Room-Temperature Synthesis of Gold Nanoparticles &Amp
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CORE Metadata, citation and similar papers at core.ac.uk Provided by Springer - Publisher Connector 275 Gold Bulletin Volume 43 No 4 2010 Room-temperature synthesis of gold nanoparticles – Size-control by slow addition Sankar K. Sivaraman, Sanjeev Kumar and Venugopal Santhanam* www.goldbulletin.org Abstract 1 Introduction We report a simple and rapid process for the room- Gold nanoparticles in the 1-10 nm size range have temperature synthesis of gold nanoparticles size-tuneable physico-chemical properties (1,2) using tannic acid, a green reagent, as both the that are useful in a wide variety of applications such reducing and stabilising agent. We systematically as cancer diagnostics (3), catalysis (4), Raman investigated the effect of pH on the size and fluorescence spectroscopy (5,6), selective distribution of nanoparticles synthesized. Based ionisation of biomolecules (7), and single electronics on induction time and Z-potential measurements, (8,9). Commercial realisation of such applications we show that particle size distribution is relies on economical synthesis of size-controlled controlled by a fine balance between the rates nanoparticles, which requires the development of of reduction (determined by the initial pH of continuous-flow versions of lab-scale processes. reactants) and coalescence (determined by the Among the various lab-scale processes, solution- pH of the reaction mixture) in the initial period phase redox reactions are currently in wide use for of growth. This insight led to the optimal batch synthesizing gold nanoparticles in the size range of process for size-controlled synthesis of 2-10 1-10 nm, due to their relative energy-efficiency and nm gold nanoparticles – slow addition (within ease of implementation (10). 10 minutes) of chloroauric acid into tannic acid. Central to the development and success of solution- phase processes for size-controlled continuous, nanoparticle synthesis are issues related to economic and environmental viability (11). Resolution of these issues warrants the development of lab-scale processes having fast process completion times, using green reagents in aqueous phase at ambient conditions. The use of various naturally available reagents as reducing agents for aqueous-phase redox synthesis of colloidal gold is well documented by Weiser (12); recent reports (13,14) exemplify the growing library of such reagents. Presently, most green processes either offer moderate control over polydispersity or require significant thermal energy input (on account of the high specific heat capacity of water, ~4.186 J/[g.K]) to realize reasonable batch Department of Chemical Engineering, Indian Institute of processing times, hindering their adaptation Science, Bangalore, India 560 012. for continuous flow processing. Typically, the use of hazardous reducing agents (e.g. sodium * Corresponding author borohydride, hydrazine) in conjunction with other Email: [email protected] stabilisers is used to achieve rapid, room- Keywords: gold nanoparticles, room-temperature, size-control, temperature synthesis of size-controlled gold slow addition, tannic acid nanoparticles (15,16). 276 Gold Bulletin Volume 43 No 4 2010 Recently (17), we demonstrated room temperature was used in all the experiments. All chemicals were synthesis of silver nanoparticles in seconds using used as received. tannic acid at alkaline pH as the reducing and stabilising agent. Tannic acid, a polyphenolic 2.2 Synthesis of gold nanoparticles compound derived from plants, has been used as The experiments were carried out in glassware a reducing agent and a protective colloid in gold cleaned with aquaregia (CAUTION: Aquaregia nanoparticle synthesis for a long time. Ostwald (18) solutions are extremely corrosive), rinsed thoroughly reported that tannin can reduce chloroauric acid at with deionized water and then dried in air prior to use. neutral pH to form stable gold nanoparticles even if For the preliminary investigations and experiments tap water is used to prepare the aqueous solutions. ‘A’ – ‘F’, the precursor solutions were prepared Tannic acid is also an important constituent of as follows. 0.25 mL of 25.4 mM chloroauric acid the widely used Slot and Geuze protocol (19) for solution was made up to 22 mL using DI water. 3 mL synthesizing gold nanoparticles in the size range of of the desired concentration of tannic acid solution 3-17 nm with nominal standard deviation (S) of 0.5- was prepared. 150 mM potassium carbonate 1.8 nm (20) at 60°C. Turkevich et al. (21) replicated solution was used to adjust the pH of tannic acid Ostwald’s protocol and reported the particle size to and/or chloroauric acid solution. If necessary, 0.1 be 12.0 ± 3.6 nm (μ ± S) based on TEM images. N hydrochloric acid was used to adjust the pH of The objective of the present work was to develop chloroauric solution. A reactor equipped with home- an optimal batch process using tannic acid, for the made Rushton turbine and baffles of standardized rapid, size-controlled synthesis of 1-10 nm gold design (25) machined out of Teflon was used to mix nanoparticles at ambient conditions. the reactants (macroscopic blending time of 2s). Typically, chloroauric acid solution was taken in the In the following, we show the effect of varying stirred vessel and the reducing agent was added as the initial molar ratio of tannic acid to chloroauric one portion (within 1 second). The reaction mixture acid (MR) on the particle size distribution was stirred till the colour of the colloidal solution did and morphology. This is followed by a not appear to change. systematic investigation of the effect of pH on nanoparticle formation rate and coalescence, For Stop Flow Module (SFM) studies (experiment as characterised by TEM, induction time, and ‘H’), stock solutions of chloroauric acid and tannic Z-potential measurements. Next, we show the effect acid were prepared as follows. Briefly, 0.25 mL of of varying the mode of contacting the reagents, 25.4 mM chloroauric acid was made up to 20 mL and mixing on nanoparticle size distribution. These using DI water. 2.25 mL of 5.9 mM tannic acid was studies revealed the importance of the dynamics made up to 5 mL using DI water. of transformation among various chloroauric acid species in solution, and the competition between Experiment ‘I’ was carried out by adding 10 mL of coalescence and stabilisation during the initial 0.64 mM chloroauric acid solution (pH-3.2) slowly period of nanoparticle formation. Such insights led (dropwise, ~1 mL/min) into 15 mL of 0.89 mM tannic us to the optimal batch process for size-controlled acid solution (pH-7.1) that was being stirred. The synthesis: slow (dropwise) addition of chloroauric total volumes and amounts of reagents used were acid into tannic acid, which is contrary to prevalent kept at the same value as experiment ‘B’ and ‘G’. For practice (22-24). This process has the potential to seeded growth experiment reported in Figure 6f, 2.5 be extended to continuous mode of synthesis with mL (from a total of 25 mL) of the solution formed in residence time of the order of minutes. experiment ‘I’ was made upto 15 mL using DI water and 10 mL of 0.32 mM chloroauric acid solution was 2 Experimental methods added slowly (dropwise) into it. The pH was adjusted to be above 6.4 by intermittently adding required 2.1 Materials quantities of 150 mM potassium carbonate solution. Hydrogen tetrachloroaurate (III) hydrate (99.999%) and tannic acid (ACS reagent) were purchased 2.3 Sample characterisation from Sigma-Aldrich Co. The replicate experiments Transmission Electron Microscopy (TEM) reported in Figure 7 were carried out using chemicals characterisation was performed using a Tecnai F30 of similar purity purchased from Acros organics Ltd. operating at 200 KV. Samples for TEM were prepared Potassium carbonate was purchased from Merck by drop casting and ImageJ (http://rsbweb.nih.gov/ Co. Deionized water obtained from a MilliQ® system ij) software was used for analysing TEM images. 277 Gold Bulletin Volume 43 No 4 2010 The mean (μ y 3di/ n) and standard deviation furnace). Standard curve method was used for 2 (S y (3(di – μ) /(n-1)) of the nanoparticle size calibration and analysis. For sample preparation, distribution are reported along with the size 5mL of colloidal gold solution was precipitated histograms displaying the actual number of particles by the addition of 5 mL of ethanol and 3 μL of counted. dodecanethiol followed by centrifugation at 3000g for 30 min. The nanoparticle precipitate was then UV-Vis spectra were collected using a Systronics dissolved in 5 mL of aquaregia. double beam UV visible spectrophotometer 2201. DI water was used as the reference. Stop flow reactor The average hydrodynamic particle size (dh) and (SFM-400, Biologic SA, Claix, France) equipped with standard deviation were measured, as per ISO xenon lamp and photomultiplier tube was used to 13321, at 90° scattering angle using a Dynamic Light monitor, with 10 μs time resolution, the absorbance of Scattering (DLS) instrument (Model BI-200SM) from the reaction mixture at the desired wavelength after Brookhaven Co. equipped with BI-9000AT correlator rapid mixing (dead time ~3 ms) of the reactants. system. It was observed that the hydrodynamic size obtained from DLS was always greater than The % yield of the synthesis was measured by TEM mean diameter indicating the presence of an carrying out Atomic Absorption Spectrometry of adsorbed layer, and the variability was very sensitive gold precipitate using GBC Avanta 3000 (graphite to the presence of non-spherical particles. Figure 1 a 1.2 b 50 MR 0.69 MR 2.08 1 40 MR 5.0 0.8 30 0.6 20 0.4 Absorbance (a.u) 0.2 10 Hydrodynamic diameter (nm) Hydrodynamic 0 0 400 500 600 700 800 0246 Wavelength (nm) Molar ratio of tannic acid to chloroauric acid c d e Effect of varying the initial molar ratio of tannic acid to chloroauric acid (MR).