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Quantum Dots Bearing Lectin-Functionalized Nanoparticles as a Platform for In ViWo Brain Imaging Xiaoling Gao,†,‡ Jun Chen,† Jiyao Chen,§ Bingxian Wu,† Hongzhuan Chen,*,‡ and Xinguo Jiang*,† Department of Pharmaceutics, School of Pharmacy, Fudan University, P.O. Box 130, 138 Yixueyuan Road, Shanghai 200032, P.R. China, Department of Pharmacology, Institute of Medical Sciences, Shanghai Jiaotong University School of Medicine, 280 South Chongqing Road, Shanghai 200025, P.R. China, and Surface Physics Laboratory (National Key Laboratory), Physics Department, Fudan University, Shanghai 200433, P.R. China. Received July 1, 2008; Revised Manuscript Received September 16, 2008

Delivery of imaging agents to the brain is highly important for the diagnosis and treatment of central nervous system (CNS) diseases, as well as the elucidation of their pathophysiology. Quantum dots (QDs) provide a novel probe with unique physical, chemical, and optical properties, and become a promising tool for in ViVo molecular and cellular imaging. However, their poor stability and low blood-brain barrier permeability severely limit their ability to enter into and act on their target sites in the CNS following parenteral administration. Here, we developed a QDs-based imaging platform for brain imaging by incorporating QDs into the core of poly(ethylene glycol)-poly(lactic acid) nanoparticles, which was then functionalized with wheat germ agglutinin and delivered into the brain via nasal application. The resulting nanoparticles, with high payload capacity, are water-soluble, stable, and showed excellent and safe brain targeting and imaging properties. With PEG functional terminal groups available on the nanoparticles surface, this nanoprobe allows for conjugation of various biological ligands, holding considerable potential for the development of specific imaging agents for various CNS diseases.

INTRODUCTION Delivery of imaging agents to the brain is highly important for the diagnosis and treatment of central nervous system (CNS) diseases (1-3), which also offers the opportunity to examine the structural, functional, and biochemical changes in the brain that will lead to new insights regarding the pathophysiology of these disorders. However, brain delivery of agents is often restricted by the blood-brain barrier (BBB), which is formed by tight junctions within the capillary endothelium, preserving brain homeostasis as well as preventing active agents against doing their jobs in the brain (4). Quantum dots (QDs), semiconductor nanocrystals with unique physical, chemical, and optical properties, are now emerging as a revolutionary means for imaging and optical detection (5-7) and achieving more and more attention in neuroscience (8, 9). Functionalized QDs obtained by attaching to ligands such as antibodies (10), peptides (11), and carbohydrates (12) have been used for in ViVo molecular and cellular imaging. TAT-conjugated quantum dots have been described for brain imaging following intra-arterial administration (13). A recent report suggested that transferrin-conjugated quantum rods as probes were efficient for transmigration across an in Vitro BBB (14). However, the major drawbacks of these previous QDs-based imaging techniques are their poor stability in aqueous media and in the biological systems (15-17). Encapsulations of QDs with amphiphilic copolymers (10) and phospholipids (18) have been previously presented to increase the solubility and stability of QDs but limited by their low payload capacity and lack of brain delivery ability. In general, the poor stability and lack of BBB * Corresponding authors. Prof. Xinguo Jiang, Tel.: 086-02154237381, Fax: 086-021-54237381, E-mail: [email protected], and Prof. Hongzhuan Chen, Tel.: 086-021-63846590-776451. Fax: 086021-64674721. E-mail: [email protected]. † School of Pharmacy, Fudan University. ‡ Shanghai Jiaotong University School of Medicine. § Physics Department, Fudan University.

permeability severely limit the ability of QDs to enter into and act on the target sites in the CNS following parenteral administration. Previous studies in our laboratory and others have demonstrated that intranasal administration offers a practical, noninvasive, alternative route for drug delivery to the brain (19-21). Our recent work suggested that it is possible to deliver agents carried by certain lectin-functionalized nanoparticles into the brain via intranasal administration (22-24). Continuing these efforts, here we developed a quantum dots-based nanoplatform for brain imaging by encapsulating QDs into the core of poly(ethylene glycol)-poly(lactic acid) (PEG-PLA) nanoparticles and conjugating the nanoparticles with wheat germ agglutinin (WGA) to improve their brain delivery. This innovative approach raises new possibilities for the development of QDs-based specific imaging agents for various CNS diseases. To address the feasibility of this nanoplatform, we evaluate the particle size, zeta potential, internal structure, biobinding activity, and stability under in Vitro settings, as well as biodistribution, brain delivery, and imaging property of the resulting nanoparticles in animals following intranasal administration.

EXPERIMENTAL PROCEDURES Preparation of WGA-Functionalized Quantum DotsLoaded Nanoparticles (WGA-QDs-NP). WGA-QDs-NP were prepared by incorporating tri-n-octylphosphine oxide-coated CdSe/ZnS QDs (TOPO-QDs) into PEG-PLA nanoparticles followed by conjugating the resulting nanoparticles with Traut’s reagent-thiolated WGA via maleimide groups at the ends of the PEG spacers (Scheme 1). TOPO-QDs, high-quality, hydrophobic QDs (size: 7.2 nm, emission wavelength: 607 nm), were synthesized as described previously (25) and kindly provided by Dr. Yang Yue (National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences). Methoxy-PEG-PLA and maleimide-PEG-PLA were synthesized by ring-opening polymerization of D,L-lactide (99.5% pure, PURAC) initiated by methyl-PEG and maleimide-

10.1021/bc8002698 CCC: $40.75  2008 American Chemical Society Published on Web 10/16/2008

2190 Bioconjugate Chem., Vol. 19, No. 11, 2008 Scheme 1. Preparation of Lectin-Functionalized Quantum Dots-Loaded Nanoparticles 80 × 99 mm2 (600 × 600 DPI)

PEG, respectively, using stannous octoate as catalyst (26). QDsloaded PEG-PLA nanoparticles were prepared with a blend of maleimide-PEG3400-PLA54000 and methyl-PEG3000-PLA49000 using an emulsion/solvent evaporation technique. Briefly, 1 mg maleimide-PEG-PLA and 9 mg methyl-PEG-PLA were dissolved in 0.5 mL of dichloromethane containing 3.2 × 10-5 mol/L QDs. The solution was then emulsified by sonication (220 w, 30 s) on ice in 2 mL of a 1% aqueous sodium cholate solution. The emulsion obtained was diluted into 8 mL of a 0.5% aqueous sodium cholate solution under moderate magnetic stirring. Five minutes later, dichloromethane was evaporated at low pressure at 30 °C using Bu¨chi rotavapor R-200 (Bu¨chi, Germany). Then, the nanoparticles were centrifuged at 21 000 g for 45 min using TJ-25 centrifuge (Beckman Counter, USA) equipped with an A-14 rotor. The supernatant was discarded, and the obtained nanoparticles were subjected to a 1.5 × 20 cm sepharose CL-4B column and eluted with 0.05 M HEPES buffer (pH 7.0) containing 0.15 M NaCl to remove the QDs un-entrapped or absorbed to the exterior of the nanoparticles. Then, the QDs-loaded nanoparticles were incubated with 2-iminothiolane-thiolated WGA at the ratio of thiolated lectin to maleimide 1:3 for 9 h for WGA conjugation. After that, the resulting WGA-QDs-NP were purified with a 1.5 × 20 cm Sepharose CL-4B column. Morphology, Particle Size, and Zeta Potential of WGAQDs-NP. Morphological examination of the nanoparticles was performed by transmission electron microscopy following negative staining with sodium phosphotungstate solution. The mean diameter and zeta potential of the nanoparticles were determined by dynamic light scattering analysis using Zeta Potential/Particle Sizer NICOMPTM 380 ZLS, respectively. Internal Structure and Payload Capacity of WGA-QDsNP. In order to determine the internal structure of WGA-QDsNP, the nanoparticles were diluted to appropriate concentration and observed under a freeze-fracture electron microscope (JEOL 2010, JEOL, Japan) as described below. First, the samples were suspended in a microperforated grid under controlled temperature (25 °C) and humidity conditions (95-99%)

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and blotted gently to form a 100-250 nm thick liquid film with a filter paper. The liquid film was then immediately plunged into liquid ethane at its freezing point (-183 °C), producing a vitrified specimen, and kept under liquid nitrogen until loaded onto a cryogenic sample holder. Vitrified samples were observed under the freeze-fracture electron microscope at 200 kV using a magnification of 40 000. Images were recorded using a Gatan 832 multiscan CCD camera. Haemagglutination Test. The retention of the bioactivity of WGA following covalent coupling to the surface of nanoparticles was confirmed by a haemagglutination test. Unmodified QDs-loaded NP and WGA-QDs-NP were incubated with 20% (v/v) suspension of fresh rat blood in 0.9% NaCl at 37 °C for 30 min, respectively. After that, the mixtures were observed under a fluorescence microscope (Olympus IX71), and the images were taken and colored with Image-Pro Plus. Stability of WGA-QDs-NP in Aqueous Solution and in Serum. WGA-QDs-NP were prepared as 5 mg/mL solution in water and stored at 4 °C for one week and in serum for 24 h, respectively. After that, the solutions were diluted to appropriate concentration with the morphology and internal structure of the nanoparticles observed under freeze-fracture electron microscopy as described above. Optical Properties of WGA-QDs-NP. The optical properties of WGA-QDs-NP were characterized by a luminescence spectrometer and compared with those of QDs at the excitation wavelength of 400 nm and emission wavelength between 550 and 650 nm. The excitation slit was 10.0 nm, Em slit 2.5 nm, scan speed 30 nm/min, and the scan times 10 with a delay of 120 s. Biodistribution of WGA-QDs-NP Following Intranasal Administration. Biodistribution of WGA-QDs-NP following intranasal administration in BALB/c nude mice was investigated by a NightOWL II LB983 NC320 in ViVo optical imaging system (Berthold Technologies, Oak Ridge, TN) consisting of a highly sensitive Peltier cooled backlit CCD camera (2184 × 1472 pixels). The animal experiments were performed according to the protocols evaluated and approved by the ethical committee of Fudan University. First, nude mice (25-30 g) were anesthetized with sodium pentobarbital (50 mg/kg, i.p.) and imaged for the signal of autofluorescence. After that, the animals were given intranasally with WGA-QDs-NP at the dose of about 40 mg/kg (QDs 3.2 × 10-8 mol/kg) in 10 µL (5 µL each nostril) and imaged at 2 min and 1, 2, 3, 4, and 8 h postadministration, respectively (n ) 6 for the time points of 2 min and 1, 2, and 3 h; n ) 3 for the time points of 4 and 8 h). For imaging, mice were placed in the light-tight chamber, the filters for excitation and emission were set at 530 and 600 nm, respectively, and the photon emission was integrated over a period of 5 s. After that, a gray scale reference image was collected under white light. For localization of the photon emission, gray scale and pseudocolor images were merged using WinLight (32) software. From the luminescent images, regions of interest (ROIs) were drawn, and the peak and integrated bioluminescence flux (photons/s) were calculated utilizing the software. To determine the biodistribution of WGA-QDs-NP in various organs, three hours following administration, three mice were euthanized and fixed by heart perfusion with their brains, hearts, livers, spleens, lungs, and kidneys collected for imaging under the NightOWL II LB 983 imaging system. After that, the brain tissues were immersed in 4% paraformaldehyde for 24 h, dehydrated in sucrose solution, embedded in Tissue-Tek O.C.T compound (Sakura Finetek, USA) and stored at -70 °C until frozen sectioning. Finally, the sections were transferred to microscope slides, counterstained with 4′,6-diamidino-2-phenylindole (DAPI), and observed under a fluorescence micro-

Quantum Dots Based Nanoplatform for Brain Imaging

Figure 1. Transmission electron micrograph of (A) quantum dots-free WGA-conjugated nanoparticles (WGA-NP) and (B) WGA-conjugated quantum dots-loaded nanoparticles (WGA-QDs-NP) negatively stained with phosphotungstic acid solution, respectively. Bar, 100 nm. 80 × 48 mm (600 × 600 DPI).

Figure 2. Internal structure of WGA-conjugated quantum dots-loaded nanoparticles (WGA-QDs-NP) exhibited by freeze-fracture electron microscopy. Bar, 100 nm. 80 × 68 mm (600 × 600 DPI).

scope (Olympus IX71) to determine the distribution of WGAQDs-NP in the CNS.

RESULTS Characterization of WGA-QDs-NP. The resulting nanoparticles exhibited a spherical shape under the examination of a transmission electron microscope (Figure 1B) with a numberbased average diameter (95.3 ( 41.0 nm) comparable to that of the QDs-free WGA-functionalized nanoparticles (Figure 1A, 92.0 ( 33.7 nm). The zeta potential of WGA-QDs-NP was -22.67 ( 1.21 mV, which was also similar to that of QDsfree WGA-functionalized nanoparticles (-18.9 ( 2.4 mV). We examined the internal structure of WGA-QDs-NP with freezefracture electron microscopy, finding out that numerous QDs were encapsulated into each nanoparticle and most of them located at the center of the particles (Figure 2). We believe this phenomenon is mainly resulted from the strong hydrophobic interaction between TOPO and PLA fragment of the polymer, which composes the core of the resulting PEG-PLA nanoparticles (27). The retention of WGA’s bioactivity following covalent coupling to the surface of the QDs-loaded nanoparticles was confirmed by haemagglutination test. It was exhibited that the

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erythrocytes agglutinated following their incubation with WGAQDs-NP and all the red fluorescence of QDs specifically stuck to the surface of the erythrocytes (Figure 3A). But in the case of the unmodified QDs-loaded nanoparticles, the fluorescent signals dispersed at random and did not show any specific affinity to erythrocytes (Figure 3B). This evidence clearly suggested that WGA, treated through the covalent coupling procedure, still retained the carbohydrate binding bioactivity and the incorporation of QDs into the nanoparticles did not alter the biobinding activity of the lectin-functionalized nanoparticles. Stability of WGA-QDs-NP in aqueous solution and in serum was evaluated under freeze-fracture electron microscopy, respectively. The photos showed that WGA-QDs-NP dispersed well in water without significant changes in particle size even after one week storage (Figure 4A). Furthermore, they retained their structures and loadings after 24 h incubation in serum (Figure 4B). Such telling results strongly suggested the excellent solubility and stability of the present nanoplatform, indicating its potential to serve as a probe for imaging in the biological systems. Optical properties of WGA-QDs-NP were characterized by a luminescence spectrometer and compared with those of QDs. The spectral curves of QDs reduced in intensity with increased light exposure, while those of WGA-QDs-NP showed a slightly blue shift but remained unchanged under the same scan condition (Figure 5), suggesting that the incorporation of QDs in the core of PEG-PLA nanoparticles also significantly improved their photostability. Biodistribution of WGA-QDs-NP Following Intranasal Administration. Biodistribution of WGA-QDs-NP following intranasal administration was investigated in BALB/c nude mice by a dedicated imaging system. It was shown that, under the present imaging settings, autofluorescence of nude mice is negligible (Figure 6A, the first photo named “Blank”) and fluorescent signals of WGA-QDs-NP in brain increased with exposure time, reaching peak at 3-4 h following administration, but almost disappeared at 8 h (Figure 6A,B). We also found that, 8 h following administration, certain amounts of fluorescent signals were detected near the urethra (photo taken when the animal was in supine position; Figure 6A, the last photo). Therefore, we deduced that at least part of WGA-QDs-NP were eliminated by filtration and excretion through the kidney. To better understand the distribution of WGA-QDs-NP in different areas of the brain, 3 h following administration, three mice were euthanized with their brain collected, frozensectioned, and observed under a fluorescence microscope following nuclear staining with DAPI. It was shown that, 3 h after intranasal administration, fluorescence signals of WGAQDs-NP were detected in the olfactory bulb, olfactory tract, cortex, caudate-putamen, hippocampus, third ventricle, dorsal third ventricle, and lateral ventricle (Figure 7), suggesting that the imaging agents distributed quickly and extensively into the CNS. In addition, we found that the imaging agents described herein presented excellent brain targeting property following intranasal administration. Three hours following the administration, three mice were euthanized and fixed by heart perfusion with their brains, hearts, livers, spleens, lungs, and kidneys collected for imaging. The data showed that the relative average fluorescent intensities of WGA-QDs-NP in the tissues at this time point followed the order of brain g lung > liver > kidney > heart > spleen (Figure 8A,B). These results clearly suggested the efficient brain targeting property of the present nanoplatform, which is very important for its safe and efficient in ViVo imaging.

DISCUSSION In the present study, we first developed a QDs-based imaging platform for brain imaging by incorporating QDs into the core

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Figure 3. Interactions between erythrocytes and (A) WGA-conjugated quantum dots-loaded nanoparticles (WGA-QDs-NP) or (B) unmodified quantum dots-loaded nanoparticles (QDs-NP): erythrocytes suspension (4%, 200 mL) incubated with WGA-QDs-NP (10 mg/mL, 50 mL) and QDs-NP (10 mg/mL, 50 mL) at 37 °C for 30 min, respectively. 80 × 31 mm (300 × 300 DPI).

Figure 4. Freeze-fracture electron micrograph of WGA-conjugated quantum dots-loaded nanoparticles (WGA-QDs-NP) after (A) one week storage in aqueous solution and (B) 24 h incubation in serum. 80 × 33 mm (600 × 600 DPI).

Figure 5. Luminescence emission spectra of quantum dots (QDs) (upper) and WGA-QDs-NP (lower) at the excited wavelength of 400 nm, respectively. The spectral curves of QDs reduced in intensity sequentially with the increase of light exposure, while those of WGAQDs-NP showed a slightly blue shift but remained unchanged under the same scan condition (Ex: 400 nm, Em scan range: 550-650 nm; Ex slit: 10.0 nm; Em slit: 2.5 nm; Scan speed: 30 nm/min; Scan times: 10; Delayed: 120 s). 80 × 83 mm (600 × 600 DPI).

of PEG-PLA nanoparticles, which was then functionalized with WGA and delivered into the brain via nasal application. The

resulting nanoparticles, with high payload capacity, are watersoluble, stable, and showed excellent and safe brain targeting and imaging properties. With the flexibility of having PEG functional terminal groups on the nanoparticles surface, this nanoprobe allows for conjugation of various biological ligands (e.g., sugars, peptides, proteins, and antibodies) as “smart nanoparticles” for targeting and imaging in various brain disorders. Brain delivery properties of nanoparticles are essential for effective brain imaging in ViVo. Our newly developed QDsbased nanoplatform showed the following advantages for in ViVo brain imaging: First, WGA-QDs-NP stayed in the brain for more than 4 h (Figure 6), offering enough time for the recognition and binding of the imaging agents with the disorder regions in the CNS. Second, the imaging agents were almost cleared 8 h after administration (Figure 6), therefore supplying a safe technique for in ViVo imaging. Third, the imaging agents distributed quickly and extensively into the CNS (Figure 7), which might provide a good opportunity for the recognition and binding of imaging agents with their targets. Furthermore, the excellent brain targeting property of WGA-QDs-NP following intranasal administration could thereby reduce potential toxicity to the peripheral organs (Figure 8). The mechanism of brain delivery of WGA-QDs-NP is not clear yet. However, our previous work demonstrated that the enhancement of brain delivery of agents carried by WGAfunctionalized nanoparticles was mediated by the substrates of WGA s N-acetyl-D-glucosamine and sialic acid, which were abundantly observed in the nasal cavity (28, 29). It is welldocumented that nasal mucosa was composed of the respiratory region, which had the highest degree of vascularization and was mainly responsible for drug absorption into the circulation, and olfactory mucosa, which played a vital role in the transport of drugs into the brain and cerebrospinal fluid (30). WGA, specifically binding to N-acetyl-D-glucosamine and sialic acid, presented a great affinity to the olfactory epithelium while a

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Figure 6. Distribution and retention of WGA-conjugated quantum dots-loaded nanoparticles (WGA-QDs-NP) in the brain following intranasal administration. (A) Representative optical images taken before, 2 min, and 1, 2, 3, 4, and 8 h after dose under a dedicated imaging system designed for small animals imaging. (B) Quantification of the luminescence signal from WGA-QDs-NP in the brain over time following intranasal administration (n ) 6 for the time points of 2 min and 1, 2, 3 h; n ) 3 for the time points of 4 and 8 h). 176 × 85 mm (300 × 300 DPI).

Figure 7. Distribution of WGA-conjugated quantum dots-loaded nanoparticles (WGA-QDs-NP, red) in the olfactory bulb, olfactory tract, cortex, caudate-putamen, hippocampus, third ventricle, dorsal third ventricle, and lateral ventricle of nude mice 3 h after intranasal application. 176 × 145 mm (300 × 300 DPI).

moderate degree to the respiratory epithelium (29, 31). Accordingly, our previous data also showed that the affinity of WGANP to the olfactory mucosa was higher than that to the respiratory one (24). It has been well-presented that, following an intranasal administration to rats, wheat germ agglutininhorseradish peroxidase (WGA-HRP) concentrated in the olfac-

tory bulb, while a negligible amount of label was detected in the same location after intravenous administration of WGAHRP or intranasal administration of unconjugated HRP (32). Therefore, we deduced that at least part of WGA-NP enter the brain via olfactory mucosa. If this is true, it would become much easier to understand why WGA-QDs-NP achieved such good

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Figure 8. Distribution of WGA-conjugated quantum dots-loaded nanoparticles (WGA-QDs-NP) in various organs following intranasal administration in nude mice. (A) Representative optical image taken 3 h after dosing under a dedicated imaging system designed for small animal imaging. (B) Quantification of the luminescence signal from WGA-QDs-NP in various organs 3 h following intranasal application (n ) 3). 176 × 81 mm (300 × 300 DPI).

brain targeting property. Previous research also strongly suggested that WGA-HRP can pass BBB following intravenous administration by absorptive-mediated mechanism (33). So, it is also possible WGA-NP achieve brain delivery via the systemic pathway. The safety of QDs is now one of the major concerns for in ViVo QDs-based imaging techniques. A recent paper suggested that cytotoxicity of QDs, which is mainly induced by the release of Cd2+ ions into the cellular environment, was greatly reduced by coating the bare QDs with polymers (34). In the present paper, QDs were entrapped into the core of nanoparticle and did not show any leakage after 24 h incubation in serum (Figure 4B). Therefore, the toxicity of QDs was considered to be low. The coating polymer PEG-PLA is a well-developed copolymer, and its safety has been well-acknowledged (35). In addition, the amount of WGA in the systemic circulation is far less than that required for the development of blood clots (36). Therefore, the application of WGA-QDs-NP is believed to be safe. Actually, no obvious side effects were observed in our experiments (data not shown). However, since the safety of the probe is now one of the major concerns for in ViVo imaging, the longterm side effects of WGA-QDs-NP still need to be considered very carefully and deserve further research before their further application. The emerging nanostructure QDs offer numerous potential applications in bioimaging in Vitro and in ViVo. However, conventional QDs-based imaging often encounters their poor stability in aqueous solution and in the biological systems, which was mainly caused by photocatalytic destruction of the mercaptoacetic acid coating and precipitation of the nanocrystals (15-17). Here, the use of TOPO-QDs encapsulation into PEGPLA nanoparticles enables their potential application in imaging in ViVo. Our data clearly suggested that QDs were encapsulated into the core of PEG-PLA nanoparticles with high loading efficiency (Figure 2) without changing the morphology, size, zeta potential, or biobinding activity of the lectin-functionalized nanoparticles (Figure 1, Figure 3). We believe that this phenomenon is mainly contributed by the strong hydrophobic interaction between TOPO and PLA fragment of the polymer, which composed the core of the resulting PEG-PLA nanoparticles (27). In the course of primary emulsification, TOPO-QDs coating with PLA remained inside the oil droplets while the hydrophilic group PEG migrated to the water surface. Upon solvent evaporation, the nanoparticles core solidified, forming

solid nanoparticles with higher stability compared with both micelles and liposomes. The advantages of this preparation procedure were obvious. By using it, we obtained nanoparticles with high payload capacity and improved stability in both water and serum (Figure 4). In addition, the photostability of the encapsulated QDs was significantly improved compared with that of free QDs. In contrast, previous works in which QDs were encapsulated in the chloroform/ethanol solvent mixture to obtain QDs-loaded micelles had several drawbacks such as low payload (one QD per micelle) and easy aggregation (10). Taking advantage of the thiolated-protein conjugating ability of maileimide group at the end of PEG, we successfully conjugated WGA to the surface of the QDs-loaded nanoparticles under mild condition without changing the biobinding ability of WGA. By incorporating PEG-PLA with other functional terminal groups such as biotin, amino (11), and carboxyl groups at the end of PEG, we can also further modify the nanoparticles with various biological ligands for specific molecular and cellular imaging (17). Therefore, WGA-QDs-NP described herein provided the following advantages to serve as a nanoplatform for bioimaging: (1) high payload capacity; (2) improved stability in aqueous solution and in biological system; (3) increased optical stability; (4) biobinding activity; and (5) flexibility of having PEG functional terminal groups available on the surface of nanoparticles, which allows conjugation of various biological ligands for specific molecular imaging. Further study is needed to fully optimize WGA-QDs-NP and to address the potential application of this nanoplatform in specific CNS disorders such as Alzheimer’s disease and brain tumors.

ACKNOWLEDGMENT The study was supported by National Key Basic Research Program (2007CB935800), National Natural Science Foundation of China (No. 30801442), and Grants from Shanghai Education Committee and Institute of Medical Sciences, Shanghai Jiaotong University School of Medicine. The authors also acknowledged Dr. Kunpeng Li and Prof. Ming Yao for their technical assistances in freeze-fracture electron microscopy and in ViVo optical imaging, respectively. Supporting Information Available: Distribution of WGAQDs-NP in different areas of the brain and various organs 4 h

Quantum Dots Based Nanoplatform for Brain Imaging

following intranasal administration in nude mice. This material is available free of charge via the Internet at http://pubs.acs.org.

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