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This chapter describes the exciton concept and exciton Bohr radius. It describes new and diverse QDs applications in the fields of photonics, quantum dot lasers, photon upconversion (PUC) and downconversion (PDC) and their applications, biosensors, environmental sensing, ratiometric fluorescence and colorimetric dual-mode sensors, food quality sensing, cancer biomarkers detection, non-photonic medical imaging including magnetic resonance imaging (MRI), radiolabeled quantum dots, positron emission tomography (PET), drug delivery, blood-brain barrier (BBB) crossing, electrochemical sensing, photocatalysis including CO2 reduction, H2 production, and environmental remediation. The chapter ends with a Conclusion and prospects section expecting crucial QDs industrial applications such as displays, solar cells, wastewater treatment, quantum computers, and biomedical applications. Heavy metal-free QDs formulations are a demand to minimize traditional QDs toxicity. There is progress in using non-toxic and eco-friendly starting materials, including carbon-based, biomolecules-based, silicon-based, and ternary I-III-VI QDs alternatives.
Faculty of Science, Chemistry Department, Tanta University, Tanta, Egypt
Ehab A. Okba
Faculty of Science, Chemistry Department, Tanta University, Tanta, Egypt
*Address all correspondence to: elzeiny.ebeid@science.tanta.edu.eg
1. Introduction
Quantum dots (QDs) are nanoscale particles that exhibit unique optical and electronic properties due to quantum confinement effects. They are characterized by small sizes that are smaller than twice the size of its exciton Bohr radius. The principles behind quantum dots are rooted in quantum mechanics, solid-state physics, and nanotechnology. The confinement of electrons and holes within the quantum dot structure leads to the quantization of energy levels. In other words, the energy levels of electrons and holes become discrete rather than continuous.
Quantum confinement in QDs leads to optical tunability based on their size. Some unique spectral properties of QDs are relatively narrow and symmetrical emission peaks spanning the UV to the IR spectral regions, high emission quantum yields, high photostability, high molar absorptivities, long emission lifetimes, and large Stokes shifts. This led to a variety of important research and commercial applications including bioimaging, solar cells, LEDs, diode lasers, transistors, phosphor displays, electrochemiluminescence, and on/off switches on electrode surfaces [1, 2, 3, 4, 5].
The small QDs size makes them suitable for biological and medical applications including bioimaging and biosensors. For instance, QDs are used in studying intracellular processes, targeting tumors, real-time observation of cell trafficking in vivo, diagnosis, and cellular imaging at high resolutions [6]. The large volume-to-size ratio renders QDs of valuable applications in catalysis, solar cells, delivery of therapeutics, and highly enhanced adsorption of chemicals [7].
More recently, electrochemical nanosensors in the form of carbon quantum dots (CQDs), graphene quantum dots (GQDs), and semiconductor quantum dots are widely applied to fabricate sensing systems [8]. This assured better redox properties allowing detection of many analytes such as ions, pharmaceuticals, small molecules, and biological macromolecules.
The high edge-to-area ratio in some QDs leads to spin polarized edge states, which can generate magnetic properties. Magnetic quantum dots (MQDs) find applications in catalysis, sensors, cancer treatment, environmental remediation, and contrast-improving agents in magnetic resonance imaging (MRI) [9].
With the number of drug-resistant infections continually rising, QDs find applications to treat antibiotic-resistant infections by producing reactive oxygen species (ROS) when exposed to light, which can result in oxidative damage to bacterial cells [10], or alternatively producing antibiotics that release a superoxide enzyme with quantum dots [11, 12].
QD lasers apply semiconductor QDs as active laser media in their light-emitting regions. QD lasers avoid some of the drawbacks associated with traditional semiconductor lasers based on bulk or quantum well (Q-well) lasers [13, 14]. In Q-well lasers, a change in the dimensions of the well changes the output wavelength, and a larger population inversion is needed for laser action to be achieved. In QD lasers, a wide range of light emission is obtained by controlling dot size, and small active volumes are needed, allowing lasing action using smaller population inversion. This leads to less temperature dependence of threshold current density, an increase in gain (2–3 times larger than Q-well), high-frequency operation, and more laser efficiency [15].
Due to the discrete nature of energy levels in QDs, QD semiconductor lasers are characterized by properties such as excellent temperature stability, tolerance to crystalline defects, low threshold current density, and reduced reflection sensitivity. They can produce wavelengths from the near-infrared (NIR) region to the visible and even into the ultraviolet (UV) range. Typical applications often operate within the near-infrared spectrum, employing wavelengths ranging from 600 to 1600 nm. The discrete nature also serves in quantum cryptographic communication as well as quantum information processing for applications such as quantum computers. The use of discrete energy level of quantum dots allows the absorption of long-wavelength light necessary for future development of high-efficiency solar cells. A number of quantum dots applications include high-sensitivity, low-dark-current infrared detectors allowing a strong carrier confinement [16]. The construction of a quantum dot laser diode is shown in Figure 1.
Figure 1.
The construction of a quantum dot laser diode with p- and n-region layers. The active layer situated between them contains quantum dots. Metal contacts connected to the p-region and n-region facilitate electrical current flow within the diode.
Wu et al. developed a theory of output optical power of QD lasers based on carrier injection from the cladding layers to the optical confinement layer (OCL) followed by carrier capture from the OCL into the QD excited state and carrier escape from the QD excited state to the OCL. Spontaneous radiative recombination in the OCL occurs with downward transition in QDs (intradot relaxation), upward transition in QDs, spontaneous and stimulated radiative recombination via QDs excited state, and spontaneous and stimulated radiative recombinations via the ground state in QDs [17]. The faces of p- and n-layers are coated with high reflectivity materials causing light reflection back and forth to increase stimulated emission and in turn, the laser emission is enhanced (Figure 2).
Figure 2.
Energy band diagram of a QD laser (the layers are not drawn to scale). The main processes (shown by arrows) are as follows: ① carrier injection from the cladding layers to the optical confinement layer (OCL), ② carrier capture from the OCL into the QD excited state, ③ carrier escape from the QD excited state to the OCL, ④ spontaneous radiative recombination in the OCL, ⑤ downward transition in QDs (intradot relaxation), ⑥ upward transition in QDs, ⑦ spontaneous and stimulated radiative recombination via the excited state in QDs, and ⑧ spontaneous and stimulated radiative recombinations via the ground state in QDs. Adapted with permission from Ref. [17]. Copyright (2015), AIP Publishing.
Figure 3.
Some common methods for (PUC) phenomenon given in Table 1.
2.2 QDs in photon upconversion (PUC) and downconversion (PDC) and their applications
Photon upconversion (PUC) refers to a nonlinear optical process whereby two or more low-energy photons are converted into one high-energy photon (i.e., anti-Stokes behavior) [18]. The discovery of photon upconversion was coined in 1966 by François Auzel who showed that an infrared photon of light could be upconverted into a photon in the visible-light region in ytterbium-erbium and ytterbium-thulium systems [19].
On the other hand, photon downconversion (PDC) is a process in which a high-energy ultraviolet (UV) photon is converted into a low-energy visible or NIR photon. The (PDC) phenomenon was first reported in YF3: Pr3+, in which one ultraviolet (UV) photon was converted into two visible (Vis.) photons of lower energy [20]. Lanthanide ions dopants are applied as activator ions since they have multiple 4f excitation levels and filled 5s and 5p shells. These outer-filled shells act as a shield for the 4f electrons, thus producing sharp f-f transition bands. The electronic configuration of Lanthanides is [Xe] 4f1–145d0–106s2 where the 4f electrons are not affected by environmental conditions due to outer orbits 5d0–106s2 electrons. The coupling of the 4f electronic excited states to the surrounding lattice is weak and this leads to long excited state lifetimes and sharp optical line shapes [21].
Both (PUC) and (PDC) phenomena are helpful in many fields, including photonics, nanomedicine, biomedical, and therapeutic applications. Table 1 summarizes some common methods to get (PUC) and Table 2 summarizes some common methods to get (PDC) phenomena [22].
Energy migration-mediated upconversion (EMU) (Figure 3f)
Simultaneous Virtual intermediate states (dotted) (Figure 3g)
Sequential Real Metastable excited states. UC emission is much more efficient (Figure 3h)
Table 1.
Some common methods for (PUC) phenomenon.
Table 2.
Some common methods for (PDC) phenomenon.
A brief description of processes in Tables 1 and 2 is given as follows:
2.2.1 Photon avalanche (PA)
In photon avalanche (PA), multiple photons combine to produce a single higher energy photon. This phenomenon was first discovered by Chivian in 1979 in Pr3+ doped LaCl3/LaBr3 systems based on infrared quantum counters [22, 23, 24, 25, 26]. Although PA is seen in some systems, it is the least observed mechanism for upconversion [26].
Cooperative sensitization upconversion implies the simultaneous decay of two lanthanide ions in their excited states to their ground states, generating a higher energy photon [22, 27, 28]. This encounters one or more elementary steps (sensitization or luminescence). Similarly, in cooperative luminescence, two excited state ions transfer their energy to a neighboring ion in one elementary step.
2.2.3 Excited state absorption (ESA)
In excited-state absorption, a previously exited electron within an ion absorbs a photon [29] and is promoted by long-lived metastable energy levels. This exists in many lanthanides including Er3+, Tm3+, and Ho3+ ions.
2.2.4 Energy-transfer upconversion (ETU)
In energy-transfer upconversion, energy transfers from one exited ion to another. An ideal energy-mediated upconversion requires a core-shell nano-system with a sensitizer ion (e.g., Yb3+) embedded in the core. Its role is to transfer energy via a ladder (e.g., Tm3+) to a migratory ion capable of promoting the energy to an activator ion located in the shell. Energy-transfer upconversion (ETU) and excited state absorption (ESA) are the two most common processes by which upconversion occurs in lanthanide-doped nanoscale materials [20, 30].
2.2.5 Simultaneous two-photon absorption (TPA)
Simultaneous two-photon absorption involves virtual intermediate states for absorption of two or more photons [22].
2.2.6 Sequential two-photon absorption (TPA)
In sequential two-photon absorption, real metastable excited states allow for sequential absorption. Therefore, a necessary condition for upconverting systems is the existence of optically active long-lived excited states. It is of more interest compared to simultaneous two-photon absorption that is based on virtual energy levels. In sequential two-photon absorption, lower energy densities are required (low-power and incoherent excitation sources) [20, 22].
2.2.7 Triplet-triplet annihilation (TTA)
Triplet-triplet annihilation (TTA)-based UC emission is common in organic molecules. Since TTA systems are usually very sensitive toward oxygen (a known triplet quencher), special experimental precautions are required. Inorganic semiconductor quantum dots (QDs) are potential candidates in TTA showing a potential role in harvesting low-energy photons, which can then be used for photon upconversion (PUC), via triplet-triplet annihilation (TTA) [31].
Wang et al. achieved a strong coupling regime in which excited carriers spatially delocalize across anthracene molecules strongly linked to silicon quantum dots via double bonds. Triplet exciton annihilation leads to photon upconversion with high efficiency (17.2%) and low threshold intensity (0.5 W cm−2) [32].
2.2.8 Energy migration upconversion (EMU)
Energy migration upconversion (EMU) is a complex process mainly observed in core-shell structures in which four kinds of lanthanide ions are embedded. It was first proposed by Liu and coworkers in 2011 [33]. A sensitizer ion (type I) located in the shell is first excited and then transfers its excitation energy to an accumulator ion (type II) that is also located in the shell. The energy transfers from the high-lying excited state of the accumulator to a migrator ion (type III) that is also located in the shell. An excited migrator ion then forwards its excitation energy across the core-shell interface via random hopping through many (type III) migrator ions that populate the core-shell interface and the core. Finally, the migrating energy is trapped by the activator ion (type IV) located in the core, resulting in upconverted emission [34, 35, 36].
2.2.9 Photon downconversion (PDC)
2.2.10 Quantum cutting (QC) phenomenon
In quantum cutting (QC) phenomenon, a high-energy photon is converted into two or more than two photons of lower energy. It was long applied to get photon downconversion (PDC) [37, 38].
2.2.11 Long persistent luminescence (LPL)
Long persistent luminescence (LPL) is a phenomenon in which light emission lasts for a very long time (hours) after excitation is ceased. Referring to Table 2, an ion is excited to a delocalized excited state (D). The delocalized electrons can be trapped for a long time. After ceasing the excitation sources, the trapped electrons can be de-trapped back to the delocalized state (D) because of thermal or photon activation. This is followed by a non-radiative relaxation pathway to the E state and then radiatively deactivates to the ground state (G) giving LPL. Representative LPL systems include SrAl2O4:Eu2+, Dy3+ (green), CaAl2O4:Eu2+, Nd3+ (blue), Y2O2S:Eu3+, Mg2+, Ti4+ (red), and Zn3Ga2Ge2O10:Cr3+ (near-infrared: NIR). Near-infrared quantum cutting long persistent luminescence (NQPL) is also a known phenomenon [38].
2.2.12 QDs in photonic technology
In the field of photonic technology, quantum dots (QDs) have become an essential component, greatly impacting the advancement of high-performance devices, especially in the LED and QLED markets. Semiconductor quantum dots (QDs) have many useful features that can be used in photonic applications. These include the ability to modify the bandgap, structural stability, cost-effectiveness, vast area coverage, and solution processability. Quantum dots (QDs) are essential for producing vivid, dynamic displays with high color accuracy, and are used in QLEDs for efficient light production. Their adaptable properties have a significant impact on various disciplines, including soft robotics, computer hardware, and medical devices [39].
Perovskite quantum dots (QDs) have greatly enhanced the performance and efficiency of light-emitting diodes (LEDs), especially in the case of QLEDs. The nanocrystals, namely the all-inorganic cesium lead halide (CsPbX3) QDs, exhibit exceptional photoelectric characteristics, rendering them very suitable for photonic applications such as LEDs. The utilization of titanium-doped cesium lead iodide quantum dots (QDs) in conjunction with yellow-emitting phosphors, blue LED chips demonstrates the promise of perovskite QDs in solid-state lighting and display technologies [40, 41, 42, 43, 44, 45, 46, 47].
2.2.13 Photon upconversion (PU) and photon downconversion (PDC) applications
He et al. reported a novel fluorescent probe based on the combination of rare earth upconversion QDs, perovskite quantum dots, Molecular Beacon as a carrier, and Black Hole Quencher-1. The probe was used for cancer early detection via specific recognition of miRNA-155 at concentrations down to a detection limit of 73.5 pM [48, 49].
Lai and Wu reported that interfacial triplet energy transfer from red-emitting CdSe QDs to surface-anchored Rhodamine B is enhanced upon coating with ZnS shell. The generated RB triplet state can be applied in singlet oxygen generation for the purpose of application in photodynamic therapy (PDT) [50].
Lu et al. reviewed the potential applications of QDs in optical energy conversion into other useful forms of energy including optical energy to electricity, photon upconversion (PUC) and photon downconversion (PDC), optical energy to chemical bonds including green hydrogen generation, QDs with cocatalysts, integrating QDs into photoelectrochemical cells, as photocatalysts in organic synthesis, CO2 reduction and N2 fixation [50].
The demand for non-toxic QDs is growing. Most existing quantum dots contain toxic metals such as cadmium and lead, which limit their electronic and medical applications. Safer core-shell quantum dots of InP core surrounded by a thin shell of ZnS that emit pure blue light in a solid state and solution were prepared. The dots emit blue light even inside the biological cells [51].
2.3 Quantum dots as sensors
Quantum dots (QDs) have emerged as promising tools in the field of medical imaging, offering unique advantages that can revolutionize diagnostics and treatment monitoring. Their size-tunable fluorescence emission, high photostability, and brightness make them ideal candidates for various imaging modalities. In fluorescence-based imaging, QDs outperform conventional organic dyes, providing enhanced image quality and reduced photobleaching. Because of their surface functionalization capabilities and exceptional optical qualities, quantum dots (QDs) have in vivo applications that allow for the real-time imaging of biological processes [52, 53].
Quantum dots probes enjoy unique properties that make them excellent fluorescent imaging agents. These include:
Their size-modulated absorbance and emission enable the fabrication of identically constructed probes with diverse optical characteristics for the aim of multiplexing [54, 55].
High photostability allows imaging for lengthy periods of time without signal loss—potentially useful for fluorescence-guided surgery [55].
A large Stokes shift makes it easier to block out the excitation light and allows the use of a single excitation wavelength for imaging in multiple colors [54].
Their high brightness is attributed to their high excitation cross-section values. This allows improved sensitivity and allows fluorescence imaging at low receptor concentration levels [55].
Despite having similar photoluminescence quantum yields (PLQYs) in the UV-visible region, QDs have superior PLQYs application in the NIR region compared to organic dyes [56].
The remarkably long lifetimes of excited states allow the development of probes with time-delay microscopy and this avoids interference with the autofluorescence of cells and other moieties [57].
High-surface area-to-volume ratio in QDs allows for efficient functionalization with other imaging agents and the creation of multi-modal probes to be used in imaging of disease states at different scale lengths and different tissue depths [58].
The unique optical characteristics and multifunctionality of nanoscale semiconductor particles, such as carbon-based, chalcogenide, black phosphorus, and cadmium containing QDs, make them valuable instruments for imaging cancer cells. Benefits of QDs include intense fluorescence, a wide excitation range, narrow emission, and excellent resistance to photobleaching. QDs are intriguing prospects for enhancing cancer cell imaging in medical research and diagnostics because of their potential for early disease identification and their ability to be adjusted for targeted imaging [59].
2.3.1 Biosensors
CQDs are utilized as biosensor carriers due to their excellent water solubility, surface modification flexibility, non-toxicity at the applied concentrations, excitation-dependent emission in different light regions, high biocompatibility, good cell permeability, and strong photostability. These CQD-based biosensors have been employed for real-time monitoring of various analytes, including glucose [60], cellular copper ions [61], phosphate ions [62], iron ions [63], potassium ions [64], pH [65], and nucleic acids [66]. In nucleic acid detection, CQDs are used as efficient fluorescent sensing platforms capable of detecting single-base mismatches. This approach involves the initial adsorption of fluorescently labeled single-stranded DNA probes onto CQDs through π-π interactions, leading to significant fluorescence quenching. Upon specific hybridization with their target sequences to form double-stranded DNA, the dsDNA detaches from the CQD surface, resulting in fluorescence recovery, enabling the detection of the target DNA [62].
Quantum dots (QDs) have emerged as one of the most cost-effective and promising indicators for viral infections. Leveraging QDs’ ultrasensitive capabilities as biological probes, combined with the direct use of physiological components, allows for the simultaneous detection of several viral indicators. The effectiveness of QDs as detection techniques for human pandemic viruses is based on incorporating quantum dot-based biomarkers into ultrasensitive probes for multiplex detection of human viral infections. This collaborative approach has great promise for improving our ability to combat viral infections effectively [67].
Quantum dots (QDs) have the unique capacity to selectively attach to biomolecules, including oligonucleotides, proteins (including peptides, antibodies, or enzymes), and polymers, due to their special characteristics. The flexibility of this technology allows for a wide range of sensing applications, as demonstrated by the quick and accurate detection of avian influenza (AIV) virus, namely the H5N1 subtype, using fluoroimmunoassay assays based on quantum dots (QDs) [68].
2.3.2 Cancer biomarkers detection
Fluorescent quantum dots are utilized in the identification of liver cancer biomarkers using aptamer-based fluorescence biosensors, which are crucial for the diagnosis of liver cancer. These biosensors employ aptamers to label fluorescent CdTe quantum dots. Aptamers, being oligonucleotides composed of either DNA or RNA, possess the capacity to selectively bind to certain biomolecules, including alpha-fetoprotein (AFP), a prominent cancer biomarker. Fluorescence resonance energy transfer (FRET) takes place when quantum dots get close to gold nanoparticles that have been modified with an anti-AFP antibody. This action results in the reduction of the fluorescence signal emitted by the quantum dots. Employing this method enables accurate and dependable assessment of AFP levels, which is essential for both the identification and ongoing surveillance of liver cancer [69]. FRET is a distance-dependent interaction between light-sensitive molecules, transferring energy non-radiatively between donor and acceptor molecules. It is efficient when within 1–10 nm, making it useful for molecular interactions and conformational changes, and is used in biological and chemical sensing. Metal-enhanced fluorescence (MEF) is a technique that uses metallic nanoparticles to enhance fluorescence intensity in fluorophores. This interaction increases excitation rate and lifetime, enhancing fluorescence sensitivity and detection limits in fluorescent-based assays, especially for biomolecule detection in diagnostic applications.
2.3.3 Quantum dots in environmental sensing
QDs facilitate the detection of pollutants and toxic substances at very low concentrations, contributing to improved environmental protection and safety. The role of QDs in sensing is pivotal, leveraging their exceptional optical characteristics to enhance the performance of sensors in terms of sensitivity, specificity, and multiplexing capabilities, thus meeting the demands of various complex sensing tasks. Si QDs exhibited good water solubility and high stability. Under the optimized conditions, fluorescent probes for the detection of Co2+ in environmental water samples with a limit of detection of 0.37 μmol/L (based on 3 s/k) were reported [70]. Graphene quantum dots (GQDs) were synthesized and incorporated with polyethylene dioxythiophene:poly(4-styrenesulfonate) (PEDOT:PSS) and carbon nanotubes (CNT) to form a composite that can be used as humidity sensors [71].
The creation of innovative nanoribbons and nanobelts using zigzag-hexagonal graphene quantum dots (ZHG) demonstrates distinct electrical, optical, and sensing characteristics, rendering them auspicious options for volatile organic compound (VOC) sensing applications, including trichloroethylene and chloromethane. The sensitivity and selectivity of the nanobelts toward gas molecules are enhanced by their structure, which has a reduced energy gap than that of planar single quantum dots or extended nanoribbons with the interacting electronic states localized at the zigzag edges. VOCs are effectively adsorbed with the help of moderate adsorption energy and charge transfer [72].
2.3.4 Ratiometric fluorescence and colorimetric dual-mode sensors
A ratiometric fluorescence and colorimetric probe is a sophisticated sensing instrument used to identify and measure specific analytes by observing alterations in fluorescence intensity and visible color. It uses two distinct fluorescence signals to react differentially to the presence of a substance, enabling more precise measurements and mitigating environmental fluctuations [73].
Quantum dots (QDs) have been used in the creation of innovative ratiometric fluorescent probes for the precise detection of Cu2+ [74], chlorothalonil (CHL) [75], and mercury ions in environmental water samples [76].
2.3.5 Quantum dots in food quality sensing
Food quality and safety are major public health concerns worldwide. Fluorescence detection methods, such as quantum dot (QD)-based fluorescent nanosensors, are gaining popularity due to their broad detection range, high sensitivity, and rapid detection [77]. Different QDs were utilized to detect volatile components, heavy metal ions, food additives, pesticide residues, mycotoxins, foodborne pathogens, humidity, and temperature [78].
Quantum dot-polymer nanocomposites represent notable progress in food packaging, providing immediate monitoring, detection of foodborne pathogens, assessment of food quality, improvement of food safety, and extension of shelf life. These sensors utilize QDs that are incorporated within polymer matrices to monitor temperature, humidity, oxygen levels, and chemical contaminants [79]. Food safety and quality are enhanced by CdS-QDs and chitosan, carboxymethylcellulose, and pectin polymer matrices [80].
3.1 QDS in magnetic resonance imaging (MRI) medical imaging
The variable magnetic characteristics of QDs allow customization for specific imaging demands, which is why QDs are now being investigated for use in magnetic resonance imaging (MRI) as contrast agents. It is possible to obtain QDs emitting in the (650–950 nm) and (1000–1400 nm) near-infrared (NIR) windows, by selectively controlling the synthetic methodologies. This allows for superior imaging properties [81].
3.2 QDS in positron emission tomography (PET)
Radiolabeled QDs are important in molecular imaging, notably positron emission tomography (PET), which is used to investigate biological processes in vivo. PET is used to diagnose diseases, track illness progression, and aid in medication discovery in both preclinical and clinical settings. QDs are adaptable radiolabeling probes that broaden the range of synthons acceptable for radiolabeling and enable the creation of PET tracers. These PET tracers have applications in a variety of clinical disciplines, including oncology, cardiology, neurology, and drug discovery, where they improve target engagement studies and enhance drug development efforts. Radiochemical innovations, such as labeling tactics and bioconjugation processes, have expanded the range of functional groups that can be radiolabeled, resulting in the development of novel radiopharmaceuticals. Despite the difficulties with biocompatibility, toxicity, clearance, and stability, QDs in PET imaging have revolutionized the industry, increasing patient care and therapeutic results in the era of precision medicine [82].
Quantum confinement effect and size tunability enable precise targeting and imaging capabilities, offering a new horizon for therapeutic interventions with enhanced efficacy and reduced side effects [83].
A novel method for high-throughput and multiplexed single nucleotide polymorphisms (SNP) genotyping for the purpose of using the Qbeads framework applies QDs to encode microspheres utilized as a stage for multiplexed examines [84]. Qbeads is defined as a family of reagents that enables capture of specific proteins on QDs. This enables multiplexed quantitation of cytokines, adhesion molecules, enzymes, growth factors receptors and more.
On the other hand, QDs are candidates in drug delivery. Specifically, the approach of labeling a conventional drug carrier with QDs, which serve as photostable fluorescent reporters. Is rewritten as: On the other hand, QDs are candidates in drug delivery. One approach is labeling a conventional drug carrier with QDs, which serve as photostable fluorescent reporters. This technique is considered an innovative approach to developing biocompatible nano formulations that can target and treat human diseases by using functionalized nanoparticles to carry medications to specific tissues or organs. Additionally, the passage mentions that QDs have been utilized as drug carriers, both in organic and inorganic forms, and there has been significant progress in the field of ODN (oligonucleotide) and siRNA (small interfering RNA) delivery, potentially even involving microorganisms and viruses [85].
Carbon quantum dots (CQDs) have surfaced as an innovative technology that exhibits great potential for implementation in drug delivery systems. They are great candidates for targeted drug delivery because they are biocompatible, have a surface chemistry that can be changed, and can carry therapeutic agents. Coupled CQDs with gold nanoparticles form an assembly, which was then conjugated with PEI-pDNA for delivering DNA to cells [86]. Functionalized CQDs have the capability to encapsulate and transport a wide range of medications, such as gene therapies, antibiotics, and anticancer agents. CQDs functionalized gold nanorods for the delivery of doxorubicin in a multi-modal fashion, including drug delivery, photothermal therapy, and bioimaging using the same platform [87]. In addition, their fluorescence characteristics facilitate the monitoring of drug distribution in the body in real time, which enables accurate administration and reduces the occurrence of adverse effects. Drug delivery systems utilizing CQDs have the capacity to fundamentally transform the medical field through the enhancement of treatment efficacy and safety across a broad spectrum of ailments. The antibiotic ciprofloxacin was attached to CQDs with bright green fluorescence allowing bioimaging and providing an efficient new nanocarrier for controlled drug release with high antimicrobial activity under physiological conditions [88].
4.2 Quantum dots (QDs) in blood-brain barrier (BBB) crossing
The BBB serves as a highly selective semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system. It restricts the passage of most chemical substances, ensuring the brain’s protection from potentially harmful agents while also posing a significant challenge for the delivery of therapeutic agents [89]. The significance of QDs in BBB lies in its potential to modify or enhance the permeability of the BBB, enabling the delivery of drugs that would otherwise be unable to reach the brain tissue. By integrating QDs into the design of drug delivery systems, the efficiency of treatments for neurological disorders can be improved, where the ability to bypass or temporarily open the BBB could significantly enhance therapeutic outcomes. This approach holds promise for advancing the treatment of a wide range of conditions, including neurodegenerative diseases, brain tumors, and psychiatric disorders, by facilitating targeted drug delivery directly to the site of pathology [90]. Several studies have investigated QDs for efficient blood-brain barrier (BBB) penetration and brain-targeted drug delivery. The successful use of asparagine-glycine-arginine peptide NGR-PEG-CdSe/ZnS QDs is suitable for imaging malignant brain tumors, where QDs could cross the BBB and target CD13-overexpressing glioma [91], CM-QDs-PEG-PAEA-DDA was used for cellular brain imaging [92]. CdSe@ZnS/ZnS core@multiple shell QDs were used as a fluorescence imaging probe with a size of 12.7 nm (CM-QDs-PEG-PAEA-DDA). The probe has high solubility, stability, and biocompatibility and enters the brain within 1 hour [93]. PEGylated Ag2S QDs exhibiting robust near-infrared (NIR) fluorescence were successfully produced and employed to dynamically visualize intracerebral hemorrhage and increased blood-brain barrier (BBB) permeability induced by hyperglycinemia, thereby facilitating the enhanced accumulation of QDs within the brain [94]. Polyethylene glycol (PEG) was utilized as a coating for MoS2 quantum dots (QDs) to enable the attachment of a mitochondria-targeted ligand, resulting in effective penetration through the blood-brain barrier (BBB) [95]. Water-soluble zinc oxide quantum dots (ZnO QDs) were employed as carriers to deliver pDNA into the brain for Alzheimer’s disease (AD) therapy. Also, glutathione (GSH) was firmly attached to the nanoparticles (NPs) to help them pass through the blood-brain barrier (BBB) more easily through a process called receptor-mediated transcytosis (RMT) [96, 97].
Both carbon quantum dots (CQDs) and graphene quantum dots (GQDs) are promising candidates capable of penetrating the BBB. Both types of QDs show promising progress in drug delivery for the treatment of CNS diseases [98, 99].
Additionally, the use of QDs in blood-brain barrier research aids in the development of diagnostic techniques and imaging modalities. QDs’ unique optical properties, including intense and stable fluorescence, enable precise visualization of BBB permeability and integrity. This imaging capability is particularly valuable in understanding the dynamics of diseases that affect the BBB, such as brain tumors or inflammatory conditions [97]. QD-based imaging provides researchers with a non-invasive and sensitive approach to detect subtle changes in the BBB structure and function, facilitating early diagnosis and monitoring of neurological disorders. The integration of QD technology in blood-brain barrier studies contributes to the advancement of both basic neuroscience research and the development of innovative diagnostic and therapeutic strategies for neurological conditions (Figure 4).
Figure 4.
The mechanisms of external materials crossing BBB.
4.3 Quantum dots in electrochemical sensing
QDs have revolutionized electrochemical sensing, particularly in biosensing. Graphene quantum dots (GQDs) are a promising immobilizing agent for cardiac biomarkers, proving useful in early diagnosis of myocardial infarction. Their size-dependent properties enable precise detection mechanisms, and their high surface-volume ratio, biocompatibility, and surface modification capabilities make them valuable in immunological electrochemical sensors [100].
QDs have proven their versatility in various sensing applications, such as multiplex immunosensors using PbS, ZnS, and CdS QDs, and cytochrome C protein detection on gold surfaces. QDs have also been used in electrochemical sensing, generating measurable currents through electrochemiluminescence. These examples demonstrate the broad applicability and innovative use of QDs in electrochemical sensing, paving the way for advanced diagnostics and analytical techniques [5].
Two-dimensional quantum dots (2D-QDs) are a versatile material with unique properties, including large surface areas, abundant active sites, excellent electrical properties, good aqueous dispersibility, and ease of functionalization. They are used in electrochemical biosensing for various types of sensors, such as DNA, immunological, enzyme, and apt sensors. 2D-QDs such as graphene, black phosphorus, nitrides, transition metal dichalcogenides (TMDCs), transition metal oxides (TMOs), and MXenes offer a broad spectrum of characteristics due to quantum confinement effects and edge effects, which are beneficial for sensing applications. Their advantages include easy immobilization of biomolecules, biocompatibility, and high multifunctionality. They also serve as co-reactants and emitters in electrochemiluminescence biosensors. Their high-surface activity, strong adsorption capacity, and improved electron transfer efficiency make them ideal for electrode modification. 2D-QDs are also used in chemical sensing, bioimaging, solar cells, and electromagnetic wave absorption, offering high sensitivity, selectivity, and low detection limits for various analytes [101].
4.4 QDs as photocatalysts
QDs have carved out a place in photocatalysis, opening new opportunities for energy conversion and environmental remediation solutions. The QDs’ high surface-to-volume ratio offers several active sites for catalytic processes, hence augmenting their activity. Furthermore, the ability to adjust the band gap of quantum dots (QDs) by modifying their size and composition enables the customized creation of photocatalysts that can efficiently engage in various processes, such as water splitting, CO2 reduction, and degradation of organic pollutants [102].
The utilization of QDs in photocatalysis and catalysis encounters obstacles, such as the vulnerability to agglomeration and photo-corrosion, which can hinder their effectiveness. Recent progress has been centered around tackling these problems by employing techniques such as surface modification [103], heterostructure with other materials, and the integration of protective layers [102]. Surface functionalization of quantum dots significantly increases the surface dipole, which affects the valence and conduction and improves the catalytic performance [104]. Quaternary photocatalyst quantum dots exhibit significant visible-light photoactivity and quick charge migration, revealing QDs’ ability to increase the photocatalytic destruction of pollutants under visible light [105].
Green QDs can be used as photocatalysts or coupled with other semiconductors to make heterojunctions, which improve charge separation and reduce electron-hole recombination. This improves the efficiency of processes such as organic pollutant degradation, CO2 conversion to useful compounds, and water splitting for hydrogen production. Green QDs adhere to the principles of green chemistry, providing a low-toxicity and ecologically friendly alternative to standard photocatalysts. Citrus grandis-derived CQDs have been explored for metal-free photocatalysis for pollution removal under sunlight, demonstrating an eco-friendly approach [106]. Palm powder-derived CQDs co-doped with sulfur/chlorine show promising applications in visible-light photocatalysis [107].
Different applications of QDS in photocatalysis include:
4.4.1 Photocatalytic CO2 reduction
Photocatalytic CO2 reduction aims to convert CO2 into valuable organic fuels, using quantum confinement effects as a low-cost and efficient catalyst [108]. An innovative approach involves encapsulating CuO QDs within a MIL-125 (Ti) MOF and integrating LiFePO4 to promote photoreduction [109]. This reduces bandgap, enhancing electron transfer and hole transfer, preventing recombination. Supporting CdS QDs on a 3D-ordered macroporous carbon skeleton increases CO2 adsorption and provides active sites for CO2 reduction [110].
4.4.2 Photocatalytic H2 production
Quantum dots (QDs) are crucial for hydrogen production through photocatalytic water splitting, a process essential for sustainable energy generation. Their tuning capability maximizes solar energy conversion efficiency, a critical factor in photocatalytic hydrogen production. Photocatalytic hydrogen evolution was improved using a p-n heterojunction of n-type NiP2 quantum dots and p-type Cu3P nanoparticles from NiAl-LDH as a powerful photocatalyst [111].
4.4.3 Photocatalytic environmental remediation
QDs can be optimized for specific remediation tasks, such as dye degradation and heavy metal ion removal from water. Anatase/rutile QDs on thin slices of g-C3N4 were employed as a photocatalyst to degrade oxytetracycline in salted seawater [112]. Surface modification can enhance selectivity and efficiency. However, practical application poses challenges, including potential toxicity and environmental impact [113].
QDs are entities of great unique optical, electronic, and chemical properties that are not observed in other materials. These unique properties are attractive for a variety of scientific and commercial applications. Photonic applications include, for example, quantum dot lasers, light-emitting diodes (LED), displays, solar cells, and biosensors. Non-photonic applications include magnetic QDs, magnetic resonance imaging (MRI), positron emission tomography (PET), radiolabeling, drug delivery, blood-brain barrier (BBB) crossing, electrochemical sensing, photocatalysis, environment remediation, photocatalytic CO2 reduction, green H2 production. Interesting potential QD applications in future perspectives encounter crucial industries such as displays, solar cells, wastewater treatment, quantum computers, and biomedical applications. However, the major drawback of traditional semiconductor QDs made of heavy metal ions is their toxicity. Core-shell structure modification of QDs applying biocompatible ligands or polymers is one way to minimize toxicity. Heavy metal-free QDs formulations using non-toxic and eco-friendly starting materials include carbon-based, biomolecules-based, silicon-based, and ternary I-III-VI QDs are more promising alternatives.
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Written By
El-Zeiny M. Ebeid and Ehab A. Okba
Submitted: 13 February 2024Reviewed: 17 February 2024Published: 25 July 2024
Open access peer-reviewed chapter - ONLINE FIRST
