O₂ Plasma Etching and Antistatic Gun Surface Modifications for CNT Yarn Microelectrode Improve Sensitivity and Antifouling Properties

Carbon nanotube (CNT) based microelectrodes exhibit rapid and selective detection of neurotransmitters. While different fabrication strategies and geometries of CNT microelectrodes have been characterized, relatively little research has investigated ways to selectively enhance their electrochemical properties. In this work, we introduce two simple, reproducible, low-cost, and efficient surface modification methods for carbon nanotube yarn microelectrodes (CNTYMEs): O₂ plasma etching and antistatic gun treatment. O₂ plasma etching was performed by a microwave plasma system with oxygen gas flow and the optimized time for treatment was 1 min. The antistatic gun treatment flows ions by the electrode surface; two triggers of the antistatic gun was the optimized number on the CNTYME surface. Current for dopamine at CNTYMEs increased 3-fold after O₂ plasma etching and 4-fold after antistatic gun treatment. When the two treatments were combined, the current increased 12-fold, showing the two effects are due to independent mechanisms that tune the surface properties. O₂ plasma etching increased the sensitivity due to increased surface oxygen content but did not affect surface roughness while the antistatic gun treatment increased surface roughness but not oxygen content. The effect of tissue fouling on CNT yarns was studied for the first time, and the relatively hydrophilic surface after O₂ plasma etching provided better resistance to fouling than unmodified or antistatic gun treated CNTYMEs. Overall, O₂ plasma etching and antistatic gun treatment improve the sensitivity of CNTYMEs by different mechanisms, providing the possibility to tune the CNTYME surface and enhance sensitivity

High Temporal Resolution Measurements of Dopamine with Carbon Nanotube Yarn Microelectrodes

Fast-scan cyclic voltammetry (FSCV) can detect small changes in dopamine concentration; however, measurements are typically limited to scan repetition frequencies of 10 Hz. Dopamine oxidation at carbon-fiber microelectrodes (CFMEs) is dependent on dopamine adsorption, and increasing the frequency of FSCV scan repetitions decreases the oxidation current, because the time for adsorption is decreased. Using a commercially available carbon nanotube yarn, we characterized carbon nanotube yarn microelectrodes (CNTYMEs) for high-speed measurements with FSCV. For dopamine, CNTYMEs have a significantly lower Δ<i>E</i>_p than CFMEs, a limit of detection of 10 ± 0.8 nM, and a linear response to 25 μM. Unlike CFMEs, the oxidation current of dopamine at CNTYMEs is independent of scan repetition frequency. At a scan rate of 2000 V/s, dopamine can be detected, without any loss in sensitivity, with scan frequencies up to 500 Hz, resulting in a temporal response that is four times faster than CFMEs. While the oxidation current is adsorption-controlled at both CFMEs and CNTYMEs, the adsorption and desorption kinetics differ. The desorption coefficient of dopamine<i>-o-</i>quinone (DOQ), the oxidation product of dopamine, is an order of magnitude larger than that of dopamine at CFMEs; thus, DOQ desorbs from the electrode and can diffuse away. At CNTYMEs, the rates of desorption for dopamine and dopamine<i>-o-</i>quinone are about equal, resulting in current that is independent of scan repetition frequency. Thus, there is no compromise with CNTYMEs: high sensitivity, high sampling frequency, and high temporal resolution can be achieved simultaneously. Therefore, CNTYMEs are attractive for high-speed applications

Cooperative Island Growth of Large-Area Single-Crystal Graphene on Copper Using Chemical Vapor Deposition

In this work we explore the kinetics of single-crystal graphene growth as a function of nucleation density. In addition to the standard methods for suppressing nucleation of graphene by pretreatment of Cu foils using oxidation, annealing, and reduction of the Cu foils prior to growth, we introduce a new method that further reduces the graphene nucleation density by interacting directly with the growth process at the onset of nucleation. The successive application of these two methods results in roughly 3 orders of magnitude reduction in graphene nucleation density. We use a kinetic model to show that at vanishingly low nucleation densities carbon incorporation occurs by a cooperative island growth mechanism that favors the formation of substrate-size single-crystal graphene. The model reveals that the cooperative growth of millimeter-size single-crystal graphene grains occurs by roughly 3 orders of magnitude increase in the reactive sticking probability of methane compared to that in random island nucleation

New Insights on Electro-Optical Response of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Film to Humidity

Understanding the relative humidity (RH) response of poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) is critical for improving the stability of organic electronic devices and developing selective sensors. In this work, combined gravimetric sensing, nanoscale surface probing, and mesoscale optoelectronic characterization are used to directly compare the RH dependence of electrical and optical conductivities and unfold connections between the rate of water adsorption and changes in functional properties of PEDOT:PSS film. We report three distinct regimes where changes in electrical conductivity, optical conductivity, and optical bandgap are correlated with the mass of adsorbed water. At low (RH < 25%) and high (RH > 60%) humidity levels, dramatic changes in electrical, optical, and structural properties occur, while changes are insignificant in mid-RH (25 < RH < 60%) conditions. We associate the three regimes with water adsorption at hydrophilic moieties at low RH, diffusion and swelling throughout the film at mid-RH, and saturation of the film by water at high RH. Optical film thickness increased by 150% as RH was increased from 9 to 80%. Low frequency (1 kHz) impedance increased by ∼100%, and film capacitance increased by ∼30% as RH increased from 9 to 80% due to an increase in the film dielectric constant. Changes in electrical and optical conductivities concomitantly decrease across the full range of RH tested

Laser Treated Carbon Nanotube Yarn Microelectrodes for Rapid and Sensitive Detection of Dopamine in Vivo

Carbon nanotube yarn microelectrodes (CNTYMEs) exhibit rapid and selective detection of dopamine with fast-scan cyclic voltammetry (FSCV); however, the sensitivity limits their application in vivo. In this study, we introduce laser treatment as a simple, reliable, and efficient approach to improve the sensitivity of CNTYMEs by threefold while maintaining high temporal resolution. The effect of laser treatment on the microelectrode surface was characterized by scanning electron microscopy, Raman spectroscopy, energy dispersion spectroscopy, and laser scanning confocal microscopy. Laser treatment increases the surface area and oxygen containing functional groups on the surface, which provides more adsorption sites for dopamine than at unmodified CNTYMEs. Moreover, similar to unmodified CNTYMEs, the dopamine signal at laser treated CNTYMEs is not dependent on scan repetition frequency, unlike the current at carbon fiber microelectrodes (CFMEs) which decreases with increasing scan repetition frequency. This frequency independence is caused by the significantly larger surface roughness which would trap dopamine-<i>o</i>-quinone and amplify the dopamine signal. CNTYMEs were applied as an in vivo sensor with FSCV for the first time, and laser treated CNTYMEs maintained high dopamine sensitivity compared to CFMEs with an increased scan repetition frequency of 50 Hz, which is 5-fold faster than the conventional frequency. CNTYMEs with laser treatment are advantageous because of their easy fabrication, high reproducibility, fast electron transfer kinetics, high sensitivity, and rapid in vivo measurement of dopamine and could be a potential alternative to CFMEs in the future

K₃Fe(CN)₆: Pressure-Induced Polymerization and Enhanced Conductivity

Recent theoretical studies indicate that applying high pressure (up to tens of gigapascals) to simple compounds with triple bonds can convert the triple bonds to conjugated double bonds, which results in these compounds becoming electrically conductive or even superconductive. This might indicate a new route for the synthesis of inorganic/organic conductors of various compositions and properties and could greatly expand the field of conductive polymers. Here, we present a study of the phase behavior and electrical properties of K₃Fe­(CN)₆ up to ∼15 GPa using Raman spectroscopy, synchrotron X-ray diffraction, and impedance spectroscopy at room temperature. In this pressure range, two new crystalline phases were identified, and their unit cells and space groups were determined. The cyanide ions react to form conjugated CN bonds in two steps, and the electronic conductivity is enhanced by 3 orders of magnitude, from 10^–7 to 10^–4 S·cm^–1. Because this material is also an ionic conductor, these studies might “shed light” on the development of new cathode materials for alkali metal batteries. Enhancing the electrical conductivity by applying high pressure to compounds containing triple bonds could provide a potential route for synthesizing multifunctional conductive materials

High-Performance Flexible Perovskite Solar Cells by Using a Combination of Ultrasonic Spray-Coating and Low Thermal Budget Photonic Curing

Realizing the commercialization of high-performance and robust perovskite solar cells urgently requires the development of economically scalable processing techniques. Here we report a high-throughput ultrasonic spray-coating (USC) process capable of fabricating perovskite film-based solar cells on glass substrates with a power conversion efficiency (PCE) as high as 13%. Perovskite films with high uniformity, crystallinity, and surface coverage are obtained in a single step. Moreover, we report USC processing on TiO₂/ITO-coated polyethylene terephthalate (PET) substrates to realize flexible perovskite solar cells with a PCE as high as 8.1% that are robust under mechanical stress. In this case, a photonic curing technique was used to achieve a highly conductive TiO₂ layer on flexible PET substrates for the first time. The high device performance and reliability obtained by this combination of USC processing with optical curing appear very promising for roll-to-roll manufacturing of high-efficiency, flexible perovskite solar cells

Synthesis of Millimeter-Size Hexagon-Shaped Graphene Single Crystals on Resolidified Copper

We present a facile method to grow millimeter-size, hexagon-shaped, monolayer, single-crystal graphene domains on commercial metal foils. After a brief <i>in situ</i> treatment, namely, melting and subsequent resolidification of copper at atmospheric pressure, a smooth surface is obtained, resulting in the low nucleation density necessary for the growth of large-size single-crystal graphene domains. Comparison with other pretreatment methods reveals the importance of copper surface morphology and the critical role of the melting–resolidification pretreatment. The effect of important growth process parameters is also studied to determine their roles in achieving low nucleation density. Insight into the growth mechanism has thus been gained. Raman spectroscopy and selected area electron diffraction confirm that the synthesized millimeter-size graphene domains are high-quality monolayer single crystals with zigzag edge terminations

Synthesis, Structure, and Pressure-Induced Polymerization of Li₃Fe(CN)₆ Accompanied with Enhanced Conductivity

Pressure-induced polymerization of charged triple-bond monomers like acetylide and cyanide could lead to formation of a conductive metal–carbon network composite, thus providing a new route to synthesize inorganic/organic conductors with tunable composition and properties. The industry application of this promising synthetic method is mainly limited by the reaction pressure needed, which is often too high to be reached for gram amounts of sample. Here we successfully synthesized highly conductive Li₃Fe­(CN)₆ at maximum pressure around 5 GPa and used in situ diagnostic tools to follow the structural and functional transformations of the sample, including in situ X-ray and neutron diffraction and Raman and impedance spectroscopy, along with the neutron pair distribution function measurement on the recovered sample. The cyanide anions start to react around 1 GPa and bond to each other irreversibly at around 5 GPa, which are the lowest reaction pressures in all known metal cyanides and within the technologically achievable pressure range for industrial production. The conductivity of the polymer is above 10^–3 S·cm^–1, which reaches the range of conductive polymers. This investigation suggests that the pressure-induced polymerization route is practicable for synthesizing some types of functional conductive materials for industrial use, and further research like doping and heating can hence be motivated to synthesize novel materials under lower pressure and with better performances

Deciphering Halogen Competition in Organometallic Halide Perovskite Growth

Organometallic halide perovskites (OHPs) hold great promise for next-generation, low-cost optoelectronic devices. During the chemical synthesis and crystallization of OHP thin films, a major unresolved question is the competition between multiple halide species (e.g., I^–, Cl^–, Br^–) in the formation of the mixed-halide perovskite crystals. Whether Cl^– ions are successfully incorporated into the perovskite crystal structure or, alternatively, where they are located is not yet fully understood. Here, in situ X-ray diffraction measurements of crystallization dynamics are combined with ex situ TOF-SIMS chemical analysis to reveal that Br^– or Cl^– ions can promote crystal growth, yet reactive I^– ions prevent them from incorporating into the lattice of the final perovskite crystal structure. The Cl^– ions are located in the grain boundaries of the perovskite films. These findings significantly advance our understanding of the role of halogens during synthesis of hybrid perovskites and provide an insightful guidance to the engineering of high-quality perovskite films, essential for exploring superior-performing and cost-effective optoelectronic devices