Assembly of gold nanoparticles on like-charge graphene oxide for fast release of hydrophobic molecules

Abstract

A strategy of employing gold nanoparticles (AuNPs) is proposed to achieve quick release of paclitaxel (PTX) from GO. Under NIR irradiation, AuNPs absorb and convert NIR lamp energy to local heat, destroying the interactions between GO and PTX, thus enabling the release of PTX from GO.

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As a member of graphene,¹ graphene oxide (GO)² has important amphiphilic properties. It can load hydrophobic molecules mainly by π–π stacking^3,4 and at the same time disperse well in water. However, the interactions between GO and such molecules are so stable that only acid has been reported to decrease the interactions to stimulate the release of drugs.⁵ Their release, especially quick release at neutral pH, is difficult to realize, which is more important for both pharmaceutics and functional materials, including drug release systems and anticorrosion coatings. Quick release can induce high release rates in the initial stage upon activation and shorten the course of treatment.⁶ In addition, although there is pH difference between normal and cancer cells, the pH range of cancer cells is about 5.6–7.6. Drugs loading on GO hardly release in this pH range. pH = 2 is far more acidic than the lowest pH (5.6 for squamous cell) of the cancer cells.⁷

In this paper, we successfully realize fast release of a hydrophobic drug, paclitaxel (PTX), from GO nanosheets at neutral pH. The key point lies in the usage of gold nanoparticles (AuNPs), which can absorb near infrared (NIR) light and convert to local heat.⁸ The heat decreases the interactions between GO and PTX and enables the fast release of PTX from GO. Since NIR light features the non-invasion and minimal absorption by the human tissues,⁹ the release stimuli is externally controllable. The AuNPs carrying negatively charges are assembled on the like-charge GO with the help of layers of positively charged polyelectrolytes by the layer-by-layer (LbL) technique.¹⁰

The GO nanosheets loading PTX (in abbr., GO + PTX, Fig. 1A) were obtained by referring to literature methods (ESI†).¹¹ The (GO + PTX) nanosheets were 1.3 nm thick (Fig. 1B). Compared to the characteristic FTIR peaks of pure PTX,¹² the ν_C=O (1735 cm⁻¹) of ester groups red shifted to 1729 cm⁻¹; the ν_C=O (1644 cm⁻¹) of acylamine groups red shifted to 1637 cm⁻¹; the ν_O–H (3476 cm⁻¹) red shifted to 3450 cm⁻¹; the ν_C–H (2946 cm⁻¹) of PTX red shifted to 2934 cm⁻¹(Fig. 1C). Compared to the characteristic Raman peaks of GO and PTX,¹³ the Raman peak (615 cm⁻¹, aromatic ring) of PTX appeared in GO + PTX, the D band (1327 cm⁻¹) and G band (1581 cm⁻¹) of GO blue shifted to 1331 cm⁻¹ and 1585 cm⁻¹. The other two Raman peaks (1000 cm⁻¹ and 1698 cm⁻¹) of PTX might be covered by the strong peaks of GO. These changes of peaks indicate on one hand the loading of PTX on GO, and on the other hand the existence of the π–π stacking interaction between GO and PTX, due to the conjugated π electrons of the sp² carbons and those in aromatic rings of PTX, as well as the hydrogen bonds¹⁴ between GO and PTX due to the polar groups such as the OH.

Fig. 1 (A) Schematic illustration of the loading of PTX on a GO nanosheet. (B) AFM height image and section analysis of GO + PTX nanosheets. (C) FTIR spectra of GO, PTX and GO + PTX. (D) Raman spectra of GO, PTX and GO + PTX. (E) The UV spectrum of PTX in dichloromethane (DCM). (F) The working curve of absorbance at 232 nm against concentration. (G) Release profile of PTX from GO + PTX at pH = 7.4.

These interactions are good for the loading of PTX on GO. The loading capacity of PTX was calculated to be 0.4 mg mg⁻¹, based on a working curve (Fig. 1F). However, PTX released very slowly from GO (Fig. 1G) and only about 6% of the total bound PTX released within 11 h at neutral pH, as observed by others for hydrophobic drugs on GO.^3,5,15

To achieve release, low pH = 2 had been used to destroy the hydrogen bonds and decrease the π–π stacking interaction.⁵ However, low pH is destructive not only for most biological organisms but also metallic materials, which limits the application of functional molecules loaded on GO. Other stimuli to trigger the release of hydrophobic molecules should be found.

AuNPs capable of absorbing NIR light and convert to local heat were thus selected. Both the AuNPs and GO + PTX carry negatively charges, which has been thought to be a problem to assemble AuNPs on the like-charge GO. Herein, we find a solution to this problem by making good use of the LbL technique. That is, a positively charged polyelectrolyte, poly (allylamine hydrochloride) (in abbr., PAH) was employed. First, we deposited on GO + PTX nanosheets a layer of PAH. Second, we deposited AuNPs. Then another PAH layer was deposited to stabilize and prevent the AuNPs against peeling off, as illustrated in Fig. 2A. The final complexes existed in the structure of nanocapsules, which was represented by (GO + PTX)/(PAH/AuNPs)/PAH.

Fig. 2 (A) Schematic illustration of the LbL assembly of PAH and AuNPs on GO + PTX nanosheets. (B) UV-Vis-NIR spectrum of AuNPs. Inset: TEM micrograph of the (GO + PTX)/(PAH/AuNPs)/PAH nanocapsule. (C) AFM height image and section analysis of the (GO + PTX)/(PAH/AuNPs)/PAH nanocapsule.

AuNPs had a broad absorption band at 700–1000 nm in the NIR region (Fig. 2B), favorable for the assembly of NIR stimuli-responsive nanocapsules. AuNPs had the lateral dimension of 20–60 nm, dispersed homogeneously in (GO + PTX)/(PAH/AuNPs)/PAH nanocapsule as observed by TEM (inset in Fig. 2B). The (GO + PTX)/(PAH/AuNPs)/PAH nanocapsules were in the shape of nanosheets, ca. 0.5–1.5 μm wide and 6.1 nm thick (Fig. 2C). AFM images can hardly distinguish AuNPs or PTX assembled inside the shell. As a result, the contrast in the AFM image (Fig. 2C) is poor. It is worthy to note that this AFM image is so far the best for GO-loading drugs. To estimate the thickness of the shell, we select a representative section across as shown in Fig. 2C. The nanosheets are smaller than what reported by others,¹⁶ which meets the size requirement as high performance carriers of drugs.⁴ Although the TEM can locate the AuNPs inside the nanocapsules, it is difficult to find the edge of the nanocapsule from the TEM image (inset in Fig. 2B). The (GO + PTX)/(PAH/AuNPs)/PAH nanocapsule is not so big but so thin that the contrast between the holy carbon film as background and all the components except AuNPs in the nanocapsule is too small. The best technique to get surface morphology for nanocapsules is AFM. So the structure information of the nanocapsule should be obtained by combining the AFM and the TEM images.

We used a low energy lamp rather than high intensity laser to produce NIR light. The irradiation intensity was about 40 mW cm⁻². The effect of AuNPs on release of PTX was obvious. The controlled experiment showed that, without AuNPs, PTX did not release from nanocapsules to the phosphate buffered saline (PBS) solution at 270 s under NIR light irradiation at neutral pH (Fig. 3). However, PTX released and reached quickly the equilibrium from capsules with AuNPs. Approximately 75% of PTX released from the (GO + PTX)/(PAH/AuNPs)/PAH nanocapsules, and reached the release equilibrium at 150 s.

Fig. 3 Release profiles of PTX from nanocapsules containing PTX with/without AuNPs at pH = 7.4 under NIR light irradiation.

The release temperature of PTX from GO was studied in the temperature range of 35–40 °C, corresponding to the time range of 300 s in the temperature–time plot (Fig. S1, ESI†). Because Köhler and co-workers indicate that, the glass transition temperature of polyelectrolyte (GTT) is around 35–40 °C.¹⁷ Above the GTT, the polyelectrolyte molecules will seriously change the conformation, resulting in the difference of permeability. As a result, the release of PTX from the GO in the nanocapsules is carried out within 300 s.

AuNPs in the nanocapsules play a crucial role for the quick release of PTX from GO: firstly unloading of PTX from GO, secondly releasing through the polyelectrolyte layers. The quick release is schematically illustrated in Fig. 4 and described as follows. Under NIR irradiation, AuNPs absorb and convert NIR irradiation into local heat,⁹ destroying the π–π stacking interaction and the hydrogen bonds.¹⁸ So the interactions between GO and PTX are decreased and PTX is released from GO. The local heat from AuNPs also leads to the conformational change of polyelectrolytes around AuNPs from extended to be more coiled. The molecular packing of polyelectrolytes changes from crystalline state to fluid-like state.¹⁹ So the permeability of the PAH layers increases, enabling the release of PTX through pores in the polyelectrolyte layers. Compared to internal acid stimuli of drug release from GO, the NIR stimuli is both mild and externally controllable. So the assembly of AuNPs in this work to trigger quick release of PTX mainly by decreasing the interactions between GO and PTX under NIR irradiation at neutral pH is a promising strategy.

Fig. 4 Schematic illustration of quick release of PTX at neutral pH from nanocapsules containing PTX and AuNPs under NIR irradiation.

Conclusions

In conclusion, the strategy of using AuNPs to stimulate fast release of PTX from GO by NIR irradiation proves successful. The negatively charged AuNPs can be assembled on the like-charge GO by the LbL technique. The quick release of hydrophobic drugs on GO at neutral pH under remote stimuli rather than slow release under local corrosive acidic condition is promising, providing helpful guidance to decrease interactions between GO and functional molecules. Worldwide researchers on GO-based pharmaceutics and functional materials will benefit from this strategy to shorten the course of treatment.

Acknowledgements

We thank the National Nature Science Foundation of China (no. 20533050, 21273135), Shandong Provincial Natural Science Foundation, China (ZR2010BM039), Independent Innovation Foundation of Shandong University (2011JC026), and the Max Planck Society, Germany. We also thank Prof. Xiyou Li for providing the access to AFM.

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Footnote

† Electronic supplementary information (ESI) available: Full details of materials, experimental methods and characterization techniques. See DOI: 10.1039/c3ra46509h

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