Hydrothermal carbonization synthesis of amorphous carbon nanoparticles (15-150 nm) with fine-tuning of the size, bulk order, and the consequent impact on antioxidant and photothermal properties

Abstract

Hydrothermal carbonization (HTC) of carbohydrates has been reported as a sustainable and green technique to produce carbonaceous micro- and nano-materials. These materials have been developed for several applications, including catalysis, separation science, metal ion adsorption and nanomedicine. Carbon nanoparticles (CNPs) obtained through HTC are particularly interesting for the latter application since they exhibit photothermal properties when irradiated with near-infrared (NIR) light, act as an antioxidant by scavenging reactive oxygen species (ROS), and present good colloidal stability and biocompatibility. However, due to the highly disordered structure, there is still a poor understanding of the mechanism of synthesis of CNPs. Consequently, the modulation of the CNP properties by controlling the synthetic parameters is still a challenge. In this work, a novel and simplified HTC synthetic strategy to obtain non-aggregated glucose derived CNPs in the 15-150 nm size range with precise control of the diameter is presented, together with an advance in the understanding of the reaction mechanism behind the synthesis. Modifications of the synthetic parameters and a post-synthesis hydrothermal process were applied to increase the bulk order of CNPs, resulting in an increase of the photothermal and ROS scavenging activities, without affecting the morphological and colloidal properties of the nanomaterial.

Fig. 1. Effect of the synthetic parameters on the CNP size distribution. (A) – upper graph – DLS size distribution (by intensity) of CNP dispersions obtained at different concentrations and synthesis times (from the smallest diameter: glucose, 0.022 M – synthesis time, 3 h (black); 0.022 M – 6 h (red); 0.022 M – 9 h (blue); 0.22 M – 3 h (green); 0.22 M – 4 h (violet); 0.22 M – 5 h (mustard)); (A) – bottom graph – CNP mean diameters determined by EM as a function of initial glucose concentration (0.022 M red circles, 0.22 M blue squares, and 2.2 M grey triangle) and synthesis time (temperature fixed at 190 °C). (B–H) TEM and FESEM images, DLS and NTA measurements of CNPs synthesised at different precursor concentrations and synthesis times. (B–D) glucose, 0.022 M: (B) synthesis time, 6 h, (C) 9 h, (D) and 15 h; (E–G) glucose, 0.22 M: (E) 3 h; (F) 4 h; (G) 5 h; (H) glucose 2.2 M – 3 h. The hydrodynamic diameter evaluated by DLS is expressed as the relative percentage in intensity. DLS and NTA were performed on suspensions of CNPs in water.

Fig. 2. Effect of the synthetic parameters on CNP ζ-potential. ζ-potential of CNPs synthesized starting from 0.022 M (red circles), 0.22 M (blue squares) and 2.2 M glucose solutions (grey triangle) at the different synthesis times tested, all measured at pH ∼4.

Fig. 3. Effect of the precursor concentration on the bulk structure of CNPs. (A) Normalized baseline-subtracted Raman spectra of CNPs synthesized starting from 0.022 M (red), 0.22 M (blue) and 2.2 M (grey) glucose aqueous solutions with a synthesis time of 3 h at 190 °C. (B) ATR-FTIR of the same samples (Greek letters refer to band assignation, see Table 3).

Fig. 4. Effect of the time of synthesis on the bulk structure of CNPs. (A and B) ATR-FTIR spectra of CNPs synthesized starting from 0.22 M (A) and 0.022 M (B) glucose at different synthesis times. (C) Normalized baseline-subtracted Raman spectra of CNPs synthesized in 3 h (red) and 15 h (green) starting from 0.022 M glucose.

Fig. 5. (A) TEM images of CNPs (0.22 M – 3h) purified and after hydrothermal treatment at 190 °C for 24 h. (B) DLS, (C) Raman, and (D) ATR-FTIR of CNPs before and after hydrothermal treatment at 190 °C for 24 h (both samples were purified by 30 kDa tangential dialysis).

Fig. 6. CNP formation process. (A) Concentration of particles in NPs per mL (blue squares) and mean D_H (orange circles) of CNPs synthesised starting from 0.22 M glucose solution at different synthesis times, measured by NTA, and concentration values reported are the average of 3 measurements. (B) Yield of reaction of CNPs prepared starting with 0.22 M glucose solution. (C) CNP mean D_H (DLS < 50 nm, NTA > 50 nm) as a function of initial glucose concentration (0.022 M red circles, 0.22 M blue squares, and 2.2 M grey triangle) and synthesis time.

Fig. 7. Photothermal activities of CNPs irradiated with NIR light: (A) CNPs obtained as follows: 2.2 M glucose in 3 h (grey); (i) 0.22 M glucose in 3 h (blue); (ii) 0.022 M glucose in 15 h (red). (B) CNPs obtained as follows: from 0.22 M glucose in 3 h (blue) and the same sample subsequently re-heated for 24 h at 190 °C (green).

Fig. 8. Scavenging activity of CNPs toward hydroxyl radicals generated by a Fenton reaction measured by the EPR spin-trapping technique. DMPO was used as the spin-trap molecule. EPR spectra recorded after 60 minutes of incubation in the absence of CNPs (red, Ctrl −); EPR spectra recorded in the presence of CNPs obtained from 0.22 M glucose in 3 h (blue) and the same sample subsequently re-heated for 24 h at 190 °C (green). The number of radicals produced is proportional to the intensity of the EPR signal. The reported signals are the average of at least 3 independent experiments.