Biological Imaging Using Nanoparticles of Small Organic Molecules with Fluorescence Emission at Wavelengths Longer than 1000 nm

Counterion-Paired Bright Heptamethine Fluorophores with NIR-II Excitation and Emission Enable Multiplexed Biomedical Imaging

Photon excitation and emission at the NIR-II spectral window enable high-contrast deep-tissue bioimaging. However, multiplexed imaging with NIR-II excitation and emission has been hampered by the limited chemical strategies to develop bright fluorophores with tunable absorption in this spectral regime. Herein, we developed a series of heptamethine cyanines (HCs) with varied absorption/emission maxima spanning from 1100 to 1600 nm through a physical organic approach. A bulky counterion paired to HCs was found to elicit substantial improvements in absorptivity (7-fold), brightness (14-fold), and spectral profiles in water, addressing a notorious quenching problem of NIR-II cyanines due to aggregation and polarization. We demonstrated the utilities of HC1222 and HC1342 for high-contrast dual-color imaging of circulatory system, lymphatic structures, tumor, and organ function in living mice under 1120 nm and 1319 nm excitation, showing HCs as a promising platform for non-invasive bioimaging.

Heat Treatment Effects for Controlling Dye Molecular States in the Hydrophobic Core of Over-1000 nm Near-Infrared (NIR-II) Fluorescent Micellar Nanoparticles

Organic molecules that emit near-infrared (NIR) fluorescence at wavelengths above 1000 nm, also known as the second NIR (NIR-II) biological window, are expected to be applied to optical in vivo imaging of deep tissues. The study of molecular states of NIR-II dye and its optical properties are important to yield well-controlled fluorescent probes; however, no such study has been conducted yet. Among the two major absorption peaks of the NIR-II dye, IR-1061, the ratio of the shorter wavelength (900 nm) to the longer one (1060 nm) increased with an increase in the dye concentration in tetrahydrofuran, suggesting that the 900 nm peak is due to the dimer formation of IR-1061. Both absorption peaks are also observed when IR-1061 is encapsulated in the hydrophobic (stearyl) core of micellar nanoparticles (MNPs) of a phospholipid-poly(ethylene glycol). The dimers in the MNP cores decreased via dimer dissociation by enhancing the mobility of the hydrophobic stearyl chains by heat treatment of the dye-encapsulating MNPs at 50-70 °C. The MNPs maintained the dissociated IR-1061 monomers in the core after recooling to 25 °C and showed a higher NIR-II fluorescence intensity than those before heat treatment. This concept will provide better protocols for the preparation of NIR-II fluorescent probes with well-controlled fluorescence properties.

Versatile Types of Inorganic/Organic NIR-IIa/IIb Fluorophores: From Strategic Design toward Molecular Imaging and Theranostics

In vivo imaging in the second near-infrared window (NIR-II, 1000-1700 nm), which enables us to look deeply into living subjects, is producing marvelous opportunities for biomedical research and clinical applications. Very recently, there has been an upsurge of interdisciplinary studies focusing on developing versatile types of inorganic/organic fluorophores that can be used for noninvasive NIR-IIa/IIb imaging (NIR-IIa, 1300-1400 nm; NIR-IIb, 1500-1700 nm) with near-zero tissue autofluorescence and deeper tissue penetration. This review provides an overview of the reports published to date on the design, properties, molecular imaging, and theranostics of inorganic/organic NIR-IIa/IIb fluorophores. First, we summarize the design concepts of the up-to-date functional NIR-IIa/IIb biomaterials, in the order of single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth-doped nanoparticles (RENPs), and organic fluorophores (OFs). Then, these novel imaging modalities and versatile biomedical applications brought by these superior fluorescent properties are reviewed. Finally, challenges and perspectives for future clinical translation, aiming at boosting the clinical application progress of NIR-IIa and NIR-IIb imaging technology are highlighted.

Stabilization of indocyanine green dye in polymeric micelles for NIR-II fluorescence imaging and cancer treatment

One of the most commonly used near infrared (NIR) dyes is indocyanine green (ICG), which has been extensively used for NIR bioimaging, photothermal and photodynamic therapy. However, upon excitation this dye can react with molecular oxygen to form singlet oxygen (SO), which can then cleave ICG to form non-fluorescent debris. In order to reduce the reaction between ICG and oxygen, we used energy transfer (ET) between the former and the NIR dye IR-1061. The two dyes were encapsulated in micelles composed of biocompatible poly(ethylene glycol)-block-poly(ε-caprolactone) (PCL-PEG). Micelles were characterized for their size using dynamic light scattering (DLS) and were found to measure about 35 nm in diameter. Fluorescence emission measurements were conducted to show that the stability of ICG against photodecomposition is increased. Moreover, this increased stability allows the encapsulated dye to generate more heat and for a longer time, compared to its free form. Studies with a SO indicator showed that as more IR-1061 is added to the micelles, less SO is produced. These results show how by changing the amount of added IR-1061 it is possible to tune the heat and SO generated by the system. Cell viability studies demonstrated that while particles were nontoxic under physiological conditions, upon 808 nm irradiation they become potent at eradicating MCF7 cancer cells. Moreover, it was demonstrated that both the increase of temperature and the creation of decomposition debris play a role in the cytotoxic efficacy of the micelles. Dye-loaded micelles that were injected to live mice showed bright fluorescence in the over 1000 nm NIR (OTN-NIR) region, allowing for visualization of blood vessels and internal organs. Most importantly, the encapsulated dyes remained stable for over 30 minutes, gradually accumulating in the liver and spleen. The presence of IR-1061 in addition to the heat-generating dye ICG allowed for simultaneous temperature modification and monitoring. We were able to assess the change in temperature by measuring the change in the fluorescence intensity of IR-1061 in the OTN-NIR region, a range with deep penetration of living tissues. These features illustrate the potential use of ICG/IR-1061 in PCL-PEG micelles as promising candidates for cancer treatment and diagnosis.

Fluorination Enhances NIR-II Fluorescence of Polymer Dots for Quantitative Brain Tumor Imaging

Here, we describe a fluorination strategy for semiconducting polymers for the development of highly bright second near-infrared region (NIR-II) probes. Tetrafluorination yielded a fluorescence QY of 3.2 % for the polymer dots (Pdots), over a 3-fold enhancement compared to non-fluorinated counterparts. The fluorescence enhancement was attributable to a nanoscale fluorous effect in the Pdots that maintained the molecular planarity and minimized the structure distortion between the excited state and ground state, thus reducing the nonradiative relaxations. By performing through-skull and through-scalp imaging of the brain vasculature of live mice, we quantitatively analyzed the vascular morphology of transgenic brain tumors in terms of the vessel lengths, vessel branches, and vessel symmetry, which showed statistically significant differences from the wild type animals. The bright NIR-II Pdots obtained through fluorination chemistry provide insightful information for precise diagnosis of the malignancy of the brain tumor.

Optical fluorescence imaging with shortwave infrared light emitter nanomaterials for in vivo cell tracking in regenerative medicine

In vivo tracking and monitoring of adoptive cell transfer has a distinct importance in cell‐based therapy. There are many imaging modalities for in vivo monitoring of biodistribution, viability and effectiveness of transferred cells. Some of these procedures are not applicable in the human body because of low sensitivity and high possibility of tissue damages. Shortwave infrared region (SWIR) imaging is a relatively new technique by which deep biological tissues can be potentially visualized with high resolution at cellular level. Indeed, scanning of the electromagnetic spectrum (beyond 1000 nm) of SWIR has a great potential to increase sensitivity and resolution of in vivo imaging for various human tissues. In this review, molecular imaging modalities used for monitoring of biodistribution and fate of administered cells with focusing on the application of non‐invasive optical imaging at shortwave infrared region are discussed in detail.

Near-Infrared-II Molecular Dyes for Cancer Imaging and Surgery

Fluorescence bioimaging affords a vital tool for both researchers and surgeons to molecularly target a variety of biological tissues and processes. This review focuses on summarizing organic dyes emitting at a biological transparency window termed the near-infrared-II (NIR-II) window, where minimal light interaction with the surrounding tissues allows photons to travel nearly unperturbed throughout the body. NIR-II fluorescence imaging overcomes the penetration/contrast bottleneck of imaging in the visible region, making it a remarkable modality for early diagnosis of cancer and highly sensitive tumor surgery. Due to their convenient bioconjugation with peptides/antibodies, NIR-II molecular dyes are desirable candidates for targeted cancer imaging, significantly overcoming the autofluorescence/scattering issues for deep tissue molecular imaging. To promote the clinical translation of NIR-II bioimaging, advancements in the high-performance small molecule-derived probes are critically important. Here, molecules with clinical potential for NIR-II imaging are discussed, summarizing the synthesis and chemical structures of NIR-II dyes, chemical and optical properties of NIR-II dyes, bioconjugation and biological behavior of NIR-II dyes, whole body imaging with NIR-II dyes for cancer detection and surgery, as well as NIR-II fluorescence microscopy imaging. A key perspective on the direction of NIR-II molecular dyes for cancer imaging and surgery is also discussed.

A targeted biocompatible organic nanoprobe for photoacoustic and near-infrared-II fluorescence imaging in living mice

Multimodal molecular imaging probes have attracted much attention, and they possess great potential to accurately diagnose diseases due to the synergistic superiorities of multiple complementary imaging. Herein, a targeted biocompatible organic nanoplatform (IR-PEG-FA) with a strong optical absorption in the near-infrared window (NIR-I) for photoacoustic imaging (PAI) and excellent second near-infrared (NIR-II) fluorescence imaging property for NIR-II imaging is fabricated. The dual-modal nanoprobe is composed of the small organic dye molecule IR-1061, water-soluble poly(ethylene glycol) (PEG) and folic acid (FA) as the targeted ligands. Depending on the strength of high temporal resolution and preeminent spatial resolution, the targeted biocompatible dual-mode nanoprobe for PAI and NIR-II imaging can provide more detailed date of cancers and diseases, and enables us to specifically diagnose them through quite a precise way.

We developed a targeted organic nanoprobe for both photoacoustic imaging and near-infrared-II fluorescence imaging in living mice.

Small-Molecule Lanthanide Complexes Probe for Second Near-Infrared Window Bioimaging

Over the past few years, significant efforts have been made to create new fluorescent probes operating at longer wavelengths, particularly in the second near-infrared (NIR-II) window from 1000 to 1700 nm, offering enhanced tissue penetration compared to light in the visible and first near-infrared window (700-900 nm). However, most of the reported NIR-II fluorophores meet such dilemmas; they are excreted slowly and largely retained within the reticuloendothelial system. Here, we report a rapidly excreted NIR-II lanthanide complex Nd-DOTA (over 50% excreted through the kidneys within 3 h postinjection) with a molecular mass only 0.54 kDa. The NIR-II imaging quality of Nd-DOTA was far superior to that of clinically approved ICG with good photostability and deep tissue penetration (7 mm). Superior tumor-to-normal tissue ratio was successfully achieved to facilitate the abdominal ovarian metastases surgical delineation. Metastases with ≤1 mm can be completely excised under NIR-II bioimaging guidance. Significantly, since the Nd-DOTA structure is same to the clinically approved magnetic resonance imaging (MRI) contrast Gd-DOTA, it will speed up the clinical translation for this novel kind of NIR-II probes in the future.

An Efficient 1064 nm NIR-II Excitation Fluorescent Molecular Dye for Deep-Tissue High-Resolution Dynamic Bioimaging

A small-molecule fluorophore FD-1080 with both excitation and emission in the NIR-II region has been successfully synthesized for in vivo imaging. A heptamethine structure is designed to shift the absorption and emission into NIR-II region. Sulphonic and cyclohexene groups are introduced to enhance its water solubility and stability. The quantum yield of FD-1080 is 0.31 %, and can be increased to 5.94 % after combining with fetal bovine serum (FBS). Significantly, 1064 nm NIR-II excitation was demonstrated with the high tissue penetration depth and superior imaging resolution compared to previously reported NIR excitation from 650 nm to 980 nm. FD-1080 is not only capable of realizing non-invasive high-resolution deep-tissue hindlimb vasculature and brain vessel bioimaging, but also quantifying the respiratory rate based on the dynamic imaging of respiratory craniocaudal motion of the liver for the awake and anaesthetized mouse.

3D NIR-II Molecular Imaging Distinguishes Targeted Organs with High-Performance NIR-II Bioconjugates

Greatly reduced scattering in the second near-infrared (NIR-II) region (1000-1700 nm) opens up many new exciting avenues of bioimaging research, yet NIR-II fluorescence imaging is mostly implemented by using nontargeted fluorophores or wide-field imaging setups, limiting the signal-to-background ratio and imaging penetration depth due to poor specific binding and out-of-focus signals. A newly developed high-performance NIR-II bioconjugate enables targeted imaging of a specific organ in the living body with high quality. Combined with a home-built NIR-II confocal set-up, the enhanced imaging technique allows 900 µm-deep 3D organ imaging without tissue clearing techniques. Bioconjugation of two hormones to nonoverlapping NIR-II fluorophores facilitates two-color imaging of different receptors, demonstrating unprecedented multicolor live molecular imaging across the NIR-II window. This deep tissue imaging of specific receptors in live animals allows development of noninvasive molecular imaging of multifarious models of normal and neoplastic organs in vivo, beyond the traditional visible to NIR-I range. The developed NIR-II fluorescence microscopy will become a powerful imaging technique for deep tissue imaging without any physical sectioning or clearing treatment of the tissue.

Zwitterionic diketopyrrolopyrrole for fluorescence/photoacoustic imaging guided photodynamic/photothermal therapy

Water-soluble zwitterionic diketopyrrolopyrrole (DPP-SPMA) for fluorescence/photoacoustic imaging guided photodynamic/photothermal therapy with favorable renal excretion and ultralow cytotoxicity.

Flavylium Polymethine Fluorophores for Near- and Shortwave Infrared Imaging

Bright fluorophores in the near-infrared and shortwave infrared (SWIR) regions of the electromagnetic spectrum are essential for optical imaging in vivo. In this work, we utilized a 7-dimethylamino flavylium heterocycle to construct a panel of novel red-shifted polymethine dyes, with emission wavelengths from 680 to 1045 nm. Photophysical characterization revealed that the 1- and 3-methine dyes display enhanced photostability and the 5- and 7-methine dyes exhibit exceptional brightness for their respective spectral regions. A micelle formulation of the 7-methine facilitated SWIR imaging in mice. This report presents the first polymethine dye designed and synthesized for SWIR in vivo imaging.

Rational Design of Molecular Fluorophores for Biological Imaging in the NIR-II Window

A new design for second near-infrared window (NIR-II) molecular fluorophores based on a shielding unit-donor-acceptor-donor-shielding unit (S-D-A-D-S) structure is reported. With 3,4-ethylenedioxy thiophene as the donor and fluorene as the shielding unit, the best performance fluorophores IR-FE and IR-FEP exhibit an emission quantum yield of 31% in toluene and 2.0% in water, respectively, representing the brightest organic dyes in NIR-II region reported so far.

Traumatic Brain Injury Imaging in the Second Near-Infrared Window with a Molecular Fluorophore

Traumatic brain injury (TBI) is a leading cause of death and disability worldwide. A bright, renal-excreted, and biocompatible near-infrared II fluorophore for in vivo imaging of TBI is designed. A transient hypoperfusion in the injured cerebral region, followed by fluorophore leakage, is observed. NIR-II fluorophores can provide noninvasive assessment of TBI.

Significant metal enhanced fluorescence of Ag2S quantum dots in the second near-infrared window

The amplification of light in NIR-II from Ag2S QDs via metal enhanced fluorescence (MEF) is reported for the first time. Significant fluorescence enhancement of over 100 times for Ag2S QDs deposited on Au-nanostructured arrays, paves the way for novel sensing and imaging applications based on Ag2S QDs, with improved detection sensitivity and contrast enhancement.

Engineering Lysosome-Targeting BODIPY Nanoparticles for Photoacoustic Imaging and Photodynamic Therapy under Near-Infrared Light

Developing lysosome-targeting organic nanoparticles combined with photoacoustic imaging (PAI) and photodynamic therapy (PDT) functions toward personalized medicine are highly desired yet challenging. Here, for the first time, lysosome-targeting BODIPY nanoparticles were engineered by encapsulating near-infrared (NIR) absorbed BODIPY dye within amphiphilic DSPE-mPEG5000 for high-performing lysosomal PAI and acid-activatable PDT against cancer cells under NIR light.

A Three-Photon Active Organic Fluorophore for Deep Tissue Ratiometric Imaging of Intracellular Divalent Zinc

Deep tissue bioimaging with three-photon (3P) excitation using near-infrared (NIR) light in the second IR window (1.0-1.4 μm) could provide high resolution images with an improved signal-to-noise ratio. Herein, we report a photostable and nontoxic 3P excitable donor-π-acceptor system (GMP) having 3P cross-section (σ3 ) of 1.78×10(-80)  cm(6)  s(2)  photon(-2) and action cross-section (σ3 η3 ) of 2.31×10(-81)  cm(6)  s(2)  photon(-2) , which provides ratiometric fluorescence response with divalent zinc ions in aqueous conditions. The probe signals the Zn(2+) binding at 530 and 600 nm, respectively, upon 1150 nm excitation with enhanced σ3 of 1.85×10(-80)  cm(6)  s(2)  photon(-2) and σ3 η3 of 3.33×10(-81)  cm(6)  s(2)  photon(-2) . The application of this probe is demonstrated for ratiometric 3P imaging of Zn(2+) in vitro using HuH-7 cell lines. Furthermore, the Zn(2+) concentration in rat hippocampal slices was imaged at 1150 nm excitation after incubation with GMP, illustrating its potential as a 3P ratiometric probe for deep tissue Zn(2+) ion imaging.

Biodegradable and crosslinkable PPF-PLGA-PEG self-assembled nanoparticles dual-decorated with folic acid ligands and rhodamine B fluorescent probes for targeted cancer imaging

Novel biodegradable and crosslinkable copolymers of hydrophobic poly(propylene fumarate)-co-poly(lactic-co-glycolic acid) (PPF-PLGA) linked with hydrophilic poly(ethylene glycol) (PEG), namely PPF-PLGA-PEG, were developed and fabricated into core-shell nanoparticles through self-assembly and photocrosslinking. A fluorescent probe, rhodamine B (RhB), was conjugated to the end of the copolymer chain (PPF-PLGA-PEG-RhB), which allows tracking of the nanoparticles through visualizing the fluorescence probe. Folic acid (FA) ligand was conjugated to another series of chains (PPF-PLGA-PEG-FA) for targeted delivery of the nanoparticles to the tumor sites by binding to the ubiquitously overexpressed FA receptors on tumor cells. Our results showed that PPF-PLGA-PEG nanoparticles incorporated with RhB fluorescence probes and FA tumor binding ligands have specific cancer cell targeting and imaging abilities. These crosslinkable nanoparticles are potentially useful to serve as a platform for conjugation of fluorescence probes as well as various antibodies and peptides for cancer targeted imaging or drug delivery.

Biodegradable nanoellipsoidal artificial antigen presenting cells for antigen specific T-cell activation

Non-spherical nanodimensional artificial antigen presenting cells (naAPCs) offer the potential to systemically induce an effective antigen-specific immune response. In this report it is shown biodegradable ellipsoidal naAPCs mimic the T-Cell/APC interaction better than equivalent spherical naAPCs. In addition, it is demonstrated ellipsoidal naAPCs offer reduced non-specific cellular uptake and a superior pharmacokinetic profile compared to spherical naAPCs.

Recombinant protein (EGFP-Protein G)-coated PbS quantum dots for in vitro and in vivo dual fluorescence (visible and second-NIR) imaging of breast tumors

We report a one-step synthetic strategy for the preparation of recombinant protein (EGFP-Protein G)-coated PbS quantum dots for dual (visible and second-NIR) fluorescence imaging of breast tumors at the cellular and whole-body level.

1D polymeric platinum cyanoximate: a strategy toward luminescence in the near-infrared region beyond 1000 nm

We report the synthesis and properties of the first representative of a new class of PtL2 complexes with ambidentate mixed-donor cyanoxime ligands [L = 2-cyano-2-oximino-N,N'-diethylaminoacetamide, DECO (1)]. Three differently colored polymorphs of "Pt(DECO)2" (3-5) were isolated, with the first two being crystallographically characterized. The dark-green complex [Pt(DECO)2]n (5) spontaneously forms in aqueous solution via aggregation of yellow monomeric complex 3 into the red dimer [Pt(DECO)2]2 (4), followed by further oligomerization into coordination polymer 5. A spectroscopic and light-scattering study revealed a "poker-chips"-type 1D polymeric structure of 5 in which units are held by noncovalent metallophilic interactions, forming a Pt---Pt wire. The polymer 5 shows a broad absorption at 400-900 nm and emission at unusually long wavelengths in the range of 1000-1100 nm in the solid state. The near-infrared (NIR) emission of polymer 5 is due to the formation of a small amount of nonstoichiometric mixed-valence Pt(II)/Pt(IV) species during synthesis. A featureless electron paramagnetic resonance spectrum of solid sample 5 recorded at +23 and -193 °C evidences the absence of Pt(III) states, and the compound represents a "solid solution" containing mixed-valence Pt(II)/Pt(IV) centers. Exposure of KBr pellets with 5% 5 to Br2 vapors leads to an immediate ∼30% increase in the intensity of photoluminescence at 1024 nm, which confirms the role and importance of mixed-valence species for the NIR emission. Thus, the emission is further enhanced upon additional oxidation of Pt(II) centers, which improves delocalization of electrons along the Pt---Pt vector. Other polymorph of the "Pt(DECO)2" complex--monomer--did not demonstrate luminescent properties in solutions and the solid state. An excitation scan of 5 embedded in KBr tablets revealed an emission only weakly dependent on the wavelength of excitation. The NIR emission of quasi-1D complex 5 was studied in the range of -193 to +67 °C. Data showed a blue shift of λmax and a simultaneous increase in the emission line intensity with a temperature rise, which is explained by analogy with similar behavior of known quasi-1D K2[Pt(CN)4]-based solids, quantum dots, and quantum wells with delocalized carriers. The presented finding opens a route to a new class of platinum cyanoxime based NIR emissive complexes that could be used in the design of novel NIR emitters and imaging agents.

In vitro nucleus nanoprobe with ultra-small polyethylenimine functionalized graphene quantum dots

Ultra-small polyethylenimine functionalized graphene quantum dots with unique nucleus labeling property are reported.