High yield synthesis of graphene quantum dots from biomass waste as a highly selective probe for Fe3+ sensing

Abstract

Graphene quantum dots (GQDs), a novel type of zero-dimensional fluorescent materials, have gained considerable attention owing to their unique optical properties, size and quantum confinement. However, their high cost and low yield remain open challenges for practical applications. In this work, a low cost, green and renewable biomass resource is utilised for the high yield synthesis of GQDs via microwave treatment. The synthesis approach involves oxidative cutting of short range ordered carbon derived from pyrolysis of biomass waste. The GQDs are successfully synthesised with a high yield of over 84%, the highest value reported to date for biomass derived GQDs. As prepared GQDs are highly hydrophilic and exhibit unique excitation independent photoluminescence emission, attributed to their single-emission fluorescence centre. As prepared GQDs are further modified by simple hydrothermal treatment and exhibit pronounced optical properties with a high quantum yield of 0.23. These modified GQDs are used for the highly selective and sensitive sensing of ferric ions (Fe³⁺). A sensitive sensor is prepared for the selective detection of Fe³⁺ ions with a detection limit of as low as 2.5 × 10^-6 M. The utilisation of renewable resource along with facile microwave treatment paves the way to sustainable, high yield and cost-effective synthesis of GQDs for practical applications.

Figure 1

A schematic illustration of the synthesis procedure for graphene quantum dots (GQDs). Spent tea waste is subjected to pyrolysis treatment and carbon rich biochar is obtained, which is then microwave treated to produce GQDs with high yield. Figure was drawn using ChemDraw v16 (https://www.perkinelmer.com), Adobe Photoshop 2019 (https://www.adobe.com) and Microsoft PowerPoint 2016 (https://www.microsoft.com).

Figure 2

Comparison of the effect of (a) microwave power at a fixed duration of 120 min and (b) processing time at a fixed power of 500 W, on the photoluminescence (PL) emission of GQDs at an excitation wavelength of 340 nm. Figures were drawn using OriginPro 2018b (https://www.originlab.com).

Figure 3

(a) Ultraviolet visible (UV–Vis) spectra of raw precursor, GQDs-500 and GQDs-900 showing a strong absorption of GQDs in the UV range (insets are the photographs of GQDs-500 and GQDs-900 under visible and 365 nm UV light). (b) PL spectra of GQDs-500 at different excitation wavelengths and (c) their normalised PL spectra exhibiting excitation independent emission. (d) PL spectra of GQDs-900 and (e) their normalised PL spectra showing no shift in PL peak position. Inset images in (a) were taken using iPhone Xs Max (https://www.apple.com) and graphics in (a)–(e) were plotted using OriginPro 2018b (https://www.originlab.com).

Figure 4

(a) TEM image, (b) HRTEM image and (c) respective particle size distribution of GQDs-500. (d) TEM, (e) HRTEM and (f) size distribution measurements of GQDs-900. Both types of GQDs were obtained after 120 min of treatment. Images in (a), (b), (d) and (e) were processed using DigitalMicrograph 3.40.2804.0 (https://www.gatan.com) and plotted using ImageJ 1.8.0_112 (https://imagej.nih.gov/ij/). Figures (c) and (e) were plotted using OriginPro 2018b (https://www.originlab.com).

Figure 5

(a) UV–Vis spectra of GQDs-500 and GQDs-500-M showing a strong absorption at 300 and 270–280 nm range, respectively. (b) The PL spectra of GQDs-500-M at a range of excitation wavelengths (insets are the photographs of GQDs-500-M solution under visible and 365 nm UV light) and (c) normalised PL spectra showing a red shift in PL emission. Figures (a)–(c) were plotted using OriginPro 2018b (https://www.originlab.com) and inset photographs in (b) were taken using iPhone Xs Max (https://www.apple.com).

Figure 6

(a) Low magnification TEM image, (b) high magnification TEM image (c) high resolution TEM (HRTEM) image and (d) corresponding size distribution of the GQDs-500-M. Images in (a), (b) and (c) were processed using DigitalMicrograph 3.40.2804.0 (https://www.gatan.com) and plotted using ImageJ 1.8.0_112 (https://imagej.nih.gov/ij/). Figure (d) was plotted using OriginPro 2018b (https://www.originlab.com).

Figure 7

(a) Raman and (b) FTIR spectra of GQDs-500-M displaying the oxygen and nitrogen functional groups. (c) X-ray photoelectron spectroscopy (XPS) survey spectrum, (d) C1s, (e) N1s and (f) O1s high resolution spectra of GQDs-500-M, showing extensive number of oxygen and nitrogen functionalities. Drawings were plotted using OriginPro 2018b (https://www.originlab.com).

Figure 8

(a) Comparison of the PL intensities of 50 μg mL⁻¹ GQDs-500-M solution in the presence of different metal ions (100 µM) at an excitation wavelength of 340 nm. (b) The comparison of the affinity of different metal ions towards GQDs-500-M (F₀ and F are the PL intensities of GQDs-500-M without and with 100 µM of different metal ions). (c) The PL spectra of GQDs-500-M at different concentrations of Fe³⁺ and (d) corresponding linear plot. Three independent experiments were carried out to obtain the mean values. Figures were drawn using OriginPro 2018b (https://www.originlab.com).