RESEARCH ARTICLE Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, 1–7, 2012 Ab-Initio Adsorption Study of Chitosan on Functionalized Graphene: Critical Role of Van Der Waals Interactions R. Rahman 1 and D. Mazumdar 2 ∗ 1 Department of aerospace engineering and mechanics, The University of Alabama, Tuscaloosa, AL 35487 2 Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487 We investigate the adsorption process of an organic biomolecule (chitosan) on epoxy-functionalized graphene using ab-initio density functional methods incorporating van-der-waals (vdW) interactions. The role of London dispersion force on the cohesive energy and conformal preference of the large molecule is quantitatively elucidated. Binding energy values are observed to increase by over an order of magnitude after including vdW corrections to the total energy, implying that dispersive interactions dominate the physisorption process. Functionalizing graphene with epoxy groups also leads to weak hydrogen-bond interactions with the hydroxyl and amine functional groups of chitosan. Detailed conformal study of functional groups reveal that binding is strongest when the molecule is oriented with the hydroxyl group approaching the functionalized graphene. At the binding distance a cohesive energy of nearly 30 kcal/mol is evaluated for this configuration which changes very slowly with increasing distance. Our study furthers advances the promise of functionalized graphene for a variety of applications. Keywords: 1. INTRODUCTION Graphene based research is moving at a breathtaking speed ever since the ideal two-dimensional flatland was dis- covered in 2004. 1 The impact is such that the discov- erers have been awarded the 2010 physics Nobel Prize. The unique structure of graphene with a zero band gap at Dirac point has attracted the physics community as the electronic motion is akin to a massless particle. 2 The high electronic mobility is equally attractive for a variety of electronic applications, and engineering ways to open and possibly tune the band gap by doping or structural modification 3 4 has been one of the major research thrust. A current active topic for graphene application is in the area of biological and electrochemical sensors where it could act as an excellent electrode material. Apart from its high electrical mobility, single layer graphene has a large surface area (theoretically 2630 m 2 /g), high mechanical strength, 5 thermal conductivity, 6 and excellent biocompatibility. Recent experiments have also demon- strated that graphene can be chemically functionalized with organic molecules, enzymes, all of which is promising for ∗ Author to whom correspondence should be addressed. novel graphene-based biosensors and nanocomposites. 7–10 Theoretical studies using graphene as an electrode has largely focussed on chemisorption processes of small molecules like NO 2 ,O 11 12 3 p-type or monovalent elements. 13 14 But research into adsorbates which are either biological or organic is also gathering much importance. Binding of large organic bio-polymers such as polysac- charides on graphene is particularly relevant for applica- tions such as immunoassay, drug-delivery and bio-sensing. However, for such sparse matter, the absorption process is of physical nature (physisorption) where the domi- nant interaction responsible for attractive forces are weak, long-ranged van-der Waals (also called dispersive London forces). 15 Traditional first-principles method implementing density functional theory use either local density or gen- eralized gradient approximation and are primarily suited to describe the ground state of dense (solid state) matter. Therefore search has intensified in recent years looking for methods at the ab-intio level which can treat both local and long-ranged interactions within the same framework. From a practical stand-point, these methods can provide valuable insight into experiments which are often difficult to interpret due to a variety of factors. J. Nanosci. Nanotechnol. 2012, Vol. 12, No. xx 1533-4880/2012/12/001/007 doi:10.1166/jnn.2012.5798 1