Assessing the evolution of oxygenated functional groups on the graphene oxide surface upon mild thermal annealing in water.

Graphene oxide (GO) is known to be a 2D metastable nanomaterial that can be reconstructed under thermal annealing into distinct oxidized and graphitic phases. Up to now, such phase transformation, mainly related to epoxide and hydroxyl functional groups, has been usually achieved by thermally treating layers of GO in the solid state. Here, we present the mild annealing of GO dispersed in an aqueous medium, performed at two temperatures, 50 °C and 80 °C, for different intervals of time. We show experimental evidences of the epoxide instability in the presence of water by means of XPS, cyclic voltammetry and Raman spectroscopy, demonstrating the reorganization of epoxide and hydroxyl moieties initiated by water molecules. In fact, at 50 °C an increase in oxygen content is detected in all annealed samples compared to untreated GO, with a transformation of epoxide groups into vicinal diols. On the other hand, at 80 °C the oxygen content decreases towards the initial value since the vicinal diols, previously formed, transform into single hydroxyls and C[double bond, length as m-dash]C bonds. Moreover, the higher temperature annealing likely favours oxygenated functional groups rearrangements and clustering, in accordance with the literature, leading to a higher electron affinity and conductivity of the graphenic network.

Resorc[4]arene Modifiers for Supramolecular Site-Directed Immobilization of Antibodies on Multi-Walled Carbon Nanotubes.

One of the main problems in developing immunosensors featuring carbon nanotubes (CNTs) is immobilizing antibodies (Abs) onto the CNT surface to afford selective binding to target antigens (Ags). In this work, we developed a practical supramolecular Ab conjugation strategy based on resorc[4]arene modifiers. To improve the Ab orientation on the CNTs surface and optimizing the Ab/Ag interaction, we exploited the host-guest approach by synthesizing two newly resorc[4]arene linkers R1 and R2 via well-established procedures. The upper rim was decorated with eight methoxyl groups to promote selective recognition of the fragment crystallizable (Fc ) region of the Ab. Moreover, the lower rim was functionalized with 3-bromopropyloxy or 3-azidopropiloxy substituents to bind the macrocycles on the multi-walled carbon nanotubes (MWCNTs) surface. Accordingly, several chemical modifications of MWCNTs were evaluated. After the morphological and electrochemical characterization of nanomaterials, the resorc[4]arene-modified MWCNTs were deposited onto a glassy carbon electrode surface to evaluate their potential applicability for label-free immunosensor development. The most promising system showed an improved electrode active area (AEL ) of almost 20 % and a site-oriented immobilization of the SARS-CoV-2 spike protein S1 antibody (Ab-SPS1). The developed immunosensor revealed a good sensitivity (23.64 µA mL ng-1 cm-2 ) towards the SPS1 antigen and a limit of detection (LOD) of 1.01 ng mL-1 .

One-pot carboxyl enrichment fosters water-dispersibility of reduced graphene oxide: a combined experimental and theoretical assessment.

Graphene, one of the allotropic forms of carbon, has attracted enormous interest in the last few years due to its unique properties. Reduced graphene oxide (RGO) is known as the nanomaterial most similar to graphene in terms of electronic, chemical, mechanical, and optical properties. It is prepared from graphene oxide (GO) in the presence of different types of reducing agents. Nevertheless, the application of RGO is still limited, owing to its tendency to irreversibly aggregate in an aqueous medium. Herein, we disclosed the preparation of water-dispersible RGO from GO previously enriched with additional carboxyl functional groups through a one-pot reaction, followed by chemical reduction. This novel and unprecedentedly reported reactivity of GO toward the acylating agent succinic anhydride (SA) was experimentally investigated through XPS, Raman, FT-IR, and UV-Vis, and corroborated by DFT calculations, which have shown a peculiar involvement in the functionalization reaction of both epoxide and hydroxyl functional groups. This proposed synthetic protocol avoids use of sodium cyanide, previously reported for carboxylation of graphene, and focuses on the sustainable and scalable preparation of a water-dispersible RGO, paving the way for its application in many fields where the colloidal stability in aqueous medium is required.

Two-phase synthesis of Fe-loaded hydrochar for As removal: The distinct effects of initial pH, reaction time and Fe/hydrochar ratio.

Hydrothermal carbonization (HTC) is a promising technology for producing char material (hydrochar) from waste biomass. In the present work, a two-stage process was applied and optimized to obtain a composite Fe-loaded hydrochar effective in removing arsenic from water. The first stage of carbonization of the biomass in acid conditions was followed by loading Fe3O4 in the second stage into the hydrochar by alkaline co-precipitation. The effect on the kinetics and on the final yield of HTC induced by a variation of the initial acid pH (5.6, 2.0, and 0.5) was tested. Biomass hydrolysis initially decreased the hydrochar yield and released soluble organic species, responsible for the observed pH variation. This effect was more remarkable at the lower initial pH tested. Soluble organic compounds eventually underwent polymerization, with secondary char formation, an inversion of the pH trend and an increase of hydrochar yield and C%. The final pH attained was linearly related to the hydrochar C%, O/C ratio, and initial pH. Better carbonization performances were achieved at pH = 0.5, 200 °C, and 30 min reaction time, which resulted in 53% mass yield and 72 C%. This value is larger than those previously reported for processes conducted at higher temperatures, and it shows how the addition of acid allows working at lower operative temperatures. Fe loading gave better yields at lower hydrochar concentrations, producing an adsorbent with up to 74% Fe3O4, which adsorbed 2.67 mg/g arsenic. Its adsorption capacity was remarkably affected by the stirring method used, indicating that particle-to-particle interactions considerably influence the process. This effect should be better studied for improved applications in fixed-bed columns.

Upcycling Real Waste Mixed Lithium-Ion Batteries by Simultaneous Production of rGO and Lithium-Manganese-Rich Cathode Material.

The direct synthesis of high-value products from end-of-life Li-ion batteries (LIBs), avoiding the complex and costly separation of the different elements, can be reached through a competitive recycling strategy. Here, we propose the simultaneous synthesis of reduced graphene oxide (rGO) and lithium-manganese-rich (Li1.2Mn0.55Ni0.15Co0.1O2 - LMR) cathode material from end-of-life LIBs. The electrode powder recovered after LIBs mechanical pretreatment was directly subjected to the Hummers' method. This way, quantitative extraction of the target metals (Co, Ni, Mn) and oxidation of graphite to graphene oxide (GO) were simultaneously achieved, and a Mn-rich metal solution resulted after GO filtration, owing to the use of KMnO4 as an oxidizing agent. This solution, which would routinely constitute a heavy-metal liquid waste, was directly employed for the synthesis of Li1.2Mn0.55Ni0.15Co0.1O2 cathode material. XPS measurements demonstrate the presence in the synthesized LMR of Cu2+, SO4 2-, and SiO4 4- impurities, which were previously proposed as effective doping species and can thus explain the improved electrochemical performance of recovered LMR. The GO recovered by filtration was reduced to rGO by using ascorbic acid. To evaluate the role of graphite lithiation/delithiation during battery cycling on rGO production, the implemented synthesis procedure was replicated starting from commercial graphite and from the graphite recovered by a consolidated acidic-reductive leaching procedure for metals extraction. Raman and XPS analysis disclosed that cyclic lithiation/delithiation of graphite during battery life cycle facilitates the graphite exfoliation and thus significantly increases conversion to rGO.

Two-Dimensional Restructuring of Cu2O Can Improve the Performance of Nanosized n-TiO2/p-Cu2O Photoelectrodes under UV-Visible Light.

p-Cu2O/n-TiO2 photoanodes were produced by electrodeposition of octahedral p-type Cu2O nanoparticles over n-type TiO2 nanotubes. The photoresponse of the composite p-n photoanodes was evaluated in photoelectrochemical cells operating at "zero-bias" conditions under either visible or UV-vis irradiation. In both operating conditions, the produced electrodes invariably followed the p-n-based photoanode operations but exhibited lower photoelectrochemical performance as compared to the bare n-TiO2 photoanode under UV-vis light. The reported experimental analysis evidenced that such decreased photoactivity is mainly induced by the scarce efficiency of the nanosized p-n interfaces upon irradiation. To overcome such limitation, a restructuring of the originally electrodeposited p-Cu2O was promoted, following a photoelectrochemical post-treatment strategy. p-Cu2O, restructured in a 2D leaf-like morphology, allowed reaching an improved photoelectrochemical performance for the p-n-based photoanode under UV-vis light. As compared to the bare n-TiO2 behavior, such improvement consisted of photoanodic currents up to three times larger. An analysis of the mechanisms driving the transition from compact (∼100 nm) octahedral p-Cu2O to wider (∼1 µm) 2D leaf-like structures was performed, which highlighted the pivotal role played by the irradiated n-TiO2 NTs.

Biosynthesis and physico-chemical characterization of high performing peptide hydrogels@graphene oxide composites.

Hydrogels based on short peptide molecules are interesting biomaterials with wide present and prospective use in biotechnologies. A well-known possible drawback of these materials can be their limited mechanical performance. In order to overcome this problem, we prepared Fmoc-Phe3self-assembling peptides by a biocatalytic approach, and we reinforced the hydrogel with graphene oxide nanosheets. The formulation here proposed confers to the hydrogel additional physicochemical properties without hampering peptide self-assembly. We investigated in depth the effect of nanocarbon morphology on hydrogel properties (i.e. morphology, viscoelastic properties, stiffness, resistance to an applied stress). In view of further developments towards possible clinical applications, we have preliminarily tested the biocompatibility of the composites. Our results showed that the innovative hydrogel composite formulation based on FmocPhe3 and GO is a biomaterial with improved mechanical properties that appears suitable for the development of biotechnological applications.

Effect of Electrolytic Medium on the Electrochemical Reduction of Graphene Oxide on Si(111) as Probed by XPS.

The wafer-scale integration of graphene is of great importance in view of its numerous applications proposed or underway. A good graphene-silicon interface requires the fine control of several parameters and may turn into a high-cost material, suitable for the most advanced applications. Procedures that can be of great use for a wide range of applications are already available, but others are to be found, in order to modulate the offer of different types of materials, at different levels of sophistication and use. We have been exploring different electrochemical approaches over the last 5 years, starting from graphene oxide and resulting in graphene deposited on silicon-oriented surfaces, with the aim of understanding the reactions leading to the re-establishment of the graphene network. Here, we report how a proper choice of both the chemical environment and electrochemical conditions can lead to a more controlled and tunable graphene-Si(111) interface. This can also lead to a deeper understanding of the electrochemical reactions involved in the evolution of graphene oxide to graphene under electrochemical reduction. Results from XPS, the most suitable tool to follow the presence and fate of functional groups at the graphene surface, are reported, together with electrochemical and Raman findings.

Designing Cascades of Electron Transfer Processes in Multicomponent Graphene Conjugates.

A novel family of nanocarbon-based materials was designed, synthesized, and probed within the context of charge-transfer cascades. We integrated electron-donating ferrocenes with light-harvesting/electron-donating (metallo)porphyrins and electron-accepting graphene nanoplates (GNP) into multicomponent conjugates. To control the rate of charge flow between the individual building blocks, we bridged them via oligo-p-phenyleneethynylenes of variable lengths by ß-linkages and the Prato-Maggini reaction. With steady-state absorption, fluorescence, Raman, and XPS measurements we realized the basic physico-chemical characterization of the photo- and redox-active components and the multicomponent conjugates. Going beyond this, we performed transient absorption measurements and corroborated by single wavelength and target analyses that the selective (metallo)porphyrin photoexcitation triggers a cascade of charge transfer events, that is, charge separation, charge shift, and charge recombination, to enable the directed charge flow. The net result is a few nanosecond-lived charge-separated state featuring a GNP-delocalized electron and a one-electron oxidized ferrocenium.

Understanding and Controlling Short- and Long-Range Electron/Charge-Transfer Processes in Electron Donor-Acceptor Conjugates.

We probed a series of multicomponent electron donor2-donor1-acceptor1 conjugates both experimentally and computationally. The conjugates are based on the light harvester and primary electron-donor zinc-porphyrin (ZnP, donor1) to whose ß positions a secondary electron-donor ferrocene (Fc, donor2) and the primary electron-acceptor C60-fullerene (C60, acceptor1) are attached. Linking all of them via p-phenylene-acetylene/acetylene bridges of different lengths to gain full control over shuttling electrons and holes between C60, ZnP, and Fc is novel. Different charge-separation, charge-transfer, and charge-recombination routes have been demonstrated, both by transient absorption spectroscopy measurements on the femto, pico-, nano-, and microsecond time scales and by multiwavelength and target analyses. The molecular wire-like nature of the p-phenylene-acetylene bridges as a function of C60-ZnP and ZnP-Fc distances is decisive in the context of generating distant and long-lived C60•--ZnP-Fc•+ charge-separated states. For the first time, we confirm the presence of two adjacent charge-transfer states, a C60-ZnP•--Fc•+ intermediate in addition to C60•--ZnP•+-Fc, en route to the distant C60•--ZnP-Fc•+ charge-separated state. Our studies demonstrate how the interplay of changes in the reorganization energy and the damping factor of the molecular bridges, in addition to variation in the solvent polarity, affect the outcome of the charge-transfer and corresponding rate constants. The different regions of the Marcus parabola are highly relevant in this matter: The charge recombination of, for example, the adjacent C60•--ZnP•+-Fc charge-separated state is located in the inverted region, while that of the distant C60•--ZnP-Fc•+ charge-separated state lies in the normal region. Here, the larger reorganization energy of Fc relative to ZnP makes the difference.

Biocompatible N-acetyl cysteine reduces graphene oxide and persists at the surface as a green radical scavenger.

We demonstrate that N-acetyl cysteine (NAC) reduces graphene oxide (GO) at room temperature. This represents a new green method to produce reduced GO (rGO). NAC adheres to the rGO surface as demonstrated by several spectroscopy techniques and avoids GO-mediated oxidation of glutathione. This method offers new opportunities for the production of green biocompatible rGO and NAC-based therapies.

Graphene oxide coatings prevent Candida albicans biofilm formation with a controlled release of curcumin-loaded nanocomposites.

AIM: Fabrication of graphene oxide (GO)-based medical devices coatings that limit adhesion of Candida albicans, a main issue of healthcare-associated infections. METHODS: The GO composites noncovalently functionalized with curcumin (CU), a hydrophobic molecule with active antimicrobial action, polyethylene glycol (PEG) that hinders the absorption of biomolecules or a combination of CU and PEG (GO-CU-PEG) were drop-casted on surfaces and antifungal efficacy was assessed. RESULTS: We demonstrate that GO-CU-PEG coatings can reduce fungal adhesion, proliferation and biofilm formation. Furthermore, in an aqueous environment, surfaces release curcumin-PEG nanocomposites that have a minimum inhibitory concentration of 9.25 µg/ml against C. albicans. CONCLUSION: Prevention of early cell adhesion and creation of a proximal environment unfavorable for growth make these GO-supported biomaterials attractive for innovative medical device manufacturing.

Functionalization of Carbon Spheres with a Porphyrin-Ferrocene Dyad.

Meso-tetraphenylporphyrin connected with a ferrocene molecule in the beta-position of the macrocycle through a triple carbon-carbon bond has been bound to carbon spheres using the Prato-Maggini reaction. The ethynyl or/and phenylene ethynylene subunits were chosen as a linking bridge to give a high conjugation degree between the donor (i. e., ferrocene), the photoactive compound (i. e., porphyrin), and the acceptor (i. e., carbon spheres). The molecular bridges have been directly linked to the beta-pyrrole positions of the porphyrin ring, generating a new example of a long-range donor-acceptor system. Steady-state fluorescence studies together with Raman and XPS measurements helped understanding the chemical and physical properties of the porphyrin ring in the new adduct. The spectroscopic characteristics were also compared with those obtained from a similar compound bearing fullerene instead of carbon spheres.

Aqueous polythiophene electrosynthesis: A new route to an efficient electrode coupling of PQQ-dependent glucose dehydrogenase for sensing and bioenergetic applications.

In this study, polythiophene copolymers have been used as modifier for electrode surfaces in order to allow the immobilization of active pyrroloquinoline quinone dependent glucose dehydrogenase (PQQ-GDH) and to simultaneously improve the direct electrical connection of the enzyme with the electrode. Polymer films are electrosynthesized in aqueous solution without the need of surfactants onto carbon nanotubes modified gold electrodes from mixtures of 3-thiopheneacetic acid (ThCH2CO2H) and 3-methoxythiophene (ThOCH3) using a potentiostatic pulse method. Polythiophene deposition significantly improves the bioelectrocatalysis of PQQ-GDH: the process starts at - 200 mV vs. Ag/AgCl and allows well-defined glucose detection at 0 V vs. Ag/AgCl with high current density. Several parameters of the electro-polymerization method have been evaluated to maximize the anodic current output after enzyme coupling. The polymer deposited by this new procedure has been morphologically and chemically characterized by different methods (SEM, EDX, FT-IR, UV-Vis, XPS and Raman spectroscopy). The bioelectrocatalytic response towards increasing glucose concentrations exhibits a dynamic range extending from 1 µM to 2 mM. The low applied potential allows to avoid interferences from easily oxidizable substances such as uric acid and ascorbic acid. Short and long-term stability has been evaluated. Finally, the PQQ-GDH electrode has been coupled to a bilirubin oxidase (BOD)- and carbon nanotube-based cathode in order to test its performance as anode of a biofuel cell. The promising results suggest a further investigation of this kind of polymers and, in particular, the study of the interaction with other enzymes in order to employ them in building up biosensors and biofuel cells.

Taurine grafting and collagen adsorption on PLLA films improve human primary chondrocyte adhesion and growth.

Biocompatible and degradable poly(α-hydroxy acids) are one of the most widely used materials in scaffolds for tissue engineering. Nevertheless, they often need surface modification to improve interaction with cells. Aminolysis is a common method to increase the polymer hydrophilicity and to introduce surface functional groups, able to covalently link or absorb, through electrostatic interaction, bioactive molecules or macromolecules. For this purpose, multi-functional amines, such as diethylenediamine or hexamethylenediamine are used. However, common drawbacks are their toxicity and the introduction of positive charges on the surface. Thus, these kind of modified surfaces are unable to link directly proteins, such as collagens, a promising substrate for many cell types, in particular chondrocytes and osteoblasts. In this work, poly(L-lactide) (PLLA) film surface was labelled with negatively charged sulfonate groups by grafting taurine (TAU) through an aminolysis reaction. The novel modified PLLA film (PLLA-TAU) was able to interact directly with collagen. The reaction was carried out in mild conditions by using a solution of tetrabutylammonium salt of TAU in methanol. ATR-FTIR, XPS and contact angle measurements were used to verify the outcome of the reaction. After the exchange of tetrabutylamonium cation with Na+, collagen was absorbed on the TAU grafted PLLA film (PLLA-TAU-COLL). In vitro biological tests with human primary chondrocytes showed that PLLA-TAU and PLLA-TAU-COLL improved cell viability and adhesion, compared to the unmodified polymer, suggesting that these modifications make PLLA substrate suitable for cartilage repair.

Improving Osteoblast Response In Vitro by a Nanostructured Thin Film with Titanium Carbide and Titanium Oxides Clustered around Graphitic Carbon.

INTRODUCTION: Recently, we introduced a new deposition method, based on Ion Plating Plasma Assisted technology, to coat titanium implants with a thin but hard nanostructured layer composed of titanium carbide and titanium oxides, clustered around graphitic carbon. The nanostructured layer has a double effect: protects the bulk titanium against the harsh conditions of biological tissues and in the same time has a stimulating action on osteoblasts. RESULTS: The aim of this work is to describe the biological effects of this layer on osteoblasts cultured in vitro. We demonstrate that the nanostructured layer causes an overexpression of many early genes correlated to proteins involved in bone turnover and an increase in the number of surface receptors for α3ß1 integrin, talin, paxillin. Analyses at single-cell level, by scanning electron microscopy, atomic force microscopy, and single cell force spectroscopy, show how the proliferation, adhesion and spreading of cells cultured on coated titanium samples are higher than on uncoated titanium ones. Finally, the chemistry of the layer induces a better formation of blood clots and a higher number of adhered platelets, compared to the uncoated cases, and these are useful features to improve the speed of implant osseointegration. CONCLUSION: In summary, the nanostructured TiC film, due to its physical and chemical properties, can be used to protect the implants and to improve their acceptance by the bone.

Calix[4]arene-Functionalised Silver Nanoparticles as Hosts for Pyridinium-Loaded Gold Nanoparticles as Guests.

A series of lipophilic gold nanoparticles (AuNPs) circa 5 nm in diameter and having a mixed organic layer consisting of 1-dodecanethiol and 1-(11-mercaptoundecyl) pyridinium bromide was synthesised by reacting tetraoctylammonium bromide stabilised AuNPs in toluene with different mixtures of the two thiolate ligands. A bidentate ω-alkylthiolate calix[4]arene derivative was instead used as a functional protecting layer on AgNPs of approximately 3 nm. The functionalised nanoparticles were characterised by transmission electron microscopy (TEM), and by UV/Vis and X-ray photoelectron spectroscopy (XPS). Recognition of the pyridinium moieties loaded on the AuNPs by the calix[4]arene units immobilised on the AgNPs was demonstrated in solution of weakly polar solvents by UV/Vis titrations and DLS measurements. The extent of Au-AgNPs aggregation, shown through the low-energy shift of their surface plasmon bands (SPB), was strongly dependent on the loading of the pyridinium moieties present in the organic layer of the AuNPs. Extensive aggregation between dodecanethiol-capped AuNPs and the Ag calix[4]arene-functionalised NPs was also promoted by the action of a simple N-octyl pyridinium difunctional supramolecular linker. This linker can interdigitate through its long fatty tail in the organic layer of the dodecanethiol-capped AuNPs, and simultaneously interact through its pyridinium moiety with the calix[4]arene units at the surface of the modified AgNPs.

A Mechanistic Explanation of the Peculiar Amphiphobic Properties of Hybrid Organic-Inorganic Coatings by Combining XPS Characterization and DFT Modeling.

We report a combined X-ray photoelectron spectroscopy and theoretical modeling analysis of hybrid functional coatings constituted by fluorinated alkylsilane monolayers covalently grafted on a nanostructured ceramic oxide (Al2O3) thin film deposited on aluminum alloy substrates. Such engineered surfaces, bearing hybrid coatings obtained via a classic sol-gel route, have been previously shown to possess amphiphobic behavior (superhydrophobicity plus oleophobicity) and excellent durability, even under simulated severe working environments. Starting from XPS, SEM, and contact angle results and analysis, and combining it with DFT results, the present investigation offers a first mechanistic explanation at a molecular level of the peculiar properties of the hybrid organic-inorganic coating in terms of composition and surface structural arrangements. Theoretical modeling shows that the active fluorinated moiety is strongly anchored on the alumina sites with single Si-O-Al bridges and that the residual valence of Si is saturated by Si-O-Si bonds which form a reticulation with two vicinal fluoroalkylsilanes. The resulting hybrid coating consists of stable rows of fluorinated alkyl chains in reciprocal contact, which form well-ordered and packed monolayers.

Assembly of gold nanoparticles on functionalized Si(100) surfaces through pseudorotaxane formation.

The assembly of gold nanoparticles (AuNPs) on a hydrogenated Si(100) surface, mediated by a series of hierarchical and reversible complexation processes, is reported. The proposed multi-step sequence involves a redox-active ditopic guest and suitable calix[n]arene-based hosts, used as functional organic monolayers of the two inorganic components. Surface reactions and controlled release of AuNPs have been monitored by application of XPS, atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM) and electrochemistry.

Negatively charged gold atoms in subnanometric particles: experimental evidence from an X-ray photoelectron spectroscopy study.

The results of an X-ray Photoelectron Spectroscopy study conducted on a series of gold nanoparticles recently reported by us, stabilized by monodentate, bidentate, tridentate and tetradentate thiolate calix[n]arene ligands, are presented here. By virtue of the different denticity of the ligands, the nuclearity of the resulting particles can be tuned down to the subnanometric range. From the present XPS results, a clear correlation among the experimental binding energy of single Au 4f peak components and the specific Au state of charge is proposed, where the smaller (i.e., nanometer) fraction of the series selectively shows negatively charged Au atoms. Our findings are relevant for the open discussion of a specific role played by negatively charged Au atoms in catalytic reactions, especially at low temperatures.