Sweet sense of GOD

A glucose sensor based on a room-temperature ionic liquid rather than conventional solvents has much better acid-resistance than other sensors and so could be developed into a much more robust sensor device for diabetes monitoring.

Xiaoying Liu, Xiandong Zeng, Nannan Mai, Yong Liu, Bo Kong, Yonghong Li, Wanzhi Wei, and Shenglian Luo of the State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, at Hunan University, in Changsha, People's Republic of China, have developed a new type of amperometric glucose biosensor based on glucose oxidase enzyme held within a colloidal gold-modified carbon ionic liquid electrode. The biosensor displays remarkable acid stability, the team says.

Glucose biosensors are widely used and equally well studied because of the important role they play in monitoring blood glucose concentration in the control of diabetes and in biomedical research into metabolic problems. Glucose oxidase (GOD) is the common enzyme used in amperometric glucose biosensors. "GOD contains two tightly bound flavine adenine dinucleotide (FAD) co-factors and can catalyze the electron transfer from glucose to oxygen with the accompanying production of gluconolactone and hydrogen peroxide," the team explains, the reactions display the transducing principle as hydrogen peroxide is released, they add. This means that the concentration of glucose can be quantified using electrochemical detection of the enzymatically liberated hydrogen peroxide.

There are always solvent effects and room for improvement so the Hunan University team has turned to room temperature ionic liquids (RTILs) to side-step some of the issues associated with conventional solvents. Ionic liquids, also known as molten salts, form when bulky and asymmetrical ions of opposite charge (usually an organic cation and various anions) are brought together. At room temperature the energy of crystallisation is too high for the salt to form a solid so, as their name suggests, they remain as a melt. Over the last decade or so, chemists have demonstrated various intriguing properties of RTILs and have developed a vast repertoire of organic cations and their associated anions with which to form these intriguing solvents.

When compared to commonly used electrolyte solvents, RTILs have a much wider potential window, good electrical conductivity, high ionic mobility, excellent chemical and thermal stability, negligible vapour pressure, a wide liquid range, low combustibility, and the ability to solvate a diverse range of compounds, the researchers add. Such a favourable range of properties makes them of great interest to those involved in electrochemistry and the team has now demonstrated the utility of RTILs in a glucose sensors.

They constructed a colloidal gold-modified carbon ionic liquid electrode by mixing colloidal gold-modified graphite powder with a solidified form of the RTIL n-octyl-pyridinium hexafluorophosphate (OPPF6) to form a paste. Glucose oxidase (GOD) was bound within this composite matrix and was shown to retain its bioactivity and be particularly stable. The team investigated the effect of changing pH, applied potential and GOD loading and found that GOD sustained its enzymatic properties even at a strongly acidic pH of 2.0 for more than sixty minutes.

"The proposed biosensor responds to glucose linearly over concentration range of 5.0 × 10^-6 to 1.2 × 10^-3 and 2.6 × 10^-3 to 1.3 × 10^-2 molar, and the detection limit is 3.5 × 10^-6 molar," the team reports. "The response time of the biosensor is fast (within 10 seconds), and the life time is over two months." The researchers also point out that using a Nafion film can inhibit the effects of electroactive interferents, such as ascorbic acid, and uric acid, making the biosensor more sensitive to glucose concentration alone. Statistical analysis was used to optimise the performance of the biosensor by analysing the effect of applied potential on the response of the biosensor to glucose.