Selective Recognition and Reversible “Turn-Off” Fluorescence Sensing of Acetate (CH3COO−) Anion at Ppb Level Using a Simple Quinizarin Fluorescent Dye

Abstract

A simple and cost-effective optical sensing system based on quinizarin fluorescent dye (QZ) for the selective and reversible sensing of CH₃COO⁻ anions is reported. The anion binding affinity of QZ towards different anions was monitored using electronic absorption and fluorescence emission titration studies in DMSO. The UV-visible absorption spectrum of QZ showed a decrease in the intensity of the characteristic absorption peaks at λ = 280, 323, and 475 nm, while a new peak appeared at λ = 586 nm after the addition of CH₃COO⁻ anions. Similarly, the initial strong emission intensity of QZ was attenuated following titration with CH₃COO⁻ anions. Notably, similar titration using other anions, such as F⁻, Cl⁻, I⁻, NO₃⁻, NO₂^−, and H₂PO₄^-, caused no observable changes in both absorption and emission spectra. The selective sensing of CH₃COO⁻ anions was also reflected by a sharp visual color change from bright green to faint green under room light. Further, the binding was found to be reversible, and this makes QZ a potential optical and colorimetric sensor for selective, reversible, and ppb-level detection of CH₃COO⁻ anions in a DMSO medium.

1. Introduction

The recognition and sensing of anions have gained immense research interest in recent years because of their vital role in various biological, environmental, and industrial processes [1]. For instance, fluoride (F⁻) ions are important for maintaining dental health and treating osteoporosis [2,3], sulphite (SO₃²⁻) ions are used in the preservation industry [4], and CH₃COO⁻ ions play a crucial role in various enzymatic and antibody functions [5]. Superoxide (O₂⁻) ions, which are reactive oxygen species (ROS), exhibit distinctive activity in cell growth and metabolism [6,7]. Phosphate (PO₄³⁻) and halide (X⁻) anions are involved in the activity of enzymes, synthesis of protein, hormone transport, and DNA regulation [8,9,10,11]. Cyanide (CN⁻) ions are frequently employed in metallurgy, electroplating, gold mining, resins, herbicides, and the manufacture of various organic chemicals [12]. Although anions are ubiquitous species and play numerous indispensable roles, overexposure to anions can cause severe disorders and be harmful to the environment [13]. Therefore, the qualitative and quantitative detection of anions is of paramount interest.

Among various anions, the CH₃COO⁻ anion in particular has drawn special research focus because of its unique role in biochemical, environmental, and medicinal sciences. It has been a common practice to use the rate of CH₃COO⁻ generation and oxidation as a gauge for the decomposition of organic materials in marine sediments [14] and the trans-metalation of tetrapyrroles [15,16]. The regulation of DNA, hormone transfer, protein synthesis, and enzyme activities are influenced by the recognition of CH₃COO⁻ ions [8,9,10,11]. Considering the vitality of CH₃COO⁻ ions in both biological and environmental aspects, developing suitable sensor systems for the selective, sensitive detection and quantification of CH₃COO⁻ is an important topic of research. Designing colorimetric and fluorescent anion sensors is of great importance in the current scenario due to their simple procedure, low cost, high sensitivity, and low detection limit [17,18]. In the recent past, several small-molecule-based chemosensors have been developed and successfully employed for selective anion recognition and sensing [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36]. For the detection of anions, the most prevalent chemosensors are polar -NH protons of urea [37] or thiourea [38], amide [39], pyrrole [40], and imidazolium [41,42] moieties. Among the different non-covalent interactions studied, hydrogen-bonding interactions are particularly beneficial and successful in the interaction of anions with a sensor. Coordinative interactions have also been extensively reported by sensors using metal ions. Sensors that use hydroxyl (-OH) groups as hydrogen bond donors have been explored to a lesser extent, although they can form effective hydrogen bonds with anions or induce deprotonation on anion coordination [43,44].

Anthraquinones are an important class of organic small-molecule fluorescent dyes that have been used extensively for the selective detection of anions, cations, and toxic small molecules [45,46]. Anthraquinone derivatives are substances of substantial chemical and biological interest. For many years, the chromophoric system of anthraquinones has been widely utilized to make textile dyes [47]. In addition, these compounds play a significant role as a class of biologically relevant and pharmaceutically active chromophores as well as an analytical tool for metal sensing [46,48,49]. Quinizarin (QZ, 1,4-dihydroxy-anthraquinone) is one type of fluorescent anthraquinone with two hydroxyl groups (See Figure 1 for structure). In this report, we were interested in examining the use of structurally simple QZ as a selective and reversible fluorescent sensor for anions. We anticipated that the presence of two -OH groups could act as receptor sites for anion recognition, either through hydrogen-bonding interactions or deprotonation of phenolic-OH protons following the binding of anions that subsequently perturbs the optical properties of the QZ sensor. Indeed, electronic absorption and fluorescence titration studies showed that QZ can selectively bind with CH₃COO⁻ anions via a fluorescence quenching-based sensing mechanism. The initial fluorescence emission of QZ in DMSO was quenched following the addition of CH₃COO⁻ anions. Notably, similar fluorescence titration studies using other anions, such as F⁻, Cl⁻, I⁻, NO₃⁻, NO₂⁻, and H₂PO₄⁻, exhibited negligible binding affinity with QZ. Therefore, QZ dye could be a potential optical sensor for the selective and reversible sensing of CH₃COO⁻ anions.

2. Materials and Methods

2.1. Materials

The reagents and chemicals were obtained from various commercial sources and used as received, without further purification. For the titration experiments, all anions were added either as tetrabutyl ammonium or sodium salts. The sensor Quinizarin is commercially available and was purchased from Sigma-Aldrich (Bangalore, India) and used as received. The solvents used for spectroscopic studies were purchased from various suppliers and were HPLC grade.

2.2. Instrumentations

The UV–visible absorption spectra were measured in quartz cuvettes using Thermo Scientific evolution 201 (Waltham, MA, USA) spectrometers. Baseline corrections were applied to all measurements. The fluorescence emission spectra were recorded using the Perkin Elmer-6500 (Waltham, MA, USA) Fluorimeter. All measurements were carried out at 298 K, and spectral data were processed using OriginPro 8.5 (Waltham, MA, USA).

2.3. Fluorescence Sensing Studies

For the titration studies, stock solutions of the QZ sensor (1 mM) and different anions (1 mM) as sodium salts, specifically F⁻, NO₃⁻, and NO₂⁻, or tetrabutyl ammonium salts, specifically CH₃COO⁻, Cl⁻, I⁻, and H₂PO₄⁻, were freshly prepared in DMSO. In a cuvette, 200 μL of 1 mM DMSO solution of QZ was taken and diluted with 1800 μL of fresh DMSO solvent. CH₃COO⁻, F⁻, Cl⁻, I^-, NO₃⁻, NO₂⁻, and H₂PO₄⁻ salts were added to this solution separately in increments of 20 µL to obtain concentrations up to 90.9 µM. The fluorescence emission intensity of QZ was monitored before and after the addition of different anions at various concentrations. For each fluorescence titration experiment, the sample was excited at λ = 437 nm and the fluorescence quenching efficiency was calculated using Equation (1):

Quenching efficiency (%) = (I₀ − I)/I₀ × 100(1)

where I₀ is the initial emission intensity of QZ in DMSO and I is the intensity after the addition of the anions. The Stern–Volmer quenching constant (K_SV) was obtained by fitting the data using the following Stern–Volmer Equation (2):

I₀/I = K_SV[Q] + 1(2)

K_SV was calculated from the slope of the linear curve of the relative changes in emission intensity (I₀/I) vs. the concentration [Q] of CH₃COO⁻ ions.

3. Results and Discussion

3.1. Anion Sensing Studies Using UV–Vis Titrations

At first, the anion-binding properties of QZ were investigated by UV–Vis absorption studies at 25 °C. Titrations were carried out using 1 mM solutions of QZ in the DMSO solution. The absorption spectra of QZ were recorded before and after the incremental addition (0.0 to 90.9 µM) of the standard solution (1 mM) of different anions: CH₃COO⁻, F⁻, Cl⁻, I⁻, H₂PO₄⁻, NO₂⁻, and NO₃⁻. As shown in Figure 2A, the DMSO solution of QZ (10 µM) exhibited typical anthraquinone characteristic absorption bands: a high-energy band at λ = 280 nm (ε = 1.2 × 10⁴ M⁻¹ cm⁻¹), a shoulder band at λ = 323 nm (ε = 3.5 × 10³ M⁻¹ cm⁻¹), and a low-energy broad peak centered at λ = 475 nm (ε = 9.6 × 10³ M⁻¹ cm⁻¹). After the addition of 10 equiv. of CH₃COO⁻ ions, the electronic absorption intensity of the bands at λ = 280 nm and 475 nm decreased significantly, which was accompanied by the appearance of a longer wavelength with a new absorption band at λ = 586 nm corresponding to the complex formation between QZ and CH₃COO⁻ anions (Figure 2A). Notably, the appearance of a clear isosbestic point at λ = 525 nm was consistent with an equilibrium existence of complexed and free QZ with CH₃COO⁻ anions in the DMSO solution (Figure 2C). The significant changes in absorption intensity of QZ after the addition of CH₃COO⁻ anions were also reflected by a sharp visual color change from bright green to faint green under room light before and after the mixing of QZ with CH₃COO⁻ anions (Figure 2A). Interestingly, similar absorption titration studies using 10 equiv. of other competing anions, such as F⁻, Cl⁻, I⁻, NO₃⁻, NO₂⁻, and H₂PO₄⁻, showed only little changes in the absorption spectra of QZ, and these results demonstrate that QZ can discriminatively and selectively bind with CH₃COO⁻ anions (Figure 2B). From the electronic absorption titration profile, a perfect linear plot of 1/ΔA vs. 1/[CH₃COO⁻] was obtained using the Benesi–Hildebrand equation and from the slope of this linear plot; moreover, the association constant K_a for CH₃COO⁻ ion binding was determined to be 7.63 × 10³ M⁻¹, which is comparable to already reported different optical sensors for CH₃COO⁻ ions (Figure 2D) [26,35,50]. The high value of K_a indicates the strong binding affinity of QZ for CH₃COO⁻ anions in the DMSO solution.

3.2. Anion Sensing Using Fluorescence Titrations Studies

The anion-binding propensity of QZ was further monitored using fluorescence titration studies in the DMSO solution. The QZ sensor was highly emissive due to the presence of emissive anthraquinone moiety and exhibited a broad emission band maximum at λ = 562 nm, which was quenched drastically after the addition of increasing concentrations of CH3COO⁻ anions (0.0 to 10 equiv.) (Figure 3C). The observed attenuation in emission intensity after the anion addition was presumably due to the deprotonation of the phenolic-OH group of QZ following CH₃COO⁻ interactions. This binding facilitates the photoinduced electron transfer (PET) from O to π* orbitals of anthraquinone moiety that results in the subsequent quenching of the fluorescence emission intensity (Scheme 1).

The proposed sensing mechanism of anion-induced deprotonation of phenolic-OH groups of QZ following CH₃COO⁻ anion addition was further validated by pH-dependent fluorescence titration studies in the DMSO solution. The fluorescence emission intensity of QZ was checked under different pH levels from 3.5 to 11.5. As given in Figure S1, the obtained results revealed that QZ is strongly emissive under acidic pH less than 5, and the fluorescence emission intensity was found to decrease under basic pH up to 7. With further increases in pH from 10 to 11.5, QZ was almost poorly emissive, presumably due to the deprotonation of phenolic-OH groups under basic pH, which resulted in significant fluorescence quenching of QZ due to the proposed PET transition from O to π* orbitals of anthraquinone. We further performed computational calculations to probe the cyclic stability of QZ and support the proposed PET transition. The energy-minimized structure of QZ verified that the proposed hydrogen-bonded cyclic structure is feasible and stable (see Figure S2). Moreover, the highest-occupied molecular orbital (HOMO) of deprotonated QZ²⁻ was higher in energy than the lowest-unoccupied molecular orbital (LUMO) of the QZ sensor, which confirms that the existence of the PET transition from O to π* orbitals of anthraquinone causes the observed fluorescence quenching. A perfectly linear Stern–Volmer plot was obtained from the fluorescence titration data, and the calculated Stern–Volmer quenching constant was found to be K_SV = 2.3 × 10³ M⁻¹. The high value of K_SV, again, confirms the strong binding affinity of QZ for CH₃COO⁻ anions (Figure 3D).

To establish the sensing selectivity of QZ towards CH₃COO⁻ ions, we performed similar fluorescence titration studies using other competing anions, such as F⁻, Cl⁻, I⁻, H₂PO₄⁻, NO₂⁻, and NO₃⁻. As depicted in Figure 3A, only CH₃COO⁻ anions elicited significant fluorescence quenching with no other spectral changes seen, whereas F⁻, Cl⁻, I⁻, NO₂⁻, and NO₃⁻ anions showed almost negligible fluorescence quenching effects. Thus, QZ can be a selective ”turn-off” fluorescent sensor for CH₃COO⁻ anions. To further corroborate the high selectivity, we carried out competitive fluorescence sensing studies on other anions under the same experimental conditions. As shown in Figure 3B, the initial fluorescence emission intensity (red bar) of QZ did not change (green bar) significantly after the addition of different anions, such as F⁻, Cl⁻, I⁻, H₂PO₄⁻, NO₂⁻, and NO₃⁻. However, the subsequent mixing of CH₃COO⁻ anions exhibited a dramatic fluorescence quenching (blue bar). These results further validated the high selectivity of QZ for CH₃COO⁻ anion detection. It is worth mentioning that in the presence of H₂PO₄⁻, CH₃COO⁻ anions showed a moderate fluorescence quenching effect, which may be due to the possible interference from H₂PO₄⁻ anions because of their similar basicity and charge density with CH₃COO⁻ anions [50].

Gratifyingly, the observed selective sensing of CH₃COO⁻ anions was also clearly visible to the naked eye. As shown in Figure 4A, the initial bright-orange color of QZ changed to colorless after adding CH₃COO⁻ anions under UV light irradiation, and a bright green solution changed to faint green under room light. However, no visible color changes were observed for QZ in presence of other anions; thus, the fluorescent dye QZ can be used as a colorimetric probe for CH₃COO⁻ ion detection. Moreover, we fabricated a filter paper-based colorimetric sensor for the detection of CH₃COO^- ions. As depicted in Figure 4B, the initial green emission of drop-casted QZ on filter paper changed to non-fluorescent after mixing a drop of the CH₃COO⁻ solution; thus, QZ can be realized as a suitable sensor for the detection of CH₃COO⁻ anions both in solution and solid phases.

To meet the practical uses, the sensor must be reversible and fluorescence sensing properties should be regenerable. To verify this, we carried out a fluorescence titration experiment employing the polar solvent H₂O as a potential competing solvent to establish the reversibility of QZ for CH₃COO⁻ sensing. As seen in Figure 5A, the addition of H₂O can effectively protonate O^- to form back the original OH groups, thereby leading to the recovery of emission intensity of unbound QZ dye. The sensitivity of QZ was additionally determined from the plot of changes in emission intensity as a function of CH₃COO⁻ anions at minute concentrations (Figure 5B). From the plot, the limit of detection (LoD) was determined to be 38 ppb, which is quite low for the detection of CH₃COO⁻ ions prevalent in many chemical and biological systems.

4. Conclusions

In conclusion, we have presented a simple quinizarin (QZ)-based small-molecule optical sensor for the selective and reversible sensing CH₃COO⁻ anions in DMSO. Electronic absorption and fluorescence titration studies demonstrated that the QZ sensor could selectively recognize CH₃COO⁻ in presence of various other competing anions (F⁻, Cl⁻, I⁻, NO₃⁻, NO₂⁻, and H₂PO₄⁻). The initial emission intensity of QZ was remarkably quenched upon the addition of CH₃COO⁻ ions, which was attributed to the deprotonation of phenolic -OH groups of QZ following CH₃COO⁻ anion addition. Moreover, the observed selective binding of CH₃COO⁻ anions was also reflected by sharp visual color changes both in solution and solid phases. In summary, the CH₃COO⁻ anion sensing properties of the fluorescent dye QZ are comparable to several reported small-molecule-based fluorescent sensors (see Table S1); hence, QZ can be a possible optical and colorimetric sensor for the detection and quantification of CH₃COO⁻ anions. Further work is in progress to functionalize QZ to make it a practically feasible water-soluble sensor for its application in the chemosensing of biologically relevant and environmentally concerned analytes.

Author Contributions

Investigation, M.K.N., A.S., B.M.; writing–original draft preparation, M.K.N., S.S.; writing–review and editing, B.M., S.S.; supervision, S.S.; project administration, S.S.; funding acquisition, S.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are thankful to the Central Instrumentation Facility (CIF) at IIT Palakkad for the research facilities.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Scheme

Figure 1. Structure of Quinizarin (QZ) fluorescent dye.

Figure 2. (A) The UV–visible absorption spectrum of QZ before and after the addition of 10 equiv. of CH3COO- anions (Inset: corresponding visual color changes). (B) The observed changes in absorption intensity of QZ after the addition of 10 equiv. of different anions. (C) Relative changes in UV–visible absorption intensity after increasing CH3COO- anion concentrations. (D) Corresponding Benesi–Hildebrand plot.

Figure 3. (A) The changes in fluorescence emission intensity of QZ (10 μM) after the addition of different anions (90.9 μM). (B) The competitive binding ability of QZ towards CH3COO− ions in the co-existence of other anions. (C) The relative changes in emission intensity of QZ after increasing the concentration of CH3COO−. (D) Corresponding Stern–Volmer plot.

Scheme 1. Proposed fluorescence quenching-based detection of CH3COO− through anion-induced deprotonation of OH groups in QZ.

Figure 4. (A) The observed visual color changes for QZ in DMSO solution after the addition of different anions. (B) The color changes observed for QZ coated on filter paper before and after the mixing of CH3COO− anions.

Figure 5. (A) Regeneration of fluorescence emission of QZ following titration with H2O at 25 °C. (B) The sensitivity plot indicates that the limit of detection is at the ppb level.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemistry4040092/s1, Figure S1. (A) pH-dependent emission titration for QZ measured in the DMSO solution; (B) corresponding plot of emission intensity (monitored at λ = 562 nm) vs. pH showing that the fluorescence intensity is low at basic pH. Figure S2. Energy-minimized structures of sensor QZ before and after binding with CH₃COO⁻ anion leads to the formation of anion-induced deprotonation of QZ²⁻. Table S1. Comparison of sensing properties of various reported small-molecule fluorescence sensors with quinizarin sensor reported in this article [50,51,52,53,54,55,56,57,58,59].

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