Tunable Plasmon Coupling in Distance-Controlled Gold Nanoparticles

Article pubs.acs.org/Langmuir

Tunable Plasmon Coupling in Distance-Controlled Gold Nanoparticles Holger Lange,*,† Beatriz H. Juárez,*,‡ Adrian Carl,§ Marten Richter,∥ Neus G. Bastús,⊥,# Horst Weller,⊥ Christian Thomsen,† Regine von Klitzing,*,§ and Andreas Knorr∥ †

Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, 10623, Berlin, Germany IMDEA Nanoscience, Campus Universitario de Cantoblanco, 28049 Madrid, Spain § Stranski Laboratorium für Physikalische und Theoretische Chemie, Technische Universität Berlin, Germany ∥ Institut für Theoretische Physik, Nichtlineare Optik und Quantenelektronik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany ⊥ Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany ‡

S Supporting Information *

ABSTRACT: Plasmons are resonant excitations in metallic films and nanoparticles. For small enough static distances of metal nanoparticles, additional plasmon-coupled modes appear as a collective excitation between the nanoparticles. Here we show, by combining poly(N-isopropylacrylamide) micro- and nanospheres and Au nanoparticles, how to design a system that allows controllably and reversibly switching on and off, and tuning the plasmon-coupled mode.

volume phase transition at around 32 °C.12 The combination of pNIPAM and metal nanoparticles leads to hybrid materials that benefit from the properties of both components, namely, the thermoresponse of the pNIPAM platform and the plasmonic properties of the metal nanoparticles.13 Nanocrystals can be arranged in superlattices with tunable properties by using pNIPAM as flexible spacer.14 Lu et al. observed a shift of the absorption peak wavelength of Ag nanoparticles bound to pNIPAM layers when changing the spacing between the metal particles.10 The plasmon of gold nanorods can be dynamically shifted in nanorod covered pNIPAM microgels.15,16 Other works about Au-pNIPAM hybrids study the dependence of the plasmon on the refractive index of the surrounding17,18 or use the core−shell configuration for direct SERS.19 In the present work, the volume phase transition of pNIPAM-based particles is used to tune directly the distance between Au nanoparticles in order to control the plasmon coupling. Two exemplary hybrid Au-pNIPAM systems that allow forming and tuning a plasmon-coupled mode are presented. In one of the hybrids, the Au nanoparticles are covering the pNIPAM sphere surface and it is named AupNIPAM in the following. The other system (pNIPAM-Au) consists of Au nanoparticles embedded within the pNIPAM

1. INTRODUCTION Metal nanoparticles have strong plasmon resonances vastly used for surface enhanced Raman scattering (SERS) of single molecules1 and biodiagnostics.2 The arrangement of metal nanoparticles leads to plasmonic structures, which can be constructed from nanoparticles combined with, for example, molecular linkers or templates. Especially, linking nanoparticles with DNA is a versatile approach to build plasmonic nanostructures.3 Other approaches employ the self-assembly of nanoparticles.4,5 The coupled plasmon modes are very sensitive to the interparticle distances and the particle nature.6 For example, monitoring the coupling between plasmons of silver and gold nanoparticles during DNA hybridization events allows studies of the hybridization kinetics.7 Other applications of plasmon coupling can be found in optical spectroscopy.8 However, most linking approaches result in static distances between the nanoparticles which reduces the flexibility of the plasmonic nanostructures. It is thus desirable to find suitable platforms in which the plasmon coupling of nanoparticles can be reversibly switched on and off. In dense systems of metal particles, coupled plasmon modes appear at longer wavelengths than the single-particle plasmon.9 These modes intensify and shift with decreasing interparticle distance and have been observed in dense layers of Ag particles.10 Recently, Zhou and Odom presented an intense strongly coupled mode in an array of large Au nanoparticles.11 Poly(N-isopropylacrylamide) pNIPAM is an established polymer to produce thermoresponsive gels, which have a © 2012 American Chemical Society

Special Issue: Colloidal Nanoplasmonics Received: January 11, 2012 Revised: February 21, 2012 Published: March 14, 2012 8862

dx.doi.org/10.1021/la3001575 | Langmuir 2012, 28, 8862−8866

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spheres were inspected in a JEOL 1010 operated at 100 kV. pNIPAMAu spheres were investigated with a Tecnai G220 S-TWIN (FEI), operated at 200 kV. UV−vis spectra of Au-pNIPAM beads were recorded with a Thermo Scientific Evolution Array UV−visible spectrophotometer. The samples were placed in a multicell holder, where temperature control was provided by a Thermo Scientific AC 150 immersion circulator. Additionally, to the Au-pNIPAM spheres plain Au nanoparticles in water, plain pNIPAM in water and pure water were measured as references. The accuracy of the set temperature was controlled by measuring the temperature in a cuvette with water. Spectra were taken at temperatures between 18 and 64 °C. DLS experiments of pNIPAM-Au beads were carried out with a ALV/ CGS-3 Compact goniometer system including an ALV/LSE-5004 correlator. UV−vis spectra of pNIPAM-Au beads were recorded with a Varian Cary 50 tablet, combined with a Thermostate K10 (Haake) and a DC 50 controller (Haake). 2.4. DDA Calculations. The absorbed light from two Au spheres at different distances was calculated. A single orientation of the incident light toward the target and a discrete dipole approximation algorithm26,27 were used to self-consistently solve the Maxwell equations, utilizing the DDSCAT program developed by Draine and Flatau.28,29 The two Au spheres are described by a grid of cubical blocks of polarizable material, which is described by the dielectric function of the material. We used a grid of at least 30 × 30 × 30 dipoles per sphere. If not stated otherwise, the medium around the Au spheres is described by an effective refractive index n = 1.37 as a mean value between the refractive indices of water and NIPAM. This value was chosen as the particles can be considered to be mostly surrounded by pNIPAM and the coupling starts after the phase transition. Please note that refractive index influences have found to be small in this regime (see Results and Discussion section below). For the Au particle, the dielectric function provided by Johnson and Christy was used.30

spheres. For the former, monodisperse citrate-stabilized Au particles20 are attached on the surface of pNIPAM spheres via their functionalization with modified poly(ethylene oxide) (PEO)-amine based ligands.21 This flexible approach allows different nanoparticle coverage densities and is also valid for semiconductor nanoparticles,22 what would enable the tuning of the fluorescence of the semiconductor in systems combining both kinds of nanoparticles.23 Fluorescence quenching and heating effects have been observed by optical tweezers in individually trapped pNIPAM beads covered with metal and semiconductor nanoparticles.24 The second system is prepared by the direct synthesis of the Au nanoparticles within the pNIPAM microgel. The diameter of the particles can be tuned in order to control the average distance between the particles at a fixed number density. The combined hybrid Au-pNIPAM systems both show a new plasmon coupled mode above the pNIPAM’s phase transition, additionally to the well known shift and broadening of the fundamental plasmon peak. The new plasmon mode depends both on the density and the size of the nanoparticles and can be switched on and off simply by changing the temperature. We employ discrete dipole approximation (DDA) calculations to characterize this resonance as a strongly coupled quadrupole Au plasmon mode.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Au-pNIPAM Spheres. The pNIPAM spheres were produced following the work of Das et al.25 The procedure is described in ref 22. Au nanoparticles were synthesized in aqueous solution. Sodium citrate 2.2 mM (150 mL) was heated with a heating mantle for 15 min under vigorous stirring. After boiling, 1 mL of HAuCl4 (25 mM) was injected. The resulting nanoparticles (∼10 nm, 3 × 1012 NPs/mL) are coated with negatively charged citrate ions and hence well suspended in H2O. The ligands used for the conjugation with Au nanoparticles are amine-terminated poly(ethylene oxide) (PEO-NH2 branched) as those used and described in ref 22. For the conjugation, 100 μL of PEO-NH2 branched aqueous solution (1 mM) was added to 50 mL of citrate-stabilized Au nanoparticles dispersion and the mixture was stirred for 30 min at room temperature. The excess of PEO molecules was removed by precipitation (15 000 rpm, 10 min) and redissolution in water. For the immobilization of PEONH2 capped gold nanoparticles on the surface of pNIPAM beads, 0.25 mL of a suspension of pNIPAM beads containing 1.6 mg/mL was incubated at room temperature for 24 h under stirring with 8 mL of Au nanoparticles (3 × 1012 NPs/mL) in a final volume of 5 mL. The Au nanoparticles dispersion was washed twice by centrifugation at 15 000 rpm for 20 min and sonicated in double deionized water prior the incubation. After incubation, centrifugation at 5000−6000 rpm for 20 min and sonication cycles were performed to discard nanoparticles in the supernatant not anchored to the pNIPAM surface. 2.2. Synthesis of pNIPAM-Au Spheres. The synthesis of Au particles within the pNIPAM microgels consisted of a seeding and a growth steps of gold particles in the presence of the microgel. In the first step, small gold seed particles of 1−3 nm size were grown by reducing HAuCl4 with the strong reducing agent NaBH4. The number of seeds (and latter Au particles) per microgel is determined by the concentration ratio between seeds and pNIPAM microgel. The conditions during the seeding step are crucial for a successful growth step. In order to obtain spherical Au particles a certain amount of microgel particles loaded with gold seed particles was added to a growth solution containing additional HAuCl4, the mild reducing agent, ascorbic acid, and C16TAB. The size of the Au nanoparticles is controlled by the concentration ratio between microgel and HAuCl4. At least 95% of the seeds grew to particles. 2.3. Microscopical and Optical Characterization. Aqueous suspensions of the spheres were dried on carbon coated copper grids and inspected by transmission electron microscopy. The Au-pNIPAM

3. RESULTS AND DISCUSION Figure 1a displays a TEM micrograph of the Au-pNIPAM system; a TEM micrograph of the pNIPAM-Au system is displayed in Figure 1b. The Au nanoparticles are distributed statistically on the top of, or within, the pNIPAM spheres. For comparison different coverage (high and low) and different size, Au nanoparticles have been studied (additional TEM micrographs can be found in Figure S1 in the Supporting Information). The thermoresponsive properties of the pNIPAM-Au spheres are displayed in Figure 1c. For the Au-pNIPAM system, the thermoresponsive properties of bare pNIPAM and pNIPAM decorated with nanoparticles were published elsewhere.22 In both cases, the hydrodynamic radius changes by a factor of at least 2 between the swollen and the collapsed state of the pNIPAM sphere, leading to a decreasing distance between the particles, schematically displayed in Figure 1d. The slight shift in the lower critical solution temperature (LCST) of pNIPAM-Au systems from the pure pNIPAM is related to the synthetic procedure and the presence of the counterions of Au nanoparticles. UV−vis absorption spectra for increasing temperatures of highly covered AupNIPAM beads are displayed in Figure 2a. The temperature was increased from room temperature (RT) up to 64 °C. The spectra feature a constant background related to the pNIPAM absorption. Below the LCST, the plasmon of individual Au particles is reproduced. Heating the sample above LCST leads to a red-shift and broadening of the plasmon peak. At 40 °C, an additional peak appears at the long wavelength shoulder of the fundamental plasmon. This peak becomes more intense at temperatures above 50 °C. It increases in intensity and shifts to longer wavelengths upon further increasing the temperature. This behavior is completely reversible as displayed in the inset 8863


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Figure 1. (a) TEM micrograph of Au-pNIPAM spheres with high Au nanoparticle coverage. The scale bar corresponds to 500 nm. Please note that the particles are densely packed and completely collapsed in the micrograph and thus appear nonspherical, deformed. (b) TEM micrograph of pNIPAM-Au microgel with embedded 23 nm Au nanoparticles. The scale bar corresponds to 200 nm. (c) Hydrodynamic behavior of the pNIPAM-Au system. (d) Sketch of the swelling behavior (note that nanoparticles could be either on the surface or in the interior). The average distance between the Au particles decreases upon pNIPAM sphere shrinkage.

of Figure 2a as dashed-line spectrum. The spectra can be well described by two Gaussian functions for the Au related peak and a Gaussian function and a linear function for the pNIPAMrelated background (not shown). The Au related peak consists of the fundamental plasmon and a long-wavelength shoulder. Figure 2b displays the temperature dependence of the two components of the Au peak for the Au-pNIPAM beads. The intensity of the fundamental plasmon is almost constant, whereas the intensity of the shoulder increases significantly for temperatures above 50 °C. The fundamental plasmon shifts gradually by about 5 nm in the whole temperature range under investigation. In contrast, the additional peak shifts by 90 nm in the temperature range from 50 to 64 °C. The gradual and not sharp shrinkage shown for these beads explains the higher temperature range required for the maximum collapse of the bead.22 The absorption spectra below and above the LCST of pNIPAM-Au containing 40 nm Au nanoparticles are depicted in Figure 2c. The corresponding spectrum for the system containing 23 nm Au nanoparticles is displayed as inset. With increasing particle size the absorption maximum of the plasmon peak is red-shifted. With increasing temperature the absorption peaks are broadened regardless the particle size, and the maximum red shifts. Additionally, a shoulder appears similar to the one observed for the Au-pNIPAM system. The appearance

Figure 2. (a) Absorption spectra of highly covered Au-pNIPAM beads at different temperatures. Inset: Comparison of the initial spectrum with that obtained after cooling back to room temperature. (b) Fundamental (squares) and coupled plasmon (circles) modes evolution with temperature for Au-pNIPAM spheres highly covered. The lines are guides to the eye. (c) Absorption spectra of Au-pNIPAM spheres with embedded Au particles of 40 nm diameter at 25 °C, 45 °C, and after cooling back to 25 °C. Inset: Corresponding spectra for Au-pNIPAM spheres with 23 nm Au particles at 25 and 45 °C. The sketches on the upper-left side of the figures illustrate the decorated (a) and embedded (c) systems.

and shape of the additional peak depends on the particle size and the peak is more intense for the larger particles. As for the Au-pNIPAM system, the optical properties change reversibly when cooling the system down (dashed-lined spectra in Figure 2c). 8864


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to the tunable average interparticle distance decrease between the Au nanoparticles during the pNIPAM phase transition. As it can be observed, the experimental results agree well with the calculated spectra. For distances corresponding to the swollen state of the pNIPAM sphere, plasmon resonances are mostly decoupled. The spectra then consist of the Au plasmon peak of isolated particles. This peak remains at around 530 nm regardless of the distance between the nanoparticles and can be attributed to the transverse one.32 For decreasing distances, a second feature appears in the calculated spectra at longer wavelengths which manifests as an independent peak. These distances are reached when the pNIPAM platforms are in the collapsed state. Figure 3b shows the absolute values of the electric field of two Au nanoparticle pairs in pNIPAM, excited in resonance with the first peak. For distant particles (d = 7 nm), plasmons almost do not couple and the resonance is dipole-like. For short distances (d = 0.5 nm), the observed feature appears as a strongly coupled quadrupole contribution to the Au absorption. In the extreme case of direct contact, this is the only contribution to the spectra (not shown in Figure 3). The strength of this coupling strongly depends on the Au particle size, as depicted in Figure S5. This result reveals the way to optimize the plasmon coupling to a specific need. By choosing appropriate particle sizes, not only the wavelength of the fundamental plasmon resonance but also the coupling strength can be set. The latter relates with the particle density which determines the coupling strength with the temperature. Thus, for a given density of the selected particles, the switching of the coupled-plasmon mode is only limited for the time required to change and stabilize the temperature of the surrounding aqueous medium, which can be reached in a minute time scale.

The changes in the absorption spectra can be explained by two separate effects: The change in refractive index and the coupling of the plasmon resonances. From ellipsometry measurements of densely packed microgel monolayers it is known that the refractive index of the microgel changes slightly from 1.34 to 1.37 below and above LCST.31 This effect leads to a slight shift and broadening of the plasmon peak. Corresponding absorption spectra of low covered Au-pNIPAM beads and calculations can be found in Figure S2. The measurements ascertain the peak shift and broadening with no signs of additional features as observed in the highly covered Au-pNIPAM beads. The comparison of experiment and theory shows that, for low covered beads, the average interparticle distance is not short enough so as to boost plasmon coupling. This also demonstrates that the coupled mode is not promoted by refractive index changes. Using statistical distances obtained from the TEM and the hydrodynamic radius, the coupling can be analyzed by means of DDA calculations. Both effects, the change in refractive index of the pNIPAM microgel and the distance dependence of the nanoparticles, were included in the calculations. Figure 3a displays a series of calculated absorption

4. CONCLUSIONS In summary, we have shown that the combination of pNIPAM and Au nanoparticles allows the preparation of hybrid structures with a plasmon-coupling mode that can be controlled by simply setting the temperature. The coupling behavior can be fine tuned by setting the Au nanoparticle density and size during the synthesis. The experimental results are in excellent agreement with DDA calculations that confirm the coupling. The dynamic control of the Au plasmon within a moderate temperature regime enables quantitative investigations of the influence of size and interparticle distance on the plasmon coupling. Besides, the presented pNIPAM platforms allow for the combination with different shaped nanoparticles, opening interesting studies about the additional influence of this parameter in the coupling.

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Figure 3. (a) Calculated absorption spectra for Au nanoparticles of 10 nm diameter in pNIPAM as surrounding medium and different average interparticle distances (see Figure S3). (b) Absolute values of the electric field for an interparticle distance of 7 and 0.5 nm, excited at 537 nm. The k-vector of the incident light is orthogonal to the image plane and its electric field is polarized in the abscissa of the image plane (see Figure S4).

ASSOCIATED CONTENT

S Supporting Information *

TEM micrographs of Au-pNIPAM spheres with low Au nanoparticle coverage and pNIPAM-Au with embedded 40 nm Au. Absorption spectra of low covered Au-pNIPAM beads and calculations of the absorption spectra for pairs of nanoparticles at different nanoparticle distances. Dependency of the coupling mode with the particle size at a fix distance. This material is available free of charge via the Internet at http://pubs.acs.org.

spectra. The spectra were created from statistical distributions of pairs of Au nanoparticles with different interparticle distances (the underlying individual spectra can be found in Figure S3). The decrease in the average distance corresponds 8865


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(18) Contreras-Cáceres, R.; Pacifico, J.; Pastoriza-Santos, I.; PérezJuste, J.; Fernández-Barbero, A.; Liz-Marzán, L. M. Adv. Funct. Mater. 2009, 19, 3070−3076. (19) Á lvarez-Puebla, R.; Contreras-Cáceres, R.; Pastoriza-Santos, I.; Pérez-Juste, J.; Liz-Marzán, L. M. Angew. Chem., Int. Ed. 2009, 48, 138−143. (20) Bastus, N. G.; Comenge, J.; Puntes, V. Langmuir 2011, 27, 11098−11105. (21) Nikolic, M. S.; Krack, M.; Aleksandrovic, V.; Kornowski, A.; Förster, S.; Weller, H. Angew. Chem., Int. Ed. 2006, 45, 6577−6580. (22) Salcher, A.; Nikolic, M. S.; Casado, S.; Vélez, M.; Weller, H.; Juárez, B. H. J. Mater. Chem. 2010, 20, 1367−1374. (23) Ratchford, D.; Shafiei, F.; Kim, S.; Gray, S. K.; Li, X. Nano Lett. 2011, 11, 1049−1054. (24) Hormeno, S.; Bastus, N. G.; Pietsch, A.; Weller, H.; AriasGonzalez, J. R.; Juárez, B. H. Nano Lett. 2011, 11, 4742−4747. (25) Das, M.; Sanson, N.; Fava, D.; Kumacheva, E. Langmuir 2007, 23, 196−201. (26) Purcell, E. M.; Pennypacker, C. R. Astrophys. J. 1973, 186, 705− 714. (27) Draine, B. T.; Goodman, J. Astrophys. J. 1993, 405, 685−697. (28) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A 2008, 25, 2693− 2703. (29) Draine, B.; Flatau, P. J. User Guide for the Discrete-Dipole Approximation code DDSCAT 7.1, http://arxiv.org/abs/1002.1505, 2010 (30) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370−4379. (31) Schmidt, S.; Motschmann, H.; Hellweg, T.; von Klitzing, R. Polymer 2008, 49, 749−756. (32) Jensen, T.; Kelly, L.; Lazarides, A.; Schatz, G. C. J. Cluster Sci. 1999, 10, 295−317.

AUTHOR INFORMATION

Corresponding Author

*(B.H.J.) E-mail: [email protected]. Telephone: 0034 91 497 8600. Fax: 0034 01 497 6855. (H.L.) E-mail: [email protected]. (R.v.K.) E-mail: klitzing@ mailbox.tu-berlin.de. Present Address #

Institut Català de Nanotecnologia (ICN), Campus UAB, 08193 Bellaterra, Barcelona, Spain. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.L. acknowledges DFG (German Research Foundation) for financial support within an individual Grant Programme. N.G.B. thanks Generalitat de Catalunya for financial support by the Beatriu de Pinos program. B.H.J. thanks the European Commission for Grant ERG FP7-PEOPLE-ERG-2008 and the Spanish Ministry of Science and Innovation for RYC-200701709 and MAT2009-13488. M.R. acknowledges support from the DFG through SPP 1391. R.v.K. and A.K. acknowledge the DFG for financial support via the Cluster of Excellence UniCat and Sfb 951. Sören Selve from Zelmi at TU Berlin is acknowledged for the TEM images of the pNIPAM-Au system, Stephanie Reich at FU Berlin for granting access to the UV/vis spectrophotometer, and Friederike Ernst at FU Berlin for help with the measurements.

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REFERENCES

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Tunable Plasmon Coupling in Distance-Controlled Gold Nanoparticles

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