Quasi one-dimensional assembly of gold nanoparticles templated by a pH-sensitive peptide amphiphile from silk fibroin

Abstract

One dimensional (1-D) assembly of gold nanoparticles (AuNPs) plays critical roles in fabricating linear optical and electronic devices. Herein, a novel pH-sensitive template formed by C₁₂-GAGAGAGY, a peptide amphiphile based on the sequence of Bombxy mori silkworm silk fibroin, was employed to generate gold nanoparticles (AuNPs) in situ and direct their aggregation. Due to the incorporation of tyrosine, C₁₂-GAGAGAGY was able to reduce Au³⁺ to AuNPs and further stabilize them at pH 11 without any external reducing or capping reagents. In addition, these AuNPs could reversibly assemble/disassemble by varying pH due to the pH-sensitive assembly of the template, C₁₂-GAGAGAGY. These templates were cylindrical nanofibers at pH 11 and stacking nanoribbons at pH 4. AuNPs were well-dispersed among the networks formed by cylindrical nanofibers of C₁₂-GAGAGAGY at pH 11, while they aggregated on both sides of the stacking nanoribbons at pH 4. Interestingly, unlike traditional pre-defined templates, the pH-induced assembly of C₁₂-GAGAGAGY and assembly/disassembly of AuNPs occurred simultaneously. These results present us with the potential to utilize such smart peptide amphiphile templates for the fabrication of 1-D inorganic nanostructures with promising applications in nano-scale optical devices.

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Introduction

The unique physical properties of gold nanoparticles (AuNPs), such as electronic, optical and catalytic features, give them versatile applications in nanotechnology, materials science and biomedicine.^1–3 Research on AuNPs was commonly classified into two fields: synthesizing nanoparticles with controlled morphology^4,5 and developing methods to direct their assembly,⁶ between which the latter has attracted more attention. Assemblies of AuNPs were significantly different from discrete nanoparticles since they displayed collective electronic and optical properties.^7,8 Among these aggregates, due to the anisotropic arrays, one dimensional (1-D) assemblies exhibited excellent inter-nanoparticle electronic, photonic and energy transfer, which made them suitable for fabricating nano-scale optical and electronic devices.^6,9 Generally, 1-D assembly architectures of AuNPs are obtained by three methods:¹⁰ 1) utilizing template techniques where nanoparticles are deposited on the pre-defined scaffolds;^11–15 2) using interactions (hydrogen bonds, electrostatic interactions and dipole–dipole interactions) between capping molecules, which could be controlled by an environmental stimulus (such as pH, temperature, solvent polarity and light, etc.);^16,17 and 3) employing external electric or magnetic fields.^18,19 Due to the complexity and anisotropy of various 1-D nanostructures, template techniques have mainly been utilized as the favourable method to direct the anisotropic assembly of AuNPs. By using linear templates assembled from synthetic polymers or natural macromolecules, such as DNA and proteins, a number of 1-D assemblies have been facilely obtained.^12,20,21

On the other hand, short peptides can be easily prepared through gene engineering or solid phase synthesis.¹⁴ Their sequences can be defined to incorporate specific amino acid residues with the ability to produce nanoparticles.^22–24 Moreover, through reasonable design, such peptides are capable of forming specific secondary structures, e.g. α-helix or β-sheet, and then aggregating to well-defined nanostructures, such as nanofibers and nanotubes.^25–28 Therefore, the templates based on short peptides provide a facile method to construct the 1-D aggregate of nanoparticles. For instance, after adding gold colloids to a peptide aqueous solution, Fu et al. found that the AuNPs aligned along the nanofiber of the peptides dominated by β-sheet conformation.²⁹ Sharma et al. also presented that the AuNPs were immobilized on the nanofibers assembled from an alanine-rich peptide with positive charges by simply mixing the nanoparticles and peptide in aqueous solution.³⁰ Other templates, such as laminated nanofibers from oligopeptides³¹ and double helix nanofibers from peptide amphiphiles composed of a hydrophobic tail and a peptide segment,¹⁵ have also been reported. However, it should be noticed that in all of these cases, either reducers (e.g. NaBH₄²⁹, sodium citrate^30,31) or stabilizers (e.g. 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)¹⁵) were additionally used to assist the preparation, and the morphology of the formed 1-D nanostructure hardly responded to the environmental stimulus.

In our previous study, an octapeptide with abundant yield, GAGAGAGY, was obtained from the hydrolysate of Bombxy mori silkworm silk fibroin.³² After coupling a dodealkyl chain to the N-terminal of the peptide, it was found that the peptide amphiphile, C₁₂-GAGAGAGY, assembled into nanofibers with different morphologies at various pH values (Scheme 1). Such a peptide amphiphile may have reducing ability under alkaline conditions since it contains a tyrosine residue. Therefore, in this work we aimed to employ this pH-sensitive peptide amphiphile to produce gold nanoparticles in situ under alkaline conditions and then control their 1-D assembly by changing the pH value of the solution.

Scheme 1 Different structures of nanofibers formed by C₁₂-GAGAGAGY at different pH values. The phenol group is the position that can reduce Au³⁺ and bind AuNPs.

Experiments and materials

In situ reduction of AuNPs

Peptide amphiphile based on Bombxy mori silk fibroin (C₁₂-GAGAGAGY) was synthesized as previously reported.³² AuNPs were prepared as follows: 4 mg C₁₂-GAGAGAGY was dissolved in 1 mL NaOH aqueous solution (0.04 mol L⁻¹). Then, 100 μL solution was diluted by 200 μL NaOH aqueous solution, followed by adding 100 μL HAuCl₄ (1 mmol L⁻¹) dropwise. The pH of the mixture was adjusted to 11, and the mixture solution was stirred at room temperature for 6 h. The molar ratio of tyrosine to Au³⁺ was 5 : 1. An obvious change of the color of the solution was observed from yellow to red, indicating the formation of AuNPs. NaBH₄-reduced AuNPs were prepared according to Fu's experiments.²⁹

Assembly of AuNPs

The peptide–AuNPs solutions at pH 7 and 4 were prepared by dropwise addition of concentrated HCl (2 mol L⁻¹) to the as-prepared C₁₂-GAGAGAGY/AuNPs mixture (pH 11). A gradual color change from red to purple was observed when the pH value decreased from 11 to 4. As a control, the AuNPs prepared by NaBH₄ were directly added to the C₁₂-GAGAGAGY solution at pH 4 and then the mixture was incubated for 6 h at room temperature.

UV-Visible spectroscopy

UV-Vis measurements were carried out on Hitachi U2910 UV-vis spectrophotometer. The sample solutions at pH 11, 7 and 4 were placed in the quartz cell. Then, the absorption of the solution was recorded in the range 400–800 nm at a scan rate of 400 nm min⁻¹.

For time-resolved UV spectra, the solution was prepared as follows: 100 μL 4 mg mL⁻¹ C₁₂-GAGAGAGY alkaline aqueous solution was diluted by 200 μL NaOH aqueous solution and then 100 μL 1 mM HAuCl₄ was added dropwise. After mixing for 1 min, the absorption of solution was recorded. The absorptions at 5, 30, 60, 90, 120, 150, 180 and 210 min were recorded by the same method.

Fourier transform infrared spectroscopy (FTIR)

20 μL of peptide–AuNPs solutions at different pH values and the peptide amphiphile solution without AuNPs were coated on a CaF₂ plate and dried in a vacuum. FTIR absorption spectra were collected on a Nicolet Nexus-470 spectrometer with a resolution of 4 cm⁻¹. Each spectrum was an average of 64 consecutive scans.

To investigate the adsorption of C₁₂-GAGAGAGY on gold nanoparticles, first, the as-prepared gold colloidal solution were centrifuged. Then, the precipitated AuNPs were collected, washed and dried. Finally, the FTIR spectrum of these nanoparticles was recorded in a KBr pellet. The sample of AuNPs reduced by NaBH₄ was also prepared as above described.

Transmission electron microscopy (TEM)

The AuNPs solution (5 μL) at pH 11, pH 7 and pH 4 was dropped on a copper grid and dried in air. The samples were imaged under a Hitachi H-600 operated at 75 kV.

Results and discussion

AuNPs produced by using C₁₂-GAGAGAGY at pH 11

In previous studies,^23,33 tyrosine was found to act as an electron donor and reduce Au³⁺ to Au⁰ due to the dissociation of the phenol group of tyrosine in alkaline solution and the relatively high oxidation–reduction potential of Au³⁺. After this oxidation–reduction reaction, the phenol group of tyrosine was oxidized to the quinones and Au³⁺ was reduced to Au⁰ atoms, which further combine to form AuNPs. Moreover, the corresponding quinones were able to stabilize the formed AuNPs through the interactions between their carbonyl groups and the nanoparticles. With the tyrosine at its C-terminal, C₁₂-GAGAGAGY was also speculated to have such reducing and stabilizing ability.

Fig. 1A displays the time-resolved UV absorption of the HAuCl₄/C₁₂-GAGAGAGY mixture without addition of any other reducing or stabilizing reagents. Neither HAuCl₄ nor C₁₂-GAGAGAGY showed any absorption in the range 400–800 nm. However, a new peak immediately appeared around 527 nm after adding HAuCl₄ to C₁₂-GAGAGAGY solution (blue curve in Fig. 1A). This was a typical Surface Plasma Resonance (SPR) band originating from the collective oscillation of the electron cloud in neighboring nanoparticles,³⁴ indicating the formation of gold nanoparticles with particle size less than 20 nm.³⁵ This band was weak and broad at the beginning and then gradually turned to intensive and sharp. Its maximum absorption was reached after 180 min post-mixing, suggesting the complete in situ reduction of Au³⁺. Meanwhile, the colour of the solution changed from yellow to red. Thus, all of the following experiments were based on such completely reduced AuNPs. TEM images confirmed the formation of spherical AuNPs with an average diameter of 10 nm (Fig. 2A and inset), consistent with the estimation from UV spectra. Since no additional capping reagents were added, it is highly possible that the formed AuNPs with uniform size were stabilized through their interaction with C=O of the oxidized phenol, i.e. quinone, in C₁₂-GAGAGAGY.³³ Such an interaction resulted in the adsorption of the peptide amphiphile on nanoparticles, verified by the FTIR results of the nanoparticles. Two peaks at 2918 and 2850 cm⁻¹, corresponding to –CH₂–, and a sharp peak, ascribed to –COO⁻ groups at 1680 cm⁻¹, were observed in the spectrum of the AuNPs reduced by C₁₂-GAGAGAGY, suggesting the presence of C₁₂-GAGAGAGY on the nanoparticle surface (Fig. 3A). In comparison, the AuNPs directly prepared by NaBH₄ didn't show any characteristic absorption at these positions (the broad band at 1650 cm⁻¹ was caused by moisture³⁶). The stabilization of the nanoparticles by the peptide amphiphiles could be further illustrated by the good dispersion of AuNPs. At pH 11, the dissociation of the carboxyl group in C₁₂-GAGAGAGY resulted in the electrostatic repulsion between these molecules. On account of such repulsion, C₁₂-GAGAGAGY adsorbed on the nanoparticles prevented the aggregation of neighbouring AuNPs. As a result, AuNPs were scattered, as observed in the TEM images.

Fig. 1 UV-Visible absorption of AuNPs reduced by C₁₂-GAGAGAGY: A) time-resolved UV-Visible spectra of AuNPs at pH 11; concentration of C₁₂-GAGAGAGY is 1 mg ml⁻¹ and AuNPs is 0.25 mmol L⁻¹. B) UV absorption for AuNPs at different pH values; the inset is a digital photograph of the peptide–AuNPs solutions at different pH values.

Fig. 2 TEM images of AuNPs reduced by C₁₂-GAGAGAGY at pH 11 : A) coexistence of cylindrical nanofibers and AuNPs were observed; the inset is a histogram analysis of AuNPs, indicating that these nanoparticles have an average diameter of 10 nm. B) the cylindrical nanofibers were reformed and the nanoparticles were re-distributed well after the pH value of the mixture solution was adjusted from 4 back to 11; concentration of C₁₂-GAGAGAGY is 1 mg ml⁻¹ and AuNPs was 0.25 mmol L⁻¹. The samples were negatively stained by uranyl acetate.

Fig. 3 A) FTIR spectra for AuNPs reduced by NaBH₄ (black curve) and by C₁₂-GAGAGAGY (red curve), indicating the absorption of peptide amphiphile on the nanoparticles. B) FTIR curves for peptide amphiphile at pH 11, AuNPs–peptide amphiphile mixture at pH 11 and AuNPs–peptide amphiphile mixture at pH 4.

On the other hand, nanofibers were also found in the TEM image due to excessive peptide amphiphile (Fig. 2A). They were around 10 nm in width and several micrometers in length.

According to our previous report,³² cylindrical nanofibers with a planar β-sheet conformation were formed at basic conditions. Herein, a peak attributed to this secondary structure appeared at around 1625 cm⁻¹ (Fig. 3B, red curve) in the FTIR spectrum of AuNPs–peptide amphiphile mixture, confirming the formation of the cylindrical nanofibers at pH 11. Compared with the curve of C₁₂-GAGAGAGY (Fig. 3B, blue curve), a new band appeared at 1725 cm⁻¹ for AuNPs–peptide amphiphile mixture, indicating the oxidization of tyrosine.³⁷ Due to the electrostatic repulsions from the adsorbed C₁₂-GAGAGAGY, the nanofibers didn't show any template effect on the AuNPs at pH 11. Although some of the nanoparticles were attached to the nanofibers, most of them were dispersed among the networks formed by these nanofibers.

Simultaneous formation of 1-D assembly of AuNPs and the nanoribbon templates triggered by decreasing pH

Although the AuNPs hardly aggregated in view of the electrostatic repulsion at pH 11, our previous results suggested that it would be possible to drive the assembly of AuNPs simply by decreasing pH values, which can eliminate the electrostatic repulsion between C₁₂-GAGAGAGY.³². It was found that the color of the peptide–AuNPs solution changed from wine red to light purple when the pH values of the solution decreased from 11 to 4 (Fig. 1B, inset). Correspondingly, the SPR band in the UV spectra red-shifted from 527 nm to 540 nm (Fig. 1B), which was associated with aggregation of AuNPs.³⁴ The assembly of these AuNPs at low pH was further supported by TEM images. At pH 4, a linear array of AuNPs along the nanoribbons was visualized (Fig. 4A). These particles get much closer than the ones at pH 11, in accordance with the observation of the solution colour change. The magnified image shows a sophisticated 1-D superstructure, where the AuNPs were immobilized on both sides of the nanoribbons, which functioned as a template (Fig. 4B). In some cases, AuNPs leaked into the solution, maybe caused by the relatively weak interaction between AuNPs and C₁₂-GAGAGAGY. Interestingly, it was found that the secondary structure of these nanoribbons at pH 4 slightly differed from that of cylindrical nanofibers formed at pH 11. The FTIR spectrum of the nanoribbons showed that the amide I band red-shifted to 1631 cm⁻¹ (Fig. 3B, black curve), indicating that C₁₂-GAGAGAGY formed stacking β-sheet laminates at pH 4 as previously reported.³² This band was rather broad, indicating the existence of other conformations, such as a random coil.³⁸ Combined with the TEM image, we reasoned that C₁₂-GAGAGAGY adsorbing on leaked AuNPs adopted a random coil, finally resulting in the broadening of the amide I band. Based on the above results, it demonstrated that decreasing the pH value of peptide–AuNPs solution both induced the 1-D aggregation of AuNPs and triggered the assembly of C₁₂-GAGAGAGY. The neutralization of the dissociated carboxyl group in C₁₂-GAGAGAGY at pH 4 resulted in the elimination of electrostatic repulsions and C₁₂-GAGAGAGY had a strong tendency to assemble into β-sheet laminates rather than cylindrical nanofibers. These β-sheet laminates further stacked to form nanoribbons due to the hydrophobic interaction between two laminates. C₁₂-GAGAGAGY adsorbing on AuNPs was involved in this process, finally driving the nanoparticles to assemble along the nanoribbons. In the case of directly adding AuNPs produced by NaBH₄ to the peptide solution at pH 4, the AuNPs were indeed randomly immobilized on the surface of nanoribbons and the 1-D assembly for AuNPs that array on both sides of the nanoribbons could rarely be obtained (Fig. 5). Thus, it was reasonable to conclude that the process of assembly of C₁₂-GAGAGAGY and the process of aggregation of gold nanoparticles occurred simultaneously. It should be noticed that both assembly of AuNPs and transition of templates in the present case were triggered by varying pH and shared one dynamic process, which differed from that of “one single step” concluded after filtering the peptide solution to remove assembled nanostructures and then reducing HAuCl₄ to AuNPs that aggregated on the nanofiber templates of peptide.¹⁵ Furthermore, when the concentration of C₁₂-GAGAGAGY and HAuCl₄ was proportionally increased, stacking and entwining of the nanoribbons became more evident (Fig. 4C). Compared to that at lower concentration, AuNPs were still attached to both sides of the nanoribbons, causing a “stain” effect on the nanoribbons. It was noteworthy that such pH-sensitive assembly was reversible. By adjusting the pH value of the peptide–AuNPs solution from 4 back to 11, the colour of the solution changed back to red, which is consistent with the change of the SPR band, which shifted from 540 nm back to around 527 nm (Fig. 1B and inset). The TEM image further confirmed that the assembly of cylindrical nanofibers and AuNPs were reversible (Fig. 2B).

Fig. 4 Assembly of AuNPs controlled by C₁₂-GAGAGAGY: A) a 1-D nanoparticles superstructure was obtained when pH was further decreased to 4; B) the magnified image showed nanoparticles were immobilized on both sides of the nanofibers, and in some area stacking of nanofibers was observed; C) a network of nanofibers “stained” by AuNPs was formed when both concentrations of HAuCl₄ and C₁₂-GAGAGAGY were proportionally increased; D) nanoparticles started to aggregate along the nanofibers when the pH was decreased to 7.

Fig. 5 A TEM image of AuNPs that were prefabricated by NaBH₄ immobilized on the surface of nanoribbons (pH 4).

An intermediate pH, i.e. pH 7, was also chosen to probe the transitional state of the assembly of AuNPs. As the pH values decreased from 11 to 7, the solution showed an intermediate colour between red and purple. Meanwhile, the UV-vis curve red-shifted to 535 nm, indicating that the AuNPs started to aggregate. This was further confirmed by the co-existence of dispersed and aggregated AuNPs in the TEM image and the latter ones mainly went along the nanoribbons (Fig. 4D). Further descent of the pH value from 7 to 4 led to the decreasing of dispersed AuNPs and a more sophisticated 1-D AuNPs. This observation was consistent with those of the UV spectra, in which the SPR band red-shifted from 527 nm at pH 11 to 535 nm at pH 7 and to 540 nm at pH 4 (Fig. 1B).

Mechanism for 1-D aggregation of AuNPs

Based on the results above, a probable mechanism of 1-D assembly for AuNPs is suggested in Scheme 2. When C₁₂-GAGAGAGY was dissolved in alkaline aqueous solution (pH = 11), cylindrical nanofibers with an alkyl core and a peptide shell were formed. Meanwhile, the phenol group of the tyrosine was dissociated, serving as a reducing agent for metal ions. Once HAuCl₄ was added to the solution, Au³⁺ cations were reduced to Au⁰ atoms, which further combine to form AuNPs. Some of these nanoparticles were formed on the surface of cylindrical nanofibers, while the others were synthesized in the aqueous environment. Meanwhile, the oxidized product of phenol groups, quinones, were able to stabilize the formed AuNPs through the interaction between C=O and the surface of gold nanoparticles.^33,39 This interaction resulted in the adsorbing of C₁₂-GAGAGAGY on nanoparticles. Consequently, a protective shell of C₁₂-GAGAGAGY was formed around the nanoparticles, contributing to the stabilization of AuNPs. Such peptide protective shell was negatively charged due to the dissociated carboxyl groups, thereby preventing the aggregation of the nanoparticles. Therefore, C₁₂-GAGAGAGY didn't have any template effect in spite of the interaction between the nanoparticles and the peptide amphiphile, namely, dispersion rather than aggregation was observed for AuNPs at pH 11.

Scheme 2 The mechanism for the aggregation of AuNPs simultaneously accompanied by the assembly of nanofibers templates from C₁₂-GAGAGAGY.

When the pH was decreased from 11 to 4, the dissociated carboxyl groups were gradually protonated, resulting in the elimination of electrostatic repulsion. Thus, C₁₂-GAGAGAGY was able to aggregate into β-sheet laminates that had a strong tendency to stack. The peptide amphiphiles absorbing on nanoparticles also participated in this process, spontaneously driving the nanoparticles to aggregate along the nanofibers. As a result, the assembly of C₁₂-GAGAGAGY and the aggregation of AuNPs were coupled into one single step. Furthermore, according to our previous model,³² C₁₂-GAGAGAGY assembled into β-sheet laminates with the hydrophobic tail aggregating in the center and the peptide segment exposed to water. Herein, the tyrosine located at both sides of these β-sheet laminates, consequently leading to immobilization of AuNPs on both sides of the laminates, as revealed by TEM. When both the concentrations of HAuCl₄ and C₁₂-GAGAGAGY were increased, the stacking and entwining between laminates became more evident and more nanoparticles were attached to the nanofibers. Finally, a network decorated by AuNPs was obtained.

Conclusions

In conclusion, a pH-sensitive template for preparing AuNPs and controlling their 1-D assembly was presented. This template, prepared from C₁₂-GAGAGAGY, a peptide amphiphile based on the sequence of Bombyx mori silk fibroin, can regulate the assembly/disassembly of AuNPs at different pH values through its pH-sensitive assembly. The nanoparticles were well-dispersed at pH 11, while they aggregated along both sides of the nanoribbons at pH 4. Moreover, the aggregation of AuNPs occurred simultaneously with the assembly of peptide amphiphiles. These results present a novel and facile method to produce stimulus responsive 1-D inorganic nanostructures using pH-sensitive templates.

Financial supports from the National Natural Science Foundation of China (NSFC 21034003) and Chinese Ministry of Science & Technology (973 Project No. 2009CB930000) are grateful acknowledged.

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