**3. Conclusions**

*Biosensors for Environmental Monitoring*

pirimiphos-methyl-oxon.

Currently, the employment of screen printed electrodes (SPEs) has boosted the scenario of AChE-based biosensors. Those electrodes promote the system miniaturization addressing the sample volume issues, combining cost effectiveness and simple manipulation. Therefore, several strategies of modification have been applied to achieve high sensitivity and low limit of detection. A smart AChE biosensor approach used homemade SPE modified with single-walled carbon nanotubes (SWCNT) derivatized with cobalt phthalocyanine to detect thiocholine at a lower overpotential in comparison to bare SPE and SPE modified with nonfunctionalized SWCNT in only 80 μL of sample [48]. Remarkably, the performance of an AChE-based biosensor was improved due to electrode modification with *N*-carbamoylmaleimide-functionalized carbon dots (*N*-MAL-CDs) as a nanostabilizer [49]. The initial electrochemical signals of thiocholine were obtained without signal loss, as a result of the Michael addition reaction functionalizing CDs with *N*-MAL. Then, *N*-MAL-CDs can react with thiol group from thiocholine, forming a thiol containing compound. The aforementioned compound cannot be easily oxidized during the detection process, avoiding the signal loss. For the fabrication of AChE*/N*-MAL-CDs/SPE biosensor, they used a commercial SPE in which all electrochemical measurements were performed in a droplet of 50 μL. One significant breakthrough offered by SPE is the simultaneous analysis performed by an array of electrodes [50]. The multiplexed analysis integrated into an automated system enables the rapid detection of OP pesticides being convenient for commercial and routine applications. Hence, an array with 12 SPEs deposited in sequence side by side on a ceramic substrate in which the working electrode was printed with a carbon ink containing cobalt phthalocyanine and Ag/AgCl/KClsat was used as reference/counter electrode. By means of using six types of recombinant AChE, it was possible to acquire qualitative and quantitative information through inhibition assay since the enzyme becomes selective among the OP pesticides, such as dichlorvos, malaoxon, chlorpyrifos-oxon, chlorpyrifos-methyl-oxon, chlorfenvinphos, and

Despite all exceptional SPE properties, they present certain drawbacks, such as the dissolution of conductive and insulating inks due to use of organic solvents, lack

Numerous immobilization strategies and fabrication methods have brought new perspectives to AChE-based biosensors. The investigations have focused on enzyme stability, reproducibility, miniaturization, and mass production [51]. The usage of smartphones in biosensing has played new horizons in environmental monitoring; however, it remains a challenge [52]. The electrochemical biosensors on smartphone use portable electrical detectors for amperometric, potentiometric, and impedimetric measurements, but environmental analyses are still scarce. Although great progress has been made with wireless biosensors, there is a lack of applications in

The continuous progress in biosensing area leads to the development of paper-based analytical devices (PADs) with electrochemical detection. The PADs have emerged as a powerful analytical tool integrating the convenience of SPEs, i.e., portability, simplicity with easy manufacturing of paper, availability, and reduced cost. Furthermore, the PADs provide singular advantages since they can be scalable manufactured from renewable sources, biocompatibility, biodegradable, and low cost. A pioneering research involving a paper-based amperometric sensor for AChE determination was based on screen printed graphene electrodes fabricated by a wax printing method to obtain the detection area. The approach was applied for blood sample analysis, but it has potential to be used for pesticide

of reproducibility, and need of pretreatment procedure.

**46**

detection.

pesticide detection.

In this chapter, we presented the state of the art of biosensors to detect phenolic compounds, with environmental and pharmacological applications. Monitoring the negative environmental impacts of phenolic compounds uses has attracted researchers' attention since these compounds are widely applied in several industrial sectors. Among the biosensors developed for environmental monitoring, enzymatic ones are the most prominent used for phenolic compound detection. By combining enzymes with electrochemical transducers, cheaper devices had been developed, which is a great advantage to environmental analytical methods. The immobilization of enzymes on the electrode surface consists in physical and chemicals interactions. The location of actives sites is important to biosensor response; nevertheless, mediators can be used to transpose this barrier and facilitate the electronic transfer needed for the detection process. Tyrosinase-based biosensor is the most common biosensor for phenolic compound detection, which is a precursor for drugs for the treatment of Parkinson's disease, as morphine-based drugs. Also, acetylcholinesterase-based biosensors are widely employed because they present high efficiency to detect organophosphate and carbamate compounds, which are used as pesticides. The design of novel AChE-based biosensors for pesticide detection concerns the application of nanomaterials offering transducing platforms with outstanding electrochemical behavior. The employment of screen printed electrodes promotes the system miniaturization, which is a new perspective to electrochemical biosensor application.
