Algae biomass as a precursor for synthesis of nitrogen-and sulfur-co-doped carbon dots: A better probe in Arabidopsis guard cells and root tissues

Comprehensive advances in the synthesis, fluorescence mechanism and multifunctional applications of red-emitting carbon nanomaterials

Red emitting fluorescent carbon nanomaterials have drawn significant scientific interest in recent years due to their high quantum yield, water-dispersibility, photostability, biocompatibility, ease of surface functionalization, low cost and eco-friendliness. The red emissive characteristics of fluorescent carbon nanomaterials generally depend on the carbon source, reaction time, synthetic approach/methodology, surface functional groups, average size, and other reaction environments, which directly or indirectly help to achieve red emission. The importance of several factors to achieve red fluorescent carbon nanomaterials is highlighted in this review. Numerous plausible theories have been explained in detail to understand the origin of red fluorescence and tunable emission in these carbon-based nanostructures. The above advantages and fluorescence in the red region make them a potential candidate for multifunctional applications in various current fields. Therefore, this review focused on the recent advances in the synthesis approach, mechanism of fluorescence, and electronic and optical properties of red-emitting fluorescent carbon nanomaterials. This review also explains the several innovative applications of red-emitting fluorescent carbon nanomaterials such as biomedicine, light-emitting devices, sensing, photocatalysis, energy, anticounterfeiting, fluorescent silk, artificial photosynthesis, etc. It is hoped that by choosing appropriate methods, the present review can inspire and guide future research on the design of red emissive fluorescent carbon nanomaterials for potential advancements in multifunctional applications.

Assessment of biomass-derived carbon dots as highly sensitive and selective templates for the sensing of hazardous ions

Access to safe drinking water and a hygienic living environment are the basic necessities that encourage healthy living. However, the presence of various pollutants (especially toxic heavy metal ions) at high concentrations in water renders water unfit for drinking and domestic use. The presence of high concentrations of heavy-metal ions (e.g., Pb2+, Hg2+, Cr6+, Cd2+, or Cu2+) greater than their permissible limits adversely affects human health, and increases the risk of cancer of the kidneys, liver, skin, and central nervous system. Therefore, their detection in water is crucial. Due to the various benefits of "green"-synthesized carbon-dots (C-dots) over other materials, these materials are potential candidates for sensing of toxic heavy-metal ions in water sources. C-dots are very small carbon-based nanomaterials that show chemical stability, magnificent biocompatibility, excitation wavelength-dependent photoluminescence (PL), water solubility, simple preparation strategies, photoinduced electron transfer, and the opportunity for functionalization. A new family of C-dots called "carbon quantum dots" (CQDs) are fluorescent zero-dimensional carbon nanoparticles of size < 10 nm. The green synthesis of C-dots has numerous advantages over conventional chemical routes, such as utilization of inexpensive and non-poisonous materials, straightforward operations, rapid reactions, and renewable precursors. Natural sources, such as biomass and biomass wastes, are broadly accepted as green precursors for fabricating C-dots because these sources are economical, ecological, and readily/extensively accessible. Two main methods are available for C-dots production: top-down and bottom-up. Herein, this review article discusses the recent advancements in the green fabrication of C-dots: photostability; surface structure and functionalization; potential applications for the sensing of hazardous anions and toxic heavy-metal ions; binding of toxic ions with C-dots; probable mechanistic routes of PL-based sensing of toxic heavy-metal ions. The green production of C-dots and their promising applications in the sensing of hazardous ions discussed herein provides deep insights into the safety of human health and the environment. Nonetheless, this review article provides a resource for the conversion of low-value biomass and biomass waste into valuable materials (i.e., C-dots) for promising sensing applications.

Preparation of nitrogen-doped carbon dots and their enhancement on lettuce yield and quality

Nanotechnology is an effective way to stimulate the yield potential of crops. Various nano-fertilizers and nano-carriers are gradually being developed to bring about a technological revolution in the agricultural industry. As a biocompatible water-soluble nanomaterial, carbon dots (CDs) have attracted the attention of researchers for applications in agriculture. In this study, we prepared nitrogen-doped CDs (N-CDs) as a type of water-soluble carbon nanofertilizer by a one-pot hydrothermal method, and investigated its effects on lettuce biomass and quality. 100 and 200 mg L-1 of N-CDs substantially promoted lettuce biomass accumulation (41.70%), elevated lettuce nutrient content, as well as promoted the accumulation of major nutrients. Moreover, 100 mg L-1 N-CDs increased the chlorophyll a content by 12.68%, significantly increased the electron transport rate (ETR) by 38.61%, significantly increased the light energy conversion efficiency (Y(II)) by 31.24% and increased the Rubisco activity by 60.61%, which are important reasons for its increase in actual photosynthesis rate. N-CDs also have a positive effect on plant nitrogen metabolism by promoting the activity of glutamine synthetase. The significant benefits of N-CDs on lettuce make them have great potential for agricultural yield increase and quality improvement.

A review on unit operations, challenges, opportunities, and strategies to improve algal based biodiesel and biorefinery

Globally, the demand for energy is increasing with an emphasis on green fuels for a sustainable future. As the urge for alternative fuels is accelerating, microalgae have emerged as a promising source that can not only produce high lipid but many other platform chemicals. Moreover, it is a better alternative in comparison to conventional feedstock due to yearlong easy and mass cultivation, carbon fixation, and value-added products extraction. To date, numerous studies have been done to elucidate these organisms for large-scale fuel production. However, enhancing the lipid synthesis rate and reducing the production cost still remain a major bottleneck for its economic viability. Therefore, this study compiles information on algae-based biodiesel production with an emphasis on its unit operations from strain selection to biofuel production. Additionally, strategies to enhance lipid accumulation by incorporating genetic, and metabolic engineering and the use of leftover biomass for harnessing bio-products have been discussed. Besides, implementing a biorefinery for extracting oil followed by utilizing leftover biomass to generate value-added products such as nanoparticles, biofertilizers, biochar, and biopharmaceuticals has also been discussed.

Biobased carbon dots production via hydrothermal conversion of microalgae Chlorella pyrenoidosa

A promising green hydrothermal process was used to produce biobased nanomaterials carbon dots (CDs) by using microalgae Chlorella pyrenoidosa (CP) and its main model compounds (i.e., glucose, glycine, and octadecanoic acid). The possible reaction pathway including hydrolysis, Amadori rearrangement, cyclization/aromatization, and polymerization was first proposed for the hydrothermal process to produce microalgae-based CDs. Interactions among carbohydrates and proteins in microalgae were vital intermediate reactions in the generation of CDs. The mass yield of CDs reached 7.2% when the CP was hydrothermally treated with 20:1 of liquid-to-solid ratio at 230 °C for 6 h. It was confirmed that nitrogen, sulfur, phosphorous, and potassium were doped onto CP-based CDs (CD-CP) successfully without additional reagents or treatments. The CD-CP yield was 4.0-24.3 times higher than that of model compound-based CDs. Regarding morphology, CD-CP was constituted by many spherical nanoparticles smaller than 20 nm. These CDs emitted blue fluorescence under ultraviolet light, and the fluorescence quantum yield of CD-CP was 4.7-9.4 times higher than that of CP model compound-based CDs. Last, CD-CP displayed broad application prospects as a sensor for Fe³⁺ detection in wastewater with high sensitivity.

A Review on Carbon Dots: Synthesis, Characterization and Its Application in Optical Sensor for Environmental Monitoring

The development of carbon dots (CDs), either using green or chemical precursors, has inevitably led to their wide range application, from bioimaging to optoelectronic devices. The reported precursors and properties of these CDs have opened new opportunities for the future development of high-quality CDs and applications. Green precursors were classified into fruits, vegetables, flowers, leaves, seeds, stem, crop residues, fungi/bacteria species, and waste products, while the chemical precursors were classified into acid reagents and non-acid reagents. This paper quickly reviews ten years of the synthesis of CDs using green and chemical precursors. The application of CDs as sensing materials in optical sensor techniques for environmental monitoring, including the detection of heavy metal ions, phenol, pesticides, and nitroaromatic explosives, was also discussed in this review. This profound review will offer knowledge for the upcoming community of researchers interested in synthesizing high-quality CDs for various applications.

Heavy metal ion detection using green precursor derived carbon dots

The discovery of carbon dots (CDs) for environmental remediation has gained awareness because of the diverse economically viable and environmental friendly green precursors generated from biowastes and biomass compared to the toxic inorganic quantum dots and CDs prepared from chemical precursors. This review presents the recent progress in green CDs, including their synthesis methods and sensing applications for the detection of heavy metal ions such as Iron (III), Mercury (II), Copper (II), Chromium (VI), Lead (II), Arsenic (III), Cobalt (II), Aluminum (III), Silver (I), and Gold (III) which are prominent environmental pollutants. The comparison based on selectivity, sensitivity, quantum yield, detection limit, linear concentration range, and sensing mechanisms are also reported. This review also covers the performance of doped green CDs using heteroatoms, toward the detection of heavy metal ions. Apart from the future perspectives, this review provides a general guide to use such environmental friendly CDs to detect harmful pollutants.

Self-doping synthesis of iodine–carbon quantum dots for sensitive detection of Fe(iii) and cellular imaging

Iodine-doped carbon quantum dots (I-CQDs) were synthesized via p-iodobenzoic acid self-doping for the detection of ferric ions (Fe3+) and cell imaging.

pH sensing and bioimaging using green synthesized carbon dots from black fungus

Biomass is regarded as an excellent candidate for the preparation of carbon nanomaterials. A pH sensor was established based on carbon dots synthesized from black fungus, and possesses good fluorescence response and reversibility for pH detection. Meanwhile, the CDs can also be applied to intra-cellular bioimaging, showing potential for bioimaging.

Carbon dots derived from black fungus were prepared and applied as a pH sensor for real water samples.

Hydrothermal synthesis of carbon nanodots from bovine gelatin and PHM3 microalgae strain for anticancer and bioimaging applications

Semi-conductor quantum dots (QDs) are favorite candidates for many applications especially for potential use as optical bioimaging agents. But the major issue of QDs is toxicity. In the present study, carbon nanodots were synthesized using a green hydrothermal approach from gelatin protein using a previously established protocol. However, the PL properties and applications of the as-synthesized CG (bovine gelatin) nanodots were remarkably different from those of previously reported gelatin carbon dots. CG (bovine gelatin) nanodots had sizes greater than the Bohr exciton radius but still had QD like fluorescence characteristics. Furthermore, the results from fluorescence spectroscopy demonstrated a tunable PL emission profile at various excitation wavelengths. Second, carbon nanodots were also synthesized from algal biomass of Pectinodesmus sp. via a green hydrothermal approach, denoted as CA (PHM3 algae) nanodots. A study of the PL properties and surface chemical composition of CG (bovine gelatin) and CA (PHM3 algae) nanodots suggested that the surface chemical composition significantly alters the surface states which directly influence their PL properties. CG (bovine gelatin) nanodots were used for imaging of plant and bacterial cells with good imaging sensitivity comparable to toxic semiconductor quantum dots. Moreover, the results from in vitro studies suggested good anticancer properties of CA (PHM3 algae) and CG (bovine gelatin) nanodots with minimum GI50 values of 0.316 ± 0.447 ng ml^-1 (n = 2) and 8.156 ± 6.596 ng ml^-1 (n = 2) for HCC 1954 (breast cancer) and 0.542 ± 0.715 ng ml^-1 (n = 2) and 23.860 ± 14.524 ng ml^-1 (n = 2) for HCT 116 (colorectal cancer) cell lines, respectively.

Microalgae biochar-derived carbon dots and their application in heavy metal sensing in aqueous systems

This research seeks a coupled solution for managing the large amounts of biochar produced by microalgae biofuel production, and the necessity for novel, economic and accurate heavy metal sensing methods. Therefore, this study evaluated the transformation of microalgae biochar (MAB) into carbon dots (Cdots) and their subsequent application as heavy metal ion sensors in aqueous systems. The experimental phase included the transformation of MAB into microalgae biochar-derived carbon dots (MAB-Cdots), MAB-Cdot characterisation and the evaluation of the MAB-Cdots as transducers for the detection of four heavy metal ions (Pb²⁺, Cu²⁺, Cd²⁺, and Ni²⁺). MAB-Cdot fluorescence was stable over a wide range of pH and resistant to photo-bleaching, making them suitable as fluorescence probes. The MAB-Cdot fluorescence was quenched by all of the metal ions and displayed different quenching levels. Depending upon the ions involved, MAB-Cdots were used to detect the presence of heavy metal ions from concentrations of 0.012 μM up to 2 mM by measuring the reduction in fluorescence intensity. Neutral and slightly alkaline pHs were optimal for Cu²⁺ Ni²⁺ and Pb²⁺ heavy metal quenching. To quantify the concentration of the heavy metal ions, linear and logarithmic functions were used to model the MAB-Cdot fluorescence quenching. The sensing mechanism was determined to be reversible and purely collisional with some fluorophores less accessible than the others. This work demonstrated the ability to produce Cdots from microalgae biochar, examined their application as a transducer for detecting heavy metal ions in aqueous systems and paves the way for novel sensing systems using MAB-Cdots.

Marine and Freshwater Feedstocks as a Precursor for Nitrogen-Containing Carbons: A Review

Marine-derived as well as freshwater feedstock offers important benefits, such as abundance, morphological and structural variety, and the presence of multiple elements, including nitrogen and carbon. Therefore, these renewal resources may be useful for obtaining N- and C-containing materials that can be manufactured by various methods, such as pyrolysis and hydrothermal processes supported by means of chemical and physical activators. However, every synthesis concept relies on an efficient transfer of nitrogen and carbon from marine/freshwater feedstock to the final product. This paper reviews the advantages of marine feedstock over synthetic and natural but non-marine resources as precursors for the manufacturing of N-doped activated carbons. The manufacturing procedure influences some crucial properties of nitrogen-doped carbon materials, such as pore structure and the chemical composition of the surface. An extensive review is given on the relationship between carbon materials manufacturing from marine feedstock and the elemental content of nitrogen, together with a description of the chemical bonding of nitrogen atoms at the surface. N-doped carbons may serve as effective adsorbents for the removal of pollutants from the gas or liquid phase. Non-recognized areas of adsorption-based applications for nitrogen-doped carbons are presented, too. The paper proves that nitrogen-doped carbon materials belong to most of the prospective electrode materials for electrochemical energy conversion and storage technologies such as fuel cells, air⁻metal batteries, and supercapacitors, as well as for bioimaging. The reviewed material belongs to the widely understood field of marine biotechnology in relation to marine natural products.