Carbon Dot–Cellulose Nanocrystal Hybrids from Biowaste for Security Encryption and Metal Ion Detection

Cellulose nanocrystals (CNCs) are of growing interest due to their tunable optical properties, which can be further enhanced through surface modification or elemental doping. In this study, we developed a multifunctional material by first decorating CNCs extracted from almond peels (CNCs_AM) with carbon dots (CDs), forming CNCs/CD_AM, and subsequently introducing boron doping to obtain superior optical properties in CNCs/CD_AM-BA. The structural and optical characteristics of the CNCs/CD hybrids were examined. Boron doping modifies the thermal stability, surface charge, and optical properties of the CNCs/CD_AM. The CNCs/CD_AM-BA exhibit enhanced optical activity while remaining invisible in daylight, making them suitable for security inks. CNCs/CD_AM-BA exhibit sensing capabilities for Fe³⁺ ions through fluorescence quenching, with minimal interference from other metal ions. At pH 8, the sensor exhibits optimal fluorescence performance with a detection limit of 50.5 μM for Fe³⁺. A morphological study revealed alterations in size and surface roughness due to doping. The potential for extensive anticounterfeiting applications in paper products and packaging is demonstrated using CNCs/CD_AM-BA as water-based security inks for spray printing. This study highlights the versatility of CNCs/CD_AM-BA in developing advanced multifunctional materials for security and sensing applications.