## 1 Introduction  
Heavy metals are a class of highly toxic pollutants that can affect human health and the environment. Antimony pollution is of increasing interest because of its wide use in various industries including semiconducting [1]. Normally, antimony occurs in different forms in environmental water as a result of the oxidation of sulfide minerals and dissolution by water [2]. Its biological function, permissible levels, and mechanisms of interaction with biological materials remain poorly understood. To answer these questions, [PERSON] et al. showed its binding by iron oxide phases [3]. In this case, compounds of antimony have a valency of 5. Due to rock disruptions and increased consumption by industry, antimony can be easily emitted into the environment [4]. Antimony basically occurs as Sb\({}_{2}\)S\({}_{3}\) and Sb\({}_{2}\)O\({}_{3}\) compounds [3]. Sb\({}^{3+}\) is more toxic than Sb\({}^{5+}\), and Sb can form bonds with methylated and other organic compounds [5].  
The importance of antimony detection is confirmed by the fact that the U.S. Environmental Protection Agency (EPA) regards it a priority pollutant, and the Council of the European Communities [6] established the maximum permissible concentration of antimony in waters at 0.01 mg/L. However, the task of widespread monitoring of antimony is not yet solved. High-performance liquid chromatography [7], electrothermal atomic absorption spectrometry [8; 9; 10], atomic fluorescence spectroscopy [8], anodic stripping voltammetry [11; 12], and multi-syringe flow injection analysis with hydride generation-atomic fluorescence detection [13] are currently the main methods for Sb3+ detection. However, the listed approaches require expensive equipment, highly qualified workers, and can be implemented only in a limited amount of laboratories.  
Currently, a range of easy and cheap colorimetric techniques has been proposed to determine the presence of antimony [7; 14; 15; 16; 17]. The first technique was based on antimony hydride generation and its detection by UV-spectrophotometry [7]. Various indicator dyes, such as pyrogallol red [15], pyridine-2,6-dicarboxylic acid [18], brilliant green, malachite green and crystal violet [16; 19], and bromopyrogallol red [16], are traditionally used for these assays. These dyes are used due to their property of form-colored complexes with target antimony ions [15] with followed assessment of absorbance. The developed colorimetric techniques are based on complexation of antimony with organic molecules, allowing the colored product to be detected spectrophotometrically.  
Alternatively, the colorimetric approaches based on modified gold nanoparticles (GNPs) are widely applied due to their unique optical properties [20]. Aggregation of GNPs modified with receptor molecules after the addition of metal ions can change the original color of GNPs from red to blue [21; 22; 23]. However, selective induction of the aggregation should be based on specific recognition of the target metal ion. Increasing the selectivity of this method can be achieved by modifying the GNPs' surface using an aptamer poly-A oligonucleotide (PAO). Aptamers are oligonucleotide receptors that can react with the target molecule with high affinity and selectivity. A number of aptamers have been used to detect heavy metal ions [24], such as Hg\({}^{2+}\)[25], Pb\({}^{2+}\)[26], Cd\({}^{2+}\)[27], Ag\({}^{+}\)[28], and As\({}^{3+}\)[29]. The majority of these solutions are based on a heterogeneous interaction on solid surfaces and therefore require additional equipment to detect induced changes [30]. Carrying out the reaction in a homogeneous medium with visually observed changes is an undoubted advantage. The aggregation of aptamer-modified GNPs after adding a target analyte allowed rapid color change. To the best of our knowledge, there has been no research on aptamer application for Sb\({}^{3+}\) detection. The study presents the first homogeneous colorimetric Sb\({}^{3+}\) massay based on aptamer-modified GNPs.