2D Materials Kill Bacteria from Within.

Early work demonstrated that some two-dimensional (2D) materials could kill bacteria by using their sharp edges to physically rupture the bacteria envelope, which presents distinct advantages over traditional antibiotics, as bacteria are not able to evolve resistance to the former. This mechano-bactericidal mode of action, however, suffers from low antibacterial efficiency, fundamentally because of random orientation of 2D materials outside the bacteria, where the desirable "edge-to-envelope" contacts occur with low probability. Here, we demonstrate a proof-of-concept approach to significantly enhance the potency of the mechano-bactericidal activity of 2D materials. This approach is in marked contrast with previous work, as the 2D materials are designed to be in situ generated inside the bacteria from a molecularly engineered monomer in a self-assembled manner, profoundly promoting the probability of the "edge-to-envelope" contacts. The rationale in this study sheds light on a mechanically new nanostructure-enabled antibacterial strategy to combat antibiotic resistance.

An Orthogonal Conductance Pathway in Spiropyrans for Well-Defined Electrosteric Switching Single-Molecule Junctions.

While a multitude of studies have appeared touting the use of molecules as electronic components, the design of molecular switches is crucial for the next steps in molecular electronics. In this work, single-molecule devices incorporating spiropyrans, made using break junction techniques, are described. Linear spiropyrans with electrode-contacting groups linked by alkynyl spacers to both the indoline and chromenone moieties have previously provided very low conductance values, and removing the alkynyl spacer has resulted in a total loss of conductance. An orthogonal T-shaped approach to single-molecule junctions incorporating spiropyran moieties in which the conducting pathway lies orthogonal to the molecule backbone is described and characterized. This approach has provided singlemolecule conductance features with good correlation to molecular length. Additional higher conducting states are accessible using switching induced by UV light or protonation. Theoretical modeling demonstrates that upon (photo)chemical isomerization to the merocyanine, two cooperating phenomena increase conductance: release of steric hindrance allows the conductance pathway to become more planar (raising the mid-bandgap transmission) and a bound state introduces sharp interference near the Fermi level of the electrodes similarly responding to the change in state. This design step paves the way for future use of spiropyrans in single-molecule devices and electrosteric switches.

Advanced Vitrimer with Spiropyran Beads: A Multi-Responsive, Shape Memory, Self-Healing, and Reprocessable Smart Material.

Thermosetting materials have limitations in terms of reshaping and recycling due to their irreversible bond structures, leading to significant plastic waste issues. Recently, epoxy vitrimers based on dynamic covalent bond exchange have been introduced as promising alternatives to traditional thermosets. Particularly, they demonstrate significant potential applications in the field of multi-responsive materials. In this research, a self-healable and mechano-responsive vitrimer (EB-V) is successfully prepared, incorporating epoxide spiropyran beads (ESP beads) derived from citric acid and epoxy derivatives. To enable self-reporting of cracks through color changes, ESP beads are covalently bonded to the vitrimer via an epoxy-carboxylic acid reaction. The photochromic properties of EB-V are demonstrated by color and fluorescence changes, and its tensile strength increased from 2.0 to 6.8 MPa compared to the control sample. Dynamic mechanical analysis confirmed the covalent exchange reaction of the vitrimer, revealing its reconfigurable behavior and stress relaxation at elevated temperatures. Furthermore, EB-V exhibited exceptional properties, including self-healing and reprocessability. As a smart material, it holds great promise for a wide range of applications, such as sensors, actuators, 4D printing, and industrial safety diagnostics.

Reversible Sensing Technologies Using Upcycled TPEE: Crafting pH and Light Responsive Materials towards Sustainable Monitoring.

Multiresponsive materials with reversible and durable characteristics are indispensable because of their promising applications in environmental change detections. To fabricate multiresponsive materials in mass production, however, complex reactions and impractical situations are often involved. Herein, a dual responsive (light and pH) spiropyran-based smart sensor fabricated by a simple layer-by-layer (LbL) assembly process from upcycled thermoplastic polyester elastomer (TPEE) materials derived from recycled polyethylene terephthalate (r-PET) is proposed. Positively charged chitosan solutions and negatively charged merocyanine-COOH (MC-COOH) solutions are employed in the LbL assembly technique, forming the chitosan-spiropyran deposited TPEE (TPEE-CH-SP) film. Upon UV irradiation, the spiropyran-COOH (SP-COOH) molecules on the TPEE-CH-SP film undergo the ring-opening isomerization, along with an apparent color change from colorless to purple, to transform into the MC-COOH molecules. By further exposing the TPEE-CH-MC film to hydrogen chloride (HCl) and nitric acid (HNO3) vapors, the MC-COOH molecules can be transformed into protonated merocyanine-COOH (MCH-COOH) with the simultaneous color change from purple to yellow.

Light-Modulated Humidity Sensing in Spiropyran Functionalized MoS2 Transistors.

The optically tuneable nature of hybrid organic/inorganic heterostructures tailored by interfacing photochromic molecules with 2D semiconductors (2DSs) can be exploited to endow multi-responsiveness to the exceptional physical properties of 2DSs. In this study, a spiropyran-molybdenum disulfide (MoS2) light-switchable bi-functional field-effect transistor is realized. The spiropyran-merocyanine reversible photo-isomerization has been employed to remotely control both the electron transport and wettability of the hybrid structure. This manipulation is instrumental for tuning the sensitivity in humidity sensing. The hybrid organic/inorganic heterostructure is subjected to humidity testing, demonstrating its ability to accurately monitor relative humidity (RH) across a range of 10%-75%. The electrical output shows good sensitivity of 1.0% · (%) RH-1. The light-controlled modulation of the sensitivity in chemical sensors can significantly improve their selectivity, versatility, and overall performance in chemical sensing.

Enhanced Photodynamic Therapy for Neurodegenerative Diseases: Development of Azobenzene-Spiropyran@Gold Nanoparticles for Controlled Singlet Oxygen Generation.

The development of durable photosensitizers is pivotal for advancing phototherapeutic applications in biomedicine. Here, we introduce a core-shell azobenzene-spiropyran structure on gold nanoparticles, engineered to enhance singlet oxygen generation. These nano-photosensitizers exhibit increased structural stability and thermal resistance, as demonstrated by slowed O-N-C bond recombination dynamics via in-situ Raman spectroscopy. Notably, the in-situ formation of merocyanine and a light-induced compact shell arrangement extend its half-life from 47 minutes to over 154 hours, significantly boosting singlet oxygen output. The nano-photosensitizer also shows high biocompatibility and notably inhibits tau protein aggregation in neural cells, even with phosphatase inhibitors. Further, it promotes dendritic growth in neuro cells, doubling typical lengths. This work not only advances chemical nanotechnology but also sets a foundation for developing long-lasting phototherapy agents for treating neurodegenerative diseases.

Spiropyran-Based Photoisomerizable α-Amino Acid for Membrane-Active Peptide Modification.

Photoisomerizable peptides are promising drug candidates in photopharmacology. While azobenzene- and diarylethene-containing photoisomerizable peptides have already demonstrated their potential in this regard, reports on the use of spiropyrans to photoregulate bioactive peptides are still scarce. This work focuses on the design and synthesis of a spiropyran-derived amino acid, (S)-2-amino-3-(6'-methoxy-1',3',3'-trimethylspiro-[2H-1-benzopyran-2,2'-indolin-6-yl])propanoic acid, which is suitable for the preparation of photoisomerizable peptides. The utility of this amino acid is demonstrated by incorporating it into the backbone of BP100, a known membrane-active peptide, and by examining the photoregulation of the membrane perturbation by the spiropyran-containing peptides. The toxicity of the peptides (against the plant cell line BY-2), their bacteriotoxicity (E. coli), and actin-auxin oscillator modulation ability were shown to be significantly dependent on the photoisomeric state of the spiropyran unit.

Photoswitching in a Liquid Crystalline Pt(II) Coordination Complex.

Photoswitching of photoluminescence has sparked tremendous research interests for super-resolution imaging, high-security-level anti-counterfeiting, and other high-tech applications. However, the excitation of photoluminescence is usually ready to trigger the photoswitching process, making the photoluminescence readout unreliable. Herein, we report a new photoswitch by the marriage of spiropyran with platinum(II) coordination complex. Viable photoluminescence can be achieved upon excitation by 480 nm visible light while the photoswitching can be easily triggered by 365 nm UV light. The feasible photoswitching may be benefited from the formed liquid crystalline (LC) phase of the designed photoswitch as a crystalline spiropyran is normally unable to implement photoswitching. Compared to the counterparts, this LC photoswitch can show distinct and reliable apparent colors and emission colors before and after photoswitching, which may promise the utility in high-security-level anti-counterfeiting and other advanced information technologies.

Fluorescent Materials Based on Spiropyran for Advanced Anti-Counterfeiting and Information Encryption.

In daily life, counterfeit and substandard products, particularly currency, medicine, food, and confidential documents, are capable of bringing about very serious consequences. The development of anti-counterfeiting and authentication technologies with multilevel securities is a powerful means to overcome this challenge. Among various anti-counterfeiting technologies, fluorescent anti-counterfeiting technology is well-known and commonly used to fight counterfeiters due to its wide material source, low cost, simple usage, good concealment, and simple response mechanism. Spiropyran is favored by scientists in the fields of anti-counterfeiting and information encryption due to its reversible photochromic property. Here, we summarize the current available spiropyran-based fluorescent materials from design to anti-counterfeiting applications. This review will be help scientists to design and develop fluorescent anti-counterfeiting materials with high security, high performance, quick response, and high anti-counterfeiting level.

New Triphenylphosphonium Salts of Spiropyrans: Synthesis and Photochromic Properties.

The most important area of modern pharmacology is the targeted delivery of drugs, and one of the most promising classes of chemical compounds for creating drugs of this kind are the photochromic spiropyrans, capable of light-controlled biological activity. This work is devoted to the synthesis and study of the photochromic properties of new triphenylphosphonium salts of spiropyrans. It was found that all the synthesized cationic spiropyrans have high photosensitivity, increased resistance to photodegradation and the ability for photoluminescence.

A Rhodamine-Spiropyran Conjugate Empowering Tunable Mechanochromism in Polymers under Multiple Stimuli.

Mechanochromic functionality realized via the force-responsive mechanophores in polymers has great potential for damage sensing and information storage. Mechanophores with the ability to recognize multiple stimuli for tunable chromic characteristics are highly sought after for versatile sensing ability and color programmability. Nevertheless, the majority of mechanophores are based on single-component chromophores with limited sensitivity, or require additional fabrication technology for multi-modal chromism. Here, we report a novel multifunctional mechanophore capable of vividly detectable and tunable mechanochromism in polymers. This synergistic optical coupling relies on strategically fusing rhodamine and spiropyran (Rh-SP), and tethering polymer chains on both subunits. The mechanochromic behaviors of the Rh-SP-linked polymers under sonication and compression are thoroughly evaluated in response to changes in force and the light-controlled relaxation process. Non-sequential ring-opening of the two subunits under force is identified, endowing high-contrast mechanochromism. Light-induced differential ring-closing reactions of the two subunits, together with the acidichromism of the SP moiety, are employed to engineer elastomers with programmable and wide-spectrum colors. Our work presents an effective strategy for highly appreciable and regulable mechanochromic functionality, and also provides new insights into the rupture mechanisms of π-fused mechanophores, as well as how the stimuli history controls stress accumulation in polymers.

Orthogonal Photoswitching in a Porous Organic Framework.

The development of photoresponsive systems with non-invasive orthogonal control by distinct wavelengths of light is still in its infancy. In particular, the design of photochemically triggered-orthogonal systems integrated into solid materials that enable multiple dynamic control over their properties remains a longstanding challenge. Here, we report the orthogonal and reversible control of two types of photoswitches in an integrated solid porous framework, that is, visible-light responsive o-fluoroazobenzene and nitro-spiropyran motifs. The properties of the constructed material can be selectively controlled by different wavelengths of light thus generating four distinct states providing a basis for dynamic multifunctional materials. Solid-state NMR spectroscopy demonstrated the selective transformation of the azobenzene switch in the bulk, which in turn modulates N2 and CO2 adsorption.

TD-DFT study of BIPS spiropyran: Effects of functionals and high-polarity solvent on Cspiro O bond dissociation and recovery.

We performed a theoretical analysis of the BIPS photochemical cycle using an extensive set of forty hybrid functionals and taking into account a highly polar solvent (methanol). The functionals with a small fraction of the exact Hartree-Fock exchange (%HF) showed the predominant S0 → S2 transition with the strengthening of the Cspiro O bond. At the same time, functionals with medium and high %HF (including those with long-range correction) gave a dominant S0 → S1 transition with weakening or breaking of the Cspiro O bond, which corresponds to the experimental results. The influence of a highly polar solvent on the photochemical electrocyclic transformations of BIPS turned out to be significant. The number of functionals causing dissociation of the Cspiro O bond decreased from 10 to 7 compared to the gas phase. The magnitude of the oscillator strength has increased by approximately one and a half times. Structural distortions of the BIPS molecule during excitation (both with and without Cspiro O bond cleavage) significantly decreased in methanol compared to the gas phase. The two strong hydrogen bonds of methanol molecules with the oxygen and nitrogen atoms of spiropyran also have a significant effect on its excitation. They lead to a change in the predominant transition from S0 → S2 to S0 → S1 for five functionals. The number of functionals giving dissociation of the Cspiro O bond decreased from seven to four (M08HX, M052X, CAM-B3LYP, and M11). After the opening of the excited BIPS molecule, both of its strong H-bonds with methanol are preserved. Of this set of four functionals, only M052X and CAM-B3LYP exhibited the dominant HOMO-1 → LUMO configuration observed in high-level computations by other authors. Therefore, both of these functionals are recommended for modeling the photochemical cycle of this spiropyran. The photochemical cycle of BIPS was theoretically analyzed. The redistribution of the electron density in this cycle was quantitatively described using the differences in NPA of the atomic charges. The most important result of this analysis was the electrostatic mechanism of the approach of Cspiro and oxygen atoms at the fourth stage, which causes further reduction of the Cspiro O bond.

Reversible Photocontrol of Microtubule Stability by Spiropyran-Conjugated Tau-Derived Peptides.

Spatiotemporal modulation of microtubules by light has become an important aspect of the biological and nanotechnological applications of microtubules. We previously developed a Tau-derived peptide as a binding unit to the inside of microtubules. Here, we conjugated the Tau-derived peptide to spiropyran, which is reversibly converted to merocyanine by light, as a reversible photocontrol system to stabilize microtubules. Among the synthesized peptides with spiropyran/merocyanine at different positions, several peptides were bound to the inside of microtubules and stabilized the structures of microtubules. The peptide with spiropyran at the N-terminus induced polymerization and stabilization of microtubules, whereas the same peptide with the merocyanine form did not exert these effects. Reversible formation of microtubules/tubulin aggregates was achieved using the peptide with spiropyran conjugated at the N-terminus and irradiation with UV and visible light. Spiropyran-conjugated Tau-derived peptides would be useful for spatiotemporal modulation of microtubule stability through reversible photocontrol of binding.

Rational Understanding of Loading and Release of Doxorubicin by UV-Light- and pH-Responsive Poly(NIPAM-co-SPMA) Micelle-like Aggregates.

A deep understanding of the interactions between micelle-like aggregates and antineoplastic drugs is paramount to control their adequate delivery. Herein, Poly(NIPAM-co-SPMA) copolymer nanocarriers were synthesized according to our previous published methodology, and the loading and release of poorly and highly water-soluble doxorubicin forms (Dox and Dox-HCl, respectively) were evaluated upon UV light irradiation and pH-variation stimuli. Capillary electrophoresis (CE) coupled to a fluorescence detector (LIF) allowed us to specifically characterize these systems and deeply study the loading and release processes. For this purpose, varying concentrations of doxorubicin were tested, and the loading/release rates were indirectly quantified thanks to the "free" doxorubicin concentration in solution. This study highlighted that Dox loading (9.4 µg/mg) was more effective than Dox-HCl loading (5.5 µg/mg). In contrast, 68 and 74% of Dox-HCl were respectively released after 2 min upon pH variation (from 7.4 to 6.0) and combined UV + pH 6.0 stimuli, while only 27% of Dox was invariably released upon application of the same stimuli. These results are coherent with the characteristics of both DoxHCl and Dox: Electrostatic interactions between Dox-HCl and the micelle-membrane structure (NIPAM) seemed predominant, while hydrophobic interactions were expected between Dox and the SP moieties at the inner part of the micelle-like aggregate, leading to different behaviors in both loading and release of the two doxorubicin forms. For doxorubicin loading concentrations higher than 3 µM, the electrophoretic profiles presented an additional peak. Thanks to CE characterizations, this peak was attributed to the formation of a complex formed between the nonaggregated copolymer and the doxorubicin molecules. This report therefore undergoes deep characterization of the dynamic formation of different micelle/drug complexes involved in the global drug-delivery behavior and therefore contributes to the development of more effective stimuli-responsive nanocarriers.

Digital holographic microscopy of spiropyran-based dynamic materials.

Spiropyran (SP)-based dynamic materials undergo structural changes in response to external stimuli. In this paper, we show that digital holographic microscopy (DHM) is an effective candidate for characterisation of SPs (embedded in polymer matrices) and for monitoring of their dynamical changes. The polymer matrices are polylactic acid (PLA) and poly(methyl methacrylate) (PMMA) films, which are decorated with SPs and immobilised on graphene quantum dots (GQDs). GQDs are modified by benzylamines prior to the loading of SP species because of the enhancement of hydrophobic characteristics. UV irradiation is used as the external stimulus and the dynamical changes of the samples before and after UV irradiation are measured. DHM is arranged on a novel self-referencing setup, which substantially reduces the sensitivity of DHM to environmental vibrations. Morphometric information for characterisation of the samples is obtained by analysis of the recorded digital holograms. The experimental results demonstrate the potential of the presented technique to serve as an alternative technique for surface measurement methodologies such as atomic force microscope and stylus profiler for surface characterisation of similar materials.

Molecular design and synthesis of methoxy-substitued spiropyrans with photomodulated NIR-fluorescence.

This study focuses on the molecular design and synthesis of salt spiropyrans with near-IR fluorescence. The structure of the obtained compounds was confirmed by NMR, IR and mass spectroscopy. In the course of studying the spectral and photoluminescent characteristics, it was possible to reveal the effect of some substituents in various positions on the properties of spiropyran dyes. Due to the structural similarity of one of the isomers to cyanine dyes, the obtained compounds are of interest as potential fluorescent probes for bioimagimg, in particular, for DNA studies. To reveal their ability of binding to DNA molecules molecular docking was carried out. Toxic effects of compounds demonstrating NIR fluorescence were studied on biofilms, as well as using bacterial lux-biosensors.

Rational design of a negative photochromic spiropyran-containing fluorescent polymeric nanoprobe for sulfur dioxide derivative ratiometric detection and cell imaging.

It is highly desirable to develop high-performance ratiometric fluorescent probes for SO2 derivative detection and realize their application in biological imaging. In this study, we report the rational design of a novel negative photochromic spiropyran derivative, spiro[azahomoadamantane-pyran] (MAHD-SP), with notable orange fluorescence in its stable ring-opened state without UV regulation. The unsaturated double bond of MAHD-SP underwent the Michael addition reaction of the SO2 derivative, making the fluorescence quenching of MAHD-SP obvious. Then, MAHD-SP, a fluorescent conjugated polymer PFO and a polymeric surfactant PEO113-b-PS49 were used to construct a ratiometric fluorescent polymeric nanoprobe (RFPN) via a coprecipitation method. The probe exhibited high sensitivity and selectivity for the ratiometric detection of SO2 derivatives in pure aqueous solutions. Moreover, the good biocompatibility of RFPN can be used to visualize exogenous and endogenous SO2 derivative generation in living cells.

Photoresponsive Spiropyran and DEGMA-Based Copolymers with Photo-Switchable Glass Transition Temperatures.

Herein, novel photoresponsive spiropyran (SP)-based P(DEGMA-co-SpMA) copolymers with variable percentages of SP fractions are synthesized. The SP group present in these polymers exhibited the abilities of reversible photoisomerism. Their photoresponsive, structural, and thermal properties have been investigated and compared using various characterization techniques. These light-responsive copolymers are found to exhibit photoswitchable glass transition temperature (Tg ), high thermal stability (Td > 250°C), instant photochromism as well as fluorescence upon exposure to UV light. It is demonstrated that the Tg of these synthesized polymers increased when irradiated with UV light (λ = 365 nm), as a consequence of the photoisomerization of incorporated SP groups into their merocyanine form. This increase in Tg is attributed to an increase in polarity and a decrease in the overall entropy of the polymeric system when it switches from the ring-closed SP form (less-ordered state) to the ring-opened merocyanine form (more-ordered state). Therefore, such polymers with a unique feature of phototunable glass transition temperatures provide the possibility to be integrated into functional materials for various photoresponsive applications.

Light-Modulated Morphological Transformation of Spiropyran Derivative from Nanosphere to Nanorod.

The construction of tunable morphological systems has important implications for understanding the mechanism of molecular self-assembly. In this study, a spiropyran derivative M1 is reported with light-responsive assembly morphology, which can be tuned from nanosphere to nanorod by ultraviolet light irradiation. The absorption spectra show that M1 molecules are transformed from closed-ring (SP) isomers into open-ring (MC) isomers and start to form H-aggregates with increasing irradiation time. Density functional theory calculations indicate that MC-MC isomers possess stronger binding energy than SP-SP isomers. The MC isomers may thus facilitate the dissociation of the SP-SP aggregates and promote the change of self-assembled morphology with the aid of stronger π-π stackings and dipole-dipole interactions. The research gives an effective method for modulating the morphology of assemblies, with great potential for applications in smart materials.