National 5 Chemistry Assignment Hydrogels In Medicine
Preparation of GSH-NP hydrogels
NP gels were synthesized from CdTe NPs, stabilized by a natural peptide l-glutamyl-l-cysteinyl-glycine (GSH) (Methods section)25. The choice of GSH as a NP surface ligand was made based on its ability to form multiple intermolecular hydrogen bonds. We also wanted the stabilizer to be relatively short so that the powerful dispersion forces, or van der Waals (vdW) interactions, between the inorganic cores of the NPs would contribute substantially to the gel formation. These NP gels can also be viewed as biomimetic analogs of dense biological gels found, for instance, in cytoskeleton, where supramolecular interactions at the protein–protein interface are augmented by forces specific to inorganic materials12. Such NP solids combining the strong core-to-core and shell-to-shell interactions were expected to show unusual viscoelastic properties that are reminiscent of many biological gels.
NPs with inorganic core diameters of 2.7, 3.2, 3.7, and 4.0 nm were made by varying the duration of the reflux stage (Supplementary Table 1 and Supplementary Figures 1–3). The four sizes of GSH-CdTe NPs differ from each other by one additional layer of the CdTe atomic crystal structure. Aqueous dispersions of NPs with diameters of 2.7–3.7 nm remained stable after 24 months but those with diameters of 4.0 nm were stable only for several days. While the 4.0 nm NPs were sufficient for some benchmark tests, most of the experimental work was carried out with 2.7, 3.2, and 3.7 nm NPs.
When 2.5 volumes of isopropanol were added to one volume of an aqueous dispersion of NPs, phase separation occurred, and produced a viscous fluid phase (Fig. 1, Supplementary Figure 1). The isopropanol-rich phase was separated by centrifugation at 3000 r.p.m./min and dried under vacuum. The dried NP solid was hydrated by adding 22 w/w % DI water, which transformed it into a voluminous, intensely luminescent gel with a high-quantum yield of about 30 % (Methods section). During the preparation of the gel, we noticed that this material was unusually stiff, yet able to flow under shear (Fig. 1).
The aqueous dispersion of NPs displayed photoluminescence (PL) peaks at 523, 566, 600, and 647 nm, whereas the prepared hydrogels had PL emission peaks at 544 nm (green), 590 nm (orange), 618 nm (red), and 657 nm (deep red), respectively (Fig. 1a–c, Supplementary Figure 4). The redshift of PL emission peaks in the gel state, as compared to the PL peaks of the initial aqueous dispersion of CdTe NPs was attributed to energy transfer between the closely spaced NPs. For brevity, we shall refer to the corresponding hydrogels as Hydrogel-544, Hydrogel-590, Hydrogel-618, and Hydrogel-657. For brevity, most of the discussion that follows is centered on the former three gels as being most representative of the GSH-CdTe NP gels; Hydrogel-657 is used as a test case for essential property trends.
Oscillatory stress/strain tests of the freshly prepared NP hydrogels with different NP sizes revealed that the stress increased linearly with the strain amplitude applied to the sample at the initial stage. Surprisingly, the gel made from the smallest NPs, Hydrogel-544, displayed the highest stiffness and shear modulus for all strains (Fig. 1d, e). We also performed static compressive tests on cylindrical specimens. Similar to hydrogels, the toughness of aerogels composed of smaller NPs are higher than those composed of larger NPs (Supplementary Figure 5). This observation contradicted our expectations and multiple previous studies of NP solids with long organic ligands26,27,28,29, as well as gels from various inorganic particles without distinct organic shells30. The reasons for such unexpected particle size dependence of viscoelastic properties should be related to the combination of strong attraction interactions between the NPs while retaining their reconfigurability that enables efficient energy dissipation.
At high strains, distinct nonlinear behavior was observed as the gels began to flow (Fig. 1d, e, g, h, j, k). This finding is unusual because the volumetric ratio of the soft organic shell made from GSH versus the hard CdTe core was the largest (3.7:1) for Hydrogel-544, as compared to 2.8:1 for Hydrogel-590, and 1.9:1 for Hydrogel-618 (Supplementary Figure 6). The critical strain whereupon the gel starts to flow can be determined by the flex point in the strain dependences for the storage modulus G′ and the loss modulus G″ (Fig. 1e, h, k). This flow point is at about 1 % strain for Hydrogel-544, while Hydrogel-590, and Hydrogel-618 begin to flow at about 2 % strain. In addition, the values of the storage moduli drop more rapidly for Hydrogel-544 than for gels from larger NPs. This means that once the structure of the gel is disrupted, the interactions between the smaller NPs are weaker than those of the larger NPs.
Additional knowledge about viscous deformations in these fluids can be obtained from the variation of dynamic mechanical properties with cyclic frequency, ω. The response of Hydrogel-544 (Fig. 1f) is remarkably different from those of Hydrogel-590 and Hydrogel-618 (Fig. 1i, l). Hydrogel-544 reveals a plateau for both moduli and over the entire range of frequencies G′ > G″. For Hydrogel-590 and Hydrogel-618, both moduli increase with ω and show a transition from mostly elastic to mostly viscous behavior at ω ~ 5 rad/s when G′ ~ G″. Hydrogel-657 again shows lower values of stiffness, shear modulus, storage moduli, and loss moduli compared to Hydrogel-618 (Supplementary Figure 7).
The nearly ideal linear stress-strain response of Hydrogel-544 in Fig. 1d is indicative of the dominance of elastic interactions between NPs at the nanoscale. Such viscoelastic behavior is uncommon for hydrogels because it is typically determined by highly dissipating intermolecular bonds. Moreover, the magnitude of G′ for Hydrogel-544 exceeds 106 Pa for some frequencies. These values of storage moduli are considerably higher than those of other hydrogels made from a wide spectrum of other chemical, biological, and nanoscale components with different structural organizations11,30, in some cases by several orders of magnitude (Supplementary Table 4); such high values of G′ support the possibility of simultaneously combining high stiffness and high-energy dissipation in these materials.
The viscoelastic properties of materials can be cumulatively characterized by the viscoelastic figure of merit (VFOM) calculated as VFOM = |G*|tanδ, where G* = (G′2 + G″2)0.5 and tanδ = G″/G′. Hydrogel-544 has exceptionally high VFOM with a value of 1.83 MPa at 10 Hz (Supplementary Table 3). Similarly high values of VFOM = 1.71 MPa were observed for Hydrogel-590, while for other NP gels made with GSH, CYS, MPA ligands and both CdTe and Au cores ranged from 0.01 to 0.67 MPa in the wide range of frequencies. Even for the smallest values of VFOM in this family of gels, they exceed VFOMs for most advanced NP hydrogels based on niobate nanosheets2 whose VFOM is < 0.001 MPa or articular cartilage3 with VFOM ~ 0.001 MPa. Such high VFOMs in NP based gels are attributed to the strong attraction between the NPs while retaining their reconfigurability. Besides practical significance associated with unusually high VFOMs for gels18, the materials that can combine high values of both G″ and G′ challenge the fundamental understanding of about the property correlations and how the materials the limits apparent limits specific to particular classes of the materials can be overcome.
Hydrogel-544 clearly stands out among other hydrogels studied here. Understanding the origin of the unique viscoelastic properties for this and other hydrogels studied here requires better understanding of the gel structure. The freeze-dry process was used to remove constituent water without the disruption of the NP networks. SEM and STEM high-angle annular dark-field (HAADF) images (Fig. 2a–c) revealed that all three hydrogels are structurally similar in that they are comprised of densely packed CdTe NPs (Fig. 2d–f). However, the hydrogels were solid with low-density porous structures indicated by Brunauer–Emmett–Teller (BET) analysis, small-angle X-ray scattering (SAXS), and X-ray tomography (Methods section and Supplementary Figures 8–10). Energy-dispersive X-ray elemental mapping (Supplementary Figure 11) confirmed that the packed NPs are distributed throughout the hydrogels. Also, evaluation of these hydrogels by thermogravimetric analysis revealed that the GSH content was higher than 31.7 w/w % (Supplementary Figure 6), which indicated that most of the stabilizer molecules around the NPs remained in place. Given the similarity of the packing within the hydrogels, the path to understanding the differences between their bulk viscoelastic properties must go through the understanding of the structure and interactions within NP–NP interfaces.
The rheology of NP hydrogels could potentially be understood using previous models of colloidal gels13,30,31 but these approaches face the well-known problem of combining the description of the NP ligand shell at the molecular level while requiring description of mechanical behavior at the macroscale level. For example, the sticky hard-sphere model is used to describe inert and rigid colloidal systems, in which the only inter-particle forces considered include infinite repulsion if the particles overlap, and a strong attractive interaction on contact32. The conceptual make-up of these models make them quite restrictive in terms of the spectrum of mechanical properties they can predict14. Most importantly, current colloidal gel models face the problem of describing the gels as having simultaneous and equally significant contributions from both the NP cores and surface ligands that represent multiple scales and complex interactions, especially in the presence of a solvent12,33.
Molecular structure of the GSH layer
The GSH surface ligands are bound to the surface of CdTe NPs via Cd2+ ions. We studied this bonding by 1H and 113Cd NMR spectroscopy using a 500 MHz spectrometer. First, as a control experiment, the 1H NMR spectra of Cdx(GSH)y complexes revealed that the cysteinyl (cys) and the glutamyl (glu) proton resonances of GSH shift to higher frequency as compared to unconjugated GSH (Fig. 3a, b). 1H NMR spectra of all three hydrogels showed two well-separated pairs of proton resonances C(α) (4.47 and 4.61 ppm) and Q(α) (3.68 and 3.58 ppm). We did not observe NMR signals attributable to considerable amount of water in the gels. The existence of pairs indicates the presence of two types of distinct coordination geometries for GSH bonding with Cd2+ (Fig. 3c–e). In addition, two-dimensional 1H–1H exchange NMR spectroscopy (EXSY) confirmed that the two bonding types are interchangeable by fast ligand exchange (Fig. 3f, Supplementary Figure 12).
To resolve molecular details of GSH bonding, we analyzed the 113Cd chemical shifts, which are sensitive to the Cd2+ coordination environment. The 113Cd NMR spectrum of Hydrogel-544 shows two distinct cadmium resonances at 324.55 ppm (dominant) and 678.3 ppm (Supplementary Figure 13). For the assignment of these two peaks, we compared the chemical shift values with reported values from the literature34. The 113Cd peak for the Cd(S-GS)4 complexes is expected at 674 ppm, whereas the 113Cd peak for the interchangeable CdS2N3O/CdSN3O2 complexes typically appears at 322 ppm. We attribute the dominant peak at 324.55 ppm to GSH moieties that are simultaneously bound via a covalent bond to Cd atoms and two coordination bonds with −NH2 and –COO groups. A similar scorpion-like configuration of GSH has been observed upon its binding to a gold surface35.
We further refer to this coordination geometry as the three-point bonding (TPB) mode. In TPB model, CdTe might include three or two of Cd atoms coordinating with S, −NH2, and −COO. To answer this question, we determined the TPB–GSH model by a quantum mechanical semi-empirical calculations using the software package Spartan (Wavefunction Inc., Irvine, CA). (Fig. 3g, Supplementary Figure 14). In contrast, we attribute the signal at 678.3 ppm to the GSH in a standard single-point bonding (SPB) mode that could also be described as the ‘crew-cut’ configuration (Fig. 3h). Bonding at three points versus one point on the NP surface makes TPB–GSH acquire an atomic configuration that is nearly parallel to the surface (Fig. 3g), as compared with the vertically aligned SPB–GSH (Fig. 3h). Furthermore, we hypothesize that making three bonds with the surface rather than one likely requires the molecule to be anchored at an edge or a corner.
This hypothesis can be evaluated using NMR spectroscopic techniques that afford quantification of the number of GSH molecules in the two bonding modes by measuring the relative intensities of signals corresponding to the C(α) protons, which resonate at 4.47 and 4.61 ppm. Accordingly, the ratios of GSH in the TPB configuration (TPB–GSH) to GSH in the SPB configuration (SPB–GSH) for Hydrogel-544, Hydrogel-590, and Hydrogel-618 are 1:0.2, 1:0.4, and 1:0.7, respectively. The relative frequency of TPB configurations is largest for the smallest NPs, which can also be seen via X-ray photoelectron spectroscopy (XPS) (Supplementary Figure 15). The dominance of the N–Cd coordination in Hydrogel-544, as evidenced by a peak at 401 eV for N 1s, cannot be detected in Hydrogel-618.
XPS data further demonstrate the difference in the structure of the interface between the NPs in Hydrogel-544 and Hydrogel-618. Hydrogel-544 and Hydrogel-618 display XPS peaks for the Cd 3d line at 404.5 and 411.3 eV, respectively, indicating a marked difference in the electron density around the Cd atoms on the NP surface. The effect of the stabilizer configuration on the hydrogen bonds can be concomitantly observed from chemical shifts and signal broadening in the 1H NMR experiment (Fig. 3a–e). The chemical shifts of GSH corresponding to the binding of Cd2+ with the functional groups –COOH, –SH, and –NH2 increase from Hydrogel-544 to Hydrogel-590 and Hydrogel-618. This is indicative of a weakening of the hydrogen bonding due to an increase of the electron density on the hydrogen atom.
Molecular effects on macroscale mechanics of nanoparticle hydrogels
One might expect that solids made from NPs with higher volume organic stabilizer shells and relatively small-crystalline cores will be easier to deform than those with a larger proportion of inorganic material. However, this is not the case here. Thermogravimetric analysis reveals that weight ratios between GSH and CdTe for Hydrogel-544, Hydrogel-590, and Hydrogel-618 are 0.90, 0.68, and 0.46, respectively. The gel with the smallest NPs, Hydrogel-544, exhibits not only the highest stiffness and highest loss modulus (Fig. 1f), but also the lowest weight ratio of CdTe.
In order to integrate the molecular scale descriptors into a theoretical models suitable for initial assessment of macroscale mechanical properties, we calculated the surface density of TPB–GSH and SPB–GSH from the weight ratios and NMR measurements under the reasonable assumption that all GSH stabilizers reside in shells around the NPs (Methods section). The reasons behind this approach to modeling of mechanical properties are governed by the desire to construct a relatively simple yet descriptive model generalizable to different NPs. More complex models with explicit description of hydrogen bonds would be difficult to parametrize because of strong dependence of hydrogen bonds on the specific configuration of GSH on the surface of NPs. While MD simulations combined with DFT or ab initio modules were successful for biomimetic NP capsids36 and NP hydration layer37, the relatively large size of the NPs in our case, large number of ligands on the surface of the NPs and the large number of NPs required to model gel mechanics make this approach difficult to realize at the moment. The model presented here can be used as a stepping-stone toward the goal of relating intrinsic particle properties and macroscopic measurements in hydrogels.
The absolute number of TPB–GSH per NP remains approximately constant at 19.8, 21.9, and 18.6, whereas the same parameter number grows for SPB–GSH from 3.9 to 8.8 to 13.1 (Methods section). We hypothesize that the relatively higher number of TPB–GSH on the surface of smaller CdTe NPs could account for the higher stiffness observed in experiments. This could be so if TPB–GSH interactions yielded stronger NP–NP interactions than SPB–GSH bindings. In order to test this hypothesis, we created a generalized interaction strength ratio parameter quantifying the degree by which TPB–GSH mediated interactions are stronger than SPB–GSH. We then analytically calculated the impact this ratio would have in the strength of an identical system of interacting NPs. Since we do not know the location of these ligands, we measure this impact for both edge-mediated interactions and facet-mediated interactions. Both upper and lower bounds, resulting from these considerations, are shown in Fig. 4a. As it can be seen, if TPB–GSH mediated interactions are twice as strong as SPB–GSH-mediated interactions and primarily located at the NP edges (upper bound), a threefold increase in stiffness of Hydrogel-544 compared to Hydrogel-618 is observed, in agreement with our experimental findings (Fig. 1d). We assumed even the smaller CdTe NPs to have tetrahedral shapes, following evidence from previous works38,39.
The fact that the number of TBP–GSH molecules is approximately constant despite the changing NP core suggests that they reside preferentially on the edges and vertices, which change less rapidly with changing NP size than surface area. Similarly, the rapid increase in number of SPB–GSH ligands with particle size, suggests a preference for the facets of the inorganic core. These assumptions also make sense in light of recent theoretical findings showing the preference of ligands for vertices, edges, and faces, respectively38,39,40, suggesting a mechanism by which the first bound ligands, attached to vertices and edges, have enough room to change configuration from SPB to TPB, whereas latecomers end up stuck in a SPB configuration on the facets (Fig. 4b). A test for this particular set of assumptions is however left for future work. Considering that the nearly horizontal arrangement of TPB–GSH molecules (Fig. 3g) means larger numbers of atoms exposed for hydrogen bonding between NPs, it is reasonable to expect TPB–GSH to result in greater attraction between neighboring NPs.
Tuning of Gel mechanics using molecular and nanoscale parameters
A step toward generalization of the observed relationship between the molecular configurations of surface ligands and the macroscale viscoelastic properties of NP hydrogels can be made if similar properties and relationships are observed for gels from similar—but not identical—NPs. More specifically, since: (i) our theoretical model suggests that the different ratio between three-point and single-point bonds are responsible for the drastic changes in gel stiffness, and (ii) the preference of three-point bonds for the NPs corners and edges is a general mechanism for the stabilization of facetted NPs38,39,40, we investigate, as positive and negative benchmarks, an additional system of facetted NP stabilized by GSH, and two other systems of CdTe, stabilized this time by two ligands not showing multiple molecular configurations (Supplementary Table 2). The hydrogels made from GSH-stabilized Au NPs (GSH-Au), as well as from cysteine- and mercaptopropionic acid-stabilized CdTe NPs, are denoted as CYS-CdTe and MPA-CdTe hydrogels, respectively.
In general, GSH-Au hydrogels displayed similar mechanical behavior to GSH-CdTe hydrogels and similar molecule structure of GSH on Au NPs with GSH-CdTe (Fig. 5a-c, Supplementary Figure 16). As expected, higher stiffness and energy dissipation was observed for small GSH-Au NPs over large ones (Fig. 5b, c). Moreover, the values of storage moduli found for GSH-Au can be even higher than those for GSH-CdTe NPs of similar size (Fig. 5b, c vs Fig. 1h, i Hydrogel-590). The increase of storage moduli for GSH-Au is associated with greater strength of the van der Waals interactions between the Au NPs than between CdTe NPs; the characteristic Hamaker constants for particles made from gold and CdTe in water are 13 × 10−20 J and 4.9 × 10−20 J, respectively. However, the VFOM value is 0.21 MPa at 10 Hz for GSH-Au hydrogels with a size of 3.0 nm of Au NPs, which is considerably lower than 3.2 nm GSH-CdTe hydrogels (Hydrogel-590) which is 1.71 MPa. These finding are significant because they clearly show that both core-to-core and shell-to-shell interactions are significant for the mechanics of these NPs gels with relatively short surface ligands.
CYS-CdTe and MPA-CdTe hydrogels display higher storage and loss moduli (G′′ and G″) when the viscoelastic solids are made from larger particles (Fig. 5d–i). These observations are commensurate with the fact that the molecular geometry of the bonding of CYS and MPA to NPs was found to change little with size. The positions of the NMR signals are commensurate with the two-point binding of both CYS and MPA to CdTe (Supplementary Figure 17)41,42. A comparison of stiffness for all three types of surface ligands, that is, GSH, CYS, and MPA, with CdTe NPs of similar size, shows that MPA-CdTe has the highest stiffness (Supplementary Figure 18), which confirms the significance of the supramolecular interactions between the surface ligands for gel mechanics. Moreover, the storage and loss moduli found for 3.2 nm CYS-CdTe hydrogels (Fig. 5d–f vs. Fig. 1g–i) can be even higher than those for GSH-CdTe. These experimental data substantiate the expectations that stiffness and energy dissipation in these systems can be increased in parallel. As expected, the VFOM value is 0.10 MPa at 10 Hz for CYS-CdTe hydrogels with a size of 3.2 nm of CdTe NPs, which is comparable to that of MPA-CdTe hydrogels with a size of 3.1 nm of CdTe NPs with VFOM = 0.16 MPa at 10 Hz.
Comparative evaluation of viscoelastic properties
The previous point can be further strengthened by the comparison of mechanical properties of all NP gels obtained in this work with those of other inorganic, polymeric, biological, and nanostructured gels (Supplementary Table 4). The characteristic structural elements of inorganic gels are particle chains organized into porous networks cross-linked by covalent and non-covalent interactions6,9,13,43 between the constituent hard particles. Similar structure of porous networks can also be found in many hydrogels from proteins44,45,46, polymers22,47,48, peptides5, and recently from aramid nanofibers49,50. The cross-links between these macromolecules are made via hydrophobic interactions, ionic bridges, and hydrogen bonds16. Unlike both inorganic and organic networked gels, NP gels described here are compact solids in which each unit retains its mobility, which leads to greater stiffness and energy dissipation per voxel.
The SQA has recently announced changes to the Course Specification for National 5 Chemistry. You can access the Revised Course Specification here.
A major which will come into effect at the start of the 2017/2018 academic session is that practical / experimental will become a mandatory feature of the assignment. SSERC has been commissioned to produce 2 Resource Packs which could be used to support teachers and students with this change.
Details of the resource packs follow but we should point out that these are exemplars, not mandatory pieces of practical work which must be done. Many other practicals currently undertaken would be equally appropriate for use to support the assignment and over the coming months we shall, within SSERC, be looking to see how we might add to the pool of exemplars.
Each Resource Pack consists of 2 elements:
- A Teacher / Technician Guide. (Incorporating generic risk assessments which might be used as the basis for the circumstances which exist in your own school / college). It also contains links to secondary data sources for comparisons.
- A Student Guide.
Resource Pack 1 - Chemical Analysis
This pack contains a series of experiments involving chemical analysis
Calcium in water, calcium in milk, Iron in tea/breakfast cereal, Chloride in seawater.
Resource Pack 2 - Electrochemistry
This will follow shortly