Colloids and Surfaces B: Biointerfaces 59 (2007) 120–127 Self-organization of guanosine 5 ′ -monophosphate on mica Klemen Kunstelj a,∗ , Francesco Federiconi b , Lea Spindler c,a , Irena Drevenˇ sek-Olenik d,a a Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia b Dipartimento di Scienze Applicate ai Sistemi Complessi, Universita Politecnica delle Marche, Via Ranieri 65, I-60131 Ancona, Italy c Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia d Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia Received 21 December 2006; received in revised form 18 April 2007; accepted 26 April 2007 Available online 5 May 2007 Abstract Self-assembled aggregates of guanosine 5 ′ -monophosphate (GMP) on the surface of muscovite mica were investigated by atomic force microscopy (AFM). Aqueous solutions of sodium, potassium and ammonium GMP salts were studied. For solution concentrations c < 0.005 wt% only small islands of deposited material are present on the surface. For c ∼ 0.02 wt%, in addition to the islands and patches, also linear aggregates called G-wires are formed. The wire-like aggregates are on average 1.9 nm high and can be several micrometers long. They exhibit a profound directional growth along the six main crystallographic axes of the basal plane of mica. For c > 0.1 wt% flat terraces with the height of 2.5 nm appear. They are formed of G-wires lying with their long axis parallel to the substrate and stacking in a hexagonal arrangement. The morphology of the adsorbates is independent of the type of salt used to prepare the initial solution. This signifies that intrinsic potassium ions from the substrate play much more important role in the GMP adsorption than cations from the solution. © 2007 Elsevier B.V. All rights reserved. PACS: 61.30.Hn (surface phenomena); 81.16.Dn (self-assembly); 68.37.Ps (atomic force microscopy, AFM); 68.08.-p (liquid–solid interfaces); 68.08.Bc (wetting) Keywords: Guanosine; G-quartet; Nanowires; AFM; Self-assembly 1. Introduction Construction of surface architectures via controllable self- assembly processes is a challenging goal, which can lead to a broad range of applications in nanoscale molecular electronic devices. Promising candidates for such exploration are gua- nine (G) and its derivatives. They are unique among the DNA constituents for their property to form highly stable molecu- lar structures, which are stabilized by non-Watson-Crick base pairing. These complex structures, denoted as G-quadruplexes or G4-DNA, can be formed from guanosine mononucleotides and their derivatives as well as from one, two or four individual G-rich DNA strands [1]. Guanine is also distinctive among the DNA bases for its low ionization potential, due to which it plays a key role in electrical conductivity of DNA-based materials [2–5]. The basic building unit of G-quadruplex structures is a molec- ular tetramer known as the G-quartet. The ability of guanine ∗ Corresponding author. Tel.: +386 14773342; fax: +386 14773998. E-mail address: klemen.kunstelj@ijs.si (K. Kunstelj). derivatives to form tetramers is known since the pioneering work of Gellert et al. [6]. The starting material is typically in the form of alkali-metal salt (Fig. 1(a)). When dissolved in water, the for- mation of hydrogen-bonded tetramers occurs (Fig. 1(b)). The next step in guanine self-organization is vertical stacking of the tetramers, which provokes the formation of helically twisted columnar aggregates called G4-wires [7] (Fig. 1(c)). The process of G4-wire formation in dilute aqueous solutions is quite well resolved [8–15]. The wires assemble above a critical concentration c*, which is in the range of 5–30 wt% at room temperature. The length of the wires in solution can be up to tens of nanometers and depends on the selected G-derivative and on solution parameters such as concentration, temperature, pH, and especially the type and concentration of the cations. The stacking process is most effectively promoted by the K + ions, while other ions assist the assembly in the stability order K + , NH 4 + ≫ Na + , Rb + ≫ Li + , Cs + [16–18]. In contrast to self-assembly in solution, much less is known about the formation of G-wire aggregates at surfaces. Recent investigations of surface structures of guanosine derivatives and G-rich oligonucleotides have shown that these molecules 0927-7765/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2007.04.022