Pulsed nanosecond laser ablation of gold in deionized water and aqueous chitosan solution Matthew Barry a , Bo Ding a , Youngsoo Jung a , B.V.K. Reddy a , Tran X. Phuoc a,b , Minking K. Chyu a,n a Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA b National Energy Technology Laboratory, U.S. Department of Energy, Pittsburgh, PA 15236, USA article info Article history: Received 20 May 2013 Received in revised form 9 October 2013 Accepted 15 October 2013 Keywords: Nanosecond laser ablation Laser irradiation Chitosan Bio-compatible surfactant abstract Gold nanoparticles (AuNP) are produced using a Nd:YAG laser in deionized water and aqueous chitosan solutions via 1064 nm nanosecond laser ablation and subsequent 532 nm nanosecond laser irradiation. AuNP produced in deionized water under single laser ablation and subsequent irradiation for 20–40 min time exhibited coalescence and the formation of nano-chains, with average particle diameters ranging from 34.3 to 18.8 nm and 37.2 to 27.3 nm, respectively. The introduction of chitosan as a bio-compatible surfactant yielded lesser average diameters and tighter size-distributions with a narrow range of UV-Vis absorption spectra values (514–525 nm). For instance, at 20 and 40 min, the single laser ablation and subsequent irradiation produced the average diameters of 12.3 and 5.3 nm and 5.7 and 4.3 nm, respectively. Increases in ablation and irradiation time decrease AuNP size. As time increased, the size- distributions of the AuNP produced in chitosan solution narrowed and the particle size monotonically decreased. Chitosan functionalized the AuNP surface, promoting hydrophobicity, preventing electrostatic attraction of the Au nanoparticles, thus inhibiting coalescence and agglomeration. UV-Vis absorption spectra, TEM and HR-TEM images are used to verify AuNP size distributions. X-ray diffraction measurements of AuNP produced in chitosan confirmed crystallite size and the absence of gold-oxide despite the high- energy laser ablation. Furthermore, samples prepared in chitosan solution showed minimal agglomeration after 24 months. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction The uses of gold nanoparticles (AuNP) range from catalysts [1,2], nanolenses [3] to biosensors [4–6]. For catalysis applications, the catalytic activity of AuNP increases as the particle size decreases [6]. Within the biomedical field, selective size and shape AuNP can be used as high-resolution biosensors for cancerous tissues and also as photothermal therapy for cancerous cells [7]. Additionally, size-dependent surface plasmon properties of AuNP allow for greater optical absorption in photovoltaics leading to greater photocurrents [8]. Thus, the size control and the subse- quent stability of AuNP produced by means of chemical precipita- tion, pulsed laser deposition and laser ablation prove imperative for the various aforementioned applications. Laser ablation of AuNP in deionized water proves problematic due to agglomeration and coalescence, and much work has been done with the use of surfactants to produce stable sub-10 nm AuNP, which are the most desirable for these applications. We are therefore motivated to find a means of producing free, selective sized AuNP with diameters less than 10 nm. Nanosecond laser ablation relies on plasma-heating of the target material surface, which causes vaporization and expulsion [9] and removal [10] of molten material. Mafuné et al. [11–13] have pursued the production of AuNP in organic, anionic sodium dodecyl sulfate via laser ablation and laser-induced size control. Laser-induced size control, or irradiation, exploits the strong surface plasmon absorp- tion of AuNP around 532 nm, which causes photofragmentation. Amendola et al. [14] used the polar protic solvents dimethyl- sulfoxide, tetrahydrofuran and acetonitrile as water-soluble sur- factants to produce stable, free AuNP under nanosecond single (1064 nm) and dual (532 nm) laser ablation and irradiation with radii less than 10 nm. Additionally, they found AuNP produced in toluene, a benzene derivative, have predominantly sub-1 nm radii due to encasement of a graphitic carbon matrix around the AuNP, presumably induced by the pyrolysis of benzene [15]. Compagnini et al. [16] prepared AuNPs in non-aqueous alkanes and thiol-alkane solutions via a 532 nm Nd:YAG laser operating with a 5 ns pulse duration and reported a reduced size distribution, attributed to S–Au bond formation and an organic alkyl shell that Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering 0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.10.019 n Corresponding author. Tel.: þ1 412 624 9720 fax: þ1 412 624 4846. E-mail addresses: mmb49@pitt.edu (M. Barry), bod6@pitt.edu (B. Ding), yoj14@pitt.edu (Y. Jung), bvkreddy680@gmail.com (B.V.K. Reddy), Phuoc.Tran@netl.doe.gov (T.X. Phuoc), mkchyu@pitt.edu (M.K. Chyu). Optics and Lasers in Engineering 55 (2014) 59–68