## 3 Results and Discussion
### Colorimetric Detection of Antimony (III)  
A simple assay protocol was developed using the obtained GNP-PAO conjugate for colorimetric determination of Sb\({}^{3+}\) in water. To appreciate the detectable minimal concentration of Sb\({}^{3+}\), different amounts of Sb\({}^{3+}\) aqueous solution were added to the solution of PAO-GNPs. At low concentrations of Sb\({}^{3+}\), the color of the solution is pale red (due to the dilution of PAO-GNPs). As the concentration of Sb\({}^{3+}\) increases, the additional amount of target ions results in an immediate color change to blue. We can observe the interaction results by the naked eye. In addition, the optical density at 620 nm was measured. This wavelength has been chosen in previous studies on colorimetry [21]. According to experimental data, when comparing the optical densities at 595 and 620 nm for the entire studied range of metal ions, it was revealed that the maximum signal (A = 0.8) for antimony ions was reached at 620 nm. The signal in the presence of other ions at 595 nm is minimized when the wavelength is changed to 620 nm. The chosen wavelength of 620 nm corresponds to the absorbance band of the formed gold aggregates. The obtained dependence of optical density intensity over the Sb\({}^{3+}\) concentrations is given in Figure 4.  
The concentration of 10 ng/mL caused sharp structural changes with a loss of gold conjugate stability in the solution. The interaction of aptamers with the target can be characterized by a gentle interaction curve if saturation occurs slowly. However, in some cases, the transition from a change in properties occurs abruptly, when saturation with the analyte leads to a loss of system stability. The same characterization of dependence was observed by [PERSON] et al. [37]. Figure 4 clearly demonstrates that sharp changes of optical density can be formally described by four-parametric equation. This fitting was done for the estimation of analytical characteristics of the developed technique. However, taking into account the absence of the working range, this technique cannot be characterized by the \"added-founded\" method. Figure 5a shows the absorbance spectra of the reaction mixture containing PAO-GNPs in the presence of different concentrations of antimony (III). As can be observed, the spectrum varies depending on the concentration of the determined ions. When the Sb\({}^{3+}\) concentration increased from 5.0 to 20 ng/mL, an additional band and shoulder between 600 and 730 nm at the absorbance spectrum occurred and optical density at 620 nm was increased, respectively. At the same time, the position of the main absorbance peak for gold nano-dispersed particles remains the same with a slow shift to the right from 529 nm for the GNP-PAOs without antimony to 542 nm for the GNP-PAOs at high antimony concentration. Figure 4: Calibration curve for Sb\({}^{3+}\) detection (with the use of GNP–PAO conjugate). The value of 0.55 indicates the absorbance of gold conjugate at 620 nm in the absence of antimony. The points above and to the right of optical density 0.5 correspond to the blue color of the final reaction mixture, and the points below and left of this level correspond to pink (y = 0.76 + (0.50 - 0.76)/(1 + (x/6.6)^{3}\).19), (R\({}^{2}\) = 0.95)). All experiments were undertaken in three replicates. Standard deviations from the mean were in the range 2.1–5.8%. If the position of the absorption peak is unchanged, an additional shoulder appears on the right side of the spectrum (600-730 nm). In a range of 0-3 ng/mL, the color solution was red. However, after 10 ng/mL Sb\({}^{3+}\) was added, the solution turned purple due to Sb\({}^{3+}\)-stimulated aggregation corresponding with absorbance and DLS data (Figure 5b). This figure indicates the similarity of the behavior of parameters. Thus, the growth of optical density (Figure 5b, blue curve) correlated well with the nanoparticle size determined by DLS (Figure 5b, red curve).