Technical Field

The invention, in general, covers the field of electrochemistry with a special accent on the specific realization of the biosensor electrode. The biosensor realized as such has potential applications in the detection of pathogenic microorganisms in contaminated food and water, i.e. nucleic acids and proteins in the sample; then in biomedicine, in the detection of tumor markers, hormones, etc.

The area of classification is wide and covers the following classes: C12Q1, G01N27, A61B5/0408, H05K1/0283, A61N1/05, and H01L23.

Background Art

The invention talks about the new construction of an electrochemical planar biosensor whose electrodes are realized using repetitive fractal geometry. The invention solves the problem of insufficient sensitivity of electrochemical sensors, which is especially important in the realization of biosensors intended for detection of biological markers or pathogens present in traces (in small quantities) in a contaminated sample in a way that new construction of repetitive geometry (specifically fractal) increases sensor sensitivity.

Increased sensitivity has been demonstrated by examples of detection of DNA of the pathogenic bacterium Campylobacter jejuni and detection of model protein (glutathione-s-transferase - GST) in milk, although the invention generally relates to the detection of different biological molecules, such as nucleic acids or proteins, present in the analyzed sample in traces. The electrode within the scope of the invention works in the same way in both examples, e.g. whether it is the detection of a nucleic acid or a protein (analyte), the only difference, in this case, is the type of bioreceptor (the molecule that recognizes the analyte) that is necessary to make the electrode functional. This information is illustrative and not limiting to the invention. The previous construction of electrodes mainly involved the realization of electrodes using circular geometry, which resulted in poor sensor sensitivity and required additional functionalization (or modification) of electrode surfaces in order to increase selectivity and sensitivity.

The invention contributes to the increase of sensitivity, because it enables a change in the design of the electrode of the biosensor and not an additional modification of the material from which the electrodes are made.

Below is given an overview of some patents and scientific papers that belong to the state of the art.

Patent US20060068381, titled “Methods for identifying a peptide that binds a geometrical shape” published on March 30, 2006, describes a new technique for immobilizing antibodies, enzymes, or aptamers on the surface of a sensor electrode, but it differs from the mentioned invention, in the way how the electrode is realized.

Patent KR20190104041 A, titled “Electrophoresis chip for electrophoretic applications” published on September 5, 2019, generally describes a set of printed electrodes with symmetrical geometry that use the electrochemical detection method for application in electrophoresis, but like the previous solution, it differs significantly from the invention in the way how the electrode is realized.

Patent EP1931248B1, titled “Biologically integrated electrode devices” published on June 18, and patent US20190030318A, titled “Fractal geometry microelectrodes and uses thereof’ published January 31, 2019. The above- mentioned patents are focused on the application of electro active, i.e. conductive polymers within the sensor, without considering specific geometries in the electrode construction. The mentioned applications refer to implantable pulse generators for stimulation of the nervous system and do not imply the use of bioreceptors.

Patent US20150380355A1, titled “Self-similar and fractal design for stretchable electronics” published on December 31, 2015, describes the application of fractals in the realization of electronic connections in flexible electronics, but does not involve the application regarding the realization of electrochemical sensors or biosensors.

Article titled “Study of fractal electrode designs for buckypaper-based micro- supercapacitors” mentions fractals, but for use in supercapacitors, whereas published article under the title “Resistance-Capacitance Gas Sensor Based on Fractal Geometry” describes the application of fractals for the realization of a resistive-capacitive gas sensor, without the use of bioreceptors.

Patent US8009053B2, titled “Rain sensor with fractal capacitor(s)” published on September 10, 2009, explains the realization of a capacitive sensor using fractal geometry, but not in accordance with the description of the invention.

Patent JP2016520986 A, titled “Self-similar fractal design for stretchable electronics” published on July 14, 2016, belongs to the state-of-the-art but does not mention that fractal geometry was applied to the planar electrode of the biosensor.

Patent ES2517919T3, titled “Nanostructured microelectrodes and biodetection devices that incorporate them” published on November 4, 2014, describes the realization of nano structured electrodes in the form of wires for biosensor applications that can be made in the form of three-dimensional fractal structures, but the principle or description how the structure is realized is not mentioned.

A similar case can be noted in patent EP3369828 titled “Bioprobes and methods of use thereof’ published on September 5, 2018, where the general application of fractals is stated, but the realization of the structure of the invention is significantly different.

Disclosure of the Invention

Today, biosensors are one of the most commonly used technological solutions in various fields of rapid diagnostics, such as medicine, agriculture, food industry, environmental protection and biology. Unlike classical methods of microbiological analysis which are slow (several hours to one week to get results), expensive (based on several stages, using larger amounts of chemicals and equipment) and require highly trained researchers to apply them and interpret the results, biosensors enable fast, simple and specific analysis at a low cost. However, the main condition for the successful commercialization of biosensors is a sufficiently sensitive level of detection to be able to directly apply the sensor for analysis of contaminated samples (food, blood and blood plasma, water, or plant material).

There are several principles of biosensor operation that are based, among others, on optical, acoustic and fluorescent methods, but electrochemical detection with all its parameters has proven to be the most reliable, fast, economically viable, and generally speaking, the most applicable technique. The physical size, geometry, number of electrodes, and construction of the electrochemical sensor usually depend on the purpose for which it is intended. Therefore, different characteristics of these sensors can be expected, in terms of sensitivity, selectivity, response time, and durability. Functionalization of sensor electrodes with different materials (nanomaterials, graphene, dendrimers, etc.) or the use of selective membranes further increases the sensitivity of the electrochemical sensors. Typical planar electrochemical sensors consist of a working electrode, an auxiliary electrode, and a reference electrode, interconnected by a thin layer of electrolyte containing an analyte (detection target). Due to the specifics of the applicability, different types of electrochemical biosensor electrodes are known today in terms of the materials from which they are made, but the circular geometry of the working electrode is mainly used, which leads to limitations in terms of detection and sensitivity. The sensitivity of the biosensor largely depends on the material from which the electrodes are made, the substrate and the geometric parameters of the electrode. Usually, metals such as gold, silver or platinum in combination with ceramics or porcelain substrates are mostly used in the construction of electrodes since these materials offer relatively easy functionalization by biological molecules (DNA probes, antibodies, enzymes, aptamers, liposomes, etc.).

The invention is based on the idea that the sensory characteristics of biosensors, in terms of sensitivity and limit of detection, can be significantly increased by changes in the geometry of the electrodes, without either changes in the material from which the electrodes are made or their additional functionalization.

The invention proposes the realization of the mentioned biosensor electrode, i.e. the design of its new geometric construction. It is a system of three electrodes consisting of a working, reference and auxiliary electrode which are structurally realized using a repeating fractal geometry of the first order, although the invention provides a range up to the third order of the repeating geometry. The realization of the invention includes repetitive geometry of the 1 st order of the Sierpine fractal, but the invention also relates to the repetitive geometry of the Koch curve, Pean curve and Hilbert curve, and of the geometric shapes includes the three most common shapes - square, circle, and triangle. The application of repetitive geometry of fractal curves in the realization of biosensors is the basic innovation of the invention since it directly increases the sensitivity of electrochemical biosensors.

The planar electrode is realized using a technology of Low-Temperature Cofired Ceramics (LTCC), where the base on which the electrode is realized is ceramic. A gold paste in the geometric shape of a fractal curve is applied to the nonsintered ceramic substrate by screen printing, over a previously prepared mask. The invention also includes platinum paste, carbon paste, and silver paste. Ceramic non-sintered strips 300 pm thick are initially cut and shaped with a laser, and after printing they are laminated and baked at 865 °C into highly electrically conductive electrodes, due to the sintering of the ceramic substrate itself and due to the sintering of the conductive paste. The process of lamination of several ceramic 1 -3 mm thick strips takes place in a uniaxial isothermal press at a temperature of 75 °C.

The invention electrode itself is not limited to the electrode fabrication technology and includes the fabrication of electrodes using inkjet printing, or the use of other thin-film or thick-layer fabrication technologies, as well as fabrication on other substrates such as paper, glass, organic, and inorganic polymers.

The planar electrode can be functionalized by various bioreceptors, such as single-stranded fragments, i.e. nucleic acid oligonucleotides (DNA and RNA probes) and antibodies, aptamers, lectins or enzymes. Bioreceptors are selected to be specific and selective for the required analyte. If functionalization is performed with antibodies, i.e. immunoglobulins, the resulting biosensor is the so-called immunosensor. Functionalization using DNA probes results in a genosensor.

Brief Description of the Drawings

Figure 1 shows the making process of an innovative biosensor electrode

Figure 2 shows an innovative geometric layout of a biosensor electrode according to the description of the invention

Figure 3 shows the innovative electrode - after fabrication: a) layout of the fabricated electrode, b) measured dimensions of the electrode, c) SEM (scanning electron microscopy) view of the gold electrode surface magnified 2000 times.

Figure 4 shows the current-voltage characteristic of the realized biosensor where Figure 4a and 4b show standard tests with a solution of 0.1 M ferrocenemethyl alcohol in 0.9M phosphate buffer and ImM ferrocene-methyl alcohol in 0.05M sulfuric acid, respectively, at a scan / ratio of 50 mV/s. Figure 4c) shows the current- voltage characteristic for 1 mM ferrocene-methyl alcohol in 0.05M sulfuric acid for different values of the scan ratio, while Figure 4d) shows the repeatability of the measurement results at different electrodes. Figure 5 shows a genosensor, i.e. a biosensor functionalized using specific DNA probes and its testing for the detection of different concentrations of DNA molecules isolated from Campylobacter jejuni, where a signal is generated when DNA sequences isolated from bacteria bind, i.e. hybridize to a specific DNA probe on the electrode surface of the invention.

Figure 6 shows the immunosensor, i.e. the testing of the immunosensor for different protein concentrations. Namely, it is a procedure of realization of the biosensor electrode, where the surface of the electrode is functionalized by applying immunoglobulins, i.e. antibodies specific for the protein (antigen) to be detected - in this case the antigen is a GST protein, and an anti-GST antibody is applied. The signal is generated when the GST protein binds to the anti-GST antibody applied to the electrode, which achieves electrochemical detection of the presence of GST protein in the sample, using ferrocene - redox indicator, which is in solution.

Figure 7 shows a comparison of the sensitivity of realized genosensors and immunosensors for the detection of Campylobacter jejuni and GST proteins in milk, respectively, as an example of the invention and a comparison with commercial sensors using circular geometry functionalized using the same electrode immobilization procedure.

Figure 8 is an example of repetitive geometry for the present invention when it comes to successive repetition of circular geometry.

Best Mode for Carrying Out of the Invention

Before disclosing the details of the invention, it is important to understand and accent that the present invention is not limited to construction details, such as dimensions, shape, and/or order of fractals, nor to electrode materials, substrate, fabrication technology, nor type of bioreceptor used for electrode functionalization which are illustrated and described below. The terms used in the description of the invention serve to understand the invention and not to limit it.

The innovative biosensor electrode consists of three electrodes whose main feature of innovation is given in the new geometric construction of the working electrode in the form of a fractal curve. The construction which the invention proposes is a construction of repeating geometry of fractal shape in order to improve the sensitivity of the sensor itself, while other electrodes follow the geometry of the working electrode. In this way, the effective area between the working and auxiliary electrodes is increased. The invention starts from the hypothesis that the sensitivity of the biosensor can be significantly increased by changing the geometry of the electrodes without changing the material from which the electrodes are made. The invention itself includes a repetitive geometry of the 1st order of the Sierpine fractal, with dimensions shown in Figure 2, but the invention also relates to the repetitive geometry of the Koch curve, Pean curve and Hilbert curve, and of the geometric shapes includes the three most common shapes - square, circle, and triangle. The application of repetitive geometry of fractal curves in the realization of biosensors represents the main innovation of the invention because it directly increases the sensitivity of electrochemical biosensors.

Figure 1 shows the process of making a sensor that begins with phase 101 where the selection of fractal geometry, fractal order, and electrode dimensions takes place, followed by phase 102 of making screen printing masks that are realized using photo-sensitive foil. The planar electrode is realized by the technology of Low-temperature Cofired Ceramics (LTCC), where the substrate on which the electrode is realized is ceramic. In phase 100, ceramic non-sintered strips 300 pm thick are first cut and shaped with a laser. In phase 103, a gold paste manufactured by Heraeus TC7102 in the geometric shape of a fractal curve is applied to the cut non-sintered ceramic substrate by a screen-printing process, over a previously prepared mask. The invention also includes platinum paste, carbon paste, and silver paste. In order to ensure the mechanical strength of the electrode in phase 104, a process of lamination of several unsintered ceramic strips of 3-7 layers is performed in order to achieve a final electrode thickness of 1-2 mm. Lamination is performed in an isothermal uniaxial press at a temperature of 75 °C for 3 minutes. The lamination is followed by phase 105, firing at a temperature of 865 °C for 10 hours, due to the sintering of the ceramic substrate itself and the sintering of the conductive paste in high-conductivity electrodes.

Repetitive geometry is realized through fractal curves whose basic characteristic is to fill the assigned space in a highly efficient way, i.e. as the order of the fractal curve grows, it changes so that its dimension increases on a limited area. Therefore, the use of such a construction contributes that the effective area between the working and auxiliary electrodes is increases 2.4 times.

The system of three electrodes, which is the basis of the biosensors which invention proposes, is realized so that the working electrode provides potential variation, auxiliary (counter) electrode balances the current of the working electrode, and the reference has a known potential so that the principle of the system is to control potential, the voltage in contact with the analytical substance while measuring the change in current. The reference electrode is half a cell with a known reduction potential. Its only role is to act as a reference in measuring and controlling the working electrode potential.

The fractal curves that the invention includes are Koch's curve, Sierpine fractal triangle, and carpet, Pean's and Hilbert's curve. The invention also uses a square, triangle and circle, whose scaled copies are periodically repeated, as a starting point for forming the fractal geometry of the biosensor electrode, where a set of parameters such as initial dimensions, scaling factors, fractal order, i.e. number of iterations of scaled copies, etc. The example of a circle best explains the importance of repetitive geometry in the application of biosensors.

Figure 8 presents the repeating geometry of the circle, where the second iteration suggests a number of parameters: the number of repetitions of the circle, the distance of small and large circles, and the scaling factor, i.e. the ratio of the diameter of the small and large circle. Finally, with a large number of iterations, we see that the line describing the whole structure increases, which is important for the biosensor in terms of increasing sensitivity.

Figure 2 shows a biosensor consisting of an innovative electrode construction consisting of a working, auxiliary and reference electrode with appropriate dimensions. The invention does not limit the dimensions of the electrode, but primarily emphasizes the dimensions below and the range of dimensions. Figure 2 shows the appearance of the electrodes, as well as the initial dimensions of the sensor.

Figure 3 shows the realized innovative electrode, an enlarged view of the surface of the realized electrode, as well as the measured dimensions of individual parts of the realized electrode.

Figure 4 shows the current-voltage characteristics of sensors tested using cyclic voltammetry. Figures 4a and 4b show standard assays with a solution of 0.1 M ferrocene-methyl alcohol in 0.9M phosphate buffer and 1 mM ferrocene-methyl alcohol in 0.05M sulfuric acid, at a scan rate of 50 mV/s, respectively. Figure 4c shows the current-voltage characteristic for 1 mM ferrocene-methyl alcohol in 0.05M sulfuric acid at different values of the scan ratio, while Figure 4d shows the repeatability of the measurement results at different electrodes.

As stated in the description of the invention, the planar fractal electrode can be functionalized with various bioreceptors, such as antibodies or single-stranded DNA probes, i.e. deoxyribonucleic acid oligonucleotides. Bioreceptors are selected to be specific and selective for the required analyte. If functionalization is performed with antibodies, i.e. immunoglobulins, the resulting biosensor is the so-called immunosensor. Functionalization through DNA testing resulted in a genosensor. The potential of developed planar electrodes for practical applications has been tested in two types of biosensor applications, genosensors and immunosensors.

For the genosensor preparation, cleaned electrodes were incubated with 10 ng ml-1 capturing DNA probe at 37° C for 1 hour. Capturing DNA probe is a single-stranded oligonucleotide with a nucleotide sequence complementary to the targeted sequence of nucleotides in the analyte. In this concrete example, DNA probes specific for Campylobacter jejuni were used. DNA bioreceptor i.e. the probe, was functionalized with thiol group on its 5 ’-end, since this allows gold-sulphur covalent bonding of the probe to the gold on the electrode surface. Functionalized electrodes were then washed three times with phosphate buffer (PBS) in order to remove all DNA molecules non-covalently bound to the surface. Methylene blue (MB) at 2 pg ml-1 in PBS was then applied to the surface for 1 hour to enable testing for DNA immobilization and hybridization. MB is a frequently used redox indicator for electrochemical measurements with genosensors, for detection of DNA hybridization since it has different electrochemical response to the single-stranded DNA i.e. single oligonucleotides and double-stranded DNA i.e. helices obtained by complementary binding of the probe and sample nucleotides. The difference stems from the interaction of MB with guanidine base in single-stranded DNA. The efficacy of DNA probe immobilization and hybridization were tested by cyclic voltammetry. Measurements were conducted between -0.3 and +0.6 vs. silver/silver chloride (Ag/AgCL) reference electrode, at 100 mV s-1 in PBS, pH 7.2 at room temperature. The efficiency was evaluated by comparing CV results obtained using complementary and non-complementary sequences. The CV values are proportional to the hybridization of the DNA molecules.

Figure 5 a shows schematics of immobilization of the DNA nucleotides specific for Campylobacter jejuni bacterium onto the electrode of the innovation. Figures 5b and 5c show results of tests with different concentration for positive and negative test control probe, while Figure 5d shows microstructure of the electrode obtained by Scanning Electron Microscopy (SEM) of the electrode functionalized with DNA.

For the immunosensor, cleaned planar electrodes were incubated with solution of 1 mM 11-mercaptoundecanoic acid (MUA) in ethanol overnight at 4 °C in the dark. Electrodes were then rinsed with ethanol and milliQ water and dried. To activate MUA self-assembled layer, the electrodes were exposed to an activation reagent solution comprising N-(3-dimethylaminopropyle)-N-ethyle- carbodiimidehydrochloride - EDC and N-hydroxysuccinimide - NHS in sodium-acetate buffer, pH 5. This treatment enables immobilization of proteins that make amide bonds with the MUA-activated surface. Next, the solution of monoclonal antibody in phosphate buffer is applied onto the MUA-layered surface of the electrode, followed by incubation of Ih on room temperature to enable formation of the amide bonds between the applied antibody and the MUA layer. The electrodes modified with antibody were sequentially rinsed with PBS buffer, pH 7.2. All biofunctionalized electrodes were kept wet at 4°C before utilization to avoid antibody denaturation. Before measurements, the remaining active sites of biofunctionalized electrodes were neutralized with 50 mM ethanolamine in PBS buffer for 30 min at room temperature, then rinsed with PBS without drying. The electrochemical measurements with electrodes functionalized with antibodies are performed in solution of 0.1M ferrocene-methanol in 0.9M PBS - ferrocene is a redox indicator and generates electrochemical signal. When the specific antigen from the sample binds to the antibody on the electrode surface, the ferrocene-generated signals changes, which is detected by cyclic voltammetry.

Figure 6 shows the method of realizing a biosensor electrode as an immunosensor, where the electrode has been functionalized with a specific antibody, in this case the monoclonal anti-GST antibody. Figure 6a schematically shows the process of antibody immobilization at the surface of the electrode, while Figure 6b shows testing for different concentrations of GST protein. Figure 6c shows testing using real milk samples. Figure 6d shows an enlarged SEM image of the electrode’s surface with GST proteins from milk, bound to the antibodies on the electrode’s surface. GST was used as a model protein to check the efficiency of the sensor. Measurements were performed in an aqueous solution (phosphate buffer -PBS) and in milk to demonstrate that the electrode could be used directly for the analysis of contaminated food (milk). In both cases, ferrocene is added to the sample solution as a redox indicator.

Figure 7 shows the sensitivity of the realized innovative sensors and the comparison of the characteristics with the test results on the circular electrode realized on the same surface. The comparison was performed for both types of sensors, i.e. both for the genosensor and the immunosensor. It can be noticed that the sensitivity is increased more than 5 times in relation to the circular geometry.

Industrial Applicability

Biosensors are used in various fields where rapid diagnostics and field applications are required, in food production and storage processes and other aspects of food technology, as well as in biomedicine and in the monitoring of environmental parameters, etc. Commercialization depends on sensitivity, since samples for analysis usually contain small concentrations of analyte (biomarkers).