A protocol for synthesizing ~12 nm diameter gold nanoparticles (Au nanoparticles) in an organic solvent is presented. The gold nanoparticles are capped with oleylamine ligands to prevent agglomeration. The gold nanoparticles are soluble in organic solvents such as toluene.
Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of tetrachloroauric acid in 3.0 g (3.7 mmol, 3.6 mL) of oleylamine (technical grade) and 3.0 mL of toluene into a boiling solution of 5.1 g (6.4 mmol, 8.7 mL) of oleylamine in 147 mL of toluene. While boiling and mixing the reaction solution for 2 hours, the color of the reaction mixture changed from clear, to light yellow, to light pink, and then slowly to dark red. The heat was then turned off, and the solution was allowed to gradually cool down to room temperature for 1 hour. The gold nanoparticles were then collected and separated from the solution using a centrifuge and washed three times; by vortexing and dispersing the gold nanoparticles in 10 mL portions of toluene, and then precipitating the gold nanoparticles by adding 40 mL portions of methanol and spinning them in a centrifuge. The solution was then decanted to remove any remaining byproducts and unreacted starting materials. Drying the gold nanoparticles in a vacuum environment produced a solid black pellet; which could be stored for long periods of time (up to one year) for later use, and then redissolved in organic solvents such as toluene.
Gold nanoparticles are an interesting and useful class of nanomaterials that are the subject of many research studies and applications; such as biology1, medicine2, nanotechnology3, and electronic devices4. Scientific research on gold nanoparticles dates back to as early as 1857, when Michael Faraday performed foundational studies on the synthesis and properties of gold nanoparticles5. The two primary “bottom up” techniques for synthesizing gold nanoparticles are the citrate reduction method6,7,8 and the organic two-phase synthesis method9,10. The “Turkevich” citrate reduction method produces fairly monodisperse gold nanoparticles under 20 nm in diameter, but the polydispersity increases for gold nanoparticles above 20 nm in diameter; whereas the “Brust-Schiffrin” two-phase method uses sulfur/thiol ligand-stabilization to produce gold nanoparticles up to ~10 nm in diameter11. Gold nanoparticle solutions that are pre-synthesized using these methods are commercially available. For applications where large volumes, high monodispersity, and large diameters of gold nanoparticles are not necessary, it may be sufficient to purchase and use these pre-synthesized gold nanoparticles from suppliers. However, gold nanoparticles that are stored in solution, such as many of those that are commercially available, may degrade over time as nanoparticles begin to agglomerate and form clusters. Alternatively, for large-scale applications, long-term projects in which gold nanoparticles need to be used frequently or over a long period of time, or in which there are more stringent requirements for the monodispersity and size of the gold nanoparticles, it may be desirable to perform the gold nanoparticle synthesis oneself. By performing the gold nanoparticle synthesis process, one has the opportunity to potentially control various synthesis parameters such as the amount of gold nanoparticles that are produced, the diameter of the gold nanoparticles, the monodispersity of the gold nanoparticles, and the molecules used as the capping ligands. Furthermore, such gold nanoparticles can be stored as solid pellets in a dry environment, helping to preserve the gold nanoparticles so that they can be used at a later time, up to a year later, with minimal degradation in quality. There is also the potential for cost savings and the reduction of waste by fabricating gold nanoparticles in larger volumes and then storing them in a dry state so that they last longer. Overall, synthesizing gold nanoparticles oneself provides compelling advantages that may not be feasible with commercially available gold nanoparticles.
In order to realize the many advantages that are possible with gold nanoparticle synthesis, a process is presented herein for synthesizing gold nanoparticles. The gold nanoparticle synthesis process that is described is a modified version of a process that was developed by Hiramatsu and Osterloh12. Gold nanoparticles are typically synthesized with a diameter of ~12 nm using this synthesis process. The primary chemical reagents that are used to perform the gold nanoparticle synthesis process are tetrachloroauric acid (HAuCl4), oleylamine, and toluene. A nitrogen glovebox is used to provide an inert dry environment for the gold nanoparticle synthesis process, because tetrachloroauric acid is sensitive to water/humidity. The gold nanoparticles are encapsulated with oleylamine ligand molecules to prevent the gold nanoparticles from agglomerating in solution. At the end of the synthesis process, the gold nanoparticles are dried out in a vacuum environment so that they can be stored and preserved in a dry state for later use, up to one year later. When the gold nanoparticles are ready to be used, they can be resuspended into solution in organic solvents such as toluene.
Performing the gold nanoparticle synthesis protocol as presented above should produce gold nanoparticles with ~12 nm diameter and fairly high monodispersity (± 2 nm). However, there are some critical steps and process parameters that can be adjusted to potentially change the size/diameter and monodispersity/polydispersity of the gold nanoparticles. For example, after injecting the precursor solution into the reaction vessel and allowing the tetrachloroauric acid, oleylamine, and toluene solution to boil for two hour…
The authors have nothing to disclose.
The authors would like to thank Frank Osterloh for assistance with nanoparticle synthesis methods. The authors would like to acknowledge financial support from the National Science Foundation (1807555 & 203665) and the Semiconductor Research Corporation (2836).
50 mL Conical Centrifuge Tubes with Plastic Caps (Quantity: 12) | Ted Pella, Inc. | 12942 | used for cleaning/storing gold nanoparticle solution/precipitate (it's best to use 12 tubes, to allow the gold nanoparticles from the synthesis process to last up to one year (e.g., 1 tube per month)) |
Acetone | Sigma-Aldrich | 270725-2L | solvent for cleaning glassware/tubes |
Acid Wet Bench | N/A | N/A | for cleaning chemical reaction glassware/supplies with gold etchant solution (part of wet chemical lab facilities) |
Aluminum Foil | Reynolds | B08K3S7NG1 | for covering glassware after cleaning it to keep it clean |
Burette Clamps | Fisher Scientific | 05-769-20 | for holding the condenser tube and reaction vessel during the synthesis process (located in the nitrogen glove box) |
Centrifuge (with 50 mL Conical Centrifuge Tube Rotor/Adapter) | ELMI | CM-7S | for spinning the gold nanoparticles in solution and precipitating/collecting them at the bottom of the 50 mL conical centrifuge tubes |
DI Water | Millipore | Milli-Q Direct | deionized water |
Fume Hood | N/A | N/A | for cleaning laboratory glassware and supplies with solvents (part of wet chemical lab facilities) |
Glass Beaker (600 mL) | Ted Pella, Inc. | 17327 | for holding reaction vessel, condenser tube, glass pipette, and magnetic stir bar during cleaning with gold etchant and then with water |
Glass Beakers (400 mL) (Quantity: 2) | Ted Pella, Inc. | 17309 | for measuring toluene and gold etchant |
Glass Graduated Cylinder (5 mL) | Fisher Scientific | 08-550A | for measuring toluene and oleylamine for injection |
Glass Graduated Pipette (10 mL) | Fisher Scientific | 13-690-126 | used with the rubber bulb with valves to inject the gold nanoparticle precursor solution into the reaction vessel |
Gold Etchant TFA | Sigma-Aldrich | 651818-500ML | (with potassium iodide) for cleaning reaction vessel, condenser tube, magnetic stir bar, glass pipette [alternatively, use Aqua Regia] |
Isopropanol | Sigma-Aldrich | 34863-2L | solvent for cleaning glassware/tubes |
Liebig Condenser Tube (~500 mm) (24/40) | Fisher Scientific | 07-721C | condenser tube, attaches to glass reaction vessel |
Magnetic Stirring Bar | Fisher Scientific | 14-513-51 | for stirring reaction solution during the synthesis process |
Methanol (≥99.9%) | Sigma-Aldrich | 34860-2L-R | new, ≥99.9% purity (for washing gold nanoparticles after synthesis) |
Microbalance (mg resolution) | Accuris Instruments | W3200-120 | for weighing tetrachloroauric acid powder (located in the nitrogen glove box) |
Micropipette (1000 µL) | Fisher Scientific | FBE01000 | for measuring and dispensing liquid chemicals such as oleylamine and toluene (if using micropipette instead of graduated cylinder for measurement) |
Micropipette Tips (1000 µL) | USA Scientific | 1111-2831 | for measuring and dispensing liquid chemicals such as oleylamine and toluene (if using micropipette instead of graduated cylinder for measurement) |
Nitrile Gloves | Ted Pella, Inc. | 81853 | personal protective equipment (PPE), for protection, and for keeping nitrogren glove box gloves clean |
Nitrogen Glove Box | M. Braun | LABstar pro | for performing gold nanoparticle synthesis in a dry and inert environment |
Non-Aqueous 20 mL Glass Vials with PTFE-Lined Caps (Quantity: 2) | Fisher Scientific | 03-375-25 | for weighing tetrachloroauric acid powder and mixing with oleylamine and toluene to make injection solution |
Oleylamine (Technical Grade, 70%) | Sigma-Aldrich | O7805-100G | technical grade, 70%, preferably new, stored in the nitrogen glove box |
Parafilm M Sealing Film (2 in. x 250 ft) | Sigma-Aldrich | P7543 | for sealing the gold nanoparticles in the 50 mL centrifuge tubes after the synthesis process is over |
Round Bottom Flask (250 mL) (24/40) | Wilmad-LabGlass | LG-7291-234 | glass reaction vessel, attaches to condenser tube |
Rubber Bulb with Valves (Rubber Bulb-Type Safety Pipet Filler) | Fisher Scientific | 13-681-50 | used with the long graduated glass pipette to inject the gold nanoparticle precursor solution into the reaction vessel |
Rubber Hoses (PVC Tubes) (Quantity: 2) | Fisher Scientific | 14-169-7D | for connecting the condenser tube to water inlet/outlet ports |
Stainless Steel Spatula | Ted Pella, Inc. | 13590-1 | for scooping tetrachloroauric acid powder from small container |
Stand (Base with Rod) | Fisher Scientific | 12-000-102 | for holding the condenser tube and reaction vessel during the synthesis process (located in the nitrogen glove box) |
Stirring Heating Mantle (250 mL) | Fisher Scientific | NC1089133 | for holding and supporting reaction vessel sphere, while heating with magnetic stirrer rotating the magnetic stirrer bar |
Tetrachloroauric(III) Acid (HAuCl4) (≥99.9%) | Sigma-Aldrich | 520918-1G | preferably new or never opened, ≥99.9% purity, stored in fridge, then opened only in the nitrogen glove box, never exposed to air/water/humidity |
Texwipes / Kimwipes / Cleanroom Wipes | Texwipe | TX8939 | for miscellaneous cleaning and surface protection |
Toluene (≥99.8%) | Sigma-Aldrich | 244511-2L | new, anhydrous, ≥99.8% purity |
Tweezers | Ted Pella, Inc. | 5371-7TI | for poking small holes in aluminum foil, and for removing Parafilm |
Vortexer | Cole-Parmer | EW-04750-51 | for vortexing the gold nanoparticles in toluene in 50 mL conical centrifuge tubes to resuspend the gold nanoparticles into the toluene solution |