Cellulose nanocrystals from waste cotton fabric and polyvinyl alcohol composite hydrogel for non-invasive colorimetric sensing of cysteine.

Cellulose nanocrystals (CNCs) were successfully extracted and purified from cotton fabric waste via alkali treatment and acid hydrolysis. The resulting nanostructure was characterized using transmission electron microscopy (TEM) and X-ray diffraction spectroscopy (XRD), confirming the successful isolation of CNCs. A PV/Cu-CNCs/CNFs/PVA hydrogel was then synthesized through chemical crosslinking of polyvinyl alcohol (PVA), cellulose nanofibrils (CNFs), cellulose nanocrystals (CNCs), and borax. The incorporation of CNCs as a reinforcing agent significantly enhanced the hydrogel's surface area and water absorption capacity, leading to a substantial improvement in the colorimetric sensitivity of the sensor. This hydrogel was employed as a non-invasive colorimetric sensor for urinary cysteine detection, utilizing an indicator-displacement assay (IDA) with pyrocatechol violet/copper ion (PV/Cu) as the indicator. Upon exposure to cysteine, the sensor exhibited a distinct color change from greenish-blue to orange-red. The sensor demonstrated a linear detection range of 0-0.6 g/L and a limit of detection (LOD) of 0.017 g/L, effectively encompassing the clinical cut-off level for cysteine in human urine (0.25 g/L), relevant for Alzheimer's disease indication. Eventually, this sensor was validated for cysteine determination in artificial urine samples, confirming its practical applicability.