Adjusting the Metrics of 1-D Helical Gold Nanoparticle

Article pubs.acs.org/Langmuir

Adjusting the Metrics of 1‑D Helical Gold Nanoparticle Superstructures Using Multivalent Peptide Conjugates Andrea D. Merg,† Joseph Slocik,‡ Martin G. Blaber,§ George C. Schatz,§ Rajesh Naik,‡ and Nathaniel L. Rosi*,† †

Department of Chemistry, University of Pittsburgh, 219 Parkman Ave., Pittsburgh, Pennsylvania 15260, United States Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433, United States § Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States ‡

Downloaded by UNIV OF CAMBRIDGE on September 4, 2015 | http://pubs.acs.org Publication Date (Web): August 20, 2015 | doi: 10.1021/acs.langmuir.5b02208

S Supporting Information *

ABSTRACT: The properties of nanoparticle superstructures depend on many factors, including the structural metrics of the nanoparticle superstructure (particle diameter, interparticle distances, etc.). Here, we introduce a family of gold-binding peptide conjugate molecules that can direct nanoparticle assembly, and we describe how these molecules can be systematically modified to adjust the structural metrics of linear double-helical nanoparticle superstructures. Twelve new peptide conjugates are prepared via linking a gold-binding peptide, AYSSGAPPMPPF (PEPAu), to a hydrophobic aliphatic tail. The peptide conjugates have 1, 2, or 3 PEPAu headgroups and a C12, C14, C16, or C18 aliphatic tail. The soft assembly of these peptide conjugates was studied using transmission electron microscopy (TEM), atomic force microscopy (AFM), and infrared (IR) spectroscopy. Several peptide conjugates assemble into 1-D twisted fibers having measurable structural parameters such as fiber width, thickness, and pitch that can be systematically varied by adjusting the aliphatic tail length and number of peptide headgroups. The linear soft assemblies serve as structural scaffolds for arranging gold nanoparticles into double-helical superstructures, which are examined via TEM. The pitch and interparticle distances of the gold nanoparticle double helices correspond to the underlying metrics of the peptide conjugate soft assemblies, illustrating that designed peptide conjugate molecules can be used to not only direct the assembly of gold nanoparticles but also control the metrics of the assembled structure.

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INTRODUCTION Nanoparticle superstructures exhibit collective properties that are dependent on their morphology and structure.1−5 Methods that allow one to tune and optimize these collective properties are important for proposed applications. Currently, challenges remain for controlling the structural metrics of nanoparticle superstructures. One method that aims to address and overcome these challenges utilizes peptides as “bonds” that serve to link and assemble nanoparticles together.6−9 Peptides are ideal molecules for controlling the assembly of nanoparticles into nanoparticle superstructures. They are highly programmable molecules that can be designed to bind to nanoparticle surfaces6,10−16 and can be designed to assemble into target structures having nanoscale features.17−19 There are numerous studies that explore soft assembly structure as a function of peptide sequence and peptide terminus modification;20−25 however, few accounts exploit these highly tunable nanoscale soft assemblies as a means for designing programmable, precisely ordered nanoparticle superstructures.26−28 We have developed peptide-based methods for assembling nanoparticles into complex architectures.8,26−29 In one example, we used the peptide conjugate C12-PEPAu (C12-PEPAu = © 2015 American Chemical Society

[C11H23CO]-AYSSGAPPMPPF) to direct the assembly of chiral gold nanoparticle double helices.8,13,29 These assemblies exhibit a collective plasmonic circular dichroism (CD) signal arising from the chiral arrangement of discrete achiral nanoparticles.28 Chiral nanoparticles and chiral nanoparticle assemblies are receiving significant interest,30−53 and tuning their chiroptical properties is important for potential applications.54−57 However, adjusting the structure of helical nanoparticle assemblies has not been extensively explored experimentally. Govorov and co-workers computationally investigated the chiroptical signal of “ideal” nanoparticle helices as a function of defects and helical structure parameters.58 They showed that adjusting the pitch and other metrics of the helix modulates the strength of the CD signal. Moreover, computational modeling of the CD behavior of double-helical gold nanoparticle superstructures prepared by our group indicates that the CD signal strength is dependent on structural parameters including helix pitch (Figure S1) and nanoparticle Received: June 30, 2015 Revised: August 10, 2015 Published: August 11, 2015 9492

DOI: 10.1021/acs.langmuir.5b02208 Langmuir 2015, 31, 9492−9501

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Langmuir size.28 Motivated by these computational studies that illustrate the need for fine-tuning helical nanoparticle superstructures, we aimed to use peptides to systematically control the structural parameters of double-helical gold nanoparticle assemblies via simple synthetic modifications to the peptide conjugate. In this work, we demonstrate that peptide conjugates can be tailored to construct a collection of helical nanoparticle superstructures with systematically tunable structural parameters. In previous research, we found that C12-PEPAu conjugates assemble into 1-D twisted fibers.8 From structure and spectroscopy studies, we determined that the handedness of the fibers derives from the chirality of the peptides,28 which pack laterally perpendicular to the fiber axis. A reported assembly model that is consistent with our observations is illustrated in Scheme 1.20,59 In this model, the peptide

(TEAA) was purchased from Aldrich (catalog number: 90358), and 4(2-hydroxyethyl)-1-piperazineethanesulfonic acid (pH = 7.3) (HEPES) buffer was purchased from Fisher (catalog number: BP299-100). Chloroauric acid (HAuCl4) was purchased from Aldrich (catalog number: 520918). 0.1 M citrate (pH 7.4) was prepared by dissolving citric acid in nanopure water and adjusting the pH to 7.4 using NaOH. Peptide conjugates were purified using an Agilent 1200 Series reverse-phase high-pressure liquid chromatography (HPLC) instrument equipped with an Agilent Zorbax 300SB-C18 column. Peptide conjugates were quantified based on their absorbance at 280 nm and using the extinction coefficient for tyrosine (1280 M−1 cm−1). Spectra were collected using an Agilent 8453 UV−vis spectrometer equipped with deuterium and tungsten lamps. Transmission electron microscopy (TEM) samples were prepared by drop casting 6 μL of solution onto a 3 mm diameter copper grid. TEM images were collected with a FEI Morgagni 268 (80 kV) equipped with an AMT side mount CCD camera system. Phosphotungstic acid (pH 7.4) was used to stain TEM sample grids for soft assembly studies. All proton nuclear magnetic resonance (1H NMR) data were collected using a Bruker Avance III 300 MHz spectrometer. All liquid chromatography mass spectrometry (LC-MS) data were collected using a Shimadzu LC-MS 2020. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) data were collected using an Applied Biosystem Voyager System 6174 MALDI-TOF mass spectrometer (positive reflector mode; accelerating voltage: 20 kV) and using α-cyano-4-hydroxycinnamic acid (CHCA) as the ionization matrix. Atomic force microscopy (AFM) samples were prepared on freshly cleaved mica (sample incubated for 5 min and washed twice with nanopure water) and analyzed in tapping-mode using an Asylum MFP-3D atomic force microscope. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectra were collected on a PerkinElmer Spectrum 100 FTIR with a universal attenuated total reflectance sampling accessory coupled to a computer using PerkinElmer Spectrum Express software; the peptide conjugates were dissolved in a solution of acetonitrile/nanopure water (1:1) and deposited onto the sample stage. A Q-Sense E4 quartz crystal microbalance with dissipation (QCM-D) was used to measure peptide conjugate binding on gold. Gold-coated QCM sensors (Q-Sense) were cleaned via UV-ozone treatment for 10 min, followed by heating in a 7.5:1:1 solution of double deionized water/30% H2O2/NH4OH at 80 °C for 10 min. The sensors were thoroughly rinsed with double deionized water and dried with N2. The clean gold-coated sensors were mounted in a Q-Sense window flow cell module. For peptide conjugate binding, a flow rate of 0.17 mL/min was used at a constant temperature of 23 °C, and the third overtone resonance was monitored. Nanopure water (NP H2O, 18.2 MΩ) was obtained from a Barnstead DiamondTM water purification system. All TEM and AFM measurements were made using ImageJ software. Preparation of the Peptide Conjugates. Alkyne-terminated aliphatic substrates were prepared according to protocols detailed in the Supporting Information. The peptide conjugates were prepared using copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC)60,61 in which N3-PEPAu was reacted with particular alkyne-terminated aliphatic substrates. A representative protocol for the preparation of C16-(PEPAu)2 is detailed here. The following stock solutions were prepared: A, 24.7 mM divalent alkyne in tetrahydrofuran (THF); B, 198.3 mM CuSO4 in NP H2O; C, 37.3 mM tris(3-hydroxypropyltriazolylmethyl)amine (THPTA) in NP H2O; and D, 101.0 mM sodium ascorbate in NP H2O. In a 2 mL glass vial, N3-PEPAu (3 mg, 2.23 μmol) was dissolved in 462.3 μL of THF and 337.5 μL of NP H2O. To this vial were added 37.7 μL of A, a mixture of B and C (14.1 μL of B mixed with 74.8 μL of C), and 73.6 μL of D. The vial was capped, wrapped in aluminum foil, and stirred for at least 4 h at room temperature. Dimethylformamide (100 μL) was added to the product solution. The resulting solution was passed through a 0.2 μm syringe filter (Whatman, catalog number 6789-1302). The reaction vial was washed with 400 μL of a 1:1 mixture of NP H2O and acetonitrile (CH3CN), and this wash was passed through the same 0.2 μm syringe filter and mixed with the DMF/product solution. This final solution containing the product peptide conjugate was purified using reverse-


Scheme 1

(a) Peptide conjugate consisting of a peptide headgroup and an aliphatic chain; (b) peptide conjugates associate in an end-to-end fashion via hydrophobic interactions between aliphatic tails and hydrophilic interactions between peptide headgroups; (c) assembly scheme illustrating the assembly of helical fibers from peptide conjugate building blocks (fiber width (w) and fiber thickness (d) are indicated).

conjugates (Scheme 1a) associate in an end-to-end fashion through hydrophobic interactions between the aliphatic tails and hydrophilic interactions between the peptide headgroups (Scheme 1b) to form layers with a certain width (w) that span the length of the fiber (Scheme 1c). These layers can also stack perpendicular to the fiber axis; the degree of stacking determines the fiber thickness (d) (Scheme 1c). Of course, this ignores restructuring that likely occurs because the structural model allows for direct exposure of the aliphatic tails to water. Based on this model, multiple factors could affect peptide assembly and the structural parameters of the resulting fibers. Two important factors are (i) the length of the aliphatic tail and (ii) the steric requirements of the peptide. Here, we present a study that explores the effect of these two factors on various structural metrics of assembled fibers and helical gold nanoparticle superstructures.

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EXPERIMENTAL SECTION

General Methods and Instrumentation. All chemicals were purchased from either Aldrich or Fisher and used without further purification. N3-C4H8CO-AYSSGAPPMPPF (N3-PEPAu, Figure S2) was synthesized by the University of Pittsburgh Peptide Synthesis Facility or New England Peptide. Triethylammonium acetate buffer 9493


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Langmuir phase HPLC eluting with a linear gradient of 0.05% formic acid in CH3CN and 0.1% formic acid in NP H2O (5/95 to 95/5 over 30 min). Preparation of Soft Assemblies. In a plastic vial, lyophilized peptide conjugates (∼3.75 × 10−8 mol) were dissolved in 250 μL of a 1:4 mixture of 0.1 M citrate and 0.1 M HEPES buffer. 2 μL of a 0.1 M CaCl2 solution in NP H2O was added to the peptide conjugate solution. The resulting solution was vortexed briefly and then allowed to sit undisturbed at room temperature. TEM and/or AFM samples were prepared after 1 day of incubation at room temperature. Preparation of Nanoparticle Assemblies. In a plastic vial, lyophilized peptide conjugates (∼1.87 × 10−8 to ∼7.49 × 10−8 mol) were completely dissolved in 250 μL of a 1:4 mixture of 0.1 M citrate and 0.1 M HEPES buffer and allowed to sit at room temperature for 30 min. A fresh stock solution of HAuCl4 in TEAA buffer was prepared by mixing 100 μL of 0.1 M HAuCl4 in NP H2O with 100 μL of 1.0 M TEAA buffer. The resulting mixture was vortexed for 1 min. After sitting 30 min at room temperature, 2 μL of the freshly prepared HAuCl4/TEAA solution was added to the peptide conjugate solution. A dark precipitate appeared 2−4 s after the addition of the HAuCl4/ TEAA solution; at this time, the vial was briefly vortexed and then left undisturbed at room temperature. TEM samples were prepared after 4 h of incubation at room temperature.

Table 1. Family of 12 Peptide Conjugate Molecules with Varying Number of Peptide Headgroups and Varying Aliphatic Tail Lengths

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RESULTS AND DISCUSSION Design and Synthesis of Peptide Conjugates. The unique peptide sequence is important for binding to the gold nanoparticle surface; therefore, instead of adding amino acids to PEPAu to adjust its steric requirement, we designed a series of peptide conjugates in which we varied the number of peptide headgroups to systematically control the sterics of the peptide portion. Within this series, we also adjusted the length of the aliphatic tail. In total, we prepared a new family of 12 peptide conjugate molecules containing either 1, 2, or 3 peptide headgroups and having either 12-, 14-, 16-, or 18-carbon aliphatic chains attached to their N-termini: monovalent conjugates C12-(PEPAu)1, C14-(PEPAu)1, C16-(PEPAu)1, and C 18 -(PEP Au ) 1 ; divalent conjugates C 12 -(PEP Au ) 2 , C 14 (PEPAu)2, C16-(PEPAu)2, and C18-(PEPAu)2; and trivalent conjugates C12-(PEPAu)3, C14-(PEPAu)3, C16-(PEPAu)3, and C18-(PEPAu)3 (Table 1). We used Cu(I)-catalyzed azide− alkyne cycloaddition “click” chemistry60,61 to efficiently synthesize the conjugates. The conjugates were prepared by reacting N-terminal azido-modified PEPAu (N3-PEPAu = N3C4H8CO-AYSSGAPPMPPF, Figure S2) with aliphatic tails (C12, C14, C16, or C18) functionalized with 1, 2, or 3 alkyne groups (Figure S3) (see Supporting Information for details). The identity of the alkyne substrates was confirmed using 1H NMR spectroscopy. The peptide conjugates were purified using reverse-phase HPLC, and their composition was confirmed using mass spectrometry (Figures S4 and S5). Soft Assembly of Peptide Conjugates. Soft assembly studies were performed to determine how the conjugates assemble in aqueous buffer. Each conjugate was dissolved in a mixture of HEPES buffer and citrate. These conditions were chosen because they are used for the nanoparticle synthesis and assembly experiments (vide inf ra). The −COOH at the Ctermini of the peptides are deprotonated at pH 7.3, so CaCl2 was added to the solutions to provide Ca2+ ions that could shield the negatively charged carboxylates and promote assembly of the conjugates.62 After each solution was allowed to sit at room temperature for 1 day, TEM was used to observe and characterize the soft assemblies (Figure 1). Depending on the conjugate, either 1-D fibers or small spheres/aggregate structures were observed. Fibers were the predominant product for all of the monovalent and divalent conjugates, with the

exception of C12-(PEPAu)2, for which no clear assembled structures were observed. Twisted ribbon fiber morphologies were clearly observed for the monovalent conjugates. In addition, some spherical structures were observed for C12(PEPAu)1 and C14-(PEPAu)2. Generally, more fibers were observed under TEM as the length of the aliphatic tail increases. Only small spherical assemblies/aggregate structures were observed for the trivalent conjugates. We concluded from these assembly studies that the propensity for a conjugate to assemble into fibers is dictated by the relative ratio of its hydrophobic (aliphatic tail) and hydrophilic (peptide) components. As the length of the hydrophobic tail increases, the likelihood of forming fibers increases. If the aliphatic tail is too short relative to the size of the peptide headgroup, the conjugates do not assemble into fibers. In these cases, to maximize the interactions between the hydrophobic tails, the peptide conjugates assemble into spherical/aggregate structures. The relative solubilities of the conjugates are also important. The peptide headgroup is relatively soluble due to numerous H-bonding sites along the peptide; therefore, increasing the peptide valency would decrease the overall hydrophobicity of the conjugate and lower the driving force for assembly. Having established that the monovalent and divalent conjugates assemble into fibers, we next examined how valency and aliphatic tail length affect fiber width, thickness, and pitch. Fiber widths were measured from the TEM images (Figure 2). Fibers assembled from divalent conjugates had narrower widths than those assembled from monovalent conjugates, and in both cases the aliphatic tail length does not affect the fiber width. This observation is consistent with the reported model (Scheme 1).20,59 The fiber width correlates with the extent of lateral packing, and the monovalent conjugates can presumably 9494


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Figure 1. Negatively stained TEM images of the peptide conjugate soft assemblies as a function of peptide valency and aliphatic chain length (scale bar = 100 nm): (a) C12-(PEPAu)1, (b) C14-(PEPAu)1, (c) C16-(PEPAu)1, (d) C18-(PEPAu)1, (e) C12-(PEPAu)2, (f) C14-(PEPAu)2, (g) C16-(PEPAu)2, (h) C18-(PEPAu)2, (i) C12-(PEPAu)3, (j) C14-(PEPAu)3, (k) C16-(PEPAu)3, and (l) C18-(PEPAu)3.

Figure 2. Nanofiber width distributions for (a) C12-(PEPAu)1, 12.6 ± 1.3 nm based on 60 counts; (b) C14-(PEPAu)1, 12.1 ± 1.7 nm based on 70 counts; (c) C16-(PEPAu)1, 12.6 ± 1.1 nm based on 100 counts; (d) C18-(PEPAu)1, 13.8 ± 1.1 nm based on 100 counts; (e) C14-(PEPAu)2, 8.9 ± 0.9 nm based on 70 counts; (f) C16-(PEPAu)2, 9.3 ± 1.2 nm based on 100 counts; and (g) C18-(PEPAu)2, 8.3 ± 0.9 nm based on 100 counts.

assemble more easily laterally (Figure 3a) compared to the divalent conjugates (Figure 3b) because their peptide headgroup has a smaller steric requirement. This allows for more favorable side-by-side packing for the monovalent conjugates and greater hydrophobic interactions between the aliphatic tails compared to the divalent conjugates. Peptide conjugate packing within the assemblies was studied using ATR-FTIR spectroscopy. IR spectroscopy can provide information about the internal structure of the fibers.63 Briefly, C18-(PEPAu)1, C18(PEPAu)2, and C18-(PEPAu)3 were dissolved in a 1:1 mixture of CH3CN and NP H2O, and the solution was directly deposited

onto the ATR-FTIR substrate. The solution was allowed to slowly evaporate to induce assembly. Both C18-(PEPAu)1 and C18-(PEPAu)2 formed fibers, while C18-(PEPAu)3 formed spherical structures (Figure S6). The signals observed at 2920, 2924, and 2925 cm−1 for C18-(PEPAu)1, C18-(PEPAu)2, and C18-(PEPAu)3, respectively, correspond to C−H vibrations (Figure 3d). The signal for C18-(PEPAu)1 is the same for what is observed for crystalline polymethylene chains (ν ∼ 2920 cm−1), and the signals for C18-(PEPAu)2 and C18-(PEPAu)3 are closer to what is observed for the liquid state (v ∼ 2928 cm−1).64−66 These data suggest a greater amount of disorder within the 9495


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Figure 3. Packing model of (a) C18-(PEPAu)1, (b) C18-(PEPAu)2, and (c) C18-(PEPAu)3 assemblies. (d) C−H vibrations bands in IR spectra of assemblies of C18-(PEPAu)1, C18-(PEPAu)2, and C18-(PEPAu)3.

Figure 4. TEM images of the nanoparticle assemblies as a function of peptide valency and aliphatic chain length (scale bar = 100 nm): (a) C12(PEPAu)1, (b) C14-(PEPAu)1, (c) C16-(PEPAu)1, (d) C18-(PEPAu)1, (e) C12-(PEPAu)2, (f) C14-(PEPAu)2, (g) C16-(PEPAu)2, (h) C18-(PEPAu)2, (i) C12(PEPAu)3, (j) C14-(PEPAu)3, (k) C16-(PEPAu)3, and (l) C18-(PEPAu)3.

aliphatic core of the divalent and trivalent peptide conjugate assemblies in comparison to the monovalent peptide conjugate assemblies, which is consistent with the assembly model and our fiber width measurements. Specifically, we expect greater order in the monovalent conjugate assemblies because the conjugates can more effectively pack together (Figure 3a) than

the divalent (Figure 3b) or trivalent conjugates (Figure 3c). This efficient packing leads to greater lateral assembly and larger fiber widths. From the TEM images the monovalent conjugates clearly assemble into twisted fibers. In these cases, the fiber thickness (d in Scheme 1) could be measured at the twist point where the 9496


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Figure 5. Nanoparticle diameters measured from TEM images: (a) C12-(PEPAu)1: 5.3 ± 0.9 nm based on 100 counts; (b) C14-(PEPAu)1: 6.6 ± 1.2 nm based on 100 counts; (c) C16-(PEPAu)1: 7.6 ± 1.5 nm based on 100 counts; (d) C18-(PEPAu)1: 10.2 ± 2.9 nm based on 50 counts; (e) C12(PEPAu)2: 2.9 ± 0.6 nm based on 100 counts; (f) C14-(PEPAu)2: 4.5 ± 1.1 nm based on 100 counts; (g) C16-(PEPAu)2: 5.9 ± 1.1 nm based on 100 counts; (h) C18-(PEPAu)2: 6.1 ± 1.4 nm based on 100 counts; (i) C12-(PEPAu)3: 2.8 ± 0.6 nm based on 50 counts; (j) C14-(PEPAu)3: 2.6 ± 0.5 nm based on 100 counts; (k) C16-(PEPAu)3: 3.0 ± 0.6 nm based on 50 counts; and (l) C18-(PEPAu)3: 2.8 ± 0.7 nm based on 100 counts.

width of the fiber is aligned perpendicular to the surface of the TEM grid (Figure S7a). The fiber thicknesses were ∼9 nm and remained relatively consistent between assemblies, regardless of aliphatic tail length (Figure S7b−e). We were not able to use this method to measure the thicknesses of the fibers constructed from the divalent conjugates, which appear cylindrical in the TEM images. However, because the AFM data indicate that the divalent fibers are twisted (vide inf ra), we conclude that the thickness (d) and the width (w) are nearly the same (∼9 nm). TEM and AFM were used to study the fiber helicity. From AFM, helical segments of the fibers were analyzed to determine their pitch. The fibers assembled from C12-(PEPAu)1, C14(PEPAu)1, C16-(PEPAu)1, and C18-(PEPAu)1 had average pitch values of 186 ± 13, 196 ± 11, 214 ± 7, and 238 ± 30 nm, respectively (Figures S9−S12). Because these twisted fibers were sufficiently wide, TEM could also be used to measure their pitch (Figure S8). For the divalent conjugates, C16(PEPAu)2 and C18-(PEPAu)2, the average pitch measured from AFM was 178 ± 20 and 184 ± 15 nm, respectively (Figures S14 and S15). We were not able to observe fibers for C14-(PEPAu)2 (Figures S13); instead, spherical structures were observed. We attribute this to the low yield of fibers formed for this conjugate compared to C16-(PEPAu)2 and C18-(PEPAu)2, based on the TEM data (Figure 1). From these data it was determined that the pitch (i) increases with increasing aliphatic chain length and (ii) decreases with increasing peptide valency. These results are consistent with previous reports where larger fiber width leads to twisted fibers having longer pitch.20,22 To summarize, monovalent conjugates, which have larger fiber widths (more

lateral packing), are less prone to twisting than their divalent counterparts, which have narrower widths. Nanoparticle Assembly Studies. After studying the soft assembly behavior of the fibers, we proceeded to prepare nanoparticle assemblies using the peptide conjugates. The peptide conjugates were dissolved in a mixture of HEPES buffer and citrate. HEPES functions as the primary reducing agent for the gold ions and assists in dissolving the peptide conjugates.13,67 An aliquot of a solution of HAuCl4 in 1.0 M TEAA buffer was added, and the resulting solution was vortexed and then left undisturbed at room temperature for several hours. Nanoparticle assemblies were observed by TEM (Figure 4). The morphology of the nanoparticle assemblies formed from the different peptide conjugates corresponds to the morphology of the respective soft assembly structures. 1-D nanoparticle assemblies were observed for the conjugates that assembled into 1-D fibers; randomly distributed nanoparticles were observed for the conjugates that did not assemble into fibers. In previous work, we found that conjugates that assemble rapidly into fibers tend to yield poorly formed nanoparticle superstructures with irregular particle shapes and diameters.26 On the other hand, conjugates that assemble more slowly tend to yield more well-defined superstructures with clear morphological features and regular particle diameters.8,28 In cases where conjugates assemble very slowly, either no nanoparticle assemblies or rather ill-formed assemblies are observed. A preponderance of evidence, both published and unpublished, suggests that the peptide conjugates must first associate with small gold particles prior to assembly in order to form a well-defined superstructure.26 Thus, there exists a delicate balance between particle growth, particle−peptide 9497


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Langmuir conjugate binding, and peptide conjugate assembly that must be realized to produce well-defined nanoparticle superstructures. In this study, the conjugate that assembles most rapidly into fibers, C18-(PEPAu)1, and the one that assembles most slowly into fibers, C12-(PEPAu)1, form poorly defined linear superstructures with many overlapping nanoparticles. Conjugates having intermediate assembly rates (e.g., C14(PEPAu)1, C16-(PEPAu)1, C16-(PEPAu)2, and C18-(PEPAu)2) tend to form more well-defined nanoparticle superstructures consisting of collinear chains of nanoparticles; in each of these cases, the nanoparticle superstructures exhibit regions of helicity (vide inf ra). Several structural parameters of the nanoparticle superstructures could potentially affect their properties.28,31,34 We first considered nanoparticle size within the assembled superstructures. Nanoparticle diameters (Figure 5) were measured from the TEM images. The data indicate that nanoparticle diameter within the 1-D nanoparticle assemblies is dependent on both the aliphatic tail length and the valency of the peptide conjugate. Nanoparticle diameter increases with increasing aliphatic chain length and decreases with increasing valency. These observations led us to hypothesize that the decrease in particle size with increased valency may be due to increased binding affinity associated with multivalency.68−70 An increased binding affinity could limit the growth of the nanoparticles. To test this hypothesis, we measured the equilibrium binding constants for mono- and divalent peptide conjugates using data from a quartz crystal microbalance (QCM) analysis (Figure 6). For this experiment, it was important to use mono- and divalent conjugates without aliphatic tails in order to prevent assembly before binding to the substrate (Figure S16). The equilibrium binding constant, Keq (Keq = Ka/Kd, where Ka is the association constant and Kd is the dissociation constant), of the monovalent conjugate was slightly higher than that of the divalent conjugate, although the values were very similar. By comparison, these are consistent to the binding constants of multivalent dendrons with metal oxides and repeating gold-binding peptides.71,72 One possible explanation for the statistically similar Keq values is that the proximity of the two peptides in the divalent conjugate prevents favorable peptide interaction with the substrate. It is known that AYSSGAPPMPPF adopts a specific conformation for optimal binding; a second peptide in close proximity may hinder this conformation.68,73 The variation in particle diameter within the nanoparticle superstructures, however, may be explained by the interplay between fiber formation and nanoparticle binding (vide supra). For the conjugates that assemble into fibers, the nanoparticle diameter increases as the propensity to form fibers increases, as illustrated in going from C12-(PEPAu)1 to C18-(PEPAu)1 (Figure 5a−d) and in going from C14-(PEPAu)2 to C18-(PEPAu)2 (Figure 5f−h). If the peptide conjugate monomers rapidly assemble into fibers, they have less time free in solution to cap the growth of the growing nanoparticles. This leads to larger particle diameters. Conversely, if the peptide conjugate monomers slowly assembly into fibers, they will have more time free in solution to cap the growth of the nanoparticles, leading to smaller nanoparticle diameters. When nanoparticle assembly was not observed (C12-(PEPAu)2, C12-(PEPAu)3, C14(PEPAu)3, C16-(PEPAu)3, and C18-(PEPAu)3), the particles are uniformly small (∼3 nm), independent of valency and tail length (Figure 5e,i−l). In these cases, the peptide conjugates

Figure 6. QCM binding data for the (a) monovalent and (b) divalent peptide conjugates. (c) The association binding constant, Ka, dissociation binding constant, Kd, and equilibrium binding constant, Keq, were calculated from the QCM experiments.

remain in solution and have adequate time to bind to the growing nanoparticles to cap their growth. We next considered structural metrics of the linear nanoparticle superstructures, including the distance between the collinear nanoparticle chains as well as the helical pitch. The interchain distances of the nanoparticle superstructures constructed using C14-(PEPAu)1, C16-(PEPAu)1, C16-(PEPAu)2, and C18-(PEPAu)2 were measured via TEM (Figure S17). These conjugates were chosen because they yielded the most welldefined superstructures in which two collinear chains can clearly be identified. The interchain distances for the C14(PEPAu)1 and C16-(PEPAu)1 superstructures were 5.9 ± 1.2 and 5.6 ± 1.2 nm, respectively, while those for the C16-(PEPAu)2 and C18-(PEPAu)2 superstructures were 4.6 ± 1.2 and 5.0 ± 1.4 9498


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Figure 7. TEM images of nanoparticle assemblies showing regions of helicity and pitch measurements from (a) C16-(PEPAu)1 and (b) C18-(PEPAu)2 (scale bar = 100 nm).

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nm, respectively. Although we cannot determine whether the particles are bound to the edge or the face of the fibers (Figure S18), the similar interchain distances suggests that the particles are bound to the fiber faces because the fiber thicknesses are approximately the same, whereas the fiber widths for the monovalent and divalent are different. Nanoparticle superstructures formed using C14-(PEPAu)1, C16-(PEPAu)1, C16(PEPAu)2, and C18-(PEPAu)2 each exhibit regions of helicity (Figure 7 and Figure S19). The helicity is most clearly defined in the C16-(PEPAu)1- and C18-(PEPAu)2-based nanoparticle superstructures (Figure 7). The average pitch for C16-(PEPAu)1 and C18-(PEPAu)2, ∼204 and ∼173 nm, respectively, are similar to the average pitch values of the C16-(PEPAu)1 and C18(PEPAu)2 nanofibers as determined by AFM (214 ± 7 and 184 ± 15 nm, respectively). These measurements suggest that the peptide conjugate assembly directs the structure of the nanoparticle assembly. Moreover, these results, when taken together with our reported results on C12-PEPAu double-helical gold nanoparticle superstructures8,28 (pitch ∼85 nm), indicate that modifications to the peptide conjugate can allow one to adjust the pitch of a double-helical nanoparticle assembly.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02208. Supplementary data and synthetic protocols detailing the preparation of the peptide conjugate molecules (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.L.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Science Foundation (DMR-0954380, N.L.R.) and the Air Force Office of Scientific Research (FA9550-11-1-0275, N.L.R. and G.C.S.). R.R.N. also acknowledges support from the Air Force Office of Scientific Research.

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CONCLUSION We have prepared a family of peptide conjugate molecules in which the aliphatic tail length and peptide valency were systematically varied. The conjugates have an increased propensity to form twisted 1-D fibers as the tail length increases and the valency decreases. In cases where twisted 1-D fibers formed, we found that the pitch of the fibers was inversely proportional to valency and proportional to tail length while the width of the fibers decreased with increasing valency. The peptide conjugates were used to direct the assembly of nanoparticle superstructures, whose morphology and structural parameters correspond to those of the soft assemblies. The metrics of the nanoparticle superstructures, including interchain distance and helical pitch, correspond closely to the metrics of the conjugate soft assemblies. These results illustrate the ability to control and tailor the metrics of 1-D helical nanoparticle superstructures through synthesis and assembly of peptide conjugate molecules, and they represent an important step forward in our ability to prepare programmable nanoparticle superstructures by design.

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Adjusting the Metrics of 1-D Helical Gold Nanoparticle

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