## 1 Introduction  
Due to the unique physical and chemical properties of gold nanoparticles (GNPs), they are widely used in biochemistry, medicine, and analytical chemistry [1; 2; 3; 4; 5; 6]. GNPs have high surface energy, which determines the possibility of their modification [7; 8]. As a rule, GNPs are modified using biological macromolecules such as proteins and nucleic acids [9; 10]. GNPs in complexes with bioreceptor molecules (antibodies, aptamers, etc.) are used as analytical markers [11; 12]. To obtain nanocomposites, a large number of covalent sewing techniques have been developed, but for biological macromolecules, physical adsorption methods remain the most popular [13; 14; 15].  
The interaction of macromolecules and nanoparticles depends on the chemical nature of the adsorbed substance, the composition of the medium, and other factors. Electrostatic, hydrophobic, van der Waals, and donor-acceptor interactions can contribute to this process [6]. The high affinity of gold for sulfur-containing groups leads to the formation of a self-organizing monolayer of thiol-containing molecules on gold surfaces [16]. Proteins are effective stabilizers for GNPs. Even physical sorption of proteins on the surface of gold is characterized by a high binding constant (K\({}_{\rm d}\) -up to 10\({}^{-9}\) M) [17; 18]. As well as proteins contain many groups with different natures, their interaction with the surfaces of particles occurs by several mechanisms.  
Despite the long history of the use of GNPs, the synthesis of their conjugates with biomolecules is not a strictly delineated procedure to this day. The syntheses often lead to unstable conjugates being unsuitable for use. Particularly relevant is the question of obtaining conjugates of GNPs with antibodies. The high variability of the composition of antibodies impedes the development of a universal technique for obtaining such conjugates. Fairly often, the conjugates are unstable and quickly lose activity during storage [19; 20], which is primarily a result of the process of particle aggregation [21].  
Aggregation of metal nanoparticles is accompanied by significant changes in their adsorption spectra. This property has been successfully used in various homogeneous biosensors [22; 23; 24]. However, for heterogeneous bioanalytical systems, for example, immunomodtoragraphical ones, aggregation of nanoconjugates leads to a lost of their mobility and analytical signals. For this reason, obtaining stable non-aggregating conjugates of metal nanoparticles with antibodies is an important task. To solve this problem, a number of covalent conjugation methods have been developed, for example, using 2-(N-morpholino) ethanesulfonic acid and 3-(N-morpholino)propanesulfonic acid [25]. However, simple physical adsorption is still the most commonly used method for obtaining such conjugates [26]. In this regard, there is a need for studying factors that affect the aggregation of nanoparticles during their conjugation with antibodies.  
In this paper we consider an example of obtaining conjugates of a monoclonal antibody preparation with GNPs with non-stable aggregating product. The reasons for reducing their stability are investigated, and a method for solving this problem is proposed.