Although optical absorption spectroscopy — a method for identifying molecules by the way they absorb light — is a common technique in chemistry and biology, it has low sensitivity and resolution. Now, researchers at the University of California, Berkeley in the US show that by conjugating biomolecules to metallic nanoparticles, the sensitivity of absorption spectroscopy is improved by several orders of magnitude.

Luke Lee and colleagues1 conjugated cytochrome c — a metalloprotein that acts as an energy acceptor and is found in the mitochondria of cells — to gold nanoparticles. Distinctive dips seen in the scattering spectra of these conjugates were explained by the transfer of plasmon resonance energy (PRET) — which comes from the oscillation of free-electrons near metal surfaces — from the gold nanoparticles to the hundreds of cytochrome c molecules adsorbed to them. Because the energy transfer is direct and the detection site is now reduced from a typical 1-cm cuvette in conventional absorption spectroscopy to a nanoparticle, PRET absorption can be used for ultrasensitive detection of molecules. Control experiments showed that for energy transfer to occur, the plasmon resonance peaks of the metallic nanoparticle and the electronic resonance peaks of the biomolecule must overlap, much like the process in fluorescence resonance energy transfer involving donor and acceptor fluorophores.

Using different metallic nanoparticles with plasmon resonance wavelengths that match a variety of biomolecules, this technique could be used in genetic analysis and pathogen detection with ultrahigh sensitivities.