Controlled release of USPIO micelles by environmental triggers. (A) Self-assembly of EcoRV-sensitive ssDNA-USPIO clusters, and subsequent enzymatic treatment results in measurable changes in R2 relaxation coefficient relative to initial values. Following EcoRV treatment, R2 values return to baseline, a phenomenon that is in significant contrast to the effects of EcoRI treatment of the same clusters (n = 6). * p < 0.05. (B) Thermocycling of DNA-crosslinked USPIO clusters results in measurable changes in R2 relaxation coefficient. Heating of the clusters melts the DNA and results in declustering of the particles, corresponding to an approximately 50% decrease in R2 coefficient. After allowing the system to cool, the R2 coefficient increases to original levels, suggesting the reclustering of the ssDNA-USPIOs (n = 3). * p < 0.05.