High green-emission carbon dots and its optical properties: Microwave power effect

Green synthesis of fluorescent carbon quantum dots from Eleusine coracana and their application as a fluorescence ‘turn-off’ sensor probe for selective detection of Cu2+

Green synthesis of carbon quantum dots from wasted enzymatic hydrolysis lignin catalyzed by organic acids for UV shielding and antioxidant fluorescent flexible film

Biomass-based carbon quantum dots (CQDs) have become a significant carbon materials by their virtues of being cost-effective, easy to fabricate and low in environmental impact. However, there are few reports regarding using cyanobacteria as a carbon source for the synthesis of fluorescent CQDs. In this study, the low-cost biomass of cyanobacteria was used as the sole carbon source to synthesize water-soluble CQDs by a simple hydrothermal method. The synthesized CQDs were mono-dispersed with an average diameter of 2.48 nm and exhibited excitation-dependent emission performance with a quantum yield of 9.24%. Furthermore, the cyanobacteria-derived CQDs had almost no photobleaching under long-time UV irradiation, and exhibited high photostability in the solutions with a wide range of pH and salinity. Since no chemical reagent was involved in the synthesis of CQDs, the as-prepared CQDs were confirmed to have low cytotoxicity for PC12 cells even at a high concentration. Additionally, the CQDs could be efficiently taken up by cells to illuminate the whole cell and create a clear distinction between cytoplasm and nucleus. The combined advantages of green synthesis, cost-effectiveness and low cytotoxicity make synthesized CQDs a significant carbon source and broaden the application of cyanobacteria and provide an economical route to fabricate CQDs on a large scale.

Luminescent quantum dots: Synthesis, optical properties, bioimaging and toxicity

When carbon quantum dots (CDs) are used to enhance photosynthesis, they inevitably enter the plant. However, the dose-dependent effects of CDs on plant growth are poorly understood. In this study, we investigated the dose-dependent effects of CDs on soybean growth. CDs were synthesized from citric acid and urea via a hydrothermal procedure. The analysis of the structure, chemical composition, and optical properties revealed that synthetic CDs have a sphere-like shape with rich hydrophilic groups on their surface. CDs exhibited superior upconverted photoluminescence properties and emitted strong fluorescence (exciting wavelength of 220 nm; emitting wavelength of 438 nm). Various concentrations of synthetic CDs (0–1000 mg L−1), as a photosynthesis enhancer, were applied to soybean plants under hydroculture for 1–10 days. CDs positively affected soybean growth at concentrations less than 200 mg L−1. However, at higher concentrations (500 or 1000 mg L−1), they exhibited significant toxicity to plant growth, which was evidenced by the mass accumulation of CDs in damaged leaves. Metabolomic and transcriptomic analyses indicated that CDs at a low concentration (100 mg L−1) could increase antioxidant and biomass accumulation in soybeans to promote plant growth. This study provided valuable insights into the impacts of CDs on plants in sustainable agricultural practices involving the use of nanomaterials.

The impact of Carbon Quantum Dots (CQDs) on the photocatalytic activity of TiO2 under UV and visible light

High quantum yield carbon quantum dots as selective fluorescent turn-off probes for dual detection of Fe2+/Fe3+ ions

Synthesis of highly photoluminescent carbon dots via citric acid and Tris for iron(III) ions sensors and bioimaging

Natural product-derived carbon quantum dots (CQDs) have recently drawn much attention owing to their excellent optical and electrical properties. This study aims to develop CQDs that can be applied to fluorescent inks and detect Fe3+ metal ions in aqueous solutions. Watermelon peel waste was converted into CQDs using a one-step, straightforward, environmentally friendly, and efficient hydrothermal process. The resulting CQDs were analyzed with various techniques, including UV-Visible absorbance spectroscopy, spectrofluorometric, X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy. According to the available literature, the CQDs exhibit strong blue fluorescence that is very photostable, good enough to allow their possible use in a fluorescent ink application. The CQDs were examined regarding their potential as sensors to detect metal ions by studying how much their fluorescence was quenched when mixed with several different metal ions in solution. Examination of the data indicates that the only metal ions that significantly quenched the fluorescence of the CQDs were the Fe3+ metal ions, and these caused about an 80% reduction in the amount of fluorescence that was observable from the CQDs. This study suggests that these watermelon peel-derived carbon dots are probably good candidates for both fluorescent inks and sensors for detecting metal ions.

This study investigates the synthesis and photoluminescent properties of carbon quantum dots (CQDs) derived from Actinidia deliciosa using the hydrothermal method. The effect of concentration and pH on the composition, structure, and optical properties of CQDs was analyzed using characterization techniques such as TEM, EDS, FTIR, UV-Vis, and photoluminescence (PL) spectroscopy. The CQDs exhibited particle sizes ranging from 1 to 10 nm, with a graphitic structure and oxygen-containing functional groups, as identified by FTIR bands corresponding to OH, C=O, and C=C. The stability analysis revealed particle agglomeration over 30 days, increasing the size up to <40 nm. Regarding the optical properties, the CQDs displayed absorption peaks at 225 and 280 nm and a bandgap of ~3.78–3.82 eV. The PL characterization demonstrated tunable emission from violet to green, depending on the excitation wavelength. CQDs synthesized at an acidic pH of 2 exhibited enhanced luminescence due to protonation effects, whereas an alkaline pH led to a reduction in emission intensity. The hydrothermal method enabled a simple and eco-friendly synthesis, using water as the sole solvent, yielding stable CQDs with a luminescence lifespan exceeding 30 days. Their optical and electronic properties make them promising candidates for photocatalysis, heavy metal detection, and bioimaging applications.

Carbon quantum dots (CQDs) have emerged in the research of optoelectronic devices as a kind of novel photoelectronic materials in recent years because of their excellent properties such as high fluorescence quantum yield, tunable bandgap, good solvent dispersion, high electron transporting performance, long hot-electron lifetime, wide optical absorption, low cost and environmental friendliness. In this paper, their synthesis, optoelectronic properties, applications and existed problems were reviewed, which will provide reference for expanding the application of CQDs in lighting, display, communication and solar cells in the future. Firstly, the synthesis, structure and optical properties of CQDs were summarized, and the emission mechanisms of CQDs from quantum confinement effect, surface defects or molecular state were systematically investigated. The CQD emission from surface defects can cause non-radiative transition, reducing the fluorescence quantum yield (QY) of CQDs. Therefore, in order to improve the electroluminescence and photoluminescence properties of CQDs, the non-radiative transition caused by surface defects should be avoided as far as possible. As for the emission of CQDs from molecular state, with the increasing of reaction temperatures, the luminescent groups of molecular state gradually are carbonized and the carbon cores of CQDs gradually are formed, the proportion of luminescence from molecular state gradually decreases while carbon cores gradually dominate the emission of CQDs. In addition, the heteroatom doping can significantly adjust the bandgap and electron cloud density of CQDs, resulting in the improvement of their QY. The impact of their particle size, structure and surface state on the emission wavelengths of CQDs were summarized. The bandgap width of CQDs gradually decrease with the increasing the particle size, sp2 conjugation degree and oxygen-containing functional groups of CQDs, which help their emission with long wavelengths. The solid-state luminescence property of CQDs is necessary for the application of CQDs in solid state lighting and full color display. However, in most cases, the CQDs in solid-state suffer obvious fluorescence quenching, which inevitably reduces the performance of optoelectronic devices. Increasing the interparticle spacing of CQDs is an effective measure for suppressing the solid-state fluorescence quenching through dispersing CQDs in a bulk matrix or long-chain passivation. Secondly, the recent developments of CQD application in optoelectronic devices including white light-emitting diode (WLED), quantum dot-based electroluminescent light-emitting diode (QLED), laser diode (LD), visible light communication (VLC) and polymer solar cell (PSC) were introduced. The emission wavelength of CQDs has been extended from deep-ultraviolet to near-infrared region, and the QY of blue, green and red emission CQDs has reached more than 80%. Moreover, the CQDs with narrow bandwidth emission were obtained, which endows CQDs with the broad application prospects in the field of LED and LD as photoluminescence phosphor powders and electroluminescence emitting layers. Owing to the excellent photoluminescence properties and short fluorescence lifetime of CQDs, they also have potential applications in VLC as photoluminescence phosphor powders. In addition, owing to their excellent electron transporting abilities and solution processability, they also can be used as electron acceptor or electron transport layer in PSC. Finally, the major problems and opportunities for CQDs in optoelectronic were analyzed and outlined. Their solid-state luminescent quenching, long-wavelength emission, high QY and thermal stability and product yield of CQDs need to be improved in the future.

We present a facile strategy for highly sensitive and selective determination of copper(II) ions and vitamin C (ascorbic acid, AA) using new amino-terminated nitrogen-doped carbon quantum dots (CQDs) synthesized from melamine as the carbon and nitrogen source by the hydrothermal method. The CQDs have superior optical features, including a pH-sensitive photoinduced electron transfer process. The CQDs form a complex with Cu2+ ions, leading to the development of naked-eye, colorimetric, and fluorometric determination. AA reduces the Cu2+ ions to Cu+ ions, which cannot form the complex. Thus the absorbance and fluorescence of the CQDs are recovered by addition of AA because of dissociation of the complex into Cu+ and CQD. The in situ generation of reactive oxygen species when AA is added to Cu-CQD complexes in the presence of dissolved oxygen leads to the sensitive determination of AA, proposed on CQDs for the first time. The in situ generation of reactive oxygen species was confirmed by a fluorescence method using a hydroxyl radical indicator (i.e., coumarin). This novel "turn-off/turn-on" sensing approach using amine-functionalized CQDs is potentially applicable to determining the concentration of Cu2+ ions and AA in the areas of materials chemistry, nanobiomedicine, nanobiotechnology, and bioengineering because of its high sensitivity, high selectivity, low cost, simple naked-eye readout, and good linearity. Graphical abstract.

This research demonstrates a carbon quantum dot (CQD) based fluorescence technique for milk quality assessment and detection of urea adulteration using a commercialized optical fiber spectrometer (OFS) and a developed color sensor device (CSD). The change in pH from the normal pH of milk (∼6.7) indicates spoilage. The CQDs were synthesized using phthalic acid (carbon source) and triethylenediamine (TED) (passivant). The CQDs were found to be sensitive in the pH range 3–10 with a quantum yield of about 18.9%. The accuracy of spoilage detection using CQDs was expressed in the form of mean percent recovery and found to be 99.2% (R2 = 0.97) by the OFS and 99.59% (R2 = 0.98) by the CSD. The mean percent recovery obtained for urea adulteration in milk was 103.0% (R2 = 0.98) by the OFS and 97.9% (R2 = 0.97) by the CSD. The results indicate that CQDs have excellent potential for use as biosensors for spoilage and adulteration detection.

This study reports the microwave-assisted synthesis and surface modification of carbon quantum dots (CQDs) from natural precursors and their evaluation as fluorescent probes for cancer cell visualization. CQDs were obtained using amino-glucose as the carbon source and betaine, marine collagen, or dopamine as surface modifiers. Further functionalization with 7-amino-4-(trifluoromethyl)coumarin enhanced their fluorescence properties. Spectroscopic analyses confirmed successful surface modification, with coumarin-modified CQDs displaying a strong emission peak at ~500 nm and approximately 1.5-fold higher fluorescence intensity compared to unmodified CQDs. Cytotoxicity testing on MG-63 osteosarcoma cells showed cell viabilities above 80% for selected samples, fulfilling ISO 10993-5 criteria for non-toxicity. In vitro bioimaging of astrocytoma 1321N1 cells demonstrated bright and uniform intracellular staining, confirming effective cellular uptake. Compared with the literature reports of green-synthesized CQDs, our results indicate comparable or superior fluorescence performance and similar levels of biocompatibility. These findings highlight the potential of surface-engineered CQDs as biocompatible nanoprobes for cancer diagnostics and represent an initial step toward their application in the detection of circulating tumor cells (CTCs).

Development of multifunctional fluorescent inks is gaining attention owing to the current technological developments in optoelectronics. The present work involves the development of a simple and highly fluorescent carbon quantum dot (CQD)‐based nano‐ink suitable for sensor and bioimaging applications. CQDs were synthesized by a one‐pot hydrothermal method using folic acid (FA) a carbon source. The synthesized CQDs were conjugated with hydroxyapatite (HAP) and sodium aliginate (SA) by covalent and ionic linkages methods. Fourier transform infrared (FT‐IR) spectra confirm the formation of nano‐inks by the ionic interaction between the HAP and SA and conjugation with CQDs through hydrogen bonding. The surface morphology of the CQDs and nano‐inks depicts that the particles exhibit in spherical and needle‐like morphological structures, respectively. The X‐ray diffraction (XRD) pattern confirms that the obtained CQDs and fluorescent nano‐ink are highly crystalline in nature. The SA and HAP provided shear thinning ability and structural stability for the fluorescent nano‐ink. The nano‐ink‐based strips displayed high fluorescence under Ultraviolet irradiation. The fluorescent nano‐ink could be used in bioimaging, flourescent coatings, and anti‐counterfeiting applications.