A fluorescent sensor constructed from nitrogen-doped carbon nanodots (N-CDs) for pH detection in synovial fluid and urea determination

Abstract

Blue luminescent nitrogen-doped carbon nanodots (N-CDs) with pH-dependent properties were prepared from citric acid (CA), glutathione (GSH), and polyethylene polyamine (PEPA) using a two-step pyrolytic route. The N-CDs showed stable and strong emission bands at approximately 455 nm under 350 nm excitation. Moreover, the fluorescence of N-CDs can be gradually decreased by gradually increasing the pH value. A good linear relationship between the fluorescence intensity of N-CDs and the pH range of 3.0-9.0 was obtained. Thus, the response mechanism of N-CDs to pH was systematically investigated. N-CDs possessed -NH₂, -COOH, and -CONH- as active functional groups, which allowed the variable protonation/deprotonation of N-CDs to regulate the fluorescence emission intensities under changed pH values. Furthermore, upon combining urease-catalyzed hydrolysis of urea with increased pH values, a simple but effective fluorescence assay for urea was developed. The analytical performance for urea detection was the linear range of 0 to 10 mM with a detection limit of 0.072 mM. Additionally, the fluorescent sensor based on N-CDs was successfully applied for pH detection in synovial fluid and urea determination in serum.

Fig. 1. Optical properties of UV/Vis and fluorescent spectra (A), appearance under visible (left) and UV (right) light (B), fluorescent spectra with different excited wavelengths (C), and the fluorescence stability with continuous irradiation at 350 nm for 4 h (D) of N-CDs.

Fig. 2. TEM image (A), FTIR spectrum (B), XPS survey spectrum (C), and high-resolution XPS survey of N1s (D) of N-CDs. Insets of (A) are the HRTEM (left) and size distribution (right) of N-CDs.

Fig. 3. Fluorescence emission spectra (A) and fluorescence images (C) of N-CDs under variable pH conditions. (B) Relationship and linear calibration equation (inset) between fluorescence intensities and pH values.

Fig. 4. Zeta-potential (A) and time-resolved fluorescence spectra (B) of N-CDs under variable pH conditions.

Fig. 5. Fluorescence emission spectra (A), relationship between fluorescence intensities and concentrations of urea (B) and linear calibration equation (C) of the detection of urea. (D) Normalized fluorescence response of the detection of urea and the addition of the interferents of Cys, Glu, Ala, NO₂⁻, S₂O₈²⁻, CH₃COO⁻, SCN⁻, and BSA. The detecting system contained 22.8 μg mL⁻¹ N-CDs and 3.6 U mL⁻¹ urease in PB in a total volume of 500 μL.