Plasmonic nanogels for naked-eye sensing of food adulterants

Cellulose based nanoplasmonic sensors gained immense attention for various applications due to their advantageous physicochemical characteristics such as ease of fabrication, low density, chirality, surface functionalization and disposal. Herein, a hydrogel based nanoplasmonic sensor probe was fabricated and evaluated for the detection of melamine (MA). Plasmonic nanomaterials (AuNPs) were synthesized by the redox reaction using a dual reducing agent (β-cyclodextrin (βCD) and citrate). The physicochemical characteristics of the synthesized AuNPs were extensively determined by various spectroscopic and microscopic techniques. The colorimetric sensing of melamine (MA) was carried out in solution and hydrogel phases. Upon the addition of MA, AuNPs tend to aggregate and exhibit color changes from orange-red to purple due to surface plasmon resonance (SPR) coupling. This nanosensor probe showed high selectivity and sensitivity for detection of MA with a detection limit of 3 × 10^-7 M. Plasmonic hydrogels were prepared using the cellulose acetate (CA) polymer and optimized for stability and interaction with melamine. The βCD-citrate stabilized AuNPs showed color changes with the CA hydrogels. The hydrogel-based sensor probe exhibits similar characteristics with respect to the selective and sensitive detection of MA under optimized conditions. The fabricated nanoreactor based sensor probe has high potential for food sensor applications.