Microscale diffusion immunoassay
Methods and apparatuses are provided for determining presence and concentration of analytes by exploiting molecular binding reactions and differential diffusion rates. Analyte particles and binding particles are allowed to diffuse toward each other, and slowing of the diffusion front is detected when they meet. From the position of the diffusion front, presence and concentration of analyte particles can be determined. One embodiment provides a competitive immunoassay in a microfluidic format. This diffusion immunoassay (DIA) relies on measuring the concentration of labeled antigen along one dimension of a microchannel after allowing it to diffuse for a short time into a region containing specific antibodies. A simple microfluidic device, the T-Sensor, was used to implement a DIA to measure the concentration of phenytoin, a small drug molecule. Concentrations of analyte over the range of 50 to 1600 nM can be measured in less than a minute. The assay is homogeneous, rapid, requires only microliter volumes of reagents and sample, and is applicable to a wide range of analytes, including therapeutic drugs, molecular biological markers, and environmental contaminants. Methods for separating particles of similar size in a diffusion separator are also provided.
1. A method for determining presence or concentration of sample analyte particles in an analyte fluid, the method comprising:
a) adding additional analyte particles labeled with a detectable marker to said analyte fluid to provide a predetermined concentration or amount of labeled analyte particles in said analyte fluid;
b) providing a diffusion fluid containing binding particles capable of binding to said sample analyte particles and said labeled analyte particles;
c) providing a laminar flow channel comprising an analyte stream inlet and a diffusion stream inlet;
d) flowing said analyte fluid into said analyte stream inlet as an analyte stream, and flowing said diffusion fluid into said diffusion stream inlet as a diffusion stream whereby said analyte and diffusion streams flow in adjacent laminar flow;
e) allowing diffusion between said analyte and diffusion streams;
f) detecting a diffusion profile in said channel formed by said labeled analyte particles;
g) determining from said diffusion profile the presence or concentration of said sample analyte particles.
2. The method of claim 1 wherein said concentration is determined by comparing said diffusion profile to a calibration profile.
3. The method of claim 1 wherein said concentration is determined by calculation.
4. The method of claim 1 wherein said concentration is continuously determined.
5. The method of claim 1 wherein said analyte fluid is whole blood.
6. The method of claim 1 wherein said binding particles are antibodies.
7. The method of claim 1 also comprising providing a reference fluid containing a known concentration of unlabeled sample analyte particles and a known concentration of labeled analyte particles and flowing said reference fluid as a reference stream in laminar flow contact with said diffusion stream.
8. The method of claim 1 also comprising providing an inert separation stream between said analyte stream and said diffusion stream.
9. The method of claim 1 wherein the concentration of binding particles is at least about two-fold greater than the concentration of unlabeled sample and labeled analyte particles.
10. The method of claim 1 wherein the concentration of binding particles is approximately equal to the concentration of unlabeled sample and labeled analyte particles.