Introduction

The generation of solid wastes (SWs) has increased dramatically as a result of urbanization and significant population growth and is anticipated to reach 3.40 billion tons by 2050 (“Solid Waste Management” n.d.). Thus, SWs treatment and disposal is a critical issue that must be addressed by both develo** and developed countries, particularly in metropolitan areas (Bui et al. 2022; Khan et al. 2021). Generally, these SWs are a collection of agricultural, paper, food, animal, yard trimming, plastics, metals, rubber, leather, textiles, wood, glass wastes, and so on (Fig. 1) (Abdel-Shafy and Mansour 2018). The continuous rise in ecologically harmful and hazardous SWs is currently a major concern, necessitating either proper trash disposal or reuse (Meyer et al. 2020). According to reports, over 70% of the global SWs are disposed of in landfills, while only 20% are recycled. As a result, it is imperative to enhance the proportion of SWs recycled into valuable goods.

Fig. 1
figure 1

Globally available common solid waste materials

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Because of the high cost, lack of organization, and difficulties, sustainable management of these SWs is the most significant impediment to increasing urbanization and an improvement in the standard of living (Chien et al. 2021; Khan et al. 2022). Thus, the proposed management method must be cost-effective, simple, sustainable, and ecologically appealing, as well as legally and socially permissible. Because most agricultural, household, human, and animal wastes are rich in proteins, minerals, and carbohydrates, thus, these SWs can be used as raw materials in material science and other related areas.

Owing to their wide range of applications in physical, chemical, biological, and materials research, nanomaterials are a revolutionary finding of the late twentieth century (Das et al. 2020; 2022; Kolahalam et al. 2019; Sasidharan et al. 2019; Talapin and Shevchenko 2016; Rajabi et al. 2020, 2018; Wei et al. 2017; Zang et al. 2017; Liu et al. 2018; Vatanpour et al. 2022; Fakhraie et al. 2023). Generally, they have different physicochemical properties compared to their bulk counterparts. Quantum dots (QDs) are a subset of nanomaterials that were envisioned in the 1980s, when charge transporters were confined to three-dimensional semiconductor materials (Das et al. 2018; Lv et al. 2020). Carbon onions (Dalal et al. 2021; Lettieri et al. 2017a, b), carbon nanotubes (Gao et al. 2014; Li and Shi 2014; Spreinat et al. 2021; Welsher et al. 2009), carbon nanoribbons (Lu et al. 2009; Wang et al. 2019b). Processed black liquid from the Kraft pul** of eucalyptus in the pulp paper industry was treated with concentrated sulfuric acid, to get the alkali lignin. The process of converting alkali lignin to GQDs was divided into two steps: the first was fractionation into lignin nanoparticles doped with nitrogen and sulphur, and the second was a hydrothermal approach to convert the produced lignin nanoparticles into GQDs. As indicated by a dynamic light scattering study, the average diameter range of the synthesized lignin-based GQDs (LGQDs) was 500–800 nm, and the QY was 21% under an excitation wavelength of 380 nm.

In another method, Ding et al. reported the synthesis of GQDs from alkali lignin (Ding et al. 2018). Here, alkali lignin was first treated with 67% nitric acid under ultrasonication to change it into the black carbon-based solution, and the resultant dispersed solution was hydrothermally treated for 12 h at 180 °C. The size range of GQDs was found to be 2–6 nm with a QY of 21%.

Table 1 displays different approaches for synthesizing GQDs from various SWs precursors. The top-down technique for GQD synthesis requires a certain type of carbon skeleton in the starting material, making it less prevalent than CQDs (Zhao et al. 2020). Since majority of SWs lack this sort of structure, therefore mostly GQDs have lower QY than CQDs.

Synthesis of graphene oxide quantum dots

Unlike CQDs and GQDs, GOQDs are produced under a strong oxidizing environment, hence a large number of COOH, -OH, and epoxy functional groups are usually available on their surfaces, making them highly water soluble (Kang et al. 2019). To the best of our knowledge, GOQDs have been reported to be synthesized from waste toner (Xu et al. 2019) and waste paper as SWs sources (Adolfsson et al. 2015) only.

Xu et al. (2019) hydrothermally synthesized GOQDs from waste toner at 180 °C for 4 h in an oxidizing atmosphere of 5% (w/w) hydrogen peroxide. They optimized the experimental conditions to achieve the best results. They further modified the synthesized GOQDs by heating them with polyethyleneimine (PEI) at 60 °C for 3 h to synthesize PEI@GOQDs. The diameter of the synthesized GOQDs, evaluated by HRTEM, was between 2 and 3.5 nm, and the QY was found to be 10.6% under illumination at 340 nm using quinine sulphate as a reference.

Adolfsson et al. developed a microwave-assisted method to synthesize GOQDs from cellulose-enriched waste-paper (Adolfsson et al. 2015). They synthesized the GOQDs in two stages: during the first stage, carbon nanospheres (CNs) were synthesized as an intermediate material directly from waste paper, followed by the conversion of CNs to GOQDs during the second stage. The second stage was sub-divided into two parts, the first being the disintegration of CNs and the second being the oxidation-degradation process. Finally, the heating was performed for various durations (30–60 min) to obtain different sizes of GOQDs. For 30 min and 60 min heating, the final HRTEM analyzed sizes for GOQDs were 3 nm and 1 nm, respectively.

Table 1 summarizes the available literature on GOQDs preparation from SWs. Due to the complexity of the synthesis process and the lower fluorescence intensity compared to the other two QDs, there have not been many reports of GOQDs from SWs. However, conjugated GOQDs are progressively gaining popularity among researchers due to the presence of various functional groups in their structure.

Figure 2 summarizes all the current methods of Cb-QDs synthesis from SWs, with hydrothermal being used in most cases. Hydrothermal synthesis is a simple and efficient approach for producing these Cb-QDs, and the QY obtained from this process is often the highest when compared to the other methods. The microwave is the second-best method for producing Cb-QDs. Both methods are environmentally sustainable but expensive. Reflux, sonochemical, pyrolysis, and other processes are still in their early stages and require further development. Some of the existing methods utilize toxic chemicals, while others demand high temperatures conditions. Thus, efforts should be directed toward the development of efficient, eco-friendly, cost-effective and low temperature procedures for producing Cb-QDs from SWs.

Fig. 2
figure 2

Various current methods towards the synthesis of Cb-QDs from solid wastes

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The temperature of the preparation, the presence of an oxidizing/reducing environment, the structure of starting material’s (in some cases), and the synthesis procedure are some vital factors that affect the formation of Cb-QDs. For example, Ye et al. (2013).

Characterization of carbon-based quantum dots

There are three types of characterization methodologies for identifying relevant Cb-QDs:

  • Chemical composition or structural characterizations,

  • Morphological characterization,

  • Optical characterization.

Chemical composition or structural characterizations

Various characterization approaches may be used to identify information about the chemical composition of the QDs and the groups present in various Cb-QDs or functionalized Cb-QDs. Among them, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), energy dispersive X-ray (EDX) analysis, and thermogravimetric analysis (TGA) are most common techniques. Nevertheless, we will limit our discussion only to the characterization of unmodified Cb-QDs as the surface modification will result in many complicated peaks of the spectra.

Fourier transform infrared (FT-IR) analysis

For CQDs

The FT-IR analysis data of CQDs prepared from lemon peel waste is shown in Fig. 3a (Tyagi et al. 2016). A broad peak at 3414 cm−1 was found in this FT-IR spectrum, which might be due to O–H stretching for water in CQDs. The vibrational frequency of C–H was found to be 2940 cm−1, C = O stretching was observed at 1715 cm−1, and the -COO− exhibited two peaks at 1605 and 1405 cm−1.

Fig. 3
figure 3

FTIR spectra of (a) lemon peel waste–derived CQDs (Tyagi et al. 2016), (b) lignin biomass–derived GQDs (Ding et al. 2018), and (c) waste toner–derived GOQDs (Xu et al. 2019)

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For GQDs

GQDs, on the other hand, produce more distinct peaks in FT-IR spectrum than GOQDs (Fig. 3b). Stretching vibration of C = C (for aromatic ring) was detected at 1590 cm−1. The vibrational frequencies of in-plane and out-plane C–H (aromatic) groups appeared at 1041, 870, and 762 cm−1. The presence of O–H, C = O, and C–O groups throughout the periphery of each GQDs sheet resulted in broad maxima at 3360, 1697, and 1190 cm−1. Ding et al. reported that the peak for C–O–C at 1261 cm−1 in the starting materials (alkali lignin) was missing in GQDs and it was replaced by a C–N bond (due to nitric acid oxidation), which appeared at 1149 cm−1(Ding et al. 2018).

For GOQDs

GOQDs are rich in oxygen containing functional groups on its surface, so due to that polar crown, GOQDs were highly dispersible in water. In FT-IR spectrum for uncoated GOQDs (Fig. 3c), O–H group on the GOQDs surface (or from the water) appeared at 3438 cm−1 and two weak C–H bending peaks were found at 2923 and 2849 cm−1. Moreover, one weak and one strong C = O stretching frequency were obtained at 1717 and 1635 cm−1, a weak C–H bending vibration was reported to be at 1385 cm−1 in addition to a broad and quite strong type of C–OH stretching frequency (Xu et al. 2019).

X-ray photoelectron spectrum (XPS) analysis

XPS determines the energy of the bonds present on the Cb-QDs. This provides information related to types of the bonding present (i.e., C–O, C = O, C = C, C–O–C) over the specific Cb-QD.

For CQDs

Prasannan and Imae (2013) described XPS of orange waste peel–derived CQDs to confirm the various functional groups. The C1s spectral analysis (Fig. 4a) exhibited total of five peaks at 284.9, 285.9, 287.3, and 288.8 eV, respectively for the C = C/C–C, C–OH/C–O–C, C = O, and O = C–O groups. On the other hand, three peaks were found from O1s spectrum (Fig. 4b), at 530.5, 531.9, and 533.2 eV for C–O, C = O, and C–OH/C–O–C species, respectively.

Fig. 4
figure 4

XPS-spectra of (a) C1s and (b) O1s type of CQDs, derived from orange waste peel (Prasannan and Imae 2013); (c) C1s, (d) N1s, and (e) O1s of GQDs-derived from spent tea (Abbas et al. 2020); (f) C1s, and (g) O1s are the and part of the XPS spectra in case of carboxymethyl cellulose–derived GOQDs (Adolfsson et al. 2015)

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For GQDs

The XPS spectra for the spent tea–derived GQDs showed three peaks at 285.08, 400.08, and 532.08 eV for C1s (Fig. 4c), N1s (Fig. 4d), and O1s (Fig. 4e), respectively, with a small peak at 347.08 eV for Ca 2p (Abbas et al. 2020). The elemental analysis data of GQDs (produced under 500 W) showed C = 56.45%, O = 36.73%, N = 4.76%, and traces of Ca. C1s spectral analysis under high resolution XPS showed that they had three peaks at 284.9, 286.3, and 288.4 eV, for C–C/C = C, C–O–C, and O-C = O groups. N1s spectral analysis revealed three peaks at 399.89, 402.01, and 406.05 eV, for C–N/N–H (pyridinic or pyrrolic), N–C (3°-amine), and N–O (nitro) bonds, respectively. O1s spectrum on the other hand possessed two peaks at 531.58 and 533.08 eV, for O = C and O–C species, respectively. They also found that no structural change occurred in GQDs after 2 months, demonstrating excellent GQDs stability.

For GOQDs

The XPS C1s spectral analysis (Fig. 4f) of carbon nanospheres showed four peaks at 285 (C–C/C = C), 286.3 (C–O), 287.5 (C–O–C), 289.2 (O = C–O), and 290.1 eV (π-π* of aromatic ring) and three for GOQDs, viz. 285 (C–C/C = C), 286 (C–O), 287.3 (C–O–C), and 288.3 eV (O = C–O), respectively (Adolfsson et al. 2015), while two O1s spectral analysis peaks (Fig. 4g) were found at 532 and 533.4 eV for C = O and C–O groups. After the conversion of CNs to GOQDs, the increase in oxygen containing groups can be attributed to the decrease in C/O ratio from 4.2 to 2.0.

All three types of Cb-QDs might contain similar or different types of groups, depending on the synthetic procedures. If highly polar groups (e.g., COOH, C = O, OH, N–H) are present at the edge or exterior surface of the Cb-QDs, then those QDs would be highly soluble in water. With modification, we can tune the solubility as well as other properties of the QDs.

Miscellaneous skeletal characterizations

Other characterization techniques include RS, EDX analysis, TGA analysis, solid state cross polarization/magnetic angle spinning 13C nuclear magnetic resonance spectroscopy (CP-MAS-13C-NMR) analysis, two-dimensional NMR (2D-NMR) analysis, heteronuclear single quantum coherence (HSQC) spectroscopic analysis, gas chromatography–mass spectrometry (GC–MS), and high-performance liquid chromatography (HPLC).

Like FT-IR, RS detects the functional groups present over Cb-QDs. Although RS is more time consuming and expensive than FT-IR, it has the advantage of not being affected by the presence of water molecules in the spectra. The CP-MAS-13C-NMR spectra can distinguish between processed and unprocessed materials (David et al. 2009). When compared to 1D-NMR, 2D-NMR provides more information on the type of protonic environment, but HSQC provides information about proton coupling with heteronuclei, i.e., we may find out the interaction of a proton and a heteronuclei using this form of NMR spectroscopy. The elemental composition of the various QDs is determined using EDX analysis. TGA confirms the deposition of various molecules, drugs, or nanomaterials on the surface of Cb-QDs sheet. GC–MS identifies the components in QDs from the starting materials/final products mixtures and clearly reveals about whether the conversion is completed or not. Unlike GC–MS, HPLC can detect the components from the same mixtures but in the liquid phase and with varied time intervals.

Morphological analysis

For morphological analysis, the preferential characterization technique is transmission electron microscopy (TEM) and its high-resolution version (HRTEM), which simultaneously furnishes the details about size, shape, layers, and thickness. Morphological analysis is discussed below for the three unmodified Cb-QDs.

Transmission electron microscopy (TEM) analysis

The CQDs synthesized from peanut shell were analyzed by TEM (Xue et al. 2016). The CQDs were spherical in shape with 2–4 nm diameter (Fig. 5a). From single crystalline structural analysis by HRTEM, the lattice spacing was estimated to be 0.338 nm (Fig. 5b), corresponding to the (002) graphitic plane.

Fig. 5
figure 5

TEM image (resolution 10 nm) (a) and corresponding HRTEM of a single CQDs crystal of peanut shell–derived CQDs (b) (Xue et al. 2016); TEM (resolution 50 nm) (c) and HRTEM (2 nm) images of rice husk–derived GQDs (d) (Wang et al. 2016c); TEM (resolution 20 nm) (e), corresponding size analysis histogram (f) (inset) and HRTEM (2 nm) image (g) (inset) of waste toner–derived GOQDs (Xu et al. 2019)

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In one study, HRTEM, employed for rice husk–derived GQDs analysis indicated its highly crystalline nature with spacing of 0.24 nm i.e., (1120) diffraction plane (Fig. 5c) (Wang et al. 2016b). TEM analysis of the GQDs revealed the size range 3–6 nm (Fig. 5d).

Xu et al. (2019) reported that the GOQDs produced from waste toner had a size range of 2 – 3.5 nm (histogram, Fig. 5f) using TEM analysis (Fig. 5e). The space in a single GOQDs crystal was estimated to be roughly 0.2 nm using HRTEM, resulting in a (102) plane of diffraction (graphitic sp2 carbon, JCPDS no.- 26–1076) (Fig. 5g).

Miscellaneous techniques related to morphological analysis

Dynamic light scattering (DLS) data partially offers an idea of the size of the QDs. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are also used to analyze the morphology of diverse nanomaterials, which includes the size, shape, thickness of the layers, and surface stability. To confirm the structures of crystalline nanomaterials, powder X-ray diffractometry (XRD) is used. Selected area electron diffraction (SAED) pattern is employed to validate the crystallinity data obtained by powder XRD. Brunauer–Emmett–Teller (BET) surface area analysis explores the specific surface area (in m2/g) of Cb-QDs via gas (inert type, viz. nitrogen) adsorption analysis, over a solid sample.

Optical characterization analysis

UV–visible and photoluminescence spectral analysis

UV–Visible spectroscopy is one of the most effective tools to obtain preliminary information for the efficient fluorescence characteristics of Cb-QDs. Furthermore, the presence of conjugation or electron excessive groups/atoms on the QDs can be concluded by UV–Vis spectral analysis (Table 2). Photoluminescence (PL) spectroscopy is used to examine the fluorescence and phosphorescence abilities of QDs, as well as the QY, which indicates the successful conversion of a carbon source into QDs, at a maximum excitation wavelength (Table 2).

Table 2 Summary of UV–visible and photoluminescence spectroscopic data for the reported SWs-derived Cb-QDs
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According to Wang and Hu (2014), the color of fluorescence varies with the size of CQDs. The red shift for emissive radiation occurs when the size of Cb-QDs increases, due to a decrease in the HOMO–LUMO gap.

Miscellaneous optical characterizations

There are a variety of different optical techniques that can be used to fully comprehend the fluorescence properties of QDs. Time-correlated photon counting (TCSPC) is an optical characterization technique that measures the fluorescence decays and directly tells us about the lifetime of a QDs. Time-resolved photoluminescence (TRPL) is an extension of normal fluorescence spectroscopy that produces an excitation spectrum as a function of time. Furthermore, Fluorocube time-correlated single photon counting (TCSPC) fluorimetry is a well-developed method of measuring fluorescence lifetime with high data accuracy and high sensitivity.

Based on all the characterization techniques, the following conclusions can be drawn:

  • UV–Vis, PL, XPS, EDX, and FTIR spectroscopy can be used to identify all three Cb-QDs.

  • Carbon sheets are organized into layers to form a complete spherical CQD crystal, whereas GQDs and GOQDs exhibit a sheet-like structure.

  • CQDs and GQDs are generally less polar than GOQDs, regardless of modification, and so the latter have distinct water solubility.

  • CQDs and GQDs, on average, have a higher C/O ratio than GOQDs.

  • When exposed to UV light, all three types of Cb-QDs emit light at the same frequency for example, blue emission occurs for all three types of Cb-QDs at 365 nm (Fig. 6).

  • All the three types of Cb-QDs would have the same surface functionalities if they were synthesized from the same carbon source.

  • In CQDs and GQDs, the numbers of conjugated mobile (π-bonded) electrons are higher than in GOQDs. As a result, GOQDs are not probably exploited as popular fluorescent materials.

Fig. 6
figure 6

(a) Peanut shell–derived CQDs before excitation (left) and after excitation at 365 nm (right) (Xue et al. 2016); (b) GQDs from rice husk biomass under visible (left) and at 365 nm UV irradiation (right) (Wang et al. 2016c); (c) waste toner–derived GOQDs under day light (left) and 365 nm UV irradiation (right) (Xu et al. 2019)

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Applications of carbon-based quantum dots

Cb-QDs, with and without surface functionalization, have gained immense attention among researchers in the past decades due to their wide range of applications. Because of their small size and perfect shape, they are used in various areas like bioimaging, in vitro sensing, drug delivery, chemical sensing, environmental applications, electrical devices, and catalysis. Herein, the various applications of Cb-QDs derived from SWs in the fields of biomedicine, electrical devices, environmental, and catalysis are discussed.

Biomedical applications

Cytotoxicity is a serious issue in the development of medications and therapeutic procedures since it can have a negative impact on both infected and normal tissues. Cb-QDs are now emerging as one of the important materials for biomedical applications because of their lower toxicity than other inorganic-based QDs. Cb-CDs at various concentrations, with or without surface modifications or do**, have been shown to be less cytotoxic, and thus more biocompatible on various cell lines. CQDs derived from alkali lignin showed more than 83% viability of HeLa cell lines even after 24 h incubation with a dosage of 50 mg/L (Zhang et al. 2019). Similarly, the cell viability of HeLa cells was above 90% when incubated with a high concentration of waste frying oil–derived sulphur-doped CQDs (600 μg/L) for 48 h. Likewise, a higher concentration of CQDs (1 mg/L) from the pseudo-stem of a banana plant on incubation with Hela and MCF-7 cell lines for 24 h revealed more than 85% cell viability. Similar types of high cell viability were observed for other cell lines (like A549, HepG2, Vero, HCT116, C6, T24) even after incubation for ≥ 24 h with a higher concentration of functionalized/doped/uncoated CQDs (Table 3). Like CQDs, functionalized/doped/uncoated GQDs also showed similar types of HeLa, L929, 3T3, HepG2, HEK293, etc., cell viability when incubated for ≥ 24 h. There is no information on the cytotoxicity of SWs-derived GOQDs on any types of cells. A live-cell imaging and sensing of cholesterol is illustrated in Fig. 7.

Table 3 Biomedical applications of SWs-derived Cb-QDs
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Fig. 7
figure 7

Living cell imaging in case of HepG2 cells incubated with CQDs at 37 °C. (a) Bright field and fluorescent image under excitation with laser at (b) 405 nm, (c) 488 nm, and (d) 514 nm.(Xue et al. 2016); (e) sensing scheme of cholesterol by diesel soot–derived CQDs (Tripathi et al. 2014)

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Drug delivery, sensors, magnetic hypothermia, photothermal therapy, in vivo imaging, and in vitro biosensing are currently well-developed techniques with Cb-QDs. Despite the fact that a nanomaterial’s ability to penetrate the blood–brain barrier (BBB) is challenging, several articles have reported that the relevant Cb-QDs can be effective in overcoming the problem. Kim et al. (2018) synthesized unfunctionalized GQDs and showed their ability to prevent synucleinopathy in Parkinson’s disease. Because of the strong fluorescence, water solubility, photo-stability, low toxicity, cell-membrane permeability, and excellent biocompatibility, CQDs produced from the pseudo-stem of the banana plant were used as fluorescent probes for imaging of HeLa and MCF-7 cells in three colors (blue, green, and red) (Vandarkuzhali et al. 2017). The highly luminous r-CQDs-GS from processed white rice has been effectively proven for in vitro and in vivo bioimaging of A549 cells (Anthony et al. 2020). Apart from that, fabricated N-doped CQDs derived from Platanus biomass have been widely used for cellular imaging of HeLa cells, L02 cells, and macrophage cells. There are other reports where CQDs derived from SWs were used for imaging different carcinoma cells like HeLa, C6, MC3T3 HUVEC, MDA-MB-231, Caco-2, DU145, and more (Zhang et al. 2019). Apart from biosensing of cells, fluorescent CQDs from diesel engine soot have been successfully utilized for the imaging of Escherichia coli (E. Coli) along with the sensing of cholesterol (Tripathi et al. 2014). Several other researchers have used SWs-derived CQDs for a variety of other applications, including bacterial cell imaging (Ang et al. 2020; Tripathi et al. 2014), drug sensing (Yu et al. 2019), drug delivery (D’souza et al. 2018), and antioxidant (Rajamanikandan et al. 2021). Unlike CQDs, applications of GQDs derived from SWs are yet to be explored in their wider range. Internalization of GQDs derived from waste molasses has also demonstrated in DF-1, HepG2, and HEK293 cells (Sangam et al. 2018). There are studies on the use of GQDs derived from SWs for bioimaging of HeLa (Wang et al. 2016b, 2016c), L929 and 3T3 (Wang et al. 2015a). When another strong chelating compound (ligand) is introduced to the metal ion − Cb-QDs complex solution, it triggers a chelation competition and, as a result, separates the “alien” metal ions by chelation. Thus, the fluorescent activity is regained (“OFF–ON” part). A description of “ON–OFF-ON” pathway is given in Fig. 9e. The total process is supported by Dexter energy transfer mechanism.

While sensing organic molecules including DNA and TNT, π* (antibonding orbital) of the acceptor molecule is involved (Demchenko and Dekaliuk 2013) in the reduction of PL intensity and causes the turn- “OFF” the fluorescent signal. Now if another molecule possing strong binding efficiency with that organic molecule added to that organic molecule − Cb-QDs complex solution, then the fluorescence signal (turn- “ON”) will be restored. Thus, as electron deficiency or electron accepting inclination increases, electron transfer from donor Cb-QDs to metal ions/organic molecules increases, resulting in a greater reduction of PL intensity. As a result, increase in sensitivity occurs with a significant increase in LOD efficiency.

The second mechanism is photoinduced electron transfer, or PET, which includes charge separation and excitation of the donor side via irradiation (Jose et al. 2017), resulting in either fluorescence quenching or amplification. Here, the electron transfer between the metal ion and the fluorophore, or inside the self-fluorophore-chelate unit, can also result in fluorescence property modulation (Fig. 9c). Direct electronic transitions between fluorophores (excited) and metal ions, containing low energy d-orbitals (empty or partially filled) are usually accompanied by quenching. PET can also enhance the fluorescence for fluorophore-electron rich metal chelate. Without metal ions, excitation leads to the separation of charges, and hence, the PET in between excited fluorophore and chelate goes in for emission, thereby PET gives rise to systematic relaxation pathway and resulting a decrease in QY for that fluorophore. When a metal ion binds to an electron-rich chelating site, a shifting of charge density occurs and thus effectively quenches the PET decay pathway, resulting in the increase in QY.

Another mechanism that involves radiation-free energy transfer through dipole–dipole coupling between a photoexcited donor and an acceptor is known as Forster resonance energy transfer, FRET (Masters 2014) (Fig. 9d). FRET efficiency of energy transfer is highly dependent on the distance (inversely and to the sixth power) between the donor and acceptor. The acceptor can be a chromophore (capable of absorbing energy) or a fluorophore, in which a photon is irradiated by a high energy molecule during relaxation to the low energy state due to sensitized emission. In general, FRET reduces donor emissions, resulting in a shorter lifetime. Metal binding results in a change in molecule structure and can affect distance and/or orientation that can assist or hinder FRET (Carter et al. 2014).

The mechanism for the fluorescence emission is still ambiguous due to the different theories among the researchers. The bioimaging within the cells are primarily due to the fluorescence emission property of CQDs. In general, two mechanisms are widely accepted for CQDs: excitation dependent and excitation independent. The excitation dependent fluorescence is primarily due to band gap emissions in π-domain where strong absorption in the UV range and a weak emission is observed. Surface defects in the structure of CQDs induce excitation independent fluorescence emission, which exhibits mild absorption and high emission in the visible region.

Defects in GQDs containing sp3 carbons are structurally comparable to those observed on the surface of tiny CQDs with an unusually high surface-to-volume ratio (Cao et al. 2013). According to Molaei group, the PL mechanism observed in GQDs is primarily due to surface defects and band gap transitions associated with conjugated π -domains (Molaei 2019). It is possible to use GQDs with near infrared (NIR) emission for bioimaging because tissues often exhibit autofluorescence and low light absorption in the NIR region (Younis et al. 2020).

Comparison of CQDs, GQDs, and GOQDs based on their synthesis, structures, physical properties, and chemical properties

The summary of SW-based Cb-QDs in terms of synthesis, structures as well as their physical and chemical properties are given in Table 5. It is evident that CQDs do not demonstrate any selectivity in their synthesis, but the structure of SWs plays a significant role during the synthesis of both GQDs and GOQDs. Though the surface functionalities of GOQDs are more compared to CQDs and GQDs, yet it is less explored because of its low fluorescent intensity than other two QDs.

Table 5 Comparison of SWs-derived CQDs, GQDs, and GOQDs based on their synthesis, structures, physical properties, and chemical properties
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Limitations and future research prospects of carbon-based quantum dots

Cb-QDs have been employed as a selective or non-selective medication, gene, and drug delivery agent for in vivo or in vitro studies on a certain kind of cell. Because of their stable PL, these materials are highly suitable for sensing in vivo and in vitro cells. Aside from medicinal uses, these materials are also employed for energy storage or batteries, as well as for the detection of toxic elements, ions or molecules, and explosives due to their high fluorescence and low and adjustable band gap.

Apart from these, there are many other applications of Cb-QDs. Lu et al. (2015) reported better photocatalytic activity of zinc-porphyrin modified GQDs compared to zinc-porphyrin towards the degradation of methylene blue (MB) under visible-light. On the other hand, Dang et al. (2022) used Fe-doped CQDs to generate methanol by CO2 reduction. A three-fold increase in photocatalytic activity of S and N co-doped CQDs with TiO2 NPs was observed towards acid red 88 degradation (Rahbar et al. 2019). Tammina et al. (2019) reported the application of N and P co-doped CQDs as dopamine sensor with a detection limit of 0.021 M. In another study, the use of N-doped GQDs, as a sensor, was reported for the detection of Fe3+ with high selectivity and sensitivity (Tam et al. 2014). Malček et al. (2022), through theoretical modelling, reported that Mn and Cr doped GQDs were highly effective at adsorbing H2 gas.

The fuel cell applications of doped CQDs and GQDs were also reported. N-doped CQDs decorated on the carbon paper surface were used to develop microbial fuel cell anode (Shaari et al. 2021). According to Yun et al. (

Conclusions

SW-derived Cb-QDs are inexpensive, reducing the cost of imaging, sensing, and detection applications. Carbon enriched SWs are the best raw materials for the production of Cb-QDs while agricultural and food wastes are top** up the list. Among the SWs-derived Cb-QDs, CQDs are the best QDs, as they can be produced with higher QY than the GQDs and GOQDs. Hence, SWs can be utilized in the form of CQDs with versatile applicability. Because of the low QY associated with them, to date, GQDs and GOQDs have not had broad utility. As of now, Cb-QDs are not very popular materials for practical applications because of their poor reproducibility during large scale synthesis. Therefore, a lot of opportunities are still there for future researchers to develop advanced pathways for their synthesis and applications. There is also a potential for converting tons of SWs generated daily in rural and urban areas into environmentally benign Cb-QDs for more practical utilization.