Imaging and Quantifying the Chemical Communication between Single Particles in Metal Alloys.

The communication within particle agglomerates in industrial alloys can have a significant impact on the macroscopic reactivity, putting a high demand on the adaptation of wide-field methodologies to clarify this phenomenon. In this work, we report the application of correlated optical microscopies probing operando both local pH and local surface chemical transformation correlated with identical location scanning electron microscopy to quantify in situ the structure reactivity of particle agglomerates of foreign elements in the Al alloy. The optical operando analyses allow us (i) to reveal and quantify the local production of OH- from proton and oxygen reduction at individual Si- or Fe-rich microparticles and (ii) to quantify (and model) the chemical communication between these active sites, within a few micrometer range, on the local chemical transformation of the material. Wide-field image analysis highlights the statistical importance of chemical communication that may introduce a new conceptual framework for the understanding of the mechanisms in related fields of charge transfer, electrocatalysis, and corrosion.

Selective Nsp2 - and Csp2 - Photografting of Au-Surface by Aryldiazonium Salts and Arylazo Sulfonates.

Arylazo sulfonates (Ar-N=N-SO3 Na) have been found to undergo photografting on gold surface through both Au-Nsp2 - and Au-Csp2 - bond formation. The functionalized materials have been fully characterized by infrared reflection absorption spectroscopy (IRRAS), Raman, XPS, DFT calculations and UV-Vis absorption spectroscopy. These methods permit to evidence aromatic substituents (IRRAS), the Au-N=N signature (Raman and XPS spectroscopy), and the bond dissociation energy values of the two linkages (DFT calculation). The grafting proceeds through two competitive paths, namely a stepwise reaction involving an aryl radical (for the formation of the Au-Ar bonds) and a concerted reaction on the surface of gold (for Au-N=N-Ar bond formation). The occurrence of an aryl radical upon irradiation has been fully evidenced by EPR spectroscopy. Finally, E/Z photoisomerisation of the N=N bonds present on prepared few layer films has been observed by means of UV-Vis absorption spectroscopy.

Long-Range Plasmon-Induced Anisotropic Growth of an Organic Semiconductor between Isotropic Gold Nanoparticles.

Plasmon-induced diazonium reduction was used to graft an organic semiconductor, namely oligo(bisthienylbenzene) (BTB), onto square arrays of gold nanoparticles (NPs) of various diameters. Grafting was evidenced by scanning electron microscopy (SEM) measurements by the extinction spectra of the localized surface plasmon resonance, as well as by Raman and energy dispersive X-ray (EDX) spectroscopies. We show that BTB is selectively deposited around the NPs. The thickness of the layer increases with increasing irradiation time and reaches a limit which depends on the size of the NPs with the thicker organic layers being generated for smaller NPs. Under polarized irradiation, BTB growth is strongly anisotropic. Starting from arrays with square gratings and spherical NPs, long-range plasmon-induced anisotropic growth makes it possible to generate in the direction of the polarized light, lines, columns, or lines and columns of NPs connected by an organic semiconductor. These results demonstrate that the growth is due to hot electrons.

Imprinted Photonic Hydrogels for the Size- and Shell-Selective Recognition of Nanoparticles.

Sensors based on responsive photonic hydrogels have recently attracted considerable attention for visual medical diagnostics, pharmaceutical bioassays, and environmental monitoring. However, the use of these promising materials for the detection of nanoparticles (NPs) has never been explored so far, although the sensing of nanoobjects is a rapidly evolving area of research. To address this issue, we have combined the concepts of inverse-opal hydrogels and nanoparticle-imprinted polymers. In this way, we could obtain a NP-imprinted photonic hydrogel consisting of a three-dimensional, highly ordered poly(methacrylic acid) macroporous array, in which nanocavities complementary to the target NPs, in this case colloidal quantum dots, are distributed. This novel type of NP-imprinted photonic hydrogel sensor was shown to display high sensitivity and selectivity, thus opening new prospects for the development of equipment-free and cost-efficient sensing devices for NPs.

Reversible Trapping of Functional Molecules at Interfaces Using Diazonium Salts Chemistry.

Developing thin polymeric films for trapping, releasing, delivering, and sensing molecules is important for many applications in chemistry, biotechnology, and environment. Hence, a facile and scalable technique for loading specific molecules on surfaces would rapidly translate into applications. This work presents a novel method for the trapping of functional molecules at interfaces by exploiting diazonium salt chemistry. We demonstrate the efficiency of this approach by trapping two different molecules, 4-nitrobenzophenone and paracetamol, within polycarboxyphenyl layers grafted on gold and glassy carbon (GC) and by releasing them in acidic medium. The former molecule was chosen as a proof of concept for its electrochemical and spectroscopic properties, and the latter one was selected as an example of a pharmaceutical molecule. Advantages of the present approach rely on the simplicity, rapidity, and efficiency of the procedure for the reversible, on demand, trapping and release of functional molecules.

Control of the grafting of hybrid polyoxometalates on metal and carbon surfaces: toward submonolayers.

A Keggin-type POM is attached to gold or glassy carbon surfaces by electro(chemical) or peptidic coupling. In addition to demonstrating the robust attachment of the POMs (by electrochemistry, XPS, and IRRAS), the surface concentration, layer thickness, and rate constant for electron transfer from the surface to the POMs have been measured. The use of such complementary techniques is mandatory to characterize the modified electrodes properly. Whatever the grafting method, experimental conditions are found to allow monolayer or submonolayer coverage. Besides covalently grafted species, additional electrostatically bonded POMs are present in the film. Cathodic polarization allows removing them to get a grafted film that is stable with time and potential, which is a requirement in the design of molecular memories.

Electrografting of diazonium-functionalized polyoxometalates: synthesis, immobilisation and electron-transfer characterisation from glassy carbon.

Polyoxometalates (POMs) are attractive candidates for the rational design of multi-level charge-storage materials because they display reversible multi-step reduction processes in a narrow range of potentials. The functionalization of POMs allows for their integration in hybrid complementary metal oxide semiconductor (CMOS)/molecular devices, provided that fine control of their immobilisation on various substrates can be achieved. Owing to the wide applicability of the diazonium route to surface modification, a functionalized Keggin-type POM [PW11 O39 {Ge(p-C6 H4 -CC-C6 H4 -${{\rm N}{{+\hfill \atop 2\hfill}}}$)}](3-) bearing a pending diazonium group was prepared and subsequently covalently anchored onto a glassy carbon electrode. Electron transfer with the immobilised POM was thoroughly investigated and compared to that of the free POM in solution.

Scanning electrochemical microscopy monitoring in microcantilever platforms.

The deflection of cantilever systems may be performed by an indirect electrochemical method that consists of measuring the local cantilever activity and deflection in a feedback generation-collection configuration of the SECM. This is illustrated during the electrochemically assisted adsorption of Br onto a gold-coated cantilever, either in its pristine state or previously coated with a thin organic barrier. It is further extended to the adsorption of an antibody in a heterogeneous immunoassay at an allergen-coated microcantilever platform. In both reactions, the cantilever deflection is qualitatively detected from the SECM tip current measurement and a quantitative estimate is obtained through modeling. This electroanalytical strategy provides an alternative approach to standard optical detection. It can overcome some limitations of the optical method by allowing electrochemical characterization of nonconductive cantilevers and appropriate use for closed systems.

Diazonium salt-derived 4-(dimethylamino)phenyl groups as hydrogen donors in surface-confined radical photopolymerization for bioactive poly(2-hydroxyethyl methacrylate) grafts.

In this paper we describe a novel methodology for grafting polymers via radical photopolymerization initiated on gold surfaces by aryl layers from diazonium salt precursors. The parent 4-(dimethylamino)benzenediazonium salt was electroreduced on a gold surface to provide 4-(dimethylamino)phenyl (DMA) hydrogen donor layers; free benzophenone in solution was used as a photosensitizer to strip hydrogen from the grafted DMA. This system permitted efficient surface initiation of photopolymerization of 2-hydroxyethyl methacrylate. The resulting poly(2-hydroxyethyl methacrylate) (PHEMA) grafts were found to be very adherent to the surface as they resist total failure after being soaked in the well-known paint stripper methyl ethyl ketone. The PHEMA grafts were reacted with 1,1'-carbonyldiimidazole to yield carbamate groups that are able to react readily with amino groups from proteins. The final surface consisted of protein-functionalized PHEMA grafts where bovine serum albumin (BSA) protein is specifically linked to the grafts by covalent bonds. We used X-ray photoelectron spectroscopy to monitor the chemical changes at the gold surface all along the process from the neat gold to the end-protein-functionalized polymer grafts: the PHEMA graft thickness ranged from 7 to 27 nm, and the activation by 1,1'-carbonyldiimidazole reached 37% of the OH groups, which was sufficient for 90% surface coverage of the grafts by BSA. This work conclusively provides a new approach for bridging reactive and functional polymers to surfaces via aryl diazonium salts in a simple, fast, and efficient approach of importance in biomedical and other applications.

Aryl diazonium salts: a new class of coupling agents for bonding polymers, biomacromolecules and nanoparticles to surfaces.

This critical review summarizes existing knowledge on the use of diazonium salts as a new generation of surface modifiers and coupling agents for binding synthetic polymers, biomacromolecules, and nanoparticles to surfaces. Polymer grafts can be directly grown at surfaces through the so-called grafting from approaches based on several polymerization methods but can also be pre-formed in solution and then grafted to surfaces through grafting onto strategies including "click" reactions. Several routes are also described for binding biomacromolecules through aryl layers in view of developing biosensors and protein arrays, while the use of aryl diazonium coupling agents is extended to the attachment of nanoparticles. Patents and industrial applications of the surface chemistry of diazonium compounds are covered. This review stresses the paramount role of aryl diazonium coupling agents in adhesion, surface and materials sciences (114 references).

Green synthesis of water splitting electrocatalysts: IrO2 nanocages via Pearson's chemistry.

Highly porous iridium oxide structures are particularly well-suited for the preparation of porous catalyst layers needed in proton exchange membrane water electrolyzers. Herein, we report the formation of iridium oxide nanostructured cages, via a water-based process performed at room temperature, using cheap Cu2O cubes as the template. In this synthetic approach, based on Pearson's hard and soft acid-base theory, the replacement of the Cu2O core by an iridium shell is permitted by the difference in hardness/softness of cations and anions of the two reactants Cu2O and IrCl3. Calcination followed by acid leaching allow the removal of residual copper oxide cores and leave IrO2 hierarchical porous structures with outstanding activity toward the oxygen evolution reaction. Fundamental understanding of the reaction steps and identification of the intermediates are permitted by coupling a set of ex situ and in situ techniques including operando time-resolved X-ray absorption spectroscopy during the synthesis.

Highly hydrophilic surfaces from polyglycidol grafts with dual antifouling and specific protein recognition properties.

Homopolymer grafts from α-tert-butoxy-ω-vinylbenzyl-polyglycidol (PGL) were prepared on gold and stainless steel (SS) substrates modified by 4-benzoyl-phenyl (BP) moieties derived from the electroreduction of the parent salt 4-benzoyl benzene diazonium tetrafluoroborate. The grafted BP aryl groups efficiently served to surface-initiate photopolymerization (SIPP) of PGL. In similar conditions, SIPP of hydroxyethyl methacrylate (HEMA) permitted the production of PHEMA grafts as model surfaces. Water contact angles were found to be 66°, 15°, and 0° for SS-BP, SS-PHEMA, and SS-PPGL, respectively. The spontaneous spreading of water drops on SS-PPGL was invariably observed with 1.5 µL water drops. PPGL thus appears as a superhydrophilic polymer. Resistance to nonspecific adsorption of proteins of PPGL and PHEMA grafts on gold was evaluated by surface plasmon resonance (SPR) using antibovine serum albumin (anti-BSA). The results conclusively show that PPGL-grafts exhibit enhanced resistance to anti-BSA adsorption compared to the well-known hydrophilic PHEMA. PPGL grafts were further modified with BSA through the carbonyldiimidazole activation of the OH groups providing immunosensing surfaces. The so-prepared PPGL-grafted BSA hybrids specifically interacted with anti-BSA in PBS as compared to antimyoglobin. It is clear that the superhydrophilic character of PPGL grafts opens new avenues for biomedical applications where surfaces with dual functionality, namely, specific protein grafting together with resistance to biofouling, are required.

Confinement Effect of Plasmon for the Fabrication of Interconnected AuNPs through the Reduction of Diazonium Salts.

This paper describes a rapid bottom-up approach to selectively functionalize gold nanoparticles (AuNPs) on an indium tin oxide (ITO) substrate using the plasmon confinement effect. The plasmonic substrates based on a AuNP-free surfactant were fabricated by electrochemical deposition. Using this bottom-up technique, many sub-30 nm spatial gaps between the deposited AuNPs were randomly generated on the ITO substrate, which is difficult to obtain with a top-down approach (i.e., E-beam lithography) due to its fabrication limits. The 4-Aminodiphenyl (ADP) molecules were grafted directly onto the AuNPs through a plasmon-induced reduction of the 4-Aminodiphenyl diazonium salts (ADPD). The ADP organic layer preferentially grew in the narrow gaps between the many adjacent AuNPs to create interconnected AuNPs. This novel strategy opens up an efficient technique for the localized surface modification at the nanoscale over a macroscopic area, which is anticipated to be an advanced nanofabrication technique.

Electrografted aryl diazonium initiators for surface-confined photopolymerization: a new approach to designing functional polymer coatings.

This article reports on the preparation of polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-hydroxyethyl methacrylate) (PHEMA) ultrathin grafts on gold substrates modified by 4-benzoylphenyl (BP) moieties derived from the electroreduction of the parent diazonium salt BF(4)(-), (+)N(2)-C(6)H(4)-CO-C(6)H(5) (DS). The grafted organic species -C(6)H(4)-CO-C(6)H(5) was found to be very effective in the surface-initiating photopolymerization (SIPP) of vinylic monomers in the presence of an aromatic tertiary amine co-initiator acting as a hydrogen donor. This novel tandem diazonium salt electroreduction/SIPP was found to be effective in grafting PS, PMMA, and PHEMA from the surface of gold-coated silicon wafers. The polymer films were characterized in terms of chemical structure and wettability by infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy, and contact angle measurements, respectively. The polymer grafts were further evaluated as adsorbents for bovine serum albumin (BSA) used as a model protein. It was found gold/PHEMA resisted BSA adsorption because of its hydrophilic character, whereas PS and PMMA grafts adsorbed BSA via interfacial hydrophobic interaction. The XPS-determined extent of adsorbed BSA was found to increase linearly with the hydrophobic character of the polymer grafts as measured by water contact angles. This work shows that this novel tandem diazonium salt electroreduction/SIPP is a facile, ultrafast, efficient protocol for grafting polymer chains to surfaces. It broadens the enormous possibilities offered by aryl diazonium salts to generate functional organic coatings.

Diazonium salt chemistry for the design of nano-textured anti-icing surfaces.

Aryl diazonium salts have emerged as a new generation of robust surface modifiers for a wide range of applications. However, their use for creating anti-icing surfaces has never been investigated so far. We fill this gap by modifying nano-textured copper surfaces with aryl diazonium salts, bearing low surface energy end groups, leading to efficient anti-icing properties.

Regioselective surface functionalization of lithographically designed gold nanorods by plasmon-mediated reduction of aryl diazonium salts.

Site-selective surface functionalization of anisotropic gold nanoparticles represents a major breakthrough for fully exploiting nanoparticle anisotropy. In this paper, we explore an original strategy for the regioselective functionalization of lithographically designed gold nanorods (AuNRs), based a combination of photo-induced plasmon excitation and aryl diazonium salt chemistry.

Water-soluble plasmonic nanosensors with synthetic receptors for label-free detection of folic acid.

We describe an original approach to graft molecularly imprinted polymers around gold nanorods by combining the diazonium salt chemistry and the iniferter method. This chemical strategy enables fine control of the imprinting process at the nanometer scale and provides water-soluble plasmonic nanosensors.