Electrochemical sensors and methods for using electrochemical sensors to detect plant pathogen infection

1. An electrochemical sensor comprising a volatile detection electrode comprising:

an electrode substrate; and

a bio-nanocomposite detection element on a surface of the electrode substrate and in electrochemical communication with the electrode substrate, the bio-nanocomposite detection element comprising:

a nanomaterial transducer material, and

an enzyme system capable of specific reaction with a target stress-induced plant volatile compound methyl salicylate, its hydrolysis product, or both, wherein the enzyme system comprises a bi-enzyme or tri-enzyme system comprising at least an enzyme pair selected from the group of enzyme pairs consisting of: salicylate hydroxylase and tyrosinase, and alcohol oxidase and horseradish peroxidase,

wherein the enzymes are immobilized on the nanomaterial transducer material via a linking group comprising an amine-terminated silane cross-linker attached to the nanomaterial and an amine-amine crosslinker attached to an amine of the amine-terminated silane cross-linker, and

wherein reaction between the enzymes and methyl salicylate, its hydrolysis product, or both generates an electrical signal, wherein detection of the electrical signal indicates the presence of methyl salicylate.

2. The electrochemical sensor of claim 1 , further comprising an electrochemical workstation to provide a current or voltage source to the electrode and for measuring and detecting the electrical signal generated at the detection electrode when the enzymes react with the methyl salicylate, its hydrolysis product, or both.

3. The electrochemical sensor of claim 1 , wherein the enzyme pair comprises salicylate hydroxylase and tyrosinase.

4. The electrochemical sensor of claim 1 , wherein the enzyme system comprises a tri-enzyme system comprising a first enzyme and the enzyme pair, the first enzyme capable of hydrolyzing methyl salicylate to salicylate and methanol, such that each enzyme in the enzyme pair reacts with the salicylate or methanol in a cascade of reactions, wherein the final reaction in the cascade generates the electrical signal.

5. The electrochemical sensor of claim 4 , wherein the first enzyme is selected from the group consisting of: tannase and esterase.

6. The electrochemical sensor of claim 1 , wherein the bio-nanocomposite detection element further comprises one or more enzymes capable of specific reaction with a second target volatile compound selected from the group consisting of: a target stress-induced plant volatile compound and a target pathogen-emitted volatile compound.

7. The electrochemical sensor of claim 6 , wherein second target volatile compound is selected from the group consisting of: ethyl phenol, ethyl guaiacol, octanone, octanol, a green leaf volatile compound, and combinations of these volatile compounds.

8. The electrochemical sensor of claim 7 , wherein the second target volatile compound is selected from ethyl phenol, ethyl guaiacol, or both, and the enzyme capable of reaction with the second target volatile compound is horseradish peroxidase or tyrosinase.

9. The electrochemical sensor of claim 7 , wherein the second target volatile compound is octanone and the enzyme capable of detecting octanone is ADH.

10. The electrochemical sensor of claim 6 , wherein the one or more enzymes capable of specific reaction with a second target volatile compound is selected from the group consisting of: tyrosinase (TYR), laccase (Lc), bilirubin oxidase (BRO), horseradish peroxidase (HRP), salicylate hydroxylase, alcohol oxidase (AO), alcohol dehydrogenase (ADH), and combinations of these enzymes.

11. The electrochemical sensor of claim 1 , further comprising a second volatile detection electrode that detects at least a second target volatile compound other than methyl salicylate, the second volatile detection electrode comprising:

an electrode substrate; and

a bio-nanocomposite detection element on a surface of the electrode substrate and in electrochemical communication with the electrode substrate, the bio-nanocomposite detection element comprising:

a nanomaterial transducer material, and

one or more enzymes capable of specific reaction with the second target volatile compound or its hydrolysis product, wherein the second target volatile compound is a stress-induced plant volatile compound or a target pathogen-emitted volatile compound,

wherein the one or more enzymes are immobilized on the nanomaterial transducer material and reaction between the one or more enzyme and the second target volatile compound generates an electrical signal, wherein detection of the electrical signal indicates the presence of the second target volatile compound.

12. The electrochemical sensor of claim 11 , wherein the second target volatile compound is selected from the group consisting of: ethyl phenol, ethyl guaiacol, octanone, octanol, a green leaf volatile compound, and combinations of these volatile compounds, and wherein the one or more enzymes capable of specific reaction with the second target volatile compound is selected from the group consisting of: tyrosinase (TYR), laccase (Lc), bilirubin oxidase (BRO), horseradish peroxidase (HRP), salicylate hydroxylase, alcohol oxidase (AO), alcohol dehydrogenase (ADH), and combinations of these enzymes.

13. The electrochemical sensor of claim 1 , wherein the nanomaterial transducer material comprises a nanomaterial selected from the group consisting of: multiwalled carbon nanotubes (MWCNTs), carbon nanoparticles, graphene nanoparticles, metal nanoparticles, and metal oxide nanoparticles.

14. The electrochemical sensor of claim 1 , wherein the crosslinkers comprise two or more crosslinkers of varying lengths to accommodate two or more layers of enzymes.

15. The electrochemical sensor of claim 1 , wherein the volatile detection electrode is a working electrode of an electrochemical cell further comprising a counter electrode and a reference electrode in electrochemical communication with the working electrode, and a potentiostat configured to supply an electric current to the electrochemical cell and monitor changes in the electric current generated at the working electrode.

16. The electrochemical sensor of claim 15 , wherein changes in the electric current in the electrochemical cell are recorded as a cyclic voltammogram, differential pulse voltammogram, or other current response to an applied potential or voltage.

17. The electrochemical sensor of claim 1 , further comprising a mediator to facilitate electron transfer from the enzyme to the electrode.

18. An electrochemical sensor comprising a volatile detection electrode comprising:

an electrode substrate; and

a bio-nanocomposite detection element on a surface of the electrode substrate and in electrochemical communication with the electrode substrate, the bio-nanocomposite detection element comprising:

a nanomaterial transducer material, and

an enzyme system capable of specific reaction with a target stress-induced plant volatile compound methyl salicylate, its hydrolysis product, or both, wherein the enzyme system comprises a bi-enzyme or tri-enzyme system comprising at least an enzyme pair selected from the group of enzyme pairs consisting of: salicylate hydroxylase and tyrosinase, and alcohol oxidase and horseradish peroxidase,

wherein the nanomaterial transducer material comprises multiwalled carbon nanotubes (MWCNTs), and the one or more enzymes are immobilized to the MWCNTs via a tethering agent selected from the group consisting of: a 1-pyrene butanoic acid succinamidyl ester (PBSE) tethering agent; 4,4′-[(8,16-dihydro-8,16-dioxodibenzo[a,j]perylene-2,10-diyl)dioxy] dibutyric acid di(N-succinimidyl ester) (DPPSE) tethering agent, and other PBSE-type heterobifunctional tethering agents, and

wherein reaction between the enzymes and methyl salicylate, its hydrolysis product, or both generates an electrical signal, wherein detection of the electrical signal indicates the presence of methyl salicylate.

19. A plant volatile detection system comprising:

(a) a volatile collection reservoir adapted to collect volatile compounds emitted from a plant;

(b) an electrochemical sensor of claim 1 further comprising a counter electrode, and a reference electrode, wherein both the counter electrode and the reference electrode are in electrochemical communication with the volatile detection electrode, and a potentiostat to supply an electric current to the electrochemical cell and monitor changes in the electric current produced at the volatile detection electrode; and

(c) a signal processing mechanism in operative communication with one or more elements of the electrochemical sensor, the signal processing mechanism having data transfer and evaluation software protocols configured to transform raw data from the electrochemical sensor into diagnostic information regarding the presence or absence or levels of the target plant volatile compound.

20. A method for monitoring a condition of a plant or crop of plants, the method comprising:

(1) periodically sampling volatile emissions from the plant or one or more crop plants using a plant volatile detection system, the plant volatile detection system comprising:

(a) a volatile collection reservoir adapted to collect volatile compounds emitted from a plant;

(b) an electrochemical sensor of claim 1 further comprising a counter electrode, and a reference electrode, wherein both the counter electrode and the reference electrode are in electrochemical communication with the volatile detection electrode, and a potentiostat to supply an electric current to the electrochemical cell and monitor changes in the electric current produced at the volatile detection electrode; and

(c) a signal processing mechanism in operative communication with one or more elements of the electrochemical sensor, the signal processing mechanism having data transfer and evaluation software protocols configured to transform raw data from the electrochemical sensor into diagnostic information regarding the presence or absence or levels of the target plant volatile compound; and

(2) determining the presence of a plant disease associated with the presence of methyl salicylate based on the information provided by the signal processing mechanism regarding the presence or absence or levels of methyl salicylate.