Graphene Oxide: Key to Efficient Charge Extraction and Suppression of Polaronic Transport in Hybrids with Poly (3-hexylthiophene) Nanoparticles.

Nanoparticles (NPs) of conjugated polymers in intimate contact with sheets of graphene oxide (GO) constitute a promising class of water-dispersible nanohybrid materials of increased interest for the design of sustainable and improved optoelectronic thin-film devices, revealing properties exclusively pre-established upon their liquid-phase synthesis. In this context, we report for the first time the preparation of a P3HTNPs-GO nanohybrid employing a miniemulsion synthesis approach, whereby GO sheets dispersed in the aqueous phase serve as a surfactant. We show that this process uniquely favors a quinoid-like conformation of the P3HT chains of the resulting NPs well located onto individual GO sheets. The accompanied change in the electronic behavior of these P3HTNPs, consistently confirmed by the photoluminescence and Raman response of the hybrid in the liquid and solid states, respectively, as well as by the properties of the surface potential of isolated individual P3HTNPs-GO nano-objects, facilitates unprecedented charge transfer interactions between the two constituents. While the electrochemical performance of nanohybrid films is featured by fast charge transfer processes, compared to those taking place in pure P3HTNPs films, the loss of electrochromic effects in P3HTNPs-GO films additionally indicates the unusual suppression of polaronic charge transport processes typically encountered in P3HT. Thus, the established interface interactions in the P3HTNPs-GO hybrid enable a direct and highly efficient charge extraction channel via GO sheets. These findings are of relevance for the sustainable design of novel high-performance optoelectronic device structures based on water-dispersible conjugated polymer nanoparticles.

Tungsten Disulfide-Interfacing Nickel-Porphyrin For Photo-Enhanced Electrocatalytic Water Oxidation.

Covalent functionalization of tungsten disulfide (WS2 ) with photo- and electro-active nickel-porphyrin (NiP) is reported. Exfoliated WS2 interfacing NiP moieties with 1,2-dithiolane linkages is assayed in the oxygen evolution reaction under both dark and illuminated conditions. The hybrid material presented, WS2 -NiP, is fully characterized with complementary spectroscopic, microscopic, and thermal techniques. Standard yet advanced electrochemical techniques, such as linear sweep voltammetry, electrochemical impedance spectroscopy, and calculation of the electrochemically active surface area, are used to delineate the catalytic profile of WS2 -NiP. In-depth study of thin films with transient photocurrent and photovoltage response assays uncovers photo-enhanced electrocatalytic behavior. The observed photo-enhanced electrocatalytic activity of WS2 -NiP is attributed to the presence of Ni centers coordinated and stabilized by the N4 motifs of tetrapyrrole rings at the tethered porphyrin derivative chains, which work as photoreceptors. This pioneering work opens wide routes for water oxidation, further contributing to the development of non-noble metal electrocatalysts.

Optical properties and carrier dynamics in Co-doped ZnO nanorods.

The controlled modification of the electronic properties of ZnO nanorods via transition metal doping is reported. A series of ZnO nanorods were synthesized by chemical bath growth with varying Co content from 0 to 20 atomic% in the growth solution. Optoelectronic behavior was probed using cathodoluminescence, time-resolved luminescence, transient absorbance spectroscopy, and the incident photon-to-current conversion efficiency (IPCE). Analysis indicates the crucial role of surface defects in determining the electronic behavior. Significantly, Co-doping extends the light absorption of the nanorods into the visible region, increases the surface defects, and shortens the non-radiative lifetimes, while leaving the radiative lifetime constant. Furthermore, for 1 atomic% Co-doping the IPCE of the ZnO nanorods is enhanced. These results demonstrate that doping can controllably tune the functional electronic properties of ZnO nanorods for applications.

Cobalt-Doped ZnO Nanorods Coated with Nanoscale Metal-Organic Framework Shells for Water-Splitting Photoanodes.

Developing highly efficient and stable photoelectrochemical (PEC) water-splitting electrodes via inexpensive, liquid phase processing is one of the key challenges for the conversion of solar energy into hydrogen for sustainable energy production. ZnO represents one the most suitable semiconductor metal oxide alternatives because of its high electron mobility, abundance, and low cost, although its performance is limited by its lack of absorption in the visible spectrum and reduced charge separation and charge transfer efficiency. Here, we present a solution-processed water-splitting photoanode based on Co-doped ZnO nanorods (NRs) coated with a transparent functionalizing metal-organic framework (MOF). The light absorption of the ZnO NRs is engineered toward the visible region by Co-doping, while the MOF significantly improves the stability and charge separation and transfer properties of the NRs. This synergetic combination of doping and nanoscale surface functionalization boosts the current density and functional lifetime of the photoanodes while achieving an unprecedented incident photon to current efficiency (IPCE) of 75% at 350 nm, which is over 2 times that of pristine ZnO. A theoretical model and band structure for the core-shell nanostructure is provided, highlighting how this nanomaterial combination provides an attractive pathway for the design of robust and highly efficient semiconductor-based photoanodes that can be translated to other semiconducting oxide systems.

Carbon Nanotube Film Electrodes with Acrylic Additives: Blocking Electrochemical Charge Transfer Reactions.

Carbon nanotubes (CNTs) processed into conductive films by liquid phase deposition technologies reveal increasing interest as electrode components in electrochemical device platforms for sensing and energy storage applications. In this work we show that the addition of acrylic latex to water-based CNT inks not only favors the fabrication of stable and robust flexible electrodes on plastic substrates but, moreover, sensitively enables the control of their electrical and electrochemical transport properties. Importantly, within a given concentration range, the acrylic additive in the films, being used as working electrodes, effectively blocks undesired faradaic transfer reactions across the electrode-electrolyte interface while maintaining their capacitance response as probed in a three-electrode electrochemical device configuration. Our results suggest a valuable strategy to enhance the chemical stability of CNT film electrodes and to suppress non-specific parasitic electrochemical reactions of relevance to electroanalytical and energy storage applications.

Bottom-Up Synthesized MoS2 Interfacing Polymer Carbon Nanodots with Electrocatalytic Activity for Hydrogen Evolution.

The preparation of an MoS2 -polymer carbon nanodot (MoS2 -PCND) hybrid material was accomplished by employing an easy and fast bottom-up synthetic approach. Specifically, MoS2 -PCND was realized by the thermal decomposition of ammonium tetrathiomolybdate and the in situ complexation of Mo with carboxylic acid units present on the surface of PCNDs. The newly prepared hybrid material was comprehensively characterized by spectroscopy, thermal means, and electron microscopy. The electrocatalytic activity of MoS2 -PCND was examined in the hydrogen evolution reaction (HER) and compared with that of the corresponding hybrid material prepared by a top-down approach, namely MoS2 -PCND(exf-fun), in which MoS2 was firstly exfoliated and then covalently functionalized with PCNDs. The MoS2 -PCND hybrid material showed superior electrocatalytic activity toward the HER with low Tafel slope, excellent electrocatalytic stability, and an onset potential of -0.16 V versus RHE. The superior catalytic performance of MoS2 -PCND was rationalized by considering the catalytically active sites of MoS2 , the effective charge/energy-transfer phenomena from PCNDs to MoS2 , and the synergetic effect between MoS2 and PCNDs in the hybrid material.

In-Situ Growth and Immobilization of CdS Nanoparticles onto Functionalized MoS2 : Preparation, Characterization and Fabrication of Photoelectrochemical Cells.

A facile strategy for the controllable growth of CdS nanoparticles at the periphery of MoS2 en route the preparation of electron donor-acceptor nanoensembles is developed. Precisely, the carboxylic group of α-lipoic acid, as addend of the modified MoS2 obtained upon 1,2-dithiolane functionalization, was employed as anchor site for the in situ preparation and immobilization of the CdS nanoparticles in an one-pot two-step process. The newly prepared MoS2 /CdS hybrid material was characterized by complementary spectroscopic, thermal and microscopy imaging means. Absorption spectroscopy was employed to register the formation of MoS2 /CdS, by observing a broad shoulder centered at 420 nm due to CdS nanoparticles, while the excitonic bands of MoS2 were also evident. Moreover, based on the efficient quenching of the characteristic fluorescence emission of CdS at 725 nm by the presence of MoS2 , strong electronic interactions at the excited state between the two species within the ensemble were identified. Photoelectrochemical assays of MoS2 /CdS thin-film electrodes revealed a prompt, steady and reproducible anodic photoresponse during repeated on-off cycles of illumination. A significant zero-current photopotential of -540 mV and an anodic photocurrent of 1 µA were observed, underlining improved charge-separation and electron transport from CdS to MoS2 . The superior performance of the charge-transfer processes in MoS2 /CdS is of direct interest for the fabrication of photoelectrochemical and optoelectronic devices.

A tool box to ascertain the nature of doping and photoresponse in single-walled carbon nanotubes.

The effect of doping on the electronic properties in bulk single-walled carbon nanotube (SWCNT) samples is studied for the first time using a new in situ Raman spectroelectrochemical method, and further verified by DFT calculations and photoresponse. We use p-/n-doped SWCNTs prepared by diazonium reactions as a versatile chemical strategy to control the SWCNT behavior. The measured and calculated data testify an acceptor effect of 4-aminobenzenesulfonic acid (p-doping), and a donor effect (n-doping) in the case of benzyl alcohol. In addition, pristine and covalently functionalized SWCNTs were used for the preparation of photoactive film electrodes. The photocathodic current in the photoelectrochemical cell is consistently modulated by the doping group. These results validate the in situ Raman spectroelectrochemistry as a unique tool box for predicting the electronic properties of functionalized SWCNTs in the form of thin films and their operational functionality in thin film devices for future optoelectronic applications.

Integrating Water-Soluble Polythiophene with Transition-Metal Dichalcogenides for Managing Photoinduced Processes.

Transition-metal dichalcogenides (TMDs) attract increased attention for the development of donor-acceptor materials enabling improved light harvesting and optoelectronic applications. The development of novel donor-acceptor nanoensembles consisting of poly(3-thiophene sodium acetate) and ammonium functionalized MoS2 and WS2 was accomplished, while photoelectrochemical cells were fabricated and examined. Attractive interactions between the negatively charged carboxylate anion on the polythiophene backbone and the positively charged ammonium moieties on the TMDs enabled in a controlled way and in aqueous dispersions the electrostatic association of two species, evidenced upon titration experiments. A progressive quenching of the characteristic fluorescence emission of the polythiophene derivative at 555 nm revealed photoinduced intraensemble energy and/or electron transfer from the polymer to the conduction band of the two TMDs. Photoelectrochemical assays further confirmed the establishment of photoinduced charge-transfer processes in thin films, with distinct responses for the MoS2- and WS2-based systems. The MoS2-based ensemble exhibited enhanced photoanodic currents offering additional channels for hole transfer to the solution, whereas the WS2-based one displayed increased photocathodic currents providing supplementary pathways of electron transfer to the solution. Moreover, scan direction depending on photoanodic and photocathodic currents suggested the existence of yet unexploited photoinduced memory effects. These findings highlight the value of electrostatic interactions for the creation of novel donor-acceptor TMD-based ensembles and their relevance for managing the performance of photoelectrochemical and optoelectronic processes.

Quantification of Signal-to-Noise Ratio in Cerebral Cortex Recordings Using Flexible MEAs With Co-localized Platinum Black, Carbon Nanotubes, and Gold Electrodes.

Developing new standardized tools to characterize brain recording devices is critical to evaluate neural probes and for translation to clinical use. The signal-to-noise ratio (SNR) measurement is the gold standard for quantifying the performance of brain recording devices. Given the drawbacks with the SNR measure, our first objective was to devise a new method to calculate the SNR of neural signals to distinguish signal from noise. Our second objective was to apply this new SNR method to evaluate electrodes of three different materials (platinum black, Pt; carbon nanotubes, CNTs; and gold, Au) co-localized in tritrodes to record from the same cortical area using specifically designed multielectrode arrays. Hence, we devised an approach to calculate SNR at different frequencies based on the features of cortical slow oscillations (SO). Since SO consist in the alternation of silent periods (Down states) and active periods (Up states) of neuronal activity, we used these as noise and signal, respectively. The spectral SNR was computed as the power spectral density (PSD) of Up states (signal) divided by the PSD of Down states (noise). We found that Pt and CNTs electrodes have better recording performance than Au electrodes for the explored frequency range (5-1500 Hz). Together with two proposed SNR estimators for the lower and upper frequency limits, these results substantiate our SNR calculation at different frequency bands. Our results provide a new validated SNR measure that provides rich information of the performance of recording devices at different brain activity frequency bands (<1500 Hz).

Carbon Nanofoam Supercapacitor Electrodes with Enhanced Performance Using a Water-Transfer Process.

Carbon nanofoam (CNF) is a highly porous, amorphous carbon nanomaterial that can be produced through the interaction of a high-fluence laser and a carbon-based target material. The morphology and electrical properties of CNF make it an ideal candidate for supercapacitor applications. In this paper, we prepare and characterize CNF supercapacitor electrodes through two different processes, namely, a direct process and a water-transfer process. We elucidate the influence of the production process on the microstructural properties of the CNF, as well as the final electrochemical performance. We show that a change in morphology due to capillary forces doubles the specific capacitance of the wet-transferred CNF electrodes.

Self-Assembled Core-Shell CdTe/Poly(3-hexylthiophene) Nanoensembles as Novel Donor-Acceptor Light-Harvesting Systems.

The self-assembly of novel core-shell nanoensembles consisting of regioregular poly(3-hexylthiophene) nanoparticles (P3HTNPs) of 100 nm as core and semiconducting CdTe quantum dots (CdTeQDs) as shell with a thickness of a few tens of nanometers was accomplished by employing a reprecipitation approach. The structure, morphology, and composition of CdTeQDs/P3HTNPs nanoensembles were confirmed by high-resolution scanning transmission microscopy and dynamic light-scattering studies. Intimate interface contact between the CdTeQDs shell and the P3HTNPs core leads to the stabilization of the CdTeQDs/P3HTNPs nanoensemble as probed by the steady-state absorption spectroscopy. Effective quenching of the characteristic photoluminescence of CdTeQDs at 555 nm, accompanied by simultaneous increase in emission of P3HTNPs at 660 and 720 nm, reveals photoinduced charge-transfer processes. Probing the redox properties of films of CdTeQDs/P3HTNPs further proves the formation of a stabilized core-shell system in the solid state. Photoelectrochemical assays on CdTeQDs/P3HTNPs films show a reversible on-off photoresponse at a bias voltage of +0.8 V with a 3 times increased photocurrent compared to CdTeQDs. The improved charge separation is directly related to the unique core-shell configuration, in which the outer CdTeQDs shell forces the P3HTNPs core to effectively act as electron acceptor. The creation of novel donor-acceptor core-shell hybrid materials via self-assembly is transferable to other types of conjugated polymers and semiconducting nanoparticles. This work, therefore, opens new pathways for the design of improved optoelectronic devices.

Covalent functionalization of single-walled carbon nanotubes with polytyrosine: Characterization and analytical applications for the sensitive quantification of polyphenols.

This work reports the synthesis and characterization of single-walled carbon nanotubes (SWCNT) covalently functionalized with polytyrosine (Polytyr); the critical analysis of the experimental conditions to obtain the efficient dispersion of the modified carbon nanotubes; and the analytical performance of glassy carbon electrodes (GCE) modified with the dispersion (GCE/SWCNT-Polytyr) for the highly sensitive quantification of polyphenols. Under the optimal conditions, the calibration plot for the amperometric response of gallic acid (GA) shows a linear range between 5.0 × 10(-7) and 1.7 × 10(-4) M, with a sensitivity of (518 ± 5) m AM(-1) cm(-2), and a detection limit of 8.8 nM. The proposed sensor was successfully used for the determination of total polyphenolic content in tea extracts.

Graphene oxide nanoribbon-based sensors for the simultaneous bio-electrochemical enantiomeric resolution and analysis of amino acid biomarkers.

In this work, a straightforward in-situ measurement of L and D-amino acids (AAs) has been developed using disposable graphene oxide nanoribbon (GON) screen printed electrodes. For that, we took advantage of the electroactivity of certain clinically relevant AAs, such as tyrosine (Tyr) and methionine (Met), which are involved in important bacterial diseases (Bacillus subtilis and Vibrio cholera, respectively). The strategy is based on a dual electrochemical and enzymatic approach. The D-AA with the class enzyme D amino acid oxidase (DAAO) generates H2O2. This H2O2 is simultaneously detected with the L-AA, electroactive molecule by differential pulse voltammetry (DPV). These GON disposable platforms use just 50 µL of sample and a total analysis time of 360 s. Both L and D enantiomers calibration and quantitative analysis were explored and were simultaneously detected with accuracy and precision in urine samples. Any interference of uric acid and other electroactive AAs was noticed. This proposed electrochemical GON-based enantiomeric bio-sensor becomes a highly promising tool as future point of care for fast and reliable early diagnosis of diseases related to the presence of D-AAs.

Study of neuron survival on polypyrrole-embedded single-walled carbon nanotube substrates for long-term growth conditions.

Cultures of primary embryonic rat brain hippocampus neurons with supporting glia cells were carried out on different substrates containing polypyrrole (PPy) and/or single-walled carbon nanotubes (SWCNTs). Neuron adhesion, neurites and dendrites branching elongation, and development of neuron networks on substrates were followed by phase-contrast optical microscopy and quantified to state cell survival and proliferation. Suspensions of as-grown and purified SWCNTs were sprayed on a glass coverslips and PPy/SWCNTs were deposited by potentiodynamic electrochemical deposition. Cell neurotoxicity revealed by neuron death was very high for purified SWCNTs substrates in good agreement with [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] (MTT) test showing lower viability on SWCNTs containing substrates compared with PPy-substrates and control samples probably due to the metal content and the carboxylic groups introduced during the purification. It is interesting to highlight that neurons grown on PPy-substrates adhere developing neurites and branching dendrites earlier even than on control cultures. On subsequent days the neurons are able to adapt to nanotube substrates developing neuron networks for 14-day cultures with similar patterns of complexity for control, PPy and PPy/SWCNT substrates. PPy/SWCNT substrates show a lower impedance value at frequencies under 1 Hz. We have come to the conclusion that glia cells and PPy added to the culture medium and substrates respectively, improve in some degree nanotube biocompatibility, cell adhesion and hence cell viability.