Nanoparticle Scintillators and Proximity Assays

Low energy β- and α- particle emitting radionuclides are useful molecular labels for binding assays and uptake studies, or as radiotherapy agents, but are challenging to detect and quantify directly in biological samples due to their low energies and short penetration depths. Activity measurements for these radionuclides are commonly made in liquid scintillation cocktail (LSC), a mixture of energy-absorbing organic solvents, surfactants, and scintillant fluorophores, which is incompatible with most functional biological samples, including living cells, and provides only end-point measurements. We developed polymer-core silica-shell scintillator nanoparticles for detecting radionuclide-labeled analytes directly in biological samples, with little to no additional sample preparation. These nanoparticles avoid the toxicity of LSC, as well as many of the experimental limitations of existing polymer and inorganic crystal scintillators. The small diameter, dispersibility, and aqueous compatibility of the nanoparticle scintillators may facilitate radiopharmaceutical design to monitor the activity, location, and degradation over time. The outer surface of the scintillating nanoparticles can be easily functionalized with specific binding molecules for scintillation proximity assays (nanoSPA^TM). This nanoparticle platform provides several advantages over other methods for monitoring analytes, for example: significantly lower background, improved target specificity, and a self-contained sensor. Proof-of-concept SPA assays demonstrated selectivity up to 30 times greater for bound ³H-analytes over unbound ³H analytes when the labeled species was covalently bound to our nanoparticles, 18 times greater for antibody functionalized nanoparticles, 8 times greater for biotin functionalized nanoparticles, and 4 times greater for ssDNA functionalized nanoparticles. We have also measured and monitored peptide phosphorylation with kinase and ³³P ɣ-ATP. Preliminary data show that nanoSPA^TM can be used to monitor uptake of radiolabeled small molecules in living cells, suggesting that these particles could potentially be used as cellular or intracellular imaging probes.