## 1 Introduction  
Modern life constantly forces humanity to confront new threats. In addition to natural disasters, people continue to face resource scarcity, biological hazards, epidemics, poor nutrition, poor sanitation, pollution, or shortages of fresh drinking water. Very often, humans are the cause, since urban growth, increased consumption, overpopulation, and the release of various chemicals lead to contamination of the most valuable source of life--drinking water [1]. In addition, various emissions end up in water sources used for agricultural and domestic purposes, which in turn leads to imminent health risks for the population due to contamination of groundwater [2]. Recently, there has been more and more talk about the dangers of plastic containers, microplastics, and dangerous chemicals leached from them [3, 4]. Investigations conducted in various countries show that compounds belonging to different classes penetrate into the environment, such as heavy metals, phthalates, alkylphenols (nonylphenol (NP), octylphenol (OP) and others), bisphenol A, brominated flame retardants, and other organic compounds [5, 6]. All listed groups of compounds are subject to regulation in accordance with regulatory documents of various countries. A fairly large proportion of additives easily penetrate from plastic into water due to the lack of chemical bonds with the polymer base [7]. One of these dangerous compounds is nonylphenol, a toxic degradation product of the corresponding ethoxylate, which is actively used by humans for various household purposes as detergents [8]. Penetrating into freshwater sources of the environment, it accumulates in them [9], as well as water bodies and soil. Their interaction with solid carriers occurs under the influence of van der Waals forces, hydrophobic, and electrostatic interaction [10, 11]. Its proven toxic effects on the human body are associated with a destructive effect on the reproductive system due to estrogen-like actions and interaction with estrogen receptors [8, 12]. The main mechanism of toxic action is associated with oxidative stress in cells [13]. Due to the abovementioned effects, both NP and OP should be determined in water [14; 15]. Usually, chromatographic methods with mass spectrometric detection are used in practical analytical laboratories to determine nonylphenol and structurally similar alkylphenols. Despite their sensitivity and widespread use in environmental monitoring, these methods are not suitable for screening a large number of samples, and the analysis requires sample preparation in a laboratory setting [16]. As an alternative, immunochemical techniques are offered, which allow the analysis of groups of samples under the same conditions, in the absence of, or with minimal, sample preparation [17]. Among them, immunoenzyme assay (EIA) provided in microplates still has not lost its popularity due to their widespread use in medical, clinical, food, and veterinary laboratories [18]. The relative ease of analysis, small sample volumes--about 50-150 uL, and the availability of portable readers, as well as the possibility of replacing labels and using them in multianalysis, make various EIA options excellent alternatives to chromatographic methods [18; 19]. There is one more important nuance. Nonylphenol is a very hydrophobic compound, which is not only washed out of plastic but is also adsorbed onto it, including onto the components of immunochemical test systems [20]. This compound has a tendency to adsorb on various polymeric materials that also have a hydrophobic surface, which has been repeatedly studied based on its properties [21; 22; 23; 24]. The tendency for spontaneous adsorption of NP on the surface, or its pronounced hydrophobicity, requires changing of the conditions for the immunoenzyme assay. The addition of blocking compounds or suitable organic solvents is typically used to retain the analyte in the reaction media. However, not all solvents can be used in immunoassays, since they lead to inactivation of antibodies [25].