The intersection of nanotechnology and urban agriculture: applications of carbon dots

Abstract

Amidst the ongoing urbanization process, the significance of urban agriculture has garnered increasing attention. With the expansion of urban population, the urban landscape broadens, and agriculture becomes more commercialized, the connection between the city and agriculture is gradually strengthened. Consequently, there is an urgent need to explore efficient, environmentally friendly, and sustainable modes of urban agricultural production. Carbon dots (CDs), emerging as a novel fluorescent nanomaterial, exhibit remarkable biocompatibility, nanoscale dimensions, low toxicity, and photoluminescence properties. These characteristics render CDs capable of safely and effectively enhancing photosynthesis efficiency, thus revealing immense application potential in the realm of urban agriculture. This paper delves into the characteristics and functionalities of CDs in various agricultural contexts, including plant photosynthesis, plant transport and storage systems, as well as biosecurity considerations. Furthermore, it explores the applications prospects of CDs in urban agriculture, aiming to provide robust support for the advancement of this vital sector.

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Environmental significance

As the urban populace expands, the urban landscape broadens, and agriculture becomes more commercialized, the nexus between the city and agriculture is gradually strengthening. Consequently, there is a pressing need to explore efficient, environmentally friendly, and sustainable modes of urban agricultural production. In this context, plant factories with artificial lighting represents an innovative and promising production system, demonstrating significant potential for stable and effective agricultural production amidst the swift pace of urbanization. However, within the plant factory energy consumption framework, lighting constitutes a significant proportion. This high reliance on artificial lighting, underscores the need for efficient energy management and optimization strategies in this domain. Carbon dots (CDs), emerging as a novel fluorescent nanomaterial, exhibit remarkable biocompatibility, nanoscale dimensions, low toxicity, and photoluminescence properties. These attributes render CDs capable of safely and effectively enhancing photosynthesis efficiency, thus revealing immense application potential in the realm of urban agriculture.

1. Introduction

1.1 History, development, and current status of urban agriculture worldwide

Since the mid-point of the last century, urban agriculture has garnered increasing attention from the global community. As far back as the late 19th and early 20th centuries, a British urban scholar Ebenezer Howard introduced the concept of the “Garden City”.¹ In the seminal book, “Agricultural Economic Geography”, published in 1935, the Japanese geoeconomist Shiro Aoshika introduced the concept of “urban agriculture”.² In 1957, the agricultural economist Thompson formally introduced the concept of urban agriculture in his paper titled “Urban Agriculture in Southern Japan”.³ With the acceleration of industrialization and urbanization, agriculture in urban and suburban has undergone profound transformations. To adapt this transformation, agriculture has embarked on a path of intensification, facilities-based production, and industrialization.⁴ In order to cater to the survival and growth of contemporary urban landscapes, urban agriculture has become a pivotal component.^5–7

Urban agriculture is a kind of modern agriculture which is formed in the suburbs of the city and within the economic circle of the city to meet the needs of the survival and development of the modern metropolis.⁸ Based on the agricultural scale, this practice can be categorized into four distinct types: greenhouse farming, plant factories, community agriculture, rooftop framing (Fig. 1). These categories not only serve diverse functions but also contribute significantly to various aspects of urban life. These functions include agricultural production, enhancement of urban landscapes, maintenance of ecological balance, promotion of cultural and educational activities, stimulation of social economy, and provision of leisure and entertainment facilities (Fig. 1).^9–11

Fig. 1 Comparison of traditional agriculture and urban agriculture.

As the urbanization process has advanced in Europe, America, Japan, Singapore, and other nations, agricultural scientists and urban geographers in these regions have consecutively conducted research on urban agriculture. As its underlying principles and significance have continuously been refined and expanded, the concept of urban agriculture has gradually gained widespread acceptance worldwide.⁹

Since the early 1970s, the emergence and advancement of high technology and novel technologies have propelled the urban agricultural sector in Europe, Japan, and other developed nations towards increasing maturity.³ Urban agriculture in Germany originated in 1919, primarily manifesting as “citizen gardens”.¹² In the 21st century, Germany has enacted an array of laws and regulations, thereby establishing the legitimate status of urban agriculture and delineating the development paradigm for civic farmers.¹² Currently, “citizen farm parks” has become a crucial component of German agriculture, with their aggregate output value comprising approximately one-third of the overall agricultural output value in Germany.¹³ After nearly five decades of progress, Japanese urban agriculture has attained remarkable advancements. Urban farmers constitute approximately 25% of the total farming population in Japan, while the operational farmland area accounts for 27% of the total cultivable land, particularly notable for the high proportion dedicated to the cultivation of vegetables, fruits, and flowers.^2,14,15 As a city-state, Singapore is renowned as the “garden city”. Despite its limited agricultural background, the urban agriculture sector has emerged as a distinct, productive, and profitable industry.¹⁶ Singapore has successfully established a comprehensive urban agriculture system, which not only ensures adequate food supply for the city but also significantly contributes to improving the degree of food self-sufficiency.

Urban agriculture has attained a certain level of maturity in several developed nations; however, globally, its development in diverse countries continues to encounter numerous challenges. These challenges not only impede the productivity and profitability of urban agriculture but also pose a significant threat to its sustainable growth.

1.2 The principal challenges in the advancement of urban agriculture

As the global population continues to expand and the expenses associated with transporting artificial fertilizers, pesticides, agricultural resources, and food itself worldwide increase, urban agriculture has emerged as a viable and beneficial complement to traditional agriculture.¹⁷ Currently, approximately 15 to 20% of global food production occurs within urban or suburban boundaries.^18,19

However, despite its numerous benefits, urban agriculture inevitably encounters several challenges in its development process: (1) the cost of production inputs remains significant, encompassing expenses related to land acquisition, labor force utilization, and irrigation water usage. Given the constraints on labor, irrigation facilities, and land resources in urban environments, the production costs of urban agricultural products tend to be relatively high.^20,21 (2) Pertaining to environmental risks, these primarily encompass soil contamination, aquatic pollution, outbreaks of diseases and insect infestations, as well as associated public health hazards. Urban agriculture necessitates significant quantities of water resources and energy inputs, yet concomitantly, agricultural waste generated during production processes poses potential environmental implications. (3) The advancement of urban agriculture is hindered by inadequate urban planning strategies that fail to adequately account for the versatility of urban agricultural practices. This oversight is compounded by the neglect of the multifaceted benefits-beyond mere food provision-offered by urban agriculture, including the preservation of cultural heritage and agricultural biodiversity, which contribute significantly to social well-being and sustainable urban development.^22,23 (4) Two primary challenges that must be addressed in the context are technological innovation and energy consumption. Science and technology serve as the vital backbone and cornerstone of urban agriculture, yet the foremost technical hurdle encountered by this sector pertains to the reduction of energy consumption.²⁴

Amidst the myriad challenges posed by rapid urbanization to urban agriculture production, ensuring a stable supply of agricultural products to urban areas relies not only on the achievement of efficient and stable output and balanced supply from farming systems but also on shortening distribution chains to guarantee continuous product availability and accessibility.²⁵ In this context, plant factories with artificial lighting represents an innovative and promising production system, demonstrating significant potential for stable and effective agricultural production amidst the swift pace of urbanization. Plant factories embody a multi-tiered indoor agricultural system that integrates green and sustainable crop cultivation technologies, encompassing vertical farming, optimized lighting recipes, energy-saving technologies, and intelligent control systems.^26–29 These features enable agricultural production to transcend the constraints of climate and geography. Furthermore, rigorous environmental control and the enclosed space effectively prevent pests and diseases from infiltrating, which reduces pesticide and fertilizer applications.²⁹ By producing fresh crops in an environmentally friendly manner, plant factories have exhibited robust adaptability and immense potential in tackling the most formidable challenges confronting urban agricultural science, including population growth, water scarcity, soil resource constraints, food safety, and supply chain disruptions.^27,30 Thus, plant factories undeniably stand to play a significant role in spearheading the agricultural revolution, safeguarding food security, contributing to carbon neutrality, and driving the development of humanity towards a brighter future. However, within the plant factory energy consumption framework, lighting constitutes a significant proportion, accounting for approximately 80% of the total power energy consumption and approximately 25% of the overall cost. This high reliance on artificial lighting, specifically LEDs (light-emitting diodes), underscores the need for efficient energy management and optimization strategies in this domain.^24,31 Hence, there remains a significant journey ahead in terms of reducing the costs and enhancing the efficiency of plant factory.

2. The significant impact of light sources and light quality on the development of plant factory

The advancement of nanotechnology presents novel opportunities to modify plant responses to light and enhance the efficacy of light absorption. Carben dots (CDs), an emerging class of carbon nanomaterials, possess distinctive characteristics. Their potential as regulators of plant photosynthesis arises from their capacity to emit red and blue light, which coincides with the primary absorption regions of plant chlorophyll.³²

Extensive research demonstrates that ultraviolet (UV), blue, red, far-red, and green light, all of which play crucial roles in various physiological and biochemical processes within plants.³³ The composition of the spectrum determines the characteristics of light quality. Among this, red light (622–760 nm) and blue light (435–470 nm) stand out as the two primary types of light quality that play a pivotal role in driving photosynthetic biosynthesis.³⁴ Exposure to red light positively affects the accumulation of photosynthetic pigments, enhances plant elongation, and promotes the accumulation of photosynthetic products.^35–37 Blue light can stimulate chlorophyll biosynthesis, modulate stomatal aperture and chloroplast maturation, and further enhance the accumulation of secondary metabolites, including phenols and flavonoids, thereby exerting profound effects on plant growth and development.^38–41 The utilization of monochromatic red or blue light alone is inadequate for supporting the requirements of typical plant growth, and any absence of these two specific light quality types results in diminished photosynthetic efficiency.⁴² Extensive research has demonstrated that the admixture of red and blue light in an optimal ratio can significantly enhance plant growth and photosynthesis.^42–44 Therefore, the red-blue composite light is considered to be the most effective light source for promoting healthy plant growth.⁴⁵ The regulation of the light source has greatly affected the development of plant factory.

Urban agriculture, whether utilizing natural or artificial lighting, has emerged as a viable solution to address the challenge posed by land scarcity.⁴⁶ In contrast to traditional agricultural, urban agriculture exhibits superior production efficiency, with light sources and light quality playing a pivotal role in its successful implementation.⁴⁷ Regarding the selection of lighting sources, urban agriculture, particularly exemplified by plant factory, predominantly relies on artificial illumination systems, encompassing LEDs, high-pressure sodium lamps, and fluorescent lamps (Table 1). These lighting systems facilitate the achievement of higher crop yields within shorter timeframes, effectively elongating the growing season to span an entire year, ultimately encompassing 365 days.⁴⁸ Nevertheless, light sources such as high-pressure sodium lamps and fluorescent lamps consume a large amount of electrical energy and may cause light stress in plants to some extent, which ultimately makes them inefficient for optimal growth.⁴⁹

Table 1 The impact of different light source on the plant growth

Light source		Wavelength	Advantages or disadvantages	Effect of plant	Ref.
Natural light source	Sunlight	Ultraviolet (UV) (320–400 nm)	Affected by season and weather	Provide various wavelengths for plant growth	50
		Blue (400–500 nm)
		Green (500–600 nm)
		Red (600–700 nm)
		Far-red (700–800 nm)
Artificial light source	High-pressure sodium lamps	Yellow (565–590 nm)	No blue light	Inhibition of hypocotyl elongation	51
		Orange (590–625 nm)
		Red (625–700 nm)
	Fluorescent lights	Red (635–700 nm)	Low light intensity	Reduced the risk of heat damage to plants	50
		Blue (400–499 nm)
	LEDs	250–1000 nm (adjustable, usually red and blue)	High luminous efficiency, low power consumption, complete spectral distribution and increased the use efficiency of space	Adjustabled wavelength promotes plant photomorphogenesis	52

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LED technologies have played a pivotal role in this context. In contrast to traditional light sources, LED technology offers a homogeneous and efficient production setting at low costs. This is attributed to their small size, low lamp surface temperature, high light utilization efficiency, and wide light spectrum. The ability to select and control light intensity and wavelength enables the production of highly functional and cost-effective plant products.⁵³ Particularly, by modulating the light spectrum, it is possible to regulate light transduction mechanisms and control specific plant traits such as flowering induction, elongation, branching, secondary metabolites, nutrient status, seedling development, and cell growth⁵⁴ (Table 2). Consequently, it provides a suitable lighting solution for plant factory. However, the adjustable wavelength capability of LED technology necessitates sophisticated materials, technology, and equipment support, resulting in higher costs at present.^33,55

Table 2 The impact of different LED light sources of different wavelengths on the plant growth

Light quality	Wavelength	Photosynthetic photon flux density (PPFD)	Plant species	Effect	Ref.
Red	660 nm	300 μmol m^−2 s^−1	Cucumber (Cucumis sativus L.)	Enhanced accumulation of P, K, Mn and Zn, reduced numbers and size of chloroplast and starch grain	56
Blue	440 nm	300 μmol m^−2 s^−1	Cucumber (Cucumis sativus L.)	Blue light exhibited more effective in maintaining chloroplast ultrastructure than red light	56
Red	660–665 nm	250 μmol m^−2 s^−1	Ginkgo biloba	Accumulated more photosynthetic pigments and increased the content of total flavonoids and flavonoids per plant	57
Blue	460–465 nm
Red	600–699 nm	250 μmol m^−2 s^−1	Eggplant	Improved the morphological index, biomass accumulation and energy yield	58
Blue	400–499 nm
Red	665 nm	95 ± 5 μmol m^−2 s^−1	Pepper (Capsicum annuum L.)	Blue light increased the photosynthetic performance under water deficit	59
Blue	445 nm
Red	660 nm	200 μmol m^−2 s^−1	Potato	Increased the synthesis of carotenoids	60
Blue	445 nm
Red	653 nm	Low 200 μmol m^−2 s^−1	Lettuce	Red light had the highest QYinc under low PPFD, green light had the highest QYinc under high PPFD	61
Blue	446 nm	High 1000 μmol m^−2 s^−1
Green	523 nm
Red	662 nm	0–1800 μmol m^−2 s^−1	Tomato	Improved the photosynthetic activity in plants	62
Blue	446 nm
Red	660 nm	135.29 ± 1.46–191.50 ± 0.94 μmol m^−2 s^−1	Chrysanthemum (C. morifolium)	Promoted anthocyanin biosynthesis by the ratio of high red light and far red light	63
Far-red	730 nm
Red	660 nm	280 μmol m^−2 s^−1	Tomato	High R/FR light-induced root exudates play a key role in chemotaxis, biofilm formation and root colonization of S. plymuthica A21-4	64
Far-red	730 nm
Red	660 nm	200 μmol m^−2 s^−1	Cucumber (Cucumis sativus L.)	Red light increased Cd sensitivity, whereas blue light enhanced Cd tolerance	65
Blue	447 nm
Red	660 nm	150 μmol m^−2 s^−1	Carnation	Increased the vase life, flower numbers	66
Blue	440 nm
Red	660 nm	70–120 μmol m^−2 s^−1	Saffron	Increased flower induction	67
Blue	450 nm
Green	500–600 nm
Red	635–700 nm	200 μmol m^−2 s^−1	Strawberry	Increased plant biomass and fruit yield	68
Blue	450–490 nm
Red	650–670 nm	150 μmol m^−2 s^−1	Tomato	Increased biomass of shoots and leaf area, displayed significantly higher rates of photosynthesis	69
Blue	455–475 nm

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The current impediment that plant factory faces in its development is the significant energy consumption required to sustain LED artificial light in the system.⁷⁰ Furthermore, photosynthesis remains the sole mechanism for plants to transform the radiation they intercept into biomass.⁷¹ However, according to reported findings, the conversion efficiency of the C₃ plants remains approximately 0.1, whereas the conversion efficiency of C₄ plants stands at around 0.13, constituting merely one-fifth of their theoretical maximum.⁷² In the realm of urban agriculture, the optimization of the ratio between red and blue light during plant photosynthesis, along with the enhancement of the photosynthetic energy conversion efficiency, have emerged as two pivotal challenges that are imperative for fostering plant growth and enhancing overall crop yield.

Recent research has demonstrated that CDs exhibit notable stimulatory effects on plant growth.^73–75 Furthermore, CDs exhibit several noteworthy properties, including their minute size, tunable photoluminescence (PL), exceptional water solubility, biocompatibility, and insignificant toxicity.⁷⁶ Notably, their PL capabilities have the potential to significantly enhance photosynthesis and thereby boost crop yield by optimizing the utilization efficiency of solar energy. Consequently, CDs emerge as effective photosynthetic enhancers with promising applications in plant factory.⁷⁵

3. Structural characteristics of carbon dots

CDs, a novel class of fluorescent nanomaterials, possess sizes smaller than 10 nm.⁷⁷ In 2004, the fluorescent nanomaterials were initially synthesized by researchers through electrophoresis experiments, specifically designed for the separation and purification of single-walled carbon nanotubes.⁷⁸ Since that time, carbon nanomaterials exhibiting fluorescent properties have garnered significant attention. In 2006, the nanoscale carbon particles produced through laser ablation were formally designated as CDs, establishing a nomenclature that has been widely adopted in subsequent academic literature.⁷⁹

The CDs exhibit a nuclear-shell-like configuration arising from the hybridization of graphite carbon cores in the sp² state and the sp³ hybridization of carbon defects, which gradually form the matrix or shell through a nucleation process.⁸⁰ The carbon core typically comprises polycrystalline nanodomains, whereas the surrounding matrix encompasses an array of functional moieties, including hydroxyl (–OH), epoxy/ether (–O–), carbonyl (–CO–), and carboxylic acid (–COOH) groups.^81,82 These hydrophilic functionalities endow CDs with water-solubility, thereby facilitating their integration and utilization in various aqueous environments.^81,82 Later, the biocompatibility of the structure of its shell was enhanced.⁸³ Currently, carbon-based nanomaterials, specifically CDs, are primarily categorized based on the hybridization states of carbon atoms, namely sp² and sp³, as well as their distribution within the core or shell regions.⁸⁴ These CDs are primarily classified into three distinct types: carbon quantum dots (CQDs), graphene quantum dots (GQDs), and carbonized polymer dots (CPDs).⁸⁴ This classification scheme enables a more nuanced understanding of their physicochemical properties and potential applications in various fields (Fig. 2).

Fig. 2 Various attributes of carbon dots include size, types, structure and properties.

CDs possess distinctive physical and chemical attributes that have garnered significant attention among researchers. These attributes include fluorescence, phosphorescence, PL, water solubility, biocompatibility, resistance to photobleaching, and low toxicity.⁸¹ The origination of these properties can be traced to quantum confinement effects, edge defects, surface states, tunable band gaps, and the presence of multiple emission centers.⁸⁵ By carefully selecting carbon precursors, adjusting reaction conditions, employing diverse synthesis methods, manipulating the solvent properties, and controlling the solution pH, CDs can be tailored precisely to meet the demands of specific applications.^86,87 Up until now, a significant amount of literature has documented its diverse applications across various fields, including bioimaging, fluorescent sensing, cancer therapeutics, catalysis, and plant growth.^88–93

4. Optical properties of carbon dots

It was demonstrated that CDs exhibit a robust absorption broadband specifically ranging from 230 nm to 270 nm within the UV absorption region, which is attributed to the π–π* transition of carbon core electrons, whereas the observed weaker absorption in the visible and near-infrared regions is ascribed to the π–π* transition of surface state electrons.^94,95 Moreover, the interconnected chemical moieties potentially contribute to the absorption properties within the UV–visible spectral region.^96,97

CDs garnered initial interest owing to their intriguing photoluminescence properties. Typically, the precursor materials used in their preparation do not exhibit fluorescence characteristics; however, during the synthesis process, a photoluminescence center gradually emerges. Regarding the underlying mechanisms, several prevailing theories have been proposed, including the quantum size effect, surface state, molecular state, and cross-linking enhanced emission (CEE) effect.^98–101 When considering the quantum size effect and surface state mechanism, the emission wavelength of CDs exhibits a redshift phenomenon as its bandgap rows become narrower.¹⁰² It was discovered that the fluorescence properties of CDs underwent a transition, shifting from blue to near-infrared regions.¹⁰³ The molecular state mechanism implies that the luminescent characteristics of CDs are primarily attributed to their organic fluorophores, rather than the chemical functional groups or carbon cores residing on the surface.⁹⁹ Furthermore, in order to meticulously articulate the photoluminescence properties of nonconjugated carbonized polymer dots, a mechanism termed cross-linking-enhanced emission effect was introduced. This mechanism proposes that the immobilization of the rotation and vibration of subfluorophores located on the surface of carbonized polymer dots, achieved through the presence of a crosslinked backbone and/or carbon core, significantly enhances radiative transitions, ultimately intensifying the photoluminescence properties of CDs.⁹⁶ Moreover, CDs possess the remarkable property of upconversion luminescence, commonly known as up-conversion photoluminescence (UCPL), which involves the conversion of low-energy photons into high-energy photons, a unique characteristic exclusive to them.¹⁰⁴ The upconversion phenomenon exhibited by CDs typically encompasses mechanisms such as multiphoton excitation or energy transfer. As an illustrative instance, when near-infrared light (NIR) of lower energy serves as the excitation source for CDs, these nanoparticles are capable of emitting visible light of higher energy.¹⁰⁵ Upconversion PL is deemed effective due to its enhanced efficiency and remarkable capacity to penetrate deeply into tissues without inducing any damage to the surrounding tissues.¹⁰⁶ This attribute renders it a promising candidate for various applications in the biological and agricultural fields. In plant factories, a large amount of energy is used to power photosynthesis, while the characteristics of CDs enhancing photosynthesis is expected to provide new solutions to solve the bottleneck of plant factories-energy consumption (Fig. 2).

5. Carbon dots and photosynthesis

5.1 Transport and accumulation of carbon dots in plants

When delving into the biosafety of CDs, it is imperative to investigate the mechanisms underlying their transportation within plants. Presently, the absorption and translocation of CDs in plants, along with their potential implications on plant growth and development, have garnered considerable attention among researchers. A thorough examination of these facets will not only enhance our comprehension of the potential applications and associated risks of CDs in plants but also furnish a theoretical foundation for their potential widespread utilization in urban agriculture.

It was demonstrated that the pore size of plant cell walls exhibits significant variations across different species, with a range spanning from 1.6 nm to 6.2 nm.¹⁰⁷ The CDs fabricated through diverse methodologies typically exhibit variations in size falling below 10 nm, enabling them to traverse plant cells and be efficiently assimilated by the plant system when their size is smaller than the aperture of the cell wall.¹⁰⁸ This assimilation process subsequently exerts a profound influence on plant growth.

In addition, leveraging the photoluminescence properties inherent in CDs, the translocation of these nanomaterials within plants can be visualized through various techniques such as fluorescence imaging, transmission electron microscopy, and Raman spectroscopy and so on.^109,110 Through fluorescence microscopy, the presence of CDs was observed in the root crown cells, root cortex, vascular bundle, and mesophyll cells of maize seedlings that were treated with CDs.¹¹¹ This finding demonstrates the localization of CDs within specific tissues of the maize seedlings, providing valuable insights into their distribution and potential interactions within the plant system. Laser confocal microscopy imaging unequivocally verified the penetration of CDs through the intercellular spaces of the seed coat, resulting in their accumulation within the cotyledons.¹¹²

Plants transport water and nutrients efficiently through two distinct pathways: the symplast pathway, which involves the continuous cytoplasmic network of cells; the exoplasmic pathway, which utilizes the intercellular spaces outside the cell walls.¹¹³ Nonetheless, CDs can traverse not only via the exoplasmic pathway but also through the symplastic pathway, thus exhibiting a versatile mode of transportation.¹¹⁴ Studies on the uptake and transport of CDs show that they can penetrate plant cells and then transport along with water and nutrients from the roots to stems and leaves, which later accumulate in different regions of the plant.^115,116 Smaller-sized CDs exhibited a greater capacity for seamless transfer and penetration into cells.¹¹⁷ Within cells, plants absorb CDs via the exoplasmic pathway, traversing the cell wall and plasmodesmata, and subsequently penetrating the cortex to gain access to the xylem via the extracellular space.¹¹⁸ Utilizing laser confocal microscopy, imaging revealed that the CDs possess the capacity to adhere onto the surface of the root and subsequently infiltrate into the vascular bundle of root.¹¹² Subsequently, these CDs were transported from the root, through the vascular system, to the stem and leaf, ultimately being detected within the leaf veins.¹¹² The transportation of CDs from rice roots through the stems to the leaves was not only confirmed, but also subcellular localization revealed the presence of CDs within the nucleus.¹¹⁹

Apart from absorption through roots, CDs can also be absorbed via leaves through foliar spraying techniques. During the gas exchange process, two guard cells at the stomatal pore form an aperture with a width ranging from 3–12 μm and a length of approximately 10–30 μm.¹²⁰ This intricate anatomical structure facilitates the direct entry of CDs into the plant system through the stomatal pores, thereby enhancing the efficiency of CDs utilization within plants. CDs initially penetrate leaf cells via pores present on the leaf surface, subsequently undergoing translocation to other parts of the plant via the intricate vascular network within the leaf, and even descending through the phloem (Fig. 3).¹²¹

Fig. 3 Absorption and transport of CDs in plants.

Furthermore, the surface characteristics of CDs have the potential to significantly influence their translocation and dissemination within plant.¹²² Upon examination, it was observed that black solid material, identified as CDs, sporadically emerged at the apical and marginal regions of maize seedling leaves.¹¹¹ Therefore, during the process of translocation within plants, CDs penetrate the root system and primarily accumulate within the nucleus of the primary root meristem.¹¹⁵ The transportation of nutrients and substances to the leaves occurs primarily via the vascular system, with accumulation predominantly occurring within the veins, rather than within the mesophyll system, and a minute amount of precipitation phenomenon can also be observed in the leaves during the accumulation process.¹¹²

In summary, the distribution of CDs in plants is influenced by their physical structure and superficial properties, resulting in their occurrence in multiple anatomical regions including the roots, stems, and leaves. Hence, the investigation of the safety profile of CDs in plants holds significant implications for their utilization in agricultural production.

5.2 Current knowledge on photosynthesis modulated by carbon dots in plants

Photosynthesis, a fundamental process in plant biology, converts solar energy into chemical energy, thereby playing a pivotal role in promoting plant growth and enhancing biomass accumulation. This complex biochemical process is primarily categorized into two distinct stages: the photoreaction stage, which was occurring on the thylakoid membrane, encompassing the absorption and transformation of light energy, electron transfer mechanisms, and photosynthetic phosphorylation; the dark reaction process within the chloroplast matrix, primarily associated with the carbon assimilation process.¹²³ Photosynthesis primarily occurs within chloroplasts, utilizing chlorophyll as the key photosynthetic pigment crucial for capturing light energy, specifically exhibiting peak absorption in the red spectrum (640–660 nm) and blue-purple spectrum (430–450 nm).¹²⁴ Chlorophyll, upon excitation by light, efficiently transfers energy to the central pigment molecule, thereby initiating a photochemical reaction. This process involves the seamless transmission of absorbed light through intricate electron transport chains, ultimately reaching the photosystem-I (PS-I) and photosystem-II (PS-II) located on the thylakoid membrane.¹²⁵ This meticulous orchestration ensures the efficient conversion of light energy into chemical energy within the photosynthetic apparatus. The light reaction ultimately generates adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH), which serve as the energy sources for the dark reaction of photosynthesis, specifically for the fixation of carbon dioxide (CO₂).¹²⁶ This process, known as CO₂ assimilation, involves the utilization of energy provided by NADPH and ATP to convert CO₂ into carbohydrates, ultimately leading to the synthesis of various carbohydrate molecules.¹²⁶

The conversion efficiency of absorbed light energy remains significantly below its theoretical potential. Consequently, optimizing the light reaction process presents a viable avenue for enhancing photosynthetic performance. This optimization involves strategies such as enhancing the absorption and transformation capabilities of light, expediting the recuperation from non-photochemical quenching, and fine-tuning the enzymes involved in the Calvin cycle.¹²⁷ By implementing these improvements, it is possible to achieve a more efficient utilization of light energy, thereby bolstering photosynthetic efficiency.

CDs possess outstanding adjustable PL properties, and it has been reported that their luminous range encompasses nearly the entirety of the visible spectrum.¹²⁸ CDs, regarded as artificial optical antennas, capture ultraviolet (UV) and infrared light spectra unabsorbable by plants, converting them into visible light wavelengths with plant photosynthesis, enabling chloroplasts to efficiently capture and assimilate this converted light energy, thus enhancing the rate of photosynthetic electron transfer and ultimately boosting the overall efficiency of the light reaction process.^{92,124,128–130} Concurrently, CDs possess the dual functionality of serving as both electron donors and acceptors, thereby expediting the photosynthetic process via the facilitation of electron transfer initially to the chloroplast and ultimately to the light-harvesting complex (Fig. 4).¹³¹

Fig. 4 Gain effects of CDs in photosynthesis.

It was reported that recombined the synthesized blue fluorescent CDs with isolated chloroplasts, it is found that CDs can strongly conjugate over the surface of the chloroplast and transfer electrons towards chloroplasts by assistance of absorbed light or photons. CDs can modulate the electron transfer process to convert light energy into electrical energy and finally to the chemical energy as assimilatory power (ATP and NADPH), thereby elucidating the role of CDs in facilitating energy transfer to chloroplasts during the process of photosynthesis.¹³² The far-red CDs, synthesized from reduced glutathione and formamide, empower chloroplasts to harness UV-A radiation and transduce it into far-red light. This capability allows chloroplasts to directly and efficiently utilize far-red light, thereby facilitating the electron transfer from PS-II to PS-I.¹³³ Consequently, it enhances the ATP content within the chloroplasts, contributing to the overall photosynthetic efficiency. The study demonstrated that CDs significantly impacted chlorophyll content, elevating it, facilitated light absorption, improved energy conversion efficiency within chloroplasts, and enhanced absorption and fluorescence emission spectra intensity in chloroplasts, thereby augmenting photosynthetic system activity by accelerating electron transfer.¹³⁴ These findings contribute to a deeper understanding of the mechanisms involved in chloroplast function and photosynthesis. Researchers successfully synthesized magnesium and nitrogen co-doped carbon dots (Mg,N-CDs), and further demonstrated their versatility beyond photoluminescence properties by utilizing rice and ex vivo chloroplasts as model systems.¹²⁸ The results revealed that CDs have the potential to enhance chlorophyll metabolism and its associated photosynthetic activity by upregulating the expression of genes involved in chlorophyll biosynthesis and turnover.¹²⁸

Furthermore, CDs are capable of enhancing not only photoreactions but also the dark reactions of photosynthesis. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) stands as a pivotal enzyme within the Calvin cycle, playing a fundamental role in carbon assimilation during the dark phase of photosynthesis.¹²⁷ Consequently, the activity of RuBisCO has a direct impact on the overall rate of photosynthesis, thus highlighting its significance in this biological process. After CDs penetrate the plant mesophyll cells, they effectively enhance the enzymatic activity of RuBisCO, catalyze the first major carbon fixation reaction in Calvin cycle of photosynthesis, thereby augmenting the rate of CO₂ assimilation.^92,135,136 Upon treatment with 30 mg L⁻¹ CDs, a noteworthy enhancement of 51.9% in RuBisCO enzyme activity was observed in lettuce, which is more conducive to the accumulation of carbohydrates and effectively promotes the formation of photosynthetic products.¹³⁴ Therefore, the content of soluble protein is increased by 60.5%, and the content of soluble sugar is increased by 16.2% in leaf tissue.¹³⁴

Researchers successfully synthesized three distinct types of blue fluorescent CDs (B-CDs) utilizing various raw materials.¹³⁷ Notably, these B-CDs demonstrated a remarkable ability to traverse leaf cell membranes, resulting in efficient intracellular organelle transport, which significantly enhanced photosynthetic parameters including chlorophyll synthesis, Fd protein activity, RuBisCO enzyme functionality, and gene expression patterns.¹³⁷ Consequently, the net photosynthetic rate of plants was substantially elevated, ranging from an impressive 138.5% to a staggering 350.0% increase.¹³⁷ The study found that CDs in plants will be catalytically degraded by horseradish peroxidase (HRP) and hydrogen peroxide (H₂O₂) into phytohormone analogs and CO₂.^138,139 These phytohormone analogues possess the capability to regulate plant growth, while CO₂ functions as a vital raw material in the Calvin cycle, contributing to the synthesis of organic matter.^138,139 This enzymatic process not only plays a significant role in plant metabolism but also underscores the intricate interplay between enzymatic catalysis and plant physiology.^138,139 The aforementioned findings suggest that CDs have the capacity to significantly enhance crop yield through the facilitation of photosynthetic processes in plants, thereby offering immense potential for utilization in plant factory (Table 3).

Table 3 The influence of CDs on plant photosynthesis

CD precursor	Size	Optimum concentration	Plant species	Mode of application	Effect on plant photosynthesis	Ref.
	Fluorescence
Glutathione and formamide	3.8 nm	9 μg mL^−1	Roman lettuce	Hydroponic	Increased photosynthetic efficiency by light conversion	133
	Far-red
Citric acid and ethanolamine	3.25 nm	300 μg mL^−1	Rice	Foliar spray	CDs in chloroplasts could convert UV into the light coverage of photosynthesis	92
	Blue and red
Citric acid and cysteine	4–8 nm	100 μg mL^−1	Mung bean	Seed pretreatment	Enhanced the photosynthesis and accumulated more carbohydrates by improving the activity of rubisco enzyme	140
	Blue
Glucose	5 nm	0.56 mg ml^−1	Rice	Seed pretreatment	Enhanced the RuBisCO activity and CDs degraded to form CO2 which is then converted into carbohydrates through the Calvin cycle of photosynthesis	139
	Blue
Natural xylose	1.9–3.5 nm	200 μg mL^−1	Isolated chloroplasts of fresh spinach	CDs-isolated chloroplasts mixing	The synergistic effect of fragmented graphitized structures embedded in the amorphous structure of carbon dots facilitates efficient electron transfer	141
	—
Withered leaves	—	0.15 mg ml^−1	Bean sprout	Seed pretreatment	Increased chlorophyll content and photosystem activity	142
	Blue
Citric acid and ethylenediamine	3.8 nm	5 mg L^−1	Soybean	Foliar spray	Scavenged ROS, protected plant photosynthesis from oxidative stress and promoted carbohydrate transport	143
	Blue
Biochar	1–4 nm	150 mg L^−1	Rice	Foliar spray	Improved the photosynthetic rate and stomatal conductance in relation to carbon assimilation and intrinsic water use efficiency of C3 and C4 plants	144
	Blue
Natural waste biomass	6–20 nm	—	Brassica chinensis L.	Foliar spray	Improved photosynthetic parameters by enhanced the ability of light energy conversion and nutrient supply can	137
	Blue
—	4–6 nm	0.02 mg ml^−1	Mung bean sprout	Seed pretreatment	Enhanced the photosystem activity by accelerating the electron transfer rate	116
	Yellow-green
Citric acid and 1,3-diaminopropane	1.8–3.8 nm	100 mg L^−1	Lettuce	Foliar spray	Increased photosynthetic pigment content and affected chlorophyll fluorescence parameters	145
	Blue
Diethylenetriamine and citric acid	2.48–3.31 nm	300 mg L^−1	Apple	Medium treatment with CDs	As electron donors, offer a PQ-9-involved complement of PETC to improve photosynthesis	146
	Blue
L-Cysteine and glucose	1.5–2.7 nm	0.066 mg ml^−1	Lettuce	Seed pretreatment	Promoted plant photosynthesis by enhanced the activity of photosystem I (PSI)	147
	Blue
Citric acid	2.5 ± 0.5 nm	5 mg L^−1	Corn	Foliar spray	Improved the optical conversion and electron supply	148
	Blue
Levofloxacin hydrochloride	4.2 nm	25 mg L^−1	C. pyrenoidosa	Incubation of C. pyrenoidosa with CDs	Reduced the recombination efficiency of electron–hole pairs in CDs and improved the electron transfer rate in the optical system by transferring photo-induced electrons to P680^+ and QA^+	149
	Blue

---

6. Systematic biotoxicity and safety evaluation on carbon dots

To delve deeper into the utilization of CDs in urban agriculture, a comprehensive biosafety assessment of CDs is imperative. Presently, the biotoxicity of CDs remains a subject of debate, and its influence on plants varies significantly depending on the specific plant species and the nature of the carbon dot material combinations employed. Consequently, future investigations exploring the application of CDs in the urban agriculture must take into account their biotoxicity and safety aspects thoroughly.

6.1 Safety of carbon dots in plants

To effectively translate the practical utilization of CDs in urban agriculture into reality, it is imperative for researchers to meticulously assess their potential hazards towards various organisms and the surrounding environment. Presently, the bulk of research exploring the toxicity of CDs in vivo primarily centers on animals and microorganisms, with a conspicuous scarcity of studies pertaining to their toxicity in plants.

In the manufacturing of CDs, the utilization of diverse raw materials, preparation techniques, and research methodologies may exert varying impacts on plants. Specifically, CDs exhibiting differing degrees of toxicity in plants have been observed, with CDs containing amino groups demonstrating low toxicity levels while CDs possessing intricate surface functional groups exhibiting high toxicity towards plants.¹⁵⁰ Therefore, it is crucial to carefully consider the choice of raw materials and preparation methods during the production of CDs in order to minimize potential negative impacts on plants.

In the context of safety evaluation of CDs in plants, bean sprouts were selected as a suitable model organism for investigating the toxicity of CDs due to their rapid growth rate and transparent nature, facilitating facile observation and analysis. Researchers conducted growth experiments using bean sprouts to compare the in vivo toxicity of CDs with graphene oxide (GO) and single-wall carbon nanotubes. The results demonstrated that CDs at a concentration of 100 mg L⁻¹ were capable of enhancing the growth of mung bean sprouts without exhibiting cytotoxic effects.¹⁵¹ It was demonstrated that CDs have the potential to enhance growth in diverse plant species (mung bean, tomato, lettuce, Arabidopsis, peanut, wheat, corn, etc.).^{74,111,114,116,152–154} These findings contribute to the understanding of the potential safety of CDs in plants, providing valuable insights for future applications in agricultural.

However, some studies have documented the adverse effects of high concentrations of CDs on plant growth, indicating their potential toxicity.⁷³ Upon investigation, it was discovered that the germination roots of mung beans underwent a significant retardation in their elongation process when the concentration of CDs exceeded 1200 mg L⁻¹.¹⁵¹ Research has demonstrated that the application of CDs to plants results in their degradation into analogs of plant hormones, subsequently enhancing plant growth.³²

Based on the aforementioned studies, it is implied that the potential impact of CDs on plants exhibits a concentration-dependent trend, specifically, low concentrations tend to promote plant growth, whereas higher concentrations exert inhibitory effects.³² Therefore, the optimization of CD dosages prior to their application is crucial for mitigating the potential risks associated with phytotoxicity.

6.2 The cytotoxicity of carbon dots

Currently, the primary approach employed for evaluating the biotoxicity of CDs involves the identification of diverse viable cells through techniques such as MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl-2H-tetrazolium bromide) assay or staining procedures designed to distinguish between live and dead cells.^155,156 Researchers have conducted comprehensive cytotoxicity investigations on carbon spots, encompassing diverse compositions and surface structures, utilizing various mammalian cell lines as the basis for their experiments.¹⁵⁷ The MTT assay further confirmed the non-toxic nature of CDs towards human lung cancer A549 cells and human hepatoma HepG2 cell line.^158,159 Upon investigation, it was observed that the cytotoxicity exhibited by carbon spots is contingent upon the choice of surface-passivated molecules, and a more pronounced cytotoxic effect was noted at elevated concentrations.¹⁶⁰

Three surface passivation molecules (PEG_1500N, PPEI-EI, PAA) were tested for cell treatment. Cytotoxicity tests on functionalized CDs found PEG_1500N-functionalized CDs non-toxic, while PPEI-EI and PAA-functionalized CDs, especially at concentrations near the cytotoxicity threshold, contained higher oligo-amino polymers, leading to greater cytotoxicity.¹⁶⁰ Functionalized CDs PEG_1500N were discovered to exhibit nontoxic properties towards human renal cell lines and numerous human cancer cell lines.^161,162 In conclusion, CDs exhibit relatively low toxicity towards cells, with certain materials demonstrating concentration-dependent impacts on cellular function.

6.3 Safety of carbon dots in animals and human

Upon application to plants, CDs inevitably permeate both the animal and human bodies via the intricate food chain. However, the precise implications of CDs on human health remain largely uncharted and necessitate further rigorous investigation.¹⁶³ A study on mice showed that CDs were not toxic even at higher exposure levels and durations than in common optical in vivo imaging studies. No abnormalities or necrosis were found in harvested organs. The liver and spleen had higher CD concentrations but accumulation was minimal, no organ damage.¹⁵⁹

The investigation of the underlying mechanism of CDs toxicity, which necessitates the utilization of suitable in vivo toxicological models, constitutes a protracted endeavor to ensure the relevance of the outcomes to human. Given their high genetic similarity to humans, coupled with their transparency during early developmental and growth stages, zebrafish have emerged as a highly favored model organism for exploring the toxicological effects of CDs.^164,165 The results demonstrated that 50% lethal dose concentration (LC₅₀) value exceeded 100 μg ml⁻¹, indicating that the CDs belonged to the practically non-toxic category.¹⁶⁶ At a concentration of 150 μg mL⁻¹, there is no obvious embryonic toxicity or teratogenic effect. However, upon exceeding a CDs concentration threshold of 200 μg ml⁻¹, zebrafish embryos will be significantly affected, including pericardial and yolk sac edema, growth retardation, spinal curvature, and death.¹⁶⁶ Further exploration of the neurotoxicity of zebrafish reveals that high-concentration CDs can cause neurotoxicity and multi-organ damage in zebrafish larvae. CDs can be safely utilized when the concentration is lower than 200 μg mL⁻¹.¹⁶⁶

After conducting a series of cytotoxicity experiments and in vivo studies, CDs were found to possess no significant biological toxicity when specific surface modifications and functional groups were introduced. Specifically, CDs functionalized with PEG_1500N and those containing surface amino groups exhibited no notable toxicity. Furthermore, the biotoxicity phenomenon associated with CDs displayed a distinct concentration-dependent pattern, demonstrating nontoxicity within a specific concentration threshold range. However, given the vast array of CDs, varying preparation methods, and the intricate and diverse biological responses to CDs, further comprehensive and systematic research is necessary to thoroughly assess their biosafety and potential risks.

7. A perspective on application of carbon dots in urban agriculture

Currently, urban agriculture, an emerging agricultural modality, is experiencing a rapid surge in development across the globe, holding a pivotal position within the agricultural landscape. Nevertheless, the progression of urban agriculture is confronted with numerous challenges, including land scarcity, environmental hazards, and energy expenditure, particularly in the realm of vertical agricultural lighting systems for plant factory, where energy consumption is particularly significant.

Prior investigations have demonstrated that CDs possess the capacity to enhance chloroplasts' ability to harness increased light energy, expedite the rate of electron transportation across the thylakoid membrane, upregulate RuBisCO enzyme activity, and bolster plant carbon fixation rates by facilitating chlorophyll synthesis and converting ultraviolet light into visible light, which effects collectively contribute to a notable elevation in the plant's net photosynthetic rate.^{92,137,167,168} Therefore, CDs have the capacity to comprehensively enhance the photosynthetic efficiency of plants, irrespective of specific crop varieties, thereby offering a novel approach to reducing costs and enhancing productivity in plant factory.

Apart from increasing photosynthetic efficiency, CDs have a wide range of agricultural science applications due to their versatility. They exhibit remarkable properties in diverse aspects, including promoting plant growth, accelerating seed germination, optimizing nitrogen nutrient uptake, effectively managing pests, and enhancing plant resilience against both biotic and abiotic stresses.⁷⁵ Furthermore, their utilization as advanced fluorescent probes and nanosensors has pioneered novel avenues for plant in bioimaging, vastly enriching the research tools and application prospects of agricultural science¹⁶⁹ (Fig. 5).

Fig. 5 Application prospects of carbon dots in agriculture.

As the global population continues to expand and the demand for food intensifies, the utilization of pesticides and fertilizers has escalated accordingly. However, the extensive application of high-throughput chemical fertilizers and pesticides has reached a saturation point in terms of enhancing agricultural productivity, leading to a range of issues including environmental pollution and a decline in crop yield and quality.¹⁷⁰ In the present context, nanotechnology has exhibited immense promise in enhancing the efficiency of pesticides and fertilizers. The controlled release and targeted delivery of nanoscale active ingredients hold significant potential for promoting sustainable agriculture and precision farming practices.^171,172

In contrast to traditional fertilizers and pesticides, CDs exhibit remarkable potential in enhancing crop yield and quality, while concurrently mitigating energy consumption and mitigating environmental pollution during agricultural production. As a case in point, the application of nitrogen-doped CDs (N-CDs), serving as a water-soluble carbon nanofertilizer, has been demonstrated to significantly boost romaine lettuce biomass accumulation, enrich nutrient content, elevate photosynthetic rates, and facilitate nitrogen metabolism in plants.⁷³ This innovative approach offers a promising alternative to conventional agricultural practices, aiming to achieve sustainable crop production with reduced environmental impact. The profound benefits associated with the application of N-CDs on romaine lettuce underscore their immense potential for fostering agricultural productivity and enhancing crop quality.

Additionally, CDs, including N-CDs, exhibit promising prospects as nano-pesticides. According to the report, N-CDs synthesized from horsetail roots demonstrated remarkable in vitro growth inhibition against plant pathogens such as Corynespora cassiicola and Phytophthora nicotianae, thus indicating their potential as environmentally friendly pesticides.¹⁷³ At the same time, CDs exhibit significant advantages as sensors, demonstrating their effectiveness in detecting harmful substances, including pesticides and herbicides, within food and soil samples.¹⁷⁴ This capability holds promise for their application in ensuring the safety of agricultural products and the preservation of soil health.

Furthermore, CDs exhibit favorable impacts in bolstering plant resilience against both abiotic and biotic stressors. Their noteworthy ROS scavenging capabilities, along with their influence on antioxidant defense mechanisms and the expression patterns of disease resistance genes, contribute significantly to enhancing the resilience of crops towards a range of abiotic or biotic challenges.^143,175 Meanwhile, CDs exhibited antibacterial activity against a wide range of bacteria and fungi, thereby establishing a solid foundation for their potential utilization in crop disease management.^176–178 All of these findings offer innovative approaches towards enhancing crop yields and managing diseases, thereby contributing to the advancement and sustainable development of urban agriculture.

However, to successfully apply carbon dots in urban agriculture, a series of challenges still need to be overcome. Primarily, it is essential to comprehensively assess their biotoxicity and optimize dosing to ensure agricultural safety. In addition, factors such as technical bottlenecks, economic costs, and policy supervision also need to be comprehensively considered. Future research should deeply explore the plant growth promotion mechanism of carbon dots, promote their deep integration with intelligent agricultural technologies, and at the same time strengthen toxicity risk assessment, cost control, and policy guidance to fully realize the potential of carbon dots in urban agriculture. By judiciously employing CDs, urban agriculture stands to better accommodate the demands of contemporary urban development and offer novel avenues for the realization of efficient, eco-friendly, and sustainable agricultural production techniques.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Author contributions

HFZ and QC proposed the project; all the authors carried out reference searching; HFZ and YW wrote the manuscript; HFZ, YW and QC made review and final editing. All authors read and approved the final manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This research was funded by National Natural Science Foundation of China (32101661); Open Project of State Key Laboratory of Cotton Biology (CB2024A35). We are grateful to BioRender for providing an intuitive and powerful tool for scientific illustration. The figures in this paper have obtained permissions from BioRender.

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