Toward development of first-principles theory for molecule-surface interactions in aqueous solutions
Yong-Hyun Kim and S.B. Zhang
National Renewable Energy Laboratory, Golden, Colorado 80401, USA
Organic and biological molecule passivation of semiconductor quantum dots has recently attracted much attention not only because the rapid advances in fabricating novel colloidal quantum dots but also because it offers the opportunity to biologically manipulate the dots into desirable assembly.Typically, the interaction between semiconductor surfaces and molecules takes place in aqueous solutions. However, little has been developed to understand such interfaces by ab initio approaches. For example, a cysteine (Cys), one of the amino acids, has multi reactive sites whose chemical activity depends critically on the pH value of the solution. If one conducts the calculation in a vacuum condition as many do today, it is practically impossible to control the reactions such that they take place at the sites known experimentally. As such, most of the calculated results are hardly useful. We have recently formulated a method to include the aqueous solutions into first-principles calculations in a surprisingly simple yet completely general way. The method has been applied to a number of molecules such as protonated and deprotonated water molecules, the Cys, the NH3 and HCOOH where the -NH2 and -COOH radicals are the elementary building blocks of the amine group, R-CH-NH2-COOH. Our calculations yield good pKa values when compared with experiments. This opens the field for direct first-principles calculations of molecule-surface interactions in aqueous solutions, with modern molecular dynamic simulation techniques, without having to make any unrealistic assumptions.