 Fluorescent nanoparticles light up the otherwise invisible processes that make our biology tick, or, in the case of diseases like cancer, that make a go rye. Some of the most powerful particles of this type are so-called carbon dots. They tend to be easier to fabricate and more compatible with our biological makeup than other similar nanoparticles used by scientists. Unfortunately, they also tend to be fairly dim, emitting relatively few photons versus the amount they absorb. Now, researchers report a new method that vastly improves the quantum yield of carbon dots, reaching a high efficiency of 94.5%. What's more, the bright dots intrinsically target certain cancer cells, paving the way toward better cancer diagnostics. The team from China formed the particles through a simple one-step method involving a single ingredient, folic acid. A B vitamin that helps regenerate healthy cells, folic acid is one way to track some cancer cells in the body. The receptor that locks onto folic acid is known to abundantly scatter on the surface of tumors associated with ovarian, breast, and lung cancers. Heating a well-dispersed solution of folic acid in water through a process known as carbonization yielded particles of carbon measuring about 5 nanometers on average, each sprinkled with folic acid residues on its surface. Tests showed that the carbon dots emitted strong violet light when pumped with higher energy light, all with extremely high efficiency. The quantum yield of 94.5% is much higher than that reported for similar carbon dots containing nitrogen, phosphorus, or sulfur. The team attributes the high quantum yield to the surface chemistry afforded by folic acid. The amino groups therein help boost the probability that electrons are excited by high energy light. As more electrons get excited, more of them relax to their lower energy ground state. That relaxation translates directly into strong sustained light emission. In fact, the dots were able to retain more than 90% of their initial intensity even after being pumped for more than two and a half hours and remained bright even after five weeks of storage. When incubated with human cells over-expressing folic acid receptor, models of certain cancers, the dots showed bright fluorescence as soon as within one hour. Comparison with cells not expressing the receptor highlighted the dot's strong target ability. This special ability makes the novel carbon dots extremely attractive for imaging applications in biological and biomedical studies, providing researchers a new tool for potentially scanning for cancer cells.