Characterization of Conventional One-Step Sodium Thiosulfate Facilitated Gold Nanoparticle Synthesis

Abstract

Gold-gold sulfide nanoparticles are of interest for drug delivery, biomedical imaging, and photothermal therapy applications due to a facile synthesis method resulting in small particles with high near-infrared (NIR) absorption efficiency. Previous studies suggest that the NIR sensitivity of these nanoparticles was due to hexagonally shaped metal-coated dielectric nanoparticles that consist of a gold sulfide core and gold shell. Here, we illustrate that the conventional synthesis procedure results in the formation of polydisperse samples of icosahedral gold particles, gold nanoplates, and small gold spheres. Importantly, through compositional analysis, via UV/vis absorption spectrophotometry, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS), we show that all of the nanoparticles exhibit identical face center cubic (FCC) gold crystalline structures, thus suggesting that sulfide is not present in the final fabricated nanoparticles. We show that icosahedrally shaped nanoparticles result in a blue-shifted absorbance, with a peak in the visible range. Alternatively, the nanoplate nanoparticles result in the characteristic NIR absorbance peak. Thus, we report that the NIR-contributing species in conventional gold-gold sulfide formulations are nanoplates that are comprised entirely of gold. Furthermore, polydisperse gold nanoparticle samples produced by the traditional one-step reduction of HAuCl4 by sodium thiosulfate show increased in vitro toxicity, compared to isolated and more homogeneous constituent samples. This result exemplifies the importance of developing monodisperse nanoparticle formulations that are well characterized in order to expedite the development of clinically beneficial nanomaterials.

The surface chemistry of near-infrared resonant gold nanotriangles obtained via thiosulfate synthesis

“…We found a significant (p < 0.001) decrease in coverage values, close to 25(2)% only for the maximum tested dose (500 µM ≈ 100 μg/ mL). These values are in accordance with previous reports that show similar results for these AuNP concentrations [11,[24][25][26]76,77].…”

The surface chemistry of near-infrared resonant gold nanotriangles obtained via thiosulfate synthesis

“…We found a significant (p < 0.001) decrease in coverage values, close to 25(2)% only for the maximum tested dose (500 µM ≈ 100 μg/ mL). These values are in accordance with previous reports that show similar results for these AuNP concentrations [11,[24][25][26]76,77].…”

Controllable Synthesis of Gold Nanoparticles in Aqueous Solution by Microwave Assisted Flow Chemistry

“…27,37 However, UV−vis spectroscopy alone is an inadequate technique for particle size and morphology analysis since the SPR of Au-NPs is highly morphology dependent; thus, the use of alternative techniques such as electron microscopy is necessary to elucidate the reason for the observed λ max shift. 9,26,27,34,35,38 Thus, transmission electron microscopy (TEM) has been extensively used to observe the mean particle size and size distribution of Au-NPs. 13,15,35 TEM analysis of Au-NPs synthesized by the SMMM system shows the presence of a mixture of spherical and high aspect ratio nonspherical Au-NPs, rather than commonly observed spherical Au-NPs formation via the Turkevich method 12,22,39−41 (see Supporting Information Figure S2 for TEM images).…”

“…Formation of citrate-stabilized Au-NPs was previously shown to follow a mechanism, including nucleation, aggregation, and spherical particle growth via Oswald ripening. ,,, For microwave assisted Au-NP synthesis, it is widely accepted that microwave irradiation accelerates the nucleation of particles due to its selective and fast heating characteristics. − , We have also recently found that an SMMM system has similar advantages including a fast temperature ramping rate and selective heating of iron oxide for its self-assembly . Change in temperature ramp rates due to varied microwave powers is therefore expected to influence the nucleation rate of nanoparticles.…”

“…Careful inspection of the absorption spectra of Au-NPs reveals that the maximum absorption (λ max ) of Au-NPs synthesized at high microwave power (e.g., 45 W) is blue-shifted compared to the absorption of a sample synthesized at low microwave power (e.g., 25 W) as shown in Figure c. For spherical Au-NPs, a blue-shift in λ max is usually attributed to the formation of Au-NPs smaller in size. , However, UV–vis spectroscopy alone is an inadequate technique for particle size and morphology analysis since the SPR of Au-NPs is highly morphology dependent; thus, the use of alternative techniques such as electron microscopy is necessary to elucidate the reason for the observed λ max shift. ,,,,, Thus, transmission electron microscopy (TEM) has been extensively used to observe the mean particle size and size distribution of Au-NPs. ,, …”

“…750 nm for Au-NPs synthesized under 25 W, that may probably be attributed to the presence of high aspect ratio (>2) Au-NPs observed in TEM images (Supporting Information Figure S2). 34,35 A typical TEM image of the Au-NPs synthesized under constant citrate-to-gold molar ratio of 6/1 and 4 mL min −1 flow rate at 36 W is shown in Figure 1b. The high magnification HR-TEM image (inset of Figure 1b) proves that Au-NPs (average aspect ratio of 1.37) with lattice spacing consistent with Au 0 (111) have been synthesized by the system.…”

The surface chemistry of near-infrared resonant gold nanotriangles obtained via thiosulfate synthesis

“…We found a significant (p < 0.001) decrease in coverage values, close to 25(2)% only for the maximum tested dose (500 µM ≈ 100 μg/ mL). These values are in accordance with previous reports that show similar results for these AuNP concentrations [11,[24][25][26]76,77].…”

Controllable Synthesis of Gold Nanoparticles in Aqueous Solution by Microwave Assisted Flow Chemistry

“…27,37 However, UV−vis spectroscopy alone is an inadequate technique for particle size and morphology analysis since the SPR of Au-NPs is highly morphology dependent; thus, the use of alternative techniques such as electron microscopy is necessary to elucidate the reason for the observed λ max shift. 9,26,27,34,35,38 Thus, transmission electron microscopy (TEM) has been extensively used to observe the mean particle size and size distribution of Au-NPs. 13,15,35 TEM analysis of Au-NPs synthesized by the SMMM system shows the presence of a mixture of spherical and high aspect ratio nonspherical Au-NPs, rather than commonly observed spherical Au-NPs formation via the Turkevich method 12,22,39−41 (see Supporting Information Figure S2 for TEM images).…”

“…Formation of citrate-stabilized Au-NPs was previously shown to follow a mechanism, including nucleation, aggregation, and spherical particle growth via Oswald ripening. ,,, For microwave assisted Au-NP synthesis, it is widely accepted that microwave irradiation accelerates the nucleation of particles due to its selective and fast heating characteristics. − , We have also recently found that an SMMM system has similar advantages including a fast temperature ramping rate and selective heating of iron oxide for its self-assembly . Change in temperature ramp rates due to varied microwave powers is therefore expected to influence the nucleation rate of nanoparticles.…”

“…Careful inspection of the absorption spectra of Au-NPs reveals that the maximum absorption (λ max ) of Au-NPs synthesized at high microwave power (e.g., 45 W) is blue-shifted compared to the absorption of a sample synthesized at low microwave power (e.g., 25 W) as shown in Figure c. For spherical Au-NPs, a blue-shift in λ max is usually attributed to the formation of Au-NPs smaller in size. , However, UV–vis spectroscopy alone is an inadequate technique for particle size and morphology analysis since the SPR of Au-NPs is highly morphology dependent; thus, the use of alternative techniques such as electron microscopy is necessary to elucidate the reason for the observed λ max shift. ,,,,, Thus, transmission electron microscopy (TEM) has been extensively used to observe the mean particle size and size distribution of Au-NPs. ,, …”

“…750 nm for Au-NPs synthesized under 25 W, that may probably be attributed to the presence of high aspect ratio (>2) Au-NPs observed in TEM images (Supporting Information Figure S2). 34,35 A typical TEM image of the Au-NPs synthesized under constant citrate-to-gold molar ratio of 6/1 and 4 mL min −1 flow rate at 36 W is shown in Figure 1b. The high magnification HR-TEM image (inset of Figure 1b) proves that Au-NPs (average aspect ratio of 1.37) with lattice spacing consistent with Au 0 (111) have been synthesized by the system.…”

The surface chemistry of near-infrared resonant gold nanotriangles obtained via thiosulfate synthesis

“…We found a significant (p < 0.001) decrease in coverage values, close to 25(2)% only for the maximum tested dose (500 µM ≈ 100 μg/ mL). These values are in accordance with previous reports that show similar results for these AuNP concentrations [11,[24][25][26]76,77].…”

Controllable Synthesis of Gold Nanoparticles in Aqueous Solution by Microwave Assisted Flow Chemistry

“…27,37 However, UV−vis spectroscopy alone is an inadequate technique for particle size and morphology analysis since the SPR of Au-NPs is highly morphology dependent; thus, the use of alternative techniques such as electron microscopy is necessary to elucidate the reason for the observed λ max shift. 9,26,27,34,35,38 Thus, transmission electron microscopy (TEM) has been extensively used to observe the mean particle size and size distribution of Au-NPs. 13,15,35 TEM analysis of Au-NPs synthesized by the SMMM system shows the presence of a mixture of spherical and high aspect ratio nonspherical Au-NPs, rather than commonly observed spherical Au-NPs formation via the Turkevich method 12,22,39−41 (see Supporting Information Figure S2 for TEM images).…”

“…Formation of citrate-stabilized Au-NPs was previously shown to follow a mechanism, including nucleation, aggregation, and spherical particle growth via Oswald ripening. ,,, For microwave assisted Au-NP synthesis, it is widely accepted that microwave irradiation accelerates the nucleation of particles due to its selective and fast heating characteristics. − , We have also recently found that an SMMM system has similar advantages including a fast temperature ramping rate and selective heating of iron oxide for its self-assembly . Change in temperature ramp rates due to varied microwave powers is therefore expected to influence the nucleation rate of nanoparticles.…”

“…Careful inspection of the absorption spectra of Au-NPs reveals that the maximum absorption (λ max ) of Au-NPs synthesized at high microwave power (e.g., 45 W) is blue-shifted compared to the absorption of a sample synthesized at low microwave power (e.g., 25 W) as shown in Figure c. For spherical Au-NPs, a blue-shift in λ max is usually attributed to the formation of Au-NPs smaller in size. , However, UV–vis spectroscopy alone is an inadequate technique for particle size and morphology analysis since the SPR of Au-NPs is highly morphology dependent; thus, the use of alternative techniques such as electron microscopy is necessary to elucidate the reason for the observed λ max shift. ,,,,, Thus, transmission electron microscopy (TEM) has been extensively used to observe the mean particle size and size distribution of Au-NPs. ,, …”

“…750 nm for Au-NPs synthesized under 25 W, that may probably be attributed to the presence of high aspect ratio (>2) Au-NPs observed in TEM images (Supporting Information Figure S2). 34,35 A typical TEM image of the Au-NPs synthesized under constant citrate-to-gold molar ratio of 6/1 and 4 mL min −1 flow rate at 36 W is shown in Figure 1b. The high magnification HR-TEM image (inset of Figure 1b) proves that Au-NPs (average aspect ratio of 1.37) with lattice spacing consistent with Au 0 (111) have been synthesized by the system.…”