A Highly Sensitive and Selective Probe for the Colorimetric Detection of Mn(II) Based on the Antioxidative Selenium and Nitrogen Co-Doped Carbon Quantum Dots and ABTS•+

Herein, selenium and nitrogen co-doped carbon quantum dots (Se/N-CQDs) were hydrothermally synthesized by using citric acid, histidine, and sodium selenite, which had sp3 and sp2 hybridized carbon atoms and showed excitation-dependent fluorescence behavior. Furthermore, due to the redox reaction of ABTS•+ and Se/N-CQDs, Se/N-CQDs had the excellent antioxidant capacity that it was demonstrated by scavenging ABTS•+ with the fading of blue. Based on the synergistic effect of Se/N-CQDs and Mn(II) on ABTS•+, Se/N-CQDs and ABTS•+, as a stable, sensitive, selective, and reproducible colorimetric sensor, was applied to the detection of Mn(II) with a detection limit of 1.69 μM and a linear range of 0 to 142.90 μM. More importantly, the probe was successfully applied to detecting Mn(II) in tap water, illustrating that it could be a promising tool for Mn(II) detection in water environments.


S1.2. Apparatus
Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were conducted with operaing voltage of 300 kV on a TECNAI transmission electron microscope. Atomic force microscope (AFM) images were collected by using a Bruker Dimension Icon AFM equipped with Scanasyst in Air peak force tapping mode AFM tips from Bruker. Raman spectroscopy measurement was carried out using Raman spectroscopy with laser excitation of 532 nm at room temperature (HORIBA Scientific, Xplora plus, France). X-ray photoelectron spectroscopy (XPS) were obtained by a Thermo ESCALAB 250Xi spectrometer (Setting parameters: monochromatic Al Ka (hv = 1486.6 eV), operated at 150 W, 500 μm beam spot, the binding energy was calibrated against the carbon 1s line at 284.8 eV, USA). Fourier-transformed infrared spectroscopy (FTIR) was taken on a IRPrestige-21 (SHIMADZU, KBr wafer technique, Japan). Fluorescence spectra were recorded on a fluorescence Spectrophotometer (HITACHI F-4600, Japan). The lifetime of fluorescence was obtained by an Edinburgh FLS980 all functional steady-state/transient fluorescence spectrometer (Edinburgh Instruments, UK).
UV-Vis spectrum was obtained a UV-3600 UV-VIS-NIR spectrophotometer (SHIMADZU, Japan). Zeta potential was measured by using the Nano-ZS90 zetasizer (Malvern Instruments Corp, UK.). Thermogravimetric (TG) analysis was performed on a TA company TGA Q500 unit in N2 at a heating rate of 10 °C min −1 from room temperature to 800 °C. XRD spectrum was measured by using the XRD-7000 X-RAY DIFFRACTOMETER (SHIMADZU, Japan). 1 H NMR and 13 C NMR spectra were recorded on the Advance Bruker 400M NMR spectrometer. Ultra-pure water was obtained from a Millipore Mingche™-D 24 UV water purification system.
The electrochemical measurement was measured by using a AUTOLAB electrochemical workstation (Autolab PGSTAT302N,Metrohm,Switzerland) in a typical three electrode, a graphite rod as the counter electrode, a glassy carbon electrode coated with Se/N-CQDs as the working electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrode was polished with polishing powder, washed with deionized water, and then dried with cold air. Then, 4 mg Se/N-CQDs powder, 4 mg carbon black were admixed in 750 μL deionized water, 250 μL ethanol, 30 μL 5 wt% Nafion solution, forming a mixed solution, which was ultrasound for at least 30 min, 5 μL the prepared solution was dropped onto the polished electrode eventually.
The redox properties of Se/N-CQDs were investigated in phosphate buffer (pH = 7.20)with the loading of 0.5 mg cm -2 on a glassy carbon (GC) electrode. CV curve was performed at a potential range of -0.5 -1.5 V vs RHE and at a scan rate of 100 mV/s.

S1.3. The measurement of quantum yield (QY) of Se/N-CQDs
The reference method was used to determine the quantum yield of Se/N-CQDs [S1]. Quinine sulfate in 0.1 M H2SO4 solution (quantum yield of 53.0% excited at 365 nm) was selected as a criterion by the following equation: Φ = Φs [(I / Is) (As / A) (n 2 / ns 2 )] (1) Where "Φ" is the quantum yield, "I" is the fluorescence peak area at the maximum excitation wavelength, "A" is the UV-Vis absorbance intensity，the subscript "S" represents the reference quinine sulfate, "n" is the different refractive index of solvent, the superscript "2" means the square of "n".

S1.4. Study on the fluorescence intensity of Se/N-CQDs
3 mL of Se/N-CQDs (1 mg/mL) were continuously measured for 20 counts to investigate the stability of the fluorescence intensity of Se/N-CQDs under the same conditions. In order to study the effect of concentration on the fluorescence intensity of Se/N-CQDs, the prepared Se/N-CQDs (1 mg/mL) were diluted to 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.05 times with ultra-pure water. The effect of pH on the fluorescence intensity of Se/N-CQDs was further discussed. The aqueous solutions at different pH (2.60, 3.46, 4.23, 5.10, 6.03, 7.25, 8.36, 9.36, 10.35, 11.16, 12.03) were prepared by using sodium hydroxide and concentrated sulfuric acid. 3 mL of the aqueous solutions at different pH and 3 mL of ultra-pure water were added into 1 mL of Se/N-CQDs, respectively. The fluorescence intensities were recorded on the fluorescence spectrophotometer (HITACHI F-4600) at the excitation 6 wavelength of 370 nm. All solutions were prepared in ultra-pure water at room temperature. added into the 800 μL ABTS •+ + Se/N-CQDs mixed solutions S1, respectively.

S1
The UV-Vis absorption spectra were recorded by a UV-3600 UV-VIS-NIR spectrophotometer under the same conditions. All solutions were prepared in ultra-pure water at room temperature.
The band at 1400 cm -1 was ascribed to C-N stretching vibrations [S3]. The results of FTIR and XPS demonstrated that there were carbon-containing groups, oxygen-containing groups and nitrogen-containing groups on the surface of Se/N-CQDs, which were very important for the formation of CQDs and had an effect on the fluorescence intensity of Se/N-CQDs [S4].    NMR spectra, sp 2 hybridized carbon atoms signals were found in the range of 7-9 ppm, suggesting the aromatic structures in CQDs [S8, S9]. In the 13 C NMR spectra, signals detected in the range of 15-70 ppm and 100-185 ppm correspond to aliphatic (sp 3 ) carbon atoms and sp 2 hybridized carbon atoms, respectively. Signals between 170-185 ppm should correspond to carboxyl/ amide groups on the surface of CQDs [S8-S12]. Fig. S8E was the 1 H NMR spectrum of Se/N-CQDs. Signals between 6 ppm and 9 ppm belong to sp 2 hybridized carbon atoms, illustrating the aromatic structures of Se/N-CQDs [S9]. 13 C NMR spectrum of Se/N-CQDs was shown in Fig. S8F, signals between 25 ppm and 70 ppm were attributable to aliphatic (sp 3 ) carbon atoms.
Signals between 100 ppm and 180 ppm were ascribed to sp 2 hybridized carbon atoms [S9, S13]. Especially, signals between 160 ppm and 180 ppm should be put down to carboxy/amide groups on the surface of Se/N-CQDs [S9]. From the 1 H NMR and 13 C NMR of spectra in Fig S7 and S8, compared with citric acid and L-Histidine, aliphatic (sp 3 ) carbon atoms, sp 2 hybridized carbon atoms and carboxyl/amide groups were retained in the Se/N-CQDs. Compared with other reported CQDs [S8-S18],the NMR results of Se/N-CQDs had almost no change.