A high yield, one-pot dialysis-based process for self-assembly of near infrared absorbing gold nanoparticles Dhruvinkumar Patel, Kurtis T. James, Martin O’Toole ⇑ , Guandong Zhang, Robert S. Keynton, André M. Gobin Department of Bioengineering, University of Louisville, Louisville, KY 40292, United States article info Article history: Received 4 September 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Gold nanoparticles Near infrared Synthesis Coating Dialysis DiaSynth abstract Hypothesis: A facile, dialysis-based synthesis of stable near infrared (nIR) absorbing plasmonic gold nano- particles (k max = 650–1000 nm) will increase the yield of nIR particles and reduce the amount of gold col- loid contaminant in the product mixture. Experiments: Chloroauric acid and sodium thiosulfate were reacted using a dialysis membrane as a reac- tion vessel. Product yield and composition was determined and compared to traditional synthesis meth- ods. The product particle distribution, yield, and partitioning of gold between dispersed product and membrane-adsorbed gold were determined. Findings: The synthesis results in polydisperse particle suspensions comprised of 70% spheroid-like par- ticles, 27% triangular plates, and 3% rod-like structures with a 3% batch-to-batch variation and a promi- nent nIR absorption band with k max = 650–1000 nm. The amount of small gold colloid (k max = 530 nm; d < 10 nm) in the isolated product was reduced by 96% compared to traditional methods. Additionally, 91.1% of the gold starting material is retained in the solution-based nanoparticle mixture while 8.2% is found on the dialysis membrane. The synthesis results in a quality ratio (QR = Abs nIR /Abs 530 ) of 1.7–2.4 (twice that of previous techniques) and 14.3 times greater OD/ml yield of the nIR-absorbing nanoparticle fraction. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction The production of gold nanostructures such as nanoplates, nanoshells, and nanorods with plasmon resonance frequencies in the near infrared (nIR) region of the electromagnetic spectrum is currently an area of growing research focus [1–4]. The importance of the nIR region (650–900 nm) in medicine is due to the high transmission and low absorption of light by native tissue compo- nents, such as water and hemoglobin [6–8]. Thus, nIR light has minimal interference with tissue and interacts strongly with exog- enous materials that absorb nIR light. This enables targeted drug delivery and biosensing, as well as combined therapeutic and imaging (theranostics) capabilities such as nIR imaging and photo- thermal treatment in situ [7,9–14]. To date, a number of methods have been employed to synthe- size gold nanoparticles (GNPs), including nanoshells [15–19], nanorods [20–27], nanocages [28–30], nanostars [31–34], and nanoplates [35–51] that absorb in the nIR spectral region. Although these methods produce nIR-GNPs, they are typically seed mediated syntheses that require multiple steps, use toxic agents, difficult to remove surfactants (i.e. CTAB) or require laborious purification steps that significantly reduce product yield. However, of the above mentioned techniques, one of the most promising approaches to synthesizing nIR particles is through the reaction of chloroauric acid (HAuCl 4 ) with a sulfur-containing reducing agent (i.e. sodium sulfide or sodium thiosulfate) using either a 1- or 2-step process [49,50,52–58]. The reaction with either of the sulfur reagents can be performed at room temperature and produce similar products. Sodium sulfide (Na 2 S) is typically ‘‘aged’’ for several days in solution, prior to the reaction, during which time sodium thio- sulfate (Na 2 S 2 O 3 ) and potentially other oxidized sulfur species http://dx.doi.org/10.1016/j.jcis.2014.11.029 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. Abbreviations: niR, near infrared; GNP, gold nanoparticle; nIR-GNP, near infrared absorbing gold nanoparticle; CTAB, cetyltrimethylammonium bromide; QR, quality ratio; Abs, absorbance; MWCO, molecular weight cutoff; SEM, scanning electron microscopy; OD, optical density. ⇑ Corresponding author at: Department of Bioengineering, University of Louis- ville, Rm. 411 Lutz Hall, Louisville, KY 40292, United States. Fax: +1 502 852 6806. E-mail addresses: dnpatel01@gmail.com (D. Patel), kurtis.james@louisville.edu (K.T. James), martin.otoole@louisville.edu (M. O’Toole), guandongzhang@hotmail. com (G. Zhang), rob.keynton@louisville.edu (R.S. Keynton), iamgobin@gmail.com (A.M. Gobin). Journal of Colloid and Interface Science 441 (2015) 10–16 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis