Abstract:

As a new fluorescent nanomaterial, carbon dots (CDs) have many advantages, such as uniform particle size distribution, good light stability, adjustable excitation-emission wavelength, and surface modification. Moreover, one of the fascinating characters of CDs is that they are considered to be low-toxic and eco-friendly alternatives in chemical and biological analyses. They have exhibited broad application prospects in the fields of analysis, detection, and bioimaging. Silkworm excrement is dried and easily available. A large number of hydroxyl and carboxyl compounds in silkworm excrement can be used as ideal starting materials for the preparation of CDs. Also, compounds containing nitrogen and sulfur in silkworm excrement can be used as nitrogen and sulfur sources; thus, when used in the preparation of CDs, silkworm excrement can impart many more unique properties to CDs. Nitrogen-containing CDs prepared by microwave synthesis have an average hydration diameter of 4.86 nm. Elemental analysis data show that the prepared CDs contained 59.84% carbon, 5.46% nitrogen, and 2.32% sulfur. XPS spectra reveals sulfur (2p), carbon (1s), nitrogen (1s), and oxygen (1s) in CDs. FTIR data demonstrate that the prepared CDs may contain hydroxyl, amino, carbonyl, sulfonic, ester, and ether functional groups as well as carbon-nitrogen structures. The XRD pattern of the CDs has a broader peak of the amorphous carbon phase at approximately 2θ = 24.6°, and only D bands (at ~1400 cm^-1) can be obviously detected in the Raman spectra of CDs. The intensity of fluorescence emission peak of CDs increases first and then decreases with the increase in excitation wavelength. The maximum intensity of fluorescence emission shifts gradually with the red shift of the excitation wavelength, and the relationship between excitation and emission wavelengths is exponential. In the pH ranging from 2.18 to 10.24, the fluorescence emission intensity of CDs decreases gradually with the increase in pH, and the maximum fluorescence emission intensity shifts gradually with the increase in pH. There is a linear relationship between pH and maximum emission wavelength. The fluorescence emission intensity of CDs decreases gradually with the increase in metal ion concentration. Under neutral conditions, CDs can selectively detect Cu²⁺. Under acidic conditions, CDs can detect Cu²⁺, Fe³⁺, Al³⁺, Ni²⁺, and Fe²⁺ separately without interference from other ions. There is a Stern-Volmer linear relationship between metal ion concentration and fluorescence intensity. The intensity of the fluorescence emission peak of CDs decreases with the increase in temperature, which may be due to the non-radiative transition process caused by molecular thermal motion. There is a linear relationship between temperature and fluorescence intensity. The maximum fluorescence emission intensity of CDs gradually shifts with the increase in polarity of the dispersed solvents. There is a linear relationship between fluorescence intensity and empirical constant E_T of solvent polarity. Compared with the reported CDs prepared from natural products, silkworm-excrement-based CDs have abundant surface groups although they do not have an obvious crystal structure, which makes them have excellent response to various environmental factors (pH, temperature, ion concentration, temperature, solvent polarity, etc.) in a wide range. Above all, the fluorescence property changes with multiple environmental parameters will facilitate a broad application of silkworm-excrement-based CDs in biodetection and imaging.

Key words: Carbon dots, Silkworm excrement, Nano-materials, Fluorescence, Sensing