Base-pair mapping by chemical force microscopy on nucleobase self-assembled monolayers

Charge propagation in "ion channel sensors" based on protein-modified electrodes and redox marker ions

The mechanism of charge propagation in "ion channel sensors" (ICSs) consisting of gold electrodes modified with a layer of charged proteins and highly charged redox-active marker ions in solution was investigated by electrochemical techniques, QCM and AFM. The study is based on seven proteins (concanavalin A, cytochrome c, glucose oxidase, lysozyme, thyroglobulin, catalase, aldolase, and EF1-ATPase) in combination with seven electroactive marker ions ([Fe(CN)6]3-, [Fe(CN)6]4-, [Ru(NH3)6]3+, mono-, di-, and trimeric viologens), as well as a series of suppressor and enhancer ions leading to the following general statements: (i) electrostatic binding of charged marker ions to the domains of the protein is a prerequisite for an electrochemical current and (ii) charge propagation through the layer consists of electron hopping along surface-confined marker ions into the pores between adsorbed proteins. It is further shown that (iii) marker ions and suppressor ions with identical charge compete for oppositely charged sites on the protein domain, (iv) electrostatically bound multilayers of marker or enhancer ions with alternating charge form on a charged protein domain, and (v) self-exchange and exergonic ET catalysis between adsorbed marker ions and marker ions in solution take place. In addition to fundamental insight into the mechanism of charge propagation, valuable information for the design, optimization, and tailoring of new biosensors based on the ICS concept is demonstrated by the current findings.

MOLECULAR RECOGNITION OF NUCLEOBASES ATTACHED TO SELF-ASSEMBLED MONOLAYERS DETECTED BY CHEMICAL FORCE MICROSCOPY AND QUARTZ CRYSTAL MICROBALANCE

In order to develop a new DNA sequencing method by using chemical force microscopy (CFM), we have investigated the interaction of the hydrogen bonding between surfaces of nucleobase self-assembled monolayers (SAMs) and AFM-tips modified with the nucleobases. The two different adhesion forces, the jump-in force and pull-off force, between the AFM-tip modified with cytosine-SAM and the surfaces of four kinds of nucleobase SAMs were measured in water (20°C) by CFM. The adsorption of poly (C) onto a nucleobase-SAM on a gold electrode of quartz crystal microbalance (QCM) was measured as resonance frequency changes. The relative relation among four bases showed similar tendency in the adhesion force measured by the cytosine AFM-tip and in the adsorption amount of poly (C) on the QCM electrode as well as in the theoretically calculated interaction energies between two nucleobases.