Fluorescent carbon nanoparticles known as carbon dots could be a low-cost, biologically friendly alternative to quantum dots for sensing chemicals inside cells. Until now, carbon dots typically have glowed in the blue to green range. But researchers have now made carbon dots that emit bright light across the visible spectrum depending on the wavelength of light that they absorb (Chem. Mater. 2014, DOI: 10.1021/cm5003669).
Minjie Li, Sean Xiao-An Zhang, and their colleagues at Jilin University, in China, used the multicolor dots to measure pH levels in solution and inside cells. The results indicate that the dots could be efficient chemical sensors for use in live cells, the researchers say.
Scientists believe that carbon dots would be better suited for biological applications than quantum dots because the carbon particles are more water soluble and don’t contain toxic heavy metals such as cadmium or lead. To get different colors with carbon dots, researchers usually tweak the particles’ sizes, crystal structures, and surface properties. But, Li says, these features also change important properties such as the dots’ color intensity, solubility, and the rate they are taken up by cells. Changing those properties can affect “the validity and reproducibility of the data obtained during their applications,” she says.
To avoid the complications of using different-sized particles, the Jilin team wanted to get bright, high-intensity, multicolor emission from a single dot. They came up with an easy, one-step method to make both blue and multicolor carbon dots. They heat a mixture of chloroform and diethylamine and separate out the nanoparticles formed as a result. Heating the mixture for one hour gives 1- to 3-nm-diameter blue dots while heating it for 60 hours yields 2- to 4-nm-diameter multicolor dots. The multicolor particles emit light across the visible spectrum: They emit blue light at 470 nm when excited by 390 nm light, and glow red at 670 nm when they absorb at 650 nm.
The researchers analyzed and compared the chemical composition of both the blue and multicolor dots using Fourier transform infrared spectroscopy. They found that the surfaces of the multicolor ones carry a large number of functional groups containing C-O and C-N double bonds. When the researchers removed these groups using sodium borohydride, the intensity of emitted light in the green to red range decreased. This suggested that the functional groups are responsible for the particles’ ability to emit across the visible spectrum.
The team made pH sensors with the multicolor dots by adding a pH-sensitive dye, and then used the particle-dye combinations to measure pH levels in solutions and in human cancer cells.
“This work is really impressive,” says Su Chen, a chemist at Nanjing Tech University. The mild one-pot process to synthesize the dots should make it accessible and easy to scale to large quantities.