Plastic Waste-Derived Carbon Dots: Insights of Recycling Valuable Materials Towards Environmental Sustainability

Abstract

Carbon dots (CDs) or carbon quantum dots (CQDs) have emerged as rising stars in the carbon family due to their diverse applications in various fields. CDs are spherical particles with a well-distributed size of less than 10 nm. Functional CDs are promising nanomaterials with low toxicity, low cost, and enormous applications in the field of bioimaging, optoelectronics, photocatalysis, and sensing. Plastic is non-biodegradable and hazardous to the environment, however extremely durable and used in abundance. During the COVID-19 pandemic, the use of plastic waste, particularly masks, goggles, face shields, and shoe cover, has increased tremendously. It needs to be recycled in a productive way as plastic wastes take hundreds or thousands of years to degrade naturally. The conversion of plastic waste into magnificent CDs has been reported as one of the key alternatives for environmental sustainability and socio-economic benefits. In this review, synthetic routes for the conversion of plastic wastes into CDs utilizing hydrothermal, solvothermal, pyrolysis, flash joule heating, and characterization of these CDs using different techniques, such as Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and transmission electron microscope, have been discussed. Furthermore, potential applications of these plastic-derived CDs in sensing, catalysis, agronomics, and LED lights are summarized herein.

Introduction

In the past decade, fluorescent nanoparticles have emerged as an important class of nanoparticles due to their several unique properties. Most notably, fluorescent nanoparticles can emit light when exposed to light, which is distinct from their non-fluorescent counterparts. Due to these properties, fluorescent nanoparticles have found applications in a wide variety of fields, including bioimaging, chemical sensing, and photonics. The rapid expansion of the field of fluorescent nanoparticles has been primarily propelled by the discovery of new types of fluorescent nanoparticles, such as graphene dots, semiconductor quantum dots, and CDs [1, 2]. The optical properties of CDs are not the only reason for their popularity but also because of simple synthesis procedures like top-down and bottom-up methods [3]. CQDs are carbon nanoparticles with an average diameter ranging from 10 to 60 nm [4]. CD is the youngest member in the family of the nanoworld.

The discovery of CDs was accidentally done by Xe et al. (2004), while purifying a single-walled carbon nanotube. Apart from this, in 2006, Sun et al. synthesized stable photoluminescent carbon nanoparticles of different sizes and named them “CQDs” [5]. CDs are highly stable, shows good conductivity, and possesses low toxicity which makes them environment-friendly and, a huge topic for research especially due to their strong and tunable fluorescence emission properties. However, CDs have not yet reached the same level of commercialization as other nanomaterials such as carbon nanotubes and graphene. The first CD to reach the market was synthesized by Schwarze et al. in 2009 using the hydrothermal method. CDs can be easily functionalized with various functional groups such as hydroxyl, sulphoxyl, carboxyl, amino, and epoxy groups over their surface. These modifications make it easier to bind CDs with inorganic and organic moieties [6]. Surface engineering is a versatile and powerful tool for tailoring the chemical, physical, and photophysical properties of CDs [7]. One of the most common surface engineering techniques is plasma-induced chemical doping, which is used to introduce dopant atoms such as bromine, fluorine, or oxygen into CDs. This process creates a strong interaction between the dopant atom and the CD framework, which modifies the chemical properties of the CD without changing its basic structure [8]. CDs can be synthesized from various types of precursors like wheat husk, rice husk, various plant leaves, wine, tea, corncob[9], coconut water [10], onion waste [11], table sugar [12], lotus root [13], cloves [14] and plastic waste [15].

Biowastes can be easily degraded and, can be used as manure for agricultural purposes. On the other hand, plastic has non-biodegradable nature with a negative impact on the environment. The world has been witnessing a dramatic increase in the use of plastics in daily life. This has led to the increasing demand for plastic products especially during the COVID pandemic, causing serious environmental problems such as plastic pollution in the oceans and landfills. Due to extremely slow degradation, they remain in nature for hundreds or thousands of years and cause a hazardous effect on the human and wildlife food chain [16]. There are different types of plastic pollutants according to size such as nanoplastic, mesoplastic, microplastic, and megaplastic [17]. Microplastic, one type of plastic with an effective size less than 5 mm, is a major cause of pollution. When humans ingest microplastics, they can accumulate in the body and potentially cause health problems such as inflammation, immune system suppression, and organ damage. Microplastics adsorb hydrophobic organic contaminants (HOCs) like polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and heavy metals on their surface which further lead to more damage to humans [18]. Microplastic releases chemicals that seep through the ground and transfer through the sewage system. Most of the plastic particles are present in sludge in the sewage and this sludge is used as fertilizers for agricultural land. These plastic particles or bacteria contained in them adversely affect the healthy bacteria and other species like earthworm present in the soil and thus affecting the flora and fauna of the ecosystem [19, 20]. About 134 species of aquatic animals are on the extinct list due to plastic ingestion [21]. Human health is adversely affected by the monomers and additives used in plastic. One of the major constituents of polycarbonate plastic is bisphenol A (BPA) which is similar to the estrogen a reproductive hormone. It can disturb the estrogen functioning in the body. With the time or generally exposure to the elevated temperature, BPA molecules from the containers leach into food and water stored in them and can lead to long-term health problems such as type 2 diabetes, cardiovascular disorder, cancer (breast, prostate), reproductive and developmental problems, obesity, and endocrine disruption [22] (Fig. 1).

Fig. 1.

Impact of plastic on human health

As per the global report by Organization for Economic Co-operation and Development (OECD) in 2019, more than 368 million metric tonnes (Mt) of plastic has been produced in the year 2019 which depicts a staggering increase compared to 1.5 Mt produced in 1950 [23]. This data shows about 250 times increase in the production of plastic waste. Asia is the largest producer and receiver of plastic that produces around 121 Mt plastic per year [24]. It is a well-known fact that diverse programs are being practiced all over the world to decrease the global impact of plastic waste. Plastic waste is utilized generally to produce energy, polyester fibers, strapping, and non-food containers [25]. However, recycle rate is less than 10% in India due to the economic unviability of their recycling processes. The process of recycling plastic can also lead to plastic irritants being released in several ways. There is an assessment of 5–13 million tons of plastic entering the oceans each year. Plastics in landfills due to weathering convert into microplastic, plastic debris, and enter the ocean. The marine ecosystem is disturbed because aquatic animals suffer from toxic impacts and it leads to biomagnification affecting humans as the final consumer [26]. In 2021, 90% of birds were stomach infected due to plastic contamination which was only 5% in 1960 and if this process continues at this rate, this number will increase up to 99% [27]. Many free-grazing animals used to die by ingesting plastic along with the food [20]. According to a report in 2015, 6300 Mt of plastic waste was generated, out of which only 9% was recycled, 79% was landfilled, and 12% was incinerated. With the current production rate and poor management system, there will be around 12,000 Mt of plastic waste in landfills or in the natural environment by 2050 [16] (Fig. 2).

Fig. 2.

Projection of rate of plastic menace [16]

One of the alternative methods to utilize plastic waste as qualitative material is by converting plastic into something more valuable product such as CQDs [28]. As plastic is synthetic carbon-based material, it can be used as raw material for synthesizing CDs. In the past decade, a number of research articles have been published on the synthesis, properties, and applications of CDs derived from biomass and from chemicals. However, a focused review of CDs from plastic waste for environmental applications still remains unavailable in the literature. Therefore, this review paper briefly discussed the synthesis route of CDs from plastic wastes and discussed their applications in various fields. Several aspects and content herein focus on the utilization of plastic waste for the synthesis of valuable CDs and their applications in sensing metal ions (e.g., Fe³⁺, Cu²⁺, and Au³⁺) catalysis and bioimaging. As the plastic-derived CDs have not yet been explored much and only a limited number of publications have appeared in the literature till now, however their utilization in various fields such as sensing, catalysis, and agronomics has paved a new direction in the area of sustainable development.

Synthesis and Characterization of Carbon Dots from Plastic Waste

Synthesis of Carbon Dots

Molecular state, surface state, and quantum confinement effects are the main elements affecting the synthesis of CDs, and these elements are controllable via altering the methods used to create CDs [5, 29, 30]. Numerous functional groups, including amine, carbonyl, epoxy, carboxyl, ether, and hydroxyl, can be introduced into CDs during and after their production [31, 32]. Additionally, employing diverse chemical, polymeric, and biological materials, it is simple and clear to add hetero-atoms like N, B, S, and P among others, to CDs in order to functionalize their surface [33, 34]. Therefore, the surface functional groups’ length and size can be changed to govern the characteristics of CDs by applying new synthesis methods or utilizing various precursors [35]. It is possible to boost the quantum yield (QY) of CDs both during and even after their production [35-38]. In the past few years, significant research has been conducted to manufacture CDs having good QY enabling improved bio-applications. Improved surface passivation of CDs can lead to increased biocompatibility, but it may also lead to a decrease in the intensity of photoluminescence (PL), and conversely. However, this is indeed not quite evident that whether CDs function as that of the ideal fluorophores for a number of medical purposes [39, 40].

Bottom-up approach or a top-down approach can be used to synthesize CDs (Fig. 3) [41]. The top-down approach involves the splitting of massive bulk material into tiny particles which have sizes under 10 nm. This route includes techniques like laser ablation [42], electrochemical synthesis [43, 44], ultrasonic treatment [45], and high-energy ball milling [46] for the fabrication of CQDs. But this route has some drawbacks such as the requirements of expensive setup and starting materials, drastic reaction conditions, and prolonged reaction times [47].

Fig. 3.

Schematic diagram of top-down and bottom-up approach

Some of the top-down methods are briefly explained as follows.

Arc Discharge Method

In this method, a high-energy electric arc is used to vaporize the graphite electrodes in an inert gas atmosphere (argon, helium). The generated plasma which contains carbon atoms and ions is allowed to cool and collected on a glass slide or metal surface. The resulting CDs are collected and purified. Xu et al. [48] synthesized the single-walled carbon nanotubes via arc discharge method and during its purification CDs were isolated [48]. The arc discharge technique typically employs a large number of composite segments, and it can be difficult to purify these segments [49, 50].

Laser Ablation Method

It is the most used method for the synthesis of CDs. The laser ablation procedure exposes the substantial carbonaceous material like graphite, graphene, or carbon nanotubes to a light beam of strong intensity in the presence of a solvent or surfactant, which produces a physicochemical condition involving high temperature and pressure. Plasma is created through evaporation and the produced vapor is then transformed into CDs by crystallization process [41, 51]. Sun et al. (2006) showed how to utilize argon as a gasifier for carbon substrate and water vapor while using the laser ablation method to make luminous CDs [52]. In another study, carbon glassy particles were immersed in polyethylene glycol 200 and exposed to laser irradiation to produce bright CDs of size ~ 3 nm. The CDs synthesized in this study can be utilized as fluorescent markers for normal and cancerous human epithelial cells in live imaging applications [53]. Different solvents, like amino-toluene, could be used to make CDs which are nitrogen- doped from powder of graphite employing a one-step laser ablation process [54]. CDs created by dual laser ablation may be more advantageous than those created by a laser beam which is single pulsed in terms of QY, size, surface/volume ratio, durability, and homogeneity. Therefore, it is preferable to use double-pulsed laser ablation to improve CDs’ catalytic and sensing properties [55, 56].

Ultrasonic Method

This technique is used to synthesize CDs by breaking down the precursor compounds using ultrasonic waves of high and low pressures. The process involves the use of high-frequency sound waves to create cavitation bubbles in a liquid medium containing the carbon source and any surface-passivating agent. The collapse of these bubbles generates high temperature and pressures, which facilitate CDs’ formation [41]. By subjecting ascorbic acid and ammonia to ultrasonic treatment, Wang et al. were able to synthesize nitrogen-doped CDs [57]. By utilizing the energy generated by the ultrasonic method, massive nanomaterials which are carbon based such as graphite, CNTs, and activated carbon could be separated into CDs which are nano-sized [58, 59]. On the other hand, the bottom-up approach involves the transformation of tiny carbon structures into CQDs of the required size. The bottom-up route consists of various techniques (e.g., microwave, hydrothermal, solvothermal, pyrolysis, thermal decomposition, and carbonization) to fabricate CQDs. The bottom-up methods present interesting possibilities to control the properties, shape, and molecular size of CQDs [60].

Thermal Method

One of the most effective ways to create CDs is through thermal breakdown, which includes carbonizing or pyrolyzing the carbonaceous raw materials at elevated temperatures [61]. The advantages of employing this process include ability to high yield, its affordability, shorter reaction times, wider tolerance of raw materials, ease of synthesis, and strategies which are solvent free [62]. Furthermore, by adjusting variables like the pH of the reaction-mixture, the length of the reflux, and the reaction temperatures, the luminous properties of CDs can be improved using the thermal approach [63]. Wang et al. synthesized CDs using this method by heating citric acid on a hot plate at a temperature of 200 °C for 30 min, followed by neutralization with a sodium hydroxide solution, and finally purified through dialysis. The size of the resulting CDs was found to be in the range of 0.7 to 1 nm [64].

Microwave-Assisted Method

Since microwaves comprise a wide spectrum of electromagnetic waves ranging 1 mm to 1 m, they provide sufficient energy for the dissociation of the chemical bonds of the raw materials [65]. The microwave-assisted method is extremely cost-effective, quite quick with a short reaction time, and generates homogeneous warmth for the uniform dispersion of CDs [66]. Exercising a single step of radiation support, Yu et al. [67] produced green fluorescent CDs using phthalic acid, and triethylenediamine hexahydrate in just 1 min [67]. In a different work using the same method, luminous CDs were created utilizing glucosamine on a PEG@chitosan co-polymer layer [66].

Hydrothermal Method

The hydrothermal carbonization (HTC) method of synthesizing CDs is cost-effective, environmentally friendly, and non-toxic. This involves taking organic solvents as the starting materials, sealing them in a hydrothermal reactor, and subjecting them to high temperatures and pressures. Different types of precursors like chitosan, citric acid, saccharides, amines, and proteins can be utilized to produce CDs through the HTC technique [68]. Hasan et al. (2021) synthesized green fluorescent CDs from different precursors like furfural, hydroxy-methyl furfural, and microcrystalline cellulose. The optical properties of CDs varied based on the precursor utilized, resulting in distinct absorption and emission characteristic [69]. In order to create CDs using the hydrothermal approach, Luo et al. [70] employed ethylenediamine, cysteine, and trisodium citrate dehydrate as precursors. The prepared CDs could exhibit intense blue emission, outstanding biocompatibility, and great fluorescence stability [70].

Carbonization

The carbonization synthesis method is considered an inexpensive, simple, and fast approach for producing CDs. This method involves the pyrolysis of organic materials in an inert atmosphere to obtain solid residues with a higher carbon content. Wei et al. [71] used glucose as a carbon source and ethylenediamine as a nitrogen source to synthesize N-doped CDs using this carbonization method within 2 min, with a QY of 48% and a size range of 1 to 7 nm [71]. Wang et al. [46, 57, 64, 72] utilized citric acid carbonization to fabricate blue luminescent thermally reduced CDs with sizes ranging from 4.8 to 9 nm. They also used a thermogravimetric analyzer to thermally reduce the CDs, resulting in a fivefold increase in QY compared to non-reduced CDs [72].

Out of the various methods, the hydrothermal method is generally preferred for the fabrication of doped CDs as it usually involves an easy, low cost, and scalable single step synthetic procedure. Song et al. [73] fabricated the CDs using a simple solvothermal process [73]. In the first step of experimentation, a transparent solution was formed by mixing waste-expanded polystyrene foam (WEPS) in dichloromethane followed by addition of HNO₃ in different ratios. The mixture thus obtained on heating in Teflon lined autoclave at 200 °C for 5 h, the solvent evaporation afforded CDs. In another report utilizing solvothermal method, Perikala and Bhardwaj [74] reported CDs synthesized from plastic wastes like gloves and masks [74]. Plastic waste after thoroughly washed with soap/detergent water was heated at around 250 °C in non-coordinating solvent octadecene in a closed round bottomed bottle. CDs formation was indicated by change in color from colorless to dark brown which were purified from ethanol. Polymer phosphors of CDs were synthesized for further applications. In seminal report employing solvothermal method, Ramanan et al. [75] synthesized nitrogen-doped multi-functionalized CDs from waste-expanded polystyrene via one step solvothermal method [75]. Ethylenediamine (EDA) was used as surface passivation agent to introduce the amine functionality to CDs because bare CDs obtained from polystyrene foam did not have detectable PL spectra [76]. Tour and co-workers developed an interesting approach for the upcycling of plastic waste into flash graphene (FG) utilizing flash joule heating (FJH). This process results in the formation of carbon oligomers, hydrogen, and light hydrocarbons in addition to FG. This process requires no catalyst and is suitable for PW mixtures. The production of 1 g of virgin PET requires 38.8 kJ of energy [77]; however, treatment of PW using this protocol consumes only 23 kJ for the upcycling of tFG.

Characterization of Carbon Dots

CDs are generally characterized by various techniques such as fluorescence spectroscopy, TEM, XRD, FTIR, UV–Vis spectroscopy, Raman, and XPS as briefly discussed in this section. CDs reported by Ramanan et al. [75] possessed spherical morphology with the diameter of ~ 4.0 nm as confirmed by TEM shown in Fig. 4a and in Gaussian fitting (Fig. 4b), 1.4 value of FWHM implies that the distribution of particles is narrow [75]. Figure 4c illustrates the XRD spectrum of a broad peak at 25° indicates 002 plane and 0.36 nm d-spacing, more than that of bulk graphite, implies less crystallinity of CDs indicating sp³ defects of carbon-based materials. In the Raman spectrum, two peaks at 1364 cm⁻¹ and 1559 cm⁻¹ imply D and G band of carbon respectively (Fig. 4d). The intensity ratio of D to G band (I_D/I_G) is 2.12 which indicates a high degree of defects [76].

Fig. 4.

a TEM image of CDs. b Particle size distribution. c XRD of CDs. d Raman spectrum of CDs [75]. Reprinted (adapted) with permission from Ramanan et al. [75]

Copyright 2017 American Chemical Society

The size of CDs reported by Kumari et al. [28] was found to be 2.5 nm as confirmed by TEM [28]. Some sharp peaks in XRD of the pyrolytic residue of plastic waste (WP) disappeared when WP residue was converted into fluorescent CDs. Broad peak at 28° in XRD of CDs indicates its amorphous nature as shown in Fig. 5a. FTIR spectra for WP residue and CDs are given in Fig. 5b. The IR of CDs shows four new peaks at around 3200 cm⁻¹, 1615, 1126, and 640 cm⁻¹. Peak at 3200 cm⁻¹ can be assigned to hydrogen-bonded OH groups whereas peaks at 1615, 1126, and 640 cm⁻¹ confirm the existence of C = O, C-O, and O-C-O groups. Moreover, the peaks of C–H at 2924 and 2850 cm⁻¹ observed in WP get decreased significantly in CDs indicating thermo-oxidative degradation of WP.

Fig. 5.

a XRD and b FTIR of pyrolytic residue (WP) and CQDs [28]

Adapted from Kumari et al. [28] with permission from Elsevier

By simply heating, polypropylene (PP) plastic waste was converted to polymer CDs by Aji et al. [78]. These CDs were found to be photoluminescent and their photoluminescent intensity was dependent on heating temperature. The size of synthesized CDs was also temperature dependent. An average particle size of CDs was ~ 15 nm at 200 °C, ~11 nm at 250 °C, and ~8 nm at 300 °C showing a decrease in size with increase in temperature. Figure 6a shows the FTIR spectrum indicating the presence of surface groups such as –C = O, –OH, –C–H, and –C–O which were not initially present in raw plastic waste. With these different functional groups, CDs can further bind to polymers, proteins, organic molecules, and DNA that change the properties of the CDs through covalent and non-covalent bonding [79]. In Fig. 6b, the emission spectrum of CDs synthesized from plastic waste is presented. The synthesized CDs emit two emission wavelengths centered at 410 nm and 440 nm along with another emission peak at 465 nm. The process of emission has higher energy during recombination as compared to the adsorption process. These electronic transitions arise from the fully charged orbital by the HOMO electron to the LUMO. Generally, one peak emission is observed but two emission peaks were observed due to stimulated emission where two electron emissions took place. These two electrons undergo recombination process by photon from UV rays via CD polymer.

Fig. 6.

a Ultraviolet emission spectrum and b FTIR of polymer CD [78]

Adapted from Aji et al. [78] with permission from Elsevier

Optical Properties

The absorption characteristics of CDs generated from diverse carbon sources or utilizing distinct synthetic techniques are always different. The extent of conjugated domains, the categories and number of surface groups, the proportion of oxygen/nitrogen in carbon cores, and other factors affect the absorption characteristics of CDs [80]. In both theory and practice, PL is one of the most attractive characteristics of CDs. The PL property of CDs refers to their ability to emit light upon excitation by a light source, typically ultraviolet or visible light. The emission wavelength depends on various factors such as the size, surface chemistry, and composition of the CDs. The fluorescence of CDs is among the most significant characteristics, that has an impact on how many different areas use them. CDs have exceptional fluorescence properties, comprising constrained emission spectrum, size-dependent fluorescence emission, wide excitation spectrum, up-conversion luminescence, excellent photobleaching resistance, and outstanding fluorescence stability [81-84]. CDs usually exhibit intense optical absorption in the UV region (230–320 nm) with a gradual decrease in absorption extending into the visible range. The carbon core of the CDs exhibits a prominent peak at around 230 nm, which is attributed to the π-π* transition of the aromatic C–C bonds. Additionally, a shoulder at 300 nm is observed, which is believed to result from the n-π* transition of C = O bonds or other associated groups. These deviations in the absorption spectra suggest variations in the compositions or structures of different hybridization derivatives [85]. Mhetaer et al. (2017) found that the position of the fluorescence emission peak varies depending upon the class of organic solvent used to recover CDs from waste polystyrene foam [81]. It was believed that different organic solvents’ surface passivation of CDs would produce distinctive physical flaws and introduce new sites of emission, resulting in variations in the PL range of CDs’ apex locations and energies. CDs are more widely used than other fluorescent materials because of their excellent biocompatibility, superior light stability, stronger QY, reduced toxicity, and numerous low-cost sources materials such as cadmium/lead containing traditional quantum dots, organic dyes, and rare-earth nanomaterials. CDs synthesized by Kumari et al. [28] from polyolefin waste were green fluorescent and used for sensing application [28]. Hu et al. [25] reported CDs synthesized from waste polyethylene terephthalate (PET) with unique PL properties and used for detection of Fe³⁺ discussed later in this review [25]. Song et al. used WEPS (waste-expanded polystyrene foam) to synthesize solid CDs using the solvothermal method which showed tunable PL. Altering the volume of solvent (HNO₃), he synthesized three types of CDs’ powders named white, yellow, and orange CDs. The fluorescence emission peaks of W-CDs, Y-CDs, and O-CDs are at 470, 530, and 630 nm, respectively. The PL quantum yields (QYs) of the solid-state W-CDs, Y-CDs, and O-CDs were measured to be 5.2%, 3.4%, and 3.1%, respectively, under 365 nm excitation [73]. Ramanan et al. [75] reported nitrogen-doped multi-functionalized CDs synthesized from waste-expanded polystyrene and it exhibited a strong blue emission when exposed to 365 nm UV radiation. At excitation of 380 nm, the emission was maximum at 456 nm. It was observed red shift of PL maxima with increase in excitation wavelength [75]. The QY of CDs synthesized from plastic polybag, plastic bottles, and plastic cups was 62%, 64%, and 65% at excitation of 310 nm.

Applications of Carbon Dots

CDs generally have surface groups like -OH, COOH, and -NH₂ which can be further functionalized enhancing their applications in various fields. CDs have wide applications in drug delivery, bioimaging, sensing, optoelectronics, radionuclide, energy storage, photocatalysis, and many other fields [86, 87]. This purpose of converting non-degradable plastic waste to valuable materials proves to be a breakthrough for environmental sustainability. Applications of plastic-derived CDs are discussed below in different sub-sections, such as (i) for sensing of metal ions and anions as well as microorganism, (ii) catalysis, (iii) in agronomics, and (iv) LED lights.

Sensing

CDs have been employed as sensing probe due to its well-defined fluorescent properties. In general, the sensing mechanism of CDs is based on the interaction between the surface of CDs and the analyte molecules or ions in the surrounding environment. By selectively functionalizing the surface of CDs with specific molecules or polymers, the sensitivity and selectivity of CDs-based sensors can be enhanced, allowing for the detection of various analytes with high accuracy and sensitivity. CDs generally have excitation-dependent emission and the alter in fluorescence properties can occur via different mechanisms—fluorescence resonance energy transfer (FRET), inner filter effect, photoinduced charge, and electron transfer [88]. Heteroatom-doped CDs enhance the fluorescence and thus sensing ability of CDs [89].

In FRET mechanism (Fig. 7A), the energy of excited fluorescent donor molecule is transferred to acceptor molecule via non-radiative dipole–dipole interactions due to which there is quenching observed in fluorescence emission of donor and acceptor fluorescence emission is increased. Depending on the excitation and emission wavelength of CDs, they can act either as donor molecule or acceptor molecule [90].

Fig. 7.

Quenching of fluorescence intensity of CDs in presence of metal ions via various mechanisms; A FRET, B IFT, C ET, and D CT [47]

Modified from Sharma and Das [47] with permission from Springer Nature under Creative Commons CC BY license

In the case of CDs, the inner filter effect (IFE) can occur when the excitation and emission wavelengths of CDs overlap with the absorption spectra of the sample matrix. For example, in a complex sample matrix containing a high concentration of organic molecules or other fluorophores that absorb light in the same range as CDs, the IFE can significantly reduce the effective excitation intensity of CDs and decrease the fluorescence emission intensity (Fig. 7B). The shift of conduction band electrons of CDs to the low-lying vacant d orbitals of metal ions can occur through a process called charge transfer (CT) (Fig. 7D). CT is a phenomenon where electrons are transferred from one species to another due to differences in their energy levels. In the case of CDs-metal ion complexes, the CT can occur between the π-electron-rich surface of CDs and the low-lying vacant d orbitals of metal ions, resulting in the formation of a new energy level. This new energy level can affect the electronic and optical properties of CDs, leading to enhanced fluorescence emission, photocatalytic activity, and sensing capabilities [47].

The interaction of CDs with metal ions can also lead to the transfer of electrons from the conduction band of CDs to the conduction band of metal ions shown in Fig. 7C. This transfer of electrons is called electron transfer (ET) and can result in the formation of CDs-metal ion complexes with unique electronic and optical properties. In the ET process, the energy levels of the conduction bands of CDs and metal ions should be well matched, with the conduction band of CDs being higher in energy than the conduction band of metal ions. This energy difference drives the flow of electrons from the higher energy level of CDs to the lower energy level of metal ions [91]. CDs synthesized from plastics have been used for sensing of metal ions which include Cu²⁺, Fe³⁺, Au³⁺, anions (SO₃²⁻), and E. coli bacteria. For the sensing of metal ions, PL responses of CDs are monitored in different metal ions solutions. The sensitivity is effective for the metal ion in which maximum quenching is observed.

Sensing of Cu (II)

Copper is used in electrical, electronic, and metallurgical industry, and hence has a high chance of accumulation in water bodies resulting in water pollution. The fluorometric detection of Cu²⁺ by CDs is quick and sensitive approach. Kumari et al. [28] synthesized CDs from polyolefin waste, using ultrasonic-assisted chemical oxidation in a simple one-step hydrothermal approach [28]. The green fluorescent amorphous CDs obtained have been reported to efficiently sense the Cu²⁺ ions with a remarkable sensitivity and selectivity limit of 6.33 nM and linear detection range of 1–8.0 µm. Moreover, these CDs have the potential to be used for triple negative breast cancer cell (MDA-MB 468 cells) imaging. The detection of Cu²⁺ from different water sources is depicted below. There is a significant decrease in PL intensity with increase in the concentration of Cu²⁺ ions as shown in Fig. 8b and c.

Fig. 8.

a Images under UV lamp (365 nm) for different concentrations of polyolefin CDs. Decrease in PL intensity of CDs with increase in [Cu²⁺] ions in different water sources: b mineral water and c tap water [28]

Adapted from Kumari et al. [28] with permission from Elsevier

Sensing of Ferric Ion, Fe (III)

Ferric ion (Fe³⁺) is an important component of different biological processes such as for the transfer of O₂ as well as the transfer of electrons. However, an irrelevant amount of Fe³⁺ in the human body shows equal adverse effects. It can cause liver injury, heart disease, and many other diseases. Hu (2019) [25] adopted low-cost and green process for the production of CDs from waste polyethylene terephthalate (PET) via air oxidation followed by hydrothermal method. The resultant CDs showed selective and sensitive detection of Fe³⁺ through a PL quenching “ON–OFF” mechanism. In this mechanism, when a specific metal ion is combined with a ligand or the surface group present on the CDs in the aqueous solution, the fluorescence intensity of CDs is lowered owing to the formation of metal-CD complex (Turn-On). However, when any other ligand or the surface group is externally provided to the metal-CD complex solution which has more binding affinity for metal ion, then the metal ion can be desorbed from CD surface and the CDs’ fluorescence is regained (Turn-off).

These reported CDs show good affinity with Fe³⁺ thereby giving rise to CD- Fe³⁺ complex. The fluorescence of CDs quenched by Fe³⁺ could be regained by utilizing PPi (pyrophosphate anion) off–on mechanism as shown in Fig. 9. The LODs of Fe³⁺ and PPi were 0.21 (linear range 0.5–400 μM) and 0.86 μM (linear range 2–600 μM), respectively. The developed sensors have found good applications in real water samples and human urine samples.

Fig. 9.

Schematic diagram of Turn “on–off-on “sensing of Fe³⁺ and PPi by CDs synthesized from PET bottles [25]

Modified from Hu et al. [25] with permission from Elsevier

Reverse osmosis membranes considered plastic wastes, used for synthesizing blue N, S-doped CDs, worked as sensing probe for Fe³⁺ in tap and pond water with a limit of detection of 2.97 μM [92]. Chan and Zinchenko [93] synthesized N-doped CDs from waste PET bottles. Simple hydrothermal treatment (HT) of PET bottles does not result in fabrication of N-doped CDs. So PET aminolysis was done before oxidative hydrothermal treatment and the synthesized CDs were successfully used for sensitive detection of Cu²⁺ and Fe³⁺ (0–10 µM) [93].

Sensing of Au (III) Ions

Ramanan et al. [75] reported nitrogen-doped multi-functionalized CDs from waste-expanded polystyrene. The resultant CDs were utilized for the selective detection of Au³⁺ in aqueous medium with a LOD of 53 nm. Further, these CDs exhibit good PL, quantum yield, long shelf life in aqueous medium that were able to sense Au³⁺ ions in aqueous medium and LOD of 53 nm is achieved. The authors assumed that out of the four most probable mechanism, viz. (i) reductive electron transfer from CDs to Au³⁺, (ii) coordination-induced aggregation, (iii) FRET between CDs and Au³⁺, and (iv) lowered pH on the addition of Au³⁺, a Lewis acid, the resultant CDs here should undergo coordination-induced aggregation. The Au³⁺ ions can bind to the amino groups present on the surface of CDs and the resulting coordination leads to self-assembly of CDs which result in aggregation of CDs thereby quenching PL.

Sensing of SO3²⁻

Anions play a major role in the human body. The imbalance in the concentration of anions causes adverse effects on human health and the environment. Sulfite ions (SO₃²⁻) are extensively used as preservative, reducing and bleaching agent in the food industry; however, it could be harmful if exceeds its safety content of 10 mg/kg. Kumari et al. [94] developed highly luminescent CDs using single use plastic cups as core precursor having 59% QY. CDs show high thermal stability ranging from 150 to 600 °C. In the fluorescence spectrum shown in Fig. 10a, out of 12 anions, the emission intensity of SO₃²⁻ was minimum as compared to pure CDs and thus, these CDs provided effective and selective detection of toxic sulfite ions. The luminescence intensity decreases with increase in the SO₃²⁻ concentration in the presence of CDs (Fig. 10b). The detection limit of developed sensor was found to be 0.34 µm [94].

Fig. 10.

a Sensing of different anions using CDs. b Concentration variation plot of sulfite ions in the presence of CDs

Adapted from Kumari et al. [94]

Sensing of E. coli Bacteria

The quick identification of pathogens has received significant interest among researchers. In this series, Kumari and Chaudhary [95] reported the utilization of CDs derived from single-use plastic waste, for the recognition of E. coli bacteria. These obtained CDs possessed fine functionalities on the spherical surface which reflects high optical properties and a high quantum yield of 60 to 70%. These CDs were used to sense E. coli bacteria in aqueous medium with a limit of detection down to 10⁸ CFU/mL. In addition, these CDs possess high level of biocompatibility and non-toxicity towards blood components [95].

Catalysis

5-hydroxymethylfurfural (HMF) is an advantageous compound used in the field of fuels, pharmaceutical industry, and polymer industry. Previous research shows the conversion of fructose to 5-hydroxymethylfurfural (HMF) with the help of many heterogenous catalysts such as zeolites, metal oxides, ion exchange resins, and homogenous catalysts (H₂SO₄, HCl). CDs due to their nano-size and different surface functional groups can be used as effective catalysts. As an efficient pathway, Hu et al. [96] synthesized the CDs from plastic waste bottles made of polyethylene terephthalate (PET) using air oxidation method and explored the catalytic efficiency of synthesized CDs for the preparation of HMF from fructose with a yield of 97.4%. This was a quasi-homogeneous catalysis where CD acted as a solid acid due to various acidic groups (-OH, -COOH, -SO₃H) present on its surface shown in Fig. 11. The catalyst was effectively used for six cycles. This work of Y. Hu opened a new era in the field of biofuels [96].

Fig. 11.

Dehydration of fructose to HMF with CDs in [BMIM]Cl/ethanol

Kumar et al. [94] synthesized a metal free catalyst from waste single use plastic. The effectiveness of carbon synthesized from plastic waste (PWC) was assessed in terms of its ability to adsorb and catalyze the removal of two specific pollutants (dyes), brilliant green (BG) and eosin yellow (EY). The PWC demonstrated low adsorption capacity for both BG (7.41 mg/g) and EY (4.93 mg/g). However, when combined with peroxymonosulfate (PMS), the PWC was able to facilitate the degradation of the dyes. The investigation into the mechanism revealed that the PWC’s surface functional groups C = O and O–H played a significant role in activating PMS, resulting in the creation of species such as SO₄^•− and OH^•. Quenching experiments confirmed that SO₄^•− was the primary radical responsible for dye degradation. This study provided valuable insights into the use of a low-cost, waste-derived, metal-free catalyst for PMS activation, offering a more environmentally friendly approach to reduce organic pollutants in wastewater [97].

Agronomics

Seed nano-priming refers to a technique in which seeds are treated with nanoparticles to enhance their growth, yield, and stress tolerance. Liang et al. [98] have synthesized CDs from PET bottles via pyrolysis method. The synthesized CDs were used as a nano-priming agent for Pea (Pisum sativum). The findings from this study demonstrate the positive impact of seed priming with varying concentrations of CDs (ranging from 0.25 to 2 mg/mL) on seed germination rate, shoot and root elongation, biomass accumulation, and root moisture levels compared to the control groups. Additionally, the interaction between CDs and plants significantly enhanced seed antioxidant enzyme activity, root vigor, chlorophyll content, and carbohydrate content [98].

Chauhan et al. (2021) used the CDs synthesized from plastic wastes (e.g., bottles, cups, and polybags) and biowaste (e.g., coconut coir, pearl millet, and agar gel) to improve the germination abilities of four different types of seeds, viz. black chick peas (Cicer arietinum), mung beans (Vigna radiata), wheat (Triticum aestivum), and barley (Hordeum vulgare) [99]. These four seeds were sprouted first using distilled water and then dipped in the solution of different concentrations of CDs synthesized from polybags. Their germination was 100% as shown in Fig. 12a–d. In chickpeas and barley (Fig. 12e), the long-germinated roots with many small seminal roots were observed. Three and 5 seminal roots were observed in the case of wheat. The coleoptile length was more in the case of the chickpeas and wheat than in the barley and mung beans. The leaves were only emerged in mung beans study and so its chlorophyll content was also determined. It was found that it was maximum in the mung beans that were dipped in 3000 mg/L concentration of CDs. The analysis was done by calculating the different parameters, namely germination index, percentage inhabitation growth, Vigor Index I, and seed Vigor II. The germination rate increased with increase in the concentration of CDs. The best results were observed at the 3000 mg/L concentration of CDs. Biomass-derived CDs showed better result in comparison to the plastic-derived CDs in the germination of seeds. These results suggest that waste-derived CDs could be a promising and sustainable nano-priming agent for improving seed germination and seedling development in a cost-effective manner and could be beneficial for agricultural purposes.

Fig. 12.

Germination activity of a chickpeas, b barley, c mung beans, and d wheat at room temperature and e digital imaging of different parts of germinated seeds [99]

Adapted from Chauhan et al. [99] with permission from Elsevier

LED Lights

One of the methods for the production of CD-LED is to first synthesize CDs and then embedded into the transparent polymer to form CD phosphor. Perikala and Bhardwaj synthesized CDs from plastic wastes using the pyrolysis technique and then embedded these CDs into transparent polymer [74]. The polymer was prepared by mixing polydimethyl siloxane (PDMS) and SYLGARD 184 curing agent in a ratio of 10:1. The synthesized white CD-LED was found to be quite stable against photodegradation. With further advancement, Song and co-workers worked on WEPS to synthesize solid CDs using the solvothermal method which showed tuneable PL. During the synthesis along with WEPS and the solvent dichloromethane, HNO₃ is added in different ratio to form three batches of CDs powder showing different luminescent colors, viz. white, yellow, and orange. CDs’ powder was further used as phosphor on UV chips to fabricate LEDs of various colors [73]. Figure 13 shows the fabrication of yellow and orange CDs in visible and UV light derived from plastic waste.

Fig. 13.

a Photographs of white, yellow, and orange CDs in visible and UV light. b Solid state fluorescence [73]

Adapted from Perikala and Bhardwaj [74] with permission from RSC Advances under the license of Creative Commons (CC BY-NC 3.0)

Apart from these CDs, Li et al. (2021) worked on co-pyrolysis of cyanobacteria and plastic waste to prepare cyanobacteria-plastic composite carbon (CPC) which showed excellent performance in the removal of water contaminants mainly focused on the high adsorption of methylene blue (maximum 490 mg/g). This result provides a useful method for using dangerous microalgae and polypropylene plastic as afunctional carbon precursors for the adsorption of macromolecular pollutants [100]. Mehta et al. [101] used low-density polyethylene (LDPE) for the synthesis of fluorescence carbon (FC). FC obtained used for the detection of Cu²⁺ (LOD-86.5 nm) and composite FCs@CuO@TiO₂ used for H₂ production proving the photocatalyst activity of CDs [101]. This study opens a new approach to utilizing plastic waste in purifying waste water and energy production. Table 1 shows the details of CDs synthesized from different plastic wastes and their applications in different fields.

Table 1.

Details of CDs synthesized from various plastic wastes

Plastic	Solvent/catalyst	Method	Size	Surface groups	Applications	Ref
Plastic bottles, cups, polybags	Deionized water	Thermal calcination	4–50 nm	Alkene, ether, carbonyl	Seed germination	[99]
Disposable gloves, masks, PPE and plastic waste	Octadecene	Solvothermal carbonization	-	Carbonyl, alkene	Fabrication of white LED	[74]
Soft drink bottles	Dimethyl sulfoxide	Hydrothermal synthesis	12.9 ± 3.9 nm	C = O, C = N, C = C	Sensitive detection of Fe3+ and Cu2+ ions in aqueous solutions	[93]
Plastic cups	Deionized water	Thermal calcination	Less than 10 nm	-OH and -COOH)	Sensing of sulfite ion (SO32−) in different water sources (LOD 0.34 μM)	[94]
LDPE	H2SO4 + HNO3	Hydrothermal process	0.5 µm (sheet-like), 0.5–1 µm (large elongated structures)	Hydroxyl, C-H, C = O, OH/NH overlapping	Cu2+ sensing (LOD 86.5 nM) and H2 production	[101]
Lunch box	K2CO3 + cyanobacteria	Pyrolysis	-	-	Macromolecular pollutants absorption	[100]
RO membranes	H2O2	H2O2-assisted hydrothermal method	1.3–6.8 nm	C = O, OH, NH, C = N, C = C, C-S, C-N	Sensing of Fe3+ ions (LOD 2.97 μM)	[92]
Polyethylene terephthalate plastic bottles	Concentrated sulfuric acid	Air oxidation	3 nm	–SO3H, –COOH, –OH	Act as a catalyst for conversion of fructose to 5-hydroxymethylfurfural	[96]
Polybags, cups, and bottles	Sulfuric acid and ethanol	Thermal calcinations	5–10 nm	–COOH, –OH	Nanosensors for the fluorescence quenching of Cu2+ ion	[102]
Polybags, cups, bottles	Deionized water	Thermal calcination followed by hydrothermal	-	–COOH, –OH	Biosensor for E. coli (LOD 108 CFU/mL)	[95]
Plastic waste	-	Flash Joule heating	16 nm	Pure carbon	High-quality graphene	[103]
Water bottles	H2O2	Air oxidation followed by hydrothermal	6 nm	Hydroxyl, epoxy, carbonyl, and carboxylic acid	Sensing of Fe3+ ions (LOD 0.21 μM) and PPi (LOD 0.86 μM) in human urine and in real water sample	[25]
Expanded polystyrene foam	Dichloromethane and nitric acid	Solvothermal method	60 nm	Aromatic C = C	White/multi-color LEDs	[73]
Polypropylene	Ethanol	Simple heating	< 20 nm	carbonyl	-	[78]
Polyolefin	H2SO4 + HNO3	Pyrolysis	2.5 nm	Aliphatic hydrocarbon, a carbonyl group	Sensing of Cu2+ (LOD 6.33 nm)	[28]
Expanded polystyrene	Chloroform and EDA	Solvothermal method	4 nm	Hydroxyl and amine	Detection of Au3+ ions (LOD 53 nm) and in vitro cytotoxic study on L132 cells	[75]
PET bottles	Ferrocene	Thermal dissociation under autogenic pressure	20 nm	-	Transformation of flat graphene to fullerene	[15]

Biocompatibility and Cytotoxicity

CDs have numerous uses in the realm of bioanalysis due to the superior optical characteristics. CDs also meet the fundamental criteria for in vivo imaging of cells and tissues, having low cytotoxicity and strong biocompatibility [104-106]. CQDs have received attention due to their water solubility, specific optical properties, effective biocompatibility, high stability, and have certain functional properties without cytotoxicity [107]. Biocompatibility refers to safety of CDs towards living cells. Graphene quantum dots have shown low cytotoxicity which were due to their small size and prominent oxygen content as revealed by in vitro studies [108]. Quantum yield and photosynthetic rate in mung bean sprouts can be increased in the presence of CDs at optimal concentration which can also be attributed to its biocompatible nature [109]. No accumulation of graphene quantum dots was observed in in vivo biodistribution assay of the main organs of mice [110]. Das et al. [111] studied the effect of CD’s on L6 (rat muscle cell line), HeLa (human cervical cancer cell line), PC3 (human prostate cancer cell line), and MDAMB 231 (human breast adenocarcinoma cell line) cell lines to assess the cyto-compatibility of the as-synthesized CDs [111]. While CDs are generally considered to be biocompatible and non-toxic but some studies revealed their cytotoxic effects on cells which is induced by disrupting the cell membrane or by inducing DNA damage. Wang et al. [112] revealed that the interventions of Pollen typhae CDs prominently reduced the extent of inflammatory reactions and oxidative stress, which may be associated with the basial potential mechanism of anti-AKI activities[112]. Cancerous cell line of HT1080 was used to study the impact of CDs for the availability of anti-cancerous CQDs to further increase the efficacy of anti-cancerous drugs[109]. Liu et al. (2021) investigated that on irradiating CDs, cytotoxicity increased with longer irradiation time and the production of reactive oxygen species accounted for the light-induced cytotoxicity[113]. In a vitro cytotoxic study, the cell viability of CDs produced from waste-expanded polystyrene on human normal lung cells (L132) was found to be more than 95% making it fit for bioimaging applications. The similar results were obtained on human lung cancer cell line (A549) [75]. Kumari and Chaudhary [95] synthesized CDs from single-use plastic have shown a remarkable level of biocompatibility and do not have any toxic effects on blood components, as confirmed by various assays such as blood clotting, hemolysis, in vitro stability, anticoagulant, thrombolytic activity, and total antioxidant assays [95]. The available evidence strongly suggests that CDs have immense potential for both in vitro and in vivo imaging studies.

Conclusion and Future Outlook

This review highlights the utilization of plastic wastes by converting them into valuable CDs. A summary on the diverse applications of plastic-derived CDs for sensing, catalysis, and LED fabrications is herein presented. The sensitivity and selectivity of the CDs have been reported to be quite remarkable. Different surface passivating agents can be added during synthesis to fabricate desired doped CDs. Doping of heteroatom increases the optical properties of CDs which can enhance the sensitivity for heavy metals. In addition, different composites for CDs can be synthesized for energy production or storage. Successful fabrication of white and multicolored LEDs from plastic CDs opens diverse future research and development opportunities highlighting the energy storage portable device and reduces the issue of global energy crisis. As discussed that the degradation of plastic wastes in a natural manner requires a very long time, so its conversion seems to be one of the most viable routes for their efficient utilization. Although some research groups have reported their applications, yet this area is still in its infancy. Furthermore, the catalytic applications of these CDs are still underexplored and these can be exploited for other organic transformations as well. Apart from the applications, the degradation routes for plastic wastes may also be explored. In nutshell, this focus review will be beneficial for the scientific community working in the area of nanomaterials, chemists and environmentalists.

Funding

Financial assistance by University Grants Commission (UGC), New Delhi, India, through Junior Research Fellowship (JRF) (UGC Ref No: 114/(CSIR-UGC NET JUNE 2017) to Ms Arpita. The authors are highly grateful to Department of Science and Technology, Government of India for providing the financial assistance under DST-PURSE scheme (SR/PURSE/2022/126).

Data Availability

Not applicable.

Compliance with Ethical Standards

Conflict of Interest

The authors declare that they have no conflict of interest.

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by any of the authors.