Free-solution response function interferometry

1. An analytical method comprising detecting a molecular interaction product between a first analyte and a second analyte, wherein the detection is performed by refractive index sensing other than backscattering interferometry, or by circular dichroism, wherein one of the analytes is immobilized, and wherein one of the analytes is present in an amount of less than about 1.0×10 −6 M.

2. The method of claim 1 , wherein the detection is performed by refractive index sensing other than forward scattering or side scattering interferometry.

3. The method of claim 1 , wherein both analytes are unlabeled.

4. The method of claim 1 , wherein detecting comprises one or more of:

(a) determining an equilibrium constant, a dissociation constant, a dissociation rate, or an association rate;

(b) calculating a change in hydrodynamic volume, entropy, or enthalpy;

(c) determining the concentration of the first and/or second analyte;

(d) identifying the presence of the first and/or second analyte; and

(e) identifying the presence of a third analyte.

5. The method of claim 1 , wherein refractive index sensing is via a RI detector based on the angle of deviation method of measurement, a RI detector based on the Fresnel method of RI measurement, a Christiansen effect detector, an interferometer detector, or a differential refractometer detector.

6. An analytical method comprising the steps of:

(a) providing a refractive index sensor for reception of a fluid sample to be analyzed;

(b) introducing a first sample comprising a first analyte to be analyzed and a second sample comprising a second analyte to be analyzed onto the sensor, wherein the first analyte is allowed to interact with the second analyte;

(c) interrogating the fluid sample with light;

(d) detecting the light after interaction with the fluid sample, wherein the detected light is not backscattered; and

(e) detecting a molecular interaction between the first and second analyte,

wherein one of the analytes is immobilized, and

wherein one of the analytes is present in an amount of less than about 1.0×10 −6 M.

7. The method of claim 6 , wherein the sample is positioned inside a channel formed in a substrate, the channel has a longitudinal direction and a transverse direction, and the light is elongated in the longitudinal direction of the channel.

8. The method of claim 7 , wherein the light is incident on at least a portion of the channel greater than 4 mm in length along the longitudinal direction.

9. The method of claim 6 , wherein the light is not scattered.

10. The method of claim 6 , wherein the first and/or second analyte is a complex.

11. The method of claim 6 , wherein the molecular interaction is the formation of one or more covalent bonds, electrostatic bonds, hydrogen bonds, or hydrophobic interactions.

12. The method of claim 6 , wherein the first and/or second analyte is one or more of an antibody, an antigen, a protein, a small molecule, a drug, a receptor, a cell, an oligonucleotide, a carbohydrate, an enzyme, a substrate, a DNA, an aptamer, a RNA, a nucleic acid, a biomolecule, a molecular imprint, a protein mimetic, an antibody derivative, a lectin, a cell membrane, an ion, a virus particle, a bacteria, and a micro-RNA.

13. The method of claim 6 , wherein the molecular interaction is a binding event between one or more of antibody-antigen, protein-protein, small molecule-small molecule, small molecule-protein, drug-receptor, antibody-cell, virus-cell, virus-protein, bacteria-cell, bacteria-protein, virus-DNA, virus-RNA, bacteria-DNA, bacteria-RNA, protein-cell, oligonucleotide-cell, carbohydrate-cell, cell-cell, enzyme-substrate, protein-DNA, protein-aptamer, DNA-DNA, RNA-DNA, DNA-RNA, protein-RNA, small molecule-nucleic acid, biomolecule-molecular imprint, biomolecule-protein mimetic, biomolecule-antibody derivatives, lectin-carbohydrate, biomolecule-carbohydrate, small molecule-cell membrane, ion-protein, and protein-protein.

14. A system comprising a refractive index sensor for detecting a molecular interaction product between a first non-immobilized analyte and a second analyte, a pressure change compensator, and a first non-immobilized analyte in an amount of less than about 1.0×10 −6 M, wherein detecting is performed by refractive index sensing other than backscattering interferometry.

15. The system of claim 14 , wherein the sensor comprises a channel formed in a substrate, wherein the channel has a longitudinal direction and a transverse direction, and a light source for generating a light, wherein the light is elongated in the longitudinal direction of the channel.

16. The system of claim 14 , wherein the first analyte is unlabeled.

17. The system of claim 14 , wherein both analytes are unlabeled.

18. The system of claim 14 , wherein the first and second analyte are introduced simultaneously.

19. The system of claim 14 , wherein the first and second analyte are introduced sequentially.