Echographic detectability of optoacoustic signals from low-concentration PEG-coated gold nanorods

Francesco Conversano,1 Giulia Soloperto,¹ Antonio Greco,¹ Andrea Ragusa,^1,2 Ernesto Casciaro,¹ Fernanda Chiriacò,¹ Christian Demitri,³ Giuseppe Gigli,^2–5 Alfonso Maffezzoli,³ Sergio Casciaro<sup>1

1</sup>National Research Council, Institute of Clinical Physiology, Lecce, Italy; ²National Nanotechnology Laboratory of CNR-NANO, Lecce, Italy; ³University of Salento, Department of Engineering for Innovation, Lecce, Italy; ⁴Italian Institute of Technology – Center for Biomolecular Nanotechnology (CBN-IIT), Arnesano, Italy; ⁵University of Salento, Department of Mathematics and Physics ‘Ennio De Giorgi’, Lecce, Italy

Purpose: To evaluate the diagnostic performance of gold nanorod (GNR)-enhanced optoacoustic imaging employing a conventional echographic device and to determine the most effective operative configuration in order to assure optoacoustic effectiveness, nanoparticle stability, and imaging procedure safety.
Methods: The most suitable laser parameters were experimentally determined in order to assure nanoparticle stability during the optoacoustic imaging procedures. The selected configuration was then applied to a novel tissue-mimicking phantom, in which GNR solutions covering a wide range of low concentrations (25–200 pM) and different sample volumes (50–200 µL) were exposed to pulsed laser irradiation. GNR-emitted optoacoustic signals were acquired either by a couple of single-element ultrasound probes or by an echographic transducer. Off-line analysis included: (a) quantitative evaluation of the relationships between GNR concentration, sample volume, phantom geometry, and amplitude of optoacoustic signals propagating along different directions; (b) echographic detection of “optoacoustic spots,” analyzing their intensity, spatial distribution, and clinical exploitability. MTT measurements performed on two different cell lines were also used to quantify biocompatibility of the synthesized GNRs in the adopted doses.
Results: Laser irradiation at 30 mJ/cm² for 20 seconds resulted in the best compromise among the requirements of effectiveness, safety, and nanoparticle stability. Amplitude of GNR-emitted optoacoustic pulses was proportional to both sample volume and concentration along each considered propagation direction for all the tested boundary conditions, providing an experimental confirmation of isotropic optoacoustic emission. Average intensity of echographically detected spots showed similar behavior, emphasizing the presence of an “ideal” GNR concentration (100 pM) that optimized optoacoustic effectiveness. The tested GNRs also exhibited high biocompatibility over the entire considered concentration range.
Conclusion: An optimal configuration for GNR-enhanced optoacoustic imaging was experimentally determined, demonstrating in particular its feasibility with a conventional echographic device. The proposed approach can be easily extended to quantitative performance evaluation of different contrast agents for optoacoustic imaging.

Keywords: photoacoustic imaging, tissue-mimicking phantom, laser, nanoparticle degradation