Review—Biomass Derived Carbon Materials for Electrochemical Sensors

Sensors have become integral part of our lives. Electrochemical sensors are the oldest and the most commercially used sensors. Biomass carbonization by pyrolysis and hydrothermal methods are discussed as a cost-effective strategy for fabrication of electrodes for electrochemical sensing applications. Porosity and surface area along with graphitic nature of bio derived carbon materials greatly affects the performance of electrochemical sensors. Various techniques are used to improve the surface properties so as to enhance the electrocatalytic behavior of working electrodes. Synthetic and bioderived carbon materials are compared for their electrochemical sensing applications.

Evolution of technology has dramatically changed our lives. Ability to fabricate and miniaturize devices have had profound impact on all scientific domains including sensors. Sensors are devices capable of detecting a given variant in input energy and converting it into same or other type of energy as output. 1 Advancement in instrumentation, progress in microelectronics and computers have made it possible to design various types of sensors by utilizing principles of physics and chemistry. 2,3 Based on the method adopted to measure the effect during sensing, sensors are classified into optical, [4][5][6][7] electrochemical, 8,9 mechanical, 10 thermometric, 11 mass sensitive, 12 magnetic [13][14][15] etc. Sensors are also classified according to the application to determine a given analyte. 1 Examples include, sensors to measure pH, 16 to determine metal ions, determining gases [17][18][19] etc. Sensors can also be classified based on the mode of application as in vivo, 20 in vitro, 21 for process monitoring and so on.
Electrochemical sensors provide robust analytical methods circumscribing the advantages of rapid, sensitive and specific determination of analytes. 22 Electrochemical sensors convert information inherent of reaction between analyte and electrodes into qualitative or quantitative signals. Unlike other methods of detection, like, chromatographic or spectroscopic, electrochemical sensors can be redesigned for detecting wide range of analytes and also remaining inexpensive. Furthermore, they are capable of integrating into resilient, portable, miniatured devices enabling tailor made applications. 23 Different class of electrochemical sensors can be classified depending on the electrical magnitude used for detection: potentiometric (change of membrane potential), conductometric (change of conductance), impedimetric (change of impedance), voltammetric and amperometric (change of current with applied voltage). 8 Irrespective of the type of detection, operating principles are same for electrochemical sensors. Figure 1, shows generalized steps involved in electrochemical sensing. 24 Newer and novel materials are researched upon extensively for application in electrochemical sensors. 25 Materials used for supporting surface or electrodes are generally inert without any electroactive species and conductive, over a wide potential window. Generally employed materials for the electrode and supporting substrate are, carbonbased materials (glassy carbon, graphite, carbon black, 0D fullerenes, 1D carbon nanotubes, 2D graphene related materials, 3D nanostructured porous carbon materials), 26 metals (Hg, Pt, Au, Ag, stainless steel, Bi, Pb, Sn, Sb, Co, Ga, Se), metal oxides (iron oxides, manganese oxide, TiO 2 , CuO and Cu 2 O, ZnO, ZrO 2 , Cobalt oxide, NiO, WO 3 , V 2 O 5 , silver oxide, indium tin oxide, ITO) new hybrid and inorganic complexes, 27 metal organic frameworks (MOFs), [28][29][30][31][32][33] organic electroconductive polymers or salts. 34,35 Two-and three-dimensional nanosz E-mail: murthyhegde@gmail.com tructured electrodes with large surface area (mesoporous silicates, metal oxides, polymers and carbons) were also recently reported. 36 The major challenge of improving the electroanalytical performance in terms of sensitivity and specificity is addressed by carbon-based electrodes. Carbon, with relatively inert electrochemistry and electrocatalytic activity for variety of redox reactions and low cost, is hugely popular in sensor applications. 37

Carbon Based Materials
Numerous forms of carbon-based materials ( Figure 2) 38 are synthesized, such as, carbon black, graphite and carbon nano allotropes, which are being extensively researched for use in electroanalytical applications. 35 The sensitivity and selectivity of these carbon materials can be engineered to create defects as well as grafting functional groups to the surface. Also, their mechanical properties make them ideal for integrating into electronic devices. Higher sensitivity can be achieved with low dimension carbon materials as most of their atoms are exposed to environment thereby rendering large surface area. Hence, grain boundary poisoning is lower in carbon based quasi onedimensional materials leading to higher stability of devices. Surface properties of low dimension crystals are easier to control and modify than polycrystalline structures. Some carbon forms such as graphene and nanotubes exhibit excellent ballistic charge transfer (high carrier mobility) which is important for achieving good transduction characteristics. 26 Synthesizing such carbon materials generally involves multiple steps associated with expensive precursor feedstock which requires pretreatment. 39,40 Commercial and lab scale production of some of the carbon forms like carbon nanofibers, CNTs and fullerenes involve electrospinning followed by carbonization and plasma enhanced chemical vapordeposition, requiring expensive instrumentation. [41][42][43][44][45][46][47][48][49][50] Graphene, which provides larger surface area than that of single walled carbon nanotubes, involves multiple steps of chemical and mechanical exfoliation, epitaxial growth and chemical vapor deposition. [51][52][53][54][55][56] Graphene is also poorly dispersed in aqueous media. To make a stable suspension, additional functionalization to graphene surface using dispersing agents such as, amphiphilic polymers, alkylamines, hydrophilic carboxyl groups, become necessary. 35 Therefore, in recent times, the efforts are on to develop carbon-based materials from renewable sources through cost effective techniques for sensor applications. With this context, biomass derived carbon materials are increasingly becoming popular due to abundantly available bio precursors and easier synthesis methods, 57-60 which altogether reduces the cost in development of novel electrochemical sensors. Generally, bio derived carbon materials are synthesized by pyrolysis and hydrothermal carbonization.
Hydrothermal carbonization.-Hydrothermal carbonization involves biomass carbonization at lower temperatures than pyrolysis. 65 Lignocellulose and hemicellulose are believed to undergo hydrolysis and decomposition as shown in Equation 1. 66 In hydrothermal method, hydrolysis of ether linkages predominates at 200°C, over free radical reactions at elevated temperatures in pyrolysis. Hydrolysis is followed by polymerization and formation of spherical particles.
Characterization.-The size, morphology and surface properties of carbonized products obtained from pyrolysis or hydrothermal methods are characterized by various techniques. The size  determination and morphological identification of carbon structures is determined by electron microscopic techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD) method is used to demonstrate the arrangement of particles in different crystal lattices. Crystallinity, disorder in the material and the extent of graphitization is understood by Raman spectroscopy. Fourier transform infrared spectroscopy (FTIR) is done to understand purity of the compound. BET studies are conducted to evaluate the surface properties like surface area, pore size, pore volume by N 2 adsorption-desorption isotherms. The representative SEM and TEM images of spherical shaped carbon structures obtained from Lablab purpureus beans, at different pyrolysis temperatures viz. 500, 700 and 800°C (coded as LP500, LP 700 and LP 800, respectively), 68 are shown in Figure 3. Figure 4, 68 depicts Raman shift (a) and XRD (b) for LP carbon nanospheres. Presence of D-band in Raman spectrum indicates defects and disorderness and graphitic nature is indicated by the presence of G-band. The relative intensity ratio, I D /I G of carbon nanospheres was reported to be ∼0.95 signifying turbostratic structure. 68 XRD peaks at 2θ = ∼29.8°and ∼40.5°are assigned to graphitic 0 2 0 and 0 0 2 planes. Figure 5 68 shows FTIR (a) and N 2 adsorption desorption isotherms (b). FTIR helps to identify the functional groups present in carbon structure. BET surface area of LP 800 carbon nanosphere was calculated from N 2 adsorption-desorption isotherm. Surface area was reported to be 42.2 m 2 /g for LP 800. Pore diameter was reported to be 2-3 nm. 68

Bio-Derived Carbon Materials for Electrochemical Sensing Applications
The low dimension carbon-based materials possess large surface area and porosity, making them ideal for the fabrication of electrochemical sensors. Mostly, activation and functionalization procedures are involved in the development of such carbon materials, as they improve the surface properties, which in turn enhance the electrochemical sensing. But attempts are in progress to use the carbonized materials directly without any further treatment.   to electrolytes (or analytes). 69 To enhance ionic migration and reduce diffusion impedance, mesopore density is increased by utilizing chemical and physical activation steps. 70 One such chemical activation step was done by Veeramani V. et al., 71 for synthesizing graphene sheetlike porous activated carbon (GPAC) using biomass, Bougainvillea spectabilis flower, assisted with 10% ZnCl 2 , for detection of catechin (CA). 71 The pre-heated flowers grounded into fine powder served as precursor material for chemical activation using ZnCl 2 followed by carbonization by pyrolysis at 700, 800 and 900°C. Pyrolyzed products were coded as, AC-700, GPAC and AC-900 respectively. The as prepared products were coated on glassy carbon electrode (GCE) for sensing catechin by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques in 0.05 M phosphate buffer saline (PBS, pH = 7).
Electrochemical response to the oxidation of CA for bare GCE and modified GCEs is shown in Figure 6a. From the graph it is clear that, GPAC exhibited sharper peak than AC-700 and AC-900, with a very low overpotential of 0.16V. Figure 6b, shows the scan rate effect (in the range of 50 mV s −1 to 500 mV s −1 ) of GPAC-modified electrode in presence of catechin (240 μM). As expected, with increase in scan rate, peak current was increased. It can be seen from Figure 6b, that, with increase in scan rate, there is a shift of oxidation peaks toward the positive potentials and the reduction potential shifted more toward the negative potentials. This kind of peak shifts, generally suggests that,  the process is diffusion-limited and kinetically controlled process, with electrode kinetics playing a predominant role. [72][73][74] However, the linear regression equations (Equations 2 and 3), and y = −0.1036x − 4.95 [3] with correlation coefficients, R 2 = 0.9931 and 0.9752 respectively, reaching unity, mathematically proves that the oxidation of catechin on the modified electrode surface is a diffusion-controlled process. Electrochemical processes also depend on the pH of the medium. is important for the material to be commercially viable. GPAC showed a CV peak potential at pH 7. A linearly increasing oxidation peak with concentration of CA was depicted by differential pulse voltammetry (DPV) as shown in Figure 6d. The linear range, sensitivity and detection limit are listed in Table II. GPAC modified GCE electrode was used in real-time scenario for detection of CA in green tea leaves extract which exhibited linear range up to 206 μM. The increased current response compared to a blank PBS solution is exhibited in Figure 7. A major factor delimiting the usefulness of therapeutic drugs is its toxicity. Drug toxicities cause undesirable pharmacological effects, manifesting a harmful functional, biochemical changes in human body. 75 Hence monitoring and determining such drug concentrations in humans, is significantly important in view of public health. Acetaminophen, commonly known as paracetamol, is a leading analgesic, antipyretic drug consumed worldwide. However, acetaminophen toxicity which can also cause hepatoxicity, is trammelling the health care system around the world. 76 Dongwon Kim and others 77 have reported a novel biomass derived carbon based electrochemical sensor for determining acetaminophen (AC) which can bypass the expensive time consuming conventional methods. The electrode material for sensing AC, was synthesized involving two step activation process. The dried kelp powder was carbonized under Ar atmosphere at 700°C. The obtained kelp carbon (KC) was mixed with ZnCl 2 followed by pyrolysis at 700°C. The product obtained (coded ZAKC) was mixed with KOH in 1:1 weight ratio and pyrolyzed at 800°C. The obtained product coded ZKAKC was used as the electrode material. The graphical synthesis scheme is illustrated in Figure 8.
ZKAKC slurry loaded on to GCE was used as the modified electrode for sensing AC. 5.0 mM K 3 Fe(CN) 6 in 1.0 M KCl was employed to find the catalytic activity of fabricated electrode through CV studies. Electrochemically active surface area was calculated from Randles-Sevick Equation 4, Where, A is electrochemical active surface area (cm 2 ), D signifies diffusion coefficient in bulk solution (6.67 × 10 −6 cm 2 s −1 for  Table II. Practical application of the electrode sensor for detecting AC was done in diluted human urine sample. The enhancement in sensory performance can be attributed to increased surface area due to two step activation. However, a two-step activation process is time consuming and expensive for synthesis and alternate routes without need of multiple steps for fabricating are necessary. Dejian Chen and others 78 have reported the synthesis of N 2 rich hierarchically porous spongy carbon materials from pig lung as biomass precursors. Pig lung was cut into 1cm 2 small pieces and freeze dried which resulted in a spongy like texture. This freeze-dried spongy lung  was pre-carbonized at 450°C. The resulting black powder was mixed with KOH (1:3 ratio) and carbonization was continued at 800°C which was coded as HPCs-800. HPCs-800 modified GCE was used to determine catechol (CC), resorcinol (RC) and hydroquinone (HQ) which are a dihydroxybenzene isomers. From CV, distinct electrochemical oxidation of hydroxybenzene isomers were reported to have been detected without any overlapping of peaks in PBS (pH = 7). This was attributed to material's large surface area and spongy porous structures.
HPCs-800 was reported to have BET surface area of 958.5 m 2 g −1 with micro and mesoporous structures with average pore size of 3.6 nm. This enhanced the transfer of electrons and molecules. HPCs-800 was reported to have different nitrogen species confirmed from XPS analysis from N 1s spectrum (not shown here) at 399.2, 400.1 and 401.1 eV. Nitrogen with higher electronegativity than carbon, affected the electronic structure of carbon atoms at the edges disturbing density of states (DOS), causing changes in the electroneutrality of graphitic  electron cloud. Effect of scan rate was reported to have exhibited shift in oxidation potential toward positive, indicating the electrode kinetics to be adsorption-controlled phenomenon. DPV was further conducted to evaluate detection limit and linear range (tabulated in Table II) to establish sensitivity of HPCs-800. The practicability of fabricated electrodes was tested for detection of dihydroxybenzene isomers in actual water samples using DPV. The standard recovery addition percentages were from 98.1 to 104.3%, indicating applicability of the modified electrode in real case scenarios.

Non activated carbon materials as electrochemical sensors..-
Porous carbon nanospheres (CNSs) were synthesized by one step pyrolysis method using waste onion peels at 800°C. This was smear coated on carbon fiber paper (CFP) as a modified electrode (CNSs/CFP) for detection of progesterone by Akshaya K. B., et al. 79 Prepared carbon nanospheres have been reported to have surface area 692.21 m 2 /g with pore diameter of 2.5 nm. The carbon nanospheres modified electrode (CNS/CFP) was used for detection of progesterone at ultra-nanomolar level with detection limit of 0.012nM. Modified CNS/CFP electrodes showed capacitive currents (Figure 11a) larger than the bare CFP of the same geometric surface. CVs of bare CFP and CNS/CFP in phosphate buffer solution (PBS) in pH-7, for progesterone analysis is shown in Figures 11c & 11d. 79 A negative poten-tial shift observed for CNS/CFP when compared with bare CFP, was attributed to increased electroactive sites for the oxidation of progesterone on CNS coated CFP. Nyquist impedance plot (Figure 11b) of bare CFP exhibits charge transfer resistance, R ct of 93.68 whereas modified CNS/CFP showed significantly low R ct of 8.85 . 79 It is evident from the graphs, that, presence of carbon nanospheres accelerated the charge transfer reactions leading to higher peak currents and lower resistance.
Variation of peak currents with scan rate was used to understand the kinetics of the process. From Figure 12, when i pa is proportional to v, the process is seen to be limited by diffusion species. The straightline equation, 5 slope value was found to be almost equal to 1. This indicates that, the electrode process was not diffusion controlled but adsorption controlled. Also, it can be observed that, as the scan rate was increased, anodic peak potential was shifted to more positive potentials further confirming the diffusion limited adsorption facilitated electrochemical oxidation of progesterone on CNS/CFP electrode.
log i pa (A) = −0.98806 log v (mV/s) + 3.03786 R 2 = 0.9951 [5] Another important aspect of electrochemical measurements is to find the electrons participating in the process. Equation 6, was used to find the number of electrons involved in the electrochemical oxidation E p is peak potential and n value (electrons involved) was calculated to be 1.915 almost equal to 2. A cogent mechanism electrochemical oxidation of progesterone is schematically illustrated in Figure 13.
The electrode stability was tested using Equation 7.
% Degradation = i pn i p 1 × 100 [7] where i pn and i pi are the n th and 1 st anodic peak current respectively. The reported degradation of the CNS only was 0.7%. A DPV response to progesterone (0.15nM) in presence of eleven interfering compounds were analyzed and is shown in Figure 14.
The results suggest selectivity of the electrode toward progesterone in presence of interfering compounds. The modified electrode was tested for detection of progesterone in human blood serum and cow milk samples. The recovery values were reported to be in the range of 99.2 to 101.0%, indicating possibility of commercial application of the electrode for progesterone detection. One step synthesis adds to the advantage of the reported method.
Mesoporous nano carbonspheres from carbon rich areca nut seeds were synthesized from one step pyrolysis (550°C) for nitrite detection. 80 In this study, the surface area was reported to be 42.1 m 2 /g. Glassy carbon electrode (GCE) was modified with carbon nanospheres (CNSs). CNSs/GCE showed enhanced peak at 0.89V with a negative shift when compared to bare GCE, indicating good catalytic activity of the electrode toward nitrite oxidation. The observed activity was predicted to be because of the increased adsorption sites and enhanced porosity of the electrode due to CNSs, leading to effective electron transfer resulting in increased selectivity and sensitivity. 80 Multifunctional electrochemical sensor was developed using carbon nanoplates by pyrolysis of ground cherry husks at 700°C to detect H 2 O 2 81 (BG-CNPs). I D /I G ratio from Raman spectroscopy was reported to be 2.69 suggesting presence of large number of defective sites on the material surface. 82 Biomass ground carbon nano plates modified glassy carbon electrode (BG-CNPs/GCE) was reported to exhibit larger electrochemical active area of 0.0842 cm 2 , 81 The R ct for bare GC was reported to be 244 and for BG-CNPs/GC was 99 indicating improvement in charge transfer. 81 BG-CNPs/GCE based H 2 O 2 sensor displayed two linear ranges of 30-285 μM and 285-1685 μM leading to lower detection limit (16 μM). BG-CNPs/GCE were also employed for detection of NADH. Dual linear concentration ranges were reported. (10-220 μM and 220-830 μM). Because of higher defective sites, BG-CNPs/GC showed faster electron transfer kinetics for sensing of acetaminophenol (APAP), epinephrine (EP), cysteine (CySH), L-tryptophan (Trp), dopamine (DA), ascorbic acid (AA), and uric acid (UA). Electrocatalytic properties of BG-CNPs/GC make it possible to detect simultaneously all the four DNA bases (G, T, A and C) at physiological pH. 81 The study tries to prove that, defec- tive sites on the carbon nanomaterials are responsible for increased electrocatalytic activities [83][84][85] which can be utilized effectively for simultaneous detection of multiple compounds, bypassing the need of single sensor-single compound detection concept.
Hierarchically porous aerogels of carbon nanoballs aggregation networks (CNANAs) from Citrus maxima peels were synthesized by a simple pyrolysis technique for detection of H 2 O 2 . 86 It is reported to have surface area of 446.39 m 2 /g with mesopores centred at 2 and 12 nm and macropore at 87 nm. Such hierarchical meso-macro-porous structures provide easy path for electrolyte diffusion, leading to better electrochemical performance. 86 Raman spectroscopy of CNANAs confirms graphitization and high density of edge-plane like defective sites (EPDSs), with I D /I G ratio of 1.99. EPDSs are reported to increase the electron transfer rates on the surface of carbon nanomaterials. 87 Because of such surface properties CNANAs are reported to have sensitivity of 42.4 μA mM −1 cm −2 toward electrochemical detection of H 2 O 2 in 0.1 M pH 7 PBS. 86 Doped carbon materials have generated lot of interest for application in electrochemical sensors. Due to the synergistic effect between doped elements and carbon matrix. Attempts have been made to obtain such materials by single step. Nitrogen and phosphorous co-doped hierarchical micro/meso porous carbon (N,P-MMC) materials were synthesized from freeze dried okra by pyrolysis at 800°C. 88 0.96% weight of N and 1.47% weight of P were added to the graphitic framework of N,P-MMC. N 2 sorption isotherm reported to feature both type I and IV, indicating formation of both micropores and mesopores. BET surface area of mesopores was found to be 423 m 2 /g with pore volume 0.30 cm 3 /g, while microporous surface area and pore volume were found to be 216 m 2 /g and 0.18 cm 3 /g. Mesopores centred at 3.6nm and micropores at 0.79 nm. 88 This porous surface area provides platform for electrochemical sensing. N,P-MMC modified GCE (N,P-MMC/GCE) was used for detection of H 2 O 2 in PBS at pH 7. The modified electrode showed fast amperometric response, where, cathode current increased and achieved 95% of the steady current within 2s. This has been attributed to graphitic structure and coexistence meso and micropores. The fabricated electrode reported to exhibit two linear detection ranges, 0.1-10 mM and 20-200 mM. Hence this can be exploited for detection of H 2 O 2 at wide concentration ranges.

Carbon based metal inorganic composite materials for electrochemical sensors.-Composite
materials have attracted wide attention in recent times for their culmination of diverse properties and ability to produce synergetic effects. Metal inorganic complexes based on biomass derived carbon for sensing application is discussed. Li Wang and others, 89 reported biomass derived macro-porous carboninorganic metal ion complexes for electrochemical and biosensing applications from kenaf stem (KS). Direct pyrolysis of dried kenaf stem at 900°C, produced a 3D porous carbon (3D-KSCs). This was functionalized by refluxing with 15% HNO 3 followed by water wash till the pH becomes neutral (coded as 3D-FKSCs). Prussian blue nanoparticles (PBNPs)-carboxylic group functionalised 3D FKSCs (PBNPs-3D-FKSCs) was chemically synthesized by treating 3D-FKSCs with FeCl 3 followed by K 3 Fe(CN) 6 and conc. HCl. CuNiNPs-3D-KSCs was prepared by immersing dried KS in soaking solution of CuCl 2 and NiCl 2 for 20 days, followed by drying the soaked KS and carbonizing at 900°C. Similarly, CoNPs-3D-KSCs was prepared with soaking solution of Co(Ac) 2 and carbonizing the soaked and dried KS at 800°C. Integrated 3D-KSCs/Inorganic Nanocomposite electrode was prepared as shown in Figure 15 using pre treated tips and copper wire.
Integrated PBNPs-3D-FKSCs electrode was used to determine 3 mM H 2 O 2 in 0.05 M PBS + 0.1 M KCl. The peak current density was reported to increase linearly with square root of scan rate, indicating a diffusion-controlled process. Detection sensitivity and linear range were reported to be better than previously published reports based on PB modified electrode. 89 Decrease of current density by 2.1% for detecting 1.0 mM H 2 O 2 was reported after keeping the electrode at 4°C for 50 days, suggesting a good stability of the electrode. Glucose sensing was done using integrated CuNiNPs-3D-KSCs electrode. The process was reported to be surface controlled phenomena as peak current density was reported to increase as scan rate was increased. CuNiNPs were transformed as Cu(OH) 2 and Ni(OH) 2 in 0.1 M NaOH solution. These hydroxides at high potentials would generate enough CuOOH and NiOOH which later oxidized glucose to gluconolactone. Stability of the electrode were assessed by using the electrode kept at 4°C for 60 days. 3.4% reduction in current density for detection of 1 mM Glucose, was reported by using these electrodes. CoNPs-3D-KSCs electrode was employed to detect amino acid cysteine and N-acetyl cysteine, which was also reported to follow surface controlled mechanism similar to CuNiNPs-3D-KSCs.
Surface properties of the electrode materials discussed are listed in Table I. All the important electrochemical sensory performance of the discussed electrodes is listed in Table II.
From Bio-derived carbon materials for electrochemical sensing applications section, it can be seen that pyrolysis technique of carbonization is predominantly employed for carbon synthesis from biomass. This could be due to partial degradation of lignocellulosic materials during hydrothermal conditions. 65,67 Not only plant but animal origin precursors were also employed for carbonization (Activated carbon materials for electrochemical sensors section). 78 Carbonization produced porous materials rich in carbon with high surface area. BET surface area for one of the discussed carbon material GPAC, 71 had very high surface area of 1197 m 2 /g. As discussed from Bio-derived carbon materials for electrochemical sensing applications section, some of the analyte detection by carbon electrodes, were reported to be governed by surface adsorption mechanism and some by diffusion controlled. Presence of large surface area is believed to provide an efficient platform for the electrochemical reaction to occur, thereby enhancing the adsorption (surface) controlled mechanism of electrode-analyte interaction. Most of the discussed porous carbons structures were either micro (d < 2 nm), Meso (d > 2 nm-50 nm) or macro-porous (d > 50 nm) or hierarchical in nature which facilitates easier diffusion of molecules and ions, enhancing surface diffusion phenomena. 69,[90][91][92][93] Variation in surface area affects the mechanism of analyte detection, which can be clearly seen from 3D-porous carbonized kenaf stem as the matrix for composites. 89 GPAC 71 which exhibits graphene like nature, shows good electrochemical response when compared with some of the reduced graphene oxide electrodes listed in Table III. Surface area of PBNPs-3D-FKSCs was reported lesser than CuNiNPs-3D-KSCs (Table I). Detection of H 2 O 2 by PBNPs-3D-FKSCs electrode with surface area 827.23 m 2 g −1 , was reported to be diffusion controlled whereas, detection of glucose using CuNiNPs-3D-KSCs with surface area 1019.4 m 2 g −1 was reported to adsorption-controlled process. When compared to carbon-metal inorganic composites, only carbon-based electrodes seem to exhibit ultra-high molar detection of analyte (Table II). Activation process was employed to obtain hierarchical porous structure in carbons, is time consuming and adds an additional step in the synthesis procedures. CNANAs 86 which were reported to be carbonized hydrothermally also show good linear range of analyte detection when compared with ZKAKC 77 which were synthesized after two-step activation procedure (Table II). Hence, there is need of simpler and less time-consuming methods of synthesis for preparing electrodes. Table III lists the analytical parameters obtained from electrochemical measurements using electrodes prepared via different synthetic routes, of the same analytes, which were detected using biomass derived carbon-based electrodes. This comparison of analytical parameters helps in better understanding of the nature of electrode materials to be used in cost effective electrochemical sensors.
Sensing performance of the synthetically prepared and carbonbased electrodes from biomass carbonization can be compared from Table II and Table III. In most of the compared cases, the detection limit is a limiting factor for synthetically prepared electrodes whereas, CNSs prepared from onion peels 79 reported to have exhibited a ultrananomolar level (0.012nM) detection of progesterone. However, in some cases, it can be observed that, the linear range of detection of analyte is better with synthetically prepared electrodes than biomass derived carbon-based electrodes. Robust chemical synthetic techniques available does add to the advantage of synthetic electrodes. [109][110][111][112] Nevertheless, Biomass derived carbons have shown excellent performance stability over long days of storage, retaining optimal sensitivity required for analyte detection. The main advantage of the biomass derived carbon electrodes is the simple one or two steps synthesis procedures. The carbon-based electrodes can also be coated on different substrates without much additional procedures to follow.

Conclusions
The methods to synthesize various carbon structures from different bio-precursors are discussed. The review throws light on the effect of activation on the electrochemical sensing performance of bio-derived carbon-based electrodes. The activation processes are conducted to improve the surface properties in order to enhance the electrocatalytic sensing. But it is observed that, activation may not guarantee a better sensing performance as few bio-precursors produced hierarchical micro/meso/macro pores depending on the lignocellulosic content without activation, were sufficient for excellent sensing performance. It is to be noted that, edge defects and turbostratic nature along with presence of pores play a pivotal role in enhancing electrochemical sensing. Hierarchically porous and doped carbon materials also show excellent electrochemical behavior due to micro/mesopores and synergetic effect of doped atoms onto carbon matrix. Biomass derived porous carbon can act as a unique host matrix for inserting metal ion inorganic complexes, noble and catalytically active metals for newer electrodes which can enhance the performance of sensor. 19 Metal organic frameworks (MOFs) derived from biomass for sensing applications needs to be explored for their effectiveness as sensors.
There are reports MOFs being used as adsorbing/catalytic sites because of their unique structures, it needs to be further studied for applications in electrochemical sensing. The bioderived carbon electrodes are environmentally friendly without any heavy metals and toxic compounds as is the case with some of the synthetically prepared electrodes. Cost effective commercial production of robust, portable, sensitive and selective electrochemical sensing devices are the need of the hour. This can be accomplished easily by bio-derived carbon structures that are synthesized by simple steps, from inexpensive abundantly available renewable biomass.