Yifei Zhang‡
a,
Xiaojing Liu‡a,
Mengfan Wang*ab,
Yanan Zhaoa,
Wei Qi*abc,
Rongxin Suabc and
Zhimin Hea
aState Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China. E-mail: mwang@tju.edu.cn; qiwei@tju.edu.cn
bTianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin 300072, P. R. China
cThe Co-Innovation Centre of Chemistry and Chemical Engineering of Tianjin, Tianjin 300072, P. R. China
First published on 9th March 2017
A facile approach was developed to immobilize gold nanoparticles (AuNPs) onto peptide nanofibers for catalytic applications. In this study, fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF) was conjuncted with histidine, arginine or cysteine at their C-terminus to provide binding sites for AuNPs. The co-assembly of Fmoc-tripeptide and AuNPs was achieved by dropping the peptide monomer solution directly into the AuNP solution, leading to the formation of nanofibers and the immobilization of AuNPs in one step. Atomic force microscopy, transmission electron microscope and circular dichroism analyses demonstrated that the presence of AuNPs does not significantly change the morphology and secondary structure of the nanofibers. The histidine-containing peptide-immobilized AuNPs were found to display favorable catalytic activity and stability for the reduction of 4-nitrophenol to 4-aminophenol. The present approach to fabricating nanomaterial-supported AuNPs may be extended to the production of other nanoparticle-containing composites in the fields of catalysis, sensing and biomedicine.
Recently, molecular self-assembly has been developed as a popular tool for constructing micro- and nano-structural supports. Low-molecular-weight gels are known as distinct soft materials that are obtained through bottom-up fabrication of small molecules into various types of nano-fibril, strand or tape structures. Therefore, the development of self-assembled architectures from small molecules, including some biological molecules, provides a promising platform for nanoscaled applications. Peptides, especially some aromatic peptides and their derivatives, are well known building blocks for the fabrication of peptide-based nanomaterials because their assembly can be easily conducted under the driving force of π–π stacking between aromatic groups.15 Because these materials combine the simplicity of small molecules with the versatility of peptides, they make excellent nano-carriers or supports in the fields of drug delivery, tissue engineering and chem-biosensing.16–18
In this study, a peptide-based nanomaterial was developed as a novel support to immobilize AuNPs. Diphenylalanine (FF) is the simplest peptide that can self-assemble into nanotubes in pure water through π–π interactions between the aromatic side chains.19 Fluorenylmethoxycarbonyl (Fmoc) is a commonly used protecting group in peptide synthesis whose aromatic fluorenyl ring can further enhance π–π stacking and promote the formation of supramolecular architectures.15,20 Therefore, Fmoc-FF was chosen as the basic scaffold to build the nano-support. Additionally, it has been reported that many amino acid residues can be attached to metal nanoparticles through different types of interactions. The histidine tag (His6) in biotechnology can directly couple to AuNPs via Nε–Au interactions.21 The positively charged amino acids (arginine and lysine) or the negatively charged amino acids (aspartic acid and glutamic acid) can easily combine with opposite-charged nanoparticles through electrostatic attraction. Moreover, the sulfhydryl group on cysteine can form a strong covalent interaction with AuNPs through the formation of a S–Au bond.22 Herein, Fmoc-FF was further modified to conjugate histidine, arginine and cysteine at their C-terminus to provide a practical binding site for AuNPs (Scheme 1). The co-assembly of AuNPs and peptide (Fmoc-FFH, Fmoc-FFR or Fmoc-FFC) was achieved by dropping the peptide monomer solution directly into the AuNPs solution, resulting in the formation of the nano-support and the immobilization of AuNPs in one step (Scheme 2). The physical properties of the peptide-immobilized AuNPs were characterized and their catalytic activities were evaluated through the reduction of 4-nitrophenol and Congo red.
Fig. 1 Images of SA-H/CoA–H–AuNPs (column 1), SA-R/CoA–R–AuNPs (column 2) and SA-C/CoA–C–AuNPs (column 3). |
AFM and TEM were used to analyze the morphologies of the as-prepared SA-X and CoA–X–AuNPs, as shown in Fig. 2. Fibrous structures were obtained for all the SA-X and CoA–X–AuNP samples. Compared to SA-H nanofibers (Fig. 2A) with diameters of approximately 20–30 nm, the diameter of the CoA–H–AuNPs increased obviously (Fig. 2B), implying that the addition of AuNPs facilitated the formation of thicker nanofibers. However, the obtained CoA–H–AuNPs fibers were shorter than SA-H, which hinders the formation of 3D scaffolds and may explain the longer gelation time to become a hydrogel. The TEM image of CoA–H–AuNPs (Fig. 2C) demonstrated the association of the AuNPs with the peptide nanofibers. Since the concentration of AuNPs is low, not all fibers were observed to be decorated with them. For SA-R, the peptide monomers assembled into fragile fibrils because of the strong hydrophilicity of the arginine moiety (Fig. 2D). The introduction of AuNPs produced anomalous clusters attributed to the rapid neutralization of the negatively charged AuNPs and the positively charged arginine side chains (Fig. 2E). This result is in agreement with the formation of precipitates observed in Fig. 1 and the TEM result that the agglomeration of AuNPs was generated as shown in Fig. 2F. Unlike the others, the CoA–C–AuNPs display a rougher surface than SA-C (Fig. 2G and H), which implied that AuNPs can be attached to the peptide nanofibers. The TEM image (Fig. 2I) indicated that the AuNPs are distributed uniformly in CoA–C–AuNPs which might due to the strong interaction of S–Au bond.
Fig. 2 AFM or TEM images of SA-H (A), CoA–H–AuNPs (B, C), SA-R (D), CoA–R–AuNPs (E, F), SA-C (G) and CoA–C–AuNPs (H, I). |
The immobilization of AuNPs on peptide nanofibers can also be determined using UV-Vis spectroscopy (Fig. 3A). For each peptide-immobilized AuNP sample, the spectrum was corrected using the corresponding SA-X nanofibers as a background, thus the observed absorbance at approximately 550 nm arises only from the localized surface plasmon resonance of the AuNPs. For CoA–H–AuNPs and CoA–C–AuNPs, a distinct and relatively sharp peak can be observed at 550 nm; however, when the arginine-containing peptide was used to bind the AuNPs, a broader and red-shifted absorbance was observed. TGA was performed to investigate the relative amount of AuNPs conjuncted to different nanofibers as shown in Fig. 3B. Equal mass of CoA–X–AuNPs samples were gradually heated from room temperature to 700 °C. Due to the loss of bound water, the weight of three samples decreased slowly before 240 °C. The weight began to drop sharply beyond 240 °C, which demonstrated the degradation of peptide nanofibers until 550 °C. Based on the TGA curve, the AuNPs immobilized on CoA–H–AuNPs, CoA–R–AuNPs and CoA–C–AuNPs could be approximately calculated as 4.2%, 3.4% and 7.2%, respectively. The low amount of AuNPs in CoA–R–AuNPs might due to the formation of agglomerations which leads to the weak binding and prone to leaking from the peptide nanofibers. These results suggest that the type of amino acid X not only affect the polydispersity of AuNPs but also the amount of AuNPs immobilized on peptide nanofibers. In addition, the UV-Vis spectrum in the 250–310 nm region is typically assigned to the hydrophobic chromophore groups in the peptide molecules. The absorbance in this range was observed to decrease when AuNPs were attached to the peptide nanofibers (Fig. S1 in ESI†). Compared to the corresponding samples without AuNPs, the maximum absorbance at 263 nm of CoA–H–AuNPs, CoA–R–AuNPs and CoA–C–AuNPs decreased 8.9%, 36.4% and 5.3%, respectively, implying that the binding of AuNPs altered the original hydrophilic–hydrophobic microenvironment of the peptide nanofibers to different extents. Circular dichroism (CD) was utilized to give deep insight into the secondary structure of the peptide-immobilized AuNPs. As shown in Fig. 3C, the structure of all the CoA–X–AuNPs exhibited a mixture of α-helix and β-sheet (positive peak at approximately 185–200 nm and negative peaks at 204 and 216 nm), which is in agreement with Ding's result that Fmoc-FF peptide self-assembled nanofibers (SA-FF) typically display a mixed structure.24 The derivation of Fmoc-FF and the presence of AuNPs did not appear to affect the basic secondary structure of the peptide nanofiber; thus, the Fmoc-FF moiety can be used as an appropriate platform to build nano-scaled supports.
Fig. 3 UV-Vis spectra (A), TGA analysis (B) and circular dichroism spectra (C) of CoA–H–AuNPs, CoA–R–AuNPs and CoA–C–AuNPs. |
Fig. 4 Time-dependent UV-Vis spectra for the reduction of 4-nitrophenol catalyzed by CoA–H–AuNPs (A), CoA–R–AuNPs (B), and CoA–C–AuNPs (C) and their corresponding −ln(C/C0) vs. time plots (D). |
The activity difference of peptide-immobilized AuNPs originates from the different amino acid residues linked to the AuNPs. In this study, histidine, arginine and cysteine were designed as the functional groups to link citrate-capped AuNPs. Histidine-containing peptides have been studied extensively because of their high affinity to metal. When histidine is added to the citrate-capped AuNPs, the citrate molecules can be partly replaced by histidine through ligand exchange. Then, the imidazole group coordinates the AuNPs through Nε–Au interactions,25 which not only fix the AuNPs but also prevent the agglomeration of nanoparticles (Scheme 3A). Therefore, CoA–H–AuNPs display favorable activity in the reduction of 4-NP. In the case of CoA–R–AuNPs, the positive arginine is directly attached to the negative citrate through electrostatic interactions, which severely weakens the electrostatic repulsion between AuNPs, leading to their agglomeration. As a result, the agglomerated AuNPs are more liable to leak from the peptide nanofibers, leading to a low AuNPs amount and the lower catalytic activity (Scheme 3B). The covalent S–Au bond between the thiol group on cysteine and the AuNPs is much stronger than Nε–Au and electrostatic interactions. Many reports have demonstrated that the activity of AuNPs is badly damaged because the active sites on the surface of the AuNPs were irreversibly occupied by the formation of multiple S–Au bonds (Scheme 3C). As a result, the AuNPs on the cysteine-containing peptide nanofibers lost almost all of their catalytic ability.
Because NaBH4 is present in a significant excess in the reaction, the concentration of NaBH4 is assumed to be constant and the rate of the reaction only depends on the concentration of 4-NP. The reaction rate can be given as:
(1) |
(2) |
Fig. 4D show the kinetic curves for the degradation of 4-NP by the peptide-immobilized AuNPs. A linear relationship between ln(C/C0) and the reaction time suggests that the reaction is in agreement with the pseudo-first-order kinetics model, and the rate constants are kCoA–H–AuNPs = 0.127 min−1 and kCoA–R–AuNPs = 0.087 min−1. Compared with free AuNPs (kAuNPs = 0.880 min−1) that used for the preparation of peptide-immobilized AuNPs, the activity yield of CoA–H–AuNPs and CoA–R–AuNPs are 14.3% and 10%, respectively. This result indicated that the activity of CoA–X–AuNPs can be further improved if more AuNPs can be attached to peptide nanofibers and favourably using the histidine-containing peptide.
Congo red is a popular used azo dye in textile, paper and plastic industries which usually causes serious damage for environment. In order to investigate the practical performance of the peptide-immobilized AuNPs, we then determined the degradation of Congo red by using peptide-immobilized AuNPs. As shown in Fig. S2,† the linear relationship between ln(C/C0) and the reaction time also indicated that the reaction is in consistent with the pseudo-first-order kinetics model, and the rate constants are kCoA–H–AuNPs = 0.276 min−1 and kCoA–R–AuNPs = 0.089 min−1. Similarly, CoA–C–AuNPs displayed null activity on Congo red. Comparing with AuNPs immobilized on Salmalia malabarica gum (k = 0.236 min−1)26 or GO-G1PAMAM (k = 0.21 min−1),27 the as-prepared CoA–H–AuNPs displayed a better catalytic performance in the degradation of Congo red.
From the point of saving cost and environmental protection, the reusability is another important parameter for catalysts especially in industrial use. The as-prepared peptide-immobilized AuNPs can be separated from the reaction system through centrifugation or filtration. Fig. 6 shows the reusability of the peptide-immobilized AuNPs under reaction condition. CoA–H–AuNPs could be recycled and reused for at least 7 times with a residual activity of 75%. The decline of activity might due to the gradual mass loss after repeated recycles. However, CoA–R–AuNPs lost almost half of its original activity after the first recycle operation, and remained only 29% after the 7 times. This might due to the low stability of the agglomerated AuNPs in CoA–R–AuNPs compared with that of CoA–H–AuNPs. Therefore, we conclude that histidine-conjuncted peptide nanofibers are a preferable support to immobilize AuNPs.
Footnotes |
† Electronic supplementary information (ESI) available: UV spectra in the range of 250–310 nm, Congo red reduction reaction, images before and after 72 h incubation. See DOI: 10.1039/c6ra28673a |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2017 |