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We have demonstrated a binary chemoselective gas sensor using a combination of plasmonic nanohole arrays and a voltage-directed assembly of diazonium chemistry. The employment of a voltage-directed functionalization allows for the realization of a multiplexed sensor. The device was read optically and was fabricated using a combination of electron-beam and conventional lithography; it contains several regions each electrically isolated from each other. We used calibrated gas dosage delivery to confirm the selectivity of the sensor and observed reversible spectral shifts of several nm upon gas exposure. The resulting spectral shift indicates the potential for use in chemical arrayed detection for low concentration gas sensing
©2012 Optical Society of America
1. Introduction
The choices for making small and compact chemical sensors based on optical approaches are limited. Since most approaches require significant optical interaction length (centimeters or even meters) this usually implies macroscopic-sized sensors. Using resonant structures, a sensor can be made compact since the effective interaction length can be many times larger than the physical dimension. Plasmonic structures have been traditionally used for chemical and biological sensing since plasmonic resonances are sensitive to small dielectric changes in the vicinity of the metal layers. These changes can be initiated by the adsorbing of an analyte to the metal surface. This sensing modality is used in a number of commercial surface plasmon resonance (SPR) sensors [1,2]. Many variants of SPR sensing have been devised; especially those that take advantage of nanofabrication [3–5]. One particular and recent variant of SPR sensing is the use of plasmonic resonances caused by nanohole arrays in thin films of noble metals.
When periodic nanohole arrays of subwavelength diameter and spacings are milled into thin layers of metals (typically Au or Ag), a number of transmission resonances are observed [6–8]. At these transmission peaks, a disproportionate amount of light is transmitted through these holes by coupling to surface plasmons (this has been termed “extraordinary optical transmission (EOT)” [6]). This transmission is sensitive to the microscopic permittivity in the vicinity of the nanoholes which can be modified using different surface chemistries and chemical binding.
To date, nanohole arrays have been used for sensing in liquid environments. For example, Tetz et al [9] have used nanohole arrays with resonances at visible wavelengths to detect changes in refractive index as low as 10−5. The sensitivity of these sensors to the refractive index in the vicinity of the nanohole array has been well characterized in previous publications using liquid samples [4,10–12]. To our knowledge, this type of plasmonic sensor has not previously been used to detect gaseous compounds. The amount of dielectric change that these molecules can cause is small, since gas molecules are relatively small and dilute.
In some sensing regimes it is sufficient to use a binary sensor, meaning a sensor that reports a positive detection once a threshold is met. This is in contrast to sensors that are designed as micro/nano balances which are meant to quantify the amount of material present. In this work we use a combination of the well described nanohole array sensor and a voltage-directed, diazonium salt chemistry as a path for a multiplexed binary gas sensor. One major advantage of voltage-directed assembly techniques is that the assembled surface chemistry can be localized to individual sensing elements by proper electrical isolation. By using a series of specific molecules designed to ligate desired analytes it is possible to fabricate an array of sensing elements designed for multiple chemistries on a single compact device. A schematic of the sensor array and the voltage-directed assembly can be seen in Fig. 1 . Each sensor element has a different molecule assembled to the surface creating a complex multiplexed sensor.
Fig. 1 (a) Schematic representation of the different steps for achieving a multiplexed chemoseletive sensor array. Each sensor area (A, B, N) has a nanohole array region and is attached to an electrical contact for the purpose of providing an electrical bias for a voltage-directed assembly process. (b) Assembly process of a series of chemoselective compounds to construct a multiplexed sensor array. Chemoselectivity is given to an individual sensing area using a voltage-directed assembly technique. Since the assembly occurs only in the presence of an applied voltage, separate sensors can be given different chemistries in subsequent steps.
2. Plasmonic sensor
Our chemical functionalization approach is compatible with almost any plasmonic sensor concept provided that there is a continuous conducting path between all of the resonators. The simplest plasmonic sensor that fulfills this requirement is the “Nanohole array”. This type of plasmonic structure can be optimized to achieve high optical transmission (EOT) while providing sufficient sensitivity through a measurable shift in the spectra of its resonances. There are a number of competing explanations for the resonances observed in transmission through a nanohole array [13] but here we will use the formalism presented in the original paper by Ebbesen [6]. In the case when the aperture area is small compared to the period, the following simple expression gives the relationship between the periodicity of the hole arrays and the plasmonic resonances responsible for EOT:
where εeff is the effective dielectric constant at the metal-dielectric interface and εm is the dielectric constant of the metal. This formula also conveys the difficulty in using plasmonic resonances for gas sensing: these resonances depend on εeff which normally would have a spatial extent of roughly the evanescent mode of the plasmonic modes. Since gas molecules are typically small and dilute, they will only induce a small change in εeff only over a layer thickness not greater than a monolayer or two. This is depicted schematically in Fig. 2(a) .Fig. 2 (a) Schematic representation of the difficulty of gas sensing using plasmonics. A few gas molecules are adsorbed near the vicinity of the nanohole causing a change in the dielectric environment. This change is minute due to the low density of adsorbed molecules. Furthermore, flexible gas chemoselectivity is quite challenging. (b) Simulated (blue-solid) and measured (dotted-red) transmission through a nano-scale perforated gold film. The simulation results were obtained using a diameter of 200nm for circular holes separated by a pitch of 400nm, in a 50nm thick gold film on a silica substrate.
Using Eq. (1) we can estimate the periodicity range necessary for a certain wavelength regime of operation. A spectral window of 400-800nm (chosen to match the responsivity of inexpensive Silicon photodiodes) requires a periodicity of the hole array in the range of 250-400nm. This simple expression provides an approximate wavelength of the plasmonic peaks or valleys measured in transmission [14]; but it does not give us any information about the peak transmission magnitude or spectral linewidth and shape of the plasmonic resonance peak which in addition to the periodicity, these characteristics depend on the hole shape and size. To predict these characteristics the transmission hole-arrays were modeled using finite-difference time-domain (FDTD) [15]. Figure 2(b) shows the modeled and measured transmission spectra as a function of wavelength for a square array of circular apertures, with a lattice constant of 400nm and a fixed aperture radius of 200nm.
3. Fabrication
3.1 Nanohole array fabrication
The multiplexed sensor was fabricated using a combination of optical lithography for the voltage-directed chemistry and nanolithography for the nanohole arrays. The nanohole array sections were fabricated using electron beam lithography (JEOL 9300FS). A layer of ~300nm of polymethyl methacrylate (PMMA - positive e-beam resist) was spin coated onto a ~150 μm glass cover slip. The thickness of the PMMA layer was controlled via dilution with cholorobenzene and spin speed, enabling wider e-beam dose latitude, and better control of the nanohole shape and size. Several devices were fabricated with small exposure dose variations to optimize the nanohole diameters. The nanohole arrays were ultimately fabricated by a lift-off process using the patterned PMMA. Figure 3(a) shows a micrograph of the multiplexed array and Fig. 3(b) shows a typical region of nanohole arrays. The positive pattern also left the device regions without the nanohole array clear of gold film to enable subsequent patterning of contact pads (by photolithography) for the electrical biasing meant to assemble the diazonium molecules.
Fig. 3 (a) Microscope image of two sensor elements each composed of multiple nanohole arrays fabricated using PMMA with a complimentary pattern. The two sensor arrays are electrically isolated and both are connected to a gold contact pad (not shown) by 10µm wide bus lines. This pad was used for the electrical bias necessary to perform the voltage-driven assembly process of the surface functionalization. (b) SEM image showing the dimensions of the perforation in the gold film. The nanoholes are 200nm in diameter and are separated by 400nm in a square lattice.
3.2 Voltage-directed surface chemistry
In order to impart chemoselectivity to our nanohole arrays we used a directed assembly technique to reductively assemble diazonium molecules designed to interact with specific classes of compounds [16–20]. A phenylene ethyneylene diazonium salt capable of hydrogen bonding was synthesized to impart the desired chemoselectivity for hydrogen bond accepting analytes such as those containing C-O, S-O, or P-O double bonds. The assembly of the protected diazonium salt was directed to regions with a negative bias large enough to reduce diazonium salt. The reduction process resulted in the formation of nitrogen and aryl radicals. The aryl radicals form a covalent bond with the biased metal. The hydrogen bonding phenol was protected during the synthesis with a BOC group. After assembly, the BOC protecting group was removed with triflouroacetic acid to unmask the hydrogen bonding portion of the molecule (Fig. 4 ).
Fig. 4 Schematic representation of the (a) assembled chemoselective compound, (b) the assembled chemoselective compound following deprotection, and (c) the detection process.
Assembling the sensor in this way provides a scalable technique to further multiplexed sensing arrays since the assembly occurs only in the regions with an applied bias. Often sensors need to be operated in non-laboratory environments and as a result many sensor interferents are often present. By imparting chemoselectivity with the use of voltage directed diazonium chemistry to the nanohole sensor array, the sensor becomes less sensitive to undesired analytes such as diesel fuel. The usage of diazonium surface functionalization has been demonstrated previously for multiplexed sensor applications using Au [16–21].
4. Experiment
As seen in Fig. 5 , a simple apparatus was used to selectively illuminate and collect light from a single sensor array. This was done by producing a small spot using a high magnification objective on the illumination side of the fabricated sample. A high magnification objective in conjunction with fiber optics was also used on the collection side to further limit the sampling area. The alignment was simplified by the use of beam splitters on either side of the objective paired to CCD cameras allowing real time imaging of both surfaces of the sample. This light was then focused into an optical fiber that lead to a spectrometer with a CCD camera to analyze the optical transmission.
Fig. 5 Schematic Representation of the gas testing assembly. A hydrogen carrier gas is used in a bubbler to transport the test molecule to the sensor array and is analyzed downstream by an HP gas chromatographer.
A gas chromatography system was used to create a vapor stream of dimethylmethyl phosphonate to quantifiably expose the sensor surface to a gaseous analyte. A schematic of the delivery system is shown in Fig. 5. A binary sensor was used, where one of the two elements was functionalized with the diazonium chemistry; the other element was used as a control. A constant flow rate of hydrogen was used as the carrier gas. A bubbler with the analyte was connected to the nanohole array test fixture using a capillary tubing 1m long and with 0.25mm internal diameter. The nanohole array was exposed for a total of 15 minutes and a 3nm shift in the transmission peak was observed, as can be seen in Fig. 6 . The experiments were repeated multiple times and the spectral shift was observable only on the functionalized sensor element. The amount of gaseous analyte delivered was quantified with the use of a vapor trap containing the adsorbant Tenex. The Tenex filled tube was loaded for 15 minutes at the same flow rate as the sensor was exposed. Then the Tenex filled tube was heated up to thermally desorb the analyte which was analyzed using mass spectroscopy. The molecular flux of vapor analyte was determined to be 374ng/min when compared to a calibration curve.
Fig. 6 Experimental results from exposing the nanohole array sensor to a dilute concentration of the test molecule. The initial resonance shift (left-hand panel) is caused, at 15 minutes of flowing the test molecule, by the selective adsorption of the analyte and the following resonance shift (middle panel), at 20 minutes, is due to desorption of the molecule. After desorption the resonance returns to the initial condition. The shift from the exposure to the analyte (right-hand panel) is compared to the lack of a shift caused by exposure to the carrier gas.
From past work using this surface chemistry, we know that the maximum density of the assembled compound corresponds to a closely packed monolayer of the diazonium molecules. The sensor area was a 100x100µm square. It follows that the maximum weight of the bound material would be ~0.5ng. This is also consistent with estimates using the dead volume of the flow cell. In the absence of the analyte the bound molecules desorbed over a period of 20 minutes and the resonance returned to the initial pre-exposure wavelength. Subsequent experiments on the same sensor element exhibited the same sensing behavior. Included on the optical device was a sensor region that had not been biased during the assembly process. Since the surface of this element was devoid of the functionalizing chemistry there would be no selective binding locally, meaning the element would behave as a control. The control region showed no resolvable shift during the gas exposure. As a second control, hydrogen gas was used in the bubbler without DMMP present; the result of this experiment is shown in the right-hand column of Fig. 6. Our previous studies focused on the sensing of vapor phase compounds and liquid analytes, using similar surface chemistries [16–21] but on different sensor platforms. There we addressed the issues of multiplexing with and without interferants. Here we utilize the same surface chemistry but on a nanoplasmonic sensor platform.
These initial results are very promising for future devices and indicate a new generation of gas sensors based on plasmonic nanohole arrays. It is the unique combination of the sensor array and voltage-directed assembly technique that enables the ability to detect very small gas molecules in a dilute concentration. It is possible to use different plasmonic structures [10,11] to achieve narrower resonances which would improve the sensitivity of the device.
5. Conclusion
In this work we used a combination of plasmonic nanohole array sensors and diazonium salt chemistry as a path for a small, multiplexed binary gas sensor. Utilizing voltage-directed assembly, an arrayed detector of discriminating elements was realized using onium salts. A dilute concentration of gas phase analyte was bound to the surface by the diazonium surface chemistry and caused a detectable shift of 3nm in the transmission resonance when compared to a control. After the exposure, the transmission resonance returned to the initial wavelength indicating that the analyte molecules had desorbed from the surface. This important finding paves a path for reusable multiplexed gas sensors using plasmonic nanohole arrays and voltage-directed surface functionalization.
Acknowledgments
This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
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