Fluorescent photoinduced electron transfer (PET) sensors for anions; from design to potential application

This mini review highlights the synthesis and photophysical evaluation of anion sensors, for nonaqueous solutions, that have been developed in our laboratories over the last few years. We have focused our research mainly on developing fluorescent photoinduced electron transfer (PET) sensors based on the fluorophore-spacer-anion receptor principle using several anthracene (emitting in the blue) and 1,8-naphthalimide (emitting in the green) fluorophores, with the aim of targeting biologically and industrially relevant anions such as acetates, phosphate and amino acids, as well as halides such as fluoride. The receptors and the fluorophore are separated by a short methyl or ethyl spacer, where the charge neutral anion receptors are either aliphatic or aromatic urea (or thiourea) moieties. For these, the anion recognition is through hydrogen bonding, yielding anion:receptor complexes. Such bonding gives rise to enhanced reduction potential in the receptor moieties which causes enhancement in the rate of PET quenching of the fluorophore excited state from the anion:receptor moiety. This design can be further elaborated on by incorporating either two fluorophores, or urea/thiourea receptors into the sensor structures, using anthracene as a fluorophore. For the latter design, the sensors were designed to achieve sensing of bis-anions, such as di-carboxylates or pyrophosphate, where the anion bridged the anthracene moiety. In the case of the naphthalimide based mono-receptor based PET sensors, it was discovered that in DMSO the sensors were also susceptible to deprotonation by anions such as F− at high concentrations. This led to substantial changes in the absorption spectra of these sensors, where the solution changed colour from yellow/green to deep blue, which was clearly visible to the naked eye. Hence, some of the examples presented can act as dual fluorescent-colorimetric sensors for anions. Further investigations into this phenomenon led to the development of simple colorimetric sensors for fluorides, which upon exposure to air, were shown to fix carbon dioxide as bicarbonate.

Dual responsive chemosensors for anions : the combination of fluorescent PET (Photoinduced Electron Transfer) and colorimetric chemosensors in a single molecule

The design and synthesis of two novel fluorescent PET anion sensors is described, based on the principle of ‘fluorophore-spacer-(anion)receptor’. The sensors 1 and 2 employ simple diaromatic thioureas as anion receptors, and the fluorophore is a naphthalimide moiety that absorbs in the visible part of the spectrum and emits in the green. Upon recognition of anions such as F⁻ and AcO⁻ in DMSO, the fluorescence emission of 1 and 2 was ‘switched off’, with no significant changes in the UV–vis spectra. This recognition shows a 1:1 binding between the receptor and the anions. In the case of F⁻, further additions of the anion, gave rise to large changes in the UV–vis spectra, where the λ_max at 455 nm was shifted to 550 nm. These changes are thought to be due to the deprotonation of the 4-amino moiety of the naphthalimide fluorophore. This was in fact found to be the case, using simple naphthalimide derivatives such as 6. Sensors 1 and 2 can thus display dual sensing action; where at low concentrations, the fluorescence emission is quenched, and at higher concentrations the absorption spectra are modulated.

Milk lactate determination with a rotating bioreactor based on an electron transfer mediated by osmium complexes incorporating a continuous-flow/stopped-flow system

The high sensitivity that can be attained using a bienzymatic system and mediated by the redox polymer [Os(bpy)₂ClPyCH₂NHpoly(allylamine)] (Os-PAA), has been verified by on-line interfacing of a rotating bioreactor and continuous-flow/stopped-flow/continuous-flow processing. When the hydrogen peroxide formed by LO_x layer reaches the inner layer, the electronic flow between the immobilized peroxidase and the electrode surface produces a current, proportional to lactate concentration. The determination of lactate was possible with a limit of detection of 5 nmol l⁻¹ in the processing of as many as 30 samples per hour. This arrangement allows working in undiluted milk samples with a good stability and reproducibility. Horseradish peroxidase [EC 1.11.1.7] and Os-PAA were covalently immobilized on the glassy carbon electrode surface (upper cell body), lactate oxidase [EC 1.1.3.x] was immobilized on a disk that can be rotated.

Conducting polymer enzyme alloys : electromaterials exhibiting direct electron transfer

Glucose oxidase (GOx) is an important enzyme with great potential application for enzymatic sensing of glucose, in implantable biofuel cells for powering of medical devices in vivo and for large-scale biofuel cells for distributed energy generation. For these applications, immobilisation of GOx and direct transfer of electrons from the enzyme to an electrode material is required. This paper describes synthesis of conducting polymer (CP) structures in which GOx has been entrained such that direct electron transfer is possible between GOx and the CP. CP/enzyme composites prepared by other means show no evidence of such “wiring”. These materials therefore show promise for mediator-less electronic connection of GOx into easily produced electrodes for biosensing or biofuel cell applications.

Controllable graphene oxide mediated efficient electron transfer pathways across self-assembly monolayers: a new class of graphene based electrodes

In this paper, the influence of chemically reduced graphene oxide sheets (CRGOs) on the electrochemical performance through methyl or carboxylic acid terminated self-assembled monolayers (SAMs) is reported. The gold electrode was initially modified with methyl or carboxylic acid terminated alkanethiols with various carbon chain lengths (n = 4, 6, 8 and 11) and subsequently immobilization of the CRGOs on a SAM surface was achieved via a hydrophobic and electrostatic interaction. By using the potassium ferricyanide as a redox probe, it was observed that CRGOs could effectively enhance the heterogeneous electron transfer (ET) of the SAM due to a tunneling effect. The assemblies based on thiol end groups with methyl head groups were observed to afford more hydrophobic interaction binding with CRGOs with a higher reduction time than the assemblies developed with thiol end groups and a -COOH group which were shown to bind more electrostatically with CRGOs, a lowering reduction time. The Nyquist plots developed show a gradual decrease of the charge transfer resistance (Rct) of [Fe(CN)6]3-/4- redox couple at the CRGOs-SAMs electrode with the controllable adsorption of different CRGO's onto the SAM. Depending on the chain length and terminal functional group the electron transfer rate kinetics were observed to differ considerably.

Targeting the mitochondrial electron transport chain in autism, a systematic review and synthesis of a novel therapeutic approach

Autism is a complex developmental disorder with an unknown etiology and without any curative treatment. The mitochondrial electron transfer chains play a major role in the production of ATP, and the generation and management of reactive oxidative stress (ROS). This paper is a systematic review of the role of the mitochondrial electron transport chain in autism, and a consequent hypothesis for treating autism is synthesized.

An electronic search with pre-specified inclusion criteria was conducted in order to retrieve all the published articles about the mitochondrial electron transport chain in autism. The two databases of PUBMED and Google Scholar were searched.

The determination of urea with hypobromite: a theme for the discussion of numerous fundamental concepts in chemistry

Over the past century, numerous aspects of the reaction between urea and hypobromite have been exploited to quantify urea in clinical and industrial process samples. A review of these analytical approaches provides an interesting illustration of changes in a chemical system that indicate a reaction has occurred-the production of a gas, a color change, the release of heat, and the emission of light-and a variety of instruments that were developed to measure these changes and quantify a reacting species. In this paper we describe how we have used this material in a tutorial class for first-year undergraduate (freshman) students and a follow-up assignment, which we have included in the supporting material. In addition to the concepts exemplified by the above phenomena, we discuss the reaction pathway, which includes examples of ion and atom transfer. These are
often overlooked in favor of electron transfer in the teaching of redox chemistry.

Anion recognition and sensing in organic and aqueous media using luminescent and colorimetric sensors

This review article focuses primarily on the work carried in our laboratories over the last few years using luminescent and colorimetric sensors, where the anion recognition occurs through hydrogen bonding in organic or aqueous solvents. This review begins with the story of the discovery of fluorescent photoinduced electron transfer (PET) sensors for anions using charged neutral urea or thiourea receptors where both fluorescent and NMR spectroscopic methods monitored anion recognition. This work led to the development of dual luminescent and colorimetric anion sensors based on the use of the ICT based naphthalimide chromophore, where ions such as fluoride gave rise to changes in both the fluorescence and the absorption spectra of the sensors, but at different concentrations. Here, the former changes were due to hydrogen bonding interactions, whereas the latter was due to the deprotonation of acidic protons, giving rise to the formation of the bifluoride anion (HF2−). Modification of the 4-amino-l,8-naphthalimide moiety has facilitated the formation of colorimetric anion sensors that work both in organic or aqueous solutions. Such charge neutral receptor motifs have also been incorporated into organic scaffolds with norbomyl and calixarene backbones, which have enabled us to produce anion directed self-assembled structures.

Demonstration of the advantages of using bamboo-like nanotubes for electrochemical biosensor applications compared with single walled carbon nanotubes

The modification of glassy carbon electrodes with random dispersions of nanotubes is currently the most popular approach to the preparation of carbon nanotube modified electrodes. The performance of glassy carbon electrodes modified with a random dispersion of bamboo type carbon nanotubes was compared with single walled carbon nanotubes modified glassy carbon electrodes and bare glassy carbon electrodes. The electrochemical performance of all three types for electrode were compared by investigating the electrochemistry with solution species and the oxidation of guanine and adenine bases of surface adsorbed DNA. The presence of edge planes of graphene at regular intervals along the walls of the bamboo nanotubes resulted in superior electrochemical performance relative to SWNT modified electrodes from two aspects. Firstly, with solution species the peak separation of the oxidation and reduction waves were smaller indicating more rapid rates of electron transfer. Secondly, a greater number of electroactive sites along the walls of the bamboo-carbon nanotubes (BCNTs) resulted in larger current signals and a broader dynamic range for the oxidation of DNA bases.

Protein electrochemistry using aligned carbon nanotube arrays

The remarkable electrocatalytic properties and small size of carbon nanotubes make them ideal for achieving direct electron transfer to proteins, important in understanding their redox properties and in the development of biosensors. Here, we report shortened SWNTs can be aligned normal to an electrode by self-assembly and act as molecular wires to allow electrical communication between the underlying electrode and redox proteins covalently attached to the ends of the SWNTs, in this case, microperoxidase MP-11. The efficiency of the electron transfer through the SWNTs is demonstrated by electrodes modified with tubes cut to different lengths having the same electron-transfer rate constant.

Parameters important in fabricating enzyme electrodes using self-assembled monolayers of alkanethiols

The fabrication of enzyme electrodes using self-assembled monolayers (SAMs) has attracted considerable interest because of the spatial control over the enzyme immobilization. A model system of glucose oxidase covalently bound to a gold electrode modified with a SAM of 3-mercaptopropionic acid was investigated with regard to the effect of fabrication variables such as the surface topography of the underlying gold electrode, the conditions during covalent attachment of the enzyme and the buffer used. The resultant monolayer enzyme electrodes have excellent sensitivity and dynamic range which can easily be adjusted by controlling the amount of enzyme immobilized. The major drawback of such electrodes is the response which is limited by the kinetics of the enzyme rather than mass transport of substrates. Approaches to bringing such enzyme electrodes into the mass transport limiting regime by exploiting direct electron transfer between the enzyme and the electrode are outlined.

Harvesting waste thermal energy using a carbon-nanotube-based thermo-electrochemical cell

Low efficiencies and costly electrode materials have limited harvesting of thermal energy as electrical energy using thermo-electrochemical cells (or “thermocells”). We demonstrate thermocells, in practical configurations (from coin cells to cells that can be wrapped around exhaust pipes), that harvest low-grade thermal energy using relatively inexpensive carbon multiwalled nanotube (MWNT) electrodes. These electrodes provide high electrochemically accessible surface areas and fast redox-mediated electron transfer, which significantly enhances thermocell current generation capacity and overall efficiency. Thermocell efficiency is further improved by directly synthesizing MWNTs as vertical forests that reduce electrical and thermal resistance at electrode/substrate junctions. The efficiency of thermocells with MWNT electrodes is shown to be as high as 1.4% of Carnot efficiency, which is 3-fold higher than for previously demonstrated thermocells. With the cost of MWNTs decreasing, MWNT-based thermocells may become commercially viable for harvesting low-grade thermal energy.

A potential-controlled switch on/off mechanism for selective excitation in mixed electrochemiluminescent systems

We demonstrate the complete, rapid, and reversible switching between the emissions from two electrogenerated chemiluminescence (ECL) systems contained within the same solution, controlled by simple modification of the applied potential. The fundamental bases of the approach are the ability to selectively 'switch on' luminophores at distinct oxidation potentials, and an intriguing observation that the emission from the well-known electrochemiluminescent complex, fac-Ir(ppy)3, (where ppy is 2-phenylpyridinato), can be selectively 'switched-off' at high overpotentials. The dependence of this phenomenon on high concentrations of the co-reactant implicates quenching of the excited [Ir(ppy)3]* state by electron transfer. Rapid spectral scanning during modulation of the applied potential reveals well resolved maxima for mixtures comprising either green and red or green and blue luminophores, illustrating the vast potential of this approach for multiplexed ECL detection.