Chitosan is a biodegradable natural polymer with many advantages such as nontoxicity, biocompatibility, and biodegradability. It can be applied in many fields, especially in medicine. As a delivery carrier, it has great potential and cannot be compared with other polymers. Chitosan is extremely difficult to solubilize in water, but it can be solubilized in acidic solution. Its insolubility in water is a major limitation for its use in medical applications. Chitosan derivatives can be obtained by chemical modification using such techniques as acylation, alkylation, sulfation, hydroxylation, quaternization, esterification, graft copolymerization, and etherification. Modified chitosan has chemical properties superior to unmodified chitosan. For example, nanoparticles produced from chitosan derivatives can be used to deliver drugs due to their stability and biocompatibility. This review mainly focuses on the properties of chitosan, chitosan derivatives, and the origin of chitosan-based nanoparticles. In addition, applications of chitosan-based nanoparticles in drug delivery, vaccine delivery, antimicrobial applications, and callus and tissue regeneration are also presented. In summary, nanoparticles based on chitosan have great potential for research and development of new nano vaccines and nano drugs in the future.

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Quasi-2D Ruddlesden-Popper (RP) halide perovskites have drawn intensive research interest because they possess superior ambient stability while retaining excellent device performance as compared to their pure 2D or 3D counterparts. By phase engineering strategy, quasi-2D perovskites can fall into three types-large- n 2D perovskite, 2D:3D mixed perovskite, and 3D/2D bilayer perovskite. This Perspective discusses the modulation of phase composition, hierarchical distribution, and crystal orientation in quasi-2D perovskites, aiming to uncover the correlation between morphological structure, band alignment, and charge recombination. A perspective of phase engineering in 2D RP-type perovskite materials is then given toward the concurrent stability and device efficiency.

2D Ruddlesden–Popper (RP) perovskite solar cells have manifested superior operation durability yet inferior charge transport compared to their 3D counterparts. Integrating 3D phases with 2D RP perovskites presents a compromise to maintain respective advantages of both components. Here, the spontaneous generation of 3D phases embedded in 2D perovskite matrix is demonstrated at room temperature via introducing S‐bearing thiophene−2−ethylamine (TEA) as both spacer and stabilizer of inorganic lattices. The resulting 2D/3D bulk heterojunction structures are believed to arise from the compression‐induced epitaxial growth of the 3D phase at the grain boundaries of the 2D phase through the Pb−S interaction. The as‐prepared 2D TEA perovskites exhibit longer exciton diffusion length and extended charge carrier lifetime than the paradigm 2D phenylethylamine (PEA)‐based analogues and hence demonstrate an outstanding power conversion efficiency of 7.20% with significantly increased photocurrent. Dual treatments by NH 4 Cl and dimethyl sulfoxide are further applied to ameliorate the crystallinity and crystal orientation of 2D perovskites. Consequently, TEA‐based devices exhibit a stabilized efficiency over 11% with negligible hysteresis and display excellent ambient stability without encapsulation by preserving 80% efficiency after 270 h storage in air with 60 ± 5% relative humidity at 25 °C.

n-layered corner-sharing [BX 6 ] 4− octahedra separated by L, where n denotes the theoretical thickness of perovskite phase. [5] The L cation acts to improve both self-assembly capability and hydrophobicity of perovskite materials. [6,7] However, the poor interlayer charge carrier transport of 2D RP halide perovskites, which is attributed to the thick double insulating L layers, remains an open issue and invokes intense investigation.In general, the photogenerated excitons in 2D perovskites likely fall between the Wannier and Frenkel regimes where the exciton binding energy (E b ) depends not only on the number of inorganic layers (n), but also the organic spacer cations, offering the dielectric contrast against the inorganic layers. [8] Still, the dissociated charge carriers encounter the challenge of interlayer carrier transportation through the long-chain spacing cation layers. [9,10] In order to achieve more efficient exciton dissociation and interphase charge transport, Dion-Jacobson (DJ) type 2D perovskites, where the linker molecules bear two active sites have recently attracted increasing attention because only one spacing cation layer (L) is needed in-between the neighboring inorganic QWs. In an initial work, Karunadasa et al. reported the use of 2,2-(ethylenedioxy)bis(ethylammonium) as an organic spacer in 2D DJ perovskites to control the lattice distortion and the intrinsic trap states in thin film by forming an ionic bond between the NH 3 + and [PbX 6 ] 4− units. [11] In another early study, Zhao and co-workers included NH 2 C 4 H 9 COOH (Ava) with two different sites of NH 2 and COOH into MAPbBr 3 perovskite, disclosing another bonding of PbCOO − between Ava spacer and [PbX 6 ] 4− unit. [12] The above-mentioned two active sites result in the closer I···I interlayer distances and hence an enhancement of interface connection compared to RP type 2D perovskites. [13] Subsequently, numerous efforts have been made on a rational design of L cation based on either aliphatic [13][14][15][16][17][18][19][20][21][22] or aromatic [23,24] units to promote charge transport across the octahedra layers. In the former case, the linear chain with a variety of carbon chain lengths [13,14,22] and the annular chain with different locations of active site on aliphatic ring [17,21] gave rise to the highest power conversion efficiency (PCE) of 17.91% (n = 5) [14] and 12.04% (n = 4), [17] respectively. In the latter respect, Grätzel and co-workers more recently showed that a symmetric conjugated 1,4-phenylenedimethanammonium (PDMA) contain ing a conjugated benzene ring and two NH 3 + sites was introduced as L cation, which shortened the interlayer distance and enhanced π-π stacking relative to their RP counterpart, Dion-Jacobson (DJ) type 2D perovskites with a single organic cation layer exhibit a narrower distance between two adjacent inorganic layers compared to the corresponding Ruddlesden-Popper perovskites, which facilitates interlayer charge transport. However, the internal crystal structures in 2D DJ perovskites ...