Gold nanoclusters modified mesoporous silica coated gold nanorods: Enhanced photothermal properties and fluorescence imaging

Highlights

• A new photothermal system (AuNRs@SiO₂@AuNCs) was designed and synthesized.

• The enhanced photothermal conversion efficiency and stability are achieved.

• The system has better biocompatibility and photothermal therapeutic effect.

• The system has fluorescent properties that can be used for cell imaging.

Abstract

Photothermal therapy (PTT) is a non-invasive therapy that is widely used in cancer treatment. Gold nanorods (AuNRs) are particularly suitable as a photothermal reagent due to their unique localized surface plasmon resonance (LSPR) properties. However, bare gold nanorods are not stable enough during radiation to collect enough energy to kill tumor cells. In addition, they showed some biologically toxic originated from the poor colloidal stability and surfactants cetyltrimethyl ammonium bromide (CTAB), making it difficult to apply them directly to clinical research. To solve these problems, a novel nanocomposite was structured by coating silica shell and gold nanocluster on the outer layer of the gold nanorod (AuNRs@SiO₂@AuNCs). Compared with the bare gold nanorod, the nanocomposite with the core-shell structure showed superior photothermal effect. The photothermal conversion temperature reached 63 °C under a lower irradiation power. The photothermal conversion efficiency was enhanced to 77.6%. Its photothermal performance remained constant after five cycles of near-infrared laser irradiation, indicating excellent photothermal stability. In vitro cell imaging experiments show that AuNRs@SiO₂@ AuNCs can effectively enter tumor cells. By 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) analysis, cancer cells can be effectively killed when exposed to a near-infrared laser. During the synthesis process, the silica and gold nanoclusters replaced the toxic CTAB molecular layer on the surface of AuNRs. Therefore, AuNRs@SiO₂@AuNCs has good biocompatibility and fluorescence characteristics. These results suggest that such AuNRs@SiO₂@AuNCs nanocomposite shows great potential in imaging guided photothermal therapy for cancer.

Introduction

Photothermal therapy is a non-invasive and topical treatment of tumors. It has high specificity, small side effects, and quick effect [1]. It mainly generates sufficient quantity of heat to ablate tumor cells by photothermal conversion agent under near-infrared (NIR) laser irradiation. However, high power laser irradiation inevitably causes damage to normal tissues in the vicinity of tumor tissue [2]. Therefore, the balance between the irradiation power and the therapeutic heat requirement should be considered carefully. Improving the photothermal performance and the conversion efficiency of a photothermal agent under low power condition becomes important.

Nanomaterials exhibit great potential as photothermal conversion agents. Thus, they have attracted a lot of attention [[3], [4], [5]]. Gold nanorod (AuNR) is a commonly used photothermal material due to its good photothermal conversion characteristics and tunable localized surface plasmon resonance (LSPR) peak [[6], [7], [8]]. However, the biocompatibility and some defects in photothermal properties limit its application [2,3,9]. In addition, surface toxic CTAB molecular layer makes it unsuitable for direct photothermal therapy [[10], [11], [12], [13]]. Moreover, its shape will collapse and deform when subjected to laser irradiation, resulting in the decrease of photothermal performance [6,10,14]. Therefore, improving the biocompatibility and photothermal stability is a major challenge for photothermal application of gold nanorods [15,16].

According to previous reports, coating photothermal materials on gold nanorods can significantly improve the photothermal conversion performance and photothermal stability of gold nanorods [15,[17], [18], [19]]. Furthermore, gold nanoclusters (AuNCs) could aggregate in the silica nanocarrier, resulting in photothermal conversion characteristics [[20], [21], [22]]. Inspired by these points, gold nanoclusters modified mesoporous silica-coated gold nanorods system (AuNRs@SiO₂@AuNCs) was designed to improve the photothermal performance and biocompatibility. The coating of mesoporous silica could enhance the photothermal stability of AuNRs and replace the surface CTAB molecular layer to reduce the biological toxicity [3,23]. The modification of AuNCs onto mesoporous silica-coated AuNRs is equivalent to coating a layer of photothermal material outside AuNRs to improve its photothermal performance. The use of core-shell type gold nanocomposite with better photothermal effect is a new idea to improve the photothermal performance of AuNRs [1,2,15,24]. In addition, gold nanoclusters are nanomaterials (1–2 nm) with fluorescent properties. They could be used in bioimaging probe [25,26]. Therefore, the nanocomposite has the application prospect for imaging-guided photothermal therapy [27].

Herein, a core-shell gold nanocomposite AuNRs@SiO₂@AuNCs was synthesized by the procedure shown in Scheme 1. The AuNRs@SiO₂@AuNCs system was characterized and demonstrated by TEM, absorption spectra, fluorescent spectra, and zeta potential. Its cytotoxicity, photothermal cell ablation ability, and fluorescence imaging ability were performed in vitro using cells. The AuNRs@SiO₂@AuNCs system showed excellent biocompatibility, light-to-heat conversion performance, and photothermal stability. It has broad application prospects in photothermal therapy and imaging.