Advanced Photoacoustic Imaging Applications of Near‐Infrared Absorbing Organic Nanoparticles

Progress of nanotechnology in recent years has stimulated fast development of nanoparticles in biomedical research. Photoacoustic (PA) imaging as an emerging non‐invasive technique in molecular imaging has improved imaging depth relative to conventional optical imaging, demonstrating great potential in clinical applications. The convergence of nanotechnology and PA imaging has enabled a broad spectrum of new opportunities in fundamental biology and translation medicine. This review focuses on the recent advances of organic nanoparticles in PA imaging applications. Near‐infrared absorbing organic nanoparticles are classified and discussed according to their different imaging applications, which include tumor imaging, gastrointestinal imaging, sentinel lymph node imaging, disease microenvironment imaging and real‐time drug imaging. The chemistry and PA properties of organic nanoparticles are discussed in details to highlight their own merits, and their challenges and perspectives in PA imaging are also discussed.     

Aqueous‐Phase Synthesis of Highly Luminescent CdTe/ZnTe Core/Shell Quantum Dots Optimized for Targeted Bioimaging

Semiconductor quantum dots (QDs) have traditionally been synthesized in organic phase and transferred to aqueous solution by functionalizing their surface with silica, polymers, short‐chain thiol ligand, or phospholipid micelles. However, these complex steps result in i) a reduction of the quantum yield (QY) of QDs, ii) partial degrdation of the QDs, and iii) a drastic increase in the hydrodynamic size of QDs, which may hinder their biomedical applications. In this work, the fabrication and applications of cysteine‐capped CdTe/ZnTe QDs, which are directly synthesized in aqueous media, as optical probes for specific targeting of pancreatic and esophageal cancer cells in vitro are reported, as well as their capability for in vivo imaging. The CdTe/ZnTe QDs are synthesized in a one‐pot method and capped with amino acid cysteine, which contains both carboxyl and amine functional groups on their surfaces for bioconjugation. The fabricated QDs have an ultrasmall hydrodynamic diameter (3–5 nm), possess high QY (52%), and are non‐toxic to cells at experimental dosages. Confocal imaging is used to demonstrate a receptor‐mediated uptake of antibody‐conjugated QDs into pancreatic cancer cells in vitro. In vitro cytotoxicity studies (MTS‐assay) show that the IC₅₀ value of these QDs is ≈160 µg mL^?1, demonstrating low toxicity. In addition, the QDs are used for small‐animal imaging where the in vivo biocompatiblity of these QDs and their clearance following systemic injection is studied.     

Biomedical Applications of Metal‐Encapsulated Fullerene Nanoparticles

The carbonaceous nanomaterials known as metallofullerenes have attracted considerable attention due to their attractive properties. The robust nature of the “Trojan Horse” fullerene cage provides an important structural component, which isolates the metal cluster from the bioenvironment. The large carbon surface area is ideally suited for multiple exo‐functionalization approaches to modify the hydrophobic cage for a more hydrophilic bioenvironment. Additionally, peptides and other agents are readily covalently attached to this nanoprobe for targeting applications. The recent progress in developing metallofullerenes for next‐generation biomedical applications is described. Of special interest are magnetic resonance imaging (MRI) contrast agents. Several recent studies reported cumulative gadolinium deposition in the brain and bones of individuals using commercial clinical MRI contrast agents. Gadolinium‐based metallofullerenes provide 2–3 orders of magnitude improvement in MRI relaxivity and potentially lower clinical levels of toxic Gd³⁺ ions deposited. Other potential biomedical applications are also reviewed herein.     

Water‐Dispersible,pH‐Stable and Highly‐Luminescent Organic Dye Nanoparticles with Amplified Emissions for In Vitro and In Vivo Bioimaging

A new strategy is presented for using doped small‐molecule organic nanoparticles (NPs) to achieve high‐performance fluorescent probes with strong brightness, large Stokes shifts and tunable emissions for in vitro and in vivo imaging. The host organic NPs are used not only as carriers to encapsulate different doped dyes, but also as fluorescence resonance energy transfer donors to couple with the doped dyes (as acceptors) to achieve multicolor luminescence with amplified emissions (AE). The resulting optimum green emitting NPs show high brightness with quantum yield (QY) of up to 45% and AE of 12 times; and the red emitting NPs show QY of 14% and AE of 10 times. These highly‐luminescent doped NPs can be further surface modified with poly(maleic anhydride‐alt‐1‐octadecene)‐polyethylene glycol (C18PMH‐PEG), endowing them with excellent water dispersibility and robust stability in various bio‐environments covering wide pH values from 2 to 10. In this study, cytotoxicity studies and folic acid targeted cellular imaging of these multicolor probes are carried out to demonstrate their potential for in vitro imaging. On this basis, applications of the NP probes in in vivo and ex vivo imaging are also investigated. Intense fluorescent signals of the doped NPs are distinctly, selectively and spatially resolved in tumor sites with high sensitivity, due to the preferential accumulation of the NPs in tumor sites through the passive enhanced permeability and retention effect. The results clearly indicate that these doped NPs are promising fluorescent probes for biomedical applications.     

Photoacoustic Imaging: Contrast Agents and Their Biomedical Applications

Photoacoustic (PA) imaging as a fast‐developing imaging technique has great potential in biomedical and clinical applications. It is a noninvasive imaging modality that depends on the light‐absorption coefficient of the imaged tissue and the injected PA‐imaging contrast agents. Furthermore, PA imaging provides superb contrast, super spatial resolution, and high penetrability and sensitivity to tissue functional characteristics by detecting the acoustic wave to construct PA images. In recent years, a series of PA‐imaging contrast agents are developed to improve the PA‐imaging performance in biomedical applications. Here, recent progress of PA contrast agents and their biomedical applications are outlined. PA contrast agents are classified according to their components and function, and gold nanocrystals, gold‐nanocrystal assembly, transition‐metal chalcogenides/MXene‐based nanomaterials, carbon‐based nanomaterials, other inorganic imaging agents, small organic molecules, semiconducting polymer nanoparticles, and nonlinear PA‐imaging contrast agents are discussed. The applications of PA contrast agents as biosensors (in the sensing of metal ions, pH, enzymes, temperature, hypoxia, reactive oxygen species, and reactive nitrogen species) and in bioimaging (lymph nodes, vasculature, tumors, and brain tissue) are discussed in detail. Finally, an outlook on the future research and investigation of PA‐imaging contrast agents and their significance in biomedical research is presented.     

Transfer of Quantum Dots from Pregnant Mice to Pups Across the Placental Barrier

Fluorescent quantum dots (QDs) have great potential for in vivo biomedical imaging and diagnostic applications. However, these nanoparticles are composed of heavy metals and are very small in diameter, and their possible toxicity must therefore be considered. As yet, no studies have reported the transfer of QDs between mother and fetus. The transfer of CdTe/CdS QDs of different sizes and dosages, and with different outer capping materials, from pregnant mice to fetuses is investigated. It is shown that QDs may be transferred from female mice to their fetuses across the placental barrier. Smaller QDs are more easily transferred than larger QDs and the number of QDs transferred increases with increasing dosage. Capping with an inorganic silica shell or organic polyethylene glycol reduces QD transfer but does not eliminate it. These results suggest that the clinical utility of QDs could be limited in pregnant women.     

Synthesis and Biomedical Applications of Copper Sulfide Nanoparticles: From Sensors to Theranostics

Copper sulfide (CuS) nanoparticles have attracted increasing attention from biomedical researchers across the globe, because of their intriguing properties which have been mainly explored for energy‐ and catalysis‐related applications to date. This focused review article aims to summarize the recent progress made in the synthesis and biomedical applications of various CuS nanoparticles. After a brief introduction to CuS nanoparticles in the first section, we will provide a concise outline of the various synthetic routes to obtain different morphologies of CuS nanoparticles, which can influence their properties and potential applications. CuS nanoparticles have found broad applications in vitro, especially in the detection of biomolecules, chemicals, and pathogens which will be illustrated in detail. The in vivo uses of CuS nanoparticles have also been investigated in preclinical studies, including molecular imaging with various techniques, cancer therapy based on the photothermal properties of CuS, as well as drug delivery and theranostic applications. Research on CuS nanoparticles will continue to thrive over the next decade, and tremendous opportunities lie ahead for potential biomedical/clinical applications of CuS nanoparticles.     

Recent Advances in Higher‐Order,Multimodal, Biomedical Imaging Agents

Advances in biomedical imaging have spurred the development of integrated multimodal scanners, usually capable of two simultaneous imaging modes. The long‐term vision of higher‐order multimodality is to improve diagnostics or guidance through the analysis of complementary, data‐rich, co‐registered images. Synergies achieved through combined modalities could enable researchers to better track diverse physiological and structural events, analyze biodistribution and treatment efficacy, and compare established and emerging modalities. Higher‐order multimodal approaches stand to benefit from molecular imaging probes and, in recent years, contrast agents that have hypermodal characteristics have increasingly been reported in preclinical studies. Given the chemical requirements for contrast agents representing various modalities to be integrated into a single entity, the higher‐order multimodal agents reported so far tend to be of nanoparticulate form. To date, the majority of reported nanoparticles have included components that are active for magnetic resonance. Herein, recent progress in higher‐order multimodal imaging agents is reviewed, spanning a range of material and structural classes, and demonstrating utility in three (or more) imaging modalities.     

Probing Cell‐Type‐Specific Intracellular Nanoscale Barriers Using Size‐Tuned Quantum Dots

The compartmentalization of size‐tuned luminescent semiconductor nanocrystal quantum dots (QDs) in four distinctive cell lines, which would be representative of the most likely environmental exposure routes to nanoparticles in humans, is studied. The cells are fixed and permeabilized prior to the addition of the QDs, thus eliminating any cell‐membrane‐associated effects due to active QD uptake mechanisms or to specificity of signaling routes in different cell types, but leaving intact the putative physical subcellular barriers. All quantitative assays are performed using a high content analysis (HCA) platform, thereby obtaining robust data on large cell populations. While smaller QDs 2.1 nm in diameter enter the nuclei and localize to the nucleoli in all cell types, the rate and dynamics of their passage vary depending on the cell origin. As the QD size is increased to 4.4 nm, penetration into the cell is reduced but each cell line displays its own cutoff size thresholds reflecting cell‐type‐determined cytoplasmic and nuclear pore penetration specificity. These results give rise to important considerations regarding the differential compartmentalization and susceptibility of organs, tissues, and cells to nanoparticles, and may be of prime importance for biomedical imaging and drug‐delivery research employing nanoparticle‐based probes and systems.     

Cell-penetrating peptide-functionalized quantum dots for intracellular delivery

Quantum dots (QDs) are luminescent semiconductor nanocrystals that are widely used as fluorescent probes in biomedical applications, including cellular imaging and tumor tracking. Cell-penetrating peptides (CPPs), also called protein transduction domains (PTDs), are short basic peptides that permeate cell membranes and are able to deliver a variety of macromolecule cargoes, such as DNAs, RNAs, proteins, and nanomaterials. Here we review strategies to couple QDs to CPPs, by either covalent linkages or noncovalent interactions, to provide a tool to study intracellular delivery. This facilitated transport of QDs by CPPs into cells is both simple and efficient. Accordingly, CPP-QD nanoparticles are likely to be of broad utility in biological research and advance the development of medical and pharmaceutical therapeutics.     

PEGylated Tantalum Nanoparticles: A Metallic Photoacoustic Contrast Agent for Multiwavelength Imaging of Tumors

Elemental tantalum is a well‐known biomedical metal in clinics due to its extremely high biocompatibility, which is superior to that of other biomedical metallic materials. Hence, it is of significance to expand the scope of biomedical applications of tantalum. Herein, it is reported that tantalum nanoparticles (Ta NPs), upon surface modification with polyethylene glycol (PEG) molecules via a silane‐coupling approach, are employed as a metallic photoacoustic (PA) contrast agent for multiwavelength imaging of tumors. By virtue of the broad optical absorbance from the visible to near‐infrared region and high photothermal conversion efficiency (27.9%), PEGylated Ta NPs depict high multiwavelength contrast capability for enhancing PA imaging to satisfy the various demands (penetration depth, background noise, etc.) of clinical diagnosis as needed. Particularly, the PA intensity of the tumor region postinjection is greatly increased by 4.87, 7.47, and 6.87‐fold than that of preinjection under 680, 808, and 970 nm laser irradiation, respectively. In addition, Ta NPs with negligible cytotoxicity are capable of eliminating undesirable reactive oxygen species, ensuring the safety for biomedical applications. This work introduces a silane‐coupling strategy for the surface engineering of Ta NPs, and highlights the potential of Ta NPs as a biocompatible metallic contrast agent for multiwavelength photoacoustic image.     

High‐Resolution SPECT Imaging of Stimuli‐Responsive Soft Microrobots

Untethered small‐scale robots have great potential for biomedical applications. However, critical barriers to effective translation of these miniaturized machines into clinical practice exist. High resolution tracking and imaging in vivo is one of the barriers that limit the use of micro‐ and nanorobots in clinical applications. Here, the inclusion of radioactive compounds in soft thermoresponsive magnetic microrobots is investigated to enable their single‐photon emission computed tomography imaging. Four microrobotic platforms differing in hydrogel structure and four ^99mTc[Tc]‐based radioactive compounds are investigated in order to achieve optimal contrast agent retention and optimal imaging. Single microrobot imaging of structures as low as 100 µm in diameter, as well as tracking of shape switching from tubular to planar configurations by inclusion of ^99mTc[Tc] colloid in the hydrogel structure, is reported.     

The Shortening of MWNT‐SPION Hybrids by Steam Treatment Improves Their Magnetic Resonance Imaging Properties In Vitro and In Vivo

Carbon nanotubes (CNTs) have been advocated as promising nanocarriers in the biomedical field. Their high surface area and needle‐like shape make these systems especially attractive for diagnostic and therapeutic applications. Biocompatibility, cell internalization, biodistribution, and pharmacokinetic profile have all been reported to be length dependent. In this study, further insights are gotten on the role that the length of CNTs plays when developing novel contrast agents for magnetic resonance imaging (MRI). Two samples of CNTs with different length distribution have been decorated with radio‐labeled iron oxide nanoparticles. Despite characterization of the prepared hybrids reveals a similar degree of loading and size of the nanoparticles for both samples, the use of short CNTs is found to enhance the MRI properties of the developed contrast agents both in vitro and in vivo compared to their long counterparts.     

Single Chain Epidermal Growth Factor Receptor Antibody Conjugated Nanoparticles for in vivo Tumor Targeting and Imaging

Epidermal growth factor receptor (EGFR) targeted nanoparticle are developed by conjugating a single‐chain anti‐EGFR antibody (ScFvEGFR) to surface functionalized quantum dots (QDs) or magnetic iron oxide (IO) nanoparticles. The results show that ScFvEGFR can be successfully conjugated to the nanoparticles, resulting in compact ScFvEGFR nanoparticles that specifically bind to and are internalized by EGFR‐expressing cancer cells, thereby producing a fluorescent signal or magnetic resonance imaging (MRI) contrast. In vivo tumor targeting and uptake of the nanoparticles in human cancer cells is demonstrated after systemic delivery of ScFvEGFR‐QDs or ScFvEGFR‐IO nanoparticles into an orthotopic pancreatic cancer model. Therefore, ScFvEGFR nanoparticles have potential to be used as a molecular‐targeted in vivo tumor imaging agent. Efficient internalization of ScFvEGFR nanoparticles into tumor cells after systemic delivery suggests that the EGFR‐targeted nanoparticles can also be used for the targeted delivery of therapeutic agents.     

Ultrasmall Near‐Infrared Non‐cadmium Quantum Dots for in vivo Tumor Imaging

The high tumor uptake of ultrasmall near‐infrared quantum dots (QDs) attributed to the enhanced permeability and retention effect is reported. InAs/InP/ZnSe QDs coated by mercaptopropionic acid (MPA) exhibit an emission wavelength of about 800 nm (QD800‐MPA) with very small hydrodynamic diameter (<10 nm). Using 22B and LS174T tumor xenograft models, in vivo and ex vivo imaging studies show that QD800‐MPA is highly accumulated in the tumor area, which is very promising for tumor detection in living mice. The ex vivo elemental analysis (Indium) using inductively coupled plasma (ICP) spectrometry confirm the tumor uptake of QDs. The ICP data are consistent with the in vivo and ex vivo fluorescence imaging. Human serum albumin (HSA)‐coated QD800‐MPA nanoparticles (QD800‐MPA‐HSA) show reduced localization in mononuclear phagocytic system‐related organs over QD800‐MPA plausibly due to the low uptake of QD800‐MPA‐HSA in macrophage cells. QD800‐MPA‐HSA may have great potential for in vivo fluorescence imaging.     

Control of the in vivo Biodistribution of Hybrid Nanoparticles with Different Poly(ethylene glycol) Coatings

Fluorescent nanoparticles containing a gadolinium oxide core are very attractive because they are able to combine both imaging (fluorescence imaging, magnetic resonance imaging) and therapy (X‐ray therapy and neutron‐capture therapy) techniques. The exploitation of these multifunctional particles for in vivo applications requires accurate control of their biodistribution. The postfunctionalization of these particles by four different poly(ethylene glycol) derivatives, which differ by chain length and end group, exerts a great influence on the ζ potential of the nanoparticles and on their biodistribution after intravenous injection to HEK‐β3‐tumor‐bearing mice. This study reveals that the behavior of PEGylated nanoparticles, which was monitored by in vivo fluorescence imaging, depends on both the chain length and the end group of the PEG chain.     

量子点在细胞以及体内生物中成像的研究进展

量子点是一种荧光半导体纳米材料,与生物分子结合成一种高亮度而稳定的荧光探针应用于生物成像。通过生物成像可观察量子点标记分子与其靶标的相互作用,实时观测其在活细胞及活体中的运行轨迹,实现对细胞水平及在活体层次的研究。利用这种生物成像技术还可以研究疾病的发生发展过程。介绍了量子点的光学特性,重点综述了量子点在细胞、体内生物成像中的应用,并展望了其发展前景。     

Albumin Nanoshell Encapsulation of Near‐Infrared‐Excitable Rare‐Earth Nanoparticles Enhances Biocompatibility and Enables Targeted Cell Imaging

The use of traditional fluorophores for in vivo imaging applications is limited by poor quantum yield, poor tissue penetration of the excitation light, and excessive tissue autofluorescence, while the use of inorganic fluorescent particles that offer a high quantum yield is frequently limited due to particle toxicity. Rare‐earth‐doped nanoparticles that utilize near‐infrared upconversion overcome the optical limitations of traditional fluorophores, but are not typically suitable for biological application due to their insolubility in aqueous solution, lack of functional surface groups for conjugation of biomolecules, and potential cytotoxicity. A new approach to establish highly biocompatible and biologically targetable nanoshell complexes of luminescent rare‐earth‐doped NaYF₄ nanoparticles (REs) excitable with 920–980 nm near‐infrared light for biomedical imaging applications is reported. The approach involves the encapsulation of NaYF₄ nanoparticles doped with Yb and Er within human serum albumin nanoshells to create water‐dispersible, biologically functionalizable composite particles. These particles exhibit narrow size distributions around 200 nm and are stable in aqueous solution for over 4 weeks. The albumin shell confers cytoprotection and significantly enhances the biocompatibility of REs even at concentrations above 200 µg REs mL^?1. Composite particles conjugated with cyclic arginine‐glycine‐aspartic acid (cRGD) specifically target both human glioblastoma cell lines and melanoma cells expressing α_vβ₃ integrin receptors. These findings highlight the promise of albumin‐encapsulated rare‐earth nanoparticles for imaging cancer cells in vitro and the potential for targeted imaging of disease sites in vivo.     

Recent Advances in Nanotechnology for Autophagy Detection

Autophagy is closely related to various diseases, and is a diagnostic and therapeutic target for some diseases. In recent years, tremendous efforts have been made to develop excellent probes for detection of autophagy. Nanostructure‐based probes are interesting and promising approaches for in vivo biological imaging due to their unique structural and functional characteristics, e.g., modulating pharmacokinetics property by biocompatible coatings, multimodality capacity by delivering multiple imaging agents and highly specific targeting by antibody ligands. In this Review, we first introduce recent advancements in the development of nanostructure‐based probes for detection of autophagy, including inorganic hybrid nanomaterials and self‐assembled peptide polymeric nanoparticles. Meanwhile, a nanoprobe based on a “in vivo self‐assembly” strategy is highlighted. The “in vivo self‐assembly” endows nanoprobes with higher accumulation, and longer and better signal stability for in vivo detection of autophagy. Furthermore, this novel strategy could be widely used for biomedical imaging/diagnostics and therapeutics, which would attract more attention to this research area.     

Nanotoxicology: Advanced Optical Imaging Reveals the Dependence of Particle Geometry on Interactions Between CdSe Quantum Dots and Immune Cells (Small 3/2011)

The biocompatibility and possible toxicological consequences of engineered nanomaterials, including quantum dots (QDs) due to their unique suitability for biomedical applications, remain intense areas of interest. We utilized advanced imaging approaches to characterize the interactions of CdSe QDs of various sizes and shapes with live immune cells. Particle diffusion and partitioning within the plasma membrane, cellular uptake kinetics, and sorting of particles into lysosomes were all independantly characterized. Using high‐speed total internal reflectance fluorescence (TIRF) microscopy, we show that QDs with an average aspect ratio of 2.0 (i.e., rod‐shaped) diffuse nearly an order of magnitude slower in the plasma membrane than more spherical particles with aspect ratios of 1.2 and 1.6, respectively. Moreover, more rod‐shaped QDs were shown to be internalized into the cell 2‐3 fold more slowly. Hyperspectral confocal fluorescence microscopy demonstrates that QDs tend to partition within the cell membrane into regions containing a single particle type. Furthermore, data examining QD sorting mechanisms indicate that endocytosis and lysosomal sorting increases with particle size. Together, these observations suggest that both size and aspect ratio of a nanoparticle are important characteristics that significantly impact interactions with the plasma membrane, uptake into the cell, and localization within intracellular vesicles. Thus, rather than simply characterizing nanoparticle uptake into cells, we show that utilization of advanced imaging approaches permits a more nuanced and complete examination of the multiple aspects of cell‐nanoparticle interactions that can ultimately aid understanding possible mechanisms of toxicity, resulting in safer nanomaterial designs.