Electrocatalytic Efficiency Analysis of Catechol Molecules for NADH

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The Electrocatalytic Efficiency Analysis of Catechol Molecules for NADH Oxidation during Nanoparticle Collision Li-Jun Zhao, Ruo-Can Qian, Wei Ma, He Tian, and Yi-Tao Long Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02365 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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The Electrocatalytic Efficiency Analysis of Catechol Molecules for NADH Oxidation during Nanoparticle Collision Li-Jun Zhao, Ruo-Can Qian, Wei Ma, He Tian and Yi-Tao Long* Key Laboratory for Advanced Materials & Department of Chemistry, School of Chemistry & Molecular Engineering, East China University of Science & Technology, 130 Meilong Road, Shanghai, 200237, P. R. China. Fax: +86 21 64250032 E-mail: [email protected]. ABSTRACT: Electrocatalysis of molecules is a hot research in biological and energy-related chemistry. Here, we develop a new system to study the electrocatalytic efficiency of single catechol molecule for NADH oxidation by single functionalized nanoparticle collision at ultramicroelectrodes (UMEs). The proposed system is composed of gold nanoparticles (AuNPs) functionalized with catechol molecules and a carbon-fiber ultramicroelectrode (UME). In the absence of NADH, when a functionalized AuNP collision with UME at a suitable voltage, a small current spike is generated due to the oxidation of catechol molecules modified on the surface of AuNP. In the presence of NADH, the current spike is significantly amplified by the combined effects of the oxidation and electrocatalysis for NADH oxidation of catechol molecules. By analyzing the variations of the average peak charges and durations without or with NADH, we calculate that around five thousands NADH molecules could be catalyzed per second by single catechol molecule, suggesting the successful establishment of this novel catalytic system. Thus, the proposed strategy could be used as a promising platform for the electrocatalytic researches of other molecular electrocatalytic systems. The electrocatalysis of molecules for various redox reactions have attracted a great deal of attention due to its important significance in biological and energy-related chemistry.1-7 Recently, the electrocatalysis studies of single nanoparticle (NP) at micro-interface between nanoparticles and ultramicroelectrodes (UMEs) has aroused many interest, and the collision of NPs at UME has been successfully employed to study the electrocatalytic efficiency of various electrocatalytic reactions based on nanoparticles (NPs), including gold,8 platinum9-11 stannic oxide,12 titanium dioxide,13 ferroferric oxide13 and iridium oxide nanoparticles.14 Moreover, it has also shown enormous potential in single-molecule detection and catalytic efficiency analysis.15-17 However, most of the previous published works are limited to use bare metal NPs without any surface modification, causing a restriction for the choice of catalytic reactions. Therefore, there is an urgent need to study and design new types of NPs with shell more adaptable for the establishment of various catalytic systems. By modifying catalytically active molecules on the surface of NPs, it would be more convenient to establish single nanoparticle catalytic systems to study typical catalytic reactions at singlemolecular level.18 In this work, we designed a novel electrocatalytic system based on nanoparticles with smart shell, which realized the electrocatalytic efficiency analysis of single molecule for NADH oxidation by collision between the modified nanoparticles and UME. The proposed AuNPs have a core-shell structure, with an inert gold core and a smart shell composed by electrocatalytic molecules. Here, we used 4-thiol-catechol to form the shell due to its significant electrocatalytic activity for NADH oxidation.19-23 Typically, as shown in Figure 1, when the NPs diffused to UMEs surfaces in the absence of NADH, the catechol molecules modified on the surface of NPs will be

first oxidized into catalytically active o-benzoquinone, which induced a small anodic current spike; while in the presence of NADH, the produced o-benzoquinone molecules could be again reduced to catechol form during the oxidation of NADH, thus generated a redox cycle which could continuously oxidize NADH until the NPs escape from the UMEs surfaces. As a result, a significantly amplified current spike would be produced by the combined effects of the oxidation and electrocatalysis for NADH oxidation of catechol molecules. By measuring the peak currents and charges in the absence and presence of NADH, the catalytic efficiency of a single catechol molecule could be calculated, which enhanced the understand

Figure 1. Schematic illustration of the electrocatalytic process occurring at micro-interface between NPs and UME.

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Figure 2. A) Raman spectra of (a) bare AuNPs, (b) CS1 in solid state, and (c) CS1@AuNPs; B) CV curves of CS1@ AuNPs modified GC electrodes in 0.10 M PBS without (a) or with (b) NADH (1 mM) at 100 mV / s; inset: TEM image of 45 nm CS1@AuNPs.

ing of molecular electrocatalysis. The proposed method successfully realized the electrocatalytic oxidation of NADH using the smart shell-functionalized AuNPs, and could be used to study the molecular electrocatalysis at the single-molecule level. For the first time, the electrocatalytic efficiency of single-molecule was studied by single functionalized NP collision on the surface of ultramicroelectrodes. Citrate-capped AuNPs of 45 ± 5 nm in diameter were synthesized according to a reported aqueous-phase method.24-26 Three thiol-catechol analogues 4-mercaptobenzene-1, 2-diol (CS1), N-(3, 4-dihydroxyphenethyl)-2-mercaptoacetamide (CS2) and N-(3, 4-dihydroxyphenethyl)-5-mercaptopentanamide (CS3) with different alkyl spacers were synthesized and then used to modify the AuNPs, respectively (Figure 1). The S-Au bond is frequently used in the junctions of molecular wire to link AuNPs to various organic molecules, especially pconjugated aromatic thiolates.27 Surface enhanced Raman scattering (SERS) was applied to characterize the three AuNPs modified by CS1, CS2 and CS3, respectively. As shown in Figure. 2A, bare AuNPs showed no characteristic Raman peaks of CS1. After the functionalization of CS1, the AuNPs showed the characteristic peaks of CS1 near 780 cm-1, 1090 cm-1 and 1600 cm-1,28-30 demonstrating the successful binding of CS1 on the AuNPs surfaces. The successful synthesis of CS2 and CS3-tagged AuNPs were also been confirmed by SERS (Figure. S1). Transmission electron microscope (TEM) showed a good dispersion of CS1-tagged AuNPs (Figure 2B) as well as bare AuNPs (Figure S2), which was especially favourable for the single NPs collision experiments. In order to determine the electrocatalysis of the three thiolcatechol analogues for the oxidation of NADH, cyclic voltammetry (CV) was performed on glassy carbon (GC) electrodes (3 mm in diameter) modified by three core-shell AuNPs, CS1-tagged AuNPs(CS1@AuNPs), CS2-tagged AuNPs (CS2@AuNPs) and CS3-tagged AuNPs (CS3@AuNPs), respectively. As shown in Figure 2B, the oxidation peak of CS1@AuNPs modified GC electrode was observed at 0.150 V, with an oxidation current of 1.6 µA (Figure 2B-a). After the addition of NADH, the oxidation peak shifted to 0.350 V, while the oxidation current increased about 7 times from 1.6 µA to 12 µA (Figure 2B-b). For comparison, the CV of GC electrodes modified with CS2@AuNPs and CS3@AuNPs were also performed and showed good elelctrocatalytic effect for NADH oxidation (Figure S1). After the addition of NADH, the oxidation peak currents increased 7, 4, and 4 times on CS1@AuNPs/GC, CS2@AuNPs/GC, and CS3@AuNPs/GC electrodes, respectively. Therefore, we chose CS1@AuNPs to

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perform the following single NP electrocatalytic and measurement experiments due to its best performance. In addition, a control experiment was performed to test the catalytic effect of bare AuNPs on glassy carbon (GC) electrodes. Little shift of the oxidation potential could be observed before and after the addition of the NADH, which indicated that bare AuNPs was inert for the electrocatalytic oxidation of NADH (Figure S3). The electrocatalytic effect of catechol for NADH oxidation was also was investigated by cyclic voltammetry in the absence of AuNPs (Figure S4). Cyclic voltammetry (CV) were performed on glassy carbon (GC) electrodes in 0.2 M PBS (pH 7.0) containing 1 mM catechol with or without 5 mM NADH, respectively. A significantly enhancement of the anodic peak current was found to occur with NADH, which indicated that catechol molecule have a good electrocatalytic effect for NADH oxidation. For the single NP catalyzed experiments, a 7 µm diameter carbon-fiber UME was used as the working electrode, a conventional Ag/AgCl electrode was served both as the reference and counter electrodes (two-electrode cell). The carbon-fiber UME was visually confirmed by dark-field microscopy (Figure S5). Before each experiment, a steady-state CV corresponding to the diffusion limited oxidation of ferrocenylmethanol (FcMeOH) was recorded to confirm the stability and quality of the UME (Figure S6). To choose the favorable experimental condition for the establishment of the proposed catechol/NADH redox cycle, the chronoamperometric curves of single CS1@AuNP collisions were recorded under different potentials. As shown in Figure. S6, few spikes were observed when the potential was below 0.500 V, which was insufficient for the oxidation of catechol molecules. When the potential increased to 0.500 V, current spikes emerged, indicating the establishment of the redox cycle. The frequency and magnitude of the collision current spikes increased with the rising potential, until reaching a maximum value at 0.700 V. After the potential passed above 0.800 V, the current spikes began

Figure 3. (A) Chronoamperometric curves for single CS1@AuNP collisions at Carbon-fiber UME in a 15 mM phosphate buffer (pH 7.0) containing various concentrations of NADH from o to 10 mM. Applied potential is 0.7 V (vs Ag/AgCl). (B) Correlation between average peak current and concentration of NADH. (C) Correlation between average peak charge and concentration of NADH. The data acquisition time is 20 s, and 3 replicate measurements.

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Figure 4. Chronoamperometric curves of a carbon fiber electrode in PBS (15 mM, pH = 7.0) without (A) and with (B) NADH (5 mM) before and after adding 100 pM CS1@AuNPs at 0.700 V (red arrows represent the injection of CS1@AuNPs). The detail of the impact spikes are shown in the boxes. The statistical distribution of the peak current (a), charge (b) and duration (c) per spike during CS1@AuNPs collision with the GC electrode in the absence (C) and presence (D) of NADH.

to decline, which could be attribute to the collapse of the redox cycle, as the NADH molecules were directly oxidized at the surface of UME thus escaped from the redox cycle. Therefore, 0.700 V was chosen as the optimum potential. Afterwards, the chronoamperometric curves were tested under 0.700 V with different concentrations of NADH. As shown in Figure 3, the frequency and magnitude of the collision current spikes kept increasing with the increasing concentration. The average peak current increased from 31 pA to 85 pA (Figure 3B) and the average peak charge increased from 6 fC to 31 fC (Figure 3C) because of the increasing NADH concentration. These increases could be due to a higher concentration of NADH resulted in a higher reaction rate according to the equation of reaction rate. However, the AuNPs began to aggregate after the concentration of NADH reached 7 mM, due to the ascending of the concentration of ions in AuNPs solutions. Therefore, 5 mM NADH was used in the following experiments. The recording of current signals incorporates the usage of a filter, which results in the deformation of the blockage shape, particularly for rapid collision events (Figure S7). We chose 5 KHz to perform the experiments.

To further discuss the effect of CS1@AuNP collision triggered NADH oxidation, we counted the collision frequency of CS1@AuNPs in the absence and presence of NADH, to be 0.5s-1 (Figure 4A) and 4.0 s-1 (Figure 4B), respectively. An obviously increased collision frequency could be observed in the presence of NADH. As mentioned above, in the presence of NADH, the smart catechol shell was firstly oxidized into obenzoquinone molecules, and then the produced obenzoquinone molecules could be again reduced to catechol during the oxidation of NADH to NAD+. Therefore, a redox cycle was generated, which enabled the reutilization of the CS1@AuNPs. However, in the absence of NADH, after the catechol shell was oxidized, it could not return to the reduction state, which stopped the formation of the redox cycle and resulted in a consumption of CS1@AuNPs. As a result, the concentration of CS1@AuNPs in the presence of NADH at the UME surface was much higher than that in the absence of NADH, which was reflected by the higher collision frequency.31, 32 In addition, catechol molecules modified on the surface of AuNPs were partly oxidized in the absence of NADH, which might induce some undistinguishable current spikes.

At the optimum conditions from the above experiments, the chronoamperometric curves were recorded in phosphate buffer solution (PBS, 15 mM, pH 7.0). As shown in Figure. 4A, only a small background current was observed with a noise signal around 15 pA. After CS1@AuNPs were injected into the above solution under the same potential, a few small spikes were observed, due to the oxidation of CS1 modified on AuNPs surfaces, into o-benzoquinone (ox-CS1). Another chronoamperogram curves were recorded at same potential in PBS (15 mM) containing 5 mM NADH without CS1@AuNPs (Figure. 4B), a background current was observed with a noise level of around 40 pA, the increased background might come from the adsorption of NADH on the electrode surface (Figure. S8). Afterwards, CS1@AuNPs were added to the above solution, which induced intensive spikes with substantially increased peak current, due to the overlay of the catechol molecular shell oxidation and the NADH oxidation. In addition, a control experiment was performed in PBS using bare AuNPs, no spikes were observed in the absence or presence of NADH, which demonstrated that the oxidation currents came from the smart shell instead of the AuNP core (Figure S9).

The rough electrocatalytic efficiency of the smart catechol shell for NADH could be directly observed from the variation of mean peak current. The mean peak currents without NADH (Figure 4C) and with NADH (Figure 4D) were counted to be 32 pA and 81 pA, respectively. In the presence of NADH, the current value increased 2.5 times, indicating a favorable electrocatalytic efficiency of catechol for the oxidation of NADH. In order to obtain more accurate electrocatalytic efficiency of single catechol molecules in the smart shell, the mean charge and durations passed per spike were subsequently analyzed. The average peak charges were 6.2 fC and 20 fC in the absence (Figure 4C) and presence of NADH (Figure 4D), respectively. On the other hand, a longer oxidation durations was also observed in the presence of NADH (Figure 4D), which increased 1.7 times of that without NADH (Figure 4C). The variations of oxidation durations was consistent with the redox cycle of CS1@AuNPs on the surface of UME, indicating the successful establishment of the single particle collision catalytic system. To obtain the number of NADH molecules catalytically ox-

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idized per second on a single catechol molecule, further calculations were performed. Since the oxidation process of NADH involved the transfer of two electrons,33 and the oxidation of catechol to o-benzoquinone was also a two-electron process.34 Therefore, there was a one-to-one correspondence between catechol molecule and one NADH. Because of the linear relationship between the oxidation charge (Q1) of single CS1@AuNPs and the number of molecules (N) tagged on the single NP surface in the absence of NADH, the average number of oxidized catechol molecules (NCatechol) modified on the surface of a single CS1@AuNP was calculated to be 1.9×104 by the following equation (1),35 Q1 = 2eNCatechol

(1)

According to the size of CS1 molecule and the surface area of AuNPs, and assuming CS1 molecules were vertically aligned on the surface of AuNPs, the number of CS1 molecules attached on the surface of a single AuNP was calculated to be 5.4×104. In this case, around 32 % of the CS1 molecules were oxidized.36 Furthermore, the average number of NADH (NNADH) electrocatalytically oxidized by a single CS1@AuNP was calculated to be 7.3×104 by equation (2): Q2 = 2eNNADH

(2)

where the Q2 represented for the oxidation charge of the smart shell with NADH. From the above equations, the catalytic efficiency of a single catechol molecule (n) for NADH oxidation was calculated to be about 5.1×103 s-1, which means one catechol molecule could oxidize about five thousands NADH molecules per second under the effect of an electric field according to equation (3): n = NNADH / tNCatechol

(3)

where the t represented for the average oxidation duration of per spike in the presence of NADH. The above results demonstrated the CS1@AuNP collision triggered NADH oxidation and the associated parameters in the absence or presence of NADH were listed in the table 1 for the convenience of comparison. Table 1. Oxidation Currents, Charges and Durations of collision Signals. CS1@AuNPs (100 pM)

Current (pA)

Charge (fC)

Durations (ms)

Signal composition

NADH (-)

32

6.2

0.45

CS1 oxidation

NADH (+)

81

20

0.76

CS1 oxidation & NADH electrocatalytic oxidation

The histograms of oxidation currents, charges and durations for CS1@AuNPs are shown in Figure 4. “-” represents that containing 0 mM NADH in 15 PBS buffer, “+” represents that containing 5 mM NADH in 15 mM PBS buffer.

CONCLUSION In conclusion, a novel single nanoparticle redox catalytic system based on AuNPs with a smart catechol shell and UMEs has been successfully established, which realized the accurate

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electrocatalytic efficiency analysis of single catechol molecule for NADH oxidation. A significantly amplified current spike has been observed as the NP impact the electrode in the presence of NADH, which reflects the favorable electrocatalytic activity of the catechol shell. The signal reading of the catalytic current spikes is apparently affected by the setting of filter frequency. Furthermore, we obtained the catalytic efficiency of a single catechol molecule by analyzing the peak charge and durations in the absence and presence of NADH. Therefore, the proposed method have a potential application for the electrocatalytic researches of other molecular electrocatalytic systems.

ASSOCIATED CONTENT Supporting Information The synthesis and characterization of compounds CS1, CS2 and CS3, electrocatalytic activity of catechol functionalized AuNPs using GC electrodes, synthesis and characterization of gold nanoparticles, fabrication and characterization of carbon fiber microelectrodes, the influences of potentials and filter frequency for chronoamperometric curves. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel/Fax: 86-21-64252339. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (No. 21327807, 21421004), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. YJ0130504), the Program of Shanghai Subject Chief Scientist (No. 15XD1501200), the Programme of Introducing Talents of Discipline to Universities (No. B16017).

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