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Laser-induced destruction of gold nanoshells: new weapons in the cell-killing arsenal

The detonation of silica-gold nanoshells at energy levels safe for normal tissues and their unprecedented low threshold for vapor bubble formation in solution highlight an advance in cell-killing nanotechnology.
11 July 2008, SPIE Newsroom. DOI: 10.1117/2.1200805.1176

Nanoshells are nanoparticles that consist of ultra-thin metal shells surrounding core materials such as silica.1 Gold nanoshells (GNSs) have already been used in various applications, such as analytical diagnostics, photothermal-based therapies, and in vivo optical visualization, based in part on their ability to diffract and absorb light of varying wavelengths. The efficiency of nanoshell technologies,2,3 however, is limited by gaps in our current understanding of the thermal interactions between nanoshell particles and laser light pulses or continuous waves in the context of complex biological environments. Irradiation, even with moderate pulses of energy, can induce melting, evaporation, and fragmentation of nanoshells. These events can drastically alter the intended therapeutic effects and lead to the formation of vapor bubbles as well as acoustic waves and shock waves.4

Here, we report on the destruction of silica-gold nanoshells after the application of a single nanosecond laser pulse. Strikingly, direct visual observation indicated a dramatic color change within the GNS solution. According to spectroscopic and transmission electron microscope (TEM) data, this phenomenon was accompanied by complete fragmentation of the GNSs into small (20–30nm) gold particles with an absorption resonance typical of colloidal gold (530nm). Furthermore, the formation of vapor bubbles was observed at remarkably low-energy thresholds, significantly lower than that associated with other types of manufactured nanoparticals.

Figure 1. Suspensions of silica-gold nanoshells before (a) and after (b) irradiation with individual laser pulses (1064nm, 10mJ) and the corresponding extinction spectra (c). It should be emphasized that the red sample (b) corresponds to multiple, single-pulse irradiations of the suspension volume followed by mixing due to diffusion.

Figure 2. TEM images of intact silica cores (a) and nanoshells (b), as well as nanoshells following irradiation (c–f).

The GNSs were fabricated as previously described5,6 to yield particles with an average silica core diameter and gold shell thickness of 140 and 15nm, respectively. Irradiation of the GNS suspension was carried out with a Nd:YAG (neodymium-doped yttrium aluminium garnet) pulse laser (Carl Zeiss, Germany) with the following parameters: wavelength, 1064nm; pulse width, 4ns; pulse energy, 1–10mJ; minimal laser beam diameter at focus 0.1mm. The suspensions of GNSs were irradiated in glass cuvettes with an optical path length of 1, 3, and 10mm, as well as in standard 1.5ml Eppendorf vials with a suspension volume of approximately 1.5ml. Sequent irradiation of different suspension parts led to a color change within the whole suspension volume, changing from the initial green-blue color—see Figure 1(a)—to purple-red: see Figure 1(b). The measured extinction spectra of nonirradiated and irradiated samples showed significant transformation after irradiation of intact GNSs, which was characterized by a drastic decrease in the absorption band maximum near 900nm and appearance of a new absorption resonance near 530nm: see Figure 1(c). Such a resonance is typical of colloidal gold nanospheres that have undergone laser-induced fragmentation.

TEM analysis confirmed the extensive destruction of the GNSs under these conditions. Figure 2(a) and (b) shows TEM images of intact silica nanospheres and GNSs prior to irradiation. After application of the single 10mJ pulse, the formation of multiple small gold spheres in solution was observed—see Figure 2(c) and (d)—with a broad size distribution ranging from a few nanometers to 50nm. The average nanosphere diameter was within the range of 20–30nm, consistent with the localization of the absorption resonance near 530nm. Close inspection of the TEM images also revealed full or partial melting of the GNSs and the appearance of solid spherical gold nanoparticles of various sizes attached to intact silica cores—see Figure 2(e) and (f)—as well as the displacement of silica cores and the subsequent formation of gold rings: see Figure 2(d).

Time-resolved monitoring of these processes, using the photothermal thermolens technique,7 demonstrated significant vapor bubble formation after the application of a single 8ns pulse at a fluence of 5mJ cm−2 and higher. On average, the bubble lifespan and size were in the range of 20ns–5μs and 0.5–20μm, respectively, in the fluence range of 4–50mJ cm−2, with a further increase in duration and size as the laser energy was increased. It is important to note that the minimal energy threshold determined in these experiments is well below the laser safety standard of 20–34mJ cm−2 in the near-IR spectral range of 650–950nm.8

These findings suggest that the explosive destruction of nanoshell particles may prove useful in biomedical applications. The dramatic color change observed in these experiments points to the utility of GNSs as potential color-based nanosensors for use in monitoring laser-pulse treatments. However, the most attractive property of GNSs may be their low-energy threshold for vapor bubble formation, a phenomenon that correlates with irreversible and fatal cell damage. Indeed, GNSs may prove very promising for the killing of targeted cells given that they have the lowest threshold for microbubble formation among the many different nanoparticles tested. Further research on the effects of GNS clustering and fabrication methods that dramatically increase cellular damage and reduce the laser-pulse energy required to produce these explosive effects may further the utility of GNSs in cell-killing applications.

Nikolai Khlebtsov
Lab of Nanobiotechnology
Institute of Biochemistry and Physiology of Plants and Microorganisms
Biophysics Chair
Saratov State University
Saratov, Russia
Garif Akchurin, Georgy Akchurin, Valery Tuchin
Saratov State University
Saratov, Russia
Boris Khlebtsov
Lab of Nanobiotechnology
Institute of Biochemistry and Physiology of Plants and Microorganisms
Saratov, Russia
Vladimir Zharov
Philips Classic Laser Laboratories
University of Arkansas for Medical Sciences
Little Rock, AR