PAPER www.rsc.org/greenchem | Green Chemistry Novel biocompatible cholinium-based ionic liquids—toxicity and biodegradability† Marija Petkovic, a Jamie L. Ferguson, b H. Q. Nimal Gunaratne, b Rui Ferreira, a Maria C. Leit˜ ao, a Kenneth R. Seddon, a,b Lu´ ıs Paulo N. Rebelo a and Cristina Silva Pereira* a,c Received 28th October 2009, Accepted 8th January 2010 First published as an Advance Article on the web 9th February 2010 DOI: 10.1039/b922247b The synthesis, characterisation and toxicological assessment of a new group of environmentally friendly ionic liquids are presented. Focussing on the toxic effect of the anion, the ionic liquids were designed by combining the benign cholinium cation, [NMe 3 (CH 2 CH 2 OH)] + , with a range of linear alkanoate anions ([C n H 2n+1 CO 2 ] - , n = 1-9), as well as two structural isomers (n = 3 or 4). The toxicity of these ionic liquids was evaluated using filamentous fungi as model eukaryotic organisms. Surprisingly, most of the tested species showed active growth in media containing extremely high ionic liquid concentrations, up to molar ranges in some cases. The biodegradability of these ionic liquids was assessed, and new biotechnological applications for them are proposed, e.g. as solvents for biopolymers. This study leads to the better understanding of the anion influence on the ionic liquid toxicity, but its core is the recognition that conscious design of ionic liquids can be used to deliver truly biocompatible salts without adversely affecting one of the most striking of their properties—their outstanding solvent ability. Introduction The chemistry of ionic liquids has developed dramatically during the last decade. Their generic, yet not universal, properties, such as negligible vapour pressure, nonflammability, chemical and thermal stability, and outstanding solvation ability, enabled rapid advance in numerous applications. 1 The potential of ionic liquids is further emphasised because their physical and chemical properties may be finely tuned by varying the cation and the anion. 2 Their dual nature is, relative to conventional molecular organic solvents, a clear advantage. In order to improve old, or to create novel, ionic liquid based processes, aimingat cost-efficiency and sustainability, an interdisciplinary approach is essential. So-called risk-conscious design and “thinking in terms of structure–activity relation- ships” should be applied before implementing any new materials on a large scale. Thus, a better, structure-based, understanding of the environmental fate of ionic liquids is critical. 3 Unfortunately, this is a complex equation which crosses numerous unknown abiotic and biotic factors. The enormous diversity of ionic liquids is also a major concern from the ecotoxicological point of view, since testing such a vast number of ionic liquids and organisms is inconceivable. a Instituto de Tecnologia Qu´ ımica e Biol ´ ogica, Universidade Nova de Lisboa, Av. da Rep´ ublica, 2780-157, Oeiras, Portugal b The Queen’s University Ionic Liquid Laboratories, QUILL, The Queen’s University of Belfast, Belfast, BT9 5AG, UK c Instituto de Biologia Experimental e Tecnol´ ogica (IBET), Apartado 12, 2781-901, Oeiras, Portugal. E-mail: spereira@itqb.unl.pt †Electronic supplementary information (ESI) available: The spectro- scopic data of the ionic liquids ( 1 H, 13 C NMR and ESI-MS) are provided. See DOI: 10.1039/b922247b In the expanding studies on the toxicity and biodegradability of ionic liquids, they were not a priori accepted as environ- mentally benign. 4 It is widely accepted that the head group of the cation has a deciding role in toxicity, 5,6 that longer side chains have a more severe impact on living cells, 7 and that incorporation of an ester group significantly increases their biodegradability. 8 The anion plays an essential role in the ionic liquid physicochemical properties, 2 and it is known to contribute to the overall toxicity, 9 but its effect is usually neglected. The predictive value of certain trends, both for the cation and the an- ion, is unquestionable; however, generalisations should be taken cautiously, given the current limited understanding of the modes of toxicity of ionic liquids, their biodegradation pathways, and their behaviour concerning biosorption, bioaccumulation, etc. This complexity can be simplified if ionic liquids are synthesised from carefully selected naturally-derived materials, which are safer, carrying reduced toxicity and enhanced biodegradability. 10 Although a wide range of cholinium ionic liquids have been known for some time, 11-17 the alkanoates have been largely ignored. We present here a new group of cholinium ionic liquids. Cholinium, a quaternary ammonium cation, [NMe 3 (CH 2 CH 2 OH)] + , which is an essential micronutrient, 18 was chosen as the benign cation and combined with alkanoates of systematically elongated chains. In order to better understand the anion influence on ionic liquid toxicity, the toxicities of the obtained ionic liquids were evaluated using filamentous fungi— previously demonstrated to be an excellent test system. 19 Experimental section Chemicals The materials used in the ionic liquid syntheses, including source and grade, were as follows: cholinium hydrogencarbonate This journal is © The Royal Society of Chemistry 2010 Green Chem., 2010, 12, 643–649 | 643