With the continuous progress in technology, understanding carbon nanomaterials (CNMs) has reached a new level of interest. Their excellent electrical conductivity, stability, potential high-throughput screening and easy-to-use assay procedure make these compounds suitable for the fabrication of realizing sensors and energy storage devices ^[1][2][1,2]. The production of carbon nanostructures has greatly increased the use of these devices, creating a wide range of applications based on their use, ranging from the creation of miniaturized printed electrodes to the development of flexible electronic devices, humidity sensors or passive sampling systems ^[3][4][5][3,4,5]. Until now, several electronic and electrochemical tools are developed and applied at the lab scale. Nevertheless, to allow the deployment of these technologies on a large scale, it is necessary for a cost-production reduction and their actual health safety. In addition, CNMs are rather energy-intensive and expensive and require long syntheses, and studies on their toxicity are not yet concluded [6]. For example, activated carbon is conventionally derived from coal, while CNMs such as carbon nanotubes and graphene can be produced through specific techniques (i.e., chemical vapor deposition, electric arc discharge, etc.) using gaseous petrochemicals (i.e., methane, acetylene, ethylene, hydrogen, etc.) at high temperatures (>800 °C) [7]. These high-temperature and resource-intensive processes can be suitable for large-scale production in industrial settings but are not compliable with a fully sustainable solution or production in smaller installations. Alternative eco-friendly carbon material, produced at low cost, competes to replace CNMs, widely used at the industrial level [8]. Recently, attention is being focalized on green recycling carbon material, called biochar. The latter is an inexpensive, solid, recycling carbonaceous material obtained by pyrolysis of renewable sources (i.e., wood chips and pellets, bark, straw, walnut shells and rice husks, bagasse, sewage sludge, etc.) ^{[9][10][11][12][13][14][15][16]}[9,10,11,12,13,14,15,16], and its production requires a lower overall energy input than activated carbon, resulting in lower net energy consumption with a low net cost. Its morphological and physicochemical properties can strongly vary with the feedstock source choice and the conditions of the pyrolysis treatment ^{[17][18][19][20][21][22][23]}[17,18,19,20,21,22,23]. In addition, the high porosity combined with its surface being rich in functional groups on one side and its excellent electrical conductivity and biocompatibility on the other expand its applications in different fields. A schematic representation of biochar’s applications and its most exploited qualities is reported in Figure 1.

Until a few years ago, biochar has been used in a large number of applications such as adsorbent for wastewater treatment and agronomic applications (i.e., immobilizing organic and inorganic pollutants), or as an amendment in agricultural soils (effects on soil, agricultural yields, and nitrogen leaching and emissions) ^{[18][20][21][22][23][24][25][26]}[18,20,21,22,23,24,25,26]. However, recently, its potential as an eco-friendly and smart nanomaterial for several electrochemical applications is increasingly exploited [27]. In 2015, Joseph et al. reported the redox properties of different biochar, which are rich in carbon (amorphous and graphitic C, labile organic compounds, and minerals) in this regard [28]. In particular, they underlined that the electrochemical properties of biochar are a function of the concentration and composition of the various redox-active minerals and organic compounds and, at the same time, of their production method ^[29][30][29,30]. Many electrochemical studies, present in the literature, reported the use of biochar as a surface modifier of electrodes (i.e., carbon paste, glassy carbon, etc.) ^[31][32][33][31,32,33], but only a few of them concern SPEs ^[34][35][36][34,35,36]. Precisely with that in mind, the present work aims to illustrate the recent applications of biochar as a sustainable nanomaterial for electrochemical applications in the field of SPEs. Before all else, its capabilities to act as an electrochemical enhancer (improving the electron transfer process when used for the modification of SPEs), as a booster of the active surface area of the electrodes and as a protein binding substrate (i.e., enzyme, antibody, etc.) in the production of biosensors were carefully studied and discussed [37].

2. Application of Biochar in ElSectrochemical Platformsnsing

Since the application of biochar as a soil conditioner, catalyst and carbon sequester has been successfully reported, other strategies based on this recycling material to promote sensors with improved analytical performances continue to be widely studied today. In 2020, an inspiring review about sustainable materials for the design of sensors, reporting how the pyrolysis and activation process can affect biochar features, was published ^[38][94]. Here, different applications of biochar underlining the potentiality and the promising features of these carbonaceous and recycled materials for electrochemical applications have been reported (Figure 2). In Figure 2, a general scheme for the modification of screen-printed and bulk electrodes using biochar is reported. Moreover, a few examples of works reported in the literature are discussed below.

2.1. Carbon-Paste and Glassy-Carbon Electrodes

In recent years, an increasing number of publications were centered on biochar-modified glassy-carbon electrodes (GCEs) and carbon paste electrodes (CPEs) ^{[39][40][41][42][43][44]}[95,96,97,98,99,100]. In these papers, biochar was used for its adsorbent properties and as a highly effective electrode modifier for the pre-concentration of organic contaminants. In these studies, the porous surface of biochar, rich in chemical groups (i.e., carboxyl, hydroxyl, phenolic groups, etc.), which can interact in several ways with the target analytes, was exploited for the pre-concentration and voltammetric determinations of inorganic ions and organic species ^{[42][45][46][47]}[98,101,102,103]. Another interesting use of biochar is that in which it was deposited alone or decorated with metallic nanoparticles (i.e., mercury, bismuth, gold, etc.) or electrocatalytic potential nanostructures (i.e., nickel hydroxide, copper hexacyanoferrate, etc.) for the construction of a powerful electrochemical sensor. These methods, based on voltammetric procedures (i.e., stripping, differential pulse, square wave voltammetry, etc.), have provided significant improvement in the performances (sensitivity and selectivity) of the sensor developed. Table 1 reports significant works in which biochar-modified CPE and GCE were used. In these works, biochar was exploited to electrochemically determine heavy metal ions and organic compounds, including hormones, insecticides, herbicides or antibiotics, in different matrices. ^54][110]. In this biosensor (BCNPs/Tyr/Nafion/GCE), a higher sensing signal (improved amperometric current responses) due to the conductivity property of biochar nanoparticles was observed. Precisely, a decreased charge transfer resistance and lowered reduction potential were obtained if compared with the biosensor based on bare GCE (Tyr/Nafion/GCE). Moreover, biochar-based biosensors showed robust analytical performances with a sensitivity of 3.18 nM and a linear range from 0.2 to 10 μM. These results were confirmed when the device was employed in real water detection, as evidenced by the high accuracy compared to that of high-performance liquid chromatography. Another attention-grabbing application of biochar is that proposed by A. Sobhan and coworkers (2021). In this work, they employed activated biochar (activated by steam-activation method) from corn stover for the development of a biosensor (ABC/PLA-based platform) selective for NH₃. The analytical performances reported in Figure 3 ^[60][116] demonstrate the robustness of the method when used in smart food packaging control. Moreover, careful characterization in terms of polylactic acid (PLA, ranging from 15 to 50%), electrical conductivity and tensile strength were reported. Q. Zhou et al. (2021) reported on the fabrication of a sensor based on corncob biochar-modified GCE for the determination of dibutyl phthalate (DBP) ^[61][117]. In this tool, functional corncob biochar (F-CC₃) and MIP were combined, achieving high sensitivity (detection limit of 2.6 nM), reproducibility and successful application in the detection of DBP in rice wine ^[61][117]. However, there are also many examples of biochar-modified CPE-based biosensors. For instance, C. Kalinke and her co-workers proposed an electronic tongue based on biochar-modified CPE for the detection of phenolic compounds using stripping voltammetry (see Figure 4). In particular, this artificial neural network (2019)-based electronic tongue used stripping voltammetry as a discrimination technique for the analysis of a mixture of several phenolic compounds, such as catechol (CAT), 4-ethylcatechol (4-EC) and 4-ethylguaiacol (4-EG), showing good sensitivity (LOD in the micromolar range) and reproducibility ^[62][118]. Furthermore, other works based on biochar have been proposed by the same research group. For example, they developed a non-enzymatic sensor based on nickel-supported activated biochar modified-CPE (NiAB-CPME) for the determination of glucose ^[52][108]. The biosensor showed good performances in terms of repeatability (RSD% = 3.84%), sensitivity (LOD 0.137 µM) and linear range from 5.0 to 100.0 µM. Moreover, they applied this feasible green analytical procedure for the determination of glucose in human saliva and blood serum, also obtaining satisfactory results when a microfluidic methodology was used, thus proving to be a simple, robust and accurate method. In addition, in a second work, they presented a quick electrochemical immunoassay for the detection of pathogens based on biochar-modified CPE ^[63][119]. In this paper, the authors developed an immunosensor to detect Hantaviruses (single-stranded RNA viruses belonging to the Hantaviridae family) ^[63][119]. Matins and his co-workers exploited the highly functionalized surface of biochar able to bind covalently (by EDC/NHS conjugation) the specific antibody for Hantavirus in the construction of the immunosensor. The device showed good analytical performances with a LOD of 0.14 ngmL⁻¹ and a linear range ranging from 5.0 to 1.0 μgmL⁻¹. Other biosensors dealing with biochar-modified CPE and GCE are reported in Table 2.