Molecular Sieves For SERS

When analytical chemists want to detect small organic molecules in biological samples, they have to contend with proteins and nucleic acids. These large biomolecules often get in the way during analysis. Now, researchers report a thin-film material that acts as a molecular sieve, filtering out the large molecules and allowing small ones through for detection by surface-enhanced Raman spectroscopy (J. Phys. Chem. Lett. 2013, DOI: 10.1021/jz4014085).

Luis M. Liz-Marzán, a physical chemist at CIC biomaGUNE, a nonprofit research organization in Spain’s Basque Country, and his coworkers developed the material while working on a project to identify molecules that bacteria secrete to communicate with each other. They wanted to detect the small organic molecules using surface-enhanced Raman spectroscopy (SERS) because of the technique’s sensitivity. In a typical SERS experiment, researchers mix their samples with metallic nanoparticles so that the molecules adsorb to the particles. When excited by light, the molecules produce a characteristic Raman signal, which is amplified by the metal surface. Unfortunately, in biological fluids like those Liz-Marzán wanted to analyze, signals from large proteins and nucleic acids tend to drown out those given off by smaller molecules.

The team’s solution to this problem was to form a porous material around gold nanoparticles so that molecules applied to the top of the resulting thin film would have to seep through a network of narrow pores to reach the metal surfaces. They hoped that only the desired small molecules could squeeze through, while the large biomolecules stayed at the top of the film.

The researchers created these films using a process they had developed earlier (Nanoscale 2012, DOI: 10.1039/C2NR11547F). They started by chemically attaching gold nanoparticles onto glass slides and then layering a porous silica (SiO₂) or titania (TiO₂) film on top with a technique called spin-coating. By adding surfactants to the spin-coating solution, they could control the size of the pores. After fixing the material with heat, the researchers immersed the slides in a solution of chloroauric acid (HAuCl₄) to deposit additional gold onto the nanoparticles. The extra gold gave the particles sharp tips, which made them more efficient at amplifying the Raman signals.

To test the films, the team created a simplified biological fluid consisting of bovine serum albumin (BSA), a protein, and 4-nitrobenzenethiol, a small molecule. For the tests, they used a film with 6-nm-diameter pores, because BSA is about 7 nm in diameter. They applied their test fluid to the film and then shined laser light on it. Based on the resulting Raman spectra, they determined that they could detect 4-nitrobenzenethiol, but not the BSA. Liz-Marzán says his group is planning to test the films with blood plasma samples from nearby hospitals.

Tuan Vo-Dinh, a biophysical chemist at Duke University, says that although many techniques for screening out large molecules have been developed, these sieves could be used as an additional option. The method for preparing the films is quite elaborate, he says, so the researchers should find a way to clean and reuse them to make the materials more practical.