Novel 2D spacer materials for surface plasmon coupled emission sensing

(Nanowerk Spotlight) Inexpensive and rapid handheld sensors for biomarkers can revolutionize healthcare, especially in resource-limited settings widespread in low and middle-income countries. The challenge in fabricating these biosensors is to detect weak emission signals when the biomarkers are at very low concentrations, as in the early stages of the disease.

Nanomaterials like graphene and fullerenes provide an excellent platform to enhance weak signals from biomarkers (see for instance: "Smartphone-based nano-biosensors for early detection of tuberculosis").

While graphene and fullerenes perform very well for detecting isolated biomarkers, their ability to amplify emission of biomarkers in a real physiological milieu is limited due to their strong interactions with other biomolecules such as proteins and lipids.

A team of researchers from Clemson University and Sri Sathya Sai Institute of Higher Learning collaboratively developed new sensing platforms that use two-dimensional materials beyond graphene.

They reported their findings in the April 4, 2016 online edition of ACS Sensors ("MoS 2 /WS 2 /BN-Silver Thin-Film Hybrid Architectures Displaying Enhanced Fluorescence via Surface Plasmon Coupled Emission for Sensing Applications").

Similar to the fullerene-based sensors, these inexpensive biosensors are fabricated by coating thin-films of silver with an overcoat of two-dimensional (2D) spacer layers including MoS 2 , WS 2 , and BN.

In these sensors, the isotropic fluorescence from dye-stained biomarkes couples with plasmons from the silver film through the overcoated spacer layers.

Silver thin-films overcoated with MoS 2 can enhance the emission from dyes such as Rhodamine by 17 times and thus improve their sensitivity at low concentrations. (Image: Pradyumna Mulpur)

"Metallic thin-films are double-edged swords that both simultaneously enhance and quench the biomarker emission," says Pradyumna Mulpur, a graduate student at Sri Sathya Sai Institute of Higher Learning and first author of this paper. "We previously found that nanocarbons are good spacer layers in addition to protecting the silver film from oxidation."

He notes that, despite their great potential, facile synthesis, and high stability, nanocarbons interact very strongly with a variety of biomolecules through delocalized pi-electron cloud.

"This limits their ability to enhance weak emissions from the desired biomarkers in real blood samples, which contain thousands of proteins and lipids (see for instance: "A nanosensor platform for direct detection of a cancer biomarker in blood").

The emergence of 2D materials beyond graphene presents new opportunities to overcome challenges posed by nanocarbons by selectively interacting with desired biomarkers.

"Following the realization of graphene, we were able to chemically exfoliate and isolate thin layers of other materials such as MoS 2 and WS 2 ," Ramakrishna Podila, an Assistant Professor in the Department of Physics and Astronomy at Clemson University and principal investigator of this study, tells Nanowerk. "Unlike graphene, MoS 2 and WS 2 do not exhibit strong π-interactions with biomolecules. We used this to our advantage to show that emission of dyes coupled to biomarkers as in the traditional ELISA tests can be enhanced by ∼20 times, even with MoS 2 and WS 2 ."