Synthesized PHAuNPs feature a sub-25-nm shell with a 50-nm hollow core. The shell is of porous nature with the pore size about 2–3 nm, as measured in the high-resolution transmission electron microscopy (HRTEM) image shown in Figure . These nanoscale pores in the shell allow ions (Fe2+ and Fe3+) to diffuse into the hollow space in the core, where precipitation of Fe3O4 takes place upon the addition of OH-. The sizes of precipitated Fe3O4 nanoparticles (5–20 nm) are larger than the pore size, resulting in the trapping of the iron oxide nanoparticles inside the PHAuNPs (Figure ).
a and b HRTEM micrographs of PHAuNPs, showing the hollow core and the porous shell with pore size about 2–3 nm.
Figure shows TEM analysis before and after loading of iron oxide nanoparticles. After loading, the hollow core of PHAuNPs is occupied by solid substances. During the precipitation, Fe3
nanoparticles also formed outside of PHAuNPs, but TEM micrographs clearly show that no small iron oxide nanoparticles were attached to the PHAuNP surface. This is in agreement with the common notion that iron oxide usually does not stick to the Au surface [11
]. Given the very different sizes of PHAuNPs (~100 nm) and non-trapped Fe3
nanoparticles (<20 nm), they can be readily separated using centrifugation.
TEM micrographs of PHAuNPs before a and after b loading iron oxide nanoparticles.
The loading of Fe3O4 to the core of PHAuNPs was confirmed by energy-dispersive X-ray (EDS) analysis of one single particle and the selected area electron diffraction (SAED) pattern from three particles. EDS shows the coexistence of Au and Fe in a single particle (Figure , Cu peak is from the TEM grid). The low intensity of Fe may be due to the shield effect of the thick Au shell. The SAED pattern is a superposition of Au and Fe3O4 lattices (Figure ), showing three distinguishable planes of (311), (511), and (731) from Fe3O4. Other Fe3O4 planes overlap with Au planes.
a EDS spectrum of one single particle, showing the coexistence of Au and Fe. b SAED pattern from three particles, showing a superposition of Au and Fe3O4 lattices.
Shown in Figure is the appearance of a bottle of particle water suspension. The cyan color indicates that the suspension absorbs red light. The absorption spectrum is shown in Figure , which has a broad peak centering at 750 nm. This absorption peak corresponds to the SPR wavelength. Compared to PHAuNPs before loading iron oxide, the absorption spectrum shows little change. For core/shell nanoparticles, it is well known that the SPR wavelength is dependent on the refractive indices of medium, shell and core. Changing core material usually causes a shift of the SPR wavelength. However, PHAuNPs have a relatively thick shell (>20 nm). Through a three-dimensional finite difference time domain (FDTD) simulation (using a commercial software from Lumerical Inc), we have proved that at this thickness, the red-shifts of SPR peaks are mainly caused by their surface roughness, and the hollow nature of these particles plays only a minor role [17
]. The simulation results show that SPR peaks for hollow particles are only slightly red-shifted compared to solid particles with the same outer diameter (100 nm). For particles with a roughness of 5 nm, SPR peak shifts to longer wavelength (~630 nm). As the roughness increases to 8 nm which is the average grain size in the shell, a much greater red-shift (to 720 nm) is observed. This roughness effect is due to the strong interaction of electric fields from adjacent bumps on the surface, similar to the plasmonic properties of the aggregates of several nanoparticles. The simulated results are in good agreement with experimental results. This unique SPR tuning mechanism makes it possible to maintain the optical properties of PHAuNPs even after the loading of iron oxide.
Figure 4 The plasmonic and magnetic properties of the Fe3O4-loaded PHAuNPs. a Appearance of a bottle of particle water suspension. The particles can be dragged toward a permanent magnet. b Absorption spectrum of the particle water suspension, showing a broad peak (more ...)
As shown in Figure , the particles can be dragged toward a permanent magnet, unequivocally indicating the magnetic characteristics of the Au nanoparticles. Hysteresis loop of dried particle powder is shown in Figure . Since the Fe3O4 nanoparticles synthesized using the above-mentioned method are normally smaller than 20 nm, we expect to see a typical superparamagnetic behavior: zero remanence, zero coercivity, and a large saturation field. The small hysteresis shown in the measurement may reflect the presence of some large Fe3O4 nanoparticles (>30 nm) inside PHAuNPs. Given the size of the hollow space (>50 nm) and the thickness of the porous shell (25 nm), the inward diffusion of OH- ions may be partially obstructed, resulting in a much slower nucleation rate. As such, the inside particles could grow large. The measured high saturation field is in consistence with the superparamagnetic characteristic. This suggests a mixture of superparamagnetic and ferromagnetic nanoparticles. Ferromagnetic nanoparticles are usually undesirable for bioapplications because of their agglomeration caused by magnetic attraction. However, for iron oxide nanoparticles-loaded PHAuNPs, the thick Au shell can effectively separate them far apart to avoid such magnetic aggregation.