Light-Activated Sensitive Probes for Amine Detection

A three-in-one versatile sensor for concise detecting biogenic amines and beef freshness

Biogenic amines (BAs), as important indicators for evaluating food spoilage caused by fermentation processes or microbial activities, present significant risks of food safety. Consequently, the development of a simple, sensitive, and selective detection method for amines is of great importance. In this study, we proposed a three-in-one sensor 3,6-bis(dimethylamino)-9-(ethylthio)xanthylium (PSE) for high sensitivity and selectivity detecting BAs with multimodal responses, including olfactory, colorimetric, and fluorescent signals, thus facilitating convenient real-time detection of BAs. Mechanism study indicated that the nucleophilic substitution of PSE with BAs induced such rapid multi-responses with a low detection limit (LOD = 0.03 μM). We further fabricated PSE loaded paper for portable detection of BAs vapors. And the accurate determination of BAs levels is achieved through analyzing the RGB color mode. Finally, we successfully applied these test strips for non-destructive assessing meat beef freshness with the assistance of a smartphone in on-site scenarios.

A Chromo-fluorogenic Probe for Selective Detection of Picric Acid Alongside Its Recovery by Aliphatic Amines and Construction of Molecular Logic Gates

Nitroaromatic compounds are illicit explosive chemicals. For environmental security and homeland safety, selective and sensitive identification of these secondary-class explosives has been a reason for the exhaustive research arena of chemists for about a decade. We introduced a sensitive optical sensor with desalted neutral red (NR) dye. After ingressing picric acid (PA) in acetonitrile, the probe becomes non-fluorescent, displaying a colorimetric change from yellow to pink. The quenched phenomena and the changed color were re-established with aliphatic amine, trimethylamine (TEA). The reversibility is produced cyclically, both in fluorimetrically and spectrophotometrically. The detection limit for PA with our probe comes out as 0.639 µM; this value is significantly lower than many chemosensors available in the literature. Also, NR-stained filter paper strips-based test kit analysis has been deployed as a displayable photonic device for in-situ detection of PA. Furthermore, the whole system was conceptualized to produce single input, single output, and double input single output logic gates, which can be applied to digital devices. The chronological input manner as NTP (NR- TEA-PA) pushed us to configure a molecular keypad lock system, the basis of digital locking devices. The repeatable & reversible detection system exhibits "Write read- Erase-read Write-read' type memory devices.

Gated Photochromism of Dithienylethene-Embedded Expanded Calixphyrins

Dithienylethene (DTE)-embedded expanded porphyrins were synthesized and confirmed to be photochemically inactive due to the lowest excited state of the expanded porphyrins. On the other hand, DTE-embedded expanded calixphyrins exhibited reversible photochromism upon UV-irradiation to form colored closed forms, which reverted to colorless open forms upon red-light irradiation. The closed forms were oxidized with DDQ or the air to lock the recorded information by converting to photochemically inactive expanded porphyrins. This was unlocked by reduction with NaBH4 to restore expanded calixphyrins with photochromism activity. These gated photochromic behaviors were demonstrated in PMMA (polymethyl methacrylate) film.

Light-Induced Dyotropic Rearrangement of Diarylethenes: Scope, Mechanism, and Prospects

Irreversible two-photon photorearrangement of 1,2-diarylethenes is a unique process providing access to complex 2a1 ,5a-dihydro-5,6-dithiaacenaphthylene (DDA) heterocyclic core. This reaction was serendipitously discovered during studies on photoswitchable diarylethenes and was initially considered as a highly undesired process. However, in recent years, it has been recognized as an efficient photochemical reaction, interesting by itself and as a promising synthetic method for the synthesis of challenging molecules. Herein, we discuss the state-of-the-art in studies on this notable process.

Light-Promoted Basic Nitrogen Unmasking in Arene Ruthenium Complexes Derived from Z-Configured 2,2'-Azobispyridine

Two novel 2,2'-azobispyridine derivatives bearing N-dialkylamino substituents at position 4,4' were synthesized and their E-Z photoswitching behavior was characterized by combination of 1 H- and 13 C NMR spectroscopy, UV-Vis absorption and DFT calculations. Both isomers act as ligands towards arene-RuII centers, leading either to E-configured 5-membered chelates (involving coordination of nitrogen atoms from N=N bond and pyridine) or to the uncommon Z-configured 7-membered chelates (involving coordination of nitrogen atoms from both pyridines). The latter show good stability in the dark, allowing single crystal X-ray diffraction study to be reported here for the first time. All synthesized Z-configured arene-RuII complexes undergo irreversible photo-isomerization to their corresponding E isomers with rearrangement of their coordination pattern. This property was advantageously exploited for the light-promoted unmasking of a basic nitrogen atom of the ligand.

Electronic Influences on the Dynamic Range of Photoswitchable Dithienylethene-Thiourea Organocatalysts

Thiourea-based organocatalysts bearing a photoswitchable dithienylethene (DTE) core and a wide range of substituents were prepared and extensively tested for their ability to accelerate the Michael reaction between acetylacetone and trans-β-nitrostyrene. There is a strong correlation between the Hammett parameter of the modulating groups and catalytic activity following UV irradiation. Electron-withdrawing groups afford the largest reactivity difference between the catalysts in their ring-open form and their ring-closed isomer, with evidence for electronic coupling between the two halves in both oDTE and cDTE.

Skeletal photoinduced rearrangement of diarylethenes: ethene bridge effects

A skeletal photorearrangement involving UV-induced 6π-electrocyclization of diarylethenes with various ethene bridges has been studied. It has been found that deprotonation is the predominant step among the three possible alternative reaction pathways (radical abstraction, deprotonation, or sigmatropic shift) following 6π-electrocyclization, and incorporation of an electronegative carbonyl group into the geminal position to the phenyl residue results in a reduction in the reaction time and an increase in the yield of the desired product. The significant increase in the reaction time in less polar solvents (toluene, TCM) also indicates a large contribution of the deprotonation step to the skeletal photorearrangement of diarylethenes. Performing the reaction in toluene in the presence of tertiary amines leads to a reduction in the reaction time and an increase in the yield of the desired product. The best results were achieved when the reaction was carried out in toluene in the presence of DIPEA. The experimental results are in good agreement with the DFT calculations.

Molecular engineering of fluorescence probe for real-time non-destructive visual screening of meat freshness

Spoiled meat poses a great challenge to food security and human health, which should be addressed by the early monitoring and warning of the meat freshness. We herein exploited a molecular engineering strategy to construct a set of fluorescence probes (PTPY, PTAC, and PTCN) with phenothiazine as fluorophore and cyanovinyl as recognition site for the facile and efficient monitoring of meat freshness. These probes produce an obvious fluorescence color transition from dark red to bright cyan in response to cadaverine (Cad) through the nucleophilic addition/elimination reaction. The sensing performances were elaborately improved to achieve quick response (16 s), low detection limit (LOD = 3.9 nM), and high contrast fluorescence color change by enhancing the electron-withdrawing strength of cyanovinyl moiety. Furthermore, PTCN test strips were fabricated for portable and naked-eye detection of Cad vapor with fluorescence color change from crimson to cyan, and accurate determination of Cad vapor level with RGB color (red, green, blue) mode analysis. The test strips were employed to detect the freshness of real beef samples, and demonstrated a good capability of non-destructive, non-contact and visual screening meat freshness on site.

A resorufin-based fluorescence probe for visualizing biogenic amines in cells and zebrafish

Biogenic amines (BAs) are a family of nitrogen-bearing natural organic molecules with at least one primary amine, which play an important role in living organisms. Elevated concentration of BAs may cause neuron disorder, Parkinson's disease and many other diseases. Therefore, it is essential to monitor BAs in living organisms. Herein, we reported a resorufin-based fluorescence probe for sensing of various BAs. Upon nucleophilic substitution reaction with BAs, the probe released resorufin, affording to strong fluorescence emission at 592 nm with rapid response (<8 min), good selectivity and a low detection limit (LOD = 0.47 μM). The probe has low cytotoxicity and good membrane permeability, and has been successfully used to visualize BAs in living cells and zebrafish with good performance.

Low-Coordinate Erbium(III) Single-Molecule Magnets with Photochromic Behavior

The structures and magnetic properties of photoresponsive magnets can be controlled or fine-tuned by visible light irradiation, which makes them appealing as candidates for ternary memory devices: photochromic and photomagnetic at the same time. One of the strategies for photoresponsive magnetic systems is the use of photochromic/photoswitchable molecules coordinated to paramagnetic metal centers to indirectly influence their magnetic properties. Herein, we present two erbium(III)-based coordination systems: a trinuclear molecule {[Er^III(BHT)₃]₃(dtepy)₂}^.4C₅H₁₂ (1) and a 1D coordination chain {[Er^III(BHT)₃(azopy)}_n·2C₅H₁₂ (2), where the bridging photochromic ligands belong to the class of diarylethenes: 1,2-bis((2-methyl-5-pyridyl)thie-3-yl)perfluorocyclopentene (dtepy) and 4,4′-azopyridine (azopy), respectively (BHT = 2,6-di-tert-butyl-4-methylphenolate). Both compounds show slow dynamics of magnetization, typical for single-molecule magnets (SMMs) as revealed by alternating current (AC) magnetic susceptibility measurements. The trinuclear compound 1 also shows an immediate color change from yellow to dark blue in response to near-UV irradiation. Such behavior is typical for the photoisomerization of the open form of the ligand into its closed form. The color change can be reversed by exposing the closed form to visible light. The chain-like compound 2, on the other hand, does not show significant signs of the expected trans–cis photoisomerization of the azopyridine in response to UV irradiation and does not appear to show photoswitching behavior.

Three-coordinate [Er^III(BHT)₃] single ion magnets undergo ligand addition reaction in pentane to form linear trinuclear photochromic nanomagnets where both functionalities persist.

Three-pole wheel paddle luminescent metal organic frameworks (LMOFs) based on the oxygen substituted triazine tricarboxylic acid ligand: recognition and detection of small drug molecules and aromatic amine molecules

Luminescent metal organic frameworks (LMOFs) are considered to be a type of promising optical sensing material due to their designable and tunable functions, and stable pore structures. Therefore, the preparation of LMOFs has become a research hotspot in recent years. As we know, triazine carboxylic acid ligands are conducive for constructing LMOF materials due to their large π electron conjugated system. In this work, two crystalline materials [Cd₃(TCPT)₂]·0.5DMF·4H₂O (1) and (H₃O)[Zn₂(TCPT)(μ₂-OH)₂]·0.5DMF·3H₂O (2) were obtained by the reaction of the triazine carboxylic acid ligand 2,4,6-tris(4-carboxyphenoxy)-1,3,5-triazine (H₃TCPT), as an extended carboxylate arm, and d¹⁰ transition metal salts. Their structures were determined by single crystal X-ray diffraction and characterized by infrared spectroscopy (IR), ultraviolet visible spectroscopy (UV-vis), fluorescence spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TG). The experimental results showed that complexes 1 and 2 show excellent fluorescent emission behavior. Thus, we explored their fluorescence sensing properties. To our delight, the results showed that they both had the ability to sense small organic drug molecules and aromatic amine molecules containing o-phenylenediamine (OPD), m-phenylenediamine (MPD) and p-phenylenediamine (PPD). In general, the practical applications of a MOF material are usually limited because of the relatively harsh synthesis methods. In this aspect, we studied the synthesis method in detail to obtain the optimal reaction conditions for the large-scale synthesis of 1 and 2. The preparation of the two LMOF materials only required about 3 hours of heating time and they could be prepared on a large scale, which is significant for the practical applications of LMOFs.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

Hybrid Molecules Based on Fullerene C60 and Dithienylethenes. Synthesis and Photochromic Properties. Optically Controlled Organic Field-Effect Transistors

Hybrid molecules based on fullerene C₆₀ and dithienylethene and its perfluorinated analog not inferior in the efficiency of phototransformations to the initial photochromic compounds were synthesized for the first time. The resulting pyrrolidinofullerenes containing photochromic moieties were used to fabricate organic field-effect transistors (OFETs) with output and transfer characteristics ten times exceeding similar characteristics of devices based on staring dithienylethenes. It was found that OFETs based on hybrid molecules with dithienylethenes are four times less efficient than devices based on hybrid molecules with perfluorinated analogs.

Photocyclization of Diarylethenes: The Effect of Electron and Proton Acceptors as Additives

The effect of electron and proton acceptors on the photocyclization of diarylethenes has been studied. Without any additives, the deprotonation reaction is predominant, although other processes, including the sigmatropic shift, are not excluded. A deuterium exchange experiment has shown that a strong base (DABCO) facilitates the deprotonation reaction, thereby limiting the sigmatropic shift. In the presence of an oxidizing agent or additional sources of radicals (O₂, I₂, TEMPO), the processes of deprotonation and rearrangement (H-shift) are practically not observed, and the reaction proceeds along a radical pathway with the formation of phenanthrene or its heterocyclic analogue.

A dual-mode visual detector for toxic hydrazine

Hydrazine (N₂H₄) is one of the commonly used chemical reagents in numerous industries and applications but its toxicity to humans poses a need to develop simple visual detection methods. Herein, we demonstrate a novel dual-mode system to detect and simultaneously consume hydrazine in vapour and solution by using a small photoresponsive molecule that has altered optical response (both colourimetric and fluorescent) after reacting with hydrazine.

A small photoresponsive molecule changes colour from blue to colourless when exposed to hydrazine vapours. It also becomes emissive providing two convenient ways of detecting the presence of this toxic chemical.

A New Compound for Sequential Sensing of Picric Acid and Aliphatic Amines: Physicochemical Details and Construction of Molecular Logic Gates

Picric acid (PA) at low concentration is a serious water pollutant. Alongside, aliphatic amines (AAs) add to the queue to pollute surface water. Plenty of reports are available to sense PA with an ultralow limit of detection (LOD). However, only a handful of works are testified to detect AAs. A new fluorescent donor-acceptor compound has been synthesized with inherent intramolecular charge transfer (ICT) character that enables selective and sensitive colorimetric quantitative detection of PA and AAs with low LODs in non-aqueous as well as aqueous solutions. The synthesized compound is based on a hemicyanine skeleton containing two pyridenylmethylamino groups at the donor and a benzothiazole moiety at the acceptor ends. The detailed mechanisms and reaction dynamics are explained spectroscopically along with computational support. The fluorescence property of the detecting compound changes due to protonation of its pyridinyl centers by PA leading to quenching of fluorescence and subsequently de-protonation by AAs to revive the signal. We have further designed logic circuits from the acquired optical responses by sequential interactions.

Silver Nanoparticles for a Colorimetric Determination of Putrescine and Cadaverine in Biological Samples

A convenient and uncomplicated scheme has been projected for the quantitative determination of essential diamines putrescine (PUT) and cadaverine (CAD) via sodium dodecyl sulfate protected silver nanoparticles (SDS-AgNPs). This scheme is based on the chemical interaction of a SDS-AgNPs probe with PUT and CAD, leading to a color change from yellow to red or reddish brown. The interaction was investigated through different techniques such as using a UV-visible spectrophotometer, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), dynamic light scattering spectroscopy (DLS) and the zeta potential. Both amines possess a close resemblance in structure (except for the addition of one more methylene group in CAD), and no any distinguishable color change was noted. However, the maximum absorption band at 580 and 600 nm was demonstrated for PUT and CAD correspondingly. The methodical response was observed at absorption ratios of 580/410 and 600/410 nm, with the linear regression within 4 - 12 and 6 - 14 μg/mL for PUT and CAD. The detection limits calculated for both the diamines PUT and CAD were 0.333 and 1.638 μg/mL. The scheme was successfully applied for determinations in biological samples, including spiked blood plasma and urine. Putrescine exhibited % recovery within 95.717 - 105.200%, while cadaverine was within 95.940 - 105.109%, respectively. The scheme was reproducible and precise with inter-day RSD (n = 5) within 1.126, 0.018% and the intraday RSD (n = 5) was within 0.005, 0.002% for PUT and CAD, respectively.

Switching the Mallory Reaction to Synthesis of Naphthalenes, Benzannulated Heterocycles, and Their Derivatives

This review analyzes the new life of a long-known reaction, the photocyclization of diarylethenes, which became a classical tool for the synthesis of phenanthrenes and their heterocyclic analogues (Mallory reaction). It has been shown in recent years that certain diarylethenes undergo photorearrangement to naphthalenes, benzannulated heterocycles, or related products with bicyclic unit. Herein, I analyze how the Mallory reaction path can be altered to obtain bicyclic rather than tricyclic products. The mechanistic aspects and scope of the reaction are discussed.

Sensor Array Based Determination of Edman Degradated Amino Acids Using Poly(p-phenyleneethynylene)s

A cross‐reactive optical sensor array based on poly(p‐phenyleneethynylene)s (PPEs) determines Edman degraded amino acids. We report a sensor array composed of three anionic PPEs P1–P3, and their electrostatic complexes with metal ions (Fe²⁺, Cu²⁺, Co²⁺). We recorded distinct fluorescence intensity response patterns as “fingerprints” of this chemical tongue toward standard phenylthiohydantoin (PTH) amino acids—degradation products of the Edman process. These “fingerprints” were converted into canonical scores by linear discrimination analysis (LDA), which differentiates all of the PTH‐amino acids. This array discriminates PTH‐amino acid residues degraded from an oligopeptide through Edman sequencing. This approach is complementary to chromatography approaches which rely on mass spectrometry; our array offers the advantage of simplicity.

Enlightening Materials with Photoswitches

Incorporating molecular photoswitches into various materials provides unique opportunities for controlling their properties and functions with high spatiotemporal resolution using remote optical stimuli. The great and largely still untapped potential of these photoresponsive systems has not yet been fully exploited due to the fundamental challenges in harnessing geometrical and electronic changes on the molecular level to modulate macroscopic and bulk material properties. Herein, progress made during the past decade in the field of photoswitchable materials is highlighted. After pointing to some general design principles, materials with an increasing order of the integrated photoswitchable units are discussed, spanning the range from amorphous settings over surfaces/interfaces and supramolecular ensembles, to liquid crystalline and crystalline phases. Finally, some potential future directions are pointed out in the conclusion. In view of the exciting recent achievements in the field, the future emergence and further development of light-driven and optically programmable (inter)active materials and systems are eagerly anticipated.

Cellulose-based fluorescent sensor for visual and versatile detection of amines and anions

It is practical and challenging to construct ultrasensitive and multi-responsive sensors for visual and real-time monitoring of the environment. Herein, a cellulose-based multi-responsive fluorescent sensor (Phen-MDI-CA) is fabricated, and realizes a visual and ultrasensitive detection of not only various amines but also three anions based on the change of the fluorescence and/or visible colors. Once exposure to various amines in both the solution and vapor state, the Phen-MDI-CA solution and test paper exhibit different fluorescence colors, which can be used to distinguish triethylamine, ethylenediamine, methylamine, aniline, hydrazine and pyrrolidine from other amines. Moreover, via combining the Phen-MDI-CA with the Phen-MDI-CA/malachite green ratiometric system, phosphate (PO₄^3-), carbonate (CO₃^2-) and borate (B₄O₇^2-) can be visually and accurately recognized depending on the change of the visible and fluorescence colors. In fluorescent mode, the LOD for B₄O₇^2-, PO₄^3- and CO₃^2- ions is as low as 0.18 nmol, 0.69 nmol and 0.86 nmol, respectively. Significantly, the Phen-MDI-CA can readily make a qualitative and quantitative detection of B₄O₇^2-, PO₄^3- and CO₃^2- anions in the mixture of anions. The state-of-the-art responsive behavior of Phen-MDI-CA originates from the amplification effect of cellulose polymer chain and the differentiated interactions between the sensor and analytes.

Time-Resolved Monitoring of Intracellular Processes with a Cyclical On-Off Photoswitchable Nanoprobe

Fluorescent microscopic imaging with the help of small-molecule probes (chemoprobes) is one of the most feasible approaches for noninvasive sensing of intracellular molecules. However, the "always on" property of current chemoprobes failed to achieve time-resolved monitoring. Here, we report the development of a supramolecular nanoassembling strategy to integrate multiple functions on one nanoscale probe (nanoprobe) with a cyclical on-off switchable sensing ability. The reversal of the nanoprobe can be rapidly achieved by converting the single-wavelength near-infrared (NIR) laser to two-way emissions by a lanthanum nanoparticle core that is sensitive to the light power density. Through regulating the NIR power density, the azobenzene derivative, which was doped in the surface of the lipid bilayer of the nanoprobe, can act as an "impeller" and "brake" for bio-benign activation and deactivation, respectively, of the nanoprobe in biological applications. A reduced nicotinamide adenine dinucleotide nanoprobe was constructed as the model to demonstrate precise and time-resolved monitoring of intracellular processes including cancerous glycolysis and ligand-induced enzymatic processes. We envision that this cyclical on-off switchable nanoprobe strategy will hold great promise for endowing universal chemoprobes with high precision and temporal resolution.

Light-responsive nanozymes for biosensing

Using light as an external stimulus plays a key role not only in modulating activities of nanozymes, but also in constructing efficient biosensing systems.

Unusual Fluorescence Behavior of Pyrene-Amine Containing Dendrimers

A new class of pyrene-based dendrimers, characterized by the presence of a 1,4,7,10-Tetraazacyclododecane (cyclen) unit as the core, was studied by SSF (steady-state fluorescence) and SPC (single-photon counting fluorescence). The photophysical behavior of these dendrimers was studied in THF, DMF and DMSO solution. The typical signals for pyrene-labeled molecules were recorded in each solvent, showing the representative fluorescence spectra: the corresponding emissions of monomer and excimer of the pyrene chromophore are observed. Unexpectedly, the typical quenching of tertiary amine on the pyrene emission was not observed in these dendrimers. Quenching studies were performed by adding up to 3 equivalents of trifluoroacetic acid (TFA). To our knowledge, this is the first report of pyrene's unquenching behavior by a tertiary amine.

A phototunable anion receptor for C-HX interactions with benzoate anions

A supramolecular receptor consisting of two anthracene moieties with binding motifs for binding of benzoate anions is reported here. NMR studies indicate that the binding involves π-π interactions and CHX interactions. Upon exposure to >350 nm light, the receptor undergoes a [4 + 4] photoelectrocyclization restricting the access to the binding site for benzoate. The reverse reaction works in the presence of the dual stimuli of 254 nm light and the benzoate anions. The work thus demonstrates a light mediated dynamic control of the binding pocket of a supramolecular anion receptor.

Photorearrangement of dihetarylethenes as a tool for the benzannulation of heterocycles

A general strategy for the preparative benzannulation of aromatic heterocycles via photocyclization of 1,2-dihetarylethenes was proposed for the first time. The strategy includes two steps, namely, modular assembly of dihetarylethenes from widely available 3-hetarylacetic acids and 2-bromo-1-hetarylethanones, and subsequent preparative photorearrangement (using a UV lamp at 365 nm as the light source). This approach is efficient for the annulation of a wide range of heterocycles and provides C-, N-, O- or S-substituents in the benzoheterocycles obtained. The photochemical step is a metal-, acid-, and oxidant-free reaction, which requires non-inert conditions, and can be easily monitored by NMR spectroscopy. Applicability of the proposed strategy was tested in the synthesis of a wide range of substituted carbazoles and benzo[b]thiophenes as well as on a gram-scale benzannulation of 3-indoleacetic acid. Our study disclosed how to overcome two notable obstacles to the successful photorearrangement of dihetarylethenes: undesired reactions associated with photogenerated singlet oxygen, and the instability of desired products. The first problem was successfully solved by the addition of DABCO, while development of an in situ alkylation protocol to trap unstable photoproducts allowed us to overcome the second issue.

A metal–organic framework constructed by a viologen-derived ligand: photochromism and discernible detection of volatile amine vapors

A Co(ii)-MOF based on a viologen-derived ligand was obtained: the Co(ii) compound exhibits photochromism, and allows the visual and differentiable detection of different volatile alkylamines.

Photochromic Paper Indicators for Acidic Food Spoilage Detection

A photoresponsive microstructured composite is fabricated through the impregnation of cellulosic filter paper (FP) with a spiropyran-modified acrylic polymer. The polymer enwraps uniformly each individual cellulose fiber, increases the thermal stability of cellulose, and ensures the preservation of the composite functionalities even upon removal of the surface layers through mechanical scratching. The photochromic spiropyran moieties of the polymer, even while embedded in the cellulosic sheet, can reversibly interconvert between the colorless spiropyran and the pink merocyanine isomeric states upon irradiation with UV and visible light, respectively. Moreover, the photochromic polymer presents a faster photochromic response and a higher resistance to photodegradation, with an outstanding reusability for more than 100 switching cycles when it is incorporated in the cellulose network. Most importantly, the acidochromism of the modified FP, attributed to the spiropyran molecules after UV activation, allows the real-time optical and visual detection of acidity changes and spoilage in food products, such as wine and milk. Spoilage due to bacterial degradation and oxidation processes generates acidic vapors that induce the protonation of the merocyanine. This results in a visually detectable chromic transition from pink to white of the treated cellulose fibers, corresponding to a blue shift in the absorption spectrum. The developed photoresponsive cellulose composite can serve as cost-effective robust optical component in integrated functional platforms and consumer-friendly indicators for smart food packaging, as well as portable on demand acidoresponsive interfaces for gas monitoring in industrial and environmental applications.

Photovoltage-triggered electrochromic tablet for visualized photoelectrochemical sensing

In this work, we designed an integrated photoelectrochemical (PEC) analytical system with photovoltage-regulated electrochromic tablet for visualized sensing. The electrochromic tablet consisted of electron-injector (EI) part and colorimetric system, which was functionalized with dye-sensitized titanium dioxide and nickel oxide nanosheet film as the recognition element and signal readout, respectively. Under irradiation of home-built white light, the EI part provided sufficient and stable photovoltage and injected the photoexcited electrons to nickel oxide nanosheet through the conductive inner side of indium tin oxide slide. Thus, the different color states of nickel oxide nanosheet were observed, resulting in a detectable signal by naked eyes for visual analysis. More importantly, the color change was identified to be dependent on the photovoltages generated from EI part. Using copper ions as a model analyte, the electrochromic tablet was successfully applied to visually detect copper ions based on its quenching effect on photovoltage due to the formation of exciton trapping. As a result, the copper ions could be simultaneously determined over a wide range through photovoltage record, average RGB value, and naked eyes. Furthermore, the device exhibited considerable stability and could be reused through simple washing steps. The smart tablet PEC system provides a new avenue to design practical PEC sensor in point-of-care diagnosis.

Thermal Isomerization of Hydroxyazobenzenes as a Platform for Vapor Sensing

Photoisomerization of azobenzene derivatives is a versatile tool for devising light-responsive materials for a broad range of applications in photonics, robotics, microfabrication, and biomaterials science. Some applications rely on fast isomerization kinetics, while for others, bistable azobenzenes are preferred. However, solid-state materials where the isomerization kinetics depends on the environmental conditions have been largely overlooked. Herein, an approach to utilize the environmental sensitivity of isomerization kinetics is developed. It is demonstrated that thin polymer films containing hydroxyazobenzenes offer a conceptually novel platform for sensing hydrogen-bonding vapors in the environment. The concept is based on accelerating the thermal cis-trans isomerization rate through hydrogen-bond-catalyzed changes in the thermal isomerization pathway, which allows for devising a relative humidity sensor with high sensitivity and quick response to relative humidity changes. The approach is also applicable for detecting other hydrogen-bonding vapors such as methanol and ethanol. Employing isomerization kinetics of azobenzenes for vapor sensing opens new intriguing possibilities for using azobenzene molecules in the future.

Fluorescent Diarylethene Photoswitches-A Universal Tool for Super-Resolution Microscopy in Nanostructured Materials

Super-resolution fluorescence microscopy allows for unprecedented in situ visualization of biological structures, but its application to materials science has so far been comparatively limited. One of the main reasons is the lack of powerful dyes that allow for labeling and photoswitching in materials science systems. In this study it is shown that appropriate substitution of diarylethenes bearing a fluorescent closed and dark open form paves the way for imaging nanostructured materials with three of the most popular super-resolution fluorescence microscopy methods that are based on different concepts to achieve imaging beyond the diffraction limit of light. The key to obtain optimal resolution lies in a proper control over the photochemistry of the photoswitches and its adaption to the system to be imaged. It is hoped that the present work will provide researchers with a guide to choose the best photoswitch derivative for super-resolution microscopy in materials science, just like the correct choice of a Swiss Army Knife's tool is essential to fulfill a given task.