Impact of aggregation on fluorescence sensitivity of molecular probes towards nitroaromatic compounds

Abstract

We have designed and synthesized three tripods TIBP4, TIBP8 and TIBP12 possessing, respectively, OC₄H₉, OC₈H₁₇ and OC₁₂H₂₅ alkoxy chains on the biphenyl units and have investigated the effect of the chain length on their ease in aggregation and their efficiency in the detection of nitroaromatic compounds. Tripods TIBP8 and TIBP12 self-assemble to form densely populated nano-spheres (60–100 nm) in a water–DMSO (98 : 2) mixture, as shown by field-emission scanning electron microscopy, transmission electron microscopy and dynamic light scattering studies. TIBP4 which has shorter n-butyl alkyl chains does not undergo aggregation under these conditions. Tripods TIBP8 and TIBP12 which remain in a self-assembled state in water reveal amplified fluorescence quenching with PA, 2,4-DNP, TNT and Cl-DNB, and are associated with NAC induced disaggregation to well-dispersed particles. TIBP8 can detect as low as 10⁻¹⁴ M PA and 2,4-DNP in solution and 2.29 × 10⁻²⁰ g cm⁻² (22.9 zg cm⁻²) PA by contact mode and is nearly 6000–16 000 times more selective towards PA and 2,4-DNP over TNT and Cl-DNB at 20% fluorescence quenching. However, the tripod TIBP12 can detect as low as 10⁻¹⁴ M of each PA, 2,4-DNP, TNT and Cl-DNB and can find application as a general probe for these NACs. TIBP4 which remains in a molecularly dissolved state shows poor sensitivity (LOD 1 nM) towards NACs.