Interfacial Mass Flux at 11-Mercaptoundecanoic Acid Linked Nanoparticle Assembly on Electrodes Jin Luo, Nancy Kariuki, Li Han, Mathew M. Maye, Laura W. Moussa, Scott R. Kowaleski, F. Louis Kirk, Maria Hepel, ² and Chuan-Jian Zhong* Department of Chemistry, State UniVersity of New York at Binghamton, Binghamton, New York 13902, and Department of Chemistry, State UniVersity of New York at Potsdam, Potsdam, New York 13676 ReceiVed: May 24, 2002; In Final Form: July 12, 2002 Thin films derived from nanocrystal cores and functionalized linkers provide a large surface area-to-volume ratio and three-dimensional ligand framework. This paper describes the results of an investigation of the interfacial mass flux and binding properties of such thin films using an electrochemical quartz crystal nanobalance technique. The hydrogen-bonding assembly from gold nanocrystals and 11-mercaptoundecanoic acid was studied as a model system. The results reveal four distinctive mass response characteristics upon pH tuning or metal ion binding. First, the protonation-deprotonation characteristic of the carboxylic acid groups in the nanostructured framework is dependent on particle core size and film thickness. Second, the pH- tunable cationic redox reaction across the electrode|film|electrolyte interface is accompanied by a large cationic electrolyte mass flux. Third, the spontaneous complexation to copper ions by the nanostructured carboxylate framework is reflected by a mass increase of the film. Fourth, the redox reaction of copper loaded in the nanostructured film is accompanied by fluxes of electrolyte cations across the electrode|film|electrolyte interface which compensate electrostatically the fixed negative charges. On the basis of the mass change detected in the presence of a series of electrolyte cations, a linear relationship was determined between the mass increase and the atomic mass of the cation, and a concurrent flux of solvent molecules was also revealed. Implications of the findings to the delineation of the design parameters of the nanostructured ligand framework for controlled release and environmental monitoring or removal of metals are also discussed. Introduction Nanoparticles with organic encapsulation are attractive build- ing blocks toward responsive materials for applications in sensors, biosensors, catalysis, and controlled drug delivery. 1,2 For example, many devices or techniques for environmental monitoring of heavy metals, which threaten both human and environmental health through their increasing presence in industrial wastewater, 3,4 desire to incorporate responsive materi- als with large surface area-to-volume ratio and high binding specificity. Current methods are mostly based on ion-exchange chromatography involving preconcentration. Multidentate amine and carboxylate functionalities are excellent ligands for improv- ing the selectivity, 3,5 as demonstrated using poly(L-aspartic acid) coated on an ion-exchange column 4 and self-assembled mono- layers of 2,2′-thiobisethyl acetoacetate on an electrode. 6 The recent progress on synthesis, processing, and characterization of core-shell nanoparticles 1-2,7-16 has led to new opportunities to develop advanced materials with large surface area-to-volume ratio and three-dimensional ligand framework for enhancing interfacial binding sensitivity and specificity. The exploration of place-exchange reactivity 11,13 has enabled the ability to engineer shell structure with a desired functionality. The stepwise layer-by-layer method has been shown to create thin film assembly of nanoparticles of a wide variety of sys- tems. 12,14,17,18 The exchange-cross-linking-precipitation route 19 using ω-functionalized alkanethiols and alkanethiolate-capped gold nanoparticles has provided a simple means for thin film assembly via intershell hydrogen bonding. The viability of using multiple carboxylate sites in the assembled nanostructure as a three-dimensional ligand frame- work with a large surface area-to-volume ratio could provide an effective environment for binding metal ions. 20 This attribute has also been demonstrated recently for exploring carboxylate- M 2+ binding chemistry as a bridge in constructing a multilayer assembly of nanoparticles, 17 and in solution format for colori- metric detection of metal ions. 21 Hydrogen bonding of amine and carboxylic acid groups has also been reported for assembling nanoparticles in several other systems. 22,23 Our aim of the present study is to probe in situ the interfacial mass flux and binding characteristics of the nanostructured films. The results will be very important for delineating the design parameters of chemi- cally sensitive or responsive nanomaterials in electrochemical detection of metals and controlled release applications. Scheme 1 illustrates in a highly idealized fashion the redox reaction and possible mass fluxes at the electrode|nanostructured film|elec- trolyte interface. A good model system of the film can be assembled on an electrode surface by hydrogen-bonding linkage of gold nanocrystals using 11-mercaptoundecanoic acid (MUA) linker molecules. 19b,c The understanding of the control and regulation of ion and solvent fluxes requires a quantitative correlation of the charge flow and the mass change. The nanostructure on the electrode surface can be considered as a membrane-like film whereby electron-transfer reaction and ionic or solvent fluxes across the membrane could lead to both * To whom correspondence should be addressed. Telephone: (607) 777- 4605. E-mail: cjzhong@binghamton.edu. ² State University of New York at Potsdam. 9313 J. Phys. Chem. B 2002, 106, 9313-9321 10.1021/jp026179c CCC: $22.00 © 2002 American Chemical Society Published on Web 08/16/2002