Phenolphthalein: From Invisible Molecules to Visible Marvels

Introduction

The narrative of Phenolphthalein's discovery is a tapestry woven with threads of ingenuity and perseverance. Originating from the laboratory of German chemist Adolf von Baeyer in 1871, its synthesis marked the dawn of a new era in organic chemistry. However, it was not until the early 20th century that researchers began to unravel its potential as an acid-base indicator, bestowing upon it the name "Phenolphthalein," derived from its phenol and phthalic acid precursors. This article will introduce other applications of Phenolphthalein.

Nanomaterials¹

In one work by Faridah M. Marsin and colleagues, synthesis and characterization of phenolphthalein-immobilized titania (T-phph) and silica–titania (ST-phph) nanomatrix is reported. The thin films are deposited by sol–gel method at low temperature. The effect of host–guest chemistry in matrices, on the surface structures, optical and sensing activity of the resultant thin films is studied. The phenolphthalein-immobilized fabricated nanoparticles/nanomatrices are analyzed by field emission scanning electron microscope, energy-dispersive X-ray spectroscopy, atomic-force microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, surface analysis, thermogravimetric analysis, and UV–Vis spectroscopy. Thermally stable and high surface area homogeneous nanoparticles, containing nanocrystalline anatase phase with low refractive index (1.58), low roughness (5.5 nm), and high transparency (95 %) are obtained for phenolphthalein-immobilized ST-phph nanomatrix. Moreover, smaller nanoparticles (56–121 nm) with good incorporation of dye and good response of sensing are obtained. The sensor response is optimized at pH 12 with 10.1 pKa value at 555 nm.

Membranes²

Poly(arylene ether sulfone)s with cardo groups, especially phenolphthalein-based cardo PESs, possess good solubilities, excellent mechanical toughness, thermal stability, and easy functionalization, and they would attract more and more attentions in the future. Besides the applications of separation membranes, including ion exchange, water purification, and gas separation membranes, more application fields could be found for cardo PESs, which may be lithium batteries, electrochemical sensors, dielectric materials, coatings, glass-reinforced plastics, and so on.

There are still problems with cardo PES that must be resolved in the future. For example, there are lactone or lactam groups in phenolphthalein-based cardo PES. It is not known if the cardo ring would be opened in specific situation or in long-term use, and cause degradation of the polymers or not. And because of the large molecular weight of the phenolphthalein-based cardo structures, the IECs of the ion-functionalized cardo PES, especially the side-chain type polymers, are relatively low, while high IECs polymers could be needed and have better performance in some situation. Introduction of multifunctional groups to the cardo PES or copolymerization with monomers of small molecular weight are possible ways to solve the problem.

Fluorescent Sensor³

In recent study by Ibrahim Yilmaz team, an easy assembled colorimetric and 'turn–on' fluorescent sensor (probe P4SC) based on phenolphthalein was developed for carbonate ion sensing in a mixture of EtOH/H2O (v/v, 80/20, pH = 7, Britton–Robinson buffer) media. The probe P4SC demonstrated high sensitive and selective monitoring toward carbonate ion over other competitive anions. Interaction of carbonate ion with the probe P4SC resulted in a significant increment in emission intensity at λem = 498 nm (λex = 384 nm) due to the strategy of blocking the photo induced electron transfer (PET) mechanism. 1H NMR titration and Job’s methods, as well as the theoretical study were carried out to support the probable stoichiometry of the reaction (1:2) between P4SC and carbonate ion. The binding constant of the probe P4SC with carbonate ion was calculated as 2.56 × 10¹⁰ M 2. The probe P4SC providing rapid response time (~0.5 min) with a satisfactorily low detection limit (14.7 nM) may be useful as a valuable realistic sensor. The imaging studies on the liver cancer cells (HepG2) shows the great potential of the probe P4SC for the sensation of intracellular CO3 2 anions. Furthermore, the satisfactory recovery and RSD values obtained for water application confirming that the probe P4SC could be applied to sensing of carbonate ion.

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