Polyphenol–gelatin nanoparticles as reductant and stabilizer for one-step synthesis of gold nanoparticles and their interfacial behavior

Abstract

A facile method for the one-step synthesis of gold nanoparticles (AuNPs) supported on tannic acid (TA) and gelatin self-assembled nanoparticles at room temperature was proposed. Tannic acid, a kind of polyphenol extracted from plants, could be self-assembled into nanoparticles with gelatin by strong hydrophobic interactions. This self-assembly nanoparticle acted as a reductant and stabilizer and was shown to be a good nano reactor for in situ synthesis of AuNPs. The stabilized AuNPs were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform-infrared Spectroscopy (FTIR). The average size of the AuNPs was about 10 nm with a spherical shape. The reduction process of AuNPs was accompanied by the oxidation of polyphenol hydroxyl groups. Surprisingly, we found the oil–water interfacial behavior of polyphenol/gelatin nanoparticles had significant changes after AuNPs synthesis on the surface. Dynamic interfacial rheology tests revealed that AuNPs decreased the interfacial tension and enhanced the moduli of surface dilatational.

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1. Introduction

Gold nanoparticles (AuNPs) have been widely used in biology and catalysis areas due to their fascinating size-related electronic, optic, magnetic and catalytic properties. The focus of nanotechnology is gradually shifting from fabrication methods to its assembly behaviour into large-scale regulated structures.¹ The organized nano/microstructures exhibit remarkably collective properties with great potential in applications such as drug delivery and control release, food supplements and nano-reactors.¹ A promising application of gold nanoparticles is to stabilize emulsions or self-assemble to form specific structures on the liquid–liquid interface. Tian et al. produced amphiphilic gold nanoparticles at the liquid–liquid interface and then fabricated hybrid hollow capsules with AuNPs on the surfaces.² Amphiphilic gold nanoparticles also have been demonstrated to effectively stabilize emulsions of alkanes in water.³

The interfaces can be stabilized by nanoparticles forming dispersions reported by Pickering about a century ago.⁴ It pointed out that the particles dispersed in one phase could accumulate to the interface in order to mediate interactions between the fluid phases. Because Pickering emulsion possesses more stabilization against flocculation and relatively well-controlled size,⁵ there is a great number of researchers focus on achieving different kinds of stabled nano/micro-particles. Therefore, inorganic particles,^6,7 polymer particles,⁸ or graphene sheets^9,10 have been reported to fabricate Pickering emulsions. However, synthetic particles have limited direct applications in food, pharmaceutical and cosmetic formulations. The development of environmental friendly particles based on natural resources is demanded. Food based macromolecules provide good material resources, their original diversity interfacial properties could be advantageously used to achieve better stability and enhance encapsulation effectively. To date, food-grade particles for stabilizing Pickering emulsion, such as zein colloids,¹¹ whey protein microgel particles,¹² chitin crystal¹³ and modified starch granules,¹⁴ are widely explored. Furthermore, many researchers have been devoted to obtaining different microarrays of these particles through controlling the particle size, hydrophobicity and their self-assembled structures.¹⁵

Polyphenols, extracted from plants, are able to reduce metal ions in an easy and safe green method. As the former reported, bayberry tannin,¹⁶ coffee¹⁷ and tea extract¹⁸ were able to reduce Au³⁺ ions to Au nanoparticles because antioxidant action of phenolic compounds. Polyphenols, always consist of catechin and epicatechin units, possess hydroxyl and carboxyl groups and have high tendency to chelate metals. Meanwhile, polyphenols and proteins could combine to form complexes by noncovalent interaction.¹⁹ Some reports believed the formation of self-assembled complexes might reduce the number of hydroxyl groups available in polyphenols, however the overall antioxidant activity of complexes would be benefited due to prolonged life of polyphenols in complexes.²⁰ They found that nanoparticles prepared using gelatin and pomegranate ellagitannis could decrease cell apoptotic through antioxidant effect.²¹ In this content, nanoparticles were fabricated by gelatin and tannic acid self-assembly and they displayed strong reducing profiles and good interfacial stability.

Herein, a one step, facile and size-controlled synthesis of stable AuNPs by using gelatin–tannic acid self-assembled nanoparticles as supported matrix was described. Subsequently, this gelatin–polyphenol nanoparticles obtained surface-modification by AuNPs and improved their emulsion stabilization. As shown in Scheme 1A, the chemical structure of tannic acid (TA) belonged to a large group member of polyphenols, which are widely distributed in plants. The AuNPs in situ synthesis processes were carried out in aqueous solution at room temperature (Scheme 1B). The basic properties of AuNPs were characterized by TEM, XPS and FT-IR. After AuNPs formation on the surface of gelatin–polyphenol nanoparticles, we found that interfacial stability improved significantly. The stabilization of Pickering emulsion and quantitative analysis of dynamic surface tensions and surface dilatational moduli were measured to evaluate the changes on the interface.

Scheme 1 Schematic plot illustrating the formation and stabilization of AuNPs with polyphenol/gelatin nanoparticles as supported matrix.

2. Experimental section

Chemicals

Gelatin (Type B) was purchased from Sigma Chemicals. TA (tannic acid), MCT (medium chain triglyceride, MIGLYOL 812N), HAuCl₄ (99%) and other agents were obtained from Sinopharm Chemical Reagent Co. (Beijing, China) without further purification. They were prepared using ultrapure water (γ: 71.8 ± 0.2 mM m⁻¹ at 25 °C; resistivity 18.2 MΩ cm).

Preparation of polyphenol–gelatin nanoparticles (PGNPs)

Polyphenol/gelatin nanoparticles were prepared by dropping 0.7 ml tannic acid (2 mg ml⁻¹) solutions into 5 ml gelatin (10 mg ml⁻¹) solutions at 25 °C and continuously stirred for 30 min.

Preparation of gold nanoparticles (AuNPs) supported by polyphenol–protein nanoparticles

In situ synthesis AuNPs was obtained by dropping 4 mM HAuCl₄ aqueous into 0.4% w/w PGNPs. AuNPs stabled at oil–water was formed though dropping HAuCl₄ after achieving stabled Pickering emulsion.

Characterization

Atomic force microscopy (AFM, Agilent 5500, USA) was used to observe the morphology of gelatin–polyphenol nanoparticles in tapping model. Dynamic Light Scattering experiments were performed using BI-9000AT autocorrelator (Brookhaven Instruments Corp., Holtsville, NY) at 25 °C and a scattering angle of 90°. For DLS analysis, the size distribution of gelatin–polyphenol nanoparticles and the autocorrelation function g (t) were achieved. UV-Vis absorbance was recorded between 400 and 800 nm using UV-visible spectrophotometer (Shimadzu UV-1700, Japan). The size and distribution of AuNPs synthesized by polyphenol–gelatin nanoparticles were determined using Transmission electron Microscopy (TEM, 120 kV, Hitachi, Japan). Fourier Transform-infrared Spectroscopy (FT-IR, Nicolet Nexus 470 USA) was used for analysing the structure changes of AuNPs formation. X-Ray Photoelectron Spectroscopy (XPS, Thermal VG Scientific, UK) analyses were conducted by employing Al-K_α radiation with a power of 300 W under a vacuum of 3 × 10⁻⁷ Pa. The binding energy scale was charge referenced to the C 1s at 284.6 eV. Peaks from high-resolution core spectra were fitted with XPSPEAK41 software, using mixed Guassian–Lorentzian functions.

Interfacial behaviour measurements

Interfacial properties of PGNPs and Au-PGNPs at oil–water interfaces were observed by stabilization of Pickering emulsion. These kinds of Pickering emulsion were prepared by vortex 3 min with volume ratios of MCT and 10 mg ml⁻¹ GTNPs 2 : 3. Then, emulsions were kept at 25 °C to observe the stability.

The axisymmetric drop shape analysis with an oscillating bubble rheometer (Tracker Teclis-IT Concept, France) was employed to study the interfacial tension (γ) and dilational properties of absorbed layers at oil–water interfaces. The volume of oil droplet was 11 μl for all measurements. For the sinusoidal volume fluctuations, the surface amplitude was chosen in the linear domain and fixed at 10%. The periods ranged from 10 s to 100 s. The viscoelastic module were defined as E* = dγ/d ln(A), which could be decoupled into real and imaginary parts E′ and E′′, respectively. Therefore, E*, E′ and E′′ were calculated as following relationship:

E′ = E* cos φ, E′′ = iE* sin φ

where φ is phase angle, E′ represents the elastic properties of adsorbed layer and E′′ indicates viscosity of this layer.

3. Results and discussion

Preparation of polyphenol–gelatin nanoparticles

In the previous papers, a strong hydrophilic interaction between gelatin and natural polyphenols leads to binding into insoluble complexes.²² Herein, the appropriated ratio of gelatin and tannic acid was optimized to prepare a kind of nano-scale particle. The morphology and size distribution of polyphenol–gelatin nanoparticles (PGNPs) were observed by AFM (Fig. 1). After calculated nanoparticle size using automated analysis software, the size of nanoparticle was determined by ΔZ. The majority number of nanoparticles is around 130 nm as shown in Fig. 1A and B. Meanwhile, a certain amount of aggregations appeared, which size values reached 280.35 nm, might due to drying process of dropping sample on mica plate, (Fig. 1C and D). The hydrodynamic radius of PGNPs in the swollen state was measured by DLS. Reliability of the DLS results was verified by repeating each run 3 times, and a single run duration was set 60 s. The auto-correlation function of PGNPs and the values of hydrodynamic radius were shown in Fig. 1E and F. The peak of size distribution was approximately 168 nm with a good dispersity (polydispersity index < 0.20), which size was larger than AFM results.

Fig. 1 AFM images of PGNPs. (A) and (C) AFM images of PGNPs; (B) and (D) ΔZ of PGNPs; (E) size distribution of PGNPs using DLS and (F) DLS auto-correlation function of PGNPs.

Preparation and characterization of AuNPs synthesized by polyphenol–gelatin nanoparticles (Au-PGNPs)

As shown in Scheme 1, gold nanoparticles were synthesized by PGNPs, which acted as a supported materials and nano-reactor. Among them, the polyphenol played a leading role in reducing agent. Recent studies indicated that polyphenols could perform capacitor functions of accepting, storing and donating protons and electrons. And tannins have been involved into capsule membranes to metal reducing.²³ In this content, tannic acid, a typical natural polyphenol, possesses multiple phenolic hydroxyls and reduces Au³⁺ ions to Au⁰ atoms on the surface of PGNPs at room temperature.

The visual absorption of gold nanoparticles is one of the characteristic signatures resulting from surface plasmon resonance (SPR), which depends on the size and shape of Au nanoparticles. Upon addition of the Au³⁺ content, the colour of the solution changes from light blue opalescence to purple red, representing the gold nanoparticles formed.¹⁶ The typical characteristic SPR absorbance at 535 nm was clearly seen in Fig. 2A. In the Fig. 2B, it was clearly observed that in situ synthesized AuNPs located on the surface of PGNPs, which were little black dots shown on a light black sphere particles. TEM image of PGNPs showed the size of PGNPs in the range of 100 to 200 nm, corresponding to AFM results. The well-dispersed Au nanoparticles were observed as dark dot with an average diameter of 10 nm. Fig. 2B clearly demonstrates that PGNPs could be acted as an effective supported material for well-dispersed Au nanoparticles synthesis.²⁴

Fig. 2 (A) UV-Vis spectra of TA/gelatin nanoparticle and Au–protein nanoparticles; (B) TEM images of Au–protein nanoparticles with 400 μM Au³⁺.

The chemical state of gold nanoparticles in Au-PGNPs were analysed by XPS. The survey spectra of PGNPs before and after AuNPs synthesis were displayed in Fig. 3A. The peaks of O1s, N1s, C1s and Au4f was clearly seen, the deconvolution of elements core level spectra was analysed in Fig. 3B and 4, respectively. Fig. 3B reveals the Au4f core-level spectra of Au-PGNPs, which curve was fitted with two pairs of doubles from spin–orbital splitting of 4f_7/2 and 4f_5/2.²⁴ The deconvolution of Au4f core level spectra was divided into four peaks, centred at 84.0, 85.3, 87.6 and 88.1 eV. Among them, the peaks at 84.0 and 87.8 eV, characteristic of inner Au atoms of AuNPs, were consistent with zero valent Au⁰ according to former reports.²⁵ Another two peaks located at 85.3 and 88.1 eV were attributed to the outer surface Au atoms bonded with polyphenol. The results were coordinated with AuNPs reduction and stabilization by EGCG.²⁴

Fig. 3 XPS spectra of PGNPs and Au-PGNPs; (A) scan spectrum, (B) Au 4f core-level spectrum.

Fig. 4 Deconvoluted C1s and O1s high-resolution XPS spectra of PGNPs (A) and Au-PGNPs (B).

Moreover, deconvolution of C1s and O1s spectrum were shown in Fig. 4. There are three separated peaks in PGNPs C1s spectra, which are located in 284.6, 286.5 and 288.3 eV respectively. The peak at 284.6 eV is assigned to C–C or C–H bonds.²⁶ The peak at 286.5 eV, corresponding to C–O bonds, is attributed to NH–CHR–CO carbons of gelatin backbone and the peak at 288.3 eV, corresponding to C=O bonds, is assigned to CO–NH– peptide carbons.²⁷ As for O1s spectrum, two peaks exhibited the binding energy at 531.5 and 532.7 eV, corresponding to O=C–N in peptide carbonyl groups and oxygen atoms in phenolic hydroxyl (HO–C).²⁸

After Au³⁺ reduction, the intensity of 284.6 eV weakened to hardly observe. Meanwhile, the intensity peak of 288.3 eV was strengthened, indicating C–H or C–C was oxidized to C=O. Accordingly, the peak intensity of O1s spectra revealed the similar trend. Compared with PGNPs, the bonds of C–O/C–OH in Au-PGNPs dramatically weakened. But the bonds of C=O strengthened at the same time. These results were coordinated with the presence of Au atom to O atom electron charge transfer.

In order to obtain the information about the functional groups contributing for reducing and stabilizing gold nanoparticles, and the secondary structure changes of gelatin, FT-IR was used for analysis (Fig. 5). In the gelatin spectra (Fig. 5A), the peak of 1638 cm⁻¹ was assigned to amino group I (C=O and C–N stretching vibration), while amino group II (mainly N–H bending vibration) absorption band at 1547 cm⁻¹.²⁹ The amino group III adsorption band at 1238 cm⁻¹ arises from N–H in-plane bending vibration. Polyphenol–gelatin nanoparticles spectrum (Fig. 5B) revealed the similar peak with gelatin, while the peak of around 3400 cm⁻¹ appeared to be broadened, indicating the strong hydrogen-bonding interactions between gelatin and polyphenol. Comparing with the FTIR spectra of PGNPs and Au-PGNPs (Fig. 5C), it is obvious that there exist three distinctive strengthened peaks. First, peaks of around 2960 and 2870 cm⁻¹ shifted to lower wavenumber locating at 2930 and 2850 cm⁻¹. The changes were ascribed to C–H stretching vibration of CH₃ to CH₂. The new peak presence at 1745 and 1159 cm⁻¹ represented the carbonyl functional group stretching, which was originated from the ester linkage. However, the adsorption band around 3400 cm⁻¹ appeared to be narrowed and shifted to long wavelength (3448 cm⁻¹) probably due to weakened interaction of hydrogen bonding between gelatin and polyphenol, implying the involvement of the O–H groups after gold nanoparticles reducing.²⁴ Those results were matched well with XPS and further revealed that formation of gold nanoparticles companied with the oxidation of polyphenol hydroxyl groups.

Fig. 5 FT-IR spectra of (A) gelatin, (B) PGNPs and (C) Au-PGNPs.

Interfacial behaviour of PGNPs and Au-PGNPs

Protein aggregations and nanocomplex always displayed good interfacial properties, which served as stabilizer for preparing Pickering emulsions or foam.³⁰ In the current work, the changes of interfacial behaviour after reducing and stabilizing Au nanoparticles on the surface of PGNPs were studied. The Pickering emulsion stabilized by PGNPs was prepared simply by vortex for 1 min. The milky colour emulsion was obtained with well stability (Fig. 6). The macro-emulsions, droplet size of which is greater than 1 μm, kept evenly distribution droplet size around 500 μm.

Fig. 6 Digital photograph and optical microscopy images of emulsions used synthesizing and stabilizing AuNPs.

With the increase of Au³⁺ ion concentration, the more Au nanoparticles were formed on the surface of PGNPs. The digital photograph of emulsions after synthesis AuNPs on the surface of PGNPs was shown in upper picture of Fig. 6. The colour of emulsions changed from light to dark red, which was in accordance with AuNPs solutions shown in Fig. 2A.

The optical microscopy images investigated the changes of emulsion droplet size after different amount of AuNPs formation. Initially, at the low Au³⁺ ion concentration, AuNPs disturbed the dynamic balance of PGNPs adsorption–desorption at oil–water interface. This is mainly reflected on the big differences among droplet sizes (Fig. 6b–d), which emulsion sizes were varied from 100 to 500 μm. Until the concentration of Au³⁺ reached 400 μM, the size of oil droplets decreased to 100 μm with an evenly distribution (Fig. 6f). It may infer that AuNPs covered the partial surfaces of PGNPs and changed the interfacial property of PGNPs. A change in surface composition will lead to a change in rheological properties of adsorbed layers.³¹ The size of oil droplets suspended in water depended on the decrease of interfacial tension. To further confirm the changes, the effect of AuNPs covering on dilational rheological properties of PGNPs adsorbed layers has been examined at low frequencies using oscillating bubble rheometry.

Interfacial tension used to express surface tension as a sum of components due to dispersion force and polar forces.³² It mainly In Fig. 7, the dynamic surface tensions demonstrated that surface tension of all solutions with or without AuNPs decreased with the time prolonging. The curve of GTNPs reached a plateau region at 21 mM m⁻¹ with a fast rate of stabilization at interfaces. The presence of AuNPs could further decrease the surface tension significantly. For 40 μM AuNPs, the change of surface tension displayed the similar trends with GTNPs, expect for lower interfacial surface tension (18 mM m⁻¹). However, in the presence of 400 μM AuNPs, the curve of transformed into a steep drop, indicating AuNPs played a dominated role in changing adsorption layer structure. This phenomenon could be explained by nanoparticles rearrangement at the interfaces due to surface functional groups changes. When single GNTPs adsorbed at an oil–water interface, the initial surface tension decreased due to the diffusion-limited transport of nanoparticles between the bulk phase and the interfaces. In the presence of massive AuNPs, the nanoparticles' size did not obviously change in TEM images, so we speculate the stabilized capacity enhanced might because AuNPs increased adsorption layer elastic property and blocked droplet merge.

Fig. 7 Adsorption kinetics of 0.4% w/w PGNPs (□) and Au-PGNPs at 25 °C. The plots show the adsorption kinetics of Au-PGNPs at two AuNPs concentrations of 40 μM (○) and 400 μM (Δ) respectively.

Studies on emulsion behaviour can be related to the physical properties of two-dimensional model interfaces.³³ As is well known, the reduction in interfacial tension favours the emulsification process, but once interfacial films formed, viscoelastic properties of these interfacial films play a remarkable role in the stabilization of the oil droplets. The influences of oscillation frequency (from 0.01 Hz to 0.1 Hz) on dilatational rheological moduli and phase angles for GTNPs, GTNPs-Au solutions are displayed in Fig. 8. As the conventional studies defined, the specific properties of interfaces, e.g. Gibbs elasticity, complicate the understanding and interpretation of the 2-D elasticity storage moduli (E′) and loss moduli (E′′) of the adsorption layer.

Fig. 8 Surface dilational moduli of PGNPs and Au-PGNPs adsorbed at the oil–water interface. The data have been measured with or without AuNPs, PGNPs (■), 40 μM (●) and 400 μM (▲). Shown are the complex surface moduli E* (A), storage moduli E′ (B), loss moduli E′′ (C) and phase angle (D).

In Fig. 8A and B, the complex surface moduli E* and E′ increased with oscillation frequencies rising. In the presence of AuNPs, a significant increase in the E*, E′ and E′′ of adsorbed GTNPs is observed. Besides, the increase pace of E′ is faster than that of E′′. It exhibited a clear elastic response with elastic modulus (E′) completely dominating the storage modulus (E′′). Therefore, the adsorption layer is considered to be solid-like properties as the previous researches reported the storage module was much higher than the loss module. Then, the formation of AuNPs results in predomination elastic property.

In turn, the loss moduli (Fig. 8C) dropped slightly companying with the frequencies increase, resulting in weakening liquid-like properties. The large phase angle displayed in Fig. 8D illustrated the dilatational properties of the adsorbed layer are dominated by the exchange of nanoparticles between the bulk phase and the interface in the range of frequency examined.

4. Conclusions

A new kind of food-grade nanoparticles was developed with the diameter of 100–200 nm and sphere morphology by gelatin and tannic acid through their self-assemble behaviour. They possess reducing property to synthesize AuNPs in situ, companying with polyphenol oxidation supporting by FTIR and C spectra of XPS results. After AuNPs synthesized, Pickering emulsion stabilized by the nanoparticles displayed smaller and more evenly droplet size, meaning they improved the interfacial stabilization by formation of strong solid-like adsorption layer. Besides, the drop of surface tension and the increase of absorption layer both illustrated the mechanism of interfacial stabilization.

Acknowledgements

This work financially supported by National Natural Science Foundation of China (no. 31371841) and the PhD Candidate Research Innovation Project of Huazhong Agricultural University (no. 2014bs38).

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