GO/ionic surfactant inspired photophysical modulation of rhodamine B in Reline with or without additives Vishwajit Chavda, Darshna Hirpara, Sanjeev Kumar ⇑ Applied Chemistry Department, Faculty of Technology & Engineering, The Maharaja Sayajirao University of Baroda, Vadodara 390 002, India article info Article history: Received 14 September 2022 Revised 6 October 2022 Accepted 13 October 2022 Available online 18 October 2022 Keywords: Fluorescence sensor Deep eutectic solvent Rhodamine B Surfactant micelle Graphene oxide Sustained release abstract Photophysical behaviour of rhodamine B (RB) in deep eutectic solvents (DES, formed by quaternary ammonium salt and hydrogen bond donor (HBD) in a specific eutectic ratio) with or without graphene oxide (GO) or ionic surfactants, is less known. The nature of Reline (choline chloride (ChCl): urea (HBD), 1:2), a well-known DES, has been designed by adding glycerol or water as the second HBD for sus- tained movement of RB. Effects of GO, surfactant, or GO + surfactant, in controlling RB movement, at var- ious sites (GO surface, surfactant micelle, DES surface, or background solvent), have been fluorometrically reported. The basic nature of Reline (pH = 10.38) causes modification of GO surface (deprotonated site) and nature of RB (cationic ? zwitter ionic). Above Reline-inspired changes have been found to modify interactions of RB with GO and/or sodium dodecyl sulphate (SDS, an anionic surfactant) or cetyl trimethyl ammonium bromide (CTAB, a cationic surfactant). SDS (10 mMdm 3 , < critical micelle concentration (CMC) in Reline) shows  2.6, 1.6, and 1.4 fold fluorescence intensity enhancement of RB (in water, pure Reline, and methanol, respectively). However, GO and/or CTAB shows quenching behaviour. Further, the fluorescence of RB shows weak dependence on changing the second HBD (water or glycerol). DES- controlled cationic vs zwitterionic form of RB is responsible for the interaction and sustained movement towards GO surface, micellar surface, or negatively charged ion-pair formation (with SDS monomers). Findings of the work have implications in searching potential fluorescent levels/sensors for photophysics, photobiology, or wider vehicle means for sustained drug delivery. Ó 2022 Elsevier B.V. All rights reserved. 1. Introduction In the last decades, fluorescent chemo-sensors have been pre- ferred over other conventional analytical tools due to their better sensitivity and selectivity [1-3]. Rhodamine dyes (Rhodamine B (RB) and derivatives) have been widely used in single-molecule detection, fluorescence leveling, or for DNA sequencing [4,5]. The choice of solvent medium and dye concentration is a pre- requisite for the specific use. In the past, the majority of the work addressed individual solvent effects with a limited concentration range [6-8]. In a later study, it has been reported that solvent polar- ity affects the linear and non-linear properties of the chro- mophores [9]. The photophysical properties of rhodamine dyes made them important members of laser dyes [10]. The association behaviour of ionic dyes is distinctly influenced by several factors such as concentration, pH, temperature, and nature of the medium [11-14]. In a few studies, it has been shown that solvent nature governs the photophysical process shown by fluorescent molecules with or without nanomaterials of the graphene family [14-17]. Therefore, a wider window exists to investigate the photophysics of the systems involving carbon-based nanomaterial, dye mole- cules, and solvents with a mechanism of fluorescence modulation. In light of the above facts, the role of solvent is decisive in molecular-level interactions of chromophores with biomolecules [18]. Recently, a new solvent system based on the process of the exchange of hydrogen bonds between hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) has been introduced as the solvent of the 21st century [19]. These solvent systems are greener and more benign and popularly known as ‘Deep Eutectic Solvent’ (DES) [20-22]. DESs have many characteristics analogous to conventional ionic liquids (ILs) [22]. Since ILs are reported to form mixed micelles with surfactants, one can expect DES- inspired modification of the solution behaviour of a surfactant https://doi.org/10.1016/j.molliq.2022.120614 0167-7322/Ó 2022 Elsevier B.V. All rights reserved. Abbreviations: DES, Deep eutectic solvent; GO, Graphene oxide; HBD, Hydrogen bond donor; RB, Rhodamine B; CMC, Critical micelle concentration. ⇑ Corresponding author at: Department of Applied Chemistry, Faculty of Tech- nology & Engineering, The Maharaja Sayajirao University of Baroda, Vadodara 390 001, Gujarat, India. E-mail address: sanjeevkumar-appchem@msubaroda.ac.in (S. Kumar). Journal of Molecular Liquids 368 (2022) 120614 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq