This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A reaction on a single electrode can tell scientists a remarkable amount about the chemical nature and number of analytes involved. The challenge comes when they want to distinguish many reactions on the same electrode.
After more than a decade of trying to overcome that limitation, Ravi Saraf and his Nebraska Engineering graduate students have demonstrated a way to quantify many reactions on the same electrode without sacrificing extraordinary sensitivity. The advancement, Saraf said, could lead to better early diagnoses of diseases such as cancer and improved assessment of injury from exposure to high-energy radiation. "It is certainly one of the more exciting things I've done," said Saraf, Lowell E. and Betty Anderson Distinguished Professor of chemical and biomolecular engineering.
In two studies, Saraf and graduate students Linh Hua, Md Tawfiq Anik and Joydip Dey describe an optical-electrochemical technique capable of detecting reactions involving just zeptomoles, each of which includes roughly 600 molecules, while probing different locations on the same electrode. The results bring the technology close to a fundamental quantum mechanical limit imposed not by the instrument, but by the statistical error in counting electrons involved in a reaction. Electrochemical sensors are everywhere, with the glucose meter perhaps the most familiar example.
But conventional sensors measure the total electrical current produced on an electrode's surface, making it difficult to distinguish multiple reactions occurring simultaneously. Their lab-built instrument, the Scanning Electrometer for Electrical Double-Layer (SEED), uses light to measure multiple reactions on a single electrode. The researchers attached methylene blue molecules to DNA on a gold electrode.
As the molecules underwent reduction and oxidation reactions, they switched between colored and colorless states. SEED measured those changes optically. That sidesteps a longstanding obstacle.
Increasing the speed of an electrochemical reaction increases the signal, but also the capacitive background that can overwhelm it. Because SEED measures an optical change instead of that electrical background, Saraf said, "this device is sort of blind to that." That allowed the team to operate at higher speeds and achieve extraordinary sensitivity. In a study published in ACS Measurement Science Au, Saraf's team reported detecting redox reactions of about eight zeptomoles (roughly 4,800 molecules) and had effectively reached what Saraf calls the "quantum mechanical wall." At that scale, unavoidable statistical fluctuations associated with counting electrons become significant.
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