Measuring the Optical Absorption Cross Sections of Au-Ag Nanocages and Au Nanorods by Photoacoustic Imaging

Department of Biomedical Engineering, Washington University, St. Louis, Missouri 63130.
The Journal of Physical Chemistry C (Impact Factor: 4.77). 06/2009; 113(21):9023-9028. DOI: 10.1021/jp903343p
Source: PubMed


This paper presents a method for measuring the optical absorption cross-sections (σ(a)) of Au-Ag nanocages and Au nanorods. The method is based on photoacoustic (PA) imaging, where the detected signal is directly proportional to the absorption coefficient (μ(a)) of the nanostructure. For each type of nanostructure, we firstly obtained μ(a) from the PA signal by benchmarking against a linear calibration curve (PA signal vs. μ(a)) derived from a set of methylene blue solutions with different concentrations. We then calculated σ(a) by dividing the μ(a) by the corresponding concentration of the Au nanostructure. Additonally, we obtained the extinction cross-section (σ(e), sum of absorption and scattering) from the extinction spectrum recorded using a conventional UV-vis-NIR spectrometer. From the measurements of σ(a) and σ(e), we were able to easily derive both the absorption and scattering cross-sections for each type of gold nanostructure. The ratios of absorption to extinction obtained from experimental and theoretical approaches agreed well, demonstrating the potential use of this method in determining the optical absorption and scattering properties of gold nanostructures and other types of nanomaterials.

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    • "where σA is the absorption cross section. σE in turn was obtained by applying the relationship between cross sections and absorption coefficients μ [42,43]: "
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    ABSTRACT: The wavelength dependence of the laser-induced photoacoustic signal amplitude has been measured for water dispersions of 10, 61, and 93 nm diameter gold nanospheres. The whole region of the localized surface plasmon resonance has been covered. This “photoacoustic excitation profile” can be overlayed with the extinction spectrum between 450 nm and 600 nm in the case of the smallest nanoparticles. At variance, the larger-sized nanoparticles display a progressive deviation from the extinction spectrum at longer wavelength, where the photoacoustic signal becomes relatively smaller. Considering that photoacoustics is intrinsically insensitive to light scattering, at least for optically thin samples, the results are in agreement with previous theoretical work predicting i) an increasing contribution of scattering to extinction when the nanoparticle size increases, and ii) a larger scattering component at longer wavelengths. Therefore, the method has a general validity and can be applied to selectively determine light absorption by plasmonic systems.
    Photoacoustics 03/2014; 2(1). DOI:10.1016/j.pacs.2013.12.001 · 4.60 Impact Factor
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    • "The release profile can also be monitored using PA amplitude since both AuNC and HPPH can function as exogenous PA contrast agents.(13, 50 As the amplitude of PA signal is proportional to the light absorption,44 extinction spectra were acquired and compared with the change of PA signal during the release. The extinction spectrum of AuNC-HPPH initially included two convoluted peaks at 665 and 685 nm (Fig. 2B) which could be attributed to the high local concentration of HPPH,51 metalation of the pheophorbide ring,52 metal-HPPH interactions,53 or a combination thereof. "
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    ABSTRACT: We have demonstrated that gold nanocage-photosensitizer conjugates can enable dual image-guided delivery of photosensitizer and significantly improve the efficacy of photodynamic therapy in a murine model. The photosensitizer, 3-devinyl-3-(1'-hexyloxyethyl)pyropheophorbide (HPPH), was noncovalently entrapped in the poly(ethylene glycol) monolayer coated on the surface of gold nanocages. The conjugate is stable in saline solutions, while incubation in protein rich solutions leads to gradual unloading of the HPPH, which can be monitored optically by fluorescence and photoacoustic imaging. The slow nature of the release in turn results in an increase in accumulation of the drug within implanted tumors due to the passive delivery of gold nanocages. Furthermore, the conjugate is found to generate more therapeutic singlet oxygen and have a lower IC50 value than the free drug alone. Thus the conjugate shows significant suppression of tumor growth as compared to the free drug in vivo. Short-term study showed neither toxicity nor phenotypical changes in mice at therapeutic dose of the conjugates or even at 100-fold higher than therapeutic dose of gold nanocages.
    Theranostics 01/2014; 4(2):163-74. DOI:10.7150/thno.7064 · 8.02 Impact Factor
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    • "Notably, the excitation peak of AuNCs was adjusted to a wavelength of 800 nm for both two-photon microscopy and photoacoustic microscopy, which could further improve the penetration depth of the two imaging modalities due to the relatively low absorption of biological tissues at such a wavelength. Given the large optical absorption cross-section of AuNCs 48, it could also improve the detection sensitivity and therefore reduce the dose and potential cytotoxicity of the nanoparticles to the cells. "
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    ABSTRACT: Stem cell tracking is a highly important subject. Current techniques based on nanoparticle-labeling, such as magnetic resonance imaging, fluorescence microscopy, and micro-computed tomography, are plagued by limitations including relatively low sensitivity or penetration depth, involvement of ionizing irradiation, and potential cytotoxicity of the nanoparticles. Here we introduce a new class of contrast agents based on gold nanocages (AuNCs) with hollow interiors and porous walls to label human mesenchymal stem cells (hMSCs) for both in vitro and in vivo tracking using two-photon microscopy and photoacoustic microscopy. As demonstrated by the viability assay, the AuNCs showed negligible cytotoxicity under a reasonable dose, and did not alter the differentiation potential of the hMSCs into desired lineages. We were able to image the cells labeled with AuNCs in vitro for at least 28 days in culture, as well as to track the cells that homed to the tumor region in nude mice in vivo.
    Theranostics 07/2013; 3(8):532-43. DOI:10.7150/thno.5369 · 8.02 Impact Factor
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