Elsevier

Renewable Energy

Volume 165, Part 2, March 2021, Pages 42-51
Renewable Energy

Quantification of water in bioethanol using rhodamine B as an efficient molecular optical probe

https://doi.org/10.1016/j.renene.2020.11.041Get rights and content

Highlights

  • Absorption and fluorescence techniques were used to quantify the water content in bioethanol.

  • Rhodamine B (RhB) was applied as an optical probe.

  • RhB absorption and emission were precisely applied for determining the water content.

  • Analytical and cross-validation results confirmed the robustness of the proposed methods.

  • MD simulations confirmed that the RhB optical features are correlated with its hydration level.

Abstract

The present study reports the use of ultraviolet–visible absorption, steady-state fluorescence, and time-resolved fluorescence to determine the water content in ethanol by using Rhodamine B as an optical probe. The experiments were performed by preparing water/ethanol blends with different percentages of water in the presence of the optical probe. The absorbance, fluorescence intensity, and fluorescence lifetime values were linearly dependent on the water content in the blends in the range between 0 and 10% (w/w). The results also revealed that the three techniques have a limit of detection for water 1.4% (w/w). Besides, molecular dynamics simulations were carried out to evaluate the interaction of RhB with water and ethanol molecules when subjected to different water content in the blends. The simulations revealed that water molecules perform well-oriented dipole-dipole interactions (hydrogen bonding) with chromophores/fluorophores groups of Rhodamine B, affecting its absorption and emission characteristics, and altering the microenvironment density and viscosity. The present findings point out that common optical techniques can be used to develop a simple, rapid, portable, and precise approach to monitor water in ethanol as far as RhB be used as a probe.

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:

References (49)

  • X. Jin et al.

    A highly sensitive and selective fluorescence chemosensor for Fe3+ based on rhodamine and its application in vivo imaging

    Sensors Actuators B Chem

    (2017)
  • W. Humphrey et al.

    VMD: visual molecular dynamics

    J. Mol. Graph.

    (1996)
  • U. Bilibio et al.

    Enhanced simultaneous electroanalytical determination of two fluoroquinolones by using surfactant media and a peak deconvolution procedure

    Microchem. J.

    (2014)
  • M.A.G. Trindade et al.

    Development of an HPLC–UV/Vis method for the determination of dyes in a gasoline sample employing different pre-treatments

    Fuel

    (2010)
  • F.L. Arbeloa et al.

    Fluorescence self-quenching of the molecular forms of Rhodamine B in aqueous and ethanolic solutions

    J. Lumin.

    (1989)
  • F. López Arbeloa et al.

    Influence of the molecular structure and the nature of the solvent on the absorption and fluorescence characteristics of rhodamines

    Chem. Phys.

    (1989)
  • F.K. Coradin et al.

    Etched fiber bragg gratings sensors for water-ethanol mixtures: a comparative study

    J. Microwaves, Optoelectron. Electromagn. Appl.

    (2010)
  • (ANP) Brazilian National Agency of Petroleum, ANP, Resolut. N°. 19 20.04

    (2015)
  • T.A. Chimenez et al.

    Fluorescence as an analytical tool for assessing the conversion of oil into biodiesel

    J. Fluoresc.

    (2012)
  • J.B. Grimm et al.

    The chemistry of small-molecule fluorogenic probes

  • J.B. Grimm et al.

    Synthesis of rhodamines from fluoresceins using pd-catalyzed c-n cross-coupling

    Org. Lett.

    (2011)
  • A.P. Demchenko et al.

    Monitoring biophysical properties of lipid membranes by environment-sensitive fluorescent probes

    Biophys. J.

    (2009)
  • F.L. Arbeloa et al.

    Adsorption of rhodamine 3B dye on saponite colloidal particles in aqueous suspensions

    Langmuir

    (2002)
  • O. Guvench et al.

    CHARMM additive all-atom force field for carbohydrate derivatives and its utility in polysaccharide and carbohydrate-protein modeling

    J. Chem. Theory Comput.

    (2011)
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