Spectroelectrochemical Sensing: Current Trends and Challenges

Spectroelectrochemistry (SEC) has been used for more than 50 years, but this set of techniques has not been widely used for quantitative analysis. For many years, no commercial instruments were available, which made very difficult to spread the use of SEC. Nowadays, only the creativity of the researchers is required to exploit the capabilities of SEC. This review is written with the aim of showing the potential of SEC, mainly for analytical chemistry. Here, we explain what SEC is, how analytical responses can be obtained, why these techniques are useful for sensors, with a brief description of its advantages in use, and, finally, we try to show the challenges that must be addressed in the next years. SEC can resolve interesting analytical problems using the high amount of data provided by this intrinsic trilinear technique. Given the quantitative analysis point of view of this review, the discussion of the SEC techniques is focused on UV/Vis absorption, photoluminescence and Raman SEC.Spectroelectrochemistry (SEC) has been used for more than 50 years, but this set of techniques has not been widely used for quantitative analysis. For many years, no commercial instruments were available, which made very difficult to spread the use of SEC. Nowadays, only the creativity of the researchers is required to exploit the capabilities of SEC. This review is written with the aim of showing the potential of SEC, mainly for analytical chemistry. Here, we explain what SEC is, how analytical responses can be obtained, why these techniques are useful for sensors, with a brief description of its advantages in use, and, finally, we try to show the challenges that must be addressed in the next years. SEC can resolve interesting analytical problems using the high amount of data provided by this intrinsic trilinear technique. Given the quantitative analysis point of view of this review, the discussion of the SEC techniques is focused on UV/Vis absorption, photoluminescence and Raman SEC.Ministerio de Economía y Competitividad (Grants CTQ2017-83935-R-AEI/FEDERUE) and Junta de Castilla y León (Grant BU033-U16 and BU297P18). J. G.–R. thanks UBU for his postdoctoral contract. J.V.P–R. thanks JCyL for his postdoctoral fellowship (Grant BU033-U16). Thanks to

Reticular synthesis of porous molecular 1D nanotubes and 3D networks

Synthetic control over pore size and pore connectivity is the crowning achievement for porous metal–organic frameworks (MOFs). The same level of control has not been achieved for molecular crystals, which are not defined by strong, directional intermolecular coordination bonds. Hence, molecular crystallization is inherently less controllable than framework crystallization, and there are fewer examples of ‘reticular synthesis’, in which multiple building blocks can be assembled according to a common assembly motif. Here we apply a chiral recognition strategy to a new family of tubular covalent cages to create both 1D porous nanotubes and 3D diamondoid pillared porous networks. The diamondoid networks are analogous to MOFs prepared from tetrahedral metal nodes and linear ditopic organic linkers. The crystal structures can be rationalized by computational lattice-energy searches, which provide an in silico screening method to evaluate candidate molecular building blocks. These results are a blueprint for applying the ‘node and strut’ principles of reticular synthesis to molecular crystals

Studies on the electrochemical deposition of niobium oxide

Various strategies for the electrodeposition of niobium oxide from both non-aqueous and aqueous solution are explored. The methods used were based on the electrochemical manipulation of the pH which leads to hydrolysis of soluble, well defined niobium precursors, leading to oxide deposition at the electrode. The reactions which proceed are believed to be closely related to those involved in conventional sol-gel processing. In non-aqueous systems containing niobium alkoxides, hydroxide ions are generated by the two-electron reduction of tertiary alcohols (at &lt; -2.5 V) and react with the niobium precursor to give a film of niobium oxide on the dropping mercury electrode. Hydroxide generation via the reduction of oxygen to superoxide in O-2-saturated, non-aqueous solvents containing NbOCl3 did not, however, lead to niobium oxide films. In acidic aqueous systems containing [NbOCl5](2-) species, raising the local pH around the electrode by hydrogen evolution does give rise to niobium oxide deposition, but not at the electrode; In aqueous alkaline solutions of the niobates K7H [Nb6O19] or (TMA)(6)[Nb10O28] (TMA = tetramethylammonium), protons generated by the electrochemical oxidation of water at &gt; + 1.5 V cause the formation of mixed niobate and niobium oxide films on the electrode after &gt; 30 min.</p

Cyclic voltammetry and spectroelectrochemical study of nickel and cobalt diphenyltetraazaannulene complexes

The absorption spectra. cyclic voltammetry and spectroelectrochemistry of [Ni(II)DPTAA] and [Co(II)DPTAA] (DPTAA = 6,13-diphenyldibenzo[b,i][1,4,8,11] tetraaza[14]annulene) complexes in DMF are reported in detail. The ligand oxidation is observed for [Ni(II)DPTAA] at +0.70 V vs. SCE whereas Ni2(+/+) occurs at - 1.60 V. For [Co(II)DPTAA], a ligand oxidation redox couple is seen at +0.56 V while the Co2+/+ and Co2+/3+ redox couples appear at -1.21 and +0.24 V, respectively. All observed redox couples are assigned to reversible one-electron processes on account of peak separations and scan-rate dependency. These processes were further investigated by spectroelectrochemistry for [Co(II)DPTAA]. For [Co(II)DPTAA], axial ligation of pyridine was found to shift the Co2+/3+ redox couple more negative. while the ligand oxidation was shifted to more positive potentials. From a spectrophotometric titration of [Co(II)DPTAA] with pyridine an equilibrium constant, K-f, was determined for the binding of pyridine to [Co(II)DPTAA]. This was found to be 10.2 dm(3) mol(-1), slightly lower than that of [Co(II)TAA], indicating the influence of the phenyl groups. From this value and shifts in the Co2+/3+ redox couple upon ligation, an equilibrium constant for the binding of pyridine to [Co(III)DPTAA], K'(f), was found to be 5.06 x 10(6) dm(3) mol(-1). (c) 2007 Elsevier B.V. All rights reserved.</p

Polyradicals:Synthetic strategies and characterization

Synthetic routes to polyradicals are briefly reviewed; in addition the synthesis and electrochemical characterization of a novel 2,2,6,6-tetramethylpiperidinyloxy-l-pyrrylacetate(I) (or, TEMPO-1-pyrrylacetate) are reported. Rather than forming a conducting polymer deposit, this monomer coats the electrode with a passivating monolayer, which is electroactive at the nitroxide potential (+0.93 V vs Ag+/0). The cyclic voltammetry of the surface-confined nitroxide not only exhibits typical monolayer behaviour (calculated coverage 0.5 nmol cm-2), but also unusual spikes caused by trapped charge.</p

Effect of meta-linkages on the photoluminescence and electroluminescence properties of light-emitting polyfluorene alternating copolymers

Alternating copolymers poly[9,9-dihexylfluorene-alt-(1,3-phenylene)(x)(1,4- phenylene) (1-x)] (x = 0, 0.05, 0.1, 0.25, 0.5, 0.75, 1.0) were prepared, and the absorption spectra showed the expected hypsochromic shift with increasing meta linkages. In the solution photoluminescence (PL) emission spectra, all but the all-meta polymer (x = 1) showed a spectrum similar to the all-para polymer, indicating rapid intrachain energy transfer to para-linked segments. Similar spectra were observed from thin films in which inter-chain energy transfer is also available. Annealing of the thin films in air above the T-g of the polymer for several hours led to an increase in long wavelength emission, but the increase was smaller for the polymers containing more meta linkages. IR experiments confirmed the formation of fluorenone defects during the annealing process. For those polymers with high meta-linkage incorporation (i.e. 75% and 100% meta) the long wavelength emission is suppressed. This suggests that when the conjugation length of the polymer is reduced then energy transfer to fluorenone defects is less likely, due to lower exciton delocalisation and mobility. The long wavelength emission was more prominent in the electroluminescence (EL) spectra of devices prepared from the polymer, and, while initially less intense in the high meta linkages polymers, became dominant even for the all-meta linked polymer after a few minutes of operation. In summary, the meta linkages had a beneficial effect in suppressing the undesirable long wavelength emission in both the PL and EL output.</p

Polyradicals:Synthetic strategies and characterization

Synthetic routes to polyradicals are briefly reviewed; in addition the synthesis and electrochemical characterization of a novel anthraquinone derivatized thiophene monomer, 1-[4-(3-thienyl)butyl]2-anthraquinone carboxylate (I) is discussed. This monomer may be electropolymerized onto Pt in CH2Cl2 by successively more anodic potential sweeps from 0.0V, finally reaching a steady growth at a limit of +2.0V vs Ag+/0. Thhe voltammogram of the resulting conducting polymer displaysa polymer waves at Epa + 1.0 and Epc + 0.84V, and quinone waves at E0′ = -0.80 and -1.24V vs Ag.+/0 The polymer is active towards O2 reduction in pH7 buffered aqueous solution.</p

Synthesis of Thiophene Substituted Ruthenium and Rhenium Bipyridyl Complexes

3-Alkyl thiophenes may be conveniently polymerised by chemical or electrochemical means and further processed to provide electroactive polymers. Here we describe a versatile synthetic strategy that provides thiophene metal complexes with potential use in the fields of redox active polymers and in electroluminescent devices. The electrochemical behavior of two complexes is established and is reported.</p

Polyradicals:Synthetic strategies and characterization

Synthetic routes to polyradicals are briefly reviewed; in addition the synthesis and electrochemical characterization of a novel anthraquinone derivatized thiophene monomer, 1-[4-(3-thienyl)butyl]2-anthraquinone carboxylate (I) is discussed. This monomer may be electropolymerized onto Pt in CH2Cl2 by successively more anodic potential sweeps from 0.0V, finally reaching a steady growth at a limit of +2.0V vs Ag+/0. Thhe voltammogram of the resulting conducting polymer displaysa polymer waves at Epa + 1.0 and Epc + 0.84V, and quinone waves at E0′ = -0.80 and -1.24V vs Ag.+/0 The polymer is active towards O2 reduction in pH7 buffered aqueous solution.</p

Effect of meta-linkages on the photoluminescence and electroluminescence properties of light-emitting polyfluorene alternating copolymers

Alternating copolymers poly[9,9-dihexylfluorene-alt-(1,3-phenylene)(x)(1,4- phenylene) (1-x)] (x = 0, 0.05, 0.1, 0.25, 0.5, 0.75, 1.0) were prepared, and the absorption spectra showed the expected hypsochromic shift with increasing meta linkages. In the solution photoluminescence (PL) emission spectra, all but the all-meta polymer (x = 1) showed a spectrum similar to the all-para polymer, indicating rapid intrachain energy transfer to para-linked segments. Similar spectra were observed from thin films in which inter-chain energy transfer is also available. Annealing of the thin films in air above the T-g of the polymer for several hours led to an increase in long wavelength emission, but the increase was smaller for the polymers containing more meta linkages. IR experiments confirmed the formation of fluorenone defects during the annealing process. For those polymers with high meta-linkage incorporation (i.e. 75% and 100% meta) the long wavelength emission is suppressed. This suggests that when the conjugation length of the polymer is reduced then energy transfer to fluorenone defects is less likely, due to lower exciton delocalisation and mobility. The long wavelength emission was more prominent in the electroluminescence (EL) spectra of devices prepared from the polymer, and, while initially less intense in the high meta linkages polymers, became dominant even for the all-meta linked polymer after a few minutes of operation. In summary, the meta linkages had a beneficial effect in suppressing the undesirable long wavelength emission in both the PL and EL output.</p