Halogen bond triggered aggregation induced emission in an iodinated cyanine dye for ultra sensitive detection of Ag nanoparticles in tap water and agricultural wastewater

Abstract

Aggregation induced emission (AIE) has emerged as a powerful method for sensing applications. Based on AIE triggered by halogen bond (XB) formation, an ultrasensitive and selective sensor for picomolar detection of Ag nanoparticles (Ag NPs) is reported. The dye (CyI) has an iodine atom in its skeleton which functions as a halogen bond acceptor, and aggregates on the Ag NP plasmonic surfaces as a halogen bond donor or forms halogen bonds with the vacant π orbitals of silver ions (Ag⁺). Formation of XB leads to fluorescence enhancement, which forms the basis of the Ag NPs or Ag⁺ sensor. The sensor response is linearly dependent on the Ag NP concentration over the range 1.0-8.2 pM with an LOD of 6.21 pM (σ = 3), while for Ag⁺ it was linear over the 1.0-10 μM range (LOD = 2.36 μM). The sensor shows a remarkable sensitivity for Ag NPs (pM), compared to that for Ag⁺ (μM). The sensor did not show any interference from different metal ions with 10-fold higher concentrations. This result indicates that the proposed sensor is inexpensive, simple, sensitive, and selective for the detection of Ag NPs in both tap and wastewater samples.

Fig. 1. Absorption (a) and emission (λ_ex = 420 nm) (b) spectra of CyI in different solvents at room temperature.

Fig. 2. Linear plot of (a) absorption and (b) emission maximum versus the E_T(30) solvent parameter for CyI.

Fig. 3. Emission spectra of CyI at different concentrations.

Fig. 4. TEM of Ag NPs prepared using (a) trisodium citrate as the reducing agent, (b) Ag⁺ and CyI as the reducing agent, and (c) CyI + Ag NPs (after 2 days).

Fig. 5. Emission spectra of 10 μM CyI and increasing concentration of Ag⁺ cations in aqueous media (pH = 7) at room temperature. The inset is the change in the emission spectra (600 nm) upon addition of different Ag⁺ concentrations.

Fig. 6. Emission spectra of 10 μM CyI and increasing concentration of Ag NPs in aqueous media (pH = 7) at room temperature. The inset is the change in the emission spectra (600 nm) upon addition of different Ag NP concentrations.

Fig. 7. (a) Calculated MEP of CyI and (b) optimized structure of CyI–Ag at the B3LYP/LanL2DZ level of theory.

Scheme 1. Microwave synthesis of Cy and CyI.

Fig. 8. Emission spectra of (a) Cy (10 μM) at different concentrations from Ag NPs, (b) CyI (10 μM) under different concentrations of CB6 and (c) CyI (10 μM) under different concentrations of CB7 in aqueous solution.

Fig. 9. Optimized structures (B3LYP/LanL2DZ) of (a) CyI–CB6, and (b) CyI–CB7 molecular adducts.

Fig. 10. Emission spectra of (a) 10 μM CyI (green), CyI + 10.3 μM CB6 (black) and CyI + 10.3 μM CB6 + 7.88 μM Ag NPs (red); (b) 10 μM CyI (green), CyI + 6.8 μM CB7 (black) and CyI + 6.8 μM CB6 + 7.88 μM Ag (red).

Fig. 11. Calibration curve for CyI emission with increasing concentrations of Ag NPs.