Fluorescent Schiff Base Chemosensors for Selective Ion Detection: A Brief Review

Dr Pritam D. Torawane ^a*

^a Department of Chemistry, Vidya Vikas Mandals, Sitaram Govind Patil Arts, Science and Commerce College, Sakri 424304 (MS), India

*Corresponding author (Pritam Torawane): E-mail: pritamtorawane@gmail.com

Abstract

In the field of supramolecular chemistry, the detection of ions by using fluorescence and absorption techniques have gained significant importance due to their simplicity, high sensitivity and selectivity, low cost, detection limit, rapid response, and applicability to bioimaging. In recent years, Schiff-based receptors have been developed for the detection of various ions. This study mainly focuses on the fluorescent sensors which are based on Schiff base. 

Keywords: Chemosensor, Schiff base, Fluorescence, Molecular recognition. 

Introduction

In chemistry, environment, medicine and biology, cations play a vital role. In biological processes such as maintaining potentials across cell membranes, triggering muscle contraction, metal cations play an important role. On the other hand, some cations, such as lead and mercury, can cause harmful effects to the human body and the environment.In medical diagnostics, catalysis, environmental chemistry and physiology, several neutral and ionic species find extensive applications [1,2]. Excess accumulation of toxic ions may cause somesevere neurodegenerativediseases, such as Parkinson’s disease, Alzheimer’sdisease, amyotrophic lateral sclerosis (ALS), and Wilson’sdisease in the human body.

From above it is clear that detection of ions is necessary either they are useful or harmful. Fluorescent molecular sensors are used for detecting ions. Since fluorescent sensors are highly selective and easy to operate, they play an important role in many areas and disciplines.  The molecular recognition of cations and anions by using absorption and fluorescence technique are receiving great interest in the field of supramolecular chemistry [3-6]. Cation complexation chemistry has played a significant role in the origin of the field of molecular recognition, since in many areas,cations play an important role. For the detection of cations, anions and biomolecules wide range of highly selective and sensitive chemosensors have been developed [7, 8].

The present review highlights recent progress in Schiff base fluorescent probes used for sensing biologically and environmentally significant ions.

Sensors for Nickel

Liu et al. reported a highly selective colorimetric chemosensor (1) for detection of Ni²⁺ ions in aqueous system DMSO-H₂O (v/v = 1:1, pH= 7.4). The addition of 10 equivalent of Ni²⁺ to the aqueous solution of probe (1) results into a dramatic colour change from yellow to red. Absorbance spectra of probe (1) showed a new peak at about 525 nm. Titration plots in UV-visible spectra revealed 1:1 stoichiometry between (1) and Ni²⁺. Interference study shows that no significant changes in the UV-visible spectra was found with and without the other competing metal ions. The detection limit was found to be 2.2×10^-7M. [9].

Fegadeet al. reported a fluorescent receptor (2) for the determination of Ni²⁺ in semi-aqueous media DMSO-H₂O (1:1, v/v) solvent system. Upon addition of Ni²⁺ ion solution prepared in distilled water to the aqueous solution of receptor (2) gave remarkable fluorescent enhancement. Also, addition of 10 equivalent of Ni²⁺ to the aqueous solution of probe (2) causes color change from colorless to yellow. Interference study showed that interference of other tested metal ion in the detection of Ni²⁺ was insignificant. Job’s plot experiment indicates the formation of 1:1 complex between probe (2) and Ni²⁺[10].

Sensors for Zinc

Khairnar et al. reported a highly selective fluorescent ‘turn on’ chemosensor (3) for the detection of Zn²⁺ in DMSO-H₂O (90:10, v/v) solvent. The weak fluorescence of probe (3) was enhanced with red shift from 360 nm to 385nm (Δλ=25). The probe (3) was successfully applied for detection of Zn²⁺ in live HeLa cells. Interference study shows that probe (3) has high selectivity toward Zn²⁺even in the presence of same concentration of other metal ions. Job’s plot indicates 1:1 binding ratio between Zn²⁺ and probe (3). The detection limit was found to be 0.67 µM [11].

Tayade et al. reported a novel chemosensor (4) based on isonicotiamide for the detection of Zn²⁺. The probe (4) also shows selectivity towards HSO₄^–. Weakly fluorescent probe (4) showed highly selective enhancement in the emission wavelength at 470 nm for Zn²⁺. Interference study showed that no significant variation was observed in the fluorescence of probe (4) with Zn²⁺in the presence and absence of other cations.  LOD of probe (4) as a fluorescent sensor for the analysis of Zn²⁺ was found to be 3.81 nM. The fluorescence properties of probe (4) were effectively clarified by two chemical input (Zn²⁺ and HSO₄^–) OR INHIBIT type logic gates at molecular level [12].

Sensors for Copper

Yeh et al. reported a coumarin-based sensitive and selective fluorescent sensor (5) for the detection of Cu²⁺. In presence of Cu²⁺ probe (5) shows significant fluorescence quenching. Probe (5) upon addition of Cu²⁺ shows visible colour change from yellow to orange. Titration of Cu²⁺ with probe (5) shows that the absorbance at 487 nm decreased and new band at 440 nm was produced. Interference study shows that in presence of other competing metal ions no significant changes were observed in fluorescence spectra of probe (5) with Cu²⁺. Job’s plot revealed that 2:1 binding stoichiometry between Cu²⁺ and probe (5). The probe has limit of detection of 0.27µM. Moreover, probe (5) could be successfully used as a fluorescent probe for detection of Cu²⁺ in living cells [13].

Yang et al. reported a colorimetric and fluorescent sensor (6) for Cu²⁺ detection in methanol-water (3/7, v/v) solvent system. Upon addition of Cu²⁺ to the probe (6) results into enhancement of the absorbance with formation of new peak at 552 nm and the color of the solution changes from colorless to pink. When more than 1.0 equivalent of Cu²⁺ was added the enhancement was saturated indicating that the binding mode was probably of 1:1 stoichiometry. Same conclusion about binding mode was obtained from Job’s plot. The limit of detection for Cu²⁺ was found to be 0.096 µM. The competition experiments showed that no visible color change was observed and no change in the fluorescence spectra of probe (6) with Cu²⁺were observed in the presence of other competing metal ions. The probe (6) was successfully applied for the fluorescence imaging in living cells [14].

Sanmartin-Metalobos et al. Synthesized a fluorescent probe (7) for detection of Cu²⁺ in aqueous samples.  Spectroscopic studies show that probe (7) has higher affinity towards copper than other tested d-block metal ions. The detection limit of probe (7) for Cu²⁺ ion was found to be 8.7 nM [15].

Zhang et al. reported a novel chemosensor (8) for detection of Cu²⁺ ion in DMSO solution. Chemosensor (8) showed visible colour change from yellow to colorless on treatment with Cu²⁺ ion. The detection limit of chemosensor (8) forCu²⁺ ion was found to be 4.87 nM. The binding constant for chemosensor (8) and Cu²⁺ ion was determined as 6.15×10¹⁰ M^-1. Job’s plot revealed that 1:2 binding stoichiometry of the complex between chemosensor (8) and Cu²⁺ ion [16].

Sensors for Magnesium

            Kao et al. synthesized a turn on Schiff base fluorescent sensor (9) for the detection of Mg²⁺. The probe (9) alone shows no significant emission after excitation at 353 nm but upon addition of Mg²⁺, the fluorescence intensity of probe (9) increases significantly at the wavelength of 487 nm. Also, the probe (9) shows weak fluorescence enhancement upon addition of Ca²⁺ and Cd²⁺. On the other hand, probe (9) shows very weak fluorescent band towards other metal ions. The probe (9) was successfully applied to detect Mg²⁺ in different sources of water such as lake, ground and tap water. Job’s plot clearly shows 1:1 binding stoichiometry between probe (9) and Mg²⁺.The detection limit for probe (9) for the analysis of magnesium was found to be 19.1 ppb. The association constant for probe (9) and Mg²⁺ was determined as 1.91×10⁷M^-1 [17].

Wang et al. reported a turn on fluorescent sensor (10) based on Schiff base derivative for the detection of Mg²⁺. Fluorescence spectra show that upon addition of Mg²⁺ ion to the probe (10) displayed significant fluorescence enhancement with emission maximum at 547 nm due to the Photo-Induced Electron Transfer (PET) effect. No other metal ion except Mg²⁺shows change in the absorption and fluorescence spectra of probe (10). Interference study shows that even in presence of other competing metal ions probe (10) shows similar spectral changes that with Mg²⁺ion. Based on Benesi-Hildebrand the association constant for complex (10)-Mg²⁺ were determine as 3.33×10⁴ M^-1. The detection limit was found to be 5.16×10^-7M [18].

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