## 1 Introduction  
The measurement of aquatic chemical substrates is important for research on water quality assessment, human health, and water pollution control. Among the chemical substrates, cadmium (Cd) is a well-known toxic and persistent pollutant that can cause both short-term and long-term health effects [1; 2]. Besides foods, drinking water is the major source for human exposure to Cd(II), and the guideline value for Cd in drinking water is 3 ug/L, as set by the World Health Organization (WHO) [3]. The concentrations of Cd in unpolluted water are low, and the reported concentrations of Cd were at (or below) the ug/L level [4; 5]. High values of tens of ug/L were also reported in some water samples [6]. A low concentration of Cd in natural water and drinking water samples requires high-sensitivity analytical methods for the precise concentration measurement. Graphite furnace atomic absorption spectrometry (GFAAS) [7], flame furnace atomic absorption spectrometry (FAAS) [8], colorimetric determination with dithidzone [9; 10], inductively coupled argon plasma mass spectrometry (ICP-MS) [11], and inductively coupled plasma atomic emission spectrometry (ICP-AES) [12] were applied to determine the trace Cd in water samples. However, the methods were not valid for the direct analysis of some samples with extremely low Cd concentrations, with the exception of ICP-MS. For example, the limit of detection (LOD) for Cd(II) by GFAAS was 3 \(\mathrm{\SIUnitSymbolMicro g}\)/L, which was larger than the concentrations found in water samples. Therefore, preconcentration is necessary during analytical procedures [5; 13]. Adsorbents, like chelating resins [14], mesoporous sorbent [15], etc., have been investigated for the preconcentration of Cd(II) in water samples. However, a desorption process was needed after preconcentration with these solid adsorbents, which made the analytic process complicated, time-consuming, and the recovery ratio was not good because of the incomplete desorption. Therefore, the development of new preconcentration methods with high sensitivity and easy operation processes for the analysis of ultra-trace Cd is necessary. For instance, slurry sampling was applied to analyze the adsorbate with adsorbents to avoid the desorption process. However, stabilizers, like agar and surfactant, are usually needed, and success of the slurry sampling technique depends on many parameters, such as the sample particle size, stabilizer ratio, etc. The addition of stabilizers could increase the background value of the matrix to some extent [16]. Differently from other inert metallic oxide nanomaterials, ZnO is soluble in acidic or alkaline solutions. Namely, the pre-concentrated solutes can be directly redissolved for its concentration analysis. However, like other metallic oxide nanomaterials, ZnO has low adsorption capacity to Cd(II). The modification of ZnO is needed to increase the binding affinity. Polydopamine (PDA) deposition has been successfully applied in the modification of metallic oxide [17; 18], silicon dioxide [19], carbon-based nanomaterials [20], hydrophobic polymer [21], and other material surfaces. PDA coated adsorbents have high adsorption capacity of Cd(II) and other heavy metal ions [22; 23]. For instance, the adsorption capacity of Cd(II) onto PDA modified meso-structured silica is 107 mg/g [22]. Thiol has a higher chelating ability with Cd(II) when compared to hydroxyl and amino functional groups [24; 25]. Dimercaptosuccin acid (DMSA) is a well-known chelating agent for Cd(II) that has been used for Cd(II) antidote [26]. For instance, Cd(II) could be chelated by DMSA in aqueous solutions at pH 7.4 [27]. Further, DMSA can react with PDA through \(-COOH\) and \(-\mathrm{NH_{2}}\). Therefore, DMSA/PDA/ZnO can have the advantages of high adsorption capacity of Cd(II) and being soluble in acidic solutions, and it can be a novel candidate for ultra-trace Cd(II) preconcentration and analysis.