A significant problem with implantable sensors is electrode fouling, which has been proposed as the main reason for biosensor failures in vivo. Electrochemical fouling is typical for dopamine (DA) as its oxidation products are very reactive and the resulting polydopamine has a robust adhesion capability to virtually all types of surfaces. The degree of DA fouling of different carbon electrodes with different terminations was determined using cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) approach curves and imaging. The rate of electron transfer kinetics at the fouled electrode surface was determined from SECM approach curves, allowing a comparison of insulating film thickness for the different terminations. SECM imaging allowed the determination of different morphologies, such as continuous layers or islands, of insulating material. We show that heterogeneous modification of carbon electrodes with carboxyl-amine functionalities offers protection against formation of an insulating polydopamine layer, while retaining the ability to detect DA. The benefits of the heterogeneous termination are proposed to be due to the electrostatic repulsion between amino-functionalities and DA. Furthermore, we show that the conductivity of the surfaces as well as the response toward DA was recovered close to the original performance level after cleaning the surfaces for 10-20 cycles in HSO on all materials but pyrolytic carbon (PyC). The recovery capacity of the PyC electrode was lower, possibly due to stronger adsorption of DA on the surface.

Carbon based materials have been frequently used to detect different biomolecules. For example high sp 3 containing hydrogen free diamond-like carbon (DLC) possesses many properties that are beneficial for biosensor applications. Unfortunately, the sensitivities of the DLC electrodes are typically low. Here we demonstrate that by introducing topography on the DLC surface and by varying its layer thickness, it is possible to significantly increase the sensitivity of DLC thin film electrodes towards dopamine. The electrode structures are characterized in detail by atomic force microscopy (AFM) and conductive atomic force microscopy (C-AFM) as well as by transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS). With cyclic voltammetry (CV) measurements we demonstrate that the new improved DLC electrode has a very wide water window, but at the same time it also exhibits fast electron transfer rate at the electrode/solution interface. In addition, it is shown that the sensitivity towards dopamine is increased up to two orders of magnitude in comparison to the previously fabricated DLC films, which are used as benchmarks in this investigation. Finally, it is shown, based on the cyclic voltammetry measurements that dopamine exhibits highly complex behavior on top of these carbon electrodes.

The electrochemical reactions of dopamine, catechol and methylcatechol were investigated at tetrahedral amorphous carbon (ta-C) thin film electrodes. In order to better understand the reaction mechanisms of these molecules, cyclic voltammetry with varying scan rates was carried out at different pH values in H 2 SO 4 and PBS solutions. The results were compared to the same redox reactions taking place at glassy carbon (GC) electrodes. All three catechols exhibited quasireversible behavior with sluggish electron transfer kinetics at the ta-C electrode. At neutral and alkaline pH, rapid coupled homogeneous reactions followed the oxidation of the catechols to the A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTcorresponding o-quinones and led to significant deterioration of the electrode response. At acidic pH, the extent of deterioration was considerably lower. All the redox reactions showed significantly faster electron transfer kinetics at the GC electrode and it was less susceptible toward surfacepassivation. An EC mechanism was observed for the oxidation of dopamine at both ta-C and GC electrodes and the formation of polydopamine was suspected to cause the passivation of the electrodes.

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Successful deployment of carbon nanomaterials in many applications, such as sensing, energy storage, and catalysis, relies on the selection, synthesis, and tailoring of the surface properties. Predictive analysis of the behavior is difficult without detailed knowledge of the differences between various carbon nanomaterials and their surface functionalization, thus leaving the selection process to traditional trial-and-error work. The present characterization fills this knowledge gap for carbon nanomaterial surface properties with respect to chemical states and functionalization. We present an overview of the chemical trends that can be extracted from soft X-ray absorption spectroscopy (XAS) spectra on an extended set of nonideal carbon nanomaterials as a function of sp2 bonded carbon and bond ordering. In particular, the surface chemical state, the presence of long-range order in the carbon matrix, and a qualitative estimation of the amount of oxygen and nitrogen and their respective functional group formation on the material surface, together with the detailed material fabrication parameters, are reported. The results expand our understanding of carbon nanomaterial functionalization, which can support material selection in practice, provided that the specifications of the application are known.

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Carbon-based materials, such as diamond-like carbon (DLC), carbon nanofibers (CNFs), and carbon nanotubes (CNTs), are inherently interesting for neurotransmitter detection due to their good biocompatibility, low cost and relatively simple synthesis. In this paper, we report on new carbon-hybrid materials, where either CNTs or CNFs are directly grown on top of tetrahedral amorphous carbon (ta-C). We show that these hybrid materials have electrochemical properties that not only combine the best characteristics of the individual "building blocks" but their synergy makes the electrode performance superior compared to conventional carbon based electrodes. By combining ta-C with CNTs, we were able to realize electrode materials that show wide and stable water window, almost reversible electron transfer properties and high sensitivity and selectivity for detecting dopamine in the presence of ascorbic acid. Furthermore, the sensitivity of ta-C + CNF hybrids towards dopamine as well as glutamate has been found excellent paving the road for actual in vivo measurements. The wide and stable water window of these sensors enables detection of other neurotransmitters besides DA as well as capability of withstanding higher potentials without suffering from oxygen and hydrogen evolution.

Unmodified and multi-walled carbon nanotube (MWCNT) modified tetrahedral amorphous carbon (ta-C) films of 15 and 50 nm were investigated as potential in vivo sensor materials for the detection of dopamine (DA) in the presence of the main interferents, ascorbic acid (AA) and uric acid (UA). The MWCNTs were grown directly on ta-C by chemical vapor deposition (designated as ta-C+CNT) and were characterized with X-ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Electroanalytical sensitivity and selectivity were determined with cyclic voltammetry. Biocompatibility of the materials was assessed with cell cultures of mouse neural stem cells (mNSCs). The detection limits of DA for both ta-C and ta-C+CNT electrodes ranged from 40 to 85 nM, which are well within the required range for in vivo detection. The detection limits were lower for both ta-C and ta-C+CNT electrodes with 50 nm of ta-C compared to 15 nm. The ta-C electrodes showed a large dynamic linear range of 0.01-100 µM but could not resolve between the oxidation peaks of DA, AA and UA. Modification with MWCNTs, however, resulted in excellent selectivity and all three analytes could be detected simultaneously at physiologically relevant concentrations using cyclic voltammetry. Based on cell culture of mNSCs, both ta-C and ta-C+CNT exhibited good biocompatibility, demonstrating their potential as in vivo sensor materials for the detection of DA.