Tuning Dye-to-Particle Interactions toward Luminescent Gold

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Tuning dye-to-particle interactions towards luminescent gold nanostars Julien Navarro, Adrien Liotta, Anne-Charlotte Faure, Frederic Lerouge, Frederic Chaput, Guillaume Micouin, Patrice L. Baldeck, and Stephane Parola Langmuir, Just Accepted Manuscript • DOI: 10.1021/la402222c • Publication Date (Web): 25 Jul 2013 Downloaded from http://pubs.acs.org on August 3, 2013

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Tuning dye-to-particle interactions towards luminescent gold nanostars Julien R.G. Navarro, Adrien Liotta, Anne-Charlotte Faure, Frederic Lerouge, Frederic Chaput, Guillaume Micouin, Patrice L. Baldeck and Stephane Parola* Ecole Normale Supérieure de Lyon, CNRS, Université Lyon 1, Laboratoire de Chimie UMR 5182, 46, allée d’Italie, F-69364, Lyon cedex 07, France Tel: +33 4 72 44 81 67; E-mail: [email protected] KEYWORDS Gold nanostars, luminescence, chromophore, fluorescence, LbL, polyelectrolyte, Surface Plasmon Resonance

ABSTRACT

Light-matter interactions are of great interest for potential biological applications (bioimaging, biosensing, phototherapy). For such applications, sharp nanostructures exhibit interesting features since their extinction bands (Surface Plasmon Resonance) cover a large bandwidth in the whole visible wavelength region due to the existence of ‘hot-spot’ located at the end of the tips. In this context, gold nanostars appear to be interesting objects. However, their study remains

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difficult mainly due to complicated synthetic methods and further functionalization. This paper reports the synthesis, functionalization and photophysics of luminescent hybrid gold nanostars prepared using a Layer-by-Layer (LbL) deposition method for the tuning of chromophore-toparticle distances together with the impact of the spectral overlap between the plasmon and the emission/absorption of the dyes. Several luminescent dyes with different optical signatures were selectively adsorbed at the nanoparticles surface. The optimized systems, exhibiting the highest luminescence recovery, clearly showed that overlap must be as low as possible. Also, the fluorescence intensities were quenched in close vicinity of the metal surface, and revealed a distance-dependence with almost full recovery of the dyes emission for 11 LbL layers, which corresponded to 15 nm distances evaluated on dried samples. The photophysics of the luminescent core-shell particles were carried out in suspension and correlated with the response of isolated single-objects.

INTRODUCTION The control of light interactions with organic luminescent molecules have received intense attention due to promising bio-applications1–10 (imaging, sensing). The lack of stability (e.g. bleaching effect) within a certain irradiation time11 (photochemical reaction altering the dyes properties) often compromises their further use in biological applications. It was recently shown that interactions between chromophores and a metallic surface may prevent this bleaching process11. The enhanced photostability is the result of an energy transfer from the probe excited state to the metal. Nonetheless, the dye optical signature is directly affected (e.g. quenching12–14 or enhancement15–17 of the fluorescence brightness). Several parameters may affect this fluorescence response such as the metal-to-dye distance18,19, the molecular dipole orientation


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versus the metallic surface20 and the metal-chromophore spectral overlap21 (absorptionemission). Metallic surfaces presenting Surface Plasmon Resonance (SPR) effects can be used to tune the optical response of organic luminescent dyes using the interactions with the local electromagnetic field. This effect can be widely investigated and controlled since metal nanoparticles can be synthesized in various shapes and size (spheres22, rods23,24, prisms25,26, bipyramids27,28 and stars29–31), with large possibilities of plasmon resonance bands in the visible and Near Infra-Red wavelengths. Thus the size, shape and surrounding environment affect the overall final optical response32-35. They become extremely interesting supports for further use as enhanced optically responsive materials. Metal nanoparticles and in particular gold nanoparticles (GNPs) can be easily functionalized by sulfur based ending groups such as thiols, due to the strong affinity between gold and sulfur36,37. This extinction efficiency (contribution of both scattering and absorption phenomena) can be so intense that their detection, even at extremely low concentration, becomes possible38. We recently reported the synthesis and functionalization of biocompatible PEGylated gold nanostars31. To extend the possibilities of surface modification of gold nanostars with photoresponsive organic moieties, we propose to use a layer-by-layer (LbL) approach18,39-42. This method consists of successive adsorptions of oppositely charged polyelectrolytes on the gold nanostars surface. The LbL structures were achieved using alternatively an anionic polyelectrolyte, the poly(sodium 4-styrenesulfonate) (PSS), and a cationic one, the poly(diallyldimethylammonium chloride) (PDD). The polymer shell gets thicker as the number of depositions increases, enabling to precisely modulate the chromophore-to-particle distances. The polyelectrolyte adsorption was monitored using UV-Visible spectroscopy and zeta potential


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measurements. The polymer shell thickness was estimated using Transmission Electron Microscopy (TEM) on dried samples. Finally, luminescent cationic dyes (Rhodamine B, Atto590 and Atto610) were deposited on the ultimate anionic polyelectrolyte layer. The fluorescence spectroscopy was investigated both in suspension and at the single-object level. To the best of our knowledge the only core shell system with gold nanostars was produced with silica shell43 and this is the first example of surface modification using polyelectrolyte on nanostars.

MATERIALS AND METHODS Materials Chloroauric acid (HAuCl4, 3H2O, 99.9%), myristyl bromide, domiphen bromide, silver nitrate (AgNO3, 99%), sodium citrate, ascorbic acid, hydrogen bromide (HBr), ruthenium (IV) chloride, sodium periodate, Poly(diallyldimethylammonium chloride) solution 35wt. % in water (Mw < 100 000 g/mol) and Poly(sodium 4-styrenesulfonate) (Mw = 70 000 g/mol) were purchased from Sigma-Aldrich and used as received. All the synthesis and functionalization were performed in milliQ water. Absorption spectra were recorded using a Perkin-Elmer UV-Vis-NIR Lambda 750 spectrometer. Zeta potential was obtained using a Malvern Zetasizer Nano Series. Transmission electron microscopy (TEM) data were obtained using a TOPCON EM-002B microscope (80 kV & 120 kV). Gold nanostars synthesis The gold nanostars were synthesized using a procedure previously reported31. In the first step, spherical 13 nm gold nanoparticles (Cpart= 4.9 nM) were obtained using the citrate procedure2,22. The prepared gold nanospheres suspension (10 mL) was slowly added drop by drop to a domiphen bromide solution (0.2 M, 10 mL) under vigorous stirring. The suspension was stirred


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for 30 min and left undisturbed for 48h in the dark before use. The particles suspension was used without further purification. 250 µL of these particles (2.5 nM, ǫ(part) = 2.4 x 108 L.mol-1.cm-1 / 520 nm) used as seeds were injected in a growth aqueous solution containing 5 ml of HAuCl4 (1 mM), 5 mL of myristyl bromide (50 mM), 450 µL of AgNO3 (4mM), 200 µL of HBr (0.2 M) and 100 µL Ascorbic acid (80 mM). The solution was stirred for 25 seconds and stored in the dark at 30 °C for 12 h. The particles were then centrifuged (8000 RPM /10 min), the supernatants were removed and the particles were dispersed in water. This operation was repeated twice for purification. Coating the gold nanostars with multilayered polyelectrolytes In a typical procedure, the purified gold nanostars were dispersed in 5 mL of pure water (Cpart= 2.5 nM, ǫ(part) = 1.79 x 1010 L.mol-1.cm-1 / 528 nm). An aqueous solution of polyelectrolyte (10 mg/mL, 0.15 mM, 3 mL) was slowly added drop by drop to the gold nanostars suspension under vigorous stirring.

The resulting suspension was stirred for 3 hours. The gold nanostars

suspensions were then centrifuged at 8000 RPM for 10 min. The supernatants were discarded and the particles were dispersed in water. This operation was repeated twice. In the case of the first layer of PSS, the solution was stirred for 24h to ensure a good adsorption on the gold surface. After each layer deposition, the purified particles were characterized with UV-Visible spectroscopy and Zeta potential measurement.

Coating of the Gold nanostars with the chromophores The purified polyelectrolytes stabilized nanostars were diluted with an HCl solution (pH=4) and the corresponding chromophores solutions (pH=4) were added.


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Staining agent and estimation of the polymer shell thickness A 0.5 wt% RuO4 solution was prepared using the oxidation of RuCl3 with sodium hypochlorite. The protected gold nanostars were cast on a TEM grid using the slow solvent evaporation technique. The deposited layer was treated with RuO4 vapor for 30 min before Transmission Electron Microscopy (TEM) analysis (80 kV).

Single particle tracking Single nanoparticle visualization in fluorescence configuration was performed using a Zeiss LSM510 confocal microscope. The excitation was achieved using an argon laser at 514 nm. Then, light was reflected by a dichroic mirror and directed toward the sample using an A-Plan 40×Zeiss objective (NA=0.65). The emitted fluorescence was collected by the objective passes through the dichroic (HFT458/514) and a spectral emission filter (554-608nm). Fluorescence images were recorded with the LSM5 software.

RESULTS AND DISCUSSION Gold nanostars were synthesized using the seed-mediated growth method31. A representative UV-Visible spectrum and the corresponding TEM picture are represented in Figure 1. The photos reveal almost uniform gold nanostars with an opposite tip-to-tip distance of 130 nm ± 10 nm, a gold core of 60 nm ± 7 nm and a tips number of 10 ± 3 per gold structure. The extinction spectrum reveals two distinct bands. The first band at 612 nm is attributed to the collective electron oscillation of the short axis of the nanostars, corresponding to the core elongation response. The second band at 808 nm corresponds to the electron oscillation along the tips-totips axis. This extinction band in the near IR region depends on the number of tips branched on


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the gold core. This explains the small broadening of the band (e.g. tips number of 10 ± 3 per gold nanostar).

Figure 1. UV-Visible spectrum (left) and TEM picture (right) of a gold nanostars suspension.

The polyelectrolyte layers were electrostatically deposited on the gold nanostars’surface using the layer-by-layer methodology44-46. Similar approach was previously used on nanorods44,45 but in the case of stars the object needed further optimization for surface modification and chromophores interactions. The initial gold nanostars are stabilized in colloidal suspension by a myristyl bromide layer which is positively charged (layer 0). The first layer of polyelectrolyte (layer 1) was deposited on the myristyl bromide stabilized gold nanostars, using a negatively charged polyelectrolyte, the Poly(sodium 4-styrenesulfonate) (PSS). Afterward, a cationic polyelectrolyte, the Poly(diallyldimethylammonium chloride) (PDD) was deposited on the anionic surface to get the layer 2. This process was repeated until getting the gold nanostars coated with up to 11 polyelectrolytes layers (See SI). The adsorption of the alternatively charged polyelectrolytes was monitored using UV-Visible spectroscopy and zeta potential measurements. It is now well established that the position and the morphology of the surface plasmon band strongly depends on the nanoparticle shape, size and the surrounding environment (dielectric constant)32-35. Thus, it is expected that the presence of polyelectrolyte layer on the gold nanostars would shift the plasmon resonance. The short band


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at 632 nm was not affected by the polyelectrolyte coating and shift was only observed for the tips-to-tips band. Interestingly, the first deposited layer (n=1, PSS, AuStar(PSS-PDD)0(PSS)1) showed a blue-shift for the absorption maximum from 808 nm to 793 nm (Figure 2). This blue shift was only observed for the first layer. A similar effect was previously reported by our group when using a thiol-ended PEG ligand31 (methoxy-PEG2000 –thiol).

Figure 2. Tips-to-tips Surface Plasmon band position of polyelectrolytes stabilized gold nanostars versus the number of deposited polyelectrolyte layers. Usually, the surface plasmon band is red shifted when the surrounding medium of the gold nanoparticles is modified, in particular with organic moieties47,48 (e.g. changing the refractive index around the nanoparticles). This red-shift can be attributed to an electron transfer from the particles to the surrounding layer, here the polyelectrolyte. Reversibly, the surface plasmon band can be blue shifted when an electron transfer occurs from the surrounding layer to the metallic surface49-51. In our case, the plasmon was red shifted while the number of deposition of the polyelectrolytes increased (n>1), until stabilization around 818 nm after 5 layers (n=5). In the other cases with n≥5, the thickness of the polymer shell was probably too large to consider a direct interaction of the gold surface with the polymer (e.g. too long distance for an electron


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transfer). This hypothesis is in good agreement with the experimental values (e.g. SPR at 821 nm ± 2 nm). The Figure 3 compares the UV-Visible spectrum of an initial suspension stabilized with a layer of myristyl bromide (n=0) and the final suspension after 11 deposited layers (AuStar(PSSPDD)5(PSS)1) on nanoparticles. For clarity and comparison, all the suspensions were not showed in the UV-Visible spectrum.

Figure 3. UV-Visible spectra of gold nanostars stabilized with 0 layer (surfactant) and 11 layers of polyelectrolytes showing the red shift of the surface plasmon band. The polyelectrolytes adsorption process (n=1 to n=11) on the gold nanostars did not destabilized the final object. Indeed, the apparition of a new absorption band in the NIR region would have been observed in the case of an aggregation52,53. No aggregation was observed since the SPR morphologies did not evolve. The gold nanostars’suspensions (n=1 to n=11) remained stable even after more than 2 months. The presence of the multilayered polyelectrolyte was also confirmed by zeta potential measurement (Figure 4).


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Figure 4. Zeta potential of polyelectrolytes stabilized gold nanostars as a function of the deposited polyelectrolyte layers. The initial gold nanostars (e.g. stabilized with a myristyl bromide layer, layer 0) showed a zeta potential of +44.2 mV which was consistent with a previous study on CTAB protected gold nanorods54. As the charge of the polyelectrolyte was alternatively inversed, due to the alternate adsorption of the polyelectrolyte layers, the zeta potential switched between roughly -58 mV and +58 mV (Figure 4). The polyelectrolyte layers thicknesses were estimated using Transmission Electron Microscopy (Figure 5). The average shell thickness of the adsorbed polyelectrolytes layers on the gold nanostars were in the range 4 to 15 nm depending on the number of adsorbed layers (Table 1). The polymer shell thickness estimation (over 10 objects per deposited layer) remained difficult since the coverage of the gold nanostars was not as homogeneous as it used to be for spherical nanoparticles55,56. After few purification steps, the gold nanostars tips morphology was affected due to their high sensitivity to oxidation (Figure 5) inducing partial dissolution of gold atoms from the tips.


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a

b

d

c

e

Figure 5. TEM pictures of gold nanostars stabilized by (a) 3, (b) 5, (c) 7, (d) 9, (e) 11 layers of polyelectrolytes. The polymers were stained with RuO4 to reveal the polymer shell thickness.

Table1. Physical properties of the nanostars suspension after the polyelectrolyte depositions Layer Polymer numbers thickness

Zeta potenti al /mV

SPB /nm

1

4.5± 1 nm

-58.3

793

3

7 ± 2 nm

-54.6

813

5

8 ± 2 nm

-57.5

820

7

10 ± 3 nm

-63.9

819

9

13.5 ± 4 nm

-56.4

819

11

15 ± 6 nm

-57.4

823

Photophysics of the functionalized fluorescent gold nanostars The desired number of polyelectrolyte layers was alternatively deposited on the gold nanostars surface using electrostatic interactions. In our case, the hybrid gold nanostars had their ultimate


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external polyelectrolyte layer made of PSS (anionic layer). Cationic chromophores (Rhodamine B, Atto590 and Atto610) were then selectively deposited through electrostatic interactions with the anionic polyelectrolyte of the external layer of the shell AuStar(PSS-PDD)x(PSS)y. The electrostatic interactions between the dyes and the polyelectrolytes allowed strong surface binding and preparation of stable fluorescent gold nanoparticles57-59. The influence of the spectral overlap (absorption-emission) of the chromophore with the extinction band of the stars, as well as the dye-to-particle distances were respectively investigated. The luminescence spectra of the nanoparticles are shown in Figure 6. The area under the emission curve was assimilated to the brightness of the fluorescent probes (rhodamine B, Atto590 and Atto610) adsorbed on the hybrid gold nanostars. For all our measurements, the gold nanostars and fluorescent dye concentrations remained the same in all samples. The concentrations were determined from their respective extinction coefficients. In each studied sample, the number of luminescent molecules per particle was estimated to be 670. The distance between the nanoparticles and the chromophores, controlled by the number of deposited polyelectrolyte layers, directly affected the photophysical properties of the dye as shown in Figure 6. The optical signature of the dyes, in each case, were strongly quenched (relative fluorescence intensity ≈16%) for a short particle-to-chromophore distance (4.5 nm / 1 layer). It is however extremely interesting to note that the fluorescence intensities were almost totally recovered with a minimal distance of 10 nm (n=7). The Atto590 and Atto610 brightness were less quenched than the Rhodamine B within a minimal distance of 7 nm.


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Figure 6. Emission spectrum of hybrid gold nanostars stabilized with several layers of polyelectrolytes. (a) Rhodamine B, (b) Atto590, (c) Atto610. “n” corresponds to number of deposited layer. The chromophores were deposited on the ultimate PSS layer of the structure through electrostatic interactions: AuStar(PSS-PDD)0(PSS)1 and AuStar(PSS-PDD)5(PSS)1. (d) Normalized fluorescence intensity as a function of the estimated dye-to-particle distances.

The photophysical properties of the chromophores are shown in Figure 8. As the absorption and emission band of the chromophores evolved in the red wavelength, the overlap with the stars core extinction band was more pronounced. As expected, to get optimal fluorescence intensity, the absorption-emission band (probe) and the extinction band (anisotropic particles) must overlap (Figure 6 & 7).


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Figure 7. Absorption (left) and emission (right) spectra of the Rhodamine B, Atto590 and Atto610. For clarity, the spectra were normalized and the extinction spectrum of the polyelectrolytes stabilized stars, AuStar(PSS-PDD)5(PSS)1, were over layered.

Fluorescence microscopy imaging A single particle tracking study, using a confocal microscope, was performed. The particles suspensions

AuStar(PSS-PDD)0(PSS)1,

AuStar(PSS-PDD)5(PSS)1,

containing

adsorbed

Rhodamine B, were deposited in a lab-tek chambered coverglass and left undisturbed one hour (sedimentation process). The resulting fluorescence images are shown in SI. For the one adsorbed layer, the luminescent spots (e.g. Rhodamine B on gold nanostars) were difficult to detect even using a low signal:noise ratio. This confirms the previous results in solution, in which the probe was totally quenched in close proximity of the metal surface (e.g. 4.5 nm). When the adsorbed polyelectrolytes thickness increased to 15 nm (n= 11), the fluorescence intensity was easily recovered with a very good signal:noise ratio.


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CONCLUSIONS In conclusion, luminescent hybrid gold nanostars were synthesized and functionalized using the LbL approach. The desired number of polyelectrolyte layers was electrostatically deposited on the gold nanostars. The thickness of the polymer shell (4.5 nm to 15 nm) was tuned by controlling the number of deposited layers. Cationic luminescent probes were linked at the ultimate anionic layer of the structure and stabilized through electrostatic interactions. The distance between the nanoparticles and the chromophores, as expected, directly affected the photophysical properties of the dyes. In close proximity of the metallic surface, the optical emission of the dyes was strongly quenched. Interestingly, the fluorescence intensities were almost recovered with a minimal distance of 10 nm. The overlap between the extinction bands of the chromophores and the nanoparticles was evidenced as an important parameter for tuning the emission of the final nano-objects. The dyes fluorescence is less quenched when the absorption and emission band overlap the extinction band of the gold anisotropic particles. Besides, contrary to gold nanospheres, gold nanostars have a strong light extinction all over the visible wavelength which made them ideal candidate for such a study on this spectral dependence effect. In our case, Atto610 were the less quenched. According to Chen et al.60, the fluorescence brightness should be more intense when the LSPR peak is positioned between the absorption and emission band of the dye. This was confirmed by our experimental results. Such approach is promising on the way to the preparation of highly luminescent nanoprobes for biomedical imaging and sensing.

ACKNOWLEDGEMENTS

This work was supported by grants from French National Research Agency (ANR) P3N project nanoPDT # ANR-09-NANO-027-04 #


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TOC Graphic Tuning dye-to-particle interactions towards luminescent gold nanostars Julien R.G. Navarro, Adrien Liotta, Anne-Charlotte Faure, Frederic Lerouge, Frederic Chaput, Guillaume Micouin, Patrice L. Baldeck and Stephane Parola*


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Tuning Dye-to-Particle Interactions toward Luminescent Gold

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