Later this year, Philips will introduce a handheld electronic device that uses magnetic nanoparticles to screen for five major recreational drugs.

The device is intended for roadside use by law enforcement agencies and includes a disposable plastic cartridge and a handheld analyzer. The cartridge has two components: a sample collector for gathering saliva and a measurement chamber containing magnetic nanoparticles. The particles are coated with ligands that bind to one of five different drug groups: cocaine, heroin, cannabis, amphetamine, and methamphetamine.

Philips began investigating the possibility of building a magnetic biodetector in 2001, two years after a team of researchers at the Naval Research Laboratory (NRL) in Washington, DC, first used magnetic sensors similar to those employed in hard drives to sniff out certain biowarfare agents. The NRL scientists labeled biological molecules designed to bind to target agents with magnetic microbeads, and then scanned for the tagged targets optically and magnetically. The latter approach used the same giant magnetoresistant (GMR) sensors that read the bits on an iPod's hard drive. They quickly developed a shoebox-sized prototype capable of detecting toxins, including ricin and anthrax.

Philips initially developed both a GMR sensor and an optical one that relies on frustrated total internal reflection (FTIR)--the same phenomenon that underlies fingerprint scanners and multitouch screens. The company decided to go the FTIR route in order to exploit its expertise in building optical sensors for consumer electronics devices, says Jeroen Nieuwenhuis, technical director of Philips Handheld Immunoassays, the division responsible for commercializing the biosensor technology, which goes by the trade name Magnotech.

Moving to an optical detection method also allowed Philips to simplify the test cartridges that the device employs, making them easier to mass-produce, says Nieuwenhuis. With the current FTIR-based system, "we can make simpler cartridges in larger quantities more easily," he adds.

Once the device's sample collector has absorbed enough saliva, it automatically changes color and can then be snapped into the measurement chamber, where the saliva and nanoparticles mix. An electromagnet speeds the nanoparticles to the sensor surface, different portions of which have been pretreated with one of the five target-drug molecules. If traces of any of the five drugs are present in the sample, the nanoparticles will bind to them. If the sample is drug free, the nanoparticles will bind to the drug-coated sensor surface instead.

The orientation of the magnetic field that first drew the nanoparticles to the sensor is then reversed, pulling away any nano-labeled drug molecules that may accidentally have stuck to the sensor surface but leaving legitimately bound ones in place. This last magnetic trick promises to reduce what Larry Kricka, a clinical chemist at the University of Pennsylvania who recently co-authored an article in Clinical Chemistry on the use of magnetism in point-of-care testing, calls "a major restraint in such assays": the unintentional capture of molecular labels on the test surface, a leading cause of both false positives and false negatives. Kricka is not involved with Philips but does serve as a consultant to T2 Biosciences, a Cambridge, MA, firm that promotes a magnetic biosensor based on MRI technology.