Developments in the field of fluorescent chemosensors for trace level determination of Al³⁺ have been described. Design, synthesis and applications of Al³⁺ selective fluorescent sensors have attracted a great deal of attention due to their versatile and emerging biological implications. A systematic description of different fluorescent probes based on their fluorophore skeleton has been made. The sensing mechanism of each fluorescent probe has been briefly discussed. Special emphasis has been given to consideration of the greenness of the methods, particularly, the use of greener solvents like water. The review covers published work on fluorescence sensing of Al³⁺ worldwide together with our own contribution in this field.

An efficient fluorescent Al³⁺ receptor, N-(2-hydroxy-1-naphthalene)-N′-(2-(2-hydroxy-1-naphthalene)amino-ethyl)-ethane-1,2-diamine (L) has been synthesized by the condensation reaction between 2-hydroxy naphthaldehyde and diethylenetriamine. High selectivity and affinity of L towards Al³⁺ in ethanol (EtOH) as well as in HEPES buffer at pH 7.4, makes it suitable to detect intracellular Al³⁺ with fluorescence microscopy. Metal ions, viz.Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, Hg²⁺ and Pb²⁺ do not interfere. The lowest detection limit for Al³⁺ is 3.0 × 10⁻⁷ M and 1.0 × 10⁻⁷ M in EtOH and HEPES buffer respectively.

An Al³⁺ selective ratiometric fluorescent probe (L) has been synthesized by condensation of 4,5-diaminopyrimidine with 1-pyrenecarboxaldehyde. The structure of L has been confirmed by single crystal X-ray analysis. In the presence of Al³⁺, L exhibits a considerable excimer emission at 445 nm along with the decrease of its monomer emission at 368 nm in DMSO–H₂O (4 : 1, v/v). The lowest detection limit for Al³⁺ is 0.24 μM. Furthermore, L can permeate through the cell membrane and detects intracellular Al³⁺ ions under a fluorescence microscope.

2-((Naphthalen-6-yl)methylthio)ethanol (HL) was prepared by one pot synthesis using 2-mercaptoethanol and 2-bromomethylnaphthalene. It was found to be a highly selective fluorescent sensor for Al³⁺ in the physiological pH (pH 7.0–8.0). It could sense Al³⁺ bound to cells through fluorescence microscopy. Metal ions like Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, Hg²⁺, Cr³⁺ and Pb²⁺ did not interfere. No interference was also observed with anions like Cl⁻, Br⁻, F⁻, SO₄ ²⁻, NO₃ ⁻, CO₃ ²⁻, HPO₄ ²⁻ and SCN⁻. Experimentally observed structural and spectroscopic features of HL and its Al³⁺ complex have been substantiated by computational calculations using density functional theory (DFT) and time dependent density functional theory (TDDFT).

A weakly fluorescent cobalt(II) complex is synthesized using 2-(2-pyridyl)-benzimidazole (PBI) as a chelating fluorescent ligand and characterized by single crystal X-ray structure. This complex serves as an efficient fluorescent probe for trace level determination of aspartic acid (AspA) and glutamic acid (GluA) in aqueous solution. Rest of the naturally occurring amino acids did not interfere. Both aspartic acid and glutamic acid replaces PBI from the coordination sphere of Co(II)-PBI complex resulting appearance of strong fluorescence signal for the released free PBI. The signal response is very fast and the interaction of both the AspA and GluA with the Co(II) is strong enough as evident from their displacement equilibrium constant values, viz. 4357.8 M⁻¹ and 8333.33 M⁻¹ respectively.

A naphthalene–thiophene hybrid molecule (Z)-1-((thiophen-2-ylmethylamino)methylene)naphthalen-2(1H)-one (HL) was prepared by condensation of 2-thiophenemethylamine and 2-hydroxy-1-naphthaldehyde. According to FTIR, ¹H NMR spectrometry and single crystal X-ray analysis, HL exists in the cis-keto-amine tautomeric form. HL behaves like a molecular AND type binary logic gate with two inputs viz. Zn²⁺ and OAc⁻ ions whereby the fluorescence of the system turns on. The structures of HL and its zinc acetate complex were also optimized by DFT calculations. HL binds Zn²⁺ in a 1 : 1 ratio with an association constant K_a = 2.05 × 10⁴ M⁻¹, and detects Zn²⁺ as low as 3 × 10⁻⁸ M. HL is useful for the detection of intracellular Zn²⁺ under a fluorescence microscope.