We present a disposable low cost paper-based metamaterial for sensing liquids based on their dielectric properties. The sensor is based on resonance shift due to the change in the effective capacitance of each resonator in the metamaterial array. Key novelty in the design is the implementation of metamaterial on low cost and ubiquitous paper substrate. This metamaterial-on-paper sensor is fabricated in a totally cleanroom-free process using wax printing and screen printing. Wax patterning of paper enables creation of microfluidic channels such that liquid analytes can be delivered to each metamaterial unit cell for sensing. Screen printing is used to implement disc shaped resonator unit cells. We demonstrate sensing of liquids: Oil, methanol, glycerol and water each showing an average resonance frequency shift of 1.12 (9.6%), 4.12 (35.4%), 8.76 (75.3%) and 11.63 GHz (100%) around the center frequency of around 94 GHz respectively. Being label-free, this approach can be expanded to sense other liquids based on their dielectric constants.
© 2017 Optical Society of America
Terahertz electromagnetic waves are non-ionizing, non-hazardous and therefore suitable for label-free detection of chemicals based purely on their unique THz and far-infrared (IR) resonance frequencies in their absorption spectra. This makes terahertz technology particularly useful for safety and surveillance applications such as concealed weapon detection . In applications related to detection of chemical explosives, it is desirable to use terahertz techniques in free-space for stand-off measurements. However, in spite of the promise, terahertz science and measurement is plagued by lack of powerful sources, detectors and other useful components to make practical devices, a well-known attribute commonly known as a terahertz gap. Metamaterials have recently shown promise in the realization of various components to fill this terahertz gap [2,3]. Victor Veselago introduced the metamaterial concept in 1968 , which are mesoscale structures created bottom up from an array of sub wavelength resonators where their combined EM properties such as permittivity and permeability depends not on the chemical composition but on the geometry and the size of the unit cells in the array. Metamaterials showed properties that were not found in nature. As an example, metamaterials have been shown to provide negative permittivity or permeability which can be used for invisibility or cloaking , amplitude and phase modulation at terahertz and IR frequencies [6,7], even serve as strain or rotation sensor by monitoring frequency shift due to mechanical movements [8,9]. There has also been an effort to use emerging materials with interesting physical properties like graphene and vanadium dioxide for active metamaterials [10–12], where the properties of the metamaterials can be tuned in time and space.
One area where metamaterials can show great promise is that of sensing. Metamaterial-inspired sensors have already been investigated by many groups [13–22]. These sensors show high sensitivity and robustness that can be used for unidentified chemical detection based on their dielectric properties. Even biosensing for identifying molecular protein monolayers with capability of measuring their thickness has been demonstrated using metamaterials . However, most approaches towards the fabrication of these metamaterial-based sensors are complex, needing high-tech instruments in expensive cleanroom environment. So the goal must be to reduce the complexity and fabrication cost of metamaterials for such applications. To address this concern, we explored the use of cellulosic paper as a sensing substrate. Paper is ubiquitous, cheap and flexible; it has been engineered for many interesting applications in low cost diagnostics. There also has been some work in using paper substrate to realize electronic [24,25] and optical devices . Although paper-based metamaterial has been proposed in  it still relied on cleanroom processing for its fabrication in order to achieve feature sizes down to few microns. Also previously proposed sensors are mostly designed to be used in waveguide configuration and do not operate in free space. Therefore, despite all the progress in the field of metamaterials, their mass production is still a challenge, and most of the demonstrations of metamaterial-based devices have been limited to laboratory level.
In this paper, we present a metamaterial-on-paper for monitoring liquids based on their dielectric constants. We propose a much simpler process towards fabrication of metamaterials on paper substrate using ambient processing without the need for expensive equipment or facilities. Our metamaterials are fabricated on chromatography paper using screen printing. The fabricated metamaterials have resonant frequency in the transmission spectrum, which is sensitive to the local dielectric properties of each unit cell. When liquid is added to the capacitive gap of the resonators in the metamaterial, the resonance frequency shifts proportional to its dielectric permittivity. The amount of shift can vary based on the dielectric constant of the added liquid in the capacitive gap. Figure 1 explains this concept of using metamaterials for sensing different liquid analytes by their effect on the resonance frequency of metamaterial.
The sensing liquid analytes however need to be delivered accurately to each resonator unit cell in the metamaterial to maximize electromagnetic coupling. This is achieved by creating microfluidic channels so that liquid analyte can be directed via capillary action towards the sensitive capacitive gap of the resonators. No special pumps or valves are needed to regulate the flow. Figure 1(a) shows conceptual metamaterial on paper sensor based on simple split-ring resonator or SRR  architecture with the analyte loaded in its split gap. First order equivalent circuit model is shown in Fig. 1(b) resembles an LC resonator, where the liquid analyte is shown in the capacitive gap of each metamaterial unit cell. Having different liquid analytes in the gap shifts the resonance frequency based on their individual permittivities. To the first order the resonance frequency is given by , where C is the effective capacitance proportional to the permittivity of liquid analyte (C α εL, where εL is the liquid perimittivity). The choice of metamaterials can influence whether the sensor responds to electric or magnetic fields or both, and at what center frequency one expects to evaluate the resonance shift to be at. There are much different architecture for metamaterials  that can be used such as helix , dumbbell and circular SRRs . We chose to have disk-shaped metamaterials with the target analyte provided via microfluidic channels implemented on the same paper substrate in between the resonators as shown in Fig. 2(c). The advantage of choosing disk-shaped resonator [Fig. 1(c)] is the tighter fabrication tolerances for implementing continuous circular structures using screen printing compared to geometries that have sharp corners such as SRRs. Moreover there is inherent simplicity in implementing a continuous microfluidic channel to deliver the analyte to each resonators. Other designs such as using SRR [Fig. 1(a)] will have to explore complex 3D microfluidic channels to deliver analytes to the capacitive gaps of metamaterial.
2. Design and fabrication
Figure 2 shows the ambient room temperature process of fabricating metamaterial on paper substrate. The paper chosen as a substrate is Whatman TM chromatography paper which has been wax printed by Xerox® ColorQube 8580, making wax-printed areas hydrophobic and the rest hydrophilic.
After wax printing, the substrate was heated so that wax can penetrate to the other side of paper, creating an impenetrable zone for any liquids. Such wax based patterning of paper enables creation of microfluidic channels through the non-wax regions that are hydrophilic causing liquid analytes to be routed using natural capillary actions to any desired regions on the substrate, which in our case would be the capacitive gaps of each resonator in the metamaterial array. Although wax does not distribute homogenously on its way to the other side of the paper, it is sufficient to create microfluidic channels of a few hundred micrometers width to enable liquid flow [Fig. 2(f)]. A stencil mask with periodic design of circles corresponding to the resonator unit cells, were cut on a polyimide tape of Nulink by using CO2 cutter Boss LS-1416 with speed of 5 mm/s and power with 10% of cutter’s maximum power. It is then adhered on to the substrate paper and aligned onto the waxed region of the paper next to the microfluidic channels but not on top of them. The alignment of polyimide sheet and paper might have been a challenge in metamaterials with lower unit cell dimensions. However the unit cells had the dimensions of 3 × 3 mm2 to avoid alignment problems. After adhering polyimide sheet on wax-printed paper, the whole space was painted by silver ink using AG-510 Silver conductive ink supplied by Applied Ink Solutions. After painting on the whole polyimide tape we used a tapered blade stainless steel scraper knife with metal end to smoothen the surface. By doing this the silver disks will be of the same height as polyimide tape (100μm). Later on, we verified it by measuring numerous amount of disks thickness. Then, polyimide sheet was peeled off the paper leaving behind circular shapes of conductive silver parts on waxed areas, which serves as our disk resonators. The resultant prototype is shown in Fig. 2(e) schematically. The unit cells of the metamaterial is shown in Fig. 2(g), with dimensions listed. In reality, there is a variation in the actual dimensions and not all of those disks are identical, an issue discussed in detail in the result section.
3. Results and discussion
3.1 Theory vs. experiment
We used CST Microwave Studio® to simulate performance of the device using FDTD. On the other hand we extracted experimental data using a continuous wave terahertz spectrometer  with set up specifications discussed in detail in [6, 31]. Transmission spectrum in both theoretical and experimental setup are presented in Fig. 3(a) (Experimental transmission spectrum is normalized due to signal loss in experimental setup using CW THz spectrometer). One should note that the conductivity of silver paste used in the simulation is lower than bulk silver. This resonator shows resonance frequency at 94 GHz with electric field distribution shown in Fig. 3(b) and surface current density shown in Fig. 3(c).The experimental transmission spectrum is drastically broadened around resonance frequency comparing to the theoretical one, this could be predicted due to relaxed tolerances of the screen printing based fabrication process which leads to variability in the size of disk resonantors. Figure 4(a) inset shows the inhomogeneity of conductive parts and justifies the broadening of experimental transmission spectrum.
Having imperfect shapes and sizes, not only has the broadening effect but also affects its loss behavior. Painted silver disks would definitely be larger than actual size due to diffusion of silver, thickness of silver-painted circles is another variable which would alter in each unit cell. All of these conditions would lead to more loss in reality. The device is fabricated with the dimensions of 11.5 × 11.5 cm2 shown in Fig. 4(a). Microfluidic Channels would also differ in width since the waxed paper is heated non-uniformly causing non-uniform spreading. We tested microfluidic channels for their reliability by flowing food dye through these microfluidic channels in Fig. 4(b) for easier visualization.
3.2 Experimental setup
The schematic figure of our setup is shown in Fig. 5. We characterized transmission spectrum using continuous wave (cw) THz spectroscopy system, TeraScan 1550 by sweeping between 50 and 150 GHz with spectral resolution of 200 MHz. Depending on the integration time, it would take longer or shorter to scan. We chose 9.83 ms as integration time. For higher frequencies we need to choose higher integration time since the setup exhibits a lower SNR at higher frequencies so the integration time is increased to 671.08 ms. The overall scan takes about 7 minutes. Fiber-coupled photo-conductive antenna generates the linearly polarized THz wave then it gets collimated by 76.2 mm diameter 90° off-axis parabolic mirror (focal length of 152.2 mm). Next, the continuous wave THz passes through the sensor and focused by another mirror on a photo-conductive mixer antenna as receiver. The sensor is oriented in a way that microfluidic channels are perpendicular to the E field direction. The detected THz photocurrent is amplified using a programmable gain amplifier module. Extraction the THz photocurrent of the sensor and comparing it with the THz photocurrent of air would give us the transmission spectrum of our device.
3.3 Experimental results
We tried different liquid analytes with a wide range of permittivity for sensing, including oil, methanol, Glycerol and water with relative permittivity of 3.1, 33.1, 57 and 80.4 respectively. The Glycerol was supplied by Aldon Corporation. Figure 6 shows the experimental results of three different samples using four different liquid analytes. Needless to say that different samples may have subtle dissimilarities in transmission spectrum due to the issues discussed in section 3.1.
The transmission spectrums in Fig. 6 show the resonance frequency shift while using the above-mentioned chemicals for sensing. Overall, 10 samples for each analyte were tested but plots show three out of 10 response for each analyte in Fig. 6 for clarity. As it appears on Fig. 6(a) the resonant frequency shift is so small owing to the low dielectric constant of oil comparing with methanol or water. Since oil gets absorbed by waxed zones, ordinary printing paper was used with thickness of 110 micrometer. Figure 7 summarizes the frequency shift for the four analytes. For error bar, we included the results of all 10 samples for each analyte in Fig. 7.
Transmission spectrum of the metamaterial shows drastic change of 11 GHz when water flows through microfluidic channels. By applying this method we could distinguish the analytes by the shift imposed on the resonance frequency. However this method is not an accurate way to ascertain analytes which have similar dielectric constants. It is noticeable that the water spectrum is more broadened than for oil and glycerol owing to more lossy nature of water at 94 GHz. The sensor is planned for single use as it is expected to foul or deform based on the analyte used for sensing.
As shown in Fig. 7, resonance frequency shift has a linear trend since dielectric constant of the chemical rises. Glycerol has less variability in resonance frequency shift due to its low viscosity which causes less structural deformation of the paper. Variability of resonance frequency shift is shown for four different analytes including oil, methanol, Glycerol and water with relative permittivity of 3.1, 33.1, 57 and 80.4 respectively. The samples showed resonance frequency between 91 and 96 GHz and the mean value of resonance frequency shift for oil, methanol, glycerol and water were 1.12, 4.12, 8.76 and 11.63 respectively showing an average shift of 0.138 GHz per unit of relative permittivity. In terms of figure-of-merit (FOM) which can be expressed as , with the frequency shift for unit of relative permittivity and to be the linewidth we have 0.01442 as figure-of-merit for our sensor. The values of permittivity shown in the figure are known low frequency permittivity of analytes. It should be noted here that high frequency permittivity of these analytes will be different than the values reported in the Fig. 7. For example, the complex permittivity for methanol and water are ε* = 5.78-0.80j and ε* = 7.98-13.06j respectively at 94 GHz. The values for oil and glycerol are not known however but is expected to result in the response trend shown in Fig. 7 at 94 GHz.
We demonstrated a low-cost metamaterial on paper substrate for detection of liquids with different dielectric constants in the microwave and terahertz spectrum. The platform used natural capillary action of the paper substrate to route analytes to the individual resonators in the metamaterial array. The metamaterial resonance depended strongly on the dielectric properties of the liquid in this channel. A continuous wave terahertz spectrometer was used to obtain the transmission spectrum from the metamaterial sensor. Results were presented for oil, glycerol, methanol and water with very linear response in frequency shift as a function of dielectric constant of the liquid. The platform is promising for detection of dielectric analytes based on their inherent dielectric permittivity. The paper provides a low cost substrate while the fabrication approach using screen and wax printing enables easy fabrication of these sensors, making this highly suitable for routine sensing applications.
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