Electrocatalytic Oxidation of Tris(2-carboxyethyl)phosphine at

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Article Cite This: Anal. Chem. 2018, 90, 6059−6063

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Electrocatalytic Oxidation of Tris(2-carboxyethyl)phosphine at Pyrroloquinoline Quinone Modified Carbon Nanotube through Single Nanoparticle Collision 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 S Supporting Information *

ABSTRACT: Inspired by the addition-elimination catalytic mechanism of natural pyrroloquinoline quinone (PQQ) containing proteins, PQQ-modified hybrid nanomaterials have been increasingly developed recently as biomimetic heterogeneous electrocatalysts. However, up until now, no existing electrochemical approach was able to assess the intrinsic catalytic activity of PQQ sites, impeding the design of efficient PQQ-based electrocatalysts. Herein, in this work, we introduced a new method to calculate the turnover frequency (TOF) of any individual PQQ functional group for electrocatalytic oxidation of tris(2-carboxyethyl)phosphine (TCEP), through the study of single PQQ-decorated carbon nanotube (CNT) collisions at a carbon fiber ultramicroelectrode by chronoamperometry. The core advantage of this approach is being able to resolve the number of PQQ catalytic sites grafted on each individual CNT, so that the charge of any CNT collision event can be accurately translated into the intrinsic activity of the respective PQQ functional groups. The resulting collision-induced current responses clearly showed that the functionalization of CNTs with PQQ could indeed enhance its catalytic performance by 3 times, reaching a TOF value of 133 s−1 at 1.0 V vs Ag/AgCl. Such a single CNT collision technique, which is proposed for the first time in this work, can open up a new avenue for studying the intrinsic (electro)catalytic performance at a molecular level.

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method at the current stage is still difficult to exhibit a reasonably high throughput.7 Alternatively, the single particle collisions enable the examination of hundreds of analytes in a few minutes by recording the transient current response for each colliding particle at a given potential.8 Since the first reported observation of the single particle collision events through electrocatalytic amplification, this approach has been extensively applied to examine the correlation between particle structure and catalytic function.9 Significant progress has been made so far to elucidate the impact of collision-induced catalytic reactions and to understand the complex mass transfer of nanocatalysts as well.10,11 The collision behaviors of a wide range of metal or metal oxide nanoparticles have been studied including platinum,12 gold,13 palladium,14 iridium oxide,15 or ruthenium oxide;16 the influence of several different ultramicroelectrodes (UME) such as carbon,17 gold,18 platinum,19 and mercury/platinum20 has also been explored. Nevertheless, there is no previous work on single carbon nanotube (CNT) collision, to the best of our knowledge, except one early attempt to attach gold nanoparticle tethered CNT on a platinum electrode.21 This work aimed to unveil the electrocatalytic activity of PQQ to oxidize tris(2-carboxyethyl)phosphine (TCEP) through the

yrroloquinoline quinone (PQQ) is a catalytic cofactor that adopts an addition-elimination pathway:1 the PQQcontaining enzymes such as alcohol dehydrogenases can efficiently oxidize substrates to aldehyde, together with a reduction of PQQ to PQQH2; the formed PQQH2 will spontaneously be reoxidized back to PQQ with the aid of other electron acceptors.2 Inspired by this mechanism, a large amount of research over the past few decades has focused on the development of bioelectrodes which are modified with PQQ-dependent dehydrogenases in order to achieve cost-effective microbial fuel cells.3 Recent years have further witnessed the increasing development of biomimetic PQQ-decorated heterogeneous electrocatalysts, for instance, the PQQ-functionalized carbon nanotubes that are capable of direct electron transfer.4 At present, however, it remains challenging to reveal the intrinsic catalytic activity of PQQ at a molecular level, which has greatly hindered the rational design of high-performance electrocatalysts with PQQ functional groups.5 Single-nanoparticle electrochemical measurement may facilitate the calculation of the turnover frequency (TOF) for heterogeneous catalytic reactions, considering that the major limitations associated with traditional ensemble methods lie in exactly the polydispersity of active sites (i.e., type and number).6 Although it is possible to attach one individual nanoparticle directly onto nanoelectrodes, and to conduct voltammetry with the immobilized particle so as to analyze its catalytic activity, this © 2018 American Chemical Society

Received: December 24, 2017 Accepted: April 27, 2018 Published: April 27, 2018 6059

DOI: 10.1021/acs.analchem.7b05405 Anal. Chem. 2018, 90, 6059−6063

Article

Analytical Chemistry

experiments. Deionized water (DI, 18.2 MΩ cm) used throughout the experiments was obtained from a Milli-Q ultrapure water system (Millipore, USA). Synthesis of PQQ-Modified CNTs (PQQ@CNT-NH2). Prior to use, CNT-NH2 was purified as reported previously.23 Briefly, the as-received CNT-NH2 was first dispersed in a 4 M HCl solution and was heated at 60 °C for 12 h, and then it was cooled to room temperature, was centrifuged, was washed with DI water three times, and was dried. After that, 2 mg purified CNT-NH2 was added to 1 mL DMF, and the suspension was placed in an ice bath. The suspension was ultrasonicated for 60 min to obtain a black colloidal solution. Meanwhile, 10 mM PQQ and 30 mM EDC DMF solutions were prepared, separately. Finally, 1 mL CNTs suspension was mixed with 0.5 mL PQQ and 0.5 mL EDC. The mixture was kept overnight, followed by centrifugation and washing with DMF three times. The solid obtained (PQQ@CNT-NH2) was dispersed in 1 mL DMF solution for further use. Preparation of Carbon Fiber Ultramicroelectrode (UME) and Glassy Carbon Electrode (GCE). A 7 μm carbon fiber UME was fabricated on the basis of our previously reported method.17 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 were 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 was washed with a large amount of DI water. To demonstrate the electrocatalytic effect of PQQ-modified CNTs for oxidizing TCEP, a conventional three electrode measurement using GCE (3 mm in diameter) was also conducted. The GCE was first chemically cleaned with a piranha solution for around 2 min to clean out any organic residues; was washed with a large amount of DI water, dichloromethane, and ethanol; and was dried with a pure N2 gas flow. It was then mechanically polished with alumina pasta (1 and 0.05 μm) and was sonicated in DI water and ethanol successively for 1 min. The electrochemical cleaning was done by obtaining a reproducible curve in the blank PBS solution (0.2 M, pH 7.0). Finally, 10 μL PQQ@ CNT-NH2 or CNT-NH2 nanoparticles suspension (1 mg mL−1) was drop casted onto the GCE and was dried at room temperature. Electrochemical Measurements. Before testing, all the supporting electrolytes were bubbled with N2 for 10 min to remove the dissolved oxygen. A two-electrode cell consisting of a carbon fiber UME as the working electrode and a homemade Ag/AgCl as both the reference and counter was used for collision experiments. DMF solutions containing either 1 mg mL−1 CNT-NH2 or PQQ@ CNT-NH2 and 1−10 mM TCEP were used as the electrolytes. To ensure sufficient electrical conductivity for measurement, 10 mM LiCLO4 was also added into the electrolytes. All the twoelectrode collision measurements were carried out in a Faraday cage. On the other hand, cyclic voltammetry (CV) measurements were performed with a three-electrode configuration using GCE, a saturated calomel electrode (SCE), and a platinum wire as the working, reference, and counter electrode, respectively. The SCE potentials were converted to the Ag/AgCl ones by a shift value of 0.044 V.

study of single PQQ-modified CNT collisions (Scheme 1). The key feature of this approach is being able to accurately translate Scheme 1. Schematic Illustration for the (a) Synthesis of PQQ@CNT-NH2, (b) Electrocatalytic Oxidation of TCEP to TCEP=O by PQQ@CNT-NH2 at Carbon Fiber UME, and (c) the Typical Current Responses at 1 V vs Ag/AgCl for Single CNT Collisions of PQQ@CNT-NH2 and CNT-NH2 in the Presence of TCEP and that of PQQ@CNT-NH2 in the Absence of TCEP

the charge of a CNT collision event into the intrinsic activity of PQQ functional groups, since the number of PQQ catalytic sites grafted on each CNT can be readily resolved. The electrocatalytic oxidation of TCEP was particularly chosen because the activity of PQQ on this reaction has seldom been reported before, so that it is likely to evaluate the capability of this new method for quantifying the performance of any unknown catalysts. Moreover, multiwalled CNTs were used as the substrates rather than single-walled ones. In that case, the intensive PQQ modification on the outer surface of CNTs would not affect the fast electron transfer through the inner graphitic walls.22 The resulting TOF of the PQQ-modified CNTs at 1 V versus Ag/AgCl was 3 times higher than that of the amino-functionalized one under the same charge-transfer controlled test condition, confirming that the enhanced electrocatalytic oxidation of TECP must be due to the superior intrinsic activity of PQQ.

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EXPERIMENTAL SECTION Reagents and Apparatus. Pyrroloquinoline quinone (PQQ), l-ethyl-3(3-(dimethylamino)propyl)carbodiimide (EDC), and lithium perchlorate (LiClO4, 98%) were purchased from Aladdin Industrial Corporation (Shanghai, China). Tris(2carboxyethyl)phosphine (TCEP) was supplied by Sigma-Aldrich (China). Dichloromethane CH2Cl2 (≥99.5%) and ethanol were obtained from Shanghai Chemical Reagent Co., Ltd. (China). Amino functionalized multiwalled carbon nanotubes with a typical diameter of 8−15 nm, length of 50 μm (CNT-NH2, 0.45 wt % NH2-doping), were purchased from XF NANO, INC., Advanced Material Supplier (China). N,N-Dimethylformamide (DMF, ≥99.5%) and hydrochloric acid (HCl, 12 M) were obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. (China). Transmission electron microscopy (TEM, JEOL JEM100 CX II) was used for the structural and morphology characterization of PQQ-modified CNTs. A CHI 660C workstation (Shanghai Chenhua Co., Ltd., China) was for electrochemical

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RESULTS AND DISCUSSION The electrocatalytic oxidation process of TCEP by PQQ-modified CNTs was first examined in a traditional three-electrode 6060

DOI: 10.1021/acs.analchem.7b05405 Anal. Chem. 2018, 90, 6059−6063

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Analytical Chemistry

(Figure 2b) affirmed the successful modification of PQQ functional groups onto multiwalled carbon nanotubes, as a clear boundary was seen between amorphous PQQ and layered-like CNTs. The low-magnification TEM image of single CNT-NH2 is also given in Figure S1. A series of chronoamperometric collision measurements was conducted at the potentials of 0, 0.5, 0.7, 0.8, 0.9, and 1.0 V versus Ag/AgCl in DMF containing 10 mM TCEP, Figure 3, and was

measurement. Figure 1 shows the CV plots of PQQ@CNT-NH2 with and without TCEP and a comparison of the catalytic activity

Figure 1. Cyclic voltammograms of (a) PQQ@CNT-NH2 in the absence of TCEP, (b) PQQ@CNT-NH2, (c) CNT-NH2, and (d) the bare GCE in the presence of TCEP, at a scan rate of 100 mV s−1 in DMF electrolytes.

for PQQ@CNT-NH2, CNT-NH2, and bare GCE in the presence of 10 mM TCEP as well, under a scan rate of 100 mV s−1 in DMF solutions. As seen in the inset of Figure 1, when there was no TCEP in the electrolyte, only a small pair of redox peak was observed within the potential range of −0.3 to 0 V versus Ag/AgCl, corresponding to the reduction and oxidation of PQQ/PQQH2. However, when there was 10 mM TCEP, a clear enhancement of current was seen when the applied potential was more positive than 0.3 V versus Ag/AgCl, which should be assigned to the catalytic oxidation of TCEP by PQQ@CNT-NH2. Moreover, also shown in Figure 1, the TCEP oxidation current of PQQ@ CNT-NH2 was much higher than that of CNT-NH2, while the bare GCE had no contribution to this reaction, confirming that the grafting of PQQ onto CNT-NH2 could indeed improve its electrocatalytic activity. Single PQQ@CNT-NH2 collision was further carried out in order to quantify the intrinsic activity of PQQ sites for electrocatalytical oxidation of TCEP. To ensure that the dispersion of individual PQQ-modified carbon nanotubes was actually achievable for collision measurement, the morphology and structural characteristics of PQQ@CNT-NH2 were obtained using TEM. A single carbon nanotube was found in the low-magnification image (Figure 2a), with another CNT attached in the middle,

Figure 3. (left) Chronoamperometric collision measurement and (middle) the respective close-ups of typical current spikes at 0, 0.5, 0.7, 0.8, 0.9, and 1.0 V vs Ag/AgCl for 1 mg mL−1 for PQQ@CNT-NH2 by carbon UME in DMF containing 10 mM TCEP. (right) The statistics analysis of duration, charge, and current for 1.0 V vs Ag/AgCl at 10 mM TCEP.

recorded with carbon UME. No current spikes were observed until at 0.7 V versus Ag/AgCl, suggesting that a greater overpotential was required in collision methods than in conventional CV, which was probably because of the reduced number of catalytic sites involved. As the applied bias further increased, both the current magnitude and the collision frequency were enhanced considerably. In particular, it was shown in the statistic analysis that PQQ@CNT-NH2 reached an average current value of 135 pA, a duration of 0.99 ms, and a charge of 0.133 pC, at 1.0 V versus Ag/AgCl. On the contrary, CNT-NH2 exhibited only a third of the catalytic current of PQQ@CNT-NH2, Figure S2. Since the number of PQQ sites on PQQ@CNT-NH2 was purposely designed to be the same as the amount of amino functional groups on CNT-NH2 (see the Supporting Information for more detailed calculations), any difference observed in collision currents should thus be able to reflect the variation of their intrinsic activity. The number of PQQ active sites on each carbon nanotube (nPQQ) can be determined by the following eq 1: nPQQ =

=

NPQQ NCNT

N = NH2 = NCNT

mNH2 × NA MNH2 VCNT V0

mNH2 × NA MNH2 mCNT / ρCNT V0

=

Figure 2. TEM images of an individual PQQ@CNT-NH2 under a (a) low and (b) high magnification. The scale bars are 50 and 5 nm, respectively.

πr 2l × NA × ρCNT mNH2 × mCNT MNH2

(1)

where NPQQ, NNH2, and NCNT are the total number of PQQ, NH2, and CNT, respectively; mNH2 and mCNT are the total mass of NH2 and CNT added (0.45 wt % NH2 in CNT-NH2); VCNT and V0 are the total and unit volume of CNT; ρCNT is the density of CNT

which would probably be due to the unavoidable aggregation during sample drying. In addition, the high-magnification image 6061

DOI: 10.1021/acs.analchem.7b05405 Anal. Chem. 2018, 90, 6059−6063

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Analytical Chemistry (2.1 g cm−3);24 r and l are the diameter and length of CNT provided (15 nm and 50 μm); MNH2 is the molar mass of NH2 groups (16 g mol−1); and NA is the Avogadro constant (6.02 × 1023). Hence, the number of PQQ on each carbon in this work, nPQQ, is calculated to be ca. 3.2 × 106. The calculation of turnover frequency (TOF) was thus based on the following equation17 (eq 2), assuming that the oxidation of each TCEP involved two electrons transferred: TOF =

QTCEP NTCEP = Δt × nPQQ 2e × Δt × nPQQ

The huge changes observed in the collision current distribution between 8 and 10 mM TCEP (bottom half of Figure 4) further confirmed that the rate-determining step (RDS) switched from the mass transfer of TCEP at a low reactant concentration to a charge-transfer-controlled process when the TCEP concentration was high enough (i.e., 10 mM).

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CONCLUSION In this work, a single carbon nanotube collision method was adopted in order to examine the intrinsic activity of the PQQ grafted on CNTs for electrocatalytic oxidation of TCEP. Compared with the traditional ensemble methods, one of the key advantages of this approach was that the exact number of PQQ functional groups involved in one single reaction current spike can be determined, so that the molecular-level intrinsic catalytic kinetics should be achievable. To prove this concept, it was first confirmed by TEM images that the dispersion of individual PQQ-modified CNT was indeed achievable. The PQQ@CNT-NH2 collision results exhibited an enhanced TOF of 133 s−1 at 1.0 V versus Ag/AgCl and in the presence of 10 mM TCEP, compared with a much smaller value of 42 s−1 for CNTNH2. The subsequent statistic analysis ensured that these results were obtained under a charge-transfer-controlled condition, and thus, the different intrinsic activity should be the only reason for the gap in reactions kinetics, given that the number of active sites in PQQ@CNT-NH2 was almost identical to that of CNT-NH2. Such an exciting observation based on a single carbon nanotube collisions method may offer a new possibility to explore the intrinsic activity of (electro)catalytic reactions.

(2)

where NTECP, QTCEP, and Δt are the total number of oxidized TCEP molecules, the charge transferred, and the duration of one single carbon nanotube collision event. In this regard, the average TOF of PQQ@CNT-NH2 to elctrocatalytically oxidize TCEP is 133 s−1 at 1.0 V versus Ag/AgCl and in the presence of 10 mm TCEP, while that of CNT-NH2 is only 42 s−1 under the same conditions. The proper measurement of intrinsic catalytic kinetics required the reaction to be conducted under a charge-transfercontrolled condition; the influence of the TCEP concentrations on the collision-induced current responses was also investigated. Different concentrations of TCEP were applied, including 2, 4, 8, and 10 mM. As seen in those typical current responses and in the corresponding current distributions, top half of Figure 4, the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b05405. The TEM image of CNT-NH2, the collision results of CNT-NH2 at 1.0 V vs Ag/AgCl in the presence of 10 mM TCEP, and the calculation of the maximum number of PQQ catalytic sites on one carbon nanotube (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel/Fax: 86-21-6425339. *E-mail: [email protected]. Tel/Fax: 86-21-6425339. ORCID

Yi-Tao Long: 0000-0003-2571-7457 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21421004, 21327807), the Program of Introducing Talents of Discipline to Universities (B16017), the 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).

Figure 4. (top) The chronoamperometric collision and the respective close-ups of typical current spikes in DMF containing 0, 2, 4, 8, and 10 mM TCEP at 1.0 V vs Ag/AgCl for 1 mg mL−1 for PQQ@CNT-NH2 by carbon UME. (bottom) The statistics analysis of current at 10 mM TCEP was done at 1.0 V vs Ag/AgCl.

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average current spikes gradually turned from 20 to over 30 mA when the TCEP concentration was increased from 2 to 8 mM; however, when the amount of TCEP further reached 10 mM, the current spikes were sharply enhanced to above 130 mA.

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