D3.3 - Colorimetric Gas Sensors for the Detection of Am¬monia, Nitrogen Dioxide and Carbon Monoxide: Current Status and Research Trends

Abstract

Colorimetric detection of gases has been used for years. Colorimetric gas sensors are based on the change in color of a chemochromic reagent incorporated in a porous matrix. Several products are available on the market, such as the well established Drager gas tubes [1]. When the gas is sucked in the tube a color change occurs and its concentration can be evaluated on a reference scale. This publication focuses on the state-of-the-art of colorimetric gas sensors and current developments for the detection of ammonia, nitrogen dioxide and carbon monoxide. For fire detection, the detection of particularly CO and NO2 as leading substances is of high interest. In special applications CO-sensors are already used to support conventional optical or heat detectors for fire detection. Although CO is a leading gas in many fire events, there are other fire events emitting mainly NO2 (for example polyurethane or n-heptane fires). Gas sensors for the detection of CO or NO2 are available on the market, yet are too expensive, the dimensions are too big or the long-term stability is too poor. Additionally the power consumption of most suitable technologies is much too high. We present our work on colorimetric films combined with a MEMS-compatible low-power technology for the CO and NO2detection in order to overcome these drawbacks of the existing technologies. Motivation The advantage of colorimetric gas sensors is their high selectivity to only one gas. The selectivity is caused by a chemical reaction between gas and dye. The reaction depends on the chromogenic material. For the detection of ammonia, pH indicators like bromophenol blue or bromocresol purple can be used. In this case, the gas acts as a Lewis-base and induces the color change due to hydrogen release. Other gasochromic materials are complexes. Their color chance is induced through changes in the ligand field. The most common gas sensors working with the gasochromic principle are the so-called Drager gas tubes. These packed glass tubes are used by fire departments in case of an unknown fire. The endings of the tube get bricked away to pump gas inside. The concentration of the detected gas can be easily read from an imprinted scale on the tube. The main disadvantage of these sensors is that always an operator is used to read out the exact gas concentration. The work presented here offers the possibility of a standalone colorimetric gas sensor system. A simple measurement set-up enables an automatic measurement. The sensor is based on a planar optical waveguide. In the easiest case this can be a microscope slide. The light of an LED is coupled into one end of this waveguide and passes through it under the conditions of total inner reflection (TIR). After decoupling at the opposite side, the light is focused on a photo detector. The dye can be deposited onto this waveguide by dipor spin coating. To obtain homogeneous films, the dye is embedded into a polymeric matrix. These polymers should offer the following properties: long term stability, no swelling due to humidity and minimal interference with the dye. Possible polymers are poly vinyl chloride (PVC), ethyl cellulose (EC) and/or poly methylmethacrylate (PMMA). The color change of the dye, due to gas exposure, leads to changes in the evanescent field on the waveguide surface. These changes can be directly detected by changes in the output voltage of the photo detector. The working principle of this sensor is shown in Figure 1. D3.3 S E N S O R + T E S T C o n f e r e n c e s 2 0 1 1 S E N S O R P r o c e e d i n g s 5 6 2 A picture of the developed measurement chamber is shown in Figure 2. Figure 2: Set-up with gas cell for the gas measurements in a waveguide configuration. Colorimetric ammonia sensor Ammonia is for example produced during the putrefaction of perishable goods like seafood, and is present in chemical compounds such as explosives or fertilizers. Colorimetric detection of ammonia might be performed by different types of chemochromic reagents such as porphyrin-based or pH indicator-based films. We present the evaluation of colorimetric films for the detection of ammonia by using pH-indicators. They were dissolved in a proper amount of solvents to obtain spin-coatable subμm thin films. They have been successfully deposited onto glass waveguides to evaluate their optical properties and gas sensing operation. The influences of the matrix composition and the pH indicator Colour change Wave guide Colour dye