Porphyrin Self-Assembled Nanostructures and Applications

Abstract

Porphyrins are a class of macromolecular heterocyclic compounds formed by the inter-carbon atoms of four pyrrole-like subunits through the submethyl bridge (〓CH∙). Porphyrin rings have 26 electrons in highly conjugated system and are easily modified peripheral structures, often serve as ideal building blocks to construct self-assembled nanostructures with excellent physical and chemical properties. Porphyrin nanostructures have excellent visible light absorption properties, which will significantly improve the efficiency of electron–hole separation, and are also commonly used in photocatalysis fields. Porphyrin photosensitizers have superior strong phototoxicity and little side effects, and are widely used in tumor photothermal/photodynamic treatment. This chapter summarizes the self-assembly methods of porphyrins, the applications progress of porphyrin self-assembled nanomaterials in photocatalysis and tumor therapy, and discusses the development trend in future of porphyrin nanomaterials.

1. Introduction

Porphyrins are widely found in animals’ blood and plants in nature. They are macromolecular heterocyclic compounds, have highly conjugated system, and are formed by the cross-carbon atoms of four pyrrole subunits through a submethyl bridge (〓CH∙) [1]. Because the rigid conjugated electron ring and adjustable geometry endow porphyrin molecule the self-assembly characteristics [2], conjugate molecular framework of delocalized aromatic electron ensure them excellent physical, chemical, photochemical, and biological characteristics, which make them applicate in supramolecular electronics, light collection, energy conversion system, chemical/biosensors, and biomedicine fields [3, 4].

Self-assembly process is that smaller structural units spontaneously assemble into nanostructures with confined morphology driven by various non-covalent intermolecular interactions, such as hydrogen bonds, π-π packing, hydrophobic interactions, electrostatic interactions, and van der Waals forces. Porphyrin self-assembled nanostructures will inherit the characteristics of porphyrin monomer, and possess electronic buffer, photoelectric conversion, photosensitivity, and high chemical stability, so they have been a hot object of research in photocatalysis and nanomedicine fields [5].

Molecular self-assembly is an effective method for preparing nanostructured materials [6]. The unique properties of porphyrin self-assembled nanostructures depend not only on the size, morphology, and composition of molecular units, but also on the large-scale spatial arrangement order of the assembly. Therefore, the construction of porphyrin self-assembled nanostructures is an important research direction in the current field of materials science. This chapter focused on the preparation methods of porphyrin self-assembly, characterization methods, and application fields of porphyrin self-assembled nanomaterials, and explore the future development trend of such materials.

2. Methods of porphyrin self-assembly

The preparation methods of porphyrin self-assembled nanostructures mainly include acid–base neutralization/micelle confined self-assembled method, surfactant-assisted mixing solvent assembly method, microemulsion-assisted/mesophase transfer self-assembly method, peptide or nanocrystal, and porphyrin coassembly method. Of course, there are other strategies, such as ion self-assembly [7] and droplet volatilization-induced self-assembly, which will not be described in this chapter.

2.1 Acid: base neutralization/micelle confined self-assembled method

Most porphyrin monomer is insoluble in water, which affects their wide application in catalysis, biotherapy, etc. In order to solve this problem, Bai developed an acid–base neutralization micelle confinement self-assembly method. They dissolved the pyridyl porphyrin in an acidic solution, or dissolved the phenol, phenylcarboxyl porphyrin, etc. in an alkaline solution, and then poured them into the alkaline or acidic solution of the emulsifier, and an acid–base neutralization reaction occurs (Figure 1A), and the porphyrin molecules are reprecipitated and self-assembled to form a regular and orderly stacked self-assembled nanostructure (Figure 1B) [8]. Therefore, the pH of the solution and the ratio of porphyrin emulsifier, and assemblies of different sizes can be obtained [9].

Figure 1.

(A) Scheme of THPP dissolved in NaOH aqueous solution and protonation process. (B) Schematic illustration of acid–base neutralization micelle confined self-assembled method. Reproduced with permission from reference [8]. Copyright 2018 American Chemical Society.

Typically, 5,10,15,20-tetrakis(4-(hydroxyl)phenyl) porphyrin (THPP) was dissolved in NaOH aqueous solution forming a water-soluble TPP⁴⁻ anion, then was quickly poured into an acidic emulsifier solution (c_surfactant > critical micelle concentration (CMC)), instantaneously occur acid–base neutralization, then TPP⁴⁻ anion protonation and forms neutral THPP reprecipitation, and entered into the emulsifier micellar, after stirring agitation and growth, THPP molecule self-assembled into nanowires driven by the non-covalent interactions, such as hydrogen bond, π-π interaction, hydrophobic interaction, and van der Waals force. Scanning electron microscopy (SEM) (Figure 2A) shows the average length and diameter of THPP nanowires and are about 4.5 μm and 110 nm, respectively. Besides, according to the transmission electron microscopy (TEM) images (Figure 2B) and high-resolution TEM (HR-TEM) image (Figure 2C), the nanowires are uniform and monodispersed without defects with well-resolved lattice fringes with interplanar distance of 1.39 nm, indicating the possible formation of multiple hydrogen bonds, such as (pyrrol) N + H···Cl and OH···H, among THPP molecules within the nanowires (Figure 2D), which are balanced favoring the formation of J-aggregates [8].

Figure 2.

Structure characterizations of the self-assembled porphyrin nanostructures by acid–base neutralization micelle confined self-assembly method. (A) SEM image of the THPP nanowires. (B) Corresponding TEM and (C) HR-TEM image of (A), the inset is FFT. (D) Crystal structure simulated of THPP nanowires (chloride, green; nitrogen, blue; oxygen, red; carbon, gray). Reproduced with permission from reference [8]. Copyright 2018 American Chemical Society. SEM images of CoTPyP nanocrystals were collected at (E) 100 d and (F) 150d. Reproduced with permission from reference [10]. Copyright 2020 The Materials Research Society. (G) SEM of the self-assembled H₂TPyP nanooctahedra. (H) SEM of the self-assembled Zn-metalated H₂TPyP nanowires. Reproduced with permission from reference [11]. Copyright 2016 American Chemical Society.

The pyridine and pyrrole groups are inclined to protonated and dissolve in large amounts in an acidic aqueous solution, lead to pyridine matrix and form MTPyP-H₄⁴⁺ soluble tetrapyridinium solution [9]. Then, the acidification porphyrin solution was injected into a basic surfactant aqueous solution (c_surfactant > CMC) under vigorous stirring at room temperature (25°C), immediately triggers an acid–base neutralization reaction. Then, the deprotonation of MTPyP-H₄⁴⁺ produced neutral insoluble MTPyP porphyrins and encapsulated into the hydrophobic cores of surfactant micelles. Driven by intermolecular non-covalent interactions, such as π-π stacking, hydrogen bonding, Zn-N axial coordination, hydrophobic-hydrophobic interactions, and so on. MTPyP molecules started the self-assembly process with the assistance of surfactant template and further initiated nucleation and growth to form uniform porphyrin nanocrystals.

Based on this, the self-assembly process of Co-tetra(4-pyridyl) porphyrin (CoTPyP) underwent very slow, and hardly get self-assembled product less than 10 days at 25°C. After continuous stirring and aging for 100 days [10], a small amount of CoTPyP regular polyhedral nanoparticles with an average size 300 nm were obtained (Figure 2E). Until continuous stirring for 150 days, the CoTPyP monomers completed the self-assembly process and formed uniform and regular hexagonal prism nanocrystals. The size of CoTPyP prism is 700 nm × 1000 nm × 750 nm (Figure 2F), and the interplanar distance of lattice fringes is 1.53 nm. Contrarily, the meso-tetra(4-pyridyl) porphine (H₂TPyP) can self-assemble into well-defined 3D octahedral morphology (Figure 2G) with no apparent defects in 12 h. Moreover, the self-assembled structures obtained by different central metalloporphyrins are also different. When Zn²⁺ (through Zn(NO₃)₂) was added in the H₂TPyP self-assembly solution, Zn-metalation into the core of H₂TPyP forming ZnTPyP [11], the morphology turned in nanowires (Figure 2H). The porphyrin self-assembled structure synthesized based on this method can be easily controlled by the solution pH, the concentration of the emulsifier, the concentration of the porphyrin, etc., and can be prepared in large amounts.

2.2 Surfactant-assisted mixing solvent assembly method

The good solvent solution of porphyrin is poured into the poor solvent of porphyrin, and the porphyrin reprecipitates and assembles to form nanostructures with the surfactant-assisted due to the change of solubility. Therefore, the size and morphology of self-assembled nanostructures can be controlled by the solvent polarity and the mole ratio of surfactant and porphyrin.

Typically, Pd (II) tetra (4-carboxylphenyl) porphyrin (PdTCPP) powder is insoluble in water but has good solubility in N, N-dimethylformamide (DMF) solution. Therefore, PdTCPP powder dissolved in DMF to obtain a PdTCPP/DMF homogeneous solution, and then quickly added into to the 1-decanesulfonic acid sodium salt (DASS) aqueous solution at one time under stirring at room temperature. Instantly, PdTCPP molecules are precipitated from the DMF system and started to self-assemble driven by hydrophobic-hydrophobic interaction between porphyrin molecules and the end of the surfactant and π-π stacking and hydrogen bonding [12]. The SEM and TEM images of the PdTCPP assemblies showed that these assemblies are two-dimensional leaf-like nanostructures with highly regular morphology and uniform size (Figure 3A), and the surface of the PdTCPP nanoleaf is smooth without obvious defects. The fine structure of TEM showed that the edge of PdTCPP nanoleaf showed a jagged irregular shape, and the structure was superimposed layer by layer (Figure 3B). The AFM results further demonstrated that the surface of the PdTCPP nanoleaf was smooth and the thickness was about 200 nm (Figure 3C and D). The long and short axes of the PdTCPP nanoleaf are 5.43 μm and 1.64 μm, respectively.

Figure 3.

The self-assembled PdTCPP nanoleaves were prepared by surfactant-assisted mixing solvent assembly method. (A) SEM image, (B) TEM image, and (C) AFM image of the nanoleaves. (D) the thickness of the vertical interface in panel (C). (E) TEM image of the cross-section, (F) HRTEM image of the corresponding (E); the inset is the corresponding FFT, (G) XRD patterns, (H) simulated crystal structure of the nanoleaves. Reproduced with permission from reference [12]. Copyright 2022 American Chemical Society.

In order to acquire the internal fine structure, the PdTCPP nanoleaves were embedded in epoxy resin and made ultrathin sections. The cross-section of the PdTCPP nanoleaves was rectangular and uniformly distributed in the slice, and the thickness of the vertical interface is about 200 nm (Figure 3E). HR-TEM of cross-sectional showed that there was 1.5 nm distinct and well-resolved lattice fringe on the cross-section of the PdTCPP nanoleaf (Figure 3F). Moreover, the obvious diffraction spots of the layered structure are obtained by fast Fourier transform (FFT), indicating that the PdTCPP nanoleaf has a good crystal structure (Figure 3F). The XRD test showed that the films and powders of PdTCPP nanoleaf had strong diffraction peaks at 5.95, 11.67, 17.39, 23.16, and 28.97° (Figure 3G), and the positions of the diffraction peaks were approximately 1:2:3:4:5, which also proved that the PdTCPP nanoleaf is a layer-by-layer structure. While the commercial PdTCPP powder has no diffraction peak as a control. The density functional theory calculation of the crystal structure shows that the PdTCPP nanoleaf is mainly formed through an orderly arrangement of porphyrin intermolecular by the hydrogen bonding and π-π stacking interaction.(Figure 3H).

2.3 Microemulsion-assisted/mesophase transfer self-assembly method

Microemulsion refers to a thermodynamically stable, isotropic, transparent, or translucent dispersion system formed by two immiscible liquids in the presence of surfactants, and their particle sizes distribution is within 10–100 nm. The microemulsion system is composed of oil phase, water phase, and surfactant [13]. Nanoparticles prepared by microemulsion technology have a controllable shape and size, relatively narrow particle size distribution, and hard to agglomerate [14, 15]. In the process of preparing microemulsion, the oil phase and the water phase are emulsified under ultrasonic probe, and microemulsion droplets with narrow size distribution are obtained. These droplets can exist stably in the aqueous phase in the presence of surfactants or stabilizers and can act as nanoreactors for the reaction. This “nanoreactor” can change the particle size according to the number of solubilized substances in the droplet. The more solubilized substances, the larger the radius of the reaction core, and the larger the radius of the generated nanoparticles, which can control the shape and size in the preparation of the nanoparticles process.

Liu et al. reported a microemulsion droplet nanoreactor using cetyltrimethylammonium bromide (CTAB) as a surfactant. Typically, the CHCl₃ solution of In (III) mesotetraphenyl porphine chloride (InTPP) was injected into CTAB aqueous solution. Then, the reaction solution was placed under an ultrasonic probe to emulsify and sonicated at 80 W for 1 min. The microemulsion then transfers in a 60°C water bath to volatilize chloroform solvent for 30 min, the InTPP molecules self-assembled into regular nanostructures driven by non-covalent interactions (Figure 4). After that, the porphyrin solution was centrifuged to separate the precipitate, then washed with water to remove excess surfactant. By changing the mole ratio of porphyrin and surfactant, the size and morphology of the resulting assemblies can be regulated. Starting from here, InTPP self-assembled nanorods were successfully synthesized depending on the concentrations of CTAB [16]. Representative TEM images show that the resulted InTPP nanocrystals are uniform and monodisperse nanowires or nanorods (Figure 5A–D). Moreover, the well-defined nanowires or nanorods with controlled aspect ratios of 81.3, 21.0, 4.6, and 1.6 can be adjusted by the CTAB concentration. These results further confirmed that the surfactant microemulsion droplets are good nanoreactors to control the size of nanoparticles.

Figure 4.

Schematic illustration of the microemulsion-assisted/mesophase transfer self-assembly process. Reproduced with permission from reference [16]. Copyright 2019 American Chemical Society.

Figure 5.

Structure characterizations of the self-assembled porphyrin nanostructures by microemulsion-assisted/mesophase transfer self-assembly method. TEM images of the self-assembled InTPP nanocrystals at different CTAB concentrations of (A) 2.5, (B) 10, (C) 15, and (D) 25 mM, respectively. Reproduced with permission from reference [16]. Copyright 2019 American Chemical Society. SEM images of the self-assembled of MnPc NPs (E) and MnPc NWs (F). Reproduced with permission from reference [17]. Copyright 2021 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021. TEM images of the self-assembled of AuTPyP NPs (G) and FeTPP@Fe₃O₄ (H). (G) Reproduced with permission from reference [18]. Copyright 2021 American Chemical Society. (H) Reproduced with permission from reference [19]. Copyright 2022 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2022.

Similarly, the microemulsion droplet approach is also applicable to the self-assembly of phthalocyanine nanocrystals. As shown in Figure 5E, when the emulsifier is sodium dodecyl sulfate (SDS), the morphology of the manganese phthalocyanine (MnPc) assemblies is mainly spherical nanoparticles with a diameter of ~62 nm, and the size changes with the SDS concentrations. When sodium dodecyl benzene sulfonate (SDBS) was used, the morphologies of the MnPc assemblies were all linear (Figure 5F) with a 1 μm in length. With the decrease of the emulsifier concentration, the MnPc assemblies gradually grew from linear (0.02 M) to sheet-like assemblies (0.01 M), further reducing the concentration of SDBS (below 0.005 M), the length of the assembly gradually decreased [17].

Comparably, the Au(III) tetra-(4-pyridyl) porphine (AuTPyP) can self-assemble into nanospheres with an average diameter of ∼65 nm (Figure 5G) using SDS aqueous solution/CHCl₃ as microemulsion droplet [18]. More interestingly, the microemulsion droplet technology can well integrate the organic–inorganic interface, and then be used to construct the organic–inorganic composite coassemblies. Typically, the CHCl₃ solution of 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPP) and oleic acid (OA) modified magnetic Fe₃O₄ NCs was injected into SDS aqueous solution to create an oil-in-water microemulsion by an ultrasonic probe [19]. After evaporation of the chloroform solvent, the FeTPP and OA-Fe₃O₄ NCs randomly coassembled into nanospheres driven by π-π stacking and hydrophobic-hydrophobic interactions (Figure 5H).

3. Photoelectric response of porphyrin self-assemblies

Optical spectroscopy has been used to study the light absorption properties of porphyrin nanostructures [20, 21]. The spectra provide information about the intermolecular interactions in porphyrins nanostructures [22, 23]. The UV–vis absorption spectra of self-assembled ZnTPyP with different morphologies show that the porphyrin nanostructure has a larger spectral absorption broadening and obvious redshift compared with the porphyrin monomer. After self-assembled into the nanostructures (the nanodisk, hexagonal nanorod, tetragonal nanorod, and nanoparticle), the absorption at 425 nm of the Soret band porphyrin monomer is split into three absorption peaks at 421 nm, 449 nm, and 475 nm, respectively. The tetragonal nanorod absorbs at 421 nm and 448 nm, and nanoparticle absorbs at 421 nm. The nanodisk has the largest redshift, which is usually associated with the spatially ordered J-aggregate arrangement of porphyrin molecules. Basically, ordered π-π stacking and long-range delocalization within porphyrin assemblies enable these nanomaterials with tailored collective optical properties for a broader visible light absorption spectrum. Since the pyridyl group in the ZnTPyP molecule has a certain angle with the plane of the porphyrin ring, the intermolecular force of the porphyrin molecule in the self-assembly process is different, which lead to different close-packed stacking and different morphologies [20]. The relationship between visible absorption spectra-nanostructures of porphyrin nanostructures will also seriously affect the photocatalytic activity (Figure 6).

Figure 6.

UV–vis absorptions of zinc-tetra(4-pyridyl) porphyrin (ZnTPyP) self-assembled nanocrystals. Reproduced with permission from reference [20]. Copyright 2014 American Chemical Society.

The self-assembled tetra(4-carboxylphenyl) porphyrin (TCPP) (SA-TCPP) exhibits a single absorption edge at 700 nm due to the spatial order aggregation of TCPP molecules, suggesting the SA-TCPP could absorb the entire visible spectrum (Figure 7A). In addition, the SA-TCPP could further utilize the UV spectrum because of the blue-shift of the Soret band. Therefore, the SA-TCPP could collect the full solar spectrum light and their theoretical spectral efficiency reached up to 44.4% [24]. Compared with the commercial TCPP powder, the photocurrent of SA-TCPP is as high as 3.52 μA cm⁻² (Figure 7B), further confirming that the ordered packing of porphyrin molecules is conducive to electron delocalization and transition and further enhances the photoelectric response performance.

Figure 7.

(A) UV-vis diffuse reflection spectroscopy of TCPP self-assembled nanostructures. (B) Photocurrent response of SA-TCPP and TCPP powder. Reproduced with permission from reference [24]. Copyright 2018 Wiley-VCH.

4. Applications of porphyrin self-assemblies

4.1 Photocatalytic degradation of methyl orange (MO)

Based on the excellent light-harvesting ability of porphyrins [25], porphyrin self-assembled nanostructures are used in the field of photocatalysis. H₂TPyP nanooctahedra, Zn-metallized H₂TPyP intermediate nanoparticles, and Zn-metallized H₂TPyP nanowires were used for photocatalytic degradation of methyl orange (MO) [26]. As shown in Figure 8, the regular 200 nm H₂TPyP nanooctahedra has a degradation efficiency of about 10% after 140 minutes of illumination (panel b). The degradation efficiency of the mixture of octahedra and long wires after part of Zn-metallized, degradation efficiency increased to nearly 80% (panel c), and the equivalent degradation efficiency of Zn-metallized completely porphyrin long wires is close to 90% (panel d). The self-degradation of MO as a control was almost unchanged under the same condition [11]. These results confirmed that the addition of center metal atoms played a very important role in the improvement of the catalytic efficiency. On the other hand, the long-range ordered π-π conjugated stacking in nanowires promoted better electron transport and electron–hole separation, thereby showing better photocatalytic degradation efficiency than that of nanooctahedra. In addition, porphyrin nanostructures have the advantages of less dosage, high activity, easy recycling, and stability in photocatalytic applications, which makes them a potential photocatalytic agent.

Figure 8.

Photocatalytic degradation of methyl orange (MO). (a) Self-degradation of MO as a control. (b) H₂TPyP nanooctahedra. (c) Zn-metallized H₂TPyP intermediate nanoparticles. (d) Zn-metallized H₂TPyP nanowires. Reproduced with permission from reference [11]. Copyright 2016 American Chemical Society.

4.2 Photocatalyzed hydrogen evolution

The molecular ordered self-assembled PdTCPP nanoleaf has strong absorption in the range of 400 to 700 nm and high photogenerated charge separation efficiency under visible light irradiation. In the absence of cocatalyst, the hydrogen production rate of PdTCPP nanoleaf (9.9 mmol g⁻¹ h⁻¹) was nearly 40 times higher than that of commercial PdTCPP powder (0.25 mmol g⁻¹ h⁻¹) [12]. After loading 1 wt% Pt cocatalyst, the hydrogen production rate of PdTCPP nanoleaf reached 138 mmol g⁻¹ h⁻¹, which was 14 times higher than that without cocatalyst (Figure 9A). This spatial arrangement of porphyrin molecules' self-assembly effectively improved electron transport to enhance photocatalytic hydrogen production. Interestingly, TEM images showed that Pt nanoparticles were mainly deposited on the edge of the PdTCPP nanoleaf after the photocatalytic reaction, while there were almost no Pt nanoparticles on the surface when Pt was cocatalyst (Figure 9B). That is, the edge of the PdTCPP nanoleaf is the active center for the photocatalytic reduction reaction. When K₂PtCl₄ and Pb(NO₃)₂ were used as precursors to conduct photodeposition experiments, TEM images showed that Pt and PbO₂ nanoparticles were deposited on the edge and surface of PdTCPP nanoleaf, respectively (Figure 9C). The element distribution maps show that Pt and Pb elements are concentrated on the edge and surface, respectively, that is, Pt nanoparticles are selectively deposited on the edge of PdTCPP nanoleaf, while PbO₂ nanoparticles tend to be deposited on the surface of PdTCPP nanoleaf (Figure 9D–G). The above experimental results indicated that photogenerated electrons and holes are separated and transported in different directions in the PdTCPP nanoleaf. Kelvin probe force microscopy (KPFM) can provide the contact potential difference (CPD) between the tip and the sample, which can detect the charge distribution inside the single crystal. The KPFM surface potential of PdTCPP nanoleaf shown that in the dark state, the edge potential of PdTCPP nanoleaf is more negative than the surface, and there is a potential difference between edge and surface. The surface potential of PdTCPP nanoleaf becomes more positive and significantly increased under 445 nm laser irradiation, the potential difference was significantly enhanced, confirming the existence of space charge regions between the edges and faces of the PdTCPP nanoleaf (Figure 9H–J). This work reported for the first time there is an electric field inside the PdTCPP nanoleaf, and the photogenerated electrons and holes were transferred and gathered to the surface under visible light irradiation, respectively. This selective transfer of photogenerated electrons and holes to specific sites on the edge and surface of PdTCPP nanoleaf immensely improved the spatial separation efficiency of photogenerated charges, thereby enhancing photocatalytic activity.

Figure 9.

(A) H₂ production photocatalyzed by PdTCPP nanoleaves without and with 1 wt % of Pt cocatalyst. (B) TEM image of the nanoleaves with Pt NPs. (C) TEM image of the nanoleaves with Pt and PbO₂ NPs. (D–G) elements mapping of the nanoleaves/Pt/PbO₂. (H) under dark (I) and light irradiation at 445 nm of kelvin probe force microscopy images of the nanoleaves. (J) surface potential image of the cross-section in (H) and (I). Reproduced with permission from reference [12]. Copyright 2022 American Chemical Society.

4.3 Photodynamic therapy

Porphyrins as a class of biocompatible and easily modified photosensitizers, have shown great application value in tumor photodynamic therapy (PDT) [27, 28, 29, 30]. The co-assembly of biomimetic Gd(III) meso-tetraphenyl porphyrin 2,4-pentane dionate (GdTPP) and zinc meso-tetraphenyl porphyrin (ZnTPP) mGZNs were used for PDT functions research of HeLa cells [31]. Firstly, singlet oxygen (¹O₂) has been shown to be the key photooxidation species for PDT that efficiently kills cancer cells [32]. The PDT efficiency and the generation of intracellular reactive oxygen species (ROS) were further evaluated by the specific green fluorescence probe (2′,7′-dichlorodihydrofluorescein diacetate, DCFH-DA). The comparison of PBS control was performed and showed mGNs have the highest fluorescence intensity of ROS including ¹O₂ for PDT (Figure 10A). Additionally, a clear, nearly linear increase in fluorescence intensity as mGZNs concentration increased was shown (Figure 10B). Cell viability indicated an irradiation time- and dosage-dependent PDT efficiency with a maximum value of 80.6% at 200 μM of mGZNs and an irradiation duration of 9 min (635 nm, 0.15 W/cm²; Figure 10C). Moreover, flow cytometry analyses of HeLa necrosis and apoptosis induced by mGZNs showed that irradiation of mGZNs induced greater cell death compared to the blank groups (Figure 10D).

Figure 10.

In vitro experiments with mGZNs. (A) Images showing intracellular production of various porphyrin self-assembled nanostructures. Scale bar: 100 μm. (B) the fluorescence intensity of ¹O₂ in cells. (C) Irradiation durations and dosages-dependent phototoxicity of HeLa cells incubated with porphyrin self-assembled nanostructures. (D) Flow cytometry analyses of annexin V-FITC/PI double-stained cells treated with various porphyrin self-assembled nanostructures. Reproduced with permission from reference [31]. Copyright 2020 Elsevier.

4.4 Synergistic chemo-photothermal therapy

The random aggregation of porphyrin molecule will limit the separation and transfer of photoelectrons and charges, which is more conducive to photothermal conversion. AuPNS showed a dose- and irradiation time-dependent photothermal effect. After 10 min of irradiation, the AuPNS solution rapidly increased to 78°C, and the photothermal conversion efficiency of AuPNS was ~48.2% (Figure 11A) [18]. The infrared thermal image of the AuPNSs droplets can reach up to 50.2°C, while the deionized water droplets hardly change as control, and the AuPNSs show excellent photothermal stability without obvious photothermal decay after five irradiation cycles (Figure 11B). Water-insoluble pyridyl porphyrins can be protonated into water-soluble monomers under acidic conditions. Moreover, high temperatures can accelerate the release of pyridyl porphyrin in an acidic solution. Therefore, the release percentage of AuPNS showed that AuPNS was rapidly released at high temperature (50°C) and low pH (5.0) with a release rate as high as 54.0% (Figure 11C). Subsequently, AuPNS was applied to the antitumor activity test. After modifying cRGD-AuPNS with its cRGD, the confocal images showed stronger red fluorescence, indicating that cRGD promoted the endocytosis of nanodrugs (Figure 11D). Time-dependent flow cytometry analysis showed that cRGD-AuPNS reached a maximum after 6 hours of incubation in the cytoplasm (Figure 11E and F). Au(III) in AuTPyP molecule can inhibit the activity of TrxR and enhance ROS production (Figure 11G). Quantitative testing of the kit revealed potent TrxR inhibition and ROS generation (Figure 11H). Since the level of TrxR in tumor cells was higher than that in normal cells, cRGD-AuPNS showed chemotherapy-killing effect on tumor cells, but no obvious killing on normal 293 T cells. In particular, the killing of HeLa cells was the most pronounced and reached to 85.2% (Figure 11I).

Figure 11.

Synergistic chemo-photothermal therapy of AuTPyP self-assembled nanospheres (AuPNSs). (A) AuPNSs concentrations-dependent photothermal effect. (B) Photothermal stability of AuPNSs. (C) pH values and temperatures stimuli-responsive released plot profile of AuPNSs. (D) Confocal images of HeLa cells treated by cRGD modified AuPNSs. (E) the corresponding flow cytometry analyses of (D). (F) the detailed time-dependent uptake in cRGD-AuPNSs group. (G) Western blot analyses of TrxR and Trx proteins in HeLa cells treated with cRGD-AuPNSs. (H) Kits test of (G). (I) Cytotoxicity comparison of various cell lines treated with cRGD-AuPNSs. Reproduced with permission from reference [18]. Copyright 2021 American Chemical Society.

4.5 Peroxidase-like activity for anti-tumor therapy

Porphyrins and phthalocyanines (Pc) are a class of N-doped carbon-based macrocyclic compounds, and the central ring can combine with different metal ions to form metal complexes. Among them, iron porphyrins are often used to simulate the catalysis of biomimetic enzymes in vivo [33]. Similar to porphyrins, Pc has been widely used in biological phototherapy. Manganese phthalocyanine (MnPc) molecules coordinated with divalent manganese ions (Mn²⁺) were selected as the building blocks, and MnPc assemblies were prepared by microemulsion droplets method. The blended assembly of emulsifier and MnPc reduced the close-packed structure of MnPc molecules, exposed more active sites, and exhibited morphology-dependent POD-like catalytic activity [17]. The kinetic curve of POD-like catalytic reaction of MnPc nanoparticles (MnPcNPs), MnPc nanowires (MnPcNWs), tetraphenyl manganese (III) porphyrin (MnTPP) assemblies (Figure 12A), tetrapyridyl manganese (III) porphyrin (MnTPyP) and N, N-diethyl-4-anilinoporphyrin (MnTNPP) single molecule shows that MnPcNPs exhibit the best POD-like catalytic performance, while MnTPPNPs have almost no catalytic activity (Figure 12B). The catalytic performance of MnTPyP molecules is better than that of MnTNPP molecules, and this difference in catalytic performance is more intuitively reflected in the color change of the reaction system (Figure 12C). These results indicate that the catalytic activity of MnPc assemblies is higher than that of Mn porphyrin assemblies, which may be related to the larger N doping ratio in the phthalocyanine ring and the different oxidation states exhibited by the central metal. MnPc assemblies can be used as a nanozyme to simulate the POD catalytic performance, and exhibits morphology, pH value, and temperature-dependent catalytic performance [34]. The nanomaterials are expected to realize catalytic properties in tumor microenvironment for antitumor therapy.

Figure 12.

(A) TEM of MnTPPNPs. (B) the time-dependent absorbance change curves of various porphyrin-assembled nanostructures. (C) Photographs of TMB peroxidation catalyzed by different materials. Reproduced with permission from reference [17]. Copyright 2021 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021.

Since the POD catalytic performance exhibits temperature-dependent catalytic performance. Similarly, Fe₃O₄ NCs have excellent POD properties (Figure 13A), but due to the limitation of insufficient hydrogen peroxide (H₂O₂) concentration in tumor cells, the generated hydroxyl radicals (•OH) from Fe₃O₄ Fenton reaction are far from enough to kill tumor cells [35, 36]. Through the self-assembly driving force of porphyrins, FeTPP@Fe₃O₄ NPs were prepared by microemulsion droplets method. Under laser irradiation, the photothermal generated by FeTPP assemblies promotes POD performance of Fe₃O₄ NPs, and instantaneously generates a large number of free hydroxyl groups to kill tumor cells (Figure 13B–F). These results clearly demonstrate that FeTPP@Fe₃O₄ NPs can effectively accelerate the catalytic decomposition of endogenous H₂O₂ in an acidic environment under laser irradiation, further accelerate the generation of •OH, and improve the CDT efficacy in vivo [19].

Figure 13.

Photo-enhanced peroxidase-like activity of FeTPP@Fe₃O₄ NPs. (A) Schematic illustration of photo-enhanced POD-like activity of FeTPP@Fe₃O₄ NPs. (B) the pH-dependent and (C) temperatures-dependent POD-like activity of FeTPP@Fe₃O₄ NPs. (D) the time-course POD-like activity of FeTPP@Fe₃O₄ NPs as a function of concentration of TMB. (E) the corresponding Michaelis–Menten kinetics and (F) photo-enhanced POD-like activity of various nanoparticles. Reproduced with permission from reference [19]. Copyright 2022 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2022.

5. Conclusions

Porphyrins possess excellent light-harvesting properties, and have tunable spectral features, functional groups, and central metals, which endow them with unique photophysical properties. Porphyrin self-assembled nanostructures will inherit the characteristics of porphyrin monomer, which make them widely used in photocatalysis, tumor therapy, and other fields. Porphyrin nanostructures have the advantages of less dosage, high activity, easy recycling, stability in photocatalytic applications, excellent photothermal and photodynamic therapy, and nanozyme catalytic performance, which makes them a potential photocatalytic agent and nanotherapeutic platform. However, the current research on the assembly mechanism is still unclear. Various in situ techniques and integrating-sphere-assisted resonance synchronous (ISARS) spectroscopy methods need to be introduced to detect the self-assembly process for a deeper understanding of the packing nucleation process [37]. Moreover, the influence of external fields on the assembly process of porphyrin molecules, such as temperature, humidity, magnetic field, electric field, ultrasound, high-speed centrifugation, need to be urgently explored in further works. More importantly, the structure–activity relationship between the porphyrin morphology structure and performance will provide important technical support and experimental basis for revealing the self-assembly process in living organisms. The exploration of the self-assembly of porphyrins with biofunctional molecules, such as amino acids, polypeptides, and nucleic acids, are very intriguing research direction to form a mature light harvest antenna [38] and self-assembled nanomedicine platform [39, 40], which may achieve major breakthroughs in life sciences. This chapter will provide an important reference for the cognition of self-assembled nanostructures.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21802032 and U21A2085), China Postdoctoral Science Foundation (2019TQ0081), and Zhongyuan high-level talents special support plan (No. 204200510009).

Conflict of interest

The authors declare no conflict of interest.