**1. Introduction**

Chemical sensors and biosensors are devices used in detection and quantification of an analyte by converting its concentration into an analytical signal. Advances in sensor technology have been important for the enrollment of sensing methods in several applications. Chemical sensor operates based on chemical principles, where the analytical signal emerges as a result of a chemical reaction between the analyte and a specific sensitive layer. Electrochemical sensors are able to detect H2, consisting of Pt, Pd, Au, Ag, and metal oxides, as reported by Korotcenkov et al. [1]. These capabilities are expected to be performed by biosensors as well, which are sensors that present a biological recognition element integrated with the transducer. The most popular biosensors are the enzymatic-based ones, successfully represented by the glucose biosensors. Biosensors have become an attractive analytical instrument for environmental monitoring because there still severe barriers through an effective, fast, and low-cost monitoring of harmful pollutants. Among the hazardous contaminants, phenolic compounds and pesticides represent potential human health and environmental risks. Regarding this, there are several studies reporting the use of horseradish peroxidase (HRP) for phenolic compounds and hydrogen peroxide detection [2]. Enzyme-based biosensors operate by indirectly detecting analytes, through detection of consumption or production of specific compounds in the biochemical reaction progress [3]. Phenolic pollutants are important due to their extensive use in several industrial products and their resulting negative environmental impacts. Also, enzymatic biosensors are applied to detect pesticides, particularly organophosphorus and carbamates. The operation of these devices, primarily designed to quantify those pesticides, is based on the inhibition of enzyme activity by these toxic compounds. Distinctly, the use of enzymes in biosensors for environmental monitoring brings considerable advantages, such as high selectivity and specificity, enhanced sensitivity, catalytic activity, and fast performance [4]. Nevertheless, they present some drawbacks associated with the high costs of obtainment and manipulation processes (extraction, isolation, and purification), denaturation during immobilization on transducer, and activity loss after a period (short shelf life) [4]. However, when enzymatic biosensors are compared to other sensing devices, such as immunosensors, they show superior characteristics because antibodies are more expensive, they do not present catalytic activity, and their binding ability depends on conditions of the assay, such as temperature and pH. Due to their advantages, the use of enzymatic biosensors to monitor environmental pollutants, as well as their applications in pharmacology and in pesticides monitoring will be discussed in this chapter.
