Biodetectors

Biodetectors, which are used to detect the presence of biological material, can be used in forensics to detect microorganisms or some of their components in material and tissue recovered after death (post-mortem samples).

More specifically, biodetectors are analytical devices that combine the precision and selectivity of biological systems with the processing power of microelectronics. These detectors typically consist of a biological recognition system, usually enzymes or binding proteins immobilized on a surface acting as a physico-chemical transducer. One typical example of a biodetector is the immunosensor, which uses antibodies as the biorecognition system. In addition to enzymes and antibodies, the recognition systems can consist of nucleic acids, whole bacteria and other single-celled organisms, and even tissues of higher organisms. Specific interactions between the target molecule or analyte and the complementary biorecognition layer produce a detectable physicochemical change, which can then be measured by the detector.

The detection system can take many forms, depending upon the parameters being measured. Electrochemical, optical, mass, or thermal changes are the most common parameters providing both qualitative or quantitative data.

In recent years, the emphasis on measures to combat terrorism has led to the development of techniques that could be useful in forensic science. For example, a microarray of fluorescent labeled nucleic acids immobilized on a support has been developed by researchers at Argonne National Laboratory. The intended application for the "bacillus microchip" is the detection of Bacillus anthracis (the anthrax agent). It would distinguish B. anthracis from other related bacteria, such as B. thuringiensis, B. subtilis, and B. cereus and also indicate whether the organism is alive or dead by detecting DNA when there are no RNA matches. However, the same technique could be applied to the detection of other microorganisms in post-mortem samples.

A number of new fast, reliable, and portable DNA detection devices have been developed that can prepare and test samples within a very short time. Devices consisting of cell disruptors, capable of breaking bacterial spores and extracting DNA that is then used to identify the species of organism, are being tried. Some companies have incorporated an automated sample preparation scheme and coupled it with a microfluidic "lab on a chip" device for detecting microorganisms on the basis of their DNA sequence. The system can reduce a laboratory preparation procedure that can take six hours to just 30 minutes. The chip contains tiny channels, valves, and chambers through which milliliters of sample can be pumped and concentrated into a microliter volume. Any bacterial cells are broken ultrasonically and their DNA is extracted, amplified by PCR (polymerase chain reaction) and sequenced.

A DNA-based biochip designed by Northwestern University detects DNA sequences that are specific for pathogenic microorganisms. The chip initially contains very short single strands of DNA between two small electrodes. The DNA strands are complementary to DNA sequences from a specific pathogen. When DNA from that pathogen comes into contact with the chip, it hybridizes with the DNA on the chip. To detect the hybridization, further pieces of DNA are added to the system and these are complementary to the sections of pathogen DNA that have not hybridized. The additional DNA pieces contain gold particles that, on successful hybridization, form a bridge of conducting metal linking the two electrodes. The bridge completes an electrical circuit and triggers a signal.