Salt-Induced Aggregation of Negatively Charged Gold Nanoparticles

Article pubs.acs.org/Macromolecules

Salt-Induced Aggregation of Negatively Charged Gold Nanoparticles Confined in a Polymer Brush Matrix Stephanie Christau,†,§ Tim Moeller,† Jan Genzer,§ Ralf Koehler,∥,⊥ and Regine von Klitzing*,‡,# †

Stranski Laboratory for Physical Chemistry, Technische Universitaet Berlin, Str. des 17. Juni 124, 10623 Berlin, Germany Department of Physics, Soft Matter at Interfaces, Technische Universitaet Darmstadt, Alarich-Weiss-Strasse 10, 64287 Darmstadt, Germany § Department of Chemical & Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695-7905, United States ∥ Institute of Soft Matter and Functional Materials (F-ISFM), Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany ⊥ Landesamt fuer Arbeitsschutz, Verbraucherschutz und Gesundheit, Muellroser Chaussee 50, 15236 Frankfurt (Oder), Germany # Joint Laboratory for Structural Research (JLSR) of Helmholtz-Zentrum Berlin fuer Materialien und Energie (HZB), Institut für Physik, Humboldt-University Berlin, Newtonstr. 15, 12489 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: We report on the salt-induced aggregation of citratecoated gold nanoparticles (AuNPs) confined within poly(Nisopropylacrylamide) (PNIPAM) brushes grafted from flat substrates. Compared to highly dispersed AuNPs, a red-shift and broadening of the surface plasmon (SP) band is observed when the AuNPs are confined by the PNIPAM brush matrix due to their close vicinity. Additional red-shifting and broadening occur upon immersion in aqueous salt solutions (1 M NaF, NaCl, NaBr, and KCl). Nanoparticle assemblies are established due to salt-induced aggregation of AuNPs and are dependent on the type of salt. In the presence of KCl, nanoparticle assemblies are built up that result in the formation a second plasmon peak at ∼700 nm. The color change of PNIPAM/AuNP is associated with (1) the collapse of the PNIPAM brushes in the presence of salt and (2) nanoparticle aggregation due to electrostatic screening of the negative charges around the AuNPs by the salt ions. Ion specificity is related to ion-pair association energies and adsorption behavior of ions at the AuNP surface. In addition, we perform a neutron reflectivity experiment to resolve the internal structure of swollen PNIPAM/AuNP hybrids and find that penetrated AuNPs cause PNIPAM chain stretching due to electrostatic repulsion between charged particles in the brush.

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INTRODUCTION Poly(N-isopropylacrylamide) (PNIPAM) is a thermoresponsive polymer that undergoes coil-to-globule transition in water above a lower critical solution temperature (LCST) of ∼32 °C. This tunable wettability is of great relevance for a variety of applications including drug delivery,1−3 analytical separation,4,5 antifouling surfaces,6−8 coating of membranes as permeationdriven filter devices,9 and the development of nanoreactors10,11 or chemical nanosensors.12,13 PNIPAM chains end-grafted to the surface form brushlike conformations when the distance between anchoring polymer chains is smaller than the coil size. Such polymer brushes can be used as matrices for the immobilization of nanosized objects such as gold nanoparticles (AuNPs).14−16 AuNPs show a distinct absorption peak in the UV/vis region of the electromagnetic spectrum due to the plasmon coupling of the particles when they are in close proximity to each other. By immobilizing these AuNPs into a brush, the color associated with the absorption of the nanoparticles will be introduced into © XXXX American Chemical Society

the optically transparent brush matrix. Polymer brush/particle hybrids are often discussed by means of their potential use as colorimetric sensors; using polymers that are responsive to external stimuli can trigger stimuli-induced shifting of the gold surface plasmon (SP) band. Thus, polymer brush/AuNP composites have been investigated in the past regarding their suitability for optical sensing using stimuli-responsive polymers as building blocks. Employing poly(2-vinylpyridine) (P2VP) brushes coated onto gold nanoislands as templates for AuNPs (∼12 nm), Tokareva et al. observed a reversible shift of the SP band upon changing the pH from 2 to 5.17 SP band red-shifting by 50 nm and peak broadening was observed in response to the pH increase. The same system was later studied by Roiter et al., who found the same SP band shift in response to pH but gave a more detailed explanation on plasmon analysis.18 They Received: April 27, 2017 Revised: August 24, 2017

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DOI: 10.1021/acs.macromol.7b00866 Macromolecules XXXX, XXX, XXX−XXX

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grafted brushes depended on the sodium sulfate concentration;47 whereas a featureless surface was obtained at low salt concentration, the AFM images revealed mushroom-like aggregates with a characteristic size of ∼100 nm at salt concentrations >0.11 M. Since the LCST of PNIPAM is affected by salt, it should be possible to induce a color change of PNIPAM/AuNP hybrids by salt without changing the temperature. In the present study, PNIPAM brushes are loaded with citrate-capped, negatively charged AuNPs of 14 nm diameter. The attachment of AuNPs to the PNIPAM brushes is facilitated by attractive hydrogen bonding interaction between the amide moiety in PNIPAM and the carboxylate groups of the AuNPs’ citrate coating. The citrate-coated AuNPs employed in this study are tunable in size and easy to synthesize, and they are dispersed in water, which is an important aspect regarding their handling in laboratories. The citrate-coated gold nanoparticles are stable in aqueous solution while they bear negative charges. Aggregation of AuNP-citrate can be induced by adding salt; aggregation phenomena may be useful for the development of optical devices because the color shift associated with the aggregation process can be observed by the naked eye. The presence of salt undermines the colloidal stability by (partially) screening the stabilizing negative charges around the particle, thus leading to irreversible particle aggregation and sedimentation.48,49 The presence of salt causes a disruption of the equilibrium between the electrostatic repulsion and van der Waals attraction and a breakdown of colloidal stability according to the DLVO theory. This is true for highly dispersed particles in solution but may not necessarily be true for particles immobilized into a polymer brush. The brush template can potentially stabilize the particles and prevent aggregation,15,22 and one may therefore suspect that the aggregation behavior upon addition of salt may be different compared to the colloidal particle solution. Previous work has shown that color variations of PNIPAM brush/AuNP can be induced by increasing the temperature above PNIPAM’s LCST,15,22 which results in variations of the interparticle distance due to shrinking of the grafted PNIPAM chains. However, it is unclear how the presence of salt affects the colorimetric properties of such hybrid materials. Salt is ubiquitous in nature, and therefore it is crucial to gain a better understanding of structure and optical properties of potential brush/particle sensors in the presence of salt. As mentioned above, salinity affects the LCST of PNIPAM, and it also affects the aggregation behavior of negatively charged AuNPs. In this work, we chose four different salts (NaF, NaCl, NaBr, and KCl) to tackle ion-specific effects on the salt-associated color change of PNIPAM/AuNP. Since any color change crucially depends on the initial assembly of AuNPs in the brush, we furthermore aimed to resolve the spatial structure of the PNIPAM/AuNP hybrid to prove whether the particles penetrate the brush. Neutron reflectivity is employed as a tool to investigate the inner structure of the pure brushes and the hybrids in a liquid environment. The salt-induced shrinking of PNIPAM brushes alone should result in a color change of the sample due to change of AuNP interparticle distance as the brush transitions from a swollen to a shrunken state. In addition, ion-specific effects can also be observed in AuNP aggregation. We find that this saltdependent AuNP aggregation results in the formation of interesting particle assemblies when the particles are confined

explained the large (50 nm) red-shift by the fact that P2VP brushes acted as responsive linkers between the gold nanoislands and the AuNPs. In the hybrid, the SP band position is a result of individual contributions of the AuNPs, the Au islands, AuNP clusters, and AuNP−Au island coupling. While this results in larger red-shifting of the SP band, the presence of gold nanoislands is not necessary to induce SP band shifting. In brush/AuNP hybrids, variations of the interparticle distance trigger SP band shifts; such changes of AuNP proximity are induced by brush shrinking/swelling in response to outer stimuli. For example, a red-shift and broadening of the SP band of 12 nm AuNPs embedded into poly(N,N′-dimethylaminoethyl methacrylate) (PDMAEMA) brushes were observed after the pH was increased from 5 to 9.19 The SP band shift can be attributed to a decrease in AuNP proximity at higher pH values due to collapse of the PDMAEMA chains. Polymers also respond to variations in solvent quality. For example, as shown by Gupta et al., a blue-shift of PS brush immobilized AuNPs could be induced by changing the solvent from air (a bad solvent for PS) to toluene (a good solvent for PS).20 The reason behind the blue-shift is an increase in particle proximity due to swelling of PS chains in toluene. The polymer used in the present study, PNIPAM, is temperature-responsive; it exhibits an LCST in aqueous solution at ∼32 °C, above which it collapses. Various studies have employed PNIPAM brush/nanoparticle hybrids for thermoresponsive sensing. Raising the temperature above the polymer’s LCST causes a red-shift and broadening of the SP band and was observed for both larger (40 nm) AuNPs21 and smaller (5, 13 nm) AuNPs.14,22 The reason behind the SP band shift is a decrease in AuNP−AuNP proximity as the PNIPAM chains collapse at higher temperatures. The presence of salt affects the LCST of PNIPAM and leads to precipitation of the polymer chains at temperatures below ∼32 °C.9,22−35 The LCST of PNIPAM can be precisely controlled by different salts and follows the Hofmeister series,36

Figure 1. Hofmeister series ranks of ions with strongly hydrated ions (kosmotropes) on the left and weakly hydrated ions (chaotropes) on the right.

which ranks the relative influence of ions on the physical behavior of a wide variety of aqueous processes.37 The series follows with strongly hydrated ions (kosmotropes) on the left to weakly hydrated ions (chaotropes) on the right. A variety of processes like protein folding and crystallization or enzymatic activity follow this general trend.23,37−45 It has been suspected that ions are capable of changing the bulk structure of water; however, nowadays it is known that ions cause negligible changes to the bulk structure of water.29,37,46 In previous studies, the effect of NaCl concentration on the thermoinduced collapse of grafted PNIPAM brush layers was evaluated by quartz crystal microbalance with dissipation (QCM-D).33 It was found that the LCST decreased with increasing salt concentration. Contrary to bulk PNIPAM chains,25,27,28 the decrease in LCST followed a nonlinear dependence with respect to the NaCl concentration. Atomic force microscopy (AFM) images of PNIPAM brushes with high molecular weight and low grafting density showed that the morphology of the B


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(HZB) at the instrument V6 (Berlin, Germany).54 A home-built liquid cell was used for measurements in deuterated water. A complex fitting model was employed to analyze the NR data; the fitting procedure is described in the Results section. Scanning (Atomic) Force Microscopy (AFM). AFM images were recorded with a CypherAFM (Asylum Research, USA) using AC160TS (Olympus, Japan) cantilevers at scan rates between 1 and 2.44 Hz. Scanning Electron Microscopy (SEM). SEM images were obtained using a Zeiss DSM 982 GEMINI at the electron facility Zentrum fuer Elektronenmikroskopie (ZELMI) located at the TU Berlin. The SEM was operated at acceleration voltages of 10 kV. Transmission Electron Microscopy (TEM). Samples were prepared using 5 μL of AuNP solution on TEM copper grids with carbon support film (200 mesh, Science Services, Munich, Germany; pretreated using 10 s of a glow discharge). Samples were allowed drying; then they were inserted into the sample holder (EM21010, JEOL GmbH, Eching, Germany) and transferred to the JEOL JEM 2100 (JEOL GmbH, Eching, Germany). TEM images were taken using an acceleration voltage of 200 kV and recorded using a bottommounted 4k × 4k CMOS camera system (TemCam-F416, TVIPS, Gauting, Germany). TEM images were processed with a digital imaging processing system (EM-Menu4.0, TVIPS, Gauting, Germany).

in the PNIPAM brush template and discuss the salt-associated color variations.

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Synthesis Procedures. Materials. N-Isopropylacrylamide (NIPAM), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), CuCl, gold chloride hydrate (HAuCl4), methanol (MeOH), and ethanol (EtOH) were purchased from Sigma-Aldrich and used without further purification. PNIPAM Brushes. Before synthesis, the substrates were etched using piranha solution (H2SO4/H2O2, 1:1 vol %). A self-assembled monolayer (SAM) of 2-bromo-2-methyl-N-(3-(triethoxysilyl)propyl)propanamide (BTPAm) was prepared by incubating the cleaned substrate into a BTPAm solution in anhydrous toluene (4 μL of BTPAm/10 mL of toluene) for ∼20 h. The synthesis of BTPAm is described elsewhere.50 The BTPAm-coated substrates were then sonicated in toluene for 15 min, rinsed with EtOH, and dried under a stream of nitrogen. The synthesis of PNIPAM brushes was slightly modified from the literature procedure.51 2.0 g of NIPAM monomer was dissolved in 17.5 mL of MeOH and 17.5 mL of ultrapure water (Milli-Q, ≥19 MΩ cm), and the mixture was stirred (500−600 rpm) under rigorous nitrogen bubbling for 30 min. Then, 150 μL of the ligand PMDETA and 0.0195 g of CuCl were added at once. After stirring for another 30 min under nitrogen bubbling, the BTPAmcoated substrates were added, and the reaction was carried out for 8 min in a nitrogen atmosphere. Then the samples were removed quickly, sonicated for ∼5 min in ultrapure water, cleaned with ultrapure water, and dried under a stream of nitrogen. The polymer brushes grown by this method typically exhibit grafting densities of ∼0.5 chains/nm.15,52 Gold Nanoparticles. Citrate-stabilized AuNPs were prepared following a procedure as first introduced by Enustun and Turkevich.53 Briefly, 100 mL of a HAuCl4 solution (5 × 10−4 M) was heated (external temperature ∼220 °C) under stirring at 500 rpm. At the time the HAuCl4 solution started boiling, 5 mL of a 1 wt % sodium citrate solution was added quickly under continuous stirring at 500 rpm. After 3 min, the solution color changed to red. At this time, the external temperature was reduced to 190 °C and the speed was reduced to 150 rpm. After 17 min, the heat was turned off, and the solution was stirred at 150 rpm overnight. The concentration of AuNPs in the final solution is 2.88 × 1012 particles/mL, or 4.79 nM (see Supporting Information for calculation of the AuNP concentration). The diameter of particles was found to be ∼14 nm from TEM images (182 AuNPs were measured, and the AuNP diameter was determined using a Gauss fit; see Supporting Information for images and size distribution graph). PNIPAM/AuNP Hybrids. The AuNPs were attached by incubating the PNIPAM brush samples into the native gold particle solution for ∼1−2 h. After incubation, the samples were removed, rinsed with Milli-Q water, sonicated in Milli-Q water for ∼5 min, and cleaned with Milli-Q water. Instruments. UV/Vis Spectroscopy. UV/vis absorbance spectra were recorded with a UV/vis spectrophotometer (Cary 50, Varian) at 20 °C using disposable PMMA cuvettes (Brand, Germany). In the case of PNIPAM/AuNP, the coated glass substrate was placed into the PMMA cuvette. Ellipsometry. A polarizer−compensator−sample analyzer (PCSA) ellipsometer (Optrel GbR, Sinzing, Germany) was used for all thickness measurements. The ellipsometer uses a wavelength of 632.8 nm, and the angle of incidence was set to 70° for measurements at ambient conditions and 60° for measurements in water/salt. Ellipsometric data (Delta and Psi) were fitted using the software Ellipsometry: simulation and data evaluation (Optrel, v 3.1). To fit PNIPAM brush thickness and refractive index, we employed a twolayer model with silicon as backing (continuum, n = 3.885−0.180i), water/salt (n = 1.33) or air (n = 1) as fronting (continuum), silicon oxide as the first layer (n = 1.46), and the PNIPAM brushes as the second layer. Neutron Reflectivity (NR). Neutron reflectivity measurements were carried out at the beamtime facility of the Helmholtz-Zentrum Berlin

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RESULTS Ion-Specific Effects on PNIPAM Brush Shrinking. PNIPAM brush thickness in the presence of 1 M NaF, NaCl, NaBr, and KCl was measured using ellipsometry at room temperature. For all studied salts, a decrease in brush thickness compared to PNIPAM brush thickness in water was observed. We can calculate the salt-induced shrinking of PNIPAM brushes using the relationship (hwater − hsalt)/hwater, where hwater is PNIPAM brush thickness in water (∼100 nm) and hsalt is PNIPAM brush thickness in aqueous solutions containing 1 M of the respective salt. The results are displayed in Figure 2 and

Figure 2. PNIPAM brushes in the presence of 1 M KCl, NaF, NaCl, and NaBr: (A) AFM height images and (B) shrinking of PNIPAM brushes, calculated with respect to PNIPAM thickness in water (see text for further information), as measured by ellipsometry. Error bars stem from measurements on five different locations of the wafer. Note that both AFM and ellipsometry measurements were carried out at room temperature. C


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Figure 3. Left: photographs of AuNP suspensions (concentration = 4.79 nM) in the presence of NaF, NaCl, NaBr, and KCl of varying salt ionic strength (first row: 0.01 M; second row: 0.05 M; third row: 0.07 M; fourth row: 0.1 M). Right: UV/vis absorbance spectra of AuNP suspensions in the presence of NaF, NaCl, NaBr, and KCl for a salt concentration of (a) 0.07 M and (b) 0.1 M. The spectra were taken immediately after addition of one volume of salt to one volume of AuNP suspension. Time-resolved UV/vis spectra of AuNPs in 0.07 M (c) NaCl and (d) NaBr. The aggregation is more pronounced and occurs faster in NaCl. Note that the AuNP suspension (4.79 nM) and the respective aqueous salt solutions were mixed at 1:1 vol % ratio.

formation of sedimenting particle aggregates. 56 AuNP aggregation evolves over time and can be followed using UV/ vis spectroscopy: As shown in Figure 3c and 3d for 0.07 M NaCl and NaBr, the SP band increasingly red-shifts and broadens over a short time frame. The aggregation occurs faster in NaCl, as indicated by the observed flattening of the SP peak after 10 min. Because of the strong time dependence at short time scales, UV/vis spectra and photographs of the AuNP solutions were also recorded after 4 days (for UV/vis spectra see Supporting Information). For all studied salts, the AuNP aggregation had progressed over the time frame of 4 days. Except for NaF, the color had vanished due to the formation of large aggregates that precipitated from the solution. To summarize, UV/vis spectra and visual observation indicate that AuNP aggregation increases in the order NaF < NaBr < NaCl < KCl. Internal Structure of PNIPAM/AuNP. Incorporation of AuNPs. SEM imaging confirms the successful attachment of citrate-coated AuNPs to the PNIPAM brushes and homogeneous particle distribution (Figure 4A). The induction of plasmonic properties into the transparent PNIPAM brushes is verified by the presence of the particle-related surface plasmon peak. Compared to AuNPs dispersed in water, the immobilization of the particles into the PNIPAM brush template causes a slight red-shift of ∼8 nm and broadening of the SP band (Figure 4B). SEM images show successful attachment of AuNPs. However, SEM is recorded in dry state at high vacuum, where the brush structure and particle assembly are different compared to the swollen state. Thus, SEM cannot reveal any information about the swollen structure of the hybrid material. For our study it is crucial to know whether the particles penetrate into the PNIPAM brushes when they are swollen because the particles are attached by incubating the PNIPAM brush samples into a dispersion of AuNPs in water. In general, knowledge about the swollen structure is of interest for

show that the magnitude of PNIPAM brush shrinking decreases in the order KCl > NaF > NaCl > NaBr. While the differences of KCl, NaF, and NaCl-induced brush shrinking are negligible (41% NaF, 40% NaCl, and 42% KCl), the PNIPAM brushes shrink noticeable less in NaBr (16%). Furthermore, we found that salt-induced shrinking was reversible for all studied salts (for ellipsometry data, see Supporting Information). In addition, we carried out AFM measurements of PNIPAM brushes in 1 M salts (Figure 2). As the images show, there is no detectable difference in brush topography related to PNIPAM brush shrinking in the different salts. Ion-Specific Effects. AuNP Suspensions. Citrate-coated AuNPs undergo a visible color change from red to blue upon addition of salt due to electrostatic screening of the negative charges by the salt cations.55,56 The color change is caused by a red-shift of the gold SP band and is mainly attributed to the formation of aggregates in the presence of salt. As the photographs in Figure 3 show, the magnitude of this red-shift depends on the salt concentration and the type of salt. No visible change of color occurs after addition of 0.01 M NaF, NaCl, NaBr, or KCl to the AuNP suspension (salt/AuNP solution, 50:50 vol %). Increasing the salt concentration from 0.01 to 0.1 M results in a color change from red to purple and blue, depending on the type of salt. NaF shows the weakest interaction with the AuNPs; even for the highest studied salt concentration of 0.1 M, the color only changes from light red to dark red. The strongest color change occurs in the presence of KCl; as the salt concentration increases, a color change from red to purple, blue, and gray is observed. In general, the saltassociated color change increases according to NaF < NaBr < NaCl < KCl and thus does not follow any Hofmeister trend. UV/vis absorbance spectra corresponding to the photographs show that the presence of the salt causes a red-shift and broadening of the SP band (Figure 3a,b). In some cases, the SP bands adapt a flattened shape (0.07 M KCl and 0.1 M NaF, NaCl, and NaBr). Strong peak broadening and parallel dropping of the SP band intensity are an indication for the D


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Figure 5. (A) Polymer brush internal structure is modeled by a layer stack consisting of layers having the same constant thickness but varying scattering length density (SLD) increasing in the z direction perpendicular to the substrate. (B) Neutron reflectivity data (symbols), fits (solid lines), and SLD profiles (inset) of PNIPAM and PNIPAM/AuNP at 20 °C in D2O.

As shown in Figure 5, the reflectivity curves of swollen PNIPAM brushes at 20 °C do not exhibit well-pronounced Kiessig fringes. The data were fitted using the steplike model. The resulting scattering length density (SLD) profiles reveal a bilayer profile with a more concentrated layer next to the substrate followed by a large dilute region where the SLD increases continuously in the z-direction. Hints for Kiessig fringes are observed in the low-q regime (q < 0.5 Å−1) after incorporation of AuNPs. The SLD profile shows a distinct decrease of the overall SLD due to the presence of AuNPs (SLDAu ∼ 4 × 10−4 nm−2) and an increase in layer thickness compared to the PNIPAM brush before AuNP attachment. Ion-Specific Effects on Structure and Colorimetric Properties of PNIPAM/AuNP. Color Change of PNIPAM/ AuNP. Comparing the UV/vis spectra of PNIPAM/AuNP in water and salt, the plasmon bands are red-shifted and broadened for all studied salts (Figure 6). This red-shift and broadening are found to be irreversible for all salts except NaF. During sample incubation in KCl, NaCl, and NaBr, a shoulder appears at higher wavelengths. In KCl, the shoulder progressively shifted to higher wavelengths over a time frame of 5 min (Figure 7) and finally transitioned into a second plasmon peak, which is distinctive for the formation of chainlike nanoparticle assemblies.60−62 However, those particle or particle-aggregate chains cannot be preserved upon reincubation in water (the second plasmon band vanishes). The formation of the red-shifted shoulder in NaCl and NaBr indicates that formation of chainlike assemblies or interconnected particle networks could also play a role in NaCl and NaBr. Interestingly, the structures formed in NaBr can be preserved upon reswelling in water; in NaCl, the shoulder vanishes after reimmersion in water. Similar to the behavior of AuNPs dispersed in water in the presence of salt, the SP band flattens and increases in intensity at higher wavelength, indicating the formation of larger irregular particle aggregates.

Figure 4. Successful AuNP attachment to PNIPAM brushes was confirmed by SEM (A); the AuNPs induce plasmonic properties as observable by a color change of the sample from transparent to purple (photos, inset). Compared to AuNPs dispersed in water, attachment of the AuNPs to the PNIPAM brush causes a slight red-shift (∼8 nm, inset) and broadening of the plasmon resonance peak (B).

applications of such hybrid materials, i.e., for use as optical sensors that work in liquid environment. Neutron Reflectivity of PNIPAM/AuNP. Neutron reflectivity, an optimal tool to investigate soft materials in liquid environment, was carried out at the reflectometer V6 (Helmholtz-Zentrum Berlin, Germany) at 20 °C to study the particle distribution in the swollen brush. In the past, neutron reflectivity (NR) was used by Yim and co-workers to study the response of surface-attached PNIPAM chains24 upon changing the temperature for varying chain length and brush grafting density.57 More recent work employed NR to investigate the effect of temperature on PNIPAM brush structure in the presence of acetate and thiocyanate ions.58 Our group has previously employed NR to investigate the structural properties of surface-grafted PNIPAM brush with immobilized AuNPs of varying hydrophobicity.15 Reflectivity data were fitted using Motofit,59 a package of IgorPro (Wavemetrics, Inc., Portland, OR). To account for the polydispersity of the brushes, we employed a steplike fitting model15 (Figure 5). The model consists of silicon (SLD = 2.07 × 10−4 nm−2) and D2O (SLD = 6.36 × 10−4 nm−2) as continuum media and SiOx (1.5 nm, SLD = 3.47 × 10−4 nm−2) as the first layer followed by a layer with varying thickness and SLD (base layer in Figure 5). In this model, the polydispersity of the brush is modeled by a layer stack consisting of individual layers with constant thickness, varying SLD, and zero interlayer roughness. E


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Figure 6. UV/vis spectra of PNIPAM/AuNP in water (blue), 1 M KCl, NaF, NaCl, or NaBr (red) and reincubated in water after salt (green). Note that UV/vis spectra were recorded after ∼5 min of incubation in the respective salt solutions or water.

Figure 7. Formation of the second plasmon band in KCl occurs over a short time frame (∼5 min). The intensity of the ∼520 nm band gradually decreases, while the intensity of the second peak progressively red-shifts and increases in intensity.

Figure 8. AFM height (top) and phase (bottom) images of PNIPAM brushes (left) and PNIPAM/AuNP hybrids (right) at ambient conditions.

AFM Measurements of PNIPAM/AuNP. To investigate the assembly of AuNPs in the brush matrix after incubation in salt, AFM imaging (ambient conditions) was employed. Comparing surface-grafted PNIPAM brushes before and after AuNP attachment (Figure 8), the particles are clearly visible in both AFM height and phase images. In the AFM phase image, the particles appear as black dots surrounded by the polymer, which suggests that those particles are located on the brush surface; particles that are embedded within the brush matrix may not be visible in the AFM images. Next, we incubated PNIPAM/AuNP in the 1 M salts for a few hours; AFM images were taken at ambient conditions after the samples were rinsed with water and dried under a stream of nitrogen. As shown in Figure 9, the presence of salt affected the assembly of AuNPs in the PNIPAM brush. From AFM height images, it appears that the particles are homogeneously distributed on the brush

surface after incubation in NaF, whereas KCl, NaCl, and NaBr cause stronger particle aggregation. This seems to correlate with the salt-induced aggregation of AuNPs in suspension, which appeared to be least pronounced in NaF. The AFM phase images reveal further information: The AuNPs seem to be excluded from the brush after incubation in KCl, NaF, and NaClin the phase images, the AuNPs appear as black dots surrounded by the polymer, suggesting that the particles are displayed on the brush surface similar to PNIPAM/AuNP before incubation in salt. This is not the case after incubation of PNIPAM/AuNP in NaBr; although the particle aggregates are clearly visible, the phase image suggests that those particles are covered by the polymer. This seems to correlate with the saltF


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Figure 9. AFM height and phase images of PNIPAM/AuNP at ambient conditions after the samples were incubated in 1 M salt for several hours and rinsed with water.

adsorb at the AuNP surface in the order F− < Cl− < Br−, which means that Br− can increase the negative charge of the AuNP more substantially than Cl− and F−. Thus, the suspensions are more stable in NaBr compared to NaCl. We can conclude that these two counteracting effects result in a minimum in dispersion stability for NaCl. There is also a cation effect, which results in stronger aggregation in KCl compared to NaCl. The smaller size of the hydrated K+ ion and its smaller hydration shell results in strong interaction with the citrate anions and substantial screening of the negative charges around the particle by K+. In addition, ionpair association strength between Cl− and K+ is lower than for Na+ (lattice energies are 701 kJ/mol for KCl and 769 kJ/mol for NaCl).64 PNIPAM/AuNP Hybrids. As discussed previously, PNIPAM brushes shrank according to NaBr < NaCl < NaF < KCl, and AuNPs aggregated following NaF < NaBr < NaCl < KCl. Thus, the pure compounds follow different trends, which raises the question of whether the ion-specific effects of the composite system are dominated by brush shrinking, or by nanoparticle aggregation. The internal swollen structure of PNIPAM/AuNP was resolved using neutron reflectometry. When comparing the SLD profiles of pure PNIPAM brushes with the SLD profiles of PNIPAM/AuNP, we found that the overall SLD decreased after immobilization of the AuNPs because the SLD of the AuNPs is lower than the SLD of D2O. In addition, the thickness of the polymer brush layer increased after AuNP immobilization. This can be explained by (1) a volume effect, (2) increase of water content in the hybrid due to hydrophilicity of immobilized AuNPs, (3) increase of osmotic pressure due to immobilized AuNPs, or (4) stretching of polymer chains due to electrostatic repulsion of negatively charged AuNPs. The SLD profile of PNIPAM/AuNP indicates the presence of a rather uniform layer, suggesting a homogeneous distribution of the AuNPs inside the PNIPAM brush. As apparent from the SLD profile, PNIPAM brushes exhibit a certain degree of polydispersity leading to a region of higher polymer density close to the substrate and a larger dilute region toward surrounding D2O. We believe that this variation of density is results from the graf ting f rom approach employed to synthesize the brushes: Initially, ATRP initiator molecules cover the substrate homogeneously with each ATRP initiator molecule providing a reaction site for the NIPAM monomer to grow into PNIPAM

induced PNIPAM brush shrinking, which was similar in KCl, NaF, and NaCl (40%) but remarkably less pronounced in NaBr (16%).

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DISCUSSION PNIPAM Brushes. The presence of salt causes a decrease in PNIPAM brush thickness compared to the thickness in water. More specifically, PNIPAM brush thickness decreased in the order KCl (42% shrinking) > NaF (41%) > NaCl (40%) > NaBr (16%). The reason for this behavior is the reduction of PNIPAM’s LCST by the salt, resulting in shrinking of the polymer chains at lower temperatures. As initially described by Zhang and Cremer,30 salt affects PNIPAM’s LCST by means of three different interactions of PNIPAM with water molecules: (1) destabilization of the amide−water hydrogen-bonding interaction through polarization by the anion; (2) modulation of hydrophobic hydration interaction; (3) direct binding of anion to amide. We refer to the literature for a more detailed discussion on the ion specificity of this behavior.29,30,43,63 Ellipsometry measurements carried out at room temperature showed that the PNIPAM brushes collapse by ∼40% in NaF, NaCl, and KCl and by ∼16% in NaBr, indicating that the brushes are only partially collapsed in NaBr. Our findings are in agreement with previous studies of free linear PNIPAM, which showed a decrease of PNIPAM’s LCST from 32 °C to temperatures below room temperature in KCl, NaF, and NaCl, whereas NaBr only caused an LCST decrease to ∼26 °C.30,31 Gold Nanoparticles. The addition of salt causes a color change of citrate-coated AuNPs from reddish to blue (which corresponds to a red-shift of the gold SP band). This color change is caused by aggregation of the AuNPs and increases in magnitude following NaF < NaBr < NaCl < KCl. This nonsystematic behavior indicates two counteracting anion effects: Because the electronegativity decreases from F− to Br−, the ion pair association strength decreases in the same order. This is indicated by the lattice energies, which are 910 kJ/mol for NaF, 769 kJ/mol for NaCl, and 732 kJ/mol for NaBr.64 Decreasing ion-pair association leads to stronger interaction of Na+ ions with the citrate anions at the AuNP surface. The electrostatic charge screening by Na+ therefore increases with NaF < NaCl < NaBr, which explains why the suspensions are more stable in NaF compared to NaCl. On the other hand, there is a counteracting effect arising from the tendency of the ions to stay in water. The anions preferably G


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Figure 10. Schematic illustration of salt-specific behavior in PNIPAM brushes and PNIPAM brush/AuNP composites: (A) The presence of salt (1 M) results in shrinking of PNIPAM brushes (∼40%: KCl, NaCl, NaF; ∼16%: NaBr). (B) AuNP aggregation increases with NaF > NaBr > NaCl > KCl and results in SP band variations, as shown by (C) UV/vis spectra of PNIPAM/AuNP in the respective salts. Shrinking of the brushes forces the AuNPs to get closer (indicated by the arrows in part B) and results in a red-shift of the SP band. Formation of AuNP clusters contribute to the UV/ vis spectra for NaBr, NaCl, and KCl (contributions to the SP band are indicated by the black arrows in part C). In addition to the SP band of individual AuNPs (∼520 nm), a red-shifted shoulder appears as result of longitudinal plasmon adsorption; in KCl, the shoulder progresses into a second plasmon band, which indicates the formation of larger particle chains or interconnected networks.

showed the weakest interaction with the particles. KCl, NaCl, and NaBr caused stronger particle aggregation of AuNPs, both in suspension and attached to the brush. Furthermore, the color change of PNIPAM/AuNP associated with KCl, NaCl, and NaBr was found to be irreversible. The salt-induced SP band shift of AuNPs in PNIPAM brushes is a result of (1) shrinking of PNIPAM brushes and (2) salt-induced AuNP aggregation. Shrinking of PNIPAM brushes can affect the gold SP band in two ways: It causes a decrease of the local refractive index and a decrease of the AuNP interparticle distance; both lead to red-shift, broadening, and intensity increase of the SP band, as observed in NaF. Since we did not observe significant intensity increase of pure PNIPAM brushes after inducing brush shrinking in the UV/vis spectra in our previous studies,15 we believe that the decrease of AuNP proximity in the shrunken state dominates the color change. In all other salts but NaF, irreversible AuNP aggregation plays a major role in shaping the SP band shift. In NaCl and NaBr, the SP band was red-shifted with a distinctive shoulder appearing at higher wavelengths. In KCl, we observed the formation of a second SP band at longer wavelengths (∼700 nm). The occurrence of two well-separated plasmon peaks is an indication for nanoparticles forming linear assemblies (chains) and aggregating into fractal structures.65,66 The transversal peak at ∼520 nm arises from excitation along the short axis, and the longitudinal one at ∼700 nm arises from excitation along the long axis. This phenomenon has been described previously where aggregation of citrate-coated AuNPs was induced by pyridine; the particles formed short chains of a few particles before growing into complex patterns.60 Chainlike nanoparticle assemblies were observed when 2-mercaptoethanol (MEA) was added to citrate-coated AuNPs; replacement of the citrate ligands with MEA molecules resulted in the formation of discrete chains, bifurcated and looped chains, or interconnected chain networks, depending on the MEA/AuNP molar ratio.61

chains. However, the polymer chains do not all grow at the exact same rate (polymers are never perfectly monodisperse). Some of the PNIPAM chains grow faster (become longer) than others and impose a barrier for the monomer to reach the shorter polymer chains. Furthermore, as the brushes grow, they occupy more space (they are flexible and form coils rather than being stretched), thus hindering the growth of neighboring chains. This leads to a structure with a denser structure close to the substrate. It is unrealistic that the AuNP penetrate the brush all the way to the substrate, and we believe that the AuNPs only penetrate the more dilute outer region. The confinement of AuNPs in the brush caused a slight redshift (∼8 nm) of the SP band compared to highly dispersed AuNPs: Gold nanoparticles in suspension can be effectively considered as single particles; according to the Mie theory, they exhibit a well-defined, single plasmon resonance peak. The absorption of a single nanosphere is caused by a dipole induced by the external electric field. Confining the gold particles within a brush matrix leads to a decrease in interparticle distance. If the particles are close enough, the individual particles start to feel the electric field of neighboring particles, which leads to a collective plasmon oscillation resulting from the superposition of the external incident field and the dipole fields of all other nanospheres and causes red-shift and broadening of the SP band. The salt-induced SP band shift for AuNPs in suspension is a result of aggregates that are formed in the presence of the different salts. Particle aggregation occurs fast and ultimately leads to clearance of the AuNP dispersion due to precipitation of larger particle clusters. Comparing the salt-induced color change (SP band shift) of AuNPs in suspension and AuNPs in PNIPAM brushes, similarities and differences were observed. In the case of NaF, the AuNPs were rather homogeneously distributed on the brush surface, and the salt-induced color change was found to be reversible. This corresponds to the result we obtained for AuNPs in suspension, where NaF H


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Macromolecules

into a stretched conformation as this will undermine the repulsion forces between negatively charged particles. Upon immersion of PNIPAM/AuNP in different salts (KCl, NaF, NaBr, and NaCl), we observed a salt-dependent color change of the hybrids due to the formation of salt-induced nanoparticle assemblies. The presence of salt affects the PNIPAM/AuNP hybrids in three ways: It causes (1) shrinking of the PNIPAM brushes, (2) screening of the negative charges around the AuNPs, which results in particle aggregation, and (3) ion-specific effects. The confinement of AuNPs within the brush matrix leads to the development of interesting particle structures; in KCl, AuNP assemblies are built up that resulted in the formation of a second well-defined plasmon band at ∼700 nm. While NaF, NaCl, and KCl caused similar PNIPAM brush shrinking (∼40% compared to the thickness in water), saltinduced AuNP aggregation was strongly salt-dependent and increased in the order NaF < NaCl < KCl. This correlates with the behavior observed for PNIPAM/AuNP. Thus, we conclude that nanoparticle aggregation seems to be the dominating factor for the salt-induced color change of PNIPAM/AuNP in those salts. Strong nanoparticle aggregation, caused by two counteracting ion-specific effects (namely, ion pairing and ion adsorption onto the AuNP surface), also leads to irreversibility of the salt-induced color change in NaCl, NaBr, and KCl. In NaF (weak nanoparticle aggregation), brush shrinking dominates the color change, which is reversible. Brush shrinking is less pronounced in NaBr (16%), which leads to lower mobility of the AuNPs and a stronger reversibility of the color change. In general, the color change associated with nanoparticle assemblies formed due to presence of different salts is a complex interplay between brush- and particle-related properties. For nanotechnological applications of such hybrid materials it will be important to be able to control these properties.

Upon immersion of the samples in NaCl and NaBr we observed the formation of a shoulder that is red-shifted with respect to the ∼520 nm gold plasmon band. Thus, shorter chainlike assemblies or interconnected networks may also play a role in NaCl and NaBr, although the shoulder is remarkably less pronounced than the second plasmon peak in KCl. Interestingly, the nanoparticle assemblies formed in NaBr could be preserved upon reswelling the sample in water, which was the case in neither NaCl nor KCl. This is also supported by the AFM images that were taken af ter incubation in salt at ambient conditions images (Figure 9). We assume that the reason for this behavior originates for the difference in brush shrinking (40% in KCl/NaCl and 16% in NaBr). Brush shrinking seems to correlate with AuNP exclusion from the brush; the AFM images show that the AuNPs were excluded from the brush and located at the brush surface after incubation in KCl, NaCl, and NaF (shrinking ∼40%). In contrast to that, AFM images reveal a different structure after incubation in NaBr (shrinking ∼16%); the partially swollen structure in NaBr seems to render the AuNPs inside the brush. Thus, AuNP assemblies formed in NaBr are stabilized by the brush and preserved when reimmersed in water (see Figure 9). This brush stabilization is missing in NaCl and KCl because AuNPs are excluded from the interior of the brush and facilitates further aggregation of particles leading to bigger clusters (as apparent in the AFM images). Figure 10 summarizes the interplay between brush shrinking and AuNP aggregation schematically: PNIPAM brushes shrink to about the same extent in KCl, NaCl, and NaF. As the brushes shrink, the proximity of the AuNPs decreases (indicated by white arrows in Figure 10B), and at the same time, the presence of salt causes irreversible AuNP aggregation in KCl and NaCl but not NaF. For PNIPAM/AuNP in water and NaF, the spectra consist of a single plasmon peak, which is the SP band of the individual AuNPs (indicated by black arrow in Figure 10C); the position of the SP band depends on the proximity of the AuNPs to each other and is red-shifted in NaF. In all other salts, the final spectrum is shaped by the superposition of the individual AuNP SP band (520 nm) and the SP bands of different-size AuNP clusters or interconnected AuNPs (>520 nm) (indicated by the black arrows in Figure 10C). In summary, our results show that the color change influenced by different types of salt is a result of the complex interplay between brush shrinking and particle aggregation. Future studies are planned regarding variations of the brush grafting density. Our previous work showed that the uptake of AuNPs is highly dependent on the grafting density.67 Assuming that the grafting density of PNIPAM brushes in this work is ∼0.5 chains/nm,15,52 it would be interesting to investigate structural properties and salt-induced color variations of hybrids at lower PNIPAM grafting densities. Furthermore, measurements of the particle dynamics and simulations could give more insight into the mechanism behind the formation of specific aggregates.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00866. TEM images of AuNPs and AuNP size distribution, ellipsometry data of PNIPAM brushes in water and salt, UV/vis spectra of AuNP suspension after 4 days in salt, calculation of AuNP dispersion concentration (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +49 6151 16-25647 (R.v.K.). ORCID

Stephanie Christau: 0000-0003-1152-5606 Jan Genzer: 0000-0002-1633-238X

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Notes

The authors declare no competing financial interest.

CONCLUSION In this work, the assembly of negatively charged AuNPs within surface-grafted PNIPAM brushes was studied. Using neutron reflectivity allows resolving the internal structure of the brushes before and after particle immobilization. By comparing the SLD profiles of PNIPAM and PNIPAM/AuNP, we propose that the presence of AuNPs forces the surface-grafted PNIPAM chains

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ACKNOWLEDGMENTS The authors gratefully thank the Helmholtz-Zentrum Berlin (HZB) for the ability to carry out neutron reflectivity measurements. SEM measurements were performed by Christoph Fahrenson at the ZELMI (Zentrale Einrichtung I


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fuer Elektronenmikroskopie) of the TU Berlin. We thank Maren Lehmann (TU Berlin) for TEM measurements and analysis. Financial support was granted by the German Science Foundation (DFG) via the International Research Training Group (IRTG) 1524 located at the TU Berlin. Jan Genzer acknowledges partial support from the National Science Foundation, Grant No. DMR-1404639.

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Salt-Induced Aggregation of Negatively Charged Gold Nanoparticles

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