Electrochemical and impedance spectroscopy studies of various diazonium salts on a glassy carbon electrode

Abstract  

The derivatization of a glassy carbon electrode surface was achieved with and without electrochemical reduction of various diazonium salts in acetonitrile solutions. The surfaces were characterized, before and after their attachment, by cyclic voltammetry and electrochemical impedance spectroscopy to evidence the formation of a coating on the carbon surface. The results were indicative of the presence of substituted phenyl groups on the investigated surface. Also, the effects of diazonium thin films at the surface of a glassy carbon electrode, modification time, and salt concentration on their electrochemical responses in the presence of the Fe(CN)₆^3−/4− probe were investigated. Electrochemical impedance measurements indicated that the kinetics of electron transfer is slowed down when the time and the concentration used to modify the glassy carbon electrode are increased. We therefore modified a glassy carbon surface via its derivatization with and without electrochemical reduction of various diazonium salts in acetonitrile solution.     

A Simple and Efficient Synthesis of Ethyl 1‐Aryl‐4‐formyl‐1H‐pyrazole‐3‐carboxylates

A new simple and convenient method of synthesis of ethyl 1‐aryl‐4‐formyl‐1H‐pyrazole‐3‐carboxylates from aromatic amines via diazonium salts has been developed. Hydrolysis and hydrazinolyization of these compounds has been investigated.     

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.     

Syntheses and modifications of bisdiazonium salts of 3,8-benzo[c]cinnoline and 3,8-benzo[c]cinnoline 5-oxide onto glassy carbon electrode and the characterization of the modified surfaces

The goal of this study was to prepare novel glassy carbon electrode surfaces using two similar bis-diazonium salts, 3,8-benzo[c]cinnoline (3,8-BCC-BDAS) and 3,8-benzo[c]cinnoline 5-oxide (3,8-BCCNO-BDAS) at the glassy carbon (GC) surface. These diazonium salts were reduced electrochemically and covalently electrografted onto the glassy carbon electrode surface to form modified electrodes. Electrochemical reduction of 3,8-BCC-BDAS and 3,8-BCCNO-BDAS salts on the electrode surface yielded a compact and stable film. The existence of BCC moieties on the GC surface was characterized by X-ray photoelectron spectroscopy, reflectance-adsorption infrared spectroscopy, cyclic voltammetry, ellipsometry, and electrochemical impedance spectroscopy. The stability and working potential range of the novel modified electrodes were also studied. The possibility of analytical application of these novel surfaces for inorganic cations and especially selectivity to copper ions was investigated. 3,8-diaminobenzo[c]cinnoline (3,8-DABCC) and its 5-oxide derivative (3,8-DABCCNO) were synthesized from the reductive cyclization of 2,2′-dinitrobenzidine and prepared their bisdiazonium salts via the tetrazotization reactions of the diamines with NaNO₂. The structures of 3,8-DABCC and 3,8-DABCCNO and their corresponding bisdiazonium salts are confirmed by spectral analysis.     

Studies with functionally substituted N‐alkylazoles: The reactivity of 1‐(3,5‐dimethylpyrazol‐1‐yl)‐acetone towards electrophilic reagents

The reaction of 1‐(3,5‐dimethylpyrazol‐1‐yl)acetone 4 with aromatic diazonium salts afforded the corresponding arylhydrazones 5a,b that were converted into pyridazines 6a,b and 8 via condensation with active methylene nitriles and dimethylformamide dimethylacetal, respectively. Condensation of 4 with phenylhydrazine afforded the phenylhydrazone 10 , which could be converted into the indolylpyrazole 11 on treatment with ethanolic hydrochloric acid. Compound 4 also reacted with nitrous acid, benzyl‐idenemalononitrile to yield a variety of substituted new pyrazoles.     

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.     

Electrochemical Reduction of a Conjugated Cinnamic Acid Diazonium Salt as an Immobilization Matrix for Glucose Biosensor

Initial results on the synthesis of a new conjugated diazonium salt of trans‐4‐cinnamic acid diazonium fluoroborate, which is used for the chemical modification of the glassy carbon (GC) electrode with trans‐4‐cinnamic acid groups through electrochemical reduction, and direct covalent immobilization of glucose oxidase (GOD) on the cinnamic acid groups are presented. The chemically modified GC electrode exhibits a good selectivity relative to the bare GC electrode for the various possible interfering compounds in glucose analysis: namely ascorbic acid and 4‐acetamidophenol. Covalent immobilization of GOD on the chemically modified GC electrode produces a biosensor which responds to glucose concentration changes in the presence of a soluble redox mediator (ferrocenemethanol). The chemical modification of GC by cinnamic acid groups is potentially useful for the attachment of other enzymes and biochemical reagents.     

Free-radical reactions of diazonium salts with α,β-unsaturated carbonyl compounds. A new synthesis of 1,4-diarylpyrazole derivatives

Free-radical decomposition of diazonium salts catalyzed by titanous or titanous and ferrous salts in the presence of β-substituted α,β-unsaturated carbonyl compounds leads to 1,4-diarylpyrazole derivatives. The reaction occurs via an intermediate azo compound ( 1 ), which can be reduced by the metal salt or can be isolated and hydrogenated to pyrazole derivatives.     

Modification of carbon electrode with aryl groups having an aliphatic amine by electrochemical reduction of in situ generated diazonium cations

The electrochemically induced functionalization of glassy carbon electrode by aryl groups having an aliphatic amine group was achieved by reduction of in situ generated diazonium cations in aqueous media. The corresponding diazonium cations of 4-aminobenzylamine, 2-aminobenzylamine, 4-(2-aminoethyl)aniline, N-methyl-1,2-phenylenediamine, and N, N-dimethyl- p-phenylenediamine were generated in situ with sodium nitrite in aqueous HCl. The kinetics of electrochemical grafting were investigated with electrochemical impedance spectroscopy and electrochemical quartz crystal microbalance measurements (with carbon-coated quartz crystal), and the barrier properties of the grafted layers were evaluated by cyclic voltammetry in the presence of electroactive redox probes such as Fe(CN)6 3-/4- and Ru(NH 3)6 (3+). The grafting efficiency of aryl groups was found to depend on the nature of the amine (primary, secondary, and tertiary), the chain length of the alkyl substituent, and the substitution position on the aromatic ring. The nitrosation of the "aliphatic" amine, in the case of secondary and tertiary amines, was also evidenced by X-ray photoelectron spectroscopy.     

The fabrication of stable gold nanoparticle-modified interfaces for electrochemistry

Forming stable gold nanoparticle (AuNP)-modified surface is important for a number of applications including sensing and electrocatalysis. Herein, tethering AuNPs to glassy carbon (GC) surfaces using surface bound diazonium salts is investigated as a strategy to produce stable AuNP surfaces. GC electrodes are first modified with 4-aminophenyl (GC-Ph-NH(2)), and then the terminal amine groups are converted to diazonium groups by incubating the GC-Ph-NH(2) interface in NaNO(2) and HCl solution to form a 4-phenyl diazonium chloride-modified interface (GC-Ph-N(2)(+)Cl(-)). Subsequently AuNPs are immobilized on the interface by electrochemical reduction to give a 4-phenyl AuNP-modified interface (GC-Ph-AuNP). For comparison, 4-aminophenyl AuNP- and 4-thiophenol AuNP-modified GC interfaces (GC-Ph-S-AuNP and GC-Ph-NH-AuNP), in which AuNPs are tethered to the surfaces by forming S-Au and NH-Au bond, respectively, were also prepared. Cyclic voltammetry, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy are used to characterize these fabricated interfaces. The AuNP on GC-Ph-AuNP surfaces demonstrate good stability under sonication in Milli-Q water, during electrochemical treatment in 0.05 M H(2)SO(4) solution, and over several weeks. By contrast, the GC-Ph-NH-AuNP and GC-Ph-S-AuNP surfaces showed significant particle losses under equivalent conditions.     

A convenient synthesis and antifungal activity of 1-aryl-1H- and 1-aryl-3-heteroaryl-1H-pyrazolo[3,4-b]quinoxalinesA Convenient Synthesis and Antifungal Activity

Novel 1-aryl-1H- and 1-aryl-3-heteroaryl-1H-pyrazolo[3,4-b]quinoxalines (flavazoles) 9a-c, 12, 13 were synthesized from 3-methyl-2-oxo-1,2-dihydroquinoxaline 5 and the 3-triazolylmethylene-2-oxo-1,2,3,4-tetrahydroquinoxaline 6, respectively, via a facile hydrazone synthesis using aryl diazonium salts. Some of the above flavazoles and their related compounds exhibited the antifungal activity in some extent. The above results are described.     

Studies with 1‐functionally substituted alkyl azoles: Novel synthesis of functionally substituted azolylbenzimidazoles and functionally substituted azolyl‐1,2,4‐triazoles

ω‐Azolylacetophenones 1 and 2 react with dimethylformamide dimethylacetal to yield enaminones 7,8 that were converted into azolylazoles via reaction with hydrazine and with hydroxylamine. Compounds 1,2 also coupled with aromatic diazonium salts to yield arylhydrazones and reacted with nitrous acid to yield corresponding oximes.     

Synthesis of 5-chloropyrazoles by chlorodediazoniation with sulfur dioxide

A facile synthesis of 5 -chloropyrazoles 4a-e from 5 -aminopyrazoles 2a-e via diazotization followed by chlorodediazoniation is described. A new application of sulfur dioxide as a catalyst was demonstrated to be the best for the chlorodediazoniation of diazonium chlorides 3a-e .     

Investigation of the Antifouling Properties of Phenyl Phosphorylcholine‐Based Modified Gold Surfaces

Low impedance, antifouling coatings on gold electrodes based on three new zwitterionic phenyl phosphorylcholine (PPC)‐based layers namely 1) reductively adsorbed PPC diazonium salt, 2) dithiocarbamate PPC SAM and 3) lipoamide PPC SAM (PPC coupled to α‐lipoic acid) were evaluated. The layers were assessed for their ability to limit nonspecific adsorption of proteins to electrode surface with some significant differences observed compared with previously studied PPC diazonium salts reductively adsorbed on glassy carbon. Fluorescence microscopy and electrochemical impedance spectroscopy results suggest that protein adsorption is sensitive to the difference in the structure of the PPC molecules and the charge neutrality of the layers. The lipoamide PPC SAM was shown to be the most effective at resisting nonspecific protein adsorption and this layer was as effective as the ‘gold standard’ of oligo(ethylene oxide) SAMs on gold and PPC diazonium salts reductively adsorbed on glassy carbon.     

Ionic liquid 1-(3-Trimethoxysilylpropyl)-3-methylimidazolium nitrite as a new reagent for the efficient diazotization of aniline derivatives and in situ synthesis of azo dyes

A new ionic liquid 1-(3-trimethoxysilylpropyl)-3-methylimidazolium nitrite was synthesized. This ionic liquid was used as a convenient nitrosonium source in diazotization of arylamines into their corresponding diazonium salts which were converted into their related azo dyes via the in situ azo-coupling with aniline derivatives or phenolic compounds. The diazotization of anilines in this ionic liquid and subsequent azo-coupling generated the related azo dyes in good to excellent yields at 0?C5 °C in short reaction times via a simple experimental procedure.     

Novel Synthesis of 1,8-Alkanopyrido[3,4-d]pyridazine: a New Ring System

Cycloalkylidenemalononitriles couple with various diazonium salts to yield the corresponding cycloalkeno[c]pyridazines, which react with trichloroacetonitrile to give the 1,8-alkanopyrido[3,4-d]pyridazines. The reaction of cycloalkenopyridazines with DMF dimethylacetal gives enamine derivatives, which can be converted to 1,8-alkanopyrido[3,4-d]pyridazines via treating with hydrazine hydrate or aromatic amines. Substituted cycloalkenopyridines react with diazoaminobenzene to afford the corresponding 1,8-alkanopyridopyridazines.     

The electrochemical behavior of some podands at a benzo[c]cinnoline modified glassy carbon electrode

This paper describes the grafting of benzo[c]cinnoline (BCC) molecules on glassy carbon (GC) electrode surface. The attachment of BCC molecules to carbon substrate is induced by the electrochemical reduction of the corresponding diazonium salt. The modification of GC with BCC diazonium salt was done in aprotic solution and proved by blocking of dopamine electron transfer. The presence of BCC at the GC surface was characterized by cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). On modified surface, the electrochemical behavior of two different types of podands and the catalytic effects of the GC-BCC surface were studied. The XPS was used to monitor element characteristics of the adsorbates on the GC surface and confirm the attachment of BCC molecules to the GC surface.     

Enaminone Incorporating a Dibromobenzofuran Moiety: Versatile Precursor for Novel Azines and Azolotriazines

2‐Acetyl‐5,7‐dibromobenzo[b]furan ( 1 ) reacted with dimethylformamide‐dimethylacetal to give the corresponding (E)‐enaminone 3 which coupled with aromatic diazonium chloride to afford the corresponding key intermediates 3‐oxo‐2‐(2‐arylhydrazono)propanals 6a , b . Compounds 6a , b were used to prepare novel 3‐imino‐2,3‐dihydropyridazines 10a , b and 6‐phenylazopyrido[2,3‐d]pyrimidin‐4(1H)‐one derivatives 15a , b through their reaction with acetonitrile derivatives 8a , b and 6‐aminopyrimidin‐4(1H)‐one 13 , respectively. On the other hand, the enaminone 3 was taken as a synthetic precursor to synthesize novel pyrazolo[5,1‐c][1,2,4]triazines 21a – c , [1,2,4]triazolo[3,4‐c][1,2,4]triazines 25 and benzimidazo[2,1‐c][1,2,4]triazine 29 containing 5,7‐dibromobenzofuran‐2‐oyl moiety via its coupling with the appropriate diazonium salts of a variety of hetero‐armomatic amines in a facile one‐step route.     

Diazonium Salts as Grafting Agents and Efficient Radical‐Hydrosilylation Initiators for Freestanding Photoluminescent Silicon Nanocrystals

The reactivity of diazonium salts towards freestanding, photoluminescent silicon nanocrystals (SiNCs) is reported. It was found that SiNCs can be functionalized with aryl groups by direct reductive grafting of the diazonium salts. Furthermore, diazonium salts are efficient radical initiators for SiNC hydrosilylation. For this purpose, novel electron‐deficient diazonium salts, highly soluble in nonpolar solvents were synthesized. The SiNCs were functionalized with a variety of alkenes and alkynes at room temperature with short reaction times.     

The Electrochemical Grafting of a Mixture of Substituted Phenyl Groups at a Glassy Carbon Electrode Surface

Two‐component substituted aryl groups are simultaneously grafted onto the surface of a glassy carbon electrode by electrochemical reduction of a binary mixture of two aryl diazonium salts in acetonitrile. The electrochemical deposition is achieved potentiostatically and two different mixtures with four different ratios of diazonium salts are used. The binary mixtures comprise: 1) 4‐nitrophenyl diazonium and 4‐bromophenyl diazonium cations and 2) 4‐bromophenyl diazonium and N,N‐diethylaniline diazonium cations. The chemical composition of the two component films is determined by cyclic voltammetry in an electrolyte inert for electroactive groups such as nitrophenyl and bromophenyl. X‐ray photoelectron spectroscopy is also used to evaluate the surface concentration of each grafted substituted phenyl group. The surface concentration of the substituted phenyl group for which the corresponding diazonium cation is the most easily reduced is higher than its concentration in the mixture of the deposition solution. The usefulness of binary films is also discussed.