Patent Application
20180275086
The present invention relates to a method for the modification of a working electrode for a sensor which is able to detect at least one dissolved trace metallic element in a liquid, such an electrode, which is for example screen-printed and obtained by this method, a sensor incorporating it and the use thereof for detecting and possibly quantifying said at least one metallic element in liquids. The invention applies in a general manner to the monitoring and control, before or after treatment, of various types of water likely to be contaminated with or polluted by metals or metalloids present in the dissolved state in the form of traces (i.e. in the form of a dispersion of elements selected from chromium, nickel, tin, lead, copper, silver, mercury, uranium, selenium, arsenic, antimony and cobalt, without limitation), or of industrial baths whose composition is to be analysed.
It is known that industrial activities can lead to increased concentrations of metals in industrial water, waste water or other water until the legal toxicity limits are reached. It is therefore necessary for manufacturers to carry out controls on or to monitor these waters, in particular as they leave the factory, by means of regular analyses of the discharge in order to minimise the pollution of the receiving environments. For this purpose, samples must be taken on-site and then transferred to the laboratory for analysis, which has the disadvantage of involving a delay that can be relatively long between each sampling and obtaining the analysis results.
In recent years, attempts have therefore been made to design sensors that can be integrated into a monitoring station for such industrial waters or baths, with the recent development of sensors with a working electrode based on carbon modified with a mercury film. In order to stop contributing to the further pollution of the environment through the harmful metal that is mercury, recent attempts have been made to develop new working electrodes for the detection of trace metallic elements which fully satisfy the criteria of sensitivity at the μg/L range, of reproducibility (i.e. repeatability) of the measurement signals, of stability over time and of robustness with respect to the specific conditions of the solution considered containing the metal to be detected which excludes mercury.
For this purpose, working electrodes modified with a bismuth, gold, copper or silver film have been proposed, but with a sensitivity and reproducibility defect which depends on the pH of the solution for a working electrode modified with bismuth, with a high underflow which is variable depending on the oxygen for a working electrode modified with gold and a limit of detection that is too high for a working electrode modified with copper or silver.
The article Davis, J., Vaughan, D. H., Cardosi, M. F., 1998, Modification of catechol polymer redox properties during electropolymerization in the presence of aliphatic amines, Electrochimica Acta 43(3-4), 291-300, discusses deposition by electropolymerisation of pyrocatechol on a carbon material-based working electrode (made from glassy carbon), using the technique of cyclic voltammetry in a phosphate buffer solution with a neutral pH containing pyrocatechol supplemented with ethylene diamine. However, this article does not relate to the use of this working electrode modified in this way to detect trace metallic elements.
The article Khoo, S. B., Zhu, J., 1999, Poly(catechol) Film Modified Glassy Carbon Electrode for Ultratrace Determination of Cerium (Ill) by Differential Pulse Anodic Stripping Voltammetry, Electroanalysis 11(8), 546-552, discusses deposition by electropolymerisation on such a glassy carbon-based working electrode of a film made from a poly(catechol) exclusively for the detection of cerium Ill. This deposition is carried out in a basic environment, by cyclic voltammetry in a soda solution at 0.1 mol/L containing pyrocatechol, and is followed by scans in another soda solution to remove undesirable monomers within the film.
A major disadvantage of the method for the modification of a working electrode according to this latter article is that it is limited to the detection of cerium, therefore not being suitable for the detection of other trace metals such as copper or lead with satisfactory sensitivity and stability in particular.
One object of the present invention is to propose a novel method for the modification of a carbon material surface of a working electrode which overcomes the aforementioned disadvantages by being able to detect, within a sensor, at least one dissolved trace metallic element in a liquid, which has a standard oxidation-reduction potential which is very different from that of cerium.
This object is achieved in that the Applicant has unexpectedly discovered that a deposition by electropolymerisation in a first environment with a pH of between 6 and 8 of pyrocatechol to form a film based on a poly(catechol) on a carbon material surface of a working electrode, followed by conditioning of the working electrode by reduction-oxidation cycles in a second environment with a pH of at least 10, makes it possible to make the working electrode electroactive in a field of electroactivity measured by cyclic voltammetry ranging from −1 V to +1 V, making it particularly suitable for detecting, within a sensor incorporating this electrode, copper and/or lead as dissolved trace metallic element(s) in a liquid with improved sensitivity, reproducibility, stability and robustness compared to existing sensors.
In other words, a method for the modification according to the invention of a carbon material surface of such a working electrode comprises a step a) of depositing a polymeric film based on a poly(catechol) on said surface by electropolymerisation of pyrocatechol in a first environment with a pH of between 6 and 8, and this method is such that it subsequently comprises a step b) of conditioning said working electrode comprising a succession of reduction-oxidation cycles in a second environment with a pH equal to or greater than 10, in order for said modified working electrode to be electroactive in this field of electroactivity ranging from −1 V to +1 V while being able to detect copper and/or lead as said at least one metallic element.
It should be noted that this detection of trace metallic elements whose oxidation/reduction couples have a standard oxidation-reduction potential of between −1 V and +1 V and which include, in particular, copper and lead, was by no means predictable in view of the aforementioned article by Khoo, S. B., Zhu, J., and provides evidence, on the contrary, of a technical prejudice being overcome in view of this article since it taught that the electropolymerisation of pyrocatechol in a basic environment on a similar working electrode could only be used for the detection of trace cerium, whose standard oxidation-reduction potential Ce4+/C3+ close to 1.72 V is located well outside the range of the invention of between −1 V and +1 V.
It should also be noted that the Applicant has discovered that step b) of conditioning according to the invention makes it possible to make the poly(catechol) film stable in use over time and to provide it with improved detection sensitivity at the same time, compared to the aforementioned films of the prior art deposited on working electrodes.
It should be further noted that the Applicant has verified that no detection signal of a metallic element such as copper is obtained if this step b) of conditioning in a very basic environment is not implemented.
The expression “based on”, used to define the composition of the polymeric film deposited on the working electrode, means, in the present description, that this film comprises poly(catechol) predominantly in mass (i.e. with a mass fraction greater than 50%, or even greater than 75%), it having been specified that this film according to the invention could comprise other monomeric or polymeric compounds as a minority in mass such as polyhydroxybenzene derivative-type monomers (not limited to pyrocatechol) and/or other additives such as, for example, ethylene diamine, as explained below.
“Cyclic voltammetry” (abbreviated to CV) means, in a known manner, a technique based on the measurement of the current which is obtained by a linear scan with a potential between the defined potentials and which results from oxidation and reduction cycles of the elements present in a solution.
According to another feature of the invention, step b) may be implemented in a soda solution which is free from said at least one metallic element and which preferably has a pH of between 11 and 13.5 and for example a concentration of 0.1 mol/L.
Advantageously, step b) may be implemented by stripping chronopotentiometry, carrying out between 5 and 15 of said reduction-oxidation cycles.
“Stripping chronopotentiometry” also means, in a manner known in the present description, an electrochemical method which essentially comprises electrodeposition on the electrode by means of a sufficiently reducing potential applied for a given period of time with stirring, and stripping by means of an oxidation current applied in such a way that the reduced metals are re-oxidised.
Even more advantageously, step b) may be implemented for each of said cycles with the application of a reducing and then oxidising potential for a period of between 30 seconds and 90 seconds, preferably of between 50 and 70 seconds.
As a variant of stripping chronopotentiometry, it is possible to use electrochemical techniques using anodic stripping voltammetry (ASV) such as SWASV (square wave anodic stripping voltammetry) or DPASV (differential pulse anodic stripping voltammetry).
This method for the modification of the working electrode according to the invention preferably further comprises, after step b), a step c) of stabilising said working electrode comprising a repetition of other reduction-oxidation cycles in a third environment which has a pH of between 4 and 5 and which comprises said at least one metallic element to be detected, to stabilise said polymeric film.
It should be noted that this step c) is required in order to optimise the stability of the detection signal emitted by the sensor for a metallic element such as copper and/or lead in particular, and therefore in order for the sensor incorporating the working electrode modified in this way to be able to be used in good conditions.
Even more preferably, said third environment is an acetate buffer solution preferably at 0.1 mol/L which comprises copper and/or lead as said at least one metallic element at a concentration of between 50 ng/L and 100 μg/L.
Also preferentially, step c) is implemented by stripping chronopotentiometry, carrying out at least 25 of said other cycles.
Advantageously, said first environment in which step a) is implemented may be a phosphate buffer solution preferably at 0.1 mol/L which has a pH of between 6.5 and 7.5 and which comprises said pyrocatechol at a concentration of between 5 mmol/L and 15 mmol/L.
Even more advantageously, step a) is implemented by cyclic voltammetry, with a plurality of scans ranging from −0.8 V to +0.8 V carried out at a speed of at least 200 mV/s.
According to another feature of the invention, step a) may further comprise an initial addition of ethylene diamine to said first environment, at a concentration of between 5 mmol/L and 15 mmol/L.
It should be noted that the Applicant has demonstrated that the ethylene diamine initially added to the pyrocatechol solution in particular makes it possible to improve the stability of the film over time (this ethylene diamine acts as a catalyst for the electropolymerisation reaction).
A working electrode according to the invention is able to detect, within said sensor, at least one dissolved trace metallic element in a liquid that can advantageously have a pH of between 2 and 8, the working electrode having a surface based on a carbon material coated with a polymeric film based on a poly(catechol), and said film defining a working surface of the working electrode.
This working electrode is such that its working surface comprises the product of reduction-oxidation reactions by stripping chronopotentiometry of the poly(catechol) with a soda solution which functionalises it, the working electrode being electroactive in a field of electroactivity, measured by cyclic voltammetry, ranging from −1 V to +1 V while being able to detect copper and lead as said at least one metallic element.
It should be noted that this functionalisation of the poly(catechol) by the soda solution added during the aforementioned step b) makes this film according to the invention based on the functionalised poly(catechol) structurally different from a film based on a poly(catechol) which has not been functionalised by this reaction, the product of which thus defines a working electrode according to the invention.
According to another feature of the invention, this working electrode according to the invention is furthermore such that said working surface also comprises the product of reduction-oxidation reactions by stripping chronopotentiometry of said poly(catechol) with an acetate buffer solution which comprises copper and/or lead as said at least one metallic element at a concentration of between 50 ng/L and 100 μg/L, the copper and/or lead having reacted with said film.
It should be noted that this reaction product of the copper and/or lead with the film is reflected in the working electrode by the presence of one or both of these metals in this film, which thereby has a structure which is different from that of a film which has not been subject to step c) mentioned above.
A sensor according to the invention is able to detect at least one dissolved trace metallic element in a liquid, the sensor comprising a working electrode, a counter electrode and a reference electrode, and it is characterised in that said working electrode is as defined above, the sensor being able to detect said metallic elements which have a standard oxidation-reduction potential of between −1 V and +1 V and which include copper and lead.
It should be noted that metallic elements (i.e. metals or metalloids) other than Cu and Pb are able to be detected and quantified by such a sensor according to the invention, such as Cr, Ni, Sn, Ag, Hg, U, Se, As, Sb and Co, without limitation.
It should also be noted that an unmodified working electrode of the prior art (i.e. based on a carbon material such as carbon black or graphite) was only able to measure metals in solution having a standard oxidation-reduction potential of between −0.4 V and +1.2 V.
According to another feature of the invention, the sensor may have a sensitivity, expressed as the limit of detection of said at least one metallic element, which is less than 1.5 μg/L in particular for copper and lead and may even be less than 70 ng/L in the case of lead, for example.
It should be noted that the limit of detection (abbreviated to LOD) corresponds in a known manner to the moment when a signal is visible but cannot be integrated:
LOD=[b+(3·σb)]/a with:
b: intercept of the equation of the trend curve (y=ax+b),
a: slope of the equation of the trend curve, and
σb: standard deviation of the intercept b.
it should also be noted that these limits of detection are as good as, if not better than, those which are usually obtained with a working electrode modified with a mercury film.
According to the invention, said sensor may be used to detect and possibly quantify in situ, for example in a monitoring station (following the example of a pH electrode), said at least one metallic element in solution in industrial waters or baths, before and/or after their treatment, which are selected, for example, from surface waters, groundwater, industrial discharge, water from sewage treatment plants, drainage and runoff water, water from polluted and unpolluted soils and leachate from landfills.
It should be noted that this sensor according to the invention has the following advantages in particular:
Other features, advantages and details of the present invention will emerge when reading the following description of several exemplary embodiments of the invention given in an illustrative and non-limiting manner, the description being completed with reference to the accompanying drawings, in which:
The sensor 1 according to the invention illustrated in
As illustrated in
Once screen printed, these electrodes 2 were air-dried for one hour, then dried in an oven at 60° C. for one hour, and the working electrodes 2 were obtained, as can be seen in
Then, and as illustrated in
Examples of Modifying the Working Electrodes 2 Using the Method According to the Invention:
A potentiostat-galvanostat marketed by METROHM under the name of pAutolab III was used, comprising, in connection with GPES software, a first part designed to switch on the system and to manage the data, a second part designed to control the stirring and the arrival of the stream of nitrogen and a third part containing the electrochemical device.
The tests were carried out in a voltammetry cell, with the electrochemical device which comprised three electrodes:
In a known manner, the cyclic voltammetry that was used for step a) of the method of the invention (i.e. the deposition of the film 5 on each working electrode 2 by electropolymerisation) is based on the measurement of a current which is generated by a linear scan with a potential between the defined potentials and which results from the oxidation and reduction of the elements present in the solution. Each cycle is represented by a trace of the current recorded as a function of the potential applied. When scanning takes place from negative to positive potentials, the signal obtained corresponds to the oxidation of these elements (in the opposite case, the potential obtained is that of the reduction thereof).
Also in a known manner, the stripping chronopotentiometry that was used for the steps b) of conditioning and c) of stabilising the method of the invention comprises:
1) Step a) of Electrodeposition According to the Invention:
To implement this step a), 7 to 50 successive scans were carried out with cyclic voltammetry from −0.8 V to +0.8 V at a speed of 200 mV/s without stirring, in a degassed phosphate buffer solution (with a pH ranging from 7 to 7.5, in particular including pH=7 and pH=7.2) containing pyrocatechol at a concentration of 10 mmol/L with or without ethylene diamine (where appropriate, used at two concentrations of 10 mmol/L and 12 mmol/L). The pyrocatechol was thus electropolymerised on the working surface of each carbon-based electrode 2.
For the solutions of pyrocatechol alone, the number of scans was 7 or 50, whereas for the solutions of pyrocatechol+ethylene diamine, the number of scans tested was 7, 10, 20, 30 and 50.
More specifically, the electropolymerisation of pyrocatechol was tested in particular by carrying out the following comparative tests:
Furthermore, speeds of 200 mV/s, 300 mV/s, 400 mV/s and 500 mV/s were tested during these scans.
And
Therefore, the addition of ethylene diamine to the phosphate buffer solution with a substantially neutral pH containing the pyrocatechol is a preferred embodiment of the invention.
2) Step b) of Conditioning the Film According to the Invention:
To implement step b), each modified electrode was subjected, after step a), to stripping chronopotentiometry in a solution of NaOH at 0.1 mol/L for a period ranging from 60 seconds to 600 seconds.
Different solutions were tested:
For each time period and solution tested, we used:
The 10 cycles of stripping chronopotentiometry for 60 seconds in the soda solution have been identified as the best conditioning, in accordance with step b) according to the present invention.
In fact, these measurements have demonstrated that the electrodes modified after step a) of the invention but which were not subjected to conditioning in soda solution do not generate a signal in stripping chronopotentiometry, according to the dt/dE=f(E) graphs obtained for conditioning in the aforementioned “blank” solutions of phosphate at 0.1 mol/L and acetate at 0.1 mol/L.
The Applicant has verified that this step b) makes it possible to make the film electroactive in a range from −1 V to +1 V, therefore advantageously making it possible to detect metallic elements with standard oxidation-reduction potentials within this range.
It seems that conditioning in soda makes it possible to functionalise the film by making it electroactive and thus facilitate the complexation of the elements to be detected, such as copper and lead.
3) Step of Drying the Film According to the Invention:
The activated film was dried after step b) using different drying times in ambient air:
0 hours, 3 hours, 4 hours and 24 hours of drying (24 hours being the best embodiment), it having been specified, however, that a lack of drying (i.e. 0 hours) did not penalise the activation or the other properties of each film obtained.
4) Stage c) of Stabilising the Film According to the Invention:
Each working electrode coated with the film obtained after step b) and having been dried for 24 hours was conditioned again by stripping chronopotentiometry in an acetate buffer solution at 0.1 mol/L (pH=4.6) containing the metallic element to be detected, i.e. containing 100 ng/L to 40 μg/L of copper and/or lead. More specifically, 30 repetitions or cycles were carried out—with prior use in step a) of two phosphate buffer solutions, with ethylene diamine respectively added and not added, it having been specified that the stability of the film was obtained in these two cases after 25 repetitions in step c).
The Applicant has verified that these two buffer solutions with and without ethylene diamine in step a) ensure stabilisation of the film by eliminating the undesirable monomers in step c) which were possibly deposited on the working electrode, except for the fact that the solution with ethylene diamine makes it possible to obtain an even better stability of the film and this is achieved more quickly. This therefore confirms the fact that the addition of ethylene diamine in step a) is a preferred embodiment of the invention.
Validation Test Report for Sensors According to the Invention Incorporating Working Electrodes Modified in this Way for the Detection of Copper and Lead:
1) Detection of Copper:
As illustrated in
For this test, we used a film obtained:
As can be seen in
A limit of detection (LOD) for copper in solution equal to 1.3 μg/L was obtained by a probability calculation, with an operating period of several weeks for the sensors incorporating the working electrodes modified in this way.
2) Detection of Lead:
As illustrated in
For this test, we used a film obtained:
As can be seen in
As can be seen in
A limit of detection (LOD) for lead in solution equal to 63 ng/L and a limit of quantification (LOQ for short, with LOQ=[b+(10·σb)]/a) of 92 ng/L were obtained by means of probability calculations for these tests, with an operating period of the sensors incorporating the working electrodes modified in this way being several weeks.
In order to verify the stability of the working electrodes 2 obtained by the method of the invention presented above for the detection of trace lead, a series of high-frequency measurements were carried out for one month, changing the solution regularly in order to maintain identical experimental conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15 59131 | Sep 2015 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/073004 | 9/27/2016 | WO | 00 |
