## 5 Lacase-Based Sensor Development  
The above-described peculiar optical properties of lacase offer remarkable possibilities for developing different biotechnological applications as assays for evaluating lacase activity. Lacases and phenol reaction products show relevant optical signals in the UV and visible range that are usually employed for developing methods for monitoring lacase activity using absorbance or fluorescence measurements [115; 116]. The optical properties of lacase are also exploited for elaborating methods for monitoring phenol degradation [117]. However, the most relevant use of optical characteristics of lacases is related to the design of optical biosensors that present significant advantages since they are small, light, chemically inert, non-toxic, and immune to electromagnetic interferences. The use of lacase allows the development of sensors for medical applications, essentially related to the detection of dopamine, catecholamines and other neuromodulator molecules [118; 119; 120; 121; 122]. Many optical lacase-based biosensors have been developed to detect and quantify phenolic compounds. Due to the dual role of phenol compounds [12], lacase optical biosensors have essentially been developed for monitoring phenol concentrations for environmental or dietary applications. In the biotechnological use of enzymes, a fundamental role is played by immobilization procedures that have been briefly summarized in Supplementary Materials. An optical biosensor presents an active element consisting of lacase suitably immobilized in a matrix interacting with a solution containing the analyte of interest and generating an optical signal proportional to the analyte concentration. In the case of optical biosensors, the matrix should be optically transparent, able to preserve the catalytic activity of the immobilized enzyme and grant the diffusion of the analyte of interest. In 2015, [PERSON] et al. [13] revised the lacase-based biosensors for the detection of phenolic compounds and described the different optical biosensors reported in the literature. In this section, we intend to update their report by describing other developed lacase-based sensors by considering the different optical sensing schemes. * Sensor based on optical absorption spectroscopy. The most used approach in developing biosensors using lacase is certainly the one related to the measurement of changes in absorption characteristics of the active element. The first optical biosensor that exploited the changes in absorption due to the interaction between immobilized lactose and phenol compound was proposed by [PERSON] and coworkers in 1996 [123]. The prepared fiber optic biosensor used a sol-gel matrix for immobilizing the enzyme and investigated the changes occurring in the absorption spectra due to the oxidation of 2,5-dimethoxy phenol in the presence of lacase. The intensity changes at 470 nm have been used to quantify the presence of the above-mentioned phenol in a concentration range equal to 1-60 mM. The authors also monitored the response time and time stability of their biosensors. It is worth noting that sol-gel technology offers the possibility to prepare optically transparent matrices useful for immobilizing enzymes. This technology offers the possibility of fabricating matrices for lacase immobilization with suitable chemical stability, optical transparency, and porosity (see Refs. [124; 125; 126], and the Supplementary Materials of the present paper for further information). [PERSON] et al. [127] designed an optical biosensor taking advantage of the ability of lacase to oxidize methoxy phenols in the presence of 3-methyl-2 benzothiazolchlorohydazone (MBTH) to produce azo-dye compounds for detecting phenols in environmental monitoring.