Reversibly pH-responsive gold nanoparticles and their applications for photothermal cancer therapy

Abstract

Microenvironment responsive nanomaterials are attractive for therapeutic applications with regional specificity. Here we report pH responsive gold nanoparticles which are designed to aggregate in acidic condition similar to cancer environment and returned to its original disassembled states in a physiological pH. The pH responsive behavior of the particles is derived by change of electrostatic interaction among the particles where attraction and repulsion play a major role in low and high pH of the environment, respectively. Since different electrostatic interaction behavior of the particles in varied pH is induced not by irreversible chemical change but by simple protonation differences, the pH responsive process of assembly and disassembly is totally reversible. The low pH specific aggregation of gold nanoparticles resulted in red shift of plasmonic absorption peak and showed higher photothermal efficacy in acidic pH than in normal physiological pH. The low pH specific photothermal effect with long wave laser irradiation was directly applied to cancer specific photothermal therapy and resulted higher therapeutic effect for melanoma cancer cells than non-pH responsive gold nanoparticles.

Figure 1

Schematic diagram of surface modification process of AuNPs with ssDNA and cytochrome c modification process (upper) and pH responsive behavior of CytC/ssDNA-AuNPs (lower).

Figure 2

(A) Absorption peak wavelengths of CytC/ssDNA-AuNPs in various pHs. The numbers on the symbols represent the ratios of AuNPs vs. ssDNA vs. cytochrome c of the modification reaction. (B) Responsive pHs vs. reaction ratio of cytochrome c over AuNPs of each CytC/ssDNA-AuNP.

Figure 3

(A) Red shift of absorption peak accordingly with reducing pH (7.4, 6.5. 6.0 to 5.5). (B) Blue shift of absorption peak with returning to higher pH (5.5, 6.0, 6.5 to 7.4). (C) Sizes, and (D) Zetapotentials of CytC/ssDNA-AuNPs measured during the pH lowering and elevating process. (E) TEM (upper panel) and SEM (lower panel) images of CytC/ssDNA-AuNPs taken at a cycle of pH 7.4 → 6.5 → 5.5 → 7.4 (scale bar: 300 nm). Error bars refer to standard deviations from three replicates.

Figure 4

(A) Reversible absorption spectrum shift of CytC/ssDNA-AuNPs accordingly with pH variations. Blue lines: 1^st cycle of pH 7.4 to 5.5 to 7.4, Green lines: 2^nd cycle of pH 5.5 to 7.4, and Red lines: 3^rd cycle of pH 5.5 to 7.4. (B) Absorption peaks at each pH cycles extracted from (A). (C) Particle sizes and (D) surface potentials of the particles at each cycles of pH. Error bars refer to standard deviations from three replicates.

Figure 5

(A) Red and (B) blue shift of absorption peak accordingly with reducing and elevation pH in DMEM media solution. (C) Solution temperature changes with 660 nm laser irradiation times. Purple: PBS blank solution, yellow: ssDNA-AuNP in pH 7.4 DMEM media, green: ssDNA-AuNP in pH 5.5 DMEM media, blue: CytC/ssDNA-AuNP in pH 7.4 DMEM media, and red: CytC/ssDNA-AuNP in pH 5.5 DMEM media. (D) IR camera images of laser irradiated nanoparticle solutions. Error bars refer to standard deviations from three replicates.

Figure 6

(A) Dark field microscope images of B16F10 cells (left), co-incubated with ssDNA-AuNP (middle), and CytC/ssDNA-AuNP (right). Photothermal destruction of the cells co-incubated with (B) ssDNA-AuNP and (C) CytC/ssDNA-AuNP for 12 h followed by laser irradiation for 5 min at different power densities (scale bar: 100 µm). Viabilities of (D) MDCK-GFP cells and (E) B16F10 cells co-incubated with each particle with no laser or with laser irradiation.