Enhanced Performance of a Molecular Photoacoustic Imaging Agent by Encapsulation in Mesoporous Silicon Nanoparticles

Abstract

Photoacoustic (PA) imaging allows visualization of the physiology and pathology of tissues with good spatial resolution and relatively deep tissue penetration. The method converts near-infrared (NIR) laser excitation into thermal expansion, generating pressure transients that are detected with an acoustic transducer. Here, we find that the response of the PA contrast agent indocyanine green (ICG) can be enhanced 17-fold when it is sealed within a rigid nanoparticle. ICG encapsulated in particles composed of porous silicon (pSiNP), porous silica, or calcium silicate all show greater PA contrast relative to equivalent quantities of free ICG, with the pSiNPs showing the strongest enhancement. A liposomal formulation of ICG performs similar to free ICG, suggesting that a rigid host nanostructure is necessary to enhance ICG performance. The improved response of the nanoparticle formulations is attributed to the low thermal conductivity of the porous inorganic hosts and their ability to protect the ICG payload from photolytic and/or thermal degradation. The translational potential of ICG-loaded pSiNPs as photoacoustic probes is demonstrated via imaging of a whole mouse brain.

Photoacoustic Imaging: Contrast Agents and Their Biomedical Applications

“…However, ICG has the disadvantages of poor light stability and degradation in water, which severely hinder its application in the biomedical field. Therefore, various studies have focused on the encapsulation of the dye into biocompatible polymers or other materials to form stabilized NPs . For example, Sailor and co‐workers found that the intensity of the PA contrast could be improved by 17 times when using mesoporous silica NPs to encapsulate ICG; the effect was ascribed to the weak thermal conductivity of the mesoporous silica coating protecting the ICG from thermal or photolytic degradation .…”

“…Therefore, various studies have focused on the encapsulation of the dye into biocompatible polymers or other materials to form stabilized NPs . For example, Sailor and co‐workers found that the intensity of the PA contrast could be improved by 17 times when using mesoporous silica NPs to encapsulate ICG; the effect was ascribed to the weak thermal conductivity of the mesoporous silica coating protecting the ICG from thermal or photolytic degradation . Huang and co‐workers designed a PA contrast agent composed of a hydrophobic MeHg + ‐responsive NIR cyanine dye (hCy7) and liposomes (LP‐hCy7; Figure D) .…”

“…PA imaging has also been widely used in brain imaging . For example, Sailor and co‐workers developed an effective PA contrast agent, Ca‐pSiNP‐ICG, in which ICG was encapsulated in porous silicon NPs via a calcium silicate precipitation method for ex vivo mouse‐brain imaging . Compared with the free ICG or PBS controls, which had weak and nonexistent PA signals, respectively, the Ca‐pSiNP‐ICG injected into the brain showed an enhanced PA signal ( Figure A).…”

“…F) The change in blood clot area in the injured region as a function of time. (A) Reproduced with permission . Copyright 2018, Wiley‐VCH.…”

Dictionary learning technique enhances signal in LED-based photoacoustic imaging

“…Photoacoustic imaging (PAI) is a non-invasive hybrid imaging modality with tremendous potential in structural, functional, and molecular imaging for pre-clinical and clinical applications such as brain mapping, tumor detection, cancer staging, tissue vasculature, and oral health [1][2][3][4][5][6][7][8][9][10]. PAI combines optical and ultrasound imaging modalities based on the photoacoustic effect to achieve the good contrast and spectral behavior of optical imaging as well as the spatial and temporal resolution of the ultrasound imaging [11][12][13][14].…”