Graphene oxide-alginate hydrogel-based indicator displacement assay integrated with diaper for non-invasive Alzheimer's disease screening

Pyrocatechol violet/copper ion-graphene oxide/alginate (PV/Cu²⁺-GO/Alg) hydrogel was fabricated and applied as a colorimetric sensor for monitoring urinary cysteine via an indicator-displacement assay (IDA) and Cu²⁺-cysteine affinity pair. The hydrogel-based sensor was formed by Ca²⁺ cations cross-linked PV/Cu²⁺-GO/Alg. The morphologies of hydrogel were characterized by field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy and Fourier-transform Raman spectroscopy. Incorporating GO into the hydrogel improved its uniformity of porosity, large surface area, and compressive strength, leading to amplified colorimetric signals of the hydrogel sensor. Under optimal conditions, this sensor offered a linear range of 0.0–0.5 g/L with a detection limit of 0.05 g/L for cysteine without interfering effects in urine. Furthermore, this hydrogel-based sensor was applied for urinary cysteine detection and validated with laser desorption ionization mass spectrometry. This platform could be used to determine cysteine at its cutoff (0.25 g/L) in human urine, which was distinguishable between normal and abnormal individuals, to evaluate an early stage of Alzheimer's disease. Eventually, this system was integrated with diapers for a wearable cysteine sensor.