Advanced Fluorescent Silsesquioxane Sensors for Dual Detection of Fluoride and PAHs

 

In an era where environmental monitoring and rapid detection of industrial pollutants are more critical than ever, the development of advanced chemical sensors is at the forefront of materials science. A recent study published in Organometallics by Siripanich et al. (2022) introduces an innovative class of dual-response fluorescent sensors based on pyrene-functionalized silsesquioxane (SQ) cages, capable of selectively detecting fluoride ions and polycyclic aromatic hydrocarbons (PAHs) with high sensitivity and speed.

Silsesquioxane: A Molecular Scaffold with Multifunctionality

Silsesquioxanes (SQs) are cage-like organosilicon compounds with the empirical formula RSiO₁.₅, representing a unique hybrid of inorganic and organic chemistry. Their well-defined structure, solution-processability, and tunable surface chemistry make them excellent candidates for sensor development.

The authors synthesized two distinct sensor architectures:

• Mono-pyrene SQ (Compound 2): A single pyrene fluorophore covalently attached to a T8 silsesquioxane cage.

• Dumbbell-pyrene SQ (Compound 5): A larger, bulkier structure with two SQ cages linked via a pyrene unit.

Both structures were synthesized via Heck coupling reactions and comprehensively characterized using NMR spectroscopy, high-resolution mass spectrometry, UV-Vis absorption, and fluorescence spectroscopy.

Dual Sensing Mechanism

These pyrene-SQ conjugates exhibit intense deep-blue fluorescence under UV excitation. Upon exposure to target analytes, they demonstrate distinct changes in optical properties:

1. Fluoride Detection:

   • Fluoride ions interact specifically with the vinylic silicon atoms in the SQ cages, forming Si–F bonds.

   • This interaction induces a significant fluorescence quenching effect and shifts in UV-Vis absorbance, attributed to intramolecular charge-transfer (ICT) processes.

   • Compound 2, with more accessible vinylic sites and lower steric hindrance, exhibited faster fluoride response kinetics than the bulkier Compound 5.

2. PAH Detection:

   • Detection is based on π–π stacking interactions between the pyrene fluorophores and PAHs such as 1-nitropyrene and 1-pyrenecarboxaldehyde.

   • These interactions result in aggregation-caused quenching (ACQ), rapidly diminishing the emission intensity.

   • The fluorescence response is immediate, enabling real-time detection of hazardous PAHs in solution.

Analytical Performance

The detection limits (LOD) for both fluoride and PAHs were impressively low, ranging between 1–3 μM. Association constants for the most responsive PAHs reached up to 4.7 × 10⁵ M⁻¹, indicating strong host–guest interactions. Notably, the sensors also enabled visual detection under UV light due to distinct color changes upon fluoride binding.

Additionally, time-resolved fluorescence studies and density functional theory (DFT) calculations provided mechanistic insights into the energy transfer and quenching pathways, further validating the sensor design.

Implications and Future Directions

The development of these dual-functional sensors opens avenues for:

• On-site environmental monitoring of fluoride emissions and PAHs in industrial and urban settings.

• Integration into smart materials or devices for real-time pollutant detection.

• Expansion into multi-analyte sensing platforms using modified fluorophores or cage structures.

This work not only highlights the potential of silsesquioxane chemistry in advanced sensing applications but also underscores the importance of molecular-level design in achieving selectivity, sensitivity, and functionality.

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Reference:
Siripanich, P.; Bureerug, T.; Chanmungkalakul, S.; Sukwattanasinitt, M.; Ervithayasuporn, V. Mono and Dumbbell Silsesquioxane Cages as Dual-Response Fluorescent Chemosensors for Fluoride and Polycyclic Aromatic Hydrocarbons. Organometallics 2022, 41, 201–210. https://doi.org/10.1021/acs.organomet.1c00460