If there is too much water in fuel, it won’t burn properly. If oil holds too much water, it may not lubricate machinery effectively.

A team from the Autonomous University of Madrid (UAM) built a color-changing sensor that can detect the presence of water to protect oil integrity, as well as support a variety of additional applications, from food-quality control to environmental monitoring.

The plastic-composite sensor changes its color from purple to blue in wet conditions.

The researchers, led by Pilar Amo-Ochoa from the Autonomous University of Madrid (UAM), used an X-ray light source to understand how water triggers the structural – and color-specific – changes within the material.

The transformation relies upon “copper-based coordination” – a compound of water molecules bound to a central copper atom.

Using high-energy X-rays from PETRA III – a research light source located at Germany’s largest accelerator center DESY – the scientists saw that heating the sample to 60 degrees Celsius removed the water molecule from the copper atoms, changing the color.

When dried, either in a water-free solvent or by heating, the material returned to its purple hue.

The scientists mixed the copper compound into a 3D printing ink and printed sensors in several different shapes, which they tested in air and with solvents containing different amounts of water.

Tests demonstrated that the printed objects are even more sensitive to the presence of water than the compound by itself, due to the material’s porous nature.

In solvents, the printed sensors detected 0.3 to 4 percent of water in less than two minutes. In air, the sensors detected a relative humidity of 7 percent.

According to the scientists’ report in the journal Advanced Functional Materials, the sensor is stable even over many heating cycles, and the copper compounds are evenly distributed throughout the printed sensors. Additionally, the material is stable in air over at least one year and also at biological relevant pH ranges from 5 to 7.

Michael Wharmby, who conducted the X-ray diffraction measurements, told Tech Briefs how the team’s discovery could lead to new ideas in 3D-printable functional materials.

Tech Briefs: How is the sensor material able to change color?

Michael Wharmby: The sensor material itself is a coordination polymer, a compound consisting of organic molecules bonded to, in this case, copper atoms. The compound forms a 1-dimensional chain (on a molecular scale).

Water molecules also coordinate the copper atoms, and it's the presence or absence of these which determines the color of the material. They attach and detach depending how much water is in the surrounding environment.

The sensor material can be printed in various forms. The workpiece shown here is about one centimeter wide. In the presence of water, for example from the air humidity, it turns blue. (Image Credit: UAM, Verónica García Vegas)

Tech Briefs: What does the sensor look like?

Michael Wharmby: The sensor material itself is a powder. It was mixed, however, with monomers to make 3d printing ink in a 10% concentration, where Digital Light Processing was used to polymerize the ink. The actual sensor can be any shape you want (with 200 µm accuracy – a limit of the printer).

Tech Briefs: Why is understanding the presence of water so important?

Michael Wharmby: Chemists often want to exclude water from organic solvents – like ethanol or ether, for example – as water may change the way a chemical reaction takes place in a solvent.

This is also important on industrial-scale production, so having an easy test for whether a solvent is good enough to use is important.

Water contamination in oils used in lubricating machines is also a big problem as it can lead to poor lubrication. Likewise, the stability of drug molecules in storage can be affected by the presence/absence of water in their packing. A lot of time and effort is also expended in long-term storage of things in dry environments. For example, some of the U.K.'s national archive is stored in salt mines due to their inherent dryness.

Tech Briefs: Where do you envision this sensor being used, and being most valuable?

Michael Wharmby: A key feature of the sensor material is that it has a low toxicity and is stable at physiological pH, which suggests an application in biologically related devices – for example, monitoring the moisture in a packet of a drug.

From my (more chemical) perspective, it would be interesting to use this as a simple test, for example, in a large chemical plant, to check that a reaction is proceeding as expected. Maybe water is produced by an undesirable side reaction.

Similarly, you could glue a monolith of this onto the side of a box before you put it into storage to see that the moisture level is OK.

Tech Briefs: What is most exciting to you about the sensor and its application possibilities?

Michael Wharmby: The presence or absence of water is really important across a huge range of fields. A simple visual indicator is crucial to having an easy-to-use sensor, and that color change by itself is very cool. Being able to shape this sensor into any form you want using 3d printing, however, means you could have a regeneraable moisture/water sensor of any shape for whatever application you want. It just seems like something which should be of interest across many different fields. For me as a chemist, this is really cool, as it's a complete story of the synthesis of a material, understanding its function through to building it into a usable device.

Where do you envision this sensor being used? Share your comments and questions below.