## 1 Introduction  
In recent decades, immunomodatographic assay (ICA) has become widespread in medical and veterinary diagnostics [1]. Immunochromatographic test strips are used for diagnostics of infectious diseases [2, 3]; detection of pathogens and toxic substances in food [4, 5, 6], and other tasks [7]. The main advantage of this technique is the possibility of rapid assaying without expensive equipment, which makes ICA convenient for off-laboratory use. Upon contact of the test strip with a liquid sample, the front of the liquid moves under the action of capillary forces and reactants applied to different zones of the strip interact with each other. These interactions result in the formation of immune complexes that are detected as staining in zones with immobilized reagents. The most commonly used labels in the ICA are gold nanoparticles (GNPs), due to stability, optical properties and well-developed methodology for modification with biomolecules [8]. The manifestation of coloration in the analytical and control zones of the immunomodatographic test strip is caused by the surface plasmon resonance and influenced by such characteristics of GNPs as their shape, size, dielectric capacity and surroundings [9]. However, even though they are preferred in commercial and research assaying, more sensitive test systems are often needed. Approaches to signal amplification in ICA can be divided into three classes: Alternative markers (i), using markers' catalytic properties (ii), and an increase in the number of markers in the analytical zone (iii) [10; 11]. For instance, to increase the sensitivity, a mixture of conjugates with small and large GNPs was used [12; 13]. Another method of signal amplification is the deposition of silver on GNPs by reduction of the silver salt [14]. [PERSON] et al. proposed an ICA with GNPs coated with a peroxidase. When adding a substrate, such as 3,3',5,5'-tetramethylbenzidine, 3-amino-9-ethylcarbazole, or 3,3'-diaminobenzate tetrahydrochloride, insoluble colored products are formed; they are concentrated in the test and control zones of the strip, contributing to their more intense staining [15]. One of the applications of ICA is serodiagnostics--the detection of antibodies against antigens of various pathogens in the blood. The use of this approach is complicated by the fact that the antibodies present in the sample usually account for just a few percent of the total amount of immunoglobulins, even in hyperimmune sera [16; 17; 18; 19]. Blocking immunoglobulin-binding sites on the surface of the label by nonspecific immunoglobulins leads to a reduction in the signal, which in turn makes it difficult to interpret the results of the assay accurately [20]. Therefore, an increase in sensitivity is most in demand for immunochromatographic serodiagnosis [21; 22]. In a number of works, the use of quantum dots (QDs) was suggested as a means to increase the sensitivity of ICA [23; 24; 25; 26; 27]. QDs are semiconductor nanocrystals that have a narrow and symmetric emission spectrum, which depends on their size and composition [28; 29]. QDs allow the reduction of the detection limit in ICA, due to their intense fluorescence and a low background signal. A combined approach in serodiagnosis in which the use of ICA with GNPs is followed by the stage of signal amplification with QDs is suggested here for the first time. At the first stage, a standard ICA with the conjugate of GNP-(protein G) is carried out. All immunoglobulins in the sample are associated with this conjugate. When the resulting complex enters the analytical zone, it binds to the immobilized antigen by means of its interaction with specific immunoglobulins, which comprise a small fraction of immunoglobulins in the blood. Thus, the binding of the bulk of immunoglobulins to GNP-(protein G) conjugate interferes with the interaction in the analytical zone. In the suggested ICA scheme, the GNP-(protein G) conjugate plays the role of seed. The second conjugate (QD-(protein G)) binds to the GNP-(protein G)-IgG complex in the analytical zone.