Monitoring the presence of contaminants in water is of general interest in order to ensure the quality of surface, ground, and drinking water. Among the several water pollutants, such as plastic or waste, chemical fertilizers or pesticides, and pathogens, heavy metal ions are known for their high toxicity. Although some of them are essential nutrients (for instance, iron, zinc, or cobalt), they can be toxic at higher concentrations. For their part, cadmium, lead, and mercury are highly poisonous even at trace levels, showing a close association to cancer or neurodegenerative diseases. Furthermore, heavy metal ions are non-biodegradable substances and they have an accumulative effect in human body, where they enter, typically, through the air, beverages, and the food chain, in which water plays a key role. There, metal ions can be found as a result of vehicle emissions, batteries, or industrial activities. Thus, their detection at low concentrations is a matter of priority for environmental protection and disease prevention as well.