Quantification of water in bioethanol using rhodamine B as an efficient molecular optical probe
Graphical abstract
Introduction
Bioethanol is a renewable fuel produced from the fermentation of a wide range of biomass. It has gained prominence among the existing biofuels because it can be used directly in automotive engines with economic and environmental advantages when compared to fossil fuels [1,2]. Nevertheless, rigorous quality control is required to guarantee a smooth operation and maintenance of the engines. Despite its amphiphilic property, ethanol is an organic compound highly miscible in water due to its polar hydroxyl group. As it can be easily adulterated by water, coloring is added to anhydrous ethanol to identify water contamination [3]. Nonetheless, this procedure is not used in hydrous ethanol fuel [4].
Karl Fisher method is usually adopted by national regulatory agencies as a standard procedure to quantify the water content in fuels. For instance, this titration method has been established by the National Petroleum, Natural Gas and Biofuels Agency (ANP) of Brazil to determine the water content in hydrous ethanol fuel, which could not exceed 4.9% (v/v) or 7.4% (w/w) [5]. However, a real-time assessment of the biofuel condition using this method is difficult since it requires extraction and chemical treatment of the samples. Consequently, the development of fast, precise, and low-cost approaches to detect water adulteration in ethanol fuel is needed. In this scenario, optical-based methods have emerged as promising analytical tools for applications in the biofuel field, such as for monitoring and evaluating the production [6], oxidative stability [7], and physico-chemical features [8,9].
Small molecules have been suggested as optical probes to investigate a wide range of chemical, physical, and biological processes, such as chemical reactions, protein inhibition monitoring, substrate catalysis, and determination of microenvironment properties [10,11]. A molecule can be used as an optical probe when its optical feature is sensitive to the changes promoted by the process to be monitored. Nowadays, it is well established that small molecules can provide details about the microenvironment surrounding themselves, such as polarity, hydration, viscosity, and others [12]. For example, the phenoxazine dye Nile Red, which presents a low emission at 657 nm in aqueous solution, has its emission drastically shifted to 565 nm in a nonpolar media. Consequently, Nile Red has been applied as an optical probe for lipid analysis in biological systems [10,13].
Rhodamine B (RhB) is widely used as a dye in molecular spectroscopy due to its high fluorescence quantum yield, rigid structure, high stability under different pH, and high resistance to oxidation [14,15]. For instance, rhodamine-based fluorescent probes have been applied for the determination of Cu2+ in cellular membranes [16], toxic thiophenols in natural water and living cells [17], and Hg2+ in biological samples [18]. In this context, the present study evaluated the potential of three optical methodologies to quantify the water content in water/ethanol blends by using RhB as an optical probe. The optical behavior of the RhB as a function of water content was investigated by UV–vis absorption, steady-state, and time-resolved fluorescence. Additionally, molecular dynamics (MD) simulations assessed the RhB solvation by water and ethanol in different blends, providing a detailed picture of the solvation shell and its impact on the optical features of RhB.
Section snippets
Chemicals
Ethyl alcohol (UV-HPLC grade, Vetec, Brazil), deionized water, and Rhodamine B (HPLC standard, Sigma-Aldrich, Brazil) were used.
Samples preparation
Water/ethanol blends were prepared, varying the water content in steps of 1% (w/w) in the range from 0 to 15% (w/w) of water. Samples ranging from 20 to 100% (w/w) were also produced at intervals of 10% (w/w). Hereafter, sample Wx represents x% (w/w) of water in the blend. Each sample was prepared with a total weight of 50 g. A stock solution of RhB in ethanol at a
Optical characterization
Fig. 1 shows the UV–vis absorption, steady-state fluorescence, and fluorescence decay curves of RhB in ethanol and how its optical features are changed with the addition of water in the solution. The increase in the water content caused an enhancement of the RhB absorbance (Fig. 1b), followed by a 10-nm redshift of the absorption spectrum, especially in the 450–610 nm range (Fig. 1a). The steady-state fluorescence spectra of RhB between 550 and 750 nm under excitation at 550 nm are displayed in
Conclusion
The present study demonstrates for the first time that the optical spectroscopy based on UV–Vis absorption, steady-state fluorescence, and time-resolved fluorescence can be applied as analytical techniques for the quantification of water adulteration in ethanol by using RhB as an optical probe. The data revealed that the changes in the optical feature of RhB are directly correlated with its hydration level (i.e., water accumulation close to the RhB surface) and consequent microenvironment
CRediT authorship contribution statement
Wilson E. Passos: Investigation, Methodology, Writing - original draft. Ivan P. Oliveira: Investigation, Software. Flávio S. Michels: Investigation, Methodology. Magno A.G. Trindade: Formal analysis, Validation. Evaristo A. Falcão: Investigation, Formal analysis. Bruno S. Marangoni: Validation, Software. Samuel L. Oliveira: Formal analysis, Resources, Writing - review & editing. Anderson R.L. Caires: Funding acquisition, Project administration, Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the Brazilian funding agencies CNPq, CAPES, and FUNDECT, FINEP (grant number: 04.13.0448.00/2013), and FAPESP (I.P.O. postdoc fellowship process number 2017/02201-4). The authors also acknowledge the financial support provided by the CAPES-PrInt funding program (grant numbers: 88887.353061/2019–00, 88881.311921/2018-01 and 88887.311920/2018–00) and the National Institute of Science and Technology of Basic Optics and Optics Applied to Life Science (grant number:
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