# Review on polydopamine supramolecular ordering – mechanism elucidation and application in 2D nanocomposites fabrication

Jakub Szewczyk, Divyasree Radhakrishnan, Zuzanna Łukasiewicz, Emerson Coy

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## To cite this version:

Jakub Szewczyk, Divyasree Radhakrishnan, Zuzanna Łukasiewicz, Emerson Coy. Review on polydopamine supramolecular ordering – mechanism elucidation and application in 2D nanocomposites fabrication. European Polymer Journal, 2024, 221, pp.113530. 10.1016/j.eurpolymj.2024.113530. hal-05136589

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Submitted on 30 Jun 2025

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Review

# Review on polydopamine supramolecular ordering – mechanism elucidation and application in 2D nanocomposites fabrication

Jakub Szewczyk<sup>a,b,*</sup>, Divyasree Radhakrishnan<sup>a</sup>, Zuzanna Łukasiewicz<sup>a,c</sup>, Emerson Coy<sup>a</sup>

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A B S T R A C T

Polydopamine has gained recognition in surface modification and construction of functional nanocomposites, among others, due to its extraordinary adhesive properties. However, despite the clear tendency to form in a supramolecularly ordered manner, this mechanism has remained almost completely overlooked. At the same time, polymerization-induced self-assembly has been a hot topic in recent years. In this review, we emphasize the laws and relationships governing the formation of polydopamine nanoparticles, functional coatings and free-standing films in an ordered or semi-ordered manner. We took advantage of both computer simulations and basic research to summarize the state of the art and to set possible guidelines for further research work aimed at full understanding and precise control of the polydopamine supramolecular ordering process. We also present examples of application studies in which the self-assembly tendency of polydopamine and reversible cross-linking through physical interactions were used to create advanced 2D nanocomposites, hydrogels, etc. Finally, we also present future perspectives and research guidelines and encourage further work on this exceptional biomimetic polymer.

1. Introduction

Supramolecular ordering constitutes the basis of self-assembled systems, which undergo spontaneous generation of a well-defined architecture from their components [1]. Polymerization-induced self-assembly (PISA) and, in general, the self-organization of polymers are still thoroughly investigated due to the extraordinary properties of layered and structured organic materials [2]. Their potential applications spread across electronics [3,4], sensing [5], filtration [6,7], catalysis [8,9], and biomedicine [10–12], among others. In recent years, the polymerization-induced self-assembled materials field has evolved significantly due to new chemical synthesis tools and characterization techniques [13]. A unique combination of biocompatibility, broad light absorption, photothermal activity and adhesive qualities has driven particular attention to polydopamine (PDA), especially in recent years [14–20]. In the initial and most common approach, PDA can be synthesized straightforwardly by dopamine autoxidation in a slightly alkaline solution (>pH 7.4). Oxidizing species trigger the formation of the PDA; however, the autoxidation term refers to the fact that the oxygen dissolved in the solution is sufficient for the synthesis [21]. Polydopamine can be also rapidly synthesized in mildly acidic or neutral solutions by incorporating redox-active transition-metal ions [22,23]. In solution, during the dopamine autoxidation process, polydopamine is

formed preferably as nanoparticles suspended in the liquid phase [24,25]. However, it can also be deposited onto solid/liquid [26–28] and gas/liquid interfaces [29,30]. The latter is commonly called air/water in the literature, and therein, thin free-standing PDA films that are formed on the surface of the solution (water) are denoted as a/w-PDA. In the first case, we are merely dealing with PDA coatings deposited on various substrates directly from the solution. The aromatic rings in PDA facilitate the attachment to various substances through π–π interaction, while hydroxyl and amino groups via hydrogen bonds.

The PDA's supramolecular structure is advantageous for applications in surface coating, which is in line with the general interest in PISA-type materials described above. What is particularly exciting is the fact that the degree of order in the structure of eumelanins, which include PDA, significantly affects their optical properties – such as the absorption spectrum [31]. Control of this process could therefore help develop dedicated coatings, e.g. for photocatalysis or radiation protection. However, polydopamine is unique in this respect because, despite its exceptionally high interest in various areas of surface functionalization, the mechanism of its self-organization and the possibilities of manipulating it have been studied and described very briefly to date. Recent discoveries, especially regarding a/w-PDA, lead us to believe that polydopamine tends to form layers composed of flat building blocks connected by physical interactions [23,32–34]. Another noncovalent

<footnote><sup>a</sup> NanoBioMedical Centre, Adam Mickiewicz University, Wszechny Piastowskiej 3, 61-614, Poznań, Poland</footnote>

<footnote><sup>b</sup> Institut Européen des Membranes, IEM, UMR 5635, Univ Montpellier, CNRS, ENSCM Place Eugène Bataillon, 34095 Montpellier Cedex 5, France</footnote>

<footnote><sup>c</sup> Faculty of Chemistry, Adam Mickiewicz University, ul. Uniwersytetu Poznańskiego 8, 61-614 Poznań, Poland</footnote>

<footnote>* Corresponding author.
E-mail address: jaksze3@amu.edu.pl (J. Szewczyk).</footnote>
interaction to consider is the cation-π interaction, which occurs between electron-rich π system (aromatic rings) and cations (alkali metals or nitrogenous cations, e.g. from protonated amino groups) [35]. The formation and structure of PDA coatings and spherical nanoparticles show certain similarities to layered a/w-PDA. This motivated us to conduct a thorough literature review.

In this review, we summarize the current state of the art on the supramolecular ordering in the polydopamine-based nanomaterials (coatings, nanoparticles, free-standing films), aiming to explain the mechanism of the layered self-assembly. We explain in a highly ordered

manner what factors influence the increase/decrease of affinity to self-assembly. We compare experimental data with computational simulations. We carefully cited and discussed the literature on various research methods used to study this phenomenon. Finally, we provide examples of PDA nanocomposites with functional materials of regular structure, multilayer or even 2D crystalline. All this is in search of a unified description of the mechanism and to outline guidelines for authors involved in the topic.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Scheme of the PDA formation mechanism and various interactions at the interface between PDA layer and the Si/SiOx, N-TiO₂ and Au substrates. Time-resolved PDA thickness on various substrates: Si/SiOx (black, squares), N-TiO₂ (red, circles) and Au (blue, triangles) as measured by SE (full symbols) and XPS (empty symbols) (b) AFM surface topography of adherent PDA layers formed on various substrates after 10, 30 and 240 min of deposition (c). Adapted with permission from: J. Svoboda, M. Král, M. Dendisová, P. Matějka, O. Pop-Georgievsk, Unraveling the influence of substrate on the growth rate, morphology and covalent structure of surface adherent polydopamine films, Colloids Surfaces B Biointerfaces. 205 (2021) 111897. https://doi.org/10.1016/j.col surfb.2021.111897. Copyright 2024 Elsevier [45].</figcaption></figure>
## 2. Coatings

Thiol-based self-assembled monolayers (SAMs) are commonly used to form anchoring coatings with precise properties such as high smoothness, tunable chemistry, known orientation, and uniformity [36-39]. Inspired by the composition of mussel adhesive proteins, Lee et al. [21] transformed surface modification techniques by developing polydopamine (PDA) surface confluent films, offering a versatile, substrate-independent modification approach. Since that time, PDA is extensively used as a biomimetic anchoring layer for modifying various substrates [40-43]. Research on the self-organization of PDA coatings has uncovered numerous aspects of their formation and properties [17,44]. The attachment of the macromolecules to substrate surfaces and the mechanisms of film growth are influenced by several factors, such as the nature of the substrate, the deposition technique, and the properties of the macromolecules themselves [17].

Within the most widespread approach, which imitates the natural conditions occurring for the sea mussels, dopamine monomers (DA) polymerize in alkaline solutions with abundant dissolved oxygen, leading to the formation of oligomers of various dopamine oxidation and thus forming nanoparticles that can adhere to nearly any solid surface [21]. PDA's adhesion comes from orthoquinone/catechol moieties and amino groups, which form physical, coordination or covalent bonds with almost all substrates, as schematically shown in Fig. 1a. In this section, we will discuss in detail the influence of substrate chemistry on the formation of PDA coating.

To investigate how substrates influence PDA conformal coatings, Svoboda et al. [45] prepared Si/Au and Si/N-TiO₂ substrates and employed various surface-sensitive techniques. The SE study characterized the growth of PDA layers on different substrates, demonstrating time-resolved increasing coating thickness (Fig. 1b). XPS data further confirmed a linear growth in the thickness of PDA on all substrates over a 4-hour polymerization period, with notably thicker layers observed on gold substrates. Significant differences in chemical composition were observed between PDA layers formed on gold and surfaces terminated with oxides/reactive hydroxides like silicon and N-doped TiO₂ during the early stages of layer formation. For the latter, the predominance of sp³ carbon species and very low intensity of π → π* shake-up structures was notable. In turn, initial PDA layers formed on the Au substrate indicated increased sp² species, thus π-π interactions. However, as the layers developed further, a uniform chemical structure emerged regardless of the substrate type, especially the uniform presence of sp² species in the thick PDA coatings on various substrates [45]. Such observations agree with the AFM topography results. Fig. 1c shows the lack of significant topological features on Si/SiOx and N-TiO₂ surfaces for the short deposition times, which was the opposite of the Au surface with an intrinsically large number of grains. However, after approx. 4 h, all surfaces look similar and are characterized by similar roughness [45]. The combined findings from XPS and GAATR-FTIR indicate that as polymerization times increase, the structure of surface-bound PDA layers remains consistent, while the substrate type primarily influences the initial stages of polymerization. The ability of PDA to form uniform coatings across various substrates, alongside its potential for further functionalization with self-assembled monolayers, underscores its significance in surface modification and advanced material applications [45].

Another interesting observations about thin PDA coatings on the Au surface, deposited at a short time (< 10 min), were provided by another experiment [46], which showed by means of electrochemical methods that intermediate dopamine oxidation species such as leucodopamine-chrome or dopaminechrome are absent or are at low concentration in the as-deposited coatings. This leads to the conclusion that the surface of a noble metal, which is not inclined to bind to the oxygen available in catechol groups, is susceptible to PDA modification through physical interactions with aromatic rings. It is also worth mentioning that the AFM tests carried out in this experiment also coincide with those

described above [45,46].

Creating nanometer-thick polydopamine coatings on surfaces is crucial for achieving precise molecular-level structures. Research indicates that forming complete PDA layers, ranging from 0.5 to 1.1 nm thickness can be achieved within 60 to 300 s. This highlights the potential of ultrathin PDA films as molecular adhesives, laying the groundwork for developing functional surfaces [47]. Fig. 2a shows the coverage ratio over various immersion times, revealing two distinct stages: an initial rapid increase within the first 150 s followed by a gradual rise over longer durations. Alongside the coating growth rate change within first minutes it is also clear, that the thicker the coating becomes, the more its surface energy decreases (Fig. 2b). This suggests structural change upon coating growth, resulting in dissimilar surface chemistry.

The process of depositing PDA layers on substrates follows the Volmer-Weber growth theory, where PDA molecules initially adsorb onto the surface (Fig. 2c, d), forming isolated clusters or islands (Fig. 2e) that eventually merge/percolte into a continuous film through strong π-π interactions and hydrogen bonding (Fig. 2f) [47].

One prominent mechanism that has been described for PDA coatings is also cation-π assembly [35]. An interesting related phenomenon is the reversible cation-π interaction stimulated by strong base cations. Na⁺ triggers dopamine amine group deprotonation, which causes instant dissociation of polydopamine into fragments, indicating the importance of cation-π bonds. However, recovery of these interactions can be achieved by adding potassium ions, as they compensate for the loss of amine cationic species in polydopamine. This is because cation-π binding force is larger for K⁺-π compared with Na⁺-π [48].

There is a compelling biomimetics-inspired argument that cation-π is the prominent self-assembly mechanism. The enhanced water solubility in aromatic pi configurations followed by broad and strong light absorption (Fig. 3a and b) and multifunctionality due to various aromatic ring structures was achieved in nature by cationic group incorporation into the aromatic system. Certainly, the complex mechanism of dopamine oxidation and subsequent self-assembly of the oxidizing species could be facilitated by the cation-π interaction, where the cation serves as a permanent bridge for the further assembly of catecholamine species generated in the synthesis pathway. In particular, cation-π progressive assembly of heterogeneous chromophores (e.g. dopamine-quinone, DHI, indolequinone, and quinone-imine) and later larger oligomeric species, leads to broad and strong UV to near-infrared (NIR) light absorption (Fig. 3c).

Computational studies affirm that PDA building-blocks tend to form higher-order aggregates of DHI units, providing insights into the structural characteristics of dimers, trimers, tetramers, and larger oligomers [32,33,47,49]. To investigate the molecular structures of PDA and eumelanin, researchers use computational methods such as isomer generation, MD (Molecular Dynamics) simulations, and DFT (Density Functional Theory) calculations [49].

Eumelanin protomolecules, measuring about 15–20 Å, form tetramers and pentamers through covalent bonding [49]. DFT-optimized flat tetramers stack into layered aggregates, essential for eumelanin’s structure, while less flat tetramers create disordered structures. Flat molecular models are crucial for accurately simulating the self-assembly of PDA and eumelanin. Based on existing literature, Fig. 4a proposes a hypothetical mechanism for catechol polymerization, which is relevant to the formation of PDA and eumelanin. Fig. 4b illustrates two possible DHI radical species involved in these reactions and their resonant structures resulting in radical electron delocalization. Fig. 4c shows the DFT-optimized geometry of the most planar tetramer. When eight tetramers are placed in a simulation box, they stack together rapidly to form a layered aggregate (as seen in Fig. 4d) during the simulation. Conversely, Fig. 4e illustrates the DFT-optimized geometry of a tetramer with a projection product of 0.29, where stacking into a layered aggregate does not occur; instead, they form an amorphous structure, shown in Fig. 4f. This result emphasizes the criticality of choosing
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Time-resolved coverage ratio (%) function for complete PDA coatings (a). Time-resolved water contact angle (°) and surface energy (mJ m⁻²) for PDA/mica (b). Scheme of the mechanism of the PDA coating deposition (c-f). [47]. Adapted from Ref. [47]. This work is licensed under CC BY 4.0.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** UV-vis absorbance spectra of natural eumelanin (a) Time-resolved UV-vis spectra during polydopamine formation in tris buffer (pH 8.5) up to 12 h. The black arrow represents the change in the solution color from dark brown to black (b). A scheme of covalent and noncovalent interactions during progressive self-assembly of PDA, resulting in a broad light absorbance (c). Adapted from Ref [35]. This work is licensed under CC BY 4.0.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Possible catechol polymerization reaction mechanism-simplified (a). DHI radical species- two possibilities, and the resonant structures ensuing from the radical electron delocalization (b). Structures of various tetramers and combinations: The molecular structure of the most stable tetramer (c). The layered aggregate formed by 8 of the most stable tetramers (d). The molecular structure of the least stable tetramer (e). The amorphous aggregate formed by 8 of the least stable tetramers (f). The brute-force algorithmic method uses multiple layers of checkerboard plates to generate various DHI oligomers (g). Adapted from Ref. [49]. This work is licensed under CC BY 3.0.</figcaption></figure>

accurate molecular models for simulations of PDA and eumelanin to ensure significant findings. Multi-layers of checkerboard plates represent various stages of DHI oligomer polymerization. For instance, Fig. 4g demonstrates the construction of a tetramer using four layers of checkerboard plates. Fig. 4h utilizes eight orientations of a DHI monomer to generate DHI oligomers. These orientations enable the arrangement of different DHI oligomers by placing two or more DHI monomers on checkerboard plates [49].

We would also like to describe the compelling methodology used and described by the authors, due to its complexity and accuracy. Reactive sites and potential reaction pathways of DHI were pinpointed using reactivity measures like the Fukui function and spin density calculations. These measures were then fed into a brute-force algorithmic generator, which created all possible chemical isomers for small DHI oligomers. From nearly 3,000 isomers, DHI's most stable dimers, trimers, and tetramers were chosen to develop molecular models that enhanced the precision of modeling PDA and eumelanin [49].

Another milestone MD simulations of the interfacial adhesion and precipitation of PDA and DHI on SiO₂ substrates were performed very recently, together with the experimental examination [50]. The precipitation and coating formation of PDA and DHI showed significant differences after a 24-hour reaction. Most importantly, the collected PDA precipitates have lost the ability of coating formation, but still maintained a certain degree of adhesion. The affinity of the SiO₂ surface for dopamine is significantly higher than that for DHI species, as evidenced by a larger number of adsorbed dopamine molecules. The experimental part of this study involved the characterization of Raman spectra, XPS, UV-vis absorbance, and ζ potential. It indicates, that the aminoethyl moieties remain in PDA coating, precipitate and its supernatant upon assembly, but are absent for DHI. All this suggests that the initial adsorption state of dissolved dopamine molecules onto the SiO₂ before oxidative coupling, plays a crucial role in the formation of a uniform PDA coating, due to the most active H-bonding interactions.

DHI did not form a film due to the lack of primary amine-participated H-bondings. In summary, it was shown, that the contribution of catechol to the PDA coating formation, lies more in the promotion of the oligomeric building blocks formation, rather than facilitating interfacial adsorption. Extraordinary adhesion should not be attributed to the DHI-based eumelanin-like chemistry but rather to the aminoethyl group of the dopamine precursor [50].

One strategy to improve the quality of PDA coatings is to slow down the oxidation process. A study used borax-protected dopamine to enhance PDA properties, resulting in uniform nanoparticles with low aggregation and surface coatings with controlled thickness and adjustable water contact angles [51]. Generally, the use of boron-based oxidation modifiers (borax, boric acid) slows down the oxidation path of dopamine to polydopamine because the reversible reaction of the synthesis of catechol-boron complexes takes place [52]. As a consequence, the formation of a supramolecular structure may occur with a greater share of hydrogen and π-π interactions [23]. This phenomenon can be explained very effectively with the previous paragraph.

Applying PDA using the spray coating method does not strictly concern the mechanism of coating formation discussed in this chapter, but it should be mentioned as spray coating has great application potential [53–55]. Moreover, as we will describe shortly, the understanding of PDA-spray coating optimization performed by Hong et al. [49] agrees well with the general model of dopamine oxidation and PDA formation described in this work. To enhance the coating kinetics, conditions were optimized by controlling the pH, dopamine concentration, and the amount of oxidizing agent. Unconverted catechol groups are essential for surface adhesion. Notably, PDA coating did not occur above pH 10.1, indicating that strong alkaline conditions completely inhibit PDA coating on material surfaces. Importantly, it was proven that the most favourable conditions for quick and effective PDA deposition were a relatively low PDA concentration (4 mM), a pH of 9.5 and the presence of an oxidant [56]. As we will describe in the next section, such
conditions favour the generation of large numbers of small PDA nano-particles (many nucleation centers). In the spray coating method, a large amount of small nucleation centers is more advantageous than a small amount of large nucleation centers because it allows for effective coverage of large surfaces with an even layer [57].

What is exciting in this context is that self-assembled-monolayer organic monolayer growth (e.g. island size, orientation) can be significantly controlled via surface chemical termination and surface energy [58]. Thin PDA coatings can, therefore, probably be applied according to precisely defined patterns or nanoarchitectures, and the influence of the substrate can be used for advanced surface engineering purposes [59].

### 3. Nanoparticles

Numerous recent studies have focused on exerting PDA's self-assembly to obtain new nanostructures [60]. In this publication, we will discuss the topic of polydopamine-based 2D-type nanocomposites in a separate chapter (see chapter 5). Here, we would like to focus on the current state of knowledge regarding the mechanism of spontaneous formation of spherical PDA nanoparticles. Spherical polymer nanoparticles with well-defined parameters (size, dispersion, zeta potential, etc.) were achieved using various strategies e.g. solvent evaporation (SORP: self-organized precipitation method) [61,62], emulsification/solvent diffusion [63], matrice-based [64], pre-baking [65] and other. It was also shown, that well-controlled and stable polydopamine particles can be obtained in the presence of proteins, e.g. human serum albumin, lysozyme (from hen egg white) and bovine α-lactalbumine [66,67]. However, for polydopamine, the formation of perfectly spherical nanoparticles occurs completely spontaneously as a result of the autoxidation of dopamine in solution, and the parameters of the obtained nanospheres can be controlled by changing the synthesis conditions. The size of PDA nanoparticles is crucial for many applications, as it influences the color of their dispersions and other critically important properties [68]. Question arises, whether attempts to control the size of nanoparticles available in the scientific literature, can provide us with more insights about the self-assembly process of the PDA.

One of the basic modifying factors is the pH of the solution. Although obtaining PDA nanoparticles is possible in an acidic environment [22,23,69,70], it is necessary to use a strong oxidant, e.g. transition metal ions [22,23,69,70], sodium periodate [71–73] or ammonium persulfate (APS) [73,74]. In this review, we focus on the autoxidation process and, therefore, concern basic pH conditions, the range usually taken into account in scientific publications, from 7.5 to 10 [70,75,76]. Wu et al. investigated the nucleation affinity and the growth dynamics of PDA spherical nanoparticles synthesized by autoxidation of DA using different amounts of NaOH, thus the $OH^-$ ions concentration [76]. They also investigated the influence of other factors, such as DA concentration, oxidant type, reaction temperature and time. The results suggest that mixing DA with NaOH is a particularly effective method for controlling the diameter and distribution of PDA spheres [76,77].

In a very recent work, the existence of crystalline regions in conformation to conventional amorphous structure was reported within the suspended PDA nanoparticles [78]. They carefully studied the behavior of nanoparticles towards differential scanning calorimetry (DSC). They observed characteristic endothermic peaks, which should be associated with the fusion of crystalline regions. Especially, a broad peak with a maximum between 98 and 146 °C was observed (Fig. 5a–c). The Authors linked it with the melting process of PDA, i.e. destroying of the $π-π$ lamellae crystals (layers). Broad maximum and melting temperatures variations upon different synthesis conditions were assigned to crystallite imperfections and non-uniformity in their size. Further, utilizing DSC, they associated TRIS buffer concentration and synthesis temperature with the crystalline fraction. In turn, XRD analysis disclose a peak located between 21.6 and 23.4 (2θ), corresponding to variation in the distance of the layers between 3.4 and 4.1 Å. (Fig. 5d) However, all peaks – regardless of the synthesis conditions- were centered at 23.1 ± 0.1 (2θ), typically for graphitic-like structure and suggesting a d-spacing of ~3.4 Å (Fig. 5e) [78].

Recently, it was shown that aromatic molecules (e.g. folic acid [79,80], curcumin [81]) could promote templating and stacking of PDA protomolecules along the z direction with a graphite-like spacing. It is on the verge of controlling the production of supramolecular ordering processes and nanocomposites. However, we examine these instances,

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Temperature dependent DSC thermograms obtained from PDA synthesized at different dopamine concentration: 1.5 mM (a), 4.5 mM (b), and 7.5 mM (c). XRD spectra of PDA nanoparticles obtained with various synthesis condition (d). 3-layers stacked model for PDA with interplanar distance derived from XRD. Atoms as represented by colors: black-carbon, blue-nitrogen, red-oxygen, and white-hydrogen (e). Adapted from Ref. [78]. This work is licensed under CC BY-NC.</figcaption></figure>
presuming high resemblance of the supramolecular PDA nanostructure ordering phenomena and π-π interactions in the organic-organic self-assembly systems. An additional important criterion is the composition of the obtained material, because in the approaches mentioned in this paragraph, the final product is chemically polydopamine, and the templating stacking agents were separated during the synthesis steps Mesoporous polydopamine nanospheres (MPDA) were fabricated as they exhibited great potential for loading with organic dyes due to their high adsorption capacity through π-π interactions [82]. Triblock copolymer Pluronic F127 (EO106PO70EO106) and 1,3,5-trimethylbenzene (TMB) were used as the organic agents. The use of F127 and TMB resulted in mesoporous PDA nanospheres (Fig. 6a), while the use of F127 alone resulted in nonporous PDA nanospheres (Fig. 6b).

The authors first presented the typical surface morphology of nanoparticles obtained using reagents in the following proportion 38577 EtOH:156067 H2O:28 TRIS:1 F127:112 TMB:12 dopamine. Further analysis were aimed at examining the usability of TMB towards creating mesoporous PDA. As prepared MPDA particles were applied for adsorption via π-π interactions of a model dye, i.e. RhB, at the ultra-high level (~1100 µg mg⁻¹).

Following this, authors describe synthesis mechanism and planar stacking of the PDA macromolecules in a highly ordered manner through π-π interactions. The kinetics of particle formation were studied by monitoring changes in the UV-vis absorption spectra of the reaction solution over time (Fig. 7a). The absorbance at 400 nm intensity increase was utilized to study the dopamine polymerization to poly-dopamine rate in MPDA and NPDA reaction solutions during 24 h (Fig. 7b). What is striking is the significant difference in the interval of 7–10 h, where the retarded yet more significant increase in the case of MPDA indicates TMB-modified interactions involved in the growth of spherical nanoparticles. To understand this phenomenon, it was necessary to conduct a DLS time resolved study (Fig. 7c). The results suggested that the TMB-rich emulsion droplets might account for the

assembly of the already nucleated PDA < 10 nm nanospheres, formed during first 4 h of the synthesis [82].

Finally, the X-ray diffractogram of MPDA revealed a peak at 2θ = 22° which corresponds to the π-stacked structures in polydopamine, with a d-spacing of approximately 4 Å [79,83,84]. This also resembles the diffractograms obtained for free-standing PDA films obtained at the water/air interface [32], which will be discussed in the next section. Considering this, a TEM study was performed for different synthesis times of MPDA nanospheres (Fig. 7e-i). Ultimately, all primary particles were diminished (after 24 h) because they have been attached to larger MPDA nanostructures via π-stacking interactions. We can clearly see here that given appropriate synthesis conditions, polydopamine has the ability and even tendency to increase in a layered-stacking manner [82].

Inspired by the uniform size distribution of eumelanin grains, efforts are underway to create stable polydopamine (PDA) nanoparticles in solution, leveraging the structural similarities between PDA and eumelanin, the pigment found in skin and hair [85,86]. At the same time, efforts are taken to gain control on ordering or disorder in synthetic eumelanins, which would have great applications in fields such as cosmetics and biomedicine. Lampel et al. used tyrosine-containing tripeptides as tunable precursors for polymeric pigments with different degree of structural ordering [87]. They investigated sequence-dependent properties, as shown schematically in Fig. 8a. They managed to identify a sequence of peptides that self-assemble into supramolecular nanostructures when oxidized enzymatically in the neutral pH. This reflected in the obtained nanoscale morphologies, as evidence by TEM micrographs (Fig. 8b). Single-crystal x-ray diffraction enabled to distinguish several interactions building the crystal lattices, as presented in the visualization for DYF sequence (Fig. 8c) and YFD sequence (Fig. 8d). Importantly, they denoted a clear evidence of stacking along the z axis, as labeled in the Fig. 8c- hydrogen bonding of the tyrosine hydroxyl groups (3) and aromatic π-π stacking (4). Generally, they showed, that peptides with paired aromatics gave rise to self-assembly in

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Scheme on the proposed mechanism for the fabrication of mesoporous PDA (a) and nonporous PDA nanoparticles (b). Adapted with permission from: F. Chen, Y. Xing, Z. Wang, X. Zheng, J. Zhang, K. Cai, Nanoscale Polydopamine (PDA) Meets π-π Interactions: An Interface-Directed Coassembly Approach for Mesoporous Nanoparticles, Langmuir 32 (2016) 12119–12128. https://doi.org/10.1021/acs.langmuir.6b03294. Copyright 2024 American Chemical Society [82].</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** MPDA reaction solution UV-vis spectra at different synthesis times (a). UV-vis absorbance at 400 nm transient evolution for NPDA and MPDA suspensions (b). DLS transient monitoring of the hydrodynamic diameter distributions evolution of the MPDA reaction suspension (c). MPDA samples X-ray diffractogram (d). TEM images of MPDA solutions during synthesis were captured at 2 h (e) to 4 h (f), 8 h (g), 11 h (h), and 24 h (i). Adapted with permission from: F. Chen, Y. Xing, Z. Wang, X. Zheng, J. Zhang, K. Cai, Nanoscale Polydopamine (PDA) Meets π-π Interactions: An Interface-Directed Coassembly Approach for Mesoporous Nanoparticles, Langmuir 32 (2016) 12119–12128. https://doi.org/10.1021/acs.langmuir.6b03294. Copyright 2024 American Chemical Society [82].</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Scheme of the selected tripeptide sequences and the controllable polymerization (a). TEM images of tripeptide structures formed by self-assembly, scale bar is 100 nm (b). Tripeptides DYF (c) and YFD (d) crystal structures exhibiting various interfaces forming the lattice. Adapted with permission from: A. Lampel, S.A. McPhee, H.-A. Park, G.G. Scott, S. Humagain, D.R. Hekstra, B. Yoo, P.W.J.M. Frederix, T.-D. Li, R.R. Abzalimov, S.G. Greenbaum, T. Tuttle, C. Hu, C.J. Bettinger, R. V Ulijn, Polymeric peptide pigments with sequence-encoded properties, Science. 356 (2017) 1064–1068. https://doi.org/10.1126/science.aal5005. Copyright 2024 Science- The American Association for the Advancement of Science [87].</figcaption></figure>

supramolecular order [87]. However, all polymers, including amor-
phous ones, exhibited some similar features, i.e. the peaks in the ranges
of 4.4 to 4.8 Å and 2.9 to 3.2 Å, corresponding with previously reported
interplanar distances in the typical eumelanin stacking [32,49].

4. Free-Standing films at the Air/Water interface

In the previous section (see Chapter 2), we described the extraordi-
nary ability of PDA to functionalize virtually any substrate. The

influence of this substrate's chemical and morphological properties on
the deposited PDA coating was also described, including the possibility
of obtaining partially or highly ordered structures. In fact, we should
denote polymerization directly onto substrate's surface as the deposition
at the solid/liquid interface. The interface-assisted polymerization
techniques towards self-organized polymers have been broadly investi-
gated in recent years [88–91]. A particular type of interface–gas/liquid
(air/water) – is an ideal candidate for obtaining highly ordered struc-
tures due to its extraordinary properties. Namely, in conditions close to
normal, the water surface is perfectly flat and limits the mobility of
molecules to 2 dimensions (2D) [92], which opens vast opportunities in
structuration strategies [93–95].

As expected, it was investigated whether it is possible to effectively
obtain polydopamine at the air/water interface (a/w-PDA). The first
successful experiment was carried out in 2014, and the assumption was
confirmed [29]. Therein, the dopamine concentration in the Tris buffer
(pH = 8.5) was of 2 mg mL⁻¹, there was no stirring of the reaction so-
lution. Fig. 9 a and b show that the resulting a/w-PDA was transferred
on a polymeric substrate, covering an area of around 0.25 cm². More-
over, it was shown that it can be transferred onto different solid sub-
strates e.g. glass, silicon and that as deposited a/w-PDA films cannot be
detached easily by using a flow of water. The dynamics of the formation
of the film at the air/water interface were monitored by drop tensiom-
etry. The authors conclude that under these synthesis conditions, for the
first 20 min, the film is not yet formed, and in the 20–40 min interval, it
consists of very small island-like fragments that do not create a contin-
uous and homogeneous structure (Fig. 7c). Importantly, the authors
provided a comparison to the PDA coatings deposited at the solid/liquid
interface under the same conditions. UV-vis absorption spectra in the
range 200–600 nm revealed a significant increase in absorption for a/w-
PDA compared to s/l-PDA [29]. From the perspective of later discov-
eries, which are described later in this section, we can conclude that the
differences in the absorption spectrum resulted from structural differ-
ences between a/w-PDA and s/l-PDA, because the level of order in the
structure of polymers (2D vs amorphous) may reflect in UV–vis ab-
sorption [96–98]. Nevertheless, the above-described experiment was the
first milestone on the way to the development of a/w-PDA films with
high application potential. Subsequently, there was interest in the topic
of obtaining and modifying a/w-PDA, which was described in numerous
publications.

It is not the purpose of this chapter to chronicle all the knowledge
collected so far on a/w-PDA as this has already been done in other
comprehensive review articles [14,17,30]. Instead, we will consider in
detail the extraordinary ability of polydopamine to form layered struc-
tures on the water surface, which was discovered relatively recently
[32,99] and represents another milestone in the development of a/w-
PDA films with high application potential. However, several more
works should be presented that contributed to the development of an
effective synthesis of a/w-PDA, which resulted in a subsequent

breakthrough in the study of their structure.

One important study to mention is the synthesis of nanocomposite of polydopamine and poly(ethylenimine) (PEI) at the air/water interface, resulting in Janus-like and mechanically stable film [100]. Therein, the Authors showed no film formation under N₂ environment and vacuum, and therefore, the oxygen from the gas phase is a critical factor for the efficient thin film formation at the air-water interface. However, they claimed the need to keep unstirred conditions to form a mechanically stable catecholamine film. Later in this section, we will show some recent experiments in which soft stirring conditions significantly improved the morphological properties of the a/w-PDA.

Another article [101] revealed that to achieve a uniform thickness of these films, the convective flows from rapid water evaporation at the water surface must be avoided during the formation of a/w-PDA, preferably by covering the vessel with a lid with a small gap, to enable essential oxygen supply, as described above. The authors were also the first to notice the need to consider the spontaneously formed spherical PDA nanoparticles in the reaction solution, which may change the morphological properties of a/w-PDA. It has been presented schematically in Fig. 9d- the reaction pathway for dopamine oxidation to PDA and Fig. 9e- the nanoparticles aggregation evolution.

Particularly important was the discovery that a/w-PDA has the
ability to reversibly adsorb and desorb moisture in a strictly quantified
manner, which affects its electrical conductivity [102]. This level of
electrical conductivity (resistance of 10⁸-10¹⁰ Ω) allows a/w-PDA to be
classified as an organic semiconductor. This would be an unexpected
behaviour for mere aggregates of PDA nanoparticles forming at the
water/air interface. This is more resembling the characteristics of con-
jugated polymers [103,104]. The ability to effectively transport charge
carriers allows the use of a/w-PDA, e.g. in the field of photocatalysis,
which will be presented later in this chapter. It is worth emphasizing
that reversible, super-fast and highly quantified moisture adsorption/
desorption was also noticed in PDA coatings obtained by electro-
chemical deposition at the solid/liquid interface [105].

Another vital observation was the fact that the speed of oxidative
polymerization of dopamine to a/w-PDA films has been changed by
enriching reaction solutions with various metallic cations (Na⁺, Ca²⁺,
Mg²⁺ and Co²⁺). This is due to the different intermediates involved in
the dopamine oxidation process, namely complexes created via coordi-
nation between dopamine and metallic cations [106]. Importantly, there

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** The a/w-PDA floating on the water surface (a) and transferred on the polymeric substrate (b). Drop tensiometry during a/w-PDA formation at the water surface, surface tension vs time (c). Adapted with permission from: F. Ponzio, P. Payamyar, A. Schneider, M. Winterhalter, J. Bour, F. Addiego, M.P. Krafft, J. Hemmerle, V. Ball, Polydopamine films from the forgotten Air/water interface, J. Phys. Chem. Lett. 5 (2014) 3436–3440. https://doi.org/10.1021/jz501842r. Copyright 2024 American Chemical Society [29]. Scheme of the reaction pathway for dopamine oxidation to polydopamine (d) and the aggregation evolution of dopamine nanoparticles in the solution and the film at the water surface (e). Adapted with permission from: T.F. Wu, J.D. Hong, Dopamine-melanin nanofilms for biomimetic structural coloration, Biomacromolecules. 16 (2015) 660–666. https://doi.org/10.1021/bm501773c. Copyright 2024 American Chemical Society [101].</figcaption></figure>
was no close relationship between the chemistry and morphology of nanoparticles and a/w-PDA films obtained using the same cations, which was further indirect evidence that the formation of a/w-PDA does not occur through agglomeration of nanoparticles. Later in this chapter, we show how the presence of various metallic ions can affect the degree of order in the laminar structure of the a/w-PDA.

The initial report on the possible quasi-ordering of the a/w-PDA structure appeared first in 2020 [99]. Utilizing High-Resolution Transmission Electron Microscopy, authors reported a transformation from amorphous to quasi-ordered structure of a/w-PDA films resulting from exposure to the electron beam. These structural changes were assigned to the pyrrole ring-closing reaction initiated by a high-energy electron beam. Interestingly, the spacing between PDA chains (about 0.3–0.4 nm) was very close to the typical graphite interlayer space, as indicated by HRTEM. Based on this, it can be concluded that they were the first to notice that a/w-PDA films have a certain degree of structure order, although this could also have been achieved in the previously described experiments. However, the authors concluded that the high-energy HRTEM electron beam induces a high level of order in the a/w-PDA structure. Yet, in 2021, a study was published in which the formation of 2D-like ordered laminar a/w-PDA films was carefully examined, described and partially explained [32]. As previously, PDA synthesis protocol was conventional (Tris pH was 8.5, dopamine concentration was around 0.5 mg ml⁻¹), but this time, a small stirring bar was rotated at 300 rpm to mix the solution gently. However, the real uniqueness of this experimental approach results from a meticulous examination of the obtained a/w-PDA structure. Namely, the analysis was aimed at examining whether a/w-PDA has a highly ordered structure resembling 2D materials.

HRTEM micrographs were taken after transferring the free-standing films onto TEM grids (grid size 204 µm). The first thing that causes notice is the fact that despite being nanometrically thin, the a/w-PDA free-standing film stretches over the entire surface of the mesh without defects or cavities (inset in Fig. 10a). Regarding the HRTEM results specifically, at high magnification, a layered section was found in Fig. 10a. Layers were identified in the image from L1 to L4. While HRTEM micrographs show the lack of ordering in the nanometers range, a selected area electron diffraction (SAED) patterns show no in-plane distinctive organization.

XRD diffractogram of the a/w-PDA transferred onto silicon substrate revealed a set of peaks at ~10.7°, ~21.6°, ~32.4°, and ~41° (Fig. 10b). Herein, the resemblance with XRD characteristic patterns of 2D carbon-based materials such as graphene oxide (GO) and reduced graphene oxide (rGO) is prominent, suggesting well-oriented 2D-like domains abundant in the a/w-PDA. GiXRD scans at different Ψ angles were examined (0–45°) as shown in the inset in Fig. 10b. As can be seen, the intensity of the peaks decreases at higher Ψ and disappears completely at Ψ ≈ 30°. This suggested that the planes were parallel to the substrate interface and, therefore, the air/water interface. This was shown in

Fig. 10c and d.

The protomolecule stacking of 3–4 layers consisting of five to seven monomers of DHI has been proposed as a PDA organization structure. Authors noticed that the interplanar spacing of the stacking proto-molecules reported in the literature for natural and synthetic eumelanins corresponds very well with the experimental data for a/w-PDA, especially d = 8.3 Å (2Θ = 10.7°) and d = 4.1 Å (2Θ = 21.6°) [32].

Another recent article aimed at optimizing the a/w-PDA synthesis conditions, using different dopamine concentrations, various pH levels, and stirring speeds to achieve centimeter-scale homogenous and very thin free-standing films [33]. A novel in-situ spectroscopic reflectometry approach enabled control of the growth with a few nanometers of precision over a large surface. However, from the point of view of this review, the most important aspect was the molecular dynamics computation studies towards understanding the molecules docking mechanism and the influence of the air/water interface.

As mentioned above [32] only one possible aggregate growing scheme was previously identified in the literature. The purpose of molecular dynamics simulations was to demonstrate whether this is the only mechanism or whether there are other energetically justified ones. First, using VINA docking analysis, the receptor-ligand interaction sites were determined. Next, 3 specific interaction sites were revealed related to three types of possible aggregate growing schemes. One previously mentioned normal stacking (stable tetramer formation) is shown in Fig. 11a and two possibly new, where tetramer interacts with one of the faces of the aggregate already grown, once the cluster exceeds the size of 5 molecules. Due to the difference in the tetramer molecule orientation concerning the side face of the growing stack, they were named 3:1 (the tetramer molecule is aligned across the stack) and 4:1 (the tetramer molecule's longer axis is joined along with the stack) as shown in Fig. 11b. Energetically, aggregation is weakly reversible, which is another justification for the self-assembly of tetramers into larger aggregates.

Examining the effect of the water surface for the aggregation of the molecules, it was concluded that following the initial orientation at the a/w interface via a single layer of smectic-C alignment, π-π interactions dominate further polymer growth. This indicates that the initial orientation of the a/w-PDA is perhaps similar to the early growth stages of PDA on Au surfaces, meaning that in this respect, the gas/liquid interface is somehow similar to the solid/liquid interface if certain criteria are met. Progressively, the layered aggregates are energetically preferable during PDA formation at the air/water interface. However, their size is varying. Hence, the stacking alignment deviates somewhat from ideal parallel ordering (Fig. 11c).

Considering that the aggregation of the PDA building blocks mechanism follows two possible pathways, namely, covalent or non-covalent self-assembly, the next article shows that it is possible to control the path of dopamine oxidation towards a more or less covalent structure [23]. For this purpose, oxidation agents were used, namely Cu²⁺ ions (a strong

<figure><img src="image_13.png" /><figcaption>Fig. 10. HRTEM images of transferred a/w-PDA on TEM grids. Labels L1 to L4 show the increment of layers found in the samples (dashed lines represent the borders). The inset shows an image of the grid taken with an optical microscope (a). XRD diffractogram for a/w-PDA transferred on a silicon substrate. Inset presents the Ψ vs 2Θ-ω scan with yellow markers pointing to diffraction peaks (b). 3D Scheme of eumelanin molecule (red - oxygen, blue - nitrogen). The scheme shows the stacking of the molecules, the interplanar distance and the triple-stacked eumelanin monomer (c). Proposed superior order - a few stacks of three protomolecules ordered with an 8.1 Å periodicity (d). Adapted from Ref. [32]. This work is licensed under CC BY 4.0.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** The total number of clusters ($N_a$) versus cluster interaction size-dependent energy, inset: visualization of normal stacking (a). Scheme of the normal stacking and 3:1/4:1 types of stacking (b). Molecular Dynamics simulation snapshots showing semi-smectic C arrangement at the a/w interface- left panel. Zoom-in of the π-π stacked layers with the interplanar distance- right panel (c). Adapted from Ref. [33]. This work is licensed under CC BY 4.0.</figcaption></figure>

oxidizer) and boric acid (BA) as an antioxidant.

Most importantly, C1s XPS high-resolution spectra (Fig. 12a-c) revealed the presence of π-π bonds for a/w-PDA and BA-modified-a/w-PDA films, especially for the latter. This constitutes important evidence of the non-covalent self-assembly mechanisms of PDA and BA-modified-a/w-PDA aggregation through π-π interactions. The deep analysis of the C1s spectrum confirmed that a/w-PDA and BA-modified-a/w-PDA subunits form the hydrogen bonds and π-π interactions via the catechol groups and aromatic skeleton. This effect was especially significant when using boric acid. In turn, strong oxidant Cu²⁺ promoted covalent oxidative polymerization of the monomer subunits, resulting in rare connections through physical interactions. The analysis of N1s high-resolution spectra further confirmed this.

XRD, UV-vis spectroscopy, DLS spectroscopy and FTIR indicated that the reversible boron-catechol mono-complexes synthesis prevents oxidative covalent crosslinking and promotes non-covalent crosslinking of the dopamine oxidation products. This translates into favorable π-π interactions thus, superior morphological and mechanical properties of the obtained BA-modified-a/w-PDA free-standing films. Young's modulus value for BA-modified-PDA exceeds almost twice the value obtained for pure a/w-PDA, reaching up to 24.1 ± 5.6 and 18.3 ± 6.4 GPa, as measured by nanoindentation (Fig. 12d) and Brillouin light scattering (Fig. 12e and f), respectively. Importantly, the literature provides evidence that polymer materials with a weak physical crosslinking level undergo deformation easily, which is caused by a reduction in molecular order under external stimuli [107]. This corresponds very well with the very low Hardness inferred from the nanoindentation experiment for the Cu²⁺-modified-a/w-PDA, it was 0.05 GPa (while for BA-modified-a/w-PDA it was 0.26 GPa).

Summarizing this chapter, it can be concluded that a certain level of structural order in a/w-PDA films is energetically reasonable and quite common but has been overlooked in numerous experiments until 2020. This is due to several factors. Firstly, X-ray Diffractometry requires the use of sophisticated methodology to observe this phenomenon in a/w-PDA [32]. Secondly, in great simplification, the mechanism of molecular self-assembly resembles previously described mechanisms for eumelanin, but detailed MD simulations indicate a much more complex

nature of this process in the case of a/w-PDA [33]. Moreover, the synthesis conditions can significantly affect the type of polymer crosslinking. Using strong oxidants can cause the covalent bonds between dopamine oxidation products to completely dominate the material growth at the a/w-interface [23]. Finally, the observed structural order may result from the unique radiation-accelerated ability of a/w-PDA to undergo structuration [109,110], e.g. through UV-vis radiation in the spectroscopic studies, X-ray radiation during XRD examination or electron beam during HRTEM observation [99]. Exploration of this issue requires the use of advanced CRYO and in-situ techniques alongside Nuclear Magnetic Resonance studies, which should be another milestone in the characterization of these promising materials.

## 5. Multilayer and multicomponent approaches with PDA self-organization

As previously mentioned, PDA can strongly attach to virtually any surface, among others, the polymer matrix, making it a great building block for more complex well-organized multicomponent structures [44,111–113]. Self-organized PDA is not only used as an efficient bonding agent but can also affect the properties of the substrates used.

One study explores the usage of PDA in combination with dynamic crosslinked polyurea (DCPU) for self-healing photoresponsive wearable electronics [114]. The Authors prove that the synthesized self-assembled PDA particles exhibiting a rough surface improved the affinity of stronger physical interactions and higher compatibility with the polymer matrix. Namely, PDA and DCPU building blocks were connected through hydrogen bonds in a highly organized manner. This results in better mechanical properties of the DCPU/PDA nanocomposite in comparison with lone DCPU (UTS of 6.74 ± 1 MPa, an elongation at breaking of 334 ± 20 %, and a toughness of 14.96 ± 2 MJ m⁻³). Not only does the incorporation of PDA particles improve the mechanical properties but also provides an excellent photothermal effect. Thanks to that ability, a photoactuator based on the DCPU/PDA nanocomposite was constructed and exhibited great photoresponsive motion under Near-Infrared light.

PDA's great photothermal effect was also one of the focuses of
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** C1s region X-ray photoelectron high-resolution spectra for the a/w-PDA (a), BA-modified-a/w-PDA (b) and DA/BA 1:3, and Cu²⁺-modified-a/w-PDA (c). Nanointentation load-unload curves. Inset- box charts of the real Young modulus and hardness of the films (d). Dispersion relations (e) f(q) and (f) v(q) of fundamental antisymmetric Lamb (flexural) waves measured with BLS spectroscopy for BA-modified-a/w-PDA (blue) and a/w-PDA (black). The dashed lines represent the fit according to the equations given in [23,108]. Inset- optical microscopy images of the BA-modified-a/w-PDA (e) and a/w-PDA (f). Adapted from Ref. [23]. This work is licensed under CC BY 4.0.</figcaption></figure>

another study on self-assembled core-shell polydopamine@MXene nanocomposite for solar-to-vapour generation [115]. The nanocomposite was prepared as shown in the scheme in Fig. 13a. The synthesized MXene flakes and PDA microspheres were combined, creating large stable layered aggregates (Fig. 13b) due to strong hydrogen bonding between the two components. Next, PDA@MXene microspheres were filtrated through a PVDF commercial membrane filter with a diameter of 4 cm (Fig. 13c). The as-prepared photothermal membrane exhibited excellent flexibility, as shown in Fig. 13d. Moreover, the original shape of as-modified membrane can be easily recovered, due to the good integrity and stability of the PVDF/PDA/MXene surface functional groups (Fig. 13e). The introduction of PDA in this nanocomposite is also essential for achieving a broad light absorption in synergy with the MXenes [115].

Yet another approach for combining PDA with MXenes was introduced for the creation of a nanocomposite for targeted photothermal therapy of melanoma [116]. The inclusion of PDA was essential for the attachment of antiCEACAM1 antibodies to MXenes through the precise control of their surface chemistry, achieving a highly targeted and efficient antibidy complex. It also improves the biocompatibility of the nanocomposite, necessary for its biomedical application. The great adhesive properties of the self-assembled PDA layer also create an opportunity for engineering more complex therapeutic systems in the future [116].

Another approach to introducing self-organized PDA and its effects on the nanocomposite was researched regarding the production of bio-friendly and regenerable emotional monitors [117]. The Authors

managed to engineer a sensor consisting of hyperbranched polyethyleneimine (HPEI), gold nanoparticles (AuNPs) and PDA thanks to the crosslinking and self-assembly of PDA at the air/water interface (Fig. 14 a and b). PDA contributes to the creation of the film during its polymerization and stacking processes by capturing the HPEI-functionalized AuNPs through crosslinking between HPEI on the surface AuNPs and the PDA chains. Due to the extraordinary mechanical properties of polydopamine films at the air/water interface, it was possible to easily transfer these nanocomposite films directly from the water surface to silicon surfaces (Fig. 14 c and d) for further morphological and physicochemical characterization (Fig. 14e–h). The film was used for RH sensing based on the humidity levels (Fig. 14 i). The moisture concentration was detected because of the large number of amine and hydroxyl groups in both HPEI and PDA, which created webs of hydrogen bonds capable of attaching to the water molecules. Such highly-organized structures formed electrical channels through the physical interactions, enabling the conductivity changes necessary in humidity detection (Fig. 14j and k). Additionally, the strong interfacial crosslinking and self-assembly of PDA prevented the escape of chemical molecules from the elastic substrate, decreasing the risk of inhalation or absorption through the skin, which is extremely important in the application of emotional/physical activity detecting masks and RH changes detecting sports watches. Finally, the photothermal performance of the PDA/HPEI/AuNPs films was also studied, providing proof of efficient light-to-heat conversion by the nanocomposite (Fig. 14l and m).

PDA exhibits great potential in medical applications as a product of
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Scheme of the PDA@MXene photothermal layer fabrication process (a). Digital photographs of MXene colloidal solution (left), PDA suspension (center), and MXene/PDA hybrid- M/P (right) (b). PDA@MXene photothermal membrane (c). A bent PDA@MXene photothermal membrane (d). The structure stability of PDA@MXene photothermal membrane against various operations when immersed in water (e). Adapted with permission from: X. Zhao, X.J. Zha, L.S. Tang, J.H. Pu, K. Ke, R.Y. Bao, Z. ying Liu, M.B. Yang, W. Yang, Self-assembled core–shell polydopamine@MXene with synergistic solar absorption capability for highly efficient solar-to-vapor generation, Nano Res. (2020). https://doi.org/10.1007/s12274-019-2608-0. Copyright 2024 Springer Nature [115].</figcaption></figure>

self-assembly on chitosan film's surface [118]. Chitosan, the second most naturally abundant biopolymer, is often combined with functional 2D nanomaterials due to the synergistic properties [119–121]. In this study, an improvement in the cell's adhesion and proliferation was detected upon the inclusion of the PDA layer. Plain chitosan can withstand greater amounts of stress, as shown in Fig. 15a, but isn't as easily stretchable as the synthesised PDA-nanocoated chitosan film. The assembled PDA plays the role of a plasticizer and increases the elongation at break. The Young's modulus of this nanocomposite decreased until 1 h of PDA's self-assembly modification (Fig. 15b), which is shown by the increased flexibility and lower stiffness. Fig. 15c displays the film's strain at break. Those measurements show the great impact on mechanical properties of the chitosan films after the self-assembly of PDA on their surface, which is crucial for applications such as integration with soft tissue.

Another group of researchers also proved that non-covalent bonding between PDA and the polymer matrix improves tensile strength and

breaking strain, positively affecting the overall mechanical properties of the PDA-doped polyacrylic acid (PAA) hydrogel [122]. A double layered sensor consisting of a polystyrene/polyacrylamide (PS/PAM) hydrogel and PDA/PAA hydrogel exhibited great self-healing abilities attributed to the many hydrogen bonds and π-π stacking from the PDA particles incorporated into the adhesive layer of the nanocomposite. Those bonds could easily be reconnected after the appearance of fractures in the structure. The hydrogen bonds between the amino and phenolic groups from PDA and hydroxyl groups from PAA also ensured the even dispersion of nanoparticles within the PDA/PAA hydrogel layer. The self-assembly of PDA particles also promotes the adhesion of the engineered hydrogel to virtually any surface, including the skin, glass, rubber, non-stick PTFE etc [122].

Yet another example of PDA's incorporation into hydrogels was explored in the creation of self-healing, shape memory conductive hydrogels consisting of poly(acrylic acid)-polyaniline (PAA-PANI), polyvinyl alcohol (PVA) and diamino oxygen derivatives (AOP) as well
<figure><img src="image_17.png" /><figcaption>**Fig. 14.** Synthesis protocol: in-situ reduction of HAuCl₄ into HPEI-functionalized AuNPs (a), formation of air/water-polydopamine film, cross-linked with HPEI/AuNPs (b). Transferring of the films onto a silicon wafer, where top (c) and bottom (d) sides are exposed. Morphological and topographical characterization via SEM for top (e) and bottom (f) sides and via TEM (g) and AFM (h). A digital photograph of the PDA/HPEI/AuNPs sensor and equivalent electronic circuit. The current changes between dry (RH ≈ 25 %) and humid air (RH ≈ 45 %) were measured at 1 V. The response time was 0.15 s and recovery time was 0.20 s (i). Time-resolved current response from the film under 1 V, as humidity was switched between the low (25 %) and high (45 %) level (j). Relative humidity-dependent resistance of the sensor (k). Temperature change function (l) and IR thermal images (m) of the samples under a NIR (808 nm) laser irradiation (2 W cm⁻²). Adapted with permission from: L. Chen, Y. Huang, L. Song, W. Yin, L. Hou, X. Liu, T. Chen, Biofriendly and Regenerable Emotional Monitor from Interfacial Ultrathin 2D PDA/AuNPs Cross-linking Films, ACS Appl. Mater. Interfaces. 11 (2019) 36259–36269. https://doi.org/10.1021/acsami.9b11918. Copyright 2024 American Chemical Society [117].</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 15.** Results of tensile test of the films, inset shows the flexible films for soak times 1 h (left) and 6 h (right), easy to handle and manipulation without any damage and breakage (a). Statisticial examination of the Young's modulus (b), and strain at break (c). Generally, 8 different films were tested for each case. Adapted with permission from: A.K. Brooks, M. Imran, S. Pradhan, J.M. Broitman, V.K. Yadavalli, Facile fabrication and nanoscale assembly of polydopamine-functionalized, flexible chitosan films, J. Bioact. Compat. Polym. 36 (2021) 380–393. https://doi.org/10.1177/08839115211046414. Copyright 2024 Sage Publications [118].</figcaption></figure>

[123]. Again the self-assembled PDA improved the mechanical properties of the hydrogel. It also provided the reversible phase for shape memory along with AOP, thanks to the many hydrogen bonds created by those components. Both of those substrates also greatly affected the

swelling process of the hydrogels, decreasing the water absorption. After the hydrogels reached their swelling equilibrium, the water molecules, that previously broke the existing hydrogen bonds, would be discarded and the physical interactions would be recovered [123].
PDA-incorporated composites can not only possess self-healing abilities but also be self-cleaning. A great example are membranes for water purification created by co-deposition of 4,4'-azidianiline (AZO) and PDA onto ultrafiltration (UF) membranes [124]. The membranes exhibit photoresponsive ability to self-clean due to the isomerization of AZO under white light and UV light irradiation. The key role of the self-assembled PDA layer is to act as a free radical scavenger increasing the nanocomposite's protection from the UV light, necessary for creation of long-lasting devices [124].

Finally, very recently, the thermoswitchable adhesive (TSA) hydrogel was prepared from dopamine and an amine-containing polymer via interfacial polymerization at the air/liquid (air/water) interface, as presented in Fig. 16a [125]. The obtained hydrogel exhibited Janus features. Namely, the upper, air-exposed side of the hydrogel was densely crosslinked (high oxidation yield), while the bottom, solution-exposed side of the hydrogel was loosely crosslinked (Fig. 16b). Thanks to the flexible and movable adhesion molecules at the bottom surface, TSA hydrogel could be attached easily to various substrates – including human skin – at temperatures above the lower critical solution temperature (LCST). Moreover, it was easily removed by cooling below LCST (Fig. 16c). Significant adhesion of the bottom side, supported by mechanical strength and operability of the upper side, enabled cardiogram monitoring instrument electrodes to be attached and

provide a stable 10-minute test, without delamination [125].

## 6. Conclusions and future perspectives

This review aimed to collect and summarize information on the supramolecular order and crystalline (layered) nature of polydopamine. By discussing 3 forms of PDA – coatings, spherical nanoparticles and free-standing films, we aimed to unify knowledge about the mechanism of aggregation of macromolecules in an ordered manner. Therefore, below we present the conclusions resulting from this analysis.

* Apart from significant modifications (use of strong oxidants, acidic pH, etc.) In all methods of polydopamine deposition from solution, the molecules tend to arrange themselves in layers using π-π interactions and hydrogen bonds. E.g for coatings, as polymerization times increase, the structure of surface-bound PDA layers remains consistent, while the substrate type primarily influences the initial stages of polymerization. Similarly for nanoparticles: although spherical nanoparticles differ in properties such as average diameter and porosity, some repeating structural properties can be observed, e.g. layered structure with d-spacing of approximately 4 Å, as revealed by XRD. It is postulated that this is why films created on the air/water interface show an actual 2D-like ordering, since the air/

<figure><img src="image_19.png" /><figcaption>Fig. 16. Solution before and after oxygen-assisted interfacial (a/w) polymerization. First, the solution became white (sol-gel transition), while the top of the sample became brown due to the exposure to the air. Next, the air-oxidation step was held at room temperature, oxygen penetrated the whole hydrogel in the gradient like manner (a). Diagram of the Janus structure of the TSA hydrogel, showcasing its tough and adhesive regions (top). The potential chemical structure of the TSA hydrogel formed through air oxidation and crosslinking reactions is displayed at the bottom (b). A schematic of the TSA hydrogel illustrates its strong thermoswitchable adhesion and gentle detachment. The presence of dopamine in the hydrogel ensures stable underwater adhesion and the presence of NIPAm enables thermoswitchability at body temperature (c). Adapted from Ref [125]. This work is licensed under CC BY 4.0.</figcaption></figure>
water interface eases molecules' organization into layered aggregates, and the first layers arrange themselves in a perfectly flat manner due to specific properties of water surface, i.e. 2-axis limited molecules mobility.

* Computational simulations confirm the phenomena observed and described in the experiments, both for completely free arrangement of molecules and for mobility influenced by the air/water interface. PDA-building blocks tend to form aggregates of a higher-order, and the basic mechanism is the arrangement of tetramers made of PDA structural units. Importantly, such aggregation is weakly reversible. Flat tetramers stack into layered aggregates, while less flat tetramers create disordered structures. However, given that the size of the aggregate varies, the stacking alignment deviates slightly from the ideal parallel ordering.

* There is a probability that the observed exceptionally high ordering of nanometrically thin a/w-PDA films is, to some extent, induced by the observation/examination process itself using a strong radiation source. Considering the previously described observations, computer simulations and combinations of many research methods, it should be assumed that a certain degree of layered ordering of molecules in a/w-PDA films is a fact. However, its degree, in some cases very high – even resembling 2D-carbon-based materials (e.g. Graphene Oxide), can be increased by *ex-situ* irradiation treatment. However, such an extremely high susceptibility to graphitization would in itself indicate great application potential.

* Given that PDA can adhere to virtually any surface through physical interactions such as π-π stacking and hydrogen bonding, its extraordinary self-assembly abilities have been used in constructing nanocomposites with 2D materials or materials with complementary properties. MXene flakes and PDA spherical particles combine MXene's chemical activity and PDA's high stability and broad spectrum of light absorption. Polydopamine can also be used as an intermediate layer. This case provided excellent crosslinking between hyperbranched polyethyleneimine and gold nanoparticles. PDA's reversible ability to create and break hydrogen bonds between layers was also used in the recent approach, namely PDA's incorporation into hydrogels.

Finally, we would like to present future perspectives and encourage to explore this topic, pointing out some guidelines where one can potentially look for further breakthrough results.

* Since the formation of the first PDA layers on the surface of the coated material depends on the characteristics of the substrate, it is necessary to investigate the formation of very thin PDA layers (few nm) or even monolayers on atomically perfectly flat substrates (oxides, noble metals), 2D materials (graphene, $MoS_2$, MXenes), and templates.

* The formation of polydopamine should be examined using *in-situ* and *cryo* methods (spectroscopy, electron microscopy, NMR) to observe the process of formation of individual layers in real time. This is a difficult task because the process of oxidation of dopamine to poly-dopamine and the subsequent formation of films and nanoparticles is relatively dynamic. Methods of slowing down the reaction, such as using boron-based modifiers (e.g., boric acid), lowering the reaction temperature, or limiting the supply of oxygen, may be crucial.

* High computing power and interdisciplinary nature of teams conducting simulations are necessary to obtain reliable results. In addition, choosing accurate molecular models for computational simulations is crucial to ensure significant findings. Modified or more complex simulations, e.g. by increasing the number of molecules or using other algorithms (e.g. brute-force algorithmic generator), may contribute to an even better understanding of this phenomenon.

* Manipulation of layered nanomaterials is currently a hot topic. In this context, polydopamine should be considered. Examples of areas of interest include: the exfoliation of multilayer PDA films (perhaps even down to a single layer), geometric deformation of single layers from a planar plane, twisting layers at different angles and reconnecting them, stabilizing the obtained monolayers, etc.

To conclude, much research on polydopamine's supramolecular ordering is still needed and very welcomed.

### CRediT authorship contribution statement

**Jakub Szewczyk:** Writing – review & editing, Writing – original draft, Visualization, Validation, Funding acquisition, Conceptualization.

**Divyasree Radhakrishnan:** Writing – review & editing, Writing – original draft. **Zuzanna Łukasiewicz:** Writing – review & editing, Writing – original draft. **Emerson Coy:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: [Jakub Szewczyk reports financial support was provided by National Science Centre Poland. Emerson Coy reports financial support was provided by National Science Centre Poland. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper].

### Acknowledgements

**J.S.** acknowledge the financial support from the National Science Centre of Poland (NCN) by the PRELUDIUM20 grant 2021/41/N/ST5/00211. **D.R.** and **E.C.** acknowledge the financial support from the National Science Centre of Poland (NCN) by the PRELUDIUM BIS 4 grant 2022/47/O/ST5/00380.

### Data availability

No data was used for the research described in the article.

### References

[1] J.-M. Lehn, Perspectives in supramolecular chemistry—from molecular recognition towards molecular information processing and self-organization, Angew. Chemie Int. Ed. English. 29 (1990) 1304–1319, https://doi.org/10.1002/anie.199013041.

[2] Z. Zhao, S. Lei, M. Zeng, M. Huo, Recent progress in polymerization-induced self-assembly: From the perspective of driving forces, Aggregate. 5 (2024) e418, https://doi.org/10.1002/agt2.418.

[3] C.R. Kagan, T. Hyeon, D.H. Kim, R. Ruiz, M.C. Tung, H.S.P. Wong, Self-Assembly for electronics, MRS Bull. 45 (2020) 807–814, https://doi.org/10.1557/mrs.2020.248.

[4] A.K. Nandi, A review on self-assembly driven optoelectronic properties of polythiophene-peptide and polythiophene-polymer conjugates, Langmuir. 40 (2024) 9385–9405, https://doi.org/10.1021/acs.langmuir.4c00713.

[5] S. Cichosz, A. Masek, M. Zaborski, Polymer-based sensors: A review, Polym. Test. 67 (2018) 342–348, https://doi.org/10.1016/j.polymTesting.2018.03.024.

[6] V. Chimisso, V. Maffeis, D. Hürlimann, C.G. Palivan, W. Meier, Self-assembled polymeric membranes and nanoassemblies on surfaces: preparation, characterization, and current applications, Macromol. Biosci. 20 (2020) 1900257, https://doi.org/10.1002/mabi.201900257.

[7] P. Kaner, P. Bengani-Lutz, I. Sadeghi, A. Asatekin, Responsive filtration membranes by polymer self-assembly, Technology. 04 (2016) 217–228, https://doi.org/10.1142/S2339547816500096.

[8] R. Luo, M. Zhu, X. Shen, S. Li, Polymer catalyst with self-assembled hierarchical access for sorted catalysis, J. Catal. 331 (2015) 49–56, https://doi.org/10.1016/j.jcat.2015.08.018.

[9] K. Zhu, D. Wang, J. Liu, Self-assembled materials for catalysis, Nano Res. 2 (2009) 1–29, https://doi.org/10.1007/s12274-009-9002-2.

[10] L. Hu, S. Zhou, X. Zhang, C. Shi, Y. Zhang, X. Chen, Self-assembly of polymers and their applications in the fields of biomedicine and materials, Polymers. 16 (2024) 2097, https://doi.org/10.3390/polym16152097.
[11] W. Qi, X. Zhang, H. Wang, Self-assembled polymer nanocomposites for biomedical application. Curr. Opin. Colloid Interf. Sci. 35 (2018) 36–41, https://doi.org/10.1016/j.cocis.2018.01.003.

[12] L.L. Osorno, A.N. Brandley, D.E. Maldonado, A. Yiantos, R.J. Mosley, M.E. Byrne, Review of contemporary self-assembled systems for the controlled delivery of therapeutics in medicine, Nanomaterials. 11 (2021) 278, https://doi.org/10.3390/nano11020278.

[13] N.J.W. Penfold, J. Yeow, C. Boyer, S.P. Armes, Emerging trends in polymerization-induced self-assembly, ACS Macro Lett. 8 (2019) 1029–1054, https://doi.org/10.1021/acsmacromolecules.9b00464.

[14] D. Aguilar-Ferrer, J. Szewczyk, E. Coy, Recent developments in polydopamine-based photocatalytic nanocomposites for energy production: physico-chemical properties and perspectives, Catal. Today. 397–399 (2022) 316–349, https://doi.org/10.1016/j.cattod.2021.08.016.

[15] J. Szewczyk, D. Aguilar-Ferrer, E. Coy, Polydopamine films: Electrochemical growth and sensing applications, Eur. Polym. J. 174 (2022) 111346, https://doi.org/10.1016/j.eurpolymj.2022.111346.

[16] E. Coy, I. Iatsunskyi, M. Bechelany, Perspectives and current trends on hybrid nanocomposite materials for photocatalytic applications, Sol. RRL. 7 (2023) 2201069, https://doi.org/10.1002/solr.202201069.

[17] V. Ball, Polydopamine films: Versatile but interface-dependent coatings, Nanotechnology Reviews 13 (1) (2024) 20230216, https://doi.org/10.1515/ntrev-2023-0216.

[18] D. Aguilar-Ferrer, T. Vasileiadis, I. Iatsunskyi, M. Ziółek, K. Żebrowska, O. Ivashchenko, P. Blaszkiewicz, B. Grześkowiak, R. Pazos, S. Moya, M. Bechelany, E. Coy, Understanding the photothermal and photocatalytic mechanism of polydopamine coated gold nanorods, Adv. Funct. Mater. 33 (2023) 2304208, https://doi.org/10.1002/adfm.202304208.

[19] Y. Kim, E. Coy, H. Kim, R. Mrówczyński, P. Torruella, D.W. Jeong, K.S. Choi, J. H. Jang, M.Y. Song, D.J. Jang, F. Peiro, S. Jurga, H.J. Kim, Efficient photocatalytic production of hydrogen by exploiting the polydopamine-semiconductor interface, Appl. Catal. B Environ. 280 (2021) 119423, https://doi.org/10.1016/j.apcatb.2020.119423.

[20] D. Damberga, V. Fedorenko, K. Grundsteins, Ş. Altundal, A. Sutka, A. Ramanavicius, E. Coy, R. Mrówczyński, I. Iatsunskyi, R. Viter, Influence of pda coating on the structural, optical and surface properties of zno nanostructures, Nanomaterials. 10 (2020) 1–11, https://doi.org/10.3390/nano10122438.

[21] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science. 318 (2007) 426–430, https://doi.org/10.1126/science.1147241.

[22] M. Salomäki, T. Ouvinen, L. Marttila, H. Kivelä, J. Leiro, E. Mäkilä, J. Lukkari, Polydopamine nanoparticles prepared using redox-active transition metals, J. Phys. Chem. B. 123 (2019) 2513–2524, https://doi.org/10.1021/acs.jpcb.8b111994.

[23] J. Szewczyk, V. Babacic, A. Krysztofik, O. Ivashchenko, M. Pochylski, R. Pietrzak, J. Gapiński, B. Graczykowski, M. Bechelany, E. Coy, Control of intermolecular interactions toward the production of free-standing interfacial polydopamine films, ACS Appl. Mater. Interf. 15 (2023) 36922–36935, https://doi.org/10.1021/acsami.3c05236.

[24] M.J. Bigaj-Józefowska, B.F. Grześkowiak, Polymeric nanoparticles wrapped in biological membranes for targeted anticancer treatment, Eur. Polym. J. 176 (2022) 111427, https://doi.org/10.1016/j.eurpolymj.2022.111427.

[25] M.J. Bigaj-Józefowska, E. Coy, K. Załęski, T. Zalewski, M. Grabowska, K. Jaskot, P. Perrigue, R. Mrówczyński, B.F. Grześkowiak, Biomimetic theranostic nanoparticles for effective anticancer therapy and MRI imaging, J. Photochem. Photobiol. B Biol. 249 (2023) 112813, https://doi.org/10.1016/j.jphotobiol.2023.112813.

[26] J.H. Ryu, P.B. Messersmith, H. Lee, Polydopamine surface chemistry: a decade of discovery, ACS Appl. Mater. Interf. 10 (2018) 7523–7540, https://doi.org/10.1021/acsami.7b19865.

[27] Y.H. Ding, M. Floren, W. Tan, Mussel-inspired polydopamine for bio-surface functionalization, Biosurf. Biotribol. 2 (2016) 121–136, https://doi.org/10.1016/j.bsbt.2016.11.001.

[28] J. Jiang, L.L. Zhu, L.L. Zhu, B. Zhu, Y. Xu, Surface characteristics of a self-polymerized dopamine coating deposited on hydrophobic polymer films, Langmuir. 27 (2011) 14180–14187, https://doi.org/10.1021/la202877k.

[29] F. Ponzio, P. Payamyar, A. Schneider, M. Winterhalter, J. Bour, F. Addiego, M. P. Krafft, J. Hemmerle, V. Ball, Polydopamine films from the forgotten Air/water interface, J. Phys. Chem. Lett. 5 (2014) 3436–3440, https://doi.org/10.1021/jz501842r.

[30] F. Ponzio, V. Ball, Polydopamine deposition at fluid interfaces, Polym. Int. 65 (2016) 1251–1257, https://doi.org/10.1002/pi.5124.

[31] C.-T. Chen, C. Chuang, J. Cao, V. Ball, D. Ruch, M.J. Buehler, Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin, Nat. Commun. 5 (2014) 3859, https://doi.org/10.1038/ncomms4859.

[32] E. Coy, I. Iatsunskyi, J.C. Colmenares, Y. Kim, R. Mrówczyński, Polydopamine films with 2D-like layered structure and high mechanical resilience, ACS Appl. Mater. Interf. 13 (2021) 23113–23120, https://doi.org/10.1021/acsami.1c02483.

[33] J. Szewczyk, M. Pochylski, K. Szutkowski, M. Kempiński, R. Mrówczyński, I. Iatsunskyi, J. Gapiński, E. Coy, In-situ thickness control of centimetre-scale 2D-Like polydopamine films with large scalability, Mater. Today Chem. 24 (2022) 100935, https://doi.org/10.1016/j.mtchem.2022.100935.

[34] J. Szewczyk, M. Ziółek, K. Siuzdak, I. Iatsunskyi, M. Pochylski, D. Aguilar-Ferrer, M. Kempiński, F. Tanos, J. Gapiński, M. Bechelany, E. Coy, Ex-situ transferring of

polydopamine films on semiconductor interface: Evidence of functional hybrid heterojunction, Eur. Polym. J. 206 (2024) 112781, https://doi.org/10.1016/j.eurpolymj.2024.112781.

[35] S. Hong, Y. Wang, S.Y. Park, H. Lee, Progressive fuzzy cation-α assembly of biological catecholamines, Sci. Adv. 4 (2018) eaat7457, https://doi.org/10.1126/sciadv.aat7457.

[36] R.G. Chapman, E. Ostuni, L. Yan, G.M. Whitesides, Preparation of Mixed Self-Assembled Monolayers (SAMs) that resist adsorption of proteins using the reaction of amines with a SAM that presents interchain carboxylic anhydride groups, Langmuir. 16 (2000) 6927–6936, https://doi.org/10.1021/la991579l.

[37] A. Tewari, K. Björström, A.M. Ghafari, E. Macchia, L. Torsi, R. Österbacka, Stability of thiol-based self-assembled monolayer functionalized electrodes in EG-OFET-based applications, FlatChem. 42 (2023) 100553, https://doi.org/10.1016/j.flatc.2023.100553.

[38] E. Cortés, A.A. Rubert, G. Benitez, P. Carro, M.E. Vela, R.C. Salvarezza, Enhanced Stability of Thiolate Self-Assembled Monolayers (SAMs) on nanostructured gold substrates, Langmuir. 25 (2009) 5661–5666, https://doi.org/10.1021/la804251a.

[39] R. Colorado, T.R. Lee, Thiol-based self-assembled monolayers: Formation and organization, Encyclopedia of Materials: Science and Technology (2001) 9332–9344, https://doi.org/10.1016/B0-08-043152-6/01682-X.

[40] Z. Qin, D. Li, Y. Ou, S. Du, Q. Jiao, J. Peng, P. Liu, Recent advances in polydopamine for surface modification and enhancement of energetic materials: a mini-review, Crystals. 13 (2023) 976, https://doi.org/10.3390/cryst13060976.

[41] S. Gao, D. Zhang, M. Pedrero, Z. Guo, J.M. Pingarrón, S. Campuzano, X. Zou, Advances and opportunities of polydopamine coating in biosensing: preparation, functionality, and applications, Coord. Chem. Rev. 501 (2024) 215564, https://doi.org/10.1016/j.ccr.2023.215564.

[42] A. Singh, D. Ramimoghadam, A. Mirabedini, The use of polydopamine coatings for timber protection against the fire: A critical review and feasibility analysis, Prog. Org. Coatings. 175 (2023) 107359, https://doi.org/10.1016/j.porgcoat.2022.107359.

[43] H. Feinberg, T.W. Hanks, Polydopamine: a bioinspired adhesive and surface modification platform, Polym. Int. 71 (2022) 578–582, https://doi.org/10.1002/pi.6358.

[44] S. Hong, Y.S. Na, S. Choi, I.T. Song, W.Y. Kim, H. Lee, Non-covalent self-assembly and covalent polymerization co-contribute to polydopamine formation, Adv. Funct. Mater. 22 (2012) 4711–4717, https://doi.org/10.1002/adfm.201201156.

[45] J. Svoboda, M. Král, M. Dendisová, P. Matejka, O. Pop-Georgievsk, Unraveling the influence of substrate on the growth rate, morphology and covalent structure of surface adherent polydopamine films, Colloids Surf. B Biointerf. 205 (2021) 111897, https://doi.org/10.1016/j.col surfb.2021.111897.

[46] R.A. Zangmeister, T.A. Morris, M.J. Tarlov, Characterization of polydopamine thin films deposited at short times by autoxidation of dopamine, Langmuir. 29 (2013) 8619–8628, https://doi.org/10.1021/la400587j.

[47] Z.H. Huang, S.W. Peng, S.L. Hsieh, R. Kirankumar, P.F. Huang, T.M. Chang, A. K. Dwivedi, N.F. Chen, H.M. Wu, S. Hsieh, Polydopamine ultrathin film growth on mica via in-situ polymerization of dopamine with applications for silver-based antimicrobial coatings, Materials (Basel). 14 (2021) 1–16, https://doi.org/10.3390/ma114030671.

[48] Q. Lu, D.X. Oh, Y. Lee, Y. Jho, D.S. Hwang, H. Zeng, Nanomechanics of cation-α interactions in aqueous solution, Angew. Chemie Int. Ed. 52 (2013) 3944–3948, https://doi.org/10.1002/anie.201210365.

[49] C.T. Chen, F.J. Martin-Martinez, G.S. Jung, M.J. Buehler, Polydopamine and eumelanin molecular structures investigated with ab initio calculations, Chem. Sci. 8 (2017) 1631–1641, https://doi.org/10.1039/C6SC04692D.

[50] X. Li, C. Wu, J. Wu, R. Sun, B. Hou, C. Liu, M. Chen, Molecular investigation of the self-assembly mechanism underlying polydopamine coatings: the synergistic effect of typical building blocks acting on interfacial adhesion, ACS Appl. Mater. Interf. 16 (2024) 51699–51714, https://doi.org/10.1021/acsami.4c10816.

[51] S.S. Ruppel, J. Liang, Tunable properties of polydopamine nanoparticles and coated surfaces, Langmuir. 38 (2022) 5020–5029, https://doi.org/10.1021/acs.langmuir.2c00719.

[52] A. Schneider, J. Hemmerlé, M. Allais, J. Didierjean, M. Michel, M. D'Ischia, V. Ball, Boric acid as an efficient agent for the control of polydopamine self-assembly and surface properties, ACS Appl. Mater. Interf. 10 (2018) 7574–7580, https://doi.org/10.1021/acsami.7b08356.

[53] A. Srikanth, G.M.T. Basha, B. Venkateshwarlu, A Brief Review on Cold Spray Coating Process, Mater. Today Proc. 22 (2020) 1390–1397, https://doi.org/10.1016/j.matpr.2020.01.482.

[54] S. Akin, S. Lee, S. Jo, D.G. Ruzgar, K. Subramaniam, J.-T. Tsai, M.-B.-G. Jun, Cold spray-based rapid and scalable production of printed flexible electronics, Addit. Manuf. 60 (2022) 103244, https://doi.org/10.1016/j.addma.2022.103244.

[55] S. Pathak, G.C. Saha, Development of sustainable cold spray coatings and 3D additive manufacturing components for repair/manufacturing applications: a critical review, Coatings. 7 (2017) 122, https://doi.org/10.3390/coatings7080122.

[56] S.H. Hong, S. Hong, M.H. Ryou, J.W. Choi, S.M. Kang, H. Lee, Sprayable ultrafast polydopamine surface modifications, Adv. Mater. Interf. 3 (2016) 1500857, https://doi.org/10.1002/admi.201500857.

[57] D.-L. Yang, R.-K. Liu, Y. Wei, Q. Sun, J.-X. Wang, Micro-sized nanoaggregates: Spray-drying-assisted fabrication and applications, Particuology. 85 (2024) 22–48, https://doi.org/10.1016/j.partic.2023.03.013.

[58] T.V. Desai, A.R. Woll, F. Schreiber, J.R. Engstrom, Nucleation and growth of perfluoropentacene on self-assembled monolayers: significant changes in island
density and shape with surface termination, J. Phys. Chem. C. 114 (2010) 20120–20129, https://doi.org/10.1021/jp107518f.

[59] S. Lamping, C. Buten, B.J. Ravoo, Functionalization and patterning of self-assembled monolayers and polymer brushes using microcontact chemistry, Acc. Chem. Res. 52 (2019) 1336–1346, https://doi.org/10.1021/acs.accounts.9b00041.

[60] M. Battaglini, M. Emenet, A. Carmignani, G. Ciofani, Polydopamine-based nanostructures: a new generation of versatile, multi-tasking, and smart theranostic tools, Nano Today. 55 (2024) 102151, https://doi.org/10.1016/j.nantod.2024.102151.

[61] H. Yabu, T. Higuchi, K. Ijiro, M. Shimomura, Spontaneous formation of polymer nanoparticles by good-solvent evaporation as a nonequilibrium process Chaos an Interdiscip. J. Nonlinear Sci. 15 (2005) 47505, https://doi.org/10.1063/1.2137621.

[62] T. Higuchi, A. Tajima, H. Yabu, M. Shimomura, Spontaneous formation of polymer nanoparticles with inner micro-phase separation structures, Soft Matter. 4 (2008) 1302–1305, https://doi.org/10.1039/B800904J.

[63] D. Quintanar-Guerrero, E. Allémann, E. Doelker, H. Fessi, Preparation and characterization of nanocapsules from preformed polymers by a new process based on emulsification-diffusion technique, Pharm. Res. 15 (1998) 1056–1062, https://doi.org/10.1023/A:1011934328471.

[64] F.Q. Pires, I.P. Gross, L.L. Sa-Barreto, T. Gratieri, G.M. Gelfuso, S.N. Bao, M. Cunha-Filho, In-situ formation of nanoparticles from drug-loaded 3D polymeric matrices, Eur. J. Pharm. Sci. 188 (2023) 106517, https://doi.org/10.1016/j.ejps.2023.106517.

[65] T. Ogura, Y. Saito, T. Higashihara, M. Ueda, Formation of spherical nanoparticles in poly(amic acid) films, Polym. Chem. 3 (2012) 2165–2169, https://doi.org/10.1039/C2PY20093G.

[66] A. Chassepot, V. Ball, Human serum albumin and other proteins as templating agents for the synthesis of nanosized dopamine-eumelanin, J. Colloid Interf. Sci. 414 (2014) 97–102, https://doi.org/10.1016/j.jcis.2013.10.002.

[67] V. Ball, Polydopamine nanomaterials: recent advances in synthesis methods and applications, Front. Bioeng. Biotechnol. 6 (2018) 109, https://doi.org/10.3389/fbioe.2018.00109.

[68] A. Carmignani, M. Battaglini, E. Sinibaldi, A. Marino, V. Vighetto, V. Cauda, G. Ciofani, In vitro and Ex vivo investigation of the effects of polydopamine nanoparticle size on their antioxidant and photothermal properties: implications for biomedical applications, ACS Appl. Nano Mater. 5 (2022) 1702–1713, https://doi.org/10.1021/acsanm.1c04536.

[69] M. Salomäki, L. Marttila, H. Kivelä, T. Ouvinen, J. Lukkari, Effects of pH and oxidants on the first steps of polydopamine formation: a thermodynamic approach, J. Phys. Chem. B. 122 (2018) 6314–6327, https://doi.org/10.1021/acs.jpcb.8b02304.

[70] S. El Yakhlihi, V. Ball, Polydopamine as a stable and functional nanomaterial, Colloids Surf. B Biointerf. 186 (2020) 110719, https://doi.org/10.1016/j. colsurfb.2019.110719.

[71] M.C. van der Leeden, Are conformational changes induced by osmotic pressure variations, the underlying mechanism of controlling the adhesive activity of mussel adhesive proteins? Langmuir. 21 (2005) 11373–11379, https://doi.org/10.1021/la0515468.

[72] B.P. Lee, J.L. Dalsin, P.B. Messersmith, Synthesis and Gelation of DOPA-Modified Poly(ethylene glycol) Hydrogels, Biomacromolecules. 3 (2002) 1038–1047, https://doi.org/10.1021/bm025546n.

[73] Q. Wei, F. Zhang, J. Li, B. Li, C. Zhao, Oxidant -induced dopamine polymerization for multifunctional coatings, Polym. Chem. 1 (2010) 1430–1433, https://doi.org/10.1039/COPY00215A.

[74] R. Ouyang, J. Lei, H. Ju, Surface molecularly imprinted nanowire for protein specific recognition, Chem. Commun. (2008) 5761–5763, https://doi.org/10.1039/B810248A.

[75] C.C. Ho, S.J. Ding, The pH-controlled nanoparticles size of polydopamine for anti-cancer drug delivery, J. Mater. Sci. Mater. Med. 24 (2013) 2381–2390, https://doi.org/10.1007/s10856-013-4994-2.

[76] M. Wu, T. Wang, L. Müller, F.A. Müller, Adjustable synthesis of polydopamine nanospheres and their nucleation and growth, Colloids Surf. A Physicochem. Eng. Asp. 603 (2020) 125196, https://doi.org/10.1016/j.colsurfa.2020.125196.

[77] S. Cho, S.-H. Kim, Hydroxide ion-mediated synthesis of monodisperse dopamine-melanin nanospheres, J. Colloid Interf. Sci. 458 (2015) 87–93, https://doi.org/10.1016/j.jcis.2015.06.051.

[78] J.C. García-Mayorga, H.-C. Rosu, A.B. Jasso-Salcedo, V.A. Escobar-Barrios, Kinetic study of polydopamine sphere synthesis using TRIS: relationship between synthesis conditions and final properties, RSC Adv. 13 (2023) 5081–5095, https://doi.org/10.1039/D2RA06669F.

[79] H. Fan, X. Yu, Y. Liu, Z. Shi, H. Liu, Z. Nie, D. Wu, Z. Jin, Folic acid–polydopamine nanofibers show enhanced ordered stacking via π–π interactions, Soft Matter. 11 (2015) 4621–4629, https://doi.org/10.1039/C5SM00732A.

[80] X. Yu, H. Fan, L. Wang, Z. Jin, Formation of Polydopamine Nanofibers with the Aid of Folic Acid, Angew. Chemie Int. Ed. 53 (2014) 12600–12604, https://doi.org/10.1002/anie.201404947.

[81] J. Xue, W. Zheng, L. Wang, Z. Jin, Scalable fabrication of polydopamine nanotubes based on curcumin crystals, ACS Biomater. Sci. Eng. 2 (2016) 489–493, https://doi.org/10.1021/acsbiomaterials.6b00102.

[82] F. Chen, Y. Xing, Z. Wang, X. Zheng, J. Zhang, K. Cai, Nanoscale Polydopamine (PDA) Meets π–π interactions: an interface-directed coassembly approach for mesoporous nanoparticles, Langmuir. 32 (2016) 12119–12128, https://doi.org/10.1021/acs.langmuir.6b03294.

[83] M. d'Ischia, A. Napolitano, A. Pezzella, P. Meredith, T. Sarna, Chemical and structural diversity in eumelanins: unexplored bio-optoelectronic materials, Angew. Chem. Int. Ed. 48 (2009) 3914–3921, https://doi.org/10.1002/anie.200803786.

[84] D.R. Dreyer, D.J. Miller, B.D. Freeman, D.R. Paul, C.W. Bielawski, Elucidating the structure of poly(dopamine), Langmuir. 28 (2012), https://doi.org/10.1021/la204831b.

[85] M. D'Ischia, A. Napolitano, V. Ball, C.T. Chen, M.J. Buehler, Polydopamine and eumelanin: from structure-property relationships to a unified tailoring strategy, Acc. Chem. Res. 47 (2014) 3541–3550, https://doi.org/10.1021/ar500273y.

[86] M. d'Ischia, K. Wakamatsu, A. Napolitano, S. Briganti, J.-C. Garcia-Borron, D. Kovacs, P. Meredith, A. Pezzella, M. Picardo, T. Sarna, J.D. Simon, S. Ito, Melanins and melanogenesis: methods, standards, protocols, Pigment Cell Melanoma Res. 26 (2013) 616–633, https://doi.org/10.1111/pcmr.12121.

[87] A. Lampel, S.A. McPhee, H.-A. Park, G.G. Scott, S. Humagain, D.R. Hekstra, B. Yoo, P.W.J.M. Frederix, T.-D. Li, R.R. Abzalimov, S.G. Greenbaum, T. Tuttle, C. Hu, C.J. Bettinger, R.V. Uljin, Polymeric peptide pigments with sequence-encoded properties, Science. 356 (2017) 1064–1068, https://doi.org/10.1126/science.aal5005.

[88] R. Dong, T. Zhang, X. Feng, Interface-assisted synthesis of 2D materials: trend and challenges, Chem. Rev. 118 (2018) 6189–6235, https://doi.org/10.1021/acs.chemrev.8b00056.

[89] P. Dallas, V. Georgakilas, Interfacial polymerization of conductive polymers: Generation of polymeric nanostructures in a 2-D space, Adv. Colloid Interf. Sci. 224 (2015) 46–61, https://doi.org/10.1016/j.cis.2015.07.008.

[90] S.K. Chondath, R.R. Poolakkandy, R. Kottayintavida, A. Thekkangil, N. K. Gopalan, S.T. Vasu, S. Athiyanathil, M.M. Menamparambath, Water-Chloroform interface assisted microstructure tuning of polypyrrole-silver sheets, ACS Appl. Mater. Interf. 11 (2019) 1723–1731, https://doi.org/10.1021/acsmami.8b18943.

[91] S.K. Chondath, M.M. Menamparambath, Interface-assisted synthesis: a gateway to effective nanostructure tuning of conducting polymers, Nanoscale Adv. 3 (2021) 918–941, https://doi.org/10.1039/DONA00940G.

[92] N. Reitzel, D.R. Greve, K. Kjaer, P.B. Howes, M. Jayaraman, S. Savoy, R. D. Mccullough, J.T. Mcdevitt, T. Bjørnholm, Self-assembly of conjugated polymers at the air / water interface: structure and properties of langmuir and langmuir - blodgett films of amphiphilic regioregular polyphiophenes, J. Am. Chem. Soc. 347 (2000) 5788–5800, https://doi.org/10.1021/ja9924501.

[93] D. Barpuzary, K. Kim, M.J. Park, Two-Dimensional growth of large-area conjugated polymers on ice surfaces: high conductivity and photoelectrochemical applications, ACS Nano. 13 (2019) 3953–3963, https://doi.org/10.1021/acsnano.8b07294.

[94] S. Majumdar, D. Mahanta, One-step preparation of freestanding polypyrrole films at air-water interface, ChemistrySelect. 2 (2017) 9930–9933, https://doi.org/10.1002/sctl.201701871.

[95] L.M. Santino, Y. Diao, H. Yang, Y. Lu, H. Wang, E. Hwang, J.M. D'Arcy, Vapor/liquid polymerization of ultraporous transparent and capacitive polypyrrole nanonets, Nanoscale. 11 (2019) 12358–12369, https://doi.org/10.1039/C9NR02771H.

[96] Y. Jo, J. Lee, C. Kim, J. Jang, I. Hwang, J. Hong, M.J. Lee, Engineered molecular stacking crystallinity of bar-coated TIPS-pentacene/polystyrene films for organic thin-film transistors, RSC Adv. 13 (2023) 2700–2706, https://doi.org/10.1039/D2RA05924J.

[97] S. Yano, K. Sato, J. Suzuki, H. Imai, Y. Oaki, Amorphous 2D materials containing a conjugated-polymer network, Commun. Chem. 2 (2019) 97, https://doi.org/10.1038/s42004-019-02012-9.

[98] M. Saito, H. Yamada, K. Kranthiraja, J. Jeon, H. Do Kim, T. Mikie, A. Saeki, H. Ohkita, I. Osaka, Ordered π-conjugated polymer backbone in amorphous blend for high efficiency nonfullerene organic photovoltaics, Commun. Mater. 4 (2023) 72, https://doi.org/10.1038/s43246-023-00395-y.

[99] H. Wang, Q. Jiang, J. Yang, D. Li, X. Zhou, L. Cai, G. Yu, Polydopamine film self-assembled at air/water interface for organic electronic memory devices, Adv. Mater. Interf. 7 (2020) 2000097, https://doi.org/10.1002/admi.202000979.

[100] S. Hong, C.F. Schaber, K. Dening, E. Appel, S.N. Gorb, H. Lee, Air/water interfacial formation of freestanding, stimuli-responsive, self-healing catecholamine janus-faced microfilms, Adv. Mater. 26 (2014) 7581–7587, https://doi.org/10.1002/adma.201403259.

[101] T.F. Wu, J.D. Hong, Dopamine-melanin nanofilms for biomimetic structural coloration, Biomacromolecules. 16 (2015) 660–666, https://doi.org/10.1021/bm501773c.

[102] T.F. Wu, B.H. Wee, J.D. Hong, An ultrasensitive and fast moisture sensor based on self-assembled dopamine-melanin thin films, Adv. Mater. Interf. 2 (2015) 1–5, https://doi.org/10.1002/admi.201500203.

[103] G.L. Grocke, B.X. Dong, A.D. Taggart, A.B.F. Martinson, J. Niklas, O.G. Poluektov, J.W. Strzalka, S.N. Patel, Structure-transport properties governing the interplay in humidity-dependent mixed ionic and electronic conduction of conjugated polyelectrolytes, ACS Polym. Au. 2 (2022) 275–286, https://doi.org/10.1021/acspolymersau.2c00005.

[104] S. Wang, G. Zuo, J. Kim, H. Sirringhaus, Progress of conjugated polymers as emerging thermoelectric materials, Prog. Polym. Sci. 129 (2022) 101548, https://doi.org/10.1016/j.progpolymsci.2022.101548.

[105] T. Vasileiadis, T. Marchesi D'Alvise, C.-M. Saak, M. Pochylski, S. Harvey, C. V. Synatschke, J. Gapinski, G. Fytas, E.H.G. Backus, T. Weil, B. Graczykowski, Fast light-driven motion of polydopamine nanomembranes, Nano Lett. 22 (2021) 578–585, https://doi.org/10.1021/acs.nanolett.1c03165.
[106] X. Han, F. Tang, Z. Jin, Free-standing polydopamine films generated in the presence of different metallic ions: The comparison of reaction process and film properties, RSC Adv. 8 (2018) 18347–18354, https://doi.org/10.1039/c8ra02930j.

[107] S.J.D. Lugger, S.J.A. Houben, Y. Foelen, M.G. Debije, A.P.H.J. Schenning, D. J. Mulder, Hydrogen-bonded supramolecular liquid crystal polymers: smart materials with stimuli-responsive, self-healing, and recyclable properties, Chem. Rev. 122 (2022) 4946–4975, https://doi.org/10.1021/acs.chemrev.1c00330.

[108] B. Graczykowski, M. Sledzinska, M. Placidi, D. Saleta Reig, M. Kasprzak, F. Alzina, C.M. Sotomayor Torres, Elastic properties of few nanometers thick polycrystalline MoS₂ membranes: a nondestructive study, Nano Lett. 17 (2017) 7647–7651, https://doi.org/10.1021/acs.nanolett.7b03669.

[109] A. Olejnik, K. Polaczek, M. Szkodo, A. Stanisławska, J. Ryl, K. Siuzdak, Laser-induced graphitization of polydopamine on titania nanotubes, ACS Appl. Mater. Interfaces. 15 (2023) 52921–52938, https://doi.org/10.1021/acsami.3c11580.

[110] K. Lee, M. Park, K.G. Malollari, J. Shin, S.M. Winkler, Y. Zheng, J.H. Park, C. P. Grigoropoulos, P.B. Messersmith, Laser-induced graphitization of polydopamine leads to enhanced mechanical performance while preserving multifunctionality, Nat. Commun. 11 (2020) 5–12, https://doi.org/10.1038/s41467-020-18654-8.

[111] N. Sadaba, A. Larrañaga, G. Orpella-Aceret, A.F. Bettencourt, V. Martin, M. Biggs, I.A.C. Ribeiro, J.M. Ugartemendia, J.-R. Sarasua, E. Zuza, Benefits of Polydopamine as Particle/Matrix Interface in Polylactide/PD-BaSO₄ Scaffolds, Int. J. Mol. Sci. 21 (2020) 5480, https://doi.org/10.3390/ijms21155480.

[112] M.J. Herman, C. Liu, C. Cady, E.B. Watkins, N.A. Miller, A.L. Duque, J.D. Yeager, Biologically inspired reinforcement using polydopamine of polymer bound composites, Compos. Part B Eng. 254 (2023) 110563, https://doi.org/10.1016/j.compositesb.2023.110563.

[113] L. Dong, M. Chen, X. Wu, D. Shi, W. Dong, H. Zhang, C. Zhang, Multi-functional polydopamine coating: simultaneous enhancement of interfacial adhesion and CO₂ separation performance of mixed matrix membranes, New J. Chem. 40 (2016) 9148–9159, https://doi.org/10.1039/C6NJ02013E.

[114] Z. Zhou, X. Wang, H. Yu, C. Yu, F. Zhang, Dynamic cross-linked polyurea/polydopamine nanocomposites for photoresponsive self-healing and photoactuation, Macromolecules. 55 (2022) 2193–2201, https://doi.org/10.1021/acs.macromol.1c02534.

[115] X. Zhao, X.J. Zha, L.S. Tang, J.H. Pu, K. Ke, R.Y. Bao, Z. ying Liu, M.B. Yang, W. Yang, Self-assembled core-shell polydopamine@MXene with synergistic solar absorption capability for highly efficient solar-to-vapor generation, Nano Res. (2020) 255–264, https://doi.org/10.1007/s12274-019-2608-0.

[116] A. Konieva, V. Deineka, K. Diedkova, D. Aguilar-Ferrer, M. Lyndin, G. Wennemuth, V. Korniienko, S. Kyrylenko, A. Lihachev, V. Zahorodna, I. Baginskiy, E. Coy, O. Gogotsi, A. Blacha-Grzechnik, W. Simka, I. Kube-Golovin, I. Iatsunskyi, M. Pogorielov, MXene-polydopamine-antiCEACAM1 antibody complex as a strategy for targeted ablation of melanoma, ACS Appl. Mater. Interf. 16 (2024) 43302–43316, https://doi.org/10.1021/acsami.4c08129.

[117] L. Chen, Y. Huang, L. Song, W. Yin, L. Hou, X. Liu, T. Chen, Biofriendly and regenerable emotional monitor from interfacial ultrathin 2D PDA/AuNPs cross-linking films, ACS Appl. Mater. Interf. 11 (2019) 36259–36269, https://doi.org/10.1021/acsami.9b11918.

[118] A.K. Brooks, M. Imran, S. Pradhan, J.M. Broitman, V.K. Yadavalli, Facile fabrication and nanoscale assembly of polydopamine-functionalized, flexible chitosan films, J. Bioact. Compat. Polym. 36 (2021) 380–393, https://doi.org/10.1177/08839115211046414.

[119] A. Naskar, S. Kilari, S. Misra, Chitosan-2D nanomaterial-based scaffolds for biomedical applications, Polymers. 16 (2024) 1327, https://doi.org/10.3390/polym16101327.

[120] P. Choudhary, B. Ramalingam, S.K. Das, Fabrication of chitosan-reinforced multifunctional graphene nanocomposite as antibacterial scaffolds for hemorrhage control and wound-healing application, ACS Biomater. Sci. Eng. 6 (2020) 5911–5929, https://doi.org/10.1021/acsbiomaterials.0c00923.

[121] P. Yan, M. Li, J. Liu, L. Song, K. Tang, Near-infrared responsive quaternized chitosan-coated MoS₂/poly(vinyl alcohol) hydrogel with improved mechanical and rapid antibacterial properties, Eur. Polym. J. 180 (2022) 111593, https://doi.org/10.1016/j.eurpolymj.2022.111593.

[122] L. Han, M. Liu, B. Yan, Y.S. Li, J. Lan, L. Shi, R. Ran, Polydopamine/polystyrene nanocomposite double-layer strain sensor hydrogel with mechanical, self-healing, adhesive and conductive properties, Mater. Sci. Eng. C. 109 (2020) 110567, https://doi.org/10.1016/j.msec.2019.110567.

[123] L. Wan, P. Li, M. Yan, J. Wang, X. Li, Strong, self-healing, shape memory PAA-PANI/PVA/PDA/AOP conductive hydrogels with interpenetrating network and hydrogen bond interaction, Eur. Polym. J. 191 (2023) 112034, https://doi.org/10.1016/j.eurpolymj.2023.112034.

[124] S.N. Ramanan, N. Shahkaramipour, T. Tran, L. Zhu, S.R. Venna, C.-K. Lim, A. Singh, P.N. Prasad, H. Lin, Self-cleaning membranes for water purification by co-deposition of photo-mobile 4,4'-azodianiline and bio-adhesive polydopamine, J. Memb. Sci. 554 (2018) 164–174, https://doi.org/10.1016/j.memsci.2018.02.068.

[125] H. Abe, D. Yoshihara, S. Tottori, M. Nishizawa, Mussel-inspired thermo-switchable underwater adhesive based on a Janus hydrogel, NPG Asia Mater. 16 (2024) 49, https://doi.org/10.1038/s41427-024-00569-1.