Luminescent Carbon Dots from Wet Olive Pomace: Structural Insights, Photophysical Properties and Cytotoxicity

Abstract

Carbon nanomaterials endowed with significant luminescence have been synthesized for the first time from an abundant, highly localized waste, the wet pomace (WP), a semi-solid by-product of industrial olive oil production. Synthetic efforts were undertaken to outshine the photoluminescence (PL) of carbon nanoparticles through a systematic search of the best reaction conditions to convert the waste biomass, mainly consisting in holocellulose, lignin and proteins, into carbon dots (CDs) by hydrothermal carbonization processes. Blue-emitting CDs with high fluorescence quantum yields were obtained. Using a comprehensive set of spectroscopic tools (FTIR, Raman, XPS, and 1H/13C NMR) in combination with steady-state and time-resolved fluorescence spectroscopy, a rational depiction of WP-CDs structures and their PL properties was reached. WP-CDs show the up-conversion of PL capabilities and negligible cytotoxicity against two mammalian cell lines (L929 and HeLa). Both properties are excellent indicators for their prospective application in biological imaging, biosensing, and dynamic therapies driven by light.

Plant-based carbon dots are a sustainable alternative to conventional nanomaterials for biomedical and sensing applications

“…In a study by Diogo A Sousa et al (2022) [43], the production of Wet Pomace-CDs was optimized by enhancing the interaction between carbon core states and O/N-containing substituents under high hydrothermal carbonization temperatures (250 °C) and an extended duration (4 h). The focus was on minimizing the contribution of molecular fluorophores to photoluminescence.…”

Are “Carbon Dots” Always Carbon Dots? Evidence for their Supramolecular Nature from Structural and Dynamic Studies in Solution and in the Pure Solid

“…The CDs were synthesized from a biomass by-product (wet olive pomace) following a reported HTC procedure. [22] The isolated materials (WP-CDs) were then treated with acid at 80 °C for removal of inorganic carbonates, yielding an aqueous solution of CDs (t-WP-CDs). Full details on the synthesis of WP-CDs and t-WP-CDs may be found in the Supporting Information.…”

“…These values are very similar to those previously reported by us for an untreated sample, WP-CDs, with sizes of 2.5 nm � 0.8 nm. [22] The highresolution TEM (HRTEM) micrographs of t-WP-CDs (Figures 1c and S3a,b) display clear lattice fringes. Analysis of the fast Fourier transform (FFT) image (Figure 1d), equivalent to an electron diffraction pattern, consistently yield lattice fringe spacings of 0.21 nm, a value that resembles the in-plane lattice spacing ((100) plane) of graphene, as those reported in CDs derived from fullerene C 60 /H 2 O 2 , [23] nitrated pyrene, [24] p-phenylenediamine/urea, [25] citric acid/EDA, [26] citric acid/L-cysteine/ glycine, [27] laser ablation of graphite/PEG, [28] lemon juice, [29] laser ablation of graphite/glycerol/H 2 O, [30] and citric acid/l-cysteine, [31] but also those appearing in diaphite-structured nanodiamonds, [32] and aggregates of small organic molecules.…”

“…Since these sizes are very similar to those obtained for the pristine sample of t-WP-CDs before any dialysis separation, and, both of them, are well beyond the cutoff of the membranes used in the sequential dialysis separation, this can only mean that most of the particles observed in the TEM images, either before or after dialysis, are formed by supramolecular assembly of low molecular weight species (< 1 kDa), likely hydrogen bond-assisted and/or π-stacking self-assembly, namely during the process of drying of the samples ongoing from solution to solid phase. The presence of carboxylic acids, amide/imide/ carbamate/urea groups, and aliphatic/aryl alcohol functionalities in WP-CDs, [22] would assist in such a process. It is thus conceivable that the nature of the nanoparticles is dynamic in nature, their formation or collapse being dependent on the conditions (e. g., solution vs solid state).…”

“…In this work, we deepen the knowledge already acquired on the structure and photophysical properties of luminescent CDs obtained from wet olive pomace (WP), an abundant biomass waste from industrial olive oil production. [22] We were particularly interested in the establishment of tangible correlations between the morphological features of CDs in solid state and their photoluminescence in solution, a fundamental issue that is largely transversal to any type of fluorescent carbon nanomaterials, and to which there isn't yet consolidated answers. On applying advanced techniques such as time-resolved fluorescence anisotropy measurements and pulsed field gradient (PFG)-nuclear magnetic resonance (NMR) spectroscopy, in connection to high-resolution TEM (HRTEM), AFM, and matrixassisted laser desorption/ionization (MALDI) mass spectrometry (MS) to the analysis of CDs samples, we have been able to establish a coherent correlation between the morphology of the CDs in solid state (TEM/HRTEM and AFM) with their sizes in solution retrieved from TRFA and PFG-NMR measurements, and their corresponding photoluminescent properties.…”