Paper-based metamaterial biosensor

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(Nanowerk Spotlight) Paper has emerged as a focus area for researchers developing innovative techniques for printed basic electronics components. Electronic paper displays are already a commercial reality and prototypes of things like paper batteries are under development. In these applications, researchers transfer thin-films, nanoparticles or other nanostructures onto the desired substrate via various processes (see "Direct-growth fabrication for paper-based electronics").

Another area where paper could lead to low-cost innovative devices and applications is lab-on-a-chip technology. Currently, these microfluidic devices are fairly expensive due to their lithography-based fabrication process with channels patterned in glass or plastic and tiny pumps and valves directing the flow of fluids.

Inexpensive paper-based sensing kits already play an important role in ready-to-use diagnostics. Researchers have even managed to create an inexpensive microfluidic platform on hydrophobic paper with laser treatments (see "New lab-on-chip advance uses low-cost, disposable paper strips").

In a further advance, scientists have now fabricated a paper-based metamaterial device which can be potentially utilized for quantitative analysis in biochemical sensing applications.

"When compared with lab-on-a-chip fabricated on conventional substrates, paper-based biosensors still need to improve in sensitivity and accuracy, in part due to the difficulty in obtaining high-resolution, small feature sizes – e.g., micrometers or less with sharp edges – on paper substrates where conventional photolithography techniques are difficult to apply," Fiorenzo Omenetto, professor of biomedical engineering at Tufts University School of Engineering, explains to Nanowerk.

In new work led by Hu (Tiger) Tao, a postdoctoral reseach associate in Omenetto's group, together with collaborators from Tufts and Boston University, have successfully interfaced metallic resonators with high resolution with paper. The team has reported their findings in a paper ("Metamaterials on Paper as a Sensing Platform") in the June 3, 2011, online edition of Advanced Materials.

This ability to simply pattern resonators on paper substrates brings together the versatility and potential sophistication of electromagnetic transduction with an abundantly available substrate such as a paper.

"Our device adds functionality to an approach that exists in practical diagnostics and has been reinvented, notably by Whitesides et al. ("Patterned Paper as a Platform for Inexpensive, Low-Volume, Portable Bioassays" and "Low-Cost Printing of Poly(dimethylsiloxane) Barriers To Define Microchannels in Paper") as a widely available lab-on-a-chip platform" says Omenetto.

While most paper-based biosensors – usually a strip of paper doped with an antibody specific to an antigen of interest – use colorimetric readout and detect the color or intensity change in the visible range, metamaterials offer a broader operating range, covering from radio frequency to optical wavelengths. These patterned papers offer more opportunities for multiplexed and quantitative analysis.

"In our device, paper acts as the dielectric substrate providing both support and a material to sample and embed analytes which then modulate the resonance of the split-ring resonators that compose the metamaterials," explains Omenetto. "This offers additional utility in the signal transduction capabilities and provides the possibility to explore label-free sensing strategies based on electromagnetic modulation."

In their work, the team led by Omenetto patterned paper substrates through selective deposition of a target material (gold) through a 500-nm-thick silicon nitride film microstencil-based shadow mask.

a) Schematic of the micrometer-sized metamaterial resonators sprayed on paper substrates with a predefined microstencil. b) Photograph of a paper-based terahertz metamaterial sample. c) Optical microscopy image of one portion of an as-fabricated paper metamaterial sample. (Reprinted with permission from Wiley-VCH Verlag)

After fabrication of the stencils, using surface micromachining technology, the entire patterning and deposition process on paper was conducted in a dry, chemical-free environment.

Tao describes the process: "Similarly to what we had previously done for silk ("Metamaterial Silk Composites at Terahertz Frequencies"), the microstencils were carefully attached to the paper substrates in contact mode. A thin layer of 150-nm-thick gold was then sprayed on the paper substrates using electron beam evaporation. Since the surface roughness of the paper substrate affects the pattern quality (including both the minimum transferable line width and sharpness), the researchers used photo paper with a surface roughness of less than 18 nm."

After fabricating their paper metamaterial, the team undertook a proof-of-concept demonstration that they would work as biosensors by coating the paper with glucose solutions of various concentrations. The solution was allowed to dry in air and the transmission spectra were then measured by THz-TDS as a function of frequency.

"With higher glucose concentration, the analyte-induced resonance should shift more since the shift is mainly due to alterations in the split ring resonator capacitance" says Tao (split ring resonators are the most commonly used elements to build MM structures and devices). "We were able to verify this by the experimental results. A resonance at 908 GHz was observed for the paper metamaterial sample without coating and this value shifted continuously to lower frequencies as the concentration of the glucose solution increased."

Potential applications of this work are label free sensors that can be widely manufactured or additional transducers to be interfaced on paper-based assays.

The team would now like to develop multifunctional devices, with multiple antennas and split-ring resonators of different kinds that can provide a multi-channel analysis of analytes that are deposited on the paper sheet and develop RF-based assays that are cheap, disposable and that minimize the use of chemicals for transduction.

By Michael Berger. Copyright © Nanowerk