Optically active phenolphthalein encapsulated gold nanodendrites for fiber optic pH sensing

Abstract

Herein, sol-gel based synthesis of poly ethanol glycol assisted gold nanodendrites (AuNDs) is reported at low-temperature. Taking advantage of its facile synthesis, phenolphthalein (phph) is encapsulated/immobilized in AuNDs for the assay of fiber optic pH sensing. Hierarchical structure is observed by FE-SEM analysis before and after phph encapsulation. EDX analysis confirms the uniform distribution of elements with smooth morphology. AFM analysis shows that phph encapsulated AuNDs have low average roughness (2 nm) and less thickness ~20 nm. The chemical bonding between phph species and Au molecules are explored by FTIR analysis. The polycrystalline nature of AuNDs matrix before and after encapsulation is confirmed by XRD. HR-TEM analysis exhibited preferential growth of dendrites structure before and after encapsulation along (111) direction. UV–Vis analysis shows that AuNDs has refractive index 1.24 at 550 nm, which reduces down to 1.18 at 550 nm after phph encapsulation. 30 cm coated middle segment of plastic clad silica fiber (PCS) is used as sensor. Variations in output intensities in terms of transmission are used to characterize the sensor response. A fast response ~0.87 s, repeatability/reproducibility and linear response (R2 = 0.8912) is obtained at 440 nm. Acidity of rainwater (pH 6) is determined by prepared sensor for validity purpose and practical applications.