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).     

A Comparative Study of the Modification of Gold and Glassy Carbon Surfaces with Mixed Layers of In Situ Generated Aryl Diazonium Compounds

In situ generated aryl diazonium cations were synthesized in the electrochemical cell by reaction of the corresponding amines with NaNO₂ in aqueous HCl. This paper reports a study of the formation of mixed layers from in situ generated aryl diazonium cations. Firstly, glassy carbon (GC) and gold electrode surfaces were modified with five single in situ generated aryl diazonium salts to obtain their corresponding reductive potential followed by the modification of GC and gold surfaces with eight binary mixed layers of in situ generated aryl diazonium salts. The difference between GC and gold surfaces in terms of in situ formation of two‐component aryl diazonium salt films was compared. The behavior of the mixed layers formed from in situ generated aryl diazonium salts relative to diazonium salts that were pre‐synthesized prior to surface modification was also investigated. Cyclic voltammetry and X‐ray photoelectron spectroscopy were used to characterize the resulting modified GC and gold surfaces. It is found that for some aryl diazonium salts the potential at which reductive adsorption is achieved on gold and GC surfaces is significantly different. For the eight sets of binary mixed layers, the species with more anodic potential are more difficult to attach to the both GC and gold surfaces. The behavior of the mixed layers formed from in situ generated aryl diazonium salts and the pre‐synthesized diazonium salts is similar; which emphasizes the advantage of the in situ approach without any apparent difference in behavior to the presynthesized diazonium salts.     

Nanostructured electrocatalysts immobilised on electrode surfaces and organic film templates

The development of new electrocatalysts with the aim of enhancing the rate of electrochemical reactions has been a long-term goal of electrochemists. In part, this is due to the great importance of electrocatalysts in energy generation and environmental concerns. In this review, various methods of the preparation of nanostructured electrocatalysts and their applications after attachment to the electrode surface are described. Diazonium chemistry has been extensively used for the preparation and attachment of nanostructured electrocatalysts and this review thus describes the recent developments and applications of this chemistry in electrocatalysis. The preparation of nanostructured electrocatalysts including grafted molecular films and metal nanoparticles physically adsorbed on electrode surfaces and those attached to the surface by molecular links using diazonium chemistry is reviewed. Two methods for the attachment of nanoparticles by simple physical adsorption and by electrochemical deposition on molecular films are described and the electrochemical response of nanostructured electrocatalysts for some of the most common electrochemical reactions is discussed.     

Synthesis of metal nanoparticles stabilized by metal-carbon bonds

A new method for the preparation of metal nanoparticles in organic media is proposed. This is based on the formation of metal-carbon bonds after reduction of the corresponding diazonium derivative of the capping ligand. The particles are very stable due to the strong metal-ligand covalent bond, and the proposed method appears to be an alternative for the preparation of monolayer-protected metal nanoparticles when the formation of metal-sulfur or metal-nitrogen bonds needs to be avoided.     

Butylphenyl-functionalized Pt nanoparticles as CO-resistant electrocatalysts for formic acid oxidation

Butylphenyl-functionalized Pt nanoparticles (Pt-BP) with an average core diameter of 2.93 ± 0.49 nm were synthesized by the co-reduction of butylphenyl diazonium salt and H(2)PtCl(4). Cyclic voltammetric studies of the Pt-BP nanoparticles showed a much less pronounced hysteresis between the oxidation currents of formic acid in the forward and reverse scans, as compared to that on naked Pt surfaces. Electrochemical in situ FTIR studies confirmed that no adsorbed CO, a poisoning intermediate, was generated on the Pt-BP nanoparticle surface. These results suggest that functionalization of the Pt nanoparticles by butylphenyl fragments effectively blocked the CO poisoning pathway, most probably through third-body effects, and hence led to an apparent improvement of the electrocatalytic activity in formic acid oxidation.     

Metallic double shell hollow nanocages: the challenges of their synthetic techniques

Hollow metallic nanoparticles have been attracting the attention of many researchers in the past five years due to their new properties and potential applications. The unique structure of the hollow nanoparticles; presence of two surfaces (internal and external), and the presence of both cavities and pores in the wall surfaces of these nanoparticles are responsible for their unique properties and applications. Here the galvanic replacement technique is used to prepare nanocages made of gold, platinum, and palladium. In addition, hollow double shell nanoparticles are made of two metal shells like Au-Pt, Pt-Au, Au-Pd, Pd-Au, Pd-Pt, and Pt-Pd. Silver nanocubes are used as templates during the synthesis of hollow nanoparticles with single metal shell or double shell nanocages. Most of the problems that could affect the synthesis of solid Silver nanocubes used as template as well as the double shell nanocages and their possible solutions are discussed in a detail. The sizes and shapes of the single-shell and double-shell nanocages were characterized by a regular and high-resolution TEM. A SEM mapping technique is also used to image the surface atoms for the double shell hollow nanoparticles in order to determine the thickness of the two metal shells. In addition, optical studies are used to monitor the effect of the dielectric properties of the other metals on the plasmonic properties of the gold nanoshell in these mixed nanoparticles.     

In situ studies of the adsorption kinetics of 4-nitrobenzenediazonium salt on gold

Self-assembled organic layers are an important tool for modifying surfaces in a range of applications in materials science. Covalent modification of metal surfaces with aryldiazonium cations has attracted much attention primarily because this reaction offers a route for spontaneously grafting a variety of aromatic moieties from solution with high yield. We have investigated the kinetics of this process by performing real-time, in situ nanogravimetric measurements. The spontaneous grafting of 4-nitrobenzene diazonium salts onto gold electrodes was studied via quartz crystal microbalance (QCM) from aqueous solutions of the salt at varying concentrations. The concentration dependence of the grafting rate within the first 10 min is best modeled by assuming a reversible adsorption process with free energy comparable to that reported for arylthiols self-assembled on gold. Multilayer formation was observed after extended grafting times and was found to be favored by increasing bulk concentrations of the diazonium salt. Modified gold surfaces were characterized ex situ with cyclic voltammetry, infrared reflection absorbance spectroscopy, and X-ray photoemission spectroscopy. Based on the experimentally determined free energy of adsorption and on the observed grafting rates, we discuss a proposed mechanism for aryldiazonium chemisorption.     

Microscale patterning of organic films on carbon surfaces using electrochemistry and soft lithography

We have demonstrated three simple strategies employing poly(dimethylsiloxane) (PDMS) molds for patterning carbon surfaces with two different modifiers in an 18 microm line pattern. The PDMS molds are patterned with microfluidic channels (approximately 22 microm wide and 49 microm deep) and form a reversible, conformal seal to the pyrolyzed photoresist film (PPF) and modified PPF surfaces. Modifiers are electrochemically grafted to the PPF surface by the reduction of aryl diazonium salts and the oxidation of primary amines. For the fill-in patterning approach, the first modifier is electrografted to the PPF surface exposed within the microchannels, and in a second grafting step after removal of the PDMS mold, the second modifier fills in the remaining surface. The selective conversion strategy involves electrografting a continuous film of the modifier to the PPF surface, sealing the PDMS mold to the modified surface and carrying out an irreversible electrochemical reaction of the modifier exposed within the microchannels. In the build-up patterning approach, the PDMS mold is sealed to the modified PPF surface, and a chemical coupling reaction is effected in the microchannels to build up the pattern. The patterns are characterized using SEM, optical microscopy, the formation of condensation figures, and SEM imaging after the assembly of Au nanoparticles.     

Toward the syntheses of universal ligands for metal oxide surfaces: controlling surface functionality through click chemistry

A new means for functionalizing metal oxide surfaces, specifically nanoparticles, is demostrated. This process involves the design of stable ligands that bind strongly to the surface of metal oxides and can undergo further chemical modification via click chemistry, with both small molecules as well as polymers, to yield metal oxide surfaces with tailored functionality. The nanoparticles are stable and easily dispersed in both polar and nonpolar solvents, a property that is controlled by the ligand. The resultant nanoparticles were characterized by TEM, TGA, FTIR, and NMR.     

Carbon microspheres with supported silver nanoparticles prepared from pollen grains

In this paper, we present a new method to fabricate carbon microspheres with supported silver nanoparticles on the surfaces. In this method, pollen grains were first treated with AgNO(3) aqueous solution, then preoxidized in air at 300 degrees C and carbonized in nitrogen at 600 degrees C, resulting in the silver/carbon nanocomposites. The silver/carbon nanocomposites were characterized by means of SEM, TEM, TG, and XRD. The size and distribution of the silver nanoparticles on the carbon microsphere surface could be controlled by tuning the AgNO(3) treatment conditions.     

SE(R)RS devices fabricated by a laser electrodispersion method

A novel laser electrodispersion (LE) technique was employed to deposit gold nanoparticles onto Si and SiO(x) surfaces. The LE technique combines laser ablation with cascade fission of liquid metal micro-drops, which results in the formation of nanoparticles upon rapid cooling. The shape and the size distribution of the Au nanoparticles prepared by LE depend on the nature of the support. Gold nanoparticles were also deposited in the channels of microreactors fabricated by wet etching of Si and used as SE(R)RS sensors. The influence of the nanoparticle surface density as well as of the nature of the substrate on the Raman response was studied. At an appropriate surface density of the deposited nanoparticles a significant enhancement of Raman signal was observed showing the possibility to create efficient SERS substrates. Application of microfluidic devices in surface enhanced Raman spectroscopy (SERS) in continuous-flow mode with sensor regeneration is described.     

Electrocatalytic Reduction of Oxygen at Glassy Carbon Electrodes Coated with Diazonium‐derived Porphyrin/Metalloporphyrin Films

In this study, the influence of the film structure was investigated on the electrocatalytic oxygen reduction at GC electrodes covered with porphyrin and metalloporphyrin rings via the diazonium modification method. For that purpose, primarily, tetraphenylporphyrin (TPP) films on GC electrode surfaces were prepared by electroreduction of in situ generated diazonium salts of 5‐(4‐aminophenyl)‐10,15,20‐triphenylporphyrin (APP) and 5,10,15,20‐tetrakis(4‐aminophenyl)porphyrin (TAPP) molecules. Next, the formation of metalloporphyrin films on the modified surfaces was accomplished through the complexation reactions of surface porphyrin rings with metal ions in the salt solutions containing Mn(II), Fe(III) and Co(II) ions. The resulting porphyrin and metalloporphyrin layers were identified with XPS and ICP‐MS. The electrochemical barrier properties of the films on GC surfaces were examined by cyclic voltammetry in K₃Fe(CN)₆ aqueous solution. The electrocatalytic abilities of the resulting films were also investigated for the oxygen electrochemical reduction by employing cyclic voltammetry in PBS solutions saturated with oxygen. The results showed that the oxygen reduction potentials on modified GC electrodes were shifted to less negative potentials compared to that of bare GC electrode. Also, it was obtained that the oxygen reduction reaction was more effective on the GC electrodes modified with TPP rings by using TAPP molecules than those prepared by using APP molecules.     

Synthesis of Octylmethoxycinamate-silica Core-shell Nanoparticles with Self-templating Method

A self-templating method was employed to synthesize core-shell nanoparticles with octylmethoxycinamate(OMC), a well-known organic UV absorber, as core and nanosilica particles as shell. The characteristic of this method is that the whole process requires neither surface treatment for nanosilica particles nor additional surfactant or stabilizer, and all the reactions could be finished in one-pot, which exempts removing template and reduces reaction steps compared to the conventional process. The morphology, structure, particle size distribution, chemical composition and optical property of OMC-SiO2 nanoparticles were characterized by scanning electron microscopy(SEM), transmission electron microscopy(TEM), dynamic light scattering(DLS), FTIR spectrometry and UV absorption spectrometry, respectively. Experiment results indicate that the resulting OMC-SiO2 nanoparticles were perfectly spherical with smooth particle surfaces, and had clear core-shell structures. The particle size could be tuned by altering reaction conditions. In addition, the mechanism of the self-templating method for forming core-shell nanoparticles was discussed.     

Au nanoparticle monolayers covered with sol-gel oxide thin films: optical and morphological study

In this work, we provide a detailed study of the influence of thermal annealing on submonolayer Au nanoparticle deposited on functionalized surfaces as standalone films and those that are coated with sol-gel NiO and TiO(2) thin films. The systems are characterized through the use of UV-vis absorption, X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and spectroscopic ellipsometry. The surface plasmon resonance peak of the Au nanoparticles was found to red-shift and increase in intensity with increasing surface coverage, an observation that is directly correlated to the complex refractive index properties of Au nanoparticle layers. The standalone Au nanoparticles sinter at 200 °C, and a relationship between the optical properties and the annealing temperature is presented. When overcoated with sol-gel metal oxide films (NiO, TiO(2)), the optical properties of the Au nanoparticles are strongly affected by the metal oxide, resulting in an intense red shift and broadening of the plasmon band; moreover, the temperature-driven sintering is strongly limited by the metal oxide layer. Optical sensing tests for ethanol vapor are presented as one possible application, showing reversible sensing dynamics and confirming the effect of Au nanoparticles in increasing the sensitivity and in providing a wavelength dependent response, thus confirming the potential use of such materials as optical probes.     

A New Sensor with Increased Lifetime Based on a Mixed Diazonium Thick Film/Gold Nanoparticles Interface for Hg(II) Trace Detection

This work describes a novel strategy for surface functionalization, the aim of which is to significantly increase the lifetime of an electrochemical sensor dedicated to Hg(II) trace determination. In order to tailor stable mixed organic/inorganic interfaces, gold nanoparticles were electrodeposited onto a glassy carbon electrode previously functionalized by a thick 4‐thiophenol diazonium film, which affords a good anchoring to the nanoparticles. AFM and FEG‐SEM were used to characterize the film thickness and the nanoparticles average size and density, respectively. By using square wave anodic stripping voltammetry, the sensor exhibited a linear response between 1 and 10 nM Hg(II) and a normalized sensitivity 0.03 μA nM^?1 min^?1. Compared to previous works, the storage lifetime of the interface was at least three times longer, being more than three weeks.     

Detailed Structural Examinations of Covalently Immobilized Gold Nanoparticles onto Hydrogen‐Terminated Silicon Surfaces

The modification of flat semiconductor surfaces with nanoscale materials has been the subject of considerable interest. This paper provides detailed structural examinations of gold nanoparticles covalently immobilized onto hydrogen‐terminated silicon surfaces by a convenient thermal hydrosilylation to form Si? C bonds. Gold nanoparticles stabilized by ω‐alkene‐1‐thiols with different alkyl chain lengths (C₃, C₆, and C₁₁), with average diameters of 2–3 nm and a narrow size distribution were used. The thermal hydrosilylation reactions of these nanoparticles with hydrogen‐terminated Si(111) surfaces were carried out in toluene at various conditions under N₂. The obtained modified surfaces were observed by high‐resolution scanning electron microscopy (HR‐SEM). The obtained images indicate considerable changes in morphology with reaction time, reaction temperature, as well as the length of the stabilizing ω‐alkene‐1‐thiol molecules. These surfaces are stable and can be stored under ambient conditions for several weeks without measurable decomposition. It was also found that the aggregation of immobilized particles on a silicon surface occurred at high temperature (> 100 °C). Precise XPS measurements of modified surfaces were carried out by using a Au–S ligand‐exchange technique. The spectrum clearly showed the existence of Si? C bonds. Cross‐sectional HR‐TEM images also directly indicate that the particles were covalently attached to the silicon surface through Si? C bonds.     

Maleimide-activated aryl diazonium salts for electrode surface functionalization with biological and redox-active molecules

A versatile and simple method is introduced for formation of maleimide-functionalized surfaces using maleimide-activated aryl diazonium salts. We show for the first time electrodeposition of N-(4-diazophenyl)maleimide tetrafluoroborate on gold and carbon electrodes which was characterized via voltammetry, grazing angle FTIR, and ellipsometry. Electrodeposition conditions were used to control film thickness and yielded submonolayer-to-multilayer grafting. The resulting phenylmaleimide surfaces served as effective coupling agents for electrode functionalization with ferrocene and the redox-active protein cytochrome c. The utility of phenylmaleimide diazonium toward formation of a diazonium-activated conjugate, followed by direct electrodeposition of the diazonium-modified DNA onto the electrode surface, was also demonstrated. Effective electron transfer was obtained between immobilized molecules and the electrodes. This novel application of N-phenylmaleimide diazonium may facilitate the development of bioelectronic devices including biofuel cells, biosensors, and DNA and protein microarrays.     

铁氧化物/金磁性核壳纳米粒子的制备及其富集与SERS研究

本文用种子生长法制备铁氧化物/金磁性核壳纳米粒子, 并利用SERS对其磁场靶向性进行了检测.     

Electrochemical immobilization of a benzylic film through the reduction of benzyl halide derivatives: deposition onto highly ordered pyrolytic graphite

The reactivity of electrogenerated benzyl radicals at carbon surfaces was examined through the cathodic reduction of the corresponding bromide derivatives. 4-Nitrobenzyl bromide and benzyl bromide were reduced in N,N-dimethylformamide (DMF) on highly ordered pyrolytic graphite (HOPG) surfaces. Electroproduced films were examined using electrochemistry, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Experiments show the formation of strongly adherent deposits and the occurrence of electrografting processes. They are based on radical generation and the reaction of the radical with the substrate. As expected, the thickness of the organic film increases with deposition time but the deposit displays a lower compactness than previously reported for the electroreduction of aryl diazonium salts. Interestingly for benzyl derivatives, the reduction potential required for the electrografting could be rendered much more positive by simply using an iodide-type supporting electrolyte.     

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes

This study describes a novel approach for the in situ synthesis of metal oxide–polyelectrolyte nanocomposites formed via impregnation of hydrated polyelectrolyte films with binary water/alcohol solutions of metal salts and consecutive reactions that convert metal cations into oxide nanoparticles embedded within the polymer matrix. The method is demonstrated drawing on the example of Nafion membranes and a variety of metal oxides with an emphasis placed on zinc oxide. The in situ formation of nanoparticles is controlled by changing the solvent composition and conditions of synthesis that for the first time allows one to tailor not only the size, but also the nanoparticle shape, giving a preference to growth of a particular crystal facet. The high‐resolution TEM, SEM/EDX, UV‐vis and XRD studies confirmed the homogeneous distribution of crystalline nanoparticles of circa 4 nm and their aggregates of 10–20 nm. The produced nanocomposite films are flexible, mechanically robust and have a potential to be employed in sensing, optoelectronics and catalysis.