Smartphone-Assisted Plasmonic Nanosensor for Visual and Specific Sensing of Toxic Cyanide Ions by β−Cyclodextrin Templated Gold-Rich/Silver Bimetallic Alloy Nanoparticles

As cyanide ion (CN^-), an ecologically harmful pollutant, has received incessant attention with growing industrialization on a global scale, the capability of on-site monitoring of CN^- contamination becomes increasingly crucial. In this work, we have fabricated a simplistic plasmonic-sensing platform for CN^-, which can be combined with the human naked eye for visual monitoring. The main sensor part consisted of β-Cyclodextrin (β-CD)-decorated gold-rich silver bimetallic alloy nanoparticles (β-CD-Ag/Au-rich alloy NPs), while a sensing analysis was performed by a spectrophotometer or smartphone, where optical data gathered by its camera were analyzed by RGB color sensing. Upon the introduction of various CN^- quantities into β-CD-Ag/Au-rich alloy NPs, the spectral peak of the surface plasmon resonance (SPR) shifted from 488 nm to 496 nm. This redshift indicated a strong etching reaction between alloy NPs and CN^-, demonstrating a ultrahigh detection sensitivity, i.e., a limit of detection (LOD) of 0.24 nM. During the formation of metal-cyano complexes in the CN^--induced etching response of β-CD-Ag/Au-rich alloy NPs, we observed a naked-eye discernible color change from brownish-red to colorless, allowing for naked-eye monitoring. The smartphone could also analyze the colorimetric response for such an etching process via RGB color sensing, demonstrating a LOD of 1.35 nM, being still less than the maximum concentration (1.91 nM) in drinking water, which is allowable by the World Health Organization (WHO). The straightforwardness and very high sensitivity of the proposed technique for CN^- detection using alloy nanoparticles with a smartphone may hold promise for simplistic, affordable in-field examinations of CN⁻ in water.