Stability of carbon quantum dots: a critical review

Abstract

Carbon quantum dots (CQDs) are fluorescent carbon nanomaterials with unique optical and structural properties that have drawn extensive attention from researchers in the past few decades. Environmental friendliness, biocompatibility and cost effectiveness of CQDs have made them very renowned in countless applications including solar cells, white light-emitting diodes, bio-imaging, chemical sensing, drug delivery, environmental monitoring, electrocatalysis, photocatalysis and other related areas. This review is explicitly dedicated to the stability of CQDs under different ambient conditions. Stability of CQDs is very important for every possible application and no review has been put forth to date that emphasises it, to the best of our knowledge. This review's primary goal is to make the readers cognizant of the importance of stability, ways to assess it, factors that affect it and proposed ways to enhance the stability for making CQDs suitable for commercial applications.

Fig. 1. (a) UV stability of CD in silica and S-CDs in NaCl, KCl and KBr salts and (b) a detailed graphic showing the thermal stability of all samples throughout the first 8 hours (this figure has been adapted/reproduced from ref. with permission from Elsevier, copyright 2014).

Fig. 2. (a) Spectra (b) a week-long test of LED PL intensity (this figure has been adapted/reproduced from ref. with permission from Elsevier, copyright 2014).

Fig. 3. Photostability of the synthesized CQDs exposed to 450 W Xe light at different times (this figure has been adapted/reproduced from ref. with permission from Elsevier, copyright 2015).

Fig. 4. 3D luminescence plots of CNDs in aqueous solution at ambient temperature after (a) 0 hour, (b) 8 hours, and (c) 17 hours of UV irradiation. From blue to green to red, the intensity rises. The luminescence spectra of CNDs in aqueous solutions upon excitation at (d) 365 nm and (e) 800 nm, respectively at ambient temperature (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2016).

Fig. 5. Kinetics of UV-induced fluorescence photobleaching of a 0.002 wt% CNDs solution in water in the presence and absence of oxygen and in the presence of ascorbic acid (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2016).

Fig. 6. (a) Fluorescence intensity as a function of concentration, normalised to CND concentration. The relationship shows that the fluorescence quantum yield of CNDs declines with increasing CND concentration greater than 0.0082 wt%. (b) The graphic shows how CND fluorescence self quenching occurs (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2016).

Fig. 7. Before-and-after UV luminescence spectra of CND@PMMA on a glass substrate (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2016).

Fig. 8. Test of the photostability of CQDs under continuous 365 nm illumination (this figure has been adapted/reproduced from ref. with permission from Nature, copyright 2016).

Fig. 9. Quantum yield of the CQDs inside the thin film under continuous irradiation (this figure has been adapted/reproduced from ref. with permission from Wiley, copyright 2017).

Fig. 10. Fluorescence intensity of N-CDs exposed to 20 mW cm⁻² UV irradiation from a 250 W mercury lamp at various time intervals (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2017).

Fig. 11. Photostability measurements of CDs solution (this figure has been adapted/reproduced from ref. with permission from American Chemical Society, copyright 2018).

Fig. 12. (a) N-doped CDs' photochemical stability under 365 nm UV light; (b) photo-stability of CDs-PVA composite under UV (365 nm) lamp irradiation at various intervals of time (this figure has been adapted/reproduced from ref. with permission from American Chemical Society, copyright 2018).

Fig. 13. Continual exposure to a 150 W Xe lamp for 4 hours results in nearly no detectable loss in PL intensity (this figure has been adapted/reproduced from ref. with permission from Nature, copyright 2019).

Fig. 14. Normalized fluorescence intensity versus time for photostability test under UV exposure (90 min) (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2021).

Fig. 15. Decay fittings of the thermal stability data (a) first order fitting for the first 24 hours and (b) first order fitting for the remaining hours (this figure has been adapted/reproduced from ref. with permission from Elsevier, copyright 2014).

Fig. 16. (a) Temperature-dependent fluorescence emission spectra (excitation 400 nm) for the range of 15 to 90 °C (b) fluorescence emission versus temperature (this figure has been adapted/reproduced from ref. with permission from Elsevier, copyright 2015).

Fig. 17. (a) Fluorescence intensity of the N-CDs at temperatures ranging from 25 to 95 °C (b) fluorescence intensity of the N-CDs at fixed temperatures of 70, 80, and 90 °C at various time intervals (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2017).

Fig. 18. (a) The PL intensity of N-CDs as a function of temperature in the range of 20 to 90 °C; (b) the distribution of PL intensity as a function of fixed temperature (70, 80, 90 °C) at various time intervals; (c) the UV-vis spectra of N-CDs at 25 and 90 °C; and (d) the decay curves of N-CDs at 20, 60, 70, 80 and 90 °C (this figure has been adapted/reproduced from ref. with permission from American Chemical Society, copyright 2018).

Fig. 19. (a) TGA study of A-solid portion of CDs, B-aqueous CDs, C-oleic acid and D-anhydrous CDs (b) nanocomposite made of A-cellulose acetate polymer and B-CDs polymer that underwent TGA analysis (this figure has been adapted/reproduced from ref. with permission from Springer, copyright 2019).

Fig. 20. Thermal stability test (a) fluorescence intensity versus temperature (b) fluorescence intensity versus time at 80 °C (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2021).

Fig. 21. Stability of CDs in presence of various metal ions (concentration of 10⁻² M) (this figure has been adapted/reproduced from ref. with permission from Elsevier, copyright 2015).

Fig. 22. (a) PL intensity of CDs at different pH values; (b) PL intensity of CDs during cyclic pH switching between 3 and 9 (this figure has been adapted/reproduced from ref. with permission from Elsevier, copyright 2015).

Fig. 23. The fluorescence spectra of as-prepared CDs (under 365 nm UV light illumination) (1) in 0 month and (2) after 1 month when stored under normal condition (this figure has been adapted/reproduced from ref. with permission from Elsevier, copyright 2015).

Fig. 24. Fluorescence intensity of CQDs in the presence of various NaCl concentrations (conducted in a pH 5.0 solution with 50 mM NaAc-HAc buffer) upon UV (330 nm) irradiation (this figure has been adapted/reproduced from ref. with permission from Nature, copyright 2016).

Fig. 25. (a) Effect of pH on CQDs fluorescence intensity (b) comparison of CQD fluorescence intensity when various ions are present at excitation wavelength of 330 nm (this figure has been adapted/reproduced from ref. with permission from Nature, copyright 2016).

Fig. 26. Data written on a filter paper using the N-CDs invisible ink under (a) 365 nm UV lamp, (b) daylight and (c) reproducibility of the data written on filter paper under 365 nm UV lamp for 90 days storage under ambient air (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2017).

Fig. 27. (a) PL intensity of N-CDs across a range of pH values (b) PL intensity of N-CDs as a function of time (this figure has been adapted/reproduced from ref. with permission from American Chemical Society, copyright 2018).

Fig. 28. Emission spectra for varied durations of exposure to an open atmosphere for (a) bare CDs excited at 250 nm, (b) bare CDs excited at 350 nm, (c) SFCDs excited at 250 nm, and (d) SFCDs excited at 350 nm (this figure has been adapted/reproduced from ref. with permission from American Chemical Society, copyright 2019).

Fig. 29. Normalized PL intensity under different KCl concentration (this figure has been adapted/reproduced from ref. with permission from Royal Society of Chemistry, copyright 2021).