Excitation-independent dual emissions of carbon dots synthesized by plasma irradiation of ionic liquids: Ratiometric fluorometric determination of norfloxacin and mercury(II)

The capacitively coupled plasma (CCP) discharge of an ionic liquid solution of citric acid produces carbon dots (CDs) with excitation-independent fluorescent dual-emissions peaking at 410nm and 480nm. The intensity of the purple photoluminescence at 410nm increases with (a) the flow rate of O-2 plasma gas supply from 2.0 to 30 standard cubic centimeters per minute (sccm), (b) the 2-h exposure of the CDs to 254nm light, and (c) the 8-h immersion of the CDs in a solution of NaBH4. The UV exposure and the hydride immersion reduce the fluorescence intensity peaking at 480nm, which is highest at 5.0 and 10 sccm. The two emissive states were revealed by UV-vis absorption, XPS spectra, and time-resolved fluorescence. Control of the O-2 flow rate can simply tune the ratiometric fluorescence of the CCP-CDs. The CDs obtained from 5 and 30 sccm O-2 supplies present a high-intensity ratio (I-480 nm/I-410 nm approximate to 3.35) and a low one (approximate to 0.48), respectively. The 480nm fluorescence of the former CDs is quenched by mercury(II) ions in the 0.2 to 50M concentration range. The 410nm fluorescence of the latter CDs is enhanced by norfloxacin in the 25nM to 1.0M concentration range. The detection limits are 75nM for Hg(II) and 7.3nM for norfloxacin.