Analytical devices built on paper could help doctors and environmental scientists detect chemicals of interest in the field at low cost. To improve the sensitivity of these devices, chemists want to make them compatible with techniques such as surface-enhanced Raman spectroscopy (SERS). A new star-shaped paper design allows scientists to separate chemicals from samples and then detect them at sub-attomolar concentrations using SERS (Anal. Chem., DOI: 10.1021/ac303567g).
In 2010, Srikanth Singamaneni, at Washington University in St. Louis, and his colleagues showed that SERS could work on a gold-infused paper device (ACS Appl. Mater. Interfaces, DOI: 10.1021/am1009875). But that square piece of paper couldn’t separate and concentrate chemicals of interest, which a diagnostic device must do to produce strong Raman signals with little background noise. To make a device that could separate chemical mixtures, Singamaneni’s team cut filter paper into eight-pointed stars—like cowboys’ spurs.
To analyze a sample, the researchers wet the entire device with a solution of gold nanorods, which help produce strong Raman signals. They also treat each finger on the star with a different solution of charged polymers. Then they spot a sample in the star’s center. Evaporation at the tips of the star’s fingers occurs faster than in the center, driving the nanorods and analytes towards the fingertips via capillary forces. Due to interactions with the charged polymers, individual chemicals in the sample travel at different speeds, leading to separations like those seen in thin-layer chromatography.
To detect chemicals, the researchers place the paper under a microscope attached to a Raman spectrometer and monitor Raman signals along each finger. They tested the device on several samples containing dye molecules, detecting one molecule, 2-napthalenethiol, at 500 zeptomolar.
The design could have medical and homeland security applications, detecting substances such as explosives or disease biomarkers, Singamaneni says.