Green Carbon Dots: Synthesis, Characterization, Properties and Biomedical Applications

Carbon dots (CDs) are a new category of crystalline, quasi-spherical fluorescence, "zero-dimensional" carbon nanomaterials with a spatial size between 1 nm to 10 nm and have gained widespread attention in recent years. Green CDs are carbon dots synthesised from renewable biomass such as agro-waste, plants or medicinal plants and other organic biomaterials. Plant-mediated synthesis of CDs is a green chemistry approach that connects nanotechnology with the green synthesis of CDs. Notably, CDs made with green technology are economical and far superior to those manufactured with physicochemical methods due to their exclusive benefits, such as being affordable, having high stability, having a simple protocol, and being safer and eco-benign. Green CDs can be synthesized by using ultrasonic strategy, chemical oxidation, carbonization, solvothermal and hydrothermal processes, and microwave irradiation using various plant-based organic resources. CDs made by green technology have diverse applications in biomedical fields such as bioimaging, biosensing and nanomedicine, which are ascribed to their unique properties, including excellent luminescence effect, strong stability and good biocompatibility. This review mainly focuses on green CDs synthesis, characterization techniques, beneficial properties of plant resource-based green CDs and their biomedical applications. This review article also looks at the research gaps and future research directions for the continuous deepening of the exploration of green CDs.

Guar gum-sodium alginate nanocomposite film as a smart fluorescence-based humidity sensor: A smart packaging material

Perishable packed foods are easily damaged by the change in relative humidity. In this work, we demonstrate that guar gum- sodium alginate blending with glucose-glycerol carbon dots nanocomposite film can be used to detect relative humidity. The fabricated nanocomposite film was an excellent smart sensor based on the fluorescence 'on-off' mechanisms against humidity. The study demonstrates that at different relative humidity conditions, such as 11 %, 33 %, 75.30 %, 84 %, and 97 %, there is a change in the fluorescence of biocomposite films under UV light. The practical feasibility of the biocomposite developed film was tested in real conditions by placing a piece of bread with high humidity conditions wrapped with the developed nanocomposite film. It was observed that under such conditions, marked quenching of fluorescence was observed and hence detection of humidity was possible. Hence, the fabricated nanocomposite film can monitor the packed food freshness using just a UV light source. Such biopolymer nanocomposite is potential materials and may find application as smart packaging materials, especially as food packaging materials.

Solid-state nitrogen-doped carbon nanoparticles with tunable emission prepared by a microwave-assisted method

Tunable emissive solid-state carbon nanoparticles (CNPs) have been successfully synthesized by a facile synthesis through microwave irradiation. Modulating microwave interaction with the sample to generate abrupt localized heating is a long-term challenge to tailor the photoluminescence properties of CNPs. This study systematically revealed that the sample temperature through microwave irradiation plays a crucial role in controlling the photoluminescence properties over other reaction conditions, such as irradiation time and microwave duty cycle. When the sample temperature reached 155 °C in less than three minutes, the CNP sample exhibited a green-yellowish emission with the highest quantum yield (QY) of 14.6%. Time-dependent density functional theory (TD-DFT) study revealed that the tunable photoluminescence properties of the CNPs can possibly be ascribed to their nitrogen concentrations, which were dictated by the sample temperature during irradiation. This study opens up a promising route for the well-controlled synthesis of luminescent CNPs through microwave irradiation.

Tunable emissive solid-state carbon nanoparticles (CNPs) have been successfully synthesized by a facile synthesis through microwave irradiation.

Precise Surface State Control of Carbon Quantum Dots to Enhance Charge Extraction for Solar Cells

Dye-sensitized solar cells are regarded as promising candidates to resolve the energy and environmental issues in recent years, arising from their solution-processable fabrication technology and high power conversion efficiency. However, there are still several problems regarding how to accelerate the development of this type of photovoltaics, including the limited light-harvesting ability and high-production cost of molecular dye. In the current work, we have systematically studied the role of nitrogen-doped carbon quantum dots (N-CQDs) as co-sensitizers in traditional dye sensitized solar cells. A series of N-CQDs have been prepared by employing chitosan as a precursor via one-pot hydrothermal technology for various times, demonstrating a maximized efficiency as high as 0.089% for an only N-CQDs-based device. Moreover, the co-sensitized solar cell based on N719 dye (C₅₈H₈₆N₈O₈RuS₂) and optimized N-CQDs shows significantly enhanced performance, yielding a solar-to-electric conversion efficiency of up to 9.15% under one standard sun (AM 1.5G) irradiation, which is much higher than the 8.5%-efficiency of the controlled device without N-CQDs. The matched characteristics of energy level, excellent up-convention, and FRET (Förster resonance energy transfer) abilities of N-CQDs are responsible for their improved power conversion efficiency.

Carbon dots derived from water hyacinth and their application as a sensor for pretilachlor

Recently,carbon-based nanomaterials have been attracted much interest among the scientific community due to its extraordinary properties and applications. Mostly the fluorescent carbon nanomaterials are prepared from commercially available precursors. In this work, develop a new strategy for producing carbon nanoparticles (carbon dots) using phosphoric acid as an activating agent from water hyacinth present in Assam, India. These carbon nanoparticles show green fluorescence under UV light, and the sizes are found below 10 nm. These carbon dots are applied as a fluorescence sensor for detecting the herbicide (pretilachlor). The developed PL sensor is exclusively selective and sensitive for detection of this herbicide, and the limit of detection is found to be 2.9 μM.This sensor is also tested for real samples like soil contaminated with pretilachlor.

Fluorescence quenching capillary analysis for determining trace-level nitrite in food based on the citric acid/ethylenediamine nanodots/nitrite reaction

We found that nitrite after protonation can react with amine radical on citric acid/ethylenediamine carbon nanodots (CA/EDA-CDs) to form nitrosamines, and fluorescence quenching of CA/EDA-CDs occurred during this process. Using the reaction mechanism a fluorescence quenching capillary analysis (FQCA) was developed. After optimized reaction conditions, the following results were obtained: the required concentration of CA/EDA-CDs was 12 mg/L, HCl concentration was 32 mmol/L, and the reaction conducted in room temperature for 20 min. Under optimized conditions, FQCA has a linear response in 20-500 μg/L in which RSD was less 4.5% (n = 11), the detection limit was 6.5 μg/L and the recovery was in 95-105%. The measured results were consistent with the national standard method. FQCA has been used for determining nitrite in foods and nature waters. The capillary in FQCA was used as the container for CA/EDA-CDs/NO₂^- reaction and NO₂^- determination, and realized trace-level analysis for micro-volume samples (<10 μL/time).

Chiral carbon dots and their effect on the optical properties of photosensitizers

In this work, we have successfully prepared intrinsically chiral carbon dots from chiral precursors. We have also demonstrated that the chirality of these carbon dots can affect the optical properties of photosensitizer molecules like azobenzene.

Functionalized graphene oxide quantum dot-PVA hydrogel: a colorimetric sensor for Fe²⁺, Co²⁺ and Cu²⁺ ions

Functionalized graphene oxide quantum dots (GOQDs)-poly(vinyl alcohol) (PVA) hybrid hydrogels were prepared using a simple, facile and cost-effective strategy. GOQDs bearing different surface functional groups were introduced as the cross-linking agent into the PVA matrix thereby resulting in gelation. The four different types of hybrid hydrogels were prepared using graphene oxide, reduced graphene oxide, ester functionalized graphene oxide and amine functionalized GOQDs as cross-linking agents. It was observed that the hybrid hydrogel prepared with amine functionalized GOQDs was the most stable. The potential applicability of using this solid sensing platform has been subsequently explored in an easy, simple, effective and sensitive method for optical detection of M(2+) (Fe(2+), Co(2+) and Cu(2+)) in aqueous media involving colorimetric detection. Amine functionalized GOQDs-PVA hybrid hydrogel when put into the corresponding solution of Fe(2+), Co(2+) and Cu(2+) renders brown, orange and blue coloration respectively of the solution detecting the presence of Fe(2+), Co(2+) and Cu(2+) ions in the solution. The minimum detection limit observed was 1 × 10(-7) M using UV-visible spectroscopy. Further, the applicability of the sensing material was also tested for a mixture of co-existing ions in solution to demonstrate the practical applicability of the system. Insight into the probable mechanistic pathway involved in the detection process is also being discussed.

Ethylene diamine mediated cobalt nanoparticle studded graphene oxide quantum dots with tunable photoluminescence properties

The present study demonstrates ethylene diamine mediated in situ synthesis of cobalt oxide nanoparticles (Co₃O₄NPs) studded on graphene oxide quantum dots (GOQDs) showing reversible on/off fluorescence switching.

Synthesis and Characterisation of Fluorescent Carbon Nanodots Produced in Ionic Liquids by Laser Ablation

Carbon nanodots (C-dots) with an average size of 1.5 and 3.0 nm were produced by laser ablation in different imidazolium ionic liquids (ILs), namely, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4 ), 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI.NTf2 ) and 1-n-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (OMI.NTf2 ). The mean size of the nanoparticles is influenced by the imidazolium alkyl side chain but not by the nature of the anion. However, by varying the anion (BF4 vs. NTf2 ) it was possible to detect a significant modification of the fluorescence properties. The C-dots are much probably stabilised by an electrostatic layer of the IL and this interaction has played an important role with regard to the formation, stabilisation and photoluminescence properties of the nanodots. A tuneable broadband fluorescence emission from the colloidal suspension was observed under ultraviolet/visible excitation with fluorescence lifetimes fitted by a multi-exponential decay with average values around 7 ns.

Property relationship of alginate and alginate–carbon dot nanocomposites with bivalent and trivalent cross-linker ions

Biocompatible alginate and alginate–carbon dot nanocomposites with superior properties and good mechanical properties.

Green chitosan-carbon dots nanocomposite hydrogel film with superior properties

In this work we report novel chitosan-carbon dots nanocomposite hydrogel films. A new green source "tea" was used as precursor for carbon dots (CDs). The electrostatic interaction of positive charge on chitosan and negative charge on CDs prepared from tea was used for the successful preparation of a stable and robust chitosan-carbon dots nanocomposite hydrogel film. The hydrogel films were characterized by UV-visible spectroscopy, X-ray diffraction (XRD), Fourier transformed infra-red spectroscopy (FTIR), scanning electron microscope (SEM), fluorescent microscopy, thermogravimetric analysis (TGA) and contact angle analysis. It was observed that chitosan-carbon dots hydrogel films are soft but tough with superior UV-visible blocking, swelling, thermal and mechanical properties in comparison to chitosan hydrogel film. Moreover chitosan-carbon dots films are more water repellent (hydrophobic) as indicated by their high contact angle values. Thus, fabrication of such green soft but tough biocompatible chitosan-carbon dots nanocomposite hydrogel films offers tremendous bio-medical and industrial applications.