spectroscopy is a powerful technique to determine the hydrodynamic sizes of 11706


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GNPs. The experimental values obtained by SCS are in good agreement with those obtained by Zetasizer with lower uncertainties. The analysis of the integrated scattering signal confirmed the calculations done by Mie theory in the case of GNSs and allows the prediction of GNFs scattering factors by comparison between both particles at the same size. The experimental data showed that GNFs have stronger scattering properties than GNSs. Thus, GNFs are considered as better candidates for cell imaging due to their branched shape, which strongly increases their optical properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02436. The gold nanoparticles concentration measurements, the calibration of the focal volume parameters by two different methods (FCS and PSF) and the calibration of the excitation power for correlation measurements in Figures S1 to S3. The calculation of nanoparticles hydrodynamic radii and the error analysis are presented in Table S1 and theoretical calculations of extinction, absorption and scattering coefficients based on Mie theory are shown in Figures S5 and S6. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +33 148387391. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Region Ile-de-France in the framework of DIM C’Nano IdF and the Campus France Agency as part of the Tassili agreement program.

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REFERENCES

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