The fluorescent carbon quantum dots (CQDs) represent an emerging subset of carbonaceous nanomaterials, recently becoming a powerful tool for biosensing, bioimaging, and drug and gene delivery. In general, carbon dots are defined as zero-dimensional (0D), spherical-like nanoparticles with <10 nm in size. Their unique chemical, optical, and electronic properties make CQDs versatile materials for a wide spectrum of applications, mainly for the sensing and biomedical purposes. Due to their good biocompatibility, water solubility, and relatively facile modification, these novel materials have attracted tremendous interest in recent years, which is especially important for nanotechnology and nanoscience expertise. The preparation of the biomass-derived CQDs has attracted growing interest recently due to their low-cost, renewable, and green biomass resources, presenting also the variability of possible modification for the enhancement of CQDs’ properties. This review is primarily focused on the recent developments in carbon dots and their application in the sensing of different chemical species within the last five years. Furthermore, special emphasis has been made regarding the green approaches for obtaining CQDs and nanomaterial characterization toward better understanding the mechanisms of photoluminescent behavior and sensing performance. In addition, some of the challenges and future outlooks in CQDs research have been briefly outlined.

In this study, an efficient utilization and valorization of mandarin peel (Citrus unshiu Marc. var. Kuno) was investigated using innovative and green extraction techniques. The first step of this study included the extraction and analysis of the volatile compounds by performing a supercritical CO₂ (SC-CO₂) extraction under different operating pressure conditions (100 and 300 bar). The analysis of volatile compounds of the obtained extracts was conducted by gas chromatography-mass spectrometry (GC-MS), and limonene was found to be the dominant volatile component (13.16% at 100 bar; 30.65% at 300 bar). After SC-CO₂ treatment, the exhausted citrus peel waste enriched with bioactive compounds was subjected to subcritical water extraction (SWE) in a wide temperature range (130–220 °C) using different solvent-solid ratio (10–30 mL/g) in time periods from 5 to 15 min, in order to obtain bioflavonoids. Identification and quantification of present bioflavonoids was conducted by high-performance liquid chromatography with a with a diode array detector (HPLC), and hesperidin (0.16–15.07 mg/g) was determined as the most abundant flavanon in mandarin peel with other polyphenolic compounds that were possible by-products of thermal degradation. At higher temperatures, the presence of 5-hydroxymethylfurfural (5-HMF) and chlorogenic acid were detected. Antiradical activity and total phenolic content in the extracts were determined using spectrophotometric methods, while the process optimization was performed by response surface methodology (RSM).

Carbon quantum dots (CQDs) have recently emerged as innovative theranostic nanomaterials, enabling fast and effective diagnosis and treatment. In this study, a facile hydrothermal approach for N-doped biomass-derived CQDs preparation from Citrus clementina peel and amino acids glycine (Gly) and arginine (Arg) has been presented. The gradual increase in the N-dopant (amino acids) nitrogen content increased the quantum yield of synthesized CQDs. The prepared CQDs exhibited good biocompatibility, stability in aqueous, and high ionic strength media, similar optical properties, while differences were observed regarding the structural and chemical diversity, and biological and antioxidant activity. The antiproliferative effect of CQD@Gly against pancreatic cancer cell lines (CFPAC-1) was observed. At the same time, CQD@Arg has demonstrated the highest quantum yield and antioxidant activity by DPPH scavenging radical method of 81.39 ± 0.39% and has been further used for the ion sensing and cellular imaging of cancer cells. The obtained results have demonstrated selective response toward Fe³⁺ detection, with linear response ranging from 7.0 µmol dm⁻³ to 50.0 µmol dm⁻³ with R² = 0.9931 and limit of detection (LOD) of 4.57 ± 0.27 µmol dm⁻³. This research could be a good example of sustainable biomass waste utilization with potential for biomedical analysis and ion sensing applications.

Citrus fruits processing results in the generation of huge amounts of citrus by-products, mainly peels, pulp, membranes, and seeds. Although they represent a major concern from both economical and environmental aspects, it is very important to emphasize that these by-products contain a rich source of value-added bioactive compounds with a wide spectrum of applications in the food, cosmetic, and pharmaceutical industries. The primary aim of this review is to highlight the great potential of isolated phytochemicals and extracts of individual citrus by-products with bioactive properties (e.g., antitumor, antimicrobial, antiviral, antidiabetic, antioxidant, and other beneficial activities with health-promoting abilities) and their potential in pharmaceutical, biomedical, and biological applications. This review on citrus by-products contains the following parts: structural and chemical characteristics; the utilization of citrus by-products; bioactivities of the present waxes and carotenoids, essential oils, pectins, and phenolic compounds; and citrus by-product formulations with enhanced biocactivities. A summary of the recent developments in applying citrus by-products for the treatment of different diseases and the protection of human health is also provided, emphasizing innovative methods for bioaccessibility enhancements (e.g., extract/component encapsulation, synthesis of biomass-derived nanoparticles, nanocarriers, or biofilm preparation). Based on the representative phytochemical groups, an evaluation of the recent studies of the past six years (from 2018 to 2023) reporting specific biological and health-promoting activities of citrus-based by-products is also provided. Finally, this review discusses advanced and modern approaches in pharmaceutical/biological formulations and drug delivery (e.g., carbon precursors for the preparation of nanoparticles with promising antimicrobial activity, the production of fluorescent nanoparticles with potential application as antitumor agents, and in cellular imaging). The recent studies implementing nanotechnology in food science and biotechnology could bring about new insights into providing innovative solutions for new pharmaceutical and medical discoveries.

Essential oils (EOs) are highly lipophilic, which makes the measurement of their biological action difficult in an aqueous environment. We formulated a Pickering nanoemulsion of chamomile EO (C_Pe). Surface-modified Stöber silica nanoparticles (20 nm) were prepared and used as a stabilizing agent of C_Pe. The antimicrobial activity of C_Pe was compared with that of emulsion stabilized with Tween 80 (C_T80) and ethanolic solution (C_Et). The antimicrobial effects were assessed by their minimum inhibitory concentration (MIC₉₀) and minimum effective (MEC₁₀) concentrations. Besides growth inhibition (CFU/mL), the metabolic activity and viability of Gram-positive and Gram-negative bacteria as well as Candida species, in addition to the generation of oxygen free radical species (ROS), were studied. We followed the killing activity of C_Pe and analyzed the efficiency of the EO delivery for examined formulations by using unilamellar liposomes as a cellular model. C_Pe showed significantly higher antibacterial and antifungal activities than C_T80 and C_Et. Chamomile EOs generated superoxide anion and peroxide related oxidative stress which might be the major mode of action of Ch essential oil. We could also demonstrate that C_Pe was the most effective in donation of the active EO components when compared with C_T80 and C_Et. Our data suggest that C_Pe formulation is useful in the fight against microbial infections.