Gold Nanoparticle Synthesis

Summary

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.

Abstract

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.

Introduction

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 method^6,7,8 and the organic two-phase synthesis method^9,10. 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.

Protocol

Chemical Amounts: ​NOTE: To obtain the appropriate chemical amounts for the nanoparticle synthesis, take the initial amounts found on the "Nanoparticle Synthesis" sheet (on the 2nd page of the supporting information from the Osterloh research article12), and multiply the amount of all doses by 3, with some slight modifications. Table 1 shows the chemical amounts that are needed for the injection solution, boiling solution, washing/purification …

Representative Results

Figure 1 shows how the gold nanoparticle synthesis chemical reaction mixture solution (tetrachloroauric acid, oleylamine, and toluene) should gradually change color over the course of several minutes as it initially boils in the reaction vessel; from clear, to light yellow (left image), to light pink (center image), to light red (right image). The changing color of the solution is an indication of the changing size of the gold nanoparticles as they begin to nucleate and grow larger over…

Discussion

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…

Materials

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