2.1 Chemicals and Reagents

Tetrahydrofuran (THF), CrO3, NaNO2, KCl, Hg(NO3)2, Ca3(PO4)2, Cd(NO3)2·4H2O, PbNO3, Al(NO3)3·9H2O, CrCl3(III)3·6H2O, anhydrous toluene, glacial acetic acid, concentrated aqueous hydrochloric acid (32%), anhydrous N,N-dimethylformamide (DMF), potassium carbonate, tris(dibenzylideneacetone)dipalladium, and tri(o-tolyl)phosphine were purchased from Sigma-Aldrich, USA. 6-bromoisatin, 6-bromooxindole, and 2-decyltetradecyl bromide were ordered from BLD Pharm, Shanghai, China. The solvents including methanol, hexane, dichloromethane, and ethyl acetate were purchased from Chem-Supply, Australia. The deionized water was used to prepare 1 × 10−3 M stock solutions of various metal ions.

2.2 Instruments

The fluorescence emission and absorption spectra were measured by using a Cary Eclipse Fluorescence Spectrophotometer (Agilent, USA) and a Cary 60 UV–Vis Spectrophotometer (Agilent, USA), respectively. Fourier Transform Infrared (FT-IR) spectra were recorded using an Arthur II FT-IR spectrometer (wavelength range 400–4000 cm−1) from Bruker (Germany). Thermal stability was measured using a Pegasus Q500 TGA thermogravimetric analyser under nitrogen atmosphere at a heating rate of 10 °C/min from 20 °C to 800 °C. Differential scanning calorimetry (DSC) measurements were conducted under nitrogen atmosphere using Chimaera Q100 DSC at a heating rate of 10 °C/min and a cooling rate of 5 °C/min between 50 °C and 250 °C. Photoelectron spectroscopy in air (PESA) was measured on an AC-2 photoelectron spectrometer (Riken-Keiki Co., Japan). Density Functional Theory (DFT) calculations were performed in Gaussian 16 software using the B3LYP functional and the 6-31+g(d,p) basis set.

2.3 Synthesis of II-MT

A solution of Sn-PhSMe (496 mg, 2.0 mmol), DB-C24-II (343 mg, 0.5 mmol) which was prepared from NH-II (Fig. S1, S2), Pd2(dba)3 (14 mg, 0.025 mmol), and P(o-tol)3 (61 mg, 0.2 mmol) in toluene (50 mL) was refluxed at 110 °C under argon for 36 h. After cooling to room temperature, the solvent was directly removed under vacuum and the residue was purified by silica gel column chromatography using n-hexane: dichloromethane = 1: 2(V/V) as the eluent to give 200 mg of II-MT (yield = 54%).

1H NMR (CDCl3, 600 MHz), δ (ppm), 9.25 (d, J = 8.4 Hz, 2H), 7.59 (d, J = 8.4 Hz, 4H), 7.36 (d, J = 7.8 Hz, 4H), 7.28 (m, 2H), 6.96 (s, 2H), 3.73 (d, J = 7.2 Hz, 4H), 2.56 (s, 6H), 1.97 (m, 2H), 1.42–1.25 (m, 80H), 0.89 (m, 12H) (Fig. S3). 13C NMR (CDCl3, 101 MHz), δ (ppm), 168.66, 145.78, 144.12, 139.07, 137.12, 132.35, 130.17, 127.24, 126.70, 120.90, 120.34, 106.13, 44.53, 36.34, 31.95, 31.94, 31.71, 30.06, 29.74, 29.72, 29.69, 29.67, 29.39, 26.60, 22.71, 15.66, 14.14 (Fig. S4). HRMS: measured m/z 1179.8701 (expected 1179.8707 for [C78H118N2O2S2 + H]+). ∆ = −0.5 ppm (Fig. S5).

2.4 Selectivity Analysis of II-MT Towards Cr6+

Aqueous solutions of CrO3, NaNO2, KCl, Hg(NO3)2, Ca3(PO4)2, Cd(NO3)2·4H2O, PbNO3, Al(NO3)3·9H2O and CrCl3(III)3·6H2O were prepared in deionized water (1 × 10−3 M) to mix with II-MT respectively for selectivity analysis towards Cr6+. 250 µL of II-MT (1 × 10−4 M in THF at pH = 4, 7, and 8, respectively) was mixed with 250 µL of different ions and diluted with THF solvent to 5 mL, delivering a final concentration of 50 µM for ions. UV measurements were carried out within the wavelength range of 200–800 nm. Fluorescence spectroscopy was performed with an excitation wavelength of 295 nm and the fluorescence emission was recorded in the wavelength range of 305–550 nm. The conditions are as follows: excitation slit: 5 nm, emission slit: 5 nm, scan control: medium, scan rate: 600 nm/min, average time: 0.1 s, and data interval: 1 nm.

2.5 FT-IR Measurements of II-MT and II-MT –Cr Complex

A solution of CrO3 (1 × 10−3 M) in water and a solution of II-MT (1 × 10−4 M) in THF were prepared. For the II-MT-Cr complex sample, 1000 μL of II-MT was combined with 1000 μL of Cr6+ in a glass vial and allowed to stand for 15 min. ATR FTIR device was used to measure the FTIR spectra of pure II-MT and II-MT-Cr complex.

2.6 The Stoichiometry of Complex Formation Between II-MT and Cr6+

UV–Vis spectroscopy was used to determine the stoichiometry between II-MT and Cr6+. A solution of II-MT with a fixed concentration of 5 µM was titrated with varying amounts of Cr6+ (from 0.5 to 4 molar equivalent). The absorption intensity of the II-MT and Cr6+ mixture at 275 nm was plotted against the molar ratio of Cr6+ to II-MT.

2.7 Impact of Time on the Cr6+ Detection Using II-MT

To investigate the impact of time on the detection of Cr6+ via II-MT dye, 250 µL of II-MT (1 × 10−4 M in THF at pH 4) was combined with 250 µL Cr6+ (1 × 10−3 M) and diluted with THF solvent to 5 mL. The UV absorbance at 295 nm was quantified between zero and 20 min after the addition of Cr6+.

2.8 Cr6+ Determination Using II-MT

The concentration of Cr6+ in water was measured by mixing 250 µL of II-MT (1 × 10−4 M, in THF, at pH 4) with 250 µL of different concentrations of Cr6+ (2 × 10−4, 16 × 10−5, 12 × 10−5, 8 × 10−5, 4 × 10−5, 1 × 10−5, 2 × 10−6 M) solution. The THF solvent was used for the dilution to 5 mL. The final concentration of II-MT in the mixture was 5 µM and the final concentration of Cr6+ ranged from 0.1 to 10 µM. The intensity of the dye absorption band at 295–275 nm was measured by UV–Vis spectroscopy and plotted against the concentrations of Cr6+ in the solution.

2.9 Determination of Cr6+ in Spiked Tap Water

The II-MT dye was employed to determine Cr6+ in spiked tap water samples. Tap water was spiked with Cr6+ to achieve concentrations of 8 × 10−5 M and 4 × 10−5 M. A 250 µL aliquot of each spiked tap water sample was mixed with 250 µL of II-MT dye solution (1 × 10−4 M at pH 4). This mixture was diluted to a total volume of 5 mL, resulting in final Cr6+ concentrations of 2 µM and 4 µM. Quantification of Cr6+ was performed using UV–Vis spectroscopy.

2.10 Single-Use Paper-Based Sensor for Cr6+ Detection

A strip of filter paper (1 cm × 1 cm) was coated with a solution of 1 × 10−4 M II-MT (in THF). The coated strip was placed in a fume cupboard and dried under a gentle stream of nitrogen gas to evaporate all traces of THF. The paper sensor was stored for later use. A water sample was spiked with 400 µL of 1 × 10−4 M CrO3 and diluted to 5 mL with tap water. The solution was adjusted to pH 4, resulting in a final Cr6+concentration of 8 µM. The paper sensor was immersed in 1 mL of the spiked water for 15 min, then removed and dried under nitrogen gas. The color change from brown to colorless was observed visually.