Fundamental Aspects of Single Molecule and Zeptomole Electroanalysis

October 1, 2018

Defense Threat Reduction Agency, Fort Belvoir, Virginia

The objective of this research program was to provide the fundamental understanding required for using the principles of electroanalytical chemistry to detect target molecules at very low concentration, including single molecules, with high specificity, simplicity, and low power.

A probe DNA sequence (red) immobilized onto a nanoscale magnetic particle is mixed with a label sequence (blue) modified with an electrocatalyst, such as a Pt nanoparticle. In the presence of the target sequence (green), a sandwich complex forms. Application of a magnetic field in the vicinity of the electrode results in concentration of the complex onto the electrode surface. When an appropriate potential is applied to the electrode, an electrocatalytic reaction occurs (here shown as hydrazine oxidation)

Attainment of this objective required fundamental investigations of reaction kinetics, catalytic activity, nanoparticle synthesis, adsorption, and signal enhancement at surfaces. The results of this study lay the groundwork necessary for detecting and responding to the presence of low levels of CB (chemical, biological) agents.

Additionally, investigations of single electrochemical events have allowed researchers to uncover phenomena and properties that are not apparent by studying processes involving large numbers of molecules, as is typical in conventional electrochemical experiments in which an ensemble average of the data is treated.

The approach in this project has been to use a novel electrochemical method to greatly amplify the presence of specific molecules or classes of molecules. The project is based on results demonstrating the detection of individual collisions between catalytic nanoparticles and electrodes, a process called “electrocatalytic amplification” (ECA). The project also relied on a high degree of expertise with the following:

Synthesis of nanoparticles having well-defined sizes, compositions, and structures that result in tailored catalytic and magnetic functions;
Functionalization of electrodes and nanoparticles with biological probes such as DNA and proteins;
New concepts in microfluidic design for manipulating very small amounts of solution and capturing small numbers of labeled targets; and
Fabrication of nanoscale electrodes having dimensions that are on the same size scale (~ 3 nm) as the catalytic labels used to signal the presence of individual molecules.

The approach to the problem is illustrated in above. Here, a target DNA sequence (shown in green) is detected when it switches on an electrochemical reaction. An approach similar to that shown in the figure was used to detect single catalytic particles and demonstrated that amplification factors as high as 109 can be achieved. Note that although it was decided to illustrate DNA detection as a model analyte in the figure, the basic approach is also applicable to proteins and small-molecules.

This work was done by Richard M. Crooks, Allen J. Bard, and Keith J. Stevenson of The University of Texas at Austin; and Bo Zhang of the University of Washington for the Defense Threat Reduction Agency. DTRA-0009