Detailed information about the physicochemical properties of a given surface is important in order to understand and predict the performance of materials in electrochemical applications. Here we present a detailed X-ray absorption spectroscopy study of two different tetrahedral amorphous-carbon (ta-C) thin films and their subsequent electrochemical characterization. The results show marked differences in ta-C surface and bulk properties, namely differences in the amount of surface functional groups and their sp 3 /sp 2 ratios, respectively. In particular, the variation in the oxygen content of the surface leads to significantly different behavior in electrochemical measurements, such as a 10-fold increase in sensitivity for dopamine and stronger response to ascorbic acid. Results of the surface properties were further analyzed by simulations carried out within the framework of density functional theory (DFT) as well as by utilizing Raman spectroscopy.

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We prepare disposable single-walled carbon nanotube network electrodes for the detection of the potent opioid fentanyl, currently a leading cause for opioid overdose deaths in the USA. We show repeatable dry transfer of single-walled carbon nanotube (SWCNT) networks to produce robust electrodes. This process directly produces highly conductive SWCNT electrodes without the need for any further modifications required for conventional carbon electrodes. The realized electrode showed low background currents combined with spontaneous enrichment of fentanyl, resulting in a high signal-to-noise ratio. With this electrode, a detection limit of 11 nM and a linear range of 0.01−1 μM were found for fentanyl. In addition, selectivity is demonstrated in the presence of several common interferents.

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.

A disposable electrochemical test strip for the quantitative point-of-care (POC) determination of acetaminophen (paracetamol) in plasma and finger-prick whole blood was fabricated. The industrially scalable dry transfer process of single-walled carbon nanotubes (SWCNTs) and screen printing of silver were combined to produce integrated electrochemical test strips. Nafion coating stabilized the potential of the Ag reference electrode and enabled the selective detection in spiked plasma as well as in whole blood samples. The test strips were able to detect acetaminophen in small 40 μL samples with a detection limit of 0.8 μM and a wide linear range from 1 μM to 2 mM, well within the required clinical range. After a simple 1:1 dilution of plasma and whole blood, a quantitative detection with good recoveries of 79% in plasma and 74% in whole blood was achieved. These results strongly indicate that these electrodes can be used directly to determine the unbound acetaminophen fraction without the need for any additional steps. The developed test strip shows promise as a rapid and simple POC quantitative acetaminophen assay.

In this study we present for the first time tetrahedral amorphous carbon (ta-C)a partially reduced graphene oxide (PRGO) hybrid electrode nanomaterial platform for electrochemical sensing of dopamine (DA). Graphene oxide was synthesized with the modified Hummer's method. Before modification of ta-C by drop casting, partial reduction of the GO was carried out to improve electrochemical properties and adhesion to the ta-C thin film. A facile nitric acid treatment that slightly reoxidized the surface and modified the surface chemistry was subsequently performed to further improve the electrochemical properties of the electrodes. The largest relative increase was seen in carboxyl groups. The HNO 3 treatment increased the sensitivity toward DA and AA and resulted in a cathodic shift in the oxidation of AA. The fabricated hybrid electrodes were characterized with scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and electrochemical impedance spectroscopy (EIS). Compared to the plain ta-C electrode the hybrid electrode was shown to exhibit superior sensitivity and selectivity toward DA in the presence of ascorbic acid (AA), enabling simultaneous sensing of AA and DA close to the physiological concentrations by cyclic voltammetry (CV) and by differential pulse voltammetry (DPV). Two linear ranges of 0−1 μM and 1−100 μM and a detection limit (S/N = 3.3) of 2.6 nM for DA were determined by means of cyclic voltammetry. Hence, the current work provides a fully CMOS-compatible carbon based hybrid nanomaterial that shows potential for in vivo measurements of DA.

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.