N-acetylserotonin and 5-methoxyindole acetic acid started at the age of 24 days. The proportion of released indoles changed with embryo age. NE caused a decrease in MLT secretion and provoked an increase in serotonin release. Incubation of the pineal organs induced the development of MLT secretory machinery and its diurnal rhythmicity. The pineal content of catecholamines increased prominently at the end of embryonic development.The embryonic ontogeny of pineal secretory activity in birds has been investigated almost exclusively in chickens. This study aimed to characterize this process in domestic geese. The pineal organs of embryos aged 18–28 days were incubated in superfusion culture under different light conditions for 4–5 days and treated with norepinephrine (NE). Melatonin (MLT) was measured by radioimmunoassay and other indoles by HPLC with fluorescence detection. Additionally, pineal organs were collected from embryos at 14–28 days of age and used to measure catecholamines by HPLC with electrochemical detection. MLT secretion increased with embryo age, most intensively between the 22nd and 24th days of life. The daily changes in MLT secretion under the 12 L:12D cycle occurred on the first day of culture, starting from an embryonic age of 24 days. MLT secretion was controlled by the light-dark cycle in all age groups studied. However, exposure to light during the scotophase did not alter the secretion of MLT. The endogenous oscillator expressed its activity in regulating MLT secretion in the pineal organs of embryos aged 24 days and older but could not generate a rhythm after one cycle. The rhythm of 5-hydroxytryptophan release during the first day of culture was found in the pineal organs of all embryos, while the rhythmic release of  N-acetylserotonin (NAS). The pineal contents of 5-HT, 5-hydroxytryptophan (5-HTRP), and 5-hydroxyindole acetic acid (5-HIAA) show daily variations from ED 14, when eggs are incubated under a 12L:12D cycle. In the second phase (ED 16), 5-HT acetylation is turned on, and in the last phase, the methylation of 5-hydroxyindoles is turned on. Melatonin (MLT) and 5-methoxyindole acetic acid (5-MIAA) are detectable in the pineal organs from ED 18 and show significant daily variations in their contents from ED 20.The embryonic development of pineal secretory activity in birds has been investigated almost exclusively in chickens ,7,8,9,10A recently published study  demonstrGiven these data, we decided to study the regulation of MLT secretion in the goose embryonic pineal organs in superfusion culture. The first objective of the study was to characterize the secretion of MLT from the pineal organs of goose embryos aged 18–28 days, incubated under a 12L:12D cycle. This goal was important from a comparative point of view because the majority of data on MLT synthesis in the embryonic avian pineal organ were obtained in vitro ,31,32,33MLT secretion from the pineal organs of ED 18, ED 20, and ED 22 embryos slightly decreased in the first hours of culture. It then remained at a very low level  until the scotophase on the second day of incubation, when the first nocturnal peak occurred . On day The decrease in MLT release in the initial phase of culture was more intense in the pineal organs of ED 24, ED 26, and ED 28 embryos than in the younger ones . In the The presence of NE in the culture medium during photophases prominently modified the daily fluctuations in MLT secretion from the embryonic pineal organs . The secIncubation under a reversed, dark-light cycle (12D:12L) abolished the nocturnal peak of MLT secretion during the first day of culture . HoweverMLT secretion from the embryonic pineal organs incubated in continuous darkness decreased during the first hours of culture . Next, tThe release of 5-HTRP by the pineal organs of ED 18 embryos showed significant daily fluctuations starting from the first day of culture, but the daily profiles and levels of 5-HTRP release changed during the experiment . During The pineal organs of ED 20 embryos also released 5-HTRP in the daily rhythm during the entire culture period . The peaThe release of 5-HTRP from the pineal organs of ED 22 embryos showed significant daily changes throughout the experiment, with the peaks of similar values, except the last one, which was smaller . The nigIn the superfusion culture of the pineal organs of ED 24 embryos, the prominent rhythm of 5-HTRP release occurred throughout the experiment . The relThe pineal organs of ED 26 and ED 28 embryos released 5-HTRP, NAS, MLT, and 5-MIAA in daily rhythm during the whole culture period . The 5-HThe concentration of 5-HT in the culture medium samples was below the quantification limit of the analytical method, independent of embryo age and incubation duration. The release of NAS, MLT, and 5-MIAA from the pineal organs increased with embryo age; however, this increase was not proportional to the embryo age and differed between compounds . During The proportion of released indoles also changed during the course of culture. The ratio of 5-HTRP to MLT release decreased during the culture in all investigated embryos , but this change was more prominent in the pineal organs of the younger embryos . The ratTreatment with NE during the photophases largely modified the release of 5-hydroxyinoles and 5-methoxyindoles. In the superfusion culture of the pineal organs of ED 18 embryos, the release of NAS and MLT did not increase continuously during the first two days of incubation, like it did in untreated cultures. Therefore, the nocturnal peaks of NAS and MLT occurring on the second day of incubation had well-defined onset points . The preThe release patterns of the investigated indoles from the pineal organs of ED 20 and ED 22 embryos were generally similar to those described above, except for the presence of significant increases in NAS and MLT release during the first day of culture . MoreoveThe prominent daily changes in NAS and MLT release were noted starting from the first day of culture of the pineal organs of ED 24 embryos . The NASThe superfusion cultures of the pineal organs of ED26 and ED28 embryos were characterized by significant daily changes in the release of 5-HT, NAS, MLT, and 5-MIAA throughout the experiment . Daily fThe release of 5-hydroxyindoles and 5-methoxyindoles from the pineal organs increased with embryo age, but there were some exceptions, such as higher levels of NAS release in the pineal organs of ED 24 embryos than in those of ED 26 embryos . SimilarAs in experiment I, the nocturnal peak of MLT secretion was absent on the first day of culture in the pineal organs of ED 18–ED 22 embryos . This peThe nocturnal peaks of MLT secretion were observed both on the first and second days of culture in the pineal organs of ED 24–ED 28 embryos . SimilarThe NE content was very low in the embryonic pineal organs until ED 24, and a large increase in the level of this catecholamine was noted in ED 26 and ED 28 embryos . VanillyThe study of the goose embryonic pineal organs in superfusion culture provided several new data on the ontogeny of pineal secretory activity in birds. Moreover, it revealed that isolation of these pineals from the in vivo environment promotes maturation of the MLT synthesis pathway and its daily rhythmicity.The results obtained showed that the pineal organs of goose embryos at 18 days of age, ten days before hatching, secrete measurable amounts of MLT when placed in superfusion culture. Our previous study where MLT content was assayed in the pineal organs in vivo showed that MLT synthesis starts in the domestic goose between ED 17 and ED 18 . HoweverThe comparative analysis of data on the ontogeny of MLT secretion in the goose and the chicken should consider the differences in the length of embryonic development  . The chiFor the first time in ontogeny, the daily changes in MLT secretion in superfusion culture under a 12L:12D cycle were observed in the pineal organs of ED 24 embryos. It should be noted that the significant daily fluctuations of the pineal MLT content in vivo were detected earlier, in ED 20 and ED 22 goose embryos, where the hormone levels were higher at the end of the scotophase than during the photophase . For chiOur results showed that the secretory activity of the embryonic pineal organs changed significantly during the course of superfusion culture performed under the 12 L:12D cycle. On the second day of culture, prominent nocturnal peaks of MLT secretion occurred in all studied pineal organs, including those of ED 18–ED 22 embryos. The levels of MLT secretion at daytime nadirs and nocturnal peaks prominently increased on consecutive days. The elevation of MLT secretion in the course of culture was larger in the pineal organs of younger embryos than older embryos. These changes cannot be explained as a simple continuation of the developmental processes in superfusion culture because they occurred much faster than in vivo. For example, the nocturnal secretion of MLT and the amplitude of daily changes in the pineal organs of ED 22 embryos on the third day of culture were much higher than in those of ED 24 embryos on the first day of culture. Therefore, it could be concluded that the isolation of the embryonic pineal organs from their niche induced the development of the MLT secretory machinery and its daily rhythmicity. To the best of our knowledge, this phenomenon has not been previously reported. It should be noted that culture was performed in a medium without the addition of serum or proteins.To characterize changes in pineal metabolism occurring during embryonic development and in the course of superfusion culture, we measured the concentrations of 5-HTRP, 5-HT, NAS, MLT, and 5-MIAA in the selected medium samples by HPLC with fluorescence detection. The levels of other pineal 5-hydroxyindoles and 5-methoxyindoles could not be estimated because their peaks were unresolved from the medium components or below the detection limit. The present study is the first to describe the release of 5-HTRP, 5-HT, NAS, and 5-MIAA from the avian pineal organ in superfusion culture. It should be emphasized that the permeability of indoles across the cell membrane differs greatly among compounds, and the proportion of indoles in the culture medium does not reflect that within pinealocytes. The analysis of indole release during the first day of culture clearly shows that the development of the 5-HT synthesis pathway is the first step in the ontogenesis of indole metabolism. The pineal organs of ED 18 embryos released 5-HTRP in the daily rhythm, and the release of this compound was approximately 10-folds higher than that of MLT. The rhythmic release of 5-HTRP also occurred in older embryos; however, there was a shift in the ratio between 5-HTRP and MLT. The predominance of 5-HTRP over MLT decreased with the age of embryos, and finally, the release of MLT in the pineal organs of ED 26 and ED 26 was much higher than that of 5-HTRP. Furthermore, the changes in the ratio between NAS and MLT released into the medium suggest that the synthesis of NAS develops faster than the capability of this indole methylation. The release of NAS was higher than that of MLT in the pineal organs of ED 18 embryos, but lower than that of MLT in the organs of ED 22–ED 28 embryos. The results obtained in superfusion culture agree with those from measurements of the indole content in the goose pineal organs  and suppThe changes in indole release occurring in the course of culture were generally similar to those related to the age of the embryos. They involved (i) the establishment of the rhythmic changes in the release of NAS, MLT, and 5-MIAA, (ii) an increase in the ratio between MLT and 5-HTRP release, and (iii) the changes in the ratio between the release of MLT and NAS. The daily rhythm of 5-HTRP release was maintained throughout the culture period in all explants, while the rhythmic changes of NAS, MLT, and 5-MIAA release developed with a delay in the pineal organs of ED 18–ED 22 embryos. The release of 5-HTRP decreased in the course of culture in the pineal organs of ED 18–ED 22 embryos or remained the same on consecutive days in the pineal organs of ED 24–ED 28, while the secretion of MLT increased more or less prominently. MLT to NAS release ratio increased during culture in the pineal organ of ED 18 and ED 20 embryos.The superfusion culture allowed us to study the factors regulating the secretory activity of the goose pineal organ during embryonic development. The comparison of MLT secretion under normal 12L:12D cycle and reversed 12D:12L cycle, and in continuous darkness, shows that the goose embryonic pineal organs are directly photosensitive, and the daily alternations of light-dark phases drive rhythmic MLT secretion. The secretory activity was entrained to the reversed light-dark cycle on the second day of culture in the pineal organs of all age groups of embryos. According to our previous study, this also takes 2 days in the pineal organs of 14-week-old turkeys  and 1-daAnseriformes birds via melanopsin, which is functionally connected with the regulation of gene expression. To date, our attempts to detect pinopsin in the goose and duck pineal organs have been unsuccessful (unpublished data). Several experiments performed in chickens demonstrated the importance of light in regulating pineal activity during embryonic development [Special attention should be paid to our results, which showed that a 4-h-long exposition to light during scotophase did not decrease MLT secretion nor changed the course of pineal secretion in the following incubation period in continuous darkness. These data demonstrate that the direct action of light on the pineal organs of goose embryos is limited in some way, possibly to specific phases of the daily cycle. A study performed on the pineal organs of 12-week-old geese in superfusion culture showed that light alone was not sufficient for the precise entrainment of MLT rhythm to the light-dark cycle . Similarelopment ,38,39,41elopment . Three hContinuous darkness incubation was performed to determine whether an endogenous oscillator was active in the embryonic pineal organs. Under these conditions, the MLT secretion peaks occurred exclusively on the first day of culture in the pineal organs of ED 24–ED 28 embryos, demonstrating that the endogenous oscillator works in these explants, but it is unstable in vitro and quickly loses its ability to affect MLT secretion. The data obtained during incubation of the pineal organs of younger embryos indicate two possibilities: (i) absence of endogenous oscillator activity or (ii) no functional connection between the oscillator and synthesis of NAS and MLT. Similar to the present data, studies on the pineal organs of 12-week-old geese showed that the endogenous oscillator lost its activity after 1 or 2 days of culture . AdditioOur study showed that embryonic goose pinealocytes are sensitive to NE, which induces a rapid and large decrease in MLT secretion. The treatment of explants with NE during the photophases of 12L:12D cycle resulted in a high-amplitude, well-entrained rhythm of MLT secretion. The effects of NE were observed even in the pineal organs of ED 18 embryos. Our results are the first to demonstrate the strong inhibitory action of NE on MLT secretion during photophase in avian embryonic pinealocytes. Lamošová et al.  found thThe analysis of data on the indole release obtained using HPLC revealed that treatment with NE induced large changes in pineal metabolism. NE evoked a large and rapid increase in the release of 5-HT in the pineal organs of all studied embryos. Such an effect has not been described in the avian pineal organs. Initially, treatment with NE abolished the rhythmic changes in 5-HTRP and 5-MIAA release; however, these rhythms were restored in the second part of the experiment. This effect was probably caused by a temporary disturbance of homeostasis in indole metabolism or the direct effect of NE on the activity of tryptophan hydroxylase.Anseriformes birds at several weeks of age [The sensitivity of pinealocytes to NE led to the question of the presence of this neurotransmitter in embryonic pineal organs. To answer this question, the contents of catecholamines and their metabolites were measured in the pineal organs of embryos by HPLC with electrochemical detection. The NE level increased significantly at the end of embryonic development; however, no daily changes were observed in the NE and its metabolite, VMA. Such variations were reported in s of age ,40. Our s of age  and ducks of age . The preAnser anser f. domestica) eggs were obtained from a local reproductive farm and incubated in an Ova-Easy 580 Advance Series II incubator  as previously described [Domestic goose  was prepared from a powder according to the manufacturer’s instructions (pH adjusted to 7.2 with NaOH) and sterilized by filtration. Before use, the sterile solution of ascorbic acid  in Medium 199 and Antibiotic-Antimycotic Solution  were added to give a final concentration 300 mg/L of ascorbic acid, 100 IU/mL of penicillin, 100 μg/mL of streptomycin, and 0.25 μg/mL of amphotericin B. A 1 mM solution (freshly prepared) of norepinephrine bitartrate  was sterilized by filtration and diluted in the culture medium to 10 μM.The pineal organs were isolated directly after decapitation between 09:00 and 11:00, wrapped in nylon mesh, and placed individually in culture chambers connected to a multichannel peristaltic pump  and a manual fraction collector by their upper pools. The medium, continuously equilibrated with a gas mixture of 95% 02 and 5% CO2, flowed through a lower pool to the culture chambers at 0.05 mL/min. Incubation was performed at 38.5 °C in a water bath . The chambers were illuminated with fluorescent lamps emitting white light with 100 lx intensity on the chamber surface during the photophase. During the scotophase, lightproof polyethylene covers were placed on the culture chambers. Fractions of medium flowing out of the superfusion culture system were collected every 30 min and frozen until assays were performed. The superfusion system enabled the simultaneous incubation of 24 explants in a separate culture chamber.The pineal organs of embryos at the age of 18, 20, 22, 24, 26, and 28 days were incubated for 4 days under a 12L:12D cycle with photophase from 07.00 to 19.00, a 12L:12D cycle, but in the culture medium containing NE 10 μM during the photophase (from 07.00 to 19.00), a reversed dark-light cycle with photophase from 19:00 to 07:00, or continuous darkness. The experiment consisted of four identical replicates with respect to the age of the embryos and the protocol .The pineal organs of embryos at the ages of 18, 20, 22, 24, 26, and 28 days were incubated for 2 days under a 12L:12D cycle with photophase from 07.00 to 19.00, then for 2 days in a continuous darkness, and then again under a 12L:12D cycle, but with photophase from 12.00 to 24.00. During the second day of culture between 02.00 and 06.00, the pineal organs were exposed to light with 100 lx intensity on the chamber surface, incubated in the medium containing at NE a concentration of 10 μM or left untreated (control). The experiment was divided into three parts; each part involved explants of all age groups and all kinds of treatment in each age group .The pineal organs were dissected immediately after the decapitation at 14.00 and 02.00 from embryos at the age of 14, 16, 18, 20, and 22 days, and at 8.00, 10.00, 12.00, 14.00, 16.00, 18.00, 20.00, 22.00, 24.00, 02.00, 04.00, and 06.00 from embryos at the age of 24, 26, and 28 days. Five pineal organs per time point were collected from each age group. As previously described, decapitation and dissection of the pineal organs during the scotophase were performed with minimal exposure to light . The pin® IQ 7003/05 purification system  and was used in all procedures.The anti-melatonin antibody Prospect 6C was kindly provided by Dr. Andrew Foldes , 3H-melatonin was purchased from PerkinElmer , gelatin from Merck , and toluene from POCH, Gliwice Poland). Chemicals of the highest purity were purchased from J. T. Baker Chemicals  and used to prepare the mobile phases: sodium acetate, sodium dihydrogen phosphate, disodium EDTA, 1-octanesulfonic acid sodium salt, citric acid, acetic acid, and phosphoric acid. Methanol and acetonitrile, both of gradient-grade HPLC purities, were purchased from Merck Millipore . 5-HTOL was provided by Santa Cruz Biotechnology . All other reagents were purchased from Sigma-Aldrich . Ultrapure water  was freshly prepared using a Milli-QThe MLT concentration in the medium samples was measured by direct RIA with Prospect 6C antiserum and 3H-melatonin (3.2 TBq/mmol) as previously described and characterized in detail . The volThe medium samples were thawed at temperatures below 8 °C, vortexed, and 450 μL of the sample was transferred to an Eppendorf tube, to which 50 μL of perchloric acid was added. The mixture was vortexed, incubated in an ice bath for 20 min, and centrifuged at 30,000 × g (4 °C) for 15 min . Next, 400 μL of liquid was transferred to an autosampler vial.v/v). Between the 7th and 20th min of separation, the methanol content was increased linearly to 30% (v/v), maintained at a constant level for 10 min, and then decreased to the initial level. The detection was performed at an excitation wavelength of 280 nm and an emission wavelength of 345 nm at a constant temperature of 38 °C. The sensitivity of the detector was changed at the 5.8th min of the separation from level 1 to 8, at 12th min of separation again to level 1 and at 15.7th min of separation to level 8. Chromatograms were analyzed using Chromeleon 7.2.10 software . The limits of quantification (S/N ratio of 10:1) for all measured indoles was below 4 pg per injection. The intra-day precision was below 2%, and the inter-day precision was below 3%.The indole content in the culture medium was measured using an HPLC method based on gradient-grade separation and fluorescence detection with programmed sensitivity changes. The assay was performed on a Vanquish Duo U/HPLC system equipped with two gradient pumps, a cooled dual-split autosampler, column compartment, and two fluorescence detectors . The autosampler was maintained at 6 °C. The injection volume was 100 µL. Hypersil Gold aQ columns with 3 µm particle size and dimensions of 150 × 4.6 mm , as well as the mobile phase prepared through on-line mixing of methanol and a water solution of 5 mM sodium acetate and 0.01 mM disodium EDTA (pH 4.5 by adding acetic acid) were used to separate the analyzed compounds. The flow rate of the mobile phase was 1 mL/min. The initial methanol concentration was 2% (g (4 °C) for 15 min , the supernatants were used to measure the catecholamines and their metabolites, which was performed by HPLC with electrochemical detection as described previously [The pineal organs were sonicated in 80 μL of ice-cold 0.1 M perchloric acid using a Vibra-Cell VC 70 ultrasonic processor with a 2-mm probe , and the homogenates were incubated for 15 min in an ice bath. After centrifugation at 30,000× eviously .p < 0.05, was considered significant in all statistical procedures. The analyses were performed using Dell Statistica 13 .The data from experiment I were analyzed using repeated-measures analysis of variance with the treatment as the main effect and the sampling time as a repeated measure. The least significant difference test (LSD) was used as a post-hoc test. The data from experiment II were subjected to a one-way analysis of variance with Duncan’s test. The data from the in vivo studies were analyzed by two-way analysis of variance with age and time as factors. Duncan’s test was applied as a post-hoc procedure. A value of Due to the specific physiological features of the goose pineal organ and the use of novel analytical methods and experimental protocols, this study provided several new data on the embryonic development of the avian pineal organs. For the first time, it has been demonstrated that the rhythmic release of 5-HTRP from the pineal organs in vitro occurs several days before the development of daily rhythm in the release of NAS, MLT, and 5-MIAA. The analysis of age-dependent changes in the indole release supports our previous hypothesis that MLT synthesis develops during ontogeny in three stages, which are turned on one after another: (1) 5-HT synthesis, (2) acetylation of 5-HT into NAS, and (3) methylation of NAS. Moreover, the results showed that isolation of the embryonic pineal organs from their niche induced the development of the MLT secretory machinery and its daily rhythmicity. This finding is important when analyzing the results obtained in long-term cultures of avian embryonic pineal cells, such as monolayer cultures. The modifications of indole metabolism occurring during the course of culture were similar to age-dependent changes. Regarding the regulatory mechanisms, this study demonstrated the strong inhibitory action of NE on MLT secretion by embryonic pineal cells, both during the photophase and the scotophase. The treatment of explants with NE during consecutive photophases resulted in a high-amplitude, well-entrained rhythm of MLT secretion. This study also demonstrated for the first time that NE induces a large increase in the release of 5-HT from the avian pineal organ. The increase in the content of this catecholamine on the last days of embryonic life suggests that adrenergic regulation of pineal activity may develop before or shortly after hatching. The study also showed a lack of effect of 4-h-long light exposure of the pineal organs during scotophase on MLT secretion. However, it should be stressed that MLT secretion was controlled directly by the light-dark cycle in all studied age groups of explants. In addition, the endogenous oscillator was active in the pineal organs of embryos at the age of 24 days and older but could not generate a rhythm after one cycle.


The activation of the gas sensitivity of semiconductor materials by both UV and visible light is considered. When activated by UV light, the typical approaches for creating materials are (i) the use of individual metal oxides, (ii) chemical modification with nanoparticles of noble metals and their oxides, (iii) and the creation of nanocomposite materials based on metal oxides. In the case of visible light activation, the approaches used to enhance the photo- and gas sensitivity of wide-gap metal oxides are (i) doping; (ii) spectral sensitization using dyes, narrow-gap semiconductor particles, and quantum dots; and (iii) addition of plasmon nanoparticles. Next, approaches to the description of the mechanism of the sensor response of semiconductor sensors under the action of light are considered.The review deals with issues related to the principle of operation of resistive semiconductor gas sensors and the use of light activation instead of thermal heating when detecting gases. Information on the photoelectric and optical properties of nanocrystalline oxides SnO S can be determined in various ways, for example, as the ratio of the resistance of the sensor in the presence of the target gas to the resistance in the background atmosphere (Equation (1)) or as the ratio of the increase in the resistance of the sensor during gas detection to the resistance of the sensor in the background atmosphere (Equation (2)). The sensor signal is calculated in such a way that its values increase with the concentration of the target gas, so if the resistance of the sensor in the presence of the detected gas decreases, the Formulas (1) and (2) change accordingly:The operation principle of semiconductor gas sensors is based on a reversible change in their electrical resistance depending on the composition of the gas phase . This chn-type semiconductor metal oxides, such as SnO2, ZnO, In2O3, and WO3. This choice is primarily related to their structural and chemical stability, since in the vast majority of cases, for the manifestation of sensor sensitivity of such materials, they need to be heated to a temperature of 200–500 °C. One of the limitations affecting the choice of semiconductor oxide as a material for a gas sensor is its electrical resistivity, which should not be too high, making it difficult to measure . In order for the processes occurring on the surface of the sensitive layer to have a significant impact on the electrophysical characteristics of the sensor, it is necessary that the material of the sensitive layer has a significant surface. High values of the specific surface area can be achieved in the case of highly dispersed metal oxides, whose crystallite sizes range from units to tens of nanometers. The electrophysical properties of materials based on nanocrystalline metal oxides depend in a complex way on the morphology and grain size, which distinguishes them, for example, from single-crystal semiconductors, whose electrical conductivity is determined only by the concentration and mobility of charge carriers.Traditional materials for gas sensors of the resistive type are wide-gap n-type semiconductor oxides occurs with the capture of the conduction electron and the preservation of its molecular form according to Equation (3):At present, considerable attention is paid to the role of electrostatic barriers that arise at the grain boundaries in the formation of the conductive and sensor properties of nanocrystalline metal oxides . Based oAt higher temperatures, dissociation of oxygen molecules and its chemisorption in atomic form is possible :(4)O2gas0 is the permittivity of the vacuum, ε is the static permittivity of the semiconductor, q is the value of the elementary charge, n and p is the concentration of electrons and holes, respectively. The effective width effW of the depleted layer with a strong near surface bands curvature is determined by the Equation (7):To shield the negative charge that occurs on the grain surface, the near-surface region of the crystallites is depleted of electrons that can be considered as an increase in potential barriers at the grain boundaries, leading to a decrease in their electrical conductivity. The average length, which characterizes the depth of the electric field penetration into the semiconductor at small perturbations (under the flat bands condition), is called the Debye length (Equation (6)) :(6)LD=εεantities .n-type semiconductor oxides will exhibit the following behavior:Although the processes that cause the appearance of the sensor response of semiconductor oxides may be of different nature, there are only two types of response of a resistive sensor that correspond to an increase or decrease in the electrical conductivity of the sensitive layer relative to its electrical conductivity in the background atmosphere . In some cases, the dualistic behavior of the sensor response can be associated with the redox properties of the detected gas molecules. For example, NO2, Cl2, and O3, cause the electrical conductivity to decrease. This effect is explained by the higher electron affinity of these gases compared to oxygen. As in the case of oxygen, the chemisorption of these gases proceeds with the capture of conduction electrons, leading to even greater depletion of oxide grains by electrons, for example:(1) A small group of oxidizing gases, such as 3, H2S, H2, CH4, etc.) and volatile organic compounds VOCs . It is believed that their interaction with chemisorbed oxygen (Equation (9)) leads to the release of electrons and the reduction of potential barriers at the boundaries of metal oxide grains, for example: (2) An increase in electrical conductivity is caused by a larger group of reducing gases  A change in the electrical conductivity of semiconductor oxides may result from a change in the oxygen concentration in the gas phase. According to the Equations (3)–(5), an increase in the oxygen concentration in the gas phase leads to an increase in the concentration of the chemisorbed oxygen and a decrease in the electrical conductivity of the sensitive layer, and vice versa.(4) The influence of humidity is a concomitant factor in the gas detection; however, the magnitude and direction of the effect may be different, depending on the chemical nature of the detected gas and the material of the semiconductor gas sensor.One of the disadvantages of resistive gas sensors is their high operating temperature, which lies in the range of 200–500 °C. On the one hand, heating is an effective way to activate chemical reactions on the sensor surface, remove the products of these reactions, and increase the electrical conductivity of the sensitive layer. On the other hand, the need for thermal action on the sensitive layer entails a number of undesirable consequences, including:(1) high power consumption, which limits the use of such sensors in portable and mobile devices;(2) degradation of the sensor material under prolonged exposure to high temperature due to grain sintering and diffusion processes, which, in the long term, causes the drift of sensor characteristics;(3) deformation of the sensor layer when the sensor is heated and cooled due to different thermal expansion coefficients of the sensor layer and the substrate;(4) danger of using the sensor heating element in places and mechanisms where there is a possibility of explosion or fire.Because of the noted shortcomings, the need to search for sensor materials that can exhibit gas sensitivity without additional heating was revealed. The use of light activation instead of heating, i.e., irradiation of the sensitive layer with UV or visible light, as a gentler and less energy-consuming method of activation, has become widespread ,7,8,9,10The purpose of this review is to discuss the current state of light-activated conductometric gas sensors based on metal oxide semiconductors. The main focus is on the analysis of the mechanisms of UV and visible light activation of the gas sensitivity of semiconductor materials, which determine possible approaches to improve their sensor characteristics. In the case of UV activation, the photo- and gas sensitivity of semiconductor metal oxides may be enhanced by chemical modification with noble metal nanoparticles and their oxides, as well as by creating nanocomposites based on various metal oxides. When activated by visible light, the main routes for improving the sensor characteristics of wide gap metal oxides are their doping, spectral sensitization using organic dyes, or particles of narrow-band semiconductors and quantum dots, as well as plasmon nanoparticles of noble metals.The use of a fundamentally different activation method caused a revision of the requirements for the sensor materials themselves, as well as the study of the mechanisms of the processes responsible for the sensor response formation. The activation of the gas sensitivity of resistive sensors under illumination is inextricably linked to the appearance of photoconductivity due to the light absorption and the generation of photoexcited charge carriers. The photovoltaic and optical properties of the most common semiconductor oxides used as gas sensor materials are discussed in this section.2, ZnO, and TiO2 due to direct transitions lie in the UV region [2O3 includes the visible region of the spectrum due to the contribution of indirect  transitions with 2.6 eV to the light absorption [gE = 2.6–2.8 eV) is not fully determined and may be a consequence of the contribution of both direct and indirect transitions [Semiconductor oxides used as materials for gas sensors are generally wide-gap . The absV region ,12,13. Tsorption . The natnsitions .n-type semiconductors and are able to significantly increase their electronic conductivity under the light of a certain range, i.e., exhibit photoconductivity. Thus, if for semiconductor sensors operating under thermal heating, it is necessary to determine the dependence of their electrical conductivity on the composition of the gas phase, then in the case of light activation, the dependence of the photoconductivity on the composition of the gas phase should be determined. Thus, the phenomena of gas sensitivity and photoconductivity are closely related to each other. The increase in photoconductivity is caused by the appearance of nonequilibrium (photoexcited) charge carriers in a semiconductor, and in the case of single-crystal semiconductors is proportional to the change in the concentration of charge carriers (Equation (10)):σ is the specific electrical conductivity; nμ and pμ are the mobilities of the electrons and holes, respectively; n and p are the concentrations of the electrons and holes, respectively.These oxides are 2 samples is approximately at 390 nm , polycrystalline SnO2 films are characterized by the appearance of a wider range of spectral sensitivity in the visible region (up to 520 nm)  and the accumulation of photoexcited charge carriers in the metal oxide grains, leading to a gradual decrease in potential barriers between the grains and an increase in the electrical conductivity of the sample. The decrease in photoconductivity in an oxygen-free medium is mainly due to the recombination of photoexcited charge carriers, which occurs through their capture on defects-traps, and therefore has a delayed character. In an oxygen-containing atmosphere, the rapid achievement of a steady state of photoconductivity is due to the dynamic equilibrium between the rate of generation of photoexcited charge carriers and the rates of oxygen adsorption and photodesorption (Equations (11) and (12)). In this case, the decrease in photoconductivity can be caused not only by the recombination of photoexcited charge carriers through internal defect levels, but also by the regeneration of the surface concentration of chemisorbed oxygen to the initial value, so it has an accelerated character.Thus, it is essential when considering the photoconductivity of materials based on nanocrystalline oxides to take into account the possibility of interaction of photogenerated charge carriers with molecules adsorbed on the surface of a semiconductor oxide, in particular, with chemisorbed oxygen molecules.The study of the influence of the composition of the gas atmosphere on the photovoltaic properties of semiconductors has long been associated with the physical studies, thus expanding the understanding of the processes of generation and recombination of photoexcited charge carriers in these materials. It was only in the early 1990s that pioneering work was carried out to study the effect of light activation on the sensor sensitivity of semiconductor oxides. It was found that the photoconductivity of polycrystalline layers of metal oxides under UV light shows sensitivity to much weaker changes in the composition of the atmosphere than previously demonstrated in physical studies. This allowed the consideration of this activation method on a par with traditional thermal heating.2 film under UV light when acetone and trichloroethylene vapors were introduced into the measuring cell. At a concentration of acetone and trichloroethylene of 760 and 270 ppm, respectively, the relative drop in the electrical conductivity of the SnO2 film reached 25–50%. The enhancement of the sensor response of polycrystalline SnO2 and In2O3 films to NO2 and CO at room temperature under UV light was demonstrated in the works of the scientific group of Prof. G. Sberveglieri, in 1996–2001 [For example, the authors  found a 996–2001 ,33,34. I2 [2O3 [3 [2 [2 by platinum clusters of 8–10 nm in size leads to a significant (40–70 times) enhancement of the sensor response when detecting hydrocarbon vapors at room temperature and illuminated by a UV radiation source . It is not uncommon to use several types of catalytic additives to influence the selectivity of UV-activated gas sensors based on semiconductor oxides [The widespread use of low-power UV LEDs to activate the sensor sensitivity of semiconductor oxides has led to increased publication activity in this area since the 2010s. To date, numerous studies have been published on the effect of UV radiation on the activation of the sensor sensitivity of semiconductor oxides, such as SnO2 ,36, In2O2 [2O3 ,41,42,432 [2O3 ,49,50,51 [2O3 [3 ,57,58,59O3 [3 [2 ,62,63,64O3 [3 [2 ,67,68,69O3 [3 [2 ,71, Pt [O3 [3 [2 ,73, Pd [O3 [3 [2 , PdO cluO3 [3 [2 , nanoparO3 [3 [2 , and carO3 [3 [2 ,78,79. FO3 [3 [2  that modr oxides . In genen-n heterojunctions among themselves. The gas-sensitive properties of nanomcoposites ZnO/In2O3 [2 [2/SnO2 [2O3/ZnO [2 [2 system has the following form  activation by blue light (in the range ~ 400–500 nm);(2) activation by green or red light (in the range ~ 500–600 nm).2O3 [3 [When activated by light in the blue range, although the wavelength of the exciting radiation formally lies in the visible region, the photon energy is very close to the band gap of metal oxides. There are two main reasons for the photosensitivity of wide-gap nanocrystalline metal oxides when exposed to blue light. First, as is known, in the nanocrystalline state, metal oxides exhibit a significant tailing of the absorption edge, due to the disordering of their crystal structure and the appearance of a significant number of defective (impurity) levels near the edges of the energy bands. As a result, the absorption edge begins to capture the visible part of the spectrum. Secondly, in the case of In2O3  and WO3 2O3 [3 ,100,105,The photoresponse and activation of the sensor properties of wide-gap semiconductor oxides under longer-wavelength radiation (corresponding to the green and red regions of the spectrum) is a more interesting and rarer situation. In this case, the absorption edge of the metal oxide and the emission spectrum of the LED do not overlap. However, these phenomena can also be explained by the photoexcitation of electrons from deeper impurity levels located inside the band gap of these metal oxides. Some authors also associate the photoresponse of wide-gap metal oxides in the long-wavelength spectral region with direct photoionization of chemisorbed oxygen (Equation (13)) :(13)O2adIn this case, the electrons from the local acceptor levels are excited into the conduction band of the semiconductor oxide, and the electron-neutral oxygen molecule is desorbed. Although this process is not associated with the direct excitation of electrons between the internal energy levels of the semiconductor, it leads to a decrease in potential barriers at the grain boundaries of the semiconductor and the appearance of photoconductivity.Despite the fact that under visible light, wide-gap semiconductor oxides are able to demonstrate the appearance of a sensor response, this effect is usually quite weak. The main reason for this is the small coefficient of absorption of visible light by these substances. This leads to the fact that only a small part of the emitted photons is absorbed and participates in the formation of electron–hole pairs. To increase the energy efficiency of light-activated sensors, the maximum overlap of the emission spectra of the radiation source and the absorption of the sensitive material is preferred.2 [2 [2 [3 [2O5 [gE = 2.2–2.4 eV) and p-type semiconductor oxides, such as HoFeO3 [There are two fundamentally different ways to create semiconductor gas sensors with visible light activation. One way is to use semiconductors with a smaller band gap, which allows the absorption of visible light photons as the material of the sensitive layer. Such semiconductors are, for example, CdSe ,107, CdS2 , WS2 [112 [2 [2 , and BiI[2 [2 [3 , and rec[2 [2 [3 ,116. Sin [3 [2O5 ,118 (Eg s HoFeO3  and NiO s HoFeO3 . Another2) showed that the synthesized materials exhibit sensitivity to formaldehyde in the concentration range of 5–100 ppm, while the sensor signal at 100 ppm detection was 287%. The optimal iron concentration, corresponding to the best photoelectric and sensor characteristics, was 1%  high absorption coefficient in the visible spectral range;(2) the ability to transfer the photoexcitation energy to the oxide matrix.E* is higher in energy than the edge of the conduction band cE of the semiconductor oxide  dye. This makes it difficult to predict the optical properties of sensitized materials. The sufficient stability of organic dyes under conditions of long-term gas detection under illumination is also questionable, since the authors usually do not provide data on the long-term stability of the sensor properties of the obtained hybrid structures.2S [2 [3NH3SnI3 [In the literature, there is information about the sensitization of wide-gap metal oxides by Ag2S , CuO [142S , CdS [142S ,152,153,2S , SnS2 [12S [2 , and CH33NH3SnI3  nanopart2 and thiourea under the powerful ultrasonic action. After separating the excess CdS particles, the nanocomposites containing from 5.9% to 24.8% CdS were obtained. According to electron microscopy data, CdS particles with an average size of 10–30 nm were formed on the surface of ZnO grains , which are isolated nanocrystals of semiconductors with dimensions less than the Bohr exciton radius in this material. The surface of colloidal QDs is stabilized by ligands preventing their aggregation. The use of semiconductor QDs to modify the optical properties of semiconductor oxides has a number of advantages:(1) Due to the effect of quantum limitation, the optical properties of QDs made of the same material can be varied widely, changing only their size, as in the case of, for example, cadmium chalcogenides .(2) Due to the fact that colloidal QDs are synthesized and isolated in a separate chemical process, their structure, size, composition, and morphology can be independently and accurately controlled. For example, existing synthesis methods allow the synthesis of CdSe QDs with a relative radius deviation of no more than 5% .(3) Colloidal synthesis methods allow us to create core/shell structures based on quantum dots, with a controlled composition, core size, and shell thickness . The use(4) Unlike dyes, whose adsorption on the surface of semiconductor oxides is often accompanied by aggregation and changes in optical characteristics, semiconductor quantum dots, as a rule, retain their inherent optical properties, which allows more accurate predicting and regulating the optical properties of the nanocomposite as a whole.2, In2O3, and CdSe colloidal QDswere studied in [2 at room temperature, under green light   of a nanocomposite based on nanocrystalline ZnO and cubic-shaped perovskite CsPbBr3 colloidal quantum dots is demonstrated in the work [The use of CdSe ,164,165 udied in ,163,164. mW/cm2) B. Under the work .Separately, we should consider the sensitization of semiconductor oxides by nanoparticles that exhibit the effect of plasmon resonance, since in this case the mechanism of sensitization is more complex. Plasmon resonance is understood as the phenomenon of collective oscillation of valence electrons relative to the atomic core, which coincides in frequency with external radiation. An example is gold nanoparticles, for which the plasmon resonance effect is observed in the range of 500–540 nm. The undoubted advantage of noble metal particles is their stability and high light extinction coefficients. This makes their inclusion in the composition of photosensitive materials for light activated gas sensors very promising. In this case, metal nanoparticles are primarily components that ensure the specificity of light absorption in the required spectral range. The nanocomposites based on semiconductor oxides and noble metals were already mentioned in In , several(1) The main effect, apparently, is related to the fact that near the particle with the plasmon resonance, an inhomogeneous electromagnetic field is formed, reaching a high intensity at certain points. In a semiconductor, the rate constant of electron–hole pair formation is proportional to the electric field strength, so the rate of generation of electron–hole pairs increases in metal oxide grains that are in the field of a plasmon nanoparticle. It should be noted that this effect is likely not lead to a shift in the photosensitivity of the material in the visible region, but enhances the photosensitivity of the original semiconductor oxide in the UV region, while the effects of radiation in the visible range required for the manifestation of the effect of plasmon resonance.(2) The sensitizing effect can be caused by direct injection of photoexcited “hot” electrons from plasmon nanoparticles into the conduction band of the semiconductor oxide. The injection of photoexcited electrons can be detected by increasing the photoconductivity of the nanocomposite material when irradiated with light corresponding to the absorption band of plasmon nanoparticles. However, in most studies, the photoconductivity of nanocrystalline oxides sensitized by plasmon nanoparticles increases slightly under such conditions. (3) The effective scattering of resonant photons between plasmon particles has an additional effect. As a result, the optical path is lengthened, and the probability of light absorption by the semiconductor matrix increases.(4) When plasmon nanoparticles absorb light, local heat is released.2H2 were investigated. The nanocomposite obtained in this work consisted of ZnO nanowires with a thickness of about 100 nm, covered with Au island particles with a size of about 80 nm (max = 532 nm), a decrease in the electrical resistance of the ZnO/Au nanocomposite from 580 to 530 MOhm, as well as an increase in the sensor response to 100 ppm C2H2 by about 10 times were observed  and a sufficiently high intensity (60 mW/cm2) was used for illumination. The authors did not provide data on the sensor sensitivity of the structure without Au plasmon nanoparticles under similar conditions. Therefore, it is possible that the observed photosensitivity of the ZnO/Au nanocomposite may be a consequence of the intrinsic photosensitivity of zinc oxide to visible light.Often, the selected experimental conditions do not allow us to identify the effects achieved by introducing plasmon nanoparticles into the oxide matrix. Thus, the authors  showed aThe role of Au plasmon nanoparticles in the formation of the sensor response is more clearly demonstrated in , where tThe literature data on the sensor signal of sensitized wide-gap oxides under visible light activation toward various gases are summarized in The interaction of the sensitive material with radiation leads to a complex of various phenomena. The most general approach explains the appearance of the light activated sensor sensitivity of semiconductors based of two types of phenomena: (i) the generation of photoexcited charge carriers and (ii) their interaction with adsorbed gas molecules. The first process leads to an increase in the concentration of free charge carriers, and as a result, an increase in the electrical conductivity of the sensitive semiconductor layer. The second process is caused by the capture and recombination of photoexcited charge carriers with chemisorbed gas molecules that lead to a change in their concentration and, consequently, the steady-state photoconductivity value. The question about the mechanism of sensor response is to determine the specific means of generation, transport of photoexcited charge carriers, and their interaction with chemisorbed gas molecules.The list of gases that can be detected by light activated semiconductor sensors is quite wide . First oThe study of the mechanism of sensor sensitivity based only on data on photoconductivity and its changes depending on the composition of the gas atmosphere is not very informative. The electrical conductivity of the sensitive layer is an integral characteristic, and in the case of nanocrystalline semiconductor oxides, its dependence on the concentration of charge carriers, their mobility, and the presence of adsorbed molecules on the surface of the oxide grains is quite complex. The application of the barrier conductivity model to real nanocrystalline materials is also associated with a number of difficulties, for example, in calculating the Debye length and the specific height of potential barriers at the grain boundary. In addition, there are other models of conductivity , the use of which can also be justified in the case of nanocrystalline metal oxides, due to their high defectiveness and disorderliness (which mainly concerns the atoms at the grain boundaries).2, In2O3, and WO3 with oxygen have not been studied by either manometric or mass spectrometric methods.As a result, when studying the mechanisms responsible for the formation of light activated sensor response, it is necessary to involve additional methods that allow, for example, to identify reacting particles on the surface of the sensitive layer in an independent way. Examples include in situ IR spectroscopy and in situ mass spectrometry. However, such specific methods, although they provide unique information about the transformations of the detected gas molecules on the surface of semiconductors, are still very limited in their use in research on this topic. The importance of using additional research methods in studying the mechanisms of photostimulated processes can be illustrated by the example of the “oxygen-ZnO” system. From numerous experimental data, it is known that the conductivity of zinc oxide, as well as ZnO based nanostructures, in an oxygen-containing atmosphere, increases under UV illumination, although there is a tendency to conductivity decrease with increasing oxygen concentration in the gas phase. The increase in photoconductivity in this case is often associated with the process of photodesorption of oxygen from the ZnO surface (Equation (12)), which leads to a decrease in potential barriers between ZnO grains. However, manometric  and mass2 (in a number of works declared at the level of tens of ppb), good reproducibility, and a high sensor response. Almost all metal oxides studied demonstrate high sensitivity to NO2, even without the need for any special modification of their composition, morphology, stoichiometry, or the size of nanocrystals. Although the mechanism of the sensor sensitivity of semiconductor oxides to NO2 is also widely covered in the literature, none of the proposed mechanisms are generally accepted to date.The largest number of works, apparently, is devoted to the study of the photoactivated sensor response of semiconductor oxides in the detection of nitrogen dioxide, which, to some extent, has become a model gas for such investigations. The reason for this is the high sensitivity to NO2 sensor response of n-type semiconductor oxides: (i) NO2 adsorption on the surface of semiconductor oxides, leading to a decrease in conductivity; (ii) desorption of NO2 from the surface of semiconductor oxides, ensuring a return of conductivity to the base level. The first process at room temperature proceeds spontaneously, the second one requires energy consumption through thermal or light activation.In general, we can distinguish two processes responsible for the appearance of NO2 from the gas phase, which proceeds with the capture of conduction electrons, can be written as Equation (8). Since the electron affinity of the NO2 molecule is significantly higher (2.27 eV) than that of the O2 molecule (0.45 eV), a possible process is also the displacement of some of the chemisorbed oxygen species from the surface of the semiconductor oxide (Equation (14)):The adsorption of NO2 is accompanied by its dissociation into NO and O2; thus, an additional amount of chemisorbed oxygen appears on the surface:There is also a point of view  that the2 molecules during chemisorption on semiconductor oxides [Other works suggest more complex processes describing the transformations of NOr oxides ,81:(16)2All the considered processes lead to the accumulation of a negative charge on the surface of metal oxide grains, which leads to an increase in the width of the depleted layer and a decrease in electrical conductivity, and correlates with the results of sensor measurements. Unfortunately, the proposed mechanisms (Equations (14)–(17)) are speculative and have not been supported by additional research.2 interaction of with the surface of semiconductor oxides by independent methods, it is necessary to note the works in which the methods of XANES and XPS were used. Thus, in [2 chemisorption on the ZnO surface occurs mainly with the formation of nitrate anions; however, the results obtained characterize this process at a relatively high (450–500 Torr) NO2 pressure, which is significantly different from the operating range of gas sensors. In [2 was investigated by XPS in the concentration range closer to the operating ranges of gas sensors, namely, when ZnO thin films were exposed to air containing 1000 and 10 ppm of NO2. For both NO2 concentrations, on the ZnO surface, two nitrogen charge states are observed, corresponding to the NO3− (407 eV) and NO2− (403.7 eV) ions , the adsorption mechanism should include the possibility of the formation of nitrate ions on the surface of ZnO, by, for example, the reversible interaction of chemisorbed NO2 molecules with the lattice oxygen of ZnO (Equation (18)):Turning to the study of NOThus, in , the XANsors. In , the cheeV) ions A. In a sadiation B. It fol2 species, which is necessary to ensure the reversible operation of the sensor, can occur under UV light due to the recombination of electrons captured by NO2 molecules with photoexcited holes (Equation (19), similar to the previously considered mechanism of O2 photodesorption, Equation (12)):Desorption of chemisorbed NO2 molecules occurs or is accompanied by dissociation into NO and O.Although it was clearly shown in  that nitMost authors believe that the response to reducing gases is due to their reaction with chemisorbed oxygen, which leads to a decrease in its surface concentration and an increase in the concentration of conduction electrons in the semiconductor oxide. Thus, the occurrence of sensor sensitivity is explained in a similar way as in the case of thermal activation. For example, when detecting formaldehyde at room temperature, this reaction can be written as Equation (20):− or O2−).It is debatable why this reaction does not occur at room temperature under dark conditions, but occurs under UV or visible light. According to a common point of view, the oxidation reactions of type (20) involve photo-adsorbed oxygen molecules, which are more loosely bound to the surface of the semiconductor oxide than “ordinary” chemisorbed oxygen molecules, and have a greater reactivity ,69. At hSimilarly, the authors  justify 2 and ZnO are most active in photocatalytic oxidation. Varying the wavelength of UV radiation allows one to change the sensitivity to different gases, providing an increase in the selectivity of the sensor [At the same time, the authors ,183,184 e sensor .When explaining the mechanism of sensor properties, some authors consider the direct interaction of chemisorbed molecules of organic substances with photoexcited charge carriers, similar to photocatalytic reactions ,186,187.2 under UV light to CO, but not to H2. During the proposed processes (Equations (24) and (25)), adsorbed CO offered electrons to TiO2, which led to an increase in surface conductivity TiO2 in accordance with the electrochemical reactions on its surface:Due to both photoexcited holes and photoexcited electrons are involved in the process of photochemical oxidation, an increase in the conductivity of the sensor occurs and a sensor signal appears. Likewise, reinforcing their reasoning with IR spectroscopy, the authors of the work  explain 2 in a similar concentration range, the sensor signal can reach 102 –103. When interpreting low values of the sensor signal, it should be taken into account that a comparable response can be observed when the partial pressure of oxygen or water vapor in the gas mixture changes during the measurements. Therefore, when studying the sensor sensitivity to reducing gases under light activation, additional efforts should be made to separate the “true” sensor response from the interfering factors.A common feature of the studies concerning the gas sensitivity of semiconductor oxides to reducing gases and volatile organic compounds under light activation is a low sensor response. Typical values of the sensor signal usually do not exceed a few units, and often even a few percent. At the same time, when detecting NOThe analysis of the literature shows that the light activation of the sensor sensitivity of semiconductors is actively developing direction. The problems to be solved lie in the plane of both fundamental science and practical applications. Fundamental questions are related to the development of theories describing the effect of light radiation on the interaction of particles on the surface of semiconductors. The prospects for practical application are related to the possibility of reducing energy consumption when detecting gases.The materials and approaches used to create light activated resistive sensors are quite diverse. The data obtained can be summarized as follows: (i) photoactivated sensors, as a rule, show the greatest sensitivity to oxidizing gases, while the sensor response to reducing gases is much less; (ii) a fruitful idea implemented for sensors with visible and UV light activation is the separation of photoexcited charge carriers between the two phases; (iii) the most promising approach to sensitize wide-gap metal oxides to visible light is the use of particles of narrow-gap semiconductors and semiconductor QDs.Despite the large number of published articles, many questions concerning the light activation of the sensor sensitivity of semiconductors remain debatable. The disadvantage of many works is incomplete information about the parameters of the radiation used: the spectrum and power of light sources, the illuminance of the sensor layer, and the method of its determination. In this regard, it should be noted that the works on the develop of integrated miniature devices that combine the sensor layer and the light-emitting element in close to each other, which allows solving the problems of both reproducibility and the most efficient use of light energy ,190,191.


The causes of proteinuria in healthy newborns have not been clearly elucidated. Many studies have focused on proteinuria in newborns and sick children and sick animals, especially dogs and cats. The present study was the first to quantify the hypothesis of the occurrence of proteinuria in healthy calves in the first week of life and to assess its intensity and dynamics, based on the analysis of changes in total protein excretion in the urine and its fractions differing in molecular weight  and high molecular weight proteins (HMW)). It should be noted that the analysis of excreted protein fractions is a sensitive diagnostic indicator of the type of kidney disease, e.g., increased HMW protein excretion may indicate damage to the renal glomeruli, and loss of LMW proteins may indicate renal tubular disease.A newborn’s survival depends on the quick adaptation of the organism to new environmental conditions. Newborn calves show high somatic maturity compared to, for example, human newborns, but their body functions with a lower efficiency than that of adult cattle. Adaptation processes concern all organs, including the kidneys, which are not morphologically mature after birth. The ongoing morphological changes imply functional alterations in the kidneys. There is an increase in blood flow through the kidneys and the glomerular filtration rate, as well as an increase in the efficiency of resorption processes of primary urine components into the blood, including proteins. Protein is present in trace amounts in the urine of healthy adults of various species. It can occur in larger amounts in the urine of sick animals, as well as in certain physiological states, e.g., in newborns. The presence of protein in the urine of newborns in the quantity exceeding 300 mg/24 h/m2 of body surface area and statistically analyzed. Neonatal proteinuria was demonstrated in healthy calves, mainly resulting from the high concentration of LMW proteins in the urine. Their percentages decreased significantly from 84.46% on the first day of calves’ life to 64.02% on day 7. At the same time, a statistically significant increase was observed in the proportion of albumin and high molecular weight proteins in urine total protein. Albumin percentage increased from 9.54% (on day 1) to almost 20% (on day 7), while the proportion of HMW proteins increased from 6.68% to 18.13%, respectively. The concentration of total protein in the urine of newborn calves amounted to 14.64 g/L and decreased statistically significantly during the first 72 h of postnatal life, stabilizing at the level of 3–4 g/L. The mean value of total protein excretion in the first week of life was 4.81 mg/min/m2 . The analysis of protein concentration in the urine and its excretion, as well as changes in urinary excretion of the tested protein fractions, indicated that neonatal proteinuria in healthy neonatal calves was tubular . In addition, research showed that there was a rapid improvement in resorptive mechanisms in tubular cells. It should be assumed that the filtration barrier in the kidneys of these animals after birth is morphologically prepared to retain high molecular weight proteins. It seems that the increased permeability of the filtration barrier in the glomeruli does not necessarily indicate the immaturity of the kidneys, but may indicate the kidneys’ adaptation to excess protein removal from the body during feeding with high-protein food (colostrum), with an open intestinal barrier enabling protein absorption from the gastrointestinal tract to the blood.Urine testing is a convenient, non-invasive method of obtaining information about body functions. Depending on the intended purpose, urine testing may be qualitative and/or quantitative. Urine analysis can also include proteins. There are no data in the literature on the occurrence of proteinuria in healthy neonatal calves. The present study was the first that aimed to quantify the hypothesis of proteinuria occurrence in these animals in the first week of life, to assess its intensity and dynamics and to understand the underlying causes of proteinuria in healthy calves. The research was carried out on 15 healthy calves in the first seven days of life. Calves were catheterized to determine minute diuresis. Total protein concentration was determined in blood plasma and urine. Urine proteins were separated by electrophoresis (SDSPAGE) and their concentration and percentage were determined by densitometry using an image archiving and analysis software. The separated proteins were divided into three groups according to molecular weight for albumin, LMW and HMW proteins. The results were standardized per 1 m Urine testing is a convenient, non-invasive method of obtaining information about body functions. Depending on the intended purpose, urine testing may be qualitative and/or quantitative. Urine analysis can also include proteins. It is known that protein in the urine of healthy individuals of different species is present in trace amounts. It can occur in larger quantities in the urine of sick animals, as well as in certain physiological states, e.g., in newborns ,2,3,4,5.Proteinuria, even in healthy individuals, is an indicator of renal dysfunction, and the type of the excreted protein fraction  or high molecular weight proteins (HMW)) may be a marker of the type of kidney disease, e.g., albumin and higher molecular mass protein excretion may indicate glomerular damage, while loss of LMW proteins may indicate tubular diseases .The main cause of proteinuria observed in newborns of various species is morphological immaturity of the kidneys and dynamic adaptive processes occurring especially in the first days postpartum, associated with both changes in renal hemodynamics and nephron function ,14,15,16Most of the proteins excreted in the urine are plasma proteins that have been excreted into the urine due to increased permeability of the filtration barrier or decreased tubular resorption. Albumin molecule size is assumed to be a limiting value for the filtration of plasma proteins. As the particle size decreases, the ease of their filtration in the glomeruli is also higher. Elevated protein filtration is also caused by increased blood protein concentration, increased blood flow through the kidneys, increased filtration pressure in the glomeruli and electrostatic charge of proteins ,17.The resorption of filtered proteins occurs mainly in the convoluted part and in the initial straight part of the proximal tubules. The efficiency of protein absorption and hydrolysis mechanisms in tubular cells determines (apart from filtration) the intensity of proteinuria. Therefore, the determination of individual protein fractions in the urine of newborns may be a sensitive diagnostic tool in assessing the maturity and functioning of the filtration barrier and resorption mechanisms in the proximal tubules ,18.It should be noted, however, that the causes of neonatal proteinuria, mechanism, frequency and dynamics have not been clearly explained. Many studies have focused on proteinuria in newborn and sick children and animals, especially dogs and cats ,19,20,21The aim of the research was to verify the hypothesis about the occurrence of proteinuria in healthy calves in the first week of life and assess its intensity and dynamics based on the following tests: (a) total protein concentration in the urine, (b) percentage of albumin and LMW and HMW proteins in the urine, (c) excretion of protein and individual fractions in the urine and (d) percentage of albumin, LMW and HMW proteins in 24 h urine. The analysis of these indices in the urine produced over a 24 h period in consecutive days of the first week of life will enable not only quantitative but also qualitative evaluation of proteinuria, i.e., distinguishing between glomerular and tubular proteinuria. It will also help to understand and elucidate the underlying causes of proteinuria in newborn calves.The research was carried out on 15 healthy Polish-Friesian var. Black-and-White full-term female calves in the first seven days of life. Directly after birth calves were fed with their dam’s colostrum, and then animals were transported to the animal facility of the Department of Animal Physiology and placed in individual stalls, under environmental conditions ensuring welfare maintenance. During the research period animals, were subject to constant observation and showed no clinical symptoms of diseases. During the first two days of life, calves were fed their mothers colostrum in total amount up to 4 L (4–5 feeds per 24 h). In the following days of the first week of their life, animals were fed mother’s milk 3 times per day, in a total amount of 5 L in the 3rd day of life and in an amount of 6–7 L per day up to the end of the experiment. As stated above, the animals were fed individually with their mother’s colostrum and milk. According to that, the composition of the colostrum and milk have not been analyzed. The first feeding of the day and weighing of the calves took place one hour before the start of the study. The analyses started in the morning at 9:00. At first, the calves had a Foley balloon catheter inserted into the bladder. Urine was collected at two 30 min time intervals into sterile polyethylene bags in order to determine minute diuresis. Blood for tests was collected from the external jugular vein via the catheter inserted at the first time of the blood sampling 15 min after the beginning of the urine collection intervals and protected against clotting with heparin . Blood and urine, immediately after collection, were centrifuged at 3000 and 2000 rpm, respectively, for 15 min at 4 °C. The concentration of total protein in the blood plasma and urine was determined by the modified Lowry method (Bio-Rad DC Protein Assay) using the bovine serum albumin standard. Urine samples (1 mL) were lyophilized for concentration. The obtained urine lyophilisate was dissolved in 100–150 μL of deionized water before performing protein electrophoretic separation. Urine samples were mixed with Laemmli buffer containing sodium dodecyl sulfate  and reducing agent 2-mercaptoethanol , and subsequently denatured for 5 min at 95 °C.Electrophoretic separation of urine proteins was carried out on a 15% SDS-polyacrylamide gel, SDS PAGE. Their concentration was determined by densitometry using the Bio Print 215 image documentation and analysis system and the Vilber Lourmat Bio-1D software. Molecular weight markers  were used to estimate the molecular weights of separated protein fractions with the following composition: Myosin 200.00 kDa, β-galactosidase 116.25 kDa, Phosphorylase b 97.40 kDa, Serum albumin 66.20 kDa, Egg albumin 45.00 kDa, Carbonic anhydrase 31.00 kDa, Trypsin inhibitor 21.50 kDa, Lysozyme 14.40 kDa, Aprotinin 6.50 kDa.TP), urine volume low molecular weight proteins (UVLMW), urinary albumin excretion rate , urinary high molecular weight protein excretion rate (UVHMW), as well as 24 h urinary protein excretion and the percentage of LMW proteins, albumin and HMW in 24 h urine ).The separated proteins were visualized in a staining solution with the following composition: 0.05 g of Coomassie Brilliant Blue R 250 , 1 mL of methanol, 50 mL of 10% acetic acid and 450 mL of distilled water. The gels were subsequently archived with a GS-800 densitometer (Bio-Rad) and analyzed using the Quantity One software . The investigated protein bands were divided into three groups: albumin  and proteins with lower (LMW < 69 kDa) and higher molecular weight (HMW > 71 kDa). The concentration of selected fractions of proteins in the urine was calculated on the basis of optical density, and their percentage in the total urine protein was calculated. In addition, the following were calculated: urinary total protein excretion rate (UV2 body surface area according to the Meeh formula:bw is body weight (kg).The results were standardized per 1 mMean values and standard deviations were calculated. The obtained results were subjected to statistical analysis in order to assess the significance of differences using the method of univariate analysis of variance with repeated measurements using Duncan’s multiple range test (Statistica 6.0 software).The concentration of total protein in the urine of newborn calves on the first day of life amounted to 14.64 g/L and decreased statistically significantly during the first 72 h of postnatal life, stabilizing at the level of 3–4 g/L . IndividDynamic changes in the proportion of individual protein fractions in the urine were demonstrated. Low molecular weight (LMW) proteins constituted the highest percentage in urine total protein. Their percentages decreased significantly, from 84.46% on the first day of life to 64.02% on day 7 . At the 2 body surface area on the first day of life and 3.96 mg/min/m2 body surface area on day 7 . Albumin excretion tended to increase and amounted to 0.50 mg/min/m2 body surface area  and 0.69 mg/min/m2 body surface area (on the seventh day of life). The excretion of high molecular weight proteins increased more than twofold, from 0.31 (on the first day of life) to 0.69 mg/min/m2 body surface area (on the seventh day).The decrease in total protein excretion in the urine was a result of a significant reduction in the excretion of LMW proteins, with an increase in the excretion of albumin and HMW proteins , Table 1The proportions of individual protein fractions in 24 h urine are presented in Proteinuria occurs in healthy newborns of many animal species and humans; however, high species and individual variability in the frequency, duration and 24 h quantity of excreted proteins is observed, especially in the first days after birth ,22,23,24The amount of protein excreted in the urine is primarily the result of glomerular filtration of proteins and their resorption in the proximal tubules. Morphological changes in nephrons occurring in the postnatal period imply functional alterations in the newborns’ kidneys. Test results for the kidney function of many neonates, including calves, indicate that renal blood flow and glomerular filtration are low at this stage of life. With age, an increase in these parameters is observed, as well as the resorptive and secretory efficiency of the renal tubules ,26,27,28Moreover, birth age and body weight are important for the occurrence of neonatal proteinuria ,32,33,34In the present experiment, neonatal proteinuria was observed in healthy full-term calves, along with a trend towards a decrease in total protein content in the urine during the first week of life. Dynamic changes in the percentage of individual protein fractions in the urine occurring at this time should be emphasized. LMW proteins constituted over 80% of all proteins in the urine of newborn calves, and this proportion decreased by 20% within one week. A convergent trend of changes in the proportion of LMW proteins was demonstrated by Ożgo et al.  in the uA significant increase was noted in albumin percentage in the urine of calves in the first 72 h after birth (from 9.54% to 15.07%). In view of the stabilized protein concentration in the blood plasma of calves, the cause of the observed changes could be a higher GFR resulting in an increase in the filtered protein load in the glomera and/or less efficient tubular resorption. In studies performed on newborn goats, an increase in the proportion of albumin in the urine was also observed in the first days of life, from 7.26% on the first day to 12.25% on the second day and 12.05% on the third day . The dirThe tendency of increasing proportion of HMW proteins in the urine of calves demonstrated in the experiment was confirmed by the results of the studies conducted by , who obs2 (on day 1) to 1.65 mg/min/m2 (on day 7) [There are no unambiguous data in the literature concerning the quantity of proteins excreted in the urine of newborn animals, including calves, and the trends of changes in the excretion of total protein and individual protein fractions. The study carried out on male kids showed a reduction in the total protein excretion rate in the urine from 3.82 mg/min/mn day 7) . These rn day 7) ,11,19. Tp < 0.01) reduction in the excretion of LMW proteins in the urine of newborn kids from 3.41 g/min/m2 BSA (on day 1) to 1.33 g/min/m2 BSA (on day 7). The authors believe that the reduction in LMW protein excretion in the urine of young male goats observed in the first week of life, despite the changes in the concentration of these proteins in the blood plasma and increasing glomerular filtration, indicated an increasing efficiency of the tubular mechanisms of protein resorption in the kidneys of newborns. Joseph and Gattineni [The species differences were also indicated by trends in the excretion of individual protein fractions. The decrease in LMW protein excretion observed in the first week of calves’ life was confirmed by other authors. Ożgo et al.  showed ap ≤ 0.01) increase in the urinary albumin excretion rate was recorded only in the first 24–48 h of life (from 0.26 to 0.43 mg/min/m2 BSA). From the third to the seventh day of life, a decrease in albumin excretion to the value of 0.12 mg/min/m2 BSA was observed [In the first week of calves’ life, an upward trend in the urinary albumin excretion rate was observed, while in kids, a highly significant  allowed us to classify neonatal proteinuria in these animals as significant, and the predominance of the LMW protein fraction in the urine proved that it was selective proteinuria. The study carried out on young male goats also showed the presence of selective proteinuria and a reduction in total protein excretion in the urine, but the 24 h amount of protein excreted was lower than in calves (from 2.38 to 5.50 g/24 h/m2). The daily excretion of individual protein fractions ranged from 1.92 to 4.92 g/24 h/m2 (LMW proteins), from 0.16 to 0.43 g/24 h/m2  and from 0.22 to 1.07 g/24 h/m2 (HMW proteins). Joseph and Gattineni [2. Higher values indicate the presence of proteinuria.The daily excretion of protein in the urine of calves . The highest protein concentration in the urine was recorded on the first day of life and it significantly decreased in the first three days. The urinary protein excretion rate showed a similar direction of change. The results indicated that reduced tubular absorption was the cause of high urinary protein concentration and increased excretion in the first days of life. Therefore, neonatal proteinuria in healthy newborn calves was tubular in nature. In view of the stable protein concentration demonstrated in the blood plasma, the decrease in urine protein concentration and stabilization at the level of about 4 g/L from day 3–4 of life indicated a rapid improvement of the resorptive mechanisms in the renal tubular cells. This was confirmed by the decreasing tendency of the percentage content and LMW protein excretion, with an increasing, albeit small, proportion of albumin and HMW proteins in the urine. The low excretion rate of HMW proteins in the first days of life (0.31 mg/min/m2) and the low percentage of total urine protein (6.68% on day 1) indicated that the filtration barrier in the kidneys of calves was morphologically prepared for the retention of proteins of high molecular weight.The research has shown the presence of neonatal proteinuria in healthy calves. It was associated with a high concentration of proteins with a molecular weight of ≤70 kDa. Albumin and proteins with lower masses constituted over 85% of all urine proteins. The mean value of total protein excretion rate in the first week of life was 4.81 mg/min/m