Multiscale Imaging of Brown Adipose Tissue in Living Mice/Rats with Fluorescent Polymer Dots

Triglyceride-Rich Lipoprotein-Mediated Polymer Dots for Multimodal Imaging Interscapular Brown Adipose Tissue Capillaries

Brown adipose tissues (BATs) have been identified as a promising target of metabolism disorders. [18F]FDG-PET (FDG = fluorodeoxyglucose; PET = positron emission tomography) has been predominantly employed for BAT imaging, but its limitations drive the urgent need for novel functional probes combined with multimodal imaging approaches. It has been reported that polymer dots (Pdots) display rapid BAT imaging without additional cold stimulation. However, the mechanism by which Pdots image BAT remains unclear. Here, we made an intensive study of the imaging mechanism and found that Pdots can bind to triglyceride-rich lipoproteins (TRLs). By virtue of their high affinity to TRLs, Pdots selectively accumulate in capillary endothelial cells (ECs) in interscapular brown adipose tissues (iBATs). Compared to poly(styrene-co-maleic anhydride)cumene terminated (PSMAC)-Pdots with a short half-life and polyethylene glycol (PEG)-Pdots with low lipophilicity, naked-Pdots have good lipophilicity, with a half-life of about 30 min and up to 94% uptake in capillary ECs within 5 min, increasing rapidly after acute cold stimulation. These results suggested that the accumulation changes of Pdots in iBAT can reflect iBAT activity sensitively. Based on this mechanism, we further developed a strategy to detect iBAT activity and quantify the TRL uptake in vivo using multimodal Pdots.

Rational Design of Asymmetric Polymethines to Attain NIR(II) Bioimaging at >1100 nm

Organic molecules having emission in the NIR(II) region are emergent and receiving enormous attention. Unfortunately, attaining accountable organic emission intensity around the NIR(II) region is hampered by the dominant internal conversion operated by the energy gap law, where the emission energy gap and the associated internal reorganization energy λ_int play key roles. Up to the current stage, the majority of the reported organic NIR(II) emitters belong to those polymethines terminated by two symmetric chromophores. Such a design has proved to have a small λ_int that greatly suppresses the internal conversion. However, the imposition of symmetric chromophores is stringent, limiting further development of organic NIR(II) dyes in diversity and versatility. Here, we propose a new concept where as far as the emissive state of the any asymmetric polymethines contains more or less equally transition density between two terminated chromophores, λ_int can be as small as that of the symmetric polymethines. To prove the concept, we synthesize a series of new polymethines terminated by xanthen-9-yl-benzoic acid and 2,4-diphenylthiopyrylium derivatives, yielding AJBF1112 and AEBF1119 that reveal emission peak wavelength at 1112 and 1119 nm, respectively. The quantum yield is higher than all synthesized symmetric polymethines of 2,4-diphenylthiopyrylium derivatives (SC1162, 1182, 1185, and 1230) in this study. λ_int were calculated to be as small as 6.2 and 7.3 kcal/mol for AJBF1112 and AEBF1119, respectively, proving the concept. AEBF1119 was further prepared as a polymer dot to demonstrate its in vitro specific cellular imaging and in vivo tumor/bone targeting in the NIR(II) region.

Molecular Tuning of IR-786 for Improved Brown Adipose Tissue Imaging

To overcome the limitations of brown adipose tissue (BAT) imaging with MRI and PET/CT, near-infrared (NIR) fluorescence imaging has been utilized in living animals because it is highly sensitive, noninvasive, nonradioactive, and cost-effective. To date, only a few NIR fluorescent dyes for detecting BAT have been reported based on the structure-inherent targeting strategy. Among them, IR-786, a commercial cyanine dye, was used firstly for quantitative NIR imaging of BAT perfusion in 2003. Owing to the high cytotoxicity, poor water solubility, and strong nonspecific background uptake of IR-786, the chemical structure of IR-786 should be redesigned to be more hydrophilic and less toxic so that it can show more BAT-specific accumulation. Here, we developed a BAT-specific NIR dye, BF800-AM, by incorporating the tyramine linker in the original structure of IR-786. After modifying the physicochemical properties of IR-786, in vivo results showed significant uptake of the newly designed BF800-AM in the BAT with improved signal-to-background ratio. Additional in vivo studies using mouse tumor models revealed that BF800-AM targeting to BAT is independent of tumor tissues, as distinct from IR-786 showing uptake in both tissues. Therefore, BF800-AM can be used for improved noninvasive visualization of BAT mass and activity in living animals.

High-Resolution Imaging of the Ocular Vasculature of Conjunctivitis in Mice Using Highly Bright Polymer Dots

Ocular diseases are mainly caused by vascular aberrations in the eye, and accurate imaging and analysis of the ocular vascular structure is crucial. In this study, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (PFBT) polymer dots (Pdots), with the advantages of easy synthesis, high brightness, and low toxicity, are used as nanoprobes to perform high-resolution imaging of the vasculature of the eyeball and optic nerve. Moreover, rapid imaging of the choroidal microvessels is carried out by stereoscopic fluorescence microscopy with a resolution of up to 1.6 µm. The comprehensive 3D vascular information of retinal aorta and optic nerve microvessels is obtained by combining tissue clearing and multiphoton microscopy. In addition, the vascular density of Schlemm's canal and iris blood vessels is compared between the conjunctivitis mice and the normal mice. These results suggest that PFBT Pdots have great application potential in the fast and accurate imaging of ocular diseases.

Multifunctional Imaging of Vessels, Brown Adipose Tissue, and Bones in the Visible and Second Near-infrared Region Using Dual-Emitting Polymer Dots

Dual-emitting polymer dots (dual-Pdots) in the visible and second near-infrared (NIR-II) region can facilitate the high-resolution imaging of the fine structure and improve the signal-to-noise ratio in in vivo imaging. Herein, combining high brightness of Pdots and multi-scale imaging, we synthesized dual-Pdots using a simple nano-coprecipitation method and performed multi-functional imaging of vessels, brown adipose tissue, and bones. Results showed that in vivo blood vessel imaging had a high resolution of up to 5.9 μm and bone imaging had a signal-to-noise ratio of 3.9. Moreover, dual-Pdots can accumulate in the interscapular brown adipose tissue within 2 min with a signal-to-noise ratio of 5.8. In addition, the prepared dual-Pdots can image the lymphatic valves and the frequency of contraction. Our study provides a feasible method of using Pdots as nanoprobes for multi-scale imaging in the fields of metabolic disorders, skeletal system diseases, and circulatory systems.

Imaging of fluorescent polymer dots in relation to channels and immune cells in the lymphatic system

Polymer dots (Pdots) have been applied to imaging lymph nodes (LNs) and lymphatic vessels (LVs) in living mice and rats. However, the mechanism of absorption, distribution, metabolism, and excretion of Pdots in LNs and LVs is still unclear. Therefore, the relationship between Pdots and immune cells, LVs and collagen fibers in lymphatics was studied by multiple in vivo and ex vivo microscopic imaging methods and detection techniques. Flow cytometry showed that Pdots could be phagocytosed by macrophages and monocytes, and had no relationship with B cells, T cells and dendric cells in LNs. Silver staining, immunofluorescence and two-photon microscope showed that Pdots gathered in collagen fibers and LVs of LNs. Furthermore, immunofluorescence imaging results verified that Pdots were distributed in the extracellular space of collecting LVs endothelial cells. In addition, Pdots in the collecting LVs were basically cleared by leaking into the surrounding tissue or draining LNs after 21 days of injection. During the long-time observation, Pdots also helped monitor the contraction frequency and variation range of LV. Our study lays a foundation on the research of Pdots as the carrier to study lymphatic structure and function in the future.

Quantification of brown adipose tissue in vivo using synthetic magnetic resonance imaging: an experimental study with mice model

BACKGROUND

The white adipose tissue (WAT) and brown adipose tissue (BAT) are associated with the development of several obesity-associated disorders. The use of imaging techniques to differentiate BAT from WAT and quantify BAT volume remains challenging, due to limitations such as spatial resolution and magnetic field inhomogeneity. This study aimed to investigate the feasibility for differentiating BAT from WAT, and quantify the BAT volume in vivo using synthetic magnetic resonance imaging (MRI).

METHODS

A total of 16 C57BL/6 mice were scanned using synthetic MRI. Quantitative longitudinal relaxation time (T1) and transverse relaxation time (T2) maps were obtained from the original synthetic MRI data using the synthetic MRI software offline. The T1 and T2 values of interscapular BAT (IBAT) and dorsal subcutaneous WAT were measured. The IBAT volume was calculated using synthetic MRI-derived T2-weighted images (T2WIs) based on its morphological characteristics and quantitative tissue values. The body weight of mice was measured, and the IBAT specimens were excised and weighted. The correlation between IBAT volume and the weight of IBAT gross specimen and between IBAT volume and mouse body weight was analyzed.

RESULTS

The T1 values of BAT (330.3±19.57 ms) were higher than those of WAT (304.42±4.14 ms) (P<0.001), whereas the T2 values of BAT (66.06±5.06 ms) were lower than those of WAT (88.23±7.68 ms) (P<0.001). The area under the curve (AUC) values of the T1 and T2 for differentiating BAT from WAT was 0.942 and 0.995, respectively. The AUC of the T2 values was higher than that of T1 (P=0.04) using the DeLong test. The optimal cut-off value for T2 was 76 ms for differentiating BAT from WAT (100% sensitivity, 93.7% specificity). A moderate correlation was observed between IBAT volume and the weight of the IBAT gross specimen (r=0.662, P=0.014), and between IBAT volume and mouse body weight (r=0.653, P=0.016).

CONCLUSIONS

The quantitative parameters derived using synthetic MRI may be used to detect and differentiate BAT from WAT in vivo. Synthetic MRI may help quantify BAT volume in vivo.

TADF-based NIR-II semiconducting polymer dots for in vivo 3D bone imaging

Intraoperative fluorescence imaging in the second near-infrared (NIR-II) region heralds a new era in image-guided surgery since the success in the first-in-human liver-tumor surgery guided by NIR-II fluorescence. Limited by the conventional small organic NIR dyes such as FDA-approved indocyanine green with suboptimal NIR-II fluorescence and non-targeting ability, the resulting shallow penetration depth and high false positive diagnostic values have been challenging. Described here is the design of NIR-II emissive semiconducting polymer dots (Pdots) incorporated with thermally activated delayed fluorescence (TADF) moieties to exhibit emission maxima of 1064–1100 nm and fluorescence quantum yields of 0.40–1.58% in aqueous solutions. To further understand how the TADF units affect the molecular packing and the resulting optical properties of Pdots, in-depth and thorough density-functional theory calculations were carried out to better understand the underlying mechanisms. We then applied these Pdots for in vivo 3D bone imaging in mice. This work provides a direction for future designs of NIR-II Pdots and holds promising applications for bone-related diseases.

A series of NIR-II fluorescent TADF-incorporated polymer dots were successfully synthesized. The function of the TADF moiety was fully studied and the bio-applications of these polymer dots including bone imaging were also demonstrated.

Molecular Design of Ultrabright Semiconducting Polymer Dots with High NIR-II Fluorescence for 3D Tumor Mapping

Fluorescence probes emitting in the second near-infrared (NIR-II, 1000-1700 nm) window with the ability for deep-tissue imaging in mammals herald a new era in surgical methodology. However, the brightness of these NIR-II probes is still far from satisfactory due to their low fluorescence quantum yields (QYs), preventing the observation of high-resolution images such as whole-organ vascular networks in real time. Described here is the molecular engineering of a series of semiconducting polymer dots (Pdots) incorporated with aggregation-induced emission moieties to exhibit the QYs as high as 14% in the NIR-II window. Benefiting from the ultrahigh brightness, a 1400 nm long-pass filter is utilized to realize in vivo 3D tumor mapping in mice. To further understand how the geometrical and electron structures of the semiconducting polymers affect their optical properties, the in-depth and thorough density-functional theory calculations are performed to interpret the experimental results. This study lays the groundwork for further molecular design of highly bright NIR-II Pdots.

Molecular Imaging of Brown Adipose Tissue Mass

Brown adipose tissue (BAT), a uniquely thermogenic tissue that plays an important role in metabolism and energy expenditure, has recently become a revived target in the fight against metabolic diseases, such as obesity, diabetes, and non-alcoholic fatty liver disease (NAFLD). Different from white adipose tissue (WAT), the brown adipocytes have distinctive features including multilocular lipid droplets, a large number of mitochondria, and a high expression of uncoupling protein-1 (UCP-1), as well as abundant capillarity. These histologic characteristics provide an opportunity to differentiate BAT from WAT using imaging modalities, such as PET/CT, SPECT/CT, MRI, NIRF and Ultrasound. However, most of the reported imaging methods were BAT activation dependent, and the imaging signals could be affected by many factors, including environmental temperatures and the states of the sympathetic nervous system. Accurate BAT mass detection methods that are independent of temperature and hormone levels have the capacity to track the development and changes of BAT throughout the lifetime of mammals, and such methods could be very useful for the investigation of potential BAT-related therapies. In this review, we focus on molecular imaging modalities that can detect and quantify BAT mass. In addition, their detection mechanism and limitations will be discussed as well.

Real-Time Wireless Monitoring of Cell Proliferation and Detachment Based on pH-Responsive Conductive Polymer Dots

In situ wireless monitoring for cell proliferation and detachment kinetics was conducted using pH-responsive zwitterionic polymer dots (Z-PDs), based on changes in electrochemical signals derived from Z-PD-coated substrates via the interaction of charges transferred between Z-PDs and cells. Z-PD-coated substrates were found to be a potent means to monitor and manipulate cell adhesion and detachment because of their high sensitivity over a wide range of pH conditions, and modification of the coated substrates was confirmed using a wireless system. At neutral pH, Z-PD-coated wireless sensors exhibited π-π stacking involving aromatic rings with hydrophobic interactions, thereby promoting cell proliferation; consequently, an increase in the measured resistance was observed. In contrast, Z-PD-coated substrates triggered by acidic and basic conditions promoted cell detachment, which induced an increase in the resistance compared with Z-PD substrates at pH 6.8, as a result of charges transferred to support Z-PD internalization through cell membranes after detachment. Therefore, as a wireless biosensor with excellent pH responsiveness that facilitates cell proliferation and detachment and whose electrochemical signals could be additionally acquired via a smartphone, Z-PD biosensors demonstrated a more favorable approach for monitoring cell-surface interactions than conventional optically based methods.

Polymethine-Based Semiconducting Polymer Dots with Narrow-Band Emission and Absorption/Emission Maxima at NIR-II for Bioimaging

Deep-penetration fluorescence imaging in the second near-infrared (NIR-II) window heralds a new era of clinical surgery, in which high-resolution vascular/lymphatic anatomy and detailed cancerous tissues can be visualized in real time. Described here is a series of polymethine-based semiconducting polymers with intrinsic emission maxima in the NIR-IIa (1300-1400 nm) window and absorption maxima ranging from 1082 to 1290 nm. These polymers were prepared as semiconducting polymer dots (Pdots) in aqueous solutions with fluorescence quantum yields of 0.05-0.18 %, and they demonstrate promising applications in noninvasive through-skull brain imaging in live mice with remarkable spatial resolution as well as signal-to-background contrast. This study offers a platform for future design of NIR-IIa or even NIR-IIb emitting Pdots.

NIR/photoacoustic imaging of multitype gallbladder cancer using carboxyl/amino functionalized polymer dots

Gallbladder cancer has high incidence and mortality and a low early diagnosis rate and requires rapid and efficient diagnosis. Herein, carboxyl/amino functionalized polymer dots (Pdots) were designed to enhance cellular internalization and tumor accumulation. The prepared Pdots were 40-50 nm in diameter, contained no toxic metal, exhibited long circulation time and high stability, and produced strong NIR emission and photoacoustic signals. Different cellular uptake and distribution of functionalized Pdots in eight gallbladder cell lines were quantitatively investigated using flow cytometry and super-resolution microscopy. In vivo NIR fluorescence imaging showed that the functional Pdots had high accumulation in the tumor after 30 minutes of injection and remained there for up to 6 days. In addition, photoacoustic imaging found that the abundant blood vessels around the tumor microenvironment and Pdots entered the tumor through the blood vessels. Furthermore, a high heterogeneity of vascular networks was visualized in real-time and high resolution by probe-based confocal laser endomicroscopy imaging. These results offer a new avenue for the development of functional Pdots as a probe for multi-modal and multi-scale imaging of gallbladder cancer in small animals.

A novel nucleic acid amplification system based on nano-gap embedded active disk resonators

Recent advances in nucleic acid based testing using bio-optical sensor approaches have been introduced but most are based on hybridization between the optical sensor and the bio-molecule and not on an amplification mechanism. Direct nucleic acid amplification on an optical sensor has several technical limitations, such as the sensitivity of the temperature sensor, instrument complexity, and high background signal. We here describe a novel nucleic acid amplification method based on a whispering gallery mode active resonator and discuss its potential molecular diagnostic application. By implanting nanoclusters as active compounds, this active resonator operates without tapered fiber coupling and emits a strong photoluminescence signal with low background in the wavelength of low absorption in an aqueous environment that is typical of biosensors. Our method also offers an extremely low detection threshold down to a single copy within 10 min due to the strong light-matter interaction in a nano-gap structure. We envision that this active resonator provides a high refractive index contrast for tight mode confinement with simple alignment as well as the possibility of reducing the device size so that a point-of-care system with low-cost, high-sensitivity and simplicity.

Near-Infrared Polymer Dots in the Portal-Hepatic Circulation Achieve Localization of Hepatic Carcinoma In Vivo

The present study aims to use polymer dots to explore whether they can visualize tumor lesions in a diethylnitrosamine (DENA)-induced hepatocellular carcinoma (HCC) model. The HCC rat model was set up, and serum liver function indexes and AFP were tested on days 0, 30, 60, and 90 of the modeling process. After characterization of the polymer dots, they were injected into the rats and mice. The liver, spleen, and kidney of rats and the gallbladder of mice were extracted to verify the metabolic pathways of the polymer dots and their capability of fluorescent localization of HCC and gallbladder by fluorescence imaging. Strong fluorescent emission from the liver appeared immediately and 15 min after the polymer dots were injected through the main portal veins and tail veins of the model rats, respectively. A satisfactory fluorescent imaging effect lasted up to 45 min. Polymer dots circulate through the bloodstream within intrahepatic vessels rather than intracellular areas and can be clearly visualized by using both the pCLE and IVIS spectrum imaging systems. Contrast imaging of HCC lesions without fluorescent emissions was due to the lack of normal portal-hepatic veins within the tumor areas. Fluorescent imaging of the gallbladder could also be detected at 15 min after the polymer dots were injected through the tail veins of mice. The polymer dots had satisfactory fluorescent localization capability for targeted intrahepatic vessels and HCC lesions in vivo and showed potential practical value in hepato-biliary surgery.

The Effect of Polymer Dots During Mammalian Early Embryo Development and Their Biocompatibility on Maternal Health

Conjugated polymer dots have excellent fluorescence properties in terms of their structural diversity and functional design, showing broad application prospects in the fields of biological imaging and biosensing. Polymer dots contain no heavy metals and are thought to be of low toxicity and good biocompatibility. Therefore, systematic studies on their potential toxicity are needed. Herein, the biocompatibility of poly[(9,9-dioctylfluorenyl-2,7diyl)-co-(1,4-benzo-{2,1',3}-thiadiazole)],10% benzothiadiazole(y) (PFBT) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) polymer dots on early embryo development as well as maternal health is studied in detail. The results show that prepared polymer dots are dose-dependently toxic to preimplantation embryos, and low-dose polymer dots can be used for cell labeling of early embryos without affecting the normal development of embryos into blastocysts. In addition, the in vivo distribution data show that the polymer dots accumulate mainly in the maternal liver, spleen, kidney, placenta, ovary, and lymph nodes of the pregnant mice. Histopathological examination and blood biochemical tests demonstrate that exposure of the maternal body to polymer dots at a dosage of 14 µg g^-1 does not affect the normal function of the maternal organs and early fetal development. The research provides a safe basis for the wide application of polymer dots.

Fast Noninvasive Measurement of Brown Adipose Tissue in Living Mice by Near-Infrared Fluorescence and Photoacoustic Imaging

Aberrant brown adipose tissue (BAT) metabolism is linked to obesity as well as other metabolic disorders. However, the paucity of imaging tools limits the study of in vivo BAT metabolism in animal models. The current work evaluated a heptamethine dye (CyHF-8) in living mice as a dual-modality BAT-avid molecular probe for two imaging approaches, including near-infrared fluorescence imaging (NIRF) and photoacoustic imaging (PAI). CyHF-8 exhibited favorable spectral properties in the near-infrared window (786/787/805 nm) and accumulated in the subcellular mitochondria of brown adipocytes. After intravenous injection of CyHF-8, NIRF and PAI were both capable of noninvasively detecting interscapular BAT at early time points in living mice. Quantitative analysis of NIRF and PAI images showed that CyHF-8 signals respond to dynamic BAT changes in mice stimulated by norepinephrine (NE) and in diabetic mice induced by streptozotocin (STZ). In summary, dual-modality NIRF/PAI probe CyHF-8 can be used for both NIRF and PAI to noninvasively assess BAT metabolism in living animals.

Nanomaterials for the theranostics of obesity

As a chronic and lifelong disease, obesity not only significant impairs health but also dramatically shortens life span (at least 10 years). Obesity requires a life-long effort for the successful treatment because a number of abnormalities would appear in the development of obesity. Nanomaterials possess large specific surface area, strong absorptivity, and high bioavailability, especially the good targeting properties and adjustable release rate, which would benefit the diagnosis and treatment of obesity and obesity-related metabolic diseases. Herein, we discussed the therapy and diagnosis of obesity and obesity-related metabolic diseases by using nanomaterials. Therapies of obesity with nanomaterials include improving intestinal health and reducing energy intake, targeting and treating functional cell abnormalities, regulating redox homeostasis, and removing free lipoprotein in blood. Diagnosis of obesity-related metabolic diseases would benefit the therapy of these diseases. The development of nanomaterials will promote the diagnosis and therapy of obesity and obesity-related metabolic diseases.

Molecular design of near-infrared fluorescent Pdots for tumor targeting: aggregation-induced emission versus anti-aggregation-caused quenching

Semiconducting polymer dots (Pdots) have recently emerged as a new type of ultrabright fluorescent probe that has been proved to be very useful for biomedical imaging. However, Pdots often suffer from serious fluorescence aggregation-caused quenching (ACQ) especially for near-infrared (NIR) fluorescent Pdots. This article compared two strategies to overcome the ACQ effect in near-infrared emissive Pdot systems: aggregation-induced emission (AIE) and anti-aggregation-caused quenching (anti-ACQ). The results show that the anti-ACQ platform outperforms the AIE system. The fluorescence quantum yield of anti-ACQ-based Pdots can be over 50% and the average per-particle brightness of the Pdots is about 5 times higher than that of the commercially available quantum dots. To help understand why the monomer conformations could greatly affect the optical properties of Pdots, molecular dynamics simulations were performed for the first time in such complicated Pdot systems. To demonstrate applications for in vivo fluorescence imaging, both microangiography imaging on living zebrafish embryos and specific tumor targeting on mice were performed. We anticipate that these studies will pave the way for the design of new highly fluorescent Pdot systems.

High resolution tracking of macrophage cells in deep organs and lymphatics using fluorescent polymer dots

Fluorescent polymer dots for tracking macrophage cells in deep organs using probe-based confocal laser endomicroscopy (pCLE).