Patent Description:
Water pollution is an increasing global concern that damages human health, aquatic ecosystems, and economic growth. Globally, <NUM> percent of municipal wastewater is discharged untreated into water bodies, and industry is responsible for dumping millions of tons of heavy metals, solvents, and other waste effluents each year. Moreover, in most high-income countries and many emerging economies, agricultural activities have become the major factor in the degradation of inland and coastal waters due to the discharge of large quantities of agrochemicals, organic matter, and drug residues (FAO and IWMI, <NUM>). Therefore, developing simple, cost-effective, and rapid analytical tools for water quality assessment is critical to monitor water pollution and make timely decisions.

For instance, chemical oxygen demand (COD) is a key parameter for the evaluation of water quality [<NPL>]. It is expressed as the amount of oxygen (in milligrams, mg) necessary to decompose all the organic matter contained in one liter (L) of a surface water or wastewater sample. Conventional detection methods evaluate COD by the oxidative degradation of the organic compounds present in a water sample with strong oxidizing agents [<NPL>; <NPL>; <NPL>; <NPL>]. However, these methods present many disadvantages, including:.

Numerous efforts have been made to overcome these disadvantages, with electrochemical methods becoming the most promising option for determining not only COD, but also other water contaminants in a rapid, sensitive, operationally simple, and cost-effective manner. An example of said electrochemical methods is the amperometric determination of COD, based on measuring the current during the electrochemical oxidation of the organic species present in the sample performed by strong oxidant hydroxyl radicals produced on the electrode surface [<NPL>].

For achieving high electrical response signals, the development of highly sensitive electrode materials specific to the analysis of different water contaminants is of paramount importance. In the case of the electrochemical determination of COD, the following electrodes were so far reported:.

Document <CIT> discloses an analytical equipment for a bioelectrochemical determination of microorganisms or other cells in a liquid sample, the equipment including a disposable element which comprises a substrate, a plurality of electrode tracks, screen-printed over the substrate, a screen-printed working electrode, a screen-printed pseudo-reference electrode, and a screen-printed auxiliary electrode; an insulating layer arranged over the conductive tracks; and a filtering element, arranged in contact with the electrochemical cell.

However, the manufacture of electrochemical sensors comprising said specific electrodes and, in general, any electrode used for detecting water pollutants, is cumbersome in some cases or cannot be scaled up for mass-production purposes in some others. Most importantly, all these approaches require sample preconditioning before carrying out the analytical measurement, including sample filtration as well as pH and conductivity adjustment. Said sample-preconditioning involves at least two initial steps that are carried out ex-situ, limiting the application of these devices in in-field testing. Therefore, analytical tools for decentralized on-site rapid determination of water contaminants that saves analysis time are in demand.

The present invention proposes a solution to the limitations mentioned above by means of a novel screen-printed electrode that allows the manufacturing of portable, sample-to-result and user-friendly electrochemical sensors for on-site detection of water pollutants.

A first object of the present invention relates to a screen-printed electrode (SPE) for detecting a pollutant in a water sample. The SPE comprises:.

Within the scope of interpretation of the present invention, the expression "enable the electrochemical degradation" will be understood as making an oxidation or reduction reaction of the pollutant present in the water sample happen or happen more quickly.

According to the invention, the SPE of the invention comprises an electrolyte-impregnated filtering element that is arranged in contact with the electrochemical cell. In addition to filter the water sample received by the electrochemical cell, this electrolyte-impregnated filtering element enables sample preconditioning and subsequent water pollutant detection by the electrochemical cell without any user intervention. Thanks to that, sample pre-processing is not needed, maintaining sample quality as it was collected. Preferably, the filtering element is impregnated with an electrolyte comprising sodium hydroxide, as this alkaline medium favors the electrochemical degradation of the pollutant present in the water sample to be analyzed.

In a preferred embodiment of the invention, the working electrode and, optionally, the auxiliary electrode comprise/s a metal nanoparticle-carbon composite-based ink, preferably, a copper nanoparticle-carbon composite-based ink. More preferably, said copper nanoparticle-carbon composite-based ink comprises a carbon bulk material, a plurality of carbon fibers and a plurality of copper nanoparticles. The carbon bulk material is beneficial for electrochemical applications due to its porosity; the carbon fibers, preferably <NUM>-<NUM> in length, enhance conductivity; and the copper nanoparticles act as a catalyst of the electrochemical degradation of the pollutant present in the water sample.

In another preferred embodiment of the invention, the electrolyte-impregnated filtering element is covered with a fixing layer containing a plurality of holes, preferably of a plastic material. Said fixing layer fixes the electrolyte-impregnated filtering element to the electrochemical cell, allowing the water sample to flow through the electrolyte-impregnated filtering element and reach the electrochemical cell through the plurality of holes. More preferably, said fixing layer is covered with a removable protective layer to prevent contamination or electrode degradation before use. Said protective layer can be easily removed just before using the SPE for water pollution analysis.

In another preferred embodiment of the invention, the electrolyte-impregnated filtering element comprises a porous paper material.

A second object of the present invention relates to an electrochemical sensor for measuring chemical oxygen demand in a water sample containing organic matter. The system comprises:.

Within the scope of interpretation of the present invention, the expression "faradaic current" will be understood as any current generated by the oxidation or reduction of the pollutant present in the water sample.

In a preferred embodiment of the invention, the means for potential application as well as the means for current measurement and recording are comprised in a portable potentiostat powered and controlled by an electronic mobile device.

A third object of the present invention relates to a method of measuring COD in a water sample containing organic matter by means of an electrochemical sensor according to any of the embodiments herein described. Advantageously, said method comprises performing the following steps:.

A fourth object of the present invention relates to a method of fabrication of the SPE herein described. Advantageously, said method comprises performing the following steps in any technically possible order:.

Optionally, said method can comprise the step of impregnating the filtering element with an electrolyte.

In a particular embodiment of said method, the working and auxiliary electrodes are screen-printed using a copper nanoparticle-carbon composite-based ink, said ink being prepared as follows:.

Preferably, any of the methods of fabrication of the SPE herein described can further comprise:.

In this case, the water sample is dispensed onto the electrolyte-impregnated filtering element through the holes of the fixing layer.

A fifth object of the present invention relates to the use of the SPE of the invention for determining COD in surface waters (e.g. lakes and rivers), wastewater, and aqueous hazardous wastes, or for analyzing other water pollutants, such as halide ions, sucralose, and chlorinated disinfection byproducts.

In order to provide a better understanding of the technical features of the invention, the referred <FIG> are accompanied by a series of numerical references which, with an illustrative and non-limiting character, are hereby represented:.

As described in the preceding paragraphs, one object of the present invention relates to a screen-printed electrode (SPE) (<NUM>) for detecting a pollutant in a water sample. In the example of the SPE chosen to illustrate the present invention (<FIG>), the electrode comprises:.

Advantageously, the SPE (<NUM>) further comprises an electrolyte-impregnated filtering element (<NUM>), preferably of a porous paper material, that is arranged in contact with the electrochemical cell (<NUM>). This electrolyte-impregnated filtering element (<NUM>) filters and preconditions the water sample received by the electrochemical cell (<NUM>), thus saving time in pre-processing samples while avoiding any possible contamination thereof by user manipulation. Preferably, the filtering element (<NUM>) is impregnated with an electrolyte comprising sodium hydroxide, as this alkaline medium favorsthe electrochemical degradation of the pollutant present in the water sample.

The electrolyte-impregnated filtering element (<NUM>) can be covered with a fixing layer (<NUM>) containing a plurality of holes, preferably of a plastic material. Said fixing layer (<NUM>) fixes the filtering element (<NUM>) to the electrochemical cell (<NUM>), allowing the water sample to flow through the filtering element (<NUM>) and reach the electrochemical cell (<NUM>) through the plurality of holes (<FIG>). Optionally, said fixing layer (<NUM>) can be covered with a removable protective layer to prevent contamination or electrode degradation before use. Said protective layer can be easily removed just before using the SPE for water pollution analysis.

The working electrode (<NUM>) and, optionally, the auxiliary electrode (<NUM>) comprise/s a metal nanoparticle-carbon composite-based ink, preferably, a copper nanoparticle-carbon composite-based ink.

Said means (<NUM>, <NUM>) for potential application and for current measurement and recording are preferably comprised in a portable potentiostat powered and controlled by an electronic mobile device (<NUM>) (<FIG>).

A third object of the present invention relates to a method of measuring chemical oxygen demand in a water sample containing organic matter by means of an electrochemical sensor according to any of the embodiments herein described. Advantageously, said method comprises performing the following steps:.

A fourth object of the present invention relates to a method of fabrication of the SPE (<NUM>) herein described (see <FIG>). Said method comprises performing the following steps in any technically possible order:.

Said copper nanoparticle-carbon composite-based ink is prepared as follows:.

Optionally, the method of fabrication of the invention can comprise the step of impregnating the filtering element (<NUM>) with an electrolyte.

Preferably, any of the methods of fabrication described above can further comprise:.

In this case, the water sample is dispensed onto the electrolyte-impregnated filtering element (<NUM>) through the holes of the fixing layer (<NUM>).

A fifth object of the present invention relates to the use of the SPE (<NUM>) of the invention for determining chemical oxygen demand in surface water (e.g. lakes and rivers), wastewater, and aqueous hazardous wastes, or for analyzing other water pollutants, such as halide ions, sucralose, and chlorinated disinfection byproducts.

<FIG> shows scanning electron microscopy images of the copper nanoparticle-carbon composite-based ink used in a preferred embodiment of the method of fabrication of the SPE of the invention, which is made of carbon bulk, carbon fibers and copper nanoparticles. Its crystal structure was examined by X-ray diffraction (XRD) against pure carbon, depicted in grey and black, respectively, in <FIG>. The broad bumps located around 2θ values of <NUM>° and <NUM>° are characteristic of pure amorphous C. The diffraction peaks observed are characteristic of face-centered cubic (fcc) crystalline copper, corresponding to the planes (<NUM>), (<NUM>) and (<NUM>), at 2θ values of ca. <NUM>°, <NUM>° and <NUM>°, respectively, which demonstrate that the synthesized copper nanoparticle-carbon composite-based ink contains metallic copper nanoparticles.

The porosity of the copper nanoparticle-carbon composite-based ink was measured by nitrogen adsorption and desorption isotherms (<FIG>). According to the Brunauer-Emmett-Teller (BET) model, the surface areas were calculated to be <NUM><NUM>/g. The adsorption uptake at low nitrogen relative pressures (P/Po=<NUM>-<NUM>) indicates that the Cu/C material presents a lot of micropores (diameter < <NUM>). The slope of the isotherms at intermediate relative pressures (<NUM><P/P<NUM><<NUM>) and the increase in the adsorbed volume at high relative pressures (<NUM><P/P<NUM><<NUM>) reveal the existence of mesopores (<NUM>-<NUM>) and macropores (<NUM>-<NUM>), respectively. The total pore volume calculation was <NUM><NUM>/g based on the N<NUM> amount adsorbed at a relative pressure P/P<NUM> of ca. The BJH pore size distribution curve acquired from the adsorption isotherm confirmed the predominant diameters in the micropore and mesopore region with the coexistence of a small number of macropores.

<FIG> shows SEM images of the copper nanoparticle-carbon composite-based ink at different grinding times showing that, as the grinding time increases, the size of the carbon particles decreases. The Cu/C composite materials with grinding times of <NUM>, <NUM> and <NUM> were selected to study their particle size distribution (<FIG>) and the conductivity of the inks made with them after painting the ink on an insulating polyethylene terephthalate (PET) substrate and letting it dry (Table <NUM>). The best conductivity was obtained when the grinding time is <NUM> and the average particle size was <NUM>. Therefore, this particle size was used for the preparation of the ink for the screen-printed electrodes.

<FIG> shows the SEM images of the rough surface of the screen-printed working electrode made of Cu/C nanocomposite. It is like the surface of any carbon (graphite) screen-printed electrode. A higher magnification SEM image reveals both the carbon and Cu particle components dispersed in the ink. Energy-Dispersive X-Ray (EDX) analysis of the electrode surface indicates the presence of <NUM> wt. % copper element in the working electrode.

<FIG> shows the chronoamperograms and the calibration curve of the SPE of the invention with the filtering element (<NUM>) not loaded with NaOH, wherein the working electrode (<NUM>) and the auxiliary electrode (<NUM>) comprise the copper nanoparticle-carbon composite-based ink whose characterization has been previously shown. In the chronoamperometric measurements, a potential of <NUM> mV vs. silver pseudo-reference electrode (<NUM>) was initially set for <NUM> by the means (<NUM>) for potential application, at which no redox reactions occurred and the current tended to zero. Then the potential was shifted to +<NUM> mV vs. silver pseudo-reference electrode (<NUM>), at which the Cu nanoparticles catalyze the electrocatalytic oxidation of organic matter and the anodic current was recorded for <NUM> by the means (<NUM>) for current measurement and recording. The total time for one measurement is <NUM> (<NUM> for allowing the sample flow to reach the electrochemical cell and <NUM> for electrochemical analysis). <FIG> displays the corresponding chronoamperometric signals for different concentrations of glucose, used as an organic standard analyte. Based on these chronoamperograms, the value of the current recorded at <NUM> time was chosen as the analytical signal. The signal increases linearly with the glucose concentration. The corresponding calibration curve is presented in <FIG>, and a linear range from <NUM> to <NUM>/L was obtained. The slope of the calibration curve was <NUM>±<NUM> nA·L/mg. The estimated limit of detection (LOD) is <NUM>/L. Water samples from effluents of wastewater treatment plants cannot show organic matter concentrations above the legal limit of COD, set to <NUM>/L, or a minimum <NUM>% reduction with relation to the organic load of the influent. Considering that, the SPE-based sensor of the invention with the filtering element (<NUM>) not loaded with NaOH results in a promising tool for the analysis of COD in wastewater.

<FIG> shows the chronoamperogram and the calibration curve of the SPE of the invention with the filtering element (<NUM>) loaded with NaOH. As before, for conducting the chronoamperometric analysis a potential of +<NUM> mV vs silver pseudo-reference electrode (<NUM>) was set by the means (<NUM>) for potential application, and the corresponding calibration curve was plotted by the means (<NUM>) for current measurement and recording. The current value at the <NUM> time was used as the analytical signal. Then a linear range from <NUM> to <NUM>/L was obtained (<FIG>), and the slope of the calibration curve was <NUM>±<NUM> nA·L/mg. The estimated limit of detection (LOD) is <NUM>/L. As mentioned above, the wastewater treatment plants have a legal COD limit in the effluents set to <NUM>/L. Thus, the SPE-based sensor with the filtering element (<NUM>) impregnated with NaOH can be applied for the analysis of COD in wastewater. Besides, said sensor is simple, convenient, and easy to operate, so it can be implemented to measure this parameter in real water samples, as shown below.

Three real samples from a wastewater treatment plant were collected and analyzed with the screen-printed electrode of the invention, having the filtering element (<NUM>) impregnated (SPE_Cu/C_filterNaOH) or not (SPE_Cu/C filter) with NaOH and compared with the performance of the same SPE but without a filtering element (SPE_Cu/C). As can be seen in Table <NUM>, the values recorded with the three approaches were quite similar, which shows that the filtering element (<NUM>) successfully performed for filtering and preconditioning the sample before the measurement. Moreover, the values recorded with the sensors are, within the error limits, consistent with the values obtained from the standard dichromate method produced by a certified laboratory. Overall, the performance of the SPE-based sensor of the invention was highly suitable to determine the COD in real water samples.

Claim 1:
A screen-printed electrode (<NUM>) for detecting a pollutant in a water sample comprising:
- a substrate (<NUM>);
- a plurality of conductive tracks (<NUM>), screen-printed over the substrate (<NUM>);
- an electrochemical cell (<NUM>), connected to said conductive tracks (<NUM>) and configured to receive the water sample, said electrochemical cell (<NUM>) further comprising:
- a working electrode (<NUM>), screen-printed over the substrate (<NUM>) and configured to enable the electrochemical degradation of the pollutant present in the water sample;
- a pseudo-reference electrode (<NUM>), screen-printed over the substrate (<NUM>) and configured to provide a stable electric potential against which the potential of the working electrode (<NUM>) is set; and,
- an auxiliary electrode (<NUM>), screen-printed over the substrate (<NUM>) and configured to provide a pathway for an electric current to flow in the electrochemical cell (<NUM>);
- an insulating layer (<NUM>), arranged over the conductive tracks (<NUM>) and adapted so as to protect said conductive tracks (<NUM>) from a liquid environment; and
- a filtering element (<NUM>), arranged in contact with the electrochemical cell (<NUM>) and configured to filter the water sample received by the electrochemical cell (<NUM>);
and characterized in that the filtering element (<NUM>) is impregnated with an electrolyte.