Novel continuous in situ measurement of photocatalyst efficiency in liquid dispersions by laser absorption method

Novel continuous in situ measurement of photocatalyst efficiency in liquid dispersions by laser absorption method Richard Dvorsky1, Ladislav Svoboda2, Jiří Bednář2, Zuzana Vilamová2 & Zuzana Šimonová1,2 This work presents a novel method and reactor design for measuring photocatalytic activity. This method allows continuous in situ monitoring of the decrease in pollutant concentration in a liquid dispersion containing the tested photocatalyst. Due to the presence of the photocatalyst in the liquid dispersion, the standard Beer-Lambert absorption law cannot be used directly to determine pollutant concentration in photocatalytic measurements. Therefore, the presented in situ measurement method also utilizes a newly derived modification of the absorption law, which, in addition to absorption, also considers the scattering effect caused by the dispersed photocatalyst. Repeated correlation analysis showed an average deviation of only 1.04% from approximately 500 measurements. At the same time, the measured points obtained by the presented method were within the uncertainty intervals of the standard method for measuring photocatalytic activity. It has been demonstrated that this novel continuous in situ measurement method can replace the current standard measurement method and can provide an even more consistent and faster way of testing photocatalytic materials. In addition, a novel and open source (patented experimental setup for the photocatalytic reactor system, consisting of a spectrometric laser and probe, is fully described in this paper. Keywords Reactor design, Photocatalysis, TiO2, In situ measurement, Light dispersion, Methylene blue In recent decades, humanity has focused mainly on the development of various industries (pharmaceuticals, chemicals, food, textiles, cosmetics)1–6 and on the quantity of production that would be able to cover the increasing demands of all consumers due to the growing world population7. The result of these efforts has been a negative impact on climatic conditions and pollution of surface and subsurface waters. With the decreasing availability of drinking water, the demand for possible sanitation solutions has increased. In addition, there has been a gradual emergence of contaminants that are very difficult to remove with conventional water treatment technologies8,9. Therefore, so-called advanced oxidation processes (AOPs), which include chemical (oxidation) and physical (separation) processes, or a combination of both, have come to the forefront of interest of many scientific groups10,11. This group of AOPs also includes photocatalysis, which utilizes the ability of semiconducting nanomaterials to generate electrons and holes and subsequently also reactive oxygen species when irradiated12,13. Among the most studied photocatalysts belong TiO2 and ZnO nanomaterials that are activated by UV light14–18. Recently, the scientific communities have developed various photocatalysts, such as g-C3N4, Fe2O3, WO3, MoS2, BiOIO3, every year19–24. Their resulting photocatalytic activity depends on several parameters such as structure, composition, presence of defects, surface area, light harvesting ability, and electronic conductivity. By controlling the synthesis of nanomaterials, these parameters can be varied to achieve optimal preparation conditions depending on the final photocatalytic activity, which results from the combination of the parameters mentioned above. Molecular simulations are still not at a level to evaluate the photocatalytic activity of such newly developed and prepared nanomaterials. Therefore, it is necessary to experimentally test the photocatalytic activity using the tested photocatalyst and simulant such as organic dyes. 1Centre for Advanced Innovation Technologies, Faculty of Materials Science and Technology, VSB – Technical University of Ostrava, 17. listopadu 2172/15, 708 00 OstravaPoruba, Czech Republic. 2Nanotechnology Centre, CEET, VSB – Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic. email: ladislav[email protected] OPEN Scientific Reports | (2024) 14:30238 1 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports Standards have been developed to test photocatalytic materials. Among the best known and most widely used are the ISO 27,447: test method for antibacterial activity of semiconducting photocatalytic materials25, ISO 27,448: test method for self-cleaning performance of semiconductor photocatalytic materials – measurement of water contact angle26, ISO 10,676: test method for water purification of semiconducting photocatalytic materials by measurement of forming ability of active oxygen27, ISO 22,197: test methods for air-purification performance of semiconductor photocatalytic materials28–30, and ISO 10,678: the determination of photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue31. The last mentioned ISO standard 10,678 requires organic dye (methylene blue, MB) to determine the photocatalytic activity. The MB is one of the most commonly used simulants since its photocatalytic degradation and concentration decrease can be easily determined by UV-VIS spectrometry by measuring its maximum absorbance at 665nm32. However, it should be noted that MB can also be degraded in the presence of semiconducting material called dye photosensitization. This is a process in which the electron-excited MB (MB*) state transfers electrons to the conduction band of the tested photocatalyst to form the oxidized dye radical ·MB+, which is unstable and decays into colourless products33–35. In addition, an electron that the conduction band of a photocatalyst has accepted can form other oxidizing agents, such as a superoxide radical or hydrogen peroxide, in the presence of oxygen. However, several necessary conditions are needed for this photosensitization process. There belongs to the adsorption of MB on the surface of the tested photocatalyst and the usage of irradiation, which would create an electron-excited state of MB36. This undesirable process is minimized in the ISO 10,678 standard by using 365nm wavelength radiation. It is because MB absorbs mainly at different wavelengths (λ = 220–340 and 450–700nm), thus, the creation of MB* is highly eliminated37,38. In the standard evaluation method of photocatalytic activity, UV-VIS absorption spectrometry is used to determine the residual dye concentration at a given reaction time. The main disadvantage of the standard evaluation method rests in the manual sampling39. Removing the suspension from the reactor changes several conditions. These include the amount of photocatalyst and the total volume of the suspension. These changes may affect the reaction rate and the final observed kinetic rate of photocatalytic degradation due to the gradual reduction of the photocatalyst during the experiment. The second problem lies in separating the photocatalyst from the dye solution. The most common separation techniques are centrifugation, filtration, or combination of both. In the case of filtration, there is a risk that some of the dye will be trapped on the membrane filter, and the obtained results will be distorted. The disadvantages of centrifugation are the time required for centrifugation and the speed limit of the equipment. Conventional laboratory centrifuges are not sufficient when centrifuging particles close to the size of quantum dots. Regarding spherical gold nanoparticles, the recommended centrifuge speed highly depends on the particle size. The required speed is 4,000rpm for spherical gold nanoparticles of 50nm, but spherical nanoparticles with a diameter of 1.8–5nm require an ultracentrifuge with speed of 150,000rpm for at least 2min40. Most laboratory centrifuges cannot achieve such high speeds, and the measured solution still contains small amounts of photocatalyst. Thus, the researcher must check the obtained UV-VIS spectra to estimate whether the absorbance values have been affected by the photocatalyst present and correct them. The time required for centrifugation is critical and can affect the data used to evaluate the observed kinetic rate. The third and significant disadvantage is the combination of the time-consuming processes described above and the required presence of a worker to carry out all these processes and carefully check each step during photocatalytic experiments41. As mentioned, a significant problem of the standard method for measuring photocatalytic activity is the timeconsuming measurement and further physical interference with the reaction system during residual simulant concentration analysis sampling. In the general case, a sample of the liquid dispersion containing the photocatalyst particles in the simulant (methylene blue dye) solution would be removed from the reactor into a cuvette and its total opacity measured without further treatment using a UV-VIS spectrometer. However, this total opacity value consists of pure absorption of the dye molecular solution, scattering, and light absorption on the dispersed photocatalyst. Up to this date, we found that only one work reported an attempt to develop different testing method for evaluating photocatalytic activity of photocatalysts. Such kind of measurement was published in the conference proceeding42. In their report, the reactor was a spectrometric cuvette directly placed in a commercial UV-VIS spectrophotometer and irradiated with an external light source. The tested dye was methyl violet 2B with a concentration of 3.5 mg·L− 1, a volume of 3 mL, and the amount of ZnTiO4 photocatalysts was 10mg. However, their results have not been carried out according to the ISO standard 10,678 and have not even been compared. Furthermore, they should have stated in their paper whether the results obtained by their proposed measurement method are consistent with the commonly used method of separating the nanoparticles from the solution and then measuring the absorbance of the residual dye solution. Moreover, whether the measured absorbance value in the presence of the particles corresponds to the actual absorbance, and therefore the dye concentration, at the time of measurement of each point. The mentioned scattering of light on the photocatalytic particles significantly distorts the standard form of the Beer-Lambert absorption law, and the standard protocol for measuring photocatalytic activity requires the removal of dispersed photocatalytic particles from the liquid sample by centrifugation, filtration, or both. This requirement directly impacts increasing the process and time required for the measurement, and, of course, is an additional source of uncertainty. In this work, we present a novel continuous in situ measurement method and design of the novel photocatalytic measurement system, including a new mathematical formula, to eliminate all the drawbacks mentioned above and time-consuming processes connected with the testing of the photocatalysts. The novelty of our contribution is also confirmed by one of the strictest criteria for novelty, which is by granting the patent, namely CZ308365B6 “Method of continuous measurement of photocatalysis of dye simulants“43. The proposed novel measurement method was tested by using methylene blue according to ISO standard 1067831 and photocatalyst (TiO2 P25) as one of the most studied and commonly used photocatalysts44–46. In addition, the results obtained were checked by additional measurements using commonly used measurement method to prove the high accuracy and Scientific Reports | (2024) 14:30238 2 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports/ reliability of our presented novel continuous in situ measurement method. Finally, the most crucial advantage of this novel method rests in its ability to measure the decrease of tested pollutant concentration immediately after the radiation source is switched on, thus achieving a sampling rate that was previously unattainable. Experimental Instrumental setup of the novel photocatalytic reactor The scheme of the novel photocatalytic reactor is illustrated in Fig.1. In the duralumin vessel of the reactor is a 250 mL beaker with an aqueous dispersion of photocatalytic particles and tested simulant. The vessel is immersed in the water bath of the CORIO CD-300F thermostat (Julabo, Germany) and maintained at a normal temperature of 20°C. A Cimarec Micro and Cimarec-telemodule 20C electromagnetic stirrer ensured permanent homogenization of the liquid dispersion at 100rpm. The cover of the reactor holds the 673nm spectrometric laser and USB4000 fibre optics spectrometer probe (Ocean Optics, Inc., USA). Excitation radiation for the photocatalytic process is emitted into the reactor by a 365nm (10V/10W) LED chip (JUSTAR, China). Photocatalytic experiments The standard photocatalytic measurements were carried out by using TiO2 P2544–47 ((purity ≥ 99.5, particle concentration cpart = 1g·L− 1) and an aqueous solution of Methylene blue (Reag. Ph. Eur., MQ = 200; volume V = 200 mL, initial concentration co = 0.001g·L− 1) under UV light irradiation using one 10W LED chip with an emission maximum at 365nm. All the reagents were used without further purification. Deionized water was used for all prepared solutions. Before irradiation, the suspension was magnetically stirred in the dark for 30min. Then, at a specific time intervals (100, 200, 300, 400, and 500s), 1 mL was sampled and filtered to remove the photocatalyst before absorbance measurements using a Shimadzu UV-1601 spectrophotometer. The two sets of such measurements were made for photocatalytic activity evaluation. The novel continuous in situ measurement method was performed under the same conditions. Methodology of the novel measurement method The presented novel measurement method is based on calibration and subsequent measurement of the time dependence of diffuse light intensities on decreasing MB concentration c in the liquid dispersion. A model time profile of the measured intensities is shown in Fig.2. The approximate duration of the calibration measurements (Δt) is chosen on the x-axis in Fig.2 so that a statistically significant number (n = Δt/τ >> 1) of measured intensity Fig. 1. Scheme (left) and photography (right) of the reactor for continuous in situ measurement of photocatalytic activity. Scientific Reports | (2024) 14:30238 3 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports/ values I is obtained at the sampling time τ. The required three-step calibration precedes the time point t = 0 when the desired initial maximum concentration of MB (co) has already been added to the particle dispersion and the radiation source has been switched on to allow the photocatalytic reaction to proceed. For this reason, the (Δt) time intervals before this point are expressed as negative values. Similarly, such negative values on the x -axis can be seen in other publications where adsorption-desorption equilibrium was also investigated prior to the actual photocatalytic reaction, the onset of which is usually also indicated by time t = 048–50. For the maximum sensitivity of the spectrophotometric measurement, it is necessary to ensure high laser absorption by the tested simulant, MB, in our case. This is ensured by selecting a laser wavelength that lies close to the absorption maximum. The absorption maxima for different simulants are 665nm (methylene blue)51, 554nm (rhodamine B)52, 592nm (methyl violet 2B)53, 610nm (indigo carmine)52, and 275nm (phenol)52. Currently, there is an extensive range of spectrometric lasers from which it is possible to select the one with the optimal wavelength for the region around the absorption maximum of the selected simulant and the intensity sufficient to make its radiation detectable by the fibre spectrometer and at the same time lower enough to avoid the photosensitization phenomenon54. The condition of minimum spectral overlap of 90% is determined by the requirement of the maximum permissible value of the relative fluctuation of the weak signal of the spectrometer at initial concentrations of the MB (c = co) at the beginning of the measurement. This condition is well satisfied in most cases for the lowest diffused laser light intensity value at initial concentration I(co), which is about 30% of the original diffused laser light intensity I(0) without MB, and is one of the three decisive factors for applying the described in situ measurement method. The second critical parameter is the volumetric concentration of dispersed particles, which must also be analysed. The depth of penetration of the laser light into the liquid dispersion depends on the volume concentration of the dispersed particles. The backscattered light is then detected by the spectrometric probe after leaving the surface (see Fig.1). Like diffuse reflectance spectroscopy, its intensity is reduced by absorption by the selected simulant in the liquid volume. The third and last critical parameter is the value of the initial concentration co of the used simulant (MB in our case) at the beginning of the photocatalytic measurement. For the maximum sensitivity of this method in the concentration interval (0, co), the dispersion should respond to the detected intensity I(c) at concentration c in the interval of maximum possible width (I(0), I(co)). In the particular case of this work, the width ΔI of this interval of measured intensities corresponds to 70% of the original value: ΔI = I(0)  I(co) = I(0)  0.3∙I(0) = 0.7∙I(0). This is realized by a concentration MB of co = 0.001g·L−1. A detailed analysis of the reproducibility of the results with this choice is given in the discussion below. Measurement procedure In the initial calibrations, the photocatalytic particles are not activated by the corresponding excitation radiation. At a time interval (-3Δt, -2Δt) and with a suitable sampling time τ ˂˂ Δt, we measure the highest level of diffuse light intensity I(0) in a pure liquid dispersion of photocatalytic particles without the presence of MB dye (1st calibration in Fig.2). After adding the first dose of the MB, the concentration of MB increases to the half of its Fig. 2. Scheme of spectrophotometric measurement of the time dependence of diffuse laser light intensities I in the novel reactor during calibration and, subsequently, the photocatalytic process (an equal duration of Δt of all three calibration phases is not necessary). Scientific Reports | (2024) 14:30238 4 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports/ maximum concentration (c = co/2). At the time interval (-2Δt, -Δt), we measure the next level of diffuse light intensity I(co/2) in the liquid dispersion (2nd calibration in Fig.2). Then, after adding a second, equal dose of the MB, the concentration rises to its maximum and initial value (c = co). At the time interval (-Δt, 0), we measure the lowest level of diffuse light intensity I(co) because the absorption of the red laser light by the MB is the highest (3rd calibration in Fig.2). In the measurement mode, the photocatalytic reaction starts at the time t = 0, with initial concentration MB of co, by irradiating the dispersion with excitation radiation. The simulant is wholly decomposed in a sufficiently long time, and the diffuse light intensity should ideally reach its original maximum lim t→∞ I ( c ( t )) = I (0) . Results and discussion Figure3 depicts the whole absorption spectrum of the MB dye, which was constructed based on the published data (Ohno et al. 2001), together with the position of the selected red laser light used in our experiments. The selection of a laser with a wavelength of 673nm is a crucial decision, as it ensures high optical absorption of its scattered light by MB dye. A similar setup should be maintained if we decide to measure some other dye as a simulant, which has an absorption maximum at a different wavelength, and therefore, a different laser must be selected. From Fig.3, it is apparent that the wavelength of the selected laser light is close to the maximum absorbance of MB. This satisfies the condition of the maximum spectral overlap. Figure4 shows the dependence of the backscattered light intensity on the concentration of TiO2 nanoparticles in aqueous dispersion. The approximate maximum intensity I(cpart) of the backscattered diffuse laser light was about 1g·L− 1. The further decrease in intensity is naturally explained by a gradual increase in diffuse light absorption in the scattering environment. As the concentration of particles increases, a saturation effect occurs (cpart = 7–10g·L− 1). The diffuse reflection has a decreasing penetration depth and only a flat reflection from a very thin layer near the surface occurs. The data in Fig.4 also point to the reason for using the particle concentration cpart= 1g·L− 1 in our experiments. Such a photocatalyst concentration leads to sufficient penetration depth and the most efficient backscattering of the measuring laser radiation. The maximum backscattering efficiency reaching the spectrometer probe is required for the practical principles of applying this method to various photocatalysts. The appropriate optimum of cpart can be most easily determined by sequentially dosing the powdered material until the approximate maximum is found, similar to Fig.4. It should be noted that even with intensive magnetic stirring, there will be at least minimal adherence of the dispersed particles to the vessel’s walls. In a direct transmission configuration through the glass walls of the vessel or in a flow cell setup, the unwanted shielding and intensity losses would occur at the inlet and outlet walls of the vessel. Naturally, these cannot be well controlled. Therefore, the novel in situ measurement method uses backscattered light diffusion on photocatalyst particles dispersed in the liquid, and the light from the primary spectrometer laser enters the photocatalytic dispersion from above through the free surface. However, the effect of slight adhesion on the volume concentration of Fig. 3. The absorption spectrum of methylene blue (blue line) and the emission peak (red line) of the selected spectrometric laser. It is clearly seen that the laser wavelength lies close to the absorption maximum of MB. Scientific Reports | (2024) 14:30238 5 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports/ particles is marginal when applying the new method. After multiple scattering and absorption by the simulant molecules, the attenuated part of the light also comes out through the free surface into the collection optics of the spectrometer probe (see Fig.1). Considering the presence of photocatalytic particles in the liquid dispersion, it was necessary to derive a new mathematical formula from the Beer-Lambert law55,56 to describe the optical attenuation of light from the primary spectrometer laser. The new formula must consider both the attenuation of light caused by MB absorption and its attenuation by absorption and scattering on the photocatalyst particles dispersed in the solution. In addition, changes in the concentration of the MB in photocatalytic experiments shall be measured under the following conditions. The UV-VIS spectrometer is used to measure the intensity of the diffuse radiation of the spectrometer laser emitted by the liquid dispersion level into the inlet of the spectrometer probe. Throughout the measurement period, the dispersion fraction of the photocatalyst particles remains constant, and the effect of the adhesion of photocatalytic particles on the walls is negligible. This has been experimentally verified by monitoring the temporal stability of the intensity without MB. The geometric arrangement also remains unchanged throughout the measurement process, and the deformation of the surface at constant mixing does not alter. For these reasons, the detailed geometry of all optical paths of the scattered light of the primary laser spectrometer may remain unknown. The new formula of the modified Beer-Lambert law for the translucent medium of liquid dispersion is derived based on the following model assumptions: The actual concentration at a specific time (c(t)) of the MB in the liquid dispersion, a crucial parameter in photocatalytic process, is determined by optical measurement of the relevant intensity I(c) of the diffuse radiation of a spectrometric laser. In the time-invariant measurement geometry, the total intensity loss by light scattering is only proportional to the output intensity Io of the laser and the constant concentration of the dispersed particles and does not depend on the MB concentration. Adsorption of MB on the particle surface does not significantly affect the light scattering. The temporal change in the intensity of diffuse radiation during photocatalysis, a key aspect of the process, is caused only by the decrease in optical absorption due to the decrease in the concentration of MB. The absorption of the spectrometric laser radiation on the particles’ surface caused by adsorbed MB is negligible (the free volume of absorbing solution significantly exceeds the total volume of the photocatalytic particles). The primary aim of spectrophotometric measurement of photocatalytic processes is to determine the time dependence of the c(t) decrease in simulant concentration by calculating the time dependence of the light absorption rate from the used laser source. Let Io be the input intensity of the spectrometric laser and I(c) be the diffused laser light intensity for MB with the actual concentration c detected by the spectrometric probe (Fig.1). After the loss of the input intensity of the laser due to absorption by MB with concentration c (ΔIabs(Io, c)), and after the loss of the input intensity of the laser due to scattering on the photocatalyst particles (ΔIdis(Io)), the intensity I(c) is given by Eq.(1): I(c)=I0−∆Iabs (I0,c)−∆Idis (I0) (1) Fig. 4. Dependence of the intensity I(cpart) of the backscattered diffuse laser light on the TiO2 P25 nanoparticles concentration cpart in aqueous dispersion. Scientific Reports | (2024) 14:30238 6 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports/ . The intensity loss by MB absorption in a clear solution (without the photocatalyst) comes from the standard Beer-Lambert law56: ∆ Iabs ( I0,c )= I0 (1− e −αc) (2) . where a is the absorption coefficient at given concentration c of the absorbing substance. The intensity loss due to dispersion on photocatalyst is an unknown time-invariant macroscopic configuration, and it is proportional only to the input intensity I0 from the primary source: ∆Idis (I0)=βIo (3) . The constant of proportionality β simultaneously characterizes the influence of the volume concentration of photocatalytic particles and the time-invariant macroscopic configuration of the dispersion system in the reactor with the spectrometric probe. The measurement can be successfully performed in different functional configurations, but these must not change during the measurement. Since it is better to take the input intensity Io of the spectrometric laser as a hidden model parameter, we introduced a relative output intensity η(c) for practical measurements of the concentration c, where the absolute intensity Io is mathematically eliminated: η (c)def =I ( c ) I(0) ⩽1 → η(c)=e −αc −β 1−β (4) . The proportionality constant β becomes zero when no photocatalytic particles are present in the solution, and then the modified formula (4) becomes the standard form of the Beer-Lambert law. The concentration of MB c(t) during photocatalysis decreases from an initial maximum c(t = 0) = co to zero at its full decomposition c(t → ∞) = 0. The relative output intensity η(c) thus increases asymptotically from an initial minimum (e‒αco – β)/(1 – β) (at maximum absorption) to its maximum value of one: lim t→∞ η(c(t)) = η(0) = 1 (5) . During spectrophotometric measurements, the time evolution of MB concentration c(t) is then expressed via the relative output intensity η(c(t)) = η(t) from Eq.(4) by the inverse function: c(t)=− 1 αln (β(1 −η(t)) + η(t)) (6) . For a particular experimental configuration, we calibrated the measurement according to Eq.(6) with utmost precision by experimentally determining the values of the previously unknown parameters α and β. We calibrated with precision by measuring the relative output intensities for the concentrations co and co/2 and we obtained a system of two equations with two unknowns α and β: η(co)=e−αco−β 1−β ,η (1 2 co ) =e−α 1 2co−β 1−β (7) . The solution of Eq.(7) then provides the calibration parameters α and β, expressed as a function of the known experimental parameters co, η(co) a η(co/2): Scientific Reports | (2024) 14:30238 7 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports/ α=2 co ln ( 1−η(co/2) η(co/2) − η(co) ) ,β=(η(co/2))2−η(co) (1 − η(co/2))2 (8) . By experimentally determining their magnitudes by measuring the diffuse light intensities I(0), I(co/2), and I(c0) for the relative output intensities η(co/2) and η(co), we obtained an absolute dependence (6) of the MB concentration decrease on irradiation time. At the beginning of the first measurement, a three-point calibration was performed: I(0) = 156 682 Cnt, I(co/2) = 60 009 Cnt, I(co) = 45 610 Cnt, which gave the values of the calibration parameters α and β based on formula (8). η(co)= I(c o ) I(0) = 45610 156682 =0.2911 η(co/2) = I(co/2) I(0) =60009 156682 =0.3830      →{α≈3808 β≈−00.3793 (9) . The second measurement was performed under the same conditions as the first one. A three-point calibration was performed: I(0) = 148 179 Cnt, I(co/2) = 58 027 Cnt, I(co) = 42 433 Cnt, which gave the values of the calibration parameters α and β based: η(co)= I(c o ) I(0) = 42433 148179 =00.2864 η(co/2) = I(co/2) I(0) =58027 148179 =00.3916      →{α≈3510 β≈−00.3594 (10) . The obtained α and β parameters show good agreement. The slight difference between them was probably due to the quite negligible difference in the initial concentration of the two MB solutions. Then, starting from time t = 0 (see Fig.2), we measured the time dependence of the relative output intensities η(c(t)) = η(t) as two independent measurements with two different data sets. The sampling time for the first measurement was τ = 10s, and the sampling time for the second measurement was τ = 2s (Fig.5). Equation(6) is applied to the known values of the calibration parameters (9) and (10), which gives real-time dependencies of the methylene blue concentrations c(t) on the irradiation time (Fig.6). The time series of c(t) concentrations in Fig.6 show good agreement between the first and the second measurements by using our novel in situ method. The average relative deviation was 1.04% with respect to the maximum MB concentration (co = 0.001g·L − 1) used in this work. This value not only confirms the good repeatability of these two independent measurements, but also instills confidence in the reliability of our method. However, to verify the validity of the novel in situ measurement method, it was necessary to confront the experimental results with those obtained by the established standard measurement method of photocatalytic activity. The two sets of measurements were performed and evaluated as well. The obtained concentration points at the specific time can be seen in Fig.6. Their relative deviation, again relative to the maximum concentration of MB (co = 0.001g·L− 1), was 1.13%, which is approximately the same as the repeatability of the results obtained by using the novel in situ measurement method. The observed kinetic rate constants obtained by the novel in situ measurement method and by the standard measurement method are summarized in Table1 with their corresponding correlation coefficients. The data presented in Table1 confirms the validity of the results obtained by the novel in situ measurement method, especially in the situation where, apart from the point at 300s, the average of the two curves lies within the uncertainty intervals of the concentration points obtained by performing the standard measurement (Fig.6). Another advantage of this novel method rests in the ability to measure the decrease in MB concentration immediately after the radiation source is switched on and with a sampling rate that has not previously been possible even in the published work of Masař et al.42. This brings additional possibilities for studying the initial kinetics of photocatalytic processes. Conclusion This work presents a novel method for evaluating the photocatalytic activity of photocatalysts during the photodegradation of pollutants, specifically methylene blue, in this study. This method enables rapid, interference-free, and continuous in situ measurements in liquid-photocatalyst dispersions by applying a newly derived formula based on the Beer-Lambert law for optical absorption in liquid dispersions. This eliminates the need for physical sampling and particle separation, which are often challenging and costly. Methylene blue Scientific Reports | (2024) 14:30238 8 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports/ photocatalytic decomposition experiments were performed according to ISO standard 10,678. The obtained data showed strong correlation and repeatability with minimal deviation and similarly observed kinetic rate constants compared with the standard method for evaluating the photocatalytic activity, proving high accuracy and reliability. Our research provides a cost-effective alternative for laboratories lacking advanced separation equipment. The following significant advantage of this novel method is its ability to measure the decrease of tested pollutant concentration immediately after the radiation source is switched on, thus achieving a sampling Fig. 6. Comparison of two independently measured dependencies of MB concentration decrease on irradiation time. Fig. 5. Dependence of the relative output intensity η(t) on the irradiation time. Scientific Reports | (2024) 14:30238 9 | https://doi.org/10.1038/s41598-024-80585-x www.nature.com/scientificreports/