Proc. SPIE. 10246, Smart Sensors, Actuators, and MEMS VIII
KEYWORDS: Gas sensors, Cameras, Sensors, Calibration, Glasses, Chemical compounds, Gases, Sensor performance, Time metrology, Biological and chemical sensing, Arsenic, Industrial chemicals, RGB color model
Colorimetric sensors based on color-changing dyes offer a convenient approach for the quantitative measurement of gases. An integrated, mobile colorimetric sensor can be particularly helpful for occasional gas measurements, such as informal air quality checks for bad odors. In these situations, the main requirement is high availability, easy usage, and high specificity towards one single chemical compound, combined with cost-efficient production. In this contribution, we show how a well stablished colorimetric method can be adapted for easy operation and readout, making it suitable for the untrained end user.
As an example, we present the use of pH indicators for the selective and reversible detection of NH3 in air (one relevant gas contributing to bad odors) using gas-sensitive layers dip coated on glass substrates. Our results show that the method can be adapted to detect NH3 concentrations lower than 1 ppm, with measure-to-result times in the range of a few minutes. We demonstrate that the color measurements can be carried out with the optical signals of RGB sensors, without losing quantitative performance.
Bad indoor environment is often the reason for health impairment of people who spend most of their time indoors.
Modern buildings are almost air tight and air exchange is too low. This problem often occurs in retrofitted buildings. A
long time result can be mold formation in buildings. To get early information about bad indoor climate or mold
formation, sensor systems which detect volatile organic compounds (VOC) are needed. The biggest challenge in
measuring VOC gases in this scenario are the small concentrations. We present a miniaturized preconcentrating gas
sensor system with two chambers for measuring organic gases. Preconcentration is realized with a thermoelectric
element to activate sampling and desorption process in one chamber, delivering temperature gradients to a highly porous
surface. The second chamber consists of a gas detecting element to indicate the preconcentrated VOC. By driving a
temperature cycle with longtime cooling and fast heating the gas is preconcentrated and then desorbed quickly.
Furthermore an electronic circuit board has been developed to control the complete system. The result is a complete
sensor system with mechanical setup, electronic control, measurement, analyzation and peripheral communication.
Measurements regarding temperature behavior of the system are performed, as measurements with VOC.
We present the development and characterization of a fiber-optic colorimetric gas sensor combined with the electronic
circuitry for measurement control and RFID communication. The gas sensor detects ammonia using a 300 μm polyolefin
fiber coated with a gas-sensitive polymer film. The spectral and time-dependent sensitivity of various polymer films was
tested in transmission measurements. Light from a standard LED at λ = 590 nm was coupled into the polyolefin fiber
through the front face. A prototype of the gas sensor with the direct coupling method was tested under realistic
measurement conditions, i.e. battery-driven and in a completely autonomous mode. The sensor system showed good
sensitivity to the ammonia concentrations and response times in the order of minutes. The achievable power
consumption was below 100μW.The films contained the pH-sensitive dyes bromocresol purple or bromophenol blue
embedded in either ethyl cellulose or polyvinyl butyral, and optionally tributyl phosphate as plasticizer. The
bromophenol blue based films showed a strong reaction to ammonia, with saturation concentrations around 1000 ppm
and response times of about 15 seconds to 100ppm. The colorimetric reaction was simulated using a simple kinetic
model which was in good agreement with the experimental results.
Infrared spectroscopy uses the characteristic absorption of the molecules in the mid infrared and allows the determination
of the gases and their concentration. Especially by the absorption at longer wavelengths between 8 μm and 12 μm, the so
called "fingerprint" region, the molecules can be measured with highest selectivity.
We present an infrared optical filter photometer for the analytical determination of trace gases in the air. The challenge in
developing the filter photometer was the construction of a multi-channel system using a novel filter wheel concept -
which acts as a chopper too- in order to measure simultaneously four gases: carbon monoxide, carbon dioxide, methane
and ammonia. The system consists of a broadband infrared emitter, a long path cell with 1.7m optical path length, a filter
wheel and analogue and digital signal processing.
Multi channel filter photometers normally need one filter and one detector per target gas. There are small detection units
with one, two or more detectors with integrated filters available on the market. One filter is normally used as reference at
a wavelength without any cross-sensitivities to possible interfering gases (e.g. at 3.95 μm is an "atmospheric window" -
a small spectral band without absorbing gases in the atmosphere). The advantage of a filter-wheel set-up is that a single
IR-detector can be used, which reduces the signal drift enormously. Pyroelectric and thermopile detectors are often
integrated in these kinds of spectrometers. For both detector types a modulation of the light is required and can be done -
without an additional chopper - with the filter wheel.
The work presented here focuses on the investigations of metallo-porphyrins and their gasochromic behavior.
Gasochromic materials change their color while they are exposed to a certain gas. So they offer the possibility to develop
very selective chemical gas sensors. In the focus of this work is the metallo-porphyrin 5, 10, 15, 20-
tetraphenylporphyrin-zinc (ZnTPP). When embedded into a polymeric matrix (PVC) the color change to the toxic gas
NO2 can be detected. During exposure to NO2 the dye changes its color from bright purple to yellow. To develop a standalone
gas sensor, the ZnTPP/PVC matrix is deposited onto a planar optical waveguide. The color change of the porphyrin
dye, due to the gas exposure, can be detected in the evanescent field of the optical waveguide. Therefore the light of a
high power LED is coupled into the waveguide. The color change of the porphyrin is detectable with photodiodes as
variations of the decoupled light intensity. The sensor shows no cross-sensitivities to other gases like CO2, NH3, EtOH,
CO or water vapor. NO2 is detectable with a limit of 1 ppm.
The fast and direct identification of possibly pathogenic microorganisms in air is gaining increasing interest due to their
threat for public health, e.g. in clinical environments or in clean rooms of food or pharmaceutical industries. We present
a new detection method allowing the direct recognition of relevant germs or bacteria via fluorescence-labeled antibodies
within less than one hour. In detail, an air-sampling unit passes particles in the relevant size range to a substrate which
contains antibodies with fluorescence labels for the detection of a specific microorganism. After the removal of the
excess antibodies the optical detection unit comprising reflected-light and epifluorescence microscopy can identify the
microorganisms by fast image processing on a single-particle level. First measurements with the system to identify
various test particles as well as interfering influences have been performed, in particular with respect to autofluorescence
of dust particles. Specific antibodies for the detection of Aspergillus fumigatus spores have been established. The
biological test system consists of protein A-coated polymer particles which are detected by a fluorescence-labeled IgG.
Furthermore the influence of interfering particles such as dust or debris is discussed.
We report on a recently developed highly sensitive instrument for label-free detection of biomolecules based on the principle of a Young interferometer. With this technology, biomolecular interactions can be detected in real-time without elaborate sample preparation using a planar waveguide as sensing element. The binding reaction of the antibody-antigen pair immunoglobulin G and protein G has been studied and an affinity constant of K=2.6*107 M-1 has been determined.
For the measurement of biomolecular interactions such as immunoreactions it is often necessary to prepare reporter molecules to detect small biomolecules. In many cases fluorescence markers are used to detect the binding between molecules. These markers, however, can influence the examined reaction. A label-free optical detection method based on the principle of a Young interferometer offers an alternative solution. This technology allows real-time, kinetic analysis of antigene-antibody reactions or the detection of a specific analyte without elaborate sample preparation. Especially reactions where it is inconvenient or impossible to use markers can be detected with this method. In this paper, an interferometric device based on a planar waveguide as sensing element is presented. The system yields a high resolution with respect to surface mass coverage and a low sensitivity towards undesired external influences. Interferometric sensors theoretically have the highest detection limits among label-free bionsensors.