Smartphone-Based Attomolar Cyanide Ion Sensing Using Au-Graphene Oxide Cryosoret Nanoassembly and Benzoxazolium-Based Fluorophore in a Surface Plasmon-Coupled Enhanced Fluorescence Interface

Photoplasmonic platforms are beingdemonstrated as excellent meansfor bridging nanochemistry and biosensing approaches at advanced interfaces,thereby augmenting the sensitivity and quantification of the desiredanalytes. Although resonantly coupled electromagnetic waves at thesurface plasmon-coupled emission (SPCE) interface are investigatedwith myriad nanomaterials in order to boost the detection limits,rhodamine moieties are ubiquitously used as SPCE reporter moleculesin spite of their well-known limitations. In order to overcome thisconstraint, in this work, a benzoxazolium-based fluorescent molecule,(E)-2-(4-(dimethylamino)styryl)-3-methylbenzo[d]oxazol-3-ium iodide (DSBO), was synthesizedto selectively detect the cyanide (CN-) ions inwater samples. To this end, the sensitivity of the fabricated SPCEsubstrates is tested in spacer, cavity, and extended cavity nanointerfacesto rationalize the configurational robustness. The performance ofthe sensor is further improved with the careful engineering of gold(Au)-graphene oxide (GO) cryosoret nanoassemblies fabricated via anadiabatic cooling technology. The unique dequenching (turn-on) ofthe quenched (turn-off) fluorescent signal is demonstrated with thehybridized metal-pi plasmon synergistic coupling in the nanovoidsand nanocavities assisting delocalized Bragg and localized Mie plasmons.The spectro-plasmonic analysis yielded highly directional, polarized(>95%), and enhanced emission attributes with an attomolar limitofdetection of 10 aM of CN- ions with high linearity(R (2) = 0.996) and excellent reliability,in addition to an exceptional correlation with the theoretically obtainedTFclac simulations. The CN- ion sensing is experimentallyvalidated with the smartphone-based cost-effective SPCE detectiontechnology to render the device amenable to resource-limited settings.We believe that the unique fluorophore-cryosoret nanoassemblagepresented here encourages development of frugal, unconventional, andhighly desirable strategies for the selective quantitation of environmentallyand physiologically relevant analytes at trace concentrations foruse in point-of-care diagnostics.