Computed tomography for in vivo deep over-1000 nm near-infrared fluorescence imaging

Controlling Molecular Dye Encapsulation in the Hydrophobic Core of Core-Shell Nanoparticles for In Vivo Imaging

Polymeric nanoparticles with a hydrophobic core are valuable biomedical materials with potential applications in in vivo imaging and drug delivery. These materials are effective at protecting vulnerable molecules, enabling them to serve their functions in hydrophilic physiological environments; however, strategies that allow the chemical composition and molecular weight of polymers to be tuned, forming nanoparticles to control the functional molecules, are lacking. In this article, we review strategies for designing core-shell nanoparticles that enable the effective and stable encapsulation of functional molecules for biomedical applications. IR-1061, which changes its optical properties in response to the microenvironment are useful for in vitro screening of the in vivo stability of polymeric nanoparticles. An in vitro screening test can be performed by dispersing IR-1061-encapsulated polymer nanoparticles in water, saline, buffer solution, aqueous protein solution, etc., and measuring the absorption spectral changes. Through the screening, the effects of the polarity, molecular weight, and the chiral structure of polymers consisting of polymer nanoparticles on their stability have been revealed. Based on the findings presented here, more methodologies for the effective application of various biomolecules and macromolecules with complex high-dimensional structures are expected to be developed.

Three Dimensional Lifetime-Multiplex Tomography Based on Time-Gated Capturing of Near-Infrared Fluorescence Images

We report a computed tomography (CT) technique for mapping near-infrared fluorescence (NIRF) lifetime as a multiplex three-dimensional (3D) imaging method, using a conventional NIR camera. This method is achieved by using a time-gated system composed of a pulsed laser and an NIR camera synchronized with a rotatable sample stage for NIRF-CT imaging. The fluorescence lifetimes in microsecond-order of lanthanides were mapped on reconstructed cross-sectional and 3D images, via back-projection of two-dimensional projected images acquired from multiple angles at each time point showing fluorescence decay. A method to select slopes (the observed decay rates in time-gated imaging) used for the lifetime calculation, termed as the slope comparison method, was developed for the accurate calculation of each pixel, resulting in reduction of image acquisition time. Time-gated NIRF-CT provides a novel choice for multiplex 3D observation of deep tissues in biology.

Changes in the Secondary Structure and Assembly of Proteins on Fluoride Ceramic (CeF3) Nanoparticle Surfaces

Fluoride nanoparticles (NPs) are materials utilized in the biomedical field for applications including imaging of the brain. Their interactions with biological systems and molecules are being investigated, but the mechanism underlying these interactions remains unclear. We focused on possible changes in the secondary structure and aggregation state of proteins on the surface of NPs and investigated the principle underlying the changes using the amyloid β peptide (Aβ_16-20) based on infrared spectrometry. CeF₃ NPs (diameter 80 nm) were synthesized via thermal decomposition. Infrared spectrometry showed that the presence of CeF₃ NPs promotes the formation of the β-sheet structure of Aβ_16-20. This phenomenon was attributed to the hydrophobic interaction between NPs and Aβ peptides in aqueous environments, which causes the Aβ peptides to approach each other on the NP surface and form ordered hydrogen bonds. Because of the coexisting salts on the secondary structure and assembly of Aβ peptides, the formation of the β-sheet structure of Aβ peptides on the NP surface was suppressed in the presence of NH₄⁺ and NO₃^- ions, suggesting the possibility that Aβ peptides were adsorbed and bound to the NP surface. The formation of the β-sheet structure of Aβ peptides was promoted in the presence of NH₄⁺, whereas it was suppressed in the presence of NO₃^- because of the electrostatic interaction between the lysine residue of the Aβ peptide and the ions. Our findings will contribute to comparative studies on the effect of different NPs with different physicochemical properties on the molecular state of proteins.

Influence of the difference in refractive index on the interface of an object and the surroundings in near-infrared fluorescence tomography

The refraction of fluorescence from the inside of a sample at the surface results in fluctuations in fluorescence computed tomography (CT). We evaluated the influence of the difference in refractive index (RI) between the sample body and the surroundings on fluorescence CT results. The brightest fluorescent point is away from the correct point on the tomograms owing to the refraction. The speculated position is determined as the exact point if the RI ratio ranges between 0.97 and 1.03 by immersing the body in an RI matching liquid. The results can help in experimental settings of fluorescence CT for acquiring three-dimensional positional information.

The frontier of live tissue imaging across space and time

What you see is what you get-imaging techniques have long been essential for visualization and understanding of tissue development, homeostasis, and regeneration, which are driven by stem cell self-renewal and differentiation. Advances in molecular and tissue modeling techniques in the last decade are providing new imaging modalities to explore tissue heterogeneity and plasticity. Here we describe current state-of-the-art imaging modalities for tissue research at multiple scales, with a focus on explaining key tradeoffs such as spatial resolution, penetration depth, capture time/frequency, and moieties. We explore emerging tissue modeling and molecular tools that improve resolution, specificity, and throughput.

Effects of Processing pH on Emission Intensity of Over-1000 nm Near-Infrared Fluorescence of Dye-Loaded Polymer Micelle with Polystyrene Core

Fluorescence imaging using the over-thousand-nanometer (OTN) near-infrared (NIR) light is an emerging method for an in vivo imaging analysis of deep tissues without physical sectioning. Polymer micelle nanoparticles (PNPs) composed of organic polymers encapsulating an OTN-NIR fluorescent dye, IR-1061, in their hydrophobic core are expected to be biocompatible probes. Because IR-1061 quickly quenches due to the vibration of polar hydroxyl bonding in its surroundings, the influence of hydroxyl ions should be minimized. Herein, we investigated the effect of the hydrogen ion concentration during the preparation process using IR-1061 and an organic polymer, poly(ethylene glycol)-block-polystyrene (PEG-b-PSt), on the emission properties of the obtained OTN-PNPs. The OTN-PNP has a hydrodynamic diameter of 20 - 30 nm and emits 1110-nm fluorescence that is applicable to angiography. The loading efficiency of IR-1061 in the OTN-PNPs increased when prepared in an aqueous solution with a low hydroxyl ion concentration. In this solution (pH 3.0), highly emissive OTN-PNPs was obtained with IR-1061 at lower nominal concentrations. Decreasing the hydroxyl ion concentration during the preparation process yields highly emissive OTN-PNPs, which may improve the in vivo imaging analysis of biological phenomena in deep tissues.

Quo Vadis, Nanoparticle-Enabled In Vivo Fluorescence Imaging?

The exciting advancements that we are currently witnessing in terms of novel materials and synthesis approaches are leading to the development of colloidal nanoparticles (NPs) with increasingly greater tunable properties. We have now reached a point where it is possible to synthesize colloidal NPs with functionalities tailored to specific societal demands. The impact of this new wave of colloidal NPs has been especially important in the field of biomedicine. In that vein, luminescent NPs with improved brightness and near-infrared working capabilities have turned out to be optimal optical probes that are capable of fast and high-resolution in vivo imaging. However, luminescent NPs have thus far only reached a limited portion of their potential. Although we believe that the best is yet to come, the future might not be as bright as some of us think (and have hoped!). In particular, translation of NP-based fluorescence imaging from preclinical studies to clinics is not straightforward. In this Perspective, we provide a critical assessment and highlight promising research avenues based on the latest advances in the fields of luminescent NPs and imaging technologies. The disillusioned outlook we proffer herein might sound pessimistic at first, but we consider it necessary to avoid pursuing "pipe dreams" and redirect the efforts toward achievable-yet ambitious-goals.