لخّصلي

The global water crisis, marked by scarcity, pollution, and mismanagement, severely threatens human health, food security, ecosystems, and economic development. Over 3.6 billion people lack access to clean water and sanitation. Heavy metal contamination, from sources like aging water pipes (Pb(II)), is a significant concern, causing various health issues. Electrochemical sensors, particularly those incorporating nanomaterials (NMs), offer a promising solution for detecting trace amounts of toxic metals (Pb(II), Hg(II), As(III), Cu(II), Cd(II), and Ag(I)) in water. These sensors leverage their high sensitivity, accuracy, and cost-effectiveness. Various electrochemical techniques (CV, LSV, DPV, SWV, CA, CP, EIS, PEC, and ECL) and NM-based modifications (metal nanoparticles, metal oxide NMs, carbon NMs, polymers, and biomaterials) enhance sensor performance. Advances in nanotechnology, such as molecular imprinting and the use of porous organic frameworks (POFs) and carbon nanodots (CNDs) from recycled materials, further improve sensitivity, selectivity, and stability. The development of portable and cost-effective sensors using these techniques is crucial for real-time water quality monitoring and pollution control.

1. Introduction
   The water crisis is a global challenge characterized by the scarcity, pollution, and mismanagement of water resources [3], posing significant threats to human health, food security [4], ecosystems, and economic development [5].Despite Earth being 97.5 % water, most of it is salty and locked away in the oceans, leaving only a small percentage of freshwater readily available.Some of the residual freshwater is easily accessible as surface water in lakes and rivers [2].Introduction All living things depend on water, which is the lifeblood of our planet, and it is vital to human society [1].Management of this valuable resource is becoming more difficult regardless of its availability.Over 3.6 billion people globally lack basic human necessities: clean water and proper sanitation [6].
   carcinogenic.Nanotechnology methods are investigated as a promising method for targeted cleaning of water and wastewater to remove toxic metals.These sensors offer advantages like tracing very small amounts of metals quickly, but [24], stated that the current modification mechanisms in making significant strides in detecting toxic metal ions by merging NMs with electrochemical sensor platforms as depicted in Fig.Heavy metals are generally metallic elements with high density, atomic weight, or atomic number [9] Metal ions such as Pb (II), Hg (II), As (III), Cu (II), Cd(II), and Ag (I) are significant pollutants that leads to various health issues after human exposure or consumption.Atomic absorption spectrometry [16], inductively coupled plasma mass spectrometry [17], X-ray fluorescence spectroscopy [18], and Electrochemical sensor technology [19] are the main heavy metals detection techniques.These sensors offer a compelling combination of excellent vulnerability, continuous evaluation, portability, cost-effectiveness, and user-friendliness, making them invaluable for environmental impact analysis and safeguarding public health [20].Leached from aging water pipes, Pb(II) poses a serious threat to children's health and severely affects their nervous and immune systems, potentially causing permanent neurological damage and hypertension.For trace analysis of heavy metals, electrochemical sensors stand out due to their invincible combination of low power consumption, high sensitivity and accuracy, user-friendly operation, and suitability for rapid on-site detection.The unique features and great sensitivity of NM-based electrochemical sensors have made them attractive instruments for spotting toxic metals [25] but sensing only focuses on the determination of metals in water.Incorporating NMs significantly amplifies the output signal, paving the way for more efficient and robust electrochemical sensors for heavy metal detection [21].According to the World Health Organization (WHO), the maximum levels allowed in residential water of Cr (VI), Pb (II), Cd (II), and Hg (II) are 0.05, 0.01, 0.003, and 0.006 mg L-1 , respectively [9].Among the numerous methods, electrochemical sensor technology has emerged as a powerful and versatile tool for detecting toxic metals in environmental samples.Most importantly, advances in nanotechnology have allowed the integration of NMs into electrochemical sensors, enhancing their performance for sensitivity and selectivity.Consequently, improper discharge of toxic metals- contaminated wastewater leads to severe health issues for living organisms and humans.Cr (VI) is carcinogenic and is typically employed in industrial operations [12].1.
   techniques.Frequently, many electroanalytical techniques used to detect pollutants, such as cyclic voltammetry (CV), linear sweep voltammetry (LSV), differential pulse voltammetry (DPV), square wave voltammetry (SWV), chronoamperometry (CA), chronopotentiometry (CP), electrochemical impedance spectroscopy (EIS), photoelectrochemical (PEC), and electrochemiluminescence (ECL) [21] lectrochemical sensors [40] have changed quickly because of NM-based molecular imprinting, which favors the sensors' compact size, high conductivity, and ease of manufacture at a cheap cost.The sensing mechanism depends on mechanical, optical, or EC pathways, and their effectiveness relies heavily on the materials

used to modify the electrode surface [30], among sensor platforms, NMs and carbon-based options, including biochar, are prime candidates for electrode fabrication due to their exceptional electrochemical properties [31].Sensors and biosensors built with cutting-edge electrochemistry [37] have employed several electrochemical techniques such as (a) amperometry/voltammetry [38], (b) potentiometry, (c) conductometry, and (d) impedimentary [39,21] to detect heavy metals in water.Nowadays research is increasingly focused on utilizing heterojunction for the efficient removal of HMIs from water, [21,29,30] there is a lack of overall discussions on their detection using electrochemical sensor technology.Nanostructured-based electrochemical sensor Any sensor that detects and monitors the physical characteristics and communicates information about nanoparticles to the macroscopic environment is referred to as a nanosensor [31].These improvements include increased catalytic activity and conductivity, a larger active surface area, and faster electrode kinetics, all of which contribute to more sensitive and efficient sensors [35].Enhanced nanoscale electrode design offers several advantages for NM-based electrochemical sensors used in HMIs detection.It highlights the benefits of this technology and emphasizes the need for further research to overcome existing challenges and unlock its full potential for environmental monitoring, protection, and future directions.In recent years, combining NM with electrochemical (EC) sensor platforms has emerged as a powerful tool for detecting HMIs [32,33].Newly developed advanced materials combined with electrochemical Talanta Open 10 (2024) 100354 techniques allow for sensors with significantly better performance.The molecular imprinting technique bases the nano modification of electrodes in2.
electrochemistry, greatly enhancing the sensor's vulnerability, selectivity, and steadiness.Two of the most widely used characteristics of an electrochemical sensor for multiple applications are its low theoretical detection limits, which stem from the variations in Faradaic and non-Faradaic currents, and its variability in reporting signals, such as voltage, current, overall power output, or electrochemical impedance [50].Diverse analytical techniques, such as potential analysis, conductometric, and colorimetric offer distinct advantages, including rapid response times, high miniaturization potential, high sensitivity, and selectivity, cost-effectiveness, and operational simplicity, making them ideal candidates for this critical application [50,51].The performance of electrochemical sensors hinges on critical properties like sensitivity, detection limit, dynamic range, selectivity, linearity, response time, and stability, specifically exploring various techniques to modify the electrode surface.This technique encompasses carbon materials [41] nanofibers, threads, fullerenes, graphene, and graphitic substances, offering diverse functionalities for sensor development [21].The investigation reveals that both external factors such as pH and the kind and chemical composition of the electrolyte and intrinsic characteristics of nano enzymes such as composition, size, and shape have a noteworthy influence on the catalytic activity of these molecules.Current developments in the fabrication of cutting-edge NMs include metal nanoparticles, metal oxide NMs, carbon NMs polymers, and biomaterials as the basis for electrochemical sensor platforms [42].These techniques include drop casting, self-assembled monolayers (SAMs),

electro-polymerization, and molecularly imprinted polymers (MIPs) [21,44].Safeguarding our water supplies demands real-time, high-resolution measurement techniques to pinpoint ultra-low levels of heavy metals like U, Pb, Cd, Cr, and As in water and food.In recent decades, electrochemical detection through sensors has transformed water pollutant monitoring by converting chemical changes into measurable electrical signals.Modifiers, coating materials, and electrode type possess a significant impact on the selectivity and efficiency of electrochemical sensors [52].
quality monitoring.These advanced materials show great promise for developing portable and cost-effective devices for monitoring water and air quality, contributing to early warning systems and effective pollution control strategies.The exceptional porosity, large surface area, and tunable functional groups of POFs make them ideal platforms for capturing and detecting a wide range of contaminants.By transforming beer industry waste into CNDs [53], researchers have developed a sustainable and efficient method for detecting multiple heavy metals simultaneously.These CND-based sensors offer both fluorometric and electrochemical detection capabilities, enabling rapid and accurate analysis of water samples.Researchers have successfully incorporated POFs into electrochemical sensors to enhance sensitivity, selectivity, and stability.This approach not only addresses environmental pollution but also promotes circular economy principles by repurposing waste materials.