Dyes and Pigments 173 (2020) 107944 Available online 1 October 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved. Spectroscopic study of fuorescein immobilized on anodic porous alumina in aqueous solutions of different pH Shohei Yamaguchi a , Kazuhiro Shimada b , Kazunori Matsui c, * a Institute of Science and Technology, Kanto Gakuin University, 1-50-1, Mutsuurahigashi, Yokohama, 236-8501, Japan b Department of Electrical and Electronic Engineering, College of Science and Engineering, Kanto Gakuin University, 1-50-1, Mutsuurahigashi, Yokohama, 236-8501, Japan c Department of Applied Chemistry, College of Science and Engineering, Kanto Gakuin University, 1-50-1, Mutsuurahigashi, Yokohama, 236-8501, Japan A R T I C L E INFO Keywords: Fluorescein Absorption Fluorescence Anodic porous alumina Adsorption ABSTRACT Fluorescein, a popular fuorescent dye, can exist in various prototropic forms, depending on the pH of the so- lution. We studied the spectroscopic properties of fuorescein immobilized on anodic porous alumina (APA) dipped in aqueous fuorescein solutions with various pH values. Fluorescein was successfully immobilized on the APA surface to obtain a golden color between pH 5 and 6. From the absorption and fuorescence studies, we concluded that fuorescein is dominantly immobilized on APA in a monoanionic form with minor contribution from the dianionic form. A surface analysis showed that fuorescein molecules can penetrate approximately 500 nm in depth from the APA surface, which is almost the surface of the pores, considering an oxide flm thickness of APA (10 μm). These results confrmed that adsorption occurs because of the electrostatic interaction between the carboxylate moiety in monoanionic/dianionic fuorescein molecules and the positively charged APA surface. 1. Introduction Fluorescein is one of the most commonly used fuorophores that has various applications because of its high fuorescence quantum yield and chemical stability [1]. Typical prototropic forms of fuorescein, as illustrated in Fig. 1, include cationic, neutral (zwitterion, quinoid, and lactone), monoanionic (carboxylate, phenolate), and dianionic forms [1–7]. Among the various species, the pKa values of each ionization of the hydroxyl and carboxyl groups indicate that the neutral form (FH 2 ) is a mixture of lactone (70%), zwitterion (15%), and quinoid (15%), and the carboxylate monoanion (CM) is the main species in FH  in aqueous solutions [5]. The fuorescent properties of fuoresceins are highly sen- sitive to the surrounding microenvironment. Therefore, the photo- physics and photochemistry of nanocomposite systems consisting of fuorescein, including its derivatives and inorganic matrices, have recently been of considerable interest owing to scientifc and techno- logical viewpoints such as fuorescent sensors and indicators [8–19]. Subsequently, numerous inorganic matrices such as sol-gel silica, layered double hydroxide (LDH), and zeolite have been investigated by various research groups. In recent years, anodic porous alumina (APA) with highly ordered nanopores has received considerable attention in connection with the preparation of functional nanomaterials and nanocomposites [19–26], as well as conventional protective and coloring aluminum [27]. How- ever, to the best of our knowledge, there is no report on the properties of fuorescein directly entrapped in the pores of APA. One of the conve- nient processes for the preparation of dye/APA nanocomposites is dip- ping the APA in dye solutions. The general trend for adsorption of the dye, with respect to dye-solution pH, shows an ideal pH value in the weakly acidic regions of 3–5 [27]. The entrapment is mainly induced by the electrostatic interactions between the positively charged APA sur- face and negatively charged dye molecules. The charge on the APA surface varies with the pH of the solution and the pore size [28], while fuorescein also changes its structure with the pH, as mentioned earlier. Therefore, it is interesting to study the effect of a bath pH on the in- teractions between the APA surface and fuorescein. In this paper, we report on the absorption and fuorescence properties of a fuorescein-immobilized APA surface prepared under various bath pH conditions. The relationship between the fuorescein immobilization on the APA surface and the properties of the prototropic species are also discussed. * Corresponding author. E-mail address: matsui@kanto-gakuin.ac.jp (K. Matsui). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig https://doi.org/10.1016/j.dyepig.2019.107944 Received 17 July 2019; Received in revised form 30 September 2019; Accepted 30 September 2019