Advances in Deep Brain Imaging with Quantum Dots: Structural, Functional, and Disease Specific Roles

1. Introduction

Achieving deep brain imaging has long been constrained by challenges in resolution, penetration depth, and the complex interaction of light with inhomogeneous biological tissues. Multiphoton microscopy (MPM) offers a solution by confining fluorescence to a focused volume through nonlinear photon absorption, which reduces background noise and photobleaching [1]. Yet, factors such as light scattering [2], tissue absorption [3], and the delicate balance of laser power required all impact outcomes of MPM [1]. To address these challenges, it is important to understand the physical principles influencing nonlinear imaging.

In MPM, fluorescence is achieved by multiphoton excitation, when multiple low-energy photons are simultaneously absorbed, causing the fluorophore to fluoresce [4]. For a n-photon excitation process, the fluorescence intensity, I_f^,, depends on both the absorption cross-section and excitation intensity as defined by:

$$I_f\propto {\sigma }_n·I^n\mathbf{(}\mathbf{Eq}.\mathbf{1}\mathbf{)}$$

where σ_n is the absorption cross-section, I is the photon density at the focal point, and n is the number of photons absorbed simultaneously. Here, σ_n is critical as a higher absorption-cross section increases simultaneous photon absorption, generating a stronger signal even at reduced laser intensities. This is beneficial for deeper tissue imaging because photon density diminishes with depth due to scattering and absorption [5]. Therefore, attenuation of ballistic photons becomes a major limitation in achieving deeper imaging as explained by Beer-Lambert’s law: [6,7]

$$I\left(z\right)=I_0{·e}^{(-{\mu }_{t}z)}\mathbf{(}\mathbf{Eq}.\mathbf{2}\mathbf{)}$$

where I₀ is the initial intensity at the brain surface, ${\mu }_t={\mu }_s+{\mu }_a$ is the total attenuation coefficient (comprising scattering ${\mu }_s$ and absorption ${\mu }_a$ coefficients) [8], and z is imaging depth. In brain tissue, scattering dominates over absorption, particularly in the near-infrared (NIR) window, which is optimal for minimizing light loss [9]. More specifically, scattering is predictably dominant in the range of 700 - 1000 nm and becomes more discrete in the 1000 - 1700 nm range. The exponential decay of ballistic photons can be expressed by:

$$P\left(z\right)=P_0{·e}^{(-{\displaystyle \frac{z}{EAL}})}\mathbf{(}\mathbf{Eq}.\mathbf{3}\mathbf{)}$$

where $P_0$ is the power at the brain surface and EAL is the effective attenuation length [10]. The exponential decay explains the rapid decrease in laser power with increasing imaging depth. While using longer excitation wavelengths in 3PM (e.g., 1300 nm and 1700 nm) can extend the effective attenuation length (EAL), fluorescence still diminishes with depth due to attenuation effects [3,11,12,13,14,15,16]. Additionally, many standard fluorophores have lower absorption cross-sections and quantum yields [17]. As a result, overcoming these combinations of challenges often requires higher laser powers to produce adequate fluorescence signals at greater depths. Yet there still must be a balance in pulse energy, repetition rate, and overall power at the brain surface to ensure damage-free deep brain imaging.

Previous studies have sought to enhance deep tissue imaging by engineering microscope systems or incorporating complex components like adaptive optics (AO). AO plays a critical role by correcting aberrations caused by the tissue’s heterogenous refractive index and scattering variations [12]. These aberrations distort the excitation wavefront, enlarging the point spread function (PSF) and reducing both resolution and fluorescence signal [18]. Both direct and indirect AO wavefront sensing methods improve resolution and signal strength at depth [19,20,21]. For example, 3PFM with AO achieved near diffraction limited performance and a fourfold resolution improvement at 1450 μm depth [22], while 2PFM with direct imaging AO reached depths of 700 μm in mouse brain tissue [23]. Even so, achieving these improvements often requires substantial system modifications, precise calibrations, and specialized expertise. Despite these efforts, the improvements gained is often still limited, making such extensive hardware modifications not only challenging, but also costly.

Achieving deeper imaging depths can be accomplished not only by modifying various aspects of the microscopy setup, but also by utilizing brighter probes. Quantum dots (QDs) have emerged as highly effective probes for this purpose, offering significant advantages over traditional fluorophores [5,24,25,26,27]. QDs are semiconducting nanoscale particles whose distinct optical properties stem from quantum confinement effects, which are directly influenced by their physical size and material composition [28,29]. A major benefit of QDs is their tunable absorption and emission spectra allowing for custom design to meet specific experimental needs [30]. Their tunable Stokes shifts, creating a clear separation between excitation and emission wavelengths, minimizes overlap, thereby enhancing the SBR, reducing scattering, and enabling more efficient multicolor imaging [30,31,32].

QDs also exhibit high quantum yields in both the UV-visible range as well as the NIR. Resch-Grenger et al., (2008) reported QD quantum yields as high as 0.65-0.85 in the visible range, exceeding most organic dyes, which typically exhibit quantum yields of 0.3-0.5 in the visible range and even lower values in the NIR range. Additionally, QDs have larger molar absorption coefficients (Figure 1A,B; Table 1), extended fluorescence lifetimes (Figure 1C) [33,34], superior photostability [35] and exceptional brightness compared to other imaging probes [32,36]. These properties make QDs particularly well-suited for long-term, in vivo imaging of deep brain regions, such as the hippocampus or subcortical brain regions [24,26,37]. Thus, by utilizing QDs, researchers can achieve unprecedented depths without the need for extensive modifications to the microscopy system.

2. Advances in Structural Brain Imaging Using Quantum Dots

QDs have propelled advancements in MPM, demonstrating effective use in structural imaging through 2PFM [33,38], 3PFM [5,24,25,26,27,39], and 4PFM [24] modalities for in vivo visualization of mouse brain vasculature (Figure 2). These applications leverage the QDs’ superior brightness, larger absorption cross-sections, and use of longer excitation wavelengths (e.g., 1700 nm to 2200 nm) [40,41,42], which minimizes scattering and optimizes signal retention [1,2,43], allowing for deeper imaging while maintaining high SBR ratios [40,41].

2PM is a widely used bioimaging tool, but its application in the NIR-II range has been limited due to the lack of suitable probes. Nonetheless, recent advances particularly in QDs with modified core and shell properties have made progress in this field possible. For instance, PbS/CdS QDs, emitting fluorescence centered at 1270 nm when excited with a 1550 nm fs laser, allowed for deep brain imaging in the NIR-II range [33]. To further enhance signal detection and resolution, the study combined MPM with fluorescence lifetime imaging to perform two-photon lifetime imaging microscopy (2PFLIM), achieving good contrast at an imaging depth of 220 μm. This marked the first use of 2PFLIM in the NIR-II range for deep-tissue imaging [33]. In a separate study, the integration of an optical parametric oscillator (OPO) in 2PFM demonstrated commercial QDs, like Qtracker655, could achieve imaging depths of approximately 630 μm, compared to 560 μm using NLO alone, (Figure 2A) [38].

QDs have also shown considerable potential in 3PFM, offering deep imaging capabilities superior to traditional fluorophores. For instance, excited at 1700 nm, a commercial QD (Qtracker655) enabled imaging depths up to 2100 μm in the mouse brain (Figure 2B) [25], far surpassing depths reached by Texas Red dextran (1340 μm) [44] (Table 1).

Figure 2C extends this insight by comparing QDs in 3PFM and 4PFM. In 3PFM, Qtracker800 excited with a 2200 nm wavelength were able to achieve imaging depths of approximately 1060 μm, while 4PFM with Qtracker655 only reached 940 μm [24]. Though theoretically, 4PF could enable deeper penetration due to higher-order nonlinearity, in practice, 3PF achieves greater depths [24]. This is because there is a lower probability of four-photons simultaneously being absorbed with allowable power levels without damaging the brain. In higher photon orders (n), the fluorescence signal decays more rapidly due to increased scattering and absorption at depth z. Thus, achieving effective signal in 4PF requires elevated pulse energy and power, often nearing or exceeding photodamage thresholds and thereby constraining the theoretical advantage of 4PF imaging.

Surgical protocols are another factor that must be considered when conducting MPM to image subcortical regions. Craniotomies completely remove the skull to allow laser interactions directly into the brain, therefore, reducing scattering due to the absence of bone [26]. In contrast, imaging through an intact skull introduces additional scattering and absorption obstacles. Figure 2D depicts a schematic of imaging with both a craniotomy (left) and intact skull preparation (right). Bone structure significantly attenuates light, but the strong signal properties of QDs, along with optimized imaging parameters, help to overcome barriers, enabling substantial imaging depth, even under challenging conditions. CdSe/CdS/ZnS QDs were used to label brain vasculature with 3PFM to achieve imaging depths of 1550 μm with craniotomy and 850 μm through an intact skull (Figure 2D) [26]. This surpasses imaging depth achieved with commercially available Qtracker800, which only reached 1060 μm with craniotomy and 460 μm through intact skull [24]. Nevertheless, 3PFM vasculature imaging with 1700 nm excited Qtracker655 achieved an imaging depth of 2100 μm with a craniotomy preparation [25] and 750 μm through an intact skull [3]. Achieving such depths, through bone, despite these obstacles is highly promising.

To the best of our knowledge, QDs have not been used in MPM for labeling specific neurons or cell types, in vivo, in subcortical regions. Common dyes like fluorescein and rhodamine-derived molecules are typically used to label blood vasculature in the brain [38,52]. However, their emission spectrum often overlaps with those of other fluorescent proteins, complicating simultaneous imaging of different cell populations. In contrast, Qtracker655, with an emission spectrum centered around 655 nm, enables multicolor imaging by using 1050 nm excitation, allowing simultaneous imaging with Enhanced Yellow Fluorescent Protein (EYFP) or DsRed2 [38]. Ricard et al., (2016) also demonstrated the potential of Qtracker655 for multicolor imaging by successfully labeling neurons with Thy1-CFP, blood vessels with QD655, and dendritic cells/microglia with CD22c-EYFP in a Thy1-CFP/CD11c-EYFP mouse model. Overall, this highlights the utility of QDs for simultaneous visualization of multiple cell types in vivo in MPM.

3. Exploring Functional Brain Imaging with Quantum Dots

Recent advances in MPM and QD technology have extended beyond structural imaging, to enable high-resolution monitoring of hemodynamic responses [24,25] and neuronal activity deep within brain tissue [53]. This progress addresses a critical need in neuroscience: the visualization of brain function at cellular resolution in subcortical regions. Through 3PFM, QDs provide unprecedented depth penetration and precise imaging of blood flow dynamics, neural function, and cellular architectures, supporting detailed studies of hemodynamic flow, neuronal activity, and NIR light-stimulated action potentials.

In hemodynamic blood flow studies, QDs allow measurements of flow rates at significant depths within the brain. For example, Qtracker800 enabled in vivo 3PFM imaging of blood flow speed at depths 200 μm and 600 μm below the brain surface, recording flow rates of 0.96 mm/s and 0.75 mm/s, respectively [24]. Additionally, Qtracker655-labeled brain vasculature was visualized up to 1600 μm deep under 1700 nm excitation, far surpassing the capabilities of conventional fluorophores and enabling precise observation of subcortical vascular structures (Figure 3A). This visualization of blood flow speeds at such depths demonstrates QDs’ robust potential in hemodynamic monitoring [25]. These measurements are important as blood flow dynamics are intricately linked with neural activity and are critical for understanding the physiological basis of various neurological disorders [24].

The brain’s intricate architecture and electrochemical network drives complex neural computations and inter-regional communication, fundamental to cognition and behavior. In the past, methods such as in-vitro patch-clamping [54,55], calcium imaging [56], and genetically encoded voltage indicators (GEVIs), [57] have helped decipher neural dynamics. Nevertheless, these approaches are either invasive or limited in temporal resolution, photostability, and response speed. Additionally, these conventional techniques often fall short in capturing rapid neural events due to slower response times and potential photobleaching at higher excitation levels [58,59]. In comparison, QDs provide high temporal resolution, enabling the detection of single action potential with fewer indicators, critical for tracking fast neural signals without risking toxicity or photodamage [53].

Using a theoretical framework for shot-noise-limited photon detection, simulations quantified the QDs’ capability to resolve neural spikes optically. For accurate representation, QDs were sampled at rates exceeding 2 kHz and were able to capture the spike waveform with high fidelity. These simulations further confirmed that QDs’ superior brightness enables single spike detection at lower labeling densities than traditional voltage-sensitive dyes (VSDs) or GEVIs (Figure 3B). Moreover, QDs are not susceptible to photobleaching and photodamage constraints, unlike conventional fluorophores whose excitation intensities are typically limited to ~1mW/mm² due to risks of intersystem crossing into long-lived triplet states [60]. Consequently, QDs can sustain high excitation intensities, allowing for efficient spike detection while minimizing the impact on membrane capacitance.

For in vivo recordings, QDs have been modified (e.g. QD-JBD1-C60) to detect and respond to electrical stimulation to capture cortical responses in widefield imaging [53]. The fluorescence traces for each region of interest (ROI) show distinct temporal profiles in response to electrical stimulation, with proximal areas displaying a more immediate, robust fluorescence change (Figure 3C). This rapid detection reflects QDs’ capacity to monitor neural activity with both spatial and temporal precision and usefulness for electrophysiology [53].

Marshall & Schnitzer (2013) explored the potential of QDs as voltage indicators by modeling their excitation and emission dynamics as a two-state system. Here, rate constants for excitation (k_a), radiative decay (k_r), and nonradiative decay (k_nr) govern the QD behavior. In semiconductor nanocrystals, photon absorption excites a bound electron-hole pair (exciton) which recombines via radiative or nonradiative processes. The radiative decay rate is given by $k_r={\displaystyle \frac{1}{{\tau }_r}}$, where ${\tau }_r$, the fluorescence lifetime, is inversely proportional to the overlap between electron and hole wave functions:

$${\tau }_r\propto \left|⟨{\psi }_e|{\psi }_h⟩\right|-2\mathbf{(}\mathbf{Eq}.\mathbf{4}\mathbf{)}\mathrm{ADDIN}\mathrm{EN}.\mathrm{JS}.\mathrm{CITE}\left[59\right]$$

Focusing on type II-VI semiconductors like CdTe QDs, the difference in effective masses of electrons ($m_e^{*})$ and holes ($m_h^{*})$ enhances sensitivity to transmembrane voltages. Effective masses create an energy differential, with energies of the charge carriers inversely proportional to their effective masses. This makes QDs highly sensitive to transmembrane voltages, especially when embedded in or near the cell membrane [59,61]. Chen et al (2019) synthesized glutathione-capped CdSe/ZnS QDs, emitting at 520 nm, that effectively labeled the plasma membrane of neurons (Figure 3D).

Neurons were co-labeled with CdSe/ZnS-GSH QDs (green) and DiD (red) that exhibit clear colocalization, with yellow indicating merged areas, highlighting the QDs’ ability to precisely label cell membrane within 30 mins (Figure 3D). For CdTe QDs, an electric field strength of approximately 5 mV/nm produces a fluorescence lifetime change of: ${\displaystyle \frac{\mathsf{\Delta }\mathsf{\tau }}{{\tau }_r}}\approx 2.5\%$ [59]. This reinforces QDs’ sensitivity to membrane potential shifts and their potential for tracking neuronal activity [53,59,61].

Beyond detection, a recent study utilized QDs in bio-interface applications that enable remote neural stimulation (Figure 3E) [58]. Utilizing NIR light, QD based bio-interfaces stimulated primary neurons by converting light into ionic currents that evoked action potentials without the need for external power sources (Figure 3E). This setup allows for deep tissue neuron stimulation without the need for batteries, highlighting potential applications in bioelectronic medicine [58,62].

4. Synthesis Mechanisms of Quantum Dots for Enhanced Multiphoton Imaging

When selecting a QD for experimentation, researchers can opt for commercially available options like Qtracker products or custom lab-synthesized QDs. Commercial QDs offer convenience and consistency, as they are pre-functionalized for general applications and span a range of excitation and emission wavelengths compatible with common imaging systems. Qtracker QDs are specifically designed to be injected into the tail vein of mice to study vascular structures. The core is coated with polyethylene glycol (PEG) to reduce nonspecific binding interactions and allows imaging for up to three hours post-injection [63]. Alternatively, lab-synthesized QDs offer greater flexibility, allowing for customization of optical and functional properties to meet specific experimental needs, and are often a more cost-effective option.

When designing QDs for biomedical imaging, it is important to consider the composition of each component: core, shell, and surface ligands. The core properties, such as size, geometry, and chemical composition play a critical role in optimizing photophysical characteristics. Additionally, QDs must be water-soluble to be effective in biological environments. For the shell, considerations include stability, biocompatibility, and effective anchoring of ligands, as the shell shields the QD core from the biological environment. Surface ligands further enhance biofunctionalization by providing stability, maintaining ligand activity, and allowing controlled attachment of functional groups, which are necessary for biological interactions [64].

The synthesis of NIR QDs typically involves methods like hot-injection, heat-up, microwave, and hydrothermal techniques, each providing specific advantages for optimization [64] (Figure 4A). The hot-injection technique enables precise control over particle size and emission peaks by rapidly injecting precursors into a hot coordinating solvent, resulting in uniform cores with high quantum yields and photostability, ideal for stable in vivo imaging [5,27,65]. Hydrothermal and solvothermal methods, which use high temperatures and pressures in sealed vessels, help to enhance QD crystallinity and control particle size [66]. These methods have been used to make biocompatible, nitrogen-doped carbon QDs that can cross the blood brain barrier (BBB) [67,68].

Other synthesis approaches, such as heat-up and microwave methods can be combined with traditional techniques. The heat-up method, which does not require a rapid injection step, involves a single pot reaction that provides a slower, more controlled nucleation phase, but necessitates the use of reagents with closely matched reactivity to ensure uniform nucleation [69]. The hydrothermal technique relies on hot pressurized aqueous solvents to produce monodisperse QDs that are compatible with aqueous media, without post-synthesis treatments [66]. This method is frequently applied for biological compatible QDs [70]. Recent studies have combined solvothermal and microwave methods to create NIR-emitting carbon QDs utilized in imaging studies [67].

Bandgap engineering further improves QD performance in 3PF applications. Recent studies have introduced an intermediate layer between a CdSe [26] or CdTe [36] core and a ZnS shell, forming a quasi-type II configuration (Figure 4B). This structure significantly improves the absorption cross-section and boosts three photon fluorescence efficiency. Likewise, the ZnS shell provides electronic passivation, enhanced quantum yield, and makes the QD more robust, all which support prolonged imaging stability [26]. Adding a CdS interlayer minimizes lattice mismatch between the core and shell, effectively reducing non-radiative recombination [26]. Intrinsic polarization effects also makes shell thickness a key factor in determining the absorption cross-section [26,36]. Consequently, precise engineering of both the composition and structure of the shell is essential for optimizing QD performance.

Furthermore, QDs can be engineered to target specific cell types for both structural and functional imaging as previously described. For example, to visualize cellular membrane potentials, QDs such as the QD-peptide-C₆₀ bioconjugate complex have been developed [53] (Figure 4C). Precise spatial interactions within the cell membrane are critical to the modular design of these type of QDs. The QDs are comprised of a core/shell scaffold that serves as a stable structure, with peptide chains modified to include C₆₀ fullerene units attached at specific positions. These peptides self-assemble onto the QD surface via an N-terminal polyhistidine sequence, allowing for controlled binding to the QD shell. By strategically placing a lysine residue within each peptide sequence, C₆₀ fullerenes can be covalently attached through EDC chemistry, creating varied QD-C₆₀ separation distances for optimized spatial positioning. Additionally, the peptide chain contain alanine- and leucine-rich regions that form helical structures, facilitating effecting insertion into the cell membrane [53]. These techniques can be utilized in additional studies to create various QDs able to track cellular membrane electrochemical processing.

In addition, surface modifications, like PEGylation are commonly applied to QDs to maintain stability in circulation and enhance biocompatibility (Figure 4D) [34,71,72,73]. These processes helps to facilitate QD

accumulation in brain tissue, which is particularly useful for long term imaging applications [34]. Targeting ligands, such as asparagine-glycine-arginine (NGR) peptides, can further direct QDs to specific receptors, such as CD13 in gliomas, supporting BBB penetration and targeted tumor imaging. There are two main surface-coating strategies: encapsulation with amphiphilic polymers, which include PEG and carboxylated polymers and cap exchange with hydrophilic ligands such as zwitterionic and carboxylate ligands. Additionally, bioconjugation methods like biotin-streptavidin binding, amide coupling, and strain promoted azide-alkyne cycloaddition (SPAAC) allow for the attachment of biomolecules of interest (BOIs).

5. Quantum Dots in the Study, Diagnosis, and Treatment of Neurological Pathologies and Diseases

In recent studies, QDs have demonstrated significant potential in targeting and treating neurological diseases, particularly for conditions such as brain tumors, Alzheimer's disease (AD) and traumatic brain injuries (TBI). The unique properties of QDs discussed thus far, combined with their capability to cross the blood-brain barrier (BBB) make them particularly valuable for both imaging and therapeutic applications (Figure 5).

Gliomas, which represent the most common primary brain tumors, infiltrate healthy brain tissue, and are notoriously challenging to treat. The rapid spread of tumor cells and resistance to conventional treatment leads to high recurrence rates and a median survival rate of less than two years for Grade 4 gliomas, which are commonly referred to as glioblastoma [79,80]. In vivo imaging has provided key insight into the dynamics of cellular heterogeneity [81,82] and more recently tumor-neuron interactions [83,84] and cell invasion [85,86]. Building on these technological platforms, advanced imaging techniques that clearly delineate tumor boundaries are therefore crucial to improve surgical outcomes [87]. Functionalized QDs, present as a promising tool, particularly PEGylated QDs conjugated with asparagine-glycine-arginine (NGR)-based peptides, which specifically recognize tumor cells. In glioma models, these NGR- functionalized QDs, administered through the rat tail vein, successfully crossed the BBB and selectively accumulated in glioma sites with minimal distribution in healthy brain tissue [75] (Figure 5B). In other models, QD ITK amino (PEG) QDs have co-localized with macrophages in glioma cells without causing significant toxicity to other cells [88]. This has led to accurate glioblastoma detection while supporting the idea that QD can be a safe tool for targeted imaging, therapy, and surgical resection in glioblastoma.

Furthermore, photodynamic therapy (PDT) using QDs is emerging as a promising approach for targeted glioma treatment (Figure 5A). Following injection, functionalized QDs accumulate within tumor cells by selectively binding through ligands that recognize tumor-specific markers [78]. Once localized, these QDs enable precise imaging and mapping of the tumor with MPM, which focuses high-intensity light on subcellular regions within brain tissue. This level of precision has allowed for image-guided laser ablation with precise subcellular targeting of glioblastomas [89]. In addition, specifically engineered QDs can be used to generate reactive oxygen species (ROS) under specific light exposure, causing direct tumor destruction via apoptosis and necrosis. This method can further induce vascular damage within tumors, limit oxygen and nutrient supplies, or stimulate anti-tumor immune responses, all which contribute to effective tumor elimination [90,91,92,93]. As research progresses, these guided QD therapies hold promise for treating hard to reach brain tumors with minimal invasiveness and reduced side effects, providing a novel, highly specific approach to target glioma cells deep within the brain [90,91,92,93].

QDs have also shown considerable potential in imaging and monitoring traumatic brain injury (TBI), providing valuable insight into injury progression and therapeutic assessment [94]. In mice, NIR-II emitting, silver indium selenide-based and PEG-capped QDs enable real-time tracking of cerebral blood flow and inflammatory responses. These are key elements in understanding TBI pathology.

The engineered QDs facilitated TBI evaluation both pre- and post-surgery. The real time imaging capability provided guidance on the evaluation of intracranial hematomas, supporting critical intraoperative decision-making. QDs further allowed precise monitoring of secondary injuries, offering a 2.5-fold increase in the signal intensity ratio of TBI to normal brain tissue. This enhanced contrast underscores QDs’ effectiveness in distinguishing injury-affected areas from healthy brain tissue, aiding in real time assessment of TBI status across different stages of intervention and recovery [94].

Graphene QDs (GQDs) have demonstrated therapeutic effects in Alzheimer's Disease (AD), notably by reducing amyloid-beta (Aβ) accumulation and neuroinflammation [77,95]. QDs conjugated with neuroprotective peptide glycine-proline-glutamate (GQDG) were administered to an AD transgenic mouse model (APP/PS1). The treatment led to a reduction in inflammatory cytokines and an increase in anti-inflammatory cytokines compared to controls. Other notable changes include increased dendritic spine numbers and improvements in learning and memory as demonstrated by an enhanced performance in the Morris Water Maze, a test where AD models typically perform poorly (Figure 5C) [77]. Complementing these results, another study demonstrated GQDs could prevent Aβ aggregation by forming electrostatically driven aggregates with Aβ peptides. These aggregates, ranging from 100-900 nm, generated a cross-linked network structure that was highly unstable and eventually collapsed, effectively inhibiting the Aβ accumulation [95]. This dual effect of GQDs, disrupting Aβ aggregation and reducing neuroinflammation, positions QDs as noteworthy treatment for addressing both the pathological and cognitive deficits of AD.

An alternative therapeutic approach by QDs is their use in cell-specific drug delivery. Biofunctionalized QDs can facilitate receptor-mediated transcytosis, enabling precise drug targeting across the BBB (Figure 5D) [78]. QDs can be functionalized with ligands like EGFR, VEGFR, and folate receptors to precisely target brain cancer cells and deliver cytotoxic agents directly to tumor sites [78,96]. After administration, QDs accumulate in targeted tissue either through receptor-mediated endocytosis or by light-induced ionic interaction that open ion channels. Various conjugated ligands such as peptides, antibodies, nucleic acids, and aptamers, enhance QD specificity for brain tumor cells, improving delivery precision and reducing off-target effects [97,98,99]. Once inside the cell, QDs can undergo lysosomal degradation, releasing drugs directly into the cytosol, with the QD’s biocompatibility and surface modifications directly influencing their biological half-life and metabolism [100].

6. Conclusions

QDs have tremendous potential in the field of neuroscience and optical imaging, offering unprecedented capabilities in both deep brain imaging and targeted treatment of neurological disorders. Their unique optical properties, including high brightness, stability, tunable excitation and emission spectra, and customizable functionalities enable applications that significantly exceed those of conventional fluorophores. Notably, QDs have shown utility in precise monitoring of cellular dynamics, hemodynamic responses, and neuronal activity at previously inaccessible depths.

In structural imaging, QDs have proven invaluable in MPM due to their large absorption cross-sections and effectiveness in the NIR range, thereby reducing scattering and enabling visualization of subcortical regions like the hippocampus. QD-based imaging has achieved depths up to 2100 μm, demonstrating a powerful approach for studying brain architecture and activity in vivo. Combining QDs with other advancement techniques such as adaptive optics could provide real time correction of tissue induced aberrations and further enhance both imaging depth and resolution. Future research might focus on developing QDs with even better optical properties and target imaging of neurons, glial cells, as well as cerebral vessels.

QDs have also been shown to provide a robust platform for capturing rapid neural events and monitoring hemodynamic flow with high temporal resolution. Their stability and compatibility with long-term imaging further enhance their utility in therapeutic applications, including targeted drug delivery, photodynamic therapy, and image guided surgery. Therefore, QDs hold promise in bridging the divide between diagnostic imaging and therapeutic intervention. In the future, QDs could enable simultaneous imaging and treatment, allowing researchers to track therapeutic responses with cellular precision and to refine interventions in real time. Integrating QD-based imaging with targeted treatment for conditions like gliomas, TBI, and AD could pave the way for personalized, image-guided therapies. With ongoing advances in QD synthesis, targeted specificity, and biocompatibility, QDs are poised to become an essential tool, offering an innovative platform for exploring and treating neurological disorders with unparalleled depth, precision, and therapeutic efficiency.