“Signal-Switching” Fluorescence Sensing of Fe3+ 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 (H₃PO₄), 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 Fe³⁺ with an intermediate bridging effect, the fluorescence intensity of SP-CDs was quenched due to the formation of the complex (SP-CDs/Fe³⁺) between SP-CDs and Fe³⁺ (“OFF” state). Subsequently, the fluorescence signal returned to “ON” due to the redox reaction between AA and Fe³⁺ when ascorbic acid was introduced into SP-CDs/Fe³⁺ solution. Therefore, the synthesized SP-CDs could be used as a “switch type” fluorescence sensor for detecting Fe³⁺ and AA. The analysis revealed that the linear range of Fe³⁺ 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 Fe³⁺ and AA in serum samples.