Nonlinear Photoacoustic Imaging by in situ Multiphoton Upconversion and Energy Transfer

Bioimaging with Upconversion Nanoparticles

Bioimaging enables the spatiotemporal visualization of biological processes at various scales empowered by a range of different imaging modalities and contrast agents. Upconversion nanoparticles (UCNPs) represent a distinct type of such contrast agents with the potential to transform bioimaging due to their unique optical properties and functional design flexibilities. This review explores and discusses the opportunities, challenges, and limitations that UCNPs exhibit as bioimaging probes and highlights applications with spatial dimensions ranging from the single nanoparticle level to cellular, tissue, and whole animal imaging. We further summarized recent advancements in bioimaging applications enabled by UCNPs, including super-resolution techniques and multimodal imaging methods, and provide a perspective on the future potential of UCNP-based technologies in bioimaging research and clinical translation. This review may provide a valuable resource for researchers interested in exploring and applying UCNP-based bioimaging technologies.

In-depth insight into the Yb3+ effect in NaErF4-based host sensitization upconversion: a double-edged sword

NaErF₄ is the most extensively studied host for self-sensitized upconversion (UC), and Yb³⁺ is the most commonly used energy absorber. It has been reported that the red luminescence of Er³⁺ can be enhanced by introducing Yb³⁺ into the NaErF₄ host lattice, where Yb³⁺ ions serve as trapping centers to confine the excitation energy. Also, it has been pointed out that the Yb³⁺ doping in the shell of NaErF₄-hosted core-shell nanocrystals can further improve the red emission intensity. Conversely, it can be argued that the Yb³⁺ doping in the shell always results in the luminescence quenching of the NaErF₄ core. These imply that the impact of Yb³⁺ on NaErF₄-based host-sensitized UC is rather complicated and must be probed deeply. In this study, we thoroughly discussed the effect of Yb³⁺ located in the core/shell on the NaErF₄-based host sensitization UC and afforded the related mechanism interpretations. In the NaErF₄ core nanocrystals, the green-dominated UCL presented an enhancement on increasing the concentration of the Yb³⁺ dopant owing to the promoted energy harvesting for luminescence. Furthermore, the emission properties of NaErF₄:10%Yb shelled with diverse inert layers were also investigated, and the intensity difference of these core-inert shell nanoparticles could be explained by the lattice mismatch and shell thickness. In NaErF₄:10%Yb@NaYF₄:Yb with variable Yb³⁺ doping in the shell, the red-dominated UCL was generally weakened with more Yb³⁺ localized in the shell, which was ascribed to the competition of energy pooling and energy dissipation of Yb³⁺ in the outer layer. Therefore, Yb³⁺ ions wield a two-sided influence (termed a "double-edged sword") on the UC emissions of the Er³⁺ host. Additionally, we demonstrated the application potential of such UCNPs in water sensing and high-level anti-counterfeiting. This work offers an in-depth insight into the UC mechanism of Yb³⁺-doped NaErF₄ nanocrystals and inspires the engineering of novel luminescent materials with distinguished properties.

Physically stimulus-responsive nanoparticles for therapy and diagnosis

Nanoparticles offer numerous advantages in various fields of science, particularly in medicine. Over recent years, the use of nanoparticles in disease diagnosis and treatments has increased dramatically by the development of stimuli-responsive nano-systems, which can respond to internal or external stimuli. In the last 10 years, many preclinical studies were performed on physically triggered nano-systems to develop and optimize stable, precise, and selective therapeutic or diagnostic agents. In this regard, the systems must meet the requirements of efficacy, toxicity, pharmacokinetics, and safety before clinical investigation. Several undesired aspects need to be addressed to successfully translate these physical stimuli-responsive nano-systems, as biomaterials, into clinical practice. These have to be commonly taken into account when developing physically triggered systems; thus, also applicable for nano-systems based on nanomaterials. This review focuses on physically triggered nano-systems (PTNSs), with diagnostic or therapeutic and theranostic applications. Several types of physically triggered nano-systems based on polymeric micelles and hydrogels, mesoporous silica, and magnets are reviewed and discussed in various aspects.

Exploiting Nanomaterials for Optical Coherence Tomography and Photoacoustic Imaging in Nanodentistry

There is already a societal awareness of the growing impact of nanoscience and nanotechnology, with nanomaterials (with at least one dimension less than 100 nm) now incorporated in items as diverse as mobile phones, clothes or dentifrices. In the healthcare area, nanoparticles of biocompatible materials have already been used for cancer treatment or bioimaging enhancement. Nanotechnology in dentistry, or nanodentistry, has already found some developments in dental nanomaterials for caries management, restorative dentistry and orthodontic adhesives. In this review, we present state-of-the-art scientific development in nanodentistry with an emphasis on two imaging techniques exploiting nanomaterials: optical coherence tomography (OCT) and photoacoustic imaging (PAI). Examples will be given using OCT with nanomaterials to enhance the acquired imaging, acting as optical clearing agents for OCT. A novel application of gold nanoparticles and nanorods for imaging enhancement of incipient occlusal caries using OCT will be described. Additionally, we will highlight how the OCT technique can be properly managed to provide imaging with spatial resolution down to 10's-100's nm resolution. For PAI, we will describe how new nanoparticles, namely TiN, prepared by femtosecond laser ablation, can be used in nanodentistry and will show photoacoustic microscopy and tomography images for such exogenous agents.

Nonlinear Photonics Using Low-Dimensional Metal-Halide Perovskites: Recent Advances and Future Challenges

Low-dimensional metal-halide perovskites have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor counterparts, thanks to their peculiar physical and electronic structures. Their exceptional nonlinear optical characteristics make them excellent candidates for revolutionizing widespread applications. However, the research of nonlinear photonics based on low-dimensional metal-halide perovskites is in its infancy. There is a lack of comprehensive and in-depth summary of this research realm. Here, the state-of-the-art research progress related to third-and higher-order nonlinear optical properties of low-dimensional metal-halide perovskites with diverse crystal structures from 3D down to 0D, together with their practical applications, is summarized comprehensively. Critical discussions are offered on the fundamental mechanisms beneath their exceptional nonlinear optical performance from the physics viewpoint, attempting to disclose the role of intrinsic attributes (e.g., composition, bandgap, size, shape, and structure) and external modulation strategies (e.g., developing core-shell structures, transition metal ion doping, and hybridization with dielectric microspheres) in tuning the response. Additionally, their potential applications in nonlinear photonics, nonlinear optoelectronics, and biophotonics are systematically and thoroughly summed up and categorized. Lastly, insights into the current technical challenges and future research opportunities of nonlinear photonics based on low-dimensional metal-halide perovskites are provided.

Aza-BODIPY-Based Nanomedicines in Cancer Phototheranostics

Cancer phototheranostics, composed of optical diagnosis and phototherapy (including photodynamic therapy and photothermal therapy), is a promising strategy for precise tumor treatment. Due to the unique properties of near-infrared absorption/emission, high reactive oxygen species generation, and photothermal conversion efficiency, aza-boron-dipyrromethene (aza-BODIPY), as an emerging organic photosensitizer, has shown great potential for tumor phototheranostics. By encapsulating aza-BODIPY photosensitizers within functional amphiphilic polymers, we can afford hydrophilic nanomedicines that selectively target tumor sites via an enhanced permeability and retention effect, thereby efficiently improving diagnosis and therapeutic efficacy. Herein, in this spotlight article, we attempt to highlight our recent contributions in the development of aza-BODIPY-based nanomedicines, which comprises three main sections: (1) to elucidate the design strategy of aza-BODIPY photosensitizers and corresponding nanomedicines; (2) to overview their photophysical properties and biomedical applications in phototheranostics, including fluorescence imaging, photoacoustic imaging, photodynamic therapy, photothermal therapy, and synergistic therapy; and (3) to depict the challenges and future perspectives of aza-BODIPY nanomedicines. It is believed that this Spotlight on Applications article would illuminate the way of developing new aza-BODIPY nanomedicines as well as other organic photosensitizer-based nanomedicines for future clinical translation.

Neodymium-Sensitized Nanoconstructs for Near-Infrared Enabled Photomedicine

Neodymium (Nd³⁺ )-sensitized nanoconstructs have gained increasing attention in recent decades due to their unique properties, especially optical properties. The design of various Nd³⁺ -sensitized nanosystems is expected to contribute to medical and health applications, due to their advantageous properties such as high penetration depth, excellent photostability, non-photobleaching, low cytotoxicity, etc. However, the low conversion efficiency and potential long-term toxicity of Nd³⁺ -sensitized nanoconstructs are huge obstacles to their clinical translations. This review article summarizes three energy transfer pathways of all kinds of Nd³⁺ -sensitized nanoconstructs focusing on the properties of Nd³⁺ ions and discusses their recent potential applications as near-infrared (NIR) enabled photomedicine. This review article will contribute to the design and fabrication of novel Nd³⁺ -sensitized nanoconstructs for NIR-enabled photomedicine, aiming for potentially safer and more efficient designs to get closer to clinical usage.

A Conformationally Restricted Aza-BODIPY Platform for Stimulus-Responsive Probes with Enhanced Photoacoustic Properties

Photoacoustic (PA) dyes, which absorb near-infrared (NIR) light to generate an ultrasonic signal, can be detected at centimeter depths in tissues with significantly higher resolution than dyes imaged with fluorescence-based methods. As such, PA agents show great promise as research tools for the study of live-animal disease models. However, the development of activatable PA probes has been hampered by the relative scarcity of appropriate PA-active molecular platforms with properties that can be manipulated in a rational manner. Herein we synthesized and evaluated six modifications to the aza-BODIPY dye platform with respect to their absorbance, fluorescence, and PA properties. We identified a promising conformationally restricted aza-BODIPY (CRaB) scaffold that prioritizes three criteria necessary for the design of stimulus-responsive dyes with optimal ratiometric PA response: absorbance at NIR wavelengths, strong PA intensity, and large Δλ upon interaction with the desired stimulus. Using this scaffold, we synthesized three chemically diverse stimulus-responsive PA probes and demonstrated between 2- and 8-fold improvements in theoretical ratiometric response in vitro. This suggests that improvements in PA parameters are generalizable. Finally, we validated the in vitro turnover of each CRaB PA probe and demonstrated the in vivo potential of the CRaB scaffold by direct comparison to an established hypoxia-responsive probe for the detection of tumor hypoxia.

Switchable up-conversion luminescence bioimaging and targeted photothermal ablation in one core–shell-structured nanohybrid by alternating near-infrared light

Upon NIR irradiation, a GdOF:Yb/Er@(GNRs@BSA)-FA nanohybrid was expected to be a potential multifunctional imaging tracer and photothermal ablation agent switched controllably for cancer theranostics.

Molecular Photoacoustic Contrast Agents: Design Principles & Applications

Photoacoustic imaging (PAI) is a rapidly growing field which offers high spatial resolution and high contrast for deep-tissue imaging in vivo. PAI is nonionizing and noninvasive and combines the optical resolution of fluorescence imaging with the spatial resolution of ultrasound imaging. In particular, the development of exogenous PA contrast agents has gained significant momentum of late with a vastly expanding complexity of dye materials under investigation ranging from small molecules to macromolecular proteins, polymeric and inorganic nanoparticles. The goal of this review is to survey the current state of the art in molecular photoacoustic contrast agents (MPACs) for applications in biomedical imaging. The fundamental design principles of MPACs are presented and a review of prior reports spanning from early-to-current literature is put forth.

NIR induced modulation of the red emission from erbium ions for selective lanthanide imaging

Upon direct excitation with green light (522 nm), Er³⁺ ion doped nanoparticles feature a number of radiative and non-radiative decay pathways, leading to distinct and sharp emission lines in the visible and near-infrared (NIR) range. Here we apply, in addition to continuous 522 nm irradiation, a modulated NIR irradiation (1143 nm) to actively control and modulate the red emission intensity (around 650 nm). The modulation of red Er³⁺ ion emission at a chosen frequency allows us to reconstruct fluorescence images from the Fourier transform amplitude at this particular frequency. Since only the emission from the Er³⁺ ion is modulated, it allows to selectively recover the lanthanide specific signal, removing any non-modulated auto-fluorescence or background emission resulting from the continuous 522 nm excitation. The modulated emission of specific lanthanides can open up new detection opportunities for selective signal recovery.

Stable ICG-loaded upconversion nanoparticles: silica core/shell theranostic nanoplatform for dual-modal upconversion and photoacoustic imaging together with photothermal therapy

We report here the design and multiple functions of a new hierarchical nanotheronostic platform consisting of an upconversion nanoparticle (UCNP) core: shell with an additional mesoporous silica (mSiO2) matrix load shell containing sealed, high concentration of ICG molecules. We demonstrate that this UCNP@mSiO2-ICG nanoplatform can perform the following multiple functions under NIR excitation at 800 nm: 1) Light harvesting by the UCNP shell containing Nd and subsequent energy transfer to Er in the Core to produce efficient green and red upconversion luminescence for optical imaging; 2) Efficient nonradiative relaxation and local heating produced by concentration quenching in aggregated ICG imbedded in the mesopourous silica shell to enable both photoacoustic imaging and photothermal therapy. Compared to pure ICG, sealing of mesoporous silica platforms prevents the leak-out and improves the stability of ICG by protecting from rapid hydrolysis. Under 800 nm laser excitation, we performed both optical and photoacoustic (PA) imaging in vitro and in vivo. Our results demonstrated that UCNP@mSiO2-ICG with sealed structures could be systemically delivered to brain vessels, with a long circulation time. In addition, these nanoplatforms were capable of producing strong hyperthermia efforts to kill cancer cells and hela cells under 800 nm laser irradiation.