Development of spiropyran-based electrochemical sensor via simultaneous photochemical and target-activatable electron transfer

Self-Immolative Electrochemical Redox Substrates: Emerging Artificial Receptors in Sensing and Biosensing

The development of artificial receptors has great significance in measurement science and technology. The need for a robust version of natural receptors is getting increased attention because the cost of natural receptors is still high along with storage difficulties. Aptamers, imprinted polymers, and nanozymes are some of the matured artificial receptors in analytical chemistry. Recently, a new direction has been discovered by organic chemists, who can synthesize robust, activity-based, self-immolative organic molecules that have artificial receptor properties for the targeted analytes. Specifically designed trigger moieties implant selectivity and sensitivity. These latent electrochemical redox substrates are highly stable, mass-producible, inexpensive, and eco-friendly. Combining redox substrates with the merits of electrochemical techniques is a good opportunity to establish a new direction in artificial receptors. This Review provides an overview of electrochemical redox substrate design, anatomy, benefits, and biosensing potential. A proper understanding of molecular design can lead to the development of a library of novel self-immolative redox molecules that would have huge implications for measurement science and technology.

Modulating the Optical Characteristics of Spiropyran@Metal-Organic Framework Composites as a Function of Spiropyran Substitution

Understanding the interactions between the single components of hybrid systems is essential to drive the development of advanced functional materials. A prerequisite for this is the systematic variation of the building blocks of such compounds. Focusing on spiropyran@metal-organic framework (MOF) composite materials with noncovalently attached spiropyran dyes, both the host scaffold and the dye molecules can be systematically tuned. In this work, a broad substitution pattern was applied to systematically elucidate the characteristics of the resulting hybrid materials as a function of the supplemental substitution on spiropyran. The newly developed 12 composites exhibit substitution and host-dependent optical characteristics, which are particularly affected by the substitution of the 6'-position on the chromene ring. Through the favorable combination of the MOF host's polarity and an adequate strength of the spiropyran's indoline_donor-chromene_acceptor pair, reversible conversion between photoisomers is efficiently accomplished, especially for nitro-substituted spiropyrans inside MIL-68(In).

Photo-Responsive Graphene and Carbon Nanotubes to Control and Tackle Biological Systems

Photo-responsive multifunctional nanomaterials are receiving considerable attention for biological applications because of their unique properties. The functionalization of the surface of carbon nanotubes (CNTs) and graphene, among other carbon based nanomaterials, with molecular switches that exhibit reversible transformations between two or more isomers in response to different kind of external stimuli, such as electromagnetic radiation, temperature and pH, has allowed the control of the optical and electrical properties of the nanomaterial. Light-controlled molecular switches, such as azobenzene and spiropyran, have attracted a lot of attention for nanomaterial's functionalization because of the remote modulation of their physicochemical properties using light stimulus. The enhanced properties of the hybrid materials obtained from the coupling of carbon based nanomaterials with light-responsive switches has enabled the fabrication of smart devices for various biological applications, including drug delivery, bioimaging and nanobiosensors. In this review, we highlight the properties of photo-responsive carbon nanomaterials obtained by the conjugation of CNTs and graphene with azobenzenes and spiropyrans molecules to investigate biological systems, devising possible future directions in the field.

The influence of surface potential on the optical switching of spiropyran self assembled monolayers

Surfaces whose macroscopic properties can be switched by light are potentially useful in a wide variety of applications. One such promising application is electrochemical sensors that can be gated by optically switching the electrode on or off. One way to make such a switchable electrode is by depositing a self-assembled monolayer (SAM) of bistable, optically switchable molecules onto an electrode surface. Quantitative application of any such sensor requires understanding how changes in interfacial field affect the composition of photostationary states, i.e. how does electrode potential affect the extent to which the electrode is on or off when irradiated, and the structure of the SAM. Here we address these questions for a SAM of a 6-nitro-substituted spiro[2H-1-benzopyran-2,2'-indoline] covalently attached through a dithiolane linker to an Au electrode immersed in a 0.1 M solution of Tetramethylammonium hexafluorophosphate in Acetonitrile using interface-specific vibrational spectroscopy. We find that in the absence of irradiation, when the SAM is dominated by the closed spiropyran form, variations in potential of 1 V have little effect on spiropyran relative stability. In contrast, under UV irradiation small changes in potential can have dramatic effects: changes in potential of 0.2 V can completely destabilize the open merocyanine form of the SAM relative to the spiropyran and dramatically change the chromophore orientation. Quantitatively accounting for these effects is necessary to employ this, or any other optically switchable bistable chromophore, in electrochemical applications.

Fabrication of an electrochemical sensor based on spiropyran for sensitive and selective detection of fluoride ion

In the past decades, numerous electrochemical sensors based on exogenous electroactive substance have been reported. Due to non-specific interaction between the redox mediator and the target, the instability caused by false signal may not be avoided. To address this issue, in this paper, a new electrochemical sensor based on spiropyran skeleton, namely SPOSi, was designed for specific electrochemical response to fluoride ions (F(-)). The breakage of Si-O induced by F(-) based on the specific nucleophilic substitution reaction between F(-) and silica would directly produce a hydroquinone structure for electrochemical signal generation. To improve the sensitivity, SPOSi probe was assembled on the single-walled carbon nanotubes (SWCNTs) modified glassy carbon electrode (GCE) through the π-π conjugating interaction. This electrode was successfully applied to monitor F(-) with a detection limit of 8.3 × 10(-8) M. Compared with the conventional F(-) ion selected electrode (ISE) which utilized noncovalent interaction, this method displays higher stability and a comparable sensitivity in the urine samples.

Concentration-dependent and light-responsive self-assembly of bolaamphiphiles bearing α-cyanostilbene based photochromophore

In this paper, a new bolaamphiphile bearing 1-cyano-1,2-bis(phenyl)ethene (CNBE) has been synthesized. The self-assembly of this molecule in aqueous solution is concentration-dependent. Two distinct morphologies, monomolecular layered lamellas and helical nanofibres have been obtained with the as-prepared molecular configuration. Note worthily, the helical nanofibres provide an experimental evidence for the pure twisted structure in the liquid crystals, which is theoretically proposed by De Gennes. Due to the photoisomerization of CNBE, the self-assembled nanostructures undergo morphological changes upon irradiation. Although various nanostructures were observed in the solution-state, only nanofibres were obtained after the solution was cast on a substrate, which was attributed to a strong dewetting effect. This work illustrates concentration-dependent and light-responsive self-assembly and provides a novel avenue for fabricating smart soft materials.