*3.5.2 Phytomass-derived AC in sensor application*

With the impressive progress in the electrochemical sensing technologies and their application in bio-analytical chemistry, it would now be possible to utilize phytomass-based AC to show that high sensitive and low-cost sensor production is commercially feasible in the near future [71]. In this context, a few important research advancements utilizing phytomass-derived AC in sensors has been presented below.

Kim et al. [72] have reported a biomass-derived carbon for the electrochemical determination which involved the initial activation of kelp powder with ZnCl2, followed by a second activation step with KOH. The above AC coated on GCE modified electrode showed high sensitivity, selectivity and a good detection limit for the determination acetaminophen with the detection limit of 0.004 μM. Also the modified electrode showed good result towards acetaminophen in the presence of ascorbic acid and dopamine with the detection limit of 0.007 μM.

Zhang et al. [73] have reported that ZnCl2 activated peel of kiwi fruit based carbon fibre (CF) provided a high sensitivity and selective signaling of ascorbic acid (AA), dopamine (DA), and uric acid (UA) with linear response ranges of 0.05– 200 μM, 2–2000 μM, and 1–2500 μM, respectively and its detection limits (S/N = 3) as 0.02 μM, 0.16 μM, and 0.11 μM, respectively and this method was successfully applied to detect AA, DA, and UA in real sample analysis. Wang et al. [74] have shown PBNPs-3D-FKSCs, CuNiNPs-3D-KSCs and CoNPs-3D-KSCs (KSC: kenaf stem carbon, 3D: three dimensional) with honeycomb structure electrodes with good electrocatalytic performances for the reduction of H2O2, oxidation of glucose and amino acid.

Oliveira et al. [75] have reported that carbon paste electrode modified with nitric acid activated biochar obtained by the pyrolysis of castor oil cake biomass at 400°C for spontaneous preconcentration of methyl parathion (MP) and for further quantitative determination in drinking water. The electrode showed good sensitivity and limits of detection of MP as 760 μA L mmol<sup>1</sup> , 39.0 nmol L<sup>1</sup> , respectively.

Kalinke et al. [76] for the first time have reported the determination of paraquat (PQ2+) by Differential Pulse Adsorptive Stripping Voltammetry (DPAdSV) using a carbon paste electrode modified (CPME) with biochar obtained from castor oil cake at different temperatures (200–600°C). The best voltammetric response was verified using biochar yielded at 400°C (CPME-BC400). Linear dynamic range (LDR) for PQ2+ concentrations between 3.0 <sup>10</sup><sup>8</sup> and 1.0 <sup>10</sup><sup>6</sup> molL<sup>1</sup> and a limit of detection of 7.5 <sup>10</sup><sup>9</sup> molL <sup>1</sup> were verified. The method was successfully applied for PQ2+ quantification in spiked samples of natural water and coconut water.

Madhu et al. [77] have reported that PSAC/Co3O4 (PSAC: Pongam seed shells derived activated carbon] modified electrodes have potential as nonenzymatic glucose sensor and supercapacitor with ultrahigh sensitivity of 34.2 mA mM<sup>1</sup> cm<sup>2</sup> with a very low detection limit of 21 nM. Shahzada et al. [78] have reported that

sulfur-doped reduced graphene oxide (SrGO) product fabricated using an ecofriendly biomass precursor "lenthionine" through a high temperature doping process have high sensitive electrochemical sensor for detection of 8-hydroxy-2 deoxyguanosine(8-OHdG) molecule. The sulfur-doping amount was regulated and a maximum sulfur content of 2.28 atom% was achieved through controlling the precursor amount. It was homogenous presence of large number of sulfur atoms in SrGO in the form of thiophenic (CSC) bond that produced robust sensitivity (1 nM), very wide detection window (20–0.002 M).

Ni et al. [79] have modified heteroatom-enriched activated carbon-nickel oxide (HAC-NiO) nanocomposite into NiO-HAC/GCE) and have built a novel glucose sensor which exhibited a wide linear concentration range of 10 μM–3.3 mM and a low detection limit of 1 μM) towards glucose oxidation. Travlou et al. [80] have treated wood-based commercial activated carbon (BAX) and its oxidized counterpart (BAX-O) with melamine and then heated at 450°C in nitrogen. Further oxidation with nitric acid was also done. The carbons were tested for ammonia sensing (45–500 ppm of NH3). Further the role of the nitrogen functionalities on the electrical performance of the carbons was investigated by testing their selectivity with respect to H2S sensing. Interestingly, pyridinic groups, acting as p-type impurities were found to be responsible for the observed opposite electrical responses of the melamine impregnated samples upon exposure to NH3/H2S. This facilitated H2S dissociation into H+ and HS ions, speculates the authors. The latter ions, either by providing ionic conductive paths through the carbon matrix or through their oxidation to SO2 may cause a decrease of the normalized resistance.

Hayat et al. [81] have deposited a TiO2 modified activated carbon on the surface of screen printed carbon electrodes (SPCEs) and used in the direct oxidation of phenols. Calibration curves showed a high sensitivity and wide linear range for each studied compounds viz., p-nitrophenol, 1-naphthol, catechol and hydroquinone. The authors say that there was no interference of Na+, K<sup>+</sup> , Cl, Br, Mg2+, Zn2+ and NO4 ions and show 96% recoveries in real sample analysis.

Koskun et al. [82] have synthesized activated carbon (AC) decorated monodisperse nickel and palladium alloy nanocomposites modified glassy carbon electrode (Ni-Pd@AC/GCE NCs) by in-situ reduction technique and they showed a very low detection limit of 0.014 μM, a wide linear range of 0.01 mM–mM and a very high sensitivity of 90 mA mM<sup>1</sup> cm<sup>2</sup> . Furthermore, monodisperse Ni-Pd@AC/GCE was utilized to detect glucose in real sample species.

Aparna et al. [83] have reported that the NiFe2O4-AC-modified glassy carbon electrode (GCE) showed excellent electrocatalytic activity towards DA (dopamine) compared to NiFe2O4/GCE and AC/GCE. This has been attributed to the synergistic action and the large surface area of the nanocomposite. Differential pulse voltammetry (DPV) was employed for the detection of DA wherein the detection limit of 0.4 μM along with a linear range of 5 μM to 100 μM was realized. Wang et al. [84] utilized DPASV (differential pulse anodic sweep voltmmetry) technique to show that the peak currents have linear relationship with Pb2+, Cd2+ and Zn2+ concentrations respectively in the range of 0.5–2.25 mg/L, 0.5–4.0 mg/L and 1.0– 4.0 mg/L with detection limits of 0.1, 0.3 and 1.0 mg/L (S/N = 3) respectively.

Thus it is hoped that the information provided in this section proves valuable and stimulates further research and developments in the promising field of phytomass-derived AC for flexi-sensors in the near future.

#### **3.6 Application of phytomass derived AC as potential antimicrobial**

Natural plant products are always interesting to explore because of their significant antibacterial, antifungal, antiviral and anticancer activity and the presence of
