One-step shell polymerization of inorganic nanoparticles and their applications in SERS/nonlinear optical imaging, drug delivery, and catalysis

Abstract

Surface functionalized nanoparticles have found their applications in several fields including biophotonics, nanobiomedicine, biosensing, drug delivery, and catalysis. Quite often, the nanoparticle surfaces must be post-coated with organic or inorganic layers during the synthesis before use. This work reports a generally one-pot synthesis method for the preparation of various inorganic-organic core-shell nanostructures (Au@polymer, Ag@polymer, Cu@polymer, Fe3O4@polymer, and TiO2@polymer), which led to new optical, magnetic, and catalytic applications. This green synthesis involved reacting inorganic precursors and poly(styrene-alt-maleic acid). The polystyrene blocks separated from the external aqueous environment acting as a hydrophobic depot for aromatic drugs and thus illustrated the integration of functional nanoobjects for drug delivery. Among these nanocomposites, the Au@polymer nanoparticles with good biocompatibility exhibited shell-dependent signal enhancement in the surface plasmon resonance shift, nonlinear fluorescence, and surface-enhanced Raman scattering properties. These unique optical properties were used for dual-modality imaging on the delivery of the aromatic photosensitizer for photodynamic therapy to HeLa cells.

Figure 1

Schemes illustrated the polymer-assisted reduction of the HAuCl₄ precursor to prepare Au@polymer NPs a), which exhibited polymer shell-dependent SERS and nonlinear optical enhancements for the observation of microscopic drug delivery in system b). c) The synthesis strategy using the different metal salt precursors to prepare Ag@polymer, Cu@polymer, TiO₂@polymer, and Fe₃O₄@polymer NPs.

Figure 2

TEM images of the Au NPs prepared by the reaction of 0.25-mL a), 0.5-mL b), and 1-mL c) HAuCl₄ (5 mM) with 10 mL of PSMA polymer (0.78 mg/mL) at 200°C for 1 h. d) UV-visible spectra for these corresponding Au NPs.

Figure 3

Time-dependent examinations of the a-c) TEM images from 1 h to 6 h to 13 h and d) UV-visible absorption spectra for the as-prepared Au@polymer NPs (~22 nm in core size). e) Au core size/polymer shell thickness plots f) Synchrotron X-ray powder diffraction pattern of the 13 h sample of the Au@polymer NPs. g) Raman spectra of the 1, 6, and 13 h samples of the Au@polymer NPs, where the * symbol shows the styrene portion of PSMA and the D/G bands refer to the sp³/sp² hybridized carbon atoms of the graphite structure.

Figure 4. a) Nonlinear optical spectra of the as-prepared Au@polymer NPs at different reaction times and b) Raman shift detection of MB-tagged Au@polymer NPs (prepared over 1–13 h) at an excitation of 632.8 nm.

Figure 5. UV-visible and fluorescence spectra of different aromatic drugs encapsulated in the Au@polymer NPs (6 h-sample).

Figure 6

a) MTT assay of HeLa cells with free MB molecules and MB-loaded Au@polymer NPs at 24 h. b) Bright-field image of the HeLa cells with MB-loaded Au@polymer NPs and two SERS mapping images (based on the 1623 cm⁻¹ vibration of MB) collected for the rectangle shown in the bright field of a HeLa cell. N indicates the nucleus. c) Fluorescence images of DCFH-DA staining in HeLa cells incubated with MB-loaded Au@polymer NPs and free MB for 4 h. The sample-treated cells were exposed to LED light at 660 nm (30 mW/cm²) for 4 min before imaging detection. d) Fluorescence intensity of MB-loaded Au@polymer NPs and free MB in HeLa cells.

Figure 7

TEM images of a) Ag@polymer (with 12 μL HCl and 100 μL N₂H₄), b) Cu@polymer (with 18 μL HCl and 100 μL N₂H₄), c) Fe₃O₄@polymer (with 100 μL N₂H₄), and d) TiO₂@polymer (with 18 μL HCl and 100 μL N₂H₄) NPs. e) XRD measurements for these inorganic-polymer nanocomposites going from a) the bottom to d) the top. The peaks for a), b), c), and d) were assigned based on the standard patterns of Ag (JCPDS 04-0783), Cu (JCPDS 85-1326), Fe₃O₄ (JCPDS 19-629), and TiO₂ (JCPDS 89-4921), respectively. f) UV-visible spectra of Ag@polymer and Cu@polymer NPs.