Water-soluble dopamine-based polymers for photoacoustic imaging

Polydopamine-containing nano-systems for cancer multi-mode diagnoses and therapies: A review

Polydopamine (PDA) has fascinating properties such as inherent biocompatibility, simple preparation, strong near-infrared absorption, high photothermal conversion efficiency, and strong metal ion chelation, which have catalyzed extensive research in PDA-containing multifunctional nano-systems particularly for biomedical applications. Thus, it is imperative to overview synthetic strategies of various PDA-containing nanoparticles (NPs) for state-of-the-art cancer multi-mode diagnoses and therapies applications, and offer a timely and comprehensive summary. In this review, we will focus on the synthetic approaches of PDA NPs, and summarize the construction strategies of PDA-containing NPs with different structure forms. Additionally, the application of PDA-containing NPs in bioimaging such as photoacoustic imaging, fluorescence imaging, magnetic resonance imaging and other imaging modalities will be reviewed. We will especially offer an overview of their therapeutic applications in tumor chemotherapy, photothermal therapy, photodynamic therapy, photocatalytic therapy, sonodynamic therapy, radionuclide therapy, gene therapy, immunotherapy and combination therapy. At the end, the current trends, limitations and future prospects of PDA-containing nano-systems will be discussed. This review aims to provide guidelines for new scientists in the field of how to design PDA-containing NPs and what has been achieved in this area, while offering comprehensive insights into the potential of PDA-containing nano-systems used in cancer diagnosis and treatment.

Photochemically-driven highly efficient intracellular delivery and light/hypoxia programmable triggered cancer photo-chemotherapy

BACKGROUND

Using nanotechnology to improve the efficiency of tumor treatment represents a major research interest in recent years. However, there are paradoxes and obstacles in using a single nanoparticle to fulfill all the requirements of complex tumor treatment.

RESULTS

In this paper, a programmed-triggered nanoplatform (APP NPs), which is sequentially responsive to light and hypoxia, is rationally integrated for photoacoustic (PA) imaging-guided synergistic cancer photo-chemotherapy. The nanoplatform is constructed by in situ hybridization of dopamine monomer in the skeleton of PCN-224 and loading prodrug banoxantrone (AQ4N). Upon first-stage irradiation with a 660 nm laser, cellular internalization was effectively promoted by a photosensitizer-mediated photochemical effect. Furthermore, under second-stage irradiation, APP NPs exhibit a notably high photothermal conversion efficiency and sufficient reactive oxygen species (ROS) production for photothermal therapy (PTT) and photodynamic therapy (PDT), respectively, which not only triggers rapid intercellular drug release but also consequently aggravates tumor hypoxia levels, and aggravated hypoxia can further active the cytotoxicity of AQ4N for chemotherapy. Both in vitro and in vivo studies confirm that the dual-stage light guided photo-chemotherapy strategy exhibits a greatly enhanced anticancer effects and superior therapeutic safety.

CONCLUSION

This work represents a versatile strategy to construct a dual-stage light induced PDT/PTT and hypoxia-activated chemotherapy nanoplatform and will be promising for the development of multistimuli-responsive nanosystems with programmable functions for precise cancer therapy.

Synthesis of soluble melanin nanoparticles under acidic conditions using Burkholderia cepacia tyrosinase and their characterization

Melanin nanoparticles (MNPs) used for biomedical applications are often synthesized via the chemical auto-oxidation of catecholic monomers such as dopamine and 3,4-dihydroxyphenylalanine (DOPA) under alkaline conditions. However, the synthetic method for the chemical synthesis of MNP (cMNP) is relatively straightforward and more robust to control their homogenous particle size and morphology than the corresponding enzymatic synthetic methods. In this study, we demonstrated that the simple enzymatic synthesis of MNPs (eMNPs) with homogenous and soluble (<20 nm diameter) properties is possible using dopamine and Burkholderia cepacia tyrosinase (BcTy) under acidic conditions (i.e., pH 3.0). BcTy was highly reactive under pH 5.0, where the natural and chemical oxidation of catechol is complex, and thus melanin was synthesized via the hydroxylation of phenolic substrates. The detailed chemical analysis and characterization of the physical properties of the eMNPs confirmed the higher preservation of the catechol and primary amine moieties in the monomer substrate such as dopamine under acidic conditions. The eMNPs showed enhanced antioxidant activity and conferred stickiness to the formed hydrogel compared to the chemical auto-oxidation method owing to the large number of hydroxyl groups remaining such as catechol and quinone moieties. Because of these advantages and characteristics, the synthesis of MNPs using BcTy under acidic conditions can open a new path for their biomedical applications.

Melanin nanoparticles (MNPs) used for biomedical applications are often synthesized via the chemical auto-oxidation of catecholic monomers such as dopamine and 3,4-dihydroxyphenylalanine (DOPA) under alkaline conditions.

A Single Wavelength Mid-Infrared Photoacoustic Spectroscopy for Noninvasive Glucose Detection Using Machine Learning

According to the International Diabetes Federation, 530 million people worldwide have diabetes, with more than 6.7 million reported deaths in 2021. Monitoring blood glucose levels is essential for individuals with diabetes, and developing noninvasive monitors has been a long-standing aspiration in diabetes management. The ideal method for monitoring diabetes is to obtain the glucose concentration level with a fast, accurate, and pain-free measurement that does not require blood drawing or a surgical operation. Multiple noninvasive glucose detection techniques have been developed, including bio-impedance spectroscopy, electromagnetic sensing, and metabolic heat conformation. Nevertheless, reliability and consistency challenges were reported for these methods due to ambient temperature and environmental condition sensitivity. Among all the noninvasive glucose detection techniques, optical spectroscopy has rapidly advanced. A photoacoustic system has been developed using a single wavelength quantum cascade laser, lasing at a glucose fingerprint of 1080 cm-1 for noninvasive glucose monitoring. The system has been examined using artificial skin phantoms, covering the normal and hyperglycemia blood glucose ranges. The detection sensitivity of the system has been improved to ±25 mg/dL using a single wavelength for the entire range of blood glucose. Machine learning has been employed to detect glucose levels using photoacoustic spectroscopy in skin samples. Ensemble machine learning models have been developed to measure glucose concentration using classification techniques. The model has achieved a 90.4% prediction accuracy with 100% of the predicted data located in zones A and B of Clarke's error grid analysis. This finding fulfills the US Food and Drug Administration requirements for glucose monitors.

Polydopamine aggregation: A novel strategy for power-free readout of loop-mediated isothermal amplification integrated into a paper device for multiplex pathogens detection

Loop-mediated isothermal amplification (LAMP) has been widely used for detecting pathogens. However, power-free and clear visualization of results still remain challenging. In this study, we developed a paper device integrated with power-free DNA detection strategy realized by polydopamine aggregation. In the presence of DNA amplicons, the polymerization of dopamine into aggregated polydopamine was hindered, while in the absence of DNA amplicons, polydopamine aggregation is facilitated. The porosity of the paper enabled the capillary flow of dispersed polydopamine for positive sample, while aggregated polydopamine remained at the bottom of the paper strip due to large size of the aggregates for negative sample. Based on this mechanism, we fabricated a slidable paper device integrating LAMP with dopamine polymerization for the naked-eye detection, operated in a seamless manner. Moreover, the introduced paper device was successfully used to detect DNA extracted from Escherichia coli O157:H7 and SARS-CoV-2 within 25 min, as well as Enterococcus faecium within 35 min. The detection limits of both Escherichia coli O157:H7 and SARS-CoV-2 were 10^-4 ng/μL. The introduced paper device can be used as a simple and sensitive tool for detecting multiple infectious pathogens, making it an ideal tool particularly for resource-limited environment.

Phenolic-enabled nanotechnology: versatile particle engineering for biomedicine

Phenolics are ubiquitous in nature and have gained immense research attention because of their unique physiochemical properties and widespread industrial use. In recent decades, their accessibility, versatile reactivity, and relative biocompatibility have catalysed research in phenolic-enabled nanotechnology (PEN) particularly for biomedical applications which have been a major benefactor of this emergence, as largely demonstrated by polydopamine and polyphenols. Therefore, it is imperative to overveiw the fundamental mechanisms and synthetic strategies of PEN for state-of-the-art biomedical applications and provide a timely and comprehensive summary. In this review, we will focus on the principles and strategies involved in PEN and summarize the use of the PEN synthetic toolkit for particle engineering and the bottom-up synthesis of nanohybrid materials. Specifically, we will discuss the attractive forces between phenolics and complementary structural motifs in confined particle systems to synthesize high-quality products with controllable size, shape, composition, as well as surface chemistry and function. Additionally, phenolic's numerous applications in biosensing, bioimaging, and disease treatment will be highlighted. This review aims to provide guidelines for new scientists in the field and serve as an up-to-date compilation of what has been achieved in this area, while offering expert perspectives on PEN's use in translational research.

Bio-Applications of Multifunctional Melanin Nanoparticles: From Nanomedicine to Nanocosmetics

Bioinspired nanomaterials are ideal components for nanomedicine, by virtue of their expected biocompatibility or even complete lack of toxicity. Natural and artificial melanin-based nanoparticles (MNP), including polydopamine nanoparticles (PDA NP), excel for their extraordinary combination of additional optical, electronic, chemical, photophysical, and photochemical properties. Thanks to these features, melanin plays an important multifunctional role in the design of new platforms for nanomedicine where this material works not only as a mechanical support or scaffold, but as an active component for imaging, even multimodal, and simple or synergistic therapy. The number of examples of bio-applications of MNP increased dramatically in the last decade. Here, we review the most recent ones, focusing on the multiplicity of functions that melanin performs in theranostics platforms with increasing complexity. For the sake of clarity, we start analyzing briefly the main properties of melanin and its derivative as well as main natural sources and synthetic methods, moving to imaging application from mono-modal (fluorescence, photoacoustic, and magnetic resonance) to multi-modal, and then to mono-therapy (drug delivery, anti-oxidant, photothermal, and photodynamic), and finally to theranostics and synergistic therapies, including gene- and immuno- in combination to photothermal and photodynamic. Nanomedicine aims not only at the treatment of diseases, but also to their prevention, and melanin in nature performs a protective action, in the form of nanopigment, against UV-Vis radiations and oxidants. With these functions being at the border between nanomedicine and cosmetics nanotechnology, recently examples of applications of artificial MNP in cosmetics are increasing, paving the road to the birth of the new science of nanocosmetics. In the last part of this review, we summarize and discuss these important recent results that establish evidence of the interconnection between nanomedicine and cosmetics nanotechnology.

Croconaine-Based Polymer Particles as Contrast Agents for Photoacoustic Imaging

In the development and optimization of imaging methods, photoacoustic imaging (PAI) has become a powerful tool for preclinical biomedical diagnosis and detection of cancer. PAI probes can improve contrast and help identify pathogenic tissue. Such contrast agents must meet several requirements: they need to be biocompatible, and absorb strongly in the near-infrared (NIR) range, while relaxing the photoexcited state thermally and not radiatively. In this work, polymer nanoparticles are produced with croconaine as a monomer unit. Small molecular croconaine dyes are known to act as efficient pigments, which do not show photoluminescence. Here, for the first time croconaine copolymer nanoparticles are produced from croconic acid and a range of aromatic diamines. Following a dispersion polymerization protocol, this approach yields monodisperse particles of adjustable size. All synthesized polymers exhibit broad absorption within the NIR spectrum and therefore represent suitable candidates as contrast agents for PAI. The optical properties of these polymer particles are discussed with respect to the relation between particle size and outstanding photoacoustic performance. Biocompatibility of the polymer particles is demonstrated in cell viability experiments.

Melanin-based nanomaterials: The promising nanoplatforms for cancer diagnosis and therapy

Melanin-based nanoplatforms are biocompatible nanomaterials with a variety of unique physicochemical properties such as strong photothermal conversion ability, excellent drug binding capacity, strong metal chelation capacity, high chemical reactivity and versatile adhesion ability. These innate talents not only make melanin-based nanoplatforms be an inborn theranostic nanoagent for photoacoustic imaging-guided photothermal therapy of cancers, but also enable them to be conveniently transferred into cancer-targeting drug delivery systems and multimodality imaging nanoprobes. Due to the intriguing properties, melanin-based nanoplatforms have attracted much attention in investigations of cancer diagnosis and therapy. This review provides an overview of recent research advances in applications of melanin-based nanoplatforms in the fields of cancer diagnosis and therapy including cancer photothermal therapy, anticancer drug delivery, cancer-specific multimodal imaging and theranostics, etc. The remaining challenges and prospects of melanin-based nanoplatforms in biomedical applications are discussed at the end of this review.

Melanin-Like Nanomaterials for Advanced Biomedical Applications: A Versatile Platform with Extraordinary Promise

Developing efficient, sustainable, and biocompatible high-tech nanoplatforms derived from naturally existing components in living organisms is highly beneficial for diverse advanced biomedical applications. Melanins are nontoxic natural biopolymers owning widespread distribution in various biosystems, possessing fascinating physicochemical properties and playing significant physiological roles. The multifunctionality together with intrinsic biocompatibility renders bioinspired melanin-like nanomaterials considerably promising as a versatile and powerful nanoplatform with broad bioapplication prospects. This panoramic Review starts with an overview of the fundamental physicochemical properties, preparation methods, and polymerization mechanisms of melanins. A systematical and well-bedded description of recent advancements of melanin-like nanomaterials regarding diverse biomedical applications is then given, mainly focusing on biological imaging, photothermal therapy, drug delivery for tumor treatment, and other emerging biomedicine-related implementations. Finally, current challenges toward clinical translation with an emphasis on innovative design strategies and future striving directions are rationally discussed. This comprehensive and detailed Review provides a deep understanding of the current research status of melanin-like nanomaterials and is expected to motivate further optimization of the design of novel tailorable and marketable multifunctional nanoplatforms in biomedicine.

Noninvasive blood glucose detection using a quantum cascade laser

A Quantum Cascade Laser (QCL) was invented in the late 90s as a promising mid-infrared light source and it has contributed to the fields of industry, military, medicine, and biology. The room temperature operation, watt-level output power, compact size, and wide tuning capability of this laser advanced the field of noninvasive blood glucose detection with the use of transmission, absorption, and photoacoustic spectroscopy. This review provides a complete overview of the recent progress and technical details of these spectroscopy techniques, using QCL as an infrared light source for detecting blood glucose concentrations in diabetic patients.

Photoacoustic Detection of Superoxide Using Oxoporphyrinogen and Porphyrin

The superoxide (O₂^•-) ion is a highly reactive oxygen species involved in many diseases; hence, its noninvasive detection is desirable to identify the onset of pathological processes. Here, we employed photoacoustic (PA) spectroscopy, which enables imaging at ultrasound resolution with the sensitivity of optical modality, for the first time to detect O₂^•-, using stimuli-responsive contrast agents. meso-(3,5-Di-tert-butyl 4-hydroxyphenyl) porphyrins and oxoporphyrinogens were used as PA contrast agents, which trap the O₂^•- and enable its detection. The trapped O₂^•- increased the PA signal amplitude of chromophores up to 9.6-fold, and induced a red-shift in the PA signal maxima of up to 225 nm. Therefore, these trigger-responsive probes may be highly valuable as smart diagnostic PA probes to investigate pathological events stimulated by O₂^•- species.

Chain length-dependent luminescence in acceptor-doped conjugated polymers

Semiconducting polymers doped with a minority fraction of energy transfer acceptors feature a sensitive coupling between chain conformation and fluorescence emission, that can be harnessed for advanced solution-based molecular sensing and diagnostics. While it is known that chain length strongly affects chain conformation, and its response to external cues, the effects of chain length on the emission patterns in chromophore-doped conjugated polymers remains incompletely understood. In this paper, we explore chain-length dependent emission in two different acceptor-doped polyfluorenes. We show how the binomial distribution of acceptor incorporation, during the probabilistic polycondensation reaction, creates a strong chain-length dependency in the optical properties of this class of luminescent polymers. In addition, we also find that the intrachain exciton migration rate is chain-length dependent, giving rise to additional complexity. Both effects combined, make for the need to develop sensoric conjugated polymers of improved monodispersity and chemical homogeneity, to improve the accuracy of conjugated polymer based diagnostic approaches.

Silver-nanoparticles as plasmon-resonant enhancers for eumelanin's photoacoustic signal in a self-structured hybrid nanoprobe

Developing safe and high efficiency contrast tools is an urgent need to allow in vivo applications of photoacoustics (PA), an emerging biomolecular imaging methodology, with poor invasiveness, deep penetration, high spatial resolution and excellent endogenous contrast. Eumelanins hold huge promise as biocompatible, endogenous photoacoustic contrast agents. However, their huge potential is still unexplored due to the difficulty to achieve at the same time poor aggregation in physiologic environment and high PA contrast. This study addresses both issues through the design of a biocompatible photoacoustic nanoprobe, named MelaSil_Ag-NPs, relying on silica-templated eumelanin formation as well as eumelanins redox and metal chelating properties to reduce Ag⁺ ions and control the growth of generated metal nanoparticles. This strategy allowed self-structuring of the system into a core-shell architecture, where the Ag core was found to boost PA signal, despite the poor eumelanin content. Obtained hybrid nanoplatforms, showed stable photoacoustic properties even under long irradiation. Furthermore, conjugation with rhodamine isothiocyanate allowed particles detection through fluorescent imaging proving their multifunctional potentialities. In addition, they were stable towards aggregation and efficiently endocytosed by human pancreatic cancer cells (BxPC3 and Panc-1) displaying no significant cytotoxicity. Such numerous features prove huge potential of those nanoparticles as a multifunctional platform for biomedical applications.

Gadolinium Doping Enhances the Photoacoustic Signal of Synthetic Melanin Nanoparticles: A Dual Modality Contrast Agent for Stem Cell Imaging

In this paper, we show that gadolinium-loaded synthetic melanin nanoparticles (Gd(III)-SMNPs) exhibit up to a 40-fold enhanced photoacoustic signal intensity relative to synthetic melanin alone and higher than other metal-chelated SMNPs. This property makes these materials useful as dual labeling agents because Gd(III)-SMNPs also behave as magnetic resonance imaging (MRI) contrast agents. As a proof-of-concept, we used these nanoparticles to label human mesenchymal stem cells. Cellular uptake was confirmed with bright-field optical and transmission electron microscopy. The Gd(III)-SMNP-labeled stem cells continued to express the stem cell surface markers CD73, CD90, and CD105 and proliferate. The labeled stem cells were subsequently injected intramyocardially in mice, and the tissue was observed by photoacoustic and MR imaging. We found that the photoacoustic signal increased as the cell number increased (R 2 = 0.96), indicating that such an approach could be employed to discriminate between stem cell populations with a limit of detection of 2.3 × 104 cells in in vitro tests. This multimodal photoacoustic/MRI approach combines the excellent temporal resolution of photoacoustics with the anatomic resolution of MRI.

Current concepts in nanostructured contrast media development for in vivo photoacoustic imaging

To overcome the endogenous photoacoustic contrast arising from endogenous species, specific contrast agents need to be developed, allowing PAI to successfully identify targeted contrast in the range of wavelength in which the interference from the biomatrix is minimized.

Engineering Metal-Organic Frameworks for Photoacoustic Imaging-Guided Chemo-/Photothermal Combinational Tumor Therapy

Imaging-guided therapy has considerable potential in tumor treatment. Different treatments have been integrated to realize combinational tumor therapy with improved therapeutic efficiency. Herein, the conventional metal-organic framework (MOF) MIL-100 is utilized to load curcumin with excellent encapsulation capacity. Polydopamine-modified hyaluronic acid (HA-PDA) is coated on the MIL-100 surface to construct engineering MOF nanoparticles (MCH NPs). The HA-PDA coating not only improves the dispersibility and stability of NPs but also introduces a tumor-targeting ability to this nanosystem. A two-stage augmented photothermal conversion capability is introduced to this nanosystem by encapsulating curcumin in MIL-100 pores and then coating HA-PDA on the surface, which confer the MCH NPs with strong photothermal conversional efficiency. After being intravenously injected into xenograft HeLa tumor-bearing mice, MCH NPs prefer to accumulate at the tumor site and achieve photoacoustic imaging-guided chemo-/photothermal combinational tumor therapy, generating nearly complete tumor ablation. Engineering MOFs is an efficient platform for imaging-guided combinational tumor therapy, as confirmed by in vitro and in vivo evaluations.

Theranostic radioiodine-labelled melanin nanoparticles inspired by clinical brachytherapy seeds

Radioiodine is widely used in nuclear medicine, mainly serving as a tracer and therapeutic agent, and benefits from its various radioactive isotopes of iodine including I-123, I-124, I-125, I-131 and so on. Melanin is a natural material widely dispersed in the human skin, hair and eyes. The excellent biocompatibility and multifunctional abilities of melanin make it a perfect carrier for biomedical applications. Here, we fabricated theranostic radioiodine-labelled melanin nanoparticles (MNPs) through a novel Ag-I two-step method. The Ag-I labelling method for MNP radioiodine-labelling has advantages including a faster labelling time, higher labelling yield, and higher stability than the chloramine-T oxidation method reported previously. The obtained MNP-Ag-¹³¹I can be used for both single-photon emission computed tomography and Cherenkov radiation imaging. The β-rays of ¹³¹I also make it a good candidate as a cancer cell killer. The theranostic properties of this nanoparticle were also proved in a xenograft tumor model in vivo. In summary, this study provides a new concept for radioiodine labelling nanoparticles, which can be further investigated in various imaging and radiotherapy applications with different radioactive isotopes of iodine.

Natural melanin-loaded nanovesicles for near-infrared mediated tumor ablation by photothermal conversion

Photothermal therapy requires a biocompatible material to absorb near-infrared (NIR) light and generate sufficient heat. Herein, we suggest natural melanin-loaded nanovesicles (melasicles) as photothermal therapeutic agents (PTA) for NIR mediated cancer therapy in vivo. The mean size of these melasicles was 140 ± 15 nm. They showed excellent colloidal stability. After irradiation from 808 nm NIR laser at 1.5 W cm^-2, the melasicles showed good photothermal conversion efficiencies both in vitro and in vivo. In drug release study, laser irradiation increased fluidity of vesicle membrane due to photothermal generation from melanin. Initial drug release in the laser irradiation group was higher than that in the no laser irradiation group. After injecting the melasicles into tail veins of CT-26 bearing mice, tumors were suppressed or eliminated after irradiation at 1.5 W cm^-2 for 5 min once or twice. These results suggest that melasicles could be used as attractive PTA for cancer therapy and localized drug release.

Polydopamine encapsulation of fluorescent nanodiamonds for biomedical applications

Fluorescent nanodiamonds (FNDs) are promising bio-imaging probes compared with other fluorescent nanomaterials such as quantum dots, dye-doped nanoparticles, and metallic nanoclusters, due to their remarkable optical properties and excellent biocompatibility. Nevertheless, they are prone to aggregation in physiological salt solutions, and modifying their surface to conjugate biologically active agents remains challenging. Here, inspired by the adhesive protein of marine mussels, we demonstrate encapsulation of FNDs within a polydopamine (PDA) shell. These PDA surfaces are readily modified via Michael addition or Schiff base reactions with molecules presenting thiol or nitrogen derivatives. We describe modification of PDA shells by thiol terminated poly(ethylene glycol) (PEG-SH) molecules to enhance colloidal stability and biocompatibility of FNDs. We demonstrate their use as fluorescent probes for cell imaging; we find that PEGylated FNDs are taken up by HeLa cells and mouse bone marrow-derived dendritic cells and exhibit reduced nonspecific membrane adhesion. Furthermore, we demonstrate functionalization with biotin-PEG-SH and perform long-term high-resolution single-molecule fluorescence based tracking measurements of FNDs tethered via streptavidin to individual biotinylated DNA molecules. Our robust polydopamine encapsulation and functionalization strategy presents a facile route to develop FNDs as multifunctional labels, drug delivery vehicles, and targeting agents for biomedical applications.

Quinone-fused porphyrins as contrast agents for photoacoustic imaging

Photoacoustic (PA) imaging is an emerging non-invasive diagnostic modality with many potential clinical applications in oncology, rheumatology and the cardiovascular field. For this purpose, there is a high demand for exogenous contrast agents with high absorption coefficients in the optical window for tissue imaging, i.e. the near infrared (NIR) range between 680 and 950 nm. We herein report the photoacoustic properties of quinone-fused porphyrins inserted with different transition metals as new highly promising candidates. These dyes exhibit intense NIR absorption, a lack of fluorescence emission, and PA sensitivity in concentrations below 3 nmol mL-1. In this context, the highest PA signal was obtained with a Zn(ii) inserted dye. Furthermore, this dye was stable in blood serum and free thiol solution and exhibited negligible cell toxicity. Additionally, the Zn(ii) probe could be detected with an up to 3.2 fold higher PA intensity compared to the clinically most commonly used PA agent, ICG. Thus, further exploration of the 'quinone-fusing' approach to other chromophores may be an efficient way to generate highly potent PA agents that do not fluoresce and shift their absorption into the NIR range.

Melanin-Based Contrast Agents for Biomedical Optoacoustic Imaging and Theranostic Applications

Optoacoustic imaging emerged in early 1990s as a new biomedical imaging technology that generates images by illuminating tissues with short laser pulses and detecting resulting ultrasound waves. This technique takes advantage of the spectroscopic approach to molecular imaging, and delivers high-resolution images in the depth of tissue. Resolution of the optoacoustic imaging is scalable, so that biomedical systems from cellular organelles to large organs can be visualized and, more importantly, characterized based on their optical absorption coefficient, which is proportional to the concentration of absorbing chromophores. Optoacoustic imaging was shown to be useful in both preclinical research using small animal models and in clinical applications. Applications in the field of molecular imaging offer abundant opportunities for the development of highly specific and effective contrast agents for quantitative optoacoustic imaging. Recent efforts are being made in the direction of nontoxic biodegradable contrast agents (such as nanoparticles made of melanin) that are potentially applicable in clinical optoacoustic imaging. In order to increase the efficiency and specificity of contrast agents and probes, they need to be made smart and capable of controlled accumulation in the target cells. This review was written in recognition of the potential breakthroughs in medical optoacoustic imaging that can be enabled by efficient and nontoxic melanin-based optoacoustic contrast agents.

Photoacoustic imaging of tumor targeting with riboflavin-functionalized theranostic nanocarriers

Photoacoustic imaging is an emerging method in the molecular imaging field, providing high spatiotemporal resolution and sufficient imaging depths for many clinical applications. Therefore, the aim of this study was to use photoacoustic imaging as a tool to evaluate a riboflavin (RF)-based targeted nanoplatform. RF is internalized by the cells through a specific pathway, and its derivatives were recently shown as promising tumor-targeting vectors for the drug delivery systems. Here, the RF amphiphile synthesized from a PEGylated phospholipid was successfully inserted into a long-circulating liposome formulation labeled with the clinically approved photoacoustic contrast agent – indocyanine green (ICG). The obtained liposomes had a diameter of 124 nm (polydispersity index =0.17) and had a negative zeta potential of −26 mV. Studies in biological phantoms indicated a stable and concentration-dependent photoacoustic signal (Vevo^® LAZR) of the ICG-containing RF-functionalized liposomes. In A431 cells, a high uptake of RF-functionalized liposomes was found and could be blocked competitively. First, studies in mice revealed ~3 times higher photoacoustic signal in subcutaneous A431 tumor xenografts (P<0.05) after injection of RF-functionalized liposomes compared to control particles. In this context, the application of a spectral unmixing protocol confirmed the initial quantitative data and improved the localization of liposomes in the tumor. In conclusion, the synthesized RF amphiphile leads to efficient liposomal tumor targeting and can be favorably detected by photoacoustic imaging with a perspective of theranostic applications.

Synthetic Melanin E-Ink

Extensive efforts have been devoted to the development of surfactant-free electronic ink (E-ink) with excellent display resolution for high-definition resolution display. Herein, we report the use of polydopamine-based synthetic melanin, a class of functional nanoparticles with similar chemical compositions and physical properties to those of naturally occurring melanin, as a new E-ink material. It was found that such E-ink displays could achieve ultrahigh resolution (>10 000 ppi) and low power consumption (operation voltage of only 1 V) in aqueous solutions. Interestingly, simple oxidation of synthetic melanin nanoparticles enables the generation of intrinsic fluorescence, allowing further development of fluorescent E-ink displays with nanoscale resolution. We describe these bioinspired materials in an initial proof-of-concept study and propose that synthetic melanin nanoparticles will be suitable for electronic nanoinks with a potential wide range of applications in molecular patterning and fluorescence bioimaging.

Nanomaterials for In Vivo Imaging

In vivo imaging, which enables us to peer deeply within living subjects, is producing tremendous opportunities both for clinical diagnostics and as a research tool. Contrast material is often required to clearly visualize the functional architecture of physiological structures. Recent advances in nanomaterials are becoming pivotal to generate the high-resolution, high-contrast images needed for accurate, precision diagnostics. Nanomaterials are playing major roles in imaging by delivering large imaging payloads, yielding improved sensitivity, multiplexing capacity, and modularity of design. Indeed, for several imaging modalities, nanomaterials are now not simply ancillary contrast entities, but are instead the original and sole source of image signal that make possible the modality's existence. We address the physicochemical makeup/design of nanomaterials through the lens of the physical properties that produce contrast signal for the cognate imaging modality-we stratify nanomaterials on the basis of their (i) magnetic, (ii) optical, (iii) acoustic, and/or (iv) nuclear properties. We evaluate them for their ability to provide relevant information under preclinical and clinical circumstances, their in vivo safety profiles (which are being incorporated into their chemical design), their modularity in being fused to create multimodal nanomaterials (spanning multiple different physical imaging modalities and therapeutic/theranostic capabilities), their key properties, and critically their likelihood to be clinically translated.

Water Soluble Melanin Derivatives for Dynamic Contrast Enhanced Photoacoustic Imaging of Tumor Vasculature and Response to Antiangiogenic Therapy

A dynamic contrast enhanced (DCE) approach for tumor photoacoustic (PA) imaging is described. Novel water soluble melanin-based derivatives are synthesized that exhibit good PA properties, stability, safety and accumulation in tumor bearing mice. This melanin derivative is capable to characterize tumor vasculature and to monitor vessel permeability changes upon antiangiogenic treatment. DCE-PA imaging can assess functional response to cancer treatments.

pH-Induced aggregated melanin nanoparticles for photoacoustic signal amplification

We present a new melanin-like nanoparticle (MelNP) and its performance evaluation results. This particle is proposed as an exogenous contrast agent for photoacoustic (PA) imaging. Conventional PA contrast agents are based on non-biological materials. In contrast, the MelNPs are organic nanoparticles inspired by natural melanin. Melanin is an endogenous chromophore that has the ability to produce a PA signal in vivo. The developed MelNPs are capable of aggregating with one another under mildly acidic conditions after introducing hydrolysis-susceptible citraconic amide on the surface of bare MelNPs. We ascertained that the physical aggregation of the MelNPs resulted in an increased PA signal strength in the near-infrared window of biological tissue (i.e., 700 nm) without absorption tuning. This phenomenon is likely because of the overlapping thermal fields of the developed MelNPs. The PA signal produced from the developed MelNPs, after exposure to mildly acidic conditions (i.e., pH 6), is 8.1 times stronger than under neutral conditions. This unique characteristic found in this study can be utilized in a practical strategy for highly sensitive in vivo cancer target imaging in response to its acidic microenvironment. This approach to amplify the PA response of MelNPs in clusters could accelerate the use of MelNPs as an alternative to non-biological nanoprobes, so that MelNPs may be applicable in PA imaging and functional PA imaging such as stimuli sensitive, multimodal, and theranostic imaging.

Glucose-Derived Carbonaceous Nanospheres for Photoacoustic Imaging and Photothermal Therapy

Carbon nanomaterials with small size and unique optical properties have attracted intensive interest for their promising biomedical applications. In this work, glucose-derived carbonaceous nanospheres (CNSs) with high photothermal conversion efficiency up to 35.1% are explored for the first time as a novel carbon-based theranostic agent. Different from most other carbon nanomaterials, the obtained CNSs are highly biocompatible and nontoxic because of their intrinsic hydrophilic property and the use of glucose as raw materials. Under near-infrared laser irradiation (808 nm, 6 W cm(-2)) for 10 min, less than 15% of PC-3M-IE8 cells exposed to CNSs aqueous dispersions (0.16 mg/mL) remained alive. After intravenous administration of CNSs aqueous dispersions into nude mice, the photoacoustic intensity of the tumor region is about 2.5 times higher than that of preinjection. These results indicate that CNSs are suitable for simultaneous photoacoustic imaging and photothermal ablation of cancer cells and can serve as promising biocompatible carbon-based agents for further clinical trials.

Copolymerization of an indazole ligand into the self-polymerization of dopamine for enhanced binding with metal ions

Synthesis and mussel-inspired polymerization of a new catechol monomer. The generated copolymer exhibits enhanced metal binding, due to the ligand nature of the new monomer, compared to polydopamine homopolymer.