Article pubs.acs.org/Langmuir
Hybrid Amyloid Membranes for Continuous Flow Catalysis Sreenath Bolisetty, Mario Arcari, Jozef Adamcik, and Raffaele Mezzenga* Department of Health Sciences and Technology, ETH Zurich, Schmelzbergstrasse 9, 8092 Zurich, Switzerland S Supporting Information *
ABSTRACT: Amyloid fibrils are promising nanomaterials for technological applications such as biosensors, tissue engineering, drug delivery, and optoelectronics. Here we show that amyloid−metal nanoparticle hybrids can be used both as efficient active materials for wet catalysis and as membranes for continuous flow catalysis applications. Initially, amyloid fibrils generated in vitro from the nontoxic β-lactoglobulin protein act as templates for the synthesis of gold and palladium metal nanoparticles from salt precursors. The resulting hybrids possess catalytic features as demonstrated by evaluating their activity in a model catalytic reaction in water, e.g., the reduction of 4nitrophenol into 4-aminophenol, with the rate constant of the reduction increasing with the concentration of amyloid−nanoparticle hybrids. Importantly, the same nanoparticles adsorbed onto fibrils surface show improved catalytic efficiency compared to the same unattached particles, pointing at the important role played by the amyloid fibril templates. Then, filter membranes are prepared from the metal nanoparticle-decorated amyloid fibrils by vacuum filtration. The resulting membranes serve as efficient flow catalysis active materials, with a complete catalytic conversion achieved within a single flow passage of a feeding solution through the membrane.
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utilizing these nanocomposite materials is via continuous flow catalytic membranes,20 which has several advantages compared to conventional mixing of catalysts within the reaction mixture.14,21 Specifically, this allows flow and catalysis reactions to occur simultaneously22,23 and avoids the need of dispersing and recovering the catalyst from the reaction mixture. In this contribution, we show that amyloid fibrils produced from inexpensive protein precursors can serve as a carrier system for catalytic metallic nanoparticles attached onto their surface by chemical reduction of metal salt precursors and that these systems can also be used as solid membranes for continuous flow catalysis, mimicking the industrial continuous flow reactor processes. Amyloids are fibrillar formations generated by protein selfassembly with a characteristic cross β-sheet structure along the fibrillar direction.24 To rely on affordable precursors, amyloid fibrils were prepared using food grade and biocompatible βlactoglobulin25 proteins and used as templates upon which gold and palladium ions were reduced into metal nanoparticles. Since these fibrils are prepared at very acidic conditions, they possess strong positive charges and are extremely stable in solution due to electrostatic repulsion. These fibrils have several beneficial properties, including the ability to stabilize and disperse water insoluble graphene oxide26 or carbon nanotubes.27 Further, amyloid fibrils can act as building blocks28,29 to prepare wide ranges of hybrid30 inorganic nanomaterials31,32
INTRODUCTION Catalysts increase the rate of the reaction via the creation of alternative reaction pathways by making or breaking chemical bonds.1 Because of the action of catalytic materials,2 it is possible to reduce the temperature or pressure of reaction conditions, which ultimately reduces the major industrial production cost. Recently, much attention has focused on metal nanoparticles3,4 for catalysis purposes.5,6 Metal nanoparticles7 provide efficient catalytic activity8,9 as they have a high surface/volume ratio.10 Sustainable metal nanoparticle catalysts11 are distinguished by features such as high catalytic activity, high stability, low preparation cost, and efficient recyclability. Furthermore, an efficient recovery of the metal nanoparticle catalysts from the reaction medium is essential for industrial applications. However, established methods for the production of catalysts such as filtration or centrifugation cannot be adapted for particles on the nanoscale. This ultimately diminishes the potential benefits of the nanocatalyst strategy. In order to improve the recovery and cost of metallic nanoparticles, these can be synthesized on the surface of less expensive carrier systems by in situ reduction of metal ions.12 The resulting catalytic properties of these nanocomposite materials depend on both the characteristic features of nanoparticles and properties of carrier systems.13 For example, carrier systems should not block surface of the nanoparticles, and no coagulation of the carrier system should take place during generation of nanoparticles.14 So far, the reported carrier systems have been mainly polymers,15,16 dendrimers,17 colloids,18 and surfactants.19 The most efficient method of © XXXX American Chemical Society
Received: August 26, 2015 Revised: November 29, 2015
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Figure 1. Schematic representation of metal nanoparticles-decorated amyloid fibrils and preparation of the catalytically active filter membrane reactor. (a) β-lactoglobulin amyloid fibrils. (b) Metal nanoparticles (gold and palladium) preparation on the surface of the fibrils by reducing corresponding salts using sodium borohydride. (c) The nanoparticle−amyloid fibrils in water, or a filter membrane reactor made thereof, catalytically convert 4-nitrophenol into 4-aminophenol by a wet catalysis or flow reaction process, respectively. The solution color changes after conversion. 100 °C. The filtered protein solution is filled in the clean dialysis membrane and dialyzed for 1 week while the water is changed daily (20-fold excess of water). After the dialysis the solution is readjusted to pH of 2 and freeze-dried. The amyloid fibrils were produced by heating the 2 wt % purified protein monomer (pH 2) at 90 °C for 5 h. Hydrogen tetrachloroaurate (HAuCl4), sodium tetrachloropalladate (NaPdCl4), 4-nitrophenol, and sodium borohydride were purchased from Sigma-Aldrich and used without any further purification. Metal Nanoparticles Synthesis. Gold and palladium nanoparticles were prepared by initial mixing of 1 mL of β-lactoglobulin fibril solution (0.2 wt %) with 20 μL of 0.1 M HAuCl4 and Na2PdCl4 dissolved salt, respectively. After an incubation of 15 min at room temperature (23 °C), 20 μL of the reducing agent (0.056 M NaBH4) was added. The reduction of the metal salts induces the formation of metal nanoparticles.39 Then the solution was centrifuged (5000 rpm for 15 min) and washed 5 times to remove the unreacted salt and unbound nanoparticles. The onset of the reduction from gold salt to gold nanoparticles was indicated by a change of color from bright yellow to dark yellow-red. Within 30 min at room temperature the solution color turned to dark purple. In the case of palladium salt, the bright yellow palladium salt solution turned into light black just after the addition of the reducing agent (0.056 M NaBH4). Within 30 min it turned to a golden color. The NaBH4 solution and the nanoparticle decorated β-lactoglobulin fibrils were freshly prepared for every sample. Nanoparticle Synthesis by Thermal Reduction Method. In the case of thermal reduction, nanoparticles of gold and palladium were obtained by burning the solid dry content of the dispersions at 750 °C for 3 h and then cooling down to room temperature. Ash and other impurities were removed and the nanoparticles were extracted by ultrasonication. Catalytic Membrane Preparation. The catalytic membrane reactors were prepared by vacuum filtering metal nanoparticledecorated fibrils on top of nitrocellulose membranes (0.22 μm
through biomineralization processes.33,34 The combination of the properties of the nanoparticles and fibrils brings to these inorganic hybrid nanomaterials extraordinary physical properties such as conductive,35,36 magnetic,37,38 and optical characteristics39 and can serve multiple technological tasks,40 ranging from optoelectronics33 to sensors36,41 and as delivery vehicles.39 Furthermore, amyloids have extremely good mechanical strength,42,43 and their adhesiveness44 is beneficial to bind to both hydrophobic45 and hydrophilic surfaces, which ultimately help in the preparation of the composite film. The composite films26,36 prepared with amyloid fibrils have been already shown to be robust enough to act as actuators and biosensors.46 In what follows, films of β-lactoglobulin−metal nanoparticles are introduced for the first time and used as active layer in continuous flow catalysis. The catalytic activity of these membranes is quantified by a model reaction, i.e., the reduction of 4-nitrophenol to 4-aminophenol, and the kinetics of the reaction is followed by time-resolved UV−vis spectroscopy.
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MATERIALS AND METHODS
BioPURE- β-lactoglobulin was purchased from the Technische Universität Munich, department of food process engineering and dairy technology, Munich, Germany. The purification of the βlactoglobulin and detailed preparation of amyloid fibrils are discussed in our earlier reports.47 Briefly, 10 wt % of the homogeneous monomer solution (pH 4.6) is centrifuged at 15 000 rpm for 15 min. The bottom aggregates are discarded, and the supernatant solution is adjusted to pH 2. The solution is filtered using a 0.22 μm Millipore syringe filter membrane. Further purification is done by a dialysis membrane (spectra/por dialysis MWCO: 6−8 kDa). Before the dialysis, the membranes are cleaned with a 1 mM EDTA solution at B
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tion of several micrometers.48 Figure 2 shows the AFM height images (Figure 2a,c) and 3D AFM images (Figure 2b,d) of the
GSWP, Merck Millipore Ltd.), a process by which the water was also removed. The unbound nanoparticles were removed by initial washing the filter membrane with 10 mL of pH 2 water. Various filter membranes with different amounts of metal nanoparticles decorated βlactoglobulin fibrils were prepared to compare their properties. For membranes with a fibril solution volume higher than 1000 μL, the 4nitrophenol solution (0.1 mM) hardly passed the membrane, while for membranes with a fibril solution volume under 500 μL the 4nitrophenol solution passed the membrane so fast that the catalytic reaction could not take place completely. The best results for the vacuum-assisted flow catalysis carried out in this work were obtained by 500 μL of gold fibrils and 750 μL of palladium fibrils. UV−vis Spectrophotometer. Absorption measurements were used to follow the time-dependent reduction 4-nitrophenol and performed using a Cary 100 UV−vis spectrophotometer (Agilent Technologies). Atomic Force Microscopy. The atomic force microscopy (AFM) tapping mode was carried out on a Nanoscope VIII multimode scanning force microscope (Bruker). The solutions were deposited onto freshly cleaved mica sheets, incubated for 2 min, rinsed with Milli-Q water, and dried by air. The catalytic filter membrane was glued to the metal substrate. For all experiments, MPP-11200-10 tips were used for the tapping mode in soft tapping conditions (Bruker) at a vibrating frequency of 300 kHz. The images were simply flattened using the Nanoscope 8.1 software, and no further image processing was performed. Scanning Electron Microscopy. Scanning electron microscope (SEM) imaging was carried out on a Zeiss SEM, LEO 1530. The catalytic filter membrane is initially glued to the metal substrate, and the surface of the amyloid hybrid membrane is scanned at an accelerating voltage of 3 kV. Energy-Dispersive X-ray Spectroscopy. The elemental analysis of β-lactoglobulin metal nanoparticle hybrid fibrils was done by energy-dispersive X-ray spectroscopy (EDX) using an EDAX Pegasus system with a Si(Li) detector (Ametek EDAX, Wiesbaden) attached to a SEM (FEI Quanta 200 FEG) operated at an accelerating voltage of 10 kV. The X-ray spectra were recorded from small areas of the samples. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was performed using an SDTA851e instrument (Mettler Toledo) to analyze the weight fractions of the nanoparticles in the hybrid protein fibrils. The freeze-dried samples were loaded in alumina oxide crucibles and heated from 40 to 800 °C at a heating rate of 10 °C min−1 under oxygen and argon gas flow.
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Figure 2. AFM image (a, c) and 3D AFM image (b, d) of βlactoglobulin amyloid fibrils decorated with metal nanoparticles. (a, b) Gold and (c, d) palladium after the respective metal salt reduction by NaBH4.
fibrils decorated with the metal nanoparticles, reduced from the corresponding salt precursors.39 The AFM images displayed in Figure 2a,b confirm binding of gold nanoparticles onto the amyloids surface and a high linear grafting density. Figure 2c,d shows the β-lactoglobulin fibrils decorated with the palladium nanoparticles. Divalence49 of the palladium ions (PdCl4−2) induced a slight aggregation of β-lactoglobulin fibrils, and clusters of large palladium nanoparticles were formed. The energy dispersive X-ray spectroscopy (EDX) measurements provide the elemental composition of the hybrid fibrils (see Figure S2), confirming the presence of gold (Figure S2a) or palladium (Figure S2b) in the corresponding hybrid fibrils. Thermogravimetric analysis on the hybrid amyloid fibrils showed that the amount of inorganic nanoparticles is 18 wt % for the gold hybrids and 19 wt % for the palladium hybrids (see Figure S3). Kinetics of the Catalytic Activity of Hybrid βLactoglobulin Fibrils. The catalytic behavior of the metal nanoparticle-decorated fibrils was demonstrated by following the conversion of 4-nitrophenol into 4-aminophenol at various concentrations of metal nanoparticles. Figure 3 shows the timedependent UV−vis absorption analysis of the conversion of 4nitrophenol with the gold and palladium-coated β-lactoglobulin fibrils dispersions. The analysis was performed as follows: Initially, 500 μL of 0.1 mM 4-nitrophenol was mixed with 20 μL of NaBH4 (56 mM). Just before starting the UV−vis absorption scan, different amounts of amyloid−nanoparticle hybrid fibrils were added to initiate the catalytic conversion of 4-nitrophenol to 4-aminophenol. Every minute, the sample was scanned in the wavelengths spectrum ranging between 450 and 270 nm. At the beginning of the reduction process, the spectrum showed a characteristic peak at 400 nm, associated with 4-nitrophenol. As the reduction processes, the peak at the 400 nm vanished, whereas a peak at 300 nm, associated with 4-
RESULTS AND DISCUSSION
Figure 1a shows schematic amyloid fibrils, transformed from native β-lactoglobulin monomers by heating upon 90 °C at pH 2. Metal nanoparticles (gold and palladium) were synthesized on the surface of the fibrils by chemical reduction of the corresponding salts (hydrogen tetrachloroaurate, HAuCl4, and sodium tetrachloropalladate, Na2PdCl4) with sodium borohydride (NaBH4). Because of the electrostatic interactions, the negatively charged salt ions form complexes with the highly positively charged fibrils, resulting in strongly coupled metal nanoparticles formation on the fibril surface (Figure 1b). The hybrids can then be used as such for wet catalysis or used to prepare filter membranes for continuous flow catalysis. Figure 1c summarizes schematically the filter membrane preparation with the nanoparticles-decorated amyloid fibrils and shows their catalytic behavior as reducing agent for 4-nitrophenol into 4aminophenol. Synthesis of Metal Nanoparticles on Protein Fibrils. Figure S1 of the Supporting Information shows the AFM image of the mature β-lactoglobulin amyloid fibrils prepared at pH 2 after heating at 90 °C for 5 h. The fibrils show semiflexible multistranded structures having broad contour length distribuC
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Figure 3. UV−vis absorption spectrogram from 270 to 450 nm of 4-nitrophenol mixed with sodium borohydride and (a) 9.6 mg/L of hybrid βlactoglobulin gold and (b) 9.6 mg/L of hybrid β-lactoglobulin palladium fibrils. Impact of different concentrations of metal nanoparticle-decorated hybrid fibrils for the catalytic reduction of 4-nitrophenol over time for (c) hybrid β-lactoglobulin gold and (d) hybrid β-lactoglobulin palladium. Role of amyloid fibrils for the catalytic reduction of 4-nitrophenol using (e) 3 μg/mL gold nanoparticles with and without 100 mg/L amyloid fibrils and (f) 3 μg/mL palladium nanoparticles with and without 100 mg/L amyloid fibrils.
Hinshelwood model.50 Because of the large excess of NaBH4 (needed to activate the reaction) compared with the 4nitrophenol concentration, the reaction can be described by a first-order rate law (eq 1):
aminophenol, emerged. Thus, the reduction of 4-nitrophenol to 4-aminophenol occurred as expected. The peak at 400 nm was followed to measure the concentration of 4-nitrophenol versus time and to monitor the kinetics of the 4-nitrophenol reduction. The 4-nitrophenol peak decreases rapidly in the presence of both the gold and palladium-coated amyloid hybrid fibrils. The reaction speed is enhanced by increasing the concentration of hybrid fibrils in the solution. In Figure 3c,d the absorbance normalized to the initial absorbance at 400 nm is presented as a function of time for different concentrations of amyloid− nanoparticle hybrid fibrils (gold, Figure 3c) and the palladium (Figure 3d). As expected, the increase of the slopes depends on the amount of nanoparticles available to catalyze the reduction. It is thus possible to characterize the rate constant of the reaction with the slope of absorbance decrease. This was achieved by fitting the data with a simple theoretical model. The reduction of 4-nitrophenol follows the Langmuir−
−ln(A) = kappt ∝ K1Ct
(1)
where A = A(t)/A0, with A(t) and A0 being the absorption intensities of 4-nitrophenol at 400 nm at time t and time zero, respectively, i.e., the concentrations of 4-nitrophenol at the time t and initial concentration at time zero. kapp is the apparent rate constant of the catalysis process. Furthermore, it has been shown before51 in heterogeneous catalysis realized by hybrid systems of organic carriers and metal nanoparticles that the apparent rate constant kapp is proportional to the concentration of total hybrid (C), in the present case amyloid−nanoparticle hybrids. In order to study whether the organization of the nanoparticles onto the fibrils surfaces plays a role in their D
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Figure 4. SEM image of (a) β-lactoglobulin−gold and (d) β-lactoglobulin−palladium filter membranes where the fibrils are visible with the metal particles attached on their surface. AFM images with corresponding 3D AFM images of (b, c) β-lactoglobulin−gold and (e, f) β-lactoglobulin− palladium filter membranes. The insets in the top left corners of panels a and d show the photographs at 1:1 scale of the filter membrane prepared with (a) gold and (d) palladium.
Figure 5. Filtration through the hybrid β-lactoglobulin gold and palladium filter membrane. UV−vis spectrum and color changes before and after filtration demonstrate the catalytic conversion of the 4-nitrophenol to 4-aminophenol. The inset in the top left corner shows a photograph of the filter membrane prepared with (a) gold and (b) palladium.
Comparison of the Catalytic Activity between Different Metal Nanoparticles. Figure S4 displays the apparent catalytic activities plotted against the concentration of hybrid fibrils (fibrils decorated with the gold and palladium particles). The rate constant kapp obtained from the slopes in Figure 3 is plotted as a function of the concentration of fibrils decorated by the gold and palladium particles, C. The slope of this function is the rate constant K1. This enables the comparison of the catalytic activity of different metal nanoparticle decorated on the same concentration of fibrils. The difference between the curves clearly indicates that at the similar concentration conditions gold possesses a higher catalytic activity compared to the palladium, possibly due to higher surface area and/or blocking of the hindrance of the nanoparticles surface by aggregation of the amyloid fibrils. Catalytic Membrane. Figure 4 shows the SEM and AFM images of the surface of the filter membranes prepared from the hybrid gold (Figure 4a−c) and palladium (Figure 4d−f) nanoparticle-coated fibrils via vacuum assisted filtration of their dispersions on top of a nitrocellulose support membrane. In order to clearly visualize the fibrillar structures coated by the nanoparticles, SEM aerogel samples were prepared by solvent exchange to ethanol and supercritical CO2 drying. Figure S5 displays the SEM of aerogels decorated with and without
heterogeneous catalysis, we designed an experiment to study whether the catalytic performance of the very same particles changes when particles are dispersed in solution or adsorbed onto the fibrils surfaces. To make sure that the very same particles are used, this control experiment was performed using preformed nanoparticles (see Methods section). The nanoparticle batches were then divided into two identical fractions: one for bare nanoparticles catalysis having concentration of 3 μg/mL and the second mixed and adsorbed to amyloid fibrils (100 mg/L) to study the same catalytic behavior in the presence of amyloid fibrils. The corresponding kinetics of the catalytic reduction is shown in Figure 3e,f. These results clearly illustrate that the apparent rate constant of the catalysis process (kapp) with hybrid fibrils is 1.2 times higher compared to bare nanoparticles, demonstrating convincingly that the amyloid fibrils in the hybrid fibrils play an important role in the catalysis process, possibly by organizing spatially the nanoparticles in a more efficient way. It should be noted that kapp is extracted by assuming a pseudolinear behavior, although the deviation from linearity is noticeable in Figure 3e,f, pointing at a slightly more complex heterogeneous catalysis mechanism. Especially in the case of the palladium, the deviation is conspicuous, possibly as a result of the divalence nature of palladium. E
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catalytic ability of the metal nanoparticles combined with the high mechanical stability and the large aspect ratio of the amyloid fibrils allows for the creation of efficient membranes for continuous flow catalysis.
nanoparticles and clearly demonstrates that the nanoparticles were strongly bounded to the fibrils. The insets in the top left corner of Figures 4a and 4d show original photographs of the filter membranes prepared from the β-lactoglobulin fibrils decorated by gold and palladium, respectively. The membranes were highly porous, and an increasing pore size of the membrane was observed with decreasing concentration of the fibrils in the solution, which allowed for an easy control of the characteristic pore size. To test the efficiency in continuous flow catalysis, a solution with 2.5 mL of 0.1 mM 4-nitrophenol mixed with 100 μL of 56 mM NaBH4 was prepared and passed through the membrane by using vacuum. During the passage of the membrane the solution color changes from light yellow to transparent. For this process there is no difference between gold and palladium nanoparticle decorated fibrils. After the filtration, the permeate was measured by UV−vis spectroscopy and compared with the unfiltered solution. In Figure 5a, the catalytic conversion of the 4-nitrophenol to 4-aminophenol is visualized. The filtration of 2.5 mL of the 4-nitrophenol solution with the reducing agent takes less than 1 min when applying a vacuum of 10−1 bar, although the filtration process can be accelerated by simply increasing vacuum. The unfiltered 4-nitrophenol solution showed the usual absorption peak at 400 nm, but in the permeate, the absorption peak at 400 nm disappeared completely, as a result of the filtration, and was replaced by the peak at 300 nm characteristic of 4-aminophenol, indicating the complete conversion of 4-nitrophenol to 4-aminophenol. A similar behavior was observed for the solution filtered by the palladium nanoparticles (Figure 5b). A β-lactoglobulin fibril membrane without any attached metal nanoparticles was used as a control experiment (Figure S6). Filtration of a 4nitrophenol solution led to no color change as confirmed by UV−vis analysis, confirming the role of the nanoparticles in the catalytic behavior. In contrast, control attempts at producing a purely inorganic films based on the nanoparticles without the fibrils were unsuccessful because the nanoparticles passed through the membrane; thus, the amyloid fibrils carrier system is required in continuous catalysis flow.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03205. AFM image of amyloid fibrils, energy dispersive X-ray (EDX), SEM images of amyloid aerogels, UV−vis spectra, and rate constant analysis plot (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail raff
[email protected]; Tel +41 44 632 9140 (R.M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support by Stephan Handschin and the Electron Microscopy Center of ETH Zurich (ScopeM). Dr. Gustav Nyström is acknowledged for SEM imaging of the aerogels. Dr. Khay Fong, from ETH Zurich, is kindly acknowledged for critical reading of the manuscript.
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CONCLUSIONS The catalytic activity of gold and palladium nanoparticles immobilized on amyloid hybrid fibrils has been demonstrated in this study. The attachment of these metal nanoparticles on the β-lactoglobulin fibrils was achieved through an in situ chemical reduction process, and the generation of the nanoparticles was confirmed by AFM. The catalytic performance of the hybrids in converting 4-nitrophenol into 4aminophenol in water was tested both in the colloidal state, by using the dispersions of the nanoparticle−amyloid fibrils, and in continuous flow catalysis, by filtering a solution through a porous membrane formed by the hybrids. In wet catalysis, the gold nanoparticle−amyloid fibril hybrid exhibited a higher catalytic activity compared with the palladium-decorated amyloid fibrils, as a result of higher specific area in the gold case, and hindrance of the nanoparticle surfaces by aggregation of the amyloid fibrils in the case of palladium. In contrast, in flow catalysis, both systems were found to perform remarkably well, with complete catalytic conversion of 4-nitrophenol into 4-aminophenol upon a single passage through the membrane. The present study clearly demonstrates that amyloid fibrils can serve as an ideal carrier system for in situ synthesis of catalytically active metal nanoparticles. The extraordinary F
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DOI: 10.1021/acs.langmuir.5b03205 Langmuir XXXX, XXX, XXX−XXX
