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Membraneless enzymatic biofuel cells using iron and cobalt co-doped ordered mesoporous porphyrinic carbon based catalyst
Applied Surface Science 511 (2020) 145449
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Membraneless enzymatic biofuel cells using iron and cobalt co-doped ordered mesoporous porphyrinic carbon based catalyst ⁎
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Jungyeon Jia,1, Jinwoo Wooc,1, Yongjin Chungd, , Sang Hoon Jooc, , Yongchai Kwona,b,
T
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a
Graduate School of Energy and Environment, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 01811, Republic of Korea Department of Chemical and Biomolecular Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 01811, Republic of Korea c Department of Energy Engineering and School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea d Department of Chemical and Biological Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju, Chungbuk 27469, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal-doped ordered mesoporous porphyrinic carbons Membraneless enzymatic biofuel cell Hydrogen peroxide oxidation reaction Oxygen reduction reaction Physiological condition
Iron and cobalt co-doped ordered mesoporous porphyrinic carbon (FeCo-OMPC) is employed as the catalyst for both electrodes in membraneless enzymatic biofuel cells (EBCs). FeCo-OMPC is used for catalyzing oxygen redox reaction (ORR), while GOx/[FeCo-OMPC/CNT] is utilized as a catalyst for a series of glucose oxidation reaction (GOR) and hydrogen peroxide oxidation reaction (HPOR). The onset potential of the ORR by FeCo-OMPC is 0.31 V vs. Ag/AgCl, while that of HPOR by FeCo-OMPC/CNT and GOR-HPOR by GOx/[FeCo-OMPC/CNT] are both 0.12 V. The activity of this catalyst is better than previously reported similar catalysts due to the presence of Co species and high metal contents. As a result, the concentration of H2O2 generated by GOR is 1.02–1.56 mM when the same glucose concentration as human blood is used. In addition, the EBC using GOx/[FeCo-OMPC/ CNT] and FeCO-OMPC shows the maximum power density of 21.3 ± 2.97 μW cm−2 with open circuit voltage (OCV) of 0.17 ± 0.016 V. These values are significantly higher than those of EBC using the competitive Fe–N/ CNT catalyst (0.11 V and 9.6 μW cm−2). Moreover, the OCV is close to the expected value by CV (0.19 V), confirming that the FeCo-OMPC catalyst can be used for implantable bioelectronics, such as biosensors and electroceuticals.
1. Introduction As the demand for wellness and health grow, interest in real time health care is rapidly increasing. Especially, the need for body implantable biosensors and electroceuticals are expected to increase explosively to prevent adult diseases, such as diabetes and hyperlipidemia, and to maintain good health condition [1–3]. Based on these demands, to date, a great deal of effort has been made to develop a solution for real time diagnosis and treatment by the use of body implantable bioelectronics [4–6]. However, their actual uses have been rarely reported, and unstable power supply is one of the main hurdles suppressing utilization. Although various attempts including secondary batteries have been suggested to overcome these issues, the use of such power sources requires periodic and continuous surgeries to replace the power supply system, giving rise to potential hazard, such as the permeation of toxic chemicals into the human body [5,7,8]. To address these problems, enzymatic biofuel cells (EBCs), which use (i) enzyme
based biomaterials as a catalyst, and (ii) glucose, hydrogen peroxide (H2O2) and oxygen as fuels, have been focused on as a device for selfgeneration of minute electricity within the human body because all of the EBC components are compatible with the human body and complicated and expensive components like a membrane are not necessary [5,9–12]. So far, researchers have attempted to develop various types of EBCs and most researchers use glucose oxidase (GOx) as the anodic biocatalyst and horse radish peroxidase (HRP) and multi copper oxidase (MCO), such as laccase and bilirubin oxidase, as the cathodic biocatalyst [13–20]. GOx can easily oxidize glucose in human blood through glucose oxidation reaction (GOR) of the flavin adenine dinucleotide (FAD), but for the redox reaction, a harmful mediator for the human body is utilized [16,21–23]. Moreover, toxic H2O2 can be produced during the redox reaction by oxygen dissolved in human blood [10,24]. Differently from GOx, MCO and HRP are known as relatively safe catalysts because when they are used, the use of a mediator is not
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Corresponding authors. Tel.: +82 438415229 (Y. Chung). Tel.: +82 438415229 (S.H. Joo). Tel.: +82 29076805 (Y. Kwon). E-mail addresses: [email protected] (Y. Chung), [email protected] (S.H. Joo), [email protected] (Y. Kwon). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apsusc.2020.145449 Received 2 October 2019; Received in revised form 14 January 2020; Accepted 17 January 2020 Available online 21 January 2020 0169-4332/ © 2020 Published by Elsevier B.V.
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2.3. Electrochemical characterizations and performance evaluations
mandatory and direct electron transfer (DET) is possible. However, since the MCO and HRP are too expensive and they have poor stability for use in EBCs, developing a new catalytic structure that has biocompatibility, a cheap price, and long term stability in the human body is needed [9,13,18,25]. To overcome the disadvantages of the conventional approaches, catalysts that mimic the structure of nature’s catalysts have been explored. For example, iron porphyrinic structures, such as hemin, was suggested to replace MCO and HRP [9,26]. According to their study, the iron porphyrinic materials, especially hemin, which is a core structure of HRP, induced a considerable magnitude of cathodic current in proper potential ranges by the hydrogen peroxide reduction reaction (HPRR) [9]. In addition, Ji et al. developed a catalytic structure that can catalyze both the HPRR and hydrogen peroxide oxidation reaction (HPOR), eliminating H2O2 from GOR of GOx [10]. In their approach, Fe–N/CNT, which consists of an iron porphyrinic structure doped onto the surface of carbon nanotubes (CNT), acted as both the bioanode and biocathode. In this paper, we introduce a new iron and cobalt co-doped ordered mesoporous porphyrinic carbon (FeCo-OMPC) structure as the catalyst for both electrodes of EBC. FeCo-OMPC was already introduced as the ORR catalyst, substituting the conventional platinum (Pt) in acidic media [27]. However, their use within physiological conditions and as catalyst for other chemical reactions have not been reported yet. For use in EBC, the natural form of FeCo-OMPC is used as the cathodic catalyst catalyzing ORR, while the GOx and FeCo-OMPC based composite is used as the anodic catalyst catalyzing HPOR. To evaluate its viability in EBC, the electrochemical performances of this new catalyst are measured and the results are compared with those using Fe–N/CNT, which is a vying catalyst with FeCo-OMPC, and, also, membraneless EBCs using this new catalyst are operated within physiological conditions.
Cyclic voltammetry (CV) was carried out by using a SP-240 potentiostat (Bio-Logic, USA). Each catalyst was loaded onto a GCE which was utilized as the working electrode, and Ag/AgCl (sat. in 3.0 M NaCl), and Pt wire were utilized as reference and counter electrode, respectively. N2, O2, and air were injected into the electrolyte solution to make specific atmosphere [15,28]. The FeCo-OMPC/CNT structure was optically confirmed by high resolution transmission electron microscopy (TEM) and high resolution scanning electron microscopy (SEM). SEM images were taken by a scanning electron microscope (S-4800, Hitachi High-Technologies) operating at 10 kV. TEM images were obtained using a transmission electron microscope (JEM-2100, JEOL) at an acceleration voltage of 200 kV. The membraneless EBC kit was fabricated to quantify the EBC performance by measuring the polarization curves. As for the fuels that were provided to the EBC kit, 30 mM glucose solution (air purge) was fed to the cell at a flow rate of 0.1 mL min−1. The preparation of electrode was previously introduced in Section 2.2. 3. Result and discussion 3.1. ORR activity of FeCo-OMPC catalyst in physiological condition It is crucial to investigate the catalytic activity of FeCo-OMPC for ORR within physiological conditions to evaluate the viability as the cathodic catalyst in EBC. For doing that, the CV curves of the FeCoOMPC loaded GCE were measured in pH 7.4 (0.01 M PBS) and O2 saturated conditions (Fig. 1). According to Fig. 1, the onset potential of FeCo-OMPC was 0.31 V (vs. Ag/AgCl), which is slightly higher than that of Fe–N/CNT(0.29 V) [10]. However, both catalysts showed the same current densities of 0.9 mA cm−2 at 0 V [10,29]. It is an intriguing result because it is known that FeCo-OMPC is useful in acidic media, while Fe–N/CNT can be mainly used in alkaline media. Namely, although Fe–N/CNT seems to be more suitable in physiological conditions (pH 7.4), FeCo-OMPC shows better electrochemical performances as an ORR catalyst within the same conditions [10,27,29]. This indicates that FeCo-OMPC can also be used for the ORR catalyst within physiological conditions. This is compatible with the previously reported results about the ORR activity of the two catalysts. According to the ORR activity measurements, the half wave potential of FeCo-OMPC was higher (0.845 V vs RHE) than that of Fe–N/CNT (0.79 V vs RHE),
2. Experimental section 2.1. Materials Glucose oxidase (GOx, Aspergillus Niger, type X-S, 100–250 units mg-1 solid), o-dianisidine, and D-(+)-glucose were purchased from Sigma-Aldrich, while all the related chemicals, such as sodium acetate (98.5%), ethyl alcohol (EtOH, 99.5%), hydrogen peroxide (30%), and formic acid (99%), were purchased from Samchun Pure Chemical. 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine iron(III) chloride (FeTMPPCl) was purchased from Porphyrin Systems and 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine cobalt(II) (CoTMPP) was purchased from Tokyo Chemical Industry. Multiwall carbon nanotubes (MWCNT, MR99) were purchased from Carbon Nano-material Technology and phosphate buffer solution (PBS, pH 7.4) came from Life Technologies.
2.2. Synthetic procedure of cathodic and anodic catalysts FeCo-OMPC was synthesized by a solid-state nanocasting method. The detailed synthetic procedure was previously introduced by the Joo group [27]. CNT/FeCo-OMPC was fabricated through the physical mixing of FeCo-OMPC and CNT. According to the detailed synthetic procedure, 10 mg of FeCo-OMPC was mixed with 10 mg of CNT in 5 mL of EtOH, and the solution was sonicated for 10 min and agitated for 2 h at room temperature. For preparing for GOx/[FeCo-OMPC/CNT] bioanode, 10 μL of FeCo-OMPC/CNT was dropped onto glassy carbon electrodes (GCE, diameter of 5 mm) and dried for 10 min at room temperature. After the process, 10 μL of GOx in 0.01 M PBS (20 mg mL−1) was loaded onto the FeCo-OMPC/CNT. After drying, 4 μL of Nafion solution (5 wt%) was further coated on the GOx/[FeCoOMPC/CNT] loaded GCE.
Fig. 1. Cyclic voltammograms (CVs) of FeCo-OMPC loaded GCE measured under pH 7.4 in both O2 (red solid line) and N2 (black dashed line) saturated states. For the CV tests, 0.01 M PBS (pH 7.4) was used as the electrolyte and the potential scan rate was 10 mV s−1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2
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Fig. 2. (a) SEM and (b) TEM images of the FeCo-OMPC/CNT composite.
even in acidic condition (0.1 M HClO4) [27,29]. Based on the above result, it can be demonstrated that the onset potential for ORR of FeCo-OMPC is better than that of Fe–N/CNT in physiological conditions, and thus, it is expected that the EBC using FeCo-OMPC catalysts can induce a better EBC performance due to a higher open circuit voltage (OCV), which is mainly attributed to the positively positioned onset potential of ORR.
Table 1 The generated current density (at 0.3 V) by HPOR of Fe–N/CNT and FeCoOMPC/CNT. The current density was calculated from the difference between the current density with H2O2 injection and without H2O2 injection. H2O2 concentration [mM]
3.2. HPOR activity of FeCo-OMPC catalyst within physiological conditions
0.5 1 2 3 4 5 10 15
For evaluating the viability of FeCo-OMFC as catalyst for the anode, FeCo-OMPC/CNT composite was fabricated as shown in Fig. 2 and its electrochemical performance for HPOR was evaluated by CV curve measurements. In this structure, CNT plays a role as the supporter in immobilizing many GOx molecules and as the electrical connector between FeCo-OMPC and the electrode, while FeCo-OMPC is adopted as the catalyst for activating the HPOR. Regarding the catalytic activity of FeCo-OMPC/CNT for the HPOR, Fig. 3a shows its CV curves, which are measured with the injection of different H2O2 concentrations (0–15 mM). As shown in Fig. 3a, a significant anodic current was observed at the onset potential of 0.12 V, which is a negatively shifted value when compared to that of Fe–N/CNT (0.15 V). This implies that a higher OCV is possible when used as the anodic catalyst for EBCs. Moreover, the current densities measured at the H2O2 concentrations of 1, 3, 5, and 10 mM were 0.240, 0.677, 0.962, and 1.830 mA cm−2, respectively, when the measured potential was as high as 0.6 V, and 0.156, 0.2286, 0.324, and 0.350 mA cm−2, respectively, when the measured potential was as low as 0.3 V. This means that, irrespective of the used potentials, the current density of FeCo-OMPC/CNT is about 10 times better than that of Fe–N/CNT (Table 1). There are two main reasons for the discrepancy in electrochemical
Current density at 0.3 V [mA cm−2]
Current density at 0.6 V [mA cm−2]
Fe–N/CNT [10]
FeCo-OMPC/ CNT
Fe–N/CNT [10]
FeCo-OMPC/ CNT
0 0.001 0.010 0.021 0.037 0.069 0.091 0.095
0.083 0.156 0.249 0.286 0.310 0.324 0.350 0.341
0.009 0.018 0.047 0.070 0.103 0.154 0.221 0.271
0.135 0.240 0.462 0.677 0.834 0.962 1.480 1.830
performance between the FeCo-OMPC/CNT and Fe–N/CNT catalysts. First, the Co element within FeCo-OMPC/CNT triggers the negative shift of the onset potential because the metal core within the Co porphyrinic structure lowers the HPOR potential more than that of a Fe based structure, such as Fe–N/CNT [10]. Namely, the onset potential for HPOR is dominated by the redox potential of Co2+/Co3+ which is known for lowering the onset potential of HPOR to 0.004 V in neutral pH [30–32]. Furthermore, according to some research results of H2O2 sensors using Co-phthalocyanine, the structure showed the onset potential of 0.1 V for HPOR. Based on that, it is proven that the Co core promotes the negative shift of the onset potential [32,33]. Second, the larger metal core contents within FeCo-OMPC/CNT can induce a higher current density. The amount of metal atoms contained in Fe–N/CNT and FeCo-OMPC were calculated to be 2.9 and 4.8 wt%, respectively,
Fig. 3. CV curves of FeCo-OMPC/CNT measured with the injection of 0, 2, 5, and 10 mM H2O2. For CV tests, 0.01 M PBS (pH 7.4) was used as the electrolyte and the potential scan rate was 20 mV s−1. 3
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(0.38–0.6 V, HPOR B) where current density continuously increases up to 15 mM. The occurrence of HPOR A is due to the participation of OH− ions in the HPOR reaction (H2O2 + OH− → H2O + O2 + H+ + 2e−), while HPOR B occurs without the use of OH− ions (H2O2 → O2 + 2H+ + 2e−). This HPOR mechanism explanation was also similarly reported by Ji et al. [10,34–36]. Meanwhile, the pH effect on HPOR A is investigated (Fig. 4). As shown in Fig. 4, there are no HPOR A peaks in the pH 3 condition, while significant HPOR A peaks were produced in the pH 7.4 and 10 conditions. By (i) the participation of OH− ions in the HPOR reaction and (ii) the pH effect of the HPOR reaction, the presence of two different HPOR mechanisms is confirmed. Furthermore, it is substantiated that the HPOR A mechanism, which contributes to gaining a high OCV and power density in EBCs, can occur even in physiological conditions under a condition of 5 mM H2O2. 3.3. GOR activity of GOx/[FeCo-OMPC/CNT] catalyst within physiological conditions It is crucial to evaluate the compatibility between GOx and FeCoOMPC/CNT to predict the performance and long term stability of the bioanode (GOx/[FeCo-OMPC/CNT]). To do that, the amount of GOx immobilized on GOx/[FeCo-OMPC/CNT] was measured by UV–Vis spectrophotometry (Fig. 5). By the calibration of the absorbance peak near 400 nm, the H2O2 concentration produced from GOR catalyzed by free GOx molecules is calculated [9]. According to the calculation, the amount of immobilized GOx was calculated to be 69.8 Unit mg−1 which is higher than that of GOx/CNT (62.1 Unit mg−1). This demonstrates that the GOx molecules are tightly immobilized onto the FeCo-OMPC/CNT substance, and FeCo-OMPC does not prevent the immobilization of GOx molecules. Next, CV curves of the GOx/[FeCo-OMPC/CNT] bioanode are measured in pH 7.4 and air saturated conditions to estimate its catalytic activity for GOR. In this structure, the upper GOx layer produces H2O2 through the GOR catalyzed by FAD within GOx, then the H2O2 further decomposes into H2O, producing the anodic current by Fe and Co cores within the FeCo-OMPC/CNT under layer. Its schematic illustration and reaction pathway are represented in Fig. 6. Regarding their CV curves (Fig. 7), the anodic current density was observed after 0.12 V and increased with the injection of glucose. The onset potential was the same with that of the HPOR of FeCo-OMPC/CNT (Fig. 3). This demonstrates that the GOR is well conducted in the upper layer consisting of GOx molecules and the combination of GOR and HPOR promotes the anodic current by the GOx/[FeCo-OMPC/CNT] bioanode. Based on the onset potential of the GOx/[FeCo-OMPC/CNT] bioanode, the expected OCV of the EBC is 0.19 V.
Fig. 4. CV curves of HPOR catalyzed by FeCo-OMPC/CNT in (a) acidic (pH 3), (b) physiological (pH 7.4), and (c) alkaline (pH 10) states.
indicating that FeCo-OMPC/CNT has the possibility of having more active site to catalyze HPOR [27,29]. In summary, FeCo-OMPC/CNT is appropriate as the enzymatic catalyst for the anode due to its lower onset potential and higher current density for HPOR, which are ascribed to the existence of the Co core and higher metal core content. Next, the HPOR reaction mechanism was investigated, as shown in Fig. 3b. The two different mechanisms can be induced according to the electrochemical behavior observed with the injection of different H2O2 concentrations. The first mechanism (red region) appears in a lower potential range (0.12–0.38 V, HPOR A) where current density is saturated in a lower H2O2 concentration range (~5mM), and the second mechanism (blue region) appears in a higher potential range
Fig. 5. Enzyme activity measurements using UV–Vis spectrophotometry representing the amount of GOx immobilized on CNT and FeCo-OMPC/CNT. 4
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Fig. 8. Polarization curves of membraneless EBCs using GOx/[FeCo-OMPC/ CNT] and FeCo-OMPC as the anodic and cathodic catalysts. In the tests, 30 mM glucose solution was provided and it was circulated from an external bottle to the anode chamber of EBC under air state.
the calculation, the glucose sensitivity was 27.5 μA cm−2 mM−1 with a linear shape in the normal glucose concentration range of normal human blood (2–10 mM). This is better than previously reported results. For instance, a GOx/ferrocene monocarboxylic acid/aminopropyl triethoxysilane enwrapping CNT catalyst result was 10.56 μA cm−2 mM−1 and a Nafion/GOx-Gold nanoparticle catalyst was at 6.5 μA cm−2 mM−1. This implies that this catalyst can be used for the dual use of bioanode and glucose sensor when it is implanted within the human body [39,40].
Fig. 6. Schematic illustration showing a) the catalytic structure of GOx/[FeCoOMPC/CNT] and b) the reaction sequence of the overall oxidation reactions using GOx/[FeCo-OMPC/CNT].
3.4. Polarization curve of membraneless EBC and long-term stability test With the catalytic performances of GOx/[FeCo-OMPC/CNT], the polarization curves of EBCs using the catalyst were measured to evaluate the effects of the FeCo-OMPC cathode and GOx/[FeCo-OMPC/ CNT] anode within actual EBC operation (Fig. 8). For doing that, an in house membraneless EBC kit was used, and all the tests were conducted under physiological conditions to mimic human bodily fluids. As shown in Fig. 8, the OCV and maximum power density (MPD) of EBCs using the catalysts were 0.17 ± 0.016 V and 21.3 ± 2.97 μW cm−2. As already expected from Fig. 7, the OCV was very close to the expected value by CV of 0.19 V, meaning that ORR and GOR -HPOR were well catalyzed by each electrode even when in actual EBC operational conditions. In addition, the OCV and MPD of this EBC were quite better than those using the Fe–N/CNT catalyst on each side (0.11 ± 0.006 V and 9.58 ± 0.69 μW cm−2), confirming that the FeCo-OMPC based catalyst showed better performance than the previously reported Fe–N/ CNT catalyst which is a competitor of the FeCo-OMPC catalyst. Finally, to evaluate the stability of the GOx/[FeCo-OMPC/CNT] bioanode, the amperometric response of the GOR was measured under continuous operational conditions (Fig. 9). As shown in Fig. 9, the response was stably preserved for 30 min under the injection condition of 10 mM glucose, indicating that the GOx molecules on CNT/OMPC were tightly immobilized and the stability of the prepared bioanode was quite good.
Fig. 7. CV curves of GOx/[FeCo-OMPC/CNT] measured in pH 7.4 with the supply of glucose. For the CV tests, 0.01 M PBS (pH 7.4) was used as the electrolyte and the potential scan rate was 20 mV s−1.
Regarding the current density, with the injection of 4, 8, 30, and 100 mM glucose, it was measured as 147.2, 206.7, 330.3, and 412.6 μA cm−2, respectively, at 0.3 V, and 117.3, 230.4, 484.0, and 718.1 μA cm−2, respectively, at 0.5 V. The current densities measured at 0.3 V were not steeply increased compared to those measured at 0.5 V and this is due to the unique attribute of HPOR A as shown in Fig. 3, meaning that the two-step reaction (GOR and HPOR) was well incorporated by GOx/[FeCo-OMPC/CNT]. In addition, with proper regression, it is expected that the local concentration of H2O2 generated by GOx molecules of the GOx/[FeCo-OMPC/CNT] will be 1.02–1.56 mM in the glucose concentration range of human blood (4–8 mM), and this can produce an anodic current of 180 μA cm−2 when the EBC using this catalyst is implanted inside human body. The glucose sensitivity of GOx/[FeCo-OMPC/CNT] was also calculated to examine its viability as a glucose sensor [37,38]. According to
4. Conclusions A FeCo-OMPC based structure was used as the catalysts for both electrodes of a membraneless EBC and their performance was compared to those measured from the Fe–N/CNT based structure. Natural FeCo was used in catalyzing ORR, while GOx/[FeCo-OMPC/CNT] was used as the catalyst for the GOR-HPOR combination. The FeCo-OMPC cathode showed an ORR onset potential of 0.31 V which is a more positively shifted value than that of Fe–N/CNT due to their unique 5
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Fig. 9. The amperometric response of the continuous operational system of GOx/[FeCo-OMPC/CNT] measured at 0.3 V vs. Ag/AgCl with the injection of 10 mM glucose.
attributes when working in a slightly alkaline pH, while the HPOR onset potential of the FeCo-OMPC/CNT anode was 0.12 V and this negatively shifted value is due to the nature of the Co core. In addition, the current density increased about 10 times due to its high metal content. The GOx/[FeCo-OMPC/CNT] showed the same onset potential as FeCoOMPC/CNT, indicating that the GOR by GOx, and the HPOR by FeCoOMPC/CNT, were sequentially conducted well. With that, it was verified that the GOx/[FeCo-OMPC/CNT] bioanode could be used within EBC. The H2O2 produced by GOx was 1.02–1.56 mM when a similar glucose concentration to human blood was used, and it is expected that when the EBC using this catalyst is implanted into the human body, a current density of 180 μA cm−2 would be produced. Regarding the performance of the membraneless EBC, its OCV and MPD were 0.17 ± 0.016 V and 21.3 ± 2.97 μW cm−2, which are quite improved values compared to those of EBCs using Fe–N/CNT (0.11 ± 0.006 V and 9.58 ± 0.69 μW cm−2). Moreover, the OCV was very close to the expected value by CV (0.19 V), confirming that the FeCo-OMPC catalyst can be used as the power source for implantable bioelectronics such as biosensors and electroceuticals. CRediT authorship contribution statement Jungyeon Ji: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft. Jinwoo Woo: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft. Yongjin Chung: Writing - review & editing, Visualization, Supervision, Project administration. Sang Hoon Joo: Writing - review & editing, Visualization, Supervision, Project administration. Yongchai Kwon: Writing - review & editing, Visualization, Supervision, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF), Republic of Korea and the Ministry of Science and ICT (MSIT), Republic of Korea (No. 2016M1A2A2937143) of the Republic of Korea, and by the NRF and the Ministry of Education (MOE), Republic of Korea (No. 2019R1A2C1005776) of the Republic of Korea. S.H.J. and J.W. were supported by the NRF and the MSIT (NRF6
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Full Length Article
Membraneless enzymatic biofuel cells using iron and cobalt co-doped ordered mesoporous porphyrinic carbon based catalyst ⁎
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Jungyeon Jia,1, Jinwoo Wooc,1, Yongjin Chungd, , Sang Hoon Jooc, , Yongchai Kwona,b,
T
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a
Graduate School of Energy and Environment, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 01811, Republic of Korea Department of Chemical and Biomolecular Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 01811, Republic of Korea c Department of Energy Engineering and School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea d Department of Chemical and Biological Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju, Chungbuk 27469, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal-doped ordered mesoporous porphyrinic carbons Membraneless enzymatic biofuel cell Hydrogen peroxide oxidation reaction Oxygen reduction reaction Physiological condition
Iron and cobalt co-doped ordered mesoporous porphyrinic carbon (FeCo-OMPC) is employed as the catalyst for both electrodes in membraneless enzymatic biofuel cells (EBCs). FeCo-OMPC is used for catalyzing oxygen redox reaction (ORR), while GOx/[FeCo-OMPC/CNT] is utilized as a catalyst for a series of glucose oxidation reaction (GOR) and hydrogen peroxide oxidation reaction (HPOR). The onset potential of the ORR by FeCo-OMPC is 0.31 V vs. Ag/AgCl, while that of HPOR by FeCo-OMPC/CNT and GOR-HPOR by GOx/[FeCo-OMPC/CNT] are both 0.12 V. The activity of this catalyst is better than previously reported similar catalysts due to the presence of Co species and high metal contents. As a result, the concentration of H2O2 generated by GOR is 1.02–1.56 mM when the same glucose concentration as human blood is used. In addition, the EBC using GOx/[FeCo-OMPC/ CNT] and FeCO-OMPC shows the maximum power density of 21.3 ± 2.97 μW cm−2 with open circuit voltage (OCV) of 0.17 ± 0.016 V. These values are significantly higher than those of EBC using the competitive Fe–N/ CNT catalyst (0.11 V and 9.6 μW cm−2). Moreover, the OCV is close to the expected value by CV (0.19 V), confirming that the FeCo-OMPC catalyst can be used for implantable bioelectronics, such as biosensors and electroceuticals.
1. Introduction As the demand for wellness and health grow, interest in real time health care is rapidly increasing. Especially, the need for body implantable biosensors and electroceuticals are expected to increase explosively to prevent adult diseases, such as diabetes and hyperlipidemia, and to maintain good health condition [1–3]. Based on these demands, to date, a great deal of effort has been made to develop a solution for real time diagnosis and treatment by the use of body implantable bioelectronics [4–6]. However, their actual uses have been rarely reported, and unstable power supply is one of the main hurdles suppressing utilization. Although various attempts including secondary batteries have been suggested to overcome these issues, the use of such power sources requires periodic and continuous surgeries to replace the power supply system, giving rise to potential hazard, such as the permeation of toxic chemicals into the human body [5,7,8]. To address these problems, enzymatic biofuel cells (EBCs), which use (i) enzyme
based biomaterials as a catalyst, and (ii) glucose, hydrogen peroxide (H2O2) and oxygen as fuels, have been focused on as a device for selfgeneration of minute electricity within the human body because all of the EBC components are compatible with the human body and complicated and expensive components like a membrane are not necessary [5,9–12]. So far, researchers have attempted to develop various types of EBCs and most researchers use glucose oxidase (GOx) as the anodic biocatalyst and horse radish peroxidase (HRP) and multi copper oxidase (MCO), such as laccase and bilirubin oxidase, as the cathodic biocatalyst [13–20]. GOx can easily oxidize glucose in human blood through glucose oxidation reaction (GOR) of the flavin adenine dinucleotide (FAD), but for the redox reaction, a harmful mediator for the human body is utilized [16,21–23]. Moreover, toxic H2O2 can be produced during the redox reaction by oxygen dissolved in human blood [10,24]. Differently from GOx, MCO and HRP are known as relatively safe catalysts because when they are used, the use of a mediator is not
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Corresponding authors. Tel.: +82 438415229 (Y. Chung). Tel.: +82 438415229 (S.H. Joo). Tel.: +82 29076805 (Y. Kwon). E-mail addresses: [email protected] (Y. Chung), [email protected] (S.H. Joo), [email protected] (Y. Kwon). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apsusc.2020.145449 Received 2 October 2019; Received in revised form 14 January 2020; Accepted 17 January 2020 Available online 21 January 2020 0169-4332/ © 2020 Published by Elsevier B.V.
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2.3. Electrochemical characterizations and performance evaluations
mandatory and direct electron transfer (DET) is possible. However, since the MCO and HRP are too expensive and they have poor stability for use in EBCs, developing a new catalytic structure that has biocompatibility, a cheap price, and long term stability in the human body is needed [9,13,18,25]. To overcome the disadvantages of the conventional approaches, catalysts that mimic the structure of nature’s catalysts have been explored. For example, iron porphyrinic structures, such as hemin, was suggested to replace MCO and HRP [9,26]. According to their study, the iron porphyrinic materials, especially hemin, which is a core structure of HRP, induced a considerable magnitude of cathodic current in proper potential ranges by the hydrogen peroxide reduction reaction (HPRR) [9]. In addition, Ji et al. developed a catalytic structure that can catalyze both the HPRR and hydrogen peroxide oxidation reaction (HPOR), eliminating H2O2 from GOR of GOx [10]. In their approach, Fe–N/CNT, which consists of an iron porphyrinic structure doped onto the surface of carbon nanotubes (CNT), acted as both the bioanode and biocathode. In this paper, we introduce a new iron and cobalt co-doped ordered mesoporous porphyrinic carbon (FeCo-OMPC) structure as the catalyst for both electrodes of EBC. FeCo-OMPC was already introduced as the ORR catalyst, substituting the conventional platinum (Pt) in acidic media [27]. However, their use within physiological conditions and as catalyst for other chemical reactions have not been reported yet. For use in EBC, the natural form of FeCo-OMPC is used as the cathodic catalyst catalyzing ORR, while the GOx and FeCo-OMPC based composite is used as the anodic catalyst catalyzing HPOR. To evaluate its viability in EBC, the electrochemical performances of this new catalyst are measured and the results are compared with those using Fe–N/CNT, which is a vying catalyst with FeCo-OMPC, and, also, membraneless EBCs using this new catalyst are operated within physiological conditions.
Cyclic voltammetry (CV) was carried out by using a SP-240 potentiostat (Bio-Logic, USA). Each catalyst was loaded onto a GCE which was utilized as the working electrode, and Ag/AgCl (sat. in 3.0 M NaCl), and Pt wire were utilized as reference and counter electrode, respectively. N2, O2, and air were injected into the electrolyte solution to make specific atmosphere [15,28]. The FeCo-OMPC/CNT structure was optically confirmed by high resolution transmission electron microscopy (TEM) and high resolution scanning electron microscopy (SEM). SEM images were taken by a scanning electron microscope (S-4800, Hitachi High-Technologies) operating at 10 kV. TEM images were obtained using a transmission electron microscope (JEM-2100, JEOL) at an acceleration voltage of 200 kV. The membraneless EBC kit was fabricated to quantify the EBC performance by measuring the polarization curves. As for the fuels that were provided to the EBC kit, 30 mM glucose solution (air purge) was fed to the cell at a flow rate of 0.1 mL min−1. The preparation of electrode was previously introduced in Section 2.2. 3. Result and discussion 3.1. ORR activity of FeCo-OMPC catalyst in physiological condition It is crucial to investigate the catalytic activity of FeCo-OMPC for ORR within physiological conditions to evaluate the viability as the cathodic catalyst in EBC. For doing that, the CV curves of the FeCoOMPC loaded GCE were measured in pH 7.4 (0.01 M PBS) and O2 saturated conditions (Fig. 1). According to Fig. 1, the onset potential of FeCo-OMPC was 0.31 V (vs. Ag/AgCl), which is slightly higher than that of Fe–N/CNT(0.29 V) [10]. However, both catalysts showed the same current densities of 0.9 mA cm−2 at 0 V [10,29]. It is an intriguing result because it is known that FeCo-OMPC is useful in acidic media, while Fe–N/CNT can be mainly used in alkaline media. Namely, although Fe–N/CNT seems to be more suitable in physiological conditions (pH 7.4), FeCo-OMPC shows better electrochemical performances as an ORR catalyst within the same conditions [10,27,29]. This indicates that FeCo-OMPC can also be used for the ORR catalyst within physiological conditions. This is compatible with the previously reported results about the ORR activity of the two catalysts. According to the ORR activity measurements, the half wave potential of FeCo-OMPC was higher (0.845 V vs RHE) than that of Fe–N/CNT (0.79 V vs RHE),
2. Experimental section 2.1. Materials Glucose oxidase (GOx, Aspergillus Niger, type X-S, 100–250 units mg-1 solid), o-dianisidine, and D-(+)-glucose were purchased from Sigma-Aldrich, while all the related chemicals, such as sodium acetate (98.5%), ethyl alcohol (EtOH, 99.5%), hydrogen peroxide (30%), and formic acid (99%), were purchased from Samchun Pure Chemical. 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine iron(III) chloride (FeTMPPCl) was purchased from Porphyrin Systems and 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine cobalt(II) (CoTMPP) was purchased from Tokyo Chemical Industry. Multiwall carbon nanotubes (MWCNT, MR99) were purchased from Carbon Nano-material Technology and phosphate buffer solution (PBS, pH 7.4) came from Life Technologies.
2.2. Synthetic procedure of cathodic and anodic catalysts FeCo-OMPC was synthesized by a solid-state nanocasting method. The detailed synthetic procedure was previously introduced by the Joo group [27]. CNT/FeCo-OMPC was fabricated through the physical mixing of FeCo-OMPC and CNT. According to the detailed synthetic procedure, 10 mg of FeCo-OMPC was mixed with 10 mg of CNT in 5 mL of EtOH, and the solution was sonicated for 10 min and agitated for 2 h at room temperature. For preparing for GOx/[FeCo-OMPC/CNT] bioanode, 10 μL of FeCo-OMPC/CNT was dropped onto glassy carbon electrodes (GCE, diameter of 5 mm) and dried for 10 min at room temperature. After the process, 10 μL of GOx in 0.01 M PBS (20 mg mL−1) was loaded onto the FeCo-OMPC/CNT. After drying, 4 μL of Nafion solution (5 wt%) was further coated on the GOx/[FeCoOMPC/CNT] loaded GCE.
Fig. 1. Cyclic voltammograms (CVs) of FeCo-OMPC loaded GCE measured under pH 7.4 in both O2 (red solid line) and N2 (black dashed line) saturated states. For the CV tests, 0.01 M PBS (pH 7.4) was used as the electrolyte and the potential scan rate was 10 mV s−1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2
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Fig. 2. (a) SEM and (b) TEM images of the FeCo-OMPC/CNT composite.
even in acidic condition (0.1 M HClO4) [27,29]. Based on the above result, it can be demonstrated that the onset potential for ORR of FeCo-OMPC is better than that of Fe–N/CNT in physiological conditions, and thus, it is expected that the EBC using FeCo-OMPC catalysts can induce a better EBC performance due to a higher open circuit voltage (OCV), which is mainly attributed to the positively positioned onset potential of ORR.
Table 1 The generated current density (at 0.3 V) by HPOR of Fe–N/CNT and FeCoOMPC/CNT. The current density was calculated from the difference between the current density with H2O2 injection and without H2O2 injection. H2O2 concentration [mM]
3.2. HPOR activity of FeCo-OMPC catalyst within physiological conditions
0.5 1 2 3 4 5 10 15
For evaluating the viability of FeCo-OMFC as catalyst for the anode, FeCo-OMPC/CNT composite was fabricated as shown in Fig. 2 and its electrochemical performance for HPOR was evaluated by CV curve measurements. In this structure, CNT plays a role as the supporter in immobilizing many GOx molecules and as the electrical connector between FeCo-OMPC and the electrode, while FeCo-OMPC is adopted as the catalyst for activating the HPOR. Regarding the catalytic activity of FeCo-OMPC/CNT for the HPOR, Fig. 3a shows its CV curves, which are measured with the injection of different H2O2 concentrations (0–15 mM). As shown in Fig. 3a, a significant anodic current was observed at the onset potential of 0.12 V, which is a negatively shifted value when compared to that of Fe–N/CNT (0.15 V). This implies that a higher OCV is possible when used as the anodic catalyst for EBCs. Moreover, the current densities measured at the H2O2 concentrations of 1, 3, 5, and 10 mM were 0.240, 0.677, 0.962, and 1.830 mA cm−2, respectively, when the measured potential was as high as 0.6 V, and 0.156, 0.2286, 0.324, and 0.350 mA cm−2, respectively, when the measured potential was as low as 0.3 V. This means that, irrespective of the used potentials, the current density of FeCo-OMPC/CNT is about 10 times better than that of Fe–N/CNT (Table 1). There are two main reasons for the discrepancy in electrochemical
Current density at 0.3 V [mA cm−2]
Current density at 0.6 V [mA cm−2]
Fe–N/CNT [10]
FeCo-OMPC/ CNT
Fe–N/CNT [10]
FeCo-OMPC/ CNT
0 0.001 0.010 0.021 0.037 0.069 0.091 0.095
0.083 0.156 0.249 0.286 0.310 0.324 0.350 0.341
0.009 0.018 0.047 0.070 0.103 0.154 0.221 0.271
0.135 0.240 0.462 0.677 0.834 0.962 1.480 1.830
performance between the FeCo-OMPC/CNT and Fe–N/CNT catalysts. First, the Co element within FeCo-OMPC/CNT triggers the negative shift of the onset potential because the metal core within the Co porphyrinic structure lowers the HPOR potential more than that of a Fe based structure, such as Fe–N/CNT [10]. Namely, the onset potential for HPOR is dominated by the redox potential of Co2+/Co3+ which is known for lowering the onset potential of HPOR to 0.004 V in neutral pH [30–32]. Furthermore, according to some research results of H2O2 sensors using Co-phthalocyanine, the structure showed the onset potential of 0.1 V for HPOR. Based on that, it is proven that the Co core promotes the negative shift of the onset potential [32,33]. Second, the larger metal core contents within FeCo-OMPC/CNT can induce a higher current density. The amount of metal atoms contained in Fe–N/CNT and FeCo-OMPC were calculated to be 2.9 and 4.8 wt%, respectively,
Fig. 3. CV curves of FeCo-OMPC/CNT measured with the injection of 0, 2, 5, and 10 mM H2O2. For CV tests, 0.01 M PBS (pH 7.4) was used as the electrolyte and the potential scan rate was 20 mV s−1. 3
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(0.38–0.6 V, HPOR B) where current density continuously increases up to 15 mM. The occurrence of HPOR A is due to the participation of OH− ions in the HPOR reaction (H2O2 + OH− → H2O + O2 + H+ + 2e−), while HPOR B occurs without the use of OH− ions (H2O2 → O2 + 2H+ + 2e−). This HPOR mechanism explanation was also similarly reported by Ji et al. [10,34–36]. Meanwhile, the pH effect on HPOR A is investigated (Fig. 4). As shown in Fig. 4, there are no HPOR A peaks in the pH 3 condition, while significant HPOR A peaks were produced in the pH 7.4 and 10 conditions. By (i) the participation of OH− ions in the HPOR reaction and (ii) the pH effect of the HPOR reaction, the presence of two different HPOR mechanisms is confirmed. Furthermore, it is substantiated that the HPOR A mechanism, which contributes to gaining a high OCV and power density in EBCs, can occur even in physiological conditions under a condition of 5 mM H2O2. 3.3. GOR activity of GOx/[FeCo-OMPC/CNT] catalyst within physiological conditions It is crucial to evaluate the compatibility between GOx and FeCoOMPC/CNT to predict the performance and long term stability of the bioanode (GOx/[FeCo-OMPC/CNT]). To do that, the amount of GOx immobilized on GOx/[FeCo-OMPC/CNT] was measured by UV–Vis spectrophotometry (Fig. 5). By the calibration of the absorbance peak near 400 nm, the H2O2 concentration produced from GOR catalyzed by free GOx molecules is calculated [9]. According to the calculation, the amount of immobilized GOx was calculated to be 69.8 Unit mg−1 which is higher than that of GOx/CNT (62.1 Unit mg−1). This demonstrates that the GOx molecules are tightly immobilized onto the FeCo-OMPC/CNT substance, and FeCo-OMPC does not prevent the immobilization of GOx molecules. Next, CV curves of the GOx/[FeCo-OMPC/CNT] bioanode are measured in pH 7.4 and air saturated conditions to estimate its catalytic activity for GOR. In this structure, the upper GOx layer produces H2O2 through the GOR catalyzed by FAD within GOx, then the H2O2 further decomposes into H2O, producing the anodic current by Fe and Co cores within the FeCo-OMPC/CNT under layer. Its schematic illustration and reaction pathway are represented in Fig. 6. Regarding their CV curves (Fig. 7), the anodic current density was observed after 0.12 V and increased with the injection of glucose. The onset potential was the same with that of the HPOR of FeCo-OMPC/CNT (Fig. 3). This demonstrates that the GOR is well conducted in the upper layer consisting of GOx molecules and the combination of GOR and HPOR promotes the anodic current by the GOx/[FeCo-OMPC/CNT] bioanode. Based on the onset potential of the GOx/[FeCo-OMPC/CNT] bioanode, the expected OCV of the EBC is 0.19 V.
Fig. 4. CV curves of HPOR catalyzed by FeCo-OMPC/CNT in (a) acidic (pH 3), (b) physiological (pH 7.4), and (c) alkaline (pH 10) states.
indicating that FeCo-OMPC/CNT has the possibility of having more active site to catalyze HPOR [27,29]. In summary, FeCo-OMPC/CNT is appropriate as the enzymatic catalyst for the anode due to its lower onset potential and higher current density for HPOR, which are ascribed to the existence of the Co core and higher metal core content. Next, the HPOR reaction mechanism was investigated, as shown in Fig. 3b. The two different mechanisms can be induced according to the electrochemical behavior observed with the injection of different H2O2 concentrations. The first mechanism (red region) appears in a lower potential range (0.12–0.38 V, HPOR A) where current density is saturated in a lower H2O2 concentration range (~5mM), and the second mechanism (blue region) appears in a higher potential range
Fig. 5. Enzyme activity measurements using UV–Vis spectrophotometry representing the amount of GOx immobilized on CNT and FeCo-OMPC/CNT. 4
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Fig. 8. Polarization curves of membraneless EBCs using GOx/[FeCo-OMPC/ CNT] and FeCo-OMPC as the anodic and cathodic catalysts. In the tests, 30 mM glucose solution was provided and it was circulated from an external bottle to the anode chamber of EBC under air state.
the calculation, the glucose sensitivity was 27.5 μA cm−2 mM−1 with a linear shape in the normal glucose concentration range of normal human blood (2–10 mM). This is better than previously reported results. For instance, a GOx/ferrocene monocarboxylic acid/aminopropyl triethoxysilane enwrapping CNT catalyst result was 10.56 μA cm−2 mM−1 and a Nafion/GOx-Gold nanoparticle catalyst was at 6.5 μA cm−2 mM−1. This implies that this catalyst can be used for the dual use of bioanode and glucose sensor when it is implanted within the human body [39,40].
Fig. 6. Schematic illustration showing a) the catalytic structure of GOx/[FeCoOMPC/CNT] and b) the reaction sequence of the overall oxidation reactions using GOx/[FeCo-OMPC/CNT].
3.4. Polarization curve of membraneless EBC and long-term stability test With the catalytic performances of GOx/[FeCo-OMPC/CNT], the polarization curves of EBCs using the catalyst were measured to evaluate the effects of the FeCo-OMPC cathode and GOx/[FeCo-OMPC/ CNT] anode within actual EBC operation (Fig. 8). For doing that, an in house membraneless EBC kit was used, and all the tests were conducted under physiological conditions to mimic human bodily fluids. As shown in Fig. 8, the OCV and maximum power density (MPD) of EBCs using the catalysts were 0.17 ± 0.016 V and 21.3 ± 2.97 μW cm−2. As already expected from Fig. 7, the OCV was very close to the expected value by CV of 0.19 V, meaning that ORR and GOR -HPOR were well catalyzed by each electrode even when in actual EBC operational conditions. In addition, the OCV and MPD of this EBC were quite better than those using the Fe–N/CNT catalyst on each side (0.11 ± 0.006 V and 9.58 ± 0.69 μW cm−2), confirming that the FeCo-OMPC based catalyst showed better performance than the previously reported Fe–N/ CNT catalyst which is a competitor of the FeCo-OMPC catalyst. Finally, to evaluate the stability of the GOx/[FeCo-OMPC/CNT] bioanode, the amperometric response of the GOR was measured under continuous operational conditions (Fig. 9). As shown in Fig. 9, the response was stably preserved for 30 min under the injection condition of 10 mM glucose, indicating that the GOx molecules on CNT/OMPC were tightly immobilized and the stability of the prepared bioanode was quite good.
Fig. 7. CV curves of GOx/[FeCo-OMPC/CNT] measured in pH 7.4 with the supply of glucose. For the CV tests, 0.01 M PBS (pH 7.4) was used as the electrolyte and the potential scan rate was 20 mV s−1.
Regarding the current density, with the injection of 4, 8, 30, and 100 mM glucose, it was measured as 147.2, 206.7, 330.3, and 412.6 μA cm−2, respectively, at 0.3 V, and 117.3, 230.4, 484.0, and 718.1 μA cm−2, respectively, at 0.5 V. The current densities measured at 0.3 V were not steeply increased compared to those measured at 0.5 V and this is due to the unique attribute of HPOR A as shown in Fig. 3, meaning that the two-step reaction (GOR and HPOR) was well incorporated by GOx/[FeCo-OMPC/CNT]. In addition, with proper regression, it is expected that the local concentration of H2O2 generated by GOx molecules of the GOx/[FeCo-OMPC/CNT] will be 1.02–1.56 mM in the glucose concentration range of human blood (4–8 mM), and this can produce an anodic current of 180 μA cm−2 when the EBC using this catalyst is implanted inside human body. The glucose sensitivity of GOx/[FeCo-OMPC/CNT] was also calculated to examine its viability as a glucose sensor [37,38]. According to
4. Conclusions A FeCo-OMPC based structure was used as the catalysts for both electrodes of a membraneless EBC and their performance was compared to those measured from the Fe–N/CNT based structure. Natural FeCo was used in catalyzing ORR, while GOx/[FeCo-OMPC/CNT] was used as the catalyst for the GOR-HPOR combination. The FeCo-OMPC cathode showed an ORR onset potential of 0.31 V which is a more positively shifted value than that of Fe–N/CNT due to their unique 5
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Fig. 9. The amperometric response of the continuous operational system of GOx/[FeCo-OMPC/CNT] measured at 0.3 V vs. Ag/AgCl with the injection of 10 mM glucose.
attributes when working in a slightly alkaline pH, while the HPOR onset potential of the FeCo-OMPC/CNT anode was 0.12 V and this negatively shifted value is due to the nature of the Co core. In addition, the current density increased about 10 times due to its high metal content. The GOx/[FeCo-OMPC/CNT] showed the same onset potential as FeCoOMPC/CNT, indicating that the GOR by GOx, and the HPOR by FeCoOMPC/CNT, were sequentially conducted well. With that, it was verified that the GOx/[FeCo-OMPC/CNT] bioanode could be used within EBC. The H2O2 produced by GOx was 1.02–1.56 mM when a similar glucose concentration to human blood was used, and it is expected that when the EBC using this catalyst is implanted into the human body, a current density of 180 μA cm−2 would be produced. Regarding the performance of the membraneless EBC, its OCV and MPD were 0.17 ± 0.016 V and 21.3 ± 2.97 μW cm−2, which are quite improved values compared to those of EBCs using Fe–N/CNT (0.11 ± 0.006 V and 9.58 ± 0.69 μW cm−2). Moreover, the OCV was very close to the expected value by CV (0.19 V), confirming that the FeCo-OMPC catalyst can be used as the power source for implantable bioelectronics such as biosensors and electroceuticals. CRediT authorship contribution statement Jungyeon Ji: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft. Jinwoo Woo: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft. Yongjin Chung: Writing - review & editing, Visualization, Supervision, Project administration. Sang Hoon Joo: Writing - review & editing, Visualization, Supervision, Project administration. Yongchai Kwon: Writing - review & editing, Visualization, Supervision, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF), Republic of Korea and the Ministry of Science and ICT (MSIT), Republic of Korea (No. 2016M1A2A2937143) of the Republic of Korea, and by the NRF and the Ministry of Education (MOE), Republic of Korea (No. 2019R1A2C1005776) of the Republic of Korea. S.H.J. and J.W. were supported by the NRF and the MSIT (NRF6
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