Light controlled receptors for heavy metal ions

Photoswitchable Photochromic Chelating Spironaphthoxazines: Synthesis, Photophysical Properties, Quantum-Chemical Calculations, and Complexation Ability

The stable and efficient photochromic and photoswitchable molecular systems designed from spirooxazines are of increasing scientific and practical interest because of their present and future applications in advanced technologies. Among these compounds, chelating spironaphthoxazines have received widespread attention due to their efficient optical response after complexation with some metal ions being of biomedical interest and environmental importance, as well as their good cycle performance and high reliability, especially by metal ion sensing. In this mini-review, we summarize our results in the design of novel photoswitchable chelating spironaphthoxazines with specific substituents in their naphthoxazine or indoline ring systems in view of recent progress in the development of such molecular systems and their applications as metal ion sensors. The design, synthesis methods, and photoresponse of such spirooxazine derivatives relevant to their applications, as well as quantum-chemical calculations for these compounds, are presented. Examples of various design concepts are discussed, such as sulfobutyl, hydroxyl, benzothiazolyl, or ester and carboxylic acid as substituents in the chelating spironaphthoxazine molecules. Further developments and improvements of this interesting and promising kind of molecular photoswitches are outlined.

Phototriggered structures: Latest advances in biomedical applications

Non-invasive control of the drug molecules accessibility is a key issue in improving diagnostic and therapeutic procedures. Some studies have explored the spatiotemporal control by light as a peripheral stimulus. Phototriggered drug delivery systems (PTDDSs) have received interest in the past decade among biological researchers due to their capability the control drug release. To this end, a wide range of phototrigger molecular structures participated in the DDSs to serve additional efficiency and a high-conversion release of active fragments under light irradiation. Up to now, several categories of PTDDSs have been extended to upgrade the performance of controlled delivery of therapeutic agents based on well-known phototrigger molecular structures like o-nitrobenzyl, coumarinyl, anthracenyl, quinolinyl, o-hydroxycinnamate and hydroxyphenacyl, where either of one endows an exclusive feature and distinct mechanistic approach. This review conveys the design, photochemical properties and essential mechanism of the most important phototriggered structures for the release of single and dual (similar or different) active molecules that have the ability to quickly reason of the large variety of dynamic biological phenomena for biomedical applications like photo-regulated drug release, synergistic outcomes, real-time monitoring, and biocompatibility potential.

Spiropyran-based chromic hydrogels for CO2 absorption and detection

By enabling rapid, cost-effective, user-friendly and in situ detection of carbon dioxide, colorimetric CO₂ sensors are of relevance for a variety of fields. However, it still remains a challenge the development of optical chemosensors for CO₂ that combine high sensitivity, selectivity and reusability with facile integration into solid materials. Herein we pursued this goal by preparing hydrogels functionalized with spiropyrans, a well-known class of molecular switches that undergo different color changes upon application of light and acid stimuli. By varying the nature of the substituents of the spiropyran core, different acidochromic responses are obtained in aqueous media that allow discriminating CO₂ from other acid gases (e.g., HCl). Interestingly, this behavior can be transferred to functional solid materials by synthesizing polymerizable spiropyran derivatives, which are used to prepare hydrogels. These materials preserve the acidochromic properties of the incorporated spiropyrans, thus leading to selective, reversible and quantifiable color changes upon exposure to different CO₂ amounts. In addition, CO₂ desorption and, therefore, recovery of the initial state of the chemosensor is favored by irradiation with visible light. This makes spiropyran-based chromic hydrogels promising systems for the colorimetric monitorization of carbon dioxide in a diversity of applications.

Singlet Oxygen Responsive Molecular Receptor to Modulate Atropisomerism and Cation Binding

In switchable molecular recognition, ¹ O₂ stimulus responsive receptors offer a unique structural change that is rarely exploited. The employed [4+2] reaction between ¹ O₂ and anthracene derivatives is quantitative, reversible and easily implemented. To evaluate the full potential of this new stimulus, a non-macrocyclic anthracene-based host was designed for the modular binding of cations. The structural investigation showed that ¹ O₂ controlled the atropisomerism in an on/off fashion within the pair of hosts. The binding studies revealed higher association constants for the endoperoxide receptor compared to the parent anthracene, due to a more favoured preorganization of the recognition site. The fatigue of the ¹ O₂ switchable hosts and their complexes was monitored over five cycles of cycloaddition/cycloreversion.

Photoresponsive reversible self-assembly of rod-coil amphiphiles containing spiropyran groups

Stimuli-responsive assembly deformation is a key feature in constructing smart soft materials, which makes them versatile and autonomous. In this study, rod-coil amphiphilic compounds containing spiropyran (SP) groups were developed and synthesized to investigate their stimuli-responsive assembly in a solution system with 99% water content. In addition to photochromic phenomena, reversible light-mediated morphological alterations occurred in these molecular aggregates. Based on the different flexible chain segments of rod-coil amphiphiles, the initial assemblies underwent a dissociation-reassembly process under ultraviolet (UV) irradiation, whereupon they deformed or disassembled to assemblies. Furthermore, as the UV source was removed, the original nanostructures were gradually recovered again via the ring-closing reaction process. These compounds, interestingly, can selectively combine with copper ions to produce cross-linked co-assembled nanostructures. The copper ion complex solution of rod-coil amphiphilic compounds emitted unique bright blue fluorescence, which allowed for the specific visual identification of copper ions in aqueous solutions.

Analyte Detection: A Decade of Progress in the Development of Optical/Fluorescent Sensing Probes

The development of selective and sensitive chemical sensors capable of detecting metal ions, anions, neutral species, explosives and hazardous substances, selectively and sensitively has attracted considerable interest of various research groups. The presence of such analytes within the permissible limits is often beneficial, but the excess amounts may lead to lethal effects to both the environment as well as the living organisms. Owing to the toxicity of the heavy metal ions, toxic anions and nitro-aromatics which are main constituents of explosives, the timely detection of these materials is most desirable to ensure safety and security of the mankind. In this personal account, we present several classes of molecular sensors that were specifically designed in our lab during the past decade for detecting several species in solutions, solid state as well as biological media. Modulation of the optical properties in response to the presence of guest species, led to selective and sensitive detection protocols, and was supported by the theoretical studies wherever possible. We have also extended the application of some of these probes for the on-site detection of analytes by developing the paper strips, glass slides and even the wool and cotton fabrics loaded with probes. One such development represents detection of palladium in human urine and blood samples collected from clinical samples. Additionally, the sensing events in some cases have successfully been reproduced in the live cancer cells. Based on the ease and cost-effective synthesis of the molecular probes, we hope that this account shall provide significant information to researchers in understanding the structure dependent sensing capabilities of the molecular probes.

Recent progress in treatment of dyes wastewater using microbial-electro-Fenton technology

Globally, textile dyeing and manufacturing are one of the largest industrial units releasing huge amount of wastewater (WW) with refractory compounds such as dyes and pigments. Currently, wastewater treatment has been viewed as an industrial opportunity for rejuvenating fresh water resources and it is highly required in water stressed countries. This comprehensive review highlights an overall concept and in-depth knowledge on integrated, cost-effective cross-disciplinary solutions for domestic and industrial (textile dyes) WW and for harnessing renewable energy. This basic concept entails parallel or sequential modes of treating two chemically different WW i.e., domestic and industrial in the same system. In this case, contemporary advancement in MFC/MEC (METs) based systems towards Microbial-Electro-Fenton Technology (MEFT) revealed a substantial emerging scope and opportunity. Principally the said technology is based upon previously established anaerobic digestion and electro-chemical (photo/UV/Fenton) processes in the disciplines of microbial biotechnology and electro-chemistry. It holds an added advantage to all previously establish technologies in terms of treatment and energy efficiency, minimal toxicity and sludge waste, and environmental sustainable. This review typically described different dyes and their ultimate fate in environment and recently developed hierarchy of MEFS. It revealed detail mechanisms and degradation rate of dyes typically in cathodic Fenton system under batch and continuous modes of different MEF reactors. Moreover, it described cost-effectiveness of the said technology in terms of energy budget (production and consumption), and the limitations related to reactor fabrication cost and design for future upgradation to large scale application.

Fluorescence detection and imaging of intracellular sulphite using a remote light activatable photochromic nanoprobe

The development of a responsive fluorescent probe for the detection of a particular biomolecule in a specific site at the desired moment is important in the fields of bioanalysis and imaging, molecular biology and biomedical research. In this work, we report the development of a remote-light activatable nanoprobe for the fluorescence detection of sulphite in pure aqueous solution and its imaging applications in living cells. The nanoprobe, Poly-Cm-SP, is fabricated simply by wrapping photochromic molecules (Cm-SP) into a polymer nanoparticle. Upon alternate UV/Vis light irradiation for several seconds, the Poly-Cm-SP nanoprobe exhibits red/blue fluorescence switch due to the inactive/active FRET processes from coumarins to the SP/MR isomers of the photochromic molecule. In the presence of sulphite, the specific reaction of sulphite with the electron deficit "CC" bond of the MR isomer occurs, resulting in an inefficient FRET process and thus exhibiting a constant "ON" blue channel fluorescence signal. After UV-light irradiation, the formation of activated Poly-Cm-MRin situ thus enables the detection of sulphite through recording the ratiometric changes of fluorescence signals at both blue and red channels. The Poly-Cm-SP nanoprobe possesses excellent biocompatibility and lysosome distribution capability, allowing it to be used for photochromic imaging and sulphite detection in the lysosomes of living macrophage cells. This work thus offers a new remote-light activatable nanoprobe for the detection and imaging of sulphite in biological systems.

Fabrication of a light-responsive polymer nanocomposite containing spiropyran as a sensor for reversible recognition of metal ions

Poly(spiropyran ethylacrylate-co-glycidyl methacrylate) grafted onto the surface of modified TiO2 (TiO2-g-P(SPEA-co-GMA)) as a novel stimuli-responsive polymer was fabricated and employed as sensor for reversible recognition of metal ions.

Application of QSAR for the identification of key molecular fragments and reliable predictions of effects of textile dyes on growth rate and biomass values of Raphidocelis subcapitata

The current quantitative structure-activity relationship (QSAR) study seeks to explore the underlying causes of fluctuations in growth rate and biomass of microalgae mainly due to textile dyes. The derived QSAR models cover two endpoints: ErC50 (growth rate) and EbC50 (biomass) of Raphidocelis subcapitata. In order to extract the structural features involved, multiple PLS (partial least squares) models have been developed with easy to interpret and uncomplicated 2D descriptors having proper physico-chemical meaning. These descriptors were calculated from Dragon, SiRMS, and PaDEL-descriptor software. Then, the models were developed initially using stepwise regression followed by partial least squares (PLS) regression, and the model development procedure for both the endpoints (ErC50 and EbC50) followed the stringent Organization for Economic Cooperation and Development (OECD) rules. Later on, the model validation was carried out with statistically significant and internationally accepted metrics (both internally and externally) in both the cases. Next, we have used the "Intelligent Consensus Predictor" tool (available from http://teqip.jdvu.ac.in/QSAR_Tools/DTCLab/) to test the prediction quality with an "intelligent" approach to select multiple models. The estimated prediction quality for the appropriate test sets reveals that the consensus models (CM) surpass the quality shown by individual models (IM) for both the endpoints (ErC50 and EbC50). Finally, the developed models were able to identify the major contributing features (hydrophobic units, unsaturation, saturation, electronegativity, branched atoms and charged fragments) related to aquatic toxicity of textile dyes.

Spiropyran dyes

This article furnishes an introduction to one of the most well-known classes of photochromic colorant. While the properties of spiropyran dyes inspired pioneering efforts to exploit photochromism for industrial applications, their lack of robustness held them back from commercialization. Nevertheless, this type of dye remains at the heart of much of the work to develop light-responsive materials upon which many potential applications in different fields of scientific and technological endeavor depend. The article describes the photochromism, synthesis, and applications of spiropyran colorants with an emphasis on the structural subtype that has attracted the greatest scrutiny. It also acts as a springboard to sources of more detail on these aspects.

The second-order NLO property of a photoswitchable heteroditpioc ion-pair receptor based on 2-pyridyl acylhydrazone linking with 2,6-pyridine bisamide: The impacts of metal cations and anions

A photoswitchable heteroditpioc ion-pair receptor E-1 and its isomeride Z-1 (without the anion binding site), that are based on the 2-pyridyl acylhydrazone linking 2,6-pyridine bisamide, have brought our attention to systematically explore the second-order nonlinear optical (NLO) properties by the density functional theory (DFT). In this work, we mainly studied the influences of metal cations (M = Na⁺, K⁺, Mg²⁺, Ca²⁺, Hg²⁺ and Pb²⁺), anions (X = Cl^-, Br^- and I^-) and ion-pair (NaCl, NaBr and NaI) on NLO responses for the receptor. In addition, the impacts of isomerization and poto-switching processes on NLO response for these systems also have been discussed detailedly. The results show that the isomerization process does not effectively adjust the NLO properties for our studied systems. But the poto-switching process that was triggered by light to capure or release ions plays an important role in improving the NLO properties. The receptors E-1 and Z-1 are excellent candidates to effectively detect metal cation Pb²⁺, because the first hyperpolarizability (β_tot) values of E∗Pb²⁺ and Z∗Pb²⁺ increased by 13 times and 20 times relative to that of receptors E-1 (188.06 a.u.) and Z-1 (270.21 a.u.), respectively. In addition, the receptor E-1 has the possibility to detect anion I^- due to the larger β_tot values compared with other anion-complexes. However, the changes of NLO responses for ion-pair complexes are not obvious compared with corresponding anion-complexes. We are looking forward to the research would be beneficial for further theoretical and experimental studies on recognizing metal cations and anions based on large second-order NLO difference.

Aromatic and heteroaromatic azacrown compounds: advantages and disadvantages of rigid macrocyclic ligands

Current approaches to the synthesis of aromatic and heteroaromatic azamacrocycles and their derivatives are summarized and systematized. The relationship between the structure of azacrown compounds and their complexation behaviour towards metal cations is analyzed. The diversity of practical applications of azamacrocyclic derivatives in medicine, biology and analytical and organic chemistry, as well as for the design of molecular devices is demonstrated.

The bibliography includes 307 references.

Cation-induced ring-opening and oxidation reaction of photoreluctant spirooxazine-quinolizinium conjugates

Two new spiroindolinonaphthoxazine derivatives with an electron-accepting styrylquinolizinium or styrylcoralyne unit, respectively, were synthesized, and the influence of such an arylvinyl substituent on the chemical and photochemical properties of the compounds was investigated. Specifically, these spirooxazines turned out to be resistant towards the photoinduced merocyanine formation, and the irradiation with light mainly led to photodegradation of the substrates. However, it was shown by colorimetric and fluorimetric screening assays as well as by detailed NMR spectroscopic and mass spectrometric studies that the addition of particular metal ions (Cu2+, Fe3+, and to a certain extent Hg2+) initially induced a ring-opening reaction that was irreversibly followed by a fast ring closure-deprotonation-oxidation sequence to give styryl-substituted naphthoxazole derivatives as the products quantitatively. For the quinolizinium-substituted spirooxazine derivative, the formation of the respective oxidation product caused the development of a broad absorption band between 425 nm and 500 nm and a new emission band at λfl = 628 nm, so that it may be employed as a selective chemosensor or chemodosimeter for the colorimetric and fluorimetric detection of Cu2+ and Fe3+.

A 4,5-quinolimide-based fluorescent sensor for sequential detection of Cu2+ and cysteine in water and living cells with application in a memorized device

In this study, a new 4,5-quinolimide-based fluorescent sensor BNC was synthesized and characterized. BNC showed single selectivity for Cu²⁺via the "turn-off" fluorescence among various common metal ions. After forming a 1:1 stoichiometric complex with Cu²⁺, the detection limit (LOD) of BNC for Cu²⁺ was measured to be 0.44 μM. Subsequently, the in situ generated BNC-Cu²⁺ complex had been used for sensing Cys with the LOD of 1.5 μM through the displacement strategy, resulting in the revivable emission of BNC. According to the "off-on-off" fluorescence cycle of BNC generated by the alternate addition of Cu²⁺ and Cys, a reversible memorized device with "read-write-read-erase" behavior was constructed at the molecular level. Furthermore, the recoveries of Cu²⁺ in lake water with BNC were in the range of 95.0-105%. And sequential fluorescence imagings of BNC for Cu²⁺ and Cys were successfully applied in living yeast cells.

Development of “dual-key-and-lock” responsive probes for biosensing and imaging

Recent advances in the development of “dual-key-and-lock” responsive probes for accurate detection of various biomolecules are reviewed.

Textile finishing dyes and their impact on aquatic environs

In the present review, we have been able to describe the different families of dyes and pigments used in textile finishing processes (Yarns, fabrics, nonwovens, knits and rugs) such as dyeing and printing. These dyes are reactive, direct, dispersed, indigo, sulphur and vats. Such that their presence in the liquid effluents resulting from the textile washing constitutes a serious risk, in the absence of their purification, for the quality of receiving aquatic environments. Indeed, the presence of these dyes and pigments can cause a significant alteration in the ecological conditions of the aquatic fauna and flora, because of the lack of their biodegradability. This has a negative impact on the equilibrium of the aquatic environment by causing serious dangers, namely the obvious dangers (Eutrophication, under-oxygenation, color, turbidity and odor), the long-term dangers (Persistence, bioaccumulation of carcinogenic aromatic products and formation of by-products of chlorination), mutagenicity and carcinogenicity.

Colorimetric and Fluorescence-Based Detection of Mercuric Ion Using a Benzothiazolinic Spiropyran

A merocyanine dye as a p-toluenesulfonate salt was synthesized. The structure of the dye was characterized using IR, NMR, HR-MS and single crystal X-ray crystallography. The X-ray crystallographic studies revealed the formation of a stacked aggregated structure of the merocyanine dye. The stacking interactions were investigated using the Crystal Explorer program, which estimated the strength of the interactions between different molecular pairs. The merocyanine dye was screened for affinity towards heavy metal ions, which revealed a color change from pink to colorless in the presence of mercuric ions, while other metal ions did not produce a similar change in color. In addition, the fluorescence spectroscopy indicated a change in the fluorescence intensity upon addition of mercuric ions. Both techniques displayed a good limit of detection value towards mercuric ions. In addition, the pixel intensity-based detection technique was also employed for the determination of limit of detection value with the help of a smartphone. The dynamic light scattering (DLS) studies indicated that the optical change occurred in the spectra of the receptor is due to the disaggregation of the receptor induced by mercuric ions. In addition, 1H-NMR studies were also used for investigating the mechanism of interaction between the receptor and the mercuric ions. The density functional theory (DFT) studies were used to investigate the formation of the complex at the molecular level, while time dependent density functional theory (TD-DFT) studies were used to understand the observed absorption spectra through the calculation of electronic excitation parameters, which indicated an increase in the energy difference between ground and the excited state.