Real-Time, Selective Detection of Copper(II) Using Ionophore-Grafted Carbon-Fiber Microelectrodes

Simultaneous detection of neurotransmitters and Cu2+ using double-bore carbon fiber microelectrodes via fast-scan cyclic voltammetry

There is a great demand to broaden our understanding of the multifactorial complex etiology of neurodegenerative diseases to aid the development of more efficient therapeutics and slow down the progression of neuronal cell death. The role of co-transmission and the effect of environmental factors on such diseases have yet to be explored adequately, mainly due to the lack of a proper analytical tool that can perform simultaneous multi-analyte detection in real time with excellent analytical parameters. In this study, we report a simple fabrication protocol of a double-bore carbon-fiber microelectrode (CFM) capable of performing rapid simultaneous detection of neurotransmitters and Cu2+via fast-scan cyclic voltammetry (FSCV) in Tris buffer. After imaging our CFMs via optical microscopy and scanning electron microscopy to ensure the intact nature of the two electrodes in our electrode composite, we performed a detailed analysis of the performance characteristics of our double-bore CFM in five different analyte mixtures, Cu2+-5HT, Cu2+-DA, Cu2+-AA, 5-HT-DA, and 5-HT-AA in Tris buffer, by applying different analyte-specific FSCV waveforms simultaneously. Calibration curves for each analyte in each mixture were plotted while extracting the analytical parameters such as the limit of detection (LOD), linear range, and sensitivity. We also carried out a control experiment series for the same mixtures with single-bore CFMs by applying one waveform at a time to compare the capabilities of our double-bore CFMs. Interestingly, except for the Cu2+-DA solution, all other combinations showed improved LOD, linear ranges, and sensitivity when detecting simultaneously with double-bore CFMs compared to single-bore CFMs, an excellent finding for developing this sensor for future in vivo applications.

A facile approach to create sensitive and selective Cu(ii) sensors on carbon fiber microelectrodes

A facile platform derived from deposition of ethynyl linkers on carbon fiber microelectrodes has been developed for sensitive and selective sensing of Cu(ii). This study is the first to demonstrate the successful anodic deposition of ethynyl linkers, specifically 1,4-diethynylbenzene, onto carbon fiber microelectrodes. Multi-scan deposition of DEB on these microelectrodes resulted in an increased sensitivity and selectivity towards Cu(ii) that persists amidst other divalent interferents and displays sustained performance over four days while stored at room temperature. This method can be extended to other ethynyl terminal moieties, thereby creating a versatile chemical platform that will enable improved sensitivity and selectivity for a new frontier of biomarker measurement.

Carbon Electrode Sensor for the Measurement of Cortisol with Fast-Scan Cyclic Voltammetry

Cortisol is a vital steroid hormone that has been known as the "stress hormone", which is elevated during times of high stress and anxiety and has a significant impact on neurochemistry and brain health. The improved detection of cortisol is critically important as it will help further our understanding of stress during several physiological states. Several methods exist to detect cortisol; however, they suffer from low biocompatibility and spatiotemporal resolution, and they are relatively slow. In this study, we developed an assay to measure cortisol with carbon fiber microelectrodes (CFMEs) and fast-scan cyclic voltammetry (FSCV). FSCV is typically utilized to measure small molecule neurotransmitters by producing a readout cyclic voltammogram (CV) for the specific detection of biomolecules on a fast, subsecond timescale with biocompatible CFMEs. It has seen enhanced utility in measuring peptides and other larger compounds. We developed a waveform that scanned from -0.5 to -1.2 V at 400 V/s to electro-reduce cortisol at the surface of CFMEs. The sensitivity of cortisol was found to be 0.87 ± 0.055 nA/μM (n = 5) and was found to be adsorption controlled on the surface of CFMEs and stable over several hours. Cortisol was co-detected with several other biomolecules such as dopamine, and the waveform was fouling resistant to repeated injections of cortisol on the surface of the CFMEs. Furthermore, we also measured exogenously applied cortisol into simulated urine to demonstrate biocompatibility and potential use in vivo. The specific and biocompatible detection of cortisol with high spatiotemporal resolution will help further elucidate its biological significance and further understand its physiological importance and impact on brain health.

N and P co-doped MXenes nanoribbons for electrodeposition-free stripping analysis of Cu(II) and Hg(II)

The present electrochemical stripping analysis (ESA) for multiple heavy metal ions (HMI) generally requires an electrodeposition process at a very low potential below -1.0 V, which inevitably makes the sensing procedures more complex, inefficient and power-wasting. Meanwhile, the emerging MXenes rising-star materials have been studied in various fields recently. While there are only few reports focusing on the heteroatom doping of MXenes, especially no doping-MXenes for electroanalysis. Based on these issues, a novel multifunctional heteroatoms-doped MXenes nanomaterial, N and P co-doped Ti₃C₂T_x MXenes nanoribbons (N,P-Ti₃C₂T_xR), was prepared herein for the first time, and then N,P-Ti₃C₂T_xR was used as electrode material to propose an electrodeposition-free ESA strategy for multiple HMI (Cu²⁺, Hg²⁺). Owing to the unique spontaneous adsorption and reducing capacities of N,P-Ti₃C₂T_xR towards Cu²⁺ and Hg²⁺ coupled with the excellent sensing performances, Cu²⁺ and Hg²⁺ can undergo self-reduction to be preconcentrated on N,P-Ti₃C₂T_xR surface with the form of Cu⁰ and Hg⁰, thus a simple and ultrasensitive electrodeposition-free ESA platform was developed successfully for the simultaneous detection of Cu²⁺and Hg²⁺. This work opened a new pathway for the detection for multiple HMI and the preparation/application of heteroatoms doping MXenes.

Analysis of Electrochemically Elusive Trace Metals with Carbon Fiber Microelectrodes

There is great interest in rapidly monitoring metals of biological and environmental interest. Electrochemistry is traditionally a powerful tool for metal analysis but can be limited by its scope and low temporal resolution. The scope is limited by the potential window of the working electrode and rapid analysis is limited, in part, by the need for nucleation/growth for preconcentration. In prior work, we showed that a rapid equilibrium mediated preconcentration process facilitated fast scan cyclic voltammetry (FSCV) responses to Cu(II) and Pb(II) at carbon fiber microelectrodes (CFMs). In this manuscript, we apply this same principle to Ca(II), Al(III), Mg(II), and Zn(II), metal ions that are traditionally difficult to electroanalyze. We demonstrate FSCV reduction peaks for these four metals whose positions and amplitudes are dependent on scan rate. The adsorption profiles of these ions onto CFMs follow Langmuir's theory for monolayer coverage. We calculate the thermodynamic equilibrium constant of metal adsorption onto CFMs and find that these constants follow the same order as those previously reported by other groups on other activated carbon materials. Finally, a real-time complexation study is performed with ligands that have preference for divalent or multivalent ions to probe the selectivity of the real-time signal. We observe a linear relationship between formation constant ( k_f) and % change in the FSCV signal and use this correlation to, for the first time, report the k_f of an Al(III)-complex. This work demonstrates the versatility of FSCV as a method with capacity to extend the scope of rapid electroanalysis.

Liquid Cladding Mediated Optical Fiber Sensors for Copper Ion Detection

We present a label-free optical fiber based sensor device to detect copper ions (Cu2+) in water. A multimode optical fiber, with its polymer cladding removed along a 1-cm length, is used for the optical sensor head, where the injected Cu2+ in the liquid phase acts as a liquid cladding for the optical mode. The various Cu2+ concentrations modulate the numerical aperture (NA) of the liquid cladding waveguide part. The degree of NA mismatch between the liquid cladding and solid cladding guided parts gives rise to an optical power transmittance change, forming the sensing principle. The presented liquid cladding fiber sensor exhibits a minimum resolvable refractive index of 2.48 × 10-6. For Cu2+ detection, we functionalize the sensor head surface (fiber core) using chitosan conjugated ethylenediaminetetraacetic acid (EDTA) which captures Cu2+ effectively due to the enhanced chelating effects. We obtain a limit of detection of Cu2+ of 1.62 nM (104 ppt), which is significantly lower than the tolerable level in drinking water (~30 µM), and achieve a dynamic range of 1 mM. The simple structure of the sensor head and the sensing system ensures the potential capability of being miniaturized. This may allow for in-situ, highly-sensitive, heavy metal sensors in a compact format.

Carbon Nanoelectrodes for the Electrochemical Detection of Neurotransmitters

Carbon-based electrodes have been developed for the detection of neurotransmitters over the past 30 years using voltammetry and amperometry. The traditional electrode for neurotransmitter detection is the carbon fiber microelectrode (CFME). The carbon-based electrode is suitable for in vivo neurotransmitter detection due to the fact that it is biocompatible and relatively small in surface area. The advent of nanoscale electrodes is in high demand due to smaller surface areas required to target specific brain regions that are also minimally invasive and cause relatively low tissue damage when implanted into living organisms. Carbon nanotubes (CNTs), carbon nanofibers, carbon nanospikes, and carbon nanopetals among others have all been utilized for this purpose. Novel electrode materials have also required novel insulations such as glass, epoxy, and polyimide coated fused silica capillaries for their construction and usage. Recent research developments have yielded a wide array of carbon nanoelectrodes with superior properties and performances in comparison to traditional electrode materials. These electrodes have thoroughly enhanced neurotransmitter detection allowing for the sensing of biological compounds at lower limits of detection, fast temporal resolution, and without surface fouling. This will allow for greater understanding of several neurological disease states based on the detection of neurotransmitters.

Single-molecule porphyrin-metal ion interaction and sensing application

It remains a significant challenge to study the interactions between metal ions and porphyrin molecules at single ion level. Here, we constructed a nanopore-based sensing for label-free and real-time analysis of the interaction between Cu²⁺ and 5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (TPPS). The results demonstrate that emerging electronic signatures of the Cu²⁺-TPPS complex that is completely different form the original free TPPS were observed in the α-hemolysin (α-HL) nanopore. Based on the distinctive electronic signal patterns between TPPS and Cu²⁺-TPPS complex, the unique nanopore sensor can achieve a highly sensitive detection of Cu²⁺ in aqueous media. The frequency of signature events showed a linear response toward the concentration of Cu²⁺ in the range of 0.03 µM - 1.0 μM, with a detection limit of 16 nM (S/N = 3). The sensing system also exhibited high selectivity against other metal ions, and the feasibility of this approach for practical applications was demonstrated with the determination of Cu²⁺ in running water.