Detection units and methods for detecting a target analyte

1. A method of detecting a target analyte in a sample, the method comprising:

a) providing a complex formed from:

i) a first probe coupled to a particle and bound to said analyte if present, and

ii) a second probe coupled to a solid support and bound to said analyte if present, so that if the target analyte is present in the sample, the particle is indirectly coupled to the solid support via a complex formed by the target analyte and the first and second probes, and wherein the complex comprises an elongated region; and

b) either i) applying a force to the indirectly coupled particle or to the complex comprising the elongated region and measuring the amount of particle displacement or the length of the complex, wherein the amount of displacement or length of the complex indicates whether or not the target analyte is present in the sample, or ii) measuring the Brownian motion of the indirectly coupled particle, wherein the amount of Brownian motion indicates whether or not the target analyte is present in the sample.

2. The method of claim 1 , wherein the first probe comprises a first antibody that binds to the target analyte and the second probe comprises a second antibody that binds to the target analyte.

3. The method of claim 1 , wherein the first probe comprises a first nucleic acid that hybridizes to a first region of the target analyte and the second probe comprises a second nucleic acid that hybridizes to a second region of the target analyte.

4. The method of claim 3 , wherein the target analyte is a nucleic acid, and the first and second probes bind to locations on the target that are at least about 500 nucleotides from each other.

5. The method of claim 1 , wherein the elongated region is located on the first probe, the second probe, or the target analyte, or any combination thereof.

6. The method of claim 5 , wherein the elongated region comprises a biomolecule or a non-biological polymer.

7. The method of claim 6 , wherein the biomolecule comprises an elongated nucleic acid, polysaccharide, polypeptide, or combinations thereof.

8. The method of claim 5 , wherein the elongated region comprises a polyethylene oxide (PEO), polyethylene glycol (PEG), poly(N-isopropylacrylamide) (PNIPAM), polyacrylamide (PAM), polyvinyl alcohol (PVA), polyethylenimine (PEI), polyacrylic acid, polymethacrylate or polyvinylpyrrolidone (PVP), or combinations thereof.

9. The method of claim 1 , which further comprises the step of coupling the first probe to the particle.

10. The method of claim 1 , which further comprises the step of coupling the second probe to the solid support.

11. The method of claim 1 , which further comprises the steps of exposing the sample to the first probe and exposing the sample to the second probe.

12. The method of claim 11 , wherein the further steps are performed in either order, or simultaneously.

13. The method of claim 12 , further comprising a washing step after one or more of the steps.

14. The method of claim 13 , wherein the first and second probes are exposed to the sample to form a solution; the particle is then added to the solution; the solution containing the particle is then added to a capillary to which the second probe binds to form a bound complex; the bound complex is washed; a first position of the particle is determined in the presence of an applied force or in the absence of an applied force; a second position of the particle is then determined in the presence of a force applied in a different direction or optionally in the absence of an applied force if the particle position was determined before in the presence of an applied force; and the particle displacement is determined from the first and second positions.

15. The method of claim 1 , wherein the force applied to the particle is a magnetic force, a fluid buoyancy force, fluid drag force, mechanical force, electrical force, centrifugal force or a gravitational force.

16. The method of claim 1 , wherein the force applied to the complex comprising the elongated region is generated with fluid flow, a receding meniscus, or a voltage gradient.

17. The method of claim 1 , wherein the elongated region ranges from about 0.15 μm to about 20 μm in length.

18. The method of claim 1 , wherein more than one target is detected in a single assay, and wherein the targets are differentiated based on different particle displacement or amount of particle Brownian motion they produce once the complex is bound to the solid support.

19. The method of claim 1 , wherein at least one probe comprises an elongated region that has at least one internal loop that can be released or formed, and wherein releasing or forming the loop changes the amount of particle displacement.

20. The method of claim 19 , wherein a change of force, or ionic strength, or temperature, or pH is used to release or form the internal loops of the probes.

21. The method of claim 19 , wherein an auxiliary molecule is used to release or form the internal loops of the probes.

22. The method of claim 1 , wherein more than one target is detected in a single assay, and wherein at least two different second probes are used which are coupled at different locations to the solid support before they bind to their targets, and wherein the targets are differentiated based on the location where the complex binds the solid support.

23. The method of claim 1 , wherein the length or diameter of the particle ranges from about 0.3 μm to about 20 μm.

24. The method of claim 1 , wherein the particle is a magnetic particle.

25. The method of claim 24 , wherein the magnetic particle is superparamagnetic.

26. The method of claim 1 , wherein the particle is fluorescent.

27. The method of claim 1 , wherein the Brownian motion of the particle is measured in step b).

28. The method of claim 1 , wherein the displacement of the particle is measured using an imaging system with a lens, or with a lens-free microscope, or with a coherent imaging technique.

29. The method of claim 1 , wherein the target analyte is a nucleic acid molecule.

30. The method of claim 29 , which further comprises controlling the temperature of the sample to produce denaturation of double stranded nucleic acids in the sample and/or specific hybridization of nucleic acids in the sample to the first and second probes.

31. The method of claim 29 , wherein the sample is initially treated with an exonuclease enzyme to convert double stranded nucleic acids into single stranded nucleic acids.

32. The method of claim 1 , wherein the target analyte comprises a protein, carbohydrate, lipid, hormone, steroid, toxin, vitamin, hapten, metabolite, drug or a combination thereof.

33. The method of claim 1 , wherein the measurement of step b) is made with respect to multiple particles.

34. The method of claim 1 , wherein the concentration of the target in the sample is estimated from the number of particles that have a particular displacement or Brownian motion.

35. The method of claim 1 , wherein step b) comprises applying a twisting or rotational force to the indirectly coupled particle or to the complex, then measuring the amount of coiling in the complex.

36. A method of detecting a target analyte in a sample, the method comprising:

a) providing a complex formed from:

i) a first probe coupled to a particle and bound to said analyte if present, and

ii) a second probe coupled to a substantially flat solid support and bound to said analyte if present,

so that if the target analyte is present in the sample, the particle is indirectly coupled to the solid support via a complex formed by the target analyte and the first and second probes, and wherein the complex comprises an elongated region; and

b) applying a force to the indirectly coupled particle using a force with a component parallel to the solid support that removes non-specifically bound particles, wherein the presence of the particle after the application of force indicates the presence of the target analyte in the sample.