Rational Design of Cu@Cu2O Nanospheres Anchored B, N Co-doped

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Rational design of Cu@Cu2O nanospheres anchored B, N codoped mesoporous carbon: A sustainable electrocatalyst to assay eminent neurotransmitters Acetylcholine and Dopamine. Paramasivam Balasubramanian, TST Balamurugan, ShenMing Chen, Tse-Wei Chen, and Tamilarasan Sathesh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04473 • Publication Date (Web): 29 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Sustainable Chemistry & Engineering

Rational design of Cu@Cu2O nanospheres anchored B, N co-doped mesoporous carbon: A sustainable electrocatalyst to assay eminent neurotransmitters Acetylcholine and Dopamine. Paramasivam Balasubramanian1, T.S.T. Balamurugan1,2, Shen-Ming Chen1*, Tse-Wei Chen1,3, Tamilarasan Sathesh4 1Department

of Chemical Engineering and Biotechnology, National Taipei University of

Technology, Taipei 106, Taiwan, ROC 2Institute

of Biochemical and Biomedical Engineering, National Taipei University of Technology,

No.1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan. Research and Development Center for Smart Textile Technology, National Taipei University of

3

Technology, No.1, Section 3, Zhongxiao East Road, Taipei 106, Taiwan (ROC). 4Department

of Energy and Refrigerating Air-conditioning engineering, National Taipei

University of Technology, Taipei 106, Taiwan, ROC. Corresponding Author: *Shen-Ming Chen, Fax: +886227025238, Tel: +886227017147, E-mail: [email protected]

ABSRACT Exploring rapid, highly sensitive, cost-effective assay platforms to diagnose neurotransmitters is crucial in clinical biology. We propose a Cu@Cu2O nanospheres embedded B, and N co-doped mesoporous carbon (BNDC) nano electrocatalyst to assay Dopamine (DA) and Acetylcholine (ACh). The Cu@Cu2O-BNDC catalyst have been prepared through a single step polymerization, followed by a carbonization. TEM results revealed that Cu@Cu2O nanoparticles appeared as nanospheres (size= 30±5) entrapped on the mesoporous BNDC. Further, the catalyst possesses a specific surface area of 1025 m2 g-1 along with a pore-size of 4 nm offers enormous active surface area for electrochemical sensing applications. The Cu@Cu2O-BNDC catalyst was employed in electrochemical sensing of DA, and ACh in a working range of 0.004 to 542 µM, and 0.3 to 2602 µM, and detection limits of 0.5 nM and 17 nM, respectively. The practicality of the developed sensor has been assessed via DA (pH 7), and ACh (0.1 M NaOH) spiked in human blood serum samples and obtained satisfactory recovery. On top of this, the proposed synthetic protocol of the catalyst can be a versatile route to achieve heteroatom doped carbon nanomaterials owing an enormous surface area and desirable morphology. 1 ACS Paragon Plus Environment

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Keywords; BNDC, heteroatom doped carbon, large surface area, nanospheres, electrocatalyst, sensor. INTRODUCTION Neurotransmitters; a group of endogenous substrates which transmit biochemical signal among neurons, and alternate cells. Dopamine (DA), and acetylcholine (ACh) are eminent central nervous system (CNS) neurotransmitters, take a part in numerous cerebral functions such as conduct, and reasoning.1,2 Disproportion of DA, and ACh in peripheral (PNS), and central (CNS) nervous system is allied with neurological, and physiological illnesses including Parkinson's, Alzheimer's, schizophrenia, depression, and anxiety.3,4 Precise, and periodical diagnoses of DA and ACh in physiological fluids is essential in the treatment of neural disorders. Thus, developing an economical, selective, high sensitive, and rapid analytical tools to determine DA, and ACh in bio samples are indeed of clinical, and biomedical analysis. In tradition a range of analytical tools have been proposed for sensing, and determination of DA, and ACh.1,2 Out of those methods, electrochemical strategies received greater interest owing the benefits of easy operative, rapid response, high sensitivity, selectivity, and, cost-effective.5 In this prospective, acetylcholinesterase immobilized biosensors have been proposed as electrochemical tool to assay ACh in real time.6 Even though, the enzyme immobilized biosensors offer high sensitivity, and selectivity, such assay platforms suffer in terms of high-cost, complex enzyme immobilization procedures, thermal and chemical instability.7,8 On the way to overcome the aforementioned obstacles, immense work is put forward to fabricate an enzymeless biosensors built on nano electrocatalyst matrices. The performance of electrocatalyst adapted biosensors depends upon the electrocatalytic ability of electrode matrix in terms of selectivity, sensitivity, and preventing electrode fouling by absorbed chemical species.9 Our goal is to synthesis a profitable nano-electrocatalyst with superior electrocatalytic activity (high sensitivity, and lower detection limit) accompanied with chemical stability, and reliability to achieve convincing results in clinical test conditions. It has been found, that most of the proposed electrocatalytic sensors have determine either DA or ACh. The attempts to detect DA, and ACh using single electrocatalytic material is not yet succeeded. For that reason, in this study we aimed to determine DA, and ACh using an efficient electrode material under different test conditions.

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Heading ahead to the design of effectual electrocatalyst, the emerging heteroatom (B,N,S,P etc.) doped carbon (HADC) nanomaterials have drained considerable attention form nano and electrochemical communities in a short tenure. The progressing interest on HADC in nanotechnology can be rationalized to their excellent electrical conductivity, tunable molecular structures and compositions,

high earth abundance, and tolerance to acidic/alkaline

environments.10,11 This superior physiochemical properties of HADC can be attributed by the replacement of carbon atom by heteroatoms (dopants) from sp2 lattice of graphitic carbon.12 An altered electronic composition of HADC nanomaterials tailor their electronic properties, which create abundant favorable active sites for the electrochemical reactions.13 In the group of, heteroatom doped materials boron, (B) and nitrogen (N) doped carbons are expected to be promising members in electrocatalytic applications.14 The bonding configuration between electron deficient B, and electron rich N in the hybrid, can conjugate into the carbon systems to alter the electrical, and physiochemical assets of carbon materials.15,16 Thus, the B, N co-doped carbon can possess more reactive sites with subsequent increment in the catalytic ability to the nanocatalyst. Successively, transition metal nanoparticles and/or metal oxides incorporated B/N codoped carbon-nanocatalysts has been developed for the various electrochemical applications in the past. For instance, Chun Cao et al., demonstrated a Fe-Fe3O4 embedded B, N co-doped carbon with enhanced ORR activity.17 Qi Liu et al., proposed Pd nanoparticles loaded B/N co-doped graphene nanoribbons to catalyze ethanol oxidation.16 Jiming Lu et al., have prepared PtRu decorated B/N co-doped graphene, as an effective catalyst for methanol oxidation.18 Sen Lin et al., studied the introduction of copper into BN nanosheets via DFT analysis; the results unveil that incorporation of Cu greatly enhance the electrochemical reactivity of BN nanosheets.19 On account of aforementioned results, it is evident that the addition of certain transition metal nanoparticles and/or metal oxides to B/N co-doped carbon is an effective way to synthesis nanocatalysts with improved electrochemical performance. In this regard, copper (Cu) was chosen as transition metal catalyst , in courtesy of its high earth abundance, and low-cost.20 However, there are only a few reports on the preparation and application of copper supported B, N co-doped carbon materials.21 Copper oxide (Cu0, Cu1+, Cu2+, and Cu3+)-derived nano substrates owing broad oxidation states were utilized as electrocatalysts; which offer variable catalytic performance in correlation with the chemical composition and dimension of the composite.22-24 The practical applicability of copper oxide (CuO, and Cu2O) nano substrate electrocatalyst is limited by the fact of its high 3 ACS Paragon Plus Environment


overpotential, and low current densities.25 Altered techniques were employed to minimize the overpotential and enhance the electrical conductivity of Cu–Cu2O hybrid foams.26,27 More recently, polymer derived heteroatom doped carbons are of great research interest, owing to their homogenous doping and identical heteroatom content of doped carbon.10,17,28,29 This work presents a versatile synthetic platform to prepare Cu@Cu2O nanospheres encapsulated B/N co-doped carbon (BNDC) mesoporous nanocatalyst owning large surface area. The two step synthesis involves a polymerization followed by a simple carbonization process. The synthesis begins with, the injection of freshly prepared copper nitrate solution into polymeric precursor of polyaniline:boric acid (Pani:BA).30 During the polymerization, Cu (II) could interact with Pani:BA and interlocked in the polymeric network of the Pani:BA via electrostatic interaction to form a copper (II)-Pani:BA framework. Later, the copper (II)-Pani:BA framework was carbonized at 900 °C under a mild current of N2, which transforms the Cu (II)-Pani:BA framework into B, N doped mesoporous carbon with simultaneous embedding of Cu@Cu2O inside the carbon matrix. The synthesized Cu@Cu2O nanosphere embedded B/N co-doped mesoporous carbon (Cu@Cu2O-BNDC) exhibited enhanced electrocatalytic activity towards the dopamine and Acetylcholine; an effective electrochemical assay was fabricated and demonstrated to explore the real time utility of the catalyst. EXPERIMENTAL Synthesis of Cu@Cu2O nanospheres decorated B/N co-doped mesoporous carbon The Cu@Cu2O nanosphere embedded B/N co-doped mesoporous carbon composite was synthesized through a single step polymerization followed by effective carbonization. In a typical synthesis, an ice cold solution of 50 mM aniline in 1 M boric acid was coupled with an ice-cooled (0–5 ◦C) equimolar solution of APS in 1 M boric acid at 0 ◦C with constant siring over 20 min. To this, a freshly prepared solution of 50 mM Cu(NO3)2 was added in drops and continued to stir over night. The polymerization was carried out in N2 ambiance over 12 h with constant stirring at ice cold temperature. After completion solvent was evaporated in vacuum. The obtained solids were rinsed with deionized water and dried in a vacuum oven at 80 ◦C. The resultant sample was carbonized at 900 ◦C over 2 h at a ramping rate of 5 ◦C/Min under N2 to yield Cu@Cu2O-BNDC. A control catalyst sample of B, N co-doped carbon (BNDC) was prepared following the same


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route excluding Cu(NO3)2. A schematic pathway to obtain Cu@Cu2O nanospheres encapsulated B, N co-doped mesoporous carbon was illustrated in Scheme 1. RESULTS AND DISCUSSIONS Morphological analysis of Cu@Cu2O-BNDC Surface morphology, shape, and dimension of the nano catalyst was visualized through SEM, and TEM spectroscopic analysis. The displayed TEM portrait (Figure 1A) of BNDC carbon matrix; clearly displays a thin sheet-like structure. The SEM illustration of Cu@Cu2O-BNDC showcased a foam-like carbon sheets uniformly embedded with a large number of nanosphere (Figure S1). TEM images of Cu@Cu2O-BNDC shown in Figure1B-D. It can be seen that spherelike Cu@Cu2O nanoparticles in a median diameter of 30 ±5 nm was homogeneously spread throughout the B/N co-doped mesoporous carbon sheets (Figure 1C); while, the amorphous carbon shows an obvious mesoporous sheet-like structure. The high-resolution TEM portrait of the composite is displayed in Figure 1E; the lattice bounds of the nanoparticles were measured with a d-spacing of 0.21 and 0.26 nm, corresponds to (111) plane of Cu,31 and (111) plane of Cu2O, indicating that the copper nanoparticle formed along with the Cu2O, which agrees with XRD pattern. Notably, the HRTEM image of the composite manifest that the larger amount of the nanoparticles is enwrapped by graphitic carbon having a d-spacing of 0.33 nm (002), indicate a close interfacial connection among the nanoparticles and graphitic carbon. The innermost connection between the Cu@Cu2O and BNDC can effectively improve the electrocatalytic ability of the nanocatalyst.32 To clarify the elemental components of the composite, EDX analysis was carried out, Figure1F shows clear elemental signals for B, N, C, Cu, and O demonstrating that the obtained catalyst mainly comprised of B, N, C, Cu, and O. Following the surface analysis, X-ray diffraction was adapted to study the crystallinity of Cu@Cu2O- BNDC catalyst. A typical diffraction patterns are shown in Figure 2A; the BNDC (a) exhibited two intense diffraction peaks of 2θ at around 25° (002) and 43° (101), which confirms the formation of B/N co-doped graphitic carbon.33 On the other hand, typical diffraction pattern of Cu@Cu2O-BNDC (b) showed peaks ≈25° (002) and 43° (10) indicating the prevalence of graphitic carbon. The remaining intense peaks can be precisely allotted to the crystal planes of Cu nanoparticles (JCPDS 85-1326),34 and cubic Cu2O (JCPDS 05-0667).25 The X-ray diffraction studies give a clear idea about the formation of the Cu@Cu2O-BNDC nano composite. High 5 ACS Paragon Plus Environment


temperature carbonization of the polymerization product is the key step of the synthesis; where insitu materialization of B, N doped carbon, and reduction of copper ions into copper nanoparticles, and partially reduced Cu2O with a strong encapsulation into the carbon background occurs instantaneously. The XRD observations, and proposed mechanism for the formation of the composite are in agreement with results of earlier morphological analysis, and previous reports.17 Raman spectra (Figure 2B) of Cu@Cu2O-BNDC (a), and BNDC (b) showed two typical peaks situated at 1355, and 1590 cm-1 corresponding to the D and G band of carbon, respectively.35 The D band exposes the gradation of anarchy, whereas the G band symbolizes the graphitization degree of a carbon material. Hence, the ID/IG which is derived from the integrated area of individual band can be employed to estimate the degree graphitic nature of carbon materials.32 The ID/IG band intensity ratio value of Cu@Cu2O-BNDC (1.17) is greater than that of BNDC (1.02), implies that the encapsulation of Cu@Cu2O into the carbon matrices of BNDC possesses more defect sites than that of pure BNDC. Subsequent to the morphological studies, the catalyst was subjected to surface area analysis. Surface area is one of the key factor that directly influence electrocatalytic activity of the catalyst, subsequently a greater surface area can offer extra interface reaction sites for electrochemical reactions. Additionally, a huge surface can formulate rapid transmission of ions, added with a mesoporous structure boosts the electrocatalytic behavior of the catalyst. Thus, the porous texture of the nanocatalyst was inspected through nitrogen (N2) adsorption-desorption experiments and given in Figure 2C&D. The specific surface area of BNDC, and Cu@Cu2OBNDC composites are 859 and 1025 m2 g-1, respectively, while the mean cavity dimensions obtained from the pore dimension distribution plots for the samples were found to be 6 and 4 nm respectively, (BJH method). The adsorption-desorption studies validate the mesoporous structure of the BNDC and Cu@Cu2O-BNDC composites; further, it attests the embedding of Cu@Cu2O into BNDC matrix enhance the superficial area and minimizing the void size of the composite. In summary, an elevated specific superficial area, and smaller cavity size of the Cu@Cu2O-BNDC composite can endow outstanding electrocatalytic performance. In sequence to the morphological and surface area analysis, the Cu@Cu2O-BNDC composite was subjected to X-ray photoelectron spectroscopy (XPS) in order to understand the elemental constitution along with the surface properties and structure of the catalyst. The whole 6 ACS Paragon Plus Environment

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XPS survey spectra of the composite (Figure 3A), displays the elemental signals for B, N and Cu; which confirm the existence of these elements in the composite, and the spectral data are in complete harmony with the outcomes of EDX analysis discussed earlier. The high resolution B 1s spectrum (Figure 3B) exhibited three peaks after deconvolution, 189.8, 191.3, and 192 eV, corresponds to B-N, B-C, and B-O bonding configuration of boron.36 Following, the Figure 3C shows a refined C 1s spectra, the peaks pinpointed at 284.9, 285.8, 286.5, and 289.2 eV, can ascribed to the sp2C-C, C-O/C-N/C-O-B, C=O, and O-C=O bonding, respectively.37 Further, the resolved N 1s spectra in Figure 3D is tallied with three peaks at 399.1, 400.3, 401.5, and 402.9 eV which are assigned to B-N, pyridinic-N, graphitic-N, and pyridinic oxide.36,38,39 The electrochemical performance of a nanocatalyst such reaction rate, and electrical conductivity can be enhanced by the presence of pyridinic-N, and Graphitic-N.40 Interestingly, the synthesized Cu@Cu2O-BNDC conjointly own pyridinic-N, and Graphitic-N which can offer an elevated catalytic efficiency to the catalyst. Moreover, when N, and B atoms are doped into carbon simultaneously polarization of both N, and B atoms will be enhanced remarkably; if the B, and N atoms are adjutant to each other to form special B-N structure, the enhanced polarization degree can upsurge the electrocatalytic ability of the resultant composite.17 On the other hand, the refined XPS spectra of Cu 2p showed (Figure 3E) signature peaks at 934.5, and 954.0 eV which indorsed to the binding energies of Cu 2p3/2 and Cu 2p1/2, correspondingly. The wide Cu 2p3/2 peak has been resolved into a dual peak designated at 933.5 and 936.1 eV and ascribed to Cu2O/Cu (Cu1+/Cu0), and (Cu2+), respectively.41 However, it is hard to distinguish Cu2O (Cu1+) and Cu0 via XPS attribute of Cu 2p3/2 as the binding energies of Cu and Cu2O are close to one another and differs by merely about 0.1–0.2 eV. Though, we can discriminate Cu2O/Cu (Cu1+/Cu0) by observing the position of their LMM auger transition of Cu at 568 eV, and Cu2O at 571 eV in the XPS spectrum.42,43 This result denotes the co-existence of Cu, and Cu2O in the composite, and the observations are in alliance with the XRD results of the composite described earlier. The O 1s spectra (Figure 3F) resolved into a triplet peak at 531.2 (O1), 532.6 (O2), and 533.4 eV (O3), and allocated to C=O, O-C=O, and C-O groups, which are coherent with that of C 1s spectra.44 Electrochemical characterization Electrochemical impedance spectroscopy (EIS)



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To explore the electrochemical functioning of the catalyst, a modified electrode was studied via EIS measurements. EIS is an effectual, non-destructive tool to investigate the electrochemical assets of a catalyst film coated electrode and examine the related interfacial properties of electrode material; and thus, letting us to understand the chemical changes associated with the electrode surface. Electron transfer kinetics of the catalyst film tailored electrode was studied through EIS spectroscopy. Randles circuit (inset: Figure 4A) was used to fit the impedance data. The diameter of the semicircle in the obtained EIS spectra is equivalent to the value of charge transfer resistance (Rct). Figure 4A shows the attained Nyquist plots (-Z’’ vs Z’) of unmodified GCE (a), BNDC/GCE (b), and Cu@Cu2O-BNDC/GCE (c) in open-circuit condition at 0.1 M KCl comprising 5 mM Fe(CN)63-/4- (1:1). From EIS spectra, a semi-circle was observed at high frequency region, and diameter of the semicircle is corresponding to the charge-transfer resistance through the electrode interface. The Rct values were acquired to be 70 Ω for unmodified GCE (a), 41 Ω for the BNDC/GCE (b) and 9.8 Ω for the Cu@Cu2O-BNDC/GCE (c). From the result, a minimized Rct were obtained at Cu@Cu2O-BNDC/GCE, indicate a prompt electron transfer kinetics over the surface of Cu@Cu2O-BNDC/GCE electrode, and it can be beneficial to electrochemical sensing applications. In addition, the heterogeneous electron transfer rate constant (ks) for various modified electrodes were estimated using equation (1),45 Rct =RT/n2F2ACks

(1)

Where, R, T, n, F, A, and C representing the gas constant (8.314 J mol−1K−1), temperature (298 K), number of electrons involved, Faraday constant (96,485 C/mol), geometric area of the electrode (cm2), and the concentration of [Fe(CN)6]3−/4− in electrolyte, correspondingly. The calculated ks value of unmodified GCE (a), BNDC/GCE (b), and Cu@Cu2O-BNDC/GCE (c) is about 1. 06×10-5, 2.85×10-5, and 7.76×10-5 cm s−1, respectively. The higher heterogeneous electron rate constant (ks) of Cu@Cu2O-BNDC/GCE (c) than that of BNDC/GCE designate he electron transfer process is quicker in presence of Cu@Cu2O. Electrochemical behavior of 5 mM [Fe(CN)6]3−/4− in 0.1 M KCl was examined by CV at GCE (a), BNDC/GCE (b), and Cu@Cu2O-BNDC/GCE (c) (Figure S2A). As can be seen in Figure S2A, the Cu@Cu2O-BNDC/GCE (Ipa= 140 µA, ∆Ep=97 mV) displays a higher redox current and lower peak separation values than that of BNDC/GCE (Ipa= 111 µA, ∆Ep=100 mV), and bare GCE 8 ACS Paragon Plus Environment

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(Ipa= 96 µA, ∆Ep=108 mV) confirmed that Cu@Cu2O doping comfort the electron transfer in [Fe(CN)6]3−/4− redox process. Further, the electroactive surface area of the modified electrodes was analyzed via CV. The CVs of various modified electrodes were performed in 0.1 M KCl having 5 mM Fe(CN)63-/4- under variable scan rates (20 to 200 mV s-1) (Figure S2B). The response current (Ipa and Ipc) has linear relationship with the square root of the scan rates (ν1/2) (Figure S2C), demonstrating the overall redox process of Fe(CN)63-/4- is a characteristic diffusion regulated practice. The Randles-Sevick equation (2) was used to calculate the electroactive surface area,46 Ip = 2.69×105 AD1/2n3/2ν1/2C

(2)

Where Ip, A, D, n, ν, and C representing the peak current, active surface area, diffusion coefficient of [Fe(CN)6]3−/4−, number of electron transfer (n=1), scan rate (mV s−1), and the concentration of [Fe(CN)6]3−/4−, respectively. By applying these values in eqn. 2, the calculated active surface area (A) GCE, BNDC/GCE, and Cu@Cu2O-BNDC/GCE is about 0.078, 0.104 and 0.138 cm2, correspondingly. The large electroactive surface area of Cu@Cu2O-BNDC/GCE can provide massive reactive sites for electrochemical sensing process. Electrochemical determination of dopamine (DA) The electrochemical reaction of DA at bare GCE (a), BNDC/GCE (b), and Cu@Cu2OBNDC/GCE (c) were examined using CV in 0.05 M PBS (pH 7.0) in a sweep rate of 50 mV s-1 in the presence of 0.2 mM DA. Figure 4B displays the cyclic voltammograms at unmodified GCE (a), a poor redox couple (Epa=0.29 V; Ipa=7 µA) was appeared. The control electrode BNDC/GCE (b), offers a redox peak (Epa=0.22 V; Ipa=19.33 µA) with increased peak current. As we expect, Cu@Cu2O-BNDC/GCE (e), produced a massive hike in the redox peak current (Epa =0.21 V; Ipa=58.4 µA), The response current observed at catalytic film modified electrode is about 8 folds higher than that of obtained at bare GCE. Obviously, the Cu@Cu2O-BNDC/GCE exhibit much better catalytic activity towards electrochemical oxidation of DA. The immense hike in the redox current response can be attributed to the unique physiochemical properties of Cu@Cu2O-BNDC; such, a large specific surface area, excellent ionic conductivity, better electron transfer kinetics, and synergistic effect of BNDC and Cu@Cu2O nanospheres. Furthermore, it is well recognized that the catechol component of DA can bind to the Cu-ion more strongly via catechol-Cu chelate.44 Thus, this electrochemical platform based on Cu@Cu2O-BNDC/GCE offered enhanced sensitivity



towards DA. The probable mechanism of DA electro-oxidation at Cu@Cu2O-BNDC/GCE is shown in Scheme. 2. Optimizing the analytical condition is the key of an assay performance, as our attention was turned to review the influence of pH on to the electrochemical signal of DA, as the selection of suitable medium is important to obtain high sensitivity with lower detection limit. Hence, the influence of pH on response current towards electro-oxidation of 0.2 mM DA on Cu@Cu2OBNDC/GCE was investigated in the pH range from 5.0 to 9.0 (PBS (0.05 M)) in CV and presented in Figure 4C. The response current of DA was found to be increased linearly with the increase in pH from 3.0-7.0; with subsequent increment in pH from 7.0 to 9.0 resulted in the drop of peak current; thus, indicate the influence of protons in the electrocatalytic oxidation of DA. The output of pH studies implies pH 7.0 had delivered better performance and optimal pH for electrochemical detection of DA in the subsequent experiments. A decent linear correlation among Epa and pH was obtained with regression equation of Epa (V) = -0.0561 pH +0.038, R2= 0.9972 (Figure 4D). The slope (Epa/pH) was -56.1±0.7 mV/pH, suggest the electro-oxidation of DA at Cu@Cu2OBNDC/GCE involved a two-electron and two-proton process.47,48 Hence, pH 7.0 phosphate buffer was chosen for the consequent systematic experiments. Next, the influence of sweep rate over electro-oxidation of DA was explored via CV at different sweep rates (20 to 200 mV s-1) in presence of 0.2 mM DA. It is apparent from Figure 4E that with rising scan rates, the redox peak current amplified linearly with a slender shift in oxidation potential signifying the existence of quasi-reversible reaction, which is consistent to the previous reports. The calibration plot (Figure 4F) of redox peak currents (Ipa & Ipc) against scan rate exhibited a linear relationship (R2= 0.9982 and 0.9983) indicated that the electro-oxidation of DA at Cu@Cu2O-BNDC modified GCE is a typical adsorption-controlled process.47 In a range of electrochemical techniques amperometry is an effective tool for the detection of analytes at very low concentrations in minimum time with maximum response. Figure 5A displays an amperometric response of the Cu@Cu2O-BNDC modified electrode on a successive addition of DA in regular time interval to 0.05 M PBS at a fixed potential of +0.22 V, on electrode revolution of 1400 rpm. The characteristic current-time plot revealed that the Cu@Cu2O-BNDC modified electrode delivered a prompt response to the alteration of DA concentration. The electrode achieves utmost steady-state current inside 6s. This fast electro-oxidation response can 10 ACS Paragon Plus Environment

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be accredited by the rapid absorption, and activation of DA on the surface of the Cu@Cu2OBNDC. The consequent calibration plot (Figure 5B) of response current against concentration of the modified electrode delivered an exceptional linear relationship among current and concentration in the range of 0.004 to 542 µM with a linear equation of I (µA) = 0.9172 [DA] (µM) + 2.6074 (R2= 0.9965). The standard method was followed to calculate the limit of detection, LOD= 3S/q, Where, S is the standard deviation of the blank solution, q is the calibration curve.49 The limit of detection was found to be 0.5 nM (S/N=3). Further, assessing the analytical accomplishment of the Cu@Cu2O-BND based DA sensor over earlier reports for DA detection sensors is summarized in Table 1. Noticeably, the Cu@Cu2O-BND based DA sensor shows much better catalytic performance over other overpriced catalysts reported to assay DA. The selectivity of the Cu@Cu2O-BNDC modified GCE towards DA detection was studied by introducing three vital interfering species which are co-exist in human fluids and have similar oxidation potential of DA. Those three key biological interferons namely; uric acid, glucose and ascorbic acid are spiked in the continually stirred PBS comprising 10 µM of DA. As presented in Figure 4C a 10-fold surplus of ACh, AA, UA, glucose and ACh does not produce any current response, understandably the addition of interfering species did not affect the DA detection. Further, the operational stability of the proposed sensor was evaluated in presence of DA in 0.05 M PBS over 2500 s and the fabricated assay upholds 97.3 % of its initial current as shown in Figure 4D which attest the reliable functioning firmness of the sensor. The results of stability and selectivity measurements indicates that the proposed sensor has decent discriminating ability and excellent stability. The practical applicability of the Cu@Cu2O-BNDC based DA sensor was showcased in human serum samples (standard addition method) following optimized test conditions via amperometry technique. The analytical outcomes were synopsized in Table S1. The obtained recovery value of the DA in serum samples fluctuated in between 97.8 % and 100.2 % and the RSD are range from 1.16 to 1.82 %. The satisfactory output of DA detection in human fluids proves that Cu@Cu2O-BNDC can be an efficient electrocatalyst for clinical analysis. Electrochemical determination of Acetylcholine (ACh) Following the detection of dopamine, our next task is to evaluate the catalytic ability of Cu@Cu2O-BNDC towards ACh detection. Preliminary cyclic voltammetric results of Cu@Cu2O11 ACS Paragon Plus Environment


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BNDC/GCE in 0.1 M NaOH solution in absence (green) and presence (red) of 1 mM ACh at a sweep rate of 50 mV s-1 was projected in Figure 6A. The cyclic voltammogram of Cu@Cu2OBNDC/GCE in 0.1 M NaOH in presence of 1 mM ACh, displays a higher anodic peak current response at a peak potential about 0.62 V, signifies that Cu@Cu2O-BNDC is a valuable catalyst for ACh detection. The received voltammetric response of ACh oxidation on Cu@Cu2OBNDC/GCE electrode in alkaline medium has good agreement with former reports.9 It should be noted that the Cu2O on the surface of Cu@Cu2O-BNDC would contain definite extent of negative charge in pH 12.7 solution (0.1 M NaOH), due to its isoelectric point of 9.5.34 The partially oxidized (negatively charged) surface of catalyst can interact with the partially positive charged amine group of ACh with ease; thus, resulted in the enhanced oxidation (i.e., enhanced anodic current) of ACh on the exterior of Cu@Cu2O-BNDC/GCE. The response current of bare GCE and BNDC/GCE not shown, since both electrodes does not show any current response for 1 mM ACh, denoting the necessity of redox mediator. Electrocatalytic oxidation mechanism ACh over Cu@Cu2O-BNDC electrode surface can be described as follows.9 Cu + 2OH- → Cu(OH)2 + 2e-

(3)

Cu2O + H2O + 2OH- → 2Cu(OH)2 + 2e-

(4)

Cu(OH)2 → CuOOH

(5)

CuOOH + ACh → Cu(OH)2 + product

(6)

Reaction (6) occurring via two steps as follows. CH3COOCH2CH2N+(CH3)3 + H2O → CH3COOH + HO(CH2)2N+(CH3)3

(7)

Followed by oxidation of the alcoholic groups: CuOOH + HOCH2CH2N+(CH3)3 + OH- → HOOCCH2N+(CH3)3 + H2O + Cu(OH)2

(8)

In detail, ACh was hydrolyzed in alkaline solution to yield acetate anion and choline, later Cu@Cu2O-BNDC was oxidized to form Cu(II)/Cu(III); further the catalytic system absorbs the alcoholic choline species which later oxidized to respective carboxylic acid by Cu(III)/Cu(II)


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redox mediator. Based on these elucidations, it is understandable that the electro-oxidation of ACh in the surface of Cu@Cu2O-BNDC/GCE was occurring via the active Cu(III) intermediate species. Furthermore, carry out cyclic voltammetry as a rationale of sweep rate changing from 50 to 400 mV s-1 were shown in Figure 6B. As the scan rate rises, the consequent anodic current response enhanced along with a steady potential shift towards positive direction. As illustrated in inset of Figure 6B. the oxidation peak current (Ipa) of ACh linearly increased with the square root of the scan rate in the range of 50 – 400 mV s-1 with a linear relationship coefficient (R2) of 0.9907, which directs that the electro-oxidation of Ach on Cu@Cu2O-BNDC/GCE is a conventional diffusion-controlled course.8 The obtained results are coherent with the preceding investigations which showcased; the copper-based biosensors would be under a diffusion control process.9 Advancing to estimate the catalytic performance of Cu@Cu2O-BNDC towards ACh, an amperometric assay was performed at the working potential of +0.60 V, with successive injection of ACh to 0.1 M NaOH with constant electrode rotation of 1400 RPM and the results are exhibited in Figure 6C. A brisk current retort was witnessed in subsequent to the addition of ACh to the solution, as the oxidation current increases rapidly and reaches a steady state within 5s, which can be ascribed to the peculiar physiochemical properties coupled with excellent synergy of the Cu@Cu2O-BNDC electrocatalyst. The amperometric current response of ACh oxidation increased linearly in a concentration span of 0.3 to 2602 µM. As depicted in Figure 6D, the oxidation current response has a good linearity against concentration of ACh with a linear regression equation of Ip (µA)= 0.177 [ACh] (µM) + 4.7713; R² = 0.9973. Evaluating from the intercept of the regression plot and standard deviation, the sensing limit of the developed sensor was determined to be 17 nM under signal-to-noise ratio of 3 (S/N=3). Notably, the physiological concentration of ACh in mammalian nervous system (synapse) is around 2.4 mM,57 which is inside the working range of the developed sensor. It can be found that the Cu@Cu2O-BNDC based ACh biosensor demonstrates a reliable resolution to the faint amounts of ACh. Moreover, the detection limit of Cu@Cu2O-BNDC based ACh sensor is much lower than the previously reported non-enzymatic ACh sensors as listed in Table 2. This superior electrocatalytic performance can be endorsed to the spherical nature of Cu@Cu2O on BNDC which possess the strait forward benefits of large surface area and good mechanical strength. Thus, the high flying conductivity of Cu@Cu2O,



awards excellent electrical contact to the immobilized nanomaterial and allowing long range electron transfer between the electrode and electrolyte. Excluding the endogenous interfering species is a challenge goal to be achieved in all nonenzymatic biosensors as the human fluids are rich of electroactive substrates which could simultaneously oxidized in the under the same potential of the bioanalytes under study to yield false positive results. Therefore, the sensor was subject to test the selectivity towards ACh (10 µM) in the consecutive addition of 100 µM dopamine (DA), ascorbic acid (AA), uric acid (UA), glucose, H2O2, L-cysteine, norepinephrine, and epinephrine in 0.1 M NaOH. As displayed in Figure. S3, there is no noteworthy response current was observed by the injection of tested interference species, designate the selectivity of the proposed sensor. In purpose of substantiating the durability, and longstanding firmness of the developed sensor, Cu@Cu2O-BNDC/GCE was exposed to amperometric measurement with ACh in 0.1 M NaOH solution (data not shown). The amperometric signals shows that the sensor retains up to 94.4% of its initial current after running time of 1500s, advocating the antifouling properties of the Cu@Cu2O-BNDC modified electrode towards ACh detection. Up next, real-world reliability of the developed sensor was assessed by determining the ACh concentration in human blood serum samples by amperometric method (Figure. S4). Fresh blood serum samples were received from three different nutritious volunteers (n=3) and the concentration of ACh present in the serum samples were ascertained with the modified electrode following standard addition protocol. The obtained retrieval rates of ACh in serum samples are in the scale between 97.8 % and 100.2 % and the RSD are range from 1.29 to 1.98 %. (Table S1). These outcomes validate the practical feasibility of the developed ACh biosensor based on Cu@Cu2O-BNDC modified electrode; which can be employed to determine the levels of ACh in blood serum samples. CONCLUSION An efficient versatile route to the design and synthesizes of metal-metal oxide nano particle embedded heteroatom doped mesoporous carbon materials was reported. The synthesized Cu@Cu2O-BNDC was systematically characterized via range of spectral and electrochemical methods. The Cu@Cu2O doping offers an enormous surface area and unique pore size, thus enhancing the electron transfer kinetics of the resultant Cu@Cu2O-BNDC nanocatalyst. The 14 ACS Paragon Plus Environment

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peculiar physiochemical assets coupled with excellent synergy of the constituents offers superior electrocatalytic activity to the material. Under optimized condition, Cu@Cu2O-BNDC based sensor offered a superior electroanalytical performance towards the detection of DA (pH 7) and ACh (0.1 M NaOH) with sensing bounds of 0.5 nM and 17 nM, respectively. The fabricated sensor was employed to assay essential neurotransmitters in physiological fluids under various test conditions with satisfactory results. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support of the Ministry of Science and Technology, Taiwan through contract no. MOST 107-2113-M-027-005-MY3. ASSOCIATED CONTENT Supplementary information Materials and methods, Fabrication of Cu@Cu2O-BNDC composite modified electrode, SEM image of Cu@Cu2O-BNDC, anti-interference study of ACh sensor based on Cu@Cu2O-BNDC modified electrode, and real sample analysis. AUTHOR INFORMATION Corresponding Author: *Shen-Ming Chen, Fax: +886227025238, Tel: +886227017147, E-mail: [email protected] Conflicts of interest There are no conflicts to declare.



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Figures and Tables

Scheme 1. A graphical synthetic route for the Cu@ Cu2O-BNDC composite.

Figure. 1. TEM image of (A) BNDC, (B-D) Cu@Cu2O-BNDC, (E) HRTEM image, (F) EDX analysis of Cu@Cu2O-BNDC.


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Figure. 2. (A) XRD of BNDC (a) and Cu@Cu2O-BNDC (b), (B) Raman spectra of BNDC (a) and Cu@Cu2O-BNDC (b), BET analysis of (C) BNDC (inset- pore size distribution) and (D) Cu@Cu2O-BNDC (inset- pore size distribution (BJH-method)).



Figure. 3. XPS spectrum of Cu@Cu2O-BNDC: (A) survey, (B) B1s, (C) C1s, (D) N1s, (E) Cu2p, (F) O1s.


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Figure. 4. (A) EIS curves of bare GCE (a), BNDC/GCE (b), Cu@Cu2O-BNDC/GCE (c). (B) CVs of bare GCE (a), BNDC/GCE (b), and Cu@Cu2O-BNDC/GCE (c) in PBS (pH 7.0) solution containing 0.2 mM DA at a scan rate of 50 mV s-1. (C) CV response obtained at Cu@Cu2OBNDC/GCE for 0.2 mM of DA at different pH solutions from 5.0 to 9.0 at a scan rate of 50 mV s-1. (D) Plots of Ipa (blue) and Epa (green) against pH (n=3). (E) CVs for Cu@Cu2O-BNDC/GCE PBS (pH 7.0) solution containing 0.2 mM DA at different scan rate in the range of 20–200 mV s1.

(F) The corresponding calibration plot for scan rate vs. Ipa and Ipc (n=3).



Scheme 2. The plausible electrocatalytic mechanism of DA and ACh at Cu@Cu2O-BNDC/GCE


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Figure. 5 (A) Typical amperometric current response of Cu@Cu2O-BNDC modified electrode upon cumulative injection of DA into 0.05 M PBS (pH 7.0) solution at an applied potential +0.22 V. (inset shows current response at lower concentrations). (B) The corresponding calibration curve of DA: [DA] vs current response (n=3). (C) Amperometric current response of Cu@Cu2O-BNDC modified electrode toward the addition of DA and various interfering species including ascorbic acid (AA), uric acid (UA), and glucose. (D) Stability test of the developed sensor.

Figure. 6. (A) CV curve of Cu@Cu2O-BNDC/GCE in 0.1 M NaOH solution in absence (green) and presence (red) of 1 mM ACh at a scan rate of 50 mV s-1. (B) CVs for Cu@Cu2O-BNDC/GCE in 1 mM ACh at different scan rate in the range of 50–400 mV s-1. Inset shows the corresponding calibration plots of anodic peak current response (Ipa) vs square root of the scan rates (n=3). (C) Typical amperometric current response of Cu@Cu2O-BNDC modified electrode upon cumulative injection of ACh into 0.1 M NaOH solution at an applied potential +0.60 V (inset shows current 27 ACS Paragon Plus Environment


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response at lower concentrations). (D) The corresponding calibration curve of ACh: [Ach] vs Ipa (n=3). Table 1. Comparison of analytical performance of the proposed with other DA sensors. Material

Linear range (µM)

LOD (nM)

ref

FePt–Fe3O4 nanoparticles

0.005–0.110

1.0

44

PABSA-rMoS2/CPE

1–60

220

47

CeO2/Au

10–100

56

48

NiO-CuO/Graphene

0.5–20

167

50

Cu2O@Pt NPs

0.01–1027.16

3.0

51

rGO–Co3O4

1–30

277

52

H-Fe3O4@C/GNS

0.1–150

53

53

rGO-poly(Cu-AMT)

0.01–0.4

3.5

54

graphene–AuNPs

5–1000

1860

55

pulverized graphite

0.005–200

1

56

Cu@Cu2O-BNDC

0.004–542

0.5

This work

Table 2. Comparison of analytical performance of the proposed with other non-enzymatic ACh sensors. Material

Linear range

LOD (µM)

Ref

(µM) NiAl-LDH/CD composites

5–6885

1.7

7

Ni–Al LDHs/OMC/GC

2–4922

0.042

8

copper nanoparticles

120–2680

39

9

hollow nickel microspheres/carbon

0.24–828

0.049

58

nickel oxide nanostructure

250–5880

26.7

59

Cu@Cu2O-BNDC

0.3–2602

0.017

This work


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For Table of Contents Use Only

Synopsis A versatile synthetic platform to prepare Cu@Cu2O nanospheres encapsulated B/N co-doped carbon (BNDC) mesoporous nanocatalyst owning large surface area is reported. The synthesized catalyst was employed in electrochemical detection of Dopamine and Acetylcholine.


Rational Design of Cu@Cu2O Nanospheres Anchored B, N Co-doped

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