Water is essential for life. Nevertheless, even small amounts of water in the wrong places – such as fuels, lubricants, or organic solvents – can cause motors to sputter, metal parts to rust, or chemical reactions to go awry. That's why one of the most common lab tests performed in industry is one that looks for traces of water in other substances, even though the test itself is complicated and time-consuming.
A new method for detection and measurement of small amounts of water, developed in the lab of Dr. Milko van der Boom in the Weizmann Institute's Organic Chemistry Department, might allow such tests to be performed accurately and quickly. Van der Boom and postdoctoral fellow Dr. Tarkeshwar Gupta created a versatile film on glass that is only 1.7 nanometers thick. The film can measure the number of water molecules in a substance even when it contains only a few parts per million.
"The problem," says van der Boom, "is that water is hard to detect and to quantify." His method is a departure from previous sensing techniques. In general, such sensor systems are based on relatively weak but selective "host-guest" interactions. In the Weizmann Institute team's sensor, metal complexes embedded in the film steal electrons from the water molecules. When the number of electrons in the metal complexes changes, so does their color, and this change can be read optically. Devices based on optical readout do not need to be wired directly to larger-scale electronics – an issue that's still a tremendous challenge for much of molecular-based electronics.
The test can be done in as little as five minutes, and the molecular film can be returned to its original state by washing it with a simple chemical. The film also remains stable, even at high temperatures and with repeated use. In addition, it can be deposited in an inexpensive, one-molecule-thick layer on glass, silicon, optical fiber, or plastic. The ease and low cost of fabrication may also make such films ideal for one-time use. Testing for water in fuel or solvents might become as simple as checking chlorine levels in a swimming pool. Optical detection and quantification by electron transfer could potentially work for numerous substances other than water. The scientists are now exploring the possibility of adapting the method to testing for trace amounts of materials or substances such as specific metal ions or gasses.