## 1 Introduction  
Given the heavy metals' (HMs) tendency to accumulate in ecosystems, careful monitoring of their content in waters is required to avoid associated health problems. Monitoring conducted by methods with high sensitivity and accuracy is desirable, in addition to online and on-site monitoring. Electrochemical sensors are an alternative that can meet all these requirements. They can be included in portable devices that allow continuous on-site tracking of HMs. Compared to classical spectroscopic and optical techniques, electrochemical sensors based on chemically modified electrodes (CMEs) have fast responses and high sensitivities [1]. The design of new structures based on complexing ligands for HMs is a current challenge for the development of new CMEs for new analytical methods.  
The goal of this study is to characterize and investigate the electrochemical behavior of 5-benzylidene-2-sulfanylidene-1,3-thiazol-4-one (benzylidenorphaline, **BR**) for the preparation of new CMEs, which are useful for the analysis of HMs.  
**BR** (Figure 1) is a derivative of rhodamine (**R**), a five-membered heterocyclic organic compound possessing a thiazoline ring (Figure 1). The rhodanines are derivatives of rhodamine with aryl and alkyl substituents in positions 3 and 5. They have complexingproperties with respect to HMs due to the heteroatoms involved in their electronic structure. These compounds are important for the production of drugs for the treatment of diabetes and cancer [2; 3]. Some rhodamine derivatives have antimicrobial properties [4; 5; 6]. Rhodanine derivatives can be used as pan-assay interference compounds for biological samples [7; 8; 9], for water treatment in efficient removal of HMs cations [10], and as fluorescence sensors for organic compounds [11] or HMs (Fe [12], Hg [13; 14; 15], and Cu [13]; Ag [15; 16], Zn [17], and Cd [17] targets). There are no reported electrochemical sensors based on CMEs by using rhodamine derivatives for the detection of HMs.  
Rhodanine derivatives act as efficient organic corrosion inhibitors because they possess high electron density heteroatoms such as sulfur, nitrogen, or multiple bonds acting as adsorption centers [18]. They form polymeric protective films against corrosion and are involved both in physical or chemical adsorption, reducing the corrosive attack in the case of mild steel [19; 20] or copper [21; 22; 23], and they can be used in the construction of photovoltaic cells [24; 25; 26].  
As mentioned above, the polymerization of rhodamine and its derivatives intended for various industrial applications has already been reported [18; 19; 20; 21; 22; 23]. Polybroadanine films have been obtained by electrochemical oxidation of rhodamine solutions in water [27; 28] or organic media [29]. Newly synthesized homogeneous polyhydonamine films proved to have good properties such as high thermal stability and suitable solubility in common organic solvents, indicating their potential utilization as polymeric films to be applied on metal surfaces [27]. In this work, it is shown that such films had the potential to be used for complexing electrochemical sensors based on CMEs. Taking into account the confirmed complexing properties of rhodamine moiety [29; 30], the ligands based on benzylidenerhodanine are good candidates to serve as HM sensors for the analysis of HM content in waters.