Parkinson’s disease is a neurodegenerative disorder that involves motor and balance functions in a slow but progressive way. It is a long-term disabling disease that mostly involves people over 50 years of age, involving the degeneration of dopaminergic neurons in the substantia nigra ^[10][136]. It has been demonstrated that the damage induced by free radicals is one of the causes that lead to the onset of a neurodegenerative disease. Accordingly, different studies investigate the role of antioxidant molecules for Parkinson’s disease treatment ^[11][137]. Therefore, the use of polyphenols and antioxidant molecules could improve the efficacy of the therapeutic treatment against Parkinson’s disease, even if their therapeutic application is often limited due to their poor bioavailability. Schisanterin, a Chinese medicinal herb, has been evaluated for its antioxidant properties and for reducing dopaminergic loss. Studies have shown that this molecule is able to cross the blood–brain barrier (BBB), but its oral bioavailability is very low. In the work of Chen et al., schisanterin NCs were developed and showed a significant improvement of drug bioavailability compared to the pure drug. Neuroprotective bioassay conducted on zebrafish revealed that schisanterin NCs reverse MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), a neurotoxic substance that produces the degenerative effects of Parkinson’s disease ^[12][138]. NCs made of ginkolide B, which is a molecule extracted from Gingko Biloba and characterized by a powerful antiparkinsonian effect, were prepared by Liu et al. The authors found that, thanks to their small size (less than 100 nm), the dissolution rate of the drug and the cellular absorption by endocytosis was significantly increased compared to the free drug. Furthermore, ginkolide B NCs showed greater bioavailability in vivo than coarse ginkolide B, leading to a potential reduction in drug dose for Parkinson’s patients ^[13][139]. In the study of Xiong et al., the authors focused on the improvement of the oral bioavailability of a molecule with strong antioxidant power, resveratrol. Resveratrol could be used as an adjuvant in the treatment of Parkinson’s diseases, which include conditions of oxidative stress. However, its huge benefits are hindered by its very low water solubility. For this purpose, resveratrol NCs were prepared with the solvent–antisolvent technique revealing an increase in drug bioavailability compared to the pure drug ^[14][140]. Very interesting results were obtained by Ghaffari et al., which investigated the effect of quercetin NCs on an induced model of oxidative stress. In this study, a parkinsonian model was induced in rats by using a toxic molecule, hydroxydopamine, capable of generating free radical stress. The administration of quercetin NCs, prepared with the evaporative precipitation of the nanosuspension technique coupled with high-pressure homogenization (HPH)HPH, reduced the levels of oxidative stress in the hippocampal area, protecting this area of the brain from degeneration induced by free radicals. The antiois study, compared to the previous one, also confirmed the improvement of drug uptake. The antioxidant activity of NCs was confirmed by different assays, such as the determination of the quercetin NC effect on superoxide dismutase (SOD) and catalase activity (CAT), and the effect of neat quercetin and quercetin NCs on the total glutathione content and malondialdehyde level, revealing that the formulation restored the levels of SOD and CAT and increased the levels of GSH with a final decrease in lipid peroxidation, offering a wide contribution of antioxidant activity ^[15][141]. Xiong et al. formulated puerarin NCs, a medical herb that has therapeutic properties for various diseases, such as diabetes, Parkinson’s disease, and ischemic stroke. This molecule is placed within the class IV of the BCS system. Several strategies have already been attempted to improve puerarin solubility, such as puerarin encapsulation into solid lipid nanoparticles (SLN)^[16] [142] or polymeric NPs ^[17][143], but none have achieved a satisfactory encapsulation efficiency, such as that obtained with NCs. In the work of Xiong et al., five different stabilizers were investigated for NC production and Pluronic F-68 was selected, as it maintained stability over time and in the fluids of the GI system and offered a higher drug dissolution rate ^[18][144]. In the recent work of Tan et al., a magnolol NC-based formulation (MAG-NCs) complexed with a hydrogel of poly(N-isopropylacrylamide) (PNIPAM) was designed for the treatment of Parkinson’s disease by intranasal injection. The formulation prolonged the residence time of the drug into the nasal cavity and decreased its clearance thanks to the processes of self-gelation at near-body temperature. Moreover, the nanometric size improved its crossing through the BBB by enhancing its delivery to the brain. In vivo tests in a mouse model of Parkinson’s disease with an intranasal injection of MAG-NCs demonstrated a reduction in neurotoxicity through the downregulation of ROS and ATP in dopaminergic neurons, typical of Parkinson’s mitochondrial dysfunction. Taking into account these findings, NC technology can be considered a useful approach to improve the antioxidant molecule bioavailability intended for Parkinson’s treatment ^[19][145].

The main feature of psychosis lies in an altered perception and interpretation of the environment, false beliefs, and disorganized patterns of speech and behavior. In clinical practice, it is seen as a severe mental illness, in which delusions and hallucinations are predominant. Research has shown that elderly patients are at a greater risk for the development of psychotic symptoms. Therefore, good patient compliance is of outmost importance, considering that the therapy is mainly addressed to geriatric patients ^[20][146]. For example, long-acting antipsychotic formulations have been recently discovered, in order to increase patient compliance, reducing the number of administrations over time. Some of these have been approved by the FDA and consist in NC-based drugs for intramuscular administration ^[21][147]. Different efforts have been made in order to improve patients’ adherence to therapy. For example, adhesive buccal films are the most recently developed dosage form that can be easily administered and promote drug release in a controlled manner. An aripiprazole NC formulation was used to impregnate a double-sided adhesive film consisting of chitosan. Aripiprazole NCs were prepared by acid–base neutralization process and then incorporated into the polymer blend (chitosan and glycerol) before being cast and forming the film. Aripiprazole is a new generation antipsychotic drug, useful for chronic and long-term therapy. The film is simple to use and can be useful for the immediate delivery of the drug into the systemic circulation, thanks to the great vascularization of the buccal mucosa ^[22][148]. The work of Gol et al. involved the preparation of risperidone NCs (a second generation of antipsychotic drug), due to its high log P and its low solubility (class II BCS system). In particular, the authors investigated the impact of the process variables, including the solvent used, the drug-to-surfactant ratio, the solvent-to-antisolvent ratio and the stirring speed on the particle size and polydispersity index of the final risperidone nanosuspension ^[23][149]. Within the second generation of antipsychotics, NCs were also considered for ziprasidone, which is characterized by a poor oral bioavailability. The study focused on the preparation of ziprasidone NCs to obtain orodispersible tablets through direct compression. The technology of oral dispersible tablets (ODTs) is useful in the case of neurodegenerative diseases and in the treatment of elderly patients and children, overcoming swallowing difficulties. In Tthis work, ODTs were prepared by microfluidization. The whole study was conducted using the Design of Experiments (DoE) approach to optimize the final formulation, investigating the ratios between disintegrants and lubricants, in order to gain a final formulation with optimal parameters of hardness, friability and disintegration time ^[24][150]. Another work on risperidone was carried out by preparing two different nanosuspensions using Pluronic F127 or PVP K30 by nanoprecipitation. The study demonstrated that the risperidone-based nanosuspension improved drug bioavailability. The Cmax and AUC curves, obtained from the in vivo investigation on rabbits, revealed a clear difference between the commercial form Risperidal^® and the risperidone nanosuspension, with a bioavailability increased by two times compared to the marketed product ^[25][151].

Alzheimer’s disease (AD) is the most common form of dementia, with progressive and irreversible loss of cognitive function, recognized by the World Health Organization as a global public health priority ^[26][152]. Currently, the drugs are available for palliative care to improve patients’ quality of life. In addition, the low bioavailability of the drugs encourages and motivates the search for new strategies to increase treatment efficacy and to enable brain targeting to achieve the desired responses. As stated previously, the main limitation for the therapeutic treatment of neurodegenerative disorders, AD included, is the difficulty to reach the target site. The application of nanomedicine that delivers the drug directly to the brain could overcome this issue. However, this field has been little explored, since projects concerning the use of nanotechnologies to avoid the passage of the BBB for AD are less than 1% ^[27][153]. NCs with calpain inhibitors have been designed. Calpain has been shown to play a role in intractable diseases, such as AD. In AD, beta amyloid plaques lead to an increase in intracellular calcium, mediated by NMDA glutamate receptors, with a consequent final increase in this molecule. In neurodegenerative diseases as well as in AD, an increase in the concentration of calcium ions leads to the final activation of calpain, responsible for the apoptotic processes of cell death. Hence, calpain inhibitors can be useful in the treatment of AD ^[28][154]. NCs based on calpain inhibitors and more precisely on calpain I and SNJ-1945 inhibitors were designed for the first time by the spray dryer method ^[29][155]. One of the most used molecules in the treatment of AD is donepezil. Its use in the form of oral tablets causes a gastro-injurious effect that can be avoided after intramuscular administration. The intramuscular administration of donepezil improves its bioavailability and reduced peripheral side effects, but at the same time, it is associated with poor patient compliance, especially for long-term therapy, which can be improved thanks to the new long-acting formulations. At the same time, the donepezil orodispersible tablets called Aricept^® show the problem of severe gastrointestinal side effects and require daily dosing, compromising the adherence to the therapy. To overcome these problems, in the work of Mittapelly et al., donepezil NCs were designed for intramuscular administration, aimed at obtaining a slow release over time, in the manner of a long-acting formulation. NCs were prepared by the HPH technique, leading to a higher loading efficiency than the conventional formulation Aricept^® (less than 17%). In the study, donepezil NCs were prepared using an intermediate hydrophobic derivative by the hydrophobic ion pair (HIP) technique, prior to homogenization. The native donepezil is the second drug approved by the U.S. FDA for the treatment of mild-to-moderate AD, but it is a hydrochloride salt that has high aqueous solubility, so it is necessary to increase its hydrophobicity prior to NC formation. Embonic acid was chosen as adjuvant molecule with the aim of improving the hydrophobicity of donepezil through insoluble salt formation with the method of hydrophobic ion pairing. The study resulted in a stable formulation with a high drug load, which revealed good efficacy to replace conventional donepezil-based therapies ^[30][156]. The up-regulation of the group A phospholipase A2 enzyme (sPLA2-IIA) was found in rats after cerebral ischemia. This can be considered a marker in inflammatory diseases, such as atherosclerosis or sepsis, and neurodegenerative diseases, such as AD and cerebral ischemic stroke. Wang et al. focused on the potential treatment of cerebral ischemia with PX-18 NCs, considering the high production of sPLA2-IIA in the AD; PX-18 NCs could also be useful in the treatment of AD. PX-18 is considered a phospholipase A2 inhibitor. PX-18 NCs were prepared by HPH to obtain a “ready-to-use” formulation for parenteral administration. The formulation exhibited excellent stability, keeping the particle size unchanged for 180 days, which is promising for neuronal damage reduction ^[31][157].

A brain infection refers to an infection caused by viruses, bacteria, fungi, or parasites that affects the brain. Encephalitis is one of the most common examples referring to brain inflammation caused by bacteria, amoebas, viruses (such as cytomegalovirus) or parasites, such as Toxoplasma Gondii ^[32][158]. It has been reported that NC functional coating can improve NC transport, absorption and accumulation in brain areas as demonstrated on in vivo studies in rats via intravenous administration. Studies showed increased brain absorption using coating agents, such as polysorbates, poloxamers and/or peptides. It has been demonstrated, for example, that the modification of NC surface with polysorbate 80 allowed to bind the lipoproteins that act as receptor ligands for LDL receptors at the BBB level, inducing a better formulation absorption in the brain ^[33][34][35][159,160,161]. In the study of Dibaei et al., the researcheuthors demonstrated an improvement of curcumin biodistribution in the brain thanks to the design of coated nanosuspension due to the presence of Tween^® 80 or TPGS as a coating agent. The results revealed that the best coating agent was found to be polysorbate 80, which allowed drug absorption and targeting to the brain, confirmed by the plasma absorption of the protein ApoE on Tween^® 80. Curcumin NCs were recognized as LDL and interacted with endothelial cells in the capillaries of the brain and were absorbed via endocytosis into the brain. The literature suggested that peptides could serve as coating agents in order to improve drug targeting to the brain ^[35][36][161,162]. Accordingly, this strategy was investigated in some studies focused on atovaquone, used for brain toxoplasmosis. Toxoplasmosis is a zoonosis caused by the Toxoplasma gondii parasite and this pathology also affects the CNS. Two different studies were performed on atovaquone by using different coating systems to improve its brain targeting. The first study involved patients with AIDS, which were affected by manifestations of abscesses in the CNS (brain and cerebellum) and neurotoxicosis (Toxoplasma brain infection) with acute symptoms, such as seizures, paresis, or comatose states. Atovaquone NCs were formulated with three different surfactants Tween^® 80, poloxamer 184 or poloxamer 338 as absorption enhancers and the same NCs formulated with ApoE as coating agent, in addition to the three surfactants, and their brain targeting efficacy was investigated. The results showed that the ApoE-coated atovaquone NCs did not achieve the desired absorption in the brain, while the Tween^® 80-coated atovaquone NCs enhanced drug absorption to the brain compartment. The researcheuthors stated that the presence of ApoE improved atovaquone NC accumulation in the brain endothelial cells, but the enhancement of drug absorption in the brain was due to the presence of Tween^® 80 and the mean size of the NCs ^[37][163]. In a following study, the same researcheuthors prepared atovaquone NCs by using sodium dodecyl sulfate (SDS) or Poloxamer 188 for the treatment of toxoplasma encephalitis compared with the commercial drug Wellvone^®. The first difference between atovaquone NCs and Wellvone^® (atovaquone micronized suspension on the market) was the particle size and polydispersity index, which was much higher for the marketed product. Significant differences were found related to the biodistribution of the drug. In particular, on day 12 of infection, mice were treated and drug distribution and absorption in organs, such as brain, lungs and liver, were assessed by HPLC analysis. Poloxamer-coated atovaquone NCs gave similar results to the commercially available Wellvone^® formulation, while SDS-Atovaquone NCs resulted in a very high concentration of the drug in the various districts considered, including the brain, compared to the commercial pharmaceutical form ^[38][164]. In the study of Lemke et al., amphotericin B NCs were investigated against encephalitis caused by the amoeba Balamuthia mandrillaris. Amphotericin B is characterized by low water solubility and nephrotoxicity. Seven nanosuspensions were prepared differing in the surfactant’s concentration. The most promising formulation was found to be that prepared with Tween^® 80 and sodium cholate (SC), promoting the high absorption of ApoE at the surface of NCs and a low absorption of opsonins, such as fibrinogen and IgG γ. The good absorption of ApoE on the NC surface and a minimization of hepatic absorption (low fibrinogen and IgG γ) indicated a high absorption in the brain ^[39][165]. A coating model was designed for Baicalin NCs, with the aim of obtaining a formulation for both ischemic stroke and Alzheimer’s disease. To improve the passage of the BBB due to the low bioavailability of the molecule, the nanosuspension, addressed to the intravenous route, was prepared using a combination of Tween^® 80 and TPGS as stabilizer agents. The final goal was to obtain a formulation able to absorbs ApoE on the surface, acting as a ligand for the LDL receptors present on the BBB, improving their crossing and leading to their brain-uptake by endocytic processes. As mentioned above, the coating with ApoE led to a lower adsorption of fibrinogen and Ig γ (opsonins) encountered in the blood that can promote phagocytosis and the removal of the drug carriers from the systemic circulation ^[40][166]. Serum albumin and polyethylene glycol 1000 were also investigated as functional coatings for neviparine NCs. Nevirapine is a non-nucleotide reverse transcriptase inhibitor used in the treatment of HIV-associated dementia. The problem with antiretroviral drugs is the difficulty of reaching the “reservoir” site or reaching it at low doses or for short periods of time. To be effective, they need high dosages, which inevitably leads to side effects. Nevirapine NCs were obtained by HPH. Dextran 60, PEG 1000 or serum albumin were absorbed on the NC surface as a functional coating to improve drug brain targeting. The best cellular absorption was obtained with serum albumin, 3.84 times greater than the pure drug and 1.39-fold greater than the uncoated NCs probably due to an increased recognition by macrophage receptors ^[41][58]. Therefore, the use of a coating agents can be defined “functional” when used to increase cellular absorption and drug concentration at the target site, avoiding the massive dosage of antiretroviral drugs.

The effective delivery of chemotherapy drugs to the brain is very difficult, and the treatment must necessarily be targeted to the specific tumor site in order to avoid the drug’s accumulation in healthy tissues. The presence of coating agents on the particle surface could be useful to promote drug delivery to a specific district. Chai et al. designed a coated docetaxel NCs, modified with tumor-targeting ligands, for the treatment of glioma. Docetaxel is an antimitotic drug, and its use is associated with severe side effects, including aplastic anemia, mouth ulcers, nausea, vomiting and hair loss. The administration of docetaxel NCs could reduce drug dosage and the peripheral side effects. After preparation, docetaxel NCs were coated with a membrane constituted by red blood cells (RBCs) modified via avidin–biotin interactions by adding a tumor-targeting ligand (Rgdyk) that binds to integrins on cancer cells. The final formulation improved drug efficiency and targeting to the brain after intravenous administration ^[42][167]. In another study, 20(S)-protopanaxadiol NCs were prepared by the solvent–antisolvent precipitation technique. The 20(S)-protopanaxadiol is a molecule that has recently been re-evaluated for its potent anti-cancer action, comparable to paclitaxel. The 20(S)-protopanaxadiol NCs were then coated with TPGS. It has been shown that TPGS, safe and approved by the FDA, can induce apoptosis and implement the effect of some chemotherapy molecules ^[43][44][168,169].

Nerves are mostly covered by a myelin sheath, which consists of lipids and proteins and can be considered a protective layer that wraps around the axons of neurons to aid in the insulation of the neurons, to increase the number of electrical signals being transferred and enhance the speed of these electrical signals, allowing all actions to be conducted quickly. The process by which myelin is formed is carried out by Schwann cells and oligodendrocytes. The process requires a lot of energy and metabolic precursors, such as acetyl-coenzyme A, pyruvate, and NADPH. Thus, in conditions of metabolic stress, the myelin formation process fails and the reduced ability to form it leads to pathological conditions ^[45][170]. Multiple sclerosis (MS) is one example, in which the myelin coating around axons and the ability to transmit neuronal signals is lost, followed by the death of neurons. Studies show that markers of metabolic stress have been found in MS. In fact, in conditions of metabolic stress, oligodendrocytes tend to reduce the production of myelin to safeguard cell survival ^[46][171]. Some remyelination strategies are discussed In the following. The administration of gold exerts beneficial effects on cellular metabolism. The study of Robinson focused on the preparation of a nanosuspension based on gold NCs with a clean and faceted surface, revealing that these systems, having a mean size of about 80 nm, can be powerfully catalytic. They catalyze an important reaction of the cellular metabolic process, namely the oxidation of NADH, becoming important for the generation of ATP, glycolysis and oxidative phosphorylation. Thus, gold NCs can be useful in cases of remyelination or diseases, such as MS, to reactivate the metabolism useful for the production of myelin. Additionally, the comparison between bulk gold and gold reduced to nanometric dimensions (NCs) revealed that, only in this latter condition, it exerted a catalytic capacity ^[47][172]. Curcumin-based NCs were prepared for administration to rat models of the MS disease. In the study of Caillaud et al., curcumin NCs were administered intraperitoneally to an animal model of the transgenic Charcot–Marie–Tooth disease (CMT type 1A). Curcumin was selected for its antioxidant power. Neurodegenerative diseases, such as CMT, are often associated with the aberrant folding of a protein derived from oxidative stress; therefore, it is advantageous to use a molecule with a powerful antioxidant effect. A previous study showed that the administration of curcumin near the damaged nerve allowed for slight repair. In addition, this molecule can stimulate the remyelination of demyelinated nerves. Curcumin NCs can be used in the treatment of the CMT disease, a disease associated with damaged nerves and damaged nerve conduction, as well as with muscle weakness and atrophy. Cellulose NCs functionalized with β-cyclodextrin were prepared. The results of the study showed that the formulation of curcumin NCs led to a decrease in reactive oxygen species (ROS), an increase in nervous performance, a decrease in creatinine (marker of muscle lysis) and an increase in antioxidant enzymes. Thus, the authors considered the formulation promising for the treatment of demyelinating pathologies [173].

Many studies concern NCs made of elements of the lanthanide series, commonly known as rare-earth elements. Among these, erbium, ytterbium and europium are the most used in the preparation of NCs for brain diagnostics. The work of Fan et al. summarizes the current landscape regarding bioimaging using luminescent probes consisting of NCs based on lanthanides, which is a non-invasive diagnostic with emissions in the NIR II region (second region of the near infrared). In particular, the researcheuthors referred to the use of nanoprobes for the diagnosis of cerebral vascularization ^[50][176]. One of the main disadvantages of most diagnostic NCs is the high toxicity due to the presence of heavy metal In the study of Choi et al., “heavy-metal-free” fluorescent NCs for intranasal administration were prepared. TAs detailed in Section 4, this administration route is gaining great attention for its feasibility to serve as a direct transport to the brain, bypassing the BBB and allowing the possibility to detect ischemic brain lesions through fluorescent NCs. This strategy could overcome the degradation and agglomeration associated with the intravenous and/or subcutaneous administration of NCs due to the superficial absorption of blood proteins. An advantage is that NIR NCs fluoresce in the near infrared region, so that they do not absorb skin or other organ-derived fluorescence. In order to improve their water solubility and transport, NIR NCs were coated with PEG in the final step, allowing for brain injury detection and damage caused by neurodegenerative diseases. Finally, these cadmium-free NCs allowed for repeated and long-term diagnoses ^[51][177]. The imaging of brain vessels through luminescence-emitting NCs is one of the main studies conducted in this field. In the work of Zhong et al., rare-earth NCs were prepared and then doped with cerium emitting luminescence at 1500 nm. In contrast to near infrared (NIR I, 500–900 nm), their use enhances tissue resolution and the quality of in vivo imaging diagnostics. They have shown promising results for their non-toxicity and photostability in aqueous fluids, which makes them stable in biological fluids. The test was performed on buffered saline solutions and bovine serum. NCs with dimensions of approximately 18 nm were synthesized by the co-thermolysis of rare-earth trifluoroacetates into oleic acid, 1-octadecene and/or oleylamine. They were doped with Ce³⁺ to reach a luminescence of 1500 nm. To make the surface hydrophilic and biocompatible, poly-(maleic anhydride-alt-1-octadecene, PMH) was used, which was bonded through van der Waals bonds using a catalyst to the oleic acid present on the surface. The formulation, injected intravenously, excited by a laser at 890 nm, allowed for the detection of blood flow in the vessels. After injection to rats, an image of the cerebral vessels was succeeded in a short time; within 10 s, the vessels of the skull were perfused, indicating an efficient material for diagnosing cerebrovascular diseases. Moreover, NC uptake in the liver and spleen was observed after 24 h, thanks to the sequestration of the NCs by the cells of the endothelial reticulum ^[52][178]. Accordingly, in the work of Yu et al., cadmium-and-lead-free NCs were exploited for brain tumor diagnosis. The nucleation of the NCs took place at low temperatures, forming CuInS2/ZnS NCs, which fluoresce in the near infrared region, prepared with diphenylphosphine sulfide (SDPP) as a precursor of elemental S. These systems were able to locate a brain tumor after administration ^[53][179]. The work of Wang et al. showed the preparation of luminescent probes, consisting of ytterbium-doped NCs, emitting at 1525 nm. Usually, the systems that emit at this wavelength cause the heating of biological tissues. The aim of thise NCs se work was the detection of the cerebral vascularization in the brain of a mouse through fluorinated NCs. These NCs sensitized by erbium Er3+ were brought into the aqueous phase by covering with polyacrylic acid (PAA) without affecting the strong luminescence. NCs with a core/shell structure were obtained by co-precipitation at high temperatures ^[54][180]. The work of Sojka et al. dealt with rare-earth NCs to evaluate the transport of GABA and glutamic acid in the brain. In particular, the work compared two types of hydrophilic NCs: pegylated NCs and NCs that were surface modified by the addition of -OH groups. These hydrophilic yttrium and sodium-fluoride-based NCs were doped with Eu3+ to achieve fluorescence and showed promise in replacing and overstepping the limits of the fluorophores that are often used in diagnostics. The study compared several nanoparticle systems, among which NCs resulted to be the least neurotoxic. NCs have proved to be neuroactive, able to absorb glutamic acid and GABA in the terminals, useful in the case of neurological treatments and neurosurgery ^[55][181].

A variety of quantum dots are made from heavy metals, such as cadmium/lead, which allow the imaging of specific areas. In the work of Morales-Narváez et al., cadmium–selenide/zinc sulfide quantum dots were prepared. These type of quantum dots are able to localize the ApoE protein and, based on its quantification, the progress of AD can be assessed, since abnormal levels of the ApoE isoforms can be found in such pathology. The performance of the cadmium–selenide/zinc sulfide quantum dots and Alexa 647 fluorescent dye was evaluated simultaneously by microarray and ELISA analyses, under the same conditions to form sandwich immune complexes. From the study emerged that NCs were able to reveal the different apo-lipoprotein isoforms and detect the ApoE to a greater extent than the Alexa 647 dye ^[56][182].

Perovskite NCs have recently aroused interest in diagnostics. They consist of a general formula, ABX3, in which A is a monovalent cation, B is a divalent metal cation and X is usually a halide. In particular, these systems have proven to be useful in the diagnostics of brain areas, with an optimal emission spectrum at 1700 nm ^[57][183]. In another work, KMnF3 NCs were prepared for glioma diagnosis. These metal fluoride-based NCs have high resolution and precision and a good contrast to distinguish between the labeled tissue and surrounding tissue. NCs were prepared using the “one-pot” method and then coated with albumin to improve their biocompatibility. The conjugation with albumin facilitated the passage of the NC system through the BBB. After intravenous administration (5 mg Mn/kg), a good distribution was found, mostly in the perfused organs. Evaluating the toxicity, important in the case of contrast media, a hemolysis less than 1% was revealed. The images presented in the work of Wang et al. demonstrated NCs’ capacity to detect tumor areas as very bright areas, being a good prospect for the diagnostic evaluation of cerebral tumors on an inflammatory basis ^[58][184].

Iron oxide NCs are also applied in brain diagnostics. In particular, these systems include magnetite and maghemite NCs coated with a stabilizer to avoid agglomeration. Their application consists of the detection of brain lesions and brain tumor or in the diagnosis of vessel infiltration following an ischemic stroke (which leads to neurodegeneration). Among these systems, two classes can be distinguished: superparamagnetic iron oxide (SPIO) and ultrasmall superparamagnetic iron oxide (USPIO) particles with dimensions below 50 nm. Their peculiar characteristic is that they are picked up by the macrophage system. Therefore, they are useful in the diagnosis of pathologies with high macrophage activity, such as in neurodegenerative diseases where the neuroinflammatory component is very high ^[59][185]. In the work of Danscher et al., it has been shown that the injection of sodium selenide or sodium selenite leads to the systemic formation of zinc–selenium NCs in the synaptic terminals of glutaminergic and GABAergic neurons. It can be used as a useful tool for brain diagnostics, but it is important that the used dose of sodium selenide injected is sufficiently high to ensure the filling of zinc-enriched synaptic vesicles (ZEN) to be detected. Larger amounts of selenide injected intracerebrally led to the increased formation of zinc–selenium molecules, which then agglomerate to form NCs. The selenium method was introduced in 1982 as a tool for zinc ion tracing in vesicular compartments, such as ZEN terminals in the CNS. With this method, it is possible to track glutaminergic and GABAergic neurons containing Zn-T3 proteins in their vesicular membrane ^[60][186].

Synaptic devices are highly reliable devices that allow the synapses of the brain to be reproduced electrically thanks to a system that modifies its characteristics based on the strength of the electrical signals. In the work by Zhao et al., a multilayer system constituted by a layer of silicon NCs, in which subsequent layers were deposited on the top of each other by thermal evaporation, was prepared. These devices have several important synaptic functionalities, including coupled impulse facilitation (PPF), short-term transition plasticity (STP) to long-term plasticity (LTP), and time-dependent plasticity to peak (STDP). The researcheuthors aimed to obtain a device that allowed the synapses to resume their function through the conduction of the impulse between the presynaptic axon and the postsynaptic dendritic terminal. Therefore, the light-emitting diodes of silicon NCs showed their ability to exhibit a series of important synaptic functionalities, mimicking biological synapses ^[61][187]. Finally, the development of a biosensor for the detection of adrenaline in biological fluids is discussed. Adrenaline is a hormone synthesized by the adrenal medulla, which acts as a neurotransmitter for the sympathetic nervous system. It is involved in neurodegenerative diseases, such as Parkinson’s. It proved to be an excellent method for medical brain diagnostics, as the biosensor was tested on the zebrafish brain. Among the constituents of the biosensor, cellulose NCs can be mentioned. The dosimetric biosensor reproduced a signal based on the amount of adrenaline detected. It showed a good selectivity towards adrenaline and could be used in the diagnosis of neurodegenerative diseases ^[62][188].