Tailoring the Electrochemical Properties of Carbon Nanotube Modified Indium Tin Oxide via in Situ Grafting of Aryl Diazonium

The Role of Aryldiazonium Chemistry in Designing Electrochemical Aptasensors for the Detection of Food Contaminants

Food safety monitoring assays based on synthetic recognition structures such as aptamers are receiving considerable attention due to their remarkable advantages in terms of their ability to bind to a wide range of target analytes, strong binding affinity, facile manufacturing, and cost-effectiveness. Although aptasensors for food monitoring are still in the development stage, the use of an electrochemical detection route, combined with the wide range of materials available as transducers and the proper immobilization strategy of the aptamer at the transducer surface, can lead to powerful analytical tools. In such a context, employing aryldiazonium salts for the surface derivatization of transducer electrodes serves as a simple, versatile and robust strategy to fine-tune the interface properties and to facilitate the convenient anchoring and stability of the aptamer. By summarizing the most important results disclosed in the last years, this article provides a comprehensive review that emphasizes the contribution of aryldiazonium chemistry in developing electrochemical aptasensors for food safety monitoring.

Electrografting of 4-Nitrobenzenediazonium Salts on Al-7075 Alloy Surfaces-The Role of Intermetallic Particles

In this work, the electrografting of Al-7075 aluminium alloy substrates with 4-nitrobenzenediazonium salt (4-NBD) films was studied on a complex aluminium alloy surface. Prior to the electrografting reaction, the substrates were submitted to different surface treatments to modify the native aluminium oxide layer and unveil intermetallic particles (IMPs). The formation of the 4-NBD films could be correlated with the passive film state and the distribution of IMPs. The corresponding electrografting reaction was performed by cyclic voltammetry which allowed the simultaneous analysis of the redox reaction by a number of complementary surface-analytical techniques. Spatially resolved thin film analysis was performed by means of SEM-EDX, AFM, PM-IRRAS, Raman spectroscopy, XPS, and SKPFM. The collected data show that the 4-NBD film is preferentially formed either on the Al oxide layer or the IMP surface depending on the applied potential range. Potentials between -0.1 and -1.0 VAg/AgCl mostly generated nitrophenylene films on the oxide covered aluminium, while grafting between -0.1 and -0.4 VAg/AgCl favours the growth of these films on IMPs.

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.

Polypyrrole-Wrapped Carbon Nanotube Composite Films Coated on Diazonium-Modified Flexible ITO Sheets for the Electroanalysis of Heavy Metal Ions

Highly sensitive multicomponent materials designed for the recognition of hazardous compounds request control over interfacial chemistry. The latter is a key parameter in the construction of the sensing (macro) molecular architectures. In this work, multi-walled carbon nanotubes (CNTs) were deposited on diazonium-modified, flexible indium tin oxide (ITO) electrodes prior to the electropolymerization of pyrrole. This three-step process, including diazonium electroreduction, the deposition of CNTs and electropolymerization, provided adhesively-bonded, polypyrrole-wrapped CNT composite coatings on aminophenyl-modified flexible ITO sheets. The aminophenyl (AP) groups were attached to ITO by electroreduction of the in-situ generated aminobenzenediazonium compound in aqueous, acidic medium. For the first time, polypyrrole (PPy) was electrodeposited in the presence of both benzenesulfonic acid (dopant) and ethylene glycol-bis(2-aminoethylether)-tetraacetic acid (EGTA), which acts as a chelator. The flexible electrodes were characterized by XPS, Raman and scanning electron microscopy (SEM), which provided strong supporting evidence for the wrapping of CNTs by the electrodeposited PPy. Indeed, the CNT average diameter increased from 18 ± 2.6 nm to 27 ± 4.8, 35.6 ± 5.9 and 175 ± 20.1 after 1, 5 and 10 of electropolymerization of pyrrole, respectively. The PPy/CNT/NH₂-ITO films generated by this strategy exhibit significantly improved stability and higher conductivity compared to a similar PPy coating without any embedded CNTs, as assessed by from electrochemical impedance spectroscopy measurements. The potentiometric response was linear in the 10⁻⁸–3 × 10⁻⁷ mol L⁻¹ Pb(II) concentration range, and the detection limit was 2.9 × 10⁻⁹ mol L⁻¹ at S/N = 3. The EGTA was found to drastically improve selectivity for Pb(II) over Cu(II). To account for this improvement, the density functional theory (DFT) was employed to calculate the EGTA–metal ion interaction energy, which was found to be −374.6 and −116.4 kJ/mol for Pb(II) and Cu(II), respectively, considering solvation effects. This work demonstrates the power of a subtle combination of diazonium coupling agent, CNTs, chelators and conductive polymers to design high-performance electrochemical sensors for environmental applications.

Highly sensitive detection of rutin in pharmaceuticals and human serum using ITO electrodes modified with vertically-ordered mesoporous silica–graphene nanocomposite films

A vertically-ordered silica–graphene nanocomposite film modified transparent ITO electrode was prepared by a one-step electrodeposition method for antifouling detection of rutin in pharmaceuticals and human serum.

Ionic Strength Gated Redox Current Rectification by Ferrocene Grafted in Silica Nanochannels

In this work, a silica nanochannel membrane (SNM) consisting of a high density of vertically aligned channels on an indium tin oxide (ITO) electrode surface was used as a stencil to confine the grafting of ferrocene (Fc) via electrochemical reduction of diazonium cations. The grafting selectively occurs on the underlying ITO surface (namely, the bottom of silica nanochannels) instead of on the nanochannel walls. Thus, the obtained electrode, designated as Fc@SNM/ITO, is composed of an array of silica nanochannels with redox activity on the bottom. Immobilized Fc molecules are able to rectify the electron transfer between the underlying ITO electrode and soluble redox species. In other words, they can promote the electron transfer in one direction while suppressing that in the opposite direction. Anodic and cathodic redox rectification was observed for Fe(CN)₆^4- and IrCl₆^2-, respectively, with the magnitude of current rectification directly proportional to the concentration of redox species. Moreover, because of strong permselectivity of silica nanochannels arising from their ultrasmall size (2-3 nm in diameter) and negatively charged surface (due to deprotonation of surface silanol groups), the access of Fe(CN)₆^4- and IrCl₆^2- to nanochannels strongly depends on the ionic strength of electrolyte solution. A higher ionic strength favors the access of anionic redox species, thus leading to a larger redox current rectification. The redox current rectification phenomenon not only proves the permselective nature of silica nanochannels but also holds potential for the development of electrochemical current rectification-based sensors.

Surface modification of CoCr alloys by electrochemical reduction of diazonium salts

Tailoring the surface chemistry of CoCr alloys is of tremendous interest in many biomedical applications. In this work, we show that CoCr can be modified by diazonium electrografting provided the surface is not homogeneously covered with an oxide layer. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) show the electrografting of a poly(aminophenylene) (PAP) layer on CoCr when treated at a reductive potential (CoCr-0.5 V), whereas no PAP film was formed on CoCrOCP and CoCr1 V, treated at open circuit and anodic potentials respectively. Based on XPS results, we attributed the electrografting to the formation of carbide bonds between PAP and the inhomogeneous thin oxide layer of CoCr-0.5 V. We then show an example of application of PAP coatings on CoCr and prove that the presence of a PAP coating on CoCr-0.5 V results in a 5-fold increase of the adherence of poly methyl methacrylate (PMMA) to PAP-coated CoCr compared to uncoated samples; this is of prime significance to improving the long-term stability of dental prostheses. These findings support the importance of reducing the oxide layer for effective functionalization of metal oxides with aryl diazonium salts and suggest a promising surface modification approach for biomedical applications.

Metal/Carbon Hybrid Nanostructures Produced from Plasma-Enhanced Chemical Vapor Deposition over Nafion-Supported Electrochemically Deposited Cobalt Nanoparticles

In this work, we report development of hybrid nanostructures of metal nanoparticles (NP) and carbon nanostructures with strong potential for catalysis, sensing, and energy applications. First, the etched silicon wafer substrates were passivated for subsequent electrochemical (EC) processing through grafting of nitro phenyl groups using para-nitrobenzene diazonium (PNBT). The X-ray photoelectron spectroscope (XPS) and atomic force microscope (AFM) studies confirmed presence of few layers. Cobalt-based nanoparticles were produced over dip or spin coated Nafion films under different EC reduction conditions, namely CoSO₄ salt concentration (0.1 M, 1 mM), reduction time (5, 20 s), and indirect or direct EC reduction route. Extensive AFM examination revealed NP formation with different attributes (size, distribution) depending on electrochemistry conditions. While relatively large NP with >100 nm size and bimodal distribution were obtained after 20 s EC reduction in H₃BO₃ following Co²⁺ ion uptake, ultrafine NP (<10 nm) could be produced from EC reduction in CoSO₄ and H₃BO₃ mixed solution with some tendency to form oxides. Different carbon nanostructures including few-walled or multiwalled carbon nanotubes (CNT) and carbon nanosheets were grown in a C₂H₂/NH₃ plasma using the plasma-enhanced chemical vapor deposition technique. The devised processing routes enable size controlled synthesis of cobalt nanoparticles and metal/carbon hybrid nanostructures with unique microstructural features.

Molecular Sieving and Current Rectification Properties of Thin Organic Films

For the purpose of preparing well-organized functional surfaces, carbon and gold substrates were modified using electroreduction of a tetrahedral-shape preorganized tetra-aryldiazonium salt, leading to the deposition of ultrathin organic films. Characterization of the modified surfaces has been performed using cyclic voltammetry, X-ray photoelectron spectroscopy, infrared absorption spectroscopy, ellipsometry, atomic force microscopy, and contact angle measurements. The specific design of the tetra-aryldiazonium salts leads to an intrinsic structuring of the resulting organic films, allowing molecular sieving and current rectification properties toward redox probes in solution.