Green DNA-based flame retardant coatings assembled through Layer by Layer Federico Carosio, Alessandro Di Blasio, Jenny Alongi, Giulio Malucelli * Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Viale Teresa Michel 5,15121 Alessandria, Italy article info Article history: Received 4 March 2013 Received in revised form 21 May 2013 Accepted 10 July 2013 Available online xxx Keywords: Layer by Layer DNA Flame retardancy abstract For the first time, DNA and chitosan are employed using the Layer by Layer technique in order to build green coatings exhibiting efficient flame retardant properties. DNA by its chemical structure can be considered as an intrinsically intumescent compound, since it contains precursor of phosphoric- polyphosphoric acid, a polyhydric char source (deoxyribose) and the nitrogen-containing bases that may release ammonia, acting as a blowing agent. When combined with chitosan, DNA layers promote the char formation of the former, by releasing phosphoric and polyphosphoric acid. Such bioarchitectures show an exponential growth as assessed by infrared spectroscopy and scanning electron microscopy. Very interestingly, these LbL assemblies are capable of i) reaching the self-extinguishment of cotton during horizontal flammability tests, ii) increasing the limit oxygen index up to 24% and iii) reducing the heat release rate by 40% during cone calorimetry tests. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, the Layer by Layer (LbL) technique has arisen as a promising surface modification technique capable of conferring flame retardant (FR) properties to several kinds of substrates [1e4]. This self-assembly technique was firstly presented by Iler in 1966 but saw a practical application only in early 90s [5,6]. In its raw description, the LbL simply consists in an alternate adsorption of chemical species on a chosen substrate exploiting different kinds of interactions, among which electrostatic attraction is the most common [7]. The process through electrostatic attraction is extremely tunable as it is highly affected by such parameters as chemistry of used polyelectrolytes [8], molecular weight [9], tempera- ture [10,11], counterions [12], ionic strength [13], and pH [14]. By the accurate selection of the layer constituents and the deposition parameters it possible to target different FR effects. The first papers that showed the potential of the LbL assembly in this field were focused on the assembly of hybrid organiceinor- ganic or totally inorganic coatings with the aim of creating an inorganic barrier on the surface, capable of shielding the substrate from heat and oxygen and thus resulting in an FR effect as demonstrated by Li et al., in 2009 [15]. Subsequently, LbL assemblies made of silica nanoparticles or zirconium phosphate were able to improve charring when applied to cellulosic textiles or even suppress incandescent melt dripping during flammability tests and reduce both heat and smoke release during combustion when deposited on synthetic fibers [16e19]. Pursuing the LbL development, the coating composition has been designed in order to obtain an intumescent-like behavior during combustion, by simply assuring the simultaneous presence, within the coating, of an acid, a carbon source and a blowing agent that, upon flame application, can develop a blown charred structure protecting the underlying substrate during combustion in a more efficient way with respect to the LbL architectures previously investigated [20e 23]. The first intumescent coating made with poly(allylamine) coupled with sodium phosphates was found able to stop the flame in a vertical flame test when applied to cotton [20]. Our research group has firstly employed chitosan (carbon source) coupled with ammonium polyphosphate (acid source and blowing agent) in an intumescent coating successfully deposited on cotton-polyester blends [21e23]. Very recently, the LbL technique has been exploited for depos- iting coatings with intumescent behavior but having a green composition and derived from renewable sources; to this purpose, chitosan and phytic acid have been used to impart flame retardant properties to cotton [24]. In the present paper we address the LbL assembly of novel and environmentally sustainable FR coatings based on deoxy- ribonucleic acid (DNA). Indeed, the double helix of DNA represents * Corresponding author. Tel.: þ39 0131229369; fax: þ39 0131229399. E-mail addresses: giulio.malucelli@polito.it, malucelli@polito.it (G. Malucelli). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.07.029 Polymer xxx (2013) 1e6 Please cite this article in press as: Carosio F, et al., Green DNA-based flame retardant coatings assembled through Layer by Layer, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.07.029