BRIEF COMMUNICATION Long-term stability of surfactant-free gold nanostars Marienette Morales Vega • Alois Bonifacio • Vanni Lughi • Stefano Marsi • Sergio Carrato • Valter Sergo Received: 6 July 2014 / Accepted: 1 November 2014 / Published online: 9 November 2014 Ó Springer Science+Business Media Dordrecht 2014 Abstract This work investigates the long-term sta- bility of suspensions of surfactant-free gold nanostars with mean diameter of 78 ± 13 nm (measured from tip to tip across the nanostar).We monitored the optical and morphological properties of the nanostars over the course of several days after synthesis by observing the changes in the UV–visible absorption spectra and mean radius of curvature of the nanostar tips. An aging process can be observed, evident in the blunting of the nanostar tips, leading to a blue shift in the absorption maximum. Stability is greatly improved by depositing on the nanostars a monolayer of mercaptopropionic acid (MPA), possibly because of the formation of the gold–sulfur (Au–S) bond that limits the mobility of the Au atoms. Capping the nanostars with MPA is an easy additional step for extending the stability of the nanostars in suspension without significantly affecting the original plasmonic resonance band. Keywords Gold nanostars Á Nanoparticles Á Surfactant-free Á Aging Á Stability Á Mercaptopropionic acid Introduction Nanoparticles of various shapes, sizes, and composi- tions have been studied for a wide variety of applica- tions, a non-comprehensive listing of which includes drug delivery (Yuan et al. 2012b; Nam et al. 2013), cancer diagnosis (Tamburro et al. 2011) and therapy (Cheng et al. 2011), contrast enhancers for imaging (Allijn et al. 2013), as well as surface-enhanced Raman spectroscopy (SERS) (Yuan et al. 2012a; Liu and Wu 2013; Jarvis and Goodacre 2004; Kastanos et al. 2009). Nanoparticles are relatively straightfor- ward and inexpensive to synthesize through wet chemical preparation techniques. Well-known routes are available for almost any desired morphology. Of particular interest for SERS are branched gold nano- particles, or nanostars, because of their peculiar plasmonic properties (Hao et al. 2007; Casu et al. 2012; Guerrero-Martinez et al. 2011; Rodriguez- Oliveros and Sanchez-Gil 2012; Kooij et al. 2012). At present, many sophisticated synthetic routes ranging from seed-mediated (Kooij et al. 2012; Wu et al. 2009; Wheeler et al. 2012; Casu et al. 2012; Yuan et al. 2012c) to one-pot methods (Hrelescu et al. 2009; Trigari et al. 2011) are widely available for nanostars. The most common seed-mediated synthesis requires the use of cetyltrimethylammonium bromide (CTAB) (Rodriguez-Lorenzo et al. 2011; Esenturk and Walker 2009; Ahmed et al. 2010), polyvinylpyrrolidone (PVP), N,N-dimethylformamide (DMF) solution (Kumar et al. 2008; Khoury and Vo-Dinh 2008), and M. M. Vega (&) Á A. Bonifacio Á V. Lughi Á S. Marsi Á S. Carrato Á V. Sergo Department of Engineering and Architecture, University of Trieste, Via Valerio 6/a, 34127 Trieste, Italy e-mail: marienettemorales.vega@phd.units.it 123 J Nanopart Res (2014) 16:2729 DOI 10.1007/s11051-014-2729-z