π-Conjugated molecules identified for reversible and visual detection of F⁻ in aqueous: Effect of heterocycle unit on sensing performance

Highlights

• Two kinds of π-conjugated molecules containing heterocycle unit or not were compared.

• Dye 1 with heterocycle is a potentially reversible ratiometric-colorimetric sensor of F⁻.

• Dye 1 was successfully applied for colorimetric or naked-eye F⁻ detection in environmental water, tea and toothpaste.

• The sensing mechanism was confirmed to be deprotonation one with a 1:1 binding stoichiometry.

Abstract

To illustrate the impact of molecular structure, especially heterocycle unit, on the sensing performance, two kinds of π-conjugated molecules containing aromatic heterocyclic (Dye 1) and aromatic ring (Dye 2) were identified and compared each other. Even with similar structures, they possessed quite different spectral and colorimetric responses to F⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, HSO₄⁻, H₂PO₄⁻, ClO₄⁻ and CH₃COO⁻, etc. The reason might result from the difference in withdraw-electron ability of aromatic and heterocyclic rings, which would lead to different acidity of active H in the target π-conjugated molecules. In acidic aqueous, Dye 1 expressed a reversible ratiometric-colorimetric response to F⁻, accompanying with a visual color change from bright yellow to purple, a nice linear range of 2.0–35.0 × 10⁻⁶ mol/L and a low detection limit of 1.60 × 10⁻⁷ mol/L. While Dye 2 did not react with any anion due to its weak acidity of active hydrogen. Under the optimized conditions, Dye 1 was successfully applied for colorimetric or naked-eye detection of F⁻ in environmental water, tea and toothpaste samples with RSD ≤ 3.1%. The recognition mechanism for Dye 1 to F⁻ was confirmed to be deprotonation one with a 1:1 binding stoichiometry.

Introduction

To efficiently construct optical sensors with obvious colorimetric change and revisable sensing property has been attracting more and more attention for on-site monitoring environmental pollution, due to their simple and convenient sensing processes [[1], [2], [3], [4], [5], [6]]. For this, a plethora of multifunctional organic molecules with different structures and tunable optical properties have been developed to monitor various analytes, such as heavy metal ions [[7], [8], [9]], anions [[10], [11], [12], [13], [14], [15]], organic pollutions [[16], [17], [18], [19], [20]] and toxic gases [15,[21], [22], [23]]. Among all the analytes, fluorion (F⁻) is often be ignored in our daily life for it is widely utilized. People, who assimilate the excess or deficient F⁻, will suffer from the diseases such as kidney failure, collagen breakdown, bone disorders, immune system disruption, gastric and kidney disorders, etc. [[24], [25], [26], [27], [28], [29]] Therefore, it is urgent for scientific workers nowadays to develop some effective probes to monitor F⁻ on-site [[30], [31], [32], [33], [34], [35], [36]]. For example, based on a diarylethene combining to a 1,8-naphthalimide Schiff base, Fu, et al. [37], developed a typical colorimetric and fluorescent dual-mode sensor for F⁻, which exhibited high selective and prominent color change from greenish yellow to blue in the presence of F⁻. Our group [38] also developed a D-A-D typed conjugated chromophore (PBHPI) and its functionalized testing-paper strips for on-site F⁻ detection with obvious color change from light yellow to dark red and a low detection limit of 7.8 × 10⁻⁷ mol/L. However, the proposed PBHPI could only be applied in pure organic phase, which will limit it to be applied in practice.

Recently, Sharma, et al. [39], reported a selective colorimetric sensor (Dye 2), combining 2-hydroxy-1-naphthaldehyde with 2-aminophenol, for F⁻ detection in mixed solvent of 1% H₂O: 99% CH₃CN. However, the characterized absorption peak for F⁻ was just a weak shoulder one, which led to not high sensitivity. By changing 2-aminophenol into 2-amino-3-hydroxypyridine (Dye 1), Ding, et al. [40], developed selective fluorescent sensor for Al³⁺ in DMSO. However, the colorimetric response of Dye 2 to anions in aqueous has not been investigated and reported. What's more, the impact of the molecular structure such as introduce of heterocycle unit on the sensing property (Dye 1 and Dye 2), as far as we known, has not been investigated and compared so far.

With this aim in mind, two kinds of π-conjugated molecules containing aromatic heterocyclic (Dye 1) and aromatic ring (Dye 2, Scheme 1) were identified in this work, whose UV–vis absorption spectral property, especially colorimetric responses to different anions were investigated and compared in detail. It is expected the resulting relationship between molecular structure and sensing performance will provide a new insight for designing colorimetric sensors with desirable property in the future.