Light-induced forces push optically trapped nanoparticles and reveal molecules’ light absorption

Light absorbed by a single molecule can reveal information about its location or structure. But since the amount of light that a single molecule absorbs is too small to detect directly with current equipment, researchers determine absorption by monitoring a molecule’s response to absorbing light: A fluorescent probe releases light or light-induced forces push on a surface, for example.

A team of researchers reports a new way to measure light absorption by molecules: by detecting light-induced forces on a gold nanoshell suspended by optical trapping. The technique, described in The Journal of Chemical Physics, starts with a glass slide coated with a light-absorbing material — either a layer of fluorescent organic molecules or a film of semiconducting metallic nanocrystals. Next, the researchers float a glass nanoparticle, coated with a gold shell, in room temperature water and capture it inside a focused laser beam. Then they move the laser beam such that the optically trapped nanoparticle hovers above the light-absorbing surface. Finally, they shine pulses of visible light onto the surface.

When the molecules or crystals on the surface absorb the light, they generate thermal or electrostatic femtonewton scale forces that push the suspended nanoparticle inside the laser beam. The distance that the nanoparticle moves correlates to known absorption spectra of the organic molecules or nanocrystals.

The researchers currently measure forces produced by a collection of molecules, showing that the gold nanoshell is an efficient force-detecting probe and, they argue, suggests that the method could eventually be extended to measuring light absorbed by a single molecule, even one in solution.

Source: “Force-detected nanoscale absorption spectroscopy in water at room temperature using an optical trap,” by Alexander Parobek, Jacob W. Black, Maria Kamenetska, and Ziad Ganim, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5017853 .