Insulating film-based biosensors
Scientists at Leeds have been working on a range of low-cost biosensors based on impedimetric measurement techniques. The technique relies on first sealing an electrode surface with a mixed self-assembled monolayer (mSAM). This monolayer consists of a standard surfactant mixed with smaller amounts of a compatible phospholipid to which a sensing biomolecule, for example an antibody, is attached.
The mSAM is a well-defined insulating layer on the electrode surface, which has reproducible impedance characteristics. When the electrode is bathed in a solution containing the analyte of interest, the analyte binds to the sensing biomolecule, and is concentrated by this binding reaction at the electrode surface.
As the width of the mSAM is very narrow - a single molecule - the addition of the bound analyte has a profound influence on the impedance characteristics of the sensor, so comparatively tiny amounts of analyte can be readily detected.
Above right: Cyclic voltammograms of a solution of ferro/ferricyanide on a gold electrode, showing strong current peaks corresponding to oxidation and reduction of the iron ions. After coating with the mSAM, these peaks disappear as the gold electrode is insulated by the ordered sensor monolayer.
Detecting heart attacks
The leading cause of death for both men and women all over the world is myocardial infarction - a heart attack - this occurs when blood flow to the heart muscle is stopped by a blockage. Part of the heart muscle is then starved of oxygen, and heart function is proportionately impaired. A wide range of symptoms may accompany the attack, and it is therefore not always evident that immediate medical treatment is necessary, or what the appropriate treatment might be. As a consequence, there is a need for a cheap, fast method for unambiguously identifying the underlying problem.
One of the key indicators of a myocardial infarction is myoglobin, which is released by the damaged muscle immediately after the infarction, and in increasing amounts as the muscle deteriorates. Free myoglobin in the bloodstream is thought to be the first molecular indicator of an infarction.
The myoglobin biosensor
To make the sensor, a clean gold electrode is coated with a mixture of 16-Mercaptohexadecanoic Acid (MHDA) and a lipid functionalised with a biotin group. This is seen to self assemble into a coherent monolayer (an mSAM) on the gold electrode surface, as demonstrated by the cyclic voltammetry results (graph top).
NeutrAvidin has a very high affinity for biotin, so binds to the functionalised lipid. NeutrAvidin is composed of four identical sub-units, each of which can bind to a biotin molecule. The myoglobin antibody is therefore also functionalised by adding the biotin group to it, and can be bound in turn to the sensor surface.
When exposed to myoglobin in plasma, the myoglobin binds to the antibody, and the impedence of the sensor film changes. The sensor shows linear response to myoglobin concentration in the range 10-6 to 10-11M.
The design of the biosensor if very flexible, any antibody can be bound to the surface, opening a wide range of specific sensor possibilities.
Right: The resistive (Z') component of the impedance at 250mHz of a thin film sensor in the presence of diffferent concentrations of myoglobin (Mb). There is a linear relationship between resistance and log concentration in the region 10-11 and 10-6 M Mb.
References
"Development of a myoglobin impedimetric immunosensor based on mixed self-assembled monolayer onto gold" Morsaline Billah, Henry C.W. Hays and Paul A. Millner, Microchim Acta (2007)
"Biosensors: Rapid preparation-free measurement" Mark Rogers and Paul Millner, Biodiscovery Technology (non-technical review).
More information on causes and symptoms of a myocardial infarction are available on Wikipedia.
Find out more about biosensors, biocatalysis and combinatorial biology on Paul Millner's website: website link.
