I recently read an article in the European journal Angewandte Chemie (“Applied Chemistry”) – which is one of my favorite science journals – which described a new method of analyzing samples for the presence of cocaine. The detection of trace amounts of illegal drugs is a an important part of forensic chemistry. Chemists have techniques such as HPLC-MS (high pressure liquid chromatography – mass spectrometry) which can find and quantify the smallest amount of an unknown sample. However, those tests take time, and they require a highly skilled operator and an expensive piece of lab instrumentation. Police officers and federal agents need to have a rapid, inexpensive, easy-to-use test for cocaine which they can take into the field and use to screen suspects and objects in a crime scene.
With that in mind, the article that I read discussed a particular type of hydrogel that exhibited a vivid color change upon exposure to small amounts of cocaine. Hydrogels consist of long polymeric molecules which become entangled in a water solution, which traps the water molecules inside a three-dimensional network; Jello is a perfect example. In the case of this particular research article, the polymer chains were decorated by short snippets of amino acids. The particular amino acid sequence was coded to be sensitive to presence of cocaine. This type of molecular design is possible because short amino acid chains (known as peptides, or in this case, aptamers) can be synthesized from scratch in the laboratory. The process is completely automated – chemists just input the sequence that they desire into the amino acid synthesizer, and computer controls do all of the chemical processing.
The water solution that gels in the presence of the aptamers is tained a dark blue with a combination of starch (amylose) and iodine. The iodine forms a complex with the starch molecules and turns a blue color. As the final preparation step, the chemists dosed the gel with amylase enzymes: tiny catalysts isolated from Nature which, when freed from the gel, can chew up the starch molecules. So we have polymer strands that connectd to aptamer molecules, aptamers which bind to cocaine. These strands are dissolved in water which contains amylose enzymes and an amylose:iodine complex. This gel is stable and can be stored in tiny disposable one-use vials.
When it comes time to use the test (maybe at a crime scene, or at a jail processing center) the vial is opened and a small sample of the unknown material is added to the vial. It could be a tiny fragment of a suspects clothing, or a particle of a suspicious powder found in a suspects possession. If the unknown sample is not cocaine, the material will simply wash around in the gel and nothing will happen. The amino acid molecules on the polymer chain are completely selective for the shape and electric charge of the cocaine molecule. A sample which doesn’t match these exact parameters will not trigger any change in the gel. However, if the unknown sample does contain cocaine – even the tiniest amount of cocaine, an amount far too small to even be seen without a microscope – then a dramatic change does take place. The cocaine molecule begins to interact with the aptamer chains. As the amino acids bind to the cocaine, they release their hold on the polymer chains to which they were originally bound. Without the aptamers providing structure and support, the hydrogel collapses. The amylase enzyme is freed to interact with the starch:iodine complex, and since an amylase is specifically designed to consume starch molecules, the deep blue color of the starch:iodine complex disappears.
This process sounds rather complicated but all of the work is done during the preparation of the gel, work that is done by trained chemists in a laboratory setting. Once the gel is in the hands of police officers, all that needs to be done is to add a sample of material to the vial and then gently shaking it for five to ten minutes. If the starting blue color disappears, cocaine is present; if not, no cocaine is present. While quantification of the amount of cocaine requires a more detailed test, the gel vials serve as a “prescreen”. If a positive test results, chemists can follow up with a longer, more accurate test in a forensics laboratory.
This technology should help law enforcement to rapidly test for the presence of illegal drugs. Cocaine isn’t the only molecule that can interact with aptamers. The amino acid sequence can be modified to test for all manner of molecules. Drug dealers now have one more thing to worry about.
The source of this article can be found at: http://www3.interscience.wiley.com/journal/123243828/abstract