Article pubs.acs.org/Langmuir
Isolation of Template Effects That Control the Structure and Function of Nonspherical, Biotemplated Pd Nanomaterials Rohit Bhandari and Marc R. Knecht* Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, Florida 33146, United States S Supporting Information *
ABSTRACT: Advances in nanotechnology have indicated that the passivant and the inorganic surface play a pivotal role in controlling the structure/function relationship of materials. Beyond standard materials-based methods, bioligands have recently demonstrated the production of unique nanomaterial morphologies for application under ambient conditions for multiple activities, such as catalysis and biosensing. We have recently demonstrated that a biotemplate technique could be employed to produce spherical and linear Pd nanostructures in water using a self-assembling peptide framework. The materials possessed high catalytic reactivity that was controlled by the three-dimensional structure of the composite materials. To investigate the effect of the peptide template on the reactivity of Pd nanostructures, an in depth analysis of the catalytic activity of Pd nanostructures fabricated via truncated templates is presented. The new templates were designed from portions of the original framework, which demonstrated unique synthetic and functionality control. Two different reactions, Stille C−C coupling and 4-nitrophenol reduction, were employed to ascertain the effect of template structure on the reactivity of synthesized Pd nanomaterials via changes in reagent diffusion through the bioscaffold. The results indicate that the peptide framework plays an important role and could be used to tune and optimize the functionality of the final composite materials for the target application.
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INTRODUCTION Extensive research toward the development and optimization of nanomaterials has been conducted due to their potential wide range of promising applications, from electronics1 and biosensors2,3 to catalysis.4−9 Noble metal nanoparticles are particularly important due to their high degree of stability and broad functionality. One specific noble metal, Pd, has already been shown to play an important role in various catalytic reactions, thus making it an attractive tool for industrial applications.10−14 Due to the high surface to volume ratio achieved at the nanoscale, reactive materials with enhanced functionality are possible for individual reactions to conserve precious metal catalyst. An important set of Pd-catalyzed systems include C−C cross coupling reactions such Stille, Suzuki, Heck, and Sonogashira couplings.8,14 These reactions are important for the synthesis of various bioactive compounds, pharmaceuticals, molecular electronics, as well as a variety of other organic structures, thus making the design and optimization of individual catalysts important for efficient application. Furthermore, most of these couplings employ harsh conditions such as high temperatures15 and organic solvents with large catalyst loadings.14 As such, efforts are being made to continuously promote a greener approach for carrying out these reactions under room temperature and aqueous conditions. Such simple changes are challenging at the chemical level; however, they would represent a significant decrease in energy consumption, which is attractive in light of the current global energy condition. © 2012 American Chemical Society
To fabricate catalytically reactive Pd nanostructures, numerous systems have been designed that exploit peptides,5,16 resins,17 dendrimers,15,18,19 alkanethiols,20 and polymers21−23 to prepare the materials. These systems can become complex, as the passivant must stabilize the materials to prevent aggregation without poisoning the metal surface. An ideal ligand would encapsulate the structure without binding to the material to maximize the solution exposed catalytic surface. Moreover, by employing these systems for the fabrication of different nanostructures, a uniform degree of shape and size control could be realized that is critical for controlling the overall activity. For instance, Feng et al. have employed functionalized poly ethylene glycol to prepare water-soluble Pd nanoparticles, which can be used as a catalyst for the aerobic oxidation of alcohols in water.24 They proposed that the size and the surface properties of the Pd nanoparticles, as dictated by the passivant, play a critical role in determining the catalytic efficiency. Many groups, notably those of Crooks and Astruc, have studied olefin hydrogenation and carbon-coupling reactions using dendrimer-encapsulated Pd nanoparticles where the size of the particles depends on the number of Pd2+ ions initially loaded into the dendrimer template.8,18,25 From this, strict control over the nanoparticle size can be achieved where the surface of the catalytic materials is highly exposed for efficient functionality. From most catalytic studies, it remains clear that while the use of passivating ligands Received: June 28, 2011 Published: May 16, 2012 8110
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Table 1. Size Analysis for the T1, T2, T1A, and T2A Peptide Templates with and without Pd2+a Pd2+/peptide aggregate size (nm)
peptide aggregate size (nm)
a
peptide
sequence
T1 T2 T1A T2A
SSKKSGSY SGSKGSKRRIL SSKKSGSYRRIL SGSKGSK
in water 389 566 561 370
± ± ± ±
Pd30
27 28 77 10
614 972 775 979
± ± ± ±
7.0 52 73 56
Pd60 728 1219 1416 1419
± ± ± ±
80 157 14 277
Pd90 3255 3643 4307 2323
± ± ± ±
140 77 299 448
Data reported using size distribution by % intensity.
motif suggested to be responsible for peptide assembly into the T2 sequence only. These truncates were chosen because of their similar amino acid composition, charge distribution, and hydrophilicity. The other two peptides (T1A and T2A) were designed by shifting the RRIL motif from the T2 sequence to the C-terminus of the T1 truncate to further elaborate the effects of peptide assembly on the fabrication and reactivity of nonspherical Pd nanomaterials. Using all of these peptides, the production of different Pd nanostructures, ranging from spherical to nanoribbons to nanoparticle networks (NPNs), was achieved that depends upon the Pd concentration employed in the reaction. The formation of nonspherical materials using the T1 and T2A peptides in the absence of the assembling RRIL motif was surprising; however, analysis of the synthesis of these materials demonstrated that peptide assembly, and thus initial template formation, was evident. The inorganic materials were fully characterized using UV−vis spectroscopy, transmission electron microscopy (TEM), and dynamic light scattering (DLS), all of which confirmed the formation of polycrystalline Pd nanomaterials, encapsulated within the bioscaffold, that possessed reactivity for both Stille coupling and 4-nitrophenol reduction. Comparing this reactivity to the materials templated using the parent R5 sequence demonstrated enhanced reactivity for nanostructures prepared with one sequence (T1) and diminished catalytic functionality for the other materials. This suggests that the peptide template could be directly manipulated to modify and enhance the structure-control and activity, which could be important for the development of energy neutral pathways for important catalytic reactions.
provides a unique pathway toward the refinement of nanomaterial size and structure, these same ligands may also sterically hinder access to the reactive metallic surface. As a result, studies are necessary that focus on the design of passivating agents that optimize catalyst surface exposure while maintaining particle stability before, during, and after the reaction. Furthermore, the passivating structure could potentially be designed to impart additional functionality that may not be achievable using a metallic particle with the ligands bound directly to the surface. Beyond traditional materials approaches, many groups have recently begun to turn to biology for inspiration in the design of functional materials that operate under green conditions.26 From this, biomacromolecules such as oligonucleotides, peptides, proteins, and viruses have been used to fabricate functional inorganic nanomaterials with applications in biomedical technologies, energy storage, catalysis, and directed assembly.4,26−33 Unfortunately, the final structure of these materials remains difficult to control beyond spherical, where shaped materials may have important applications in various technologies, including photonics and catalysis;34−36 however, recent efforts have begun to demonstrate the formation of linear, cubic, and tetrahedral nanostructures using peptides.37,38 In that regard, we have recently shown that, by employing a bioinspired approach, nonspherical, linear-like nanostructures can be produced using a peptide template.4,5 Here, the R5 peptide, (SSKKSGSYSGSKGSKRRIL) isolated from the diatom Cylindrothica f usiformis,39 has been shown to selfassemble via the C-terminal RRIL motif to generate amine rich aggregates40 that can sequester Pd2+ ions and encapsulate zerovalent Pd materials. On the basis of the concentration of Pd in the reaction, the scaffold directs the formation of nonspherical nanoparticle networks (NPNs) that demonstrated reactivity for the C−C coupling Stille reaction and the reduction of nitro functional groups.5 The overall reactivity of the system is controlled by two factors dictated by the peptide template: the inorganic surface area and reagent diffusion through the bio framework. To further enhance the reactivity of this biobased system, optimization of these two structural factors could be employed. While modification of the inorganic materials may be possible, they are already 100 nanostructures was used to determine individual size distributions. Numerous particle images from multiple surfaces of the TEM grid were used for size determination. 1H NMR measurements were performed using a Varian Inova 400 MHz spectrometer with a quadruply tuned switchable probe or a Bruker 400 MHz NMR spectrometer with multinuclear inverse and broadband 5 mm probes. For UV−vis analysis, an Agilent 8453 UV−vis spectrophotometer was 8111
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employed for the characterization of the Pd nanomaterials and the 4nitrophenol reduction reactions. For all studies, a 1.00 cm path length quartz cuvette (Starna) was employed. DLS analyses were conducted using a Malvern Zetasizer Nano ZS instrument with size detection range of 0.3 nm to 10 μm, where 500 μL of the sample was used in a folded capillary clear disposable ζ cell. Synthesis of Pd Nanostructures. Truncate peptides T1, T2, T1A, and T2A were synthesized using standard FMOC synthetic protocols employing a TETRAS peptide synthesizer (CreoSalus; Louisville, KY).41 The peptides were purified using reverse phase HPLC and characterized using MALDI-TOF. For the fabrication of Pd nanostructures using the T1 peptide, 2.06 μL of an aqueous peptide solution (10.0 mg/mL = 11.8 mM) was diluted in 3.00 mL of deionized water. To individual reactions, 7.32 μL, 14.70 μL, or 22.05 μL of a freshly prepared, aqueous K2PdCl4 solution (100 mM) was added, resulting in the formation of reactions with a Pd/peptide ratio of 30, 60, and 90, termed the T1-Pd30, T1-Pd60, and T1-Pd90 samples, respectively. The solution was allowed to stir for 15.0 min, followed by the addition of 75.0 μL of a freshly prepared NaBH4 solution (100 mM). The materials were allowed to fully reduce while stirring for 1.00 h to ensure Pd reduction. The reactions were then dialyzed using cellulose dialysis tubing with a molecular weight cutoff of 14 kDa for 24.0 h against deionized water. Identical procedures were used for the synthesis of Pd materials employing the T2, T1A, and T2A peptides; however, the volume of the peptide solution added to the reaction was adjusted on the basis of the stock concentration to reach the same reaction concentration for materials synthesis. The materials were used for the catalytic reactions immediately following dialysis. Catalytic Stille Coupling. The template-encapsulated Pd nanostructures were employed as catalysts for the Stille reaction using standard procedures.4,5 Briefly, 124 mg of 4-iodobenzoic acid (0.50 mmol) was dissolved in 8.00 mL of 2.25 M KOH followed by the addition of 98.6 μL of PhSnCl3 (0.60 mmol) as the transmetalation reagent. The solution was allowed to fully dissolve under vigorous stirring, followed by the addition of the dialyzed Pd nanomaterials with loadings ranging from 0.001 mol % Pd to 0.500 mol % Pd. The reaction was allowed to proceed for 24.0 h, after which it was quenched with 50.0 mL of 5.00% HCl. The product was then extracted three times with diethyl ether followed by a saturated NaCl wash. The organic layer was dried with Na2SO4, and 75.0 mg (0.50 mmol) of ptert butyl phenol was added as an internal standard. The ether was evaporated, from which ∼1−2 mg of the solid material was dissolved in 1.00 mL of CDCl3 and analyzed for product yield using 1H NMR spectroscopy. For turnover frequency (TOF) measurements, the reaction was scaled up by 5-fold, from which 4.00 mL aliquots were analyzed at 30.0 min time intervals. Reduction of 4-Nitrophenol. The reduction of 4-nitrophenol using the peptide-based Pd nanostructures was studied using UV−vis spectroscopy employing previously described methods.5,42 For the materials prepared using the T1 peptide, 1.00 mL of T1-Pd30, 0.50 mL of T1-Pd60, and 0.335 mL of T1-Pd90 solutions were added to separate cuvettes respectively, followed by the addition of 0.500 mL of a freshly prepared NaBH4 solution (63.0 mM). Such volumes of the Pd nanomaterials were selected so that the total Pd concentration in solution remained constant at 79.0 μM. The mixture was allowed to stand for 15.0 min, after which 2.00 mL of an 85.0 μM 4-nitrophenol solution was added to the cuvettes with the T1-Pd60 and T1-Pd90 materials, while 1.50 mL of the 112.0 μM 4-nitrophenol solution was added to the cuvette with T1-Pd30 materials. On the basis of this procedure, the 4-nitrophenol reagent concentration was 56.0 μM in all reaction mixtures. Immediately after the addition of 4-nitrophenol, UV−vis spectra were obtained at time intervals of 7.00 s at temperatures ranging from 10.0 °C to 60.0 °C. All spectra were background subtracted against their respective Pd nanomaterial solution at the reaction concentration in deionized water. Identical analyses were conducted for the Pd materials synthesized using the T2, T1A, and T2A peptides.
Article
RESULTS AND DISCUSSION Nanostructure Synthesis and Characterization. To elucidate the bioframework effect on the reactivity of peptidetemplated, nonspherical Pd nanomaterials to optimize their catalytic functionality, truncations of the peptide sequence were probed. The scaffold is known to significantly affect reagent diffusion, where smaller templates could generate nonspherical structures that present the reactive materials closer to the solution interface.4,5 Here, the T1 and T2 truncates, which represent the first and second half of the parent peptide sequence, respectively, were employed. Furthermore, the T1A and T2A peptides were generated that swapped the RRIL assembling motif from the T2 peptide to the C-terminus of the T1 sequence. Using these four peptides, separate templatebased nanomaterial syntheses were studied that varied the Pd/ peptide ratio from 30 to 120, where the peptide concentration remained constant. In this setup, Pd2+ ions were incubated with the peptide, followed by NaBH4 reduction in water. For all reactions regardless of the template at ratios ≤90, a pale yellow solution was generated prior to reduction. The solution color turned deep brown after BH4− addition, consistent with the formation of stable Pd nanomaterials. For reactions with higher Pd/peptide ratios, Pd black precipitation was noted; thus, such samples were not studied further. The synthetic scheme and final materials were then fully characterized. Figure 1 presents the UV−vis spectra of the nanomaterials postreduction. Figure 1a specifically presents the analysis for
Figure 1. UV−vis spectra of the (a) T1-, (b) T2-, (c) T1A-, and (d) T2A-encapsulated Pd nanostructures postreduction.
the structures stabilized by the T1 peptide. For these materials, a broad absorbance spectrum is noted that increases in intensity toward lower wavelengths. In general, such a spectrum is consistent with colloidal suspensions of Pd nanomaterials,43 including the Pd materials formed using the parent R5 peptide,4 suggesting such structures could result by using the truncated sequence. Furthermore, the intensity of the absorbance across the spectrum increases for the three different samples proportional to the Pd:peptide ratio used in the synthesis. Such results are likely due to the concentration of inorganic materials in the sample, where higher concentrations would result in increased absorbance. Similar observations were noted 8112
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of zerovalent, face centered cubic (fcc) Pd. Finally, using Pd/ peptide ratios >90 employing the T1 sequence resulted in bulk Pd black precipitation, suggesting that the system becomes oversaturated with Pd2+, leading to unstable structures. Note that this value is lower than the Pd/peptide ratio where bulk precipitation is observed for the parent R5 peptide template (150).4 This suggests that a smaller peptide scaffold is present that can sequester fewer Pd2+ ions, consistent with the peptide truncation. Analysis of the three different systems generated using the T2 peptide is presented in Figure 2b. Using this template, which possesses the C-terminal RRIL motif of the parent R5 peptide, similar Pd nanostructures to those prepared using the T1 peptide are generated. Using the T2-Pd30, T2-Pd60, and T2-Pd90 reaction systems, nanospheres, short nanoribbons, and NPNs are generated, respectively. Interestingly, the dimensions of the materials are also quite similar to those prepared in the T1 system. For instance, the nanoparticles fabricated in the T2-Pd30 reaction possessed an average diameter of 3.0 ± 0.5 nm, which is quite similar to the average diameter of the nanoparticles prepared in the T1-Pd30 system (3.2 ± 0.8 nm). Considering the T2-Pd60 and T2-Pd90 materials, nanoribbons and NPNs with average widths of 3.7 ± 0.6 nm and 4.2 ± 0.6 nm, respectively, are observed, which are again reminiscent of the values for the materials prepared at the same Pd/peptide ratio in the T1-based system. Finally, for the NPN-based structures, polycrystalline materials are noted, which is consistent with a nanoparticle-based aggregation method, suggesting that similar processes as compared to the mechanism of the peptide-template approach of the parent R5 peptide4 are employed using the truncates. Transferring the RRIL motif from the T2 to the T1 sequence to generate the T1A and T2A peptides demonstrated minimal changes to the resultant material morphologies. For the T1Abased system (Figure 2c), TEM analysis noted the production of spherical particles for both the T1A-Pd30 and T1A-Pd60 materials, with an average diameter of 3.4 ± 0.7 nm and 4.0 ± 0.5 nm, respectively. For the T1A-Pd60 system, short nanoribbons were also observed along with the spherical structures, suggestive of partial aggregation of the nanoparticles. For the T1A-Pd90 materials, dense networked structures were observed with an average width of 5.2 ± 0.7 nm. Figure 2d presents the TEM analysis of T2A-encapsulated Pd nanomaterials, where spherical particles of 3.9 ± 0.8 nm were observed. For the T2A-Pd60 materials, larger spherical nanoparticles were noted with an average size of 4.3 ± 0.9 nm, along with the presence of short nanoribbons. Finally, for the highest ratio, T2A-Pd90, dense NPNs were observed with an average width of 4.4 ± 0.7 nm. Taken together, highly similar Pd nanostructures with complementary dimensions were generated for all four peptide truncates at the selected Pd/peptide ratios, suggesting that a similar synthetic mechanism is driving the materials fabrication process. The generation of highly similar structures using the multiple peptides was initially quite surprising. Previous studies using the native R5 peptide demonstrated the production of nanospheres, nanoribbons, and NPNs via a self-assembling peptide template.4,5 In this mechanism, the C-terminal RRIL motif is known to drive R5 peptide assembly,40 which generates a peptide scaffold that displays a large number of binding sites to sequester Pd2+ from solution and direct the final inorganic morphology postreduction. Immediately upon reduction, Pd nanoparticles are formed within the bioscaffold that aggregate
for the Pd nanomaterials generated using the T2, T1A, and T2A peptides, as shown in parts b−d, respectively, of Figure 1, where broad absorbances that increased on the basis of the Pd/ peptide ratio were observed. Interestingly, when comparing the spectra of the materials prepared at the same Pd/peptide ratio using the different peptide templates, for instance the Pd30 samples, nearly identical UV−vis spectra were achieved. This suggests that similar inorganic structures may be prepared for each sample; however, their bioscaffold is likely different on the basis of the different peptide sequences used to template the materials. To characterize the structures for all of the materials fabricated using the four different peptides, TEM analysis was conducted, as presented in Figure 2. Figure 2a presents the
Figure 2. TEM analysis of the materials fabricated using the (a) T1, (b) T2, (c) T1A, and (d) T2A peptide templates at selected Pd/ peptide ratios. Selected inserts present the corresponding highresolution TEM image for the particular material. Scale bar = 20 nm; insert scale bar = 2 nm.
TEM images for the T1-Pd30, T1-Pd60, and T1-Pd90 materials. As is evident, spherical materials were prepared at the lowest Pd/peptide ratio of 30 that possessed an average diameter of 3.2 ± 0.8 nm. For the materials prepared at a Pd/ peptide ratio of 60, T1-Pd60, elongated and nonspherical structures are observed. Here, short nanoribbons are generated with an average width of 3.1 ± 0.7 nm. Finally for the T1-Pd90 sample formed at the highest Pd/peptide ratio, highly networked, linear NPN structures are prepared. In this sample, the length of the NPNs increases as compared to the T1-Pd60 nanoribbons; however, the width of the materials increased only slightly to an average value of 4.3 ± 0.6 nm. The inset of the T1-Pd90 figure presents a high-resolution analysis of the NPNs, which displays polycrystalline materials with lattice fringes of 2.4 Å that are consistent with the {111} plane spacing 8113
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Scheme 1. Proposed Scheme for NPN Formation Using the Peptide Templatesa
a
Initially the peptide aggregates bind the Pd2+ ions, which can become cross-linked. Upon reduction, spherical nanoparticles form where linear assembly within the scaffolds occurs at high Pd concentrations leading to the formation of template-encapsulated, networked nanostructures.
via metal ion coordination. This likely occurs from Pd/peptide template cross-linking, which arises from the formation of Pd2+−amine bonds between two bioscaffolds, as shown in Scheme 1. The extent of template cross-linking depends on the amount of Pd2+ present in solution, where greater concentrations result in the formation of larger aggregates; this would be anticipated due to the increased number of cross-linking Pd−amine bonds in the sample. Subsequent reduction by NaBH4 then results in the formation of the linear-based structures at high Pd2+ concentrations due to template-directed nanoparticle aggregation. On the basis of the peptide assembly analysis and the differences observed in template sizes, it is highly likely that the peptide scaffold for all of the materials is quite different. As such, the different biological frameworks may affect the catalytic reactivity of the encapsulated, nonspherical materials. From this, the variations in functionality observed could be directly correlated to the peptide scaffold and reagent diffusion, as nearly identical inorganic structures with comparable surface area are prepared using the different templates. Catalytic Analysis. All of the materials fabricated using the T1, T2, T1A, and T2A templates were employed as catalysts for both the Stille coupling reaction and the reduction of 4nitrophenol. Previous studies of the R5-based materials have shown that the Pd structures can be used as efficient catalysts for these reactions under ambient conditions in water.4,5 The reactivity of the R5-templated nanostructures was dependent upon the composite structure of the materials, which controlled the surface area and the diffusion of the reagents within the peptide framework. By using the shorter truncated sequences, significant alterations in the reagent diffusion are anticipated within the extended peptide framework, which could be used to enhance and/or optimize materials functionality. Furthermore, as the shapes of the inorganic structures are quite similar for the samples prepared using the different peptides at the same Pd/ peptide ratio, the effects of metallic nanoparticle structure are anticipated to be nearly equal; thus, changes in the reactivity can be attributed to the peptide template diffusion effects. To study the Stille catalytic activity of the materials, a model coupling reaction of 4-iodobenzoic acid with PhSnCl3, presented in Figure 3a, was employed. The reaction was
to form the linear-based materials. Similar structures were expected for the T2 and T1A truncates that were anticipated to self-assemble via the RRIL component; however, production of comparable structures by the T1 and T2A truncates was quite surprising. These sequences do not possess the self-assembling RRIL motif, and therefore, they were anticipated to remain independent in solution. To determine the assembly states of all four peptides, DLS analyses were conducted. Table 1 presents the average size of the peptide aggregates in water and in the presence of Pd2+ ions for the different ratios employed. All analyses were preformed in triplicate, from which the Zaverage size is reported. For the T1 and T2 peptides, aggregate sizes of 389 ± 27 nm and 566 ± 28 nm were observed, respectively, both of which are smaller than aggregates for the parent R5 sequence (825 nm).40 For the T1 peptide, when the system was then studied in the presence of Pd2+ metal ions, progressively larger aggregate sizes were noted. For instance, for the T1-Pd30 sample, an aggregate size of 614 ± 7.0 nm is observed. The aggregate size then increases with increasing amounts of Pd2+ in solution such that aggregates of 728 ± 80 nm and 3255 ± 180 nm were observed for the T1-Pd60 and T1-Pd90 ratios, respectively. Similar size increases were observed for T2 peptide with Pd2+, where aggregates of 972 ± 52 nm, 1219 ± 157, and 3643 ± 77 nm were noted for the T2−30, T2−60, and T2−90 materials, respectively. For the T1A and T2A peptides, similar effects were observed where peptide aggregation was noted for both samples. For instance, aggregates with dimensions of 561 ± 77 nm and 370 ± 10 nm were observed for the T1A and T2A peptides in the absence of metal ions. Similar to the T1 and T2 systems, when increasing concentrations of Pd2+ were added to these initial templates, larger aggregates were noted. Taken together, these sizing results suggest that all of the peptides can assemble into biological frameworks, even sequences without the RRIL motif. Interestingly, the peptides that do possess the RRIL component do assemble to form larger templates, supportive of its role in the assembly process; however, these results suggest that other factors in the sequence are also able to drive peptide assembly. This is evident by template formation for the T1 and T2A peptides that do not possess the RRIL motif. Furthermore, the presence of the Pd2+ ions drives additional peptide aggregation 8114
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for T2A-Pd60 and T2A-Pd90 materials at a Pd loading of ≥0.005 mol % Pd. Interestingly, substantially higher Pd loadings are required for the T2A-Pd30 materials to generate quantitative product yields (0.05 mol % Pd). The inset in Figure 3e displays the product yield variation at low catalyst concentrations, which demonstrates the diminished catalytic activity for the T2A-Pd30 materials. For a comparison, when Pd nanoparticles, nanoribbons, and NPNs are generated via the R5 peptide, quantitative product yields are observed at a Pd loading of ≥0.01 mol %, respectively.4 Note that these structures are generated at different Pd/peptide ratios of 60, 90, and 120, respectively, due to the larger bioscaffold generated by the parent template. For a more detailed analysis of the reactivity and the catalytic ability of the different nanostructures, determination of the catalytic TOF for each sample was conducted. For this study, the reaction was scaled up 5-fold and aliquots were extracted at selected time intervals that were quantitated to monitor the progress of the reaction. Figure 4 presents a bar chart
Figure 3. Loading analysis for the Stille coupling reaction, presented in part (a), catalyzed by the materials fabricated with the (b) T1, (c) T2, (d) T1A, and (e) T2A peptide templates. The plots present the product yield as a function of the concentration of the Pd catalyst employed in the reaction.
processed at selected Pd loadings, calculated on the basis of the total Pd concentration in solution, from which the product was extracted and quantitated after 24.0 h. Figure 3b presents the loading analysis for the materials prepared using the T1 template. Here, it is evident that quantitative yields were achieved for the T1-Pd30, T1-Pd60, and T1-Pd90 materials at Pd loadings as low as 0.005 mol %, while the inset demonstrates that lower product yields were achieved at 0.001 mol % Pd under identical conditions. At this lowest Pd loading, a percent yield of 12 ± 1.7% was achieved using the T1-Pd90 system, while slightly higher values of 16 ± 3.5% and 16 ± 2.5% were observed for the T1-Pd30 and T1-Pd60 systems, respectively. Figure 3c presents the loading analysis plot for the T2-based materials, where somewhat different results were observed as compared to the T1-based nanostructures. For these materials, quantitative product yields were achieved at a Pd loading of 0.005 mol % for the T2-Pd60 and T2-Pd90 materials; however, for the T2-Pd30 structures, Pd loadings of 0.010 mol % are required to achieve such results. Furthermore, minimal to no activity is observed for the T2Pd30 system at Pd loadings
