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    Topic review

    Photoacoustic Imaging Techniques

    Subjects: Others
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    Submitted by: Dong-Wook Han


    2D materials can be used as carriers for delivering therapeutic agents into a lesion, leading to phototherapy. Various optical imaging techniques have been used for the monitoring of the treatment process. 

    1. Introduction

    Two-dimensional nanomaterials with a well-ordered 2D planar structure (thickness > 100 nm) have been widely developed. Thanks to their beneficial biocompatible and biodegradable characteristics, various types of 2D nanomaterials, such as graphene derivatives; LDH, layered double hydroxide; TMD, transition metal dichalcogenide; TMO, transition metal oxide; and BP, black phosphorus, have been used for biomedical applications including drug delivery, tissue engineering, bio-imaging, and bio-sensing[1] (Figure 1). Because such materials have beneficial physicochemical properties of biocompatibility and degradability, they are suitable for biomedical applications including drug delivery, tissue engineering, bio-imaging, and biosensors[2][3][4][5][6]. Two-dimensional nanomaterials can cause a photothermal effect that generates heat by converting light energy into thermal energy when they are irradiated with near-infrared (NIR) light, and can then be used for phototherapy[7].

    Figure 1. Schematic illustration of representative 2D nanomaterials. LDH, layered double hydroxide; TMD, transition metal dichalcogenide; TMO, transition metal oxide; BP, black phosphorus.

    The representative phototherapy methods are photothermal therapy (PTT) and photodynamic therapy (PDT)[8]. In PTT, the local heating of NIR-absorbing agents is triggered by NIR light illumination. As tumor cells have difficulty dissipating heat, the NIR-triggered photothermal effect causes selective death of cancerous cells, which can be ablated more than 42 °C through necroptosis and apoptosis, which is programmed cell death, while endowing little damage to normal cells[9]. In contrast, PDT is an indirect method using photosensitizers that generate harmful singlet oxygen (1O2) when they absorb light. As PDT does not generate heat, nanomaterials in PDT usually perform as carriers that transfer photosensitizers using their surface properties[10]. To assess the therapy, drug delivery, and biodegradability, visualization of the internal biodistribution is essential.

    Various biomedical imaging techniques, such as X-ray computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET), have been used for visualizing the distribution of nanomaterials, monitoring the delivery of nanomaterials, and assessing the efficacy of treatments[11][12][13][14]. Particularly, optical imaging techniques have been widely used as they provide high optical contrast, rich functional information, and excellent spatiotemporal resolution[15]. Compared with other biomedical imaging techniques, optical imaging systems can be implemented with low cost and simple configuration. In addition, optical imaging does not create harmful ionizing radiation, which makes the system favorable for future clinical translation. However, despite the advantages set out above, optical imaging is not widely used in clinics. The primary limitation of optical imaging is shallow imaging depth due to photon scattering in biological tissue[16].

    Photoacoustic (PA) imaging is a biomedical imaging technique that combines the principles of ultrasound (US) and optical imaging[17]. The principle of PA imaging is based on the PA effect, which involves energy transduction from laser to acoustic waves through thermoelastic expansion (Figure 2). The typical procedure of PA imaging is as follows: (1) illumination of a short (typically a few nanoseconds) pulsed laser beam to target tissue, (2) light absorption and heat release by the optically absorbing chromophores, (3) acoustic wave (i.e., PA wave) generation through rapid thermal expansion and relaxation, (4) signal reception using US transducer, and (5) image generation and display. Because PA imaging inherits the principles of optical and US imaging techniques, it can provide both strong optical contrast and high ultrasound resolution in deep tissue[18][19]. In addition to intrinsic chromophores (oxy- and de-oxy-hemoglobins, melanin, lipids, and water), external agents (organic dyes, liposomal nanoformulations, nanoparticles, and nanostructures) have been widely used to obtain contrast-enhanced PA images[20][21][22][23]. Moreover, using multiple wavelengths of the excitation laser, molecular functional information of biological tissue can be obtained, which can be used for investigating the bio-distribution of external agents in vivo[24][25][26].

    Figure 2. Schematic explanation of the procedure of PA imaging. PA, photoacoustic; US, ultrasound. The inset image is reproduced with permission from[27].

    2. Phototherapy Using 2D Nanomaterials

    2.1. Types and Characteristics

    Currently, various phototherapy agents including gold nanostructures[28][29], mesoporous silica nanoparticles[30][31], and 2D nanomaterials have been widely investigated for clinical application. Among them, 2D nanomaterials are attracting the most attention because they can be used not only for phototherapy, but in other biomedical fields thanks to their excellent biochemical properties (Table 2). Typical 2D nanomaterials consist of one or a few atomic layers. As a representative 2D nanomaterial, graphene has a honeycomb structure consisting of carbon atoms. By changing the shape, the number of layers, and chemical modifications, several derivatives can be formed. Among them, graphene oxide (GO)[28] and reduced graphene oxide (rGO)[29][30] have been widely utilized for biomedical applications because of their superior electrical and thermal conductivities, large surface area, chemical versatility, and biocompatibility[31][32][33]. Layered double hydroxides (LDHs) are inorganic 2D nanomaterials with structures in which metal atoms are sandwiched by hydroxide layers. Because of their high charge density with excellent biocompatibility, LDHs are widely used as nano-carriers for theranostic agents in drug or gene delivery, phototherapy, and immunotherapy[34][35][36]. Transition metal dichalcogenides (TMDs) have hexagonal lattices consisting of monolayers of transition metal atoms between chalcogen atom layers. In addition to thin, flexible, and strong characteristics, TMDs are excellent optical absorbers, thus producing photolimunescence and photothermal effects [37][38][39][40]. TMDs can be formed with various materials including molybdenum disulfide (MoS2)[41][42][43][44], tungsten disulfide (WS2)[45][46][47][48], and molybdenum diselenide (MoSe2)[49][50][51]. Transition metal oxides (TMOs) are compounds of oxygen atoms and transition metals such as titanium (TiO2)[52][53] and manganese (MnO2)[54][55][56]. Their wide bandgap results in excellent photochemical and electric properties[57][58]. They can also directly interact with drugs, genes, or other biomolecules by surface modification and thus can be utilized in biomedical applications including drug delivery, cancer therapy, tissue engineering, bio-imaging, and biosensing[59][60][61]. Mxenes are the most recently discovered 2D materials and have been applied in various biomedical applications because of their extreme thinness, large surface area, high surface–volume ratio, and mechanical strength[62][63][64][65][66]. Black phosphoruses (BP) are the most stable allotropes of phosphorus with zigzag or armchair bilayer structures. BP have shown potential in biomedical applications with their strong optical absorption in the ultraviolet (UV) and NIR regions [67][68][69][70][71]. They have also demonstrated promising biocompatibility and biodegradation[72].

    2.2. Phototherapy Using 2D Nanomaterials

    By taking advantage of their strong optical absorption and thermal transition properties, 2D nanomaterials have been used for PTT. Liu et al. successfully prepared a doxorubicin (DOX)-loaded MoS2-PEG nanosheet for combined PTT–chemo cancer therapy[73]. In that study, nanosheets were analyzed for drug delivery and photothermal effects by NIR irradiation. MoS2-PEG/DOX nanosheets exhibited synergistic anti-cancer effects, inhibiting tumor growth in in vivo experiments. Zeng et al. reported on ultrathin MnO2 nanosheets of polyethylene glycol-cyclic arginine-glycineaspartic acid tripeptide (PEG-cRGD) and encapsulated chlorin e6 (Ce6) for PTT/PDT synergistic cancer therapy (Figure 5)[74]. In that study, the nanosheets showed photothermal efficiency of 39% and could be reduced by overexpressed acidic H2O2, which could efficiently generate O2 and further enhance the therapeutic efficiency of PDT. Moreover, the MnO2-PEG-cRGD/Ce6 exhibited pH-controlled and NIR-induced Ce6 release, and showed favorable therapeutic outcomes under a single 660 nm NIR laser.

    Figure 5. (A) Schematic illustration for the preparation of MnO2-PEG-cRGD/Ce6 for synergistic photothermal/photodynamic (PTT/PDT) therapy. (B) Relative viabilities of PC3 cells after incubation with free Ce6, MnO2-PEG-cRGD with 660 nm light irradiation, or MnO2-PEG-cRGD/Ce6 with or without 660 nm light irradiation (0.6 W cm−2, 10 min, *** p < 0.001, ** p < 0.01). (C) Fluorescence images of calcein acetoxymethyl ester (Calcein-AM, green)/propidium iodide (PI, red) double stained cells after different treatments. The images are reproduced with permission from[74].

    A number of PDT studies have investigated the development of 2D nanomaterials that release photosensitizers. Moosavi et al. prepared N-TiO2 nanoparticles to generate reactive oxygen species (ROS) and induce autophagy [52]. They showed the dose-dependent capability of well-dispersed photo-activated N-TiO2 NPs to induce terminal megakaryocyte differentiation and cell death in K562 leukemia cells. In cellular experiments, N-TiO2 nanoparticles increased ROS levels with light irradiation. Yang et al. fabricated covalently, incorporating both chlorin e6 (Ce6) and triphenyl phosphonium (TPP) onto BP@PDA NSs for dual-modal imaging-guided synergistic photothermal and photodynamic therapy (Figure 6)[75]. BP@PDA–Ce6&TPP NSs can produce considerable heat, mainly owing to BP@PDA NSs, and can generate sufficient ROS for PDT. With these results, BP@PDA–Ce6&TPP NSs can be used for PTT/PDT therapy of cancers.

    Figure 6. (A) Schematic illustrations of the preparation, therapeutic uses, and peaks in functions of BP@PDA–Ce6&TPP NSs. (B) Ex vivo fluorescence imaging of main organs as well as tumor at 24 h post-injection. (C) Relative viabilities of HeLa cells incubated with BP@PDA NSs, BP@PDA–Ce6 NSs, and BP@PDA–Ce6&TPP NSs at different BP@PDA concentrations with laser illumination (660 nm, 0.5 W cm−2, 5 min). Data represent mean ± SD (n = 4). The images are reproduced with permission from[75].

    Table 2. Phototherapy using 2D nanomaterials. PTT, photothermal therapy; PDT, photodynamic therapy.
    2D Nanomaterials Photothermal Conversion Therapy Applied Forms Ref.
    Graphene derivatives 63% (G), 35% (GO) [30] PTT, PDT GO-UCNPs-ZnPc [32]
    PTT GO/MnFe2O4/DOX [33]
    TiO2 40.8% [57] PTT Ag@TiO2 [53]
    PDT N-TiO2 [52]
    MoS2 0.84% [38] PTT MoS2-HA-DTPA-Gd [42]
    PTT, PDT AuNBPs@MoS2 [43]
    PTT MoS2-Gd-BSA [44]
    BP 30.84% [68] PTT BP-Au NSs [69]
    PTT BP-PEG-FA/Cy7 NSs [70]
    PTT, PDT BP@PEG/Ce6 NSs [71]
    Mxene(Ti3C2) ≒100% [63] PTT Ti3C2@Au [64]
    PTT, PDT Ti3C2-SP [65]
    PTT, PDT Ti3C2-DOX [66]
    WS2 35% [39] PTT, PDT BSA-WS2@MB [46]
    PTT WS2-PEG [47]
    PTT WS2-IO/S@MO-PEG [48]
    MoSe2 54.3% [40] PTT, PDT MoSe2/Fe3O4 [50]
    PTT, PDT MoSe2@PEG-Dox [51]
    2D Boron 42.5% [75] PTT, PDT B@Ce6–PAH–PAA  
    MnO2 62.4% [58] PTT MnO2-PEG-FA/DOX [55]
    PTT BSA-MnO2 NPs [56]

    The entry is from 10.3390/biomedicines9010080


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