## 3 Results and Discussion
### Characterization of PAO-GNPs Conjugate  
Conjugation of GNPs was provided with the use of 3 uM of PAO. This concentration was chosen in accordance with our previous study [21] to stabilize GNPs. The prepared native and PAO-modified GNPs were characterized by absorbance spectroscopy and TEM (Figure 2). Maximal absorption for native GNPs was at 525 nm, which corresponds to the GNPs at around 30 nm in diameter [34]. With the abovementioned techniques, we estimated that the average diameter was 27.3 \(\pm\) 5.7 nm, confirmed by TEM. For this analysis, 127 particles were counted. The same value for GNP-PAOs was 29.7 \(\pm\) 1.6 nm (TEM). The difference between the average sizes determined by these techniques could be explained by the drying of the oligonucleotide hydrophilic layer on GNP-PAOs allocated to the mesh grid before TEM. [PERSON] et al. [35] studied similar effects when preparing GNPs stabilized with bovine serum albumin. In our research, oligonucleotides of small size (around 5 kDa) were used to modify GNPs, whereby a hydrophilic layer formed due to the presence of hydrophilic groups of adenine. Moreover, GNPs and PAO were conjugated in a buffer with a high ionic strength, which caused linear conformation of the oligonucleotide and necessary stabilization of the GNPs [36]. According to the modeling and experimental work of [PERSON] et al. [35], the difference in the size of the modified nanoparticles is due to differences in the environment when analyzing dissolved (DLS) and dried (TEM) preparations.  
Figure 1: Principle of colorimetric determination of Sb\({}^{3+}\) based on aggregation of modified gold nanoparticles (GNPs).  
Characterization of gold nanoparticles modified with PAO as well as native GNPs was provided by DLS technique. To show the impact of the oligonucleotide sequence, nanoparticles were added to the water solution with a known concentration of antimony (III), and all the samples were then measured in three replicates. Unmodified GNPs showed no sufficient increase in size before (28.8 \(\pm\) 5.0 nm) and after (38.6 \(\pm\) 8.2 nm) the addition of antimony ions (Figure 3). This indicates that in the absence of surface modification, interaction did not take place. Contrary to this result, the presence of the oligonucleotide sequence on the gold surface leads to the formation of complexes detected by this technique. Thus, modification of GNPs by PAO led to an increase in the diameter of a carrier to 57.8 \(\pm\) 5.3 nm compared to freshly synthesized nanoparticles (28.8 \(\pm\) 5.0 nm). The addition of antimony (III) to the reaction media led to an increase in the average diameter due to the formation of aggregates (128.9 \(\pm\) 4.2 nm). This difference indirectly confirms the contribution of the oligonucleotide sequence to the interaction with antimony (III) ions and demonstrates the possibility of applying this type of modification for the detection of target ions.  
Figure 3: Results of dynamic light scattering (DLS) characterization of native and PAO-modified gold nanoparticles in absence and presence of antimony at 10 ng/mL in the solutions (n = 3).  
Figure 2: Transmission electron microscopy (TEM) images of unmodified GNPs (**a**) and PAO-modified GNPs (**b**).