Synthesis and Characterization of Biodegradable Lignin Nanoparticles with Tunable Surface Properties Alexander P. Richter, †,‡ Bhuvnesh Bharti, † Hinton B. Armstrong, †,‡ Joseph S. Brown, † Dayne Plemmons, † Vesselin N. Paunov, § Simeon D. Stoyanov, ∥,⊥ and Orlin D. Velev* ,† † Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States ‡ BENANOVA Incorporated, Raleigh, North Carolina 27606, United States § Department of Chemistry, University of Hull, Hull, HU67RX, U.K. ∥ Physical Chemistry and Soft Matter, Wageningen University, 6708 PB Wageningen, The Netherlands ⊥ Department of Mechanical Engineering, University College London, Torrington Place, London, WC1E 7JE, U.K. * S Supporting Information ABSTRACT: Lignin nanoparticles can serve as biodegradable carriers of biocidal actives with minimal environmental footprint. Here we describe the colloidal synthesis and interfacial design of nanoparticles with tunable surface properties using two different lignin precursors, Kraft (Indulin AT) lignin and Organosolv (high-purity lignin). The green synthesis process is based on flash precipitation of dissolved lignin polymer, which enabled the formation of nanoparticles in the size range of 45−250 nm. The size evolution of the two types of lignin particles is fitted on the basis of modified diffusive growth kinetics and mass balance dependencies. The surface properties of the nanoparticles are fine-tuned by coating them with a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). We analyze how the colloidal stability and dispersion properties of these two types of nanoparticles vary as a function of pH and salinities. The data show that the properties of the nanoparticles are governed by the type of lignin used and the presence of polyelectrolyte surface coating. The coating allows the control of the nanoparticles’ surface charge and the extension of their stability into strongly basic regimes, facilitating their potential application at extreme pH conditions. 1. INTRODUCTION Engineered nanomaterials have the potential to solve challenges in the fields of environmental remediation, 1 food and agriculture, 2 and health care. 3 However, the possible risk associated with the use of many synthetic inorganic nanoparticles and their postutilization activity have limited their large-scale application. 4 The deactivation of waste nanoparticles in solid- waste incineration plants, 5 or the recovery of persistent nanoparticles in wastewater treatment systems, 6 is a nontrivial problem. The potential postutilization activity of such nano- particles, 7 combined with their possible persistence, 5,8 could lead to long-term environmental impact. 9 It has been demonstrated previously that, by applying green chemistry principles 10 at the early stage of nanomaterial engineering, one can solve or mitigate some of these problems 11 and synthesize sustainable functional nanomaterials. 12 Biodegradable nanoparticles prepared from renewable feed- stocks could be used for safer delivery of active ingredients in molecular or ionic form. 13,14 After cellulose, lignin is the second most abundant biopolymer in nature and the first most abundant aromatic one. 15,16 It has an amorphous structure, 17 where the polymeric building block units form a three-dimensional network. 18 Lignin is an antioxidant 19 and lacks cytotoxicity toward human cells. 20,21 Since lignin is of plant origin, 22 it is naturally degradable and biocompatible. 23 Microbial decom- position of lignin in the environment 24 transforms it to form the bulk component of soil humus or natural compost. 25 This biodegradability of lignin makes it an ideal precursor for developing environmentally friendly nanoscale materials. The most common industrial-scale method to extract lignin is the Kraft pulping process 26 from which alkali lignin, such as Indulin AT lignin from MeadWestvaco Corporation, is recovered. This lignin contains several hydrophilic functional groups such as carboxylic, phenolic, and aliphatic hydroxyl groups as well as a small number of thiol groups, whose relative fractions are listed in Table 1. 27 These functional groups have chelating properties toward metallic micronutrients, e.g., Fe, Zn, and Mn. Due to the environmentally sustainable nature of lignin, 28 it has been used for the delivery of such micronutrients in agronomic products. 29,30 Lignin obtained via the Organosolv process, such as high-purity lignin (HPL) from Lignol Innovations Limited is Received: March 21, 2016 Revised: June 2, 2016 Published: June 6, 2016 Article pubs.acs.org/Langmuir © 2016 American Chemical Society 6468 DOI: 10.1021/acs.langmuir.6b01088 Langmuir 2016, 32, 6468−6477