Two benzothiadiazole-based fluorescent sensors for selective detection of Cu2+ and OH– ions

Effects of donor and acceptor substituents on the photophysics of 4-ethynyl-2,1,3-benzothiadiazole derivatives

The present work explores the photophysical, electrochemical, and fluorescence polarization properties of a group of π-conjugated phenylethynyl-2,1,3-benzothiadiazole derivatives (BTDs) bearing different electron-donating (ED) or electron-withdrawing (EW) substituents at the para position of the phenylethynyl moiety. The BTDs were synthesized through the Sonogashira cross-coupling reaction between 4-bromo-2,1,3-benzothiadiazole and the respective para-substituted phenylethynyl derivatives. The BTDs with the EW-substituents show relatively weak solvatochromic behavior, while the BTDs with the strong ED-substituents like methoxy and N,N-dimethylamino-based substituents (BTDPhOMe and BTDPhNMe2) exhibit a pronounced solvatochromic behavior. The change in dipole moments in the excited states of the derivatives was calculated using Lippert-Mataga plots. The conclusions drawn on the spectral behavior of the molecules could be rationalized by TD-DFT calculations involving electron density difference (EDD) maps that correlate with the ICT characteristics of the molecules. The experimental and theoretical calculations reveal that the BTDs with the strong ED-substituents (strong push-pull type BTDs) have a strong ICT character in the excited state. These strong push-pull type BTDs show high fluorescence quantum yield (ΦF) in apolar solvents and low ΦF in polar solvents. In contrast, the BTDs with the weak ED-substituents (weak push-pull type BTDs) and EW-substituents (pull-pull type BTDs) have a weaker ICT character with low ΦF in apolar and high ΦF in polar solvent media. There is good a agreement among the HOMO-LUMO band gaps obtained from absorption spectroscopy and electrochemical studies and theoretical calculations. The fluorescence anisotropy measurement in the glycerol medium shows that the studied BTDs generally exhibit higher sensitivity towards microviscosity than the traditional DPH fluorescence anisotropy probe.

Synthesis of a quinoxaline-hydrazinobenzothiazole based probe-single point detection of Cu2+, Co2+, Ni2+ and Hg2+ ions in real water samples

A novel quinoxaline-hydrazinobenzothiazole based sensor was synthesized and characterized using NMR, FTIR, and Mass spectroscopy techniques. The sensor achieves the distinct "single-point" colorimetric and fluorescent detection of Cu²⁺, Co²⁺, Ni²⁺ and Hg²⁺ ions with distinguishable color changes from yellow to red, pale red, pale brown and orange, respectively. The UV-visible and fluorescence emission spectral investigation revealed the excellent single-point sensing ability of the probe towards four different heavy metal ions with a ratiometric response. Nanomolar levels of detection of about 1.16 × 10^-7 M, 9.92 × 10^-8 M, 8.21 × 10^-8 M, and 1.14 × 10^-7 M for Cu²⁺, Co²⁺, Ni²⁺ and Hg²⁺ ions, respectively, were achieved using our sensor, which are below the US-EPA permissible limits. Additionally, the sensor was utilized for naked eye detection under normal daylight. Quantitative determination of the metal ions in real water samples was also demonstrated.

Selective recognition of Hg2+ ions in aqueous solution by a CdII-based metal-organic framework with good stability and vacant coordination sites

A novel water-stable Cd^II-based metal-organic framework, namely {[Cd(BIBT)(TDC)]·2H₂O}_n (JXUST-28, BIBT = 4,7-bi(1H-imidazol-1-yl)benzo-[2,1,3]thiadiazole and H₂TDC = 2,5-thiophenedicarboxylic acid), was synthesized using a mixed-ligand strategy. Structural analysis demonstrates that JXUST-28 exhibits a two-dimensional layer structure with 4-connected sql topology. Intriguingly, JXUST-28 presents good stability in boiling water (at least 5 days), common organic solvents and aqueous solutions with different pH values of 2-12 (more than 24 hours). Furthermore, fluorescence experiments revealed that JXUST-28 could sense Hg²⁺ ions in aqueous solution via a quenching effect with a detection limit of 0.097 μM. Meanwhile, JXUST-28 can also be regenerated at least 5 times to detect Hg²⁺ ions. In addition, light-emitting diode lamps, luminescent films, and test papers of JXUST-28 have been successfully developed for practical applications.

Shifting the Triangle-Square Equilibrium of Self-Assembled Metallocycles by Guest Binding with Enhanced Photosensitization

Shifting a triangle-square equilibrium in one direction is an important problem in supramolecular self-assembly. Reaction of a benzothiadiazole-based diimidazole donor with a cis-Pt(II) acceptor yielded an equilibrium mixture of a triangle ([C₁₈H₂₄N₁₀O₆S₁Pt₁]₃≡ PtMC^T) and a square ([C₁₈H₂₄N₁₀O₆S₁Pt₁]₄≡ PtMC^S). We report here the shifting of such equilibrium toward a triangle using a guest (pyrene aldehyde, G1). While both benzothiadiazole and pyrene aldehyde can form reactive oxygen species (ROS) in organic solvents, their therapeutic use in water is restricted due to aqueous insolubility. The enhanced water solubility of the benzothiadiazole unit and G1 by macrocycle formation and host-guest complexation, respectively, enabled enhanced ROS generation by the host-guest complex (G1' ⊂ PtMC^T) in water (G1' = hydrated form of G1). The guest-encapsulated metallacycle (G1' ⊂ PtMC^T) has shown synergistic antibacterial activity compared to the mixture of macrocycles upon white-light irradiation due to enhanced ROS generation. The mechanism for such enhanced activity was established by measuring the oxidative stress and relative internalization of PtMCs and G1' ⊂ PtMC^T.

Amino acid triggered water-soluble NBD derivatives for differential organelle staining and the role of the chemical moiety for their specific localization

Apart from being the unit of protein, amino acids have diverse roles. Here we have shown that amino acids guide the differential transportation of the dye molecule to the cellular organelles depending upon the property of their intrinsic functionality. We have conjugated nitrobenzofurazan (NBD) moiety with two amino acids (lysine and histidine derivatives) with a linker. Both the derivates are water-soluble and biocompatible in nature. Surprisingly we found that lysine conjugated NBD (NBD-Lys) stains lipid droplets whereas the histidine conjugated NBD (NBD-His) stains lysosomes. We also measured the spectral properties of these two NBD conjugates. Results depict that both the conjugates are extremely stable both in air and inert atmosphere and the fluorescence of the derivatives almost remain unaltered at different pH. Further by altering the functionality of the side chain, we established the contribution of each functional group towards this differential organelle targeting.

A Benzothidiazole-Based Reversible Fluorescent Probe with Excellent Performances in Selectively and Sensitively Sensing of Hg2+ in Water

The development of mercury ion selective fluorescent probe is significant because it is one of toxic heavy metals and poses great risks and hazards to human health. Herein, we develop a mercury ion-selective fluorescent probe, namely IB, based on imidazole decorated benzothiadiazole that obtained by a facile palladium catalytic C-N coupling reaction. IB exhibits high selectivity and sensitivity towards mercury ion in water with nearly 32-fold fluorescent enhancement. The detection limit is calculated to be 0.93 nmol/L. In addition, the sensing of mercury ion can be conducted in a wide pH scope ranging from 4.0 to 10.0. Subsequently, the mercury ion elicits fluorescence of IB solution can be quenched by the addition of cyanide anions, showing "off-on-off" fluorescence transformation with at least 5 cycles, demonstrating the reversible sensing ability of IB. Furthermore, an INHIBIT logical detector has been developed using mercury ion and cyanide anions as inputs and fluorescence of IB as output. Significantly, IB can be utilized for mercury ion detection in real water sample.

Facile fabrication and luminescence properties of a new ZnII coordination polymer-based fluorescent sensor toward antibiotics

A new ZnII-based coordination polymer could selectively and sensitively recognize NFT and DCN via turn-off effect. Interestingly, a mixed matrix film for visualizable sensing has been successfully developed.

N-alkylation briefly constructs tunable multifunctional sensor materials: Multianalyte detection and reversible adsorption

A series of N-alkyl-substituted polybenzimidazoles (SPBIs), synthesized by simple condensation and N-alkylation, act as functional materials with tunable microstructures and sensing performance. For their controllable morphologies, the formation of nano-/microspheres is observed at the n(RBr)/n(PBI) feed ratio of 5:1. Products with different degrees of alkylation can recognize metal ions and nitroaromatic compounds (NACs). For example, SPBI-c, obtained at the feed ratio of 1:1, can selectively detect Cu2+, Fe3+, and NACs. By contrast, SPBI-a, obtained at the feed ratio of 0.1:1, can exclusively detect Cu2+ with high sensitivity. Their sensing mechanisms have been studied by FT-IR spectroscopy, SEM, XPS, and DFT calculations. Interestingly, the SPBIs can adsorb Cu2+ in solution and show good recyclability. These results demonstrate that polymeric materials with both sensing and adsorption applications can be realized by regulating the alkylation extent of the main chain, thus providing a new approach for the facile synthesis of multifunctional materials.

Fluorescent Benzothiadiazole Derivatives as Fluorescence Imaging Dyes: A Decade of New Generation Probes

The current review describes advances in the use of fluorescent 2,1,3-benzothiadiazole (BTD) derivatives after nearly one decade since the first description of bioimaging experiments using this class of fluorogenic dyes. The review describes the use of BTD-containing fluorophores applied as, inter alia, bioprobes for imaging cell nuclei, mitochondria, lipid droplets, sensors, markers for proteins and related events, biological processes and activities, lysosomes, plasma membranes, multicellular models, and animals. A number of physicochemical and photophysical properties commonly observed for BTD fluorogenic structures are also described.

A fluorescence red-shift and turn-on sensor for acetylacetone derived from ZnII-based metal–organic framework with new topology

JXUST-4 with a rare (3,3,7)-c topology net has been prepared, acting as a fluorescent sensor for acac through obvious fluorescence red-shift and enhancement with excellent properties.

A multifunctional benzothiadiazole-based fluorescence sensor for Al3+, Cr3+ and Fe3+

A benzothiadiazole-based Zn^II MOF (JXUST-3) has been synthesized, which is a good multifunctional chemosensor for the detection of Al³⁺, Cr³⁺ and Fe³⁺.

Deciphering the Dynamics of Organic Nanoaggregates with AIEE Effect and Excited States: Lipophilic Benzothiadiazole Derivatives as Selective Cell Imaging Probes

An aggregation-induced emission enhancement (AIEE) effect in fluorescent lipophilic 2,1,3-benzothiadiazole (BTD) derivatives and their organic nanoaggregates were studied. A set of techniques such as single-crystal X-ray, dynamic light scattering (DLS), electron paramagnetic resonance (EPR), UV-vis, fluorescence, and density functional theory (DFT) calculations have been used to decipher the formation/break (kinetics), properties, and dynamics of the organic nanoaggregates of three BTD small organic molecules. An in-depth study of the excited-state also revealed the preferential relaxation emissive pathways for the BTD derivatives and the dynamics associated with it. The results described herein, for the first time, explain the formation of fluorescent BTD nanoaggregate derivatives and allow for the understanding of their dynamics in solution as well as the ruling forces of both aggregation and break processes along with the involved equilibrium. One of the developed dyes could be used at a nanomolar concentration to selectively stain lipid droplets emitting an intense and bright fluorescence at the red channel. The other two BTDs could also stain lipid droplets at very low concentrations and were visualized preferentially at the blue channel.