A novel one-step synthesis of gold nanoparticles in an alginate gel matrix by solution plasma sputtering

Abstract

We report a novel strategy to produce stable colloidal gold nanoparticles (AuNPs) in alginate aqueous solution which can be done in one step and without any chemicals. The AuNPs were produced by applying a voltage across a pair of gold electrodes which were immersed in alginate aqueous solution. Since the generation of AuNPs was caused by the sputtering of gold electrodes, the process was named the solution plasma sputtering (SPS) process. We utilize the alginate polymer in order to meet three important requirements: (1) to promote the generation of plasma in a liquid environment, (2) to endow biocompatibility to the AuNPs, and (3) to provide colloidal stability to the AuNPs-alginate aqueous suspensions. The alginate concentrations were varied as 0.2, 0.5, and 0.9 %w/v. The concentration-dependent effect on the particle size of AuNPs, the physical absorption property and the stability of the AuNPs-alginate suspensions were studied. Results indicate that preparation of chemical-free colloidal AuNPs-alginate aqueous suspension is successful by the SPS process. The obtained colloidal suspensions were stable and able to retain their strong plasmon absorption bands within a reasonable time period. As a consequence, this is a high-potential technique to produce AuNPs suspended in alginate aqueous solution appropriate for biomedical applications.

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1. Introduction

The advancement of knowledge in nanotechnology enables gold nanoparticles (AuNPs) to offer great potential for diagnosis and therapy of disease in a variety of medical fields.^1–4 Owing to their high surface-to-volume ratios which offer lower detection limits and higher selectivity than conventional strategies,⁵ the AuNPs have been studied to detect specific targets such as nerve gases,⁶ ions,⁷ DNAs,⁸ and proteins.⁹ For their therapeutic potential, AuNPs are utilized as drug delivery vehicles. The flexibility in their size and shape as well as their versatility for functionalization have facilitated approaches for encapsulation of numerous therapeutic agents such as small drug molecules,^10,11 insulins,¹² and nucleic acids or genes.^13,14 As drug molecule carriers, they have been widely studied for application in anticancer therapy due to the enlarge requirement for new treatments in this area.¹⁵ To consider AuNPs for any in vivo biomedical applications, it is important to confirm their biocompatibility. The particles should be passivated with a layer acting as biocompatible interface before administration into living organisms. Apart from chemical functionalization on the AuNP surface,^16–18 one common strategy to wrap the AuNPs with biocompatible layer is to disperse them in a natural substance such as herbs,¹⁹ sugars,²⁰ and biopolymers, including: porphyran,²¹ chitosan,^12,22 and sodium alginate.^23,24 However, the pathway to produce AuNPs before dispersion is based on chemical reaction in which gold precursor (e.g., HAuCl₄) and/or reducing agent (e.g., NaBH₄, C₆H₈O₇, and H₂O₂) are required. This may be associated with biological toxicity. There is also some level of environmental impact occurring in the manufacturing process. Although the physical methods such as gamma irradiation^25,26 and UV photo-activation²⁷ are utilized to eliminate the use of reducing agents, the HAuCl₄ precursor is still required.

In the present contribution, we propose a one-step preparation of colloidal AuNPs in sodium alginate matrix (which thereafter called alginate). The AuNPs were produced from sputtering of gold electrodes, which were immersed in alginate aqueous solution, by the application of a potential difference. The process was named solution plasma sputtering (SPS) process.²⁸ We select alginate polymer as a matrix since it is one of the most commonly used materials for the encapsulation of biologicals. Its naturally block fashion is assumed to have an effect on the appearance of the encapsulated AuNPs. Apart from its potential safety issue, the alginate polymer can promote the generation of plasma in water²⁹ and provide colloidal stability to the AuNPs.^23,24 No chemicals are required and the process can be operated at room temperature and atmospheric pressure. Effect of the alginate concentration on the appearance of AuNPs and the optical absorption properties and the stability of AuNPs-alginate aqueous suspensions were studied. Finally, proposed mechanism for the production of colloidal AuNPs-alginate aqueous suspensions by the SPS process is demonstrated.

2. Experimental section

2.1. Materials and sample preparation

Sodium alginate was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). High purity gold rod (1 mm diameter, 99.95%) was purchased from Nilaco Corp., (Tokyo, Japan). Sodium alginate aqueous solutions were prepared at concentrations of 0.2, 0.5, and 0.9 %w/v. Ultra-pure water from Aquarius water distillation apparatus, RFD250NB, Advantec (Tokyo, Japan) was used for preparation of the alginate solutions and as a control-study.

2.2. Experimental setup and production procedure

The SPS process experimental setup used for the production of AuNPs-alginate aqueous suspensions is schematically shown in Fig. 1. The setup consists of two gold-rod electrodes inserted in a 100 mL glass beaker having an inner diameter of 43 mm and a height of 75 mm. The distance between the tips of electrodes was set to 0.3 mm. Alginate aqueous solution of 90 mL volume was added into the glass reactor and the plasma was generated, at atmospheric pressure and room temperature, using a bipolar-DC pulsed power supply (SPIK 2000A/KT-IDP-1010S). The pulse width and frequency employed during the generation of plasma were 2 μs and 15 kHz, respectively. The plasma discharge time was varied as 1, 2, 5, and 10 min. Solutions were stirred continuously during the operation in order to maintain homogeneity at any instant. During the generation of plasma, optical emission spectra were collected using an optical emission spectroscope (AvaSpec-3648, Avantes, USA), operated within the wavelength range from 200 to 1000 nm, with an integration time of 10 ms and average 10 scans.

Fig. 1 Experimental setup of the SPS process used for the production of AuNPs-alginate aqueous suspensions.

2.3. Characterizations

The shape and size of AuNPs were observed with a transmission electron microscope (TEM) JEM-2500SE (JEOL, Japan) operated at 200 kV. Samples were prepared by adding ethanol into the suspension which was then dropped onto a copper grid before let it dry by exposure to the air for 24 h. Particle size and size distribution were obtained by measuring diameters of 500 particles viewed in the TEM images.

The formation of AuNPs was monitored by observing optical property of the AuNPs suspensions with a UV-3600 spectrophotometer (Shimadzu, Japan) in the spectral range from 200 to 900 nm. For AuNPs-alginate aqueous suspension, concentration of the test-samples was 0.1 %w/v which was calculated based on the concentration of alginate solutions.

The stability of AuNPs-alginate aqueous suspensions was observed by letting the suspension settle, up to 6 months, before observing for their appearance. Zeta potential analysis was carried out using a zeta potential analyzer ELS-7300K (Photal Otsuka Electronics, Japan). Data were observed from samples which were left standing for 6 months. The pH measurement was conducted using twin pH waterproof B-212 (Horiba, Japan).

FTIR spectra were taken on a Shimadzu, IRPrestige-21 FTIR-8400s (Kyoto, Japan) at the wavenumber range from 4000 to 400 cm⁻¹. Samples were prepared by dropping about 40 mL of the solution on a silicon plate and letting it dried at room temperature prior to the detection recorded with 128 scans at a resolution of 4 cm⁻¹.

Note that samples of AuNPs produced in water were immediately prepared after the synthesis for TEM analysis and UV-vis measurement.

3. Results and discussion

3.1. Effect of alginate concentration on shape, size, and size distribution of AuNPs

TEM images of the AuNPs produced in water and alginate solution with different concentrations, along with their size distributions are shown in Fig. 2. Data were collected from samples prepared at sputtering time of 5 and 10 min for AuNPs dispersed in water and alginate solutions, respectively. From Fig. 2a, while AuNPs produced in water had non-uniform shape and exhibited the particles stick to each other to form aggregates; the AuNPs produced in alginate aqueous matrix were spherical and dispersed well. Their size and size distribution (Fig. 2b) were dependent on alginate concentration. When the concentration of alginate solution was increased, the size of AuNPs decreased. The average particle diameters of AuNPs were found to be 4.31 ± 0.85, 3.54 ± 1.04, and 2.87 ± 0.77 nm for 0.2, 0.5, and 0.9 %w/v alginate solutions, respectively. These sizes were smaller than those prepared by other methods reported in previous studies.^22,25,30,31 The obtained smaller size of AuNPs is a merit of the SPS method since it attributes to the broadening applicability of the particles. The size of nanoparticles is an important factor in medical applications, in which, penetration through a pore structure of a cell membrane is required. It was reported that distribution of AuNPs in tissues/organ was size dependent, for example; only AuNPs with the size smaller than 50 nm could pass blood-brain barrier, as evidenced from Au concentration detected in the brain of mice after 24 h of administration.³¹ Hence, the smaller size of the AuNPs reported here can indicate their ability to pass through several target cells.

Fig. 2 (a) TEM images of the AuNPs produced in water and alginate aqueous solution with different concentrations, along with (b) their size distributions.

3.2. Optical absorption properties of AuNPs-alginate aqueous suspensions

An important property of AuNPs is their ability to interact with light at a specific wavelength that leads to a phenomenon called surface plasmon resonance (SPR). The particular wavelength of light where the SPR occurs is strongly dependent on the AuNP size and shape. The rod-shaped AuNPs exhibit SPR spectra with two characteristic absorption bands, while the sphere-shaped AuNPs exhibit SPR spectra with only one band.³⁰Fig. 3 shows UV-vis absorption spectra of AuNP suspension produced in water and AuNPs-alginate aqueous suspensions prepared from different alginate concentrations and SPS discharge times. Spectra of the AuNPs dispersed in water exhibited an absorption band with maximum absorption wavelength (λ_max) at ∼532 nm. However, the absorbance from 600 to 800 nm did not appear at the absorption values equal to zero. This is caused by the presence of agglomerates which can absorb light at longer wavelengths. Besides, the production rate and amount of sputtered-Au ions from the discharge in water was larger than that in alginate solutions. We found that the sputtering of electrodes with a length of 1 mm (see Fig. 1b) discharged in water was terminated within 5 min. For AuNPs produced in alginate solution, spectra of the AuNPs dispersed in 0.2 and 0.5 %w/v alginate solutions exhibited an absorption band with λ_max at ∼510 nm. The observed single λ_max is assigned to the dipole resonance of the gold nanosphere. Previously reported results evidenced that the λ_max shifted to a shorter wavelength (blue shift) when the size of the nanoparticles decreased. For example, the λ_max were found at 534, 530, and 527 nm when the average diameter of the AuNPs were ∼14.4, ∼12.4, and ∼8.6 nm, respectively.³² Our optical absorption measurements are in agreement with this report in which the observed λ_max was at a lower wavelength (510 nm) because the average sizes of the particles were smaller (∼3–5 nm). Furthermore, intensities of the absorption bands remarkably decreased when the alginate concentrations were increased, suggesting the reduction in amount of sputtered-Au atoms. For the AuNPs prepared in 0.9 %w/v alginate solution, the λ_max was not clearly observed. This could be caused by a smaller amount of AuNPs as compared with that produced in 0.2 and 0.5 %w/v alginate solutions (see also Fig. S1†). We have shown in our previous report that the solution plasma could also depolymerize alginate polymer.²⁹ It should be postulated that the reduction in amount of sputtered-Au atoms when concentration of alginate solutions was increased could be attributed to the subsidiary reaction of the reactive species with alginate polymer.

Fig. 3 UV-vis absorption spectra of AuNP suspensions produced in water and alginate aqueous solution with different concentrations and SPS discharge times.

Hu et al. has reported the synthesis of AuNPs by the SPS process in various solvents including liquid nitrogen (LN₂), ethanol, and water.³³ In this study they can produce colloidal AuNPs in water with average diameter of spherical AuNPs of ∼3 nm. As a consequence, the λ_max of UV-vis spectra was observed at 511 nm. This achievement was made by controlling the temperature during sputtering to be 0 °C. For AuNPs produced in LN₂, average diameter of the AuNPs was found to be ∼1.25 nm. Therefore, they concluded that the particle size was dependent on the solvent temperature. Compared with our study, sputtering of gold electrodes in water at room temperature cannot lead to the formation of colloidal AuNP suspension while the presence of alginate gel matrix can accomplish this goal. Furthermore, the size of the AuNPs can be tuned by varying the concentration of the alginate matrix, leading to particles with tailored optical properties for different applications.

3.3. Stability of AuNPs-alginate aqueous suspensions

To determine whether or not the colloidal AuNPs-alginate aqueous suspensions would remain stable within a reasonable time period, we carried out stability study by letting the suspension settle at room temperature, up to 6 months before observing for their appearance and zeta potential analysis. Fig. 4 shows the appearance of AuNPs discharged in water and alginate aqueous solution (0.5 %w/v, 10 min) before and after allowing them to be settled. It was found that while the AuNPs discharged in water formed aggregates and precipitated since 2 days of the observation, the AuNPs discharged in alginate aqueous solution still dispersed well. Furthermore, the AuNPs-alginate aqueous suspensions produced in all alginate concentrations exhibited good stability until the suspensions were left standing for 6 months. In order to indicate the magnitude of a potential stability of the suspensions, zeta potential measurement was additionally carried out. Results are shown in Fig. 5a. Clearly, the SPS discharge time did not significantly affect the zeta potential value and the values became larger negative when the alginate concentrations were decreased. Average zeta potential values of AuNPs-alginate aqueous suspensions produced from 0.9, 0.5, and 0.2 %w/v alginate solutions were −44.52 ± 0.11, −50.65 ± 2.23, and −58.57 ± 3.03 mV, respectively. Note that, an important factor that affects zeta potential value is the pH. Observed pH values of pure water and AuNPs-alginate aqueous suspensions produced in alginate solution with different concentrations and SPS discharge times are shown in Fig. 5b. Both the alginate concentration and the SPS discharge time did not affect the pH value of the suspensions. This could be the reason why the SPS discharge time did not affect the zeta potential value. It is noteworthy that if a colloidal system has a large negative or positive zeta potential value, particles in the system tend to repel each other and thus flocculation and precipitation tend not to be occurred. The general dividing line between stable and unstable suspensions is generally taken at either +30 or −30 mV. Specifically, suspensions with zeta potential values more positive than +30 mV or more negative than −30 mV are normally considered stable.¹² According to our results, zeta potential values of all AuNPs-alginate aqueous suspensions were more negative than −30 mV. The negative charge indicated that the AuNPs were properly passivated with anionic sodium alginate. An effective driving force of the suspension stability could be the electrostatic repulsion forces between –COO⁻ groups of alginate chains. These results indicate that the AuNPs-alginate aqueous suspensions are highly stable over reasonable period of times and the alginate matrix is a potent stabilizing agent.

Fig. 4 AuNP suspensions produced in water and alginate aqueous solution, before and after allowed to be settled.

Fig. 5 (a) Zeta potential measurement results and (b) pH values of AuNPs-alginate aqueous suspensions produced in alginate solution with different concentrations and SPS discharge times.

3.4. Reactive species generated during the SPS process and proposed mechanism of AuNP formation

The formation of AuNPs by the SPS process is possible due to the fact that electrode materials can be sputtered by the plasma; in which highly reactive species are produced.^28,34 Reactive species presented during the discharge, in water and alginate aqueous solutions, were investigated by optical emission spectroscopy (OES). Data for alginate solution was conducted from alginate solution with a concentration of 0.5 %w/v. Results are shown in Fig. 6. Generally, the process of plasma generation in aqueous system starts with the production of hydrogen and oxygen gas between electrodes due to the decomposition of water caused by ionic current.³⁵ Thus, predominant reactive species for the solution plasma in aqueous system are H and O species—constituent elements of H₂O. As can be seen in Fig. 6, emission peaks corresponding to excited state of atomic hydrogen (i.e., H_α, at λ = 658 nm; and H_β, at λ = 487 nm) and atomic oxygen (i.e., O I, at λ = 778 nm) were observed in both media. Atomic O II emission line (at λ = 313 nm) was additionally observed during discharged in water. These results demonstrate that reactive species responsible for the production of AuNPs can be H˙, O˙, OH˙, H*, and O*. Considering the emission line of atomic gold, emission lines corresponding to both Au I and Au II were observed when the discharge was done in water, but, only the Au I emission lines were observed when the discharge was done in alginate solutions. Generally, intensity of each emission spectrum depends on the concentration of the element in the sample. As mentioned above, reactive species generated by the plasma in aqueous system are originated from the dissociation of water molecules. The increase in the alginate concentration relates to the less amount of water in the system. As a result, intensity of OES spectrum of the plasma generated in water was remarkably higher than that in alginate aqueous solution. Effect of alginate concentration on the intensity of OES spectra was shown in our previously reported work.²⁹ Although the alginate solutions are transparent, its gelling form can obstruct the intensity of the emission spectrum. These could be the reasons why atomic O II and Au II emission lines were not observed in alginate aqueous solution.

Fig. 6 OES spectra of the plasma generated in (a) water and (b) alginate aqueous solution.

Schematic representation of the proposed AuNP formation mechanism by the SPS process—in this study—is shown in Fig. 7a. Briefly, after the gold ions are ejected from the surface of gold electrodes, they tend to diffuse to the part having lower energy. Assemblies of them subsequently occur in order to gain more stability. Fig. 7b is a schematic representation of the AuNP suspensions produced in water and alginate aqueous solution. Considering in water, assemblies of gold ions without any support template lead to the growth of the particles in an uncontrollable way. This causes the aggregate formation. The aggregates will then continuously grow in size with non-uniform shape, and as a consequence, they may settle to the bottom of the container after a period of time. In the presence of alginate, we hypothesize that the alginate polymer will form cavities and function as a template for the growth of the gold clusters. The formation of AuNPs may therefore composes of five main steps: (1) ion bombardment on gold surface, (2) ejection of gold ions, (3) ion diffusion, (4) adsorption of gold ions on the G block of alginate copolymer and reduction of gold ions to form AuNPs, and (5) growth and stabilization of the AuNPs by the alginate polymer. To find a more detail description about the particular type of interaction between AuNP and alginate, samples were analyzed by FTIR spectroscopy. Fig. S2† shows FTIR spectra of sodium alginate aqueous solution and AuNPs-alginate aqueous suspensions produced in 0.2, 0.5, and 0.9 %w/v alginate solutions. Results revealed immeasurable change in the FTIR peak of the AuNPs-alginate aqueous suspensions from that of the pure alginate solution, suggesting that the adsorption of the gold ions on the alginate chains was physical adsorption. The observation in size reduction of AuNPs when the alginate concentration was increased could confirm our proposed mechanism. The more closely alginate chains in alginate aqueous solution at the higher concentration could form smaller cavities that may function as a template for the production of AuNPs during sputtering. The concentration dependence on the size of AuNPs was also observed for the production of AuNPs in alginate solution using UV photo-activation,³² as well as in other block copolymer such as poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers.³⁶

Fig. 7 Schematic representation of (a) AuNP formation mechanism by the SPS process and (b) the AuNP suspensions produced in water and alginate aqueous solution.

4. Conclusions

A one-step method for preparing AuNPs stabilized in alginate aqueous solution named the SPS process is proposed. The synthesized AuNPs were spherical and their size was dependent on the alginate concentration. A gradual size reduction of the AuNPs was observed when the alginate concentration was increased. The average particle diameters of the AuNPs produced by this method were ∼3–5 nm which were smaller than those prepared by other previously reported methods. The colloidal AuNPs-alginate aqueous suspensions had zeta potential values in the range of −45 to −59 mV. The negative charge indicated that the AuNPs were properly passivated with anionic sodium alginate and, as a result, they were stable for more than 6 months. This method is simple, fast, and chemical free. Therefore, the colloidal AuNPs-alginate aqueous suspensions produced by the SPS process may be useful for biological applications owing to the mild process conditions and the biocompatibility of alginate surrounding the particles. The present method can furthermore be extended to produce other colloidal metal nanoparticles encapsulated in various matrices.

Acknowledgements

We greatly appreciate the financial support from the Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology (JST) Agency. Assistance in TEM analysis from Mr Hoonseung Lee, a Ph.D. student of Graduate School of Engineering, Nagoya University, Japan, is also gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45029e

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