Portable protein detector diagnoses infectious diseases

A portable protein detector powered by a nine-volt battery could provide a cost-effective solution to diagnosis problems of HIV and other infectious diseases, especially in developing countries where there may not be trained personnel or the infrastructure to support power distribution to large-scale hospital machinery. Dubbed "POCKET" ("portable and cost-effective"), the hand-held instrument uses a combination of silver-based chemistry, a diagnostic chip, and an optical sensor to do the same job as bulky, expensive hospital machinery at a fraction of the time and cost, according to researchers at Harvard University (Cambridge, MA), where the system was developed.

The POCKET immunoassay system demonstrates an analytical performance approaching that of enzyme-linked immunosorbent assays (ELISA) performed in a benchtop format in clinical laboratories. While immunoassays such as ELISA are the most reliable and widely used methods for detecting antigens and antibodies, they require expensive and bulky instruments for optical detection, hours of incubation for diffusion-limited reactions on the surface and many pipetting steps. Such constraints make it difficult to use ELISA in settings that require low-cost or compact equipment and in environments that lack electricity or trained personnel.

A prototype of the device, which has taken 18 months to develop, has been tested at Harvard with the HIV virus. George Whitesides, professor of chemistry at Harvard, lead researcher on the POCKET project, and a pioneer of micro- and nanotechnology, claims that the process is the same for any other infectious disease that can be tested by the system. He also believes that POCKET could be helpful in other applications, such as point-of-care testing and detection of biological warfare agents in the field.

"POCKET consists of a diagnostic chip for immunoassays only a few square millimeters in size, and a small, simple detector unit," Whitesides said. "Current laboratory diagnostic processes are mostly not suitable for developing nations; aside from the high cost and complex equipment, the necessary infrastructure, such as a power supply, is often lacking."

HIV studies

In the HIV studies, a technician pipetted a blood sample into 2-mm channels in the diagnostic chip, which was specially prepared with a "stripe" of protein fragments from the HIV virus (see figure). As the blood moved past the stripe, HIV antibodies in the blood sample recognized and bound tightly to the protein fragments on the stripe. A second type of antibody was then sent through the channel to mark the bound HIV antibodies. These second antibodies, which were coupled to tiny gold spheres, recognized the HIV antibodies and stuck to them.

The next step involves applying a solution of silver nitrate and an oxidizing agent. Where the gold spheres bind to the HIV molecules, the gold catalyzes the oxidation of the silver ions to metallic silver, which deposits onto the walls of the channels. A red laser diode then shines light through the channels and an integrated circuit with a photodetector registers how much the light is diminished by the silver layer. This allows the number of HIV antibodies to be quantified, just as precisely as laboratory methods but much faster. A liquid-crystal display shows the results.

The researchers took a comprehensive approach to the design of the assay by miniaturizing and integrating both the immunoassay and the detection device. The battery-powered optical detector is reusable and, with pulse modulation, it can be operated under field conditions, such as direct sunlight. The analytical performance of this integrated miniaturized device approaches that of ELISA using relatively expensive benchtop equipment. Whitesides says that the components of POCKET can be purchased for a total of $45.

"The chip is produced by 'soft lithography,' a microtechnical method well suited to mass production," Whitesides said. Soft lithography, which involves a series of patterning techniques in which a "stamp" of a nanostructure can be made and then used repeatedly to imprint new structures, produced an inexpensive, miniaturized platform that is compatible with microfluidics.

In addition, detection of silver reduction catalyzed on gold colloids, which has been used to analyze proteins and DNA, offers many advantages, according to Whitesides. It avoids photobleaching problems, and its stability enables results of immunoassays to be kept for long-term use. It also allows the use of a wide variety of laser diodes and photodetectors of any wavelength.