Kerstin Hans and Markus W. Sigrist
The research in our group is part of the project IrSens in the nano-tera network. One goal of the project is to design a laser-based sensing plat form for substances of interest in liquids. As an example a compact and easy-to-use sensor for detecting cocaine in saliva is being developed.
Since their development lab-on-a-chip-sensors have become powerful tools in diagnostics. They are of special interest because of their low costs and quick results. Micro fluidic diagnostic fulfils the need for small sample volumes and can be realized with minimal equipment. These are the conditions encountered in developing countries or in the field (e.g. concerning police or army day-to-day work) .
Very often blood or urine is used in these tests, but now a new biological matrix is gaining more attention. Many indicators for drugs or sicknesses can be found not only in blood but also in saliva. Using saliva as a matrix offers the opportunity to use ”non-expert” staff for a diagnosis based on a lab-on-a-chip-sensor.
Researchers concentrating on drug detection have chosen different approaches. However, most of the commercially available tests are immunoassay tests [2,3]. Immunoassays use the bonding between Antigens (substance of interest) and specially patterned antibodies which are localized on a surface to indicate the presence of a drug or its metabolite. Some of the commercially available products need sample preparation as well. As a consequence staff who use this approach need to be trained carefully to ensure the right outcome of the test . Still, many of those immunoassays show potential for improvement when it comes to reliability .
Our approach is to develop a test that is based on a combination of mid-infrared spectroscopy and micro fluidic diagnostic technologies. Drugs like cocaine have very defined absorption lines in the mid-infrared. This fact will very likely enable a user to determine the concentrations of the drug or a metabolite in saliva.
Saliva is a human fluid that changes on various conditions (cf. ). Hence, the first step is to determine the influence of food, drinks and mouthwashes on the infrared spectra of saliva samples. In addition various substances that might occur in saliva need to be checked for interferences with the possible detection ranges for cocaine.
The most promising method tested so far is attenuated total reflectance (ATR) spectroscopy. The basic principle involves the generation of an evanescent field by total reflectance in a crystal (cf. figure 1). The dampening or attenuation of this evanescent field by the absorption of the sample under investigation is detected by recoding the ratio between the exiting light intensity with and without sample.
ATR is mainly used to analyse strongly absorbing substances as the penetration depth into the sample is small (e.g. water in the mid-infrared 0.3 - 2 μm).
To finally obtain the absorption of a substance, the different penetration depths of the evanescent wave into the sample need to be taken into account. They depend on the wavelength used for the analysis, the refractive index of the sample and the refractive index of the crystal .
The absorption of cocaine in the infrared spectral range is very characteristic. Unfortunately, water absorbs strongly in the same spectral range. Therefore, narrow spectral ranges where cocaine absorbs well and the water absorption is not too strong were chosen. Thereafter, possible interference with substances that can be found in saliva were analyzed. As an example the absorption spectrum of glucose is also depicted in fig. 2 (cyan). It is clear that detecting cocaine in the spectral range from 1000 cm−1 to 1100 cm−1 would be rather difficult since the interference with the glucose absorption is strong.
Further measurements are made to study the spectral interferences with numerous substances that might occur in saliva.
Saliva under different circumstances as well as masking substances, diluents, adulterant and common medicine (like pain killer and birth control pill) were studied .
This research is supported by the Swiss National Science Foundation and the ETH Zurich. It is part of the nano-tera project IrSens. For more information about the collaboration partners please visit www.nano-tera.ch/projects/80.php
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