Individual Modified Carbon Nanotube Collision for Electrocatalytic

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Individual Modified Carbon Nanotube Collision for Electrocatalytic Oxidation of Hydrazine in Aqueous Solution Fato Tano Patrice, Kaipei Qiu, Li-Jun Zhao, Essy Kouadio Fodjo, Da-Wei Li, and Yi-Tao Long ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00018 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Nano Materials

Individual Modified Carbon Nanotube Collision for Electrocatalytic Oxidation of Hydrazine in Aqueous Solution Fato Tano Patrice, Kaipei Qiu*, Li-Jun Zhao, Essy Kouadio Fodjo, Da-Wei Li, and Yi-Tao Long* Key Laboratory for Advanced Materials, Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China. KEYWORDS: Individual carbon nanotube collision, Single molecule analysis, Electrocatalytic amplification, Hydrazine oxidation, Pyrroloquinoline quinone redox cycling

ABSTRACT: Collision at a single molecule level was achieved based on the nano-impact of an individual pyrroloquinoline quinone (PQQ) modified multi-walled carbon nanotube (MWCNT) at the carbon fiber ultramicroelectrode (C UME). Electrocatalytic amplification of the current responses was observed when the PQQ-modified MWCNT collided with C UME in the presence of hydrazine (N2H4) in a Tris-HCl buffer solution, which was also supported by the conventional cyclic voltammetry and chronoamperometry techniques. The enhanced catalytic oxidation of N2H4 was due to the “addition-elimination” redox cycling mechanism of PQQ/PQQH2, where the oxidation of N2H4 occurred together with the reduction of PQQ under an external bias, and

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the formed PQQH2 intermediate would be re-oxidized back to PQQ simultaneously. The average collision current, duration and charge for PQQ-modified MWCNT at 1.0 V vs. Ag/AgCl were 105 pA, 0.45 ms, and 49 fC, respectively. As a result, the turnover frequency of electrocatalytic oxidation of N2H4 by PQQ was calculated to be 54 s-1. In this regard, the proposed individual carbon nanotube collision method can not only serve as a promising sensing technique to detect biochemical species, but more importantly provide a robust approach to determine the intrinsic catalytic activity as well.


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1. Introduction Since their discovery, nanoparticles have been attracting much attention thanks to those unique properties that cannot be achieved by their bulk counterparts. It is thus imperative to develop new analytical tools based on the specific characteristics of nanomaterials. In particular, the electrochemical methods that study the interaction between an individual nanoparticle (NP) and electrode surfaces are of great interest. These investigations characterize the electrochemical signals from single NP in a colloidal dispersion which are hardly detectable by the traditional ensemble measurements.1–7 A wide range of single NP detection techniques have been developed over the last few years through the study of current blockage,8 electrocatalytic amplification,9–12 photocatalysis,13 electrochemical redox reaction of the NP,14 transformative nano impact analysis,15 and electrodissolution of single NP.16 Among the above single NP electrochemical sensing methods, the electrocatalytic amplification has been most extensively studied because it can lead to significantly enhanced current response, detection sensitivity and throughput.17 In addition, functionalization of NPs with well-characterized molecules enables to determine their intrinsic catalytic activity at a single molecule level, and furthermore to accurately interpret the biological and biochemical process in various catalytic systems.11 Carbonaceous nanomaterials, especially the surface-modified carbon nanotubes, have been increasingly applied for electroanalytical chemistry recently due to their exceptional electrical, optical, thermal, mechanical, chemical properties, as well as the flexibility to be associated with numerous chemical compounds to form new nanocomposites.19-23 Moreover, since the pristine CNTs are insoluble in most aqueous solutions and certain organic solvents24, it is also essential to conduct surface modification to improve their solubility.25 The biomolecule-tethered CNTs nanocomposites have been previously reported for electrocatalysis, biomedical imaging, drug


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delivery, and photocatalysis.21,26 The modification of multi-walled CNTs (MWCNTs) has been especially focused, compared with the single-walled ones, as the intensive functionalization on the outer surface of CNTs may not affect the fast electron transfer through the inner graphitic walls. 25,27,28 The collision behaviors of catalytic molecules modified NPs can be used to analyze the intrinsic activity at a single molecule level. For example, the electrochemical collision of goldmodified catechol molecule at the carbon fiber ultramicroelectrode (C UME) surface for electrocatalytic oxidation of NADH has been previously studied.11 Besides, the detection of single dopamine through the electrochemical coupling on the electroanalytical performance of electrolyte-backfilled C UME in single-cell measurements has also been investigated.29 However, to the best of our knowledge, the study of individual (modified) carbon nanotube collisions has never been reported before. Herein, the electrocatalytic activity of pyrroloquinoline quinone (PQQ) functionalized MWCNTs hybrids (MWCNT-NH2@PQQ) to oxidize hydrazine (N2H4) in a Tris-HCl buffer was studied through the single CNTs collision behaviors at C UME surfaces, Scheme 1, along with other commonly adopted electrochemical measurement such as the cyclic voltammetry (CV) and chronoamperometry (CA). The catalytic oxidation of N2H4 by MWCNT-NH2@PQQ was clearly confirmed by the (dis)appearance of current spikes for single MWCNT-NH2@PQQ collisions in the absence or presence of N2H4. The turnover frequency (TOF) of a single PQQ molecule to oxidize N2H4 was calculated to be 54 s-1, at 1.0 V vs. Ag/AgCl, in accordance to its excellent electrocatalytic performance caused by the rapid “addition-elimination” redox cycling between PQQ and PQQH2.30 The N2H4 oxidation was chosen in this work mainly as a model reaction to demonstrate the capability of this new method. The wide applicability of the single (PQQ-


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)modified CNTs collision method proposed in this work was also examined by oxidation of hydrogen peroxide. As a result, this robust new electroanalytical method should be able to offer a great potential, not only to apply PQQ in nanobiosensors, but more importantly to investigate a broad range of chemical and biochemical species at a single molecule level.

Scheme 1. The schematic illustration of a) the chemical synthesis route of PQQ-modified MWCNT-NH2; b) the single MWCNTNH2@PQQ collision event for electrocatalytic oxidation of N2H4 at a carbon fiber ultramicroelectrode and the typical current responses in the absence or the presence of N2H4, respectively.

2. Experimental section 2.1. Reagents and apparatus Functionalized multi-walled carbon nanotubes with a typical diameter of 8 ~15 nm, length of 50 µm (MWCNT-NH2, > 95% carbon purity) were purchased from Nanjing XFNANO Materials Tech Co., Ltd. Pyrroloquinoline quinone (PQQ), l-ethyl-3(3-dimethylaminopropyl)carbodiimide (EDC), lithium perchlorate (LiClO4, 98%), and Tris(hydroxymethyl)aminomethane were purchased from Aladdin Industrial Corporation (Shanghai, China). Dichloromethane CH2Cl2 and ethanol were obtained from Shanghai Chemical Reagent Co., Ltd (China). Hydrazine hydrate


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(N2H4.H2O) and hydrochloric acid (HCl, 12 M) were obtained from Shanghai Ling Feng Chemical Reagent Co., Ltd. (China). The electrochemical experiments were carried out using a workstation CHI 660C, which was obtained from Shanghai Chenhua Co., Ltd. (China). The Fourier transform infrared (FTIR) spectroscopy was obtained by Nicolet-6700 spectrophotometer (Thermo Fisher Scientific, USA). Milli-Q system (Millipore, USA, 18.2 MΩ cm) for deionized water was used throughout the experiments. 2.2. Synthesis of MWCNT-NH2@PQQ Prior to use, the received MWCNT-NH2 was first purified in a concentrated acid solution. The MWCNT-NH2 dispersion was heated at 60 °C for 12 h, then cooled to room temperature, centrifuged, washed with deionized water several times, and dried. Then, 2 mg purified and functionalized MWCNTs were added to 1 mL Tris-HCl buffer and placed in an ice bath to maintain the exothermic processing temperature of the suspension. Afterward, the dispersion was ultrasonicated for 60 min to obtain a black colloidal solution. Meanwhile, 10 mM PQQ and 30 mM EDC were prepared in Tris-HCl buffer, separately. Subsequently, a mixture consisted of 1 mL dispersed MWCNT, 0.5 mL PQQ, and 0.5 mL EDC was made and kept for 24 h to immobilize PQQ onto CNT. The mixture was then centrifuged, repeatedly washed with Tris-HCl buffer. Finally, the obtained precipitate was dispersed in 1 mL Tris-HCl buffer solution for further use (Scheme 1a). 2.3. Electrode preparation Commercial glassy carbon electrode (GCE, 3 mm in diameter) was used in a three electrode system for cyclic voltammetry (CV) and chronoamperometry (CA). Before use, GCE was first soaked at a room temperature for 1 min in piranha solution (6 mL of concentrated H2SO4: 2 mL of H2O2) to clean out any organic matters on its surface. [Caution! Piranha solution must be


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handled under a fume hood with extreme care as it is very corrosive and its reactivity with organic components is quite violent]. To ensure that GCE was completely cIeaned and was not damaged, it was then rinsed with a large amount of deionized water, dichloromethane, and ethanol successively, and dried with a pure N2 flow. After that, the GCE was polished with alumina powder (1 and 0.05 µm), followed by a cleaning in an ultrasonic deionized water bath (1 min) and ultrasonic ethanol bath (1 min), and, finally electrocycled in a blank 0.1 M Tris-HCl buffer (pH 7.0) to obtain a reproducible scan. A desired amount of MWCNT-NH2 or MWCNTNH2@PQQ nanoparticles was dropped onto the cleaned electrode and dried at room temperature. A 7 µm carbon fiber ultramicroelectrode (C UME) was fabricated based on our previously reported method.30 Briefly, a small piece of carbon fiber was inserted into a glass capillary after pulling it by a heating coil puller (PP-2000, Narishige, Japan). Then, the tip surfaces were cleaned using polishing pads and coated with alumina (3.5 µm in diameter). Eventually, a carbon UME was obtained after polishing it with a hard drive disk. Before use, the UME was cleaned by immersing it in piranha solution (6 mL of concentrated H2SO4: 2 mL 30% H2O2 v/v) for about 1 min and washed with a large amount of DI water. 2.4. Electrochemical measurements All experiments were conducted at ambient temperature (25 °C) and the solutions were bubbled with pure N2 gas for 10 min prior to each measurement in order to completely remove the dissolved oxygen. For CV and CA measurement of a three-electrode configuration in a onecompartment cell, the bare or modified GCE was used as the working electrode, a platinum wire as the counter, and a saturated calomel electrode (SCE) as the reference. On the other hand, for chronoamperometric single nanoparticle collisions experiments, a two-electrode cell consisting of a C UME working electrode and a homebuilt Ag/AgCl (saturated KCl) as both a reference and counter were employed.31 The positions of those two electrodes were fixed each time in the


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electrochemical cell to ensure the measurement were comparable. Tris-HCl buffer (0.1 M, pH 7.0) was used as an electrolyte. Here, all measurements were performed by adding 0.5 mL (10 mM) LiClO4 to the two-electrode cell containing 5 mL Tris-HCl buffer to ensure the high conductivity. Subsequently, the two-electrode cell was put in the Faraday cage followed by the complete immersion of the electrodes in the electrolyte. The Faraday cage was then closed, and a constant potential was applied. The electrochemical measurements were carried out either in the blank Tris-HCl buffer or by injecting 2 mg/mL MWCNT-NH2 or MWCNT-NH2@PQQ nanoparticles into the static electrolyte. Finally, the current signals are recorded through a Digi Data 1550A converter and a PC running PClamp 10.5 (Axon Instruments, Forest City, CA, USA). The collision measurement generally took only five minutes to capture the sufficient data points (i.e. several hundred to over a thousand) for statistic analysis, and the well-dispersed electrolyte remained uniform during this process with no apparent participation being observed. The conversion of potential values from SCE to Ag/AgCl (sat. KCl) is based on the following equation: SCE + 0.044 V = Ag/AgCl (sat. KCl).

3. Results and discussion 3.1. Voltammetric behavior investigations of PQQ-modified MWCNT-NH2 The electrochemical behavior of PQQ-modified MWCNT-NH2 was first examined by the cyclic voltammograms (CVs) using glassy carbon electrode (GCE) in a three electrode configuration. Figure 1 showed the CVs of bare GCE (curve a), MWCNT-NH2 (curve b), and MWCNTNH2@PQQ (curve c) in 0.1 M Tris-HCl buffer (pH 7.0) at a scan rate of 100 mV s-1. No electrochemical response was seen at either bare GCE or MWCNT-NH2 within the potential range of -0.6 to 0.6 V, but the more evident double layer capacitance shown in the CV of MWCNT-NH2 indicated that MWCNT-NH2 should have been anchored on the electrode surface.


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Furthermore, the CV plot of MWCNT-NH2@PQQ electrode showed a well-defined pair of redox peaks with the anodic peak potential at -0.1 V vs. SCE and a cathodic peak at -0.22 V vs. Ag/AgCl, which corresponded to the redox reaction of PQQ/PQQH2 couple, as shown in the following equation.32,33 The peak separation ∆E = 120 mV suggested that electron transfer in this reaction was fast and thus no overpotentials were required. This result strongly confirmed the integration of PQQ on MWCNT-NH2, which was also supported by the FTIR spectrum of MWCNT-NH2@PQQ shown in Figure S1. PQQ + 2H+ + 2e- ↔ PQQH2

Figure 1. Cyclic voltammograms (CVs) of bare GCE (curve a), CNT-NH2 (curve b), and CNT-NH2@PQQ (curve c) in 0.1 M Tris-HCl buffer (pH 7.0). The scan rate was 100 mV s-1.

Furthermore, the CV behavior of MWCNT-NH2@PQQ on GCE surface was carried out to examine the impact of the scan rate on the heterogeneous electron-transfer kinetics (Figure S2a). It can be observed that an increase in scan rate led to an increase in peak current and the peak potential separation, further denoting that the electron transfer was kinetically controlled.34,35 Also, the reaction on the electrode surface became electrochemically irreversible.36 Figure S2b indicated the relationship between peak current and scan rate of the CVs obtained at MWCNTNH2@PQQ modified GCE in the range from 10 to 150 mV s-1. It was seen that the anodic and cathodic peak currents varied linearly with the scan rate rather than the square root (not shown).


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This result further suggested a direct electron transfer through a surface-controlled electrochemical process.23 In order to evaluate the electron transfer for the irreversible electrode process, Laviron’s method37 was investigated (Figure S3). According to this approach, the standard rate constant of the surface reaction ks was found to be 3.0 s-1 which was in good agreement with the previous reported values31 and corroborating again the fast electron transfer. The electron-transfer coefficient α was evaluated to be 0.53, and the number of electrons involved in the electrochemical process n was found to be around 2, confirming the two electron process for PQQ. 3.2. Electrocatalytic oxidation of N2H4 at MWCNT-NH2@PQQ The voltammetric behavior of MWCNT-NH2@PQQ in the presence of N2H4 in 0.1 M Tris-HCl buffer (pH 7.0) was investigated in order to demonstrate its electrocatalytic effect to oxidize N2H4. Figure 2a showed the CVs from MWCNT-NH2@PQQ at different set of series of N2H4 concentrations. From this Figure, an enhancement of the oxidation current and disappearance of the cathodic peak was observed when increasing the concentration of N2H4. This phenomenon was characteristic of an electrocatalytic process and also indicated that the N2H4 oxidation reaction was concentration-dependent.

Figure 2. a) CVs of MWCNT-NH2@PQQ in the presence of N2H4 at different concentrations (0, 1, 2, 4, 6, 8, and 10 mM) in 0.1 M Tris-HCl buffer (pH 7.0) at a scan rate of 100 mV s-1; b) Calibration plot of the peak currents vs. N2H4 concentrations resulting from Figure 2a. The error bars designated the standard deviation on at 3 replicate measurements.


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In fact, MWCNT-NH2@PQQ exhibited a remarkable electrocatalytic oxidation current of 0.154 mA at an anodic peak potential 0.36 V vs. Ag/AgCl when the concentration of N2H4 was 10 mM. It is worth noting that the CV from MWCNT-NH2@PQQ modified GCE showed an oxidation current of 0.005 mA in the absence of N2H4. Thereby, the oxidation current increased around 31 times in the presence of 10 mM N2H4. Consequently, these changes demonstrated that PQQ on MWCNT-NH2 surface retained its excellent bioelectrocatalytic activity. The calibration plot of oxidation peak current against N2H4 concentration showed a linearity with an excellent correlation coefficient R2 = 0.9997 (Figure 2b). Chronoamperograms (CAs) were adopted to further examine the catalytic activity of PQQ toward N2H4 oxidation. Figure 3a showed the CAs of MWCNT-NH2@PQQ in the presence of various N2H4 at ca. 0.45 V vs. Ag/AgCl. It was seen that the anodic currents increased stepwise as the concentration of N2H4 increased from 0 to 10 mM, which was in good agreement with the CV results. In addition, a linear plot for calibration data of anodic current versus different concentrations of N2H4 was also obtained with an excellent correlation coefficient of 0.9978 (Figure 3b). These results further confirmed the biocatalytic activity of PQQ modified MWCNTNH2 towards the oxidation of N2H4

Figure 3. a) CAs of MWCNT-NH2@PQQ in the presence of N2H4 at different concentrations (0, 1, 2, 4, 6, 8, and 10 mM) in 0.1 M Tris-HCl buffer (pH 7.0) at a potential of ca. 0.45 V vs. Ag/AgCl. b) Calibration plot of the peak currents vs. N2H4


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concentrations read at 120 s and recorded at ca. 0.45 V vs. Ag/AgCl resulting from Figure 3a. The error bars designated the standard deviation on at 3 replicate measurements.

To confirm the excellent electrocatalytic effect of PQQ modified MWCNT-NH2 toward N2H4 oxidation, two control experiments including both CV and CA were performed with MWCNT-NH2 and the bare GCE (Figure S4). Upon comparison the peak currents of the three different types of electrodes for CV, the bare GCE did not display any current, MWCNT-NH2 displayed 0.025 mA, while MWCNT-NH2@PQQ indicated 0.154 mA (Figure S5). Besides, for CA, the bare GCE, MWCNT-NH2, and MWCNT-NH2@PQQ exhibited the values of 0.0004, 0.0006, and 0.01 mA, respectively (Figure S6). These results suggested that the contribution of the catalytic effect from MWCNT-NH2 was clearly negligible, compared with that of MWCNTNH2@PQQ. The excellent catalytic activity of PQQ modified MWCNT-NH2 may be due to the mediating effect of PQQ for numerous organics substances.38 3.3. Impacting C UME using MWCNT-NH2@PQQ 3.3.1. Effect of the applied potential on the single molecule collision To reveal the origin of its superior activity and to determine the exact intrinsic activity, single carbon nanotube collisions of MWCNT-NH2@PQQ were conducted with a C UME. The influence of applied potentials was firstly studied. Figure 4a showed the collisions behaviors of 2 mg mL-1 MWCNT-NH2@PQQ at C UME under different oxidation potentials of 0, 0.2, 0.5, 0.7, 0.9, and 1.0 V vs. Ag/AgCl and in a Tris-HCl buffer solution containing 10 mM N2H4. No current spike was observed when the potential was at 0.0 V vs. Ag/AgCl. While increasing the oxidation potentials from 0.2 to 1.0 V vs. Ag/AgCl, both the frequency and magnitude of the detection traces gradually increased. The close-ups of these peak currents in Figure 4b showed an asymmetric pulse shape and the peak current increased from 16 to 123 pA. The representative peak currents illustrated the feature of a single nanoparticle collision at UME surfaces.11 Note


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that the onset potential for the occurance of collision current signals was more positive but still close to the anodic peak potential for the oxidation of PQQH2, this indicated the redox cycling of PQQ/PQQH2 could occur and the overpotential of N2H4 by PQQ must be quite small. In addition, the contribution of oxygen evolution reaction (OER) to the overall current responses should also be trivial since the maximum applied overpotential of ca. 400 mV (the equilibrium potential of OER is 0.62 V vs. Ag/AgCl)39 was not sufficient for most of the metal-free electrocatalysts (i.e. PQQ) to oxidize water.40

Figure 4. a) Chronoamperometric results for the collisions of 2 mg mL-1 MWCNT-NH2@ PQQ at C UMEs in 0.1 M Tris-HCl buffer (pH 7.0) containing 10 mM N2H4 at different potentials (0, 0.2, 0.5, 0.7, 0.9, and 1.0 V vs. Ag/AgCl); b) The close-ups of the representative current spike from each impacting events.

Figure 5 showed the histograms of the collision results at 1.0 V vs. Ag/AgCl for 2 mg mL-1 MWCNT-NH2@PQQ in Tris-HCl (0.1 M, pH 7.0) containing 10 mM N2H4. It was seen that, the average charge, current, and duration were 49 fC, 105 pA and 0.45 ms, respectively, compared with 25 fC, 19 pA and 1.46 ms for MWCNT-NH2 under the same testing condition (Figure S8).


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The observed shorter duration but higher current of MWCNT-NH2@PQQ was caused by the rapid redox reactions between PQQ and PQQH2. More specifically, when a single MWCNTNH2@PQQ diffused to the C UME surface, it would oxidize the surrounding N2H4 molecules with the assistance of external applied bias; the formed PQQH2 on CNTs would be re-oxidized back to PQQ simultaneously so that it can further oxidize other N2H4 molecules (Scheme 1b). The faster electron transfer from N2H4 to [PQQ/PQQH2] and then to UME explained the higher electrocatalytic activity (greater current) of MWCNT-NH2@PQQ than MWCNT-NH2 to oxidize N2H4.41 In addition, it was also likely that most of the current response of MWCNT-NH2@PQQ ended when no more surrounding N2H4 could be oxidized (i.e. the duration reflected the reaction time), while those of MWCNT-NH2 would not stop until it moved far away from the electrode surface (i.e. the duration equaled the collision time). That’s why a longer average duration was observed for the MWCNT-NH2@PQQ collisions.

Figure 5. Histograms of the a) charge, b) current, and c) duration for the signals detected in the collision behaviors of 2 mg mL-1 MWCNT-NH2@PQQ at 1.0 V vs. Ag/AgCl in the presence of 10 mM N2H4 in 0.1 M Tris-HCl buffer at C UMEs.

3.3.2. Collisions of MWCNT-NH2@PQQ under different N2H4 concentrations The chronoamperometric collision behaviors of MWCNT-NH2@PQQ were further performed at a series of N2H4 concentrations (0, 2, 4, 8, and 10 mM) to assess the influence of reactant concentration on the impacting events, Figure 6. It was found that the current responses exhibited a gradually increase in the frequency and magnitude of the detection traces by raising the N2H4


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concentration, consistent with the previous observations in CVs and CAs. The close-ups of the representative peak stemmed from each Faradaic event also showed the typical patterns of a single NP collision event (Figure 6b). Moreover, the average charge of the collision events possessed a linear relationship with the concentration of N2H4, Figure 6c, indicating its potential to be used as biosensors, despite the detection limit of the current system was only in the mM level. Other collision parameters such as the frequency of collision events42 may be adopted in the future so as to enhance the sensitivity.

Figure 6. a) Chronoamperometric collisions results of 2 mg mL-1 MWCNT-NH2@ PQQ in the presence of different N2H4 concentrations (0, 2, 4, 8, 10 mM) in 0.1 M Tris-HCl buffer (pH 7.0) at 1.0 V vs. Ag/AgCl; b) Representative current peaks (from left to right) recorded in each impacting event (from 0 to 10 mM); and c) Linear relationship between the average charge of the collision measurement and the N2H4 concentrations. The measurements were done in triplicate.

3.4. Determination of intrinsic catalytic activity and applicability to other biochemical systems Since the addition of PQQ was in excess during the synthesis, the MWCNT-NH2 was very likely to be fully covered by PQQ molecules. Therefore, it was reasonable to estimate the actual number of PQQ active sites on each carbon nanotube (nPQQ) based on the maximum amount of PQQ functional groups (nmax) using the following equation:30


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Where f was the fractional filling efficiency (0.91), SPQQ was the projection size of PQQ (66.1 Å2), and S0, r and l were the unit surface area, radius (7.5 nm) and length (50 µm) of MWCNT, respectively. In this regard, the maximum number of PQQ molecules that can be loaded on one carbon nanotube was 3.2 × 106. Furthermore, it was thus possible to calculate the turnover frequency (TOF) of PQQ to oxidize N2H4 based on the following equation30, assuming that the oxidation of each N2H4 involved four electron transferred:

Where NN2H4, QN2H4 and ∆t were the total number of oxidized N2H4 molecules, the charge transferred, and the duration of one single carbon nanotube collision event, respectively. In this regard, the average TOF of MWCNT-NH2@PQQ to elctrocatalytically oxidize N2H4 was 54 s-1 at 1.0 V vs. Ag/AgCl and in the presence of 10 mM N2H4. In fact, the proposed single nanotube collision method to determine the intrinsic catalytic activity at a single-molecule level should be broadly applicable to a wide range of organic compounds such as amines, amino acids, alcohols, aldehydes, glucose, NADH and TCEP, or under many other operation conditions. To prove this concept, a typical collision result of 2 mg mL-1 MWCNT-NH2@PQQ in the absence or presence of 10 mM H2O2 was given in Figure S9, using C UME in 0.1 M Tris-HCl buffer (pH 7.0) at 1.0 V vs. Ag/AgCl. No detection events were observed in the absence of H2O2. However, once 10 mM H2O2 was added into the electrolyte, significant amount of current response appeared, which corresponded to the oxidation of H2O2.


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Note that, the average current, duration and charge were 31 pA, 1.55 ms, and 45 fC, respectively, the TOF of PQQ to oxidize H2O2 was calculated to be 29 s-1.

4. Conclusion In this study, the electrocatalytic oxidation of hydrazine was explored in dynamic of single molecule collision at carbon fiber ultramicroelectrode surfaces using pyrroloquinoline quinone modified functionalized multi-walled carbon nanotubes in Tris-HCl buffer. It was found that the hydrazine oxidation was a function of a continuous redox cycling of PQQ. Moreover, the electrocatalytic effect of PQQ towards hydrazine oxidation was dependent on the applied potential and hydrazine concentration. Additionally, the ensemble CV and CA studies also confirmed the electrocatalytic activity PQQ for oxidation of N2H4. The successful determination of the reaction TOF revealed the intrinsic activity of PQQ at the single molecule level. The wide applicability of the proposed single carbon nanotube collision method to other biochemical systems demonstrated great potentials in single molecule sensing and (electro)catalysis analysis.

ASSOCIATED CONTENT Supporting Information. Fourier transform infrared (FTIR) spectra of MWCNT-NH2@PQQ and MWCNT-NH2, voltammetric study of MWCNT-NH2@PQQ at GCE (effect of the scan rate, Laviron’s approach), electrocatalytic oxidation of N2H4 at MWCNT-NH2 and GCE, collision using MWCNT-NH2, collision measure for detection of H2O2.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] / [email protected]. Tel/Fax: 86-21-64252339. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (21421004, 21327807), the Program of Introducing Talents of Discipline to Universities (B16017), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-02-E00023), the Fundamental Research Funds for the Central Universities (222201718001, 222201717003), and the China Scholarship Council (2015DFH452) as well. ABBREVIATIONS PQQ, pyrroloquinoline quinone; MWCNT, multi walled carbon nanotube; GCE, glassy carbon electrode. REFERENCES (1)

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Individual Modified Carbon Nanotube Collision for Electrocatalytic

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