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

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 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.

Electrochemical Immunosensors with PQQ-Decorated Carbon Nanotubes as Signal Labels for Electrocatalytic Oxidation of Tris(2-carboxyethyl)phosphine

“…Previous investigation has demonstrated that a carbon-fiber ultramicroelectrode modified with PQQ–CNTs exhibits high catalytic activity toward the electrochemical oxidation of TCEP through single-nanoparticle collision [ 29 ]. We first attempted to record the electrochemical signal by the direct electron transfer reaction between PQQ–CNTs and the electrode ( Figure 1 A).…”

“…Addition of TCEP to the electrolyte solution induced a clear enhancement in the anodic current within the potential range of 0.2–0.6 V (curve b). Note that the sensor electrode without capture of PQQ–CNT-Ab 2 labels had no contribution to this reaction (curve c); thus, the enhancement of the anodic current in curve b can be attributed to the electrocatalytic oxidation of TCEP by PQQ–CNTs [ 29 ]. We also found that no significant signal enhancement was observed when the antibody–PQQ conjugate was used as the label instead of PQQ–CNT-Ab 2 for signal output, demonstrating that the signal was amplified by using CNTs to load large quantities of PQQ.…”

“…CNT-NH 2 was modified with antibody and PQQ through an amino covalent coupling reaction [ 29 ]. Prior to the modification, CNT-NH 2 was dispersed in 4 M HCl and heated at 60 °C for 12 h. The treated CNT-NH 2 was centrifuged, washed with ultrapure water, and then dried under vacuum overnight.…”

“…Pyrroloquinoline quinone (PQQ) is a redox cofactor adopting an addition–elimination catalytic mechanism in many PQQ-dependent enzymes. Recently, PQQ-decorated nanomaterials and electrodes have been exploited as biomimetic heterogeneous electrocatalysts [ 28 , 29 , 30 , 31 , 32 , 33 ]. The PQQ tags exposed on the nanomaterials’ surface are able to transfer electrons directly to the electrode’s surface.…”

“…The PQQ tags exposed on the nanomaterials’ surface are able to transfer electrons directly to the electrode’s surface. For instance, Long et al suggested that an electrode modified with PQQ-decorated carbon nanotubes (denoted as PQQ–CNTs) exhibits high electrocatalytic activity toward tris(2-carboxyethyl)phosphine (TCEP) oxidation, with a turnover frequency (TOF) of 133 s −1 [ 29 ]. Inspired by this study, herein, we developed a sandwich-type immunosensor with PQQ–CNTs as the electrocatalytic label.…”