Synthesis and Organization of Gold-Peptide Nanoparticles for Catalytic Activities.

A significant development in the synthesis strategies of metal-peptide composites and their applications in biomedical and bio-catalysis has been reported. However, the random aggregation of gold nanoparticles provides the opportunity to find alternative fabrication strategies of gold-peptide composite nanomaterials. In this study, we used a facile strategy to synthesize the gold nanoparticles via a green and simple approach where they show self-alignment on the assembled nanofibers of ultrashort oligopeptides as a composite material. A photochemical reduction method is used, which does not require any external chemical reagents for the reduction of gold ions, and resultantly makes the gold nanoparticles of size ca. 5 nm under mild UV light exposure. The specific arrangement of gold nanoparticles on the peptide nanofibers may indicate the electrostatic interactions of two components and the interactions with the amino group of the peptide building block. Furthermore, the gold-peptide nanoparticle composites show the ability as a catalyst to degradation of environmental pollutant p-nitrophenol to p-aminophenol, and the reaction rate constant for catalysis is calculated as 0.057 min-1 at a 50-fold dilute sample of 2 mg/mL and 0.72 mM gold concentration in the composites. This colloidal strategy would help researchers to fabricate the metalized bioorganic composites for various biomedical and bio-catalysis applications.

Introduction

The development of simple strategies to make biocomposites at the nano- to micro-scale is rapidly increasing in the fields of biomedicine and catalysis for the degradation of pollutants.[1,2] Biocomposites are usually defined as materials where biological building blocks play a crucial role in the synthesis and remain a part of materials because of certain interactions. Recently, metal, especially gold and silver, composites with peptides and proteins where they act as stabilizing and reducing agents have been developed.[3,4] The applications of gold nanoparticles depend on their size, shape, and composition as well as their arrangement and self-organization.[5,6] However, the preparation of gold nanoparticles with defined dimensions and controlled size and morphology and their random aggregation and compromised biocompatibility have remained to be significant challenges.[7] To address these challenges, one strategy is to use microorganisms like bacteria and algae and those from plant extracts for the formation of gold nanoparticles, which could potentially tackle the biocompatibility concerns of gold nanoparticles.[8] This provides a solution for problems that arise from the toxicity of reducing and stabilizing agents but the aggregation and controlled formation of gold nanoparticles remain to be a challenging task.

Other than the microbe-based synthesis of metal nanomaterials, self-assembly, a natural and spontaneous process, is a promising bottom-up approach. It depends on the noncovalent interactions (electrostatic interactions, hydrogen bonding, pi–pi stacking, and cation–pi interactions) between the components and these interactions can be controlled by various factors like the inclusion of functional groups, pH, solvents, and temperature.[9−11] Recently, self-assembling biomolecules, such as peptides, proteins, and oligonucleotides, have gained enormous attention in creating metallic composite nanoparticles because of their biocompatibility and physicochemical advantages.[4,12−14] Therefore, self-assembling building blocks like amyloid-like peptides,[15] surfactant-like peptides,[16] and peptide amphiphiles[17] have the potential to tune the physicochemical properties of metals and ability to control the size in a complexation process. In this strategy, peptide building blocks act not only as reducing and stabilizing agents for gold nanoparticles but also as a template for the synthesis of gold nanoparticles in a composite material. These peptide building blocks have versatile physical properties to control the aggregation of metallic nanoparticles because of their distinctive self-assembling and recognition capabilities.[18]

Contrary to the self-assembly approach for the formation of gold-peptide composites, generally, the metal nanoparticles have been prepared using different methods, for example, sol–gel, hydrothermal, and precipitation methods where the metal salt is mixed with some reducing agents like hydrazine, sodium citrate, sulfonic acid, and borohydrides, which could have somehow compromised the biocompatibility and could lead to severe detrimental side effects for environmental pollution.[19] Other than this, often, the size and aggregation cannot be controlled because of the lack of chemical and physical interactions and the undetermined effects of reducing agents. This leads us to use the photochemical reduction methods with the help of ultrashort peptides and mild UV light. The photochemical reduction of metal ions using peptide building blocks is an interesting approach to fabricate metal-based composite nanomaterials. Ultrashort amphiphilic peptides, a class of peptides containing three to seven amino acids, can self-assemble into a well-defined nanofibrous network, mimicking the native extracellular matrix.[20,21] Such kinds of short peptides have been used in many applications in medicine and tissue engineering, such as bioprinting,[22,23] drug delivery,[24] engineered tissue models,[25,26] and wound healing,[27] which reveal the biocompatibility of peptides. In our previous works, we have reported the photochemical synthesis of size-controlled biocompatible silver nanoparticles in the absence of any chemical reducing agents for antibacterial applications.[28,29] However, to see the versatility of this photochemical synthesis of metal-peptide nanoparticle composites, we used the tetramer peptide (IVFK) and gold metal salt for the reduction of small molecule pollutants. Interestingly, this ultrashort peptide not only reduces the gold salt into nanoparticles but provides a template of nanofibers for the organization of gold nanoparticles. Small molecule organic pollutants cause severe environmental and health concerns in recent days, and metal nanoparticles including the gold one have shown great efficiency in the degradation of different hazardous molecules.[30−36] Rather than using the inorganic reagents in the synthesis of gold nanoparticles, we proposed a green and biological, simple, and mechanistically understandable approach for the catalytic reduction of pollutants.

Herein, we report a simple strategy using an ultrashort peptide that generates gold-peptide nanoparticle (GPNP) composites without any reducing agents through a photochemical reduction mechanism. The gold nanoparticles are arranged on peptide nanofibers through the multitude of noncovalent interactions and possible interactions with the amino group of the lysine amino acid in the sequence. The photoionization activity of the peptide is due to the UV light exposure of the aromatic residue, which allows the reduction of gold ions. Interestingly, the peptide acts as a reducing, capping, and stabilizing agent at the same time. The arrangements of nanoparticles over the peptide nanofibers are presented in Scheme , which are confirmed by different characterizations. The crystallinity of the generated gold nanoparticles was then investigated by high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD), which demonstrate that gold nanoparticles are face-centered cubic in nature. The d-spacing was consistent in both techniques. The gold nanoparticles are well-aligned over the peptide nanofibers due to electrostatic interactions and certain binding with the amine group of the peptide, which is divulged from previous Fourier transmission infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) studies. Finally, we show the promising catalytic activity of gold-peptide composite nanomaterials as catalysts for the reduction of small molecule pollutants p-nitrophenol to the least toxic compound p-aminophenol in a very short time (less than 2 min) at high concentrations. The reaction rate constant for catalysis is 0.057 min–1 at a 50-fold dilute sample of 2 mg/mL and 0.72 mM gold concentration in the composites. These selective bio-mineralized peptide composites via a green synthetic approach will lead to new directions for biomedical and green catalytic applications.

Scheme 1

Chemical Structure of the Tetramer Peptide and Fabrication of GPNP Composites with UV (254 nm) Light

Results and Discussion

Preparation of Gold-Peptide Nanoparticle Composites

The functional groups amine and thiol as interacting sites have been reported, which can support polymers and carbon nanotubes to bind with gold nanoparticles.[37,38] A bio-inspired tetramer oligopeptide Ac-IVFK-NH2-based alternative pathway is proposed to fabricate colloidal composite nanoparticles by triggering the biomineralization process. The peptide was designed with the amidated C-terminus and acetylated N-terminus to avoid electrostatic repulsion among the molecules in their assembled state. The short peptide sequence has hydrophobic and hydrophilic amino acids, and there are equilibria between polar and nonpolar characters, which make it water-soluble even at a very high concentration of 100 mg mL–1. The natural and spontaneous process of self-assembly in peptides and proteins is ubiquitous and depends on the physical properties of building blocks to construct the supramolecular nanostructures.[39] These physical noncovalent interactions, such as van der Waal forces, hydrogen bonding, π–π stacking, and electrostatic forces, play a decisive and fundamental role in supramolecular chemistry for the diversification of nanomaterials.[11] In light of supramolecular chemistry, attractive and repulsive electrostatic forces can tune the self-assembly of short peptides on the demand of the application.[40] For example, Xing et al. reported an injectable collagen-gold hybrid hydrogel constructed through electrostatic attraction for combined antitumor therapy.[41] The role of hexamer oligomer peptide hydrogels in reducing the silver ions to form silver nanoparticles with the assistance of UV light was also reported.[28] However, the relatively small oligopeptide is not explored for the synthesis of gold-peptide nanoparticle composites, and focus was diverted to gold nanoparticles as they have been used in many applications with promising results.

Lyophilized peptide Ac-IVFK-NH2 (2 mg/mL, 1.83 mM) was dissolved in Milli-Q water in three glass vials and mixed with HAuCl4 solutions of concentrations of 0.18, 0.36, and 0.72 mM. All three samples were then exposed to UV light for 30 min for in situ synthesis and fabrication of the gold nanoparticles on the peptide nanofibers, without the addition of toxic reducing and capping reagents. We used the peptide in a bit low concentration because of its strong self-assembling propensity to form the self-supporting hydrogels at 4 mg/mL or higher concentrations immediately, and it was not convenient to characterize the gold nanoparticle-embedded hydrogel with traditional and commonly used techniques, such as UV–Vis spectroscopy. However, after photochemical reduction by UV exposure, the peptide hydrogel at high concentrations changed its transparent color to reddish, confirming the formation of gold nanoparticles, which are most probably trapped within the interstices of nanofibers. Inspired by the sophisticated approach of self-assembly, electrostatic complexation between the positively charged tetrapeptide motif and negatively charged [AuCl4]− ions, which were produced under UV light, was subsequently converted into GPNP composites. The formation of GPNP composites was confirmed by UV–Vis spectroscopy, as shown in Figure a, showing the appearance of a relatively broad surface plasmonic resonance (SPR) absorption peak of gold nanoparticles at 530 nm.[42] The intensity of the absorption peak increases with increasing concentrations of the HAuCl4 precursor, and the change in the color of the peptide and gold salt mixture from transparent to reddish also reveals the formation of gold nanoparticles with a bottom-up self-assembling approach, as illustrated in Figure b. The GPNP composites were also shown to be stable for up to 14 days (Figure S1). Furthermore, a concentration of 0.72 mM gold salt in water, as a control, was treated by UV light at 254 nm for 30 min and then analyzed by UV–Vis spectroscopy and there was no classical peak for the gold nanoparticles that appeared. The graphical representation shows the importance of peptides as reducing agents to address the absence of reducing/capping agents, as given in Figure S2a,b. From a mechanistic point of view, here, UV light assisted the photochemical reduction process to form nanoparticles over the nanofibers of the peptide. In another study, Bent and Hayon systematically investigated the ejection of a hydrated electron (e) from the aromatic ring of a phenylalanine residue during the photoionization process.[43] This hydrated electron is believed to play an important role in reducing the gold ions to gold nanoparticles, and the presence of phenylalanine in IVFK critically helps the reduction process. However, the mechanism of the reduction process requires further investigation to gain greater insights from a broad perspective.

Figure 1

UV–Vis spectra and pictures after UV light exposure at different gold concentrations. (a) Absorption spectra of GPNP composite suspension composed of 1.83 mM IVFK peptide and gold salt at various concentrations of 0.18, 0.36, and 0.72 mM. (b) Pictorial representation of color changing of the same concentrations of GPNPs used in (a) after UV irradiation at 254 nm wavelength.

Characterization of GPNP Composites

Generally, the metal incorporation in a biomolecular assembled material in different morphologies can be investigated by transmission electron microscopy (TEM). We used TEM to see the formation of gold nanoparticles and self-organize over the peptide nanofibers, as demonstrated in Figure a. The average size of gold nanoparticles distributed on the peptide nanofibers was approximately 5.16 nm, as given in Figure b. Furthermore, energy-dispersive X-ray spectroscopy (EDS) analysis confirms the presence of a gold element in GPNP composites, as can be seen in Figure c. The size distribution and arrangements of the gold nanoparticles could be due to promising biomineralization of peptides where the formation process can be in kinetic control and metal nucleation.[44,45] In darkfield TEM, some peptide nanofibers have not shown the gold nanoparticles over the surface of nanofibers, which is attributed to the weak molecular interactions of the two components in the assembled composites. However, it also depends on the concentration of gold salt used for the synthesis of the composites and it would be possible to obtain densely populated and fully covered nanofibrous composites; we intentionally use the low concentration of components, as shown in Figure c,d. Additionally, we used atomic force microscopy (AFM), which also identified the gold nanoparticles (Figure e). The size is also consistent with TEM, and most importantly, it showed the arrangement of gold nanoparticles similar to that given in the TEM images in Figure d.

Figure 2

Morphology characterization of GPNP composites. (a) Transmission electron microscope image indicating the formation and self-arrangement of gold nanoparticles around the nanofibers. (b) TEM size distribution of GPNPs. (c) EDS spectrum confirming the presence of gold elements on the surface of the nanoparticle. (d) TEM image to show the nanofibers with and without gold nanoparticles. (e) Dark mode TEM image indicating the alignment of gold nanoparticles over the peptide nanofibers. (f) Self-organization presented by an atomic force microscope image (AFM).

The amine group of the side chain of the lysine residue has previously been reported as the reduction and nucleation sites of the template-directed biomineralization reaction under acidic conditions.[44] In a similar way, we did not use any base or buffer but the pH was around 6.0 but not too acidic, which also confirms the amine role of the lysine residue for reduction along with phenylalanine. More importantly, the lysine residue helps the nucleation and arrangement of gold nanoparticles through noncovalent interactions. Furthermore, the crystalline nature of gold nanoparticles was investigated by selected area electron diffraction (SAED) and HR-TEM, which identified the face-centered cubic (fcc) structure with the majority of the d-spacing value of 2.3 Å from the (111) plane, as shown in Figure a,b. To validate the crystalline nature of gold nanoparticles in a bulk quantity of composites, X-ray powder diffraction (XRD) was used. The XRD spectrum of GPNP powder is in excellent agreement with the crystal database (COD: 1100138) of gold nanoparticles, as demonstrated in Figure d.[46]

Figure 3

Crystallinity investigation by HR-TEM and XRD. (a) HR-TEM image of nanoparticles, (b) HR-TEM lattice fringe image of GPNPs showing the (111) plane with the lattice distance 0.23 nm, (c) selected area electron diffraction pattern to represent the lattices, and (d) XRD pattern of GPNPs.

To determine the specific interaction between the peptide and gold nanoparticles to better understand the arrangement of gold nanoparticles over nanofibers, FTIR and XPS analyses were carried out. The tetramer peptide is positively charged because the lysine residue and in situ gold nanoparticles are negatively charged, as shown by the zeta potential in Figure S3. This explains the electrostatic interaction between the two components. Furthermore, the FTIR spectra of GPNP composites confirm the interaction between the peptide template and gold after UV irradiation, as it can be seen from the blue shift of the amide A region (3275 to 3273 cm–1) in −NH stretching vibration mode of the peptide. The NH bending mode (out of plane) in the amide II region (1548 to 1544 cm–1) is also blue-shifted, as shown in Figure a. Other characteristic peaks that remain unchanged could be due to the low concentration of gold nanoparticles being used in the sample. These shifts in NH vibrations support the hypothesis of interactions between the lysine amino acid and gold nanoparticles for their alignment on the peptide nanotemplates. Furthermore, the XPS spectra were recorded to analyze the elemental composition of GPNP composites. The peaks of the survey spectrum at 531.8, 398.3, and 284.8 eV, resulting from the peptide template, confirmed the presence of oxygen, nitrogen, and carbon, respectively, as shown in Figure S4. The high-resolution XPS spectrum of Au 4f7/2 shows three peaks at 83.8, 85.2, and 85.9 eV, which are attributed to the binding energies of Au(0), Au(I), and Au(III), respectively.[47] This suggests that metallic gold is dominant in GPNPs over other oxidation states. The high-resolution spectrum of N 1s of GPNPs was then compared to Ac-IVFK-NH2 to determine the interaction between the gold and amine group of a lysine residue. Deconvolution of the N 1s spectrum of peptide powder shows two distinct peaks of N1 at 399.7 eV and N2 at 401.5 eV, which are attributed to the amide bond[48] and protonated amine of lysine,[49] respectively, as given in Figure c. In GPNP composites, the N2 peak shifted to a lower binding energy (401.1 eV), which might be due to the coordination between gold and amine groups, as shown in Figure d.[29,50] This result implies that Ac-IVFK-NH2 can be used as both reducing and capping agents for gold nanoparticle formation through a simplistic facile strategy of photochemical reduction.

Figure 4

FTIR and XPS for binding studies. (a) Fourier transmission infrared FTIR spectra and high-resolution XPS and deconvolution of (b) Au4f@GPNP, (c) N1s@IVFK, and (d) N1s@GPNP.

Catalytic Activity of GPNP Composites

The catalytic activity of GPNP composites was investigated for the reduction of small organic molecule pollutant p-nitrophenol into p-aminophenol under ambient conditions, as given in Figure a. The reaction conditions, for example, the pH and concentration, of the components of materials affect the rate of reaction, as reported by Chen and Li. They used metallic nanoparticles to reduce p-nitrophenol in the presence of sodium borohydride (NaBH4) at a lower pH; the catalytic reduction occurred within 2 min.[51] On the first attempt, the tetrapeptide along with NaBH4 was used to reduce p-nitrophenol to p-aminophenol; however, after 2 h, the presence of p-aminophenol was not observed by UV–Vis spectrophotometry, as shown in Figure S5. The experiment was then repeated by introducing GPNP composites as catalysts to a mixture of p-nitrophenol and NaBH4, and the formation of p-aminophenol was detected simultaneously at around 400 nm, as given in Figure b, indicating the catalytic activity of GPNP composites. We calculated the rate of reaction by using the gold-peptide composites as catalysts for the reduction of small molecule pollutant p-nitrophenol to p-aminophenol and the rate constant for catalysis is 0.057 min–1 at a 50-fold dilute sample of 2 mg/mL and 0.72 mM gold concentration in the composites. However, the rate of reaction was dependent on the concentration of peptide-gold nanocomposites because when we used them without dilution, then the conversion of the organic pollutant was completed in less than 2 min, as demonstrated in Figure S6. This catalytic reduction of p-nitrophenol to p-aminophenol is in good agreement with the previously reported literature.

Figure 5

Catalytic activity of GPNPs for the reduction of p-nitrophenol. (a) Chemical structure of p-nitrophenol and p-aminophenol. (b) Catalytic reduction of p-nitrophenol into p-aminophenol in the presence of 2 mg/mL peptides and 0.72 mM GPNP composites (50× diluted). (c) Absorbance of p-nitrophenol at 400 nm as a function of time. (d) Reaction rate constant for catalysis.

Conclusions

In summary, we introduce a facile strategy to fabricate GPNP hybrids composites via self-assembly of ultrashort peptide Ac-IVFK-NH2 and gold salt with the help of UV without any additional capping and reducing agents through a photochemical reduction approach. The phenylalanine and lysine amino acids in the sequence play a role in the formation of gold nanoparticles, while the lysine amino acid is mainly responsible to hold the nanoparticles on the peptide nanofibers. This attachment of gold nanoparticles is due to noncovalent interactions between two components as revealed by FTIR and XPS results. The crystallinity of nanoparticles was investigated by HR-TEM, SAED, and XRD, which demonstrate that GPNPs are face-centered cubic in nature and the d-spacing is consistent in all techniques. Furthermore, gold-peptide composites have shown a promising fast reduction of small molecule pollutant p-nitrophenol to p-aminophenol, and the reaction rate constant for catalysis is 0.057 min–1 at a 50-fold dilute sample of 2 mg/mL and 0.72 mM gold concentration in the composites. However, the rate of reaction was dependent on the concentration of peptide-gold nanocomposites because when we used them without dilution, then the conversion of the organic pollutant was completed in 2 min. These peptide-metal hybrid composites via a green synthetic approach will pave the way for new approaches in biocatalysis and environmental applications.

Experimental Section

Materials

The tetrapeptide Ac-IVFK-NH2 was synthesized in the laboratory using the previously reported method.[23,52] HAuCl4, p-nitrophenol, and sodium borohydride were purchased from Sigma Aldrich. Water of pH 6.8 with resistivity 18.2 Ω from the Milli-Q water system was used. All chemicals were used as received, unless otherwise stated here.

Gold-Peptide Nanoparticle (GPNP) Formation

Two milligrams of purified peptide was dissolved in Milli-Q water under vortex until complete dissolution. This peptide solution was then homogeneously mixed with 0.18, 0.36, and 0.72 mM HAuCl4 solution. The sample mixtures were vortexed for 1 min, and the samples were exposed to UV light using a UVP CL-1000s UV Crosslinker at 254 nm wavelength with an intensity of 2.4 W/cm2 for 30 min. The stability of the GPNP suspension was observed for up to 14 days.

UV–Vis Spectroscopy

The formation of gold nanoparticles was characterized by ultraviolet–visible spectroscopy (Perkin Elmer UV/Vis/NIR Spectrometer Lambda 1050) using a wavelength window of 200–800 nm in 10 mm-thick quartz cuvettes.

Transmission Electron Microscopy (TEM)

Transmission electron microscopy analysis was carried out using an FEI Titan G2 80-300 CT, fitted with a 300 kV emission gun. A 2 μL sample solution was dropped onto a carbon-coated copper grid (EMS CF300-Cu) without any additional staining reagent. The TEM grids were then dried under vacuum overnight before imaging. The SAED pattern and EDS were taken with the same instrument. The average diameter of a GPNP was measured from 5014 NPs using ImageJ and Origin software.

Atomic Force Microscopy (AFM)

Atomic force microscope (AFM) characterization of sample morphology was carried out on a freshly cleaved mica substrate. The viscous peptide solution (5 μL) was dropcast on mica and then blotted with filter paper after 2 min. The samples were dried under a low vacuum overnight. The AFM images were taken on a Dimension Icon SPM Vecco using a tapping mode under ambient conditions. Scans were rastered using silicon-coated aluminum probes (Asylum research AC240TS-R3) with a tip radius of 9 ± 2 nm and 70 kHz resonant frequency.

Zeta Potential Measurements

The zeta potential of gold-peptide nanoparticle composites was measured using the Zetasizer Nano series HT Malvern at 25 °C.

Fourier Transform Infrared (FTIR)

The measurements were taken using a Thermo Scientific FTIR-ATR iS10. A background scan was measured before the sample. The spectrum was collected in a range of 500–4000 cm–1, with a 1 cm–1 interval. Both background and sample measurements were taken as an average over 10 scans.

X-ray Powder Diffraction (XRD)

The crystal structure of the samples was determined using a Bruker D2 Phaser X-ray diffractometer. The lyophilized peptide-gold powders were scanned in a range of 2θ = 10–90° with a step size of 0.02036°. The result was then compared to a gold reference with a face-centered cubic structure (COD 1100138).[46]

X-ray Photoelectron Spectroscopy (XPS)

The gold nanoparticles were lyophilized to form dry powder for XPS analysis. The XPS experiments were performed on a Kratos Axis Ultra DLD instrument equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) operated at a power of 150 W under UHV conditions with ∼10–9 mbar. All spectra were recorded in hybrid mode using electrostatic and magnetic lenses and an aperture slot of 300 μm × 700 μm. The survey and high-resolution spectra were acquired at fixed analyzer pass energies of 160 and 20 eV, respectively. The samples were mounted in floating mode to avoid differential charging. The peak fitting was performed using CasaXPS version 2.3.15 with Shirley background subtraction and the standard 70% Gaussian/30% Lorentzian line (GL30). No preliminary smoothing was conducted during analysis.

Catalytic Performance of Gold-Peptide Nanoparticles (GPNPs)

The catalytic reduction of p-nitrophenol to p-aminophenol by GPNP composites was conducted in a solution containing 100 μL of 0.1 mM aqueous p-nitrophenol, 100 μL of 50-fold dilute GPNP composite from the initial stock concentration of 2 mg/mL peptide and 0.72 mM gold concentration, and 100 μL of 0.1 M aqueous NaBH4, which was freshly prepared under ambient conditions. As a control, the reduction of p-nitrophenol was also conducted using a high concentration of GPNPs (2 mg/mL IVFK and 0.72 mM gold concentration) with the same ratio of p-nitrophenol and NaBH4. Catalytic performance was carried out inside a UV–Vis spectroscope (Perkin Elmer UV/Vis/NIR Spectrometer Lambda 1050) to monitor the concentration change of the reactant (i.e., p-nitrophenol) and the product (i.e., p-aminophenol).