Synthesis and Comparative Evaluation of Photoswitchable Magnetic Resonance Imaging Contrast Agents

Mapping light distribution in tissue by using MRI-detectable photosensitive liposomes

Characterizing sources and targets of illumination in living tissue is challenging. Here we show that spatial distributions of light in tissue can be mapped by using magnetic resonance imaging (MRI) in the presence of photosensitive nanoparticle probes. Each probe consists of a reservoir of paramagnetic molecules enclosed by a liposomal membrane incorporating photosensitive lipids. Incident light causes the photoisomerization of the lipids and alters hydrodynamic exchange across the membrane, thereby affecting longitudinal relaxation-weighted contrast in MRI. We injected the nanoparticles into the brains of live rats and used MRI to map responses to illumination profiles characteristic of widely used applications of photostimulation, photometry and phototherapy. The responses deviated from simple photon propagation models and revealed signatures of light scattering and nonlinear responsiveness. Paramagnetic liposomal nanoparticles may enable MRI to map a broad range of optical phenomena in deep tissue and other opaque environments.

A Modern Look at Spiropyrans: From Single Molecules to Smart Materials

Photochromic compounds of the spiropyran family have two main isomers capable of inter-switching with UV or visible light. In the current review, we discuss recent advances in the synthesis, investigation of properties, and applications of spiropyran derivatives. Spiropyrans of the indoline series are in focus as the most promising representatives of multi-sensitive spirocyclic compounds, which can be switched by a number of external stimuli, including light, temperature, pH, presence of metal ions, and mechanical stress. Particular attention is paid to the structural features of molecules, their influence on photochromic properties, and the reactions taking place during isomerization, as the understanding of the structure-property relationships will rationalize the synthesis of compounds with predetermined characteristics. The main prospects for applications of spiropyrans in such fields as smart material production, molecular electronics and nanomachinery, sensing of environmental and biological molecules, and photopharmacology are also discussed.

Preparation of Hydrothermal Carbon Quantum Dots as a Contrast Amplifying Technique for the diaCEST MRI Contrast Agents

The discovery of exogenous contrast agents (CAs) is one of the key factors behind the success and widespread acceptability of MRI as an imaging tool. To the long list of CAs, the newest addition is the chemical exchange saturation transfer (CEST)-based CAs. Among them, the diaCEST CAs are the safer metal-free option constituted by a large pool of organic and macromolecules, but the tradeoff comes in terms of smaller natural offset. Another major challenge for the CEST CAs is that they need to operate in the tens of millimolar concentration range to produce any meaningful contrast. The quest for high efficiency diaCEST agents has led to a number of strategies such as use of hydrogen bonding, use of equivalent protons, and use of diatropic ring current. Here, we present carbon quantum dot formation using hydrothermal treatment as a new strategy to amplify diaCEST contrast efficiency. We show that while the well-known analgesic drug lidocaine hydrochloride when repurposed as a diaCEST CA produces no contrast at the physiological pH and temperature, the carbon dots prepared from it elevate the physiological contrast to a sizable 11%. Also, the maximum efficiency at an acidic pH gets amplified by a factor of 2 to 46%. The study showed that the enhancement in CEST efficiency is reproducible and the pH response of these carbon dots is tunable through variation in synthesis conditions such as temperature, duration, and precursor concentration.