Quantum Dots for Medicine Current Applications, Challenges, and Future Directions: A Comprehensive Review

The objective of precision medicine is to provide a diagnosis and treatment that is tailored to the individual patient’s molecular profile. This requires the use of technologies that are sensitive and specific enough to investigate biological systems at the molecular or even atomic level. Quantum particles and quantum-inspired nanomaterials have been identified as promising candidates for this purpose, as they possess properties that are fundamentally different from those of conventional materials.[1]

Quantum nanomaterials, such as quantum dots, nanocrystals, and nanoscale defects, rely on quantum confinement phenomena for their distinctive properties. As the size of a material approaches the de Broglie wavelength of charge carriers, energy levels become discrete, resulting in size-dependent optical and electronic properties. In the biomedical field, this means that fluorescence emission can be designed to be tunable, with small spectral bandwidths, high quantum yields, and high resistance to photobleaching. This is especially useful in bioimaging, biosensing, and multiplexed diagnostics.[2]In addition to material aspects, quantum medicine also involves the use of quantum-sensing and imaging capabilities that rely on quantum phenomena such as spin coherence and superposition. Quantum-enhanced technologies in magnetic resonance imaging, magnetometry, and functional imaging are being developed to attempt to detect very weak signals that are often below the level of detection of conventional instruments. Notably, the goal of these technologies is not innovation, but improvement.The early and precise diagnosis of diseases is still a major challenge in the fields of oncology, neurodegenerative diseases, cardiovascular diseases, and infectious diseases. Conventional imaging techniques and biomarker analysis may not be sensitive enough for the early diagnosis of diseases or for the detection of minimal residual disease, especially when multiple targets need to be evaluated together. Quantum nanomaterials can help overcome some of these challenges as they allow multiplexed detection, high-contrast imaging, and real-time analysis of biological processes.[3,4] The many quantum nanomaterials, quantum dots have been the most investigated for their applications in biomedicine. Quantum dots have been used as fluorescent labels for in vitro and in vivo imaging, finding applications in immunoassays, nucleic acid detection, tumor imaging, and cell tracking. The high photostability of quantum dots makes it possible to monitor cellular functions for extended periods, and their ability to be functionalized enables them to target specific biomarkers for diseases.On the other hand, quantum nanomaterials are also being investigated as therapeutic agents. In theranostic applications, a quantum-based platform is used to combine diagnosis and therapy, enabling real-time monitoring of treatment. This is especially promising in photodynamic and photothermal therapy applications, where feedback from imaging can be utilized to optimize light exposure. Although there has been considerable progress, there are still challenges in the use of quantum nanomaterials for therapeutic applications.[4,5]

The purpose of this review is to give a full account of quantum nanomaterials in precision medicine, including materials, diagnostic and therapeutic uses, recent advances, challenges, and the future outlook. Through the integration of nanotechnology, quantum physics, and biomedical knowledge, this article will discuss the potential and challenges of translating quantum nanomedicine from the laboratory to the clinic.

2. Types of Quantum Nanomaterials

Table 1. Application of Quantum Dots in Biomedical

2.1 Semiconductor Quantum Dots

Semiconductor quantum dots are nanocrystals that are usually between 1-10 nm in size and are made of II-VI or III-V semiconductors such as CdSe, CdTe, ZnS, or InP. At this size, there is strong quantum confinement, which results in the ability to tune the fluorescence wavelength by size. Quantum dots have high quantum yields, sharp emission bands, and high photostability, making them excellent fluorescent markers for biomedical applications.[6,7,8,]However, for biological applications, semiconductor quantum dots are typically made water-dispersible and biocompatible by surface modification with polymers, silica shells, or lipid layers. Biological functionalization with antibodies, peptides, aptamers, or small molecules allows specific targeting of disease biomarkers. Consequently, quantum dots have been applied extensively for tumor marker conjugation, nucleic acid sensing, cell imaging, and in vivo imaging of cancer, vascular diseases, and infections. The major advantage of quantum dots is their ability to perform multiplex analysis because several quantum dots with different emission wavelengths can be applied simultaneously with very little spectral overlap.[8,9,10]

2.2 Carbon Quantum Dots

Carbon quantum dots are fluorescent nanoparticles, generally smaller than 10 nm, composed primarily of carbon and its derivatives. Unlike traditional semiconductor quantum dots, carbon quantum dots do not rely on heavy metals, which significantly improves their biocompatibility profile. Their photoluminescence arises from a combination of quantum confinement and surface-state effects, and their optical properties can be tuned through size control, heteroatom doping, and surface functionalization.[10,11,13]Carbon quantum dots can be synthesized using a wide variety of top-down and bottom-up approaches from inexpensive and abundant carbon sources. They typically exhibit good water solubility, chemical stability, and resistance to photobleaching. These properties have led to extensive exploration of carbon quantum dots as probes for cell imaging, tumor localization, and biosensing of ions, metabolites, and proteins. Furthermore, functionalization with targeting ligands and therapeutic cargos enables their use as platforms for combined imaging and drug or gene delivery.[14,15]

2.3 Graphene Quantum Dots and Related Carbon-Based Nanostructures

Graphene quantum dots are nanoscale fragments of graphene or graphene oxide that combine quantum confinement with edge effects arising from their finite size. These materials have discrete energy levels, size-dependent fluorescence properties, and a high density of surface functional groups, which are beneficial for chemical modifications. The sp²-hybridized carbon lattice is responsible for the distinctive electronic and optical properties, such as high light absorption and efficient photothermal conversion. [16, 17, 18,] In the field of biomedical research, graphene quantum dots have been explored for fluorescence imaging, photothermal and photodynamic therapy, and biosensing. The high near-infrared absorption of graphene quantum dots makes them highly desirable for photothermal therapy, and their surface chemistry allows for sensitive biomolecule detection in electrochemical and optical biosensors. These properties make graphene quantum dots versatile building blocks for multifunctional theranostic systems.[19,20]

2.4 Emerging Quantum Nanomaterials

Besides traditional quantum dots, there are a number of newly emerging nanomaterials that have been investigated for their potential use in biomedicine based on their quantum properties. Black phosphorus quantum dots have high near-infrared absorption and efficient photothermal conversion, but their chemical instability in aqueous and oxygen-rich media requires special stabilization. Perovskite quantum dots have very high efficiencies of photoluminescence and a narrow emission spectrum, but the presence of lead and stability issues have so far hindered their in vivo applications.[21,22]Transition metal dichalcogenide quantum dots, for example, those based on MoS? or WS?, are materials that exhibit the quantum size effect together with the layer properties of two-dimensional materials.These systems are under investigation for multimodal imaging, responsive drug delivery, and theranostic applications that exploit light or redox-triggered mechanisms.[23,24,25]

2.5 Quantum Sensing for Medicine

Based on the point defects generated by a substitutional nitrogen atom adjacent to a lattice vacancy, the nitrogen-vacancy (NV) defects in diamonds are a specific type of quantum sensing architecture. Spin-dependent fluorescence can be used for the optical initialization and readout of these defects, which are quantum spin systems.[26]The main advantage of NV centers is that they can detect extremely weak thermal, electric, and magnetic signals in the nanoscale under physiological conditions. In addition, NV-based sensing can detect high-resolution images of biological processes, including neuronal activities and intracellular communication. This is in contrast to other imaging techniques, including magnetic resonance imaging (MRI), which require huge magnetic fields and have low spatial resolution.[27]Moreover, real-time and continuous monitoring of dynamic biological media is facilitated by the long spin coherence time and photostability of NV centers. Due to this, quantum sensing platforms have the potential to emerge as significant tools for future diagnostics that will be much more sensitive and spatially precise compared to conventional techniques.[28]

2.6 Quantum Confinement Regimes and Optical Behavior

Quantum confinement regimes in semiconductor nanostructures are sometimes categorized depending on the size of the quantum dot in relation to the exciton Bohr radius as weak, medium, and strong quantum confinement, depending on the quantum dot radius in relation to the exciton Bohr radius. In the weak quantum confinement regime, the quantum dot radius is larger than the exciton radius, whereas in the strong quantum confinement regime, the quantum dot radius is smaller than the exciton radius. This changes the optical properties of the semiconductor nanostructures dramatically, with the quantum dot size causing a blue shift in the excitation peaks when the size is smaller.[68]Recent developments indicate that excitons may be electrically tuned to confine in 2D semiconductors, enabling exact control over excitonic states and opening the door for quantum photonic devices.[69]In addition, in PbS quantum dots, the absorption coefficients are quite different from bulk values due to quantum confinement effects. In these quantum dots, quantum confinement effects cover wide spectrum regions 5. By using confinement potentials and external fields, these quantum confinement regimes can also be used to control refractive index changes and nonlinear optics.[70]

3. Applications in Diagnostics

Table 2. Diagnostic applications of quantum Dots in  medicine

3.1 In -Vitro Diagnostics and Biosensing

In-vitro diagnostics (IVD) refers to diagnostic tests performed on biological samples such as blood, serum, plasma, urine, or saliva outside the human body to detect diseases, monitor health conditions, or assess therapeutic responses. Biosensing technologies represent the primary analytical basis for contemporary IVD systems, which allow selective, sensitive, and rapid detection of clinically significant biomarkers.[32,33]Biosensors in IVD combine a biological recognition component (such as enzymes, antibodies, nucleic acids, or aptamers) with a physicochemical transducer that translates the biorecognition process into a measurable signal. This signal is then processed and analyzed to give diagnostic results. The specificity of biosensing in IVD is mainly due to highly selective biological interactions such as enzyme-substrate reactions, antigen-antibody interactions, or complementary nucleic acid hybridization.[34,35]Even though there are several studies showing good sensitivity and specificity for quantum dot-based systems, there are differences due to differences in surface functionalization, synthesis, and biological settings, which again highlight the need for standard evaluation procedures.

3.2 Cellular and Subcellular Bioimaging

Cell/subcellular bioimaging can be described as the utilization of cutting-edge imaging tools that allow the real-time imaging of biological processes at the single cell/subcellular level in living organisms. This technique is largely driven by the use of fluorescent labels, particularly genetically encoded fluorescent proteins, which allow the specific labeling of organelles, proteins, or signaling pathways, all without affecting the biological conditions. With the use of these labels in conjunction with high-resolution imaging tools such as confocal, two-photon, or intravital microscopes, it is possible to image real-time biological processes such as protein transport, cell-cell interactions, gene expression, or signaling.[39]In subcellular analysis, bioimaging methods have been very important for understanding the location and movement of cell components such as the nucleus, mitochondria, cytoskeleton, and endoplasmic reticulum. Notably, bioimaging techniques have limitations in histology investigations as they are capable of monitoring living organisms in their lifetime. In addition to this limitation, bioimaging techniques are facing challenges in terms of the scattering of light, depth penetration, as well as photobleaching. However, continuous improvements in probes for bioimaging analysis have solved this limitation. In this way, bioimaging analysis in cells or subcellular components can no longer be underestimated in understanding biological systems.[40]

3.3 In Vivo Imaging and Early Disease Detection

In vivo imaging finds immense applications in the early diagnosis and treatment of diseases because it enables the visualization of structural, biochemical, and functional alterations within living tissues prior to the manifestation of symptoms. Modern imaging modalities, including optical imaging (autofluorescence and narrow band imaging), molecular imaging, MRI, PET, and optical coherence imaging, are useful for real-time analyses of morphological, vascular, and metabolic alterations within living tissues resulting from early malign transformation. The techniques find applications in the diagnosis of lesions with early malign transformation and cancers.[41]Significantly, in vivo imaging is a complementary aspect of traditional approaches to diagnosis because it increases the efficiency of screening, biopsies, and follow-up on the progression of the disease. Although existing imaging technologies increase the visibility of early lesions, there remains uncertainty with regard to sensitivity and specificity, making them not entirely adequate for confirming diagnoses histopathologically. Breakthroughs in the coming era can therefore be anticipated to increase the effectiveness of in vivo imaging by virtue of enhanced sensitivity, specificity, and its utility in early diagnosis and preventative medicine.[42]

3.4 Multiplexed and Personalized Diagnostics

A new trend in today’s healthcare industry is multiplexed and personalized diagnostics. This can be regarded as a paradigm shift in today’s healthcare industry because for the first time in medical diagnostics, multiple biomarkers can be detected together to create a personalized disease profile for patients. Unlike before when medical diagnostics involved testing for a single analyte in a sample, today’s advanced platforms detect groups of proteins, nucleic acids, and/or metabolites simultaneously. This helps in understanding complex diseases like cancer, cardiovascular disease, and inflammation in a more differentiated manner.[43,44]Electrochemical, optical, and microfluidic biosensing platforms are at the heart of a multiplexed diagnostic system, which provides the sensitivity, detection limits, and miniaturization required by a point of care diagnostic platform. There is a tremendous advantage with the incorporation of nanomaterials, including nanoparticles, carbon tube structures, and nanostructured electrodes, which can provide a boost to the process of detection, sensitivity, and biosensing platforms. There is a lot of hope with the concept of personalized diagnostics, which can be achieved by deciphering the data from the multiplexed biomarkers, allowing the physician to prescribe treatments according to the individual biomarker of the patient, assess the response, and predict relapse.[45,46]While various studies have demonstrated satisfactory sensitivity and specificity for quantum dot-based systems, differences arise due to differences in surface functionalization, synthesis methods, and biological systems, thus emphasizing the importance of standardization in assessment procedures.

4. Applications in Treatment

Table 3. Therapeutic  applications of quantum nanomaterials.

4.1 Targeted Drug and Gene Delivery

Targeted drug and gene delivery is a major breakthrough in cancer therapy, which allows for the selective accumulation of drugs at the tumor site with minimal systemic toxicity. The article highlights that nanocarriers made of polymers can be designed to facilitate both passive and active targeting. Passive targeting is mainly based on the EPR effect, which enables nanoparticles to selectively accumulate in the tumor tissue due to its characteristic leaky vasculature and poor lymphatic drainage.[47]Besides the passive targeting approach, active targeting is realized by modifying the surface of polymer nanocarriers with particular ligands such as antibodies, peptides, folic acid, or aptamers that target the overexpressed receptors on the surface of cancer cells. The ligand-receptor interaction increases the cellular uptake of drugs and genetic materials through receptor-mediated endocytosis.[48]egarding gene delivery, the role of polymer nanocarriers in shielding nucleic acids (siRNA, miRNA, plasmid DNA) from enzymatic degradation and their transport across biological barriers is essential. The current topic is emphasized to improve gene transfection efficiency in the tumor microenvironment by using cationic and stimuli-responsive polymers. In addition, co-delivery approaches that combine drugs and genes in a single polymeric platform are also mentioned as a promising strategy to overcome multidrug resistance and achieve synergistic anticancer effects.[47,48]

4.2 Photodynamic Therapy

Photodynamic therapy, or PDT, represents a minimally invasive treatment modality using a photosensitizing agent activated by light of specific wavelengths. This, in turn, generates reactive oxygen species that cause targeted cell destruction. The medical domain in which this therapy finds wide application includes oncology in the treatment of various cancers and dermatology for conditions such as actinic keratosis and acne. The localized delivery of light in instances of the selective accumulation of a photosensitizer in diseased tissues enables precise targeting while limiting damage to surrounding normal cells . Similarly, singlet oxygen and free radicals generated under PDT conditions induce apoptosis and necrosis, vascular shutdown, and an immune system attack, making the treatment more effective. The benefits of PDT include reduced systemic toxicity, repeatability, and a potential for combination with other treatment modalities.[49,50]One key aspect that defines the specificity of PDT is the double specificity of the preferential uptake of the photosensitizer by abnormal cells combined with the focused application of the activating photons. As a result, one gets specificity in the treatment due to reduced systemic toxicity that is often associated with conventional therapeutic methods such as chemotherapy and radiation. Ideally, the procedure of conducting a treatment in photodynamic therapy is considered to be comprised of three essential parts.[51,52]

4.3 Photothermal Therapy and Combined Approaches

Photothermal therapy presents a very precise and minimally invasive technique for cancer treatment through the use of near-infrared light to heat up and ablate tumor cells locally. In general, the combination of PTT with other therapies increases the overall efficacy by using complementary mechanisms. For example, in combination with chemotherapy, PTT increases tumor permeability, thus facilitating higher intratumoral intake of the drug and overcoming resistances, including reducing drug dosages and side effects. Similarly, combining photothermal therapy with immunotherapy enhances systemic anti-tumor immune responses by releasing antigens associated with tumors, leading to reduced metastasis and recurrence risks.[55]To overcome these issues, a combined therapeutic approach that combines PTT with other treatments has attracted considerable attention. Combining PTT with chemotherapy can enhance thermally triggered drug release, cellular uptake, and overcome multidrug resistance mechanisms to achieve a synergy of cytotoxicity at low doses of drugs. Similarly, PTT/photodynamic therapy (PDT) combinations can combine oxygen perfusion by heat-treated tissues and ROS production to achieve a synergy of tumor killing even under a hypoxic environment. Combining with radiotherapy has been demonstrated to make cancer tissues sensitive to the treatment by DNA damage induced by heat sensitivity and increased vascular permeability.[56]

4.4 Antimicrobial and Anti-Infective Applications

Natural products are now resurfacing as an important source of antimicrobial and anti-infective agents in an attempt to counteract the ever-increasing danger of antimicrobial resistance (AMR). The latest findings underscore a wide range of bioactive compounds with a broad spectrum of activity against bacteria, fungi, viruses, and parasites, including multidrug-resistant strains, discovered in plants, microbes, and other natural sources. The mechanisms by which natural agents target microbes are varied and include actions on microbial cell membrane, interference with crucial enzymatic processes, modulation of nucleic acid synthesis, and reduction of virulence factors and biofilms[57].Notably, many natural products have been shown to have synergistic effects with conventional antibiotics, which can improve treatment outcomes while possibly lowering the dose and toxicity of drugs. Recent improvements in in silico simulations, molecular docking, and high-throughput screening have also helped to expedite the discovery and optimization of promising antimicrobial agents. Taken together, the above observations highlight the immense potential of natural products as complementary or alternative approaches for anti-infective therapy, and their inclusion in the development of next-generation antimicrobial therapies[58].

4.5 Theranostic Platforms

Recent research has revealed that Metal-Organic Frameworks (MOFs) and their derived nanomaterials have emerged as an attractive candidate material that has high potential for theranostic applications owing to their high surface area, ease of pore tuning, ease of structural design, as well as their adjustable chemical compositions. This provides an effective means to combine therapeutics, imaging, or biological molecules through a single platform.[59]Theranostic systems derived from MOFs, including porous carbon materials, metal or metal oxide nanoparticles, and single-atom nanozymes, show improved stability, catalytic ability, and responsiveness to stimuli in physiological conditions. Theranostic systems based on MOFs have been investigated extensively in multimodal imaging (flammency, magnetic resonance imaging, and photoacoustic imaging) and in combination with therapeutic modalities like chemotherapy, photothermal therapy (PTT), photodynamic therapy (PDT), and chemodynamic therapy. More specifically, drug liberation and activation in response to tumor-specific stimuli, including pH values, redox potential, and enzymatic activity, can be finely controlled in MOF-based systems with minimal systemic toxicity and greater efficacy than traditional systems.[60]

Figure 1. Quntum Dot and Application