Microstructured Optical Fibers and Live Cells: A Water-Soluble, Photochromic Zinc Sensor

Spiropyran based recognitions of amines: UV-Vis spectra and mechanisms

As one of the important photochromic molecules, spiropyran (SP) compounds are widely used as detectors and fluorescence probes in the environment and bio-imaging field. Although great achievements have been attained for various sophisticated spiropyrans in metal ion sensing, less success is achieved in sensing organic molecules due to the weak interaction between the spiropyran and the target of the organic molecule. In this study, a spiropyran derivative containing a hydroxyl group (SPOH) was employed for the recognition of four kinds of amines via ultraviolet-visible (UV-Vis) spectra. The aliphatic primary amines, aromatic primary amines, aliphatic secondary and tertiary amines, aromatic secondary and tertiary amines were successfully distinguished according to the shapes and trends of their UV-Vis absorption spectra. The chemical reaction between aliphatic, aromatic primary amines and SPOH as well as alkalinity are two vital interaction mechanisms for the recognition process which are testified by Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR). Although SP is generally water-insoluble, it is easy to achieve soluble by fixing SPOH inside micelle or vesicle and thus the results in this study is meaningful for amine recognition utility in environments and biological systems.

Spiropyran-Based Nanocarrier: A New Zn2+ -Responsive Delivery System with Real-Time Intracellular Sensing Capabilities

A new spiropyran-based stimuli-responsive delivery system is fabricated. It encapsulates and then releases an extraneous compound in response to elevated levels of Zn²⁺ , a critical factor in cell apoptosis. A C₁₂ -alkyl substituent on the spiropyran promotes self-assembly into a micelle-like nanocarrier in aqueous media, with nanoprecipitation and encapsulation of added payload. Zn²⁺ binding occurs to an appended bis(2-pyridylmethyl)amine group at biologically relevant micromolar concentration. This leads to switching of the spiropyran (SP) isomer to the strongly fluorescent ring opened merocyanine-Zn²⁺ (MC-Zn²⁺ ) complex, with associated expansion of the nanocarriers to release the encapsulated payload. Payload release is demonstrated in solution and in HEK293 cells by encapsulation of a blue fluorophore, 7-hydroxycoumarin, and monitoring its release using fluorescence spectroscopy and microscopy. Furthermore, the use of the nanocarriers to deliver a caspase inhibitor, Azure B, into apoptotic cells in response to an elevated Zn²⁺ concentration is demonstrated. This then inhibits intracellular caspase activity, as evidenced by confocal microscopy and in real-time by time-lapsed microscopy. Finally, the nanocarriers are shown to release an encapsulated proteasome inhibitor (5) in Zn²⁺ -treated breast carcinoma cell line models. This then inhibits intracellular proteasome and induces cytotoxicity to the carcinoma cells.

A Liposomal Platform for Sensing of Extracellular Analytes Near Cells

Cell-permeable fluorescent chemosensors (calcein, monochlorobimane, and a recently reported spiropyran-based sensor SP2) have been incorporated into yeast total lipid extract-based liposomes to suppress inherent cell permeability to allow the detection of extracellular Ca²⁺, GSH, and Zn²⁺, respectively. The repurposed sensors have enhanced aqueous solubility and the ability to quantitatively measure biologically relevant concentrations of Ca²⁺ (0.25 mM–1 mM), Zn²⁺ (6.25 µM–50 µM), and GSH (0.25 mM–1 mM) by fluorescence in aqueous media. In addition, the liposomal sensors are nontoxic to HEK293 cells and have the ability to detect exogenously added Zn²⁺ (1 mM), Ca²⁺ (1 mM), or GSH (1 mM) near cells without internalisation. This new sensing platform provides a means to repurpose a range of intracellular fluorescent sensors to specifically detect extracellular analytes, while also improving biocompatibility for overall enhanced use in a wide range of biomedical applications.

Rationally Designed Probe for Reversible Sensing of Zinc and Application in Cells

Biologically compatible fluorescent ion sensors, particularly those that are reversible, represent a key tool for answering a range of fundamental biological questions. We report a rationally designed probe with a 6'-fluoro spiropyran scaffold (5) for the reversible sensing of zinc (Zn²⁺) in cells. The 6'-fluoro substituent overcomes several limitations normally associated with spiropyran-based sensors to provide an improved signal-to-background ratio and faster photoswitching times in aqueous solution. In vitro studies were performed with 5 and the 6'-nitro analogues (6) in HEK 293 and endothelial cells. The new spiropyran (5) can detect exogenous Zn²⁺ inside both cell types and without affecting the proliferation of endothelial cells. Studies were also performed on dying HEK 293 cells, with results demonstrating the ability of the key compound to detect endogenous Zn²⁺ efflux from cells undergoing apoptosis. Biocompatibility and photoswitching of 5 were demonstrated within endothelial cells but not with 6, suggesting the future applicability of sensor 5 to study intracellular Zn²⁺ efflux in these systems.

A highly selective and sensitive fluorescent chemosensor for Zn2+based on a diarylethene derivative

A promising photochromic fluorescent chemosensor1olinked with Schiff base unit was synthesized and the sensitivity test of1otoward Zn²⁺has been performed with detection limit up to 8.10 × 10⁻⁸M without any interference from Cd²⁺.

Microstructured Optical Fiber-based Biosensors: Reversible and Nanoliter-Scale Measurement of Zinc Ions

Sensing platforms that allow rapid and efficient detection of metal ions would have applications in disease diagnosis and study, as well as environmental sensing. Here, we report the first microstructured optical fiber-based biosensor for the reversible and nanoliter-scale measurement of metal ions. Specifically, a photoswitchable spiropyran Zn(2+) sensor is incorporated within the microenvironment of a liposome attached to microstructured optical fibers (exposed-core and suspended-core microstructured optical fibers). Both fiber-based platforms retains high selectivity of ion binding associated with a small molecule sensor, while also allowing nanoliter volume sampling and on/off switching. We have demonstrated that multiple measurements can be made on a single sample without the need to change the sensor. The ability of the new sensing platform to sense Zn(2+) in pleural lavage and nasopharynx of mice was compared to that of established ion sensing methodologies such as inductively coupled plasma mass spectrometry (ICP-MS) and a commercially available fluorophore (Fluozin-3), where the optical-fiber-based sensor provides a significant advantage in that it allows the use of nanoliter (nL) sampling when compared to ICP-MS (mL) and FluoZin-3 (μL). This work paves the way to a generic approach for developing surface-based ion sensors using a range of sensor molecules, which can be attached to a surface without the need for its chemical modification and presents an opportunity for the development of new and highly specific ion sensors for real time sensing applications.

Crowned spiropyran fluoroionophores with a carboxyl moiety for the selective detection of lithium ions

The absorbance and fluorescence spectra of carboxylated spiropyrans containing methyl-1-aza-12-crown-4, methyl-1-aza-15-crown-5, methyl-1-aza-18-crown-6 moieties are compared in the presence of alkali metal ions.

The ultrafast reactions in the photochromic cycle of water-soluble fulgimide photoswitches

High polarity and protic nature of the solvent strongly influence the reaction dynamics of a photochromic water-soluble indolylfulgimide.

Photoregulation of α-Chymotrypsin Activity by Spiropyran-Based Inhibitors in Solution and Attached to an Optical Fiber

Here the synthesis and characterization of a new class of spiropyran-based protease inhibitor is reported that can be reversibly photoswitched between an active spiropyran (SP) isomer and a less active merocyanine (MC) isomer upon irradiation with UV and visible light, respectively, both in solution and on a surface of a microstructured optical fiber (MOF). The most potent inhibitor in the series (SP-3 b) has a C-terminal phenylalanyl-based α-ketoester group and inhibits α-chymotrypsin with a Ki of 115 nM. An analogue containing a C-terminal Weinreb amide (SP-2 d) demonstrated excellent stability and photoswitching in solution and was attached to the surface of a MOF. The SP isomer of Weinreb amide 2 d is a competitive reversible inhibitor in solution and also on fiber, while the corresponding MC isomer was significantly less active in both media. The ability of this new class of spiropyran-based protease inhibitor to modulate enzyme activity on a MOF paves the way for sensing applications.

A facile synthesis of a highly water-soluble and selective fluorescent sensor towards zinc ions derived from β-cyclodextrin based on an unexpected sensing process

A highly selective and sensitive fluorescent sensor for Zn²⁺ derived from a β-cyclodextrin derivate was fabricated. Through fluorescence micrograph experiments, the sensor showed an excellent image effect on onion epidermal cells.

Dual sensor for Cd(II) and Ca(II): selective nanoliter-scale sensing of metal ions

The first selective, dual sensor for Ca(2+) and Cd(2+) capable of detection at 100 pM concentrations was designed and synthesized. The experimental observations made for the MC-cation complexes and the selectivity of compounds 1 and 2 with Ca(2+) and Cd(2+) ions were further explored using density functional theory. A first step toward a nanoliter-scale dip sensor for the dual sensing of Ca(2+) and Cd(2+) was demonstrated using microstructured optical fiber as the sensing platform which is important for ion sensing in confined spaces such as the medium surrounding cell clusters. In addition, this system displays picomolar sensitivity for these ions, with an added ability to reproducibly turn ion-binding on/off.