In the present study, the syntheses of crystalline silver and gold nanoparticles (NPs) has been demonstrated in a green and environmentally friendly approach using citrus fruit extracts (lemon, Citrus limon (Cl-1) and sweet orange, Citrus limetta (Cl-2)). In addition, potentially hazardous metal ion sensing properties of these NPs in aqueous solution has been explored. Cl-1 and Cl-2 that predominantly contained citric and ascorbic acid, exhibited different reducing abilities towards silver and gold ions into NPs. Cl-1 reduces silver ions into AgNPs only in the presence of sunlight whereas AuNPs from gold ions were formed without exposure to sunlight. In contrast, Cl-2 converts both silver and gold ions into the corresponding NPs in the absence of sunlight. The colorimetric sensor studies of these green synthesized AgNPs (Cl-1–AgNPs) showed selective sensing of the potentially hazardous Hg²⁺ ion in water at micromolar concentrations. More importantly, green synthesized Cl-1–AgNPs sensor systems detected Hg²⁺ ions in water in a wide pH range (3.2 to 8.5).

Smart fluorescent materials exhibit a controlled fluorescence response to different external stimuli such as pressure, heat, light, pH, etc. Molecular structure and supramolecular assembly via weak intermolecular interactions strongly influence the fluorescence efficiency and colour. In particular, non-planar molecular structures play a significant role in developing solid state fluorescent materials. Triphenylamine (TPA), a typical non-planar propeller molecule with interesting optoelectronic properties, provides an excellent opportunity for developing a variety of molecular fluorescent materials by taking advantage of synthetic feasibility. For example, a donor–acceptor (D–A) aggregation induced emissive (AIE) fluorophore can be developed by integrating an acceptor group into the phenyl unit of a TPA donor. In this review, we focus exclusively on TPA-based smart fluorescent materials that exhibit distinct and reversible fluorescence switching towards different stimuli. Molecular engineering of the TPA fluorophore resulted in the development of different types of stimuli-responsive materials and the conformational flexibility of non-planar phenyl groups often produced polymorphism induced fluorescence tuning. A detailed discussion of the structure–property relationships of TPA derivatives could provide insights for developing new TPA-based smart fluorescent materials with versatile properties.

Selective colorimetric sensing of potentially health and environmentally hazardous heavy metal ions in aqueous solution was demonstrated using green synthesized silver nanoparticles (AgNPs) from plant extracts, by taking advantage of metal coordinating organic functional groups present in the extracts. AgNPs synthesized using fresh neem leaf extracts (NF-AgNPs) selectively detected Hg²⁺ whereas sun-dried neem leaf extract based AgNPs (ND-AgNPs) were observed to selectively sense Hg²⁺ and Pb²⁺ at micromolar concentrations. Neem bark extract based AgNPs exhibited selective colorimetric sensing of Hg²⁺ and Zn²⁺. Similarly, AgNPs synthesized from mango leaf (fresh, MF-AgNPs; sun-dried, MD-AgNPs) and green tea extracts (GT-AgNPs) showed selective colorimetric sensing of Hg²⁺ and Pb²⁺ ions. AgNPs obtained using pepper seed extracts exhibited Hg²⁺, Pb²⁺ and Zn²⁺ selective colorimetric sensor properties. Importantly these green synthesized AgNPs selectively detected the presence of hazardous metal ions in aqueous solutions across a wide range of pH values (2.0–11), which is a highly desirable attribute from the perspective of different sources of water pollution. The selective colorimetric sensing of Hg²⁺, Pb²⁺ and Zn²⁺ ions by green synthesized AgNPs over a wide pH range demonstrates the multi-functional utility of plant extracts in green nanotechnology and environmental sensor applications.

Substitutional change and controlling intra and intermolecular interactions of ESIPT molecules resulted in realizing multifunctional fluorescence properties.