Abstract
Highly complex nanoparticles combining multimodal imaging with the sensing of physical properties in biological systems can considerably enhance biomedical research, but reports demonstrating the performance of a single nanosized probe in several imaging modalities and its sensing potential at the same time are rather scarce. Gold nanoshells with magnetic cores and complex organic functionalization may offer an efficient multimodal platform for magnetic resonance imaging (MRI), photoacoustic imaging (PAI), and fluorescence techniques combined with pH sensing by means of surface-enhanced Raman spectroscopy (SERS). In the present study, the synthesis of gold nanoshells with Mn-Zn ferrite cores is described, and their structure, composition, and fundamental properties are analyzed by powder X-ray diffraction, X-ray fluorescence spectroscopy, transmission electron microscopy, magnetic measurements, and UV-Vis spectroscopy. The gold surface is functionalized with four different model molecules, namely thioglycerol, meso-2,3-dimercaptosuccinate, 11-mercaptoundecanoate, and (11-mercaptoundecyl)-N,N,N-trimethylammonium bromide, to analyze the effect of varying charge and surface chemistry on cells in vitro. After characterization by dynamic and electrophoretic light scattering measurements, it is found that the particles do not exhibit significant cytotoxic effects, irrespective of the surface functionalization. Finally, the gold nanoshells are functionalized with a combination of 4-mercaptobenzoic acid and 7-mercapto-4-methylcoumarin, which introduces a SERS active pH sensor and a covalently attached fluorescent tag at the same time. 1H NMR relaxometry, fluorescence spectroscopy, and PAI demonstrate the multimodal potential of the suggested probe, including extraordinarily high transverse relaxivity, while the SERS study evidences a pH-dependent spectral response.
Highlights
Magnetic nanoparticles have attracted considerable attention as so-called negative contrast agents for magnetic resonance imaging (MRI), which were popularized mainly through the commercial agents Feridex® /Endorem® and Resovist® /Cliavist®
The present study aims to employ virtually the same approach for sensing pH, namely the functionalization of a nanosized SERS substrate with mercaptobenzoic acid (MBA), whose different levels of protonation will be determined by SERS measurements
The ferrite concentration was determined by magnetometric analysis to be 2.94 mmol(f.u.) L−1, where f.u. stands for the formula unit of Mn0.61 Zn0.42 Fe1.97 O4, which corresponds to a yield, primarily of the fractionation, of 41% with respect to starting
Summary
Magnetic nanoparticles have attracted considerable attention as so-called negative contrast agents for magnetic resonance imaging (MRI), which were popularized mainly through the commercial agents Feridex® /Endorem® and Resovist® /Cliavist®. Nanomaterials 2022, 12, 428 magnetic nanoparticles in the superparamagnetic state have enabled the establishment of an entirely new imaging method–magnetic particle imaging (MPI), which was experimentally demonstrated for the first time in 2005 [1]. The logical effort to combine magnetic imaging techniques, above all MRI, with other imaging modalities in order to increase the accuracy of diagnostics has led to multimodal contrast agents [2]. Much more rare are the reports on trimodal and multimodal contrast agents that would offer reasonable contrast effects both in traditional methods, such as MRI, and in the newly emerging modalities of MPI and photoacoustic imaging (PAI) (see, e.g., [5,6,7,8])
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