**1. Introduction**

Fluoride is the inorganic anion of Fluorine. As all the halogens in �1 state, it generates colorless salts and can be classified as a weak base due to the *pK*HF *<sup>a</sup>* ¼ 3*:*2. Fluorine is in the 13th position of abundance in the earth, and it is present only in the anion form as fluorite (CaF2), the most abundant fluoride mineral.

The most relevant use for fluoride is cavity prevention, due to the presence of fluoride in water, toothpaste, and fluoride therapy in the form of sodium fluoride (NaF) or sodium monofluorophosphate (Na2PO4F). Water fluoridation is considered by the U.S. Centers for Disease Control and Prevention (CDC) as *"one of 10 great public health achievements of the twentieth century"* [1].

Fluoride is present in dental products, food, and drinking water. Fluoride content in dental products is between 1.0 and 1.5 mg kg–<sup>1</sup> . Vegetables and fruits have a low content (0.1–0.4 mg kg–<sup>1</sup> ), while rice and barley can contain higher fluoride levels (2 mg kg–<sup>1</sup> ). Meat and fish can have higher concentrations, but it is accumulated in bones, which does not represent a risk. The dietary recommendations for adults in U.S.A. are between 3.0 and 4.0 mag day–<sup>1</sup> , while in Europe are between 2.9 and 3.4 mag day–<sup>1</sup> . The major known risk of fluoride deficiency is the risk of tooth cavities.

On the other hand, excess fluoride can conduce to health problems. World Health Organization (WHO) settled the recommended upper limit for fluoride in drinking water to 1.5 mg kg–<sup>1</sup> [2]. Prolonged exposure to higher levels of fluorides above the recommended limit can cause dental fluorosis (1.5–3 mg kg–<sup>1</sup> ), which exhibits defects in enamel formation, mottling, browning, and severe teeth deterioration. Higher concentrations (4–8 mg kg–<sup>1</sup> ) can cause skeletal fluorosis, where the bones are hardened and less elastic, increasing the frequency of fractures. Even higher concentrations can cause crippling deformities of the spine and major joints, reducing body mobility and can also cause neurological defects and compression of the spinal cord.

The incidence of fluorosis is low in urban populations but more frequent in rural populations. The most affected areas are located in the south of South America, Southwest North America, north and east coast of Africa, India, and China [3]. In the case of east coast of Africa, fluoride concentration is related to geological formation, like volcanic activity, (East African Rift through Sudan, United Republic of Tanzania, Uganda, Ethiopia, and Kenya). Kenyan Lakes of Najura and Elmentaita presented 2800 and 1630 mg kg–<sup>1</sup> fluoride, respectively, and Tanzanian Momella soda lakes presented 690 mg kg–<sup>1</sup> fluoride.

As the contamination of natural waters with fluoride are mainly geogenic than anthropogenic, and thus the distribution of fluorides levels is determined by the geological formation of the riverbeds. This scenario generates an inhomogeneous distribution of fluoride levels in the water sources, even in small areas. In **Figure 1**, the distribution of fluoride levels in Arusha, Tanzania is shown. The red spots represent water sources with fluoride concentration above WHO recommendations and the blue ones below this level. The figure shows that safe and unsafe water sources can be closed and with adequate information, the local populations can choose the safer water source and avoid health risks [4].

Recently, the water fluoridation effectiveness against teeth decay was strongly questioned [5]. Countries without water fluoridation systems, like Denmark, exhibit tooth decay rates similar to US communities with fluoridation. This observation makes it necessary to rethink the need for water fluoridation to prevent cavity prevention. The amount of fluorides in toothpaste and rinses seems to be enough to protect the teeth enamel. When fluoride ions are in the mouth, they are incorporated into plaque. When the pH decreases, the fluoride ions are released from the plaque and participate in the remineralization process, which slows down the tooth decay rate. The fact that the cells involved in the remineralization process, the ameloblasts, are affected by the presence of fluorides, suggests that other cells in the body can also be affected. The relationship

*Fluoride Detection and Quantification, an Overview from Traditional to Innovative… DOI: http://dx.doi.org/10.5772/intechopen.102879*

#### **Figure 1.**

*Distribution of fluoride on different water sources near Arusha region. Red dots correspond to water sources with fluoride levels higher than WHO recommendations. Blue dots are water sources with fluoride levels lower than 1.5 mg kg–<sup>1</sup> . Reprint with permission from ACS Sens. 2021, 6, 1, 259–266 publication date: January 8, 2021, https://doi.org/10.1021/acssensors.0c02273 copyright © 2021 American Chemical Society.*

between IQ and water fluoridation was recently reported, opening the possibility of pointing to fluoride as being a developmental neurotoxin [6].

Due to severe risks on human health, it is vital to study the long-term effects of fluorides in the population, have strict control on the fluoride intake, and develop techniques to provide reliable quantification of fluoride on drinking water.

The analytical methodologies for fluoride quantification range from electrochemical approaches to colorimetric methodologies, by using naked eye detection or by means of spectrophotometric measurements. The more reliable quantification methodologies performed in laboratories, require trained operators to perform the quantification and to accurately interpret the results. However, these instruments are often out of reach for the majority of the communities in developing countries.

In order to develop accessible, reliable, and sustainable fluoride quantification methodologies, it is important to understand the chemistry involved and how new technologies like 3D printing, low-cost electronics using Arduino, and the use of smartphones as interfaces can make a major contribution to the improvement of the user experience.

In this chapter, an overview of the traditional fluoride quantification methodologies and those emerging from the use of advanced materials like Metal–Organic Frameworks will be found. Also, the implementation of smartphones as user interfaces for analytical determinations will be discussed, prioritizing the easiness, fast response, and accessibility of the methodology.

In some cases, a compromise between the accuracy or the application range will be found, but always keep in mind the convenience of the final user and the democratization of science and tech.
