## 4 Conclusions and Outlook  
Clarification of the general regulations of GNPs aggregation by antibodies requires additional studies, including the characterization of various aggregating monoclonal preparations. Differences between aggregating and non-aggregating preparations could be directly related to the amino acid composition and its influence on the structure of proteins, as well as to the peculiarities of  
Figure 7: Immunochromatographic testing of the binding of GNP conjugates to Mab and _M. tuberculosis_ antigen MPT64.  
synthesis and transformation of immunoglobulins, leading to the presence of fragments of antibodies in the preparations.  
Thus, high affinity of sulfur-containing chemical groups to gold surfaces [37; 38], structural changes of antibodies in the course of their immobilization on gold surfaces and GNPs [39], the effect of immobilized proteins on the electrical properties of metal surfaces, potentially manifesting itself in decreased stability of colloidal solutions [40; 41] were described.  
[PERSON] et al. [42], studying the aggregation of GNPs by nanobodies (molecules with an incomplete structure of immunoglobulins), suggest two possible mechanisms for the aggregation phenomenon: (1) sorption of the protein on GNP leads to its denaturation, the appearance of hydrophobic groups on the surface and the aggregation provoked by these groups, and (2) cross-linking of GNPs by positively charged antibody fragments. Different isoelectric points of whole antibodies and their fragments can affect the variability of the aggregation stability.  
The presence of free light chains immunoglobulins in the preparation can be caused by either violations of the ratio of heavy and light chain synthesis in antibody-producing cells, or fragmentation of native molecules in contaminated preparations. The variation in synthesis of heavy and light chains for a significant part of monoclones was noted in [43; 44]. Control of the absence of antibodies destruction during their storage should be considered as an important element in assessing the reactants quality in the manufacture of immunochromatop graphic test systems. Such techniques as asymmetrical flow field-flow fractionation, size-exclusion chromatography, AgNPs-based array [24], etc. could be used for this purpose.  
Aggregation instability of conjugates of GNPs and antibodies is a frequent problem when developing analytical systems based thereon. Often, the problem is solved by screening various antibody clones to find the most stable conjugates. Our work demonstrated that a partial destruction of antibodies might be the reason for the decrease in the stability of conjugates. Therefore, in the case of a problem of conjugate aggregation, antibody preparations should be purified from fragments of their decay. This proposal can help developers build nanocomposite-based systems with enhanced functionality.  
Conceptualization, D.V.S., A.V.Z. and B.B.D.; investigation, D.V.S., I.V.S., V.G.A., I.V.K., S.S.B., V.Y.G.; methodology, D.V.S., A.V.Z., I.V.S., V.G.A., I.V.K., S.S.B., V.Y.G. and B.B.D.; resources, D.V.S., I.V.S., V.G.A., I.V.K., S.S.B. and V.Y.G.; validation, D.V.S., I.V.S., V.G.A.; visualization, D.V.S. and A.V.Z.; writing (original draft), D.V.S., I.V.S., V.G.A., A.V.Z. and B.B.D.; Writing (review and editing), D.V.S., A.V.Z. and B.B.D. All authors have read and agreed to the published version of the manuscript.  
This study was financially supported by the Ministry of Science and Higher Education of the Russian Federation (grant agreement No. 14.613.21.0080 on 22.11.2017, unique identifier RFMEFI613170080).  
The authors declare that they have no known competition for financial interests or personal relationships that could have appeared to influence the work reported in this paper.