Biomass Derived Biofluorescent Carbon Dots for Energy Applications: Current Progress and Prospects

Biomass resources are often disposed of inefficiently and it causes environmental degradation. These wastes can be turned into bio‐products using effective conversion techniques. The synthesis of high‐value bio‐products from biomass adheres to the principles of a sustainable circular economy in a variety of industries, including agriculture. Recently, fluorescent carbon dots (C‐dots) derived from biowastes have emerged as a breakthrough in the field, showcasing outstanding fluorescence properties and biocompatibility. The C‐dots exhibit unique quantum confinement properties due to their small size, contributing to their exceptional fluorescence. The significance of their fluorescent properties lies in their versatile applications, particularly in bio‐imaging and energy devices. Their rapid and straight‐forward production using green/chemical precursors has further accelerated their adoption in diverse applications. The use of green precursors for C‐dot not only addresses the biomass disposal issue through a scientific approach, but also establishes a path for a circular economy. This approach not only minimizes biowaste, which also harnesses the potential of fluorescent C‐dots to contribute to sustainable practices in agriculture. This review explores recent developments and challenges in synthesizing high‐quality C‐dots from agro‐residues, shedding light on their crucial role in advancing technologies for a cleaner and more sustainable future.


Introduction
Engineered nanoparticles have evolved over the past decade, emerging as a significant class of innovative biomaterials with characteristics that make them attractive for commercial development. [1]Carbon Quantum Dots (C-dots), a class of nanoparticles (< 10 nm)composed of carbon, hydrogen, and oxygen, represent zero-dimensional fluorescent carbon nanomaterials with discrete or quasi-spherical structures.Recently, these particles have garnered attention in the fields of biotechnology, biomedicine energy, and environment. [2]Biomass, defined as biodegradable and non-fossilized organic material, offers a substantial opportunity to meet the growing energy demand. [3]In India, annual biomass production reaches approximately 500 million tonnes (MT), with 120 to 150 MT considered surplus. [4]Biomass, a significant source has the potential to yield a diverse range of products for energy and environmental applications.Reusing surplus biomass becomes imperative to close the loop from solid waste generation to greenhouse gases (GHGs) emissions.Effective utilization of biomass resources could potentially offset 446 MT of GHG emissions by 2030.In India, issues such as air pollution and emission of hazardous substances, like particulate matter results from the direct burning of surplus biomass (92 metric tonnes/ year) in agricultural lands, emphasizing the need for utilization of biomass. [5]The gross biomass generation is predicted to be 500 MT, with nearly 121 MT of crop residues available as surplus after various applications.This surplus can be effectively used to synthesize either biofuels or biomaterials via suitable conversion process. [6]Biomass-based C-dots are now preferred for applications due to the abundant availability, low-cost raw materials, simple synthesis, and favourable physiochemical, optical, and electronic properties. [7]Conversely, the C-dots derived from chemical precursors are unsuitable for biological applications due to their high toxicity.Bio-conjugated C-dots derived from biomass find significant applications in in vitro and in vivo imaging, drug delivery, and bio-sensing, despite their utility in electro-optic devices within the biological domain. [8]he primary benefit lies in the coating of shell material on the core's surface, improving colloidal stability and inhibiting photo-degradation. [9]Colloidal core-shell C-dots, with emission colours tuneable from ultraviolet to near-infrared, hold promise for applications such as light-emitting diodes (LEDs). [10]Thin film LEDs incorporating C-dots in charge transport layers provide prospects for low cost device production of thin film solar cells through solution-based methods like spin or dip-coating.Due to their excellent and controllable photoluminescence (PL), high quantum yield (QY), low toxicity, small size, notable biocompatibility, and abundant low-cost sources, C-dots, a rising star in the carbon family, have drawn significant attention for vast applications biosensors, optoelectronic systems, solar cells and more. [11]owever, the development of C-dots is still in its earlier stages compared to quantum dots (QDs) and other carbon compounds.A fundamental challenge in this field is the absence of a systematic and scalable synthesis methodology to generate high-quality C-dots with desirable architecture, including size, shape, crystallinity, and number of functional groups, type, and the location of defects.Unconventional synthetic pathways and impurities pose challenges in deducing precise reaction mechanism, nucleation mechanism, and formation processes.Systematic exploration of the effect of precursors and reaction conditions (temperature, time, and pH) on Cdots' performance is necessary.Developing an approach to purify C-dots based on size or polarity is crucial for large-scale production with high performance via an efficient route. [12]Cdots, generally photoluminescent, find a variety of uses despite their stability in the presence of light and potential harmful effects (Figure 1).Light-irradiated, laboratory-made C-dots break down into harmful chemicals affecting human cells due to the production of free radicals and oxygenated species. [13]To address toxicity concerns, synthesis of C-dots through green route, utilizing biomass as a green source is proposed.The article aspires to discuss various methods for synthesizing green C-dots from different carbon precursors derived from biomass, exploring their fluorescent properties and applications in the energy sector.

Synthesis of C-dots
Due to its distinct photoluminescence properties, chemical stability, nearly non-existent toxicity, ease of synthesis, and environmental friendliness, this novel carbon material has motivated the scientists to identify a cost-effective method for the synthesis of C-dots. [14]C-dots derived from green precursors (i.e., biomass feedstocks), eliminate the negative aspects associated with metal-based quantum dots, such as toxicity, photo bleaching, or photo blinking, and high manufacturing costs. [15]These green precursors were classified based on sources into crop residues, fruit wastes, vegetable wastes, flower waste, leaves, waste seeds, fungi/bacteria species, and other organic wastes.The carbon material can be synthesized from organic or inorganic carbon precursors through either a top-down or bottom-up approach (Scheme 1).The top-down technique involving chemical oxidation, arc discharge, laser ablation, electrochemical oxidation, and ultrasonic procedures to disassemble large carbon structures, became a less attractive method. [16]This may be due to expensive resource materials and reactants, higher processing costs, and time-consuming with fewer safety levels (Table 1). [17]n contrast, the bottom-up approach utilizes hydrothermal, ultrasonic, thermal decomposition, pyrolysis, carbonization, microwave synthesis, and solvothermal methods to produce Cdots from smaller carbon precursors. [18]Among these methods, the bottom-up approach is considered the best for the green synthesis of C-dots due to excellent process control, simplicity, cost-effectiveness, use of non-toxic precursor molecules and suitability for green materials. [19,20]iomass, encompassing all living things produced via photosynthesis, including animals, plants and bacteria, plays a significant role in the total energy system.Biomass is a driving force because of its good environmental qualities and abundant renewable resource; making it a potential substitute for fossil Figure 1.Schematic diagram of types, properties, and application of different quantum dots. [13]cheme 1. Methods for the synthesis of C-dots. [16]

P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D
fuels in various applications. [21]The production of C-dots from carbon precursors generally involves three steps.Firstly, C-dots preparation involves carbonaceous accumulation during carbonization, which can be prevented using electro-catalytic synthesis, restricted pyrolysis, or solution chemistry techniques.Secondly, size control and uniformity are crucial for consistent properties and mechanistic study, and this can be optimized via post-treatment, such as gel electrophoresis, centrifugation, and dialysis.Thirdly, surface properties, crucial for solubility and specific applications, can be tuned during preparation. [17]he selection criteria for effectively synthesizing nanomaterials are based on the type of reactor, process cost, efficiency, bulk quantity and environmental safety. [22]Various methods for synthesizing C-dots using biomass are further discussed.

Hydrothermal Carbonization (HTC)
In this method, green precursors were mixed with water, placed in an airtight reactor, and subjected to a polymerization or exfoliation process, allowing reactants to produce C-dots at higher pressures and temperatures.Throughout the process, thermochemical decomposition of biomass occurs in the presence of aqueous media (water).In HTC, water is the medium with water to biomass mixing ratio ranging from 5 : 1 to 75 : 1. [23] HTC can be conducted at either very high temperature (400-800 °C) [24] or high temperature (200-300 °C) [25] for the synthesis of C-dots from biomass (Table 2).Scheme 2 illustrates the step-by-step process involved in hydrothermal carbonization-based C-dots production using biomaterials as feedstocks.[28] Furthermore, the HTC method offers several advantages over other synthesis methods, including green precursors as feedstocks, quick process, mild reaction conditions, non-toxicity, non-pollution, lower reaction temperature, cost-efficiency, and suitability for large-scale synthesis. [29]Different hydrothermal treatment parameters are required for green precursors, such as reaction temperature, duration, and reaction. [30]For instance, C-dots produced through one-pot HTC of Rottboellia cochinchinensis and Leucas aspera leaves reduced the cytotoxicity of cancer cells, enhancing anticancer efficacy and antioxidant properties. [31]

Microwave Method
Microwaves are electromagnetic waves with frequencies ranging between 0.3 and 300 GHz and wavelengths between 1 m and Table 1.Comparison of top-down approach and bottom-up approach. [17]8]

P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D
been synthesized under microwave digestion, yielding respective C-dots of approximately 19.4 %, [71] 54 %, [72] 14 %, [73] 13 %, [74] and 17.1 % (Table 3). [75]The microwave power used for the C-dot synthesis ranged from 250 to 700 W, with a reaction time spanning5 to 45 min.For example, C-dots produced through microwave-assisted synthesis, using water as solvent and palm kernel shell as raw material exhibited sharp absorption spectra between 240-250 nm and broad absorption spectra between 300-400 nm, with a 44 % quantum yield at an excitation wavelength of 350 nm. [76]

Thermal Decomposition
One of the most popular methods for producing fluorescent C-dots involves thermal breakdown as depicted in Scheme 4.Although the reaction route is somewhat complicated, a better understanding of the specific details is still required.For example, several intermediates are generated during synthesis, resulting in the formation of a fluorescent species. [81]Aconitic acid (AA), also known as Trans (cis)propene-1, 2, 3-tricarboxylic acid, is among the intermediates produced during thermal decomposition.It undergoes dehydration and decarboxylation to yield methyl-maleic acid when solvents like dimethylformamide, acetic acid are used. [82]The use of catalysts such as Al 2 O 3 , TiO 2 , etc., in limited proportions with biomass has been effectively explored to enhance quantum yields. [83]Low-temperature carbonization of watermelon peel has been shown to produce C-Dots (< 2.0 nm), with bright blue luminescence, tolerable fluorescence lifetime, and high stability.These particles exhibit promise as cellular optical imaging probes with excellent performance. [84]Magneto fluorescent C-dots, with good potential as a T1 contrast agent in magnetic resonance imaging were synthesized from waste crab shell chitin and doped with Gd 3 + , Mn 2 + , and Eu 3 + .They could also be utilized as theranostic and diagnostic probes. [71]

Pyrolysis Carbonization
Pyrolysis is the thermochemical decomposition of organic materials at high temperature (450-650 °C) in an inert environment.The method offers considerable flexibility in modifying operational parameters, such as heating rate, temperature, residence duration, etc., to obtain the desired products. [85]Various unit operations, including carbonization, grinding, sonication and filtration are employed in this process (Scheme 5).The particle size of C-dots produced by pyrolysis using biomass as the carbon source typically ranges between 0.4 and 6 nm, and the quantum yield ranging from 3 to 25 %.However, the reactor type, heating temperature, pressure, presence of catalysts, and reaction time, are the most influential pyrolysis parameters affecting the final product's quality and quantity.Furthermore, these parameters have a direct impact on the composition and C-dots yield. [86]For example, sago waste was converted into highly fluorescing Cdots using a straight-forward thermal pyrolysis process at 400 °C without surface passivation.The pyrolysis heating Banana peel [77] 700 W, 5 min 365 2-6 -Lignocellulose [78] -400-500 2 ~3 -Palm kernel shell [76] 358 W, 1-5 min 370 6-7 44.00 Starch from potatoes [79] 250 W, 2-14 min 350 2-4 30.00 Cow manure [80] 180 W, 20 min -5-10 -Coconut water [72] 800 W, 1 min 390 4 � 2 -temperature was found to influence the conversion of bulk sago waste into carbonaceous residues. [87]Biomass sources like horticultural fruit peels, grass, and peanut skins were exclusively investigated for producing C-dots via pyrolysis, showing a varied quantum yield range of 3 to 30 % (Table 4).Neem leaves and bagasse yielded graphene QDs with a quantum yield (QY) of 1-2 %, while the peak absorption indicated the presence of OH groups. [86]ice husk underwent a series of treatments, including tube furnace treatment at 700 °C for 2 h in a nitrogen gas environment, excess sodium hydroxide reaction at 900 °C for 2 h, and ultrasonication treatment.The sample was then transferred to an autoclave lined with Teflon and heated to 200 °C for 10 h.These C-dots have a fluorescence QY of 8.1 % and a relatively narrow size distribution, spanning 3 to 6 nm. [88]The C-dots exhibited good fluorescence without additional surface treatment.Following treatment with plasma/microwave-assisted technology, the fluorescence intensity of these C-dots was further increased in water and organic solvents.The ideal temperature for pyrolysis was determined to be 350 °C after various plant leaves were placed in a quartz boat and heated in a tube furnace in a nitrogen environment between 250 and 400 °C.][90][91][92] Pyrolytic methods require higher reaction temperatures compared to hydrothermal methods.

Solvothermal Method
In this method, organic solvents are used and water is mostly preferred as the solvent in the reaction.The temperature used for reaction exceeds the solvent boiling point, increasing the pressure in the airtight reactor.The hydrothermal method, in which the reaction takes place in an airtight reactor, is the most popular and widely used for the synthesis of straightforward, doped, and supported C-dots. [59,99,100]The benefits of this route include environmentally friendliness, mild reaction conditions, a one-step synthetic process, and good solution dispersion.The critical process parameters for this approach are type of carbon precursor used, solvent, reaction conditions (temperature, pressure and duration), which may impact Cdots' quality aspects.A wide range of precursors, including papaya, [101] apple juice, [102] water hyacinth, [103] corn stalk, [104] and milk, [105] have been tested and found to be effective in creating C-dots.C-dots made from palm kernel shells, [106] a by-product of the biomass used to make palm oil, are intended for sensing and biosensing applications.

Ultrasonic Process
[111] Wu et al. (2019) utilized an ultrasonic process to create amine-decorated C-dots from graphite rods.The resulting C-dots were effective in sensing nucleic acids in living cells and detect cobalt (II) ions in actual samples. [111]C-dots produced through ultrasonic processing from citric acid, and polyethylene glycol have found applications as lubricants.The modest size of the carbon core and its rolling impact were attributed to the lubricating action. [112]Using miscanthus, [113] a common energy crop, as the carbon precursor pyrolysis at 1200 °C was followed by edge-carboxylation using ball-milling in a CO 2 -induced environment.The resulting functionalized charcoal was exfoliated into C-dots ultrasonically using N-methyl-2-pyrrolidone (NMP) and water.UV-visible and fluorescence (FL) spectroscopy investigations revealed distinct photo-physical behaviours.C-dots with particles of size 4.6 nm were synthesized from lemon juice using 20 kHz of sonication power within a reaction time of 1 h, achieving a quantum yield of28 %. [114] When exposed to UV light, these C-dots exhibit intense blue fluorescence with an outstanding quantum yield ranging from 4 to 27 %.They are also highly water soluble. [115]C-dots synthesized using soybean under 50 °C ultrasonication for 6 to 12 h displayed an average diameter of 2.4 nm (Scheme 7). [116]onochemical preparation is considered a green method compared to many other technologies, offering benefits such as easy and gentle experimental conditions, the use of green energy sources, and the capability to produce C-dots and doped C-dots with controlled physicochemical qualities and lower toxicity. [109]

Energy-Specific Characteristics
C-dots possess numerous key advantages, including easy synthesis, low cost, good renewability, multi-coloured photoluminescence (PL), and outstanding biocompatibility.These advantages have inspired scientists to develop new materials with novel applications and minimal environmental impact.Spectroscopic techniques such as UV-visible and PL spectroscopy, are commonly employed to study the linear optical absorption behaviour of C-dots, which is a critical parameter for energy applications.

Optical Attributes
C-dots were initially identified by their vibrant fluorescence emissions. [117]Conversely, the same tiny carbon nanoparticles within the dot cores exhibit only modest emissions, with reported quantum yields reaching at best up to 1-2 %.This is due to the absence of surface functionalization by organic molecules or any appreciable surface passivation effects.Experimental evidence, [101,118] highlights the crucial role of excellent surface passivation in C-dots is for the intense fluorescence emissions.This can be achieved through dot samples created using carbonization synthesis and purposeful chemical functionalization approaches.In reality, both synthetic methods have yielded several C-dots samples with reported fluorescence quantum yields near unity. [119]The optical absorption spectrum of C-dots typically peaks between 230 and 340 nm, with a tail extending into the visible spectrum. [120]The absorption at 300-340 nm is typically caused by the n-* transition of C=O surface groups, while the UV absorption peak of C-dots is attributed to the n-* transitions of C=C bonds in the carbon core. [121,122]C-Dots usually exhibit a stunning photoluminescence based on excitation, and the fluorescent emission displays excitationdependent behaviour.Recent studies have demonstrated that PL spectroscopy can accurately predict the PL lifespan of C-Dots.Figure 2 depicts the UV-vis absorption and PL spectra at particular wavelengths of C-Dots made from Ocimum sanctum leaves.It is worth noting that the QY of C-Dots can also be assessed using the PL and UV-vis absorption spectra.A detailed calculation of the QY of a C-Dot sample was performed using a method that compared it to the QY of quinine sulphate (standard reference).The QY of C-Dots is determined by a straight-forward ratio of the total PL intensities of the two solutions. [123]heme 7. Ultrasonic synthesis for C-dots production from green precursors. [113]

Photoluminescent Attributes
The photoluminescent (PL), or fluorescent quality of C-dots, has significantly expanded their range of applications, marking a remarkable distinguishing attribute.Surface chemistry, quantum size effect, and the molecular states of the carbon core influence the PL of C-dots.The Stokes PL emission of Cdots exhibits a shorter wavelength than their excitation wavelength.C-dots showcase exceptional up-conversion, tuneable PL emission, excitation wavelength-dependent PL emission, strong fluorescence stability, and effective photobleaching resistance.The diversity in the structural properties of C-dots, essentially categorized into two types, influences their PL emissions.One type is linked to electron transitions corresponding to emissions dominated by internal causes, such as the conjugation effect, surface state, and synergistic impact.This paradigm explains the PL of GC-dots with lattice structures or a high level of graphitization.With an average lattice parameter of 0.24 nm, [124] C dots exhibit a significant degree of crystallinity.In simpler terms, the sizes of conjugated -domains being less than their exciton Bohr radii enable the regulation of their PL emission rather than the size of the individual particles.The properties of CDs from lignocellulosic biomass vary with treatment temperature.For instance, hemicellulose and cellulose hydrolyze as low as180 °C. [125]Lignin depolymerizes above 260 °Cat hydrothermal conditions but the monomers do not cleave until 275-280 °C. [126]According to Quaid et al., (2022), [127] higher temperatures can encourage the development of sp 2 hybridized carbon structures inside C-dots.A reduced band gap as a result of the electrons' delocalization affects the emission wavelength and may increase the fluorescence intensity.There is a range of temperatures that maximize quantum yield.Overheating can cause C-dots to grow or degrade uncontrollably, which lowers fluorescence efficiency. [128]The surface functionalities of C-dots are influenced by temperature and can have an impact on the fluorescence properties of the dots.Increased temperature may cause some functional groups to disappear, which would affect emission behaviour. [129]Because of the more thorough breakdown of precursor materials, more power can result in smaller C-dots.On the other hand, uncontrolled fragmentation resulting from extremely high power may affect the size distribution. [127]The size, surface chemistry, and fluorescence intensity of C-dots can all be affected by reaction time.Longer reaction periods often encourage full carbonization, which results in larger C-dots but may also lead to higher fluorescence intensity because of more fluorescent moieties being formed. [128]On the other hand, prolonged exposure periods may cause over carbonization and decreased fluorescence.The key to creating C-dots with the correct size, shape, and fluorescence properties is figuring out the best power and reaction time ratio.This frequently entails conducting experiments using the precursor materials and synthesis method of choice.
Natural-source C-dots have become increasingly attractive as fluorescent probes because of their low toxicity, biocompatibility, and eco-friendly manufacturing techniques. [2]Due to their exceptional fluorescence qualities and capacity to function as contrast agents, these C-dots are perfect for use in fluorescence microscopy and in vivo imaging, among other biomedical imaging applications [130] Furthermore, because of their sensitivity, selectivity, and quick reaction times, they can be used for biosensing and environmental monitoring, [131] in addition to sensing and detecting a variety of analytes, including biomolecules, heavy metal ions, and environmental contaminants. [132]Additionally, C-dots have demonstrated promise in drug delivery systems.Their biocompatibility guarantees that they are compatible with biological systems, and their functionalization [3] and loading capabilities allow for targeted drug administration and controlled release. [133]he described C-dots emit blue fluorescence when exposed to UV light. [134]Given that biological tissues and cells fluoresce blue, the use of blue-emitting C-dots in biological analysis is evident and may cause spectral interference (Figure 3).The optimal solution to this issue involves generating C-dots with Figure 2. UV absorption spectrum of C-dots derived from selected biomass feedstocks. [121,122]gure 3. Emission wavelength of C-dots. [134]

P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D
extended wavelength fluorescence.Various treatments, including surface modification, [91] size control, [135] and pH control, [136] were applied to achieve this.Experimental results demonstrate that C-dots exhibit brilliant luminosity in the visible after two-photon stimulation with a femto second pulsed laser in the near-IR (800-900 nm). [137,138]The reported two-photon absorption cross sections are among the best of all two-photon fluorescent materials, reaching approximately 40000 GM (1 GM = 1050 cm 4 s/photon) at 800 nm. [137]

Photo-Stimulated Electron Transfer Attributes
The conventional role of C-dots involves serving as both an electron acceptor and donor.In a fascinating study, C-dots functionalized with an ionic liquid were developed for utilization as nano-fluids in energy storage, capitalizing on the excellent electron mobility feature of C-dots at normal temperatures.These organic/inorganic hybrid nano-fluids exhibit potential applications as electrolytes and separators in energy storage systems.Mechanistically, the photoexcitation of the core carbon nanoparticles within C-dots induces effective charge separation, resulting in the formation of electrons and holes trapped at various surface sites of the nanoparticles.The observed fluorescence emissions arise from their radiative recombination. [139]In this framework, photo-induced redox activities may result from the initially created electrons and holes, becoming associated with processes involving electron transfer within the emissive excited states subsequently produced.Direct ultrafast spectroscopic probing for mechanistic investigations of photo-induced charge separation and the associated electrons and holes in C-dots, remains challenging, particularly concerning the participating redox species and activities.However, available results suggest that both the initially formed electrons and holes and those linked to subsequently formed emissive excited states are likely play roles in the observed redox activities (Figure 4).Numerous indirect experimental pieces of evidence consistently support this mechanistic picture. [140,141]C-dots with long-wavelength PL emission linked to the amine sites have also been synthesized. [142]Interaction of metal ions with amine func-tional groups led to a corresponding quenching of C-dots PL emission.The non-radiative transfer of an electron from an amine group to a metal cation, proportional to the metal's reduction potential, was proposed.Ghosh et al. thoroughly documented positron emission tomography (PET) interactions between several aniline derivatives and GQDs. [143]Radical cations were identified using these derivatives as electron donors. [143]With their photo-excited states, the transient species responsible for strong fluorescence emissions, and redox processes applicable in energy conversion systems, C-Dots evidently possess the capability to capture photons across the solar spectrum.

Catalytic Attributes
C-dots have been extensively utilized in the field of photocatalysis to enhance the activity of catalysts owing to their remarkable photocatalytic properties.For instance, C-dots efficiently synthesized from pear juice demonstrated effective degradation of methylene blue under visible-light. [18]The Cdots exhibit excellent light absorption and electron transport properties contributing to the enhancement of photocatalytic activity.In just 130 min, the degradation ratio of methylene blue reached 95 %.Furthermore, C-dots derived from leftover orange peels were employed in creating a C-dots/ZnO composite catalyst. [49]The photocatalytic activity of this composite catalyst surpassed that of the pure ZnO catalyst.This enhancement is attributed to the prevention of electronhole pairs recombination through the transfer of excited electrons from ZnO to C-dots.The interaction between ZnO and water resulted in the production of hydroxyl radicals, while the interaction between oxygen and electrons on C-dots produced superoxide ions.The generated superoxide ions were effective in degrading azo dye.In another instance, a C-dots/ TiO 2 composite catalyst was developed using C-dots derived from lemon peel waste.The developed C-dots/TiO 2 catalyst demonstrated effective degradation of methylene blue dye. [46]hese findings underscore the crucial role of C-dots in the realm of photocatalysis.

Applications
C-dots have been extensively explored for applications in solar and other energy conversion systems, leveraging their superior optical features and photo-induced redox characteristics (Figure 5).This discussion emphasizes their significant role in optoelectronic devices and as photocatalysts.

Photocatalytic Applications
Numerous studies have investigated C-dots as photocatalysts for various processes, with a particular focus on their The solar photocatalytic conversion of CO 2 into small molecule fuels, in addition to being and alternative for photovoltaics, offers an effective method for carbon sequestration.To extend photon harvesting into the visible spectrum, colloidal TiO 2 , and dye-sensitized TiO 2 have been predominantly used as photocatalysts. [75]Given that C-dots exhibit strong absorption over the visible wavelength range and share characteristics with ordinary nanoscale semiconductors, they emerge as robust candidates for efficient photocatalysts for CO 2 reduction.

Solar Cells
The inherent chemical and electrical conductivity of carbonbased materials, including various C-dots make them ideal for lithium and sodium ion battery electrode materials.Semiconducting C-dots can stabilize the surface of active electrode materials, as demonstrated by a graphene quantum dot-coated VO 2 (Vanadium dioxide) array electrode showing excellent lithium capacity retention after 1500 cycles with 94 % efficiency. [144]C-dots' unique characteristics, such as excitation-dependent or excitation-independent fluorescence emission and size-dependent fluorescence emission, have been harnessed in solar cells. [145]The inclusion of grass in C-dots' synthesis significantly increased the conversion efficiency of Cdots sensitized aqueous solar cells.Functional groups added during the synthesis process impact the performance of solar cells, with hybrid materials replacing traditional rutheniumbased dye in dye-sensitized solar cells (Figure 6). [146]Nitrogen self-doped C-dots, created using Allium fistulosum, have been explored as quantum dots sensitizers in solar cells, with rigorous examinations revealing the influence of particle size, energy level structure, doping atoms, and C-Dot manufactur-ing method on the photoelectric characteristics of solar cells. [147]

Energy Storage
The rising demand for electro-mobility has spurred a crucial quest for high-energy storage technologies to overcome traditional super capacitor energy density restrictions.Graphene, with its exceptionally high surface area and excellent conductivity has garnered significant interest for charge storage. [148]owever, the practical, specific capacitance of graphene is hindered by the aggregation of graphene layers, leading to increased packing density and hindered ion accessibility.Cdots emerge as a solution by acting as spacers, resulting in high specific capacitance and cycling stability in electrodes made from C-dots/reduced graphene oxide (rGO) composites derived from natural carbon precursors like orange juice, [149] and cauliflower leaf waste. [37]The C-Dots' ability to act as efficient spacers prevents the restacking of graphene nanosheets, increasing surface area and pore volume, which crucially contributes to the high capacitance.Nitrogen doping further enhances the electrochemical properties of rGO by impacting active sites for pseudo-capacitance caused by oxygen/nitrogen functional groups, charge transfer resistance, and proton diffusivity in the acidic electrolyte. [150]ntroducing C-dots as dopant, such as those derived from leftover durian peel through pyrolysis, significantly improves electrode characteristics and super capacitor performance. [151]A composite electrode created using polyvinylidene fluoride demonstrated a specific capacitance of 60 F g À 1 , surpassing activated carbon based electrode. [151]The large surface area, surface functional groups, and pseudo-capacitive behaviours of C-dots are attributed to the improvement in capacitance.Applications of C-dots in the renewable energy field. [11]gure 6.A solar cell enhanced with C-dots enhancement. [146]

Light Emitting Diodes (LED)
C-dots present substantial potential to enhance optoelectronic device and smart electronic display systems.Practical methods for incorporating C-dots into light-emitting diodes involve simple surface alterations, tuneable PL emissions, and the use of doped chemical components.In contrast to commercial LEDs relying on expensive and potentially harmful rare-earth elements for phosphors, LEDs constructed form C-dots offer a promising, green alternative with excellent stability and low cost. [152]The synthesis of multi-colour, highly luminous C-Dots for LEDs can be achieved through the combustion of food waste. [153]C-Dots obtained from mango leaf extract, exhibit multi-emissive properties towards white LEDs, emitting colours from blue to yellow through one-step microwave heating. [11]The photoluminescence characteristics and stability can be further improved using N-doping, surface passivation, and functionalization techniques.

Fuel Cell
C-dots play a crucial role in fuel cell applications, especially in MFC (Microbial fuel cells).MFCs, known as microbecatalysed electrochemical systems, leverage specific electroactive bacteria to metabolize organic compounds in wastewater and generate bioelectricity.In MFCs, where high electron transfer at the anode is essential, C-dots serve this purpose effectively.The innovative systems depend on the ability of electroactive bacteria to transfer electrons extracellularly near a solid electrode, facilitating the oxidation of organic substances and subsequent electron transfer to the anode surface. [154]MFC technology holds promise in wastewater treatment applications because it offers a viable method for generating electric current from diverse materials, including natural organic matter and complex organic waste. [155]The effectiveness of these cuttingedge systems relies on the capability of specific electroactive bacteria to extracellularly transfer electrons as part of their metabolism near a solid electrode.In the anode chamber, microorganisms oxidize organic substrates, releasing electrons, protons, and carbon dioxide.Redox-active proteins or cytochromes facilitate the movement of electrons generated by microbial metabolism to the anode surface.Subsequently, these electrons are carried toward the cathode through an electrical circuit within the cathode chamber, where electron transfer occurs. [156]Hence, a material that facilitates high electron transfer at the anode is essential.The utilization of cdots meets the requirement.

CO 2 Reduction
Addressing the pressing need for CO 2 conversion due to greenhouse gas emissions, C-dots offer a potential solution.
While green plants and algae efficiently convert CO 2 through photosynthesis, noble metal catalysts' high cost and energy input hinder their use in synthetic leaf technologies. [157]Cdots, as a suitable alternative, can effectively utilize the photocatalytic process to transform CO 2 into non-polluting compounds.For instance, PEGylated C-dots coated with Pt or Au demonstrated efficient CO 2 conversion under visible light irradiation. [105]The area of CO 2 conversion using C-dots as catalysts requires further research to fully understand and optimize this promising solution.

Challenges and Prospects
The production of low-cost carbon dots through green synthesis becomes feasible when utilizing carbon precursors derived from biological wastes.Typically, C-dots are synthesized from various biomass exhibits unique properties attributed to their structures, under which carbon precursors are generated, and types of synthesis employed.Additionally, the yield of biomass-derived C-dots may vary, ranging from low to high due to compositional differences. [158]Remarkably, it has been observed that quantum yield of C-dots can be altered by selecting appropriate carbon sources and employing specific production methods. [159]This is significant as each carbon precursor contributes distinct characteristics and intrinsic properties.The potential to manufacture tailored C-dots for specific applications arises from the ability to select different combinations biomass.For instance, when designing C-dots for energy storage applications, electrochemical properties are considered leading to the selection of two or more suitable green carbonaceous precursors.Consequently, a comprehensive analysis is essential to understand the reaction mechanisms, process conditions and influential parameters that ensure the production of high quality C-dots with properties suitable for energy applications.Moreover, exploring opportunities in lignocellulosic bio-refineries to valorize their by-products for C-dots synthesis is a promising avenue.This approach aligns with sustainable practices and can contribute to the development of efficient and environmentally friendly C-dots with diverse applications.

Conclusions
The recent developments show that various technologies are available for converting biomass into useful products such as biofuels, biomaterials, and biochemicals.Among these products, C-dots represent potential nanomaterials produced from renewable biomass feedstocks.This approach has more scope and potential to substitute existing C-dots.The reasons include low priced feedstocks, abundance, bulk availability, renewable nature and greater suitability for C-dots production.Moreover, these C-dots exhibit unique optical properties,

P e r s o n a l A c c o u n t T H E C H E M I C A L R E C O R D
including high quantum yields, photo-stability, and tuneable fluorescence emissions, enabling a wide range of applications such as biomedicine, environmental monitoring, and energy conversion.Furthermore, the bottom-up approach is wellsuited for C-dots production from organic carbonaceous precursors i. e., biomass wastes.This approach involves building up material from the bottom: atom-by-atom, molecule-by-molecule, or cluster-by-cluster.In the case of the bottom-up approach, different methods have been employed, including hydrothermal, microwave-assisted, and solvothermal methods.Each method imparts unique characteristics of Cdots.In conclusion, the C-dots from hydrothermal, microwave-assisted, and solvothermal methods may exhibit high quantum efficiency, a uniform size distribution and mass production with low yield, respectively.Bio-fluorescent C-dots find applications in energy conversion technologies, ranging from power generation to energy storage.For example, C-dots can enhance the efficiency of light harvesting and electron transfer, improving the stability and performance of solar cells.Other potential applications of C-dots in energy conversion include photocatalysis, serving as photocatalysts for water splitting and CO 2 reduction, as well as luminescent markers in LEDs.Future research should focus on the techno-economics of C-dots synthesis.Cross-disciplinary approaches for assessing the Cdots lifecycle, starting from synthesis to application aspects need to be studied in depth.This holistic understanding will contribute to advancing the field and exploring the full potential of C-dots in various technological applications

e r s o n a l A c c o u n
. 2024, 24, e202400030 (8 of 17) © 2024 The Authors.The Chemical Record published by The Chemical Society of Japan and Wiley-VCH GmbH

Figure 5 .
Figure5.Applications of C-dots in the renewable energy field.[11]

P e r s
o n a l A c c o u n t T H E C H E M I C A L R E C O R D Chem.Rec.2024, 24, e202400030 (12 of 17) © 2024 The Authors.The Chemical Record published by The Chemical Society of Japan and Wiley-VCH GmbH

Table 2 .
Optimal reaction conditions for synthesizing C-dots from different bio-waste.

Table 3 .
Microwave-assisted synthesis for C-dots production.

Table 4 .
Results of pyrolysis process for synthesizing C-dots from different agro wastes.