Optical sensing of insulin using liquid crystal orientational transitions triggered by insulin-aptamer interactions

Abstract

Diabetes mellitus is a significant global health concern, with insulin playing an essential role in its pathogenesis. Early and sensitive detection of insulin is vital for effective diabetes management, necessitating an economic, facile, and sensitive, diagnostic method. Herein, we propose a label-free optical immunoassay for naked-eye and sensitive detection of insulin via specific target-aptamer binding event modulating the birefringence of nematic liquid crystals (LCs) of 4-cyano-4-pentylbiphenyl (5CB). The glass slide surface were functionalized with a combination of (3-Glycidoxypropyl)methyldiethoxysilane (GPTMS) and N,N-dimethyl-N-(3-(trimethoxysilyl)propyl)-1-octadecanaminium chloride (DMOAP), a modification that induces a perpendicular orientation of the LC film and introduces active epoxy groups. These epoxy groups on the surface exhibit an advantage of covalently binding to the amino-modified aptamer without the need for an additional linker. The specific recognition probes were immobilized onto the sensing substrate without disrupting the LCs orientation. Upon the addition of insulin, the aptamer preferentially binds to it through hydrogen bonding, resulting in a distinct bright LC texture that is visibly different from the pattern observed in the absence of insulin. Under optimized conditions, the method demonstrated a linear detection range for insulin from 0.05 to 3.0 nM. This approach is highly economical, requiring only 10 μL of reagent, and it exhibits excellent specificity, stability, and reproducibility. Additionally, the recovery rates, ranging from 95.43 % to 112.29 % in diluted human urine samples, show the potential of this LC-based aptasensor for practical applications in clinical and diagnostic settings.