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Review
. 2017 Nov 30;10(12):1372.
doi: 10.3390/ma10121372.

Small Gold Nanorods: Recent Advances in Synthesis, Biological Imaging, and Cancer Therapy

Lu An  1 , Yuanyuan Wang  2 , Qiwei Tian  3 , Shiping Yang  4
Affiliations
Review

Small Gold Nanorods: Recent Advances in Synthesis, Biological Imaging, and Cancer Therapy

Lu An et al. Materials (Basel). .

Abstract

Over the past few decades, the synthetic development of ultra-small nanoparticles has become an important strategy in nano-medicine, where smaller-sized nanoparticles are known to be more easily excreted from the body, greatly reducing the risk caused by introducing nano-theranostic agents. Gold nanorods are one of the most important nano-theranostic agents because of their special optical and electronic properties. However, the large size (diameter > 6 nm) of most obtained gold nanorods limits their clinical application. In recent years, more and more researchers have begun to investigate the synthesis and application of small gold nanorods (diameter < 6 nm), which exhibit similar optical and electronic properties as larger gold nanorods. In this review, we summarize the recent advances of synthesis of the small gold nanorods and their application for near-infrared light-mediated bio-imaging and cancer therapy.

Keywords: biological imaging; cancer therapy; seedless; small gold nanorods.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Transmission electron microscopy (TEM) images of the small Au nanorods samples (GmSn) obtained with different molar ratio of seed-to-Au(III) in the growth solution. (A–C) G1S9, G2S8, and G4S6 were grown with cetyltripropylammonium bromide (CTPAB), and (D–F) G6S4, G8S2, and G9S1 were grown with CTAB [38].
Figure 2
Figure 2
Typical TEM images of the small gold nanorods obtained by Jana et al. [42] ((A) 1 mM HAuCl4, 0.2 M CTAB, 0.2 mM AgNO3, 2 mM ascorbic acid, 0.25 μM BH4), and El-Sayed et al. [37] ((B) 5.0 mL HAuCl4, 5.0 mL CTAB, 270 μL AgNO3, 8 μL HCl, 70 μL ascorbic acid, 15 μL NaBH4).
Figure 3
Figure 3
(A) TEM images obtained from gold nanorods synthesized at 25 °C, 50 °C, and 97 °C from left to right. The scale bars indicate 50 nm. (B) Particle dimension as obtained from TEM analysis. The error bars represent the error in the mean value of the distribution of the respective dimension. (C) Evolution of the integrated absorbance vs. time for nanorods synthesized at different temperatures. The solid lines are sigmoidal fits to the experimental data points [44].
Figure 4
Figure 4
Schematic illustration of the surface modification of small gold nanorods by surface coating and ligand exchange methods.
Figure 5
Figure 5
(A) Sketch of the setup used for the photoacoustic experiments (O, objective; L, focusing lens; BS, beam splitter; EM, energy meter); (B) trend of Fth as a function of effective nanoparticle radius (reff). In the inset: comparison of PA response with a single pulse excitation at fluence F < Fth for samples containing gold nanorod (GNR)5 (black line), GNR8 (green line), GNR11 (red line), GNR15 (dark blue line), and GNR22 (light blue line). GNR5, GNR8, GNR11, GNR15, and GNR22, with the numbers denoting their average effective radii (radius of a sphere having the same volume as the rod) in nanometers [71].
Figure 6
Figure 6
(A) Schematics of the two-color photothermal imaging microscopy with a near infrared probe beam at 785 nm and excitations beam at 532 or 640 nm; (B) White light (a) and PhI images of COS 7 cells incubated with nanorods under (b) 532 and (c) 640 nm excitation. Photothermal imaging microscopy recorded under red excitation shows very weak mitochondrial background signals compared to those acquired under green excitation [78].
Figure 7
Figure 7
In vivo non-invasive near-infrared (NIR) absorption images of real-time tumor specificity of cRGD-PGNRs. (A) In vivo time-dependent brain region biodistribution of cRGD-PGNRs and cRAD-PGNRs as a control; (B) relative photon counts of in vivo tumor target specificity of cRGD-PGNRs (square) and cRAD-PGNRs (circle) was recorded; and (C) relative quantification of in vivo biodistribution of cRGD-PGNRs and cRAD-PGNRs in different tissues [81].
Figure 8
Figure 8
(A) Intracellular Au contents of the small (white) and large (pink) silica-coated Au nanorods samples in U-87 MG, MDA-MB-231, and MDA-MB-435S cells; and (B) cell viability upon photothermal therapy with small (white) and large (pink) silica-coated Au nanorod samples in U-87 MG, MDA-MB-231, and MDAMB-435S cells [38].
Figure 9
Figure 9
(A) Dark-field images of pure HSC-3 cells, cell incubated AuNRs@PEG, and cells incubated in AuNRs@NLS for 24 h. Scale bar = 20 μm; cell viability (B) and apoptosis/necrosis assay (C) for the HSC-3 cells treated with PPTT at different times; Q1 (necrosis), Q2 (apoptosis), Q3 (early apoptosis) and Q4 (early apoptosis) [86].
Figure 10
Figure 10
Schematic illustration of the whole procedure for the activatable ultrasmall GNR-based “off–on” fluorescence imaging-guided PTT in tumor cells [28].
Figure 11
Figure 11
In vivo photothermal ablation of tumor after intravenous injection of Au nanorod vesicles followed by laser irradiation. (A) Infrared thermographic maps and (B) temperature changes of the tumor region treated with small AuNRs and AuNR Ve and irradiated with a 808 nm laser at different power densities; (C) tumor growth curves and (D) survival curves of tumor-bearing mice treated with phosphate-buffered saline (PBS), small AuNRs and AuNR Ve and laser irradiation; (E) photographs of the tumor-bearing mice at days 0, 1, 5, and 8 d after being treated with the AuNR Ve; and (F) hematoxylin and eosin (H&E) staining of the tumor tissue after different treatments [31].
Figure 12
Figure 12
(A) Diagram highlighting the difference between the treatment of free small gold nanorods and macrophage-loaded small gold nanorods; (B) temperature profile of tumor under 808 nm light irradiation for 10 min; and (C) growth of tumors in the different groups of mice after the irradiation treatments [99].
Figure 13
Figure 13
Schematic illustration of sequential DOX release triggered by (i) remote NIR laser irradiated photothermal effect and (ii) acidic environment of the cancer cell [102].
Figure 14
Figure 14
In vivo biodistribution and clearance of (A) BSA-bAuNRs and (B) BSA-sAuNRs together with Au concentrations at the different time points of 1, 5, 10, 15, and 30 days after intravenous injection (5 mg Au/kg). The inset are typical TEM images of the bAuNRs and sAuNRs, respectively [34].
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