An ultrasonic-assisted synthesis of leather-derived luminescent graphene quantum dots: catalytic reduction and switch on-off probe for nitro-explosives

Abstract

The current research effort demonstrates the ultrasonic-assisted synthesis of highly fluorescent graphene quantum dots (GQDs) of ∼5 nm diameter. First, acid pyrolysis with ultrasonic hydrothermal co-cutting breaks down the coarse graphite into nanometric graphene sheets (GS) and graphene oxide sheets (GOS) with oxygen-rich functionalities. These functionalities were then used to break GOS into graphene oxide nanofibers (GONFs) and graphene oxide quantum dots (GOQDs). Finally, upon reduction, GOQDs lose oxygen linkages to produce fluorescent GQDs (quantum yield up to 27%). The as-developed GQDs were characterized with detailed optical and spectral studies through UV, PL, FTIR, TEM, AFM, XPS, XRD and other techniques. Notably, the synthesized GQDs were catalytically active to serve as a ratiometric fluorescence switch on-off probe for the reduction of toxic nitrophenols. Moreover, the GQDs detected nitrophenol derivatives at lower concentrations than previously reported analytical values. During the real sample analysis of spiked industrial water and exposed soil samples, a high selectivity and sensitivity of the applied method was achieved with a recovery of 99.7% to 101.3% at spiked concentrations of 400 nM to 100 nM, respectively. The detection limit of the photoluminescent probe for paranitrophenol was as low as 10 pM.

Scheme 1. Schematic synthesis of GQDs with AFM images of different stages, along with their potential catalysis, as well as sensing ability.

Fig. 1. (a) UV-visible spectra of GQDs (acid oxidation) through a hotplate, microwave and ultrasonication treatment, and their corresponding PL spectra. (b) UV-visible spectra of GQDs synthesized with different solvents, and their PL spectra are shown in the inset. (c) The representative UV-visible and PL spectra of GQDs (the inset shows the photograph of GQDs solution under 350 nm UV lamp). (d) The PLE spectrum of the as-synthesized GQDs with a detection wavelength of 420 nm and excited PL spectrum at 330 nm, suggesting the irradiation decay of activated electrons from the HOMO → LUMO transition (of free zigzag sites having carbene like triplet ground state energy level).

Fig. 2. TEM images of GOQDs (a) and GQDs (b) synthesized at 25 min and 40 min ultrasonication, respectively. (c) XPS spectra of GOQDs and GQDs. (d) XRD patterns of GOQDs and GQDs.

Fig. 3. UV-visible (a, c and e) and PL spectra (b, d and f) of the catalytic performance of GQDs (concentrations = 10–60 μL GQDs for GQD1-GQD6) for 0.1 mM PNP (a and b) and 0.1 mM DNP (c and d) and 0.1 mM TNP (e and f) in the presence of NaBH₄ (10 μL of 1 mM). The PL quenching spectra of GQDs are shown with 10–550 nM PNP (b), 20–310 nM DNP (d), and 10–500 nM TNP (f). The insets (b, d and f) represent the linear regression plots for the detection of PNP, DNP and TNP, respectively.