Yolk-shell smart polymer microgels and their hybrids: fundamentals and applications

Abstract

Yolk-shell microgels and their hybrids have attained great importance in modern-day research owing to their captivating features and potential uses. This manuscript provides the strategies for preparation, classification, properties and current applications of yolk-shell microgels and their hybrids. Some of the yolk-shell microgels and their hybrids are identified as smart polymer yolk-shell microgels and smart hybrid microgels, respectively, as they react to changes in particular environmental stimuli such as pH, temperature and ionic strength of the medium. This unique behavior makes them a perfect candidate for utilization in drug delivery, selective catalysis, adsorption of metal ions, nanoreactors and many other fields. This review demonstrates the contemporary progress along with suggestions and future perspectives for further research in this specific field.

Fig. 1. Temperature and pH-responsive behavior of PMAA@void@PNIPAM yolk–shell microgels.

Fig. 2. Classification of yolk–shell polymeric systems on the basis of the nature of the yolk. (A) Metallic yolk, (B) smart polymeric yolk, (C) magnetite yolk, (D) Au-coated magnetite yolk, (E) silica-magnetite yolk, (F) sulfur yolk.

Fig. 3. Yolk–shell microgels with different morphologies: (A) raspberry-shaped, (B) elliptical, (C) lemon-like morphology, (D) double walled, (E) rattle-like morphology.

Fig. 4. Fabrication of Fe₃O₄/SiO₂/air/P(BIS-co-MAA) yolk–shell microspheres from Fe₃O₄/SiO₂/PMAA/P(BIS-co-MAA) tetra-layer microspheres by the removal of non-crosslinked poly(methacrylic acid) layer using ethanol.

Fig. 5. Synthesis of raspberry-shaped cross-linked poly(methacrylic acid)@cross-linked poly(N-isopropylacrylamide) [CPMAA@CPNIPAM] yolk–shell microspheres from CPMAA@PNIPAM@CPNIPAM by the self-removal of non-cross-linked PNIPAM.

Fig. 6. Synthesis of gold–poly(N-isopropylacrylamide) [Au–PNIPAM] yolk shell particles via free radical polymerization.

Fig. 7. (A) FeCl₃-triggered Friedel–Crafts reaction for the cross-linking of polystyrene. (B) Synthesis of hollow porous polymeric nanospheres having metallic yolk via RAFT polymerization.

Fig. 8. Synthesis of poly(methacrylic acid-co-ethyleneglycoldimethacrylate)@poly(N-isopropylacrylamide) [(PMAA-co-EGDMA)@PNIPAM] yolk–shell microspheres via the distillation precipitation polymerization method.

Fig. 9. Fabrication of yolk–shell magnetite@poly(methacrylic acid) [(Fe₃O₄@PMAA)] composite microspheres using seeded emulsion polymerization.

Fig. 10. Synthesis of magnetite@poly(methacrylic acid) [Fe₃O₄@PMAA] yolk–shell microspheres via the reflux-precipitation polymerization method.

Fig. 11. Synthesis of poly(methacrylic)@void@poly(N-isopropropyl-acrylamide) [PMAA@void@PNIPAM] yolk–shell microgels through emulsion precipitation polymerization.

Fig. 12. Thermoresponsive behavior of PAA@void@PHEMA yolk–shell microgels; below VPTT, the outer shell is in a swollen state and intermolecular H-bonding dominates in PHEMA, while above VPTT, intramolecular H-bonding dominates and the polymeric system is in a deswollen state.

Fig. 13. The pH-responsive behavior of the PMAA@void@PMAA polymeric system; with an increase in the pH of the medium, the size of the polymeric system is also increased. The hydrodynamic volume also increases with an increase in the pH of the medium.

Fig. 14. Effect of increasing salt concentration on the size of the polymeric network. With an increase in the salt concentration, the size decreases due to the presence of oppositely charged ions to counter the repulsive effect.

Fig. 15. (A) The mechanism of reduction of 4-NP to 4-AP using the Au@void@P(BIS) hybrid polymeric system. The reduction of 4-NP follows the Langmuir–Hinshelwood mechanism. (B) The thermo-responsive selectivity of Au@void@PNIPAM yolk–shell hybrid microgels. Below VPTT, the reduction of 4-NP is favorable due to the hydrophilic nature of the polymeric network and above VPTT, the reduction of NB is favorable because of the hydrophobic nature of the polymeric network.

Scheme 1. The mechanism of reduction of Au(iii) with ascorbic acid for the growth of Au yolk of Au@void@PNIPAM in the presence of CTAB.

Fig. 16. The fabrication of acrylic acid inside the polymeric yolk–shell microgels by thermal polymerization in the presence of Zn²⁺ as a physical cross-linker.

Fig. 17. Pictorial view of the antibacterial activity of Ag-based yolk–shell polymeric systems and the kinetics of oxidation of Ag NPs to Ag⁺ ions, the actual warriors.

Fig. 18. The drug loading and drug releasing of the yolk–shell polymeric network P(MAA-co-EGDMA)@void@P(NIPAM-co-MAA) at different pH values and temperatures.

Iqra Sajid

Ahmad Hassan

Robina Begum

Shuiqin Zhou

Ahmad Irfan

Aijaz Rasool Chaudhry

Zahoor H. Farooqi