Plant-based carbon dots are a sustainable alternative to conventional nanomaterials for biomedical and sensing applications

Carbon dots are small carbon-based particles with unique properties that make them useful in various applications. Some advantages include low toxicity, bio-compatibility, excellent photo luminescence, high stability, and ease of synthesis. These features make them promising for biomedical imaging, drug delivery, and optoelectronic devices. Carbon dots derived from plants have several advantages, including their low toxicity, biocompatibility, and renewable sources. They also have excellent water solubility and high stability and can be easily synthesized using simple and low-cost methods. These properties make them promising candidates for various biomedicine, sensing, and imaging applications. Plant-based carbon dots have shown great potential in metal sensing and bio-imaging applications. They can act as efficient sensors for detecting heavy metals due to their strong chelation and fluorescence properties. This article showcases plant-based carbon dots, emphasizing their low toxicity, biocompatibility, renewability, and potential in metal sensing and bio-imaging. It aims to illustrate their versatile applications and ongoing research for broader use. The current investigation explores their full potential and develops new synthesis and application methods.


Introduction
In recent years, nanotechnology has witnessed a revolution with the rise of plant-based carbon dots (CDs)these tiny nanoparticles [1][2][3].Composed of carbon and boasting functional surface groups, it has a diameter of fewer than 10 nanometers.What sets them apart is their minuscule size and their origin from abundant and renewable plant sources [4,5].This shift towards sustainability is pivotal, given the environmental challenges posed by conventional nanomaterials relying on non-renewable resources like fossil fuels [6,7].Plant-based CDs present a promising solution, offering.
Unique optical and chemical properties that can reshape the landscape of nanotechnology [8,9].The journey of plant-based CDs begins with their facile synthesis from various plant sources, such as tulsi, coriander, aloe vera, turmeric, and more [10,11].These plants serve as natural precursors rich in phytocompounds, allowing for the creation of non-toxic CDs with inherent therapeutic properties [12,13].The synthesis is both cost-effective and environmentally friendly, aligning with sustainability principles and reducing the carbon footprint associated with traditional nanomaterial production [14,15].
The versatility of plant-based CDs results from their surface functional groups, which are highly adaptable through different synthetic techniques [16,17].This adaptability opens up a world of opportunities for diverse applications in nanotechnology, ranging from bio imaging and sensing to drug delivery [16,17].These CDs demonstrate high emitting light efficiency and are readily compatible with biological systems, enhancing their appeal in various biomedical applications [18,19].
One of the standout qualities of plant-based CDs is their potential to replace toxic fluorescent materials in pesticide analysis, significantly contributing to environmental safety [20].By utilizing natural biomass as a precursor, researchers are championing a more sustainable and eco-friendly synthesis approach [21].Carbon sources like lemon juice, pear juice, gooseberry, and banana juice underscores this shift towards sustainable practices, promoting a greener future [22].
In the realm of sensors, plant-based CDs have showcased their superiority over conventional materials [23].Their small size, large surface area, and tunable surface chemistry make them highly sensitive to various analytes [24].They are biocompatible and exhibit exceptional photoluminescence properties, enabling visual and fluorometric detection with enhanced sensitivity and versatility [25].Additionally, their stability and resistance to photobleaching render them long-lasting sensors, providing a reliable and sustainable solution for sensing technologies [26].
The novelty of plant-based materials to create CDs introduces a new era of eco-friendly nanotechnology [27].By relying on nature's abundance, we can move away from harmful chemicals and reduce the strain on non-renewable resources [28].These plant-based CDs offer a glimpse into a promising future where sustainable technology harmonizes with our environment [29].
The potential applications of plant-based CDs are vast and exciting [30].They could play pivotal roles in sustainable energy solutions, advanced healthcare, and numerous other fields [31].Imagine a world where plant-based CDs revolutionize eco-friendly technology, ensuring a brighter and more sustainable future for future generations [32,33].The era of plant-based carbon dots has just begun, and the possibilities are boundless [34,35].

Plant-based carbon dots
Using plant-based carbon dots as highly effective fluorescent probes for metal detection has gained significant attention.These carbon dots offer exceptional sensitivity, selectivity, and stability, preventing unwanted precipitation during detection.Researchers have successfully employed them to identify metals like iron, lead, mercury, chromium, and arsenic in real-life samples such as water and soil.This innovative application has provided practical solutions to pressing environmental and societal issues.The environmentally benign properties of plant-based carbon dots further enhance their versatility, especially in biological sensing.A diverse range of plant sources has been harnessed in creating carbon dots tailored for metal detection.For instance, Yearwise reported Plant based carbon dots (figure 1) For instance, scientists have utilized carbon dots derived from T.
Angustata Bory infused with sulfur and nitrogen atoms, T cordifolia leaves, S japonica leaves, and O sanctum leaves, showcasing the potential of plant-based carbon dots in metal detection.Additionally, researchers have explored various biomass sources like honey, watermelon peel, sugarcane bagasse, taro peels, garlic peels, the flower of winter sweet, and B. flabellifer to synthesize carbon dots designed for detecting metals.These plantbased carbon dots have consistently exhibited high selectivity and sensitivity in detecting a wide array of metals.Their application underscores the viability of sustainable and environmentally friendly materials in cutting-edge technologies and signifies a hopeful path toward tackling challenges associated with metal contamination.The ongoing research and advancement of plant-based carbon dots for metal detection hold immense promise for enhancing environmental monitoring and remediation endeavors.Specified plant parts choosen for synthesising carbon dots are mentioned in figure 2.

Green synthesis of carbon dots
Green synthesis entails environmentally friendly and sustainable carbon dots (CDs) production by utilizing renewable and biodegradable materials, such as plant extracts, as the primary precursors.Unlike conventional synthesis methods that employ toxic and hazardous chemicals, green synthesis methods strictly avoid their use.In synthesizing CDs using green practices, the precursors undergo a reduction in an aqueous solution containing a stabilizing agent.This reduction results in the formation of carbon-rich nanostructures, which are subsequently treated with acid to yield the final CDs.The adoption of green synthesis methods for CDs offers numerous advantages.Firstly, it eliminates harmful chemicals, contributing to a safer and more environmentally conscious approach.Secondly, these methods lead to a reduction in overall energy consumption and a decrease in production costs.
Additionally, CDs produced through green synthesis often exhibit enhanced quality and stability compared to traditional synthesis routes.Moreover, employing plant extracts as precursors in the green synthesis of CDs results in unique properties not typically observed in CDs synthesized using standard methods.Hence, green synthesis of CDs is a highly desirable alternative to conventional synthesis methods, particularly for the largescale production of CDs intended for diverse applications, such as biomedicine, environmental sensing, and energy storage.

Hydrothermal method
The hydrothermal method has emerged as a prevalent approach for synthesizing carbon dots (CDs) from plants within a high-temperature and high-pressure aqueous solution.This method facilitates the production of carbon-rich nanostructures, which are further.Figure 3 explain the major synthesis technique for plant based carbon dots.
It is subjected to acid treatment to eliminate impurities and yield purified CDs.By adjusting various reaction conditions such as temperature, pressure, and precursor concentration, one can precisely control the size and composition of the resulting CDs.CDs synthesized through the hydrothermal method exhibit impressive photoluminescence properties, rendering them highly suitable for various applications including bio-imaging, sensing, and photovoltaics.The hydrothermal method represents a bottom-up approach to the synthesis of various nanomaterials.It involves utilizing water as a reaction medium within a closed system, maintaining the temperature below 200-300 °C and the pressure up to 2000 psi for 2-14 h.This method is extensively utilized to achieve a high quantum yield and desired surface morphology across a spectrum of temperature and pressure conditions.A significant advantage of the hydrothermal method lies in its capability to generate nanomaterials from diverse sources that may not be stable at elevated temperatures without requiring pretreatment steps.This feature underscores the versatility and efficacy of the hydrothermal approach in synthesizing CDs and other nanomaterials for various applications.
From the table 1 Plantwise Hydrothermal conditions and quantum yield are referred below In a study by Yan Mei Liang et al (2022) [36], biomass waste in the form of tobacco leaves was utilized to synthesize carbon dots (CDs) using the hydrothermal method.This approach yielded CDs with an impressive quantum yield of 54%, emphasizing the optimization of the CDs' fluorescence properties.The study also delved into chemometric studies to further enhance the understanding of these synthesized CDs.In another research conducted by Samran Durrani et al (2022) [37], CDs were synthesized using roots of Salvadora persica, incorporating Ca, N,  and S dopants.The objective was to enable fast universal cell imaging in real-time samples such as fish tissues and Industrial water, particularly for fluorescent detection in toxic dyes.The CDs achieved a quantum yield of 19%, showcasing their potential in efficient fluorescence applications.In a study by Hui Wang et al (2022) [38], the potential of carbon dots derived from Scutellaria baicalensis in inhibiting UV-B stress in lettuce was proposed.Thesecarbon dots were found to promote an antioxidant system in vivo and exhibit scavenging activity in in vitro studies.Specific procedures based on precursor materials are imperative for synthesizing carbon dots.For instance, Sharmistha et al (2022) [39] combined 1.0 g of Typha angustata Bory powder with 0.5 g of thiourea in 50 ml of deionized water within an autoclave at 200 °C for 10 h, resulting in a high quantum yield of 83%.Anjali et al (2022) [40] carbonized various parts of the Bael petra plant at 150 °C for 2 h, followed by mixing with water and stirring.This mixture was then subjected to the hydrothermal method at 180 °C for 12 h to synthesize CDs.Rinki kumari et al (2022) [41] Green pigments were extracted from Trachelospermum Jasmine plant leaves through a process involving washing, drying, and extraction with DMF.The extracted pigments underwent treatment at 190 °C for 8 h and purification by column chromatography.The researchers obtained R-CDs by extracting pigments using different chloroform/ethyl acetate mixtures, resulting in a quantum yield of 19.80%.For G-CDs, zinc acetate was added, while B-CDs were synthesized in a water medium.The resulting mixture was then filtered and centrifuged, yielding a quantum yield of 19.80%.In another study, Arugula Saravanan et al (2021) [42] synthesized CDs from 5 g of turmeric leaves using a one-step hydrothermal method at 180 °C for 10 h.After cooling, the brown extract obtained was centrifuged, filtered, dialyzed, lyophilized, and reconstituted in water, resulting in yellow-colored CDs.
In a study by Diogo A Sousa et al (2022) [43], the production of Wet Pomace-CDs was optimized by enhancing the interaction between carbon core states and O/N-containing substituents under high hydrothermal carbonization temperatures (250 °C) and an extended duration (4 h).The focus was on minimizing the contribution of molecular fluorophores to photoluminescence.Velusamy Arul et al (2018) [44] heated 30 ml of Actinidia deliciosa fruit extract and 1 ml of aqueous NH3 solution (25%) at 180 °C for 12 h to synthesize N-CDs.The resulting dark brown solution underwent filtration and centrifugation, then stored at 10 °C for future use.Hydrothermal methods were utilized by Haipeng Diao et al (2018) [45] to synthesize B-CDs and G-CDs from Syringa using Lindl as the carbon precursor.The resulting CD solution underwent filtration, centrifugation, pH neutralization, and 48-hour dialysis against ultrapure water using a 1000 MWCO dialysis

Solvothermal method
The solvothermal method employs solvents like acids, alcohols, ammonia, and water as the reaction medium.In this technique, the precursor material is heated to a specific temperature, either in an oil bath, sand bath, oven or by direct heating, to obtain the desired product within the solvent medium.Before initiating the process, it's common to pretreat the material, which may involve neutralization and centrifugation.The solvothermal method utilizes a more efficient solvent medium than hydrothermal methods, enabling the attainment of the desired product through heating alone, without additional conditions such as pressure and time.Nonetheless, some research endeavors integrate the solvothermal method with the hydrothermal approach.Figure 4 is provided the advantages of the solvothermal method.
The solvothermal process represents a promising approach for producing high purity and crystallinity materials.It allows precise control of reaction conditions and facilitates the creation of a broad spectrum of materials that can be readily scaled up for commercial production.Moreover, the solvents employed in this process contribute to functionalization.However, it's essential to consider drawbacks such as prolonged reaction times, safety concerns, and specialized equipment requirements.

Microwave-assist method
In this technique, the precursor material undergoes reduction through an external electric field, and the reaction is carried out within a microwave oven.This process allows for achieving a narrow size distribution of C-dots.Compared to alternative heating methods, this technique swiftly synthesizes C-dots in a relatively short timeframe, typically 30 seconds to 30 min.This method can utilize both natural and chemical precursors for C-dot synthesis.Microwave irradiation facilitates solvent-free water-mediated reactions and controlled heating.Ankita Deb et al (2022) [62].They conducted a carbonization pretreatment of brewery spent grain, wherein they dissolved 5 g of the material in 10 ml of water and exposed it to microwave irradiation at various temperatures and durations, ranging from 200 °C to 300 °C for 10 to 30 min.In figure 5 explain the advantages of Microwave assisted Method.
Following the microwave irradiation, they centrifuged the resulting black solution.Subsequently, they subjected it to 24 h of dialysis and freeze-drying to obtain C-dots with a quantum yield of 14%.Microwaveassisted processes offer several advantages, including accelerated reaction times, high-purity products with enhanced crystallinity production, and scalability for industrial production.Moreover, they contribute to a reduction in the use of hazardous solvents.However, this method has limitations, such as specific equipment requirements, safety concerns arising from high temperatures and pressures, and impurities or adverse reactions due to uneven heating.Additionally, microwave radiation can heat the reaction vessel, increasing pressure and necessitating additional safety precautions.

Pyrolysis method
Pyrolysis is a fundamental method for synthesizing carbon dots (CDs), tiny carbon particles with unique optical and physical properties.This process involves subjecting organic precursors like sugars, polymers, or biomass to high temperatures without oxygen, forming CDs comprising a blend of graphitic and amorphous carbon.CDs created through pyrolysis showcase a high surface area.They can be functionalized with various moieties, rendering them versatile for various applications in biomedicine, sensing, and energy fields.The size and composition of CDs synthesized via pyrolysis can be finely controlled by adjusting the temperature and the type of precursor employed.According to the pyrolysis method the advantages are mentioned in below figure 6 During carbonization, the precursors undergo high temperatures ranging from 400 °C to 1000 °C within a heating muffle furnace.
This process entails swiftly reaching a specific temperature that might take a few hours.This differs from the gradual heating involved in hydrothermal synthesis.For instance, Murugan, N et al (2018) [49] utilized Borassus flabellifer flowers, which were dried under sunlight, crushed into powder, and placed in an oven at 800 °C for 10 h.Subsequently, the powder was transferred to a furnace and heated to 300 °C for 2 h while maintaining various temperatures up to 400 °C, following a heating rate of 50 °C/min.They dissolved 0.5 g of the resulting black powder in 50 ml of water and filtered it to obtain C-dots.

Size and optical properties 4.1. Size of the carbon dots
Carbon dots represent a distinct class of carbon nanoparticles with exceptional optical and physical properties.These properties stem from differences in size and surface area, giving rise to their unique optical and physical attributes.Carbon dots find applicability in diverse fields, serving as imaging probes, photothermal agents, and sensing tools due to their high photoluminescence efficiency and excellent biocompatibility.Furthermore, carbon dots hold promise as anode materials in lithium-ion batteries, making strides in energy conversion and storage technologies.Researchers have demonstrated the synthesis of green carbon dots within a size range of 0.04 to 9.41 nanometers, and this size variation determines their specific application areas.Carbon dots within this range are commonly utilized for real-time metal detection in samples, cellular imaging, free radical scavenging, and in vitro and in vivo research.The compounds detected typically comprise elements from the D and P blocks of the periodic table, encompassing metals like iron, copper, and chromium, post-transition metals like mercury and lead, and non-metals and metalloids.Carbon dots measuring 0.04 nm to less than 2 nm exhibit many applications, including in vitro and in vivo cytotoxicity studies, cell imaging, fluorescence, and photoluminescence.The major properties of carbon dots are mentioned in below figure 7.
Those measuring 2-3 nm are primarily employed for metal sensing and cell imaging, with detected metals often including chromium, lead, iron, and mercury.The real-time applications of these carbon dots are contingent on the specific detected metals.Carbon dots measuring 4-5 nm are exclusively applied in sensing applications.In comparison, those ranging from 5 nm to less than 10 nm have various applications, such as heavy metal ion detection and optical sensor studies.Overall, the size of carbon dots delineates their application field, with smaller carbon dots possessing a broader range of applications, and larger carbon dots are more specialized in their uses.The pyrolysis process offers several advantages, including ease of use and costeffectiveness in producing a diverse array of materials, notably those with high purity, controlled porosity, and specific surface areas.This method can be implemented in varied environmental conditions, resulting in products with distinct properties.However, the process does present limitations, such as a relatively low yield, extended reaction times, and challenges in controlling the final product's morphology and size distribution.Additionally, safety concerns may arise due to the high temperatures involved and the potential use of hazardous materials.

Optical properties
Carbon dots derived from natural sources possess unique optical properties, making them suitable for various applications.By adjusting the synthetic process and source material, researchers can tailor the fluorescence emission spectra of these carbon dots.They exhibit high fluorescence intensity and excellent quantum yields, making them ideal for imaging applications.Carbon dots also demonstrate good photostability, enhancing their suitability for long-term imaging.Furthermore, they have good biocompatibility, ensuring their safety for biological applications.Consequently, carbon dots from natural sources are highly desirable for diverse applications due to their optical properties.The benefits of plant-derived carbon dots over their chemical counterparts are elucidated in the depicted figure 8.
Mejía Avila et al (2022) [48] developed carbon dots (CDs) from avocado seeds, showcasing distinct properties at varying synthesis temperatures (250 °C, 400 °C, and 600 °C).CDs created at 600 °C exhibit remarkable sensitivity as fluorescent turn-off sensors, achieving low detection limits of 0.04 nM for Cr 6+ and 0.09 nM for Cu 2+ ions.These CDs highlight their fluorescence and aptitude for precisely detecting heavy metal ions, particularly Cu 2+ and Cr 6+ .Quan Qi et al (2022) [50] introduced sulfur and nitrogen co-doped carbon dots (SNCDs) derived from carrageenan, showcasing impressive optical characteristics and suitability for sensing applications.Surface modification of SNCDs with active groups further enhances their optical properties.These SNCDs serve as highly efficient "on-off-on" fluorescent sensors, enabling sequential detection of Ag + and Lime Sulfur (LS), underscoring their exceptional optical capabilities.Ashok Varman et al (2022) [52] developed carbon dots (CDs) with a particle size of 11.2 nm, emitting blue light under UV exposure at 365 nm.These CDs exhibit excitation-dependent fluorescence, boasting a notable quantum yield of 7.01%.Their fluorescence quenching is linear within a concentration range of 0.6-3.3mM for various metal ions, as evidenced by a high correlation factor (R2) of 0.9977.These optical properties render the CDs ideal for selectively detecting Fe 3+ ions amidst a mix of metal ions and catalyzing the reduction of harmful textile dyes Azure B (AB) and Toluidine Blue (TB) with NaBH4 assistance.González-Reyna et al (2023) [51] demonstrated that Cinchona Pubescens Vahl extract-derived carbon quantum dots (CQDs) display exceptional fluorescence, ideal for imaging and visualization.These CQDs feature semispherical shapes and measure approximately 5 nm, contributing to their distinctive optical attributes.Moreover, they exhibit a noteworthy drug loading capacity and a controlled drug release profile, suggesting their promise as precise and controlled cancer nanocarriers for targeted therapies.CQDs have unique optical characteristics, making them highly adaptable in various applications.These small carbon-based nanoparticles are extremely sensitive fluorescence turn-off sensors, making them excellent for precisely identifying and measuring specific compounds.Their excitation-dependent fluorescence, causing varying light intensities based on the excitation wavelength, sets them apart from other entities.This quality makes them highly effective in emitting fluorescent light, coupled with a high quantum yield.They are suitable for detecting changes in fluorescence intensity in response to environmental conditions or analyte concentration, enabling accurate selective detection and catalysis.The size and the optical properties of synthesised carbon dots are given in table 2.

Biocompatibility
Carbon dots from natural sources possess unique optical properties, rendering them suitable for various applications.By adjusting the synthetic process and source material, one can tailor the fluorescence emission spectra of these carbon dots.They exhibit high fluorescence intensity with excellent quantum yields, making them ideal for imaging applications.Carbon dots demonstrate good photostability, enhancing their suitability for long-term imaging applications.Furthermore, they possess good biocompatibility, ensuring safety for biological applications.Therefore, carbon dots derived from natural sources are highly desirable for diverse applications due to their optical properties.
Mejía Avila et al (2022) [48] developed carbon dots (CDs) from avocado seeds, showcasing distinct properties using varying synthesis temperatures (250 °C, 400 °C, and 600 °C).CDs created at 600 °C exhibit remarkable sensitivity as fluorescent turn-off sensors, achieving low detection limits of 0.04 nM for Cr 6+ and 0.09 nM for Cu 2+ ions.This study primarily emphasizes the fluorescence and aptitude of these CDs for precise detection of heavy metal ions, particularly Cu 2+ and Cr 6+ .Quan Qi et al (2022) [50] introduced sulfur and nitrogen co-doped carbon dots (SNCDs) derived from carrageenan, highlighting impressive optical characteristics and suitability for sensing applications.The authors modified the surface of SNCDs with active groups, further enhancing their optical properties.These SNCDs serve as highly efficient "on-off-on" fluorescent sensors, enabling sequential detection of Ag + and Lime Sulfur (LS), underscoring their exceptional optical capabilities.Ashok Varman et al (2022) [52] synthesized carbon dots (CDs) with a particle size of 11.2 nm, emitting blue light under UV exposure at 365 nm.These CDs exhibit excitation-dependent fluorescence, boasting a notable quantum yield of 7.01%.Their fluorescence quenching is linear within a concentration range of 0.6-3.3mM for various metal ions, as evidenced by a high correlation factor (R2) of 0.9977.These optical properties render the CDs ideal for selectively detecting Fe 3+ ions amidst a mix of metal ions and catalyzing the reduction of harmful textile dyes Azure B (AB) and Toluidine Blue (TB) with NaBH4 assistance.González-Reyna et al (2023) [51] demonstrated that Cinchona Pubescens Vahl extract-derived carbon quantum dots (CQDs) display exceptional fluorescence, ideal for imaging and visualization.These CQDs feature semispherical shapes measuring approximately 5 nm, contributing to their distinctive optical attributes.Moreover, they exhibit a noteworthy drug loading capacity and a controlled drug release profile, suggesting their promise as precise and controlled cancer nanocarriers for targeted therapies.The authors outlined a method for obtaining and characterizing CQDs, involving extraction from Cinchona Pubescens Vahl extract and subsequent analysis of their optical and drug-carrying properties.
CQDs have a unique combination of optical characteristics, making them highly adaptable in various applications.These small carbon-based nanoparticles are extremely sensitive fluorescence turn-off sensors, making them excellent for precisely identifying and measuring specific compounds.Their excitation-dependent fluorescence, causing varying light intensities based on the excitation wavelength, sets them apart from other entities.This quality makes them highly effective in emitting fluorescent light, coupled with a high quantum yield.They are suitable for detecting changes in fluorescence intensity in response to environmental conditions or analyte concentration, enabling accurate selective detection and catalysis.

Metal sensing
Plant-based carbon dots offer superior performance as fluorescent sensing probes compared to other chemical sensors.They can be produced on a large scale for Sensing purposes in an environmentally friendly manner, making them suitable for various applications, such as biological sensing.The nano-sized carbon dot material, with its fluorescent and photostable properties, is ideal as a fluorescent probe for metal detection, resulting in precise metal sensing due to its sensitivity and selective nature.The fluorescent emission range of these carbon dots is between 10 nm and 20 nm, and the limit of detection (LOD) ranges from 10 nm to 200 nm.The detected metals, including iron, lead, mercury, chromium, and arsenic, are present in samples such as water and soil, providing solutions for social problems.Metal detection technology is also used in optical sensor studies and dye detection.Plant-based carbon dots have exceptional stability and lack precipitation in the metal and carbon dot systems.Their pH stability and reversibility of the detected metal are noteworthy for metal sensing.Yan Mei Liang et al (2022) [36] utilized biomass waste tobacco leaves for Fluorescence intensity of CDs (WTL) to detect tetracycline (TC) presence via the Stern-Volmer equation, and a quantitative fluorescence model (QFM) enhanced TC quantification accuracy, validated against HPLC for real sample analysis.Samran Durrani et al (2022) [37] CDs synthesized from Salvadora persica roots were used to quantitatively assay Congo red (CR) in fish tissues and wastewater.Mis-mPD-CDs showed high sensitivity and selectivity by detecting CR through fluorescence quenching, enabling successful probing of CR in living cells and zebrafish.Sharmistha Samota et al (2022) [39] used Typha angustata Bory doped with sulfur and nitrogen atoms based on carbon dots (N, S-CDs) as a fluorescence probe in their study.They showed fluorescence quenching properties towards mercury ions.They observed low detection limits between 3.1 and 8 nm, a more than 5% decrease in the fluorescence signal, and good reversibility up to 10 cycles.(N, S-CDs) recovery ranged from 95% to 102% and 97% to 104% in the actual water samples.Rinki Kumari et al (2022) [41] Red emissive carbon dots (CDs) derived from Star Jasmine leaves demonstrated the capability to visually and fluorometrically detect water and chloroform across a wide range.Their effectiveness in detecting these substances through visual observation and fluorescence measurements highlights their potential for versatile sensing applications.Debadatta Mohapatra et al (2022) carbon dots derived from Tinospora cordifolia leaves were effective sensing probes for detecting Fe 3+ .The researchers utilized fluorescence titration to estimate the concentration of metal ions by adding 2 ml of TCLCDS and 2 ml of aqueous Fe 3+ to attain concentrations ranging from 20 to 1000 m.The system absorbed a quantity of 0.704 mg/ml metal ions.
Jingzhou Hou et al (2020) used carbon dots derived from Sophora japonica leaves to detect Fe 3+ .The electron transfer system of CD/Fe 3+ was formed through dynamic quenching, resulting in fluorescence recovery on glyphosate detection.This system was applied for real-time potato samples, favoring glyphosate detection in food analysis and environmental safety.Lan Xia et al (2022) conducted a study on the detection of Cr(VI) and Fe 3+ using carbon dots derived from the biomass of the flower of wintersweet (FWCD).The researchers achieved parallel visual detection of metals through the dual functional integration and dual masking method.The latent mutual the interference nature of the metals resulted in high specificity.The FWSD responded to parallel and quasi-detection of water samples, industrial effluent, and iron addition with high accuracy and sufficient recovery.Murugan et al (2018) [49] investigated using carbon dots derived from the flowers of Borassus flabellifer as a highly selective and sensitive fluorescent probe for detecting Fe 3+ ions.They applied this probe for real-time sensing of Fe 3+ ions in water samples, achieving a low detection limit (LoD) of 10 nm.Bipin Rooj et al (2018) fabricated carbon dots from Polianthes tuberose L. petals and found that the resulting carbon dots could sense both Fe 2+ and Cu 2+ ions.They applied this sensing ability to real-time applications in drinking water, meeting the limits set by the WHO.The carbon dots had a LOD of 200 nm for copper ions, but the reactivity of EDTA towards the CD-Fe 2+ system was not responsible for reversibility.Lu-Shuang Li et al (2018) [46] used Hongcaitai vegetables to prepare carbon dots in both ethanol-soluble (CDs-A) and ethanol-insoluble (CDs-B) forms.CDs-B was more active in Hg 2+ detection, allowing real-time sample applications in river water.CDs-A were more sensitive towards Hg 2+ detection in the presence of ClO-ions, with an LOD of 0.015 m, while CDs-B had an LOD of 0.06 m.Rajkumar Bandi et al (2018) fabricated carbon dots from Lantana camara berries, which showed high selectivity and fluorescence sensitivity for detecting Pb 2+ ions.They successfully caught lead in real-time water and human sera samples, with an LOD of 9.64 nm at 0-200 nM lead concentrations.Figure 9 outlines the benefits of utilizing plant-derived carbon dots in sensor applications.

Bio-imaging
Synthesized carbon dots (CDs) from various natural sources, including Ginkgo fruits, algal blooms, peanut shells, and citrus fruit peels, were investigated for their potential in cell imaging.Krishna Kanthi Gudimella et al (2021) synthesized carbon dots from citrus fruit peels and explored their potential in cell imaging.They utilized the MTT assay method to detect MCF-7 breast cancer cells using CDs and folic acid-induced carbon dots (FA-CDs) under multiple excitations, including bright light, UV light (330-385 nm), blue light (450-480 nm), and green light (510-550 nm).The results demonstrated the effectiveness of both CDs and FA-CDs in imaging the MCF-7 cells, indicating their potential as fluorescent imaging probes for biological applications.This study underscores the importance of exploring new sources for carbon dot synthesis and their potential applications in biomedicine.
Zahra Fatahi et al (2019) synthesized carbon dots from orange juice and evaluated their biocompatibility and cytotoxicity for biomedical applications.They utilized MTT colorimetric assay and LDH assay to determine the cytotoxicity of CDs on SKBR3 and NIH3T3, and the results were analyzed to ascertain the extent of cytotoxicity.The study aimed to determine the safety and effectiveness of CDs for biomedical applications by assessing their biocompatibility and cytotoxicity in cell lines.Debadatta Mohapatra et al (2022) developed green fluorescent carbon dots Nanomaterials have multiple surface functions, are biocompatible, and exhibit excitation-dependent emission properties.They were tested on melanoma and cervical cancer cells and showed promise as imaging agents, with the imaging properties varying based on the dosage.The TCLCDs can potentially be used for cancer cell imaging, free radical scavenging, and sensing Fe 3+ ions in pharmaceutical and biomedical fields.The fabrication method is simple and one-step, making it cost-effective and efficient.The TCLCDs also demonstrated excellent stability and biocompatibility, highlighting their potential for various applications.Figure 10 delineates the benefits of employing carbon dots derived from plant sources in bioimaging applications.
The study offers a promising avenue for further research and development of these nanomaterials.Haipeng Diao et al (2018) [45] used Syringa obtained Lindl derived carbon dots (G-CDs) to image MCF-7 cells and observed that the fluorescence intensity of G-CDs stained cells is pH dependent.The fluorescence intensity increased at pH 8.95 and decreased at pH 4.10.Brightfield images confirmed the biocompatibility and low toxicity of G-CDs with the cells.The results suggest using G-CDs for pH imaging in living cells.Green sourcesderived carbon dots (CDs) have gained attention as a possible candidate for bio-imaging applications because they are synthesized from various biowaste sources such as fruit peels, tea leaves, and plant biomass, making them environmentally benign and sustainable.Green CDs offer several benefits over conventional dyes and fluorescent probes, including minimal cytotoxicity, excellent photostability, and outstanding biocompatibility.They have demonstrated effectiveness in various imaging modalities, including fluorescence microscopy and bio imaging in vivo.They can be easily functionalized to target specific biomolecules, making them a valuable tool for biomedical research and clinical applications.
Linlin Li et al (2022) developed carbon dots (CDs) from kiwi, pear, and avocado fruits.The CDs exhibited a high fluorescent yield and low toxicity in human epithelial cancer cells and zebrafish embryos, making them ecofriendly and biocompatible.Avocado CDs demonstrated in vivo fluorescence bio imaging in the eyes and yolk sac.However, further investigation is required to comprehend the biological mechanisms underlying the differential toxicity of CDs derived from various food sources.Yan-Mei Liang et al (2022) [36] synthesized carbon dots from waste tobacco leaves (WTL) and utilized them as a fluorescent probe to analyze tetracycline quantitatively in actual samples.They found that the QFM technique exhibited superior performance over PLS and TQ methods, as it could overcome errors induced by the peak shift effect, ultimately enhancing fluorescence quantification accuracy.In the same year, Rinki Kumari et al (2022) [41] synthesized optically tuned carbon dots from Star Jasmine leaves that covered the full visible color and emitted at wavelengths of 466 nm, 521 nm, and 625 nm.These green fluorophores have photostable emission properties and are suitable for LED use.They have a CCT of 4283 K and CIE coordinates of (0.36,0.32) for WLEDs.Additionally, these R2-CDs can act as both visible and fluorometric detectors for water and chloroform.The discovery of additional green precursors could expand the range of applications for these highly stable carbon dots, benefiting the field of LEDs.Xiang Li et al (2022) created biocompatible and green photoluminescent Carbon dots (PCDs) with promising applications in bio imaging and reducing UHMWPE particle-induced osteolysis by using sour apples as a source for carbon dots.The PCDs inhibited osteoclastogenesis and bone resorption caused by UHMWPE wear particles and decreased pro-inflammatory cytokine release and ROS stress.They were effective.

The future direction of carbon dots
A promising avenue of substantial advancements and transformative potentials marks plant-based carbon dots (CDs) trajectory in biomedical and sensing applications.Rigorous validation efforts and the translation of CDs into clinical applications are pivotal steps to guarantee their safety and efficacy, thereby facilitating their seamless integration into mainstream medical practices.A collaborative interplay with emerging technologies, notably gene editing and microfluidics, holds immense promise for pioneering therapeutic and diagnostic solutions that could redefine the landscape of modern healthcare.In addition to their burgeoning role in medicine, CDs' unique attributes position them as invaluable tools in addressing pressing environmental challenges through effective environmental monitoring.Their capacity to offer precise and real-time data holds immense potential for monitoring and mitigating environmental issues, aligning with the global endeavor toward sustainable and eco-friendly solutions.Figure 11 outlines the prospective pathways for future developments in the application of carbon dots derived from plant sources.
Furthermore, bio-imaging is on the cusp of significant advancements propelled by enhanced CD optical properties.These properties offer unparalleled precision in disease detection and monitoring, revolutionizing how diseases are diagnosed and treated.CDs' improved optical characteristics will augment bio-imaging quality and enable a deeper understanding of biological processes at a cellular level, fostering breakthroughs in biomedical research.Looking ahead, the exploration of combination therapies and the scaling up of production processes are crucial strategies to expedite the commercialization of CDs.Streamlining production and exploring synergistic therapies will enhance their accessibility and affordability, potentially leading to a revolution in healthcare delivery and environmental sustainability.As CDs continue to evolve and find wider applications, their impact is poised to extend far beyond the laboratory, influencing and reshaping the way we approach healthcare and environmental stewardship in the years to come.

Conclusion
The review underscores the superior properties and versatile applications of plant-based carbon dots compared to their chemical counterparts.These carbon dots are derived from natural sources, primarily herbal plants, and are synthesized using eco-friendly methods with effective precursors.The reviewed literature consistently demonstrates that plant-based carbon dots exhibit a high quantum yield, a crucial factor for their application efficacy.In the realm of synthesis methods, researchers frequently employ the hydrothermal method as a onestep approach to achieve small-sized carbon.Dots, widening their range of potential applications.Notably, these applications include sensing and cell imaging, focusing on real-time applications for eliminating harmful metals from the environment.However, the author observes an absence of reported sensing applications beyond metal sensing.Notably, there needs to be more exploration of biosensors and other sensing avenues.Shifting the spotlight to biomedical applications, the research primarily centers on cell imaging and bio imaging.It's worth noting the repetition in cell use across various studies and the need for more extensive clinical trials to validate these applications.Despite the abundant biological activities exhibited by plants, such as antioxidant, antibacterial, and cytotoxic properties, there's a relative scarcity of reported biological applications for plantbased carbon dots.The review mentions in vitro and in vivo cytotoxicity studies and cell imaging related to plantbased carbon dots.The inherent fluorescent properties of plant-based carbon dots enhance their optical characteristics, paving the way for applications in photocatalytic dye degradation.However, the review emphasizes the necessity for more substantial evidence concerning real-time sample applications in photocatalysis and fluorometric applications.In conclusion, this review accentuates the vast potential of plantbased carbon dots across diverse fields.Researchers are urged to delve deeper, conducting further studies to unlock the full spectrum of their applications and capitalize on their promising properties.

Figure 2 .
Figure 2. Synthesis of carbon dots from various plant parts.

Figure 3 .
Figure 3.The green synthetic routes of carbon dots.
membrane.Lu-Shuang Li et al (2018)[46] synthesized CDs with a quantum yield of 7.60% from Hongcaitai using a hydrothermal method at 240 °C for 20 h.The CDs were filtered, extracted, dialyzed against ultrapure water for 24 h, and dried at 80 °C before dissolved in anhydrous ethanol to produce CDs-A and CDs-B.Xiaohan Sun et al (2018)[47] prepared water-dispersible CDs via hydrothermal treatment of 1 g Lycii Fructus mixed with 30 ml water and 1 ml 25% ammonia solution at 200 °C for 5 h.After centrifugation and dialysis, a brown-yellow CD solution was obtained, followed by Avila et al (2022)[48] synthesized CDs from avocado seeds through carbonization at 250 °C, 400 °C, and 600 °C, resulting in CDs with varying colors.Murugan Nagaraj et al (2022)[49] synthesized CQDs via hydrothermal processing using Borassus flabellifer.The resulting brownish-yellow solution containing CQDs was filtered and stored at 4 °C for further use.Quan Qi et al (2022)[50] synthesized SNCDs by hydrothermal carbonization of a glutathione and carrageenan solution at 180 °C for 18 h.Transparent SNCDs were obtained through filtration, centrifugation, and vacuum freeze-drying.CDs were also prepared similarly using citric acid.Gonzalez-Reyna et al (2023)[51] synthesized CQDs using Cinchona Pubescens Vahl aqueous extract via hydrothermal treatment at 200 °C for 4 h.The resulting solution was dialyzed, vacuumdried, and re-dispersed in distilled water.Ashok Varman et al (2023) [52] prepared CDs from canon ball fruit by heating the pulp at 200 °C for 5 h.Sariga et al (2023)[53] Synthesized p-CQDs via a hydrothermal method using Polyalthia longifolia leaves.The leaves were washed, dried, and sonicated in deionized water, and the resulting solution was filtered and then heated at 150 °C for 6 h.The synthesized p-CQDs were stored in solution at 4 °C for further characterization and applications.Renu et al (2023) [54] synthesized highly fluorescent TCDs using holy Krishna Tulsi leaves through a hydrothermal method.Fresh Tulsi leaves were hydrothermally treated at 180 °C for varying durations (2-6 h), resulting in brown solid TCDs stored at 4 °C for future applications.Korah et al (2023) [55] synthesized carbon dots from Vitex Segundo leaves via hydrothermal carbonization.5 g of leaves were dispersed in 150 ml of water and treated at 200 °C for 7 h, resulting in a brown solution, which was then centrifuged and filtered to obtain blue fluorescent CDs stored at a low temperature.Navneet Chaudhary et al (2020) [56] synthesized NS-CQDs via a one-step hydrothermal method using banana pulp.Banana paste mixed with ethanol was heated at 150 °C for 4 h, resulting in a dark brown product.Water-soluble NS-CQDs were obtained and stored at 4 °C.Yuyan Wan et al (2019) [57] synthesized CDs using Abelmoschus manihot flowers via a one-step hydrothermal method.Flower pieces were mixed with water, sonicated, and heated at 220 °C for 4 h.After cooling.The solution was centrifuged, filtered, and stored at 4 °C for further use.Canan Baslak et al (2023) [58] synthesized CQDs using Sideritis vuralii leaf powder via a hydrothermal method at 140 °C for 4 h.The resulting purified sample was air-dried for 48 h.Tony Elizabeth et al (2023) [59] synthesized N-CQDs by crushing Crescentia cujete fruit pulp, mixing it with water, and heating it in a Teflon-lined autoclave at 200 °C for
10 h.The resulting brown solution was filtered, stored at 5 °C, and diluted for further analysis.Santorini Patra et al (2023)[60] synthesized CQDs using Tagetes patula flower petals through one-step hydrothermal carbonization.Dried and sonicated flower pieces were treated at 220 °C for 5 h, resulting in a yellowish-orange solution, which was then purified and stored at 4 °C for further characterization and experiments.Akanksha G Kolekar et al (2023) [61] synthesized Zn-Cn-CDs by mixing crushed cinnamon with zinc acetate and subjecting the mixture to hydrothermal conditions to get carbon dots.

Figure 5 .
Figure 5. Advantages of the microwave assist method.

Figure 6 .
Figure 6.Advantages of the pyrolysis method.

Figure 7 .
Figure 7.The properties of carbon dots.

Figure 9 .
Figure 9. Advantages of plant-based carbon dots in sensors.

Figure 11 .
Figure 11.Future directions of carbon dots.

Table 1 .
The hydrothermal conditions in synthesis of plant-based carbon dots.

Table 2 .
The size and optical properties of carbon dots.