Unusual multiscale mechanics of biomimetic nanoparticle hydrogels

Abstract

Viscoelastic properties are central for gels and other materials. Simultaneously, high storage and loss moduli are difficult to attain due to their contrarian requirements to chemical structure. Biomimetic inorganic nanoparticles offer a promising toolbox for multiscale engineering of gel mechanics, but a conceptual framework for their molecular, nanoscale, mesoscale, and microscale engineering as viscoelastic materials is absent. Here we show nanoparticle gels with simultaneously high storage and loss moduli from CdTe nanoparticles. Viscoelastic figure of merit reaches 1.83 MPa exceeding that of comparable gels by 100-1000 times for glutathione-stabilized nanoparticles. The gels made from the smallest nanoparticles display the highest stiffness, which was attributed to the drastic change of GSH configurations when nanoparticles decrease in size. A computational model accounting for the difference in nanoparticle interactions for variable GSH configurations describes the unusual trends of nanoparticle gel viscoelasticity. These observations are generalizable to other NP gels interconnected by supramolecular interactions and lead to materials with high-load bearing abilities and energy dissipation needed for multiple technologies.

Fig. 1

Optical and mechanical characterization of hydrogels. Optical images and fluorescence spectra of CdTe NP solutions (dashed line) and hydrogels (solid line), Hydrogel-544 (a), Hydrogel-590 (b), and Hydrogel-618 (c). Measurement of oscillatory stress/strain of Hydrogel-544 (d), Hydrogel-590 (g), and Hydrogel-618 (j). Measurement of continuous step moduli/strain of Hydrogel-544 (e), Hydrogel-590 (h), and Hydrogel-618 (k). Each measurement was performed at least twice on two different disc specimens from the same sample. Rheological dynamic oscillatory frequency sweep tests of Hydrogel-544 (f), Hydrogel-590 (i), and Hydrogel-618 (l). The solid data point and dark color lines in e, f, h, i, k, and l correspond to the storage moduli G′ (ω), which is associated with gel elasticity (stiffness). The open data point and light color lines in e, f, h, i, k, and l correspond to the loss moduli G″ (ω), which describes energy dissipation in the gel. The shear dynamics for each NP gel was measured from low to high-strain starting at 0.01–20 % at a frequency of 1 Hz or 6.28 rad/s. The rheological dynamic oscillatory frequency sweep measurements were performed with a parallel fixed plate geometry (diameter 25 mm) at a strain value of 0.01 %

Fig. 2

Electron microscopy of hydrogels. a–c SEM and STEM HAADF images of hydrogels. The scale bar of inset HAADF images is 50 nm. d–f High-magnification HAADF images of hydrogels. The images show Hydrogel-544 (a, d), Hydrogel-590 (b, e), and Hydrogel-618 (c, f). Scale bar in a–c: 5 μm, Scale bar in d–f: 20 nm

Fig. 3

Molecular structure of GSH stabilizers at the CdTe NP surface. ¹H NMR spectra of a GSH at pH = 10.0, b Cd_x(GSH)_y complexes, c Hydrogel-544, d Hydrogel-590, and e Hydrogel-618. Amino acid residues in GSH are indicated by a single-letter code: Glutamyl, Q; Glycinyl, G; Cysteinyl, C. f Two-dimensional ¹H–¹H ROESY spectrum of Hydrogel-544. Schematic representations of g the three-point bonding (TPB) mode and h the single-point bonding (SPB) mode between GSH and the edge and face of a CdTe NP, respectively. The two bonding modes are clearly visible in the NMR spectra (b–e) as indicated by the symbol (star) for TPB and symbol (triangle) for SPB. Measurements of the density of GSH indicate that only one layer of GSH is present around each NP, so ligands attached to the particle surfaces are also participating in the gel network

Fig. 4

Molecular origin of NP hydrogel mechanics. a Theoretical bounds for the stiffness of Hydrogel-544 (blue) and Hydrogel-590 (orange) compared to Hydrogel-618 as a function of the interaction strength ratio between TPB–GSH and SPB–GSH. If TPB-mediated interactions between NPs are twice as strong as SPB-mediated ones and mainly present in the edges (upper bound), a threefold increase in stiffness is expected for smaller CdTe hydrogels, b schematic representation of a 2.7 nm CdTe NP showing three-point bonds covering corners and edges and single-point bonds are covering the surfaces. Our models suggest that the higher stiffness in hydrogels from smaller NPs results from the relative higher number of three-point bonds. Note: GSH Colors follow legend in Fig. 3

Fig. 5

Mechanical characterization of GSH-Au hydrogels, CYS-CdTe hydrogels and MPA-CdTe hydrogels. Oscillatory stress/strain (a), continuous step moduli/strain (b) and rheological dynamic oscillatory frequency sweep (c) of GSH-Au hydrogels (black line: Au, ~ 3 nm, red line: Au, ~ 8 nm). Oscillatory stress/strain (d), continuous step moduli/strain (e) and rheological dynamic oscillatory frequency sweep (f) of CYS-CdTe hydrogels (black line: ~ 3.2 nm, red line: ~ 3.7 nm; solid symbol: G′, empty symbol: G″). Oscillatory stress/strain (g), continuous step moduli/strain (h) and rheological dynamic oscillatory frequency sweep (i) of MPA-CdTe hydrogels (black line: ~ 2.7 nm, red line: ~ 3.1 nm; solid symbol: G′(ω), empty symbol: G″(ω))