Role of Ion-Pair and H-Bonding in Colorimetric Detection of Insulin Fibrils with Picomolar Sensitivity

Ultrasensitive detection of amyloid fibril, a biomarker of several neurological diseases, has been achieved using a hemicyanine dye. Amyloid fibril induces a remarkable change in the absorption spectral position (~115 nm) of the perchlorate salt of the probe leading to a naked-eye detection of these neurotoxic protein aggregates. Such a large shift in the absorption spectra has been utilised to develop a simple and cost effective paper based detection of insulin aggregates with a limit of detection (LOD) of less than 2 pM. Besides the changes in the absorption spectra, the emission intensity of the probe shows almost two orders of magnitude increase in the presence of insulin amyloid fibrils. Such dual sensing properties of the present hemicyanine molecule make it a noble probe for amyloid fibrils. Our detailed investigation on the fundamental mechanism responsible for the fibril-induced spectral changes in the dye has established the role of amyloid induced dissociation of ion-pair and solute-solvent hydrogen bonding in the colorimetric detection of amyloid fibrils. Quantum chemical calculations and blind molecular docking studies have also been performed to strengthen our experimental observations. This result will open up a new frontier in the field of amyloid probes based on the unique mechanism proposed in this work.