Three strategies to stabilise nearly monodispersed silver nanoparticles in aqueous solution
© Stevenson et al; licensee Springer. 2012
Received: 22 August 2011
Accepted: 22 February 2012
Published: 22 February 2012
Silver nanoparticles are extensively used due to their chemical and physical properties and promising applications in areas such as medicine and electronics. Controlled synthesis of silver nanoparticles remains a major challenge due to the difficulty in producing long-term stable particles of the same size and shape in aqueous solution. To address this problem, we examine three strategies to stabilise aqueous solutions of 15 nm citrate-reduced silver nanoparticles using organic polymeric capping, bimetallic core-shell and bimetallic alloying. Our results show that these strategies drastically improve nanoparticle stability by distinct mechanisms. Additionally, we report a new role of polymer functionalisation in preventing further uncontrolled nanoparticle growth. For bimetallic nanoparticles, we attribute the presence of a higher valence metal on the surface of the nanoparticle as one of the key factors for improving their long-term stability. Stable silver-based nanoparticles, free of organic solvents, will have great potential for accelerating further environmental and nanotoxicity studies.
PACS: 81.07.-b; 81.16.Be; 82.70.Dd.
Metal nanoparticles have generated great interest for applications in physics, materials, chemistry and biomedical sciences in plasmonics , biosensing [2–4], nanomedicine [5–7], nanoelectronics , catalysis [9, 10], magnetic fluids  and dye-based solar cells  due to their chemical, electronic, optical and magnetic properties. These applications depend on the availability of homogeneous nanoparticles of controlled size and shape, which remain stable in their complex target environments [13, 14]. For example, metal nanoparticles exhibit surface plasmon resonance in the visible spectrum range, resulting in light scattering and characteristic absorbance peaks whose location and width depend on the type of metal, size and shape of the nanostructure and the medium they are immersed in [15–19]. Interactions between nanoparticles and biological matter will also depend on their size, shape and surface charge as they interact with different organisms .
Silver and gold nanoparticles have attracted great interest for many applications due to their strong plasmonic properties and to the availability of methods for synthesis [21, 22]. The colloidal method has been extensively used due to the ability to synthesise nanoparticles directly in aqueous solution . However, controlling the size and shape of metal nanoparticles remains challenging; nanoparticles are often heterogeneous in size and shape unless multiple reaction parameters are carefully regulated [22, 24]. Synthesis via multiple steps, seed-mediated growth or via organic solvents has overcome several of these problems, although these synthesis methods increase in complexity with the number of steps involved and will limit potential biomedical applications when organic solvents are used [25–27].
The long-term stability of nanoparticles critically depends on the medium they are immersed in. The liquid influences interparticle forces and chemical reactivity, which affect aggregation, size and shape of the nanoparticles and long-term stability ('aging'), especially if the nanoparticles are applied or stored in aqueous conditions . These effects are particularly relevant to more reactive metals such as silver. Stable silver nanoparticles in solution are necessary to apply and assess their interactions with biological matter and living cells. It is difficult to determine the effect of silver nanoparticle solutions where variable sizes, aggregates, surfactants and especially free silver ions are present .
A critical step to control the stability of inorganic nanoparticles lies in their surface modification and functionalisation [30, 31]. Here, we compare three strategies to functionalise nearly monodispersed silver-based nanoparticles directly in aqueous solution. We have synthesised silver nanoparticles with a narrow shape and size distribution (15 nm quasi-spherical nanoparticles), functionalising them with polyethylene glycol (PEG) [32–34], gold (core-shell) and chromium (alloy). We have characterised the different silver-based nanostructures and established their stability by UV-Visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM) and amplitude modulation atomic force microscopy (AFM) in liquid. We have tested the validity of our results using statistical bootstrapping tools, demonstrating a close relationship between homogeneity of nanoparticle size, shape and particle sphericity. Finally, this study has made it possible to produce stable silver-based nanoparticles that can be used for further applications and studies of toxicity and environmental impact, where the effect of the nanoparticle-containing solution can be attributed to nanoparticles of a controlled size and stability and not to by-products of synthesis (free ions, toxic surfactants), aggregation and/or degradation due to aging effects.
Materials and methods
Unless otherwise stated, all reagents were purchased from Sigma-Aldrich (Dorset, UK), of analytical grade, and were used as received. Milli-Q water (Millipore Co., Billerica, MA, USA; specific resistivity of 18 MΩ cm) was used for the preparation of all solutions. Glassware was cleaned with Milli-Q water prior to the synthesis processes. Muscovite mica was purchased from SPI Supplies (West Chester, PA, USA).
Silver (Ag) nanoparticles capped with citrate were obtained using the Turkevich method . A solution of silver nitrate (125 ml, 1 mM) was heated to 95°C, and a solution of trisodium citrate (7.75 mM) was added. Nearly 15 min later, a colour change was observed indicating the formation of nanoparticles, and the solution was cooled to room temperature and stored at 5°C. During synthesis, we maintained a constant pH of 6.5.
PEG-functionalised silver nanoparticles (AgPEG) were obtained via direct PEGylation  by immediately adding a solution of PEG-6000 (80 mM) to freshly synthesised (as mentioned) silver nanoparticles (50 ml). The sample was cooled to -5°C for 8 h before storing at 5°C.
Silver-gold (Ag-Au) nanoshells were obtained by the successive reduction of two metal salts. A fresh solution of silver nanoparticles was prepared as previously described, then immediately heated to 90°C, and a solution of hydrogen tetrachloroaurate (1.2 mM) was added. The reaction mixture was then heated to 95°C, and at this point, a solution of trisodium citrate (7.75 mM) was added. When a colour change was observed, the mixture was cooled gradually to room temperature and stored at 5°C. During the second reaction, chloride ions produced from the reduction of hydrogen tetrachloroaurate reacted with unbound silver forming a silver halide which sedimented rapidly and was removed from the product.
Silver-chromium (AgCr) alloy nanoparticles were synthesised by co-reducing two metal salts. A solution of potassium dichromate dissolved in sulphuric acid (5 ml, 42 mM) was diluted twice in water (10 ml) and added to a solution of silver nitrate (125 ml, 1 mM) before heating to 95°C. A solution of trisodium citrate (7.75 mM) was then added. When a colour change was observed, the mixture was cooled to room temperature and stored at 5°C.
UV-Vis absorption spectra of nanoparticle aqueous solutions were obtained using an MDR-23 monochromator (LOMO, St. Petersburg, Russia). TEM images were obtained using a Tecnai 12 TEM (FEI Company, Eindhoven, The Netherlands) and recorded with a Gatan US1000 2 K CCD camera (Gatan, Inc., Pleasanton, CA, USA). All samples were deposited on formvar-coated 200 mesh copper grids, and the excess liquid wicked off with filter paper. The samples were air-dried and examined unstained. TEM images were analysed in ImageJ 1.43 u (NIH, Bethesda, MD, USA). The area and perimeter of each nanoparticle was extracted before calculating circularity and diameter , estimating particles to be spherical. Both metrics were sensitive to pixel density, with smaller particles yielding larger rounding errors due to the lower number of pixels available.
For atomic force microscopy imaging in liquid (Ag, AgCr), nanoparticle samples (2 μl) were diluted in NaCl buffer (48 μl, 20 mM with 50 mM HEPES, pH 6.5) and bath sonicated (Ultrawave U300, Ultrawave, Cardiff, UK) for 15 min. Samples were incubated on freshly cleaved mica for 20 min at room temperature before imaging in NaCl buffer. Due to PEG hydration, the height of AgPEG nanoparticles characterised by AFM in liquid (data not shown) was found to be twice the value of the diameter from TEM. Consequently, AgPEG nanoparticles were characterised in air by adding 2 μl of nanoparticle sample onto freshly cleaved mica and air-dried before imaging. Ag-Au nanoparticles were imaged in air after an incubation time of 2 h on freshly cleaved mica coated with poly-L-lysine (0.01%). Samples were imaged with a commercial MFP-3D AFM (Asylum Research, Santa Barbara, CA, USA) operating in amplitude modulation mode. Olympus AC240 (k = 2 N m-1) silicon cantilever tips in air and Olympus TR800/400 (k = 0.57 N m-1/0.08 N m-1) (all from Olympus Europa Holding GmbH, Hamburg, Germany) silicon nitride cantilevers in liquid were used. Low scan rates (0.3 Hz) further minimised tip sample forces from displacing deposited nanoparticles. Post-scan processing included image flattening and particle size analysis using the manufacturer's provided software (MFP-3D 080501 + 1429), based in Igor Pro 6.04 (WaveMetrics, Inc., Portland, OR, USA). Nanoparticle heights were calculated by thresholding height in topography images and extracting the z range of each particle.
where a and b are half of the ellipse's major and minor axes (diameter and height), respectively. As the eccentricity value ranges from 0 to 1, the corresponding nanoparticle shape will range from spherical (ε = 0) to ellipsoidal (ε = 1). The sphericity of nanoparticles that resulted from different stabilisation strategies were then compared using the Kruskal-Wallis hypothesis test [extensive details are provided in Additional file 1].
Results and discussion
Silver nanoparticles functionalised with PEG: organic stability
To explore the effects of non-steric stabilisations, we functionalised citrate-capped silver nanoparticles with gold to create bimetallic silver-gold nanoshells. Gold nanoparticles synthesised by the Turkevich method are known for their homogeneity , which suggests an increased stability between gold and citrate compared with silver and citrate.
Silver-gold nanoshells: bimetallic stability I
To investigate whether a weak silver-citrate affinity at the nanoparticle surface is responsible for nanoparticle instability, we synthesised silver-chromium alloy nanoparticles through co-reduction, where chromium also has a higher valence (+3).
Silver-chromium alloy nanoparticles: bimetallic stability II
Improved stability in shapes and sizes of nanoparticles
Second, we combined TEM diameters with AFM heights to ascertain particle sphericities in three dimensions, where a higher particle sphericity indicates a more stable, homogenous growth process. We found (Figure 5B, with details in Additional file 1) that the nanoparticle sphericities increased with each stabilisation strategy, i.e. unfunctionalised nanoparticles were the least stable, followed by PEG-functionalised, silver-gold nanoshells and silver-chromium alloy nanoparticles.
Due to the highly reactive nature of silver ions and the multiple roles that citrate plays during synthesis, it has been difficult to control the size and shape of citrate-reduced silver nanoparticles without further functionalisation. In this work, we synthesise a 15 nm quasi-spherical silver nanoparticle directly in water without organic solvents. After 100 days in solution, the nanoparticles exhibited increased heterogeneity due to their inherent instability. We used PEG as an organic capping agent and gold as an inorganic capping agent. In both cases, we are effectively able to control further growth and stabilisation of the nanoparticles. We found that the addition of PEG not only stabilises the nanoparticles by steric repulsion and trapping of seeds, but also allows controlled further growth of nanoparticles, improving homogeneity in nanoparticle size, shape and stability. In the case of Ag-Au nanoshells, the higher electron affinity of gold may lead to a stronger gold-citrate interaction at the nanoparticle surface compared with that in the citrate-capped silver nanoparticles, consequently improving size and shape homogeneity and long-term stability. Similarly to gold, the higher valence of chromium may lead to a stronger chromium-citrate interaction at the nanoparticle surface. The alloying process enables the production of stable silver-based nanoparticles without the need for additional functionalisation/capping.
The findings of this work enable further fundamental research on the effects of material, size and shape on nanoparticle behaviour. The ability to understand and control the nanoparticle/liquid interface allows the possibility of studying the role of nanoparticles in further applications where electrolytes are an important consideration for e.g. water treatment and biomedical use (due to the presence of salts in physiological solutions), which is particularly timely for quantitative toxicity studies, given the current and future widespread production and application of silver nanostructures.
This project was funded through the Oxford Martin School. ST was supported by the Oxford Martin School. APZS was supported by the EPSRC Life Sciences Interface Doctoral Training Centre. SC was supported by the grant BFU2009-06974 from the Ministry of Science and Innovation of Spain. SAC was supported by RCUK. DBB and AIC were supported by CNIC, Cuba. We thank Dr. Mike Shaw of Sir William Dunn School of Pathology, Oxford for the help in acquiring TEM data. We further thank Dr. Kislon Voitchovsky of EPFL, Switzerland for his invaluable comments on the manuscript.
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