Electrochemical functionalization of glassy carbon electrode by reduction of diazonium cations in protic ionic liquid

Electrografting a Hybrid Bilayer Membrane via Diazonium Chemistry for Electrochemical Impedance Spectroscopy of Amyloid-β Aggregation

Herein, a novel hybrid bilayer membrane is introduced as a platform to study the aggregation of amyloid-β_1–42 (Aβ_1–42) peptide on surfaces. The first layer was covalently attached to a glassy carbon electrode (GCE) via diazonium electrodeposition, which provided a highly stable template for the hybrid bilayer formation. To prepare the long-chain hybrid bilayer membrane (lcHBLM)-modified electrodes, GCE surfaces were modified with 4-dodecylbenzenediazonium (DDAN) followed by the modification with dihexadecyl phosphate (DHP) as the second layer. For the preparation of short-chain hybrid bilayer membrane (scHBLM)-modified electrodes, GCE surfaces were modified with 4-ethyldiazonium (EDAN) as the first layer and bis(2-ethylhexyl) phosphate (BEHP) was utilized as the second layer. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to characterize the bilayer formation. Both positively charged [Ru(NH₃)₆]³⁺ and negatively charged ([Fe(CN)₆]^3-/4-) redox probes were used for electrochemical characterization of the modified surfaces using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). EIS results showed a decrease in charge transfer resistance (R_ct) upon incubation of Aβ_1–42 on the hybrid bilayer-modified surfaces. This framework provides a promising electrochemical platform for designing hybrid bilayers with various physicochemical properties to study the interaction of membrane-bound receptors and biomolecules on surfaces.

Tailoring the performance of electrochemical biosensors based on carbon nanomaterials via aryldiazonium electrografting

Carbon nanomaterials (CNs) offer some of the most valuable properties for electrochemical biosensing applications, such as good electrical conductivity, wide electrochemical stability, high specific surface area, and biocompatibility. Regardless the envisioned sensing application, endowing CNs with specific functions through controlled chemical functionalization is fundamental for promoting the specific binding of the analyte. As a versatile and straightforward method of surface functionalization, aryldiazonium chemistry have been successfully used to accommodate in a stable and reproducible way different functionalities, while the electrochemical route has become the favourite choice since the deposition conditions can be readily controlled and adapted to the substrate. In particular, the modification of CNs by electrochemical reduction of aryl diazonium salts is established as a powerful tool which allows tailoring the chemical and electronic properties of the sensing platform. By outlining the stimulating results disclosed in the last years, this article provides not only a comprehensively review, but also a rational assessment on contribution of aryldiazonium electrografting in developing CNs-based electrochemical biosensors. Furthermore, some of the emerging challenges to be surpassed to effectively implement this methodology for in vivo and point of care analysis are also highlighted.

Strategies for the Formation of Monolayers From Diazonium Salts: Unconventional Grafting Media, Unconventional Building Blocks

Pioneered by J. Pinson and coll. in 1990s, the reductive grafting of aryldiazonium salts has become a powerful method for surface functionalization. Highly robust interfaces result from this surface attachment, resistant to heat, chemical degradation and ultrasonication. Importantly, this approach can be applied to many materials, ranging from conducting, semi-conducting, oxides to insulating substrates. In addition, either massive, flat surfaces or nanomaterials can be functionalized. The method is easy to process and fast. The grafting process involves the formation of highly reactive aryl radicals able to attack the substrate. However, the generated radicals can also react with already-grafted aryl species, leading to the formation of loosely-packed polyaryl multilayer films, typically of 10-15 nm thick. It is thus highly challenging to control the vertical extension of the deposited layer and to form well-ordered monolayers from aryldiazonium salts. In this mini review, we briefly describe the different strategies that have been developed to prepare well-ordered monolayers. We especially focus on two strategies successfully used in our laboratories, namely the use of unconventional solvents, i.e., room temperature ionic liquids (RTILs), as grafting media and the use of calixarene macrocycles by taking benefit of their pre-organized structure. These strategies give large possibilities for the structuring of interfaces with the widest choice of materials and highlight the potential of aryldiazonium grafting as a competitive alternative to self-assembled monolayers (SAMs) of alkyl thiols.

Robust hydrophobic gold, glass and polypropylene surfaces obtained through a nanometric covalently bound organic layer

The (electro)chemical grafting of a polyfluorinated calix[4]arene on gold, polypropylene and glass is reported. The modified surfaces were characterized by ellipsometry, atomic force microscopy (AFM), and by X-ray photoelectron spectroscopy (XPS). A nanometric, robust and uniform monolayer of covalently surface-bound calix[4]arenes was obtained on the three different materials. For all surfaces, contact angles higher than 110° were recorded, highlighting the hydrophobic character given by this ∼2 nm thin organic monolayer. Remarkably, the contact angle values remained unchanged after 18 months under a laboratory atmosphere. The results presented herein thus present an attractive and sustainable strategy for bringing hydrophobic properties to the interface of a wide range of materials.

The grafting of a polyfluorinated calix[4]arene-tetradiazonium derivative on various surfaces led to the formation of very robust and stable hydrophobic monolayers.

Use of calixarenes bearing diazonium groups for the development of robust monolayers with unique tailored properties

Calixarene-based diazonium salts can be easily synthesized in a few steps. This review surveys recent examples that illustrate the key advantages of these highly reactive molecular platforms for surface modification.

Interplay of non-uniform charge distribution on the electrochemical modification of graphene

Graphene is considered a model material for surfaces because it is stable despite being composed of a single layer of carbon atoms. Although the thermal and electronic properties of graphene are well reported, the behavior of graphene sheets with the addition of charges to the structure is not well understood. Combining infrared spectroscopy, electrochemical analysis, and computational simulations, we report the effect of an electrochemically induced covalent anchoring of 4-carboxyphenyl (4-CP) units on the optical and electronic properties of graphene. Charges in graphene become concentrated at specific sites of the sheet when electrochemically perturbed and the functionalization occurs inhomogeneously along the graphene sheet. We observed that, when graphene is covalently functionalized, the resistance to heterogeneous electron transfer is increased by a factor of 1.4. Furthermore, scattering-type scanning near-field optical microscopy and atomic force microscopy show that the covalent functionalization affects drastically the optical and physical properties of the graphene/SiO2 system, especially the plasmon-phonon coupling after the functionalization. In addition, from these we infer that a comparatively higher degree of functionalization occurs near the electrode edges. These results are supported by computational simulations, which show that the covalent anchoring of 4-CP units weakens electron transfer because the charges are retained on the sp3-hybridized carbon atoms generated upon functionalization, suggesting that graphene properties are deeply influenced by the way the molecules are immobilized on its structure.

Advances on Aryldiazonium Salt Chemistry Based Interfacial Fabrication for Sensing Applications

Aryldiazonium salts as coupling agents for surface chemistry have evidenced their wide applications for the development of sensors. Combined with advances in nanomaterials, current trends in sensor science and a variety of particular advantages of aryldiazonium salt chemistry in sensing have driven the aryldiazonium salt-based sensing strategies to grow at an astonishing pace. This review focuses on the advances in the use of aryldiazonium salts for modifying interfaces in sensors and biosensors during the past decade. It will first summarize the current methods for modification of interfaces with aryldiazonium salts, and then discuss the sensing applications of aryldiazonium salts modified on different transducers (bulky solid electrodes, nanomaterials modified bulky solid electrodes, and nanoparticles). Finally, the challenges and perspectives that aryldiazonium salt chemistry is facing in sensing applications are critically discussed.

Controlling Grafting from Aryldiazonium Salts: A Review of Methods for the Preparation of Monolayers

Surface modification by grafting from aryldiazonium salts has been widely studied and applied to many substrates as a simple and versatile method for preparing strongly adherent organic coatings. Unless special precautions or conditions are used, the usual film structure is a loosely packed disordered multilayer; however, over the past decade, attention has been paid to establishing strategies for grafting just a monolayer of modifiers to the surface. To date, four general approaches to monolayer preparation have emerged: use of aryldiazonium ions with cleavable protection groups; use of aryldiazonium ions with steric constraints; grafting in the presence of a radical scavenger; and grafting from ionic liquids. This review describes these approaches, illustrates some of their applications, and highlights the advantages and disadvantages of each.

Evidence of monolayer formation via diazonium grafting with a radical scavenger: electrochemical, AFM and XPS monitoring

AFM monitoring of controlled surface modification with a radical scavenger.