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.

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.

Carbon Nanotube Fibers for Neural Recording and Stimulation.

Recordings and stimulations of neuronal electrical activity are topics of great interest in neuroscience. Many recording techniques, and even treatment of neurological disorders, can benefit from a microelectrode that is flexible, chemically inert, and electrically conducting and preferentially transfers electrons via capacitive charge injection. Commercial electrodes that currently exist and other electrodes that are being tested with the purpose of facilitating and improving the electron transport between solid materials and biological tissues still have some limitations. This paper discusses carbon nanotube (CNT)-based microelectrodes to record and stimulate neurons and compares their electron transport capabilities to noble metals such as Au and Ag. The recording ability of electrodes is tested through electroretinography on Sarcophaga bullata fly eyes by using Au and Ag wires and CNT fibers as electrodes. Stimulation is demonstrated through the implantation of Au wire and CNT fibers into the antennas of the Madagascar hissing cockroach (Gromphadorhina portentosa) to control their locomotion. Our results demonstrate that a particular property of the CNT fiber is its high rate of electron transfer, leading to an order of magnitude lower impedance compared to Au and Ag and an impressive 15.09 charge injection capacity. We also established that this carbon nanomaterial assembly performs well for in vivo electrophysiology, rendering it a promising prospect for neurophysiological applications.

Understanding the effect of nanowire orientation on time evolution of Raman spectra from laser irradiated InAs nanowire surface.

We investigate differences observed in the time evolution of Raman spectra for differently oriented (in plane) InAs nanowires (NWs), using polarized Raman spectroscopy. Specially designed polarized Raman spectroscopy experiments elucidate that laser irradiation leads to the formation of an oriented crystalline oxide film on the InAs NW surface. Both the formation of oriented crystalline oxides and Raman selection rules leading to the presence/absence of oxide peaks in the unpolarized Raman spectra are uncommon occurrences and can lead to incorrect interpretations of the oxidation process, if not looked into carefully. Further, the specially designed heating and cooling experiments for a mixed phase (wurtzite + zinc blende) InAs NW revealed the formation of specific allotropes of elemental As, i.e. gray-As (rhombohedral) and black-As (orthorhombic: metastable) at low (700-950 K) and high simulated temperatures (1000-1300 K) on the InAs NW surface, respectively. Both have high electrical conductivity due to a layered structure and control over the growth of only a few layers using laser irradiation envisages properties similar to graphene. This kind of surface of InAs NWs has the potential for novel device applications, where a semiconductor-insulator-metal heterostructure is required.