A photoluminescent and electrochemiluminescent probe based on an iridium(III) complex with a boronic acid-functionalised ancillary ligand for the selective detection of mercury(II) ions

Abstract

Exposure to mercury(II) ions (Hg²⁺) can cause various diseases such as Minamata disease, acrodynia, Alzheimer's disease, and Hunter–Russell syndrome, and even organ damage. Therefore, real-time and accurate monitoring of Hg²⁺ in environmental samples is crucial. In this study, we report a photoluminescent (PL) and electrochemiluminescent (ECL) probe based on a cyclometalated Ir(III) complex for the selective detection of Hg²⁺. The introduction of a reaction site, o-aminomethylphenylboronic acid, on the ancillary ligands allowed a prompt transmetalation reaction to take place between Hg²⁺ and boronic acid. This reaction resulted in significant decreases of the PL and ECL signals due to the photo-induced electron transfer from the Ir(III) complex to the Hg²⁺ ions. The probe was applied to the selective detection of Hg²⁺, and the signal changes revealed a linear correlation with Hg²⁺ concentrations in the range of 0–10 μM (LOD = 0.72 μM for PL, 8.03 nM for ECL). The designed probe allowed the successful quantification of Hg²⁺ in tap water samples, which proves its potential for the selective detection of Hg²⁺ in environmental samples.

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Introduction

Rapid industrialisation has resulted in the frequent release of toxic chemicals, which can cause serious environmental problems. Among various released chemicals, the mercury(II) ion (Hg²⁺) is classified as a persistent, bio-accumulative, and toxic chemical, and is considered a threat to humans and the environment.¹ Although Hg²⁺ is colourless, tasteless, and odourless, it can be easily absorbed into the body through the cell membranes of the skin, respiratory tract, and gastrointestinal tract.² The assimilation of Hg²⁺ in the human body can damage the central nervous system, endocrine glands, and organs.^3,4 Furthermore, chronic exposure to Hg²⁺ has been associated with various diseases, such as Minamata disease, acrodynia, Alzheimer's disease, and Hunter–Russell syndrome.⁵ Consequently, the U.S. Environmental Protection Agency (EPA) has recommended that the maximum level of inorganic mercury in drinking water is 10 nM.⁶ The common methods employed for the detection of Hg²⁺ include inductively coupled plasma atomic emission spectroscopy (ICP-AES),⁷ atomic absorption spectroscopy,^8,9 electrochemical analysis,^10–12 surface-enhanced Raman spectroscopy (SERS),^13,14 piezoelectric detection,¹⁵ and colourimetric and fluorometric detection.¹⁶ In particular, fluorescent chemosensors for Hg²⁺ have been developed based on the Hg²⁺-catalysed reaction and coordination of heteroatoms with Hg²⁺.¹⁷ However, these detection methods are very expensive and time-consuming, and require skilled laboratory professionals. Furthermore, they require bulky and fragile instrumental setup, including an additional optical source.

Electrochemiluminescence (ECL) is a luminescence process involving sequential electron transfers between electrogenerated species on an electrode surface.¹⁸ Compared with the photoluminescence (PL) process, ECL offers several advantages due to its low background signal and high sensitivity. In addition, ECL rarely requires complicated procedures, complex conditions, or bulky equipment for analysis.^18–20 Consequently, ECL has been considered a powerful tool for real-time, on-site sample analysis, and several studies have reported systems that combine the ECL technique with chemosensors.^21–41 So far, most ECL molecular probes have been developed using tris(2,2′-bipyridine)ruthenium(II) complexes ([Ru(bpy)₃²⁺]) with their characteristically low metal-centred state (MC state), making it challenging to adjust the wavelength with changes in the molecular structure.⁴² Among various luminophores considered as replacements for Ru(II) complexes, Ir(III) complexes are regarded as promising ECL luminophores due to their high ECL quantum yields and colour tunability through ligand modification by high ligand field stabilization energies.^33,43–48 Hong and co-workers reported the first ECL analysis system for the detection of Hg²⁺ ions based on the reaction of a small molecular probe with these ions.³⁵ Unfortunately, this system could not be applied in field monitoring because large amounts of organic solvents were required. Consequently, the development of new ECL probes with high selectivity and sensitivity toward Hg²⁺ in aqueous solutions is greatly desired.

Herein, we report a new PL and ECL chemodosimetric probe based on an iridium(III) complex for the determination of Hg²⁺ (Scheme 1). The design strategy of probe 1 is based on the following: (i) [Ir(piq)₂(bpy)][PF₆] (piq = 1-phenylisoquinoline, bpy = bipyridine) was selected as a luminophore, and (ii) o-aminomethylphenylboronic acid was introduced into the ancillary ligand as a PL and ECL signalling modulator, as well as a reaction site for the Hg²⁺ ions.^49–53 Probe 1 was expected to exhibit significant PL and ECL changes towards Hg²⁺ ions in aqueous solutions when compared to other Ir(III)-based probes because this probe has cationic charge and hydrophilic o-aminomethylphenylboronic acid, which imparts high solubility in aqueous solutions. Probe 1 promptly reacted with Hg²⁺ ions through the transmetalation reaction to generate 1-Hg²⁺, resulting in decreased PL and ECL signals. Probe 1, as analysed through PL and ECL, exhibited high sensitivity and selectivity in detecting Hg²⁺ ions within 2 min in aqueous solutions with small amounts of organic solvent (10% acetonitrile). These results imply significant potential for on-site Hg²⁺ detection, compared with other PL-based chemodosimeters, as illustrated in Table S1.† In addition, probe 1 was successfully applied to ECL analysis of Hg²⁺ ions in tap water samples.

Scheme 1 The detection mechanism of probe 1 for Hg²⁺ ions.

Experimental

Probes 1 and 2 were synthesised as shown in Scheme 2. Detailed synthetic procedures for compounds 1–11 are provided in the ESI.†

Scheme 2 Synthetic routes of (a) compound 7, (b) compound 8, and (c) probes 1 and 2.

Synthesis of 1

A mixture of crude compound 11 (17 mg, 20 μmol), compound 8 (7 mg, 25 μmol), and potassium carbonate (13 mg, 94 μmol) in 0.2 mL of acetonitrile was stirred at room temperature for 12 h. The solvent was evaporated under reduced pressure and extracted with dichloromethane three times. The organic phase was washed with water and brine, dried over anhydrous sodium sulphate, and evaporated. The residue was purified by silica gel column chromatography (acetonitrile/H₂O/sat. KNO₃(aq) = 40 : 4 : 1). The product was re-dissolved in dichloromethane/methanol (1 : 1, 1 mL) and stirred at room temperature for 2 h with potassium hexafluorophosphate (37 mg, 0.2 mmol). The solution was filtered, and the filtrate was evaporated under reduced pressure. The residue was triturated with dichloromethane and diethylether. The resulting precipitate was filtered, washed with diethylether and dried in vacuo to afford 1 (10 mg, 47%). ¹H NMR (300 MHz, CDCl₃) δ 8.99–8.84 (m, 2H), 8.78 (d, J = 8.2 Hz, 1H), 8.60 (s, 1H), 8.44 (s, 2H), 8.28–8.20 (m, 2H), 8.10 (t, J = 7.3 Hz, 1H), 7.89 (dd, J = 6.2, 3.0 Hz, 2H), 7.80–7.69 (m, 6H), 7.64 (d, J = 5.6 Hz, 1H), 7.40–7.28 (m, 8H), 7.22 (d, J = 7.1 Hz, 1H), 7.10 (t, J = 7.6 Hz, 2H), 6.88 (t, J = 7.0 Hz, 2H), 6.25 (d, J = 7.3 Hz, 2H), 3.82 (d, J = 5.5 Hz, 4H), 2.28 (s, 3H). ¹³C NMR (126 MHz, CD₃CN) δ 168.49, 168.45, 156.01, 155.25, 153.47, 153.38, 150.68, 150.61, 145.57, 145.53, 140.80, 140.68, 139.47, 137.09, 137.07, 135.92, 131.98, 131.87, 130.86, 130.83, 130.75, 130.57, 130.50, 129.14, 129.02, 129.01, 128.64, 128.48, 127.56, 127.54, 127.45, 126.76, 126.75, 126.17, 126.15, 125.63, 124.86, 122.32, 122.30, 121.98, 121.90, 62.75, 58.55, 40.79. LRMS (MALDI-TOF) m/z: [M]⁺ calc. for C₄₉H₄₀BIrN₅O₂ 934.290, found 934.063. HRMS (FAB) m/z: [M − H₂O]⁺ calc. for C₄₉H₃₈BIrN₅O 916.2799, found 916.2793.

Synthesis of 2

A mixture of crude compound 11 (9 mg, 11 μmol), benzyl bromide (4 mg, 23 μmol), potassium iodide (4 mg, 24 μmol), and potassium carbonate (3 mg, 22 μmol) in 0.2 mL of acetonitrile was stirred at room temperature for 12 h. The solvent was evaporated under reduced pressure and extracted with dichloromethane three times. The organic phase was washed with water and brine, dried over anhydrous sodium sulphate, and evaporated. The residue was re-dissolved in dichloromethane/methanol (1 : 1, 1 mL) and stirred at room temperature for 2 h with ammonium hexafluorophosphate (18 mg, 0.11 mmol). The solution was filtered and evaporated, and the residue was purified by silica gel column chromatography (CH₂Cl₂/MeOH = 50 : 1) to afford 2 (6 mg, 53%). ¹H NMR (400 MHz, CDCl₃) δ 8.92 (dd, J = 5.9, 3.1 Hz, 2H), 8.73 (d, J = 8.2 Hz, 1H), 8.58 (s, 1H), 8.26 (d, J = 8.0 Hz, 2H), 8.15 (t, J = 7.8 Hz, 1H), 7.90 (dd, J = 6.3, 3.1 Hz, 2H), 7.77 (dd, J = 6.4, 3.2 Hz, 4H), 7.72 (d, J = 4.9 Hz, 1H), 7.63 (d, J = 5.4 Hz, 1H), 7.49 (d, J = 20.2 Hz, 1H), 7.36 (m, J = 6.5 Hz, 7H), 7.28 (d, J = 7.3 Hz, 2H), 7.20 (d, J = 6.8 Hz, 1H), 7.11 (t, J = 7.6 Hz, 2H), 6.89 (t, J = 7.4 Hz, 2H), 6.27 (t, J = 6.6 Hz, 2H), 3.70 (d, J = 72.2 Hz, 4H), 2.26 (s, 3H). ¹³C NMR (125 MHz, CDCl₃) δ 169.04, 168.99, 155.65, 155.47, 153.55, 153.41, 150.11, 145.47, 145.41, 140.21, 139.71, 136.99, 132.17, 131.70, 130.87, 130.85, 130.68, 130.66, 128.70, 128.49, 127.99, 127.59, 127.57, 126.93, 126.88, 126.31, 126.26, 125.38, 122.29, 121.98, 121.96, 62.03, 29.68. HRMS (FAB) m/z: [M]⁺ calc. for C₄₉H₃₉IrN₅ 890.2835, found 890.2834.

Results and discussion

Spectroscopic properties of 1

We checked the changes in the UV-vis and PL spectra of probe 1 in the presence and absence of Hg²⁺ in an aqueous medium (pH 7.4, HEPES buffer/CH₃CN = 9 : 1, v/v). Probe 1 displayed an intense absorption peak at 288 nm, which was attributed to the spin-allowed ligand-centred (¹LC) transition states (Fig. S1†). Furthermore, the broad absorption peak detected around 330–510 nm was attributed to the mixing of metal-to-ligand charge-transfer (¹MLCT and ³MLCT) transitions, ligand-centred (³LC) transitions, and ligand-to-ligand charge-transfer (³LLCT and ¹LLCT) transitions.⁵⁴ When the concentration of Hg²⁺ ions in solution increased, the absorption bands of probe 1 showed a slight red-shift. Subsequently, the PL response of probe 1 to Hg²⁺ was evaluated. As the amount of Hg²⁺ increased, the PL signal (λ_ex = 400 nm) gradually decreased until it reached a plateau after the addition of 10 μM Hg²⁺ (Fig. 1a). This indicated that probe 1 reacted in 1 : 1 stoichiometry with Hg²⁺. The limit of detection (LOD) was 0.73 μM from the linear correlation (R² = 0.99) between the concentration of Hg²⁺ ions (in the range of 0–10 μM) and the PL intensity (Fig. 1b).

Fig. 1 (a) PL emission spectra (λ_ex = 400 nm) of probe 1 (10 μM) with increasing Hg²⁺ ion concentrations in CH₃CN/H₂O (1 : 9 v/v, pH 7.4, 10 mM HEPES). Inset: photographs of the probe solutions without and with Hg²⁺ under 365 nm UV irradiation. (b) PL intensity changes of probe 1 at 590 nm upon the addition of Hg²⁺ ions. Inset: a linear correlation between the PL intensity at 590 nm and the concentration of Hg²⁺.

To further understand the quenching behaviour of probe 1 in the presence of Hg²⁺, its fluorescence lifetimes before and after the addition of Hg²⁺ ions were measured by time-correlated single-photon counting, whereby this probe exhibited single-exponential decay with a lifetime of 0.82 μs (Fig. S2a†). However, in the presence of Hg²⁺ ions, an additional shorter lifetime of ∼0.1 μs was observed, thus revealing double-exponential decay in the PL decay curve. This can be attributed to the generation of the reaction product, 1-Hg²⁺, via the transmetalation reaction between Hg²⁺ and boronic acid on the ancillary ligands. Upon the addition of Hg²⁺, the lifetimes decreased, thus indicating that the quenching mechanism was highly dependent on dynamic quenching (Fig. S2b†).^55–57 These changes were observed with the addition of up to 10 μM Hg²⁺ ions, which were consistent with the signal change trends observed in the PL response experiments.

To determine the selectivity of probe 1 in an aqueous solution, the PL spectra were measured in the presence of various metal ions. As expected, the PL of probe 1 decreased effectively only in the presence of Hg²⁺, whereas other metal ions exhibited negligible PL signal changes (Fig. 2). This selectivity can be attributed to the transmetalation between phenylboronic acid and Hg²⁺ ions.^49–53 The PL of 1-Hg²⁺ generated through the reaction between probe 1 and Hg²⁺ was quenched by the electron transfer from the excited Ir(III) complex to Hg²⁺.⁵⁸

Fig. 2 (a) PL emission spectra (λ_ex = 400 nm) of probe 1 (10 μM) in the presence of various analytes (40 μM each; Hg²⁺, 20 μM) in CH₃CN/H₂O (1 : 9 v/v, pH 7.4, 10 mM HEPES). (b) PL emission intensities at 590 nm of probe 1 (10 μM) upon the respective addition of various metal ions (40 μM).

To investigate the probe's pH susceptibility, the PL intensity of probe 1 was measured in the pH range of 4–10 after the addition of Hg²⁺ (Fig. 3a). The probe showed a significant change in PL in the presence of Hg²⁺ over a wide pH range of 5–9, thus confirming its high stability in the detection of Hg²⁺. The relatively weaker Hg²⁺ responses observed at pH 4 and 10 were presumably due to inefficient formation of boronate and generation of mercury(II) hydroxide, respectively.^59,60 Subsequently, the sensing ability toward Hg²⁺ ions in different ratios of CH₃CN–HEPES buffer solution was evaluated (Fig. 3b). Upon increasing the ratio of the HEPES buffer solution in the CH₃CN–HEPES buffer solution, a remarkable PL signal change in the presence of Hg²⁺ was observed since in an aqueous solution o-aminomethylphenylboronic acid undergoes H₂O insertion to generate boronate.⁶⁰ Since the formation of boronate is essential in a transmetalation reaction,^61–63 the reaction of probe 1 with Hg²⁺ ions occurred more efficiently in solutions with high H₂O content. Therefore, a significant PL response of probe 1 toward Hg²⁺ was induced in a 1 : 9 CH₃CN–HEPES buffer solution. The PL intensity of probe 1 was saturated within 2 minutes in the presence of 10 μM Hg²⁺ ions, indicating that Hg²⁺ ions could be detected quickly under our experimental conditions (Fig. S3†).

Fig. 3 PL intensity changes of probe 1 (10 μM, λ_em = 590 nm, λ_ex = 400 nm) towards 20 μM Hg²⁺ ions (a) in CH₃CN/H₂O (1 : 9 v/v, pH 7.4, 10 mM HEPES) under varying pH conditions and (b) in different ratios of CH₃CN to 10 mM HEPES buffer solution (pH = 7.40).

Proposed sensing mechanism

¹H nuclear magnetic resonance (NMR) experiments with probe 1 in CD₃CN/D₂O (1 : 1) were then conducted (Fig. S4†). Upon the addition of Hg²⁺ ions to probe 1, the signals at 3.75 ppm corresponding to the two methylene protons connected to the tertiary nitrogen were slightly downfield-shifted because of the binding of Hg²⁺ and the tertiary amine. However, after the addition of an excess amount of Hg²⁺ ions, the proton signals significantly decreased, and two new methylene proton peaks appeared at 3.72 and 3.53 ppm due to the reaction between boronic acid and Hg²⁺ ions. Similarly, the methyl proton signal at 2.25 ppm moved gradually downfield with the addition of Hg²⁺ ions with a decreased intensity, while a new peak appeared at 2.21 ppm with an increased intensity. High-performance liquid chromatography coupled with Thermo Scientific linear ion trap-orbitrap mass spectrometry (HPLC-LTQ-orbitrap) analysis was performed to confirm the reaction product generated after the addition of Hg²⁺ ions (Fig. S5†). Before the addition of Hg²⁺ ions, an m/z signal appeared at 934.6, which was attributed to probe 1. Upon the addition of 20 μM Hg²⁺ ions to probe 1, the m/z signal of probe 1 disappeared, and a major peak at 1115.6 appeared, which was attributed to 1-Hg²⁺. Both NMR and HPLC-LTQ-orbitrap analyses confirmed that probe 1 reacted effectively with Hg²⁺ through transmetalation to generate 1-Hg²⁺.

To confirm the role of boronic acid in the PL and ECL signal changes with Hg²⁺ ions, probe 2 without boronic acid was designed. The PL of probe 1 significantly decreased in the presence of 20 μM Hg²⁺ ions, while probe 2 hardly showed any PL changes, even after the addition of an excess amount of Hg²⁺ ions (Fig. 4). These results indicated that the transmetalation of boronic acid with Hg²⁺ ions is essential in the PL/ECL sensing mechanism.

Fig. 4 PL intensity changes of (a) probe 1 and (b) probe 2 (10 μM) upon the addition of Hg²⁺ ions in CH₃CN/H₂O (1 : 9 v/v, pH 7.4, 10 mM HEPES).

To verify the signal changes in PL, an electrochemical analysis with DPV was performed (Fig. S6 and Table S2†). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of probe 1 were calculated to be −5.78 and −3.12 eV, respectively. Meanwhile, a mixture of probe 1 and 10 μM Hg²⁺ ions revealed a new peak at −0.54 V (vs. Fc/Fc⁺), which was attributed to the LUMO energy level (−4.26 eV) of –Hg⁺, while the HOMO and LUMO energy levels of the Ir(III) complex were similar to those of probe 1. The LUMO energy level of –Hg⁺ lies between the HOMO and LUMO levels of the Ir(III) complex (Fig. S7†), which indicated that the PL signals of probe 1 were restrained by the photo-induced electron transfer (PeT) process from the Ir(III) complex to –Hg⁺. Therefore, probe 1 showed decreased PL signals in the presence of Hg²⁺. Meanwhile, 1-Hg²⁺ showed a more stabilised LUMO energy level by 0.13 eV compared to probe 1 due to the mercury cation introduced into probe 1 through the transmetalation reaction. These results are in accordance with the red-shift tendency observed in the UV-vis absorption spectra of probe 1 with Hg²⁺ ions (Fig. S1†).

ECL properties of 1

Fig. 5a shows the ECL emission profile during cyclic voltammetry in aqueous media (pH 7.4, HEPES buffer/CH₃CN = 9 : 1 v/v, 0.1 M 2-(dibutylamino)ethanol (DBAE), 0.1 M HEPES, 0.1% polidocanol, and 0.1 M tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte). Probe 1 produced a strong ECL signal around 1.05 V. However, the ECL signals gradually decreased as the amounts of Hg²⁺ increased. Interestingly, the ECL signals showed a linear correlation with Hg²⁺ concentrations in the range of 0–10 μM (Fig. 5b, R² = 0.98) with a LOD of 8.03 nM, which was much lower than the LOD determined by the PL method. This value falls below the maximum allowable contaminant level of mercury in drinking water recommended by the U.S. EPA. The high sensitivity of probe 1 demonstrates its ability to effectively detect Hg²⁺ ions in waste streams, which can have significant adverse effects on the environment.

Fig. 5 (a) ECL intensities of probe 1 (10 μM) upon the addition of Hg²⁺ in CH₃CN/H₂O (1 : 9 v/v, pH 7.4, 100 mM DBAE, 0.1 M HEPES, 0.1% polidocanol and 0.1 M TBAP as the supporting electrolyte) while the potential is swept at a GC disk electrode (diameter 2 mm) in the range of −0.3–1.3 V (scan rate: 0.1 V s⁻¹). (b) ECL intensity changes of probe 1 at 1.05 V upon the addition of Hg²⁺. Inset: a linear correlation between the ECL intensity at 1.05 V and the concentration of Hg²⁺.

As shown in Fig. 6, the ECL intensity of probe 1 did not show any significant changes upon the addition of other metal ions; however, a significant decrease in the ECL signal (∼20% that of probe 1) was observed after the addition of 20 μM (2 equiv.) of Hg²⁺. The designed system showed rapid reaction rate, high sensitivity, and remarkable selectivity toward Hg²⁺ ions, thus offering a new practical method for field monitoring of Hg²⁺ ions.

Fig. 6 (a) ECL intensities of probe 1 (10 μM) in the presence of various metal ions (40 μM each; Hg²⁺ 20 μM) in CH₃CN/H₂O (1 : 9 v/v, pH 7.4, 100 mM DBAE, 0.1 M HEPES, 0.1% polidocanol, and 0.1 M TBAP as the supporting electrolyte) while the potential is swept at a GC disk electrode (diameter 2 mm) in the range of −0.3–1.3 V (scan rate: 0.1 V s⁻¹). (b) ECL intensities at 1.05 V of probe 1 (10 μM) upon the respective addition of various metal ions (40 μM) and Hg²⁺ ions (20 μM).

Consequently, probe 1 can be used as a highly selective Hg²⁺ probe based on ECL analysis (Scheme 3). Probe 1 was directly oxidised on glassy carbon (GC) electrodes to form 1˙⁺. Subsequently, it was turned into the excited state, 1*, by one-electron transfer from the DBAE radical (DBAE˙), which emitted red ECL. In the presence of Hg²⁺, probe 1 rapidly reacted with Hg²⁺via an irreversible transmetalation reaction to generate 1-Hg²⁺. The excited state of 1-Hg²⁺ was formed via a mechanism similar to that of probe 1 when oxidation occurs on the GC electrode. However, (1-Hg²⁺)* was rapidly consumed through an immediate PeT process from (1-Hg²⁺)* to –Hg⁺, resulting in a significant decrease in the ECL intensity (Fig. S7†).

Scheme 3 The proposed sensing mechanism of probe 1 for detection of Hg²⁺ ions through ECL analysis.

Quantification of Hg²⁺ ions in tap water

To confirm the reliability of the ECL sensing system in real samples, probe 1 was used to quantify Hg²⁺ ions in tap water using the standard addition method. Analytical solutions were prepared by diluting 1 mL of tap water containing varying concentrations of Hg²⁺ ions with 1 mL of CH₃CN/H₂O (1 : 4 v/v, pH 7.4, 20 μM probe 1, 200 mM DBAE, 0.2 M HEPES, 0.2% polidocanol, and 0.2 M TBAP). Hg²⁺ ions in the diluted analytical solutions were quantified by comparing ECL values with the calibration curve shown in Fig. 5b. Then, the actual concentrations of Hg²⁺ ions in tap water samples were accurately determined by multiplying the values obtained from the calibration curve by the dilution factor of 2. First, the ECL emission of probe 1 in tap water was assessed and almost no change in the ECL signal occurred prior to the addition of Hg²⁺ (Table 1). This observation was made in comparison with the ECL intensity of probe 1 in CH₃CN/H₂O (as shown in the inset of Fig. 5b), suggesting that the tap water sample contained a negligible amount of Hg²⁺. As the concentration of Hg²⁺ ions increased, the ECL signal of probe 1 decreased. The recovery rates for Hg²⁺ ions ranged from 97.34% to 105.93%, indicating the high reliability and feasibility of this system for the detection of Hg²⁺ in actual environmental samples.

Table 1 Quantification of Hg²⁺ in tap water samples^a

[Hg^2+]added ^b	Rel. ECL intensity (experiment, n = 3)	[Hg^2+]calc'd (μM)	Recovery (%)
a All ECL intensities were measured along with the cyclic voltammetry (CV) scan in CH3CN/H2O (1 : 9 v/v, pH 7.4, 100 mM DBAE, 0.1 M HEPES, 0.1% polidocanol, and 0.1 M TBAP) while the potential was swept at a GC working electrode in the range of −0.3–1.3 V vs. Fc/Fc^+. b Concentrations of Hg^2+ ions in tap water samples.
0 μM	1 ± 0.0167	—	—
3 μM	0.7514 ± 0.0028	3.1780 ± 0.0347	105.93 ± 1.10
5 μM	0.6371 ± 0.0071	5.0356 ± 0.1145	100.71 ± 2.27
7 μM	0.5277 ± 0.0040	6.8137 ± 0.0655	97.34 ± 0.96
10 μM	0.3409 ± 0.0068	9.8500 ± 0.1107	98.50 ± 1.12

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Conclusions

In summary, we developed a novel photoluminescent (PL) and electrochemiluminescent (ECL) dual signalling probe based on an Ir(III) complex with boronic acid as the reaction site for the selective detection of Hg²⁺ ions. PL and ECL analyses with the probe showed it had remarkable selectivity for Hg²⁺ ions with limits of detection of 0.72 μM and 8.03 nM, respectively. Furthermore, the successful quantification of Hg²⁺ ions in tap water samples using the ECL analysis demonstrated the effectiveness of this probe. Consequently, our system serves as a new proof-of-concept for real-time and on-site sample analysis. We believe that our strategy would be of great help to the advancement of developing ECL-based probes and other fields concerning ECL-based analysis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NRF grant funded by the Korean Government (MSIT) (No. 2023R1A2C2006049).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3an01266b

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