Reports: DNI554905-DNI5: Controlling Polymerization Reactions at the Nanoscale
Ke Zhang, PhD, Northeastern University
Noble metal nanoparticles (NPs) exhibit localized photothermal effect near the surface by resonant absorption of light. Beyond the applications in hyperthermal therapy, drug delivery, and biological imaging, a concept that has only marginally been explored is the ability to control chemical reactions at the nanoscale by light.
Our previous study focused on how Atom Transfer Radical Polymerization (ATRP) reactions can be controlled by light on isotropic NPs (spherical AuNPs, phase 1 and 2, Scheme 1A). The results of our initial phases of experiments indicate that controlled polymerization can be obtained with the assistant of plasmonic NPs via photothermal activation. A key lesson learned from these studies is that the monomer must be carefully selected – those with high activation energy toward ATRP polymerization, such as acrylate derivatives, are not suitable for this system due to moderate surface temperature of the AuNPs under typical photochemistry conditions. While more intensive light sources including lasers can be used to generate more heat, the excessive irradiation causes ligand desorption from the AuNP surface and particle aggregation. Therefore, to achieve controlled light-mediated ATRP on the AuNP surfaces, several factors including solvent, AuNP size, surface ATRP ligand, monomer, and light intensity must be carefully optimized.
In the third phase of the study, we focus on anisotropic nanoparticle substrates. Light-driven, nanoscale synthesis of complex macromolecules near the NP surface holds strong promise in generating anisotropic structures with spatially-selective surface chemistries, which are very difficult to access previously. In recent years, a growing interest has been placed upon anisotropic NPs as building blocks for studying higher order organization. The light-mediated photothermal living polymerization offers a unique opportunity to prepare NPs with regiospecific polymer functionalization. This is because anisotropic NPs do not exhibit homogeneous near-field intensity enhancements. Particle geometry plays a critical role in controlling the localized fields. Highly curved regions, such as the termini of rods and the tips of prisms, often exhibit the greatest level of field localization (Scheme 1B). Based on these findings, we hypothesize the anisotropic fashion in which light interacts with the NP can be used to control regiospecific modification of anisotropic NPs, which would allow polymer growth at specific regions (Phase 3, Scheme 1C). In this phase, we conducted the photothermal ATRP on anisotropic gold nanorods (AuNRs), which exhibit much stronger photothermal effect compared with spherical AuNPs under near-infrared light, and may possess tip-selectivity on polymer functionalization due to regioselective heating.
Synthesis of AuNRs. To investigate light-mediated ATRP on AuNRs, we first synthesized the AuNRs via seed-mediated growth method based on a modified El-Sayed method. Briefly, AuNP seeds were first synthesized by reducing HAuCl4 with NaBH4 in ice-cold water in the presence of hexadecyltrimethylammonium bromide (CTAB). Then, the AuNRs growth solution was prepared by adding ascorbic acid to a high concentration CTAB solution with AgNO3 and HAuCl4. Upon the addition of AuNP seeds into the growth solution at 27-30 °C, AuNRs was obtained within 20 mins. The as prepared AuNRs were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and Ultraviolet–visible spectroscopy. As can be seen in Figure 1A, the DLS images show two distribution profiles of AuNRs, which is attributed to the anisotropy of the particles. The two distributions exhibit a number-average size of 12±1 nm and 43±1 nm, which are consistent with the width and length of the AuNRs as corroborated by TEM images (Figure 1B). The UV-vis spectroscopy shows that the AuNRs have strong absorption at 532 nm and 640 nm, again due to the high aspect ratio of the particles (Figure 1C).
Phase transfer of AuNRs. Next, the AuNRs were functionalized with ATRP initiators and transferred to organic solvent, following a similar protocol as that for spherical AuNPs which we have developed, using the phase transfer catalyst tetraoctylammonium bromide (TOAB). Briefly, the CTAB-stabilized AuNRs were first centrifuged and resuspensed 3x (14000 rpm, 20 min) to remove the excess CTAB in the aqueous solution. Then, a chloroform solution of the the ATRP initiator, bis[2-(2- bromoisobutyryloxy)undecyl] disulfide was used to extract the AuNRs from the aqueous phase. Excess ATRP initiators was removed by 3x centrifugation-resuspension cycles. Thereafter, chloroform was evaporated, and the dried particles can be suspended in other organic solvents, such as dichloromethane, dimethylformamide, and dimethyl sulfoxide. The modification process does not affect the uniformity and morphology of the AuNRs as evidenced by the TEM images (Figure 2). The ATRP initiator-modified AuNRs are stable in organic solvents for several months.
ATRP on the surface of AuNRs. We anticipate that the stronger photothermal effect associated with anisotropic AuNRs can activate ATRP reactions more easily under the near-infrared light irradiation (640 nm). An facilely polymerizable monomer, n-isopropylacrylamide, was selected as the model monomer for initial testing. By using reduced bulk temperatures (0 °C), we reduced the polymerization reaction rate to an inappreciable rate. A high-power NIR light source was used for the reaction. However, it is difficult to adjust the intensity of the near-infrared light, with the light being either too intense, which jeopardizes the particle stability, or too weak, which fails to initiate the polymerization. Therefore, we switched to the previous 532 nm wavelength laser to test the AuNRs, as the AuNRs also exhibit strong absorption near the 532 nm wavelength. Similar results to spherical AuNP-based ATRP was observed, with polymers with MW from 700-3000 Da found on the particle surface by MALDI-TOF analysis. However, TEM failed to visualize any regioselectivity of the polymerization. It is possible that the characterization technique is not appropriate for this system, as the polymer and gold exhibit very different contrast, making it difficult to observe the polymer in the presence of the AuNR. Alternatively, the ATRP reaction under these conditions may not produce the regioselective polymer growth due to particle internal heat transfer. Therefore, additional work is required to optimize the choice of the light source, reaction solvent, and ligand, as well as to characterize the polymer-coated AuNPs more effectively, using for e.g. AFM and SEM.