Researchers in Germany have developed a water-based fluorescent paint that can be used to simultaneously image pressure and temperature distribution on aeroplanes and cars. The paint is environmentally friendly and easy to clean up. 

Visualising pressure and temperature distribution on aircraft using wind tunnel tests is an important part of aerodynamic research. Pressure or temperature-sensitive paints that produce fluorescent signals are used, but they are based on volatile and toxic organic solvents that require extensive clean up and special protective clothing. Now, Otto Wolfbeis at the University of Regensberg, Germany, and colleagues have developed luminescent water-soluble paints that measure pressure and temperature simultaneously. 

Wolfbeis took fluorescent platinum(II)porphyrin probes, which detect oxygen partial pressure and therefore barometric pressure, and made them soluble in water by impregnating them into water-soluble, oxygen-permeable polymer core-shell nanoparticles. These nanoparticles acted as binder for temperature-sensitive oxygen impermeable polymer microparticles dyed with a luminescent iridium(II)complex that acts as a probe for temperature, but is not affected by pressure as oxygen doesn't reach it through the polymer. 

Spraying the luminescent polymer spheres onto aluminium surfaces creates an even layer with high mechanical adhesion and excellent stability. This surprising strength of adhesion means that no added binder polymer and no toxic solvents are needed, explains Wolfbeis. 

Both indicator probes in the paint can be excited using LEDs or diode lasers emitting at a wavelength of 405 nm and two independent signals are obtained allowing imaging of barometric pressure and of temperature. The resulting sensor paint is environmentally friendly, safer to work with and easy to wash off as well as having remarkable sensing ranges and response, adds Wolfbeis. 

Robert Crutchley, an expert in oxygen sensing thin films at Carleton University in Ottawa, Canada, says, 'dual-sensing technology already exists, but no-one's done it like this before - this is a really innovative way of making pressure-sensitive paints.' 

In the future Wolfbeis plans to improve the probes' response time, which is currently limited by the rate of diffusion of oxygen through the polymer sphere to the probe. 'The automobile industry wants sensors with a response time in the order of milliseconds,' he says, 'currently we can achieve a response time of just under one second.' 

Catherine Bacon

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