Abstract
Photoacoustic imaging, an emerging modality, provides supplemental information to ultrasound imaging. We investigated the properties of polypyrrole nanoparticles, which considerably enhance contrast in photoacoustic images, in relation to the synthesis procedure and to their size. We prepared polypyrrole nanoparticles by water-based redox precipitation polymerization in the presence of ammonium persulphate (ratio nPy:nOxi 1:0.5, 1:1, 1:2, 1:3, 1:5) or iron(III) chloride (nPy:nOxi 1:2.3) acting as an oxidant. To stabilize growing nanoparticles, non-ionic polyvinylpyrrolidone was used. The nanoparticles were characterized and tested as a photoacoustic contrast agent in vitro on an imaging platform combining ultrasound and photoacoustic imaging. High photoacoustic signals were obtained with lower ratios of the oxidant (nPy:nAPS ≥ 1:2), which corresponded to higher number of conjugated bonds in the polymer. The increasing portion of oxidized structures probably shifted the absorption spectra towards shorter wavelengths. A strong photoacoustic signal dependence on the nanoparticle size was revealed; the signal linearly increased with particle surface. Coated nanoparticles were also tested in vivo on a mouse model. To conclude, polypyrrole nanoparticles represent a promising contrast agent for photoacoustic imaging. Variations in the preparation result in varying photoacoustic properties related to their structure and allow to optimize the nanoparticles for in vivo imaging.
Highlights
Photoacoustic (PA) imaging is an emerging modality [1]asserting its place among non-invasive imaging methods in biomedicine and clinics [2,3].The method is based on the photoacoustic effect described as early as in the 19th century [4].When light interacts with a material, a part of its energy is absorbed
The overoxidation softens particle surfaces, which causes a coalescence of particles due to the capillary forces that occur during particles drying on the TEM grid deposition
Our study focused on polypyrrole nanoparticles, reputed
Summary
Photoacoustic (PA) imaging (or optoacoustic imaging) is an emerging modality [1]asserting its place among non-invasive imaging methods in biomedicine and clinics [2,3].The method is based on the photoacoustic effect described as early as in the 19th century [4].When light interacts with a material (charged particles), a part of its energy is absorbed. Almost any electromagnetic radiation may induce a PA effect, most applications require radiation in the ultraviolet to infrared wavelength range [5]. Excitation by near infrared (NIR) light (750–3000 nm) is preferred in in vivo imaging due to the higher tissue penetration of NIR light [6], because shorter wavelengths are absorbed or scattered to a greater extent. Both endogenous and exogenous chromophores may absorb excitation light and produce a PA effect. Photoacoustic contrast agents, may substantially widen the range of potential PA imaging applications [7]
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