Nanoparticle-Hydrogel Composites: Concept, Design, and Applications of These Promising, Multi-Functional Materials

Abstract

New technologies rely on the development of new materials, and these may simply be the innovative combination of known components. The structural combination of a polymer hydrogel network with a nanoparticle (metals, non-metals, metal oxides, and polymeric moieties) holds the promise of providing superior functionality to the composite material with applications in diverse fields, including catalysis, electronics, bio-sensing, drug delivery, nano-medicine, and environmental remediation. This mixing may result in a synergistic property enhancement of each component: for example, the mechanical strength of the hydrogel and concomitantly decrease aggregation of the nanoparticles. These mutual benefits and the associated potential applications have seen a surge of interest in the past decade from multi-disciplinary research groups. Recent advances in nanoparticle-hydrogel composites are herein reviewed with a focus on their synthesis, design, potential applications, and the inherent challenges accompanying these exciting materials.

Keywords: hydrogel‐nanoparticle composites; multi‐functional materials; polymer networks; stimuli responsive hydrogels.

Figure 1

Concept for combination of nanoparticles and hydrogel to form new functional materials. Three different structural designs exist: a) micro‐ or nano‐sized hydrogel particles stabilizing inorganic or polymer nanoparticles, b) nanoparticles non‐covalently immobilized in a hydrogel matrix, and c) nanoparticles covalently immobilized in hydrogel matrix.

Figure 2

Five main approaches used to obtain hydrogel‐nanoparticle conjugates with uniform distribution: 1) hydrogel formation in a nanoparticle suspension, 2) physically embedding the nanoparticles into hydrogel matrix after gelation, 3) reactive nanoparticle formation within a preformed gel, 4) cross‐linking using nanoparticles to form hydrogels, 5) gel formation using nanoparticles, polymers, and distinct gelator molecules.

Figure 3

a) Schematic of titania nano sheets (Ti) acting as photo catalyst for gelation (difference in energy level of valence band and conduction band is shown). UV radiation with wavelength below 260 nm will produce hydroxyl free radicals causing gel formation reaction with vinyl monomers. b) List of vinyl monomers used for photo induced hydrogelation. c) Pictures before and after the hydrogelation using IPAAm (10.0 wt%) as monomer. The examples show hydrogel formation in a nanoparticle suspension. The nanoparticles are retained in the hydrogel by non‐covalent interactions. Reproduced with permission.15 Copyright 2013, Macmillan Publishers Ltd.

Figure 4

The construction of a gold‐nanoparticle/hydrogel composite at the electrode interface by switching between its swollen and shrunken states. a) Electrochemical formation of acrylamide hydrogel film at the gold electrode showing its shrinking behavior in acetone and swelling behavior in water. b) Use of alternate cycles of swelling of the hydrogel gold nanoparticle suspension in water followed by shrinking in acetone to physically entrap nanoparticles in the hydrogel. Adapted with permission.6

Figure 5

a) Preparation of Ag/PAAm hydrogel nanocomposite without the use of thiols. Reproduced with permission.19 Copyright 2007, Elsevier. b) Hydrogel formation and functionalization with Au NPs to obtain catalytic hydrogels by redox active catechol groups. Adapted with permission.5 Copyright 2014, American Chemical Society.

Figure 6

Crosslinking using nanoparticles to form nanoparticle‐hydrogel composites. Cross‐linking using clay nanostructure (Clay‐NS) to form nanoparticle‐hydrogel composites with enhanced mechanical properties. The semiconductor NPs, monomer, and Clay‐NS are homogeneously dispersed in water. Upon photo activation semiconductor nanoparticles produce free radicals initiating polymerization and crosslinking through clay‐NS. The photograph of the vials depicts optical images of the hydrogelation process. A mixture solution of ZnO nanoparticles, DMAA (N,N‐dimethylacrylamide), and Clay‐NS before gelation, hydrogelation after 1 h of irradiation and the resultant elastic ZnO nanocomposite hydrogel taken out of the vial are shown. Adapted and reproduced with permission.15 Copyright 2014, Macmillan Publishers Ltd.

Figure 7

Nanoparticle‐Hydrogel composite with gelator molecules to form electrodes. a) Each (silicon‐nanoparticle) Si‐NP is encapsulated within a conductive polymer surface coating and is further connected to the highly porous hydrogel framework. b) Dispersion of Si‐NPs in the hydrogel precursor solution containing the crosslinker (phytic acid), the monomer aniline and the initiator ammonium per sulphate. c) Cross‐linked viscous gel formed after several minutes of chemical reaction, d) the hydrogel gel bladed onto a 520 cm² copper foil current collector and dried to form the electrode. Reproduced with permission.29 Copyright 2013, Macmillan Publishers Ltd.

Figure 8

a) Overview of potential medical applications for Ag NP‐hydrogel composites. Reproduced with permission.37 Copyright 2013, Elsevier. b) Ag NPs incorporated into pH‐responsive hydrogel based glucose oxidase activity sensor. c) Shifts in silver nanoparticle absortion maxima (Δλ_max) as functions of glucose concentration for the plasmonic sensing device. d) Spectrophotometric glucose sensing. Change in absorption at 470 nm. Adapted with permission.48

Figure 9

a) Photo‐thermal effect of laser irradiated Au‐nano rods, causing localized collapse of hydrogel. White arrow indicates position of Au‐nano rods and laser irradiation. Reproduced with permission.57 Copyright 2007, American Chemical Society. b) T‐junction of a microfluidic device with Au‐colloid hydrogel (right valve) and Au nano shell hydrogel (left valve) with 100 μM wide channels i) When the device is illuminated with 532nm green light the left valve opened ii) when the device is illuminated with 832nm IR light the right valve opened. iii) The absorption spectra of gold nano particles and gold nano shells. Adapted and reproduced with permission.50

Figure 10

a) Removal of Cu2+ ions in aqueous medium, i) Ferrogel in solution of Cu2+ ions, ii) Magnetic rod attracts ferrogel, iii) Removal of ferrogel is accompanied by the removal of Cu²⁺ ions as well. Reproduced with permission.65 Copyright 2010, Elsevier. b) Bending of ferrogel under varying magnetic field. i) No bending under no field. (ii–iv) Increasing bending observed as magnetic field is increased from 0.5T to 1.3T. Magnetic field strength is indicated in the pictures. Reproduced with permission.66

Figure 11

a) Left: Physical demonstration of extreme elasticity of Si‐NPs hydrogels by stretching. Right: Stress‐strain plots showing excellent toughness by Si‐NP hydrogels. SNP 0.09 indicate gels made with 0.09mg/mL surface treated silica nanoparticle, SNP0.009^a indicate gels made with same amount of silica without surface treatment. CC1 is the hydrogel without silica nanoparticles. Reproduced with permission.69Copyright 2013, Royal Society of Chemistry. b) Demonstration of cellulose networks in the hydrogels protecting the CdSe/ZnS structure and preserving the quantum dots characteristics. Appearances of the Quantum dot (QD) ‐cellulose hydrogels under a 302 nm UV lamp (left), and Photoluminescence spectra of CdSe/ZnS (core/shell) QDs with average diameter 2.8 nm (green), 3.0 nm (yellowish‐green), 3.2 nm (yellow) and 3.6 nm (red) respectively in buffer solution. QD‐cellulose hydrogel emission peaks are matching with (right, bottom) with emission peaks of the free quantum dots in buffer. Adapted and reproduced with permission.77 Copyright 2009, Royal Society of Chemistry.

Figure 12

a) Mechanical compression of bilayer gels with and without poly styrene nanoparticles (PSNPs) Gel 1: without PS NP (top) and Gel 2: with 26% PS NP (bottom part of the cylindrical gel) are attached together and compression is given from top (arrow). b) Time profile of deformation ΔL₁/L and ΔL₂/L (top) and electric potential ΔV generated (bottom) upon deformation. Reproduced with permission.89 Copyright 2007, Royal Society of Chemistry.