N-Doped Carbon Dots Synthesized from Ethylene Glycol and β-Alanine for Detection of Cr(VI) and 4-Nitrophenol via Photoluminescence Quenching

We present here a facile method for nonaqueous synthesis of highly crystalline nitrogen-doped carbon dots (N-CDs) via oligomerization of ethylene glycol while heating with beta-alanine at 170 degrees C for 30 min. It exhibited bright bluish photoluminescence (PL) at lambda(emission) = 461 nm, corresponding to a relative PL quantum yield of 14.3% and an average PL lifetime of 7.4 ns. High-resolution transmission electron microscopy of N-CDs revealed systematically self-assembled transverse oligomeric chains of ethylene glycol doped with beta-alanine to form hexagonal symmetry, and their crystalline nature was revealed from electron diffraction measurement. The structure and composition of N-CDs are deduced from C-13 nuclear magnetic resonance, high-resolution mass spectrometry, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy measurements, which collectively suggested oligomerization of beta-alanine-doped ethylene glycol. The N-CDs are then demonstrated as an efficient probe for quantitative detection of Cr(VI) species at different pHs and 4-nitrophenol (4-NP) via PL quenching. The limits of detection (LoD) are deduced from respective linear Stern-Volmer plots, e.g., 1.1 mu M for H2CrO4 (at pH 0.9), 2.5 mu M for HCrO4- (at pH 3.5), and 0.29 mu M for CrO42- (at pH 9.0). The detection of Cr(VI) is highly selective against several cationic (e.g., Na+, K+, Mg2+, Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+) and anionic (Cl-, F-, PO43-, SO42-, NO3-, CO32-, HCO3-, and AsO43-) interfering agents. Similarly, the LoD of 4-nitrophenol was determined to be 0.4 mu M, which is significantly better than structurally similar compounds, e.g., phenols, benzene, and their derivatives. The mechanism of detecting Cr(VI) and 4-NP by N-CDs has been attributed to a combination of static-type PL quenching and an inner filter effect. Finally, practicality of the detection strategy by our method was assessed by analyzing spiked real samples, e.g., municipality tap water and river water. The accuracy and precision of measuring Cr(VI) and 4-nitrophenol by our method are comparable with those measured by inductively coupled plasma optical emission spectroscopy and high-performance liquid chromatography, respectively.