Tailored fluorescent polyionic nanoclays for enhanced sensing applications
Authors:   , Ishtaweera Piyuni Shakila 
Published: September 2024

Skin-interfaced wearable electronics capable of real-time monitoring of vital biophysiological signals have gained significant attention. However, traditional wearable devices often face challenges such as the use of costly materials, intricate fabrication processes, and poor stability under mechanical stress and prolonged wear. Moreover, the limited breathability of substrates can compromise comfort and cause inflammation over long-term use. My research primarily addresses these challenges by innovating materials, adapting fabrication technologies, and modifying devices to enhance breathability, stretchability, affordability, and other unique characteristics essential for on-skin wearables. The dissertation starts from the development of cost-effective, eco-friendly, and breathable wearable electronics. Direct writing technique to apply conductive graphite patterns onto cellulose paper, facilitating the development of on-skin electronics (chapter 1). Then laser-scribed molybdenum dioxide (LSM) with high electrical conductivity, biocompatibility, chemical stability, and MRI compatibility.is developed to achieve mask-free, high-resolution, and large-scale fabrication of highly conductive materials on flexible substrates and Janus devices capable of monitoring both bodily and environmental signals (Chapter 2) is also obtained. Using phase-separation technology, we develop porous composites of silver nanowires (Ag NWs) with an ultralow percolation threshold. These composites enable strain-resilient near-field communication (NFC), facilitating wireless powering and data transmission for both skin-interfaced and implantable bioelectronics (Chapter 3). Last work is focus on the engineering cellulose nanofiber interfaces (CNFI) on porous substrates to achieve surface flatness for high-quality bioelectronics printing and to construct mechanical heterogeneity for strain-resilient bioelectronics. Additionally, CNFI for microfluidic channel (MFC) is also constructed for continuous and real-time collection, transportation, and discharge of sweat (Chapter 4).We demonstrate the integration of fluorescent species into onium ion-functionalized polyionic nanoclays (PINCs) via covalent modification methods, facilitating the creation of solvent-responsive materials with sensing capabilities. By co-incorporating functional targets such as 3-aminopropyl or 3-mercaptopropyl groups, subsequent conjugation with reactive fluorophores enhances quantum yields and enables solvent-responsive properties. For instance, dansyl chloride and acrylodan conjugation generates solvent-responsive fluorescent PINCs capable of sensing the local environment (polarity, acidity) and engaging in guest-host interactions. Dansyl-labelled nanoclay exhibits a [approx]7-fold increase in quantum yield compared to free probe, while maintaining a consistent pKa. This work lays a foundation for engineered nanosheets suitable for imaging, theranostics, and signal-amplified sensing applications. In parallel, we successfully incorporate pyrene probes, known for their structured emission and prolonged excited state lifetimes, into imidazolium-functionalized PINCs via covalent modification with 3-mercaptopropyl groups, followed by thiolene click chemistry. Pyrene functionalization levels range from 0.1 percent to 15 percent of total silane groups on the clay surface. Our investigation reveals composition-dependent selective excimer and monomer formation in ethanolic solutions, with increased pyrene modification correlating with enhanced excimer formation, indicating uniform surface modification. Pyrene-conjugated PINCs (Py-PINCs) demonstrate promising capabilities in detecting explosives such as TNT, RDX, and PETN, with efficient quenching of excimer emission observed, particularly with TNT. The Stern-Volmer quenching constant for TNT is 15600[plus or minus]134 M-1. Additionally, Py-PINCs exhibit sensitivity to halide ions, with higher quenching efficiency observed for iodide compared to bromide, facilitated by enhanced electrostatic interactions with the positively charged PINC surface. Solid-state Py-PINCs serve as oxygen sensors, displaying fluorescence quenching upon exposure to oxygen gas, with a linear response to increasing oxygen pressure and a Stern-Volmer quenching constant of 5.29[plus or minus]0.07 bar-1. This research establishes a foundation for advanced sensing technologies applicable in security, environmental monitoring, and biomedical research. Furthermore, we report the synthesis of magnesium phyllo(organo)silicate nanosheets with a 2:1 phyllosilicate structure, characterized by a negative surface charge from covalently attached carboxylate and sulfonate groups, forming polyanionic nanoclays. Pyrene probes are successfully integrated into these nanoclays via covalent modification with 3-mercaptopropyl groups, enabling subsequent conjugation through thiol-ene click chemistry. These pyrene-tagged polyanionic nanoclays exhibit a superquenching effect towards lead ions (Pb2+), with Stern-Volmer quenching constants of (8.4 [plus or minus] 0.4) x 107 M-1 and (5.7 [plus or minus] 0.3) x 107 M-1 for pyrene-tagged carboxylate and sulfonate polyanionic nanoclays, respectively. This phenomenon is attributed to the high anionic charge density on the surface, which attracts Pb2+ ions through electrostatic interactions. Selective and efficient quenching is observed for Pb2+ compared to other metal ions, significantly greater than neutral and anionic free pyrene probes and pyrene-tagged polycationic nanoclays. This research provides a framework for the development of next-generation engineered nanosheets tailored for advanced sensing technologies, offering selective and efficient quenching compared to other metal ions and nanoclay compositions.