Metamaterial-enhanced vibrational absorption spectroscopy for the detection of protein molecules.

Tung S Bui,Tadaaki Nagao,Lam D Vu,Akihiko Ohi,Luu H Dang,Thang D Dao,Youngpak Lee,Toshihide Nabatame,Chung V Hoang

doi:10.1038/srep32123

Abstract

From visible to mid-infrared frequencies, molecular sensing has been a major successful application of plasmonics because of the enormous enhancement of the surface electromagnetic nearfield associated with the induced collective motion of surface free carriers excited by the probe light. However, in the lower-energy terahertz (THz) region, sensing by detecting molecular vibrations is still challenging because of low sensitivity, complicated spectral features, and relatively little accumulated knowledge of molecules. Here, we report the use of a micron-scale thin-slab metamaterial (MM) architecture, which functions as an amplifier for enhancing the absorption signal of the THz vibration of an ultrathin adsorbed layer of large organic molecules. We examined bovine serum albumin (BSA) as a prototype large protein molecule and Rhodamine 6G (Rh6G) and 3,3′-diethylthiatricarbocyanine iodide (DTTCI) as examples of small molecules. Among them, our MM significantly magnified only the signal strength of bulky BSA. On the other hand, DTTCI and Rh6G are inactive, as they lack low-frequency vibrational modes in this frequency region. The results obtained here clearly demonstrate the promise of MM-enhanced absorption spectroscopy in the THz region for detection and structural monitoring of large biomolecules such as proteins or pathogenic enzymes.

Highlights

IntroductionBecause the presence of trace molecules deposited on the SRR induces a change in the resonant frequency, the quantity of deposited molecules can be sensitively detected
Frequency can be well described by an equivalent LC circuit
An ultrathin layer of bovine serum albumin (BSA) molecules deposited on the MM is detected with a signal strength almost comparable to those of submicron-thick bulky materials owing to the strong field enhancement of the resonant MM

Summary

Introduction

Because the presence of trace molecules deposited on the SRR induces a change in the resonant frequency, the quantity of deposited molecules can be sensitively detected. This method obviously offers a sensitive approach to molecular detection, but it does not offer high selectivity for organic molecules because the dynamic range of the variation in their dielectric constant is narrow (approximately 3–4), which limits application of the method. For small organic molecules of other materials that do not exhibit any distinct signals in the THz region, no enhanced signals are obtained This clearly shows the selectivity of this method for the detection of large protein molecules, the vibrational fingerprints of which appear mainly in the THz region.

Methods

Results

Conclusion