Detection of Selected Heavy Metal Ions Using Organic Chromofluorescent Chemosensors

Abstract

Heavy and transition metal (HTM) ions have significant harmful effects on the physical environment and play crucial roles in biological systems; hence, it is crucial to accurately identify and quantify any trace pollution. Molecular sensors which are based on organic molecules employed as optical probes play a crucial role in sensing and detecting toxic metal ions in water, food, air, and biological environments. When appropriate combinations of conduction and selective recognition are combined, fluorescent and colorimetric chemosensors are appealing instruments that enable the selective, sensitive, affordable, portable, and real-time investigation of the possible presence of heavy and transition metal ions. This feature article aims to provide readers with a more thorough understanding of the different methods of synthesis and how they work. As noted in the literature, we will highlight colorimetric and fluorometric sensors based on their receptors into multiple categories for heavy metal ion detection, such as Hg²⁺, Ag²⁺, Cd^2+, Pb²⁺, and In³⁺, and simultaneous multiple-ion detection.

Keywords: biological detection; chemosensor design; colorimetric chemosensors; environmental monitoring; fluorescent chemosensors; heavy and transition metal (HTM) ions.

Figure 1

Diagrammatic illustration of an analyte’s chemosensor binding. Reproduced from ref. [12]. Analyte is added in the forward reaction, due to which the fluorescence is “on”. The analyte is detached from the analyte-receptor complex in the backward direction indicating that the fluorescence is “off”.

Scheme 1

(a) Synthesis of solid-phase peptide-based sensor 6; (b) Proposed binding mode.

Figure 2

The fluorescence emission spectra of compound 6 (3 × 10⁻⁶ M) are shown. (A) Compound 91 dissolved in 10 mM phosphate at a pH of 7.4; (B) compound 6 was dissolved in 10 mM phosphate and 1 mM NaCl at a pH of 7.4; (C) compound 6 was visible under UV light (λem = 365 nm) in the presence of different metal ions. Reproduced from ref. [93] with permission from American Chemical Society, copyright 2016.

Scheme 2

(a) Synthesis of sensor 11; (b) Proposed binding mode.

Scheme 3

(a) Synthesis of thiourea-based sensor 17; (b) Proposed binding mode.

Scheme 4

(a) Synthesis of phenothiazine-thiophene hydrazone-based sensor 20; (b) Proposed binding mode.

Scheme 5

(a) Synthesis of rhodamine-derived fluorescent sensor 26; (b) Proposed binding mode.

Figure 3

(A) Suggested mechanism of receptor 26 towards Ag⁺ and iodide I⁻ ions. Yellow colored arrow indiacte ICT mechanism and blue colored arrow indicate silver ion and iodide ion released as AgI (B) Practical naked-eye application of receptor 26 for recognition of Ag⁺ ion using solid-supported silica methods. (C) Fluorescence spectrum of receptor 26 (4 × 10⁻⁶ M). (D) Fluorescence spectrum of receptor 26+Ag⁺ (4 × 10⁻⁶ M). The arrows indicate the fluorescence spectral lines of the respective ions. Copied from ref. [112] with permission from Elsevier, copyright 2021.

Scheme 6

(a) Synthesis of naphthyl thiourea-based chemosensor 31. (b) Proposed binding mode.

Scheme 7

(a) Synthesis of triazole-imidazole-based sensor 36. (b) Proposed binding mode.

Scheme 8

(a) Synthesis of optical chemosensor 39. (b) Proposed binding mode.

Figure 4

(A) The chemosensor 39’s UV–visible spectrum. (B) The [CM1 + Cd²⁺] complex fluorescence emission spectrum. Copied from ref. [129] with permission from MDPI, copyright 2023.

Scheme 9

(a) Synthesis of Schiff-based chemosensor 43. (b) Proposed binding mode.

Scheme 10

(a) Synthesis of Schiff-based chemosensor 46. (b) Proposed binding mode.

Figure 5

(A) The UV–Vis absorption spectrum of the chemosensor 46 in the presence of different cations. (B) The UV–Vis spectrum of 46 titration against various concentrations of Pb²⁺. The arrow indicate the UV-Vis spectrum of the chemosensor 46 titration against various concentrations of lead ion. (C) Sensor 46 color change in the presence of various cations in an ethanol–water solution (v/v, 90, 10). Copied from ref. [151] with permission from Elsevier, copyright 2023.

Scheme 11

(a) Synthesis of near-infrared fluorescent probe 49. (b) Proposed binding mode.

Scheme 12

Synthesis of pyridine-2,6-dicarboxamide-based chemosensor 54.

Scheme 13

(a) Synthesis of pyridinecarbohydrazide-based fluorescent chemosensor 59. (b) Proposed binding mode.

Figure 6

(A) Fluorescence spectrum responses of 59 in DMSO: H₂O when it is excited using different metal ions at 337 nm. (B) Probe 59’s fluorescence spectrum titration in DMSO:H₂O (2:8, v/v) medium. (C) Under UV light, the fluorescence of 59 in a DMSO:H₂O solution containing different metal ions changes. Copied from ref. [161] with permission from Elsevier, copyright 2022.

Scheme 14

(a) Synthesis of AIE-based fluorescence sensor 64. (b) Proposed binding mode.

Figure 7

Bis-TPE’s (64) emission intensities with various metal ions in an H₂O-THF solution. Copied from ref. [178] with permission from Elsevier, copyright 2020.

Scheme 15

(a) Synthesis of naphthyl thiourea-based chemosensor 69. (b) Proposed binding mode.

Scheme 16

(a) Synthesis of Schiff-based chemosensor 72. (b) Proposed binding mode.

Scheme 17

(a) Synthesis of Schiff-based chemosensor 75. (b) Proposed binding mode.