Eco-Friendly Sustainable Synthesis of Graphene Quantum Dots from Biowaste as a Highly Selective Sensor

Abstract

Graphene quantum dots (GQDs) have generated a great deal of scientific interest due to their bright fluorescence, good biocompatibility, minimal toxicity and fascinating physicochemical features. However, the ultimate issues regarding the acidic contaminations and high synthesis cost of GQDs remain open challenges for their real-world applications. Herein, we report an eco-friendly, acid-free and sustainable method for the preparation of GQDs using a cost-efficient, and renewable carbon source, 'biomass-waste', which simultaneously solves the risk of contamination from strong acids and high expenditure initiated by expensive precursors. The results demonstrate that GQDs possess a size range of 1-5 nm with an average size of ~3 ± 0.4 nm and a thickness of ~1 nm consisting of 1-3 layers of graphene. As-prepared GQDs demonstrate fascinating size-dependent optical properties and considerable surface grafting. Due to their intriguing optical properties, these GQDs are employed as fluorescence probes to detect ferric ions. A focused and sensitive sensor is developed with a detection limit down to 0.29 µM. This study emphasizes the need for using a reasonably green process and an inexpensive biomass precursor to create high-value GQDs that hold great potential for use in photocatalytic, bioimaging and real-world sensing applications.

Keywords: biowaste; fluorescence sensors; graphene quantum dots; sustainable synthesis.

Figure 1

Schematic illustration of the development of GQDs from biomass waste. Biomass waste is first converted into biochar via pyrolysis and then treated at 200–250 °C for several hours in the presence of Oxone and DMF to produce GQDs.

Figure 2

Ultraviolet–visible (UV–Vis) spectra of GQDs-200-8, GQDs-200-12, GQDs-250-8 and GQDs-250-12 showing a noticeable absorption peak at about 280 nm.

Figure 3

Photoluminescence emission spectra at a range of excitations from 310–400 nm for (a) GQDs-200-8, (b) GQDs-200-12, (c) GQDs-250-8 and (d) GQDs-250-12. (e) The comparison of emission peak position for various GQDs at same excitation of 330 nm.

Figure 4

Contour maps of (a) GQDs-200-8, (b) GQDs-200-12, (c) GQDs-250-8 and (d) GQDs-250-12 for photoluminescence emission at a range of excitations from 310 to 400 nm.

Figure 5

Transmission electron microscopy images of GQDs-250-12 (a) at low magnification, (b) at high magnification and (c) particle size distribution of GQDs. (d) HRTEM image of GQDs, (e) magnified image of lattice fringes and (f) line profile showing the distance between adjacent lattice fringes.

Figure 6

(a) AFM image of GQDs-250-12 and (b) corresponding height profiles of the lines marked in (a).

Figure 7

FTIR spectra of GQDs-250-12 and biochar precursor indicating the growth of functional groups on the surface of GQDs during synthesis.

Figure 8

(a) Fluorescence spectra of GQDs-250-12 solution in the presence of different metal ions at 340 nm excitation. (b) Comparison of the emission intensity of GQDs in the absence and presence of various metal ions. (c) Comparison of the affinity of a range of metal ions towards GQDs.

Figure 9

(a) Fluorescence spectra of GQDs (10 mg mL⁻¹) with Fe³⁺ concentrations ranging from 0 to 100 µM, with an excitation wavelength of 340 nm and (b) Linear regression plot of fluorescence intensity and Fe³⁺ concentration in low concentration range of 0.5–6 µM.