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A conceptual challenge toward more versatile direct methanol fuel cell (DMFC) is the design of a single material electrocatalyst with high activity and durability for both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). This requires to conciliate methanol tolerance not to hinder ORR at the cathode with a good MOR activity at the anode. This is especially incompatible with Pt materials. We tackled this challenge by deriving a supramolecular concept where surface-grafted molecular ligands regulate the Pt-catalyst reactivity. ORR and MOR activities of newly reported Pt-calix[4]arenes nanocatalysts (PtCF3NPs/C) are compared to commercial benchmark PtNPs/C. PtCF3NPs/C exhibit a remarkable methanol tolerance without losing the MOR reactivity along with outstanding durability and chemical stability. Beyond designing single-catalyst material, operable in DMFC cathodic and anodic compartments, the results highlight a promising strategy for tuning interfacial properties.

A conceptual challenge toward more versatile direct methanol fuel cells (DMFCs) is the design of a single material electrocatalyst with high activity and durability for both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). This requires to conciliate methanol tolerance not to hinder ORR at the cathode with a good MOR activity at the anode. This is especially incompatible with Pt materials. We tackled this challenge by deriving a supramolecular concept where surface‐grafted molecular ligands regulate the Pt‐catalyst reactivity. ORR and MOR activities of newly reported Pt‐calix[4]arenes nanocatalysts (Pt NPs/C) are compared to commercial benchmark PtNPs/C. Pt NPs/C exhibit a remarkable methanol tolerance without losing the MOR reactivity along with outstanding durability and chemical stability. Beyond designing single‐catalyst material, operable in DMFC cathodic and anodic compartments, the results highlight a promising strategy for tuning interfacial properties., SCOPUS: ar.j, info:eu-repo/semantics/published

A series of surface-functionalized gold nanocatalysts are synthesized. Through diazonium chemistry, calix[4]arene molecules are grafted at the gold surface. Sharing the same anchoring function and a common backbone, the calix[4]arene ligands possess distinct terminal chemical functions where the length, hydrophobic nature, and oxygen affinity are varied. Thanks to the versatility of the calix[4]arene platform, this paper only focuses on the molecular tuning of the solid/liquid interface through the change of the tail groups while electronic effects that are regulated by the anchoring function will be similar for all catalysts. Electrocatalytic tests demonstrate the influence of the tails group on the oxygen reduction reaction performances, including efficiency, selectivity, and durability. Beyond a pure geometric steric effect, it is shown that chemical properties of the ligands can effectively tune the interactions between the metallic catalyst and the electrolyte and reactants., SCOPUS: ar.j, info:eu-repo/semantics/published

Since their first report in 2012, calix[4]arene tetradiazonium derivatives have experienced a growing interest. They now represent a favored method to obtain robust post- functionalizable monolayers with controlled composition on a wealth of surfaces (conductive, semi-conductive or insulating, as well on large surfaces as on nanomaterials). These compounds are easily synthesized and handled and, so far, have been used to functionalize surfaces for applications in (bio)sensing, catalysis, as well as for the development of hydrophobic or antifouling coatings. This chapter describes the current synthetic methods, applications and limitations of these polydiazonium salts and discusses the potential of the field.

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The development of efficient, reliable, and easy-to-use biosensors allowing early cancer diagnosis is of paramount importance for patients. Herein, we report a biosensor based on silver nanoparticles functionalized by peptide aptamers for the detection of a cancer biomarker, i.e. the Mdm2 protein. Silver nanoparticles (AgNPs) were produced and stabilized with a thin PEGylated-calix[4]arene layer that allows (i) the steric stabilization of the AgNPs and (ii) the covalent conjugation of the peptide aptamers via the formation of an amide bond. These peptide-conjugated AgNPs were then used to detect Mdm2 via a dual trapping strategy that was previously reported with gold nanoparticles (AuNPs). Our results showed that replacing AuNPs by AgNPs allows to improve the detection limit by nearly one order of magnitude, down to 5 nM, while the high selectivity of the system and the stability of the particles provided by the calixarene coating allow the detection of Mdm2 in human serum., SCOPUS: ar.j, SCOPUS: ar.j, info:eu-repo/semantics/published

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.

Surface modification represents an active field of research that finds applications, amongst others, in the development of medical devices, sensors and biosensors, anti-biofouling materials, self-cleaning surfaces, surfaces with controlled wettability, corrosion resistance, heterogeneous catalysis and microelectronics. For some applications, surface functionalization with a nanometric-size monolayer is desired. In this review, efforts to covalently functionalize a wide array of surfaces with calixarenes bearing diazonium groups are described. More specifically, methodologies to obtain monolayers of calix[4 or 6]arene derivatives on conductive, semi-conductive or insulating surfaces as well as on nanoparticles are presented. The main advantages of this general surface modification strategy (i.e. formation of true monolayers that can be post-functionalized, high robustness and control over the composition of the calixarene-based coating) and its current scope of applications and future challenges are discussed.

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 deliberate surface modification of nanocatalysts with organic ligands has recently emerged as a promising strategy to boost their efficiency, durability, and/or selectivity in key electrocatalytic processes. The interface between the metallic interface and the immobilized ligands promotes high electrocatalytic activity. Herein, the oxygen reduction reaction activity of gold nanoparticles functionalized with a covalently bound monolayer of calix[4]arenes is compared with commercially available gold nanoparticles, classically stabilized through electrostatic adsorption of citrates onto the gold surfaces. Nanocatalysts coated by calix[4]arenes show enhanced selectivity and stability compared to their citrate-stabilized counterparts. These nanohybrids exhibit excellent activity with a dominant 4-electron reduction of O2 with good electrocatalytic performances along with a high robustness under operation as revealed by X-ray photoelectron spectroscopy analyses., SCOPUS: ar.j, info:eu-repo/semantics/published

Surface modification represents an active field of research that finds applications, amongst others, in the development of medical devices, sensors and biosensors, anti-biofouling materials, self-cleaning surfaces, surfaces with controlled wettability, corrosion resistance, heterogeneous catalysis and microelectronics. For some applications, surface functionalization with a nanometric-size monolayer is desired. In this review, efforts to covalently functionalize a wide array of surfaces with calixarenes bearing diazonium groups are described. More specifically, methodologies to obtain monolayers of calix[4 or 6]arene derivatives on conductive, semi-conductive or insulating surfaces as well as on nanoparticles are presented. The main advantages of this general surface modification strategy (i.e.formation of true monolayers that can be post-functionalized, high robustness and control over the composition of the calixarene-based coating) and its current scope of applications and future challenges are discussed., SCOPUS: re.j, info:eu-repo/semantics/published

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., SCOPUS: re.j, info:eu-repo/semantics/published

The grafting of a polyfluorinated calix[4]arene-tetradiazonium derivative on various surfaces led to the formation of very robust and stable hydrophobic monolayers., 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., info:eu-repo/semantics/published

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Photoactive ruthenium-based complexes are actively studied for their biological applications as potential theragnostic agents against cancer. One major issue of these inorganic complexes is to penetrate inside cells in order to fulfil their function, either sensing the internal cell environment or exert a photocytotoxic activity. The use of lipophilic ligands allows the corresponding ruthenium complexes to passively diffuse inside cells but limits their structural and photophysical properties. Moreover, this strategy does not provide any cell selectivity. This limitation is also faced by complexes anchored on cell-penetrating peptides. In order to provide a selective cell targeting, we developed a multivalent system composed of a photoreactive ruthenium(II) complex tethered to a calix[4]arene platform bearing multiple RGD-containing cyclopentapeptides. Extensive photophysical and photochemical characterizations of this Ru(II)–calixarene conjugate as well as the study of its photoreactivity in the presence of guanosine monophosphate have been achieved. The results show that the ruthenium complex should be able to perform efficiently its photoinduced cytotoxic activity, once incorporated into targeted cancer cells thanks to the multivalent platform.

Reduction of calix[4]arene-tetradiazonium salts at the vicinity of a surface, notably a carbon surface, leads to the formation of a dense and robust covalently bound monolayer. To expand the versatility of this unique methodology, the efficient synthesis of a calix[4]arene bearing a single diazonium group and thus a single anchoring point was achieved. The introduction of the diazonium group was developed through an original approach involving a copper-catalyzed reaction for the preparation of an intermediate azido-calixarene. The grafting ability of the calix[4]arene-monodiazonium cation was evaluated by investigating its electrochemical reduction at glassy carbon electrodes. Studies of the blocking properties of the modified surfaces toward redox probes evidenced the formation of compact covalently bound layers at the carbon substrates, opening interesting perspectives in the field of surface modification by calixarene-based diazonium salts.

Biosensors that can determine protein concentration and structure are highly desired for biomedical applications. For the development of such biosensors, the use of Fourier transform infrared (FTIR) spectroscopy with the attenuated internal total reflection (ATR) configuration is particularly attractive, but it requires appropriate surface functionalization of the ATR optical element. Indeed, the surface has to specifically interact with a target protein in close contact with the optical element and must display antifouling properties to prevent nonspecific adsorption of other proteins. Here, we report robust monolayers of calix[4]arenes bearing oligo(ethylene glycol) (oEG) chains, which were grafted on germanium and gold surfaces via their tetradiazonium salts. The formation of monolayers of oEGylated calix[4]arenes was confirmed by AFM, IR, and contact angle measurements. The antifouling properties of these modified surfaces were studied by ATR-FTIR spectroscopy and fluorescence microscopy, and the nonspecific absorption of bovine serum albumin was found to be reduced by 85% compared to that of unmodified germanium. In other words, the organic coating by oEGylated calix[4]arenes provides remarkable antifouling properties, opening the way for the design of germanium- or gold-based biosensors.

Proteins are involved in a very large number of biological processes. Biosensors that enable fast and selective identification and structural characterization of proteins in complex mixtures represent an important target for research. A major challenge in the development of such biosensors is the modification of surfaces by a robust organic monolayer able to specifically interact with a protein and displaying anti-fouling properties to prevent nonspecific adsorption phenomena. In this context, there is an increasing interest in the use of germanium-based surfaces because this material can be readily used for FTIR spectroscopy. Contrary to the other reliable detection methods, FTIR spectroscopy allows to simultaneously detect proteins and collect a wealth of information such as secondary structure and post-traductional modifications (e.g. phosphorylation, glycosylation level, etc.). Oligo(ethylene glycol) (oEG) films are well known to be efficient at reducing the nonspecific adsorption of biomolecules. However, self-assembled monolayers of thiolated oEG present important limitations related to their lack of thermal and long-term stability, narrow electrochemical window and stochastic behaviour.To prevent the nonspecific adsorption of the many proteins and biomolecules present in biological fluids, we use a strategy that consists in the modification of surfaces through the grafting of calix[4]arene-tetra-diazonium salts.1 More precisely, we have investigated the grafting on Ge and Au surfaces of monolayers of calix[4]arenes bearing oEGs substituents of different lengths. The anti-fouling properties of these modified surfaces were evaluated by FTIR spectroscopy and confocal laser scanning fluorescence microscopy upon exposure to bovine serum albumin (BSA) in phosphate (PBS) buffer., info:eu-repo/semantics/published

Biosensors that enable fast and selective identification and structural characterization of proteins in complex mixtures represent an important target for research. A major challenge in the development of such biosensors is the modification of surfaces by a robust organic monolayer able to specifically interact with a protein and displaying anti-fouling properties to prevent nonspecific adsorption phenomena. In this context, there is an increasing interest in the use of germanium-based surfaces because this material can be readily used for FTIR spectroscopy. Contrary to the other reliable detection methods, FTIR spectroscopy allows to simultaneously detect proteins and collect a wealth of information such as secondary structure and post-traductional modifications (e.g. phosphorylation, glycosylation level, etc.). Oligo(ethylene glycol) (oEG) films are well known to be efficient at reducing the nonspecific adsorption of biomolecules. However, self-assembled monolayers of thiolated oEG present important limitations related to their lack of stability. To prevent the nonspecific adsorption of the many proteins and biomolecules present in biological fluids, we use a strategy that consists in the modification of surfaces through the grafting of calix[4]arene-tetra-diazonium salts.1 More precisely, we have investigated the grafting on Ge and Au surfaces of monolayers of calix[4]arenes bearing oEGs substituents of different lengths. The anti-fouling properties of these modified surfaces were evaluated by FTIR spectroscopy andconfocal laser scanning fluorescence microscopy upon exposure to bovine serum albumin (BSA) in phosphate (PBS) buffer., info:eu-repo/semantics/published

Photoactive ruthenium-based complexes are actively studied for their biological applications as potential theragnostic agents against cancer. One major issue of these inorganic complexes is to penetrate inside cells in order to fulfil their function, either sensing the internal cell environment or exert a photocytotoxic activity. The use of lipophilic ligands allows the corresponding ruthenium complexes to passively diffuse inside cells but limits their structural and photophysical properties. Moreover, this strategy does not provide any cell selectivity. This limitation is also faced by complexes anchored on cell-penetrating peptides. In order to provide a selective cell targeting, we developed a multivalent system composed of a photoreactive ruthenium(II) complex tethered to a calix[4]arene platform bearing multiple RGD-containing cyclopentapeptides. Extensive photophysical and photochemical characterizations of this Ru(II)-calixarene conjugate as well as the study of its photoreactivity in the presence of guanosine monophosphate have been achieved. The results show that the ruthenium complex should be able to perform efficiently its photoinduced cytotoxic activity, once incorporated into targeted cancer cells thanks to the multivalent platform., SCOPUS: ar.j, info:eu-repo/semantics/published

Biosensors that can determine protein concentration and structure are highly desired for biomedical applications. For the development of such biosensors, the use of Fourier transform infrared (FTIR) spectroscopy with the attenuated internal total reflection (ATR) configuration is particularly attractive, but it requires appropriate surface functionalization of the ATR optical element. Indeed, the surface has to specifically interact with a target protein in close contact with the optical element and must display antifouling properties to prevent nonspecific adsorption of other proteins. Here, we report robust monolayers of calix[4]arenes bearing oligo(ethylene glycol) (oEG) chains, which were grafted on germanium and gold surfaces via their tetradiazonium salts. The formation of monolayers of oEGylated calix[4]arenes was confirmed by AFM, IR, and contact angle measurements. The antifouling properties of these modified surfaces were studied by ATR-FTIR spectroscopy and fluorescence microscopy, and the nonspecific absorption of bovine serum albumin was found to be reduced by 85% compared to that of unmodified germanium. In other words, the organic coating by oEGylated calix[4]arenes provides remarkable antifouling properties, opening the way for the design of germanium- or gold-based biosensors., SCOPUS: ar.j, info:eu-repo/semantics/published

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Labile ligands such as thiols and carboxylates are commonly used to functionalize AuNPs, though little control over the composition is possible when mixtures of ligands are used. It was shown recently that robustly functionalized AuNPs can be obtained through the reductive grafting of calix[4]arenes bearing diazonium groups on the large rim. Here, we report a calix[4]arene-tetradiazonium decorated by four oligo(ethylene glycol) chains on the small rim, which upon grafting gave AuNPs with excellent stability thanks to the C–Au bonds. Mixtures of this calixarene and one with four carboxylate groups were grafted on AuNPs. The resulting particles were analyzed by infrared spectroscopy, which revealed that the composition of the ligand shell clearly reflected the ratio of calixarenes that was present in solution. This strategy opens the way to robustly protected AuNPs with well-defined numbers of functional or postfunctionalizable groups., info:eu-repo/semantics/published

An attractive methodology based on diazonium chemistry has been developed for the surface modification of polymers such as polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS). The grafting procedure involves the in situ formation of diazoates in basic aqueous solution. The reactivity of calix[4]arene-tetradiazonium salts and a classical aryldiazonium salt was examined through comparative studies on gold and polymer surfaces. The surfaces were analyzed with a combination of techniques such as AFM, XPS, and ellipsometry. The results highlighted the fact that the calix[4]arene molecules are grafted as a robust and uniform monolayer both on gold and polymer surfaces, allowing a fine control over surface modification. Furthermore, the chemical postfunctionalization of the grafted calixarene platforms equipped with carboxylic-pendant groups was successfully performed with either an amine or an alcohol. These results open real possibilities in the controlled immobilization on polymers of a wide variety of molecules of interest such as biomolecules or chromophores and in the tailoring of polymer properties.

Gold nanoparticles stabilized with a thin layer of post-functionalizable calix[4]arenes were prepared through the reductive grafting of a calix[4]arene-tetra-diazonium salt. These particles show exceptional stability towards extreme pH, F−, NaCl, and upon drying. Post-functionalization of the calix-layer was demonstrated, opening the way to a wide range of applications.

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An attractive methodology based on diazonium chemistry has been developed for the surface modification of polymers such as polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS). The grafting procedure involves the in situ formation of diazoates in basic aqueous solution. The reactivity of calix[4]arene-tetradiazonium salts and a classical aryldiazonium salt was examined through comparative studies on gold and polymer surfaces. The surfaces were analyzed with a combination of techniques such as AFM, XPS, and ellipsometry. The results highlighted the fact that the calix[4]arene molecules are grafted as a robust and uniform monolayer both on gold and polymer surfaces, allowing a fine control over surface modification. Furthermore, the chemical postfunctionalization of the grafted calixarene platforms equipped with carboxylic-pendant groups was successfully performed with either an amine or an alcohol. These results open real possibilities in the controlled immobilization on polymers of a wide variety of molecules of interest such as biomolecules or chromophores and in the tailoring of polymer properties., SCOPUS: ar.j, info:eu-repo/semantics/published

Gold nanoparticles stabilized with a thin layer of post-functionalizable calix[4]arenes were prepared through the reductive grafting of a calix[4]arene-tetra-diazonium salt. These particles show exceptional stability towards extreme pH, F-, NaCl, and upon drying. Post-functionalization of the calix-layer was demonstrated, opening the way to a wide range of applications., SCOPUS: ar.j, info:eu-repo/semantics/published

Calixarenes are widely used as molecular platforms in supramolecular chemistry. Their efficient modification is key for appending functional moieties such as binding, sensing, chiral, or hydrophilic subunits. Due to the presence of multiple identical functional groups, selective functionalization of such macrocyclic compounds is highly challenging. Indeed, the control of the chemo-, regio-, stereo- as well as iteroselectivity is required. This review describes rational methods leading to a high degree of regio- and/or iteroselectivity and classifies them into specific strategies. Many of these strategies are conceptually very general and hopefully will be a source of inspiration for developing selective methods for other macrocyclic platforms., SCOPUS: re.j, FLWNA, info:eu-repo/semantics/published

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