The measurement of the concentration of oxygen is important in a wide range of environmental and industrial contexts and requires sophisticated scientific instrumentation. These instruments are expensive and require specially trained operators to ensure they function correctly. Colorimetric sensors are available, but provide only low resolution quantitation, or demand complex data processing to extract meaningful data on oxygen concentrations. Now, researchers from China1 have developed an oxygen sensor strip which combines the oxygen sensitivity of a metal-porphyrin-based oxygen probe with the photoluminescence of cadmium telluride quantum dots to allow straightforward higher resolution determination of oxygen concentrations by the naked eye or using a camera.

Fig. 1: Top: A schematic of the construction of the sensing strip with electron micrographs of the final structure. Bottom: Photoluminescence of the oxygen sensor (the “oxygen ruler”). The top line of dots shows output without the green photoluminescence of the quantum dot network, while the bottom line shows emission with quantum dot network.

The metal-porphyrin probe emits red but the emission is quenched when exposed to increasing levels of oxygen, thus it is difficult to differentiate the color change at high oxygen concentrations. So the researchers decided to introduce a background green photoluminescence using quantum dots. This created a transition from red to green emission at high concentrations of oxygen which could be more easily identified (Fig. 1).

This was achieved by laying a uniform crosslinked network of core-shell quantum dots of cadmium sulphide on a glass substrate on top of which an organosilica matrix was deposited containing a uniform distribution of the metal-porphyrin molecules— an even distribution of the components of the sensor was important to ensure accurate quantitation (Fig.1). As the oxygen concentration increased, the sensor changed colour from red to green in a linear relationship on exposure to only a single wavelength of excitation. Oxygen concentrations could be resolved by eye or CCD camera to levels of 0.5 %.

The sensor was stabilized by coating the quantum dots with a cadmium sulphide shell and depositing them in a rigid matrix to prevent aggregation and oxidation. Temperature affected the accuracy of very high resolution oxygen sensing, but the sensor could be calibrated to adjust for this, the authors say.

And, appealingly, the luminescence could be recovered by placing the sensors in an oxygen-free atmosphere so the sensor can be re-used. Based on these findings, colorimetric glucose, heavy-metal-ion, pH, temperature and other gas sensing with impressive resolution could be achieved for daily life usage.