High-Performance Near-Infrared Fluorescent Secondary Antibodies for Immunofluorescence

Steric protection of near-infrared fluorescent dyes for enhanced bioimaging

Near-fluorescent (NIR) dyes that absorb and emit light in the wavelength range of 650-1700 nm are well-suited for bioimaging due to the improved image contrast and increased penetration of the long-wavelength light through biological tissue. However, the imaging performance of NIR fluorescent dyes is limited by several inherent photophysical and physicochemical properties including, low fluorescence quantum yield, high chemical and photochemical reactivity, propensity to self-aggregate in water, non-specific association with off-target biological sites, and non-optimal pharmacokinetic profiles in living subjects. In principle, all these drawbacks can be alleviated by steric protection which is a structural process that surrounds the fluorophore with bulky groups that block undesired intermolecular interactions. The literature methods to sterically protect a long-wavelength dye can be separated into two general strategies, non-covalent dye encapsulation and covalent steric appendage. Illustrative examples of each method show how steric protection improves bioimaging performance by providing: (a) increased fluorescence brightness, (b) higher fluorophore ground state stability, (c) decreased photobleaching, and (d) superior pharmacokinetic profile. Some sterically protected dyes are commercially available and further success with future systems will require experts in chemistry, microscopy, cell biology, medical imaging, and clinical medicine to work closely as interdisciplinary teams.

A strategy to enhance the water solubility of luminescent β-diketonate-Europium(III) complexes for time-gated luminescence bioassays

Luminescent β-diketonate-europium(III) complexes have been found a wide range of applications in time-gated luminescence (TGL) bioassays, but their poor water solubility is a main problem that limits their effective uses. In this work we propose a simple and general strategy to enhance the water solubility of luminescent β-diketonate-europium(III) complexes that permits facile synthesis and purification. By introducing the fluorinated carboxylic acid group into the structures of β-diketone ligands, two highly water-soluble and luminescent Eu3+ complexes, PBBHD-Eu3+ and CPBBHD-Eu3+, were designed and synthesized. An excellent solubility exceeding 20 mg/mL for PBBHD-Eu3+ was found in a pure aqueous buffer, while it also displayed strong and long-lived luminescence (quantum yield φ = 26%, lifetime τ = 0.49 ms). After the carboxyl groups of PBBHD-Eu3+ were activated, the PBBHD-Eu3+-labeled streptavidin-bovine serum albumin (SA-BSA) conjugate was prepared, and successfully used for the immunoassay of human α-fetoprotein (AFP) and the imaging of an environmental pathogen Giardia lamblia under TGL mode, which demonstrated the practicability of PBBHD-Eu3+ for highly sensitive TGL bioassays. The carboxyl groups of PBBHD can also be easily derivatized with other reactive chemical groups, which enables PBBHD-Eu3+ to meet diverse requirements of biolabeling technique, to provide new opportunities for developing functional europium(III) complex biolabels serving for TGL bioassays.

Progress in Procalcitonin Detection Based on Immunoassay

Procalcitonin (PCT) serves as a crucial biomarker utilized in diverse clinical contexts, including sepsis diagnosis and emergency departments. Its applications extend to identifying pathogens, assessing infection severity, guiding drug administration, and implementing theranostic strategies. However, current clinical deployed methods cannot meet the needs for accurate or real-time quantitative monitoring of PCT. This review aims to introduce these emerging PCT immunoassay technologies, focusing on analyzing their advantages in improving detection performances, such as easy operation and high precision. The fundamental principles and characteristics of state-of-the-art methods are first introduced, including chemiluminescence, immunofluorescence, latex-enhanced turbidity, enzyme-linked immunosorbent, colloidal gold immunochromatography, and radioimmunoassay. Then, improved methods using new materials and new technologies are briefly described, for instance, the combination with responsive nanomaterials, Raman spectroscopy, and digital microfluidics. Finally, the detection performance parameters of these methods and the clinical importance of PCT detection are also discussed.

Safety assessment of fluorescently labeled anti-EGFR Nanobodies in healthy dogs

Introduction: Surgical resection is one of the main treatment options for several types of cancer, the desired outcome being complete removal of the primary tumor and its local metastases. Any malignant tissue that remains after surgery may lead to relapsing disease, negatively impacting the patient's quality of life and overall survival. Fluorescence imaging in surgical oncology aims to facilitate full resection of solid tumors through the visualization of malignant tissue during surgery, following the administration of a fluorescent contrast agent. An important class of targeting molecules are Nanobodies® (Nbs), small antigen-binding fragments derived from camelid heavy chain only antibodies. When coupled with a fluorophore, Nbs can bind to a specific receptor and demarcate tumor margins through a fluorescence camera, improving the accuracy of surgical intervention. A widely investigated target for fluorescence-guided surgery is the epidermal growth factor receptor (EGFR), which is overexpressed in several types of tumors. Promising results with the fluorescently labeled anti-EGFR Nb 7D12-s775z in murine models motivated a project employing the compound in a pioneering study in dogs with spontaneous cancer. Methods: To determine the safety profile of the study drug, three healthy purpose-bred dogs received an intravenous injection of the tracer at 5.83, 11.66, and 19.47 mg/m2, separated by a 14-day wash-out period. Physical examination and fluorescence imaging were performed at established time points, and the animals were closely monitored between doses. Blood and urine values were analyzed pre- and 24 h post administration. Results: No adverse effects were observed, and blood and urine values stayed within the reference range. Images of the oral mucosa, acquired with a fluorescence imaging device (Fluobeam®), suggest rapid clearance, which was in accordance with previous in vivo studies. Discussion: These are the first results to indicate that 7D12-s775z is well tolerated in dogs and paves the way to conduct clinical trials in canine patients with EGFR-overexpressing spontaneous tumors.

Investigating Uncertainties in Single-Molecule Localization Microscopy Using Experimentally Informed Monte Carlo Simulation

Single-molecule localization microscopy (SMLM) enables the visualization of cellular nanostructures in vitro with sub-20 nm resolution. While substructures can generally be imaged with SMLM, the structural understanding of the images remains elusive. To better understand the link between SMLM images and the underlying structure, we developed a Monte Carlo (MC) simulation based on experimental imaging parameters and geometric information to generate synthetic SMLM images. We chose the nuclear pore complex (NPC), a nanosized channel on the nuclear membrane which gates nucleo-cytoplasmic transport of biomolecules, as a test geometry for testing our MC model. Using the MC model to simulate SMLM images, we first optimized our clustering algorithm to separate >106 molecular localizations of fluorescently labeled NPC proteins into hundreds of individual NPCs in each cell. We then illustrated using our MC model to generate cellular substructures with different angles of labeling to inform our structural understanding through the SMLM images obtained.

Doubly Strapped Zwitterionic NIR-I and NIR-II Heptamethine Cyanine Dyes for Bioconjugation and Fluorescence Imaging

Heptamethine cyanine dyes enable deep tissue fluorescence imaging in the near infrared (NIR) window. Small molecule conjugates of the benchmark dye ZW800-1 have been tested in humans. However, long-term imaging protocols using ZW800-1 conjugates are limited by their instability, primarily because the chemically labile C4'-O-aryl linker is susceptible to cleavage by biological nucleophiles. Here, we report a modular synthetic method that produces novel doubly strapped zwitterionic heptamethine cyanine dyes, including a structural analogue of ZW800-1, with greatly enhanced dye stability. NIR-I and NIR-II versions of these doubly strapped dyes can be conjugated to proteins, including monoclonal antibodies, without causing undesired fluorophore degradation or dye stacking on the protein surface. The fluorescent antibody conjugates show excellent tumor-targeting specificity in a xenograft mouse tumor model. The enhanced stability provided by doubly strapped molecular design will enable new classes of in vivo NIR fluorescence imaging experiments with possible translation to humans.

Method To Diversify Cyanine Chromophore Functionality Enables Improved Biomolecule Tracking and Intracellular Imaging

Heptamethine indocyanines are invaluable probes for near-infrared (NIR) imaging. Despite broad use, there are only a few synthetic methods to assemble these molecules, and each has significant limitations. Here, we report the use of pyridinium benzoxazole (PyBox) salts as heptamethine indocyanine precursors. This method is high yielding, simple to implement, and provides access to previously unknown chromophore functionality. We applied this method to create molecules to address two outstanding objectives in NIR fluorescence imaging. First, we used an iterative approach to develop molecules for protein-targeted tumor imaging. When compared to common NIR fluorophores, the optimized probe increases the tumor specificity of monoclonal antibody (mAb) and nanobody conjugates. Second, we developed cyclizing heptamethine indocyanines with the goal of improving cellular uptake and fluorogenic properties. By modifying both the electrophilic and nucleophilic components, we demonstrate that the solvent sensitivity of the ring-open/ring-closed equilibrium can be modified over a wide range. We then show that a chloroalkane derivative of a compound with tuned cyclization properties undergoes particularly efficient no-wash live cell imaging using organelle-targeted HaloTag self-labeling proteins. Overall, the chemistry reported here broadens the scope of accessible chromophore functionality, and, in turn, enables the discovery of NIR probes with promising properties for advanced imaging applications.

Sterically Shielded Hydrophilic Analogs of Indocyanine Green

A modular synthetic process enables two or four shielding arms to be appended strategically over the fluorochromes of near-infrared cyanine heptamethine dyes to create hydrophilic analogs of clinically approved indocyanine green. A key synthetic step is the facile substitution of a heptamethine 4'-Cl atom by a phenol bearing two triethylene glycol chains. The lead compound is a heptamethine dye with four shielding arms, and a series of comparative spectroscopy studies showed that the shielding arms (a) increased dye photostability and chemical stability and (b) inhibited dye self-aggregation and association with albumin protein. In mice, the dye cleared from the blood primarily through the renal pathway rather than the biliary pathway for ICG. This change in biodistribution reflects the much smaller hydrodynamic diameter of the shielded hydrophilic ICG analog compared to the 67 kDa size of the ICG/albumin complex. An attractive feature of versatile synthetic chemistry is the capability to systematically alter the dye's hydrodynamic diameter. The sterically shielded hydrophilic ICG dye platform is well-suited for immediate incorporation into dynamic contrast-enhanced (DCE) spectroscopy or imaging protocols using the same cameras and detectors that have been optimized for ICG.

Ratiometric fluorescence enzyme-linked immunosorbent assay based on carbon dots@SiO2@CdTe quantum dots with dual functionalities for alpha-fetoprotein

Molecular tags such as fluorophores are increasingly being replaced with nanoparticles thanks to their superior optical properties, substantial chemical stability, and stability against photobleaching. Herein, we innovatively constructed a new ratiometric fluorescence enzyme-linked immunosorbent assay (RF-ELISA) for the screening of alpha-fetoprotein (AFP) in early hepatocellular carcinoma in vitro diagnostics using carbon dots@SiO₂@CdTe quantum dots (CDs@SiO₂@CdTe QDs). Carbon dots with blue fluorescence were initially encapsulated into SiO₂ nanospheres through the typical Stöber method. Thereafter, CdTe QDs with red fluorescence were modified onto the surface of CDs@SiO₂ nanospheres. Dual-emission nanotags with blue and red fluorescent signals were utilized to design a RF-ELISA method for the determination of AFP on the anti-AFP capture antibody-coated microplate using glucose oxidase (GOx)-labeled anti-AFP secondary antibody. After the formation of the sandwiched immunocomplex, GOx catalyzed glucose to generate hydrogen peroxide (H₂O₂), which could quench the red fluorescence of CdTe QDs on the surface of nanotags. Meanwhile, the encapsulated carbon dots in the nanotags could still maintain the initial blue fluorescence intensity. The ratio between red fluorescence intensity and blue-emission intensity could be used for the quantitative monitoring of AFP concentration under optimum conditions. The experimental results indicated that CDs@SiO₂@CdTe QDs-based RF-ELISA could exhibit a good fluorescence signal with a dynamic linear range of 0.05-60 ng mL^-1 at a low detection limit of 8.7 pg mL^-1. Moreover, the fluorescence color of the solution including CDs@SiO₂@CdTe QDs changed from pink to purple to blue with the increasing AFP level when viewed by the naked eye. Good reproducibility, high specificity, and acceptable stability were achieved for the analysis of target AFP. Importantly, the accuracy of ratiometric fluorescence immunoassay was evaluated to determine human serum samples, giving well-matched results relative to commercially usable human AFP ELISA method.

A Series of PSMA-Targeted Near-Infrared Fluorescent Imaging Agents

We have synthesized a series of 10 new, PSMA-targeted, near-infrared imaging agents intended for use in vivo for fluorescence-guided surgery (FGS). Compounds were synthesized from the commercially available amine-reactive active NHS ester of DyLight800. We altered the linker between the PSMA-targeting urea moiety and the fluorophore with a view to improve the pharmacokinetics. Chemical yields for the conjugates ranged from 51% to 86%. The K_i values ranged from 0.10 to 2.19 nM. Inclusion of an N-bromobenzyl substituent at the ε-amino group of lysine enhanced PSMA⁺ PIP tumor uptake, as did hydrophilic substituents within the linker. The presence of a polyethylene glycol chain within the linker markedly decreased renal uptake. In particular, DyLight800-10 demonstrated high specific uptake relative to background signal within kidney, confirmed by immunohistochemistry. These compounds may be useful for FGS in prostate, renal or other PSMA-expressing cancers.

Application of CuS/Au Heterostructure with peroxidase-like activity in immunosensors

Early detection of tumour markers is essential for the timely treatment of cancer to realise the sensitive detection of Carcinoembryonic antigen (CEA). We chose electrochemical immunosensor as a simple analytical...

Comparison of cRGDfK Peptide Probes with Appended Shielded Heptamethine Cyanine Dye () for Near Infrared Fluorescence Imaging of Cancer

Previous work has shown that the sterically shielded near-infrared (NIR) fluorescent heptamethine cyanine dye, s775z, with a reactive carboxyl group produces fluorescent bioconjugates with an unsurpassed combination of high photostability and fluorescence brightness. This present contribution reports two new reactive homologues of s775z with either a maleimide group for reaction with a thiol or a strained alkyne group for reaction with an azide. Three cancer-targeting NIR fluorescent probes were synthesized, each with an appended cRGDfK peptide to provide selective affinity for integrin receptors that are overexpressed on the surface of many cancer cells including the A549 lung adenocarcinoma cells used in this study. A set of cancer cell microscopy and mouse tumor imaging experiments showed that all three probes were very effective at targeting cancer cells and tumors; however, the change in the linker structure produced a statistically significant difference in some aspects of the mouse biodistribution. The mouse studies included a mock surgical procedure that excised the subcutaneous tumors. A paired-agent fluorescence imaging experiment co-injected a binary mixture of targeted probe with 850 nm emission, an untargeted probe with 710 nm emission and determined the targeted probe's binding potential in the tumor tissue. A comparison of pixelated maps of binding potential for each excised tumor indicated a tumor-to-tumor variation of integrin expression levels, and a heterogeneous spatial distribution of integrin receptors within each tumor.

Generalizable synthesis of bioresponsive near-infrared fluorescent probes: sulfonated heptamethine cyanine prototype for imaging cell hypoxia

Continued advancement in bioresponsive fluorescence imaging requires new classes of activatable fluorescent probes that emit near-infrared fluorescence with wavelengths above 740 nm. Heptamethine cyanine dyes (Cy7) have suitable fluorescence properties but it is challenging to create activatable probes because Cy7 dyes have a propensity for self-aggregation and fluorescence quenching. A new synthetic strategy is employed to create a generalizable class of hydrophilic bioresponsive near-infrared fluorescent probes with appended sulfonates that provide excellent physiochemical properties. A prototype version is triggered by nitroreductase enzyme to undergo self-immolative cleavage with a large enhancement in fluorescence signal at 780 nm and the probe enables microscopic imaging of cell hypoxia with "turn on" fluorescence. Near-infrared fluorescence imaging of hypoxia is potentially useful in many different areas of biomedical research and clinical treatment.