Versatile Synthesis of Amine-Reactive Microgels by Self-Assembly of Azlactone-Containing Block Copolymers

Transient regulation of gel properties by chemical reaction networks

Transient regulation of gel properties by chemical reaction networks (CRNs) represents an emerging and effective strategy to program or temporally control the structures, properties, and functions of gel materials in a self-regulated manner. CRNs provide significant opportunities to construct complex or sustainable gels with excellent dynamic features, thus expanding the application scope of these materials. CRN-based methods for transiently regulating the gel properties are receiving increasing attention, and the related fields are worth further studying. This feature article focuses on the CRN-mediated transient regulation of six properties of gels, which are transient gelation, transient liquefaction of gels, transient assembly of macroscopic gels, temporary actuation of gels, transient healing ability of kinetically inert gels, and cascade reaction-based self-reporting of external stimuli. Recent advances that showcase the six properties of gels controlled by CRNs are featured, the characterization and structural elucidation of gels are detailed, and the significance, achievements, and expectations of this field are discussed. The strategy of transient regulation of gel properties via CRNs is potentially useful for building the next generation of adaptive functional materials.

Dual-reactive nanogels for orthogonal functionalization of hydrophilic shell and amphiphilic network

Amphiphilic nanogels (NGs) combine a soft, water-swollen hydrogel matrix with internal hydrophobic domains. While these domains can encapsulate hydrophobic cargoes, the amphiphilic particle surface can reduce colloidal stability and/or limit biological half-life. Therefore, a functional hydrophilic shell is needed to shield the amphiphilic network and tune interactions with biological systems. To adjust core and shell properties independently, we developed a synthetic strategy that uses preformed dual-reactive nanogels. In a first step, emulsion copolymerization of pentafluorophenyl methacrylate (PFPMA) and a reduction-cleavable crosslinker produced precursor particles for subsequent network modification. Orthogonal shell reactivity was installed by using an amphiphilic block copolymer (BCP) surfactant during this particle preparation step. Here, the hydrophilic block poly(polyethylene glycol methyl ether methacrylate) (PPEGMA) contains a reactive alkyne end group for successive functionalization. The hydrophobic block (P(PFPMA-co-MAPMA) contains random methacryl-amido propyl methacrylamide (MAPMA) units to covalently attach the surfactant to the growing PPFPMA network. In the second step, orthogonal modification of the core and shell was demonstrated. Network functionalization with combinations of hydrophilic (acidic, neutral, or basic) and hydrophobic (cholesterol) groups gave a library of pH- and redox-sensitive amphiphilic NGs. Stimuli-responsive properties were demonstrated by pH-dependent swelling and reduction-induced degradation via dynamic light scattering. Subsequently, copper-catalyzed azide-alkyne cycloaddition was used to attach azide-modified rhodamine as model compound to the shell (followed by UV-Vis). Overall, this strategy provides a versatile platform to develop multi-functional amphiphilic nanogels as carriers for hydrophobic cargoes.

Architecture- and Composition-Controlled Self-Assembly of Block Copolymers and Binary Mixtures With Crosslinkable Components: Chain Exchange Between Block Copolymer Nanoparticles

Chain exchange behaviors in self-assembled block copolymer (BCP) nanoparticles (NPs) at room temperature are investigated through observations of structural differences between parent and binary systems of BCP NPs with and without crosslinked domains. Pairs of linear diblock or triblock, and branched star-like polystyrene-poly(2-vinylpyridine) (PS-PVP) copolymers that self-assemble in a PVP-selective mixed solvent into BCP NPs with definite differences in size and self-assembled morphology are combined by diverse mixing protocols and at different crosslinking densities to reveal the impact of chain exchange between BCP NPs. Clear structural evolution is observed by dynamic light scattering and AFM and TEM imaging, especially in a blend of triblock + star copolymer BCP NPs. The changes are ascribed to the chain motion inherent in the dynamic equilibrium, which drives the system to a new structure, even at room temperature. Chemical crosslinking of PVP corona blocks suppresses chain exchange between the BCP NPs and freezes the nanostructures at a copolymer crosslinking density (CLD) of ∼9%. This investigation of chain exchange behaviors in BCP NPs having architectural and compositional complexity and the ability to moderate chain motion through tailoring the CLD is expected to be valuable for understanding the dynamic nature of BCP self-assemblies and diversifying the self-assembled structures adopted by these systems. These efforts may guide the rational construction of novel polymer NPs for potential use, for example, as drug delivery platforms and nanoreactors.

Influence of Nanogel Amphiphilicity on Dermal Delivery: Balancing Surface Hydrophobicity and Network Rigidity

Polymeric nanogels are promising nonirritating nanocarriers for topical delivery applications. However, conventional hydrophilic networks limit encapsulation of hydrophobic therapeutics and hinder tailored interactions with the amphiphilic skin barrier. To address these limitations, we present amphiphilic nanogels containing hydrophilic networks with hydrophobic domains. Two competing factors determine favorable nanogel-skin interactions and need to be balanced through network composition: suitable surface hydrophobicity and low network rigidity (through physical hydrophobic cross-links). To ensure comparability in such investigations, we prepared a library of nanogels with increasing hydrophobic cholesteryl amounts but similar colloidal features. By combining mechanical and surface hydrophobicity tests (atomic force microscopy (AFM)) with dermal delivery experiments on excised human skin, we can correlate an increased delivery efficacy of Nile red to the viable epidermis with a specific network composition, i.e., 20-30 mol % cholesterol. Thus, our nanogel library identifies a specific balance between surface amphiphilicity and network rigidity to guide developments of advanced dermal delivery vehicles.

Amphiphilic Nanogels: Fuzzy Spheres with a Pseudo-Periodic Internal Structure

Amphiphilic polymer nanogels (NGs) are promising drug delivery vehicles that extend the application of conventional hydrophilic NGs to hydrophobic cargoes. By randomly introducing hydrophobic groups into a hydrophilic polymer network, loading and release profiles as well as surface characteristics of these colloids can be tuned. However, very little is known about the underlying internal structure of such complex colloidal architectures. Of special interest is the question how the amphiphilic network composition influences the internal morphology and the "fuzzy" surface structure. To shine light into the influence of varying network amphiphilicity on these structural features, we investigated a small library of water-swollen amphiphilic NGs using small-angle X-ray scattering (SAXS). It was found that overall hydrophilic NGs, consisting of pure poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA), display a disordered internal structure as indicated by the absence of a SAXS peak. In contrast, a SAXS peak is present for amphiphilic NGs with various amounts of incorporated hydrophobic groups such as cholesteryl (CHOLA) or dodecyl (DODA). The internal composition of the NGs is considered structurally homologous to microgels. Application of the Teubner-Strey model reveals that hydrophilic PHPMA NGs have a disordered internal structure (positive amphiphilicity factor) while CHOLA and DODA samples have an ordered internal structure (negative amphiphilicity factor). From the SAXS data it can be derived that the internal structure of the amphiphilic NGs consists of regularly alternating hydrophilic and hydrophobic domains with repeat distances of 3.45-5.83 nm.

Tailoring Azlactone-Based Block Copolymers for Stimuli-Responsive Disassembly of Nanocarriers

Stimuli-responsive nanoparticles based on a reactive block copolymers (BCPs) of poly(ethylene glycol)-b-poly(2-vinyl-4,4-dimethylazlactone) (PEG-b-PVDMA) have been fabricated for loading and controlled release of molecular cargoes. Microphase segregation of PEG-b-PVDMA BCPs enables the construction of well-defined nanoparticles in aqueous solutions. The azlactone groups in VDMA repeat units offer active sites for hydrophilization of the BCPs and functionalization by primary amines. The hydrophilization of PEG-b-PVDMA BCPs induces gradual reconstruction and dissociation of the BCP nanoparticles. Functional primary amines can be conjugated to PEG-b-PVDMA BCPs, yielding azobenzene- and pyridine-containing BCPs. The self-assembled nanoparticles made from the functionalized BCPs can disassemble in response to different external stimuli (e.g., addition of β-cyclodextrin and pH changes). The gradual reconstruction of functionalized PEG-b-PVDMA BCP nanoparticles caused by hydrolysis of residual azlactone groups provides a novel method to engineer sub-50 nm, well-dispersed, stimuli-responsive nanoparticles. These nanoparticles can incorporate molecular cargoes and release them upon external stimuli, making the azlactone-containing BCPs attractive platforms for the development of controlled delivery vehicles.

Highly Selective and Sensitive Detection of Pb2+ in Aqueous Solution Using Tetra(4-pyridyl)porphyrin-Functionalized Thermosensitive Ionic Microgels

Tetra(4-pyridyl)porphyrin (TPyP)-functionalized thermosensitive ionic microgels (TPyP5-MGs) were synthesized by a two-step quaternization method. The obtained TPyP5-MGs have a hydrodynamic radius of about 189 nm with uniform size distribution and exhibit thermosensitive character. The TPyP5-MG microgel suspensions can optically respond to trace Pb²⁺ ions in aqueous solution with high sensitivity and selectivity over the interference of other 19 species of metal ions (Yb³⁺, Gd³⁺, Ce³⁺, La³⁺, Bi³⁺, Ba²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, Cr³⁺, K⁺, Na⁺, Li⁺, Al³⁺, Cu²⁺, Ag⁺, Cd²⁺, and Fe³⁺) by using UV-visible spectroscopy. The sensitivity of TPyP5-MGs toward Pb²⁺ can be further improved by increasing the solution temperature. The limit of detection for TPyP5-MG microgel suspensions in the detection of Pb²⁺ in aqueous solution at 50 °C is about 25.2 nM, which can be further improved to be 5.9 nM by using the method of higher order derivative spectrophotometry and is much lower than the U. S. EPA standard for the safety limit of Pb²⁺ ions in drinking water. It is further demonstrated that the TPyP5-MG microgel suspensions have a potential application in the detection of Pb²⁺ in real world samples, which give consistent results with those obtained by elemental analysis.