Impact of physical and chemical parameters on square wave anodic stripping voltammetry for trace Pb2+ detection in water.

Exposure to lead, a toxic heavy metal, in drinking water is a worldwide problem. Lead leaching from lead service lines, the main contamination source, and other plumbing materials is controlled by the plumbosolvency of water. Square wave anodic stripping voltammetry (SWASV) has been greatly explored as a rapid and portable technique for the detection of trace Pb2+ ions in drinking water. However, the impact of water quality parameters (WQP) on the SWASV technique is not well understood. Herein, SWASV was employed to detect 10 µg L-1 Pb2+ and determine trends in the stripping peak changes in simulated water samples while individually varying the pH, conductivity, alkalinity, free chlorine, temperature, and copper levels. The pH and conductivity were controlled using the buffer 3-(N-morpholino)propanesulfonic acid (MOPS), and NaNO3, respectively and kept at pH = 7.0 and conductivity = 500 µS cm-1 when exploring other WQPs. The working electrode, a gold-nanoparticle-modified carbon nanotube fiber cross-section (AuNP-CNTf-CS) electrode provided sufficiently sharp and prominent peaks for 10 µg L-1 Pb2+ detection as well as good reproducibility, with a relative error of 5.9% in simulated water. We found that conductivity, and temperature had a proportional relationship to the peak height, and pH, alkalinity, free chlorine, and copper had an inverse relationship. In addition, increasing the copper concentration caused broadening and shifting of the Pb2+ stripping peak. At extremely low conductivities (<100 µS cm-1), the voltammograms became difficult to interpret owing to the formation of inverted and distorted peaks. These trends were then also observed within a local drinking water sample in order to validate the results.

Hydrophilic Micro- and Macroelectrodes with Antibiofouling Properties for Biomedical Applications.

Implantable neural electrodes are generally used to record the electrical activity of neurons and to stimulate neurons in the nervous system. Biofouling triggered by inflammatory responses can dramatically affect the performance of neural electrodes, resulting in decreased signal sensitivity and consistency over time. Thus, long-term clinical applications require electrically conducting electrode materials with reduced dimensions, high flexibility, and antibiofouling properties that can reduce the degree of inflammatory reactions and increase the lifetime of neural electrodes. Carbon nanotubes (CNTs) are well known to form flexible assemblies such as CNT fibers. Herein, we report the covalent functionalization of predefined CNT fiber and film surfaces with hydrophilic, antibiofouling phosphorylcholine (PC) molecules. The electrochemical and spectroscopic characteristics, impedance properties, hydrophilicity, and in vitro antifouling nature of the functionalized CNT surfaces were evaluated. The hydrophilicity of the functionalized CNT films was demonstrated by a decrease in the static contact angle from 134.4° ± 3.9° before to 15.7° ± 1.5° after one and fully wetting after three functionalization cycles, respectively. In addition, the extent of protein absorption on the functionalized CNT films was significantly lower than that on the nonfunctionalized CNT film. Surprisingly, the faradic charge-transfer properties and impedance of the CNT assemblies were preserved after functionalization with PC molecules. These functionalized CNT assemblies are promising for the development of low-impedance neural electrodes with higher hydrophilicity and protein-fouling resistance to inhibit inflammatory responses.

Carbon Nanotube Microelectrode Set: Detection of Biomolecules to Heavy Metals.

An ultrasensitive electrochemical microelectrode set (µ-ES), where all three electrodes are made of highly densified carbon nanotube fiber (HD-CNTf) cross sections (length ∼40 µm), embedded in an inert polymer matrix, and exposed open-ended CNTs at the interface, is presented here. Bare open ends of HD-CNTf rods were used as the working (∼40 µm diameter) and counter (∼94 µm diameter) electrodes, while the cross section of a ∼94 µm diameter was electroplated with Ag/AgCl and coated with Nafion to employ as a quasi-reference electrode. The Ag/AgCl/Nafion-coated HD-CNTf rod quasi-reference electrode provided a very stable potential comparable to the commercial porous-junction Ag/AgCl reference electrode. The HD-CNTf rod µ-ES has been evaluated by electrochemical determination of biologically important analytes, i.e., dopamine (DA), ß-nicotinamide adenine dinucleotide (NADH), a diuretic drug, i.e., furosemide, and a heavy metal, i.e., lead ions (Pb2+). Different voltammetric techniques were employed during the study, i.e., cyclic voltammetry (CV), square wave voltammetry (SWV), amperometry, and square wave anodic stripping voltammetry (SWASV). The direct metallic connection to CNTs gives access to the exceptional properties of highly ordered open-ended CNTs as electrochemical sensors. The distinct structural and electronic properties of aligned HD-CNTf rods in the µ-ES demonstrate fast electron transfer kinetics and offer excellent detection performance during testing for different analytes with wide linear ranges, excellent sensitivity, and very low limits of detection.

Parts per trillion detection of heavy metals in as-is tap water using carbon nanotube microelectrodes.

Heavy metal contamination of drinking water is a major global issue. Research reports across the globe show contamination of heavy metals higher than the set standards of the World Health Organization (WHO) and US Environmental Protection Agency (EPA). To our knowledge, no electrochemical sensor for heavy metals with parts per trillion (PPT) limits of detection (LOD) in as-is tap water has been reported or developed. Here, we report a microelectrode that consists of six highly densified carbon nanotube fiber (HD-CNTf) cross sections called rods (diameter ∼69 µm and length ∼40 µm) in a single platform for the ultra-sensitive detection of heavy metals in tap water and simulated drinking water. The HD-CNTf rods microelectrode was evaluated for the individual and simultaneous determination of trace level of heavy metal ions i.e. Cu2+, Pb2+ and Cd2+ in Cincinnati tap water (without supporting electrolyte) and simulated drinking water using square wave stripping voltammetry (SWSV). The microsensor exhibited a broad linear detection range with an excellent limit of detection for individual Cu2+, Pb2+ and Cd2+ of 6.0 nM, (376 ppt), 0.45 nM (92 ppt) and 0.24 nM (27 ppt) in tap water and 0.32 nM (20 ppt), 0.26 nM (55 ppt) and 0.25 nM (28 ppt) in simulated drinking water, respectively. The microelectrode was shown to detect Pb2+ ions well below the WHO and EPA limits in a broad range of water quality conditions reported for temperature and conductivity in the range of 5 °C-45 °C and 55 to 600 µS/cm, respectively.

True Picomolar Neurotransmitter Sensor Based on Open-Ended Carbon Nanotubes.

Neurotransmitters are important chemicals in human physiological systems for initiating neuronal signaling pathways and in various critical health illnesses. However, concentration of neurotransmitters in the human body is very low (nM or pM level) and it is extremely difficult to detect the fluctuation of their concentrations in patients using existing electrochemical biosensors. In this work, we report the performance of highly densified carbon nanotubes fiber (HD-CNTf) cross-sections called rods (diameter ∼ 69 µm, and length ∼ 40 µm) as an ultrasensitive platform for detection of common neurotransmitters. HD-CNTf rods microelectrodes have open-ended CNTs exposed at the interface with electrolytes and cells and display a low impedance value, i.e., 1050 Ω. Their fabrication starts with dry spun CNT fibers that are encapsulated in an insulating polymer to preserve their structure and alignment. Arrays of HD-CNTf rods microelectrodes were applied to detect neurotransmitters, i.e., dopamine (DA), serotonin (5-HT), epinephrine (Epn), and norepinephrine (Norepn), using square wave voltammetry (SWV) and cyclic voltammetry (CV). They demonstrate good linearity in a broad linear range (1 nM to 100 µM) with an excellent limit of detection, i.e., 32 pM, 31 pM, 64 pM, and 9 pM for DA, 5-HT, Epn, and Norepn, respectively. To demonstrate practical application of HD-CNTf rod arrays, detection of DA in human biological fluids and real time monitoring of DA release from living pheochromocytoma (PC12) cells were performed.