"Signal-Switching" Fluorescence Sensing of Fe and Ascorbic Acid by Songaria Cynomorium Polysaccharide-Doped Carbon Dots of the Tibetan Medicinal Herb.

The Songaria Cynomorium polysaccharide-doped carbon dots (SP-CDs) were synthesized by carbonization-heating method. Songaria Cynomorium polysaccharide (PS) was the carbon source. Ethylenediamine (EDA), phosphoric acid (HPO), and concentrated hydrochloric acid (HCl) were dopants, respectively. The SP-CDs exhibited the maximum fluorescence emission wavelength of 540 nm under the excitation wavelength of 485 nm ("ON" state). After the introduction of Fe with an intermediate bridging effect, the fluorescence intensity of SP-CDs was quenched due to the formation of the complex (SP-CDs/Fe) between SP-CDs and Fe ("OFF" state). Subsequently, the fluorescence signal returned to "ON" due to the redox reaction between AA and Fe when ascorbic acid was introduced into SP-CDs/Fe solution. Therefore, the synthesized SP-CDs could be used as a "switch type" fluorescence sensor for detecting Fe and AA. The analysis revealed that the linear range of Fe extended from 0.05 to 500.00 μM, with the limit of detection (LOD) of 0.02 μM (S/N = 3). The scope of analysis for AA was determined to be within the range of 0.65-400.00 μM, with the limit of detection (LOD) of 0.09 μM (S/N = 3). Furthermore, the recommended fluorescence sensor was successfully employed for quantitatively detecting Fe and AA in serum samples.