WO2008003999A1

WO2008003999A1 - Electrochemical detection of amine compounds

Info

Publication number: WO2008003999A1
Application number: PCT/GB2007/050373
Authority: WO
Inventors: Richard G. Compton; Craig E. Banks
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2006-07-03
Filing date: 2007-07-03
Publication date: 2008-01-10
Anticipated expiration: 2009-01-03
Also published as: GB0823048D0; GB2452208B; GB0613182D0; GB2452208A

Abstract

Method of detecting an amine compound in a sample, which comprises contacting the sample with a second compound in the presence of a working electrode and an electrolyte, wherein said second compound is capable of undergoing a reaction with the amine compound, and wherein the second compound and/or a product of that reaction is capable of undergoing a redox reaction at the working electrode having a detectable r edox couple; and determining the electrochemical response of the working electrode thereto. Electrochemical sensors and electrode materials for use in said method are also provided.

Description

ELECTROCHEMICAL DETECTION OF AMINE COMPOUNDS

Field of the Invention

The present invention relates to the detection of amine compounds, in particular amphetamine compounds.

Background to the Invention

Amphetamines such as 3,4-methylenedioxymethamphetamine (ecstasy) are recreational drugs of abuse due to their stimulant and euphoric effects. The physiological effects of ecstasy and related derivatives include confusion, paranoia, depression and sleeplessness. Adverse physical effects include muscle tension, involuntary teeth clenching, nausea, blurred vision, feeling faint, tremors, rapid eye movement, and sweating or chills.

Driving under the influence of drugs is thought to be a rising epidemic in certain countries. For example, amphetamines and related compounds are used by some long distance lorry drivers as a stimulant to increase their state of alertness/awakedness. Oral fluid, which contains saliva and other liquid substances present in the oral cavity, is of great interest to police forces for roadside drug testing; testing of oral fluid is noninvasive and more convenient compared with testing of urine samples. There remains a need for cheap methodologies, which are both sensitive and portable and can be used on site, and hand-held equipment that can be carried and used by law enforcement officers at road traffic accidents to detect drug drivers.

Amines may be detected colorimetrically by labelling with sodium 1 ,2-naphthoquinone-4- sulphonate in a solution-based reaction (Campins-Falco et al, Journal of Chromatography. B, Biomedical applications, 1996, 687, 239; Hashimato et al, Mikrochimica Acta, 1978, 2, 493; and Nakahara et al, J. Chrom., 1989, 489, 371 ). Detection may be achieved using spectrophotometric analysis coupled with high- performance liquid chromatography (HPLC). However, these techniques are of limited practical utility outside the laboratory owing to their complexity, and the size and cost of the equipment needed.

Summary of the Invention The present invention is based at least in part on a discovery that the reaction between an amine compound and 1 ,2-naphthoquinone-4-sulphonate can be detected electrochemically in situ. In particular, it has been found that, by virtue of a detectable redox couple, 1 ,2-naphthoquinone-4-sulphonate and amine-substituted derivatives thereof can be detected electrochemically, thereby providing a means for detecting amine compounds indirectly. The invention allows amphetamines and other amine compounds to be detected simply and rapidly compared with techniques such as HPLC, which are time consuming and expensive. The invention is therefore particularly relevant to the roadside testing of amphetamines in drug drivers.

In a first aspect, the invention provides a method of detecting an amine compound in a sample, which comprises contacting the sample with a second compound in the presence of a working electrode and an electrolyte, wherein said second compound is capable of undergoing a reaction with the amine compound, and wherein said second compound and/or a product of said reaction is capable of undergoing a redox reaction at the working electrode having a detectable redox couple; and determining the electrochemical response of the working electrode thereto.

In a second aspect, there is provided an electrochemical sensor for the detection of an amine compound, which comprises a working electrode, a counter electrode, an electrolyte solution and a second compound, wherein said second compound is capable of undergoing a reaction with the amine compound, and wherein said second compound and/or a product of said reaction is capable of undergoing a redox reaction at the working electrode having a detectable redox couple.

A further aspect of the invention concerns the use of a 1 ,2-naphthoquinone-4- sulphonate compound or a salt thereof, for the electrochemical detection of an amine compound.

The invention also provides an electrode material comprising a 1 ,2-naphthoquinone-4- sulphonate compound.

Brief Description of the Drawings Fig. 1 shows linear sweep voltammetry at an edge plane pyrolytic graphite before (dotted line) and after 800 μM addition of D-amphetamine sulphate 2 mins (dotted and dashed line) and 10 minutes (solid line). Solution: 1 mM sodium 1 ,2-naphthoquinone-4- sulphonate with 0.01 M sodium carbonate and 0.1 M sodium hydrogen carbonate (pH 9.1 ).

Fig. 2 shows linear sweep voltammetry (A) at an edge plane pyrolytic graphite electrode resulting from 80, 160, 240, 320, 400, 480, 560, 640, 720, and 800 μM additions of D- amphetamine sulphate into an aqueous solution containing 1 mM sodium 1 ,2- naphthoquinone-4-sulphonate with 0.1 M sodium hydrogen carbonate and 0.01 M sodium carbonate (pH 9.1 ). The dotted line is the initial linear sweep voltamogram before the addition of any amphetamines.

Fig. 3 depicts linear sweep voltammetry (A) at an edge plane pyrolytic graphite electrode resulting from 80, 160, 240, 320, 480, 560, 640, 720, 800, 880 and 960 μM additions of

D-amphetamine sulphate into artificial saliva containing 1 mM sodium 1 ,2- naphthoquinone-4-sulphonate with 0.1 M sodium hydrogen carbonate (pH 8.2). The dotted line is the initial linear sweep voltamogram before the addition of any amphetamines. The voltammetric waves are from additions of 0, 80, 160, 240, 320, 480, 560, 640, 720, 880 and 960 μM. B shows the analysis of peak height versus added D- amphetamine sulphate additions.

Fig. 4 shows linear sweep voltammetry at an edge plane pyrolytic graphite electrode in 6 ml_ of oral (saliva) fluid containing 1 mM sodium 1 ,2-naphthoquinone-4-sulphonate with 0.1 M sodium carbonate. The dotted line is the initial linear sweep voltamogram before the addition of 1 mM D-amphetamine sulphate. Linear sweep voltammetry was recorded at 0, 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 minutes.

Fig. 5A depicts linear sweep voltammetry at an edge plane pyrolytic graphite electrode resulting from 240, 320, 400, 480, 560, 640, 720, 800, 880, 960 and 1040 μM additions of pseudoephedrine into artificial saliva containing 1 mM sodium 1 ,2-naphthoquinone-4- sulphonate with 0.1 M sodium hydrogen carbonate (pH 8.2). The dotted line is the initial linear sweep voltamogram before the addition of any pseudoephedrine. Voltammetric curves shown are from additions of 240, 400, 560, 720, 880, 1040 μM. Fig. 5B shows the analysis of peak height (from A) versus added concentration. Description of Various Embodiments

The amine compound may comprise a primary, secondary or tertiary amine group, usually a primary or secondary amine group. The amine compound may be in the form of a salt, typically obtained by mixing the compound with an acid addition salt. The salt may be a pharmaceutically acceptable salt.

The amine compound may be an amphetamine compound. The term "amphetamine compound" as used herein includes reference to compounds comprising a phenylethylamine moiety, which may be substituted or unsubstituted. For example, amphetamine compounds may be obtained in the form of single enantiomer or diastereomer, or a racemic mixture. Amphetamine compounds, especially those illustrated below, may be obtained in salt form, e.g. in the form of a hydrochloride or sulphate salt.

In a particular embodiment, the amine compound is an amphetamine compound of the formula (I):

(I) wherein

R¹ is hydrogen or a moiety comprising 1 to 30 plural valent atoms selected from C, N, O and S;

R² and R³ are each independently selected from hydrogen, halogen and moieties comprising 1 to 30 plural valent atoms selected from C, N, O and S;

R⁴, R⁵, R⁶, R⁷ and R⁸ are each independently selected from hydrogen, halogen and moieties comprising 1 to 30 plural valent atoms selected from C, N, O and S; or any of R⁴ and R⁵, R⁵ and R⁶, R⁶ and R⁷, and R⁷ and R⁸ taken together with the atoms to which they are attached may form a cyclic group optionally substituted with 1 , 2, 3, 4 or 5 R⁹; and

each R⁹ is independently selected from hydrogen, halogen and moieties comprising 1 to 30 plural valent atoms selected from C, N, O and S;

or a salt, e.g. a pharmaceutically acceptable salt, thereof.

With regard to the above formula, R¹ and R² are often each independently selected from hydrogen and Ci_-6 alkyl (e.g. C₁ , C₂, C₃ or C₄ alkyl). In one class of compounds, R¹ and R² are each independently selected from hydrogen and methyl. Of mention are compounds in which R¹ is hydrogen or methyl, and R² is methyl.

R³ is often selected from hydrogen, hydroxy, Ci_-6 alkyl (e.g. C₁ , C₂, C₃ or C₄ alkyl) and Ci-₆ alkoxy (e.g. C₁ , C₂, C₃ or C₄ alkoxy). In particular, R³ may be selected from hydrogen or hydroxy.

R⁴, R⁵, R⁶, R⁷ and R⁸ are usually each independently selected from hydrogen, halogen, hydroxy, Ci_-6 alkyl (e.g. C₁ , C₂, C₃ or C₄ alkyl) and Ci_-6 alkoxy (e.g. C₁ , C₂, C₃ or C₄ alkoxy). In certain compounds, R⁴, R⁵, R⁶, R⁷ and R⁸ are each hydrogen. In other compounds, one or more of R⁴ and R⁵, R⁵ and R⁶, R⁶ and R⁷, and R⁷ and R⁸ taken together with their attached atoms forms a cyclic group optionally substituted with 1 , 2, 3, 4 or 5 R⁹; and the others are each hydrogen. In certain compounds, R⁵ and R⁶ taken together form a methylenedioxy bridge, in which case R⁴, R⁷ and R⁸ are often each hydrogen.

Examples of amphetamine compounds include:

amphetamine D-amphetamine L-amphetamine

methamphetamine D-methamphetamine L-methamphetamine

phenylpropanolamine ephedrine pseudoephedrine

3,4-methylenedioxymethamphetamine

or, in each case, a salt, e.g. a pharmaceutically acceptable salt, thereof.

The presence of an amine compound in the sample may be detected by contacting the sample with a second compound in the presence of a working electrode and electrolyte, wherein the second compound is capable of reacting with the amine compound in an electrochemically detectable reaction. The sample may be a liquid sample, for example an aqueous sample. The sample may comprise a body fluid, such as blood, urine or an oral fluid (e.g. saliva).

The amine compound may be detected by determining the electrochemical response of the working electrode to the consumption of a reactant and/or the formation of a product of the reaction with the second compound, wherein said reactant or product is capable of undergoing a redox reaction at the working electrode having a detectable redox couple. For example, the second compound may have an electrochemically detectable redox couple; in this case, the amine compound can be detected by determining the response of the working electrode to the consumption of the second compound. Alternatively or additionally, a product of the reaction may have an electrochemically detectable redox couple; in this case, the amine compound can be detected by determining the response of the working electrode to the formation of that product. The product may be an amine- substituted derivative of the second compound.

In a particular process, the amine compound is contacted with a second compound which is 1 ,2-naphthoquinone-4-sulphonate compound or a salt thereof. For example, the amine compound may be contacted with 1 ,2-naphthoquinone-4-sulphonate or a salt thereof. Of particular mention is sodium 1 ,2-naphthoquinone-4-sulphonate.

The reaction of an amine compound (NHR₂) with 1 ,2-naphthoquinone-4-sulphonate is illustrated in Scheme 1 below:

Pathway B

+ 2e- + 2H⁺ Pathway A 2e- 2H⁺ Pathway C

Scheme 1

As shown in Pathway A of Scheme 1 , 1 ,2-naphthoquinone-4-sulphonate compound is capable of undergoing a reduction to a 1 ,2-naphthohydroquinone-4-sulphonate via a reversible 2-proton, 2-electron process. Since 1 ,2-naphthoquinone-4-sulphonate has a detectable redox couple, the amine compound may be detected by determining the electrochemical response of the working electrode to the consumption of the compound. This may be achieved, for example, by detecting a reduction in the magnitude of the voltammetric peak corresponding to Pathway A. In Pathway B, the 1 ,2-naphthoquinone-4-sulphonate compound is reacted with an amine to form an amine-substituted 1 ,2-naphthoquinone compound. This reaction preferably takes place in the presence of one or more carbonates, e.g. selected from sodium carbonate and hydrogen carbonate. The reaction of a 1 ,2-naphthoquinone-4-sulphonate compound with an amine may produce a variety of other products, each of which may be electrochemically detectable. For example, sodium 1 ,2-naphthoquinone-4-sulphonate is known to react with primary amines at room temperature to form a complex mixture of products (Hartke et al, Chem. Lett., 1983, 693).

The amine-substituted 1 ,2-naphthoquinone compound can be reduced via a 2-proton, 2- electron process to form an amine-substituted 1 ,2-naphthohydroquinone compound; this process is shown as Pathway C in Scheme 1 and is analagous to Pathway A. Again, since the amine-substituted 1 ,2-naphthoquinone compound has a detectable redox couple, the amine compound may also be detected by determining the electrochemical response of the working electrode to the formation of the amine-substituted 1 ,2- naphthoquinone compound or another product. This may be achieved by detecting growth of a voltammetric peak corresponding to the reaction of Pathway C.

The amine compound may be detected using an electrochemical sensor containing a working electrode with which the sample may be contacted. Typically, electrochemical sensors are based upon the configuration of an electrochemical cell, comprising a working electrode, a counter electrode and an electrolyte, for example. The sensor may further comprise a reference electrode. Suitable sensor designs are well known in the art.

The working electrode may be any suitable electrode known in the art, for example a metallic or carbon electrode. Examples of metallic electrodes include gold, silver and platinum electrodes. Examples of carbon electrodes include an edge plane pyrolytic graphite electrode, a basal plane pyrolytic graphite electrode, a glassy carbon electrode, a boron doped diamond electrode, a highly ordered pyrolytic graphite electrode, carbon powder and carbon nanotubes. Of particular mention are edge plane pyrolytic graphite working electrodes. The working electrode may be a microelectrode or a macroelectrode, and may be screen printed. The counter electrode may be any suitable electrode, for example, a platinum or graphite electrode. The second compound may be present in solution, for example the electrolyte solution. Alternatively or additionally, the working electrode may comprise the second compound. Thus, the second compound may be present in the electrolyte solution and/or on the surface of the working electrode and/or in the working electrode.

The second compound may act as the electrolyte. An electrolyte solution may therefore be obtained simply by adding the second compound to a liquid sample, e.g. an aqueous sample. Thus, for example, an electrolyte solution may be obtained by dissolving a 1 ,2- naphthoquinone-4-sulphonate compound or a salt thereof in an aqueous liquid. In this case, the electrolyte preferably also contains one or more carbonate compounds, for example selected from sodium carbonate and hydrogen carbonate. The liquid to which the electrolyte is added may be the sample itself. Alternatively, the electrolyte solution may be obtained using a liquid other than the sample. If the second compound is not capable of acting as an electrolyte, then a supporting electrolyte can be used to form the electrolyte solution.

A working electrode comprising the second compound may be obtained by immobilising the second compound on the electrode from solution. Alternatively or additionally, the second compound may be comprised in the bulk of the electrode material.

A potential may be applied across the electrodes using a potentiostat, and the response of the cell to the sample determined. For determination of the voltammetric response, the applied potential may be varied relative to a reference electrode; in this way, a cyclic voltammogram may be obtained. Alternatively, the amperometric response of the cell can be determined by applying a fixed potential across the electrodes, optionally controlled relative to a reference electrode. The reference electrode may be, for example, a saturated calomel electrode (SCE) or a silver electrode.

In one embodiment, the current is measured using linear sweep or cyclic voltammetry. In another embodiment, said current is measured using square wave voltammetry. In an alternative embodiment, the current is measured using a pulsed voltammetry technique, in particular differential pulse voltammetry.

The following Examples illustrate the invention.

Materials and Methods All Chemicals were of analytical grade and used as received without any further purification. These were sodium 1 ,2-naphthoquinone-4-sulphonate (Fluka, >97%), D- amphetamine sulphate (Sigma), pseudoephedrine (Sigma), sodium hydrogen carbonate (BDH, Analar grade, 99.5%) and sodium carbonate (BDH, Analar grade, 99.5%)

Solutions were prepared with deionised water of resistivity not less than 18.2 M Ohm cm (Millipore water systems). Voltammetric measurements were carried out using a μ- Autolab Il potentiostat (Eco-Chemie) with a three-electrode configuration. Edge plane pyrolytic graphite (eppg; Le Carbone, Ltd.) was used as the working electrode, and was constructed as described in Moore et al, Anal. Chem., 2004, 76, 2677. The eppg electrodes were polished on alumina lapping compounds (BDH) of decreasing sizes (5 - 0.1 μm ) on soft lapping pads. The counter electrode was a bright platinum wire, with a saturated calomel electrode completing the circuit. All experiments were typically conducted at 20 ± 2 °C. Before commencing experiments, nitrogen (BOC) was used for the deaeration of solutions.

Oral (saliva) fluid was provided by a volunteer. Prior to the sample being collected, the subject conducted two oral rinses with water. It is was requested that the subject did not eat for up to two hours prior to obtaining the sample to reduce the likelihood of the analysis being contaminated with particulate food.

Example 1 : Detection of sodium 1 ,2-naphthoquinone-4-sulphonate and D-amphetamine sulphate

Detection of sodium 1 ,2-naphthoquinone-4-sulphonate

Linear sweep voltammetry was first used to explore the voltammetric response of an edge plane pyrolytic graphite electrode in an aqueous solution containing 1 mM sodium 1 ,2-naphthoquinone-4-sulphonate with 0.1 M sodium hydrogen carbonate and 0.01 M sodium carbonate (pH 9.1 ).

As depicted in Fig. 1 (dotted line), a single reduction peak is observed at ca. - 0.17 V (vs. SCE), which corresponds to the electrochemically reversible reduction of sodium 1 ,2-naphthoquinone-4-sulphonate to sodium 1 ,2-naphthohydroquinone-4-sulphonate via a 2-proton, 2-electron process. This process is shown as Pathway A in Scheme 1 . Detection of D-amphetamine sulphate in buffer solution

An addition of 800 μM of D-amphetamine sulphate was then made into the solution, after which the voltammetric response was sought as shown in Fig. 1 (dot-dash line). After 2 minutes, a new voltammetric peak at ca. - 0.52 V was observed with an associated reduction in the voltammetric peak at ca. - 0.17 V. After 10 minutes, the new voltammetric feature was observed to grow further with an associated decrease in the magnitude of the peak at ca. - 0.17 V. The reduction in the first voltammetric peak (ca. - 0.17 V) is due to the loss of sodium 1 ,2-naphthoquinone-4-sulphonate since it reacts with D-amphetamine sulphate to form 4-dialkylamino-quinone as depicted in the first step of Scheme 1 . The new peak at ca. - 0.52 V is likely to be due to the electrochemical reduction of the 4-dialkylamino-quinone to 4-dialkylamino-hydroquinone via a 2-proton, 2-electron process as shown in Pathway C of Scheme 1 .

The effect of additions of D-amphetamine sulphate into a solution containing 1 mM sodium 1 ,2-naphthoquinone-4-sulphonate with 0.1 M sodium hydrogen carbonate and 0.01 M sodium carbonate (pH 9.1 ) was explored using linear sweep voltammetry at an edge plane pyrolytic graphite electrode.

Fig. 2 depicts the voltammetric profiles (Fig. 2A) before (dotted line) and after additions of D-amphetamine sulphate, along with analysis of the peak height versus added concentration shown in Fig. 2B. The increasing magnitude of the voltammetric peak at ca. - 0.52 V provides an analytically useful signal from which to determine D- amphetamine sulphate. Reduction in the voltammetric peak height at ca. - 0.17 V may also be conveniently used.

Detection of D-amphetamine sulphate in artificial saliva

The protocol in artificial saliva was then explored. The recipe for artificial saliva is described elsewhere (West et al, Electroanalysis, 2002, 14, 1470), and reflects both the mineral and mucin content of human saliva.

Fig. 3 shows the response of additions of amphetamines into artificial saliva containing 1 mM sodium 1 ,2-naphthoquinone-4-sulphonate with 0.1 M sodium hydrogen carbonate

(pH 8.2); the presence of sodium hydrogen carbonate altered the pH from 6.8 to 8.2. The magnitude of voltammetric peak heights, as evidenced in Fig. 3B decreased slightly compared to Fig. 2 which was recorded in just a buffer solution, likely reflecting the change in viscosity (presence of mucin) of the solution.

Detection of D-amphetamine sulphate in oral saliva

The response of the protocol was examined in oral fluid.

Fig. 4 shows the linear sweep voltammetry response before any amphetamine addition (dotted line). No second peak was observed, indicating that the presence of proteins in the oral fluid did not affect the voltammetric response, thus minimising the possibility of false positives in testing.

Next, an addition of 1 mM D-amphetamine sulphate into the oral (saliva) fluid was made with the voltammetric response monitored over a 20-minute period with the linear sweep voltammetric wave taken at 2-minute intervals. As observed in Fig. 4, the first voltammetric wave at ca. - 0.18 V decreases with time as the D-amphetamine sulphate in the oral (saliva) fluid reacts with the sodium 1 ,2-naphthoquinone-4-sulphonate with the increase in the new voltammetric peak occurring at ca. - 0.53 V. The baseline after this peak increases, indicating that this peak is indeed growing in magnitude with time.

Example 2: Detection of pseudoephedrine

The tagging of pseudoephedrine with sodium 1 ,2-naphthoquinone-4-sulphonate was explored using linear sweep voltammetry.

Fig. 5 depicts the linear sweep voltammetric response of 1 mM sodium 1 ,2- naphthoquinone-4-sulphonate with 0.1 M sodium hydrogen carbonate (pH 8.2) in artificial saliva towards additions of pseudoephedrine. Again, analytically useful signals were observed which can be used for monitoring the presence of amphetamines. Comparison of the analysis of peak in Fig. 3 with Fig. 5 reveals that the tagging of D- amphetamine sulphate with sodium 1 ,2-naphthoquinone-4-sulphonate is more sensitive than compared with the tagging of pseudoephedrine; this is likely reflected by the fact that pseudoephedrine is a secondary amine while D-amphetamine sulphate is a primary amine.

Claims

1 . A method of detecting an amine compound in a sample, which comprises contacting the sample with a second compound in the presence of a working electrode and an electrolyte, wherein said second compound is capable of undergoing a reaction with the amine compound, and wherein the second compound and/or a product of said reaction is capable of undergoing a redox reaction at the working electrode having a detectable redox couple; and determining the electrochemical response of the working electrode thereto.

2. A method according to claim 1 , wherein the amine compound comprises a primary or secondary amine group.

3. A method according to claim 1 or claim 2, wherein the amine compound comprises an aliphatic amine group.

4. A method according to any preceding claim, wherein the amine compound is an amphetamine compound.

5. A method according to claim 4, wherein the amine compound is amphetamine, methamphetamine, phenylpropanolamine, 3,4-methylenedioxymethamphetamine, or a salt thereof.

6. A method according to claim 5, wherein the amine compound is 3,4- methylenedioxymethamphetamine or a salt thereof.

7. A method according to any preceding claim, wherein the sample is a liquid sample.

8. A method according to claim 7, wherein the sample comprises a body fluid.

9. A method according to claim 8, wherein the sample comprises an oral fluid, for example saliva.

10. A method according to any preceding claim, wherein the second compound is capable of undergoing a redox reaction at the working electrode having a detectable redox couple.

1 1 . A method according to any preceding claim, wherein a product is capable of undergoing a redox reaction at the working electrode having a detectable redox couple.

12. A method according to any preceding claim, wherein the second compound and a product are each capable of undergoing a redox reaction at the working electrode having a detectable redox couple.

13. A method according to any preceding claim, wherein the second compound is a 1 ,2-naphthoquinone-4-sulphonate compound or a salt thereof.

14. A method according to claim 13, wherein the second compound is 1 ,2- naphthoquinone-4-sulphonate or a salt (e.g. a sodium salt) thereof.

15. A method according to claim 13 or claim 14, wherein the contacting takes place in the presence of one or more carbonate compounds.

16. A method according to claim 15, wherein the contacting takes place in the presence of one or both of sodium carbonate and sodium hydrogen carbonate.

17. A method according to any preceding claim, wherein the second compound is present in solution.

18. A method according to claim 17, wherein the solution comprises the electrolyte.

19. A method according to claim 18, wherein the second compound acts as an electrolyte.

20. A method according to any preceding claim, wherein the working electrode comprises the second compound.

21 . An electrochemical sensor for the detection of an amine compound, which comprises a working electrode, a counter electrode, an electrolyte solution and a second compound, wherein said second compound is capable of undergoing a reaction with the amine compound, and wherein said second compound and/or a product of said reaction is capable of undergoing a redox reaction at the working electrode having a detectable redox couple.

22. A sensor according to claim 21 , wherein the second compound is capable of undergoing a redox reaction at the working electrode having a detectable redox couple.

23. A sensor according to claim 21 or claim 22, wherein a product is capable of undergoing a redox reaction at the working electrode having a detectable redox couple.

24. A sensor according to any of claims 21 to 23, wherein the second compound and a product are each capable of undergoing a redox reaction at the working electrode having a detectable redox couple.

25. A sensor according to any of claims 21 to 24, wherein the second compound is 1 ,2-naphthoquinone-4-sulphonate compound or a salt thereof.

26. A sensor according to claim 25, wherein the second compound is sodium 1 ,2- naphthoquinone-4-sulphonate.

27. A sensor according to claim 25 or claim 26, wherein the solution also contains one or more carbonate compounds.

28. A sensor according to claim 27, wherein the solution contains one or both of sodium carbonate and sodium hydrogen carbonate.

29. A sensor according to any of claims 21 to 28, wherein the working electrode is a metallic or carbon electrode.

30. A sensor according to any of claims 21 to 29, wherein the second compound is present in solution.

31 . A sensor according to claim 30, wherein the electrolyte solution comprises the second compound.

32. A sensor according to claim 31 , wherein the second compound acts as an electrolyte.

33. A sensor according to any of claims 21 to 32, wherein the working electrode comprises the second compound.

34. An electrode material comprising a 1 ,2-naphthoquinone-4-sulphonate compound or a salt thereof.

35. A material according to claim 34, wherein the compound is comprised on a surface of the material.

36. A material according to claim 34 or claim 35, wherein the compound is comprised in the bulk of the material.

37. A material according to any of claims 34 to 36, wherein the compound is sodium 1 ,2-naphthoquinone-4-sulphonate.

38. Use of a 1 ,2-naphthoquinone-4-sulphonate compound or a salt thereof, for the electrochemical detection of an amine compound.

39. Use according to claim 38, wherein the 1 ,2-naphthoquinone-4-sulphonate compound is sodium 1 ,2-naphthoquinone-4-sulphonate.