Animal magic

A European research group has discovered that proteins found in mouse urine could help create powerful biosensors for environmental monitoring and security applications.

According to researchers from Manchester University, the University of Pavia in Italy, and Sweden-based KTH and Linköping University, mouse major urinary proteins (MUPs) coated on a standard piezoelectric crystal enabled a one thousand-fold increase in sensor sensitivity compared with existing electronic sensing methods. The new biosensor could allow in situ monitoring of river water, which is currently tested manually in a laboratory.

The four organisations carried out the research as part of the EU-funded General Olfaction and Sensing Projects on a European Level (GOSPEL) research network.

'We started to look at biological olfaction to see what we can learn from nature, not only in terms of absolute sensitivity and selectivity to single compounds, where natural systems are still perhaps six orders of magnitude ahead of what technological systems can do, but also what we can learn about how nature distinguishes single species [of odorant gas molecules] from very complex mixtures which might be dominated by another gas species,' said Rebecca Simpson, director of AO Action, a GOSPEL spin-out.

'For the single species, current technology is in the range of parts per million [PPM] and we have been able to reach parts per billion with the mouse urine.' PPM refers to the number of odorant molecules in a million of every other molecule in a sample.

MUPs are part of a protein family with a stable, cage-like structure that traps odorant molecules and then releases them slowly. This is what mice use to mark their territory, and is also what inspired the artificial olfaction researchers to explore other uses of MUPs, using a synthetic version of mouse urine in the experiments. According to the scientists, mice secrete a relatively large amount of protein — up to 40mg per millilitre.

'Other types of urine can also be used, but mice are particularly accessible and there is quite a lot already known about their urine,' said Simpson.

'But these proteins exist in most animals, not just mammals. insects have them too, where they are no urinary proteins. They are part of a bigger class of proteins — odorant binding, which are the proteins required.'

Simpson said the stability of MUPs, which allows it to release odorant molecules slowly, has other benefits for it as a biosensor.

'because MUPs are stable, you get a relatively long lifetime from them,' she said. 'The problem with conventional biosensors is that the cell has to be kept alive for a given period, and these proteins are inherently pretty stable, so they have good potential for being in a sensor for years rather than days.'

To use the proteins as a sensor, they need to be attached to a transducer so any changes in the odorant-binding protein can be measured when it captures an odorant molecule. When the MUP binds with an odorant molecule, by depositing it onto a quartz microbalance sensor, the scientists have already found that its mass increases. They suspect that the MUPs can also change in other ways, however.

'We also think it will change the electrical properties, so there is a potential to use it in conductor sensors as well, so you can use a variety of different transducer types to read out the change and to detect that it has captured an odorant molecule,' said Simpson.

'From a sensor developer's point of view, a material like that can be easily deposited on different transducer platforms, allowing you to attack a number of different applications. Different transducers have different inherent properties that make them more suitable for one type of measurement than another; some require a very low power, for example.'

Simpson expects the technology to reach commercialisation in the next couple of years, in the form of a cigarette box-sized biosensor suitable for air-quality or water-quality monitoring, since they work directly in water as well as in the air directly above it.

'The good thing about these is we can get them to work on known transducer platforms, such as quartz microbalances,' said Simpson. 'What we are talking about is the sensitive layer you put on top of the transducer, which means it has the potential for very quick commercialisation.' She admitted that, because of longer validation processes, the security applications could take longer.

The gas species on which the new biosensor has been tested so far are 2-isobutyl-3-methoxypyrazine, 3,7-dimethyl octanol and lauric acid.