Ultrasensitive colorimetric detection of fluoride and arsenate in water and mammalian cells using recyclable metal oxacalixarene probe: a lateral flow assay

Abstract

Globally 3 billion people are consuming water with moderately high concentrations of fluoride and arsenic. The development of a simple point of care (PoC) device or home device for the detection of fluoride/arsenic ensures safety before consuming water. Till date, lateral flow assay (LFA) based PoC devices can detect nucleic acids, viruses and diseases. An aluminium complex of rhodamine B functionalized oxacalix[4]arene (L) was designed to execute the LFA-based PoC device. Initially, Al³⁺ and Fe³⁺ ions were involved in complexation with the rhodamine B functionalized oxacalix[4]arene (L), resulting C₁ (L-Al³⁺) and C₂ (L-Fe³⁺) complexes respectively. The receptor L, as well as the probes (C₁, C₂), were characterized thoroughly using mass spectroscopy, FTIR, NMR, and EA. C₁ and C₂ were further utilized as recyclable probes for the detection of aqueous fluoride (21 ppb) and arsenate (1.92 ppb) respectively. The computational calculation indicates that upon complexation, the spirolactam ring opening at the rhodamine B site leads to optoelectronic changes. The consistency of LFA-based portable sensing device has been tested with water samples, synthetic fluoride standards and dental care products like toothpaste and mouthwash with concentrations ≥ 3 ppm. Moreover, fixed cell imaging experiments were performed to ascertain the in-vitro sensing phenomena.

Figure 1

(a) Al³⁺/Fe³⁺ induced sensing of fluoride and arsenate respectively; (b) Schematic illustration of LFA-based PoC device prototype; (c) Representation of positive and negative results in LFA toward the detection of fluoride.

Scheme 1

Synthetic scheme of the preparation of receptor L.

Figure 2

ORTEP of compound 1 with 50% ellipsoid probability (a) top view and (b) front view.

Figure 3

(a) Change in absorption maxima of the receptor L toward various metal ions, Inset: colorimetric changes; (b) Fluorescence responses of L with different metal ions (1–13) Fe³⁺ (14) and Al³⁺ (15), Inset: color changes under UV light; (c) Schematic representation for the binding of Al³⁺ and Fe³⁺.

Figure 4

Change in color of the receptor L upon increasing concentration of Al³⁺ (a, Inset: concentrations 1 → 0 nM, 2 → 100 nM, 3 → 200 nM, 4 → 300 nM, 5 → 400 nM, 6 → 500 nM, 7 → 600 nM, 8 → 700 nM, 9 → 800 nM, 10 → 900 nM) and Fe³⁺ (b, Inset: concentrations 1 → 0 nM, 2 → 25 nM, 3 → 50 nM, 4 → 75 nM, 5 → 100 nM, 6 → 150 nM, 7 → 200 nM, 8 → 300 nM, 9 → 400 nM, 10 → 500 nM) in the top panel. Addition of equal proportion of methylene blue dye onto each test solution (bottom panel).

Figure 5

Variation in emission responses of C₁ and C₂ against a series of common anions in aqueous medium (top); a pictorial representation of fluoride and arsenate anion interacts C₁ and C₂ respectively.

Figure 6

Fluorescence titration (a) Change in emission intensity of C₁ with the addition of an increasing amount of fluoride (0.1–47 μM), (b) Change in emission intensity of C₂ with the addition of an increasing amount of arsenate (5–500 nM), (c) Stern–Volmer plot for fluoride, Inset: LOD, (d) Stern–Volmer plot for arsenate, Inset: LOD; Change in color of C₁ (e) and C₂ (f) upon incremental addition of fluoride and arsenate respectively (top), the addition of an equal proportion of methylene blue dye onto each solution (bottom).

Figure 7

Ground state optimized geometries of (a) L and (b) L-Al³⁺ complex (Hydrogens are eliminated for better visualization).

Figure 8

Electron density maps of the Frontier Molecular Orbitals (FMO) of L-Al³⁺ complex representing the electronic transitions of absorption and emission band on the optimized geometries of (a) L and (b) L-Al³⁺ at HF/3-21G*.

Figure 9

Plausible sensing mechanism of L toward Al³⁺/Fe³⁺, C₁ toward Fluoride and C₂ toward AsO₄³⁻.

Figure 10

(a) Schematic representation of the recycle experiment (i) addition of Al³⁺ to the receptor L, (ii) addition of fluoride, (iii) extraction of organic layer with chloroform, (iv) removal of aqueous layer and drying of organic layer, (v) dissolve the crude in acetonitrile; (b) Recyclability of the probe L toward fluoride.

Figure 11

C₁-based LFA for detection of fluoride in aqueous media.

Figure 12

LSCM fluorescence imaging of SUM159 cell line with (a) C₁, (b) C₁ + F⁻, (c) C₂, (d) C₂-AsO₄³⁻.