Optochemical ion sensor having an antifouling coating

Abstract

The invention relates to optochemical ion sensors having an antifouling coating containing zinc ethylenebis(dithiocarbamate) as a biocidal active substance in a matrix of a water-insoluble, water-swellable hydrogel which is ion-permeable in the swollen state, and to a method for the production thereof.

Description

[0001] Optochemical ion sensor with antifouling coating

[0002] Description:

[0003] The invention relates to optochemical ion sensors with antifouling coatings.

[0004] Optochemical ion sensors are increasingly used to monitor water quality in natural bodies of water, particularly along coastlines and in the open ocean. The adhesion and growth of microorganisms on these sensors ("fouling") is a significant factor that negatively impacts reliability and data quality, and increases maintenance requirements. The pH sensors used in this application are predominantly optochemical pH sensors.

[0005] Optochemical ion sensors are sensors for measuring the ion concentration in a liquid or gaseous medium using an optical measurement principle. Optochemical ion sensors are preferably used to measure the sodium, potassium, calcium, magnesium, and ammonium ion concentrations, and particularly preferably the hydrogen ion concentration or pH value. Optochemical sensors can be used to apply various optical measurement principles, such as reflection, absorption, and luminescence (fluorescence and phosphorescence). Such optochemical sensors are also called optrodes or optodes.Optochemical sensors consist of a measuring unit comprising at least one light source for emitting light and at least one photodetector for detecting light, and a sensor element, usually connected to the light source and photodetector via one or more optical fibers. This sensor element contains an indicator whose optical properties depend on the ion concentration. The indicator is immobilized in a matrix, typically made of a polymer, and the sensor layer, composed of the indicator and the matrix, is generally mounted on a substrate.

[0006] The carrier layer is a transparent, i.e., light-transmitting, film or a thin glass layer, preferably a transparent plastic film made of a non-water-absorbing plastic, in particular a film of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polymethyl methacrylate (PMMA), or polystyrene (PS). The thickness of the carrier layer is preferably 10 µm to 1 mm, in particular 50–200 µm.

[0007] The problem of fouling has long been known in the shipping industry, and a variety of antifouling coatings exist for ships. However, antifouling coatings for ships are unsuitable for the antifouling coating of optochemical ion sensors because they have excessive thicknesses and are impermeable to ions, especially hydrogen ions, so their application would render the optochemical ion sensors inoperable.

[0008] Furthermore, many of these antifouling coatings contain biocides whose effect is based on non-specific toxicity to marine organisms. Such biocides are considered problematic from an environmental perspective, and their use is increasingly regulated. For example, the use of tributyltin compounds, formerly commonly used as biocides, in antifouling coatings for ships is now prohibited.

[0009] There is therefore a great need for optochemical ion sensors with an antifouling coating whose composition is sufficiently permeable to ions to ensure the proper functioning of the optochemical ion sensor and which contains a biocide that is as environmentally unproblematic as possible. Suitable biocides are generally known, but coating compositions suitable for use as antifouling coatings for optochemical ion sensors are lacking. The object of the invention is therefore to provide optochemical ion sensors with an antifouling coating that contains a biocide that is not environmentally unproblematic and whose composition is sufficiently permeable to ions to ensure the proper functioning of the optochemical ion sensor over a longer, practical period.

[0010] According to the invention, this problem is solved by an optochemical ion sensor, in particular a pH sensor, with the features of independent claim 1. Advantageous embodiments are described in dependent claims 2-10. The problem is further solved by a method for producing the optochemical ion sensor according to the invention as described in claim 11 and dependent claim 12.

[0011] The antifouling coating of the optochemical ion sensor according to the invention contains zinc ethylenebis(dithiocarbamate) ("Zineb") as a biocidal agent. Zinc ethylenebis(dithiocarbamate) is significantly more environmentally friendly than tributyltin compounds. Furthermore, it was surprisingly found that zinc ethylenebis(dithiocarbamate), in a matrix of a water-insoluble, water-swellable hydrogel, exhibits a significantly longer antifouling effect than copper-based biocides such as copper thiocyanate or copper(I) oxide at the same layer thickness.

[0012] Another essential feature of the antifouling coating of the optochemical ion sensor according to the invention is the embedding of the zinc ethylene bis(dithiocarbamate) in a matrix of a water-insoluble, water-swellable hydrogel that is ion-permeable when swollen. This matrix distinguishes the antifouling coating of the optochemical ion sensor according to the invention from antifouling coatings used in the shipping industry. The matrices used there are neither ion-permeable nor water-swellable.

[0013] The ion permeability, particularly the hydrogen ion permeability, of the matrix is ​​essentially determined by the water content of the swollen, water-insoluble hydrogel. Conversely, the swollen, water-insoluble hydrogel must not have an excessively high water content to ensure sufficient strength for the production of an antifouling coating. According to the invention, hydrogels are therefore preferably suitable whose water content, measured using the "tea bag method" as described, for example, in Zhang et al., MethodsX. 2020; 7: 100779, is a maximum of 80 wt.%, preferably 30 to 80 wt.%, and particularly 40 to 60 wt.% in the swollen state.In the water-insoluble hydrogels preferred according to the invention, the longitudinal expansion determined in a tensile test when the swollen hydrogel tears is also 10 to 100%, preferably 30 to 70% and particularly 40 to 60% higher than the longitudinal expansion of the unswollen hydrogel.

[0014] According to the invention, the water-insoluble, water-swellable hydrogel can be based on a polymer whose water insolubility is due to dipolar / dipolar interactions. Particularly suitable polymers from this group according to the invention are polyhydroxyethyl methacrylates (PolyHEMA). According to the invention, the water-insoluble, water-swellable hydrogel can also be based on a polymer whose water insolubility is due to a composition of blocks of hydrophilic and blocks of hydrophobic monomers. Particularly suitable polymers from this group according to the invention are polyurethane block copolymers, such as those commercially available under the name "HydroMed®". Furthermore, according to the invention, the water-insoluble, water-swellable hydrogel can also be based on a cross-linked polymer that does not contain any hydrolyzable groups. Particularly suitable polymers from this group according to the invention are polyacrylamides.According to the invention, the water-insoluble, water-swellable hydrogel is preferably based on a polymer selected from the group consisting of polyhydroxyethyl methacrylates (PolyHEMA), polyurethane block copolymers, and polyacrylamides, as well as copolymers containing blocks of two or three of the aforementioned polymers. The water-insoluble, water-swellable hydrogel is particularly preferably based on a polyurethane block copolymer or, most preferably, on polyhydroxyethyl methacrylates (PolyHEMA). For the purposes of this invention, "based" means that the hydrogel consists predominantly of the polymer but may contain small amounts of conventional excipients.

[0015] In a preferred embodiment, the optochemical ion sensor according to the invention has an ion-permeable antifouling coating composed of 20 to 80 wt.%, preferably 40 to 60 wt.% zinc ethylene bis(dithiocarbamate), 20 to 80 wt.%, preferably 40 to 60 wt.% water-insoluble, unswelled, swellable hydrogel and 0 to 20 wt.% additives, wherein the sum of the components each equals 100 wt.%.

[0016] The optochemical ion sensor according to the invention is manufactured by applying the antifouling coating as an external layer, i.e., a layer directly in contact with water during application of the optochemical ion sensor, by spin coating, dispensing, screen printing, dip coating or by doctor blade application to the part of the optochemical ion sensor that comes into contact with water during application.

[0017] In an optochemical ion sensor according to the invention, it is generally sufficient to apply the antifouling coating only to the sensor layer. The production of an optochemical ion sensor according to the invention can advantageously be achieved by coating the composite of the substrate layer and the sensor layer with the antifouling coating. This can be done by applying one of the coating methods described above.

[0018] The coating is particularly advantageously achieved by applying a dispersion of zinc ethylene bis(dithiocarbamate) in a solution of the matrix material, consisting of a water-insoluble, water-swellable hydrogel, in a suitable solvent using a doctor blade. This dispersion may optionally contain conventional excipients. Suitable solvents according to the invention are all solvents in which the matrix material is soluble and the zinc ethylene bis(dithiocarbamate) is dispersible, in particular mixtures of lower alcohols and water, with mixtures of ethanol and water being particularly preferred. After a drying step, sensor elements of a size required for the production of optochemical ion sensors are preferably cut out of the resulting composite by die-cutting.These sensor elements are then glued or clamped onto the end face of a light guide to produce the optochemical ion sensor, or preferably attached to a sensor holder or sensor cap by gluing.

[0019] Further advantages, special features and expedient further developments of the invention will become apparent from the dependent claims and the following presentation of preferred embodiments with reference to the illustrations.

[0020] The illustrations show:

[0021] Fig. 1 Schematic representation of the structure of an optochemical ion sensor with antifouling coating.

[0022] Fig. 2 View of the composite antifouling coating / sensor layer / support layer for a sensor element of an optochemical ion sensor according to the invention.

[0023] Figs. 3a, 4a and 5a Calibration curves of pH sensor elements without antifouling coating after ideal storage

[0024] Fig. 3b Calibration curves of pH sensor elements without antifouling coating after 4 weeks of seawater storage

[0025] Fig. 4b Calibration curves of pH sensor elements with CuSCN antifouling coating after 4 weeks of seawater storage

[0026] Fig. 5b Calibration curves of pH sensor elements with zinc ethylene bis(dithiocarbamate) antifouling coating after 4 weeks of seawater storage

[0027] Fig. 1 shows a schematic representation of the structure of an optochemical ion sensor (1) according to the invention with an antifouling coating. This sensor comprises a sensor element (2) consisting of an antifouling coating (3), a sensor layer (4), and a carrier layer (5). The sensor element (2) is connected to the carrier layer (5) at one end of the optical fiber (6), the other end of which is connected to the light source (8) for emitting light and to the photodetector (9). The light source (8) and the photodetector (9) are arranged in a measuring unit (7) which contains electronic components for exciting the light source to emit light (8) and for processing and evaluating the signals from the photodetector (9) (not shown here).

[0028] Fig. 2 shows a view of the composite antifouling coating (3) / sensor layer (4) / carrier layer (5) for a sensor element (2) of an optochemical ion sensor (1) according to the invention, from which the sensor element (2) of an optochemical ion sensor (1) according to the invention is produced by punching or cutting.

[0029] Figures 3a to 5a and 3b to 5b show the calibration curves of pH sensor elements without antifouling coating (Fig. 3b), with CuSCN antifouling coating (Fig. 4b), and with zinc ethylene bis(dithiocarbamate) coating (Fig. 5b), each after 4 weeks of seawater storage, compared to the calibration curves of pH sensor elements without antifouling coating after ideal storage (Figs. 3a, 4a, 5a). The ratio R = measured signal / reference signal is plotted against the pH value.

[0030] For the measurements, three sections of a film composite consisting of a carrier layer and a sensor layer, commonly used in commercial pH sensor elements, were provided. One section was left unchanged; an additional layer (15 pm thick) of CuSCN dispersed in a 1:1 weight ratio in PolyHEMA was applied to the second section; and an additional layer (15 pm thick) of Zineb dispersed in a 1:1 weight ratio in PolyHEMA was applied to the third section. Each of the three film sections was then subdivided into several smaller sections and stored in the Mediterranean Sea near Rovinj for four weeks.

[0031] After storage, the fouling that had occurred was visually assessed. The film composite without an antifouling layer showed the heaviest fouling. In contrast, the fouling on the film composite coated with CuSCN was somewhat reduced, and the fouling on the film composite coated with Zineb was significantly reduced.

[0032] Two sensor elements were punched out from each of the stored films, and the calibration curves of pH sensors fitted with these elements were measured. For comparison, calibration curves were also measured using sensor elements without the additional coating, which were stored under ideal conditions and not exposed to water. The measured calibration curves are shown in Figures 3b, 4b, and 5b compared to Figures 3a, 4a, and 5a. After water immersion, the sensor elements without the additional antifouling coating exhibit a significant drift in both the top (maximum value) and slope (curve slope) (Figures 3a and 3b). This drift is reduced with the additional CuSCN coating (Figures 4a and 4b). In contrast, the sensor elements with the additional zineb coating show no drift in the slope and only a slight drift in the top (Figures 5a and 5b).

[0033] Example implementation:

[0034] A solution of 150 mg polyHEMA in 1350 mg of a 6:4 mixture of ethanol and water was prepared and mixed with 150 mg zinc ethylene bis(dithiocarbamate) powder. This mixture was spread over an approximately 10 pm thick sensor layer (4) containing a pH indicator dispersed in a polyurethane block copolymer-based dispersed in a sensor layer (4) using a doctor blade with a 25.4 pm gap height. This sensor layer was applied to a 75 pm thick PET film substrate (5). After drying the applied antifouling coating (3), sensor elements of the required size for the fabrication of the optochemical pH sensors were die-cut from the resulting composite. These sensor elements are then bonded to the end face of an optical fiber to fabricate the optochemical pH sensor.

[0035] The embodiments shown here are merely examples of the present invention and should therefore not be interpreted as limiting. Alternative embodiments considered by a person skilled in the art are likewise covered by the scope of protection of the present invention. List of reference numerals:

[0036] 1 Optochemical ion sensor 2 Sensor element

[0037] 3 Antifouling coating

[0038] 4 Sensor layer

[0039] 5 Carrier layer

[0040] 6 optical fibers 7 measuring unit

[0041] 8 Light source for emitting light

[0042] 9 Photodetector

Claims

Patent claims:

1. Optochemical ion sensor, characterized in that the optochemical ion sensor has an antifouling coating containing zinc ethylene bis(dithiocarbamate) as a biocidal agent in a matrix of a water-insoluble, water-swellable hydrogel that is ion-permeable in the swollen state.

2. Optochemical ion sensor according to claim 1, characterized in that the optochemical ion sensor is an optochemical pH sensor.

3. Optochemical ion sensor according to claim 1 or 2, characterized in that the water content of the swollen hydrogel is a maximum of 80%, preferably 30 to 80% and particularly 40 to 60%.

4. Optochemical ion sensor according to one of claims 1 to 3, characterized in that the linear expansion of the swollen hydrogel is 10 to 100%, preferably 30 to 70% and particularly 40 to 60% higher than the linear expansion in the unswollen state.

5. Optochemical ion sensor according to one of claims 1 to 4, characterized in that the water-insoluble, water-swellable hydrogel is based on a polymer whose water insolubility is based on dipolar / dipolar interactions.

6. Optochemical ion sensor according to one of claims 1 to 4, characterized in that, that the water-insoluble, water-swellable hydrogel is based on a polymer whose water insolubility is based on a composition of blocks of hydrophilic and blocks of hydrophobic monomers.

7. Optochemical ion sensor according to one of claims 1 to 4, characterized in that the water-insoluble, water-swellable hydrogel is based on a cross-linked polymer that does not contain any hydrolyzable groups.

8. Optochemical ion sensor according to one of claims 1 to 4, characterized in that the water-insoluble, water-swellable hydrogel is based on a polymer selected from the group consisting of polyhydroxyethyl methacrylates (PolyHEMA), polyurethane block copolymers and polyacrylamides and their copolymers.

9. Optochemical ion sensor according to claim 8, characterized in that the water-insoluble, water-swellable hydrogel is based on a polyurethane-based hydrogel or, more preferably, on polyhydroxyethyl methacrylate (PolyHEMA).

10. Optochemical ion sensor according to any one of claims 1 to 9, characterized in that the ion-permeable antifouling coating contains 20 to 80 wt.%, preferably 40 to 60 wt.% zinc ethylene bis(dithiocarbamate), 20 to 80 wt.%, preferably 40 to 60 wt.% water-insoluble, unswellable, swellable hydrogel and 0 to 20 wt.% excipients, wherein the sum of the components each equals 100 wt.%.

11. Method for producing an optochemical ion sensor according to any one of claims 1 to 10, characterized in that, that the antifouling coating is applied by spin coating, dispensing, screen printing or dip coating and especially by squeegeeing.

12. Method for producing an optochemical ion sensor according to claim 11, characterized in that the antifouling coating is applied to a composite of carrier layer and sensor layer by applying a dispersion of zinc ethylene bis(dithiocarbamate)s in a solution of a matrix material consisting of a water-insoluble, water-swellable hydrogel in a suitable solvent.

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