Effect of CaP nanoparticles on storage moduli of colloidal composite gels

The effect of CaP and C-CaP nanoparticles on viscoelastic properties of Gel-CaP colloidal composite gels (gelatin solid content 10 w/v%) is shown in Fig. 5. By adding citrate-free CaP nanocrystals (C-CaP), storage moduli G’ (Fig. 5A) strongly increased from 0.5-0.8 kPa for CaP-free colloidal gelatin gels to 4-9 kPa for gels at a low CaP/Gel ratio of 0.1 up to 8-48 kPa for gels at a CaP/Gel ratio of 1. This reinforcing effect upon addition of a nanostructured CaP phase was observed to a much less extent for colloidal gels comprising negatively charged C-CaP nanoparticles (especially at a high CaP/Gel ratio of 1.0). Finally, it was observed that Gel-CaP composite gels containing GelB nanospheres displayed higher storage moduli than gels containing GelA nanospheres.

Figure 5. Storage moduli (G’) of colloidal dispersions composed of GelA or GelB nanospheres mixed with CaP or C-CaP nanoparticles as a function of (A) CaP/Gel weight ratio (gelatin solid content 10 w/v%, dispersions in deionized water) or (B) ionic strength (gelatin content of 10 w/v%, CaP/Gel ratio of 0.1) (n=3).

Gel elasticity of colloidal gels made of citrate-free CaP nanoparticles and gelatin nanospheres was hardly dependent on the ionic strength (Fig. 5B), whereas colloidal gels made of C-CaP and gelatin nanoparticles revealed a more pronounced dependency of gel elasticity on ionic strength as reflected by a continuous decrease of G’ from 4.5 to 2.5 kPa or from 6.0 to 3.5 kPa for gels containing GelA or GelB nanospheres, respectively, with NaCl concentrations increasing from 10 to 1000 mM.

without addition of CaP and C-CaP nanoparticles as a function of time (Fig. 2A and C). For GelA or GelB nanosphere suspensions, the particle size did not change in time, indicating that repulsive interparticle forces between similarly charged nanospheres stabilized the suspension by preventing particle aggregation (Fig. 2B and D (left vials)). Upon addition of CaP nanoparticles, on the contrary, particle sizes increased considerably irrespective of gelatin nanoparticle charge, resulting into heteroaggregation of organic and inorganic nanoparticles (Fig. 2B and D (middle vials)). The addition of C-CaP nanoparticles to oppositely charged GelA nanoparticles resulted into a similar increase in particle size and corresponding sedimentation of large aggregates (Fig. 2B (right vial)), whereas a constant particle size without any sedimentation was observed upon mixing of C-CaP with similarly charged GelB nanospheres.

Corresponding TEM characterizations confirmed the observations made using DLS by revealing severe clustering of spherical GelA nanoparticles with oppositely charged needle-like C-CaP (Fig. 3C) as well as CaP nanoparticles with GelA or GelB nanospheres (Fig. 3B and E). Such clusters were absent in suspensions containing solely GelA or GelB nanospheres (Fig. 3A and D) and hardly present in suspensions containing spherical GelB nanospheres and C-CaP nanoparticles (Fig. 3F).

Since the charge difference between organic and inorganic nanoparticles and resulting particle aggregation was highest for C-CaP and GelA nanoparticles (see Table 1 and Fig. 2B), this combination of nanoparticles was selected for further investigations on the nature of the interparticle forces. The ionic strength did not affect the formation of heteroaggregates between C-CaP and GelA nanoparticles at NaCl concentrations of up to 100 mM, as reflected by the constant increase in particle size (Fig. 4A) and rapid sedimentation of the aggregates (Fig. 4B). Only the highest ionic strength of 1M NaCl was effective to inhibit sedimentation of C-CaP and GelA nanoparticles. Fig. 4D shows the self-assembly process of C-CaP and GelA nanoparticles as a function of CaP/Gel ratio. The particle size increased considerably in suspensions containing C-CaP and GelA nanoparticles with moderate CaP/Gel ratios of 0.5 and 1 (w/w) resulting into rapid sedimentation of aggregates, whereas at either low or high CaP/Gel ratios (i.e. 0.1 or 5), self-assembly between organic and inorganic nanoparticles was hampered.

3.3. Gel formation

3.3.1. Effect of CaP nanoparticles on storage moduli of colloidal composite gels

The effect of CaP and C-CaP nanoparticles on viscoelastic properties of Gel-CaP colloidal composite gels (gelatin solid content 10 w/v%) is shown in Fig. 5. By adding citrate-free CaP nanocrystals (C-CaP), storage moduli G’ (Fig. 5A) strongly increased from 0.5-0.8 kPa for CaP-free colloidal gelatin gels to 4-9 kPa for gels at a low CaP/Gel ratio of 0.1 up to 8-48 kPa for gels at a CaP/Gel ratio of 1. This reinforcing effect upon addition of a nanostructured CaP phase was observed to a much less extent for colloidal gels comprising negatively charged C-CaP nanoparticles (especially at a high CaP/Gel ratio of 1.0). Finally, it was observed that Gel-CaP composite gels containing GelB nanospheres displayed higher storage moduli than gels containing GelA nanospheres.

Figure 5. Storage moduli (G’) of colloidal dispersions composed of GelA or GelB nanospheres mixed with CaP or C-CaP nanoparticles as a function of (A) CaP/Gel weight ratio (gelatin solid content 10 w/v%, dispersions in deionized water) or (B) ionic strength (gelatin content of 10 w/v%, CaP/Gel ratio of 0.1) (n=3).

Gel elasticity of colloidal gels made of citrate-free CaP nanoparticles and gelatin nanospheres was hardly dependent on the ionic strength (Fig. 5B), whereas colloidal gels made of C-CaP and gelatin nanoparticles revealed a more pronounced dependency of gel elasticity on ionic strength as reflected by a continuous decrease of G’ from 4.5 to 2.5 kPa or from 6.0 to 3.5 kPa for gels containing GelA or GelB nanospheres, respectively, with NaCl concentrations increasing from 10 to 1000 mM.

Figure 6. Effect of CaP/Gel ratios on storage moduli (A) and corresponding tan(delta) values (B) of the colloidal composite gels containing gelatin and CaP nanoparticles dispersed in deionized water at a total solid content (Gel+CaP) of 20 w/v% (n=3).

3.3.2. Effect of CaP/Gel ratio on viscoelastic properties of colloidal composite