Fluorescence based biosensor

1. A method of detecting the presence of an ion in the presence of other ions, in a sample, comprising:

forming a mixture comprising:

(1) a nucleic acid enzyme,

(2) the sample, and

(3) a substrate, the substrate and the nucleic acid enzyme comprising separate nucleic acid strands,

to produce a cleavage product from the mixture; and

determining the presence of the cleavage product;

wherein the enzyme comprises at least one quencher and is dependent on the ion to produce the cleavage product from the substrate,

the substrate comprises a ribonucleotide, at least one fluorophore and at least one quencher,

when the substrate is hybridized to the nucleic acid enzyme, the first quencher quenches the at least one fluorophore, and

when the substrate is not hybridized to the nucleic acid enzyme, the second quencher quenches the at least one fluorophore.

2. The method of claim 1 , wherein a 5′ end of the substrate comprises a first fluorophore and a 3′ end of the substrate comprises a first quencher for the fluorophore and wherein a 3′ end of the enzyme comprises a second quencher for the fluorophore.

3. The method of claim 2 , wherein the fluorophore is 6-carboxyfluorescein (FAM) and wherein the first and second quenchers are 4-(4′-(dimethylaminophenylazo)benzoic acid) (DABCYL).

4. The method of claim 1 , wherein the enzyme is linked to a support.

5. The method of claim 1 , wherein the substrate comprises a nucleic acid of SEQ ID NO: 2.

6. The method of claim 1 , wherein the enzyme comprises a nucleic acid of SEQ ID NO: 1.

7. The method of claim 1 , wherein the ion comprises a member selected from the group consisting of, Mg(II), Ca(II), Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Pb(II).

8. The method of claim 1 , wherein the ion is selected from the group consisting of Mg 2+ , Ca 2+ , Mn 2+ , Co 2+ , Ni 2+ , Zn 2+ , Cd 2+ , Pb 2+ .

9. The method of claim 8 , wherein the ion is Pb 2+ .

10. The method of claim 1 , wherein the ion is a metal ion.

11. The method of claim 1 , wherein the sample suspected of containing the ion comprises a bodily fluid.

12. The method of claim 11 , wherein the bodily fluid is blood.

13. The method of claim 1 , wherein an array of nucleic acid enzymes comprises the nucleic acid enzyme.

14. The method of claim 1 , wherein the ion is selected from the group consisting of Cu 2+ , Cd 2+ , Zn 2+ , Mg 2+ , Pb 2+ , Co 3+ , Mn 2+ , Ni 2+ and Ca 2 +.