Highly Selective Phenanthroline‑Based Fluorescent Probe for Detecting Extreme Alkalinity (pH > 14) in Water

Abstract

Detecting extreme alkalinity (pH > 14) in aqueous media remains a formidable challenge for conventional pH indicators and electrodes. We report a water‑soluble phenanthroline derivative, BMIP, that exhibits an 8 mg mL⁻¹ solubility in pure water and a rapid, highly selective fluorescence response to hydroxide concentrations between 3 and 6 mol L⁻¹. Within 10 s, BMIP converts from a colourless, non‑luminescent state to bright yellow fluorescence (λ_em ≈ 525 nm) upon exposure to pH > 14, while remaining inert to a broad panel of competing ions and to all other pH values (2 ≤ pH ≤ 13.9). The sensor’s performance—high selectivity, low detection limit, excellent anti‑interference, and quantitative linearity (R² = 0.996)—is rooted in a deprotonation–aggregation‑induced‑enhanced‑emission (AIE) mechanism confirmed by absorption, fluorescence, and ¹H‑NMR studies. BMIP therefore establishes a new benchmark for real‑time, aqueous detection of extreme alkalinity in industrial and environmental settings.

Introduction

Industries such as paper manufacturing, nuclear fuel reprocessing, waste‑water treatment, leather processing, and mineral extraction routinely operate under extreme alkaline conditions (pH > 14). Accurate, real‑time monitoring of hydroxide levels in these processes is essential for safety, process control, and environmental compliance. Traditional pH test strips fail in this range, turning uniformly dark blue, while glass‑electrode pH meters produce unreliable readings due to junction potential and ion‑exchange limitations. Fluorescent probes offer a compelling alternative, but most reported systems target pH 2–13, leaving the alkaline regime largely unaddressed. Existing alkaline probes often require organic cosolvents, exhibit poor sensitivity, or suffer from complex photophysical mechanisms that limit quantitative analysis.

1H‑imidazo[4,5‑f][1,10]phenanthroline (IP) is a rigid, planar heterocycle with strong charge‑transport properties and intrinsic fluorescence. Its NH functionality can react with hydroxide, while the aromatic scaffold confers good photostability. By appending two triethylene glycol monomethyl ether (TEG‑MeO) groups, we designed BMIP—a fully water‑soluble derivative that combines the recognition capability of IP with the solubilizing power of PEG chains. The resulting probe exhibits an unprecedented combination of features: (i) 25 mg mL⁻¹ aqueous solubility, (ii) selective hydroxide detection, (iii) AIE‑driven fluorescence turn‑on, and (iv) rapid response times suitable for process monitoring.

Results and Discussion

Synthesis, Solubility, and Optimal Probe Concentration

BMIP was synthesized in three steps via a convergent route starting from triethylene glycol monomethyl ether and 1,10‑phenanthroline‑5,6‑dione (Scheme 1). The final product was isolated as a light‑red, gelatinous solid (yield 83 %) and dissolved readily in water, achieving a solubility of 25 mg mL⁻¹ (≈ 0.4 mmol L⁻¹). We evaluated BMIP concentrations ranging from 2 × 10⁻⁵ to 4 × 10⁻³ mol L⁻¹ and determined that 1 mmol L⁻¹ (10⁻³ mol L⁻¹) provided the optimal balance between signal intensity and dynamic range for hydroxide detection.

Selective Recognition of Extreme Alkalinity

To assess ion selectivity, we added 3 mol L⁻¹ of a diverse set of cations and anions—including transition metals, alkali and alkaline earth ions, halides, and oxyanions—to a 1 mmol L⁻¹ BMIP solution. Fluorescence spectra (λ_em ≈ 525 nm) and visual inspection under UV (365 nm) revealed that only the addition of NaOH at pH > 14 induced a pronounced colour change from colourless to orange‑yellow and a 1000‑fold fluorescence enhancement. All other ions produced negligible spectral perturbations (Fig. 2), confirming exceptional selectivity toward hydroxide.

Anti‑Interference Capability

We further challenged the probe by introducing common industrial salts (KCl, Na₂SO₄, NaNO₂, NaNO₃, NaClO₄, NaBr, KI) into a BMIP–NaOH mixture. Fluorescence intensity varied by less than 5 % upon addition of each salt, demonstrating robust anti‑interference performance (Fig. 2d).

pH‑Dependent Response

BMIP exhibited a strictly pH‑selective response: solutions with pH < 14 remained non‑luminescent, whereas a sudden fluorescence turn‑on occurred only when the solution reached extreme alkalinity (pH > 14, achieved with 3 mol L⁻¹ NaOH). Across the entire pH range (2–14), the probe remained photochemically silent, ensuring that any detected signal unambiguously reports extreme hydroxide presence (Fig. 3).

Quantitative Detection and Kinetics

We established a calibration curve by measuring fluorescence intensity of BMIP (1 mmol L⁻¹) in the presence of 3–6 mol L⁻¹ NaOH. The plot of I vs [OH⁻] displayed excellent linearity (R² = 0.996) (Fig. 4b), enabling precise quantification of hydroxide concentration in the 3–6 mol L⁻¹ range. Time‑resolved measurements showed that the fluorescence response reached a plateau within 10 s, satisfying the rapid‑response criterion for industrial monitoring (Fig. 4c).

Repeatability

Repeated addition of NaOH, followed by neutralization with sulfuric acid and re‑addition of NaOH, produced identical fluorescence profiles, confirming that BMIP’s sensing cycle is fully reversible and highly reproducible (Fig. 4d).

Detection Mechanism

Deprotonation of the IP NH group by hydroxide generates a negatively charged species (BMIP⁻) that precipitates and aggregates in aqueous media. Aggregation‑induced enhanced emission (AIE) accounts for the bright yellow fluorescence observed at pH > 14. ¹H‑NMR in D₂O and DMSO‑d₆ confirmed the disappearance of the NH signal upon NaOH treatment, corroborating the deprotonation pathway. The emergence of a new absorption band near 385 nm at pH ≥ 12.26 further supports the formation of BMIP⁻ (Fig. 5). The combined photophysical data confirm that hydroxide recognition proceeds via a two‑step process: deprotonation followed by AIE.

Conclusion

We have developed BMIP, a phenanthroline‑based fluorescent probe that offers superior selectivity, sensitivity, and quantitative capability for detecting extreme alkalinity (pH > 14) in pure water. Its high solubility, rapid response (≤ 10 s), robust anti‑interference, and clear linear calibration make it a practical tool for real‑time monitoring in industrial and environmental applications. The probe’s AIE‑driven detection mechanism opens avenues for designing next‑generation alkaline sensors.

Methods

General Information

All reagents were purchased commercially and used without further purification. NMR spectra were recorded on a Bruker Avance 400 MHz instrument; LC‑MS data were obtained on a Shimadzu LCMS‑2020. Fluorescence measurements employed a Shimadzu RF‑5301 PC spectrometer. All synthesis steps were performed under inert nitrogen atmosphere using Schlenk techniques. Aqueous detection experiments were conducted at ambient temperature (22 °C) in deionized water.

Synthesis of BMIP

See Scheme 1 for the synthetic route. The final product was purified by column chromatography (EtOAc/MeOH 10:1) to give a light‑red gelatinous solid (621.29 Da, HRMS). Detailed synthetic procedures and spectral data are provided in the Supplementary Information.

Photophysical Measurements

All fluorescence and absorption spectra were recorded using a 1 mm quartz cuvette. For ion‑selectivity tests, 3 mol L⁻¹ solutions of each salt were added to a 3 mL BMIP (1 mmol L⁻¹) solution and the spectra recorded. pH‑dependence studies used freshly prepared buffers ranging from 1 mol L⁻¹ HCl to 3 mol L⁻¹ NaOH. Calibration curves were generated by plotting fluorescence intensity at 525 nm versus hydroxide concentration.

Reproducibility and AIE Experiments

All experiments were performed in triplicate; standard deviations were < 5 %. For AIE confirmation, a BMIP–NaOH solution was titrated with incremental NaOH (0.1 mL of 3 mol L⁻¹) while monitoring fluorescence changes.