FIELD OF THE INVENTION
The present invention lies within the technical field of electrochemical sensors. More specifically, the invention relates to a screen-printed electrode, its manufacturing method, the electrochemical sensor comprising said electrode for detecting water pollutants, and the operating method of said sensor.
BACKGROUND OF THE INVENTION
Water pollution is an increasing global concern that damages human health, aquatic ecosystems, and economic growth. Globally, 80 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 20 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, 2017). 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 [Li et al. Crit. Rev. Anal. Chem. 2018, 48 (1), 47-65]. 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 [Moore et al. Anal. Chem. 1956, 28, 164-167; Korenaga et al. Anal. Chim. Acta 1982, 141, 301-309; Tian et al. Anal. Chim. Acta 1992, 261, 301-305; Lee et al. Electroanalysis 2000, 12, 1334-1338]. However, these methods present many disadvantages, including:
- low sensitivity and precision;
- large sample volume;
- use of hazardous materials, such as silver sulphate, mercury sulphate, and dichromate;
- chloride interference;
- long analysis time, since it requires a time-consuming sample reflux process to achieve complete oxidation (2-4 hours); and
- professional operation.
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 [Comninellis. C. Electrochim. Acta 1994, 39, 1857-1862].
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:
- activated copper electrode [Silva et al. J. Solid State Electrochem. 2009, 13 (5), 665-669];
- copper electrode modified with copper nanoparticles [Yang et al. Sens. Actuators B Chem. 2011, 153 (1), 78-82];
- glassy carbon electrode coated with nickel (Ni) nanoparticles [Cheng et al. J. Phys. Chem. C 2011, 115 (46), 22845-22850], nickel-copper (NiCu) alloy [Zhou et al. Electrochim. Acta 2012, 74, 165-170], or cobalt oxide (CoO) film [Wang et al. Anal. Chim. Acta 2012, 736, 55-61];
- boron-doped diamond electrode [Yu et al. Electrochem. Commun. 2007, 9 (9), 2280-2285];
- platinum electrode modified with lead dioxide (PbO2) [Ai et al. Electroanal. 2004, 16 (5), 404-410];
- titanium electrode coated with nano-TiO2 [Li et al. Electroanalysis. 2006, 18 (10), 1014-1018], Rh2O3[Li et al. Meas. Sci. Technol. 2006, 17 (7), 1995-2000], or TiO2/PbO2 [Li et al. Electroanal. 2006, 18 (22), 2251-2256];
- graphite-polystyrene composite electrode containing silver (II) oxide and copper (II) oxide catalysts (AgO—CuO) [Orozco et al. Anal. Chim. Acta. 2008, 607 (2), 176-182]; and,
- carbon nanotube-polystyrene composite containing different inorganic electrocatalysts (Ni, NiCu alloy, CoO, and CuO/AgO nanoparticles) [Guti6rrez-Capiten et al. Anal. Chem. 2015, 87 (4), 2152-2160].
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.
BRIEF DESCRIPTION OF THE INVENTION
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:
- a substrate;
- a plurality of conductive tracks, screen-printed over the substrate;
- an electrochemical cell, connected to said conductive tracks and adapted for receiving the water sample, said electrochemical cell further comprising:
- a working electrode, screen-printed over the substrate and adapted for providing an electric potential to the water sample received by the electrochemical cell;
- a pseudo-reference electrode, screen-printed over the substrate and adapted for providing a reference electric potential in relation to the potential of the working electrode; and,
- an auxiliary electrode, screen-printed over the substrate and adapted for providing a pathway for electric current to flow in the electrochemical cell; and,
- an insulating layer, arranged over the conductive tracks and adapted so as to protect said conductive tracks from a liquid environment.
Advantageously, 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 enablessample 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 for COD measurements.
Within the scope of interpretation of the present invention, the expression “electrochemical degradation” will be understood as an oxidation or reduction reaction of the pollutant present in the water sample.
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 5-10 μm 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 or under conditions of storage. 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:
- a screen-printed electrode (SPE) according to any of the embodiments herein described;
- means for applying an electric potential between the working electrode and the pseudo-reference electrode of said SPE; and,
- means for measuring and recording a faradaic current at said working electrode.
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:
- dispensing the water sample onto the electrolyte-impregnated filtering element;
- applying an electric potential between the working electrode and the pseudo-reference electrode by the means for potential application;
- measuring and recording a faradaic current generated at the working electrode by the means for current measurement and recording; and,
- determining the COD of the water sample from the measured faradaic current.
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:
- providing a substrate, preferably polymer-based;
- screen-printing a plurality of conductive tracks and a pseudo-reference electrode over the substrate using an electrically conductive material, preferably comprising silver paste;
- screen-printing a working and auxiliary electrode over the substrate using a metal nanoparticle-carbon composite-based ink;
- screen-printing an insulating layer, preferably using a photocurable dielectric paste, and arranging said insulating layer over the conductive tracks;
- providing an electrolyte-impregnated filtering element; and,
- arranging said electrolyte-impregnated filtering element in contact with the electrochemical cell.
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:
- preparing an aqueous sample comprising 30-35 wt % resorcinol, 0.2-0.6 wt % sodium carbonate and 64-70 wt % formaldehyde (sample A);
- preparing an aqueous sample comprising copper (II) nitrate hydrate in a concentration comprised between 0.6-0.7 mol/L (sample B);
- mixing samples A and B in a volume ratio A:B comprised between 3:1 and 3.5:1 for a period between 60-75 minutes (sample C);
- dissolving 450 to 600 mg sodium carbonate in three additions of 150 to 200 mg in sample C and stirring for a period between 1-1.5 hours until the pH is comprised between 8 and 9 (sample D);
- heating sample D at a temperature comprised between 55-65° C. for a period between 20-24 hours;
- after heating sample D, placing the resulting wet gels in a fume hood at room temperature for at least 2 days;
- carbonizing the resulting copper-carbon composite powder under an argon flux comprised between 80-120 cm3/min at a temperature between 1000-1055° C. for a period between 110-130 minutes; and,
- mixing the copper nanoparticle-carbon composite powder with nitrocellulose, preferably 15-20 wt %, prepared in 2-butoxyethyl acetate in a weight molar ratio comprised between 3:1 and 3.5:1 until the resulting paste presented a honey-like texture.
Preferably, any of the methods of fabrication of the SPE herein described can further comprise:
- providing a fixing layer containing a plurality of holes; and,
- fixing the electrolyte-impregnated filtering element to the electrochemical cell by means of said fixing layer.
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.
DESCRIPTION OF THE FIGURES
FIG. 1a shows the screen-printed electrode (SPE) of the invention according to one of its preferred embodiments. Said electrode is comprised of a substrate, a plurality of conductive tracks, an electrochemical cell, an insulating layer, and a electrolyte-impregnated filtering element, wherein said electrochemical cell further comprises a working electrode, an auxiliary electrode, and a pseudo-reference electrode. FIG. 1b shows the SPE of the invention according to another of its preferred embodiments, wherein the electrolyte-impregnated filtering element is covered with a fixing layer containing a plurality of holes. FIG. 1c shows the electrochemical sensor of the invention according to one of its preferred embodiments, said sensor comprising the SPE of the invention shown in FIG. 1a and a potentiostat powered and controlled by an electronic mobile device.
FIG. 2 shows the stepwise fabrication of the SPE of the invention according to one of its preferred embodiments: (a) providing a substrate; (b) screen-printing a plurality of conductive tracks and a pseudo-reference electrode over the substrate using an electrically conductive material; (c) screen-printing a working and auxiliary electrode over the substrate using a metal nanoparticle-carbon composite-based ink; (d) screen-printing an insulating layer, preferably using a photocurable dielectric paste, and arranging it over the conductive tracks; (e) providing an electrolyte-impregnated filtering element and arranging it in contact with the electrochemical cell; and (f) providing a fixing layer containing a plurality of holes and fixing the electrolyte-impregnated filtering element to the electrochemical cell by means of said fixing layer.
FIG. 3 shows scanning electron microscopy (SEM) 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.
FIG. 4 shows X-ray diffraction patterns of pure carbon and copper nanoparticle-carbon composite-based ink used in a preferred embodiment of the method of fabrication of the SPE of the invention.
FIG. 5 shows nitrogen (N2) adsorption and desorption isotherms of the copper nanoparticle-carbon composite-based ink used in a preferred embodiment of the method of fabrication of the SPE of the invention. Inset shows the pore size distributions determined using the BJH method.
FIG. 6 shows SEM 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 grinding at different times using Retsch Mixer Mill MM 400 at the frequency of 15 Hz.
FIG. 7 shows the particle size copper nanoparticle-carbon composite-based ink used in a preferred embodiment of the method of fabrication of the SPE of the invention grinding at different times: (A) 20 min, (B) 30 min, (C) 60 min.
FIG. 8 shows SEM images of the surface of a screen-printed electrode (SPE) using the copper nanoparticle-carbon composite-based ink used in a preferred embodiment of the method of fabrication of the SPE of the invention. The inset in (A) is the photograph of the SPE and the dimension is 1 cm×1.1 cm.
FIG. 9a shows the chronoamperometric responses of the SPE of the invention in one of its preferred embodiments; that is, with a filtering element but not loaded with NaOH. FIG. 9b shows the corresponding calibration curve. Values are the mean of three consecutive measurements and the standard deviation is drawn as error bars.
FIG. 10a shows the chronoamperometric responses of the SPE of the invention in one of its preferred embodiments; that is; with a filtering element loaded with NaOH. FIG. 10b shows the corresponding calibration curve. Values are the mean of three consecutive measurements and the standard deviation is drawn as error bars.
NUMERICAL REFERENCES USED IN THE DRAWINGS
In order to provide a better understanding of the technical features of the invention, the referred FIGS. 1-10 are accompanied by a series of numerical references which, with an illustrative and non-limiting character, are hereby represented:
|
1
Screen-printed electrode
|
2
Substrate
|
3
Conductive track
|
4
Electrochemical cell
|
5
Working electrode
|
6
Pseudo-reference electrode
|
7
Auxiliary electrode
|
8
Insulating layer
|
9
Electrolyte-impregnated filtering element
|
10
Fixing layer
|
11
Means for potential application
|
12
Means for current measurement and recording
|
13
Electronic mobile device
|
|
DETAILED DESCRIPTION OF THE INVENTION
As described in the preceding paragraphs, one object of the present invention relates to an electrochemical device, hereafter referred to as screen-printed electrode (SPE) (1), for detecting a pollutant in a water sample. In the example of the SPE chosen to illustrate the present invention (FIG. 1a), the electrode comprises:
- a polymer substrate (2);
- a plurality of conductive tracks (3), screen-printed over the substrate (2);
- an electrochemical cell (4), connected to said conductive tracks (2) and adapted for receiving the water sample, said electrochemical cell (4) further comprising:
- a working electrode (5), screen-printed over the substrate (2) and adapted for providing an electric potential to the water sample received by the electrochemical cell (4);
- a pseudo-reference electrode (6), screen-printed over the substrate (2) and adapted for providing a reference electric potential in relation to the potential of the working electrode (5);
- an auxiliary electrode (7), screen-printed over the substrate (2) and adapted for providing a pathway for electric current to flow in the electrochemical cell (4); and,
- an insulating layer (8), arranged over the conductive tracks (3) and adapted so as to protect said conductive tracks (3) from a liquid environment.
Advantageously, the SPE (1) of the invention further comprises an electrolyte-impregnated filtering element (9), preferably of a porous paper material, that is arranged in contact with the electrochemical cell (4). This electrolyte-impregnated filtering element (9) not only filters the water sample received by said electrochemical cell (4), but also preconditions the sample without any user intervention (i.e. adjusting the pH and conductivity of the water sample so that the electrochemical measurement is carried out under optimized conditions). Thus, the simple addition of a water sample drop is required to carry out water pollutant detection by the SPE of the invention. This means saving time in pre-processing and preconditioning samples while avoiding any possible contamination thereof by user manipulation, maintaining the quality of samples as they were collected. Preferably, the filtering element (9) 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 for COD measurements.
The electrolyte-impregnated filtering element (9) can be covered with a fixing layer (10) containing a plurality of holes, preferably of a plastic material. Said fixing layer (10) fixes the filtering element (9) to the electrochemical cell (4), allowing the water sample to flow through the filtering element (9) and reach the electrochemical cell (4) through the plurality of holes (FIG. 1b). Optionally, said fixing layer (10) can be covered with a removable protective layer to prevent contamination or electrode degradation before use or under conditions of storage. Said protective layer can be easily removed just before using the SPE for water pollution analysis.
The working electrode (5) and, optionally, the auxiliary electrode (7) comprise/s a metal nanoparticle-carbon composite-based ink, preferably, a copper nanoparticle-carbon composite-based ink.
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:
- a SPE (1) according to any of the embodiments herein described;
- means (11) for applying an electric potential between the working electrode (5) and the pseudo-reference electrode (6) of said SPE (1); and,
- means (12) for measuring and recording a faradaic current at said working electrode (5).
Said means (11, 12) for potential application and for current measurement and recording are preferably comprised in a portable potentiostat powered and controlled by an electronic mobile device (13) (FIG. 1c).
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:
- dispensing the water sample onto the electrolyte-impregnated filtering element (9);
- applying an electric potential between the working electrode (5) and the pseudo-reference electrode (6) by the means (11) for potential application;
- measuring and recording the faradaic current generated at the working electrode (7) by the means (12) for current measurement and recording; and,
- determining the COD of the water sample from the measured faradaic current.
A fourth object of the present invention relates to a method of fabrication of the SPE (1) herein described (see FIG. 2). Said method comprises performing the following steps in any technically possible order:
- providing a substrate (2), preferably polymer-based;
- screen-printing a plurality of conductive tracks (3) and a pseudo-reference electrode (6) over the substrate (2) using an electrically conductive material, preferably comprising silver paste;
- screen-printing a working (5) and auxiliary (7) electrode over the substrate (2) using a metal nanoparticle-carbon composite-based ink, preferably comprising a copper nanoparticle-carbon composite;
- screen-printing an insulating layer (8) using a photocurable dielectric paste, and arranging said insulating layer (8) over the conductive tracks (3);
- providing an electrolyte-impregnated filtering element (9); and,
- arranging said electrolyte-impregnated filtering element (9) in contact with the electrochemical cell (4).
Said copper nanoparticle-carbon composite-based ink is prepared as follows:
- preparing an aqueous sample comprising 30-35 wt % resorcinol, 0.2-0.6 wt % sodium carbonate and 64-70 wt % formaldehyde (sample A);
- preparing an aqueous sample comprising copper (II) nitrate hydrate in a concentration comprised between 0.6-0.7 mol/L (sample B);
- mixing samples A and B in a volume ratio A:B comprised between 3:1 and 3.5:1 for a period between 60-75 minutes (sample C);
- dissolving 450 to 600 mg sodium carbonate in three additions of 150 to 200 mg in sample C and stirring for a period between 1-1.5 hours until the pH is comprised between 8 and 9 (sample D);
- heating sample D at a temperature comprised between 55-65° C. for a period between 20-24 hours;
- after heating sample D, placing the resulting wet gels in a fume hood at room temperature for at least 2 days;
- carbonizing the resulting copper-carbon composite powder under an argon flux of 80-120 cm3/min at a temperature between 1000-1055° C. for a period between 110-130 minutes; and,
- mixing the copper nanoparticle-carbon composite powder with 15-20 wt % nitrocellulose prepared in 2-butoxyethyl acetate in a weight molar ratio comprised between 3:1 and 3.5:1 until the resulting paste presented a honey-like texture.
Optionally, the method of fabrication of the invention can comprise the step of impregnating the filtering element (9) with an electrolyte.
Preferably, any of the methods of fabrication described above can further comprise:
- providing a fixing layer (10) containing a plurality of holes; and,
- fixing the electrolyte-impregnated filtering element (9) to the electrochemical cell (4) by means of said fixing layer (10).
In this case, the water sample is dispensed onto the electrolyte-impregnated filtering element (9) through the holes of the fixing layer (10).
A fifth object of the present invention relates to the use of the SPE (1) 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.
Characterization of Microstructure and Properties of Copper Nanoparticle-Carbon Composite-Based Ink
FIG. 3 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. 4. The broad bumps located around 26 values of 23.5° and 43.5° are characteristic of pure amorphous C. The diffraction peaks observed are characteristic of face-centered cubic (fcc) crystalline copper, corresponding to the planes (111), (200) and (220), at 26 values of ca. 43.2°, 50.4° and 74.1°, 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. 5). According to the Brunauer-Emmett-Teller (BET) model, the surface areas were calculated to be 45 m2/g. The adsorption uptake at low nitrogen relative pressures (P/P0=0.0-0.1) indicates that the Cu/C material presents a lot of micropores (diameter <2 nm). The slope of the isotherms at intermediate relative pressures (0.3<P/P0<0.8) and the increase in the adsorbed volume at high relative pressures (0.9<P/P0<1.0) reveal the existence of mesopores (2-50 nm) and macropores (50-7500 nm), respectively. The total pore volume calculation was 0.043 cm3/g based on the N2 amount adsorbed at a relative pressure P/P0 of ca. 0.995. 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. 6 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 20 min, 30 min and 60 min were selected to study their particle size distribution (FIG. 7) 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 1). The best conductivity was obtained when the grinding time is 30 min and the average particle size was 10.86 μm. Therefore, this particle size was used for the preparation of the ink for the screen-printed electrodes.
TABLE 1
|
|
Values of the conductivity of inks prepared
|
with Cu/C of different particle sizes.
|
Grinding time
Particle size
Conductivity
|
(min)
(μm)
(S/cm)
|
|
20
12.62
0.69 ± 0.27
|
30
10.86
1.32 ± 0.26
|
60
7.44
0.39 ± 0.06
|
|
FIG. 8 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 4.3 wt. % copper element in the working electrode.
Example of the Electrochemical Performance of the SPE-Based Sensor of the Invention for Measuring COD in a Water Sample
FIG. 9 shows the chronoamperograms and the calibration curve of the SPE of the invention with the filtering element (9) not loaded with NaOH, wherein the working electrode (5) and the auxiliary electrode (7) comprise the copper nanoparticle-carbon composite-based ink whose characterization has been previously shown. In the chronoamperometric measurements, a potential of 0.0 mV vs. silver pseudo-reference electrode (6) was initially set for 30 s by the means (11) for potential application, at which no redox reactions occurred and the current tended to zero. Then the potential was shifted to +800.0 mV vs. silver pseudo-reference electrode (6), at which the Cu nanoparticles catalyze the electrocatalytic oxidation of organic matter and the anodic current was recorded for 60 s by the means (12) for current measurement and recording. The total time for one measurement is 690 s (600 s for allowing the sample flow to reach the electrochemical cell and 90s for electrochemical analysis). FIG. 9a 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 90 s time was chosen as the analytical signal. The signal increases linearly with the glucose concentration. The corresponding calibration curve is presented in FIG. 9b, and a linear range from 0 to 394 mg/L was obtained. The slope of the calibration curve was 5.9±0.2 nA L/mg. The estimated limit of detection (LOD) is 24.4 mg/L. Water samples from effluents of wastewater treatment plants cannot show organic matter concentrations above the legal limit of COD, set to 125 mg/L, or a minimum 75% reduction with relation to the organic load of the influent. Considering that, the SPE-based sensor of the invention with the filtering element (9) not loaded with NaOH results in a promising tool for the analysis of COD in wastewater.
FIG. 10 shows the chronoamperogram and the calibration curve of the SPE of the invention with the filtering element (9) loaded with NaOH. As before, for conducting the chronoamperometric analysis a potential of +800.0 mV vs silver pseudo-reference electrode (6) was set by the means (11) for potential application, and the corresponding calibration curve was plotted by the means (12) for current measurement and recording. The current value at the 90 s time was used as the analytical signal. Then a linear range from 0 to 394 mg/L was obtained (FIG. 10b), and the slope of the calibration curve was 1.62±0.04 nA·L/mg. The estimated limit of detection (LOD) is 25.99 mg/L. As mentioned above, the wastewater treatment plants have a legal COD limit in the effluents set to 125 mg/L. Thus, the SPE-based sensor with the filtering element (9) 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.
Analysis of Wastewater Samples Using the SPE-Based Sensor of the Invention
Three real samples from a wastewater treatment plant were collected and analyzed with the screen-printed electrode of the invention, having the filtering element (9) 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 2, the values recorded with the three approaches were quite similar, which shows that the filtering element (9) 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.
TABLE 2
|
|
COD analysis of real water samples using SPEs and standard dichromate method.
|
Sample 2
Sample 3
|
Sample 1
Primary
Initial
|
Effluent
treatment
process
Soaking
Pre-
|
Electrodes
(mgL−1O2)
(mgL−1O2)
(mgL−1O2)
time
filtered
|
|
SPE_Cu/C
38.9 ± 4.6
98.8 ± 2.7
220 ± 10.8
10 min
Yes
|
SPE_Cu/C_filter
39.6 ± 3.8
96.4 ± 6.6
218 ± 11.2
10 min
Yes
|
SPE_Cu/C_filter
41.2 ± 4.2
99.2 ± 4.8
222.4 ± 13.6
10 min
No
|
SPE_Cu/C_filterNaOH
40.1 ± 5.4
97.8 ± 6.7
221.1 ± 14.5
10 min
No
|
SPE_Cu/C_filterNaOH
42.1 ± 6.2
100.3 ± 2.1
228.5 ± 18.4
10 min
No
|
Dichromate method
37.2 ± 7.8
84.9 ± 17.8
210 ± 25
Yes
|
|
Patent Application
20250012773
