Molecularly near-infrared fluorescent theranostics for in vivo tracking tumor-specific chemotherapy

In this review, we summary the design concepts and strategies of NIR fluorescent theranostics for the senserelease in living systems. In particular, molecularly NIR fluorescent theranostic prodrug is elucidated to address current challenges of real-time bioimaging and tumor-specific chemotherapy for personalized treatment.     

Structural and process controls of AIEgens for NIR-II theranostics

Aggregation-induced emission (AIE) is a cutting-edge fluorescence technology, giving highly-efficient solid-state photoluminescence. Particularly, AIE luminogens (AIEgens) with emission in the range of second near-infrared window (NIR-II, 1000–1700 nm) have displayed salient advantages for biomedical imaging and therapy. However, the molecular design strategy and underlying mechanism for regulating the balance between fluorescence (radiative pathway) and photothermal effect (non-radiative pathway) in these narrow bandgap materials remain obscure. In this review, we outline the latest achievements in the molecular guidelines and photophysical process control for developing highly efficient NIR-II emitters or photothermal agents with aggregation-induced emission (AIE) attributes. We provide insights to optimize fluorescence efficiency by regulating multi-hierarchical structures from single molecules (flexibilization) to molecular aggregates (rigidification). We also discuss the crucial role of intramolecular motions in molecular aggregates for balancing the functions of fluorescence imaging and photothermal therapy. The superiority of the NIR-II region is demonstrated by fluorescence/photoacoustic imaging of blood vessels and the brain as well as photothermal ablation of the tumor. Finally, a summary of the challenges and perspectives of NIR-II AIEgens for in vivo theranostics is given.

Structural and process controls of NIR-II AIEgens realize manipulating of radiative (R) and nonradiative (NR) decay for precise theranostics.     

Near-infrared fluorescence spectroscopy of single-walled carbon nanotubes and its applications

Semiconducting single-walled carbon nanotubes (SWCNTs) emit fluorescence at near-infrared (NIR) wavelengths that are characteristic of the specific diameter and the chiral angle. While providing a convenient method for structural identification of semiconducting SWCNTs, NIR fluorescence of SWCNTs also offers a powerful approach for sensor development and in vivo or real-time imaging of biological systems.This article provides an introductory overview of the approaches to obtaining individually dispersed semiconducting SWCNTs with reasonably good purity, which is a critical step in acquiring NIR fluorescence spectra. It also summarizes the progress since 2002 in sensor design and applications in bioimaging in vitro and in vivo using NIR fluorescence of semiconducting SWCNTs.     

D-A-D structured selenadiazolesbenzothiadiazole-based near-infrared dye for enhanced photoacoustic imaging and photothermal cancer therapy

Near-infrared (NIR) small molecular organic dyes as photothermal agents for cancer photothermal therapy (PTT) have attracted considerable research attention. Herein, two donor-acceptor-donor (D-A-D) structured NIR dyes, BBTT and SeBTT, are rationally designed, where the only difference is one heteroatom within the acceptor unit varying from sulfur to selenium (Se). More importantly, SeBTT NPs exhibit stronger NIR absorbance and higher photothermal conversion efficiency (PTCE ≈ 65.3%). In vivo experiments illustrate that SeBTT NPs can be utilized as a high contrast photoacoustic imaging (PAI) agent, and succeed in tumor suppression without noticeable damage to main organs under NIR photoirradiation. This study presents an effective molecular heteroatom surgery strategy to regulate the photothermal properties of NIR small molecules for enhanced PAI and PTT.     

New directions of activity-based sensing for in vivo NIR imaging

In vivo imaging is a powerful approach to study biological processes. Beyond cellular methods, in vivo studies allow for biological stimuli (small molecules or proteins) to be studied in their native environment. This has the potential to aid in the discovery of new biology and guide the development of diagnostics and therapies for diseases. To ensure selectivity and an observable readout, the probe development field is shifting towards activity-based sensing (ABS) approaches and near-infrared (NIR) imaging modalities. This perspective will highlight recent in vivo ABS applications that utilize NIR imaging platforms.

In vivo imaging is a powerful approach to study biological processes.     

Facile synthesis and in vivo bioimaging applications of porphyrin derivative-encapsulated polymer nanoparticles

Fluorescence (FL) imaging guided photodynamic therapy (PDT) is becoming highly desirable for personalized therapy and precision medicine. In this study, fluorescent polymer nanoparticles TCPP@PEI/PGA were facilely synthesized through electrostatic interaction-mediated self-assembly of porphyrins tetra(4-carboxyphenyl)porphine (TCPP) and polyethylenimine (PEI), and subsequent surface modification with γ-poly(glutamic acid) (γ-PGA). TCPP served a dual function as the FL imaging probe and the photosensitizer. The as-prepared TCPP@PEI/PGA nanoparticles showed excellent water-solubility and biocompatibility, while having outstanding capabilities of in vivo bioimaging and ¹O₂ generation. FL bioimaging of mice and effective killing of CT 26 cells as well as CT 26 tumor-bearing mice upon laser irradiation were successfully demonstrated when using TCPP@PEI/PGA as theranostic nanoprobes. This study provides a simple but robust method to design and synthesize porphyrin-based polymer nanoparticles for theranostics.     

Polarity-active NIR probes with strong two-photon absorption and ultrahigh binding affinity of insulin amyloid fibrils

Amyloid fibrils are associated with many neurodegenerative diseases. In situ and in vivo visualization of amyloid fibrils is important for medical diagnostics and requires fluorescent probes with both excitation and emission wavelengths in the far-red and NIR region, and simultaneously with high binding-affinity to amyloid fibrils and the ability to cross the blood–brain barrier, which, however, remain a challenge. Here, we rationally design and synthesize an excellent polarity-sensitive two-photon excited NIR fluorophore (TZPI) based on a donor (D)–acceptor (A)-ion compound. The electron-rich carbazole group and the ionic pyridinium bromide group, linked by an electron-poor π-conjugated benzothiadiazole group, ensure strong near infrared (NIR) emission. Furthermore, the lipophilic carbazole together with the benzothiadiazole group facilitates docking of the probe in the hydrophobic domains of amyloid aggregates with the dissociation constant K_d = 20 nM and 13.5-fold higher binding affinity to insulin fibrils than the commercial probe ThT. On association with the amyloid fibrils, the tiny decrease in polarity leads to a large increase in its NIR emission intensity with an on–off ratio > 10; meanwhile, the TZPI probe exhibits a quantum yield of up to 30% and two-photon absorption cross-section values of up to 467.6 GM at 890 nm. Moreover, the application of TZPI in two-photon imaging is investigated. The ultrahigh binding affinity, the strong NIR emission, the good two-photon absorption properties, the high photo-stability, the appropriate molecular mass of 569 Da and the lipophilicity with log P = 1.66 ± 0.1 to cross the BBB make TZPI promising as an ideal candidate for detecting amyloid plaques in vivo.

A polarity-active NIR probe based on the transformation from the CT state to the LE state for two-photon imaging of amyloid fibrils.     

Semiconducting polymer dots as near‐infrared fluorescent probes for bioimaging and sensing

In recent years, semiconducting polymer dots (Pdots) have emerged as a new type of ultrabright fluorescent probes, which have been proved to be very useful for biomedical imaging. Pdots possess several exceptional advantages including high fluorescence brightness, fast radiative rate, excellent photostability, and negligible cytotoxicity. Among these new types of Pdots, the near‐infrared (NIR) fluorescent Pdots appear to be the most urgent and important owing to their promising deep‐tissue imaging in the clinic. This mini‐review highlights the recent progress in the design of NIR‐emitting Pdots and their biomedical applications both in vitro and in vivo.     

Near-infrared dyes,nanomaterials and proteins

Taking the advantage of reduced scattering and low autofluorescence background, the NIR fluorescence probes, such as fluorescence proteins, organic molecules and nanoparticles, not only hold the promise of in vivo imaging of biological processes in physiology and pathology with high signal-to-noise ratio, but also for clinical diagnosis. In this review, we provide an overview of the recent progress on NIR probes, focusing on fundamental mechanisms of NIR dyes and nanoparticles, and protein engineering strategies for NIR proteins.     

Photocaged prodrug under NIR light-triggering with dual-channel fluorescence: <Emphasis Type="Italic">in vivo</Emphasis> real-time tracking for precise drug delivery

Light-triggered drug delivery system is an effective strategy for precise diagnosis and therapy in cancer treatment. However, it suffers from difficultly balancing the dosimetry of drug with light dose and a lack of in vivo models for validating their clinical benefits. Here we report an unprecedented near-infrared (NIR) light photocaged cyanine-based prodrug Cy-CPT-Biotin with dual-channel fluorescence mode, enabling NIR light to precisely regulate where, when and how the intact and active prodrugs are delivered. The synergy of photochemical reaction and modulation in π-conjugated polyene backbone of cyanine can fully perform distinct dual-channel fluorescence changes in a NIR light-mediated manner. The prodrug has striking characteristics of excellent tumor-targeting ability, real-time monitoring of the in vivo behaviors by dual-channel mode and NIR-light triggering, especially for achieving fine regulation and on-demand drug release in the precise dosimetry of drug with light dose in living animals. This optical orthogonality strategy that conjuncts with NIR light-triggered and dual-channel fluorescence in vivo imaging provides a powerful tool for in vivo real-time tracking and finely tuning the prodrug release for precise drug delivery.     

Biocompatible organic dots with aggregation-induced emission for in vitro and in vivo fluorescence imaging

Fluorescent probes play a key role in modern biomedical research. As compared to inorganic quantum dots (QDs) composed with heavy metal elements, organic dye-based fluorescent nanoparticles have higher biocompatibility and are richer in variety. However, traditional organic fluorophores tend to quench fluorescence upon aggregation, which is known as aggregation-caused quenching (ACQ) effect that hinders the fabrication of highly emissive fluorescent nanoparticles. In this work, we demonstrate the synthesis of organic fluorescent dots with aggregation-induced emission (AIE) in far-red/near-infrared (FA/NIR) region. A conventional ACQ-characteristic fluorescent dye, 3,4:9,10-tetracarboxylic perylene bisimide (PBI), is converted into an AIE fluorogen through attaching two tetraphenylethylene (TPE) moieties. The fluorescent dots with surface folic acid groups are fabricated from PBI derivative (DTPEPBI), showing specific targeting effect to folate receptor-overexpressed cancer cells. In vivo studies also suggest that the folic acid-functionalized AIE dots preferentially accumulate in the tumor site through enhanced permeability and retention (EPR) effect and folate receptor-mediated active targeting effect. The low cyto-toxicity, good FR/NIR contrast and excellent targeting ability in in vitro/in vivo imaging indicate that the AIE dots have great potentials in advanced bioimaging applications.     

DNA nanostructure-based nucleic acid probes: construction and biological applications

In recent years, DNA has been widely noted as a kind of material that can be used to construct building blocks for biosensing, in vivo imaging, drug development, and disease therapy because of its advantages of good biocompatibility and programmable properties. However, traditional DNA-based sensing processes are mostly achieved by random diffusion of free DNA probes, which were restricted by limited dynamics and relatively low efficiency. Moreover, in the application of biosystems, single-stranded DNA probes face challenges such as being difficult to internalize into cells and being easily decomposed in the cellular microenvironment. To overcome the above limitations, DNA nanostructure-based probes have attracted intense attention. This kind of probe showed a series of advantages compared to the conventional ones, including increased biostability, enhanced cell internalization efficiency, accelerated reaction rate, and amplified signal output, and thus improved in vitro and in vivo applications. Therefore, reviewing and summarizing the important roles of DNA nanostructures in improving biosensor design is very necessary for the development of DNA nanotechnology and its applications in biology and pharmacology. In this perspective, DNA nanostructure-based probes are reviewed and summarized from several aspects: probe classification according to the dimensions of DNA nanostructures (one, two, and three-dimensional nanostructures), the common connection modes between nucleic acid probes and DNA nanostructures, and the most important advantages of DNA self-assembled nanostructures in the applications of biosensing, imaging analysis, cell assembly, cell capture, and theranostics. Finally, the challenges and prospects for the future development of DNA nanostructure-based nucleic acid probes are also discussed.

In recent years, DNA has been widely noted as a kind of material that can be used to construct building blocks for biosensing, in vivo imaging, drug development, and disease therapy because of its advantages of good biocompatibility and programmable properties.     

Preparation and Characterization of Uniform Near IR Polystyrene Nanoparticles

Biomaterials for in vivo fluorescence imaging are required to be biocompatible, nontoxic, photostable and highly fluorescent. Fluorescence must be in the near infrared (NIR) region of the electromagnetic spectrum to avoid absorption and autofluorescence of endogenous tissues. NIR fluorescent polystyrene nanoparticles may be considered ideal biomaterials for in vivo imaging applications. These NIR nanoparticles were prepared by a swelling process of polystyrene template nanoparticles with a hydrophobic NIR dye dissolved in a water‐miscible swelling solvent, a method developed for preparation of nonbiodegradable nanoparticles, for NIR fluorescent bioimaging applications. This method overcomes common problems that occur with dye entrapment during nanoparticle formation such as loss of fluorescence and size polydispersity. Fluorescence intensity of the nanoparticles was found to be size dependent, and was optimized for differently sized nanoparticles. The resulting NIR nanoparticles were also found to be more fluorescent and highly photostable compared to the free dye in solution, showing their potential as biomaterials for in vivo fluorescence imaging.     

Rational design of iridium–porphyrin conjugates for novel synergistic photodynamic and photothermal therapy anticancer agents

Near-infrared (NIR) emitters are important probes for biomedical applications. Nanoparticles (NPs) incorporating mono- and tetranuclear iridium(iii) complexes attached to a porphyrin core have been synthesized. They possess deep-red absorbance, long-wavelength excitation (635 nm) and NIR emission (720 nm). TD-DFT calculations demonstrate that the iridium–porphyrin conjugates herein combine the respective advantages of small organic molecules and transition metal complexes as photosensitizers (PSs): (i) the conjugates retain the long-wavelength excitation and NIR emission of porphyrin itself; (ii) the conjugates possess highly effective intersystem crossing (ISC) to obtain a considerably more long-lived triplet photoexcited state. These photoexcited states do not have the usual radiative behavior of phosphorescent Ir(iii) complexes, and they play a very important role in promoting the singlet oxygen (¹O₂) and heat generation required for photodynamic therapy (PDT) and photothermal therapy (PTT). The tetranuclear 4-Ir NPs exhibit high ¹O₂ generation ability, outstanding photothermal conversion efficiency (49.5%), good biocompatibility, low half-maximal inhibitory concentration (IC₅₀) (0.057 μM), excellent photothermal imaging and synergistic PDT and PTT under 635 nm laser irradiation. To our knowledge this is the first example of iridium–porphyrin conjugates as PSs for photothermal imaging-guided synergistic PDT and PTT treatment in vivo.

Iridium–porphyrin conjugates assembled in nanoparticles are photosensitizers that exhibit excellent photothermal imaging and synergistic PDT and PTT in vivo.     

Near-infrared fluorescent molecular probes for imaging and diagnosis of nephro-urological diseases

Near-infrared (NIR) fluorescence imaging has improved imaging depth relative to conventional fluorescence imaging in the visible region, demonstrating great potential in both fundamental biomedical research and clinical practice. To improve the detection specificity, NIR fluorescence imaging probes have been under extensive development. This review summarizes the particular application of optical imaging probes with the NIR-I window (700–900 nm) or the NIR-II window (1000–1700 nm) emission for diagnosis of nephron-urological diseases. These molecular probes have enabled contrast-enhanced imaging of anatomical structures and physiological function as well as molecular imaging and early diagnosis of acute kidney injury, iatrogenic ureteral injury and bladder cancer. The design strategies of molecular probes are specifically elaborated along with representative imaging applications. The potential challenges and perspectives in this field are also discussed.

Near-infrared fluorescent molecular probes with improved imaging depth and optimized biodistribution have been reviewed, showing great potential for diagnosis of nephro-urological diseases.     

Novel photo-theranostic GdB6 nanoparticles for fluorescence imaging and NIR-photothermal therapy

The development of multifunctional theranostic nano-agents is an important resolution for personalized treatment of cancer. In this work, we synthesized a new kind of gadolinium boride nanoparticles (GBN) by a microwave-assisted chemical etching method, and discovered their optical characteristics including fluorescence imaging and near-infrared (NIR) photothermal conversion capability. Bright greenishyellow fluorescence enabled for intracellular localization, while effective NIR-photothermal conversion supported photothermal therapy (PTT). In vitro and in vivo results indicated that GBN exhibited a superior antitumor performance and high biocompatibility. This study demonstrated a promising multifunctional theranostic nanoplatform for cancer treatment.     

Mo2C‐Derived Polyoxometalate for NIR‐II Photoacoustic Imaging‐Guided Chemodynamic/Photothermal Synergistic Therapy

To overcome the current limitations of chemodynamic therapy (CDT), a Mo₂C‐derived polyoxometalate (POM) is readily synthesized as a new CDT agent. It permits synergistic chemodynamic and photothermal therapy operating in the second near‐infrared (NIR‐II) biological transparent window for deep tissue penetration. POM aggregated in an acidic tumor micro‐environment (TME) whereby enables specific tumor targeting. In addition to the strong ability to produce singlet oxygen (¹O₂) presumably via Russell mechanism, its excellent photothermal conversion enhances the CDT effect, offers additional tumor ablation modality, and permits NIR‐II photoacoustic imaging. Benefitting from the reversible redox property of molybdenum, the theranostics based on POM can escape from the antioxidant defense system. Moreover, combining the specific responsiveness to TME and localized laser irradiation, side‐effects shall be largely avoided.     

Near‐Infrared Light and pH‐Responsive Polypyrrole@Polyacrylic acid/Fluorescent Mesoporous Silica Nanoparticles for Imaging and Chemo‐Photothermal Cancer Therapy

We have rationally designed a new theranostic agent by coating near‐infrared (NIR) light‐absorbing polypyrrole (PPY) with poly(acrylic acid) (PAA), in which PAA acts as a nanoreactor and template, followed by growing small fluorescent silica nanoparticles (fSiO₂ NPs) inside the PAA networks, resulting in the formation of polypyrrole@polyacrylic acid/fluorescent mesoporous silica (PPY@PAA/fmSiO₂) core–shell NPs. Meanwhile, DOX‐loaded PPY@PAA/fmSiO₂ NPs as pH and NIR dual‐sensitive drug delivery vehicles were employed for fluorescence imaging and chemo‐photothermal synergetic therapy in vitro and in vivo. The results demonstrate that the PPY@PAA/fmSiO₂ NPs show high in vivo tumor uptake by the enhanced permeability and retention (EPR) effect after intravenous injection as revealed by in vivo fluorescence imaging, which is very helpful for visualizing the location of the tumor. Moreover, the obtained NPs inhibit tumor growth (95.6 % of tumors were eliminated) because of the combination of chemo‐photothermal therapy, which offers a synergistically improved therapeutic outcome compared with the use of either therapy alone. Therefore, the present study provides new insights into developing NIR and pH‐stimuli responsive PPY‐based multifunctional platform for cancer theranostics.     

Design of an intelligent sub-50 nm nuclear-targeting nanotheranostic system for imaging guided intranuclear radiosensitization

Clinically applied chemotherapy and radiotherapy is sometimes not effective due to the limited dose acting on DNA chains resident in the nuclei of cancerous cells. Herein, we develop a new theranostic technique of “intranuclear radiosensitization” aimed at directly damaging the DNA within the nucleus by a remarkable synergetic chemo-/radiotherapeutic effect based on intranuclear chemodrug-sensitized radiation enhancement. To achieve this goal, a sub-50 nm nuclear-targeting rattle-structured upconversion core/mesoporous silica nanotheranostic system was firstly constructed to directly transport the radiosensitizing drug Mitomycin C (MMC) into the nucleus for substantially enhanced synergetic chemo-/radiotherapy and simultaneous magnetic/upconversion luminescent (MR/UCL) bimodal imaging, which can lead to efficient cancer treatment as well as multi-drug resistance circumvention in vitro and in vivo. We hope the technique of intranuclear radiosensitization along with the design of nuclear-targeting nanotheranostics will contribute greatly to the development of cancer theranostics as well as to the improvement of the overall therapeutic effectiveness.