Synthetic Strategies for the Selective Functionalization of Carbon Nanodots Allow Optically Communicating Suprastructures.

The surface of Carbon Nanodots (CNDs) stands as a rich chemical platform, able to regulate the interactions between particles and external species. Performing selective functionalization of these nanoscale entities is of practical importance, however, it still represents a considerable challenge. In this work, we exploited the organic chemistry toolbox to install target functionalities on the CND surface, while monitoring the chemical changes on the material's outer shell through nuclear magnetic resonance spectroscopy. Following this, we investigated the use of click chemistry to covalently connect CNDs of different nature en-route towards covalent suprastructures with unprecedent molecular control. The different photophysical properties of the connected particles allowed their optical communication in the excited state. This work paves the way for the development of selective and addressable CND building blocks which can act as modular nanoscale synthons that mirror the long-established reactivity of molecular organic synthesis.

The Importance of the Purification Step and the Characterization of the Products in the Synthesis of Carbon Nanodots.

In the synthesis of carbon nanodots (CNDs), the critical step of the purification from the starting materials and unwanted side products is faced. In the exciting race toward new and interesting CNDs, this problem is often underestimated, leading to false properties and erroneous reports. In fact, on many occasions, the properties described for novel CNDs derive from impurities not completely eliminated during the purification process. Dialysis, for instance, is not always helpful, especially if the side products are not soluble in water. In this Perspective, the importance of the purification and characterization steps, in order to obtain solid reports and reliable procedures, is emphasized.

An Electroactive and Self-Assembling Bio-Ink, based on Protein-Stabilized Nanoclusters and Graphene, for the Manufacture of Fully Inkjet-Printed Paper-Based Analytical Devices.

Hundreds of new electrochemical sensors are reported in literature every year. However, only a few of them makes it to the market. Manufacturability, or rather the lack of it, is the parameter that dictates if new sensing technologies will remain forever in the laboratory in which they are conceived. Inkjet printing is a low-cost and versatile technique that can facilitate the transfer of nanomaterial-based sensors to the market. Herein, an electroactive and self-assembling inkjet-printable ink based on protein-nanomaterial composites and exfoliated graphene is reported. The consensus tetratricopeptide proteins (CTPRs), used to formulate this ink, are engineered to template and coordinate electroactive metallic nanoclusters (NCs), and to self-assemble upon drying, forming stable films. The authors demonstrate that, by incorporating graphene in the ink formulation, it is possible to dramatically improve the electrocatalytic properties of the ink, obtaining an efficient hybrid material for hydrogen peroxide (H2 O2 ) detection. Using this bio-ink, the authors manufactured disposable and environmentally sustainable electrochemical paper-based analytical devices (ePADs) to detect H2 O2 , outperforming commercial screen-printed platforms. Furthermore, it is demonstrated that oxidoreductase enzymes can be included in the formulation, to fully inkjet-print enzymatic amperometric biosensors ready to use.

Luminescent Carbon Nanodots Doped with Gadolinium (III): Purification Criteria, Chemical and Biological Characterization of a New Dual Fluorescence/MR Imaging Agent.

Carbon Dots (CDs) are luminescent quasi-spherical nanoparticles, possessing water solubility, high biocompatibility, and tunable chemical and physical properties for a wide range of applications, including nanomedicine and theranostics. The evaluation of new purification criteria, useful to achieve more reliable CDs, free from the interference of artifacts, is currently an object of debate in the field. Here, new CDs doped with gadolinium (Gd (III)), named Gd@CNDs, are presented as multifunctional probes for Magnetic Resonance Imaging (MRI). This new system is a case of study, to evaluate and/or combine different purification strategies, as a crucial approach to generate CDs with a better performance. Indeed, these new amorphous Gd@CNDs display good homogeneity, and they are free from emissive side products. Gd@CNDs (7-10 nm) contain 7% of Gd (III) w/w, display suitable and stable longitudinal relaxivity (r1 ) and with emissive behavior, therefore potentially useful for both MR and fluorescence imaging. They show good biocompatibility in both cellular and in vivo studies, cell permeability, and the ability to generate contrast in cellular pellets. Finally, MRI recording T1 -weighted images on mice after intravenous injection of Gd@CNDs, show signal enhancement in the liver, spleen, and kidney 30 min postinjection.

Single-Step Functionalization Strategy of Graphene Microtransistor Array with Chemically Modified Aptamers for Biosensing Applications.

Graphene solution-gated field-effect transistors (gSGFETs) offer high potential for chemical and biochemical sensing applications. Among the current trends to improve this technology, the functionalization processes are gaining relevance for its crucial impact on biosensing performance. Previous efforts are focused on simplifying the attachment procedure from standard multi-step to single-step strategies, but they still suffer from overreaction, and impurity issues and are limited to a particular ligand. Herein, a novel strategy for single-step immobilization of chemically modified aptamers with fluorenylmethyl and acridine moieties, based on a straightforward synthetic route to overcome the aforementioned limitations is presented. This approach is benchmarked versus a standard multi-step strategy using thrombin as detection model. In order to assess the reliability of the functionalization strategies 48-gSGFETs arrays are employed to acquire large datasets with multiple replicas. Graphene surface characterization demonstrates robust and higher efficiency in the chemical coupling of the aptamers with the single-step strategy, while the electrical response evaluation validates the sensing capability, allowing to implement different alternatives for data analysis and reduce the sensing variability. In this work, a new tool capable of overcome the functionalization challenges of graphene surfaces is provided, paving the way toward the standardization of gSGFETs for biosensing purposes.

N,N-Diphenyl Dihydrophenazines: Using π-Extension to Access Dicationic Multifunctional Materials.

Dihydrophenazines are receiving increasing attention due to applications in numerous fields of chemistry, from light emission to organo-photocatalysis. Despite this growing interest and numerous works involving the preparation of radical cations based on this scaffold, the isolation and study of the aromatic dications obtained by 2 electron oxidation of dihydrophenazines is still mostly unexplored. From this point of view, along with the substitution at the N atoms generally used to tune dihydrophenazine properties, the π-extension of the phenazine core could play a crucial role in making dicationic states accessible. This could result in an extension of the knowledge on these elusive dications and in potentially highly interesting applications ranging from material science to molecular actuators.

Photoinduced Cascade Reactions of 2-Allylphenol Derivatives toward the Production of 2,3-Dihydrobenzofurans.

A light-driven protocol for the synthesis of 2,3-dihydrobenzofurans under mild conditions is reported. Specifically, the cascade process is initiated by the photochemical activity of allyl-functionalized phenolate anions, generated in situ upon deprotonation of the corresponding phenols. The reaction proceeds rapidly with reaction times as low as 35 min, delivering a wide range of densely functionalized products (20 examples, yields up to 69%). Mechanistic studies have also been performed providing convincing evidence for the photochemical formation of carbon-centered radical species. A cascade reaction pathway involving a tandem atom transfer radical addition (ATRA) and an intramolecular nucleophilic substitution (SN) process is proposed to occur.

Neonatal rat ventricular myocytes interfacing conductive polymers and carbon nanotubes.

Carbon nanotubes (CNTs) have become promising advanced materials and a new tool to specifically interact with electroresponsive cells. Likewise, conductive polymers (CP) appear promising electroactive biomaterial for proliferation of cells. Herein, we have investigated CNT blends with two different conductive polymers, polypyrrole/CNT (PPy/CNT) and PEDOT/CNT to evaluate the growth, survival, and beating behavior of neonatal rat ventricular myocytes (NRVM). The combination of CP/CNT not only shows excellent biocompatibility on NRVM, after 2 weeks of culture, but also exerts functional effects on networks of cardiomyocytes. NRVMs cultured on CNT-based substrates exhibited improved cellular function, i.e., homogeneous, non-arrhythmogenic, and more frequent spontaneous beating; particularly PEDOT/CNT substrates, which yielded to higher beating amplitudes, thus suggesting a more mature cardiac phenotype. Furthermore, cells presented enhanced structure: aligned sarcomeres, organized and abundant Connexin 43 (Cx43). Finally, no signs of induced hypertrophy were observed. In conclusion, the combination of CNT with CP produces high viability and promotes cardiac functionality, suggesting great potential to generate scaffolding supports for cardiac tissue engineering.

Functional rewiring across spinal injuries via biomimetic nanofiber scaffolds.

The regrowth of severed axons is fundamental to reestablish motor control after spinal-cord injury (SCI). Ongoing efforts to promote axonal regeneration after SCI have involved multiple strategies that have been only partially successful. Our study introduces an artificial carbon-nanotube based scaffold that, once implanted in SCI rats, improves motor function recovery. Confocal microscopy analysis plus fiber tracking by magnetic resonance imaging and neurotracer labeling of long-distance corticospinal axons suggest that recovery might be partly attributable to successful crossing of the lesion site by regenerating fibers. Since manipulating SCI microenvironment properties, such as mechanical and electrical ones, may promote biological responses, we propose this artificial scaffold as a prototype to exploit the physics governing spinal regenerative plasticity.

Chiral Carbon Nanodots Can Act as Molecular Catalysts in Chemical and Photochemical Reactions.

In this work, a microwave synthesis followed by a simple purification process produces a new type of chiral Carbon Nanodots (CNDs). These CNDs are soluble in organic solvents, exhibit amino groups on their surface and display interesting absorption and emission properties along with mirror image profiles in the electronic circular dichroism spectrum. All these features set the stage for CNDs to act as multifunctional catalytic platforms, able to promote diverse chemical transformations. In particular, the outer shell composition of CNDs was instrumental to carry out organocatalytic reactions in an enantioselective fashion. In addition, the redox and light-absorbing properties of the material are suitable to drive photochemical processes. Finally, the photoredox and organocatalytic activations of CNDs were exploited at the same time to promote a cross-dehydrogenative coupling. This work demonstrates that CNDs can be used as catalysts to promote multiple reactivities, previously considered exclusive domain of molecular catalysts.

The Covalent Functionalization of Surface-Supported Graphene: An Update.

In the last decade, the use of graphene supported on solid surfaces has broadened its scope and applications, and graphene has acquire a promising role as a major component of high-performance electronic devices. In this context, the chemical modification of graphene has become essential. In particular, covalent modification offers key benefits, including controllability, stability, and the facility to be integrated into manufacturing operations. In this Review, we critically comment on the latest advances in the covalent modification of supported graphene on substrates. We analyze the different chemical modifications with special attention to radical reactions. In this context, we review the latest achievements in reactivity control, tailoring electronic properties, and introducing active functionalities. Finally, we extended our analysis to other emerging 2D materials supported on surfaces, such as transition metal dichalcogenides, transition metal oxides, and elemental analogs of graphene.

Singlet-Triplet Energy Inversion in Carbon Nitride Photocatalysts.

Time-resolved EPR (TR-EPR) demonstrates the formation of well-defined spin triplet excitons in carbon nitride. This permits to experimentally probe the extent of the triplet wavefunction which delocalizes over several tri-s-triazine units. Analysis of the temperature dependence of the TR-EPR signal reveals the mobility of the triplet excitons. By employing monochromatic light excitation in the range 430-600 nm, the energy of the spin triplet is estimated to be ≈0.2 eV above the conduction band edge, proving that the triplet exciton lies above the corresponding singlet. Comparison between amorphous and graphitic forms establishes the singlet-triplet inversion as a general feature of carbon nitride materials.

Direct C2-H alkylation of indoles driven by the photochemical activity of halogen-bonded complexes.

A light-driven metal-free protocol for the synthesis of sulfone-containing indoles under mild conditions is reported. Specifically, the process is driven by the photochemical activity of halogen-bonded complexes formed upon complexation of a sacrificial donor, namely 1,4-diazabicyclo[2.2.2]octane (DABCO), with α-iodosulfones. The reaction provides a variety of densely functionalized products in good yields (up to 96% yield). Mechanistic investigations are reported. These studies provide convincing evidences for the photochemical formation of reactive open-shell species.

Phenanthrene-Extended Phenazine Dication: An Electrochromic Conformational Switch Presenting Dual Reactivity.

The synthesis and isolation of one of the few examples of a π-extended diamagnetic phenazine dication have been achieved by oxidizing a phenanthrene-based dihydrophenazine precursor. The resulting dication was isolated and fully characterized, highlighting an aromatic distorted structure, generated by the conformational change upon the oxidation of the dihydrophenazine precursor, which is also correlated with a marked electrochromic change in the UV-vis spectrum. The aromaticity of the dication has also been investigated theoretically, proving that the species is aromatic based on all major criteria (structural, magnetic, and energetic). Moreover, the material presents an intriguing dual reactivity, resulting in ring contraction to a π-extended triarylimidazolinium and reduction to the dihydrophenazine precursor, depending on the nature of the nucleophile involved. This result helps shed light on the yet largely unexplored reactivity and properties of extended dicationic polycyclic aromatic hydrocarbons (PAHs). In particular, the fact that the molecule can undergo a reversible change in conformation upon oxidation and reduction opens potential applications for this class of derivatives as molecular switches and actuators.

Enhancing Oxygenic Photosynthesis by Cross-Linked Perylenebisimide "Quantasomes".

As the natural-born photoelectrolyzer for oxygen delivery, photosystem II (PSII) is hardly replicated with man-made constructs. However, building on the "quantasome" hypothesis ( Science 1964, 144, 1009-1011), PSII mimicry can be pared down to essentials by shaping a photocatalytic ensemble (from the Greek term "soma" = body) where visible-light quanta trigger water oxidation. PSII-inspired quantasomes (QS) readily self-assemble into hierarchical photosynthetic nanostacks, made of bis-cationic perylenebisimides (PBI2+) as chromophores and deca-anionic tetraruthenate polyoxometalates (Ru4POM) as water oxidation catalysts ( Nat. Chem. 2019, 11, 146-153). A combined supramolecular and click-chemistry strategy is used herein to interlock the PBI-QS with tetraethylene glycol (TEG) cross-linkers, yielding QS-TEGlock with increased water solvation, controlled growth, and up to a 340% enhancement of the oxygenic photocurrent compared to the first generation QS, as probed on 3D-inverse opal indium tin oxide electrodes at 8.5 sun irradiance (λ > 450 nm, 1.28 V vs RHE applied bias, TOFmax = 0.096 ± 0.005 s-1, FEO2 > 95%). Action spectra, catalyst mass-activity, light-management, photoelectrochemical impedance spectroscopy (PEIS) together with Raman mapping of TEG-templated hydration shells point to a key role of the cross-linked PBI/Ru4POM nanoarrays, where the interplay of hydrophilic/hydrophobic domains is reminiscent of PSII-rich natural thylakoids.

Amine-Rich Carbon Dots as Novel Nano-Aminocatalytic Platforms in Organic Synthesis.

The development of novel and effective metal-free catalytic systems, which can drive value-added organic transformations in environmentally benign solvents (for instance, water), is highly desirable. Moreover, these new catalysts need to be harmless, easy-to-prepare, and potentially recyclable. In this context, amine-rich carbon dots (CDs) have recently emerged as promising nano-catalytic platforms. These nitrogen-doped nanoparticles, which show dimensions smaller than 10 nm, generally consist of carbon cores that are surrounded by shells containing numerous amino groups. In recent years, organic chemists have used these surface amines to guide the design of several synthetic methodologies under mild operative conditions. This Concept highlights the recent advances in the synthesis of amine-rich carbon dots and their applications in organic catalysis, including forward-looking opportunities within this research field.

Nuclear Magnetic Resonance Reveals Molecular Species in Carbon Nanodot Samples Disclosing Flaws.

Carbon nanodots are currently one of the hot topics in the nanomaterials world, due to their accessible synthesis and promising features. However, the purification of these materials is still a critical aspect, especially for syntheses involving molecular precursors. Indeed, the presence of unreacted species or small organic molecules formed during solvothermal treatments can affect the properties of the synthesized nanomaterials. To illustrate the extreme importance of this issue, we present two case studies in which insufficient purification results in misleading conclusions regarding the chiral and fluorescent properties of the investigated materials. Key to identify molecular species is the use of nuclear magnetic resonance, which proves to be an effective tool. Our work highlights the need to include nuclear magnetic resonance as a standard characterization technique for carbon-based nanomaterials, to minimize the risk of observing properties that arise from molecular species, rather than the target carbon nanodots.

Transfer of Axial Chirality to the Nanoscale Endows Carbon Nanodots with Circularly Polarized Luminescence.

We report the synthesis, purification and characterization of chiral carbon nanodots starting from atropoisomeric precursors. The obtained atropoisomeric carbon nanodots are soluble in organic solvents and have good thermal stability, which are desirable features for technological applications. The synthetic protocol is robust, as it supports a number of variations in terms of molecular doping agents. Remarkably, the combination of axially chiral precursors and 1,4-benzoquinone as doping agent results in green-emissive carbon dots displaying circularly polarized luminescence. Dissymmetry factors of |3.5|×10-4 are obtained in solution, without the need of any additional element of chirality. Introducing axial chirality expands the strategies available to tailor the properties of carbon nanodots, paving the way for carbon nanoparticles that combine good processability in organic solvents with engineered advanced chiroptical properties.

Morphology and Light-Dependent Spatial Distribution of Spin Defects in Carbon Nitride.

Carbon nitride (CN) is a heterogeneous photocatalyst that combines good structural properties and a broad scope. The photocatalytic efficiency of CN is associated with the presence of defective and radical species. An accurate description of defective states-both at a local and extended level-is key to develop a thorough mechanistic understanding of the photophysics of CN. In turn, this will maximise the generation and usage of photogenerated charge carriers and minimise wasteful charge recombination. Here the influence of morphology and light-excitation on the number and chemical nature of radical defects is assessed. By exploiting the magnetic dipole-dipole coupling, the spatial distribution of native radicals in CN is derived with high precision. From the analysis an average distance in the range 1.99-2.34 nm is determined, which corresponds to pairs of radicals located approximately four tri-s-triazine units apart.

Turning the Light on Phenols: New Opportunities in Organic Synthesis.

Phenols (I) are extremely relevant chemical functionalities in natural, synthetic and industrial chemistry. Their corresponding electron-rich anions, namely phenolates (I), are characterized by interesting physicochemical properties that can be drastically altered upon light excitation. Specifically, phenolates (I) become strong reducing agents in the excited state and are able to generate reactive radicals from suitable precursors via single-electron transfer processes. Thus, these species can photochemically trigger strategic bond-forming reactions, including their direct aromatic C-H functionalization. Moreover, substituted phenolate anions can act as photocatalysts to enable synthetically useful organic transformations. An alternative mechanistic manifold is represented by the ability of phenolate derivatives I to form ground state electron donor-acceptor (EDA) complexes with electron-poor radical sources. These complementary scenarios have paved the way for the development of a wide range of relevant organic reactions. In this Minireview, we present the main examples of this research field, and give insight on emerging trends in phenols photocatalysis towards richer organic synthesis.