Reusable Colorimetric Biosensors on Sustainable Silk-Based Platforms

In biosensor development, silk fibroin is advantageous for providing transparent, flexible, chemically/mechanically stable, biocompatible, and sustainable substrates, where the biorecognition element remains functional for long time periods. These properties are employed here in the production of point-of-care biosensors for resource-limited regions, which are able to display glucose levels without the need for external instrumentation. These biosensors are produced by photopatterning silk films doped with the enzymes glucose oxidase and peroxidase and photoelectrochromic molecules from the dithienylethene family acting as colorimetric mediators of the enzymatic reaction. The photopatterning results from the photoisomerization of dithienylethene molecules in the silk film from its initial uncolored opened form to its pink closed one. The photoisomerization is dose-dependent, and colored patterns with increasing color intensities are obtained by increasing either the irradiation time or the light intensity. In the presence of glucose, the enzymatic cascade reaction is activated, and peroxidase selectively returns closed dithienylethene molecules to their initial uncolored state. Color disappearance in the silk film is proportional to glucose concentration and used to distinguish between hypoglycemic (below 4 mM), normoglycemic (4-6 mM), and hyperglycemic levels (above 6 mM) by visual inspection. After the measurement, the biosensor can be regenerated by irradiation with UV light, enabling up to five measurement cycles. The coupling of peroxidase activity to other oxidoreductases opens the possibility to produce long-life reusable smart biosensors for other analytes such as lactate, cholesterol, or ethanol.

Light-controlled pKa value of chiral Brønsted acid catalysts in enantioselective aza-Friedel-Crafts reaction

Bis(dithienylethene)-based BINOL-derived phosphoric acid (DTE-BPA) has been developed as a light-controlled chiral organocatalyst for the first time. The photoinduced modulation of the reactivity and selectivity via the open/close isomerization of the DTE scaffold led to superior light-controlled ability in the enantioselective aza-Friedel-Crafts reaction of aldimines with indoles. DFT studies showed that photoisomerization is accompanied by a shift of 1.1 pKa units between the open and closed isomers.

Large, Tunable, and Reversible pH Changes by Merocyanine Photoacids

Molecular photoswitches capable of generating precise pH changes will allow pH-dependent processes to be controlled remotely and noninvasively with light. We introduce a series of new merocyanine photoswitches, which deliver reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form, the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determined the quantum yields for the ring-closing reaction and found that the lifetimes of the transient cis-merocyanine isomers ranged from 30 to 550 ns. Quantum yields did not appear to be a limitation for bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related merocyanine photoacid.

Combined NMR and UV-Vis Spectroscopic Studies of Models for the Hydrogen Bond System in the Active Site of Photoactive Yellow Protein: H-Bond Cooperativity and Medium Effects

Intramolecular hydrogen bonds in aprotic media were studied by combined (simultaneous) NMR and UV-vis spectroscopy. The species under investigation were anionic and featured single or coupled H-bonds between, for example, carboxylic groups and phenolic oxygen atoms (COO···H···OC)^-, among phenolic oxygen atoms (CO···H···OC)^-, and hydrogen bond chains between a carboxylic group and two phenolic oxygen atoms (COO···H···(OC)···H···OC)^-. The last anion may be regarded as a small molecule model for the hydrogen bond system in the active site of wild-type photoactive yellow protein (PYP) while the others mimic the corresponding H-bonds in site-selective mutants. Proton positions in isolated hydrogen bonds and hydrogen bond chains were assessed by calculations for vacuum conditions and spectroscopically for the two media, CD₂Cl₂ and the liquefied gas mixture CDClF₂/CDF₃ at low temperatures. NMR parameters allow for the estimation of time-averaged H-bond geometries, and optical spectra give additional information about geometry distributions. Comparison of the results from the various systems revealed the effects of the formation of hydrogen bond chains and changes of medium conditions on the geometry of individual H-bonds. In particular, the proton in a hydrogen bond to a carboxylic group shifts from the phenolic oxygen atom in the system COO^-···H-OC to the carboxylic group in COO-H···(OC)^-···H-OC as a result of hydrogen bond formation to the additional phenolic donor. Increase in medium polarity may, however, induce the conversion of a structure of a type COO-H···(OC)^-···H-OC to the type COO^-···H-(OC)···H-OC. Application of these results obtained from the model systems to PYP suggests that both cooperative effects within the hydrogen bond chain and a low-polarity protein environment are prerequisites for the stabilization of negative charge on the cofactor and hence for the spectral tuning of the photoreceptor.

Photo-Lewis Acid Generator Based on Radical-Free 6π Photo-Cyclization Reaction

We present here a new photo-active molecule which acts as a photo-Lewis acid generator (PLAG) based on photo-chemical 6π-percyclization. Photo-illumination of the PLAG molecule produces a condensed aromatic carbocation with a triflate counteranion, which exhibits Lewis acid chemical catalytic reactivity such as initiation of the polymerization of epoxy monomers and catalysis of Mukaiyama-aldol reactions. The terminal-end structure in the epoxy polymerization was modified with the Lewis acid fragment. The carbocation induced the Mukaiyama-aldol reaction as a new photo-gated system with remarkably high catalytic reactivity and turnover numbers higher than 60. The photo-chemical quantum yield of the carbocation generation is 50%, which is considerably higher than obtained with most Brønsted photo-acid generators.

Efficient Light-Induced pKa Modulation Coupled to Base-Catalyzed Photochromism

Photoswitchable acid-base pairs, whose pK_a values can be reversibly altered, are attractive molecular tools to control chemical and biological processes with light. A significant, light-induced pK_a change of three units in aqueous medium has been realized for two thermally stable states, which can be interconverted using UV and green light. The light-induced pK_a modulation is based on incorporating a 3-H-thiazol-2-one moiety into the framework of a diarylethene photoswitch, which loses the heteroaromatic stabilization of the negatively charged conjugate base upon photochemical ring closure, and hence becomes significantly less acidic. In addition, the efficiency of the photoreactions is drastically increased in the deprotonated state, giving rise to catalytically enhanced photochromism. It appears that protonation has a significant influence on the shape of the ground- and excited-state potential energy surfaces, as indicated by quantum-chemical calculations.

Aromatic sulfonium polyoxomolybdates: tuning the photochromic properties through substitutions on the counter ion moiety

A series of aromatic sulfonium Keggin hybrids have been developed which exhibited excellent photochromic properties under UV irradiation. The photo-coloration speed of these hybrids could be fine-tuned by changing the substituents on the sulfonium counter ion moiety.

DNA gated photochromism and fluorescent switch in a thiazole orange modified diarylethene

Thiazole orange-modified diarylethene (1) shows weak fluorescence but no photochromism in aqueous solution. When binding with DNA, the fluorescence of 1 is enhanced drastically and the photochromic reactivity is unlocked. This kind of DNA-responsive photoswitchable system can be used for imaging nucleic acids within cells.

CB[8] gated photochromism of a diarylethene derivative containing thiazole orange groups

Photochromism in a diarylethene derivative (1O) can be gated by a host–guest interaction where the thiazole orange groups are bound into the hydrophobic cavity of CB[8] in water.

Large protonation-gated photochromism of an OPE-embedded difurylperfluorocyclopentene

Double protonation (red) increases the ring-closing quantum yield Q⃑ of a molecular switch more than tenfold.

A photoresponsive glycosidase mimic

Azobenzene-3,3'-dicarboxylic acid exists in photoisomerizable (E) and (Z)-forms. Deprotonation of the carboxylic acid groups from the (E)-form occurs simultaneously, whereas in the (Z)-form it occurs in a stepwise fashion. The mono anionic form of the (Z)-isomer acts as a glycosidase mimic that proceeds through a general acid-general base catalytic mechanism. This is the first example of a photoresponsive glycosidase mimic.

A photo- and halochromic multicolor switching system consisting of diarylethene and malachite green moieties

A diarylethene with a malachite green moiety was synthesized, and it showed multicolor states by photochromic and halochromic reactions.

Photocontrol of luminescent inorganic nanocrystals via an organic molecular switch

The photoluminescence of ZnO nanocrystals is reversibly modulated via a diarylethene photoswitch, and the photophysics mechanism was unraveled.