A highly selective ‘turn-on’ fluorescent sensor for Zn2+ based on fluorescein conjugates

Synthesis and photophysical investigation of Schiff base as a Mg2+ and Zn2+ fluorescent chemosensor and its application

In view of the fluorescent switching properties and anti-fatigue properties of diarylethene, a diarylethene fluorescent chemosensor for the immediate detection of zinc ion (Zn²⁺ ) and magnesium ion (Mg²⁺ ) in acetonitrile was synthesized in this article. The structure of 1o was determined by performing spectroscopy and elemental analysis. The presence of Zn²⁺ or Mg²⁺ made the chemosensor 1o show an obvious "turn-on" fluorescent signal (bright yellow-green for Mg²⁺ and bright cyan for Zn²⁺ ). The fluorescent change caused by the 1:1 binding of 1o and Zn²⁺ or Mg²⁺ might be due to hindering the excited-state intramolecular proton transfer (ESIPT) process, which were bolstered by Benesi-Hildebrand analysis, Job's plot curves, proton nuclear magnetic resonance (¹ H-NMR) titration and mass spectrometry. The limits of detection were acquired from the standard curve plots for Mg²⁺ at 44.6 nM and for Zn²⁺ at 14 nM. Based on the fluorescent behaviors, a logic gate was constructed with the emission intensity at 528/518 nm as output signal, the ultraviolet-visible (UV-vis) lights, Mg²⁺ /Zn²⁺ and EDTA as input signals. Exogenous Zn²⁺ and Mg²⁺ fluorescent bioimaging were performed on Hela cells with 1o, indicating its potential application in biodiagnostic analysis. In particular, 1o was manufactured into test paper, and Zn²⁺ or Mg²⁺ can be conveniently, efficiently and qualitatively identified by the fluorescent color variation of the test strips.

Design of a Highly Selective Benzimidazole-Based Derivative for Optical and Solid-State Detection of Zinc Ion

A novel series of benzimidazole-based molecules mimicking biological receptors, which exhibit selective coordination with zinc ions, were designed and synthesized. The photochromic behavior of these derivatives with various metal ions suggests a selective interaction of one of the receptors 2-(pyridin-2-yl)-4,7-di(thiophen-2-yl)-3H-benzo[d]imidazole (2c) with zinc ion. The lower limit of detection by photoluminescence quenching was determined to be 16 nM. The mechanism of selective complexation was elucidated by ¹H nuclear magnetic resonance titrations and dynamic light scattering analysis. The stoichiometry of the formation of the Zn(2c)₂ complex was evaluated by single-crystal X-ray diffraction and mass spectral techniques and calculated to be 2:1 (L:M). A change in the electronic energy levels on the sensor analyte interaction was observed by both ultraviolet photoelectron spectroscopy analysis and by density functional theory calculations, suggesting an electroactive semiconductor behavior. A symmetric Schottky structured sensor device was fabricated using the receptor 2c as the active sensing layer. A distinct change in current-voltage characteristics between the receptor and the complex suggests that the fabricated device could be used as a solid-state sensor for detecting zinc ion.

Fluorescein Based Fluorescence Sensors for the Selective Sensing of Various Analytes

Fluorescein molecules are extensively used to develop fluorescent probes for various analytes due to their excellent photophysical properties and the spirocyclic structure. The main structural modification of fluorescein occurs at the carboxyl group where different groups can be easily introduced to produce the spirolactam structure which is non-fluorescent. The spirolactam ring opening accounts for the fluorescence and the dual sensing of analytes using fluorescent sensors is still a topic of high interest. There is an increase in the number of dual sensors developed in the past five years and quite a good number of fluorescein derivatives were also reported based on reversible mechanisms. This review analyses environmentally and biologically important cations such as Cu2+, Hg2+, Fe3+, Pd2+, Zn2+, Cd2+, and Mg2+; anions (F-, OCl-) and small molecules (thiols, CO and H2S). Structural modifications, binding mechanisms, different strategies and a comparative study for selected cations, anions and molecules are outlined in the article.

A novel rhodamine-based colorimetric and fluorometric probe for simultaneous detection of multi-metal ions

Effective and simultaneous detection of multi-metal ions has been achieved by a colorimetric and fluorometric probe (REHBA) synthesized from rhodamine hydrazide and polyhydroxyl aromatic aldehyde. REHBA can serve as a colorimetric detector for Cu²⁺ and Co²⁺, and a fluorometric probe for Pb²⁺. The colorless solution of REHBA changes to pink for Cu²⁺/Co²⁺ and shows a remarkable fluorescence for Pb²⁺. The further differentiation of Cu²⁺ and Co²⁺ depends on whether the colorimetric response of REHBA is reversible upon addition of ethylene diamine tetraacetic acid. The response is reversible for Cu²⁺, while it is not for Co²⁺. The spirolactam ring-opening in REHBA and the formation of REHBA-metal complexes with binding stoichiometric ratio of 1:1 are responsible for the UV-visible and fluorescence behaviors. REHBA shows excellent selectivity, anti-interference and good sensitivity. The limit of detection of Cu²⁺, Co²⁺ and Pb²⁺ is 0.11 μM, 0.88 μM and 0.73 μM, respectively. In addition, REHBA has been applied to recognize Pb²⁺ in living cells by fluorescence image and Cu²⁺, Co²⁺ and Pb²⁺ in real water samples, indicating that REHBA is a potential candidate for multi-metal-ions detection.

Fluorescein derived Schiff base as fluorimetric zinc (II) sensor via 'turn on' response and its application in live cell imaging

A novel Schiff base L composed of fluorescein hydrazine and a phenol functionalized moiety has been designed and prepared via cost-effective condensation reaction. The L is utilized for selective sensing of Zn²⁺ over other environmental and biological relevant metal ions in aqueous alcoholic solution under physiological pH range. The binding of Zn²⁺ to the receptor L is found to causes ~23 fold fluorescence enhancement of L. The 1:1 binding mode of the metal complex is established by combined UV-Vis, fluorescence, and HRMS (high-resolution mass spectroscopy) spectroscopic methods. The binding constant (K_a) for complexation and the limit of detection (LOD) of Zn²⁺ is calculated to be 2.86 × 10⁴ M^-1 and 1.59 μM, respectively. Further photophysical investigations including steady-state, time-resolved fluorescence analysis and spectral investigations including NMR (nuclear magnetic resonance), IR (infrared spectroscopy) suggest introduction of CHEF (chelation enhance fluorescence) with the suppression of CN isomerization and PET (photo-induced electron transfer) mechanism for the strong fluorescent response towards Zn²⁺. Finally, the sensor L is successfully employed to monitor a real-time detection of Zn²⁺ by means of TLC (thin layer chromatography) based paper strip. The L is used in the cell imaging study using African green monkey kidney cells (Vero cells) for the determination of exogenous Zn²⁺ by Immunofluorescence Assay (IFA) process.

Selective and Sensitive Fluorescein-Benzothiazole Based Fluorescent Sensor for Zn2+ Ion in Aqueous Media

An efficient fluorescent sensor based on fluorescein-benzothiazole (FB) for Zn²⁺ ion was synthesized and characterized systematically. FB exhibited selective and sensitive recognition toward Zn²⁺ in MeCN-H₂O (v/v = 2/1) over other cations due to the spirolactam ring-opening power of Zn²⁺. The complexation property of FB with Zn²⁺ ion was examined by ¹H NMR, ¹³C NMR and FTIR experiments. The stoichiometric ratio of the FB-Zn²⁺ complex was determined from a Job plot to be 1:1. The binding constant (K _a) of Zn²⁺ binding to FB was found to be 4.22 × 10⁴ M^- 1, with a detection limit of 5.64 μM. In addition, the practical utility of FB was explored in the form of test strips. Graphical Abstract.

Development of a coumarin-furan conjugate as Zn2+ ratiometric fluorescent probe in ethanol-water system

In this study, a novel coumarin-derived compound bearing the furan moiety called 7-diethylamino-3-formylcoumarin (2'-furan formyl) hydrazone (1) has been designed, synthesized and evaluated as a Zn²⁺ ratiometric fluorescent probe in ethanol-water system. This probe 1 showed good selectivity and high sensitivity towards Zn²⁺ over other metal ions investigated, and a decrease in fluorescence emission intensity at 511nm accompanied by an enhancement in fluorescence emission intensity at 520nm of this probe 1 was observed in the presence of Zn²⁺ in ethanol-water (V : V=9 : 1) solution, which provided ratiometric fluorescence detection of Zn²⁺. Additionally, the ratiometric fluorescence response of 1 to Zn²⁺ was nearly completed within 0.5min, which suggested that this probe 1 could be utilized for sensing and monitoring Zn²⁺ in environmental and biological systems for real-time detection.