A new biosensor that allows point-of-care monitoring of stress hormone cortisol levels has been developed by scientists in the US. It could transform diagnostics and treatment of stress disorders, they claim.

Abnormal cortisol levels have been linked to several stress-related diseases including chronic fatigue syndrome, irritable bowel syndrome, and post-traumatic stress disorder. Detection at point-of-care using a non-invasive method, such as saliva sampling, provides easy correlation of levels with clinical observations, since salivary cortisol levels correlate well with those found in blood.

Now Sunil Arya and colleagues at the University of South Florida in Tampa, have designed a disposable sensor that is rapid, highly sensitive and can accurately detect cortisol in the range of 1pM to 10nM within 40 minutes. Importantly, it can analyse real human saliva samples and gives results comparable to those obtained using current commercially available, but more complex, techniques such as enzyme-linked immunosorbent assays, which also often require pre-treatment of the saliva.

The impedance (resistance to current) based sensor uses interdigitated microelectrodes - a series of parallel band electrodes with alternate bands connected together, like two sets of comb fingers. This type of electrode gives an increased signal-to-noise ratio and offers rapid reaction kinetics, says Arya. To create their sensor the team immobilised a cortisol-specific antibody on the surface of the electrode by assembling a monolayer of a reactive linker molecule on the electrode.

For analysis, the device is incubated with solutions containing varying concentrations of cortisol. Then a solution containing a mixture of ferrocyanide (Fe²⁺) and ferricyanide (Fe³⁺) is used as a redox probe for measuring the resistance to charge transfer from the solution to the electrode, and thus the amount of cortisol in the solution. 'As [cortisol] is insulating in nature, its binding makes the electrode surface insulating and hence results in an increase in resistance,' explains Arya.

Nongjian Tao, director of the Center for Bioelectronics and Biosensors at the Biodesign Institute at Arizona State University, US, comments that this relatively simple method is 'certainly more attractive than other more complex detection methods,' adding that using saliva 'separates the work from many others that omit real sample testing'.

Arya says they are now exploring commercial development of the technique, and imagines it could be developed into a 'user-friendly test like glucose sensors'.

Frances Galvin

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