The detailed understanding of fluorescence emission processes is still unclear. This study demonstrates Aegle marmelos derived luminescent heteroatoms (N, Ca, K) doped carbon quantum dots (CQDs) using an economically and ecologically sustainable synthesis process without the necessity for any doping precursors due to its phytochemical, vitamin and mineral content. Carboxyl functionalization was done by adding lemon juice to the fruit extract. The morphological, physiochemical, compositional, crystallinity, and surface functional groups having heteroatom doped CQDs were analysed by HRTEM, EDX, XPS, XRD, FTIR etc. Besides, CQDs exhibited pH and solvent-dependent tuneable fluorescence characteristics. In fact, beyond pH 7.77, a protonation-deprotonation-driven red-shift was observed together with a decrease in the contribution of prominent peaks. Meanwhile, the features of solvatochromic fluorescence were examined in a range of aprotic and protic solvents with low and high polarity. Based on the studied Kamlet–Taft parameters and the obtained spectroscopic characterizations, a suitable fluorescence emission mechanism is provided. The observed solvatochromic fluorescence is thought to be caused by a combination of dipole moment polarisation, intramolecular charge transfer processes with or without H-bond stabilisation via the interaction of heteroatoms doped CQDs with solvent mediated by electron donation and acceptance from various surface functional groups such as hydroxyl, carboxyl with solvent molecules. Hence, this study is believed to promote the development of eco-tuneable fluorescent heteroatom doped CQDs and provide further insights into the fundamental fluorescence mechanisms, which include the relationship between morphology, surface properties and plausible quantum effects between CQDs and solvents.

Composites of carbon quantum dots (CQDs) are important materials to utilize the optical properties of CQDs in diverse applications including photoluminescence-based sensing and LED phosphors. Combining pre-prepared CQDs with a polymeric matrix usually causes changes in the optical properties of CQDs due to unavoidable aggregation. Recently, the preparation of composites based on in-situ formed CQDs has been debated to overcome the aggregation limits of the conventional mixing methods. Herein, we have demonstrated the synthesis of homogeneous CQDs composites by simple thermal annealing blends of aluminum hydroxide (AlOH), citric acid (CA), and urea (URA). Transmission electron microscopy (TEM), X-ray diffraction, and Raman spectroscopy studies revealed the formation of individual CQDs with a diameter of about 2–9 nm dispersed homogeneously over the AlOH matrix. The composites have a broad excitation band centered at about 360 nm and exhibit excitation-dependent photoluminescence which was similar to that of hydrothermally synthesized CQDs from CA and URA. The photoluminescent intensity of the composite was stable to UV irradiation and responded selectively to Cu(II) ion demonstrating its potential application in Cu(II) sensing.

Carbon quantum dots (CQDs), the newest member of carbonaceous nanomaterials, have drawn many considerations since the past two decades. A vast number of researchers made their efforts to demystify optical behavior of these materials despite being demanding. Nevertheless, their emission origin is still a controversial issue and this area suffers from a lack of hypothesis to explain the radiative transitions of these materials. White emissive CQDs are more prized among the other ones since it has provided an affordable warm white light source for many applications. In this paper, white emissive CQDs samples were prepared through a one-step hydrothermal synthesis approach. By using the advantage of possessing cellulosic networks in the Aloe Vera gel an in-situ matrix was created to encase CQDs particles. During the formation of CQDs particles, they were entrapped and created RGB nanoemitters in the cellulosic units. The leakage of the emitted photons during the radiative transitions followed by inner-filter effect (IFE) and self-/re-absorption acted as white light emissive sources. To scrutinize the validity and possibility of the hypothesis given in this paper, a series of spectroscopic analyses, including transmission electron microscopy (TEM), surface-enhanced Raman scattering (SERS), Fourier Transform Infrared (FT-IR), ultraviolet–visible (UV–Vis), and photoluminescence (PL) were conducted.

Novel ionic liquid-functionalized carbon quantum dots (IL-CDs) were prepared by hydrothermal method, and characterized with FT-IR, TEM and XPS. The IL-CDs exhibited narrower particle size distribution with more uniform dispersion and the surface potential changes from negative to positive due to the function of IL. IL-CDs could be quenched (“turned off”) after adding ascorbic acid (AA), and as an “on–off”, fluorescent probe could be established for direct analysis AA. The linear range of AA was 0.34–30.00 μg/mL and the LOD was 0.11 μg/mL. The method was successfully applied to the determination of AA in real samples with satisfactory results.

Since soil salinization imposes various adverse effects on plants, research on how to relieve salt stress from plants is extremely urgent. We synthesized a new type of cerium-doped carbon quantum dots by a hydrothermal synthesis method. Characterization shows that the carbon quantum dots have a small and uniform particle size, high stability, high water solubility and biocompatibility. Mung bean seeds were soaked in CDs:Ce solutions under a concentration gradient (0.25, 0.5, 1, 2, 3 mg/ mL) and germinated under salt stress (150 mM NaCl). Compared with salt stress, the addition of CDs solutions effectively enhanced the ability of plants to relieve salt stress. The relieving effect on mung bean plants was the most significant after treatment with 2 mg/mL CDs:Ce, and the main root length, plant height and leaf length in comparison with the case of salt stress increased by 83%, 80%, and 60%, respectively. Chlorophyll content, peroxidase activity, superoxide dismutase activity and catalase activity, total protein content increased by 90%, 77%, 76%, 77% and 76%, respectively, malondialdehyde and proline The content decreased by 83% and 77%. Inductively coupled plasma mass spectroscopy proved mung bean plants absorbed CDs:Ce, but the absorption of NaCl decreased by 21.8%. Fluorescence imaging showed CDs:Ce was absorbed by roots, and transferred from the vascular system and apoplastic pathways to stems and leaf veins, and mainly aggregated in intercellular gaps, the vascular system, leaf veins, cilia and stomata. Stereomicroscopy showed that CDs:Ce induction increased the stomatal opening by 15.7%, and improved metabolic efficiency and NaCl excretion from the plants. Hence, CDs:Ce shows great potential in protecting crops from abiotic stress.

Food toxins are regarded as a major source of health risks, serious illnesses susceptible to even death. These dangerous pathogens may lead to significant economic impact worldwide. The food production chain undergoes different stages like harvesting, processing, storage, packaging, distribution, and lastly preparation, and consumption. Therefore, each step is susceptible to risks of environmental contamination. Nowadays, the carbon quantum dots (CDs) are regarded as one of the most widely used hybrid carbon nanomaterials due to their different magical physical and chemical properties. The CDs have a size below 10 nm and show the fluorescent property. The CDs find vast applications in different fields like sensing, food safety, drug delivery, bioimaging, catalyst, energy conversion, etc. Compared to other available methods, the fluorescence detection techniques have low cost, easy handling, and safe operating system. There is a need for a review to compile the fluorescence properties of carbon nanodots used to detect food pathogens. This brief review is addressed in that direction and mostly focused on the synthesis of carbon dots-based fluorescence sensors for detecting pathogens and toxins in foods and beverages. The detailed mechanisms and origin of fluorescence properties of carbon quantum dots are also highlighted herewith.

Herein, a facile bottom–up approach for producing nitrogen-doped carbon quantum dots (N-CQDs) was carried out by the hydrothermal treatment of microcrystalline cellulose, in the presence of different nitrogen sources (blank/urea/ammonia water/ethanediamine(EDA)/Hexamethylenetetramine). The result showed that the fluorescence intensity and quantum yields (QYs) of N-CQDs with different nitrogen sources are all higher than that without nitrogen source. Compared with the other three nitrogen sources, N-CQDs prepared by EDA not only have the highest fluorescence intensity but also the largest QYs of 51.39%. Therefore, EDA was chosen as the nitrogen source to prepare N-CQDs. The obtained N-CQDs are uniform spherical particles with a diameter of 2.76 nm. The N-CQDs also exhibit excitation-dependent and long-wave emission properties. The emission range of N-CQDs is 470–540 nm. Moreover, N-CQDs as fluorescent agents successfully acted on purple LEDs (λem = 365 nm) to achieve white LEDs light emission. At the same time, a fluorescent thin layer chromatography plate was successfully prepared using N-CQDs, silica gel G and Sodium carboxymethylcellulose as raw materials. The separation trajectory of mixed sample of Sudan red III and kerosene on the fluorescent TLC plate is obviously clearer than that of the TLC plate.

An extract of fresh guava leaves (Psidium guajava) was used as a green carbon precursor to fabricate blue fluorescent carbon quantum dots (GCQDs) by hydrothermal process. The GCQDs show bright blue fluorescence emission under UV light with an excitation wavelength of 350 nm and emission at 450 nm. The physical structure of GCQDs was characterized by Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray diffraction (XRD), High-resolution transmission electron microscope (HR-TEM) and atomic force microscopy (AFM). GCQDs 80 μg inhibited the growth of waterborne pathogens Escherichia coli and Salmonella typhi. We also investigated the catalytic activity of the GCQDs on the removal of two azo dyes, namely Congo red and bromophenol blue, with and without NaBH4. The GCQDs showed an excellent reduction of color intensity of both dyes without NaBH4 within 30 min of treatment.

In the present study, a novel electrochemical sensor for acetaminophen (AMP) which included quantum graphitic carbon nitride dots, g-C3N4QDs, was designed and conducted with molecular imprinted polymer (MIP). First, bulk g-C3N4 was generated with direct thermal polycondensation of melamine. After the treatment of the acidic solution containing H2SO4: HNO3 (1:1, v:v), the heating treatment at 200 °C on the dispersion provided g-C3N4QDs. In this respect, for nanomaterial characterization, some spectroscopic approaches were performed including Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) as well as electroanalytical methods such as electrochemical impedance (EIS) and cyclic voltammetry (CV). In accordance with the aims of the study, AMP imprinted electrode was formed after high electrocatalytic performance and linear range of 1.0 × 10– 11–2.0 × 10– 8 M and the LODs of 2.0 × 10– 12 was achieved. Eventually, an AMP-printed sensor was also used for AMP identification in pharmaceutical samples.

Highly luminescent carbon quantum dots (CQDs) are developed as fluorescent probes for selective detection of the heavy-ion Fe3+, where the CQDs exhibit excellent nontoxicity, functionalizability, sensitivity, and selectivity. Biomass-based CQDs and nitrogen-doped CQDs (N-CQDs) are synthesized for the selective detection of Fe3+ by using H2O2 as an oxidant and polyetherimide (PEI) as a nitrogen precursor by a green hydrothermal synthesis method. The prepared CQDs and N-CQDs exhibit an elliptical morphology and with an average particle size of 7 and 4 nm, respectively, and emit blue photoluminescence at 445 and 468 nm under excitation at 367 and 343 nm, respectively. The CQDs and N-CQDs exhibit good water solubility because of the abundant hydroxyl and carboxyl/carbonyl groups and graphic/pyrrolic/pyridinic nitrogen on the surfaces, giving rise to a quantum yield of about 24.2% and 30.7%, respectively. Notably, the Matrimony vine-PEI-based CQDs exhibit excellent Fe3+ selectivity and sensitivity relative to the Matrimony vine-based CQDs due to complexation of the numerous phenolic hydroxyl groups and nitrogen-containing groups with Fe3+, leading to increased fluorescence quenching, which greatly improves the sensitivity of detection. The minimum detection limit was 2.22 μmol L− 1 with a complexation constant of 44.7.

Nanostructured ZnO materials have been studied extensively because of their functional properties. This paper presents a composite material of zinc oxide quantum dots (ZnO QDs) and porous carbon using a one-step carbonization process. The direct carbonization of a metal–organic complex generates mesostructured porous carbon with a homogeneous distribution of ZnO QDs. The structural and morphological properties are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The resulting ZnO QDs@porous carbon composite delivers a high specific capacity of 990 mAh g−1 at 100 mA g−1, 357 mAh g−1 at 2 A g−1, and high reversibility when evaluated as an anode for lithium ion batteries.