Development and characterization of gold nanoparticle-based colorimetric sensing assay functionalized with L-cysteine for the detection of calcium carbide in ripened mango and banana fruits

Calcium carbide (CaC₂) is generally used as an artificial ripening agent in the agricultural and food sectors but has been prohibited due to its negative impacts on the environment and human. Therefore, in the present study, a novel and cost-effective detection technique was developed for the detection of CaC₂ using L-cysteine functionalized gold nanoparticles (AuNPs). In this work, a rapid, simple, environment friendly and portable colorimetric nanosensing assay was developed using two different sizes of L-cysteine functionalized gold nanoparticles (AuNP₂₀ and AuNP₄₀). The sensing strategy relied on the fact that the wherein introduction of CaC₂ initiates aggregation, resulting in detectable color change (red to purple) that helps in detection of CaC₂. The efficiency of the developed sensor was investigated on the artificially ripened mangoes and bananas for selective detection of CaC_2. The characterization of synthesized AuNPs was done using Uv-vis spectroscopy, FTIR, particle size, HR-TEM, and XRD analysis. The developed and optimized nanosensing assay was effectively utilized to detect the presences of CaC₂ in carbide treated mango and banana fruits. The 20 nm and 40 nm size and stability (-32.6 mV for 20 nm and -27 mV for 40 nm) of the developed AuNPs were confirmed by TEM analysis, particle size and zeta potential analysis. The application of gold nanoparticle-based sensing assay confirms the presence of CaC₂ in carbide treated mango and banana fruits by turning their color from red to purple. The developed nanosensing assay showed good sensitivity and selectivity towards CaC₂ with LOD of 5 × 10^-3 mL^-1, reinforcing the suggestion that, this sensing assay enables rapid detection of CaC₂ in fruit samples without need of a complicated laboratory setup. This simple detection method has the potential to ensure food safety, public health concerns, international standards for fruits exports, and promotes sustainable ripening methods.