Photoacoustic Properties of Polypyrrole Nanoparticles

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.

Keywords: contrast agents; nanoparticles; photoacoustic imaging; polypyrrole.

Figure 1

Transmission electron microscopy (TEM) micrographs of the polypyrrole nanoparticles.

Figure 2

UV-Vis spectra of the polypyrrole nanoparticles prepared by oxidation of Py monomer with different molar ratios of APS (samples PPyA05, PPyA10, PPyA20, PPyA30 and PPyA50) dispersed in physiological PBS (c = 55 µg/mL).

Figure 3

Photoacoustic spectra of polypyrrole nanoparticles prepared with APS oxidant. Different pyrrole and APS ratios are presented at 680–970 nm (a) and 1200–2000 nm (b) ranges. NP concentration was 3 mg/mL.

Figure 4

Photoacoustic spectra of polypyrrole nanoparticles prepared with FeCl₃ oxidant. Different pyrrole and oxidant ratios are presented at 680–970 nm and 1200–2000 nm ranges. Spectra are recalculated per one nanoparticle. Note logarithmic scale on y-axis. NP concentration was 3 mg/mL.

Figure 5

Dependence of the relative PA signal intensity (per one nanoparticle) on the nanoparticle diameter (a) or cross-sectional area (b) measured at the same concentration (3 mg/mL) at selected wavelengths (705, 800, 915 nm).

Figure 6

Multimodal US-PA imaging of the mouse heart in its long axis in B-Mode (left panels) and PA-Mode (right panels) before intravenous PPyF4 nanoparticle administration (a) and 5 s after bolus injection (180 µL) (b). The PA detection was at single wavelength mode obtained at 800 nm. RA—right atrium, A—aorta, LA—left atrium.