Amines electroactivity

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

Voltammetry has been adapted to HPLC (when the mobile phase is conducting), and CE as a detection technique for electroactive compounds. In this usage, the voltammetric cell has been miniaturised (to about 1 p.L) in order not to dilute the analytes after separation. This method of amperometric detection in the pulsed mode is very sensitive. However, this device makes it possible to detect few analytically important molecules besides phenols, aromatic amines and thiols. 

The first step is so fast that it can hardly be measured experimentally, while the second step is much slower (probably as a result of the repulsion of negatively charged species, R and R2-, in the negatively charged diffuse electric layer). The reduction of an isolated benzene ring is very difficult and can occur only indirectly with solvated electrons formed by emission from the electrode into solvents such as some amines (see Section 1.2.3). This is a completely different mechanism than the usual interaction of electrons from the electrode with an electroactive substance. 

Biogenic amines, such as histamine [131], adenine [132], dopamine [133] and melamine [134], have been determined using chemosensors combining MIP recognition and PM transduction at QCM. Electronically conducting MIPs have been used in these chemosensors as recognition materials. Initially, functional electroactive bis(bithiophene)methane monomers, substituted either with the benzo-18-crown-6 or 3,4-dihydroxyphenyl, or dioxaborinane moiety, were allowed to form complexes, in ACN solutions, with these amines as templates. Subsequently, these complexes were oxidatively electropolymerized under potentiodynamic conditions. The resulting MIP films deposited onto electrodes of quartz resonators were washed with aqueous base solutions to extract the templates. 

The histamine [131], adenine [132] and dopamine [133] amines are electroactive in the positive potential range, in which the thiophene is electropolymerized. Therefore, these amines could be oxidized at the electrode surface in the course of deposition of the MIP film. That way, products of these oxidations might be available in the electrode vicinity for imprinting rather than the desired pristine... 

Pre-column OPA derivatization was also employed to analyze biogenic amines prior to MEKC separation on a PDMS chip [654]. Pre-column OPA derivatization and MEKC were also performed on a glass chip to analyze amino acids. Usually, OPA was used for fluorescent detection. However, in this report, amperometric detection was used as the OPA derivatives were also electroactive. Voltage (needed for separation) programming was used to decrease the migration time of late migrating species [655]. 

Iodide in vitamin tablets can be found by amperometric detection [88]. Nonaqueous eluents of methanol-containing ammonium perchlorate, which are relatively oxidant resistant, have been used in conjunction with a silica column to detect a wide range of drugs [89]. The use of higher potentials not possible in totally aqueous mobile phases allows for the detection of secondary and tertiary aliphatic amines 462 drugs have been detected in this manner. For compounds that are not electroactive, a procedure using a postcolumn photolysis can generate electroactive species [90] for penicillins [91], proteins [92], and barbiturates [93]. 

In another report published near the same time, this group demonstrated the electrochemical detection of electroactive enantiomers, d- and L-3,4-dihydroxypheny-lalanine (DOPA) and (R)- and (.S )-A, A /-dimcthylfcrroccnylcthylamine (FcN) in imprinted sol-gel-derived thin films.68 To improve response times, the imprinted films were made to be very thin, — 70 nm, which is about 3 to 10 times thinner than typical sol-gel-derived films. The functional monomer used to imprint DOPA was PTMOS (Fig. 20.2) because it was expected to exhibit tt-tt and hydrophobicity interactions. For the ferrocene derivative, both PTMOS (Fig. 20.2) and carboxyethylsila-netriol sodium salt (CTES) were used for hydrophobicity and tt-tt interactions and electrostatic interactions between the amine and carboxy groups, respectively.68... 

Although hydronium ion (H30+) (Chapter 8) and dioxygen (02) (Chapter 9) are the most studied of the molecules and ions without metal atoms, several of the molecules that contain sulfur, nitrogen, or carbon also are electroactive. The results for representative examples are presented to illustrate the utility of electrochemical measurements for die evaluation of the redox thermodynamics and bond energies for non-metal-containing molecules. In particular, die electrochemistry for several sulfur compounds [S8, S02, HS(CH2)3SH], nitrogen compounds [-NO, HON=0, N20, H2NOH, hydrazines (/ NHNH/ ), amines, phenazine], and carbon compounds (C02, CO, NCT) is summarized and interpreted. 

Figure 6.31 Molecular structure of a typical electroactive aromatic amine, with a redox potential well into the band-gap region...

Figure 6.32 Cyclic voltammogram of a nanocrystalline TiC electrode modified with a electroactive aromatic amine, measured at a scan rate of 2 mV s 1. The electrolyte is acetonitrile containing 0.5 M lithium bis(trifluoromethylsulfonyl)imide. Reprinted with permission from P. Bonhote, E. Gogniat, S. Tingry, C. Barbe, N. Vlachopoulos, F. Lenzmann, P. Comte and M. Gratzel, /. Phys. Chem., B, 102,1498 (1998). Copyright (1998) American Chemical Society...

The amine function in Pcs was also found to be versatile as it allows for the facile incorporation of other photoactive or electroactive species. In particular such amino-functionalized Pcs can be easily reacted with perylene dianhydride to undergo formation of the corresponding diimides such as 18 (Fig. 13) [90-92], The Pc building blocks, carrying either a spacered amino or an anilino group, respectively, were thus implemented into the perylene structure by melting of the two components in imidazole to result in the construction of the corresponding multicomponent assemblies. The pathway toward such structures can also be performed by first introduction of the phthalonitrile to the perylene followed by the formation of the macrocycle [93],... 

Ether solutions based on TAA salts are not reduced on noble metal electrodes. The major cathodic reaction of these solutions involves the cation reduction to trialkyl amine, alkane, and alkene (which are the stable disproportion products of the alkyl radical formed by the electron transfer to the cation) [3], Electrolysis of ethers such as THF or DME containing TBAP, formed in the catholyte tributyl amine, butane and butene, were unambiguously identified by NMR and GCMS analysis [3], In the presence of water (several hundred ppm and more), the electrolysis products were found to be tributyl amine and butene (butane was not detected) [3], The potential of this reduction reaction is higher than that of the dry solution, and it is clear that the initial electroactive species in this case is the... 

A very different detection approach was used [40] based on the electro-catalytic oxidation of sugars and amines at a copper electrode surface. The ssDNA and dsDNA were detected in the picomolar concentration range. The electrochemical signal due to dsDNA was higher than due to ssDNA owing to the larger number of easily accessible sugars on the outer perimeter of dsDNA double helix compared to those on a ssDNA of the same size, in contrast to most electrochemical studies based on the electroactivity of the bases. 

The previous parts of Sect. 5 testify to the fact that development of sensors for a given chemical species is dependent on the availability of receptors with suitable selectivity, electroactivity and optical transparency. These criteria were most easily satisfied with amine derivatives with various structural modifications for the binding of protons, alkali and alkaline earth cations. Such is the ubiquity of the amine motif in fluorescent PET research that several of the examples to be discussed in this section also contain it within their receptors. 

Electrochemical studies with mesoporous materials are only beginning to be explored [194]. Electroactive species can be introduced via ion exchange or by covalently attaching species in the mesopores. Balkus and co-workers were the first to examine the electrochemistry of anchored amine-ligated Co + complexes [191]. 

The magnitudes of the standard redox potentials (e.g., —1.56 and -I-1.53 V vs. SCE in acetonitrile) and the relatively low energy of excited Mo6Clfj (1.9eV) have also made possible observation of ECL from the annihilation of Mo6Cl] 4 and MofiClf with a variety of electroactive donors D (aromatic amines forming stable... 

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