Anodic Desorption Monitored by Voltammetric and Gravimetric Measurements for Fast Estimation of Surface Coverage of Complex Organic Molecules on Au Electrodes

Abstract

1IntroductionComplex molecular nanostructures and biomolecules assembled att he surface of functional electrodes are key objectsi nr apidly growing areas of electro-catalysis, where reactants are selectively oxidizedo rr educed at the electrode surface,i nt he development of efficient (bio)sensors or (bio)-fuel cells,f or example.In depth investigation of catalyticp roperties,m olecular interactions or other phenomena in nano-objects immobilized at the electrodes urface require precise information about the surface coverage.F or "hard"i norganicp articles,avariety of powerful techniques including, transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning tunneling microscopy (STM), or atomic force microscopy( AFM) allows direct visualization of the formed structures and straightforward determination of the surface concentrations.W ith "softer" organic or biological nano-objects such techniques can still be useful, especially for imagingo rdered superstructures. However, with dense monolayers (and/or disordered layers) or highlyf lexible structures[ 1],t he accuracyo f surface coverage determination by means of these commonm ethodsi soften very limited.If the molecular nanostructures are electroactive,t he electrochemical techniqueo fp otentiodynamic cyclic voltammetry (CV) can be used to control their assembly and characterizet he resultant layers in as traightforward manner. This technique uses ac yclic linear change in electrode potential asaprobing signal and analyses the direct current flowing through the system. This provides informationa bout the number of electroactivec entres [2][3][4].H owever, the redox centersi naccessible to the electront ransfer will not be detected.A lternatively,f or electrocatylic centers,s uch as redoxe nzymes,m ediatede lectront ransfer may be exploited to quantify the catalytic properties whichi ndirectly provide information on the surface coverageo ft he system.F or this purpose the catalytica ctivity is assumed to be equal to that of the freely diffusing enzyme [5,6].I na ddition, two robust and wellestablished techniques can be usedt oc omplement electrochemical methods and determine surface coverages for non-redox active monolayers [7].T hese techniques are in-situ gravimetry using aq uartz crystalm icrobalance (QCM) [8,9] and surface plasmon resonance (SPR). The SPR methodp rovides the dry weight of the modifiers and has become one of the standard methods to investigate protein adsorption [10].H owever, interpretation of the data withr espect to surface mass requires preliminary calibrations and assumptions to be made [11].Abstract:Am ethodology for fast determinationo fs urface coverage of molecules or nanostructures assembled in monolayers at the surface of gold electrodes has been developed. Thea pproach is based on anodic oxidative desorptiono ft he molecules which is controlled by synchronized electrochemical gravimetric and voltammetric measurements.S uccessivea nodic scans induce degradation and oxidative desorption of the assembled molecu...