Hydrothermal synthesis of modified lignin-based carbon dots derived from biomass waste for fluorescence determination of valsartan

Abstract

Recently, carbon dots (CDs) have been extensively investigated as potential tools for numerous applications. Modified lignin-based CDs have been synthesized and used in the field of drug detection. They were found to be highly selective and sensitive to valsartan (VAL). Using a simple hydrothermal method, phosphorus and chlorine co-doped CDs were synthesized using lignin extracted from date seeds. The fluorescence properties of the synthesized CDs are influenced by several factors, which were investigated in detail. The optimal synthesis conditions were 1.50 g of lignin, 18 mL of 2 M NaOH, 1 mM H₃PO₄, 3 mM HCl and the mixture was heated at 220 °C for 16 hours. The synthesized lignin-based CDs have excellent FL properties and are well soluble in water with reasonable stability. Characterization of the prepared CDs revealed that they have various functional groups with a graphene oxide-like structure. The developed CDs show a good quantum yield of 37.7%. The FL of the CDs is quenched by VAL at λ _em 313 nm after λ _ex at 275 nm by a combination of static and dynamic quenching mechanisms. The response of VAL was linear in the range of 4.0-100.0 μg mL^-1. The detection and quantification limits of VAL were 1.23 and 3.71 μg mL^-1, respectively. The nanoprobe was successfully used to analyze VAL in drug samples and provided satisfactory results.

Scheme 1. Schematic illustration of the formation and purification of CDs.

Scheme 2. The possible mechanism for the synthesis of lignin-based CDs.

Fig. 1. FL spectra of lignin after hydrothermal treatment in different solvents at 200 °C for 12 h.

Fig. 2. Size distribution of lignin after hydrothermal treatment in different solvents at 200 °C for 12 h.

Fig. 3. (a) FL spectra of lignin after hydrothermal treatment with different concentrations of NaOH at 200 °C for 12 h (b) TEM image when 2 M NaOH was used as the solvent at 200 °C for 12 h.

Fig. 4. FL spectra of (a) lignin-based CDs with different amounts of lignin at 200 °C for 12 h, (b) lignin-based CDs with different temperatures for 12 h, and (c) lignin-based CDs with different durations of hydrothermal treatment at 220 °C.

Fig. 5. FL spectra of modified lignin-based CDs with different amounts of dopant precursors (a) H₃PO₄ and (b) HCl.

Fig. 6. Factors affecting the stability of synthesized lignin-based CDs: (a) effect of NaCl, (b) effect of pH, (c) effect of irradiation time, and (d) effect of storage time.

Fig. 7. (a) TEM image, (b) average particle size distribution using ImageJ software, (c) SEM image, and (d) AFM 3D image of PClCDs.

Fig. 8. (a) UV-vis absorption spectra of lignin and PClCDs, (b) FL spectra of PClCDs at different λ_ex values, (c) FT-IR spectra of lignin and PClCDs, and (d) XRD spectrum of PClCDs.

Fig. 9. (a) XPS survey scan, (b) C 1s, (c) O 1s, (d) P 2p, and (e) Cl 2p of PClCDs.

Fig. 10. EDX analysis of the synthesized PClCDs.

Fig. 11. (a) FL spectra of different concentrations of VAL solutions with and without the presence of PClCDs; effects of (b) pH, (c) buffer volume, (d) PClCDs volume, (e) response time and (f) temperature on the F₀ − F of 10 μg mL⁻¹ VAL in the presence of PClCDs.

Fig. 12. Calibration curve for VAL determination in the presence of PClCDs.

Fig. 13. Selectivity of the PClCDs-VAL system for different species at a concentration of 10 μg per mL VAL.

Fig. 14. Nonlinear Stern–Volmer plots for PClCDs-VAL system.