Aqueous synthesis and stabilization of highly concentrated gold nanoparticles using sterically hindered functional polymer Rayoung Kim a , Ho Seok Park a , Taekyung Yu a , Jongheop Yi b , Woo-Sik Kim ⇑,a a Department of Chemical Engineering, College of Engineering, Kyung Hee University, Youngin 446-701, Republic of Korea b School of Chemical and Biological Engineering, College of Engineering, Seoul National University, Seoul 151-744, Republic of Korea article info Article history: Received 1 January 2013 In final form 30 April 2013 Available online 14 May 2013 abstract This letter describes a simple and bulk route for synthesizing Au nanoparticles in a sub-10 nm regime using branched polyethyleneimine (BPEI) as a stabilizer. BPEI, a sterically hindered functional polymer, was found to play a key role in stabilizing the Au nanoparticles without any aggregation, even at a high chloroauric acid concentration of 200 mM. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The large-scale synthesis of high-quality nanoparticles has at- tracted tremendous attention, not only in fundamental studies, but also for various industrial applications [1–3]. In particular, the aqueous-phase synthesis of highly concentrated colloidal nanoparticles is extremely useful for controlling a broad range of nanoparticle concentrations to satisfy application specifications [4–6]. However, while numerous aqueous-phase synthetic routes have already been developed for metal nanoparticles, the resulting products often suffer from a low nanoparticle concentration and poor stability due to nanoparticle agglomerization [7–10]. Thus, developing a simple and bulk route for synthesizing highly concen- trated uniform metal nanoparticles remains a significant challenge. Gold (Au) nanoparticles have been used in a wide range of applica- tions, including optical, catalytic, electrochemical, biomedical, and fluidic applications [11–15]. As the physical and chemical proper- ties of Au nanoparticles are highly sensitive to the nanoparticle size and size distribution, the synthesis of uniform Au nanoparticles in a sub-10 nm regime is really important. Conventional methods for preparing colloidal Au nanoparticles achieve a small size and nar- row size distribution using a variety of stabilizers, such as small or- ganic molecules, oligomers, polymers, ionic liquids, liquid crystals, DNA, peptides, and polysaccharides [16–21]. However, in most cases, the Au nanoparticle concentrations in the resulting solutions are very low (below 1 mM) and many aggregates occur when the synthesis is conducted in a high concentration of the Au precursor. Accordingly, this report presents a simple and aqueous-phase route for synthesizing highly concentrated Au nanoparticles in a sub-10 nm regime based on introducing branched polyethyleneim- ine (BPEI) as a stabilizer. The sterically hindered and functional polymer serves as both a reducing agent to form the Au nanopar- ticles and a capping agent to protect the Au nanoparticles from agglomerization. Thus, the resulting uniform Au nanoparticles exhibited an average size of 9.4 nm. Importantly, the as-synthe- sized Au nanoparticles also maintained their size and a good dis- persion when the synthesis was conducted with a high chloroauric acid concentration of 200 mM. Thus, the proposed ap- proach provides a simple, environmentally benign, and readily scalable route for the synthesis of highly concentrated Au nanoparticles. 2. Experimental 2.1. Materials The branched polyethyleneimine (BPEI, MW = 750 000) and 1- dodecanethiol (98%) were purchased from Aldrich and used with- out further purification. The linear PEI (LPEI, MW = 250 000) was purchased from Polysciences. The chloroauric acid (HAuCl 4 ÁnH 2 O (n = 3.6)) was purchased from Kojima. 2.2. Synthesis of Au nanoparticles For the typical synthesis of Au nanoparticles, the BPEI (0.25–2 g) was dissolved in 5 mL of deionized water and heated to 50 °C. An aqueous 1 M HAuCl 4 solution (0.125–1 mL) was then added to the reaction solution using a pipette. After maintaining the reac- tion solution at 50 °C for 2 h under magnetic stirring, the reaction mixture was cooled down to room temperature. Synthesized Au nanoparticles were separated by centrifugation and re-dispersed in water. 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.04.074 ⇑ Corresponding author. Fax: +82 312732971. E-mail address: wskim@khu.ac.kr (W.-S. Kim). Chemical Physics Letters 575 (2013) 71–75 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett