Article pubs.acs.org/Langmuir
Optical Properties of Self-Organized Gold Nanorod−Polymer Hybrid Films Ulrich Tritschler,† Igor Zlotnikov,‡ Philipp Keckeis,† Helmut Schlaad,*,§,∥ and Helmut Cölfen*,† †
Physical Chemistry, University of Konstanz, Universitätsstraße 10, D-78457 Konstanz, Germany Department of Biomaterials and §Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany ∥ Institute of Chemistry, University of Potsdam, Karl-Liebknecht-Straße 24-25, D-14476 Potsdam, Germany ‡
ABSTRACT: High fractions of gold nanorods were locally aligned by means of a polymeric liquid crystalline phase. The gold nanorods constituting >80 wt % of the thin organic− inorganic composite films form a network with side-by-side and end-to-end combinations. Organization into these network structures was induced by shearing gold nanorod−LC polymer dispersions via spin-coating. The LC polymer is a polyoxazoline functionalized with pendent cholesteryl and carboxyl side groups enabling the polymer to bind to the CTAB stabilizer layer of the gold nanorods via electrostatic interactions, thus forming the glue between organic and inorganic components, and to form a chiral nematic lyotropic phase. The self-assembled locally oriented gold nanorod structuring enables control over collective optical properties due to plasmon resonance coupling, reminiscent of enhanced optical properties of natural biomaterials.
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INTRODUCTION Natural organic−inorganic materials exhibit an ingenious hierarchical structuring of mineral crystals within an organic matrix as well as a controlled coupling at the interface between both components and often feature well-tuned optical properties.1−5 In brittle stars, for example, microlenses act as an optical device due to a precise control of shape, arrangement, and orientation of the calcite crystals.6 In glass sponges the composition of optical fibers, which are composed of a silica core and two different surrounding organic layers, and their individual refractive index characteristics lead to an outstanding optical activity.7 Another example is the iridescence of nacre originating from the periodic stacking of aragonite platelets possessing a thickness of up to 0.5 μm within an organic chitin matrix.8 Many research groups have worked on the synthesis of these challenging, often CaCO3, biomineral structures by using templating techniques9−15 or via methods involving lithography.16−21 The main drawback of the obtained materials is often their time-consuming fabrication, but also limitations such as their synthesis yields in small scales or flat surfaces that are required for lithographic techniques. Nevertheless, striking results were achieved with these methods, among others the fabrication of artificial nacre via polymer-mediated mineral growth and layer-by-layer deposition of the organic matrix leading to the mimicking of nacre’s optical iridescence.22 Remarkable, CaCO3 microlens arrays replicating arrays with uniform size and focal length as found in brittle stars were recently produced under ambient conditions without using any template.23 © 2014 American Chemical Society
An optically interesting nanoparticle system is gold, in particular gold nanorods due to their tunable, aspect-ratiodependent longitudinal surface plasmon resonance (LSPR) and, to a lower extent, transversal surface plasmon resonance (TSPR).24 Ensembles of gold nanorods may possess collective optical properties differing from the optical properties of individual gold nanorods and bulk samples. An important tool to control the structure and optical properties of gold nanorods is the self-assembly of the gold nanoparticles.25 The anisotropic shape of high-aspect-ratio gold nanorods allows them to form liquid crystalline (LC) phases in the presence of certain amounts of the stabilizing agent cetyltrimethylammonium bromide (CTAB) in concentrated aqueous dispersions.26,27 Alignment of CTAB-coated gold nanorods with lower aspect ratios of 80 wt%. Binding between the carboxyl-functionalized side chains of the LC polymer and gold nanorods schematically illustrated in Scheme 2 was confirmed by TGA and zeta potential measurements as well as AUC and UV−vis spectroscopy analysis. The latter analytical techniques also revealed that the gold nanorods coated with CTAB and LC polymer mainly exist as single particles in DMF before initiating self-organization by shearing the dispersion, inducing the formation of hybrid films with long-range orientation of the polymeric LC phase. Electron microscopy, atomic force microscopy, and SAXS analysis of the composite films revealed a homogeneous distribution of gold nanorods within the polymeric lyotropic phase on the length scale of several tens of micrometers and the formation of locally oriented gold nanorod network structures. These network structures exhibit both end-to-end and side-byside arrangements of gold nanorods, attributable to the influence of the LC polymer (see Scheme 2). These structures influence the surface plasmon resonance of the gold nanorods, showing that our approach allows to control collective optical properties of the gold nanorods within the polymeric LC phase. This concept represents a universal preparation technique to tune collective optical properties of manifold anisotropic nanoparticle systems, such as CdSe or TiO2, and ultimately allows to investigate structure−optical property correlations, making this system interesting for a variety of technical applications. 13787
dx.doi.org/10.1021/la503507u | Langmuir 2014, 30, 13781−13790
Langmuir
Article
(br, SCH2 MPA), 3.3−3.6 (br, NCH2 backbone), 4.9−5.1 (m, CH2 BOx), 5.5 (s, CH Chol), 5.8−5.9 (−CH BOx). Sample Preparation for Investigating the Binding between CTAB-Coated Gold Nanorods and LC “Gluing” Copolymer. For TGA measurements, gold nanorod/LC polymer hybrid materials were prepared by adding a 0.1 wt % dispersion of PBOx-Chol-MPNa obtained after few minutes of ultrasonication to an aqueous dispersion of gold nanorods using ratios of gold nanorods to polymer of 1:3, 1:1, 2:1, and 7:1 w/w. After vigorously shaking overnight, the dispersions were centrifuged at 5000−6000 rpm (with a speed at which the polymer remained in dispersion). The supernatant solutions were removed, and the sedimented hybrid materials were freeze-dried before submission to TGA. As a reference, an aqueous dispersion of pure gold nanorods was diluted accordingly, shaken overnight, and centrifuged, followed by freeze-drying. Binding between gold nanorods and polymer was investigated in aqueous medium as well as after phase-transfer to DMF by AUC and UV−vis spectroscopy. Analyses of an aqueous dispersion of gold nanorod/LC polymer = 1:1 w/w, prepared as described above, and an aqueous dispersion of the same gold nanorod batch as a reference were performed after diluting the samples to an absorbance of ca. 0.5−1. Phase transfer to DMF was performed by centrifuging aqueous dispersions of gold nanorod/LC polymer = 1:1 w/w and pure gold nanorods, removing the supernatant solution and adding DMF. After repeating this procedure once, the absorbance was adjusted properly for AUC and UV−vis measurements. Zeta potential measurements were carried out with as-prepared, purified gold nanorod dispersion in water as well as aqueous dispersions of gold nanorod/LC polymer = 1:1 w/w after centrifuging the hybrid dispersion at 6000 rpm and redispersing in water to remove residual, nonbound polymer. Lyotropic Gold Nanorod−LC Polymer Composite Films. After adding a 0.1 wt % dispersion of 10.5 mg LC “gluing” copolymer to an aqueous dispersion containing equal amounts of gold nanorods (gold nanorods/LC polymer = 1:1 w/w) and adjusting the pH to ∼8−9 by using 0.1 M NaOH, the mixture was shaken vigorously overnight, enabling the polymer to bind to the gold nanoparticles. The sample was centrifuged at 6000 rpm for 20 min, a centrifugation speed at which the polymer did not sediment. After removing the slightly reddish aqueous supernatant (the residual amount of water was determined to ca. 155 mg by gravimetric analysis), DMF was added, finally obtaining a ∼4.1 wt % hybrid dispersion consisting of a solvent mixture with equal volumes of DMF and water. 5 μL droplets of the hybrid dispersion were placed on a glass slide, and the solvents were allowed to evaporate slowly under a partially closed environment for 3.5 h by covering the droplet with a glass vial (volume of ca. 10 mL). The resulting, viscous hybrid dispersion mainly consisting of DMF was sheared via spin-coating at a rotation speed of 2000 rpm, affording thin composite film (spin-coating at 1000 rpm was not possible due to the high viscosity of the dispersion). Analytical Instrumentation and Methods. 1H NMR measurements were performed at room temperature using a Bruker Avance III 400 operating at 400 MHz. CDCl3 (purchased from Deutero GmbH, Germany) was used as solvent, and signals were referenced to δ = 7.26 ppm. Gel permeation chromatography (GPC) was performed on a PL GPC 50 using THF (stabilized with 0.0125% BHT) as eluent at 40 °C with a flow rate of 1 mL/min. Calibration was done with polystyrene standards. MALDI-ToF mass spectrometry was performed on a Bruker Microflex MALDI-TOF by using 10 μL of polymer precursor solution (10 mg/mL in CHCl3), 10 μL solution of dithranol (10 mg/mL in acetone), and 1 μL of sodium trifluoroacetate (0.1 mg/mL in acetone). Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449 F3. The respective sample was heated from room temperature to 1000 °C with a heating rate of 10 K/min under oxygen. Analytical ultracentrifugation (AUC) was performed on a Optima XL-I ultracentrifuge (Beckman Coulter, Palo Alto, CA) in 12 mm Ti-double sector centerpieces (Nanolytics, Potsdam, Germany) at 25 °C using UV/vis absorption optics. The sedimentation coefficients s were converted into s20,w, eliminating the influence of density and viscosity of the different solvents water and DMF. (v ̅ of the hybrid materials and the CTAB-coated gold nanorods was calculated by taking the inverse
of the sum consisting of the mass fraction of the respective component (obtained from TGA) multiplied with its density.) Light micrographs were taken in transmission and reflective mode with a Zeiss Axio Imager.M2m microscope and a birefringence microscope (Abrio). TEM samples were prepared on conventional TEM grids (carbonfilmed copper grids, 300 mesh, supplied by Quantifoil Micro Tools GmbH) and TEM images were acquired on a Zeiss Libra 120 microscope operating at 120 kV. Scanning electron microscopy (SEM) analyses of hybrid films on glass slides fixed onto standard aluminum stubs by means of double-sided adhesive carbon tape and subsequently sputtered with gold were performed on a Zeiss CrossBeam 1540XB microscope at an acceleration voltage of 3 kV. SAXS measurements were carried out using a NanoSTAR diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with a Cu Kα X-ray source and two crossed Goebel mirrors, resulting in a wavelength of 0.154 nm and a beam size of approximately 400 μm in diameter. The Bruker Hi-STAR area detector was mounted at a distance of 1050 or 260 mm from the sample, which was later calibrated using crystalline silver behenate powder. The intensity was determined as a function of the scattering vector q, and corrected for background and dark current. UV−visible spectroscopy analysis of dispersions was carried out on a Varian Cary 50 Bio UV/vis Spectrophotometer and UV−vis measurements of hybrid films were conducted using a TIDAS MCS UV/NIR spectrometer (J&M Analytik AG).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail [email protected] (H.S.). *E-mail [email protected] (H.C.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the German Research Foundation DFG through the Priority Programme 1420 “Biomimetic Materials Research: Functionality by Hierarchical Structuring of Materials” for financial support. The authors thank Prof. Dr. Peter Fratzl for help with SAXS data analysis, Dr. Marina Krumova for help with AFM measurements, Rose Rosenberg for AUC measurements, and the Proteomics Center (Konstanz), especially Andreas Marquardt, for MALDI-ToF MS measurements.
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REFERENCES
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dx.doi.org/10.1021/la503507u | Langmuir 2014, 30, 13781−13790
Optical Properties of Self-Organized Gold Nanorod−Polymer Hybrid Films Ulrich Tritschler,† Igor Zlotnikov,‡ Philipp Keckeis,† Helmut Schlaad,*,§,∥ and Helmut Cölfen*,† †
Physical Chemistry, University of Konstanz, Universitätsstraße 10, D-78457 Konstanz, Germany Department of Biomaterials and §Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany ∥ Institute of Chemistry, University of Potsdam, Karl-Liebknecht-Straße 24-25, D-14476 Potsdam, Germany ‡
ABSTRACT: High fractions of gold nanorods were locally aligned by means of a polymeric liquid crystalline phase. The gold nanorods constituting >80 wt % of the thin organic− inorganic composite films form a network with side-by-side and end-to-end combinations. Organization into these network structures was induced by shearing gold nanorod−LC polymer dispersions via spin-coating. The LC polymer is a polyoxazoline functionalized with pendent cholesteryl and carboxyl side groups enabling the polymer to bind to the CTAB stabilizer layer of the gold nanorods via electrostatic interactions, thus forming the glue between organic and inorganic components, and to form a chiral nematic lyotropic phase. The self-assembled locally oriented gold nanorod structuring enables control over collective optical properties due to plasmon resonance coupling, reminiscent of enhanced optical properties of natural biomaterials.
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INTRODUCTION Natural organic−inorganic materials exhibit an ingenious hierarchical structuring of mineral crystals within an organic matrix as well as a controlled coupling at the interface between both components and often feature well-tuned optical properties.1−5 In brittle stars, for example, microlenses act as an optical device due to a precise control of shape, arrangement, and orientation of the calcite crystals.6 In glass sponges the composition of optical fibers, which are composed of a silica core and two different surrounding organic layers, and their individual refractive index characteristics lead to an outstanding optical activity.7 Another example is the iridescence of nacre originating from the periodic stacking of aragonite platelets possessing a thickness of up to 0.5 μm within an organic chitin matrix.8 Many research groups have worked on the synthesis of these challenging, often CaCO3, biomineral structures by using templating techniques9−15 or via methods involving lithography.16−21 The main drawback of the obtained materials is often their time-consuming fabrication, but also limitations such as their synthesis yields in small scales or flat surfaces that are required for lithographic techniques. Nevertheless, striking results were achieved with these methods, among others the fabrication of artificial nacre via polymer-mediated mineral growth and layer-by-layer deposition of the organic matrix leading to the mimicking of nacre’s optical iridescence.22 Remarkable, CaCO3 microlens arrays replicating arrays with uniform size and focal length as found in brittle stars were recently produced under ambient conditions without using any template.23 © 2014 American Chemical Society
An optically interesting nanoparticle system is gold, in particular gold nanorods due to their tunable, aspect-ratiodependent longitudinal surface plasmon resonance (LSPR) and, to a lower extent, transversal surface plasmon resonance (TSPR).24 Ensembles of gold nanorods may possess collective optical properties differing from the optical properties of individual gold nanorods and bulk samples. An important tool to control the structure and optical properties of gold nanorods is the self-assembly of the gold nanoparticles.25 The anisotropic shape of high-aspect-ratio gold nanorods allows them to form liquid crystalline (LC) phases in the presence of certain amounts of the stabilizing agent cetyltrimethylammonium bromide (CTAB) in concentrated aqueous dispersions.26,27 Alignment of CTAB-coated gold nanorods with lower aspect ratios of 80 wt%. Binding between the carboxyl-functionalized side chains of the LC polymer and gold nanorods schematically illustrated in Scheme 2 was confirmed by TGA and zeta potential measurements as well as AUC and UV−vis spectroscopy analysis. The latter analytical techniques also revealed that the gold nanorods coated with CTAB and LC polymer mainly exist as single particles in DMF before initiating self-organization by shearing the dispersion, inducing the formation of hybrid films with long-range orientation of the polymeric LC phase. Electron microscopy, atomic force microscopy, and SAXS analysis of the composite films revealed a homogeneous distribution of gold nanorods within the polymeric lyotropic phase on the length scale of several tens of micrometers and the formation of locally oriented gold nanorod network structures. These network structures exhibit both end-to-end and side-byside arrangements of gold nanorods, attributable to the influence of the LC polymer (see Scheme 2). These structures influence the surface plasmon resonance of the gold nanorods, showing that our approach allows to control collective optical properties of the gold nanorods within the polymeric LC phase. This concept represents a universal preparation technique to tune collective optical properties of manifold anisotropic nanoparticle systems, such as CdSe or TiO2, and ultimately allows to investigate structure−optical property correlations, making this system interesting for a variety of technical applications. 13787
dx.doi.org/10.1021/la503507u | Langmuir 2014, 30, 13781−13790
Langmuir
Article
(br, SCH2 MPA), 3.3−3.6 (br, NCH2 backbone), 4.9−5.1 (m, CH2 BOx), 5.5 (s, CH Chol), 5.8−5.9 (−CH BOx). Sample Preparation for Investigating the Binding between CTAB-Coated Gold Nanorods and LC “Gluing” Copolymer. For TGA measurements, gold nanorod/LC polymer hybrid materials were prepared by adding a 0.1 wt % dispersion of PBOx-Chol-MPNa obtained after few minutes of ultrasonication to an aqueous dispersion of gold nanorods using ratios of gold nanorods to polymer of 1:3, 1:1, 2:1, and 7:1 w/w. After vigorously shaking overnight, the dispersions were centrifuged at 5000−6000 rpm (with a speed at which the polymer remained in dispersion). The supernatant solutions were removed, and the sedimented hybrid materials were freeze-dried before submission to TGA. As a reference, an aqueous dispersion of pure gold nanorods was diluted accordingly, shaken overnight, and centrifuged, followed by freeze-drying. Binding between gold nanorods and polymer was investigated in aqueous medium as well as after phase-transfer to DMF by AUC and UV−vis spectroscopy. Analyses of an aqueous dispersion of gold nanorod/LC polymer = 1:1 w/w, prepared as described above, and an aqueous dispersion of the same gold nanorod batch as a reference were performed after diluting the samples to an absorbance of ca. 0.5−1. Phase transfer to DMF was performed by centrifuging aqueous dispersions of gold nanorod/LC polymer = 1:1 w/w and pure gold nanorods, removing the supernatant solution and adding DMF. After repeating this procedure once, the absorbance was adjusted properly for AUC and UV−vis measurements. Zeta potential measurements were carried out with as-prepared, purified gold nanorod dispersion in water as well as aqueous dispersions of gold nanorod/LC polymer = 1:1 w/w after centrifuging the hybrid dispersion at 6000 rpm and redispersing in water to remove residual, nonbound polymer. Lyotropic Gold Nanorod−LC Polymer Composite Films. After adding a 0.1 wt % dispersion of 10.5 mg LC “gluing” copolymer to an aqueous dispersion containing equal amounts of gold nanorods (gold nanorods/LC polymer = 1:1 w/w) and adjusting the pH to ∼8−9 by using 0.1 M NaOH, the mixture was shaken vigorously overnight, enabling the polymer to bind to the gold nanoparticles. The sample was centrifuged at 6000 rpm for 20 min, a centrifugation speed at which the polymer did not sediment. After removing the slightly reddish aqueous supernatant (the residual amount of water was determined to ca. 155 mg by gravimetric analysis), DMF was added, finally obtaining a ∼4.1 wt % hybrid dispersion consisting of a solvent mixture with equal volumes of DMF and water. 5 μL droplets of the hybrid dispersion were placed on a glass slide, and the solvents were allowed to evaporate slowly under a partially closed environment for 3.5 h by covering the droplet with a glass vial (volume of ca. 10 mL). The resulting, viscous hybrid dispersion mainly consisting of DMF was sheared via spin-coating at a rotation speed of 2000 rpm, affording thin composite film (spin-coating at 1000 rpm was not possible due to the high viscosity of the dispersion). Analytical Instrumentation and Methods. 1H NMR measurements were performed at room temperature using a Bruker Avance III 400 operating at 400 MHz. CDCl3 (purchased from Deutero GmbH, Germany) was used as solvent, and signals were referenced to δ = 7.26 ppm. Gel permeation chromatography (GPC) was performed on a PL GPC 50 using THF (stabilized with 0.0125% BHT) as eluent at 40 °C with a flow rate of 1 mL/min. Calibration was done with polystyrene standards. MALDI-ToF mass spectrometry was performed on a Bruker Microflex MALDI-TOF by using 10 μL of polymer precursor solution (10 mg/mL in CHCl3), 10 μL solution of dithranol (10 mg/mL in acetone), and 1 μL of sodium trifluoroacetate (0.1 mg/mL in acetone). Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449 F3. The respective sample was heated from room temperature to 1000 °C with a heating rate of 10 K/min under oxygen. Analytical ultracentrifugation (AUC) was performed on a Optima XL-I ultracentrifuge (Beckman Coulter, Palo Alto, CA) in 12 mm Ti-double sector centerpieces (Nanolytics, Potsdam, Germany) at 25 °C using UV/vis absorption optics. The sedimentation coefficients s were converted into s20,w, eliminating the influence of density and viscosity of the different solvents water and DMF. (v ̅ of the hybrid materials and the CTAB-coated gold nanorods was calculated by taking the inverse
of the sum consisting of the mass fraction of the respective component (obtained from TGA) multiplied with its density.) Light micrographs were taken in transmission and reflective mode with a Zeiss Axio Imager.M2m microscope and a birefringence microscope (Abrio). TEM samples were prepared on conventional TEM grids (carbonfilmed copper grids, 300 mesh, supplied by Quantifoil Micro Tools GmbH) and TEM images were acquired on a Zeiss Libra 120 microscope operating at 120 kV. Scanning electron microscopy (SEM) analyses of hybrid films on glass slides fixed onto standard aluminum stubs by means of double-sided adhesive carbon tape and subsequently sputtered with gold were performed on a Zeiss CrossBeam 1540XB microscope at an acceleration voltage of 3 kV. SAXS measurements were carried out using a NanoSTAR diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with a Cu Kα X-ray source and two crossed Goebel mirrors, resulting in a wavelength of 0.154 nm and a beam size of approximately 400 μm in diameter. The Bruker Hi-STAR area detector was mounted at a distance of 1050 or 260 mm from the sample, which was later calibrated using crystalline silver behenate powder. The intensity was determined as a function of the scattering vector q, and corrected for background and dark current. UV−visible spectroscopy analysis of dispersions was carried out on a Varian Cary 50 Bio UV/vis Spectrophotometer and UV−vis measurements of hybrid films were conducted using a TIDAS MCS UV/NIR spectrometer (J&M Analytik AG).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail [email protected] (H.S.). *E-mail [email protected] (H.C.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the German Research Foundation DFG through the Priority Programme 1420 “Biomimetic Materials Research: Functionality by Hierarchical Structuring of Materials” for financial support. The authors thank Prof. Dr. Peter Fratzl for help with SAXS data analysis, Dr. Marina Krumova for help with AFM measurements, Rose Rosenberg for AUC measurements, and the Proteomics Center (Konstanz), especially Andreas Marquardt, for MALDI-ToF MS measurements.
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