Poly[2-(3-butenyl)-2-oxazoline] (2 different batches) with number-average molecular weight of Mn = 7 200-7 300 g mol-1 (MALDI-ToF MS) and dispersity Mw/Mn = 1.1-1.2 (GPC) was decorated with pendant cholesteryl and carboxylic side chains via simultaneous coaddition of thiocholesterol (Chol-SH) and 3-mercaptopropionic acid (3-MPA). The composition of the
123 purified LC ’gluing’ polymer, PBOx-Chol-MPA, was [C=C]/[Chol]/[COOH] = a/b/c = 0.53-0.49:0.18-0.22:0.31-0.35 (determined by 1H NMR spectroscopy; see Scheme 6.2).
Scheme 6.2: LC ‘gluing‘ statistical copolymer obtained by simultaneous modification of oxazoline] with thiocholesterol and 3-mercaptopropionic acid (PBOx-Chol-MPA). The poly[2-(3-butenyl)-2-oxazoline] exhibited an average number of repeat units of n ~ 58. The composition of the modified polymers was found to be [C=C]/[Chol]/[COOH] = a/b/c = 0.53-0.49:0.18-0.22:0.31-0.35.
The Chol units allow the polymer to form lyotropic phases with structural long-range orientation on the length scale of several hundreds of micrometers in organic solvents like DMF upon shearing (cf. Chapter 3 and Figure S18, in which the polymeric LC pattern was obtained upon shearing a 30+ wt% solution of PBOx-Chol-MPA in DMF by spin-coating).
The MPA units are negatively charged at a pH of ~8-9, and thus allowing the polymer to bind to positively charged nanoparticle faces (cf. Chapter 4.4). By converting the carboxyl units into the respective sodium salt, the polymer can be dispersed in water, forming worm-like aggregates with a dimension of ca. 15 × 3 nm2 and spherical aggregates with dimensions between ca. 15 and 30 nm (cf. Chapter 3.4.2). In the following, the LC ‘gluing’ polymer was used to bind to positively charged CTAB-coated gold nanorods (GNRs), which was investigated by using complementary thermogravimetric analysis (TGA), analytical ultracentrifugation (AUC), UV-visible spectroscopy and zeta potential measurements.
The synthesis of GNRs according to Murray et al.291 allows fabricating gold nanorods on the scale of few tens of milligrams. These GNRs are positively charged due to their CTAB bilayer. A 0.1 wt% aqueous dispersion of LC ‘gluing’ copolymer was added to an aqueous dispersion of CTAB-coated GNRs in a ratio of GNR to polymer of 1:3, 1:1, 2:1 and 7:1 w/w, and the final dispersions were adjusted to a pH of ~8-9 by adding 0.1 M NaOH. After allowing the polymer to bind to the nanoparticles by shaking vigorously overnight, the mixtures were centrifuged at a speed of 5000-6000 rpm at which the dispersed, non-bound
124 polymer aggregates remained in dispersion, followed by freeze-drying of the isolated precipitates.
TGA of the precipitates (Figure 6.4) reveals that polymer binds to the CTAB-coated gold particles, probably mainly via Coulomb interaction. However, binding of the polymer was not quantitative. When using a ratio of GNR:LC polymer of 1:1 w/w, the organic-inorganic hybrid material consists of ca. 83% gold nanoparticles and 17% organic components, i.e. a ratio of gold particles to organics of 5:1 w/w. By taking the GNR reference sample into account, the organic part was determined more exactly to 8% CTAB and 9% LC polymer.
The relatively low amount of polymer attaching to the CTAB-coated gold particles might be attributed to the limited access of the sterically demanding polymer aggregates to the CTAB double layer. The amount of polymer binding to the gold particles when using higher amounts of gold nanorods and consequently ratios of GNR:LC polymer of 2:1 and 7:1 w/w was 5%
and 4%, respectively, and thus in both cases lower compared to using an initial feed ratio of GNR:polymer of 1:1 w/w. The loading of polymer was in these cases considerably lower than the CTAB coating of the gold nanorods and hence disadvantageous for the preparation of composite films consisting of gold nanorods with a polymeric LC phase in between. The amount of polymer binding to the nanoparticles was further increased by using an excess of polymer, e.g. a GNR:polymer ratio of 1:3 w/w, leading to a polymer loading of about 27%.
This indicates that the nanoparticle surface is not saturated with LC polymer in case of an addition ratio of GNR:polymer of 1:1 w/w.
Figure 6.4: Thermogravimetric analysis of GNR–LC polymer hybrid materials and of gold nanorod and polymer reference (measurements under O2 atmosphere).
125 An initial ratio of GNR:polymer of 1:1 w/w, finally leading to hybrid materials consisting of a ratio of GNR:organics of 5:1 w/w, fits well to the aim of this work, i.e. to prepare composite materials consisting of a high ratio of gold nanorods compared to the organic parts.
Additionally, the polymeric LC-driven structuration concept (cf. Chapter 4) can be applied when using this GNR:polymer ratio, as a significant fraction of the organic layer consists of the LC polymer (LC polymer:CTAB ~ 1:1 w/w). Consequently, the binding behavior between LC polymer and GNRs with an aspect ratio of 2.8 (55.9 ± 4.7 nm in length and 19.9 ± 2.9 nm in width; amount of by-product with shape impurities ca. 6.9%) by using an initial ratio of GNR:polymer of 1:1 w/w was further investigated via UV-visible spectroscopy and AUC sedimentation velocity experiments using absorption optics. AUC analysis provides information about the sedimentation coefficients s (in units of S (Svedberg) = 10-13 seconds), which are proportional to the size and density of the sedimenting species. The s values are converted to s20,w (standard conditions) to eliminate the difference in density and viscosity of the used solvents (water and DMF), allowing a better comparability between the s values obtained in the different solvents (Figure 6.5).300
Figure 6.5: AUC sedimentation coefficient distribution ls-g*(s) of CTAB-coated gold nanorods (black) and GNRLC polymer hybrid particles with initial ratio of 1:1 w/w (red) in aqueous dispersion (a) and after phase-transfer to DMF (c), obtained from sedimentation velocity experiments. Corresponding UV-visible spectra of CTAB-coated gold nanorods (black) and hybrid particles (red) in aqueous dispersion (b) and after phase-transfer to DMF (d).
126 Compared to the peak of the CTAB-coated gold nanorods without polymer, the peak maximum of the GNRLC polymer hybrid particles, both dispersed in aqueous medium, possess a similar s value, indicating that the hybrid particles exist mainly as single particles (Figure 6.5a). Broadening of the peak in case of the hybrid dispersion toward higher s values indicates the presence of gold particle agglomerates, mainly dimers, probably formed upon interaction between gold nanorod and LC polymer. The UV-visible spectrum of CTAB-coated gold nanorods in water (Figure 6.5b) reveals an LSPR of 690 nm, which agrees well the estimated LSPR of 690 nm of gold nanorods with an aspect ratio of 2.8 dispersed in aqueous medium based on the discrete dipole approximation method.301 UV-visible spectroscopy reveals no significant shift of the LSPR of the GNRLC polymer hybrid particle dispersion compared to the CTAB-coated GNR reference dispersion, confirming the result obtained from AUC that the GNRpolymer hybrid particles mainly exists as single particles (Figure 6.5b). The broadening of the LSPR towards higher wavelength in case of the GNRpolymer hybrid dispersion indicates the presence of gold nanorod agglomerates, which are preferentially linked in an end-to-end fashion.288 The tendency for phase-selective polymer adsorption at the ends of the nanorods is probably due to the fact that the sterically demanding polymer agglomerates tend to attach to the CTAB molecules at the ends of the nanorods, where the density of the CTAB layer is supposed to be lower than at the edges.287 Additionally, binding of polymer on the gold surface in aqueous medium was investigated by means of zeta potential measurements. As-prepared, purified CTAB-coated GNRs exhibit a positive zeta potential of ca. +37 mV due to the presence of the bilayer of CTAB molecules on the particle surface. After polymer coating and removal of non-bound polymer by centrifugation and re-dispersion of the GNRpolymer hybrid particles in water, the zeta potential changed to a negative value of ca. -13 mV, suggesting that the positively charged CTAB bilayer is shielded by negatively charged LC polymer.302,303 The change of zeta potential suggests that interactions between the CTAB bilayer and polymer are mainly due to Coulomb interactions, although the relative contribution of interactions that may take place in this complex system is difficult to determine exactly. Phase transfer of the hybrid particles from water to DMF, in which the cholesteryl moieties of the LC polymer form lyotropic phases, was performed by centrifugation of the aqueous hybrid dispersion and redispersion of the hybrid particles in DMF. An AUC sedimentation velocity experiment of the hybrid dispersion in DMF (Figure 6.5c) reveals a slightly narrower sedimentation coefficient distribution compared to CTAB-coated gold nanorods in DMF (FWHM of ~2700 S, peak of hybrid particles, vs. ~4600 S, peak of GNR reference), indicating the presence of relatively
127 monodisperse hybrid particles with less agglomeration compared to the GNRs in DMF possessing only a CTAB layer. After phase-transfer of the hybrid particles to DMF, partial agglomeration was not anymore observed via AUC analysis and UV-visible spectroscopy (Figure 6.5c and d), probably owing to completely dissolving the polymer in DMF compared to bridging of gold particles by polymer agglomerates in aqueous medium. Additionally, the sedimentation coefficient peak of the hybrid particles dispersed in DMF is shifted toward lower s values compared to the peak of the CTAB-coated GNRs dispersed in DMF, probably due to the higher organic loading of the layer on the GNRpolymer particle surface that may slightly decelerate the sedimentation of the hybrid particles. In agreement to this observation the LSPR and TSPR of the hybrid particles in DMF are slightly red-shifted compared to the CTAB-coated gold nanorods in DMF (~13 nm, Figure 6.5d), probably attributable to the change of the local refractive index due to the polymer coating. The GNR reference samples dispersed in water (Figure 6.5a) and dispersed in DMF (Figure 6.5c) exhibit both a similar peak maximum at ~7900 S, indicating that the CTAB-coated nanoparticles without polymer exist well-dispersed in both solvents.
Coating of the CTAB-capped gold nanorods with LC polymer by adding it to the aqueous gold dispersion dissolved in DMF, analogously as described for the synthesis of Laponite/LC polymer hybrid particles in Chapter 3, led to precipitation of the gold particles during shaking the dispersion overnight. Therefore, this procedure was not applied to synthesize gold particle/polymer hybrids.
A coating exchange of CTAB with LC polymer by simultaneously removing CTAB and coating the gold particles with LC polymer, similar as described by Umadevi et al.,304 did not work. This approach involves a phase-transfer of GNRs from aqueous medium to chloroform upon ligand exchange. After the successful phase-transfer, the organic phase containing the GNRs coated with the respective ligand is separated from the water phase. In case of working with the LC polymer, a phase-transfer of gold nanorods was not observed as binding of the polymer to the gold particles, probably via the thioether functionalities of the polymer side chains, might be too weak compared to the affinity of CTAB to the gold nanorods, preventing a successful coating exchange.