Recycling Motorcycle Exhaust Soot into Fluorescent Graphene Oxide Quantum Dots for Sensing Ferrocyanide Ions and Bioimaging Cells: A Method for Waste Utilization

Abstract

Graphene oxide quantum dots (GOQDs) with a high quantum yield (50%) were synthesized using soot collected from a motorcycle (petroleum vehicle) exhaust pipe and applied as sensors for ferrocyanide ([Fe(CN)₆]^4-) ions and as bioimaging agents in a cancer cell line. X-ray photoelectron spectroscopy (XPS) data for the GOQDs revealed a C/O ratio of 2.49, which was close to that of graphene oxide (GO). The synthesized GOQDs exhibited strong blue fluorescence. High sensitivity to detect [Fe(CN)₆]^4- was reported in GOQDs with a detection limit of 0.46 nmol mL^-1, and a strong linear relationship was achieved in the concentration range of 100-1100 μg L^-1. The results demonstrate the utility of GOQDs for detecting [Fe(CN)₆]^4- in a real scenario. The GOQDs exhibited almost negligible cytotoxicity in cells and were internalized within 4 h of incubation, emitting blue fluorescence in the cytoplasm. This suggests that the GOQDs are promising bioimaging agents for biomedical applications. In general, these waste-derived GOQDs appear to be good chemo- and biosensing probes for real-life applications.

Figure 1

Emissive light under 365 nm for the synthesized GOQDs at various time length: (a) 3 h, (b) 4 h, (c) 5 h, (d) 7 h and temperatures: (e) 150 °C, (f) 180 °C, (g) 200 °C.

Figure 2

(a) UV–vis, excitation, and emission spectra; (b) PL spectra. (c) Aqueous suspension before (left vial) and after (right vial) irradiation with UV-365 nm light. (d) Integrated fluorescence intensity vs absorbance plots for GOQDs and quinine sulfate (reference).

Figure 3

Variation in fluorescence intensity with (a) pH, (b) temperature, and (c) irradiation (by 365 nm UV light).

Figure 4

(a) FTIR spectra, (b) XPS spectra of motorcycle exhaust soot (upper one), and GOQDs (lower one); (c, d) are high resolution C 1s and O 1s spectra of exhaust soot-derived GOQDs.

Figure 5

(a) Multilayer, (b) Single layer, and (c) Lattice fringes in HRTEM images under resolution of 5 nm. (d) Area chosen from a HRTEM image (under 50 nm resolution) for size analysis, and (e) corresponding histogram for the GOQDs.

Figure 6

(a) Decreasing fluorescence intensity with increasing concentration of the [Fe(CN)₆]^4– ion-containing solution. (b) Relative fluorescence intensity calibration curve as a function of [Fe(CN)₆]^4– concentration (Stern–Volmer plot) at ambient temperature. (c) Selectivity study of GOQDs for sensing various anions (100 μmol).

Figure 7

Cell viability data for (a). HeLa and (b) HepG2 cells treated with various concentrations of GOQDs.

Figure 8

Cellular uptake of GOQDs (10 μM, 4 h incubation) by HeLa cells was observed using confocal microscopy. (a) HeLa cells incubated with GOQDs (blue), DRAQ5 (red), and merged image (right third), (b) HeLa cells incubated with GOQDs (blue), or MitoTracker (red), and the merge image (right third), scale bar: 10 μm.