The test, which is faster than existing whole-blood immunoassays, uses gold nanoshells, tiny optically active gold-coated glass particles that are so small about 700 could fit in the diameter of a human hair.
In laboratory tests at Rice, the nanoshell immunoassay was capable of detecting less than one billionth of a gram of the glycoprotein immunoglobulin G, or IgG, per milliliter of whole blood.
"To our knowledge, this is the first in situ whole-blood immunoassay to report sensitivities on this order in under 30 minutes," said Jennifer West, associate professor of bioengineering and chemical engineering.
Immunoassay technology capitalizes on the characteristic way that antibodies attach themselves to invading pathogens in the body. Antibodies are proteins produced by white blood cells. Their job is to recognize and bind to invading antigens in the body, and they do this with great specificity. Scientists and doctors have discovered numerous ways to harvest these antibodies and use them diagnostically to identify a multitude of different chemicals.
One of these diagnostic applications is the conventional immunoassay, in which a solution that a doctor wants to test, such as blood plasma, is exposed to a tray containing antibodies that bind with a specific antigen under investigation. When the antibodies bind to the antigen, the test changes color. These systems are used to identify and diagnose various conditions ranging from HIV to a heart attack.
Unfortunately, existing technologies haven't produced a fast, reliable whole-blood immunoassay, in part because blood is so viscous and murky that it interferes with the chemical reactions in the test solution and makes it difficult to get accurate readings. Instead, clinicians must purify the blood to remove these contaminants before proceeding with the immunoassay, a time-consuming step that typically takes an hour or more.
The new nanoshell immunoassay was able to overcome these obstacles by coupling antibodies to nanoshells that absorb near-infrared light. The study was conducted by West; nanoshell inventor Naomi Halas, the Stanley C. Moore Professor in Electrical and Computer Engineering and professor of chemistry; and graduate students Leon Hirsch, Joe Jackson and Allen Lee. It is published in the May 15 issue of the journal Analytical Chemistry.
Similar in structure to a hard-shelled chocolate candy, nanoshells are layered colloids that consist of a core of non-conducting material -- silica in this case -- covered by a thin metallic shell. By varying the thickness of the metal shell, researchers in Halas' group can precisely tune the color of light to which the nanoshells respond. Because near infrared light penetrates whole blood very well, it is an optimal wavelength for a whole blood immunoassay.
When the antibody-nanoshell particles are placed into a solution of blood containing the test molecule, the antibody-nanoshells bind to the test molecule, which causes slight changes the optical properties of the nanoshells. By monitoring these changes, Halas and West were able to monitor very slight concentrations of antigens in the blood, without any time-consuming sample preparation.