Protein Adsorption and Its Effects on Electroanalytical Performance of Nanocellulose/Carbon Nanotube Composite Electrodes.

Protein fouling is a critical issue in the development of electrochemical sensors for medical applications, as it can significantly impact their sensitivity, stability, and reliability. Modifying planar electrodes with conductive nanomaterials that possess a high surface area, such as carbon nanotubes (CNTs), has been shown to significantly improve fouling resistance and sensitivity. However, the inherent hydrophobicity of CNTs and their poor dispersibility in solvents pose challenges in optimizing such electrode architectures for maximum sensitivity. Fortunately, nanocellulosic materials offer an efficient and sustainable approach to achieving effective functional and hybrid nanoscale architectures by enabling stable aqueous dispersions of carbon nanomaterials. Additionally, the inherent hygroscopicity and fouling-resistant nature of nanocellulosic materials can provide superior functionalities in such composites. In this study, we evaluate the fouling behavior of two nanocellulose (NC)/multiwalled carbon nanotube (MWCNT) composite electrode systems: one using sulfated cellulose nanofibers and another using sulfated cellulose nanocrystals. We compare these composites to commercial MWCNT electrodes without nanocellulose and analyze their behavior in physiologically relevant fouling environments of varying complexity using common outer- and inner-sphere redox probes. Additionally, we use quartz crystal microgravimetry with dissipation monitoring (QCM-D) to investigate the behavior of amorphous carbon surfaces and nanocellulosic materials in fouling environments. Our results demonstrate that the NC/MWCNT composite electrodes provide significant advantages for measurement reliability, sensitivity, and selectivity over only MWCNT-based electrodes, even in complex physiological monitoring environments such as human plasma.

Electrochemical Detection of Oxycodone and Its Main Metabolites with Nafion-Coated Single-Walled Carbon Nanotube Electrodes.

Oxycodone is a strong opioid frequently used as an analgesic. Although proven efficacious in the management of moderate to severe acute pain and cancer pain, use of oxycodone imposes a risk of adverse effects such as addiction, overdose, and death. Fast and accurate determination of oxycodone blood concentration would enable personalized dosing and monitoring of the analgesic as well as quick diagnostics of possible overdose in emergency care. However, in addition to the parent drug, several metabolites are always present in the blood after a dose of oxycodone, and to date, there is no electrochemical data available on any of these metabolites. In this paper, a single-walled carbon nanotube (SWCNT) electrode and a Nafion-coated SWCNT electrode were used, for the first time, to study the electrochemical behavior of oxycodone and its two main metabolites, noroxycodone and oxymorphone. Both electrode types could selectively detect oxycodone in the presence of noroxycodone and oxymorphone. However, we have previously shown that addition of a Nafion coating on top of the SWCNT electrode is essential for direct measurements in complex biological matrices. Thus, the Nafion/SWCNT electrode was further characterized and used for measuring clinically relevant concentrations of oxycodone in buffer solution. The limit of detection for oxycodone with the Nafion/SWCNT sensor was 85 nM, and the linear range was 0.5-10 µM in buffer solution. This study shows that the fabricated Nafion/SWCNT sensor has potential to be applied in clinical concentration measurements.

Electrochemical Fouling of Dopamine and Recovery of Carbon Electrodes.

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 H2SO4 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.

Nanoscale geometry determines mechanical biocompatibility of vertically aligned nanofibers.

Vertically aligned carbon nanofibers (VACNFs) are promising material candidates for neural biosensors due to their ability to detect neurotransmitters in physiological concentrations. However, the expected high rigidity of CNFs could induce mechanical mismatch with the brain tissue, eliciting formation of a glial scar around the electrode and thus loss of functionality. We have evaluated mechanical biocompatibility of VACNFs by growing nickel-catalyzed carbon nanofibers of different lengths and inter-fiber distances. Long nanofibers with large inter-fiber distance prevented maturation of focal adhesions, thus constraining cells from obtaining a highly spread morphology that is observed when astrocytes are being contacted with stiff materials commonly used in neural implants. A silicon nanopillar array with 500 nm inter-pillar distance was used to reveal that this inhibition of focal adhesion maturation occurs due to the surface nanoscale geometry, more precisely the inter-fiber distance. Live cell atomic force microscopy was used to confirm astrocytes being significantly softer on the long Ni-CNFs compared to other surfaces, including a soft gelatin hydrogel. We also observed hippocampal neurons to mature and form synaptic contacts when being cultured on both long and short carbon nanofibers, without having to use any adhesive proteins or a glial monoculture, indicating high cytocompatibility of the material also with neuronal population. In contrast, neurons cultured on a planar tetrahedral amorphous carbon sample showed immature neurites and indications of early-stage apoptosis. Our results demonstrate that mechanical biocompatibility of biomaterials is greatly affected by their nanoscale surface geometry, which provides means for controlling how the materials and their mechanical properties are perceived by the cells. STATEMENT OF SIGNIFICANCE: Our research article shows, how nanoscale surface geometry determines mechanical biocompatibility of apparently stiff materials. Specifically, astrocytes were prevented from obtaining highly spread morphology when their adhesion site maturation was inhibited, showing similar morphology on nominally stiff vertically aligned carbon fiber (VACNF) substrates as when being cultured on ultrasoft surfaces. Furthermore, hippocampal neurons matured well and formed synapses on these carbon nanofibers, indicating high biocompatibility of the materials. Interestingly, the same VACNF materials that were used in this study have earlier also been proven to be capable for electrophysiological recordings and sensing neurotransmitters at physiological concentrations with ultra-high sensitivity and selectivity, thus providing a platform for future neural probes or smart culturing surfaces with superior sensing performance and biocompatibility.

Integrated Carbon Nanostructures for Detection of Neurotransmitters.

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.

Improving the function of dopamine electrodes with novel carbon materials.

For therapeutic purposes, an accurate measurement of dopamine level in situ would be highly desirable. A novel strategy for the selective determination of dopamine concentration based on the diamond-like carbon (DLC) electrode is presented in this abstract. The developed DLC electrode is able to detect 10 µM dopamine and has improved sensitivity compared to platinum. Compared to carbon fiber electrodes, the DLC electrode is more stable because the background current is much lower.