Gilded vaterite particles: Synthesis, optical characterization, and label-free imaging

Inorganic nanoparticles are capable of accommodating multiple biomedical modalities, making them prime candidates for realizing light-responsive theranostic carriers. Within this realm, merging the benefits of diverse platforms promises the development of new functionalities, broadening the range of applications. While vaterite, a polymorph of calcium carbonate, is known for its structural versatility and chemical modifiability, gold nanoparticles are recognized for their unique optical properties. Here we explore a new mesoscopic particle, gilded vaterite, which encompasses the advantages of both platforms. A flexible synthesis method is developed and optimized, demonstrating the capability to obtain high-yield fabrication of vaterite particles with controllable form factors alongside tailored surface coverage with gold. Uniform reproducible distributions of gold nanoparticles of up to 80 nm in diameter, with coverage above 40%, were demonstrated. Optical properties of gilded vaterite particles were explored, revealing strong peaks, which built up owing to the resonant hybridization of near-field coupled plasmonic resonances. Confocal microscopy of cells was utilized to explore potential applications of gilded vaterite with tunable optical properties in the realm of label-free imaging. Strong light scattering from vaterite carriers, taken up by cells, alongside the fluorescence from stained cells, was observed via reflectance confocal microscopy. This dual-channel approach allows for simultaneous observation of the particles and their biological environment. Tunable optical and biochemical properties of gilded vaterite promote this perspective platform towards accommodating new modalities and thus becoming an intrinsically optical-responsive multifunctional system.