Biocompatible and Water-Soluble Shortwave-Infrared (SWIR)-Emitting Cyanine-Based Fluorescent Probes for In Vivo Multiplexed Molecular Imaging.

Extending molecular imaging into the shortwave-infrared (SWIR, 900-1400 nm) region provides deep tissue visualization of biomolecules in the living system resulting from the low tissue autofluorescence and scattering. Looking at the Food and Drug Administration-approved and clinical trial near-infrared (NIR) probes, only indocyanine green (ICG) and its analogues have been approved for biomedical applications. Excitation wavelength less than 800 nm limits these probes from deep tissue penetration and noninvasive fluorescence imaging. Herein, we present the synthesis of ICG-based π-conjugation-extended cyanine dyes, ICG-C9 and ICG-C11 as biocompatible, and water-soluble SWIR-emitting probes with emission wavelengths of 922 and 1010 nm in water, respectively. Also, ICG-, ICG-C9-, and ICG-C11-based fluorescent labeling agents have been synthesized for the development of SWIR molecular imaging probes. Using the fluorescence of ICG, ICG-C9, and ICG-C11, we demonstrate three-color SWIR fluorescence imaging of breast tumors by visualizing surface receptors (EGFR and HER2) and tumor vasculature in living mice. Furthermore, we demonstrate two-color SWIR fluorescence imaging of breast tumor apoptosis using an ICG-conjugated anticancer drug, Kadcyla and ICG-C9 or ICG-C11-conjugated annexin V. Finally, we show long-term (38 days) SWIR fluorescence imaging of breast tumor shrinkage induced by Kadcyla. This study provides a general strategy for multiplexed fluorescence molecular imaging with biocompatible and water-soluble SWIR-emitting cyanine probes.

[Case of a Patient with Cancer of Unknown Primary in the Poor Prognosis Group Who Had a Experienced a Beneficial Treatment Course].

A 47-year-old woman with general malaise and abdominal pain presented with multiple liver tumors and lymph node metastasis. She was diagnosed with small cell carcinoma on the basis of a lymph node biopsy; however, the primary lesion was not identified. Finally, we diagnosed her with cancer of unknown primary lesion and placed her in the poor prognosis group. Although her general condition was poor, she experienced a relatively good response to treatment for small cell carcinoma.

Shortwave-infrared (SWIR) emitting annexin V for high-contrast fluorescence molecular imaging of tumor apoptosis in living mice.

Recently, shortwave infrared (SWIR) fluorescence imaging over 1000 nm has attracted much attention for in vivo optical imaging because of the higher signal to background ratios in the SWIR region. For the application of SWIR fluorescence imaging to biomedical fields, the development of SWIR fluorescent molecular probes with high biocompatibility is crucial. Although many researchers have designed a variety of SWIR emitting probes based on organic dyes, the synthesis of biocompatible SWIR fluorescent molecular imaging probes is still challenging. In this work we synthesized indocyanine green (ICG) and π-conjugation extended ICG (ICG-C11) labelled annexin V as SWIR fluorescent probes for tumor apoptosis. Annexin V is an endogenous protein with binding ability to phosphatidylserine (PS) which appears on the outer monolayer of apoptotic cell membranes. Although there are many types of visible and NIR fluorescent annexin V, there are no SWIR emitting fluorescent probes that can be used for high contrast fluorescence imaging of apoptosis in vivo. Herein, we report the synthesis and application of ICG and ICG-C11 conjugated annexin V for SWIR fluorescence imaging of tumor apoptosis. The presented fluorescent annexin V is the first SWIR emitting probe for in vivo optical imaging of tumor apoptosis. We demonstrate that SWIR emitting ICG- and ICG-C11 conjugated annexin V enable high-contrast fluorescence imaging of tumor apoptosis in living mice. We further demonstrate that ICG-C11-annexin V can be used for long-term (ca. two weeks) SWIR fluorescence imaging of tumor apoptosis. The SWIR fluorescent annexin V will greatly contribute not only to the study of tumor-apoptosis induced by anti-cancer drugs, but also to the study of apoptosis-related diseases in a living system.

In Vitro and In Vivo Fluorescence Imaging of Antibody-Drug Conjugate-Induced Tumor Apoptosis Using Annexin V-EGFP Conjugated Quantum Dots.

Antibody-drug conjugates (ADCs) are conjugates of a monoclonal antibody and a cytotoxic drug that induce tumor apoptosis. The evaluation of ADC-induced tumor apoptosis is crucial for the development of ADCs for cancer therapy. To evaluate the efficacy of ADCs, we present in vitro and in vivo fluorescence imaging techniques for ADC-induced tumor apoptosis using annexin V-EGFP (EGFP: enhanced green fluorescent protein) conjugated quantum dots (annexin V-EGFP-QDs). This probe emits visible (VIS) and near-infrared (NIR) dual fluorescence at 515 nm (EGFP emission) and 850 nm (QD emission), which can be used for the detection of tumor apoptosis at the cellular and whole-body levels. By using annexin V-EGFP-QDs, we achieved VIS and NIR fluorescence imaging of human epidermal growth factor receptor 2-positive breast tumor apoptosis induced by an ADC, Kadcyla (trastuzumab emtansine). The results show that the in vitro and in vivo fluorescence imaging of ADC-induced tumor apoptosis using annexin V-EGFP-QDs is a useful tool to evaluate the efficacy of ADCs for cancer therapy.

Shortwave-Infrared Fluorescent Molecular Imaging Probes Based on π-Conjugation Extended Indocyanine Green.

Recently, shortwave-infrared (SWIR) fluorescence imaging for the optical diagnostics of diseases has attracted much attention as a new noninvasive imaging modality. For this application, the development of SWIR molecular imaging probes with high biocompatibility is crucial. Although many types of biocompatible SWIR fluorescent probes based on organic dyes have been reported, there are no SWIR-emitting molecular imaging probes that can be used for the detection of specific biomolecules in vivo. To apply SWIR-emitting molecular imaging probes to biomedical fields, we developed a biocompatible SWIR fluorescent dye based on π-conjugation extended indocyanine green (ICG), where ICG is the only approved near-infrared dye by the US Food and Drug Administration (FDA) for use in the clinic. Using the π-conjugation extended ICG, we prepared SWIR molecular imaging probes that can be used for in vivo tumor imaging. Herein, we demonstrate noninvasive SWIR fluorescence imaging of human epidermal growth factor receptor 2 (HER2)-positive and epidermal growth factor receptor (EGFR)-positive breast tumors using π-conjugation extended ICG and monoclonal antibody conjugates. The presented π-conjugation extended ICG analog probes will be a breakthrough to apply SWIR fluorescence imaging in biomedical fields.

Comparison of clinicopathological features and long-term prognosis between mixed predominantly differentiated-type and pure differentiated-type early gastric cancer.

BACKGROUND: Recent studies have shown that mixed predominantly differentiated-type (MD) early gastric cancer (EGC) might have more malignant potential than pure differentiated-type (PD) EGC. However, no study has analyzed all differentiated-type EGC cases treated endoscopically and surgically. This study aimed to compare the differences in clinicopathological features and long-term prognosis between MD- and PD-EGC. METHODS: We evaluated all patients with differentiated-type EGCs who were treated endoscopically and surgically in our hospital between January 2010 and October 2014. The clinicopathological features and long-term prognosis of MD-EGC were compared with those of PD-EGC. RESULTS: A total of 459 patients with 459 lesions were evaluated in this study; of them, 409 (89.1%) and 50 (10.9%) were classified into the PD and MD groups, respectively. Submucosal invasion was found in 96 (23.5%) patients of the PD group and in 33 (66.0%) patients of the MD group (p < 0.01). The rates of positive lymphatic and vascular invasion and ulceration were significantly higher in the MD group than in the PD group (p < 0.01). The proportion of patients with lymph node metastasis was also significantly higher in the MD group than in the PD group (5 (10%) vs 6 (1.5%), p < 0.01). The 5-year overall and EGC-specific survival rates in the PD group were 88.3 and 99.5%, respectively, while they were 94.0 and 98.0% in the MD group, respectively. CONCLUSIONS: MD-EGC has more malignant potential than PD-EGC. However, the long-term prognosis of MD-EGC is good and is not significantly different from that of PD-EGC when treated appropriately.

Monte Carlo Modeling of Shortwave-Infrared Fluorescence Photon Migration in Voxelized Media for the Detection of Breast Cancer.

Recent progress regarding shortwave-infrared (SWIR) molecular imaging technology has inspired another modality of noninvasive diagnosis for early breast cancer detection in which previous mammography or sonography would be compensated. Although a SWIR fluorescence image of a small breast cancer of several millimeters was obtained from experiments with small animals, detailed numerical analyses before clinical application were required, since various parameters such as size as well as body hair differed between humans and small experimental animals. In this study, the feasibility of SWIR was compared against visible (VIS) and near-infrared (NIR) region, using the Monte Carlo simulation in voxelized media. In this model, due to the implementation of the excitation gradient, fluorescence is based on rational mechanisms, whereas fluorescence within breast cancer is spatially proportional to excitation intensity. The fluence map of SWIR simulation with excitation gradient indicated signals near the upper surface of the cancer, and stronger than those of the NIR. Furthermore, there was a dependency on the fluence signal distribution on the contour of the breast tissue, as well as the internal structure, due to the implementation of digital anatomical data for the Visible Human Project. The fluorescence signal was observed to become weaker in all regions including the VIS, the NIR, and the SWIR region, when fluorescence-labeled cancer either became smaller or was embedded in a deeper area. However, fluorescence in SWIR alone from a cancer of 4 mm diameter was judged to be detectable at a depth of 1.4 cm.

Association of second surveillance colonoscopy findings with index and first surveillance colonoscopy results.

OBJECTIVE: Although there have been established guidelines for first surveillance colonoscopy (FSC) after a polypectomy, there is no consensus on performing a second surveillance colonoscopy (SSC), especially in Asian countries. This study aimed to investigate the association of SSC findings with index total colonoscopy (TCS) and FSC results. METHODS: This was a single-center retrospective cohort study involving 1928 consecutive Japanese patients who had received three or more colonoscopies. High-risk colonoscopic findings were defined as advanced adenoma (≥10 mm in size, with a villous histology or high-grade dysplasia) or more than three adenomas, whereas low-risk findings were defined as one to two non-advanced adenomas. On the basis of index TCS results, the patients were divided into three groups: no adenomas (NA) (n = 888), low-risk (LR) (n = 476), and high-risk (HR) (n = 564) groups, respectively. RESULTS: In the NA group, the rate of high-risk findings on SSC was significantly higher in patients with high-risk or low-risk findings on FSC than in those with no adenoma (7.7% and 7.9% vs 2.2%, P < 0.05). Patients in the LR and HR groups with high-risk findings on FSC had a significantly higher risk on SSC than those with low-risk findings or no adenoma on FSC (LR group: 28.6%, 9.4%, and 5.9%, respectively, P < 0.01; HR group: 34.5%, 18.8%, and 7.9%, respectively, P < 0.01). CONCLUSIONS: Index TCS and especially FSC findings were predictive of SSC results. The study results may be useful for determining appropriate intervals for surveillance colonoscopy in Asian countries.

Endocuff-Assisted versus Cap-Assisted Colonoscopy Performed by Trainees: A Retrospective Study.

BACKGROUND/AIMS: The adenoma detection rate (ADR) of screening colonoscopies performed by trainees is often lower than that of colonoscopies performed by experts. The effcacy of cap-assisted colonoscopy (CAC) in adenoma detection is well documented, especially that of CACs performed by trainees. Endocuff, a new endoscopic cap, is reportedly useful for adenoma detection; however, no trials have compared the effcacy of Endocuff-assisted colonoscopy (EAC) and CAC conducted by trainees. Therefore, the present study retrospectively compared the effcacy between EAC and CAC in trainees. METHODS: This was a single-center, retrospective study involving 305 patients who underwent either EAC or CAC performed by three trainees between January and December 2018. We evaluated the ADR, mean number of adenomas detected per patient (MAP), cecal intubation rate, cecal intubation time, and occurrence of complications between the EAC and CAC groups. RESULTS: The ADR was significantly higher in the EAC group than in the CAC group (54.3% vs. 37.3%, p=0.019), as was the MAP (1.36 vs. 0.74, p=0.003). No significant differences were found between the groups with respect to the cecal intubation rate or cecal intubation time. No major complications occurred in either group. CONCLUSION: Our results suggest that EAC exhibits increased ADR and MAP compared to CAC when performed by trainees.

Bioluminescence Resonance Energy Transfer (BRET) Coupled Near-Infrared Imaging of Apoptotic Cells.

Detection of apoptotic cells is crucial for understanding the mechanism of diseases and for therapy development. So far, visible-emitting fluorescent probes such as FITC-labeled Annexin V has been widely used for the detection of apoptotic cells. However, such probes cannot be applied to noninvasive imaging in the near-infrared (NIR) region. Compared with visible light, NIR light is highly permeable in turbid biological samples and tissues. In addition, NIR optical imaging has several advantages such as lower autofluorescence and scattering from biological samples, leading to clearer images with high signal to background ratios. Here, we describe the synthesis and application of bioluminescence resonance energy transfer (BRET)-coupled quantum dots (QDs) for the NIR optical imaging of apoptotic cells.

Dual-colour (near-infrared/visible) emitting annexin V for fluorescence imaging of tumour cell apoptosis in vitro and in vivo.

Indocyanine green (ICG) labelled recombinant annexin V proteins (ICG-EGFP-Annexin V and ICG-mPlum-Annexin V) were synthesized for dual-colour fluorescence imaging of tumour cell apoptosis in vitro and in vivo. The ICG-labelled fluorescent annexin V proteins showed dual (near-infrared and visible) fluorescence emissions with binding ability to phosphatidylserines on the plasma membranes of apoptotic cells. Although several types of fluorescence labelled annexin V (e.g. FITC-annexin V, Cy3- and Cy5-annexin V) have been reported, there are no dual-colour (near-infrared/visible) emitting apoptosis-detection probes which can be used in vitro and in vivo. In this paper, the utilities of the dual-colour fluorescent annexin V are demonstrated for in vitro and in vivo fluorescence imaging of the apoptosis of human breast tumour cells induced by an antibody-drug conjugate, Kadcyla. The results suggest that the present annexin V probes will be useful to visualize the action of anti-cancer drugs against tumours both at the cellular and whole-body level.

Shortwave-infrared (SWIR) fluorescence molecular imaging using indocyanine green-antibody conjugates for the optical diagnostics of cancerous tumours.

Recently, shortwave-infrared (SWIR, 1000-1400 nm) fluorescence imaging has attracted much attention due to the higher contrast and sensitivity with deeper penetration depths compared to conventional visible and near-infrared (NIR) fluorescence imaging. For the SWIR fluorescence imaging, the development of fluorescent probes emitting over 1000 nm is necessary. So far, a variety of SWIR fluorescent probes based on single-walled carbon nanotubes, quantum dots, rare-metal doped nanomaterials, and organic dyes have been developed. However, there are a very limited number of biocompatible SWIR fluorescent probes, which can be used to biomedical applications. Among NIR and SWIR fluorescent probes, indocyanine green (ICG) is the only fluorescent dye approved by US Food and Drug Administration (FDA) for clinical use. Although ICG has a fluorescence maximum at a NIR region (ca. 830 nm), ICG emits in the SWIR region over 1000 nm. Here, we present ICG-based SWIR fluorescence molecular imaging for the highly-sensitive optical detection of breast and skin tumours in mice. As SWIR fluorescent molecular-imaging probes, we synthesized ICG-antibody conjugates, which prepared from anti-HER2 antibody (Herceptin), anti-EGFR antibody (Erbitux), anti-VEGFR-2 antibody (Cyramza), and anti-PD-L1 antibody (anti-PD-L1 ab). The present SWIR molecular imaging probes specifically accumulated to the breast and skin tumours, and their SWIR fluorescence images (>1000 nm) showed 1.5-2.0 times higher contrast than NIR tumour images taken at 830 nm. We show that the SWIR fluorescence imaging using ICG-antibody conjugates can be used for the elucidation of expression level of cancer-specific membrane proteins, HER2, EGFR, VEGFR-2, and PD-L1 in vivo. We also show that the SWIR fluorescence imaging enables quantitative analysis of the change in the size of tumour treated with an anti-cancer drug, Kadcyla. Our findings suggest that the SWIR fluorescence molecular imaging using ICG-antibody conjugates has potential to use for the optical diagnostics of cancerous tumors in medical and clinical fields.

Fluorescent, Recombinant-Protein-Conjugated, Near-Infrared-Emitting Quantum Dots for in Vitro and in Vivo Dual-Color Molecular Imaging.

Near-infrared (NIR)-emitting fluorescent probes are widely used for molecular imaging at the whole-body level. However, NIR-emitting fluorescent probes emitting over λ=700 nm are not suitable for molecular imaging at the cellular level, because most of the conventional fluorescence microscopes have very low optical sensitivity in the NIR region. Thus, to achieve fluorescence imaging at the cellular and whole-body levels by using single probes, visible and NIR-emitting dual-color fluorescent probes are desirable. For dual-color fluorescence molecular imaging, we synthesized fluorescent, recombinant-protein-conjugated, NIR-emitting quantum dots (QDs), in which the recombinant protein consists of enhanced green fluorescent protein (EGFP) and the immunoglobulin binding domain (B1) of protein G. This dual-color fluorescent QD probe binds the Fc region of immunoglobulin G (IgG) through its B1 domain at the QD surface and acts as a molecular-imaging probe at both the cellular and whole-body levels. In this paper, we present the synthesis of fluorescent, recombinant protein (HisEGFP-GB1)-conjugated, NIR-emitting QDs and their application to the dual-color molecular imaging of breast cancer cells in vitro and in vivo.

BRET based dual-colour (visible/near-infrared) molecular imaging using a quantum dot/EGFP-luciferase conjugate.

Owing to its high sensitivity, bioluminescence imaging is an important tool for biosensing and bioimaging in life sciences. Compared to fluorescence imaging, bioluminescence imaging has a superior advantage that the background signals resulting from autofluorescence are almost zero. In addition, bioluminescence imaging can permit long-term observation of living cells because external excitation is not needed, leading to no photobleaching and photocytotoxicity. Although bioluminescence imaging has such superior properties over fluorescence imaging, observation wavelengths in bioluminescence imaging are mostly limited to the visible region. Here we present bioluminescence resonance energy transfer (BRET) based dual-colour (visible/near-infrared) molecular imaging using a quantum dot (QD) and luciferase protein conjugate. This bioluminescent probe is designed to emit green and near-infrared luminescence from enhanced green fluorescent protein (EGFP) and CdSeTe/CdS (core/shell) QDs, where EGFP-Renilla luciferase (RLuc) fused proteins are conjugated to the QDs. Since the EGFP-RLuc fused protein contains an immunoglobulin binding domain (GB1) of protein G, it is possible to prepare a variety of molecular imaging probes functionalized with antibodies (IgG). We show that the BRET-based QD probe can be used for highly sensitive dual-colour (visible/near-infrared) bioluminescence molecular imaging of membrane receptors in cancer cells.

Recombinant Protein (Luciferase-IgG Binding Domain) Conjugated Quantum Dots for BRET-Coupled Near-Infrared Imaging of Epidermal Growth Factor Receptors.

For the highly sensitive near-infrared (NIR) optical detection of epidermal growth factor receptors (EGFRs) expressed on cancer cells, bioluminescence resonance energy transfer (BRET) coupled NIR quantum dots (QDs) are prepared by direct conjugation of his-tagged Renilla luciferase (RLuc) recombinant protein (HisRLuc·GB1) to glutathione-coated CdSeTe/CdS QDs (GSH-QDs). The recombinant protein has two functional groups consisting of a luciferase enzyme and an immunoglobulin binding domain (GB1) of protein G. Recombinant protein (HisRLuc·GB1) conjugated QDs (GB1·RLuc-QDs) show BRET-coupled NIR emission, which results from energy transfer from luciferin to QDs with a high BRET efficiency of ca. 50%. Since the GB1·RLuc-QDs have the GB1 domain at their surface, the QDs have an ability to bind the Fc moiety of immunoglobulin G (IgG). The resulting IgG bound QDs can be used as a molecular imaging probe with NIR fluorescence and BRET-coupled NIR emission. For NIR optical detection of EGFRs on cancer cells, we conjugated anti-EGFR monoclonal antibody to the GB1·RLuc-QDs. Herein, we show that the detection sensitivity of EGFRs by BRET-coupled NIR emission of GB1·RLuc-QDs is at least three times higher than that of the NIR fluorescence of the QDs. The conjugates between anti-EGFR antibody and GB1·RLuc-QDs make it possible to perform BRET-based highly sensitive NIR imaging of EGFRs in living cells.

Bioluminescence Resonance Energy Transfer (BRET)-coupled Annexin†V-functionalized Quantum Dots for Near-Infrared Optical Detection of Apoptotic Cells.

Deregulation in apoptosis induces numerous diseases such as cancer, cardiovascular, and neurodegenerative diseases. Detection of apoptotic cells is crucial for understanding the mechanism of these diseases and for therapy development. Although optical imaging using visible-emitting fluorescent probes, such as FITC-labeled annexin V, is widely used for the detection of apoptotic cells, there are very limited probes that can be used in the near-infrared region (NIR) over 700 nm. Compared with visible light, NIR light is highly permeable in turbid biological samples and tissues. In addition, optical imaging in the NIR region shows low autofluorescence from biological samples, leading to clearer images with high signal to background ratios. Here, we report the synthesis of bioluminescence resonance energy transfer (BRET)-coupled annexin V-functionalized quantum dots (QDs) and their application to NIR optical detection of apoptotic cells.

Immunoglobulin binding (B1) domain mediated antibody conjugation to quantum dots for in vitro and in vivo molecular imaging.

A facile method for the preparation of antibody-quantum dot (QD) conjugates using the immunoglobulin binding (B1) domain of protein G is presented. The utility of antibody-QD conjugates using the B1 domain is demonstrated for fluorescence imaging of breast tumor cells in vitro and in vivo.

Reconstructing 3D deformation dynamics for curved epithelial sheet morphogenesis from positional data of sparsely-labeled cells.

Quantifying global tissue deformation patterns is essential for understanding how organ-specific morphology is generated during development and regeneration. However, due to imaging difficulties and complex morphology, little is known about deformation dynamics for most vertebrate organs such as the brain and heart. To better understand these dynamics, we propose a method to precisely reconstruct global deformation patterns for three-dimensional morphogenesis of curved epithelial sheets using positional data from labeled cells representing only 1-10% of the entire tissue with limited resolution. By combining differential-geometrical and Bayesian frameworks, the method is applicable to any morphology described with arbitrary coordinates, and ensures the feasibility of analyzing many vertebrate organs. Application to data from chick forebrain morphogenesis demonstrates that our method provides not only a quantitative description of tissue deformation dynamics but also predictions of the mechanisms that determine organ-specific morphology, which could form the basis for the multi-scale understanding of organ morphogenesis.Quantifying deformation patterns of curved epithelial sheets is challenging owing to imaging difficulties. Here the authors develop a method to obtain a quantitative description of 3D tissue deformation dynamics from a small set of cell positional data and applied it to chick forebrain morphogenesis.

The Beneficial Effects of ETS-GS, a Novel Vitamin E Derivative, on a Rat Model of Crush Injury.

Crush syndrome is a devastating condition leading to multiple organ failure. The mechanisms by which local traumatic injuries affect distant organs remain unknown. ETS-GS is a novel water-soluble, stable anti-oxidative agent composed of vitamin E derivative. Given that one of the main pathophysiological effects in crush syndrome is massive ischemia-reperfusion, reactive oxygen species (ROS) generated from the injured extremities would be systemically involved in distant organ damage. We investigated whether ETS-GS could suppress inflammatory response and improve mortality in a rat model of crush injury. Crush injury was induced by compression of bilateral hindlimbs for 6 h followed by release of compression. Seven-day survival was significantly improved by ETS-GS treatment. To estimate anti-oxidative and anti-inflammatory effects of ETS-GS, serum was collected 6 and 20 h after the injury. ETS-GS treatment significantly dampened the up-regulation of malondialdehyde and reduction of superoxide dismutase in the serum, which were induced by crush injury. Serum levels of interleukin 6 and high mobility group box 1 were significantly decreased in the ETS-GS group compared with those in the control group. Lung damage shown by hematoxylin-eosin staining at 20 h after the injury was ameliorated by the treatment. Ex vivo imaging confirmed that ETS-GS treatment reduced ROS generation in both the lung and the muscle following crush injury. The administration of ETS-GS could suppress ROS generation, systemic inflammation, and the subsequent organ damage, thus improving survival in a rat model of crush injury. These findings suggest that ETS-GS can become a novel therapeutic agent against crush injury.

Compact and stable SNAP ligand-conjugated quantum dots as a fluorescent probe for single-molecule imaging of dynein motor protein.

Compact SNAP ligand-conjugated quantum dots (<10 nm) with high colloidal stability over a wide range of pH (5-9) have been synthesized as fluorescent probe for the single-molecule imaging of dynein motor protein.