## 1 Introduction  
Lead is a highly toxic heavy metal that causes serious environmental problems due to its non-biodegradability. It is commonly released into the environment because of mining activities, natural processes, and the development of new technological devices [1, 2], being frequently used by the automotive, plastics, paints, and ceramics industries for its corrosion resistance [3].  
Since the nitrate and chloride salts of lead show excellent solubility in water [4, 5], lead is normally present in soil and aquatic ecosystems in ionic form, as Pb(II). According to the World Health Organization (WHO), a Pb(II) concentration as low as 0.24 \(\upmu\)mol L\({}^{-1}\) can cause decreased intelligence in children, behavioral difficulties, and learning problems. For this reason, the concentration of lead in water and soils should always be below the WHO limit and, consequently, must be monitored.  
Nowadays, several analytical methods are employed for lead detection, such as spectroscopy [6], optical colorimetry [7], inductively coupled plasma mass spectrometry (ICP-MS) [8], atomic absorption spectrometry (AAS) [9], and fluorescence spectrometry [10]. However, these analytical methods are expensive (they require trained operators, complex equipment, solvents or gases, and so on) and, in some cases, sample preparation procedures are required. In this context, electrochemical techniques have been investigated because of their significant advantages such as simplicity of operation, high sensitivity, low cost, and easy handling [11; 12; 13; 14; 15]. In general, electrochemical sensors are rapid, portable, inexpensive, and highly sensitive and offer a low limit of detection, good reproducibility, good signal-to-noise ratio, and selective detection [12]. Consequently, electrochemical sensors have been applied for the determination of heavy metals in the environment, industrial products, food matrices, electronic waste, and clinical materials [16; 17; 18].  
Among the electrochemical sensors used, graphite-modified electrodes have been extensively developed due to their higher selectivity, sensitivity, high specific area, unique electrical conductivity, self-assembly behavior, mechanical flexibility, extreme resistance to oxidation, natural origin, and low cost [19; 20; 21; 22]. However, these properties can be improved by including other modifiers in their composition.  
Recently, cork has emerged as a promising low-cost and efficient green material for various environmental applications (e.g., compound detection [13; 23; 24], soil and water remediation [25; 26]). Cork is a natural organic polymeric material, which has modest electrical, magnetic, and optical properties and exhibits self-cleaning behavior and antibacterial activity. For raw cork (RAC), electrical conductivity (\(\sigma\)) values of approximately \(1.2\times 10^{-10}\) and \(1.67\times 10^{-13}\) S m\({}^{-1}\) were registered at 25 and 50 \({}^{\circ}\)C, respectively [27]. Based on the existing literature, two types of cork are often used: raw cork (RAC) and regranulated cork (RGC). Their differences are mainly due to their composition, which depends on the thermal pretreatment applied to RAC to produce RGC.  
In the present communication, cork-graphite composite electrodes to be used as electrochemical sensors for the detection of lead ions are discussed. The effects of the cork composition, the cork-graphite ratio, and the supporting electrolyte for detecting Pb(II) were investigated. The performance in Pb(II) detection of two voltammetric techniques (differential pulse adsorptive stripping voltammetry (DPASV) and square-wave adsorptive stripping voltammetry (SWASV)) was also evaluated. Finally, the applicability of the cork-graphite voltammetric device was successfully demonstrated by detecting Pb(II) in real water matrixes (groundwater, tap water, and \"produced water\") as well as verifying the selectivity, repeatability, reproducibility, and stability of the sensors.