They can determine in real time whether specific enzymes are activated or particular genes are expressed, all with unprecedented resolution within a single living cell. This could lead to a new era in molecular imaging with implications for cell-based drug discovery and biomedical diagnostics.
The researchers tackled this challenge by improving upon conventional optical absorption spectroscopy, a technique by which light is passed through a solution of molecules to determine which wavelengths are absorbed. Cytochrome c, for instance, is a protein involved in cell metabolism and cell death that has several optical absorption peaks of around 550 nanometers.
The researchers came up with a novel solution to this problem by coupling biomolecules, the protein cytochrome c in this study, with tiny particles of gold measuring 20-30 nanometers long. The electrons on the surface of metal particles such as gold and silver are known to oscillate at specific frequencies in response to light, a phenomenon known as plasmon resonance. The resonant frequencies of the gold nanoparticles are much easier to detect than the weak optical signals of cytochrome c, giving the researchers an easier target.
Gold nanoparticles were chosen because they have a plasmon resonance wavelength ranging from 530 to 580 nanometers, corresponding to the absorption peak of cytochrome c.
The researchers repeated the experiment matching the protein hemoglobin with silver nanoparticles and achieved similar results. "Our technique kills two birds with one stone," Lee said. "We're reducing the spatial resolution required to detect the molecule at the same time we're able to obtain chemical information about molecules while they are in a living cell. In a way, these gold particles are like 'nano-stars' because they illuminate the inner life of a cellular galaxy."