Antioxidant quantum dots for spinal cord injuries: A review on advancing neuroprotection and regeneration in neurological disorders

1.1 QDs: Synthesis and properties

QDs are semiconductor nanocrystals exhibiting distinctive optical and electrical characteristics attributable to quantum confinement processes. These nanomaterials often measure between 2 and 10 nm in diameter, positioning them within a size range where their physical and chemical characteristics markedly diverge from those of bulk materials. The size tunability of QDs serves as a significant advantage in controlling their light-emission properties. This enables simultaneous excitation of various-sized QDs using a single light source, along with component-tunable broad spectral windows as shown in Figure 1 [58]. The elevated molar extinction coefficients of QDs contribute to the enhanced brightness of their fluorescence, in conjunction with a high quantum yield (QY) [59]. The elevated stability against photobleaching of QDs facilitates prolonged dynamic imaging [60]. The observed blinking of QDs is categorized into states of “dimmed” or “grey” (intermediate) intensities, ensuring the detection of a single dot event, such as the observation of an individual protein [61]. The distinctive size-dependent characteristics of QDs render them indispensable in domains such as optoelectronics, bioimaging, photovoltaics, and sensing technologies.

Figure 1

The photophysical features of QDs made in biological or biomimetic systems. (a) Size-tunable fluorescence output and multiple QDs being excited by the same light source at the same time. (b) The QDs made up of various components have wide spectrum ranges that go from ultraviolet to infrared [58].

Regarding the properties mentioned, QDs have established an excellent track record in the fields of medical and biological sciences. QDs are presently utilized as luminescent tools and labels in drug delivery and targeting, as well as in the sensing of DNA and oligonucleotides. They play a significant role in various scientific imaging methods, including molecular histopathology, flow cytometry-based identifications, disease identification, and biomedical imaging. Numerous studies have demonstrated the utility of QDs in clinical applications, such as sentinel lymph node visualization, micrometastasis detection, and photodynamic therapy (PDT) [62]. Furthermore, several applications have been suggested for QDs, such as carriers for drug delivery and light indicators for biological coding. Like other nanomaterials, there are concerns regarding the potential toxicity of QDs that need to be addressed prior to their clinical application [63].

Significant advancements have been achieved in novel synthesis pathways for QDs. The formation of QDs is primarily classified into top-down and bottom-up techniques, each utilizing different methodologies to attain the nanoscale size and specific features [64,65]. Top-down synthesis methods entail the disintegration of bulk materials into nanoscale particles via physical or mechanical procedures. These techniques are very effective for attaining accurate morphologies and structural configurations. Electron beam lithography (EBL) is a prevalent top-down method that employs a highly focused electron beam to intricately shape nanoscale objects with remarkable accuracy [66]. Despite its great efficacy, EBL is costly and time-consuming, constraining its scalability. A prevalent technique is mechanical milling, which reduces bulk materials to nanoparticles via high-energy grinding. This approach is economical and appropriate for large-scale manufacturing; nevertheless, it frequently yields particles with broad size dispersion and possible surface imperfections. Alternative top-down approaches, such as laser ablation and ion implantation, are utilized but frequently need advanced equipment and specialized knowledge. Conversely, bottom-up synthesis techniques entail the chemical or physical construction of QDs from atomic or molecule precursors. These approaches are preferred for their capacity to regulate the dimensions, shape, and surface characteristics of the QDs. Colloidal synthesis is the predominant method employed owing to its flexibility and accuracy. This technique entails high-temperature reactions utilizing surfactants or stabilizing chemicals to regulate particle development and inhibit aggregation. Colloidal synthesis is highly efficient for generating monodisperse QDs with adjustable optical and electrical characteristics. Additional notable bottom-up techniques encompass hydrothermal and solvothermal methods, which entail high-pressure, high-temperature reactions in either aqueous or organic solvents [67,68]. The methods employed are environmentally sustainable, scalable, and produce QDs with outstanding crystallinity. Table 1 represents a comprehensive classification of the synthesis methodologies for QDs, including their respective benefits and drawbacks. Bottom-up techniques, including hydrothermal, microwave-assisted, soft-template, and stepwise organic synthesis, provide benefits such as scalability, controllable particle dimensions, and doping potential. But they may be tedious, expensive, and occasionally susceptible to aggregation or need extensive purification. Top-down methods, such as oxidative/reductive cutting, electrochemical cutting, and pulsed laser ablation (PLA), are frequently more appropriate for large-scale production and expedited synthesis. Nonetheless, these methods may lead to inconsistent particle sizes, necessitate the use of aggressive chemicals or costly instruments, and are less favorable for doping methods. The synthesis of nanomaterials by microwave methods, a bottom-up approach offers benefits such as reduced reaction time, swift and uniform heating, and enhanced yield and purity. Phytic acid, which is rich in phosphorus, was combined with ethylenediamine in water and subjected to treatment in a domestic microwave oven for a duration of 8 min by Wang et al. [69]. Figure 2(a) illustrates the synthesis method employed for the phosphorus-containing carbon dots (CDs). The products acquired were subsequently purified through acetone extraction, resulting in green fluorescent CDs (as shown in Figure 2b) with a QY of 21.65%. Lower excitations showed two distinct peak emissions, whereas at higher excitations, a single peak emission was observed for these CDs. The authors demonstrated that a covalent linkage of the phosphorus functional groups to the graphite-like structure was evident in the CDs. Tang and coworkers demonstrated a microwave-assisted hydrothermal synthesis (Figure 2c) of graphene quantum dots (GQDs) derived from glucose, aiming to produce GQDs with diameters spanning from 2.9 to 3.9 nm. This study illustrated that the size of GQDs can be controlled within the range of 1.65–21 nm by adjusting the reaction time from 1 to 9 min. The GQDs exhibited excitation-dependent emission characteristics, with QYs calculated to range from 7 to 11%. The GQDs demonstrated the ability to convert blue light into white light, which was further confirmed by applying GQDs onto a blue-light emitting diode. The application of these GQDs enables the conversion of blue light into white light when they are applied to a blue-light-emitting diode.

Figure 2

(a) Schematic representation of the production of phosphorus-containing CDs. (b) Emission in the presence of natural light (on the left) and 365 nm UV radiation (on the right) [69]. (c) Synthesis of glucose-derived GQDs using microwave-assisted hydrothermal synthesis method.

Non-metal QDs have gained significant attention as environmentally friendly and biocompatible alternatives to traditional metal-based QDs. Among these, CDs, GQDs, and polymer dots (PDs) have emerged as promising materials due to their unique optical, chemical, and structural properties [70,71,72]. CDs are zero-dimensional nanomaterials composed primarily of carbon, typically synthesized via hydrothermal, solvothermal, or microwave-assisted methods. These methods involve the carbonization of organic precursors such as citric acid, glucose, or chitosan [73,74,75,76]. CDs exhibit strong PL, high photostability, low toxicity, and excellent water dispersibility, making them suitable for biomedical imaging, sensing, and photocatalysis. The emission properties of CDs can be tuned by controlling the precursor composition, synthesis temperature, and surface functionalization. Notably, surface passivation with polymers or organic ligands can significantly enhance their QY and improve their stability. GQDs are small fragments of graphene sheets with lateral dimensions below 10 nm, often synthesized via top-down (e.g., oxidative cutting of GO) or bottom-up (e.g., carbonization of organic molecules) approaches [36,65]. GQDs possess unique sp² carbon structures that provide excellent PL, superior chemical stability, and strong antioxidant properties [77]. Their tunable bandgap enables broad absorption across the visible to near-infrared spectrum. GQDs also demonstrate high biocompatibility, making them ideal candidates for bioimaging, drug delivery, and PDT [78]. Their fluorescence behavior is strongly influenced by surface functional groups, edge states, and quantum confinement effects. PDs are fluorescent polymeric nanoparticles that combine the advantages of organic dyes and QDs [79]. Typically synthesized through emulsion polymerization, nanoprecipitation, or self-assembly methods, PDs offer tunable fluorescence, high photostability, and low cytotoxicity. Their structure allows precise engineering of size, shape, and surface properties, making them versatile platforms for biosensing, imaging, and optoelectronic applications [80,81]. The emission properties of PDs can be tailored by modifying their polymer backbone or incorporating fluorophores with distinct electronic structures [82].

Their application in biological systems largely depends on their biocompatibility and colloidal stability, both of which are significantly influenced by the synthesis route employed [83]. The method of synthesis governs not only the size and surface chemistry of the QDs but also the presence of impurities, surface defects, and the nature of surface ligands all of which determine how QDs interact with biological environments.

Hydrothermal synthesis is a widely used method that involves the synthesis of QDs under high pressure and temperature in aqueous or non-aqueous solvents [84]. This technique often results in QDs with high crystallinity and fewer surface defects, which improves photostability and reduces the generation of ROS under illumination which is an important factor for biocompatibility. However, if not properly capped or functionalized, QDs synthesized via this route may aggregate in physiological media due to limited water dispersibility [85]. To improve biological stability, hydrophilic surface coatings such as polyethylene glycol, carboxylic acids, or zwitterionic ligands are often introduced.

Microwave-assisted techniques allow for rapid and uniform heating, resulting in highly monodisperse QDs with controlled sizes. The rapid reaction kinetics can limit the formation of impurities and reduce batch-to-batch variability. When paired with suitable precursors and stabilizing agents, QDs produced by this method exhibit enhanced colloidal stability and lower toxicity due to reduced defect densities [86]. Moreover, the ability to perform synthesis in aqueous media makes this approach more environmentally friendly and compatible with biological systems. Green or biosynthesis methods utilize natural reducing agents such as plant extracts, amino acids, or polysaccharides to produce QDs under mild conditions. These routes avoid toxic reagents and heavy metals, thereby enhancing the inherent biocompatibility of the resulting nanomaterials [58]. Furthermore, biomolecules used in synthesis often remain on the QDs surface as capping agents, improving water solubility and enabling interactions with biological targets. However, green synthesis may offer limited control over particle size and uniformity unless carefully optimized [87]. Traditional organometallic synthesis involves high-temperature reactions in organic solvents and typically produces high-quality QDs with excellent optical properties. However, these QDs are often capped with hydrophobic ligands like trioctylphosphine oxide, making them poorly dispersible in aqueous environments [88,89,90]. To render them biologically compatible, additional ligand exchange or encapsulation in amphiphilic polymers or liposomes is required [91]. These post-synthesis modifications are crucial to reduce cytotoxicity and improve in vivo stability [92]. Regardless of the initial synthesis method, surface functionalization plays a vital role in governing QD behavior in biological systems. Functionalization with biocompatible ligands not only enhances solubility and colloidal stability but also reduces protein adsorption and immune recognition [89].

1.2 Properties of QDs

Quantum confinement effects provide distinctive optical and electrical characteristics in QDs, providing them with several benefits over existing fluorophores, including organic dyes, fluorescent proteins, and lanthanide chelates [93]. Factors that significantly affect fluorophore behavior, and consequently their suitability for various applications, encompass the breadth of the excitation spectrum, the width of the emission spectrum, photostability, and the decay lifespan. Conventional dyes exhibit limited excitation spectra, necessitating illumination by light of a specific wavelength, which differs among individual dyes. QDs exhibit broad absorption spectra, enabling excitation across a wide range of wavelengths. This characteristic can be utilized to simultaneously excite multiple differently colored QDs using a single wavelength [94]. Traditional dyes exhibit broad emission spectra, indicating that the spectra of various dyes may significantly overlap. This restricts the quantity of fluorescent probes that can be utilized to label various biological molecules and be spectrally distinguished at the same time. Conversely, QDs have narrow emission spectra, which may be manipulated relatively easily by altering core size and composition, as well as by modifying surface coatings. They can be designed to emit light throughout a range of specific wavelengths, from ultraviolet (UV) to infrared (IR). The narrow emission and broad absorption spectra of QDs render them highly suitable for multiplexed imaging, where various colors and intensities are integrated to encode genes, proteins, and small-molecule libraries [95]. Due to their excellent photostability, they may facilitate the monitoring of prolonged interactions among multiple-labeled biological molecules within cells.

Photostability is an essential characteristic in several fluorescence applications, in which QDs provide a distinct advantage. In contrast to organic fluorophores that degrade after few minutes of light exposure, QDs exhibit remarkable stability, allowing for prolonged cycles of excitation and fluorescence for hours while maintaining high brightness and resistance to photobleaching. QDs have demonstrated greater photostability compared to several organic dyes [96,97], including Alexa488, which is noted as the most stable organic dye [98]. Chan and Nie reported semiconductor QDs with bright luminescence (zinc sulfide-capped cadmium selenide) that have been covalently attached to biomolecules for highly sensitive biological identification. In comparison to organic dyes such as rhodamine, these QDs demonstrate a 20 times greater brightness, a stability against photobleaching that is 100 times higher, and a spectral line width that is one-third narrower [97].

The characteristics of the PL of QDs are their size and excitation-dependent emission features. The study by Liu et al. stated the properties of polyethylenimine (PEI)-passivated carbon quantum dots, which exhibited stable multicolor luminescence dependent on the excitation wavelength [99]. The aqueous solution of these CDs exhibited blue, green, and red fluorescence when subjected to UV light excitation, blue, and green, respectively. The PL spectra of CD solution demonstrated a red-shifted emission characteristic, transitioning from 450 to 550 nm as the excitation wavelengths varied from 340 to 500 nm. This notable behavior was attributed to the size and surface state non-uniformity of the CDs passivated by PEI. Moreover, various studies have also indicated the presence of excitation wavelength independent CDs. Dong et al. documented the synthesis of nitrogen and sulfur co-doped CDs derived from citric acid and l-cysteine as precursors, which exhibited emission characteristics independent of excitation [100]. The authors elucidated that the PL of the CDs is dependent on surface states rather than shape, and that these surface states are homogeneous. In a prior study, similar excitation-independent emissions were achieved for nitrogen and zinc co-doped CDs [101].

The PL property has been observed to exhibit pH dependence. Pan et al. proposed hydrothermally prepared GQDs that exhibited strong emission under alkaline conditions, while in acidic environments, the PL emission was substantially quenched [102]. The PL intensity exhibited reversibility; specifically, when the pH of the GQD solution alternated between 12 and 1, the PL demonstrated a reversible change as shown in Figure 3a. It is crucial to observe that the pH of the solution affected only the PL intensity of GQDs, while the PL emission wavelength remained unchanged. Furthermore, the concentrations of the CDs or GQDs also affected their PL intensity or wavelength. The CDs synthesized from banana juice, as published by De and Karak, exhibited concentration-dependent photoluminescent characteristics [103]. The spectra presented in Figure 3b revealed that the PL intensity diminishes with rising concentrations of the CDs. The author elucidated that at low concentrations, the interaction among polar groups reduces, and at large concentrations, the abundance of polar functional groups tends to create agglomerates. Das et al. have observed a similar phenomenon for the nitrogen and sulfur co-doped CDs produced from kappa carrageenan and urea [104].

Figure 3

(a) pH-dependent PL spectra of the GQDs. The inset illustrates the relationship between PL intensity and pH, ranging from 12 to 1 [102]. (b) Change in photoluminescent intensity with varied concentrations [103].

QDs are nanoscale semiconductor particles, often measuring 2–10 nm, exhibiting distinctive optical and electrical characteristics due to quantum confinement phenomena. Their dimensions directly affect their emission wavelength, with smaller QDs generating blue light and bigger ones emitting red [105]. QDs are composed of a crystalline semiconductor core, often fabricated from materials such as CdSe, PbS, or InP [106]. They may be categorized as core-only, core–shell (including a protective shell like ZnS to improve stability and efficiency), or alloyed QDs (incorporating mixed elements for customized features). QDs are typically spherical in morphology; however, they may also adopt morphologies such as rods or tetrapods, depending upon the synthesis conditions [107]. Surface functionalization with organic or inorganic ligands enhances solubility, stability, and versatility for applications in optoelectronics, photovoltaics, and biomedicine. On the other hand, CDs and GQDs are carbon-derived nanomaterials exhibiting exceptional optical, electrical, and structural characteristics, positioning them as sustainable substitutes for conventional semiconductor QDs. CDs are generally spherical nanoparticles, measuring less than 10 nm, characterized by amorphous or partially crystallized structures [108]. Their outstanding PL, minimal toxicity, and remarkable biocompatibility facilitate their use in bioimaging, sensors, and photocatalysis [109,110]. GQDs have significant quantum confinement and edge effects, providing adjustable fluorescence, elevated conductivity, and exceptional chemical stability [77]. Both CDs and GQDs are economical, eco-friendly, and exceptionally adaptable, with applications in energy storage, optoelectronics, antimicrobial agents, bioimaging, and environmental monitoring [111,112]. Table 2 summarizes recent advancements in CDs utilized as fluorescent probes for detecting neurological biomarkers in biological fluids. The table highlights key features such as synthesis methods, surface functionalization, target biomarkers, and detection limits, emphasizing the versatility and sensitivity of CDs in neurodiagnostic applications.

2.1 SCI OS hypothesis: Excess ROS generation

The production of excessive ROS is crucial in the secondary injury processes after SCI, greatly contributing to cellular and tissue damage. ROS are byproducts of normal cellular metabolism, mostly generated in mitochondria via oxidative phosphorylation [149]. Under physiological conditions, ROS levels are meticulously managed by antioxidant defense mechanisms [150]. Following SCI, there is a significant increase in ROS generation due to mitochondrial malfunction, infiltration of inflammatory cells, and the degradation of cellular structures, which surpasses the cellular antioxidant defenses. This ROS overload initiates a series of harmful events, resulting in neuronal death, tissue necrosis, and inflammation, which worsen the initial injury. In the context of SCI, ROS formation is predominantly heightened by OS processes, wherein excessive ROS compromise cellular membranes and organelles [151]. The mitochondria, due to their heightened sensitivity to stress, are a principal generator of ROS following injury. Spinal cord trauma disrupts mitochondrial activity, decreasing the electron transport chain (ETC) and increasing electron leakage, resulting in the formation of superoxide anions through reactions with oxygen [152]. Superoxide is then transformed into more reactive ROS, such as hydrogen peroxide and hydroxyl radicals, which target cellular lipids, proteins, and DNA. This oxidative damage undermines cellular integrity and facilitates apoptotic and necrotic cell death in neurons, oligodendrocytes, and astrocytes, resulting in additional functional and structural deterioration in the damaged spinal cord [153]. Moreover, ROS overload induces neuroinflammation by activating immune cells, including microglia and invading macrophages, which secrete pro-inflammatory cytokines and exacerbate ROS generation. The inflammatory response to SCI can be advantageous in many respects, as it eliminates debris and fosters a regenerative milieu; however, the overproduction of ROS by activated microglia and macrophages perpetuates a chronic inflammatory cycle that obstructs tissue regeneration. Pro-inflammatory cytokines, including tumor necrosis factor-alpha (TNF-α) and IL-1β, intensify the inflammatory response, establishing a detrimental milieu that obstructs axonal regeneration and remyelination [154]. This chronic inflammatory condition sustains a cycle of ROS production and oxidative harm, resulting in enduring functional impairments and ongoing pain in SCI patients [155].

The degradation of cellular and extracellular components caused by ROS overload further intensifies excitotoxicity, a phenomenon in which impaired neurons release excessive amounts of glutamate [156]. The increased glutamate concentration excessively activates N-methyl-d-aspartate receptors on neighboring neurons, resulting in intracellular calcium overload that subsequently enhances ROS generation and mitochondrial impairment [157]. The interaction between calcium and ROS intensifies cellular toxicity, leading to extensive neuronal death. Therapeutic approaches aimed at ROS and OS possesses considerable potential for alleviating SCI damage [158]. Antioxidants, including N-acetylcysteine (NAC), edaravone, and resveratrol, have demonstrated potential in experimental models by neutralizing ROS and diminishing lipid peroxidation. These drugs seek to restore the redox equilibrium, safeguard cellular integrity, and mitigate inflammation, thus maintaining neural functionality [159]. The intricacy of SCI pathology and the constraints of systemic antioxidant administration highlight the necessity for focused therapeutics that precisely regulates ROS levels within the spinal cord. Advancements in nanotechnology, including antioxidant-loaded nanoparticles, present exciting opportunities for precise control of ROS, thereby enhancing therapeutic outcomes in SCI patients [160]. The overproduction of ROS after SCI greatly contributes to subsequent damage mechanisms, such as neuronal death, inflammation, and excitotoxicity [161]. Mitigating excess ROS using specific antioxidant therapy and novel delivery mechanisms may enhance functional recovery and diminish long-term damage in SCI patients [162]. Nonetheless, additional study is required to comprehensively comprehend ROS dynamics and formulate effective, tailored therapies that may be used in clinical practice.

Liu et al. synthesized aldehyde-scavenging polypeptides (PAH)-curcumin conjugate nanoassemblies (PFCN) for neuroprotection in SCI by a straightforward in situ reaction-induced self-assembly method. Oxidative and acidic microenvironments in SCI may generate PAH and curcumin from synthesized PFCN [163]. PFCN mitigated neuroinflammation by eliminating harmful aldehydes and reactive nitrogen and oxygen species in neurons (Figure 6), controlling microglial M1/M2 polarization, and decreasing inflammation-related cytokines. In the contusive SCI rat model, intravenous PFCN was able to mitigate the detrimental spinal cord microenvironment, safeguard neurons, and enhance motor performance.

Figure 6

Schematic representation of the preparation of PFCN for integrated treatment of SCI [163].

Tauroursodeoxycholic acid (TUDCA) is a polar derivative of bile acid that has shown neuroprotective properties in various nervous illness models. Nonetheless, the impact and fundamental pathway of TUDCA on SCI remain inadequately clarified. This study seeks to examine the defensive benefits of TUDCA in the SCI mice model and the associated mechanisms involved [164]. TUDCA treatment may mitigate secondary injury and enhance functional recovery by diminishing OS, seditious reaction, and apoptosis resulting from primary injury, while also facilitating axon regeneration and remyelination, presenting a possible therapeutic option for human SCI recovery. From a molecular perspective, OS and inflammatory pathways are primary orchestrators of interconnected dysregulated pathways subsequent to SCI. It emphasizes the necessity of developing multitarget therapy for SCI sequelae. Polyphenols, as secondary metabolites generated from plants, possess the potential to serve as alternative physiological factors for the therapy of SCI. These secondary metabolites exhibited intonation consequences on neuronal OS, neuroinflammation, and aberrant extrinsic axonal tracts in the development and course of SCI. This review elucidates the significant significance of phenolic compounds as essential phytochemicals in modulating dysregulated OS and inflammatory signaling mediators, as well as extrinsic processes of axonal regeneration following SCI, based on preclinical and clinical investigations [165]. The activation of OS and apoptosis-induced cell death considerably contributes to the advancement of SCI. Current data indicate that maltol has natural antioxidative effects via inhibiting OS and apoptosis. Nonetheless, the substantial impact of maltol on SCI therapy is yet to be assessed. This work investigated maltol administration, which may induce Nrf2 expression and facilitate the retranslocation of Nrf2 from the cytosol to the nucleus, thereby inhibiting OS signaling and apoptosis-related neuronal cell death after SCI. Moreover, maltol administration augments PINK1/Parkin-mediated mitophagy in PC12 cells, promoting the restoration of mitochondrial functions [166]. The transplantation of bone marrow mesenchymal stem cells (BMSCs) has surfaced as a prospective therapy for SCI. The poor survival and differentiation rates of BMSCs in the spinal cord milieu considerably restrict their therapeutic efficacy. TUDCA, an active compound derived from bear bile, has shown neuroprotective, antioxidant, and antiapoptotic properties in SCI. The current work aims to investigate the potential advantages of merging TUDCA with BMSC transplantation in an animal model of SCI. The findings indicated that TUDCA markedly improved BMSC survivability while decreasing apoptosis and OS in both in vitro and in vivo settings. TUDCA expedited tissue regeneration and enhanced functional recovery post-BMSC implantation in SCI. The effects were mediated via the Nrf-2 signaling tract, as shown by the increase in Nrf-2, NQO-1, and HO-1 expression levels [167]. A thioketal-based, ROS-scavenging hydrogel was synthesized for the encapsulation of BMSCs, facilitating neurogenesis and axon re-formation by mitigating excessive ROS and reconstructing a regenerative milieu [168]. The hydrogel effectively encapsulated BMSCs and exhibited significant neuroprotection in vivo by diminishing endogenous ROS production, mitigating ROS-induced oxidative impairment, and downregulating inflammatory cytokines including interleukin-1 beta (IL-1β), interleukin-6 (IL-6), and TNF-α, thereby reducing cell apoptosis in spinal cord tissue. The BMSC-conjugated ROS-scavenging hydrogel decreased scar formation and promoted neurogenesis in spinal cord tissue, therefore significantly improving motor efficient regaining in SCI mice. To investigate the impact of photobiomodulation (PBM) on axon renewal and alterations in secretion from dorsal root ganglion (DRG) under OS after SCI, and to further examine how PBM-induced changes in DRG secretion influence macrophage polarization [169]. The PBM-DRG model was developed to conduct PBM on neurons subjected to OS in vitro. The outcome yielded energy of 4 J. Approximately 100 μM H₂O₂ was introduced to the culture medium to induce OS after SCI. A ROS test kit was applied to quantify ROS levels in the DRG. The neuronal survival rate was assessed by the CCK-8 assay, and axonal regeneration was examined via immunofluorescence techniques.

2.2 How QDs act as antioxidants?

QDs serve as effective antioxidants owing to their distinctive electrical configuration and adjustable surface characteristics, allowing them to neutralize ROS and avert oxidative harm. The antioxidant efficacy of QDs is mostly contingent upon their composition and surface changes, often accomplished by doping with elements such as sulfur, nitrogen, or cerium, which augment ROS scavenging capabilities [170]. These alterations allow QDs to emulate natural antioxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT), facilitating the conversion of superoxide radicals ( O 2 − ) and H₂O₂ into benign byproducts like water and oxygen, thereby diminishing ROS buildup. QDs interact with ROS via electron transfer mechanisms, whereby they either give or take electrons to stabilize ROS. This electron transfer may neutralize ROS, alleviating cellular damaging mechanisms such as lipid peroxidation, protein oxidation, and DNA breakage. Furthermore, QDs may influence essential cellular antioxidant processes, particularly by stimulating the Nrf2 signaling pathway, which increases the production of endogenous antioxidants such as glutathione peroxidase and CAT. By blocking the nuclear factor kappa B (NF-κB) pathway, QDs diminish the production of pro-inflammatory cytokines, thereby further reducing OS and inflammation. The combined mechanism of direct ROS neutralization and pathway regulation allows QDs to behave as multifunctional antioxidants, positioning them as prospective agents for addressing OS in neurodegenerative disorders and traumas.

GQDs seem to be among the brightest antioxidants due to their suitable antioxidant properties, distinctive structure, superior cytocompatibility, and little toxicity. Nonetheless, the comparatively diminished antioxidant activity compared to inorganic semiconductor materials and the ambiguous antioxidant mechanism restricted their cellular applications. This article investigates the antioxidant process by examining the correlation between antioxidant behavior and oxygenated surface groups. The overall oxygen fraction was regulated by post-preparation reduction with NaBH4, while the specific types of oxygen functional groups were modified by free radicals during the synthesis of GQDs [171]. Hemmateenejad et al. developed a novel QD-based test to assess antioxidant and polyphenolic activity [172]. This experiment measures the inhibitory impact of antioxidant/polyphenolic substances on the UV-induced bleaching of l-cysteine-capped CdTe QDs. QDs demonstrated remarkable photostability in the absence of UV exposure, although they underwent fast bleaching when subjected to UV irradiation. The production of ROS during UV irradiation is likely the primary factor contributing to the photobleaching of QDs. The comparison of the photostability of QDs in buffer solution, both with and without sodium azide, a recognized quencher of singlet oxygen, corroborated the role of singlet oxygen in the photobleaching of QDs. The photobleaching impact caused by ROS may be mitigated by the presence of antioxidant or polyphenolic substances. We evaluated several antioxidant and polyphenolic substances, in addition to established antioxidants like trolox and four distinct varieties of tea. Chong et al. showed that GQDs may effectively scavenge various free radicals, therefore protecting cells from oxidative injury. Upon exposure to blue light, GQDs demonstrate considerable phototoxicity by elevating intracellular ROS levels and diminishing cell viability, due to the production of free radicals under light stimulation [173]. They also affirmed that the light-induced generation of ROS arises from the electron-hole pair and, crucially, demonstrate that singlet oxygen is produced by photoexcited GQDs via both energy-transfer and electron-transfer mechanisms (Figure 7). Furthermore, following light stimulation, GQDs enhance the oxidation of non-enzymatic antioxidants and facilitate lipid peroxidation, hence adding to the light induced-toxicity of GQDs. Our findings indicate that GQDs exhibit antioxidant and pro-oxidant properties, contingent upon light exposure, which will inform the protective application and advancement of significant anticancer and bactericidal uses for GQDs.

Figure 7

GQDs scavenging various free radicals. ESR spectra of DMPO/˙OH adducts were acquired using 20 μM Fe²⁺, 50 mM DMPO, 20 μM H₂O₂, 10 mM PBS buffer (pH 7.27), and various GQD doses after 1 min of incubation. GQD concentration affects ˙OH-scavenging activity. After 1 min incubation, 25 mM BMPO, 10% DMSO, 2.5 mM KO2, 0.35 mM 18-crown-6, 10 mM PBS buffer (pH 7.27), and varied GQD doses yielded ESR spectra of BMPO/˙OOH adducts. GQD concentration affects O 2 ⋅ − scavenging. (e) Samples with 0.1 mM DPPH˙, 10 mM PBS buffer (pH 7.27), and varied GQD doses were used to generate ESR spectra. Data were taken 5 min after incubation. (f) GQD concentration affects DPPH˙-scavenging [173].

Recent investigations have shown that GQDs are anti- and pro-oxidant. Their efficiency is poor. We present chlorine-doped GQDs (Cl-GQDs) with variable Cl doping and enhanced anti- and pro-oxidant activity. Cl-GQDs had 7-fold and 3-fold greater scavenging and free radical-produced efficiency than undoped GQDs [174]. The production of ROS from GQDs under light irradiation was validated by ESR spectroscopy. TEMP was chosen as a spin trap for singlet oxygen, capable of selectively reacting with ¹O₂. Upon the entrapment of ¹O₂, a stable compound, 2,2,6,6-tetramethylpiperidine-1-oxyl, was generated, resulting in a distinctive ESR signal. Figure 8a demonstrates a pronounced ESR signal of TEMP in the Cl-GQDs-7.5V solution during irradiation. Furthermore, no ESR signal was seen in the control sample under dark conditions. The findings indicated that ¹O2 might be produced by Cl-GQDs-7.5V under irradiation, consistent with prior studies. Upon photoexcitation, ¹O₂ was generated by energy transfer from the excited triplet state of the Cl-GQDs to the ground-state oxygen (Figure 8b). Furthermore, no O 2 ⋅ − or ˙OH was produced by GQDs under irradiation. In comparison to GQDs, the elevated defect levels caused by the highly electronegative Cl atoms promoted energy transfer between Cl-GQDs and O₂, hence augmenting their pro-oxidant characteristics.

Figure 8

(a) ESR spectra from Cl-GQDs-7.5V and TEMP samples with (black) or without (red) light. (b) ¹O₂ generating schematic [174].

Composite nanoparticles of naringenin-loaded β-cyclodextrin and CQDs were effectively synthesized. The findings indicated that the integration of CQDs not only augmented the antioxidant activities of nanoparticles but also boosted the encapsulation efficiency of naringenin. The creation of composite nanoparticles was validated by several characterization techniques. The zeta potential and Fourier transform infrared spectroscopy results demonstrated that electrostatic interactions and hydrogen bonding are the predominant factors in nanoparticle formation. The X-ray diffraction experiment indicated that the naringenin-β-CD-CQDs nanoparticles are in an amorphous state, contrasting with the crystalline states of naringenin, β-CD, and the naringenin-β-CD inclusion complex. Ultimately, tests of antioxidant activity against DPPH, ABTS⁺, and Fe²⁺ chelating demonstrated that the resultant composite nanoparticles exhibited superior antioxidant activity relative to their individual components [175].

Leveraging the antioxidant properties of QDs, their use in neuroprotection seems particularly advantageous, especially regarding neurodegenerative illnesses driven by OS and acute traumas like SCI. By neutralizing ROS and modifying cellular antioxidant pathways, QDs mitigate oxidative damage and contribute to the stabilization of neuronal cell settings. This decrease in OS inhibits subsequent consequences such as apoptosis, inflammation, and mitochondrial malfunction, all of which are essential for neuronal survival and function. Moreover, the capacity of QDs to selectively target and concentrate in brain tissues has further benefits, as they may provide localized antioxidant effects exactly where required. The inherent antioxidant characteristics of QDs provide neuroprotective advantages, indicating their potential as therapeutic agents in SCI, traumatic brain traumas, and chronic neurodegenerative disorders.

3.1 QD-based modulation of inflammatory responses

The control of inflammatory responses by QDs and CDs is a potential approach for addressing SCI, where excessive inflammation aggravates tissue damage and hinders healing [202]. In SCI, initial mechanical damage triggers a series of subsequent injury processes, with inflammation being a pivotal factor [203]. This inflammatory response, although initially helpful, often turns harmful as it extends tissue damage, disturbs cellular homeostasis, and triggers apoptotic pathways, resulting in neuronal death. Nanomaterials, including QDs and CDs, provide distinct benefits as they may be tailored to target and modulate certain inflammatory pathways, providing targeted and regulated anti-inflammatory actions that may alleviate secondary harm [204].

QDs, particularly those doped or functionalized with heteroatoms, such as nitrogen, sulfur, or cerium, have demonstrated significant antioxidant potential in biological settings, including in vivo models. For example, Yang et al. synthesized cerium-doped CQDs that exhibited strong ROS-scavenging ability in a rat model of SCI. The treatment reduced malondialdehyde (MDA) levels and increased SOD activity, resulting in improved neuronal survival and reduced lesion volume [205]. In a separate study, Guo et al. reported that GQDs effectively attenuated OS in a transgenic mouse model of AD [51]. These QDs restored mitochondrial membrane potential, decreased intracellular ROS, and upregulated antioxidant enzymes such as CAT and glutathione peroxidase, ultimately improving cognitive function. Similarly, Luo et al. demonstrated that sulfur-doped carbon dots reduced ROS accumulation and inflammation in a murine model of IRI, contributing to decreased tissue necrosis and enhanced functional recovery [206].

QDs, due to their adjustable surface chemistry and functionalization potential, may engage with and influence critical inflammatory pathways in SCI. QDs may inhibit the activation of NF-κB, a pivotal regulator of inflammation that, when excessively active, facilitates the secretion of pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6. By suppressing NF-κB, QDs may diminish the synthesis of these cytokines, hence lowering immune cell recruitment and mitigating tissue damage [207]. Furthermore, QDs may enhance anti-inflammatory mechanisms, including the activation of peroxisome proliferator-activated receptor gamma (PPAR-γ), which contributes to the resolution of inflammation by inhibiting pro-inflammatory gene expression. The simultaneous reduction of pro-inflammatory signaling and enhancement of anti-inflammatory pathways may stabilize the spinal tissue environment, therefore protecting neurons and glial cells against chronic inflammation. CDs are becoming recognized for their ability to influence inflammatory responses in SCI. CDs, often sourced from organic materials and infused with components such as nitrogen and sulfur, have intrinsic anti-inflammatory and antioxidant characteristics. CDs may eliminate ROS, so mitigating OS and indirectly attenuating inflammation, as ROS are principal activators of the NF-κB pathway [208]. This ROS-scavenging mechanism inhibits the subsequent activation of pro-inflammatory cytokines, hence diminishing cellular damage. Furthermore, CDs may influence macrophage polarization, a mechanism essential for SCI recovery. Macrophages occur in two primary states: the M1 (pro-inflammatory) and M2 (anti-inflammatory) phenotypes [209]. CDs have shown the capacity to transition macrophages from the M1 to the M2 phenotype, therefore facilitating tissue repair, enhancing cell viability, and mitigating neurotoxic inflammation. By enhancing M2 macrophage activity, CDs may provide a more conducive milieu for repair and regeneration in SCI.

In addition to these direct effects, QDs and CDs may be modified with targeting ligands, such as peptides or antibodies, to facilitate targeted distribution to spinal cord damage locations. This focused strategy improves their therapeutic effectiveness, enabling precise modulation of inflammatory responses without impacting adjacent healthy tissues. Functionalizing QDs or CDs with antibodies that identify inflammatory markers allows these nanomaterials to accurately target inflammation sites, hence enhancing the concentration of anti-inflammatory medicines at the damage location and reducing off-target effects. The targeting ability, together with their intrinsic anti-inflammatory and antioxidant properties, renders QDs and CDs very adaptable for SCI therapy [210]. The control of inflammation by QDs and CDs provides a comprehensive strategy for mitigating secondary damage in SCI. By inhibiting critical inflammatory pathways, altering macrophage morphologies, and scavenging ROS, these nanomaterials foster an environment conducive to brain repair and regeneration. With enhanced targeting capabilities and sustained biocompatibility, QDs and CDs possess significant promise as therapeutic agents for managing inflammation and facilitating healing in SCI.

Ayaz et al. examined the immunological response to CDs and assessed the impact of surface passivation agents on their immunomodulatory properties. Carbon dots passivated with poly(vinyl alcohol) had significant anti-inflammatory properties, whereas carbon dots passivated with alginate demonstrated pro-inflammatory effects. CDPEG exhibited modest anti-inflammatory properties, indicating its potential as an inert carrier in drug delivery research. The findings indicated that the kind of activated group present on the surface influenced the differential effects of CDs on the inflammatory capacity of macrophages by altering the production levels of pro-inflammatory cytokines TNFα and IL6 [211]. Tosic et al. showed the capacity of GQD to mitigate inflammatory CNS injury in the rat model of experimental autoimmune encephalomyelitis (EAE). Our findings demonstrate that GQD mitigates demyelination and clinical manifestations of EAE by disrupting the peripheral activation and proinflammatory activity of CNS-reactive Th1 cells, in addition to directly safeguarding CNS cells from immune-mediated damage [212]. A GelMA hydrogel infused with FTY720 CDs (FTY720-CDs@GelMA) was developed to facilitate the integration of NSCs for the purpose of filling the capsular cavity of SCIs by in situ injection, hence enhancing spinal cord regeneration (Schematic 1). The resulting hydrogel exhibits excellent biocompatibility and ROS scavenging properties, demonstrating significant protective effects on astrocytes and neural stem cells (NSCs) in vitro. Moreover, the implantation of FTY720-CDs@GelMA hydrogel enhanced neuronal differentiation of transplanted NSCs, reduced glial scar formation, and facilitated axonal regeneration and neural circuit restoration. The findings indicated that the integration of FTY720-CDs@GelMA hydrogel with NSCs significantly enhanced motor function recovery after SCI in rats, proposing a novel approach for comprehensive SCI therapy [213]. Yang et al. synthesized an amine-functionalized aspirin-derived carbon quantum dot (NACQD) with a nominal diameter of 6–13 nm. The NACQD has strong iron-binding and antioxidative properties. NACQD treatment, administered intrathecally, significantly reduced iron accumulation and OS in cerebral tissue, mitigated meningeal inflammation, and enhanced neurological recovery in a mouse model of intracerebral hemorrhage (ICH) (Figure 13). The intrathecal injection of NACQD serves as a viable therapeutic method to mitigate ICH harm, demonstrating its potential as a proof of concept [214].

Figure 13

Protective function of NACQD in murine ICH models [214].

Following ICH, the excessive generation of ROS and iron ion accumulation are the primary contributors to subsequent injury. Eliminating surplus iron ions and ROS in the meningeal system may significantly mitigate subsequent damage after ICH. Tang et al. produced ginsenoside Rb1 CQDs (RBCQDs) using ginsenoside Rb1 and ethylenediamine by a hydrothermal process. RBCQDs demonstrate strong efficacy in scavenging ABTS+  free radicals and iron ions in solution. Following intrathecal injection, the distribution of RBCQDs is mostly confined to the subarachnoid region [215]. RBCQDs may neutralize ROS and bind iron ions in the meningeal system. RBCQD therapy markedly enhances blood circulation in the meningeal system, safeguarding degenerating neurons, ameliorating neurological function, and offering a novel therapeutic strategy for the clinical management of ICH.

3.2 Protection of neuronal cells and support of survival pathways

Antioxidant QDs have attracted considerable interest for their prospective neuroprotective effects in SCI models. The capacity of QDs to alleviate OS, a major factor in neuronal injury after SCI, renders those interesting candidates for enhancing neuroprotection and regeneration approaches. These nanomaterials, usually produced with doping elements like sulfur (S) and nitrogen (N) or integrated into polymeric matrices, have improved antioxidative properties and adjustable biocompatibility, thereby enabling their use in both in vitro and in vivo research. Understanding how different types of QDs correlate with specific biomedical strategies is crucial for advancing their clinical potential. Among the most widely studied are cadmium-based QDs, such as cadmium selenide (CdSe) and cadmium telluride (CdTe) [216]. These exhibit excellent optical properties, including high QY and narrow emission spectra, making them ideal for bioimaging, cell tracking, and diagnostic applications. However, their inherent cytotoxicity due to the release of heavy metal ions poses significant challenges for in vivo applications. To mitigate this, surface passivation with biocompatible polymers or encapsulation within inert shells like ZnS is often employed, although concerns about long-term safety persist [217]. In contrast, silicon quantum dots (Si QDs) offer superior biocompatibility, low toxicity, and biodegradability, making them attractive for drug delivery, PDT, and regenerative medicine. Their favorable pharmacokinetics and clearance profiles support their application in chronic disease models and long-term therapies [218]. Additionally, their tunable surface allows for conjugation with therapeutic molecules or targeting ligands, which is particularly advantageous in site-specific delivery systems for SCI and cancer therapy. Carbon-based QDs, including CQDs and GQDs, are increasingly explored due to their inherent antioxidant, anti-inflammatory, and neuroprotective properties. These QDs have shown significant promise in neurological applications, especially SCI, where secondary injury mechanisms such as OS and inflammation play a critical role [219]. CQDs can scavenge ROS, regulate inflammatory signaling, and support neuronal regeneration by facilitating neurotrophic factor delivery (e.g., brain-deducted neurotrophic factor [BDNF], nerve growth factor [NGF]). Their minimal cytotoxicity, tunable size, and surface functionalization make them ideal candidates for multifunctional nanocarriers in regenerative therapies. Furthermore, doped QDs, such as nitrogen, sulfur, phosphorus, or cerium-doped CQDs, exhibit enhanced therapeutic performance due to improved ROS-scavenging and enzyme-mimicking capabilities [220]. For example, cerium-doped QDs mimic CAT and SOD, enhancing their ability to mitigate oxidative damage in SCI and other neurodegenerative models [221]. These tailored QDs not only improve cellular viability but also modulate intracellular signaling pathways involved in apoptosis, autophagy, and neurogenesis. Other types, such as indium phosphide (InP) QDs, are emerging as less toxic alternatives to Cd-based QDs, with growing applications in bioimaging and targeted drug delivery [222]. Similarly, ZnO and Ag-doped QDs offer antimicrobial and anti-inflammatory benefits, which could be leveraged in SCI-associated infection management or wound healing. By aligning specific QD types with corresponding therapeutic functions whether imaging, ROS scavenging, neuroprotection, or drug delivery, researchers can better tailor nanomedicine platforms to disease-specific requirements [223]. A deeper understanding of these correlations not only enhances the translational potential of QDs but also aids in the rational design of future therapeutic systems targeting SCI and beyond.

In vitro models of SCI provide a regulated setting to assess the protective mechanisms and effectiveness of antioxidant QDs [224]. Neural cells, such as neurons and astrocytes, exposed to OS-related damage, are often used to evaluate the antioxidative capacity of QDs. Research indicates that QDs, including sulfur- and selenium-doped CDs, markedly decrease ROS levels in neural cells. These QDs mitigate free radicals via their many surface-active sites, such as hydroxyl, carboxyl, and amino groups, thereby inhibiting lipid peroxidation and preserving mitochondrial function. CQDs obtained from natural sources such as citric acid and nitrogen precursors have shown the capacity to maintain cell viability in models of OS-induced damage. They do this by regulating the activity of cellular antioxidant defense mechanisms, including SOD and CAT [225]. Furthermore, QDs functionalized with biomolecules, like neurotrophins or peptides, augment their selectivity for neuronal cells, promoting axonal expansion and synaptic repair. These data highlight the capability of QDs to mitigate the secondary damage phase of SCI, marked by inflammation and OS. The neuroprotective properties of antioxidant QDs have been confirmed in vivo using mouse models of SCI. These experiments often include the induction of SCI using contusion or compression methods, followed by the administration of QDs via localized injection or systemic distribution [226]. QDs have shown the ability to reduce neuronal death and maintain white matter integrity, thereby enhancing functional recovery. Selenium-doped QDs have shown significant effectiveness in decreasing OS indicators like MDA and increasing glutathione (GSH) levels in damaged spinal cord tissues [227]. The antioxidative effect is often enhanced by the control of inflammatory responses, since QDs diminish the activation of microglia and astrocytes, hence decreasing the production of pro-inflammatory cytokines like TNF-α and IL-6. Furthermore, the integration of QDs into hydrogels or alternative biomaterials facilitates sustained release, hence enhancing the duration of the therapeutic impact. Behavioral tests, such the Basso et al. locomotor rating scale [228], substantiate the effectiveness of QD-based therapies. Rodents administered antioxidant QDs have enhanced motor performance and decreased hindlimb paralysis relative to untreated controls. Histological research demonstrates less cavity development and increased neuronal survival, indicating strong neuroprotective effects [229]. Despite the great promise of antioxidant QDs in SCI treatment, several hurdles remain, including the optimization of biocompatibility, targeted distribution, and long-term safety. Integrating QDs with sophisticated delivery technologies, such as nanofiber scaffolds or injectable hydrogels, may enhance their therapeutic efficacy. Moreover, using these insights clinically requires thorough preclinical validation and the resolution of regulatory issues.

Regulating glial cell activation is essential for the treatment or prevention of neuroinflammation-related brain disorders. Ultrasmall 2D vanadium carbide QDs (V2C QDs) capable of traversing the BBB are synthesized, demonstrating remarkable anti-neuroinflammatory properties. The synthesized 2D V2C QDs, averaging 2.54 nm in size, exhibit commendable hydrophilicity, physiological stability, and proficient BBB permeability. The pharmacological impact of V2C QDs on inflammatory responses reveals intriguing outcomes in mitigating acquiring and storage deficits in BALB/c mice induced by LPS [230]. A separate study examined the possible neuroprotective properties of Ferulic acid against ischemia/repertory (I/R)-accelerated cerebral trauma both in vivo and in vitro, utilizing hematoxylin and eosin (H&E) and Nissl staining assays, flow cytometry, Hoechst 33258 staining, quantitative PCR, western blot analysis, and fluorescence microscopy [231]. Identifying the amyloid form that are critical promoters of the clinical hallmarks of AD is vital for comprehending subsequent processes, including tauopathies that facilitate neuroinflammation and result in neurological impairments. Chiang et al. documented the pattern and fabrication of a QDs imitative for greater orbicular oligomeric amyloid variety, serving as an endogenously fluorescent proxy for this cytotoxic amyloid fabrication to examine its role in eliciting inflammatory and stress response states in neuronal and glial cell types [232]. Figure 14 illustrates that the fomite and poly(ethylene glycol)-coated quantum dot treated cultures exhibit no discernible change in the amplitude of calcium transients, as quantified by the total fluorescence intensity normalized to the region of interest. Conversely, amyloid-beta quantum dots (ABQDs) elicit a much greater calcium response, detectable at the 200 s mark (Figure 14b–d) with the normalized intensiveness shown in Figure 14a. The elevated calcium inflow in QD-processed neuronal cells indicates excitotoxic stress in neurons, accompanied by observable neurite destruction.

Figure 14

Calcium transients from neuroglial cultures in reaction to amyloid. (a) Plotted calcium transients from neuron-astrocyte co-cultures grounded with an equivalent volume dilution of vehicle in medium (b), 10 nM PEGQDs (c), or 10 nM ABQDs (d) followed by 30 mM KCl calcium induction [232].

Wu et al. noted that CdTe QDs induced cell death and apoptosis in rat elementary cultured hippocampus neurons in a manner dependent on dosage, duration, and size. Neurons exposed to QD exhibited elevated levels of ROS and intracellular calcium, resulting in neuronal apoptosis and potential mortality, which may be entirely or partly mitigated by the antioxidant NAC. Future studies must examine the underlying processes by exploring the extrinsic and intrinsic channels via which CdTe QDs elicit neurotoxic effects [233].

Figure 15 illustrates the complex function of antioxidant QDs in alleviating secondary damage mechanisms linked to SCI. After SCI, six principal pathogenic processes are activated: OS, neuroinflammation, apoptosis, glial scar formation, poor angiogenesis, and neurodegeneration. Antioxidant QDs specifically target each route, halting the damage cascade and facilitating neuroprotection and regeneration. OS is an initial reaction to SCI, caused by excessive synthesis of ROS that surpasses cellular antioxidant defenses. This leads to significant harm to lipids, proteins, and DNA. The figure illustrates that QDs function as scavengers for ROS [234]. They emulate the function of antioxidant enzymes, including SOD and glutathione peroxidase (GPx), diminishing ROS levels and reinstating redox equilibrium. The heightened activity of endogenous antioxidants such as GSH enhances their protective effects, reducing oxidative damage at the molecular level [235]. Neuroinflammation, characterized by microglial activation and macrophage infiltration, ensues after OS and exacerbates secondary injury. Pro-inflammatory cytokines, including TNF-α and IL-6, are produced, perpetuating a feedback loop of inflammation. The graphic illustrates that QDs impede critical inflammatory pathways, notably NF-κB, thereby diminishing cytokine synthesis [236]. This inhibits the activation of inflammatory cells, reducing tissue damage and promoting a reparative environment. Apoptosis, or programmed cell death, significantly contributes to tissue loss after SCI. It is facilitated by both internal (mitochondrial) and extrinsic (death receptor) mechanisms, leading to the activation of caspases-3 and 9. The graphic demonstrates that QDs block apoptosis by obstructing caspases and activating the PI3K/Akt signaling pathway, hence enhancing cell survival. This intervention safeguards neurons and oligodendrocytes, sustaining the structural integrity of the spinal cord. The creation of glial scars, largely induced by reactive astrocytes, is identified as a significant impediment to axonal regeneration. The graphic illustrates that QDs influence astrocytic phenotypes, altering the equilibrium from neurotoxic A1 astrocytes to reparative A2 astrocytes [237]. This shift diminishes the inhibitory influence of the glial scar, promoting axonal regeneration and aiding brain repair. Impaired angiogenesis, resulting from vascular damage and hypoxia, is a significant concern in SCI. The figure illustrates that QDs facilitate angiogenesis by increasing vascular endothelial growth factor levels and fostering microvascular repair. The reestablishment of blood circulation and oxygen delivery facilitates tissue regeneration and mitigates the hypoxic conditions that impede healing. Ultimately, neurodegeneration, marked by axonal degradation and synaptic impairment, is a prolonged outcome of SCI [238]. The graphic highlights that QDs enhance neuroprotection by elevating the degrees of neurotrophic components, admitting BDNF and NGF. These elements facilitate axonal regeneration, synaptic reorganization, and neuronal viability, hence enhancing functional results. It clearly illustrates the pivotal function of QDs in targeting these pathways, with arrows connecting the SCI-induced degenerative processes to their corresponding QD-mediated therapies. This integrated architecture illustrates how QDs tackle several facets of secondary damage, offering a comprehensive therapy strategy. Their multifunctionality positions them as a revolutionary tool in the therapy of SCI, connecting oxidative damage, inflammation, cell death, and regeneration processes.

Figure 15

Diagram illustrating the pathways and mechanisms of the neuroprotective effects of antioxidant QDs in SCI models.

QDs have emerged as promising nanomaterials with significant potential in advancing neuroprotection and neuroregeneration, particularly in the context of SCI [180,239,240]. Their unique optical, electronic, and surface properties enable them to interact with neural cells in ways that can mitigate OS, promote cell survival, and enhance neuronal regeneration. As OS is a major contributor to secondary injury mechanisms in SCI, the antioxidant capabilities of QDs play a crucial role in protecting neurons from damage. One of the key neuroprotective mechanisms of QDs is their ability to scavenge ROS, which are excessively produced following SCI [149]. ROS accumulation can trigger lipid peroxidation, protein degradation, and DNA damage, ultimately leading to neuronal apoptosis and glial scarring. Antioxidant QDs, particularly those doped with elements such as selenium, cerium, or europium, have demonstrated potent ROS scavenging abilities. For instance, cerium oxide QDs exhibit regenerative redox properties, where their surface cerium ions (Ce³⁺/Ce⁴⁺) dynamically switch oxidation states to neutralize ROS [241]. Similarly, carbon-based QDs such as CDs and GQDs possess intrinsic antioxidant properties attributed to their abundant surface functional groups, including hydroxyl, carboxyl, and amino moieties [242]. These functional groups facilitate electron transfer processes that stabilize ROS, reducing OS-induced neuronal damage. In addition to their neuroprotective roles, QDs contribute to neuroregeneration by promoting cell proliferation, axonal growth, and synaptic repair. QDs can serve as carriers for neurotrophic factors such as BDNF, NGF, or glial cell line-derived neurotrophic factor [243]. By facilitating the sustained and localized release of these biomolecules, QDs create a microenvironment conducive to neuronal survival and axonal sprouting. Furthermore, surface-modified QDs can interact directly with cell surface receptors, activating intracellular signaling pathways that enhance neuronal growth. For example, QDs functionalized with peptides or bioactive ligands can stimulate the PI3K/Akt or MAPK/ERK pathways, which are crucial for promoting cell survival and inhibiting apoptosis. Moreover, QDs have shown potential in modulating the inflammatory response following SCI. The inflammatory cascade triggered by activated microglia and astrocytes often exacerbates neural damage. Studies have demonstrated that antioxidant QDs can suppress the release of pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6 while promoting the expression of anti-inflammatory mediators [244]. This immunomodulatory effect reduces glial scarring and creates a permissive environment for axonal regeneration [245]. Another key advantage of QDs in neuroregenerative applications is their potential for targeted delivery and bioimaging. Due to their tunable fluorescence and stable emission properties, QDs enable real-time tracking of neuronal repair processes. Functionalized QDs can selectively bind to injured neurons or regenerating axons, providing valuable insights into cellular dynamics during recovery [246]. While both SCI and neurodegenerative diseases share common secondary pathophysiological features such as OS, neuroinflammation, and mitochondrial dysfunction, it is critical to distinguish the underlying primary mechanisms that define each condition [247]. SCI is characterized by an acute mechanical trauma leading to immediate axonal disruption, vascular compromise, and subsequent formation of glial scars that impede axonal regeneration [30,248–250]. In contrast, neurodegenerative diseases such as Alzheimer’s and Parkinson’s are chronic progressive disorders driven by pathological protein aggregation, including amyloid-beta plaques, tau tangles, and α-synuclein accumulation, which result in gradual neuronal loss [251]. Although QDs may offer therapeutic advantages in contexts due to their ROS-scavenging, anti-inflammatory, and neuroregenerative properties, their application must be disease-specific, considering the distinct microenvironment and progression dynamics of each condition [252]. For instance, in SCI, QDs can be engineered to target the acute inflammatory milieu and promote axonal regrowth through neurotrophic factor delivery, whereas in AD, their utility may extend to imaging and modulating protein aggregation [253]. Therefore, a clear mechanistic understanding is crucial to guide the precise and effective application of QDs based involvements, ensuring they are specifically aligned with the unique pathophysiological features of SCI rather than being broadly applied within a generalized neurodegenerative context. Additionally, QDs integrated with responsive drug delivery systems allow for controlled release of therapeutic agents at the injury site, enhancing the precision and efficacy of neuroprotective treatments.

3.3 Advancements in regenerative medicine using antioxidant QDs

Improving stem cell treatment results in SCI with QDs demonstrating a significant advance in regenerative medicine. Stem cell treatment seeks to substitute impaired brain cells, facilitate axonal regeneration, and re-establish functional connection [254]. Nonetheless, obstacles such as diminished cell viability, suboptimal differentiation, and insufficient incorporation into host tissues limit its therapeutic efficacy. QDs may mitigate these constraints by providing multifunctional assistance to stem cell treatment via their distinctive optical, electrical, and antioxidant characteristics. QDs improve the viability of transplanted stem cells by reducing OS in the injury microenvironment [255]. ROS caused by SCI often result in diminished cell viability; nevertheless, QDs function as effective antioxidants, mitigating ROS and safeguarding transplanted cells from oxidative harm. This stabilization enhances stem cell viability and retention at the damage location. Moreover, QDs may alter the inflammatory milieu by suppressing pro-inflammatory pathways like NF-κB, diminishing cytokine-induced stem cell death, and establishing a more conducive habitat for cell engraftment [256]. Furthermore, QDs enhance the development and functional integration of stem cells via modulating cellular signaling pathways. QDs may facilitate the differentiation of stem cells into neurons and glial cells, essential for spinal cord regeneration, by releasing bioactive ions or delivering targeted stimulation. Their fluorescent characteristics allow for real-time monitoring of stem cell fate and distribution, aiding in the refinement of therapy procedures. Moreover, QDs may improve axonal guidance and synaptic connection by delivering bioelectrical signals, facilitating the functional integration of transplanted cells into the host brain network.

However, to support the feasibility of such integrated strategies, it is essential to understand and explain how QDs mechanistically interact with stem cells and influence their behavior both in vitro and in vivo. QDs can modulate stem cell fate through several mechanisms. Their surface functionalization enables the controlled release of bioactive molecules, such as growth factors and small-molecule drugs, which can direct stem cell differentiation toward specific cell types [257]. For example, carbon-based QDs functionalized with neurotrophic factors like BDNF or NGF have been shown to enhance neuronal differentiation of MSCs and NSCs. In one study, nitrogen-doped CQDs promoted neuronal lineage commitment of NSCs by modulating intracellular ROS levels and activating the PI3K/Akt pathway, a critical regulator of cell survival and differentiation [258]. This indicates that QDs can act as both carriers and bioactive modulators in a stem cell microenvironment. Moreover, QDs influence cell survival and engraftment, which are two critical factors for the success of stem cell therapy. OS at the injury site is a major barrier to stem cell survival. QDs with antioxidant properties, such as cerium-, sulfur-, or nitrogen-doped carbon dots, can neutralize ROS in the microenvironment and protect transplanted stem cells from apoptosis. In a recent in vivo study, cerium oxide QDs were co-administered with stem cells in a SCI model, significantly improving cell viability and functional recovery compared to stem cells alone. QDs also enhance the homing and tracking of transplanted cells [259]. Their tunable fluorescence properties allow for real-time, non-invasive imaging of stem cells post-transplantation. This capability enables precise monitoring of cell migration, integration, and survival over time, providing critical feedback for optimizing therapeutic protocols. For instance, CdSe/ZnS QDs have been successfully used to label MSCs without affecting their differentiation potential, allowing for in vivo tracking in animal models of neurodegeneration. Another avenue is QDs mediated gene delivery [260]. Functionalized QDs can deliver genetic material or siRNA to stem cells to promote specific phenotypic outcomes. For example, QDs conjugated with neural lineage-specific transcription factors or microRNAs have been used to steer stem cell differentiation toward neurons or oligodendrocytes, which are vital for remyelination and synaptic restoration in SCI repair [261]. Despite these promising attributes, QD–stem cell combinations must be carefully optimized for biocompatibility, long-term stability, and controlled release. The choice of QD type, surface chemistry, and dosage all critically influence stem cell responses. Non-toxic alternatives such as silicon QDs or carbon-based QDs are preferred over cadmium-based QDs due to lower cytotoxicity and better biodegradability.

Ma et al. investigated the novel use of DA-functionalized QDs to assess and alleviate neurotoxicity induced by DA quinone [262]. This finding is crucial for improving stem cell treatment results in SCI. The research emphasizes that DA-functionalized QDs may sensitively detect OS and neurotoxic consequences, offering real-time insights into cellular responses during stem cell therapies. The researchers used the optical features of these QDs to monitor the health of dopaminergic neurons and their oxidative states under neurotoxic environments. This meticulous monitoring may enhance the milieu for transplanted stem cells, assuring better survival, differentiation, and integration into the impaired brain networks. Moreover, the antioxidant characteristics of QDs might mitigate ROS, hence diminishing secondary injury in SCI. Within the framework of SCI, the combined role of sensitive detection and neuroprotection establishes DA-functionalized QDs as significant assets for improving the effectiveness of stem cell therapy (Figure 16). Their administration may enhance neuroprotection measures, diminish inflammation, and increase brain regeneration results, so advancing SCI therapy procedures. Another research highlighted the promise of combining nanotechnology with regenerative medicine to tackle intricate brain problems. Ensuring a conducive microenvironment is crucial for effective nerve regeneration. Cell-based treatment provides the potential for cell replacement and the enhancement of axonal development. Adipose tissue-derived mesenchymal stromal cells (Ad-MSC) are of interest due to their neuroregenerative and anti-inflammatory characteristics [263]. This research aimed to assess the impact of canine and murine Ad-MSC transplantation on the regeneration of the sciatic nerve.

Figure 16

(a) Self-assembly and FL quenching/recovery of N-(3,4-dihydroxyphenethyl)-5-mercaptopentanamide (DAs) functionalized CdTe/ZnS QDs. Schematic of DA oxidation and Cys residue-DA quinone irreversible interaction and (b) DAs quinone-QD bright field pictures, FL micrographs, and merged images from different N-ethylmaleimide concentration [262].

Agarwal et al. revealed that CdSe/ZnS core/shell QDs, surface-functionalized with a zwitterionic compact ligand, can transport a cell-penetrating lipopeptide to the developing chick embryo brain without observable damage [264]. The QD intensity was widespread over the brain, peaking between E8 and E11, with fluorescence concentrating in the choroid plexus before diminishing before hatching (E21/P0). No anomalies were identified in embryonic patterning or embryo survival, or in mRNA in situ hybridization. It was proposed that QDs may facilitate the identification and tracking of migrating NSCs, that the choroid plexus eliminates these injected QDs/nanoparticles from the brain post-E15, and that they can transport medicines and peptides to the developing brain. Maximum brain labeling was found at E8, during which QDs were widely and consistently dispersed throughout the whole brain, including the forebrain, midbrain, and hindbrain, creating different labeling patterns as shown by longitudinal brain sections (Figure 17).

Figure 17

E8 QD-peptide conjugate distribution. E8 embryonic chick brain QD-CL4-JB577 (red) distribution following spinal canal injection at E4. Representative coronal slices (40 μm) of the embryo’s forebrain (a), midbrain (b), and hindbrain (c) demonstrate similar QD distribution at 4 days post-injection (E8). A neuronal layer heavily labeled by QDs is indicated by the bracket in B. In panel B, the small white bracket indicates a neuronal layer that is heavily labeled with QDs.

CdSe/ZnS QDs may be conveyed from the olfactory tract to the brain by inhalation. Adult C57BL/6 mice were subjected to a 1 h nasal inhalation of aerosolized QDs, with nanoparticles identified 3 h post-exposure in the olfactory tract and olfactory bulb using several methods [265]. Subsequent to brief inhalation of solid QD nanoparticles, there is fast olfactory absorption and axonal conveyance to the brain/olfactory bulb, accompanied by the activation of microglial cells, indicating a pro-inflammatory response. Shen et al. synthesized 0D black phosphorus QDs modified with the antioxidant β-carotene and thoroughly examined them in SCs to clarify their potential for peripheral nerve regeneration [266]. In vitro studies revealed that BPQD@β-carotene exhibits little toxicity and excellent biocompatibility, promoting brain regeneration, angiogenesis, and the modulation of inflammation in stem cells. Additionally, the PI3K/Akt and Ras/ERK1/2 signaling pathways were active in SCs at the genetic, protein, and metabolite levels. The BPQD@β-carotene-embedded GelMA/PEGDA scaffold improved functional recovery by enhancing axon remyelination and regeneration, as well as increasing intraneural angiogenesis in peripheral nerve damage models including rats and beagle dogs.