Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2004, 43 (16), 2158—2161
Needle in a Haystack
Magnetic-bioelectronic protein diagnosis is super-sensitive and has potential for use on lab-on-chips
The selective identification and quantification of proteins is a fundamental requirement for medical diagnostics as well as biomedical research and the development of new drugs. Researchers at New Mexico State University in the USA have now developed an ultra-sensitive immunological protein test based on a DNA label that may also be suitable for lab-on-a-chip applications.
Like many current immunological protein tests, this test is based on the binding of the desired protein to two antibodies directed against it. The first antibody fishes the protein out of the sample and the second marks it, making later detection possible. In order to make such a test extremely specific and sensitive, all disruptive components of the sample and excess marker must be very effectively removed. In addition, a good "amplifier" is required in order to produce a detectable signal from the extremely small quantities of the protein present. A team headed by Joseph Wang has solved both of these problems in a clever way.
Here’s how it works: antibody number one, the "grabber", is bound to tiny magnetic spheres. The protein to be detected sticks to it like glue. In the subsequent washing process, these complexes are magnetically removed from all the other components of the sample. Now the second antibody, the marker, comes into play, attaching itself to the magnetic sphere/antibody/protein complexes. This second type of antibody is coupled to tiny plastic spheres , which are also loaded with a large number of short DNA strands, acting as the tag and providing a dramatic signal amplification. These strands contain exclusively the nucleotide base guanine. Unbound marker is then thoroughly removed in another magnetically assisted washing step. Now, the detection can finally take place: the DNA strands are separated from the plastic spheres and then the individual guanine bases are released from the DNA strands. Guanine can be very accurately detected by electroanalytical methods. When guanine molecules are adsorbed onto an electrode, its electrical properties are altered. It is thus possible to attain detection limits of two picograms (1 pg = one trillionth of a gram) of the sought-after protein per milliliter.
The nucleotide base adenine is also easily detectable by electroanalytical methods—and gives a signal different from that of guanine. It is even possible to identify markers consisting of different DNA strands with predefined ratios of adenine to guanine. It would thus be possible to detect multiple proteins in parallel.