Mediator Chemistry and Electron Transfer

Mediator Chemistry and Electron Transfer

A test strip measures glucose by catching the electrons an enzyme pulls off glucose and reading them as a tiny electric current. What carries those electrons to the electrode is the story of this page. The first glucose sensors handed them to oxygen; later strips hand them to a small molecule called a mediator [1][2]. Reviews call the oxygen designs "first generation" and the mediated ones "second generation" [1][2].

"Third generation" is less settled. Both reviews we read define it as direct electron transfer from enzyme to electrode, with no mediator at all [1][2]. They part ways on "wired" enzymes, though: one lists wiring among the ways to improve mediated sensors [2], while the other describes osmium-polymer wires as a route toward third-generation designs [1]. When a product page calls a strip "3rd Gen," it's fair to ask which definition it means.

The Problem

Early glucose sensors used oxygen as the electron acceptor. The enzyme glucose oxidase passed glucose's electrons to oxygen, producing hydrogen peroxide, and the sensor measured either the oxygen used up or the peroxide made [1][2]. Reading peroxide took a high electrode voltage. At that voltage, ascorbic acid (vitamin C), uric acid and some drugs also reacted and added false current [2]. Oxygen also dissolves poorly in blood, so swings in oxygen level, which researchers called the "oxygen deficit," shifted results [2].

Here's what surprised us: electrochemical strips for home use started out mediated. The first such meter, MediSense's pen-sized ExacTech, launched in 1987 with a ferrocene mediator and the enzyme GDH-PQQ, whose reaction doesn't depend on oxygen [2]. Oxygen still matters for strips built on glucose oxidase, because oxygen carried by red blood cells can compete with the mediator for electrons [2]. You can read more about that split in Enzyme Chemistry: GOx vs. GDH.

The Standard Solution

A mediator stands in for oxygen. It takes electrons from the enzyme, gives them up at the electrode, and is ready to go again; the electrode reads each handoff as current [2]. Ferrocene and potassium ferricyanide are the two mediators Yoo and Lee name for self-monitoring strips, and most commercial strips still work much like the ExacTech did [2]. These mediators diffuse: they dissolve in the blood sample and drift between enzyme and electrode rather than being fixed in place [2]. Ferricyanide does need a higher working voltage than the osmium mediators described below.

The Advanced Solution

The other main family is transition-metal complexes, built around metals such as osmium. The clearest strip example we found is Abbott's FreeStyle line, which came from TheraSense: its strips pair a glucose dehydrogenase (GDH) enzyme with an osmium (Os2+/3+) mediator [3]. Abbott first used the PQQ form of the enzyme and later switched to the FAD form [3]. The FDA's 2010 clearance of FreeStyle Lite strips describes FAD-GDH passing electrons "via a co-factor and mediator" to the meter [4]. These strips are coulometric, too: instead of sampling current at one instant, they count the total charge from a tiny, fixed volume of blood, about 300 nanoliters in the original design [3]. Our page on Coulometry vs. Amperometry in Biosensors covers that difference.

Polymer "wiring" is a separate design. In a wired enzyme, the osmium complex is chemically tied to a polymer gel that holds the enzyme, so electrons hop through the gel instead of riding a free-floating mediator. Abbott's documented use of wiring is in a CGM, not a strip. The FreeStyle Navigator, an earlier Abbott continuous glucose monitor that reached US users in 2008, used glucose oxidase crosslinked into a redox hydrogel with a polymer-bound osmium complex [3]. See Wired Enzyme Technology for more on that approach.

Roche often gets named alongside Abbott here, but the FDA decision memo for Roche's Accu-Chek Guide strips names only the enzyme, an FAD-dependent GDH, and doesn't identify a mediator [5]. We didn't find a primary source tying Roche strips to an osmium or ruthenium mediator, or to a wired enzyme. Ruthenium complexes appear in some descriptions of strip chemistry, but we couldn't confirm one in a specific product.

Key Benefits of Advanced Mediators

• Low Potential: Abbott picked an osmium mediator whose formal potential sits between 0.0 and -0.2 V relative to the strip's silver/silver chloride electrode [3]. Its stated reason was to stop electrons from shuttling between the two closely spaced electrodes [3]. A low potential helps with interference in principle, because a drug such as acetaminophen adds false current only when it reacts at the voltage the sensor runs at [2].
• Speed: FreeStyle Lite strips report a result in 5 seconds [4]. Fast results aren't unique to osmium. In Yoo and Lee's 2010 survey, most commercial meters returned a result in under 10 seconds [2], and we found no source that credits the mediator alone for test speed.
• Stability: Binding osmium to a polymer keeps it in the sensing layer instead of letting it diffuse away, which matters for a sensor that stays under the skin for days; the Navigator sensor was worn for 5 days [3]. For single-use strips, we found no primary source showing that wired enzymes improve shelf life.