Metal Nanoparticle Growth within ClayâPolymer Nacre-Inspired

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Metal Nanoparticle Growth within Clay – Polymer Nacre – Inspired Materials for Improved Catalysis and Plasmonic Detection in Complex Biofluids Eric H. Hill, Christoph Hanske, Alexander Johnson, Luis Yate, Hans Jelitto, Gerold A. Schneider, and Luis M. Liz-Marzán Langmuir, Just Accepted Manuscript • Publication Date (Web): 13 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017

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Metal Nanoparticle Growth within Clay – Polymer Nacre – Inspired Materials for Improved Catalysis and Plasmonic Detection in Complex Biofluids Eric H. Hill1,4*, Christoph Hanske1, Alexander Johnson1, Luis Yate1, Hans Jelitto2, Gerold A. Schneider2, Luis M. Liz-Marzán1,3,4* 1 2

Bionanoplasmonics Laboratory, CIC biomaGUNE, 20014 Donostia-San Sebastián, Spain Hamburg University of Technology, Institute of Advanced Ceramics, 21073 Hamburg,

Germany 3

Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain

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Biomedical Research Networking Center in Bioengineering Biomaterials and Nanomedicine,

Ciber-BBN, 20014 Donostia-San Sebastián, Spain

Abstract Recent studies have shown that layered silicate clays can be used to form a nacre-like bioinspired layered structure with various polymer fillers, leading to composite films with good material strength, gas barrier properties, and high loading capacity. We go one step farther by in situ growing metal nanoparticles in nacre-like layered films based on layered silicate clays, which can be used for applications in plasmonic sensing and catalysis. The degree of anisotropy of the nanoparticles grown in the film can be controlled by adjusting the ratio of clay to polymer, gold to clay, and reducing agent concentration, as well as silver overgrowth, which greatly enhances the surface-enhanced Raman scattering activity of the composite. We show the performance of the films for SERS detection of bacterial quorum-sensing molecules in culture medium, and catalytic properties are demonstrated with the reduction of 4-nitroaniline. These films serve as the first example of seedless, in situ nanoparticle growth within nacre-mimetic materials, and open the path to basic research on the influence of different building blocks and polymeric mortars on nanoparticle morphology and distribution, as well as applications in catalysis, sensing, and anti-microbial surfaces using such materials.

Keywords: Bioinspired, functional materials, plasmonic sensing, catalysis, nacre-like, layered silicate clays, composite materials, metal nanoparticles

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Introduction Nacre, also known as mother of pearl, is a composite material formed by mollusks which is composed of 95% aragonite (a form of CaCO3), and 5% proteins and polysaccharides.1–4 The hierarchical brick-and-mortar structure of nacre gives it exceptional material properties with significantly higher strength and toughness than either of the polymeric or mineral components alone.5–9 Nacre protein has a fracture toughness of 0.02 MPa m1/2, aragonite has ~0.2 MPa m1/2, while Nacre has a fracture toughness of up to 10 MPa m1/2, representing a 40x increase in toughness, which corresponds to roughly a 2000 times increase in energy terms.9 These properties make nacre-mimetics interesting systems for research of layered composites with robust material properties for structural materials.10–18 As such, nacre-mimetic materials have been recently established as an attractive model for imparting artificial materials with good mechanical properties and functionality.19–22 Nacre-mimetic materials are commonly lamellar or brick-and-mortar assemblies of inorganic building blocks with an organic “mortar” to bind the system.23 Several studies have presented methods for producing nacre-like layered materials via the assembly of different inorganic building blocks with polymers.24,25 Anisotropic inorganic particles such as layered double hydroxides,26,27 ceramics,28,29 graphene oxide,28–31 and biominerals such as nanofibrillar cellulose,33 have been studied as potential building blocks for the assembly of nacre-like materials with a variety of polymers and biopolymers to bind them. Layered silicate clays have been recently put forth as inorganic building blocks of significant interest, with a wide range of aspect ratios and diameters (25 nm – 3.5 µm), and desirable properties including fireretardant,12,34 high gas-barrier,35–37 transparency in the visible and NIR,38,39 high surface area,40– 42

and cation-exchange capacity for functionalization or loading of molecules.43–47 In a recent

study we showed that a layered silicate clay could be used as a template for seedless growth of a variety of particle morphologies.48 Inspired by these studies, a simple route towards transparent nacre-like materials with plasmonic properties was envisioned. Gold nanoparticles have been incorporated into nacre-like films for plasmonic or catalytic applications, but usually based on seed nanoparticles rather than growing the particles in situ, with fabrication methods involving sequential spray-coating or dip-coating steps.27,49 While these studies shed light on the

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capabilities of NP-loaded nacre-like films for potential applications, the ability to influence nanoparticle growth and resulting morphology was not established. In the approach herein (Scheme 1), Au ions are complexed with the layered silicate clay Laponite® RD, which is small compared to other layered silicate clays such as montmorillonite and serves as a model for these other systems though they are of larger diameter. The clay – metal complex is then mixed with polyvinyl alcohol (PVA) at varying clay:polymer ratios and dried under air at ambient conditions. Following drying, chemical reduction of the metal salt leads to nanoparticle growth within the nacre-like structure of the film. The catalytic degradation of 4-nitroaniline by these films was compared with that by an array of AuNPs, and the overgrowth of Ag on the AuNP-loaded films was studied as a means to enhance their plasmonic sensing capabilities in complex biological media.

Scheme 1. Photograph and scanning electron micrographs of H. tuberculata showing nacre structure, obtained in Zurriola Beach (top-left). Summary of approach for in situ growth of anisotropic metal nanoparticles within nacre-like films (bottom).

Materials and Methods Materials Tetrachloroauric(III) acid trihydrate (HAuCl4· 3H2O), sodium borohydride (NaBH4), L- ascorbic acid (ACS reagent 99%), 4-nitroaniline (>99.9%), silver nitrate (AgNO3, ACS reagent 99.0%), pyocyanin from Pseudomonas aeruginosa, sodium hydroxide (98%), and poly(vinyl)acid (PVA)

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of average MW 40,000 and 90% hydrolyzed were obtained from Sigma-Aldrich and used without further purification. Laponite® RD was graciously provided by BYK additives, and is abbreviated as “Lap” throughout the article. All solutions and dispersions were prepared using Millipore-filtered water with a resistivity of 18.2 MΩ·cm. Dispersions of Lap were prepared at 5 mg/mL, sonicated in a bath-type sonicator (200W, Ultrasons-H, JP SELECTA) for an hour, allowed to sit 24 hours, and sonicated for 10 minutes prior to use. PVA was prepared at 5 mg/mL, and stirred for 24 hours at 21 ºC prior to use.

Methods – Film Casting and AuNP Growth To a dispersion of Lap (5 mg/mL), equimolar amounts of HAuCl4 (0.127 M) and NaOH (0.1 M) were added under magnetic stirring. The resulting dispersion was sonicated for 5 minutes, resulting in a color change from slight yellow to colorless. This change indicates the reduction of Au (III) to Au (I) by interaction with the clay, as confirmed by UV-vis spectroscopy (Figure S1, SI). PVA (5 mg/mL) was then added to the resulting dispersion, followed by sonication for 10 minutes. Numerous weight ratios were tested, from 1:3 to 3:1 Lap:PVA. Solutions were then poured into clean polystyrene Petri dishes and allowed to dry in a well-ventilated area at room temperature for 24-72 hours, depending on the volume of liquid cast. Reduction of Au (I) to Au (0) was carried out by immersing the dried films in ascorbic acid, ranging in concentration from 1 to 20 mM for 10 minutes. Following reduction, the films were rinsed with water three times to remove excess ascorbic acid and then soaked for 10 minutes prior to drying in air or a vacuum oven.

Silver Overgrowth for Plasmonic Enhancement In order to study the plasmonic effects of silver overgrowth on the Au-NP containing films, a set of experiments was carried out. Following rinsing after gold reduction, overgrowth of silver on the gold in the film was performed by soaking the Au-NP loaded film in AgNO3 (1 mM and 5 mM) for 5 minutes. The films were then rinsed of excess AgNO3 with water several times, and then reduction was carried out by soaking with ascorbic acid (20 mM) as above. Films were then rinsed, soaked with water for 5-10 minutes, and allowed to dry under air or with the aid of a vacuum oven.

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Structural Characterization Characterization was carried out to understand the internal structure and mechanical properties of the films, and the dispersion as well as size/shape of nanoparticles within the film and on its surface. Optical characterization was carried out by UV-vis-NIR spectroscopy with a Cary 5000 spectrophotometer and an Agilent 8453 spectrophotometer. Transmission electron microscopy (TEM) analysis was performed by using a JEOL JEM 1010 microscope operating at an acceleration voltage of 100 kV. Scanning electron microscopy (SEM) was performed with a JEOL JSM-6700F or with a FEI Quanta 250 ESEM at an acceleration voltage of 5 kV. Energy dispersive X-ray (EDX) spectroscopy was performed using a FEI Titan TEM. Atomic Force Microscopy (AFM) images were recorded with a Bruker Multimode V using the Tapping Mode™ in air and aluminum-coated cantilevers (Bruker TESP-V2, f0: 320 kHz, k: 42 N/m). Data processing was conducted within the free software Gwyddion by applying the automated line- and plane-fitting. Occasional scanning artifacts were corrected with the built-in functions for marking and removing scars.

Additional characterization of elemental composition was carried out by X-ray Photoelectron Spectroscopy (XPS) to determine the variation of elemental composition as a function of film depth. UV-vis absorption spectroscopy was used to assess the plasmonic response of the films before reduction, after reduction, and after silver-overgrowth. Elastic modulus and hardness were measured by nano-indentation, using an Agilent Nano Indenter G200. The maximum indentation depth was set to 2,000 nm.

Bacterial Growth The application of plasmonic sensing to detect the bacterial quorum sensing molecule pyocyanin was carried out by performing aerobic growth of P. aeruginosa PA14.51 Bacteria were streaked onto Luria-Bertani (LB) agar (1.5% w/v) plates and incubated overnight at 37 ºC. A single colony was used to inoculate a 10 mL culture in LB medium and grown at 37 ºC with agitation (210 rpm). The optical density (OD) of the liquid culture was measured at OD600 nm for monitoring bacterial growth. Cells from a bacterial culture grown in LB were pelleted by centrifugation (8,000 x g/ 3min) and the supernatant was filtered (0.2 µm filter pore size). 5 µL

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of bacteria-free supernatants of P. aeruginosa PA14 obtained from a late stationary phase culture grown for 20 hours were incubated for 10 minutes with the different plasmonic films.

Surface-Enhanced Raman Scattering Detection Raman spectroscopy experiments were conducted with a Renishaw InVia Reflex system. The spectrograph used a high-resolution grating (1200 grooves mm−1) with additional band-pass filter optics, a confocal microscope, and a 2D-CCD camera. 785 nm excitation from a diode laser was used. Preparation for a surface-enhanced Raman scattering (SERS) measurement was carried out by first applying 5 µL of analyte solution to the film, which is wicked off with filter paper after 5 minutes and dried under air, followed by measurement with the Raman microscope (785 nm, 1.5 mW, 5s acquisition time). Films were also analyzed with a portable Raman spectrometer (BWTEK i-Raman, 785 nm, 435 mW max power, 2 second acquisition used). To study the detection of the bacterial quorum-sensing molecule pyocyanin in complex media, pyocyanin was diluted in Luria-Bertrani broth to a concentration of 1 µM, and 5 µL of this and a filtered supernatant after 18 hours of bacterial growth were dropped onto the film and allowed to dry prior to measurement. In order to assess the ability of the polymer network of the films to filter out large molecules in the nutrient broth or bacterial supernatant, the Lap-PVA films were compared with a glass slide coated with anisotropic AuNPs following the same silver-coating procedure as outlined above. These control samples were fabricated using a previously established method.52

Catalytic Degradation of 4-nitroaniline Lap-PVA free-standing films were adhered onto a glass cover slip by first wetting the cover slip with several microliters of water and then placing the film onto the cover slip, followed by briefly drying in air and room temperature. The catalysis experiments were carried out as follows: 100 µL of 4-NA (6.5 mM) was added to 6 mL of water under magnetic stirring, followed by 1.5 mL of freshly-prepared NaBH4 (0.1 M). The films were then added to the solution and aliquots were taken at 0, 1, 2, 5, 7, 10, 15, 20, and 25 minutes for measurement by UV-vis spectroscopy. Beer’s law was used to convert the absorbance at 380 nm to 4-NA concentration, using the molar extinction coefficient of 13,500 L mol-1 cm-1.53 Since the reaction follows first order kinetics, a linear regression of the natural log of concentration vs. time gives

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the kinetic rate of catalysis for conversion of 4-NA to the product p-phenylene diamine (p-PDA), which can be visually observed as the yellow solution fading to colorless.54 As a control to study the influence of the internal structure of the film on catalysis rate, a film of anisotropic AuNPs on a cover slip was also tested to compare with the Lap-PVA films (Figure S2).52

Results and Discussion Film Structure The layered structure of the films was confirmed by both SEM and TEM, where the layers can be clearly observed. Furthermore, the presence of nanoparticles inside and atop the layered structure of the film was clearly visible by electron microscopy (Figure 1).

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Figure 1. SEM (a) and TEM (b) of 1:1 Lap:PVA film after AuNP growth. The size of the layers was estimated by the difference in gray values across the layers using ImageJ, with 40 distances used to generate the average (c); (d) An example of a transmission electron micrograph of a microtomed 100 nm section where the layered structure is visible and free of particles, used to generate the distance in (c).

As can be observed in Figure 1, the films exhibit a well-defined layered structure. The presence of particles is also observed within the layered structure and at the edge of the film, by SEM and by TEM imaging of a 100 nm microtome slice (Figure 1a,b). The particles within the layered structure of the film (Figure 1b) were confirmed to be composed of Au by EDX spectroscopy

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(Figure S3). In both SEM and TEM, the layered structure has a similar appearance and resembles previously observed structure in Lap-PVA nacre-like films.39 In addition, the view of the layered structure directly parallel to the direction of the film orientation allows estimation of the layer spacing. The average layer spacing was calculated using the profile of grey values in a line across multiple layers using ImageJ to be 2.05 ±0.46 nm (Figure 1c). Das and coworkers estimated the spacing to be 1.88 nm for a layered silicate sumecton, of similar thickness but with a diameter between 35 and 600 nm, using image analysis of TEM characterization.39 In their study, they also measured a d-spacing of 3.03 nm for sumecton and 3.70 nm for Lap by XRD, a trend which is corroborated by the estimated spacings of 1.88 and 2.05 nm for sumecton and Lap as measured by TEM. Interestingly, since metal growth was carried out on films that have been dried onto a petri dish, the top of the film has a coating of nanoparticles or nanoparticle aggregates but the bottom of the film is very smooth. As can be seen by SEM and AFM, the appearance of gold particles on the bottom of the film is rare (Figure 2a,b).

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Figure 2. Bottom of artificial nacre film after Au reduction, characterized by SEM (a), and AFM (b). Top of the same film by SEM (d) and AFM (e). Photograph of a film after Ag overgrowth, facing bottom side (c), and top side (f).

We hypothesize that these particles are able to form in clay-free defects at the bottom of the film where the nacre-like composite has a small gap, either from mismatches during self-assembly or defects in the petri dish where the casting was carried out. AFM imaging shows that the bottom side is very smooth, and some very small particles of roughly 1-2 nm which are likely polymer grains from PVA are observed (Figure 2b). In the case of the top of the film, the grains are homogenously distributed Au nanoparticles, which appear as quasi-spherical or branched (Figure

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2d,e). In the AFM images, particles with a height of up to 75 nm were found. Keeping the film attached to the surface during Ag overgrowth can also allow restriction of the deposition of small Ag nanoparticles to one side of the film, giving increased reflectance on one side (Figure 2c,f). This provides a means to allow particle growth out of the film on only one side, allowing the fabrication of one-sided mirrors and tunable surface roughness.

Control of NP Morphology by Reaction Parameters The morphology and corresponding plasmon resonance in AuNPs can generally be controlled through the stabilizer, metal salt, and reducing agent concentrations.55 In the case of hybrid AuLap nanoparticles, it has been shown that an increase in the gold chloride to Lap ratio results in larger particles with greater anisotropy, as can be observed by a redshift in the plasmon resonance.48 This trend also holds true with the nanoparticles formed in Lap:PVA films, where more clay and more gold per clay result in increased anisotropy.

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Figure 3. UV-visible spectra of Lap:PVA films after Au reduction, with variation in Lap:PVA ratio and 14.2 µmol Au (a), and Au:Lap ratio with 1:1 Lap:PVA(b). (c),(d) Transmission electron micrograph of a 2:1 Lap:PVA film with 14.2 µmol Au taken perpendicular to the surface near the edge of a film, showing homogenous distribution of anisotropic AuNPs.

The distribution of the particles in the film is homogeneous, as is apparent in the TEM of the edge of a film taken facing the top of the film. In the absorption spectra shown in Figure 3, several salient relationships between the plasmon resonance and the various reactant concentrations and ratios can be clearly established. Firstly, as is expected from previous studies on the hybrid Lap-Au nanoparticles, an increase in Au:Lap ratio results in a strong red-shift and broadening of the longitudinal plasmon band (for a 1:1 Lap:PVA ratio, increasing Au from 4.8

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µmol to 15 µmol gives a shift from 550 nm to 800 nm). The Lap:PVA ratio also drastically changes the plasmon resonance of the grown Au nanoparticles. As observed in Figure 3a, for a given Au amount (10 µmol), the increase in Lap:PVA ratio from 1:2 to 2:1 results in a change of the plasmon resonance from 560 nm to ~780 nm and increased broadening, similar to that observed for an increase in Au:Lap ratio. Additionally, the ratio of clay to PVA was found to be critical for control of the plasmon resonance and moderated the redshift in the plasmon resonance observed with changing Au:Lap ratio. As can be observed in Figure 3b, whereas the AuNPs grown in 1:1 Lap:PVA can undergo a 250 nm redshift with Au increasing from 5 to 20 µmol, those grown in 2:3 Lap:PVA only witness a change in the plasmon resonance from 560 nm to 630 nm as the gold amount is changed from 5 µmol to 40 µmol (Figure S1a). Another trend observed in the UV-vis spectra shows the dependence of the plasmon resonance of the particles in the film, on the concentration of AA used for Au reduction (Figure S1b,c). With 15 µmol of Au in a film with a 2:3 Lap:PVA ratio, a redshift of the plasmon resonance from 560 to 600 nm is observed when increasing [AA] from 1 mM to 20 mM. The Lap:PVA ratio again plays a significant role in the resulting plasmon resonance at different AA concentrations, as a 1:1 Lap:PVA film with the same amount of Au undergoes a shift from 560 nm to 690 nm as [AA] is increased from 1 to 20 mM. Considering the results obtained, one could envision a variety of different compositions to reach similar plasmon resonances, depending on the ratios of Au:Lap and Lap:PVA, as well as the AA concentration used for reduction of Au[1] to Au[0]. The anisotropic morphology of the particles is clearly visible by TEM, where the majority of observed particles are non-spherical, with protrusions giving a partial star-like shape (Figure 3c,d). The anisotropy of the particles in the film is not totally uniform (Figure 3d), correlating with the broad plasmon band observed in the UV-Visible spectra (green line, Figure 3a).

Silver Overgrowth on AuNP-Loaded Films We additionally studied the ability to overgrow another metal atop the Au nanoparticles inside the lap-PVA nacre-inspired film, by soaking the films in a solution of AgNO3, rinsed, and then soaked in an ascorbic acid solution as described in the methods section. The overgrowth of Ag atop the AuNPs was clearly observed by a color change, and the UV-vis spectra provide initial

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indication of plasmon resonances from both Ag and Au.

Figure 4. (a) UV-visible absorbance spectra of a 1:1 Lap:PVA film with varying amounts of Au following soaking in 5 mM AgNO3 and reduction in AA. (b) Comparison of films with 2:3 and 1:1 Lap:PVA ratios, with 9.5 µmol of Au and no Ag or overgrowth with AgNO3 solutions of 1 mM and 5 mM. (c,d) SEM images of films without (c) or with (d) Ag overgrowth following soaking in 5 mM AgNO3 and reduction in 20 mM AA (scale bars are 1 µm). (e,f) AFM images of films without (e) or with (f) Ag overgrowth following soaking in 5 mM AgNO3 and reduction in 20 mM AA.

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In Figure 4b it is clear that with 1 mM AgNO3 there is a contribution to the absorbance of the plasmon resonance of the AuNPs in the film, however once the [AgNO3] is raised to 5 mM the plasmon resonance of free Ag particles is observed as a narrow absorption band at 460470 nm. An increase in Ag overgrowth is observed with an increasing amount of gold in the films, with the plasmon resonance of pure Ag nanoparticles at ~460 nm reducing in intensity as the amount of gold increases (Figure 4a). Silver overgrowth resulted in the formation of silver nanoparticles which were largely confined to the surface of the film. As can be observed in the SEM and AFM images in Figure 4c-f, the samples where the film is soaked in AgNO3 following reduction by ascorbic acid show a drastic increase of small (

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