Introducing Design Strategies to Preserve N-Heterocycles Throughout the On-Surface Synthesis of Graphene Nanostructures.

Despite the impressive advances in the synthesis of atomically precise graphene nanostructures witnessed during the last decade, advancing in compositional complexity faces major challenges. The concept of introducing the desired functional groups or dopants in the molecular precursor often fails due to their lack of stability during the reaction path. Here, a study on the stability of different pyridine and pyrimidine moieties during the on-surface synthesis of graphene nanoribbons on Au(111) is presented. Combining bond-resolved scanning tunneling microscopy with X-ray photoelectron spectroscopy, the thermal evolution of the nitrogen dopants throughout the whole reaction sequence is tracked. A comparative experimental and ab initio electronic characterization confirms the presence of dopants in the final structures, revealing also that the pyridinic nitrogen leads to a significant band downshift. The results demonstrate that, by using synthetic strategies to lower the reaction temperatures, one can preserve specific N-heterocycles throughout all the reaction steps of the synthesis of graphene nanoribbons and beyond the interibbon coupling reaction that leads to nanoporous graphene.

Ultratrace PFAS Detection Using Amplifying Fluorescent Polymers.

Per- and poly(fluoroalkyl) substances (PFAS) are environmentally persistent pollutants that are of growing concern due to their detrimental effects at ultratrace concentrations (ng·L-1) in human and environmental health. Suitable technologies for on-site ultratrace detection of PFAS do not exist and current methods require complex and specialized equipment, making the monitoring of PFAS in distributed water infrastructures extremely challenging. Herein, we describe amplifying fluorescent polymers (AFPs) that can selectively detect perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) at concentrations of ng·L-1. The AFPs are highly fluorinated and have poly(p-phenylene ethynylene) and polyfluorene backbones bearing pyridine-based selectors that react with acidic PFAS via a proton-transfer reaction. The fluorinated regions within the polymers partition PFAS into polymers, whereas the protonated pyridine units create lower-energy traps for the excitons, and emission from these pyridinium sites results in red-shifting of the fluorescence spectra. The AFPs are evaluated in thin-film and nanoparticle forms and can selectively detect PFAS concentrations of ∼1 ppb and ∼100 ppt, respectively. Both polymer films and nanoparticles are not affected by the type of water, and similar responses to PFAS were found in milliQ water, DI water, and well water. These results demonstrate a promising sensing approach for on-site detection of aqueous PFAS in the ng·L-1 range.

On-Surface Synthesis of Nanographenes and Graphene Nanoribbons on Titanium Dioxide.

The formation of two types of nanographenes from custom designed and synthesized molecular precursors has been achieved through thermally induced intramolecular cyclodehydrogenation reactions on the semiconducting TiO2(110)-(1×1) surface, confirmed by the combination of high-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) measurements, and corroborated by theoretical modeling. The application of this protocol on differently shaped molecular precursors demonstrates the ability to induce a highly efficient planarization reaction both within strained pentahelicenes as well as between vicinal phenyl rings. Additionally, by the combination of successive Ullmann-type polymerization and cyclodehydrogenation reactions, the archetypic 7-armchair graphene nanoribbons (7-AGNRs) have also been fabricated on the titanium dioxide surface from the standard 10,10'-dibromo-9,9'-bianthryl (DBBA) molecular precursors. These examples of the effective cyclodehydrogenative planarization processes provide perspectives for the rational design and synthesis of molecular nanostructures on semiconductors.

Circumventing the stability problems of graphene nanoribbon zigzag edges.

Carbon nanostructures with zigzag edges exhibit unique properties-such as localized electronic states and spins-with exciting potential applications. Such nanostructures however are generally synthesized under vacuum because their zigzag edges are unstable under ambient conditions: a barrier that must be surmounted to achieve their scalable integration into devices for practical purposes. Here we show two chemical protection/deprotection strategies, demonstrated on labile, air-sensitive chiral graphene nanoribbons. Upon hydrogenation, the chiral graphene nanoribbons survive exposure to air, after which they are easily converted back to their original structure by annealing. We also approach the problem from another angle by synthesizing a form of the chiral graphene nanoribbons that is functionalized with ketone side groups. This oxidized form is chemically stable and can be converted to the pristine hydrocarbon form by hydrogenation and annealing. In both cases, the deprotected chiral graphene nanoribbons regain electronic properties similar to those of the pristine nanoribbons. We believe both approaches may be extended to other graphene nanoribbons and carbon-based nanostructures.

Lactate biosensing based on covalent immobilization of lactate oxidase onto chevron-like graphene nanoribbons via diazotization-coupling reaction.

We have designed and prepared an electrochemical biosensor for lactate determination. Through a diazotation process, the enzyme lactate oxidase (LOx) is anchored onto chevron-like graphene nanoribbons (GNR), previously synthesized by a solution-based chemical route, and used as modifiers of glassy carbon electrodes. In a first step, we have performed the grafting of a 4-carboxyphenyl film, by electrochemical reduction of the corresponding 4-carboxyphenyl diazonium salt, on the GNR-modified electrode surface. In this way, the carboxylic groups are exposed to the solution, enabling the covalent immobilization of the enzyme through the formation of an amide bond between these carboxylic groups and the amine groups of the enzyme. The biosensor design was optimized through the morphological and electrochemical characterization of each construction step by atomic force microscopy, scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy.The cyclic voltammetric response of the biosensor in a solution of hydroxymethylferrocene in presence of l-lactate evidenced a clear electrocatalytic effect powered by the specific design of the biosensing platform with LOx covalently attached to the GNR layer. From the calibration procedures employed for l-lactate determination, a linear concentration range of 3.4 · 10-5- 2.8 · 10-4 M and a detection limit of 11 µM were obtained, with relative errors and relative standard deviations less than 6.0% and 8.4%, respectively. The applicability of the biosensor was tested by determining lactate in apple juices, leading to results that are in good agreement with those obtained with a well-established enzymatic spectrophotometric assay kit.

Atomically Sharp Lateral Superlattice Heterojunctions Built-In Nitrogen-Doped Nanoporous Graphene.

Nanometer scale lateral heterostructures with atomically sharp band discontinuities can be conceived as the 2D analogues of vertical Van der Waals heterostructures, where pristine properties of each component coexist with interfacial phenomena that result in a variety of exotic quantum phenomena. However, despite considerable advances in the fabrication of lateral heterostructures, controlling their covalent interfaces and band discontinuities with atomic precision, scaling down components and producing periodic, lattice-coherent superlattices still represent major challenges. Here, a synthetic strategy to fabricate nanometer scale, coherent lateral superlattice heterojunctions with atomically sharp band discontinuity is reported. By merging interdigitated arrays of different types of graphene nanoribbons by means of a novel on-surface reaction, superlattices of 1D, and chemically heterogeneous nanoporous junctions are obtained. The latter host subnanometer quantum dipoles and tunneling in-gap states, altogether expected to promote interfacial phenomena such as interribbon excitons or selective photocatalysis.

Magnetic Interactions Between Radical Pairs in Chiral Graphene Nanoribbons.

Open-shell graphene nanoribbons have become promising candidates for future applications, including quantum technologies. Here, we characterize magnetic states hosted by chiral graphene nanoribbons (chGNRs). The substitution of a hydrogen atom at the chGNR edge by a ketone effectively adds one pz electron to the π-electron network, producing an unpaired π-radical. A similar scenario occurs for regular ketone-functionalized chGNRs in which one ketone is missing. Two such radical states can interact via exchange coupling, and we study those interactions as a function of their relative position, which includes a remarkable dependence on the chirality, as well as on the nature of the surrounding ribbon, that is, with or without ketone functionalization. Besides, we determine the parameters whereby this type of system with oxygen heteroatoms can be adequately described within the widely used mean-field Hubbard model. Altogether, we provide insight to both theoretically model and devise GNR-based nanostructures with tunable magnetic properties.

An on-surface Diels-Alder reaction.

The Diels-Alder reaction is one of the most popular reactions in organic chemistry. However, its use in the field of on-surface synthesis is hampered by the spatial restrictions of this cycloaddition reaction. Herein we selected a cyclic strained triyne to demonstrate an on-surface hexadehydro-Diels-Alder reaction in a single molecule. The reaction was studied in detail by means of atomic force microscopy (AFM) with CO-functionalized tips. Our results pave the way to use this iconic pericyclic reaction for on-surface synthesis, introducing the concept of atom economy in the field.

Challenges in the synthesis of corannulene-based non-planar nanographenes on Au(111) surfaces.

The on-surface synthesis of non-planar nanographenes is a challenging task. Herein, with the aid of bond-resolving scanning tunneling microscopy (BRSTM) and density functional theory (DFT) calculations, we present a systematic study aiming at the fabrication of corannulene-based nanographenes via intramolecular cyclodehydrogenation on a Au(111) surface. The formation of non-planar targeted products is confirmed to be energetically unfavorable compared to the formation of planar/quasi-planar undesired competing monomer products. In addition, the activation of intermolecular coupling further inhibits the formation of the final targeted product. Although it was not possible to access the corannulene moiety by means of on-surface synthesis, partial cyclodehydrogenation of the molecular precursors was demonstrated.

Chemical Stability of (3,1)-Chiral Graphene Nanoribbons.

Nanostructured graphene has been widely studied in recent years due to the tunability of its electronic properties and its associated interest for a variety of fields, such as nanoelectronics and spintronics. However, many of the graphene nanostructures of technological interest are synthesized under ultrahigh vacuum, and their limited stability as they are brought out of such an inert environment may compromise their applicability. In this study, a combination of bond-resolving scanning probe microscopy (BR-SPM), along with theoretical calculations, has been employed to study (3,1)-chiral graphene nanoribbons [(3,1)-chGNRs] that were synthesized on a Au(111) surface and then exposed to oxidizing environments. Exposure to the ambient atmosphere, along with the required annealing treatment to desorb a sufficiently large fraction of contaminants to allow for its postexposure analysis by BR-SPM, revealed a significant oxidation of the ribbons, with a dramatically disruptive effect on their electronic properties. More controlled experiments avoiding high temperatures and exposing the ribbons only to low pressures of pure oxygen show that also under these more gentle conditions the ribbons are oxidized. From these results, we obtain additional insights into the preferential reaction sites and the nature of the main defects that are caused by oxygen. We conclude that graphene nanoribbons with zigzag edge segments require forms of protection before they can be used in or transferred through ambient conditions.

Chemically synthesized chevron-like graphene nanoribbons for electrochemical sensors development: determination of epinephrine.

We employ chevron-like graphene nanoribbons (GNRs) synthesized by a solution-based chemical route to develop a novel electrochemical sensor for determination of the neurotransmitter epinephrine (EPI). The sensor surface, a glassy carbon electrode modified with GNRs, is characterized by atomic force microscopy, scanning electron microscopy and Raman spectroscopy, which show that the electrode surface modification comprises of bi-dimensional multilayer-stacked GNRs that retain their molecular structure. The charge transfer process occurring at the electrode interface is evaluated by electrochemical impedance spectroscopy. The sensor is applied to the determination of EPI, employing as an analytical signal the reduction peak corresponding to the epinephrinechrome-leucoepinephrinechrome transition (E = - 0.25 V) instead of the oxidation peak usually employed in the literature (E = + 0.6 V) in order to minimize interferences. The results obtained demonstrate that chevron-like nanoribbons synthesized by solution methods exhibit reliable electrocatalytic activity for EPI determination. Using differential pulse voltammetry, we obtain a linear concentration range from 6.4 × 10-6 to 1.0 × 10-4 M and a detection limit of 2.1 × 10-6 M. The applicability of the sensor was evaluated by determining EPI in pharmaceutical samples with satisfactory results.

Uncovering the Triplet Ground State of Triangular Graphene Nanoflakes Engineered with Atomic Precision on a Metal Surface.

Graphene can develop large magnetic moments in custom-crafted open-shell nanostructures such as triangulene, a triangular piece of graphene with zigzag edges. Current methods of engineering graphene nanosystems on surfaces succeeded in producing atomically precise open-shell structures, but demonstration of their net spin remains elusive to date. Here, we fabricate triangulenelike graphene systems and demonstrate that they possess a spin S=1 ground state. Scanning tunneling spectroscopy identifies the fingerprint of an underscreened S=1 Kondo state on these flakes at low temperatures, signaling the dominant ferromagnetic interactions between two spins. Combined with simulations based on the meanfield Hubbard model, we show that this S=1 π paramagnetism is robust and can be turned into an S=1/2 state by additional H atoms attached to the radical sites. Our results demonstrate that π paramagnetism of high-spin graphene flakes can survive on surfaces, opening the door to study the quantum behavior of interacting π spins in graphene systems.

Transferring axial molecular chirality through a sequence of on-surface reactions.

Fine management of chiral processes on solid surfaces has progressed over the years, yet still faces the need for the controlled and selective production of advanced chiral materials. Here, we report on the use of enantiomerically enriched molecular building blocks to demonstrate the transmission of their intrinsic chirality along a sequence of on-surface reactions. Triggered by thermal annealing, the on-surface reactions induced in this experiment involve firstly the coupling of the chiral reactants into chiral polymers and subsequently their transformation into planar prochiral graphene nanoribbons. Our study reveals that the axial chirality of the reactant is not only transferred to the polymers, but also to the planar chirality of the graphene nanoribbon end products. Such chirality transfer consequently allows, starting from adequate enantioenriched reactants, for the controlled production of chiral and prochiral organic nanoarchitectures with pre-defined handedness.

Building a 22-ring nanographene by combining in-solution and on-surface syntheses.

A nanographene formed by the fusion of 22 benzene rings has been prepared by combining an in-solution Pd-catalyzed cycloaddition reaction and on-surface Au-promoted cyclodehydrogenation. The structure and electronic properties of the resulting three-fold symmetric C66H24 molecule have been characterized by scanning probe microscopy with atomic resolution and corroborated by theoretical modelling.