• Universitat de València

• Ana Isabel Argente García 
   

Volatile sulphur compounds such as hydrogen sulphide (H2S), methyl mercaptan (CH3SH) and dimethyl mercaptan ((CH3)2S) are toxic gases, harmful to the environment and health. The largest amounts of hydrogen sulphide and other volatile sulphur compounds are produced as a consequence of industrial activities such as oil/natural gas processing and refining, wastewater treatment plants, landfills, piping, etc. They can also be found in low concentrations in biological samples such as breath, being responsible for halitosis, also known as bad breath.

For the determination of volatile sulphur compounds in situ and in real time, using active samplers such as commercial colorimetric tubes is a possibility. This type of sampling requires an external energy source, which will imply additional energy and economic consumption due to the use of specific equipment. Although passive samplers are also available, their application is limited, since they present drawbacks such as excessively high detection limits, low reproducibility and in some cases toxicity by the used reagents. In the specific case of halitosis detection, a heliometer is commonly used. It is a portable instrument that quantitatively measures volatile sulphur compounds found in the mouth, but is inaccessible for consumers due to its high cost.

Researchers at the Universitat de València have developed a passive colorimetric sensor for the in situ detection of volatile sulphur compounds with detection limits of 45 ppb, making it possible to use it for the detection of sulphur compounds in real atmospheres. The sensor is passive and therefore it requires no pre-treatment, no power supply or external instrumentation. In addition, it is environmentally friendly, stable over a wide temperature range, stable against humidity and solar radiation, and resistant to reversion, so that the sensor response remains stable over time. The sensor can be employed for the determination of volatile sulphur compounds in any type of matrix where the formation of this type of compounds are present or generated and allows in situ, real-time monitoring with good sensitivity. Moreover, the sensor presents an adequate selectivity since it has been observed that other volatile compounds such as amines, ethanol or acetone do not interfere. Once the sensor is coloured, the colour intensity can be monitored by several methods: i) visual inspection, ii) diffuse reflectance absorbance measurement, and iii) digital analysis of the red, green and blue (RGB) colour values of sensor’s photograph obtained by a recording or image capture device such as a mobile phone.

The developed sensor is applicable as a point-of-need method for the detection of bad breath that may be related to periodontitis or gingivitis and in environmental monitoring systems for the detection of hydrogen sulphide in critical locations (wastewater treatment, landfills, drains, pipes, oil processing, etc.).

The main advantages provided by the invention are:

• Simplicity and easiness of use: it is a passive colorimetric sensor that does not require any kind of pre-treatment, power supply or external instrument.
• Low cost: simple manufacturing process without high costs.
• Low detection limits: in the order of 45 ppb v/v.
• Stability: the sensor remains stable for a period of 3 months. Reversion resistance.
• Quantitative detection: quantitative detection can be carried out by direct measurement of the diffuse reflectance of the sensors.
• In situ and real-time monitoring: sensor response is obtained in just 10 minutes.

• Patent granted