WO2021149024A1

WO2021149024A1 - Antibiofouling device for optical sensors, methods and uses thereof

Info

Publication number: WO2021149024A1
Application number: PCT/IB2021/050549
Authority: WO
Inventors: Graça Maria HENRIQUES MINAS; Luís Miguel VALENTE GONÇALVES; Vânia Cristina GONÇALVES PINTO; Paulo Jorge TEIXEIRA DE SOUSA
Original assignee: Universidade Do Minho
Priority date: 2020-01-24
Filing date: 2021-01-25
Publication date: 2021-07-29
Also published as: EP4093903A1

Abstract

The present disclosure relates to an electrolytic cell used as an antibiofouling device for optical sensors comprising: an anode of glass coated with conductive fluorine tin oxide and platinum nanoparticles, for the production of active chlorine upon seawater electrolysis; and a cathode. The present disclosure also relates to the use of the electrolytic cell as a biofouling preventer. Another aspect of the present disclosure relates to an optical sensor comprising the electrolytic cell as an antibiofouling device. The disclosure also comprises a method to obtain the anode.

Description

ANTIBIOFOULING DEVICE FOR OPTICAL SENSORS, METHODS

AND USES THEREOF

TECHNICAL FIELD

[0001] The present disclosure relates to an electrolytic cell used as an antibiofouling device for optical sensors, comprising an anode, made of glass coated with conductive fluorine tin oxide and platinum nanoparticles, and a cathode. The anode is transparent to visible light and therefore suitable for application in optical marine sensors. The prevention of biofouling is achieved by the local production of active chlorine upon seawater electrolysis.

BACKGROUND

[0002] The increasing concern about the health of marine ecosystems lead to the development of sensors for continuous monitoring of various parameters, such as the pH changes, dissolved oxygen, conductivity, turbidity, phytoplankton species, among others.

[0003] Most of the sensors used in monitorization of oceanography are based on optical devices, however, they are strongly affected by biofouling which decreases their sensitivity and life-time. Therefore, it is vital to limit as best as possible the biofouling on the sensor detection area.

[0004] Several techniques have been used for biofouling prevention: mechanical devices (wipers or scrapers), antifouling paints [tributyltin (TBT) and copper compounds], UV irradiation and biocide generation systems. Mechanical devices need moving parts and are not suitable for use in remote sensors, due to the needed maintenance after several operation cycles, otherwise the risk of their degradation is high. Protection based on TBT paints has proved to be extremely efficient, however, they have been shown to have harmful effects on the environments and, consequently, these compounds were banned for use in antifouling paints. The UV irradiation is a suitable solution for biofouling protection, but the high energy consumption needed for their use make it incompatible with underwater, and mainly with long-term monitoring systems (1).

[0005] Biocide generation systems are a widely used alternative for biofouling prevention. Their operation is based on the local production of oxidants from seawater, such as active chlorine that acts as a toxic agent for microorganisms preventing their growing in the sensors surfaces. The main advantages of the localised chlorine generation are the low-cost production, high reactivity and easiness of synthesis. It allows to control the amount of chlorine biocide to the minimum required to avoid biofouling, maintaining the sensor area cleaned without negative impact on marine environment. This technology is also used in treatment systems to produce potable water. Usually, in this approach, the used electrodes are based on platinum (Pt), dimensionally stable anodes (DSAs) of a Ti-support coated by noble metal oxides such as ruthenium, iridium, tantalum and zirconium, and doped diamond electrodes.

[0006] Pt electrodes possess excellent properties, such as high electrical conductivity, a porous surface and high catalytic activity and stability. However, the high cost of these electrodes has limited their application. The use of conventional electrodes based on noble metal oxides for chlorine generation is not suitable for optical sensors protection due to its opacity. So, the biofouling prevention methods based on chlorine generation for use in aquatic optoelectronic devices require electrodes which simultaneously offer optical transparency and electrical conductivity.

[0007] Some solutions related with the preparation of antibiofouling devices using biocide agents were already developed.

[0008] The work of Xue et al. (2) proposes a platinum/indium tin oxide (Pt/ITO) surface to electrochemically generate chlorine, that avoid biofouling in glass substrates. The ITO surface is coated with Pt via an electrodeposition approach. It was shown that Pt/ITO is more sustainable and efficient than the bare ITO in natural seawater, and the electrochemical chlorination effectively prevents the glass from fouling. Nevertheless, the Pt nanoparticles aggregate and form a thin film, decreasing the surface area available for seawater electrolysis. [0009] The work of Debiemme-Chouvy etal. (3) reports an efficient oxidation of chloride and bromide ions present in seawater to form biocidal hypohalogenous acids, namely HOCI and HOBr. Underwater optical instruments can be protected against biofouling with this approach, as the glass part is coated with a transparent antimony doped tin dioxide layer. The environmental damages are reduced because hypohalogenous acids are produced at a low level and on the window itself. Additionally, the surface to be protected can be pre-treated with organic molecules as bovine serum albumin. In such conditions, a chlorinated and brominated organic deposit is formed on the tin oxide surface that inhibits the bacterial growth at the surface. Although this strategy showed a great efficiency, the process needed to prepare the films is rather expensive. Also, no data is provided regarding the optical properties of the electrodes after production and upon electrolysis.

[0010] Laurent et a!., (4) reported an antibiofouling strategy to protect sensors and optical devices. In this work, biofouling protection is based on the coating of the optical window by a transparent conductive layer. The electrode is prepared from a transparent film of tin dioxide doped with fluorine and antimony, using a process of pyrolysis at 545 °C. Although the antibiofouling properties are maintained for up to 3 years, the fabrication process is expensive and requires high temperatures.

[0011] The document WO/2016/009166 comprises an electrochlorination apparatus for generation of a sodium hypochlorite solution. The apparatus is intended to be used for water treatment, and comprises an electrolytic cell and a brine tank that is located above the electrolytic cell. Regardless, this apparatus is not intended to be used with optical sensors, as none of the electrodes is transparent.

[0012] The document US/2017/0101329 refers to a hydrogen peroxide generator to decrease biofouling in water sensors. Hydrogen peroxide is generated electrochemically at the cathode of an electrochemical cell and can be integrated in the water sensor. Although the cathode is transparent to visible light, the generated biocide is hydrogen peroxide. Hydrogen peroxide in seawater is rapidly degraded and inactivated by bacterial enzymes, and thus, a higher dose and low temperatures are needed to avoid biofouling.

B [0013] The document US4488945 describes the electrochemical production of chlorine from seawater as a method to prevent the biofouling. However, the electrodes used are not transparent and, therefore, cannot be applied to protect optical sensors.

[0014] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.

GENERAL DESCRIPTION

[0015] The present disclosure relates to an electrolytic cell used as an antibiofouling device for optical sensors comprising an anode (electrode), and a cathode (electrode), wherein the anode is made of a glass substrate coated with conductive fluorine tin oxide and platinum nanoparticles. The anode is used for the production of active chlorine upon seawater electrolysis.

[0016] In an embodiment, the advantageous features of the Pt material and the transparent conductive oxides, lead to a convenient method for biofouling protection of optical sensors based on localised chorine generation from seawater.

[0017] An aspect of the present disclosure relates to a simple and low-cost method to prepare transparent anodes with high catalytic efficiency using equipment and procedures usually available in most laboratories. Anodes comprising fluorine tin oxide (FTO) coated with Pt nanoparticles (Pt-FTO) supported on glass substrates were chosen for electrochlorination. The coating of Pt nanoparticles was used to improve the corrosion resistance and increase substantially the catalytic activity and stability.

[0018] The antibiofouling device related to the present disclosure can be used on optical sensor's surfaces, wherein antibiofouling refers to the protection of wetted surfaces from the accumulation of biological agents as microorganisms, plants, algae or small animals.

[0019] In an embodiment, the operation principle of the antibiofouling device is based on chlorine generation by seawater electrolysis, using the anode of the present disclosure. [0020] The present disclosure relates to an electrolytic cell for antibiofouling of optical sensors comprising an anode, and a cathode. The anode further comprises a glass substrate coated with conductive fluorine tin oxide and platinum nanoparticles, for the production of active chlorine upon seawater electrolysis.

[0021] In an embodiment, the conductive fluorine tin oxide supported on glass substrate is coated with dispersed platinum nanoparticles, or platinum nanoparticle aggregates, or mixtures thereof.

[0022] In an embodiment, the platinum nanoparticles used to prepare the anode have a size between 10 and 20 nm, preferably 11-15 nm, more preferably 11.5-11.7 nm, measured by scanning electron microscopy.

[0023] In an aspect of the present disclosure, the size of platinum nanoparticle aggregates ranges between 188-210 nm, and have a dispersion of 4 x 10^s - 6 x 10^s nanoparticle aggregates/cm², preferably 4.25 x 10⁸ - 5 x 10⁸ nanoparticle aggregates/cm².

[0024] In an embodiment, the cathode of the electrolytic cell is preferably made of stainless-steel, preferentially 316 type.

[0025] In an embodiment, the anode and the cathode of the electrolytic cell are connected by a copper wire. The wire is bonded to the electrodes with silver conductive paste, and the connection area is impermeabilized with a resin, preferably epoxy.

[0026] In an embodiment, the surface of the glass substrate coated with conductive fluorine tin oxide is rough and presents a crystalline structure. The surface has a roughness between 15 to 25 nm, preferably 20.1 nm, measured by atomic force microscopy.

[0027] In an embodiment, the anode is transparent to visible light.

[0028] In an aspect, the present disclosure also relates to the use of the electrolytic cell, related to the present disclosure, as a biofouling preventer.

[0029] The present disclosure also relates to an optical sensor comprising the use of the electrolytic cell described in the present disclosure as an antibiofouling device, preferably a sensor, an actuator, or a transducer for marine applications, preferably for quantification of oxygen, chlorophyll, sediments, turbidimetry, salinity and phytoplankton, water currents measuring, energy harvesting, underwater communications, among others.

[0030] Another aspect of the present disclosure comprises a method to obtain an anode for the electrolytic cell described in the present disclosure, comprising the following steps: (i) cleaning the glass substrate coated with conductive fluorine tin oxide using an ultrasonic bath in ethanol, preferably for 15 minutes, followed by an ultrasonic bath in deionized water, preferably for 15 minutes; (ii) drying the glass substrate coated with conductive fluorine tin oxide with nitrogen flow; (iii) depositing a solution of hexachloroplatinic acid hexahydrate with a concentration between 1 and 30 mM, preferably 5-20 mM, by spin coating, using 2500 to 3500 rpm for 10 to 30 seconds at room temperature. Preferably, 3000 rpm for 20 seconds; (iv) annealing the anode for 10 to 20 minutes in an oven at 400 to 500 °C, preferably 440 to 460 °C for 15 minutes; (v) cleaning the anode with acetone; and (vi) drying the anode under nitrogen flux.

[0031] In an embodiment, the present disclosure also relates to a method to prevent the biofouling on optical devices, using the electrolytic cell described in the present disclosure, comprising seawater electrolysis on the described anode, preferably applying an electric current between 150-500 mA, more preferably an electric current between 200-300 pA; and chlorine generation from the seawater electrolysis process, wherein chlorine acts as a biocide agent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention.

[0033] Figure 1: Schematic representation of an embodiment of the new approach for biofouling prevention by seawater electrochlorination. The electrode used as anode is constituted by a Pt nanoparticle coated transparent conductive oxide thin film supported on glass substrates, responsible for chlorine generation in the sensitive area of the optical sensor. [0034] Figure 2: Photographic representation of an embodiment of a setup for chlorine generation.

[0035] Figure 3: Illustration of results of an embodiment comprising FTO surfaces. Scanning electron microscopy (SEM) images of (A) native FTO surface, (B) Pt-FTO (5 mM) and (C) Pt-FTO (20 mM). The zone 1 represents the area where Energy-dispersive X-ray spectroscopy (EDS) analysis was made. The values presented in Figure 3B show the individual nanoparticle size and the values of Figure 3C show the nanoparticle aggregates size.

[0036] Figure 4: Illustration of results of an embodiment comprising ITO surfaces. SEM images of (A) native ITO surface, (B) Pt-ITO (5 mM) and (C) Pt-ITO (20 mM). The zone 1 represents the area where Energy-dispersive X-ray spectroscopy (EDS) analysis was made. The values presented in Figure 4C show the nanoparticle aggregates size.

[0037] Figure 5: Illustration of results of an embodiment comprising FTO surfaces. Atomic force microscopy (AFM) images of (A) native FTO surface, (B) Pt-FTO (5 mM) and (C) Pt-FTO (20 mM).

[0038] Figure 6: Illustration of results of an embodiment comprising ITO surfaces. AFM images of (A) native ITO surface, (B) Pt-ITO (5 mM) and (C) Pt-ITO (20 mM).

[0039] Figure 7: Illustration of results of an embodiment comprising FTO surfaces. Energy-dispersive X-ray spectroscopy (EDS) spectrum of: A) native FTO (zone 1 of the Figure 3A) and B) Pt-FTO (zone 1 of the Figure 3C).

[0040] Figure 8: Illustration of results of an embodiment comprising ITO surfaces. EDS spectrum of: A) native ITO and B) Pt-ITO (zone 1 of the Figure 4C).

[0041] Figure 9: Illustration of results of an embodiment comprising the optical transmission spectra of: (A) ITO and (B) FTO based electrodes.

[0042] Figure 10: Illustration of results of an embodiment related to the linear sweeping voltammetry (LSV) responses of different electrodes in (A) NaCI solution of 30 g/L and (B) natural seawater. [0043] Figure 11: Illustration of results of an embodiment related to the linear sweeping voltammetry (LSV) responses of different electrodes in NaCI solution of 30 g/L after a week of chlorine generation.

[0044] Figure 12: Illustration of results of an embodiment comprising the concentration of chlorine as a function of current and time for seawater electrolysis using: (A) native FTO, (B) Pt-FTO (5 mM), (C) Pt-FTO (20 mM) and (D) Pt-ITO (20 mM).

[0045] Figure 13: Illustration of results of an embodiment comprising the current efficiency (CE) of different anodes for chlorine generation with the electrolysis time of 60 minutes in seawater (~28 g/L) at room temperature for different currents.

[0046] Figure 14: Illustration of results of an embodiment related to (A) Optical transmission of the solutions before and after chlorine generation; (B) Photograph representation of an embodiment of the solutions with seawater and algae culture before (Bl) and after (B2) chlorine generation.

[0047] Figure 15: Illustration of results of an embodiment related to the optical transmission of the Pt-FTO electrode and control glass slide during 3 months.

[0048] Figure 16: Illustration of results of an embodiment comprising microscope images (magnification of 10x) of: (A) Pt-FTO (20mM) surface and (B) glass slide surface (control) after 3 months of tests.

[0049] Figure 17: Illustration of results of an embodiment comprising (A) SEM image and (B) EDS spectrum of Pt-FTO (20 mM) electrode after chlorine generation during 3 months.

[0050] Figure 18: Photographic representation of an embodiment of a setup comprising a turbidity optical sensor integrating the chlorine generation system.

[0051] Figure 19: Photographic representation of an embodiment of a setup comprising the spatial distribution of the produced chlorine by the developed system.

DETAILED DESCRIPTION

[0052] The present disclosure relates to an electrolytic cell used as an antibiofouling device for optical sensors comprising: an anode of glass coated with conductive fluorine tin oxide and platinum nanoparticles, for the production of active chlorine upon seawater electrolysis; and a cathode. The present disclosure also relates to the use of the electrolytic cell as a biofouling preventer.

[0053] Another aspect of the present disclosure relates to an optical sensor comprising the electrolytic cell used as an antibiofouling device. The disclosure also comprises a method to obtain the anode.

[0054] In an embodiment, the electrocatalytic effect of native fluorine tin oxide electrodes (FTO) supported on glass substrate was improved by impregnation of platinum (Pt) nanoparticles onto its surface, using a low-cost and easy fabrication process while keeping the high optical transparency. Native FTO as well as indium tin oxide (ITO)-based electrodes were used as comparative examples. Pt-FTO electrodes showed a better performance to chlorine generation with a high binding stability of Pt nanoparticles on its surface.

[0055] An aspect of the present disclosure comprises the roughness of the Pt-FTO surfaces that provides a high surface area, allowing more active sites for chlorine generation and a significant improvement of the catalytic properties.

[0056] In an embodiment, Pt nanoparticles were impregnated on the surface of conductive FTO, ensuring a high surface area for catalysis and, consequently, a higher density of catalytic sites. The transparent electrodes of the present disclosure can be easily integrated on top of optical sensors without interference on measurements due to their high and stable optical transparency.

[0057] In an aspect of the present disclosure, an electrolytic cell comprising the Pt-FTO anode generates sufficient biocide concentration with a low electrical current. An advantage of the present disclosure is their low power consumption (200-500 mA) that allows continuous biofouling protection and, consequently, could be used for several months in autonomous submerged operation. An embodiment of the working principle and advantages of this approach is reported in Figure 1.

[0058] In an embodiment, the Pt-FTO electrodes related to the present disclosure can be easily assembled on optical windows of marine sensors, and the system can produce an adjustable chlorine concentration with a low power consumption (0.5-1.75 mW), allowing a continuous biofouling protection during several months in autonomous submerged operation.

[0059] In an embodiment, the antibiofouling property is given by electrolysis of seawater at the anode surface producing active chlorine, as illustrated in Figure 1 and Figure 2. Chlorine then acts as a toxic agent for microorganisms, preventing their growing in the sensors surface.

[0060] In an embodiment, chlorine is produced using a current between 200 and 500 mA, preferably 250 - 350 mA. The current efficiency for 60 minutes of electrolysis is higher than 72% and the antibiofouling properties are preserved for more than one month.

[0061] In an aspect of the present disclosure, the antifouling properties of the electrolytic cell comprising the Pt-FTO electrode of the present disclosure were maintained up to 3 months.

[0062] In a preferred embodiment, the anode preparation started by cleaning the glass substrates, coated with conductive FTO or ITO, using ultrasonic bath in ethanol for 15 min, followed by 15 minutes in deionized water, and drying with nitrogen flow. Then, hexachloroplatinic acid hexahydrate (H₂PtCl₆.6H₂0) solutions (5 and 20 mM) as a Pt precursor were deposited on FTO and ITO-coated glasses by spin coating at 3000 rpm for 20 seconds at room temperature. These electrodes were next annealed in the oven at 450 °C for 15 minutes to form the Pt nanoparticles. Finally, the electrodes were subsequently cleaned with acetone and dried under nitrogen flux. The electrical connection was made using a copper wire that was bonding to the electrodes with silver conductive paste. The connection area was also protected with impermeable epoxy.

[0063] In an embodiment, the cathode of the electrolytic cell of the present disclosure does not need to be transparent, since the chlorine is only produced at the anode. For the purpose of the present disclosure, the selected material for the cathode was the stainless-steel (316 type), due to its high mechanical strength and high corrosion resistance against seawater (5). In an embodiment, the effective area of the anodes and cathodes used for all experiments was 5 cm². [0064] In an embodiment, electrodes of FTO and Pt-FTO were prepared and characterized by scanning electron microscopy (SEM; Auriga Compact-Zeiss) and atomic force microscopy (AFM; Veeco, USA). Pt nanoparticles were obtained by spin coating and annealing of 5 mM and 20mM FhPtCle.SFhO solution. Figure S and Figure 5 illustrate the results of the present embodiment. As can be seen in Figures SA and 5A, the native FTO electrode surface, before Pt deposition, shows a typical roughness (Ra, arithmetic mean high) of a crystalline structure (Ra of 20.1 nm). After deposition and thermal treatment, it is verified that the Pt nanoparticles size and distribution are strongly influenced by the concentration of FhPtCle.SFhO solution (5 and 20 mM) used for Pt deposition onto FTO surface (Figure 3B and 3C). For the lower concentration (5 mM of H₂PtCl₆.6H₂0 solution), the Pt-FTO surface (Figures 3B and 5B) shows a uniformly distribution of the individual Pt nanoparticles strongly anchored in the FTO crystalline structure, with a size range of a few nanometres (approximately 12 nm, Figure 3B). The higher concentrations (20 mM of E^Rΐϋ_d.dE^O solution) lead to formation of agglomerates of Pt nanoparticles with a size between 188-210 nm, and a uniform distribution of them over the FTO surface of 4 x 10^s to 6 x l0⁸ nanoparticle aggregates/cm² (Figures 3C and 5C). Individual Pt nanoparticles were also founded on these surfaces. A small increase of the roughness of Pt-FTO surfaces was verified (Ra = 26.5 nm) for the two depositions processes, since the Pt nanoparticles are impregnated in the crystalline structure (Figure 3B and 3C).

[0065] In another embodiment, indium tin oxide (ITO) and ITO coated with platinum nanoparticles (Pt-ITO) were prepared and also characterized by means of SEM and AFM. Pt nanoparticles were obtained by spin coating and annealing of 5 mM and 20mM H₂PtCl₆.6H₂0 solution. Figures 4 and 6 illustrate the results of the present embodiment. The surface of ITO is very flat with a Ra of 0.531 nm (Figure 4A). However, a non-uniform distribution of Pt nanoparticles was verified on the Pt-ITO surface (Figure 4B e 4C). Since ITO substrate has a very flat surface and, consequently, a smaller surface area, the Pt nanoparticles were only adsorbed on its surface. AFM images of Figure 6B and 6C show a small change in the ITO surface roughness to 0.702 and 1.55 nm when was used a Pt solution of 5 and 20 mM, respectively. [0066] In an embodiment, a chemical composition analysis was performed to confirm the Pt deposition onto FTO and ITO surfaces, via energy-dispersive X-ray spectroscopy (EDS, INCA Energy). Figures 7 and 8 illustrate the results of the present embodiment. Figure 7A and 8A show the EDS spectra of the native FTO and ITO surface, respectively, without Pt on their composition. The spectra of Figures 7B and 8B successfully confirmed the presence of the Pt element, after the deposition process with a Pt concentration of 20 mM.

[0067] In an aspect of the present disclosure, the optical transmission of the fabricated anodes was studied by Ultraviolet-Visible (UV-VIS) spectroscopy, using a spectrophotometer composed by a 170 W tungsten light source (Newport NRC- 6334NS), a monochromator (Newport 74125), an optical fiber (Newport Standard Grade FS Fiber Optic), used to guide the light through the photodiode (Hamamatsu, S1336- 5BQ), and a picoammeter (Keithley 487) to measure the photodiode current. The results of the present embodiment are depicted in Figure 9, that shows the optical transmission of the electrodes composed by: FTO, Pt-FTO, ITO and Pt-ITO as well the Pt-ITO and Pt- FTO after chlorine generation. The optical transmission spectra show a good transparency in the range of 400 - 750 nm with a maximum decrease of ~6% when the Pt nanoparticles were deposited in FTO or ITO films. The ITO show a higher transmittance value comparatively to the FTO.

[0068] In a preferred embodiment, the electrolytic cell comprising the transparent anodes and the stainless-steel cathode is placed in a 3D printed support that ensures a fixed distance between both electrodes (2.6 cm). The support with the electrolytic cell is placed inside a glass container in which solutions of seawater and algae culture are present.

[0069] In an embodiment, the transmittance value of Pt-ITO increased after chlorine generation at the surface. This fact can relate to the release of Pt nanoparticles from ITO surface as the optical transmittance is very similar to the native ITO.

[0070] In an aspect of the present disclosure, the optical transmission of the Pt-FTO after chlorine generation was preserved proving the optical stability of the electrode (line with circle markers in Figure 9B). [0071] In an embodiment, the electrocatalytic properties of the fabricated electrodes were evaluated by linear sweeping voltammetry (LSV; Gamry Reference 600TM potentiostat/galvanostat) using a NaCI aqueous solution (30 g/L) and seawater (~28 g/L) with a current of 500 mA and a sweeping rate of 100 mV/minute. Figure 10 illustrates the results of the present embodiment. The anodic current of the Pt-FTO electrode increased significantly after 1.2 V, which represents the chlorine evolution potential. These values are very close to the theorical value for chlorine evolution potential (1.17 V) (6). The electrodes with a higher Pt nanoparticles concentration (20 mM) allow a higher current at the same electrical potential, demonstrating the high catalytic activity. The native FTO only started the catalysis for chlorine generation at 1.6 V and with a lower current. This potential is closer to the oxygen evolution (2 V) (6), which can affect the efficiency of chlorine generation.

[0072] In an aspect of the present embodiment, for the ITO and Pt-ITO (5 mM) anodes, the LSV curves showed the inflection point at 1.75 V. This potential is similar to the oxygen evolution potential, which can compromise the chlorine production. In the Pt- ITO (20 mM) anode it was verified a reduction of potential to 1.4 V. In all ITO-based anodes, the current obtained was considerably lower comparatively to the FTO-based anodes.

[0073] In an aspect of the present disclosure, the results demonstrated that the Pt nanoparticles work as the favourable catalyst for chlorine generation by decreasing the electrical potential needed for its production. A reduction of 0.4 V in the chlorine evolution potential and an increase in the current was verified in the Pt-FTO comparatively to the native FTO anodes. The gap between chlorine and oxygen evolution potential increased with Pt nanoparticles impregnation, allowing a more specific chlorine generation. These parameters proved the better electrocatalytic activity of Pt-FTO anodes. For the ITO case, the Pt effect was less evident which can be explained by the increase of the electrical resistance resulting from the deposition process.

[0074] In an embodiment, LSV analysis tested with other current values, as 50, 100, 200 and 300 mA, demonstrated that the potential for starting the chlorine generation remains the same with different current values.

IB [0075] Figure 11 represents the results related to an embodiment where the LSV analysis was performed after a week of chlorine generation, to prove the stability of electrodes related to the present disclosure. The Pt-FTO electrodes showed a similar electrical behaviour to the results obtained before chlorine generation (Figure 10A). On the other hand, in the Pt-ITO electrodes it is evident the loss in the catalytic activity (less 50%). This fact proves that Pt-ITO electrodes deteriorate during the chlorine generation with a significantly decrease of the current.

[0076] In an embodiment, the chlorine concentration generated from seawater electrolyse with different currents (200, 300 and 500 mA) is given in Figure 12. The comparison was performed using native FTO, Pt-FTO (5 mM) and Pt-FTO (20 mM). In the case of ITO anodes, only Pt-ITO (20 mM) was used, since the native and Pt-ITO (5 mM) showed, by previous tests, weak performance for chlorine generation due its lower catalytic activity. The ITO did not show a high stability for long-term chlorine production since a fast degradation of the anode was verified after a week of chlorine production (Figure 11). Free chlorine produced from electrolysis of seawater (as function electrolysis time and applied current) was quantified by DPD (N,N diethyl-p-phenylene diamine) colorimetric methods. The DPD reacts with free chlorine producing a colour change to pink, which intensity is proportional to the concentration of the available free chlorine. The colour intensity, and respective chlorine concentration, was measured using spectrophotometric techniques.

[0077] Figure 12 depicts the results of an embodiment, wherein a higher current and longer electrolysis time are associated with a higher concentration of chlorine generated. The Pt-based electrodes generate a higher concentration comparatively to the native FTO. A current of 500 pA during 60 min, generated 3.09 mg/L of chlorine using native FTO, that is similar to the concentration obtained with the same current but for 15 minutes and 10 minutes using Pt-FTO (5 mM) and Pt-FTO (20 mM), respectively. The Pt-ITO (20 mM) showed a slight increase compared to the native FTO, demonstrating its low potential to the intended application.

[0078] An aspect of the present disclosure relates to the current efficiency (CE) for 60 minutes of electrolysis of the fabricated anodes, calculated based on Faraday's law, as depicted in Figure 13. In an embodiment, the Pt-FTO (20 mM) anode related to the present disclosure present a CE of 93.89% at 200 mA and 92.16% at 300 pA. For a current of 500 pA, the CE was reduced to 72.97%. This decrease results from the excessive current which can lead to the production of oxygen during the NaCI electrolysis, reducing the chlorine generation. Relatively to the ITO and native FTO anodes, it was verified a low CE resulting from its lower catalytic activity.

[0079] In an embodiment, the biofouling prevention tests were performed with Pt-FTO (20 mM) anodes with a current of 300 pA, considering their excellent electrocatalytic properties, good CE as well as a low energy consumption. The antibiofouling prevention was performed by submerging the electrodes in a solution of 100 ml of seawater with 100 ml of a mixture of live algae, comprising Nannochloropsis gaditana, Isochrysis galbana and Tetraselmis Suecica. Figure 14B1 illustrates an embodiment of the electrodes inside the test solution, before chlorine generation. After in immersion, a current of 300 pA is applied between the anode (Pt-FTO) and the stainless-steel cathode, for 24 hours. Figure 14B2 illustrates the results of the present disclosure, wherein the biocide effect is confirmed through the loss of colour intensity caused by the decline of the algae.

[0080] In an embodiment, the antibiofouling capacity of the Pt-FTO (20 mM) anodes is confirmed using optical transmission measurements of the solutions before and after chlorine generation. Figure 14A shows the results related to the present embodiment, wherein the optical transmission is increased when the chlorine generation system is active.

[0081] In an aspect of the present disclosure, a solution of seawater with a mixture of live algae without the submerged electrodes was used as comparison. After 24 hours, the solution colour did not change, as illustrated in Figure 14B.

[0082] In an embodiment, the Pt-FTO (20 mM) anodes were tested during 3 months, changing the testing solution every 24 hours. As comparative, a glass slide was submerged in the control solution, also renewed at the same frequency. Figure 15 shows the results of optical transmission of the Pt-FTO (20 mM) anodes and the control sample during this time, evaluating the biofouling formation on the electrodes surface. The optical transmission of the electrodes remained unchanged, showing that the biofouling did not develop on the electrodes surface. On the other hand, the control sample presented a significant decrease of optical transmission, due to the development of biofouling.

[008B] Figure 16 shows the microscope images of the embodiment wherein the Pt-FTO (20 mM) electrode and a glass slide were immersed in test solutions for 3 months. The surfaces of the Pt-FTO (20 mM) electrode remained clean, while the control sample showed a high density of microorganisms.

[0084] In an embodiment, SEM and EDS analysis were performed to verify the stability of the Pt-FTO (20 mM) electrodes after chlorine generation from seawater. Figure 17A shows that the Pt nanoparticles remain impregnated in FTO crystalline structure without modification, comparatively to the same electrode before its use (Figure 3C can be used for comparison). The EDS spectrum, plotted in Figure 17B, also confirms the presence of Pt in the FTO surface. The detection of Na and Cl chemical groups results from the analysis of the electrodes after chlorine generation.

[0085] In an embodiment, Table 1 summarizes the features, advantages and disadvantages of the Pt-FTO anode, as compared to FTO, ITO and Pt-ITO. The performance was evaluated considering the main required characteristics for biofouling prevention by seawater electrochlorination, which are described on the first column.

[0086] Table 1: Summary table of results comprising a comparison between the developed anode's performance.

n.a.: not applicable; — : very bad; bad; +: good; ++: very good; +++: excellent. [0087] In a preferred embodiment, the electrolytic cell comprising the Pt-FTO anode and a stainless-steel cathode were assembled in a turbidity optical sensor as showed in Figure 18. The Pt-FTO (20mM) anode were placed in front of the LEDs and photodiodes, and the stainless electrodes were positioned in parallel on the outer sides of the Pt-FTO anodes. The assembled system was tested inside an aquarium with seawater applying a current of 300 mA during 20 min.

[0088] In an embodiment, the spatial distribution of the generated chlorine was confirmed dropping several chlorine-reagent drops (Orthotolidine - OTO) into the aquarium. The reagent produces a yellow colour when it reacts with free chlorine, which intensity is proportional to the chlorine concentration. It was used the OTO reagent instead of the DPD reagent, because it is more suitable for high volumes of water and provide a faster response time when compared with the DPD reagent. In another embodiment, the concentration of free chlorine in each zone of aquarium was quantified using the same method followed by spectrophotometric measurements, against a calibration curve with known chlorine concentrations. Figure 19 illustrates the results related to the spatial distribution of the chlorine generated during the test, wherein the higher concentration (~1.104 mg/L) was verified in front of the Pt-FTO anodes, proving the localised chlorine generation.

[0089] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0090] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above described embodiments are combinable.

[0091] The following claims further set out particular embodiments of the disclosure.

[0092] The following references, should be considered herewith incorporated in their entirety: Whelan, A. and F.JJ.o.E.M. Regan, Antifouling strategies for marine and riverine sensors. 2006. 8(9): p. 880-886. Xue, Y., Zhao, J., Qiu, R., Zheng, J., Lin, C, Ma, B., & Wang, P. (2015). In situ glass antifouling using Pt nanoparticle coating for periodic electrolysis of seawater. Applied Surface Science, 357, 60-68. Debiemme-Chouvy, C., Hua, Y., Hui, F., Duval, J. L., & Cachet, H. (2011). Electrochemical treatments using tin oxide anode to prevent biofouling. Electrochimica acta, 56(28), 10364-10370. Laurent, D., et al. Biofouling protection by electro-chlorination on optical windows for oceanographic sensors and imaging devices in OCEANS 2015- Genova. 2015. IEEE. Sanchez-Aldana, D., et al., Hypochlorite generation from a water softener spent brine. 2018. 10(12): p. 1733. Shao, D., et al., High-performance Ti/Sb-Sn02/Pb304 electrodes for chlorine evolution: Preparation and characteristics. 2014. 267: p. 238-244.

Claims

C L A I M S

1. An electrolytic cell for antibiofouling of optical sensors comprising: a cathode, and an anode comprising a glass substrate coated with conductive fluorine tin oxide and platinum nanoparticles, for the production of active chlorine upon seawater electrolysis, wherein the conductive fluorine tin oxide supported on glass substrate is coated with dispersed platinum nanoparticles or platinum nanoparticle aggregates or mixtures thereof, wherein the platinum nanoparticles have a size between 10-20 nm and/or the size of platinum nanoparticles aggregates ranges between 188-210 nm.

2. The electrolytic cell according to the previous claim, wherein the platinum nanoparticles have a size between 11-15 nm, measured by scanning electron microscopy.

3. The electrolytic cell according to the previous claim, wherein the platinum nanoparticles aggregates have a dispersion of 4 x 10^s - 6 x 10^s nanoparticle aggregates/cm², preferably 4.25 x 10⁸ - 5 x 10⁸ nanoparticle aggregates/cm².

4. The electrolytic cell according to any of the previous claims, wherein the cathode is preferably made of stainless-steel.

5. The electrolytic cell according to any of the previous claims, wherein the anode and cathode are connected by a copper wire bonded to the electrodes with silver conductive paste, and the connection area is impermeabilized with a resin, preferably epoxy.

6. The electrolytic cell according to any of the previous claims, wherein the surface of the glass substrate coated with conductive fluorine tin oxide is rough and presents a crystalline structure.

7. The electrolytic cell according to any of the previous claims, wherein the surface of the glass substrate coated with conductive fluorine tin oxide has a roughness between 15 to 25 nm, preferably 20 to 21 nm, measured by atomic force microscopy.

8. The electrolytic cell according to any of the previous claims, wherein the anode is transparent to visible light.

9. Use of the electrolytic cell, described according to any of the previous claims, as a biofouling preventer.

10. An optical sensor comprising the use of an electrolytic cell as described in any of the previous claims as an antibiofouling device preferably a sensor, an actuator, or a transducer for marine applications.

11. A method to obtain an anode for the electrolytic cell described in any of the previous claims, comprising the following steps: cleaning a glass substrate coated with conductive fluorine tin oxide using an ultrasonic bath in ethanol, preferably for 15 minutes, followed by an ultrasonic bath in deionized water, preferably for 15 minutes; drying the glass substrate coated with conductive fluorine tin oxide with nitrogen flow; depositing a solution of hexachloroplatinic acid hexahydrate with a concentration between 1 and 30 mM, preferably 5-20 mM, by spin coating, using 2500 to 3500 rpm for 10 to 30 seconds at room temperature. Preferably, 3000 rpm for 20 seconds; annealing the anode for 10 to 20 minutes in an oven at 400 to 500 °C, preferably 440 to 460 °C for 15 minutes; cleaning the anode with acetone; and drying the anode under nitrogen flux.

12. A method to prevent the biofouling on optical devices using the electrolytic cell described in any of the previous claims, comprising: seawater electrolysis on the anode described in any of the previous claims, preferably applying an electric current between 150-500 mA, preferable between 200-300 pA; and chlorine generation from the seawater electrolysis process, wherein chlorine acts as a biocide agent.