Sensors for detecting an analyte using silver nanoparticles

1. A sensor for biosensing comprising discrete silver nanoparticles that are shaped and substantially plate-like in structure dispersed in an aqueous based system in which substantially all of the surfaces of the silver nanoparticles are available for interaction with an analyte or for functionalisation with a receptor which is capable of interacting with an analyte, wherein the silver nanoparticles provide a detectable change of from 1 to 150 nm in the position of their surface plasmon resonance in the UV-visible absorption spectrum in response to the binding of an analyte.

2. The sensor as claimed in claim 1 wherein a receptor specific to a target analyte is attached to the surface of the nanoparticles.

3. The sensor as claimed in claim 2 wherein the receptor is bonded directly to the surface of the nanoparticles.

4. The sensor as claimed in claim 2 wherein a linker is provided between the receptor and the silver nanoparticles.

5. The sensor as claimed in claim 4 wherein the linker incorporates an organic or an inorganic functional group.

6. The sensor as claimed in claim 5 wherein the functional group comprises a thiol group or an amine group.

7. The sensor as claimed in claim 1 wherein more than one type of receptor is attached to the silver nanoparticles.

8. The sensor as claimed in claim 1 wherein the silver nanoparticles have dimensions in the range of from 5 to 100 nm.

9. The sensor as claimed in claim 1 wherein the silver nanoparticles have dimensions in the range of from 18 nm to 32 nm.

10. The sensor as claimed in claim 1 wherein the morphology of at least some of the silver nanoparticles is hexagonal and/or triangular in shape.

11. The sensor as claimed in claim 1 wherein at least some of the silver nanoparticles display an SPR peak in the 400 nm region.

12. The sensor as claimed in claim 1 wherein at least some of the silver nanoparticles display an SPR peak in the 470 to 600 nm region.

13. The sensor as claimed in claim 1 wherein at least some of the silver nanoparticles display an SPR peak in the 340 nm region.

14. The sensor as claimed in claim 1 wherein the silver nanoparticles show predominantly non-spherical morphology.

15. A method for detecting an analyte comprising contacting the analyte with a sensor as claimed in claim 1 and observing a detectable change.

16. The method as claimed in claim 15 wherein the detectable change is a change in the absorption spectrum.

17. The method as claimed in claim 15 wherein the detectable change is a qualitative or quantitative change.

18. The method as claimed in claim 15 wherein the change is a colour change observable with the naked eye.

19. The method as claimed in claim 18 wherein the change in the absorption spectrum is a shift which is detected in the range from 200 nm to 900 nm.

20. The method as claimed in claim 19 wherein the shift is from 1 to 150 nm.

21. The method as claimed in claim 19 wherein the shift is from 5 to 50 nm.

22. A method for preparing silver nanoparticles which comprises the step of forming the nanoparticles in the presence of a polymeric stabiliser.

23. The method as claimed in claim 22 comprising the step of controlling the optical response of the silver nanoparticles by varying the concentration of the polymeric stabiliser.

24. The method as claimed in claim 22 wherein the polymeric stabiliser has a molecular weight of greater than 10 kDa.

25. The method as claimed in claim 24 wherein the molecular weight of the polymeric stabiliser is less than 1300 kDa.

26. The method as claimed in claim 22 wherein the polymeric stabiliser is water soluble.

27. The method as claimed in claim 22 wherein the polymeric stabiliser is selected from one or more of poly(vinyl alcohol), poly(vinylpyrollidone), poly(ethylene glycol), and poly(acrylic acid).

28. The method as claimed in claim 22 wherein the polymeric stabiliser is poly(vinyl alcohol).

29. The method as claimed in claim 22 wherein the method comprises reducing a silver salt.

30. The method as claimed in claim 29 wherein the silver salt is silver nitrate.

31. The method as claimed in claim 22 wherein the reaction is carried out in the presence of seed silver nanoparticles.

32. The method as claimed in claim 30 wherein the reaction is carried out in the presence of seed silver nanoparticles.

33. The method as claimed in claim 32 wherein the ratio of [silver nitrate] to [silver seed] is greater than or equal to 50:1.

34. The method as claimed in claim 32 wherein the ratio of [silver nitrate] to [silver seed] is between 50:1 and 200:1.

35. The method as claimed in claim 32 wherein the ratio of [silver nitrate] to [silver seed] is between 50:1 and 100:1.

36. The method as claimed in claim 22 wherein the reaction is carried out in an aqueous medium.

37. The method as claimed in claim 29 wherein the reduction is carried out at a temperature of from 10° C. to 60° C.

38. The method as claimed in claim 37 wherein the reduction is carried out at a temperature of about 40° C.

39. The method as claimed in claim 22 wherein the reaction is carried out in the dark.

40. The method as claimed in claim 22 wherein the reaction is carried out in ambient light conditions.

41. The method as claimed in claim 22 wherein the reaction is carried out under controlled irradiation conditions.

42. The sensor as claimed in claim 1 wherein the silver nanoparticles have an average size of 20 nm±8 nm.

43. The sensor as claimed in claim 1 wherein the silver nanoparticles have an average size of 22 nm±8 nm.

44. A sensor for biosensing comprising discrete silver nanoparticles that are stable and that are shaped and substantially plate-like in structure dispersed in an aqueous based system in which substantially all of the surfaces of the silver nanoparticles are available for interaction with an analyte or for functionalisation with a receptor which is capable of interacting with an analyte, wherein the silver nanoparticles provide a detectable change of from 1 to 150 nm in the position of their surface plasmon resonance in the UV-visible absorption spectrum in response to the binding of an analyte.

45. The sensor as claimed in claim 44 wherein the silver nanoparticles remain dispersed in the aqueous-based system.

46. The sensor as claimed in claim 44 wherein the silver nanoparticles remain dispersed in the aqueous-based system for a period of at least 8 months.