Electron Transfer Mediators

Electron Transfer Mediators: Revolutionizing [Diabetic Test Strips](/sell-diabetic-supplies/)

An electron transfer mediator is a small molecule on a test strip that carries electrons from the glucose-reacting enzyme to the strip's electrode, where the meter reads them as current [1]. Mediators are a big part of why glucose testing could leave the clinical lab. They freed the reaction from its dependence on oxygen in the blood and let strips run at lower voltages [1].

First Generation Sensors

Early glucose sensors used oxygen from the blood to carry electrons. The enzyme glucose oxidase turned glucose and oxygen into gluconic acid and hydrogen peroxide, and the sensor measured either the oxygen used up or the peroxide produced [1]. That design had two weak spots. Oxygen dissolves poorly in blood, so its level fluctuated, a limit researchers called the "oxygen deficit" [1]. Measuring peroxide also took a high operating potential (the voltage applied to the electrode), and at that voltage other substances in blood, including ascorbic acid (vitamin C), uric acid and some drugs, reacted at the electrode too and inflated the signal [1]. Acetaminophen (Tylenol) is a well-documented case: at high doses it is oxidized directly at the electrode and pushes the glucose reading up [1].

Second Generation Sensors

Second-generation sensors replaced oxygen with synthetic mediators. The enzyme hands its electrons to the mediator; the electrode then reoxidizes the mediator, which produces the measured current and readies the mediator for another round [1]. Ferrocene and ferricyanide became the usual choices for self-monitoring strips [1]. When we read why ferrocene caught on, the reasons were practical: it doesn't react with oxygen, and it stays stable whether oxidized or reduced [1]. The first electrochemical meter for home self-testing, MediSense's pen-sized ExacTech, launched in 1987 with a ferrocene derivative [1].

Advanced Mediators

Later designs turned to metal complexes that work at lower potentials. TheraSense, now part of Abbott Diabetes Care, built its FreeStyle meter (available from 2000) around the enzyme glucose dehydrogenase and an osmium mediator with a formal potential between 0.0 and -0.2 V, measured against the strip's silver/silver chloride electrode [2]. The same team "wired" glucose oxidase to the electrode with an osmium complex bound to a polymer. That approach went into a sensor worn under the skin rather than a strip: the FreeStyle Navigator CGM, which became available in the US in 2008 [2]. Hexaammineruthenium(III), a ruthenium complex, is another mediator strip designers value for its low redox potential and its stability in storage [3].

Lower potentials reduce interference. They don't remove it. The TheraSense team reported that the substances that often interfere did not affect FreeStyle readings at their normal physiological levels [4], and that qualifier matters. Interference testing is still part of FDA clearance: in its 2010 decision summary for Abbott's FreeStyle Lite strips, the FDA noted that xylose interfered above 6 mg/dL and that the strip insert warns against using the system during xylose absorption testing [5].

Mediator chemistry is one ingredient in accuracy, not the whole recipe. ISO 15197:2013, the international accuracy standard for blood glucose meters, requires at least 95% of results to fall within ±15 mg/dL of a laboratory reference below 100 mg/dL and within ±15% at or above 100 mg/dL, with at least 99% in zones A and B of the consensus error grid [6]. Whether a meter meets that bar also depends on factors no mediator controls, such as hematocrit (the share of blood made up of red cells), temperature and humidity [1].