ESR micro-imaging of LiNc-BuO crystals in PDMS: spatial and spectral grain distribution

Microcrystals of lithium octa-n-butoxynaphthalocyanine (LiNc-BuO) in a bio-compatible and oxygen-permeable polymer matrix of poly-dimethyl-siloxane (PDMS) can be used for repetitive non-invasive imaging of oxygen in live specimens by means of mm-scale electron spin resonance (ESR) imaging. This probe denoted as "oxychip" was characterized by high-resolution mum-scale ESR microcopy to reveal the fine details of its spatial and spectral properties. The ESR micro-images of a typical oxychip device showed that while the spatial distribution of the microcrystals in the polymer is fairly homogenous (as revealed by optical microscopy), the ESR signal originates only from a very few dominant crystals. Furthermore, spectral-spatial analysis in a microcrystal and a sub-microcrystal spatial resolution reveals that each crystal has a slightly different g-factor and also exhibits variations in linewidth, possibly due to the slightly different individual crystallization process.