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MEMS device integrated with vertically aligned carbon nanotubes for virus capture and detection

Point-of-care virus diagnosis is highly desirable in worldwide infectious disease control. Here we report a hand-held device for capturing viruses by applying physical size based exclusion inside a point-of-care device integrated with vertically aligned carbon nanotube (VACNT) nanostructures to achieve label-free and high throughput virus capture. The microfluidic device is constructed from a VACNT channel wall synthesized bottom-up via chemical vapor deposition (CVD). The VACNT has ~117nm average gap size and ~97% porosity. By bonding with a polydimethylsiloxane (PDMS) cover sealing the top, the aqueous sample containing virus particles filter through the VACNT channel wall under negative pressure applied at the outlet end. We have demonstrated that the device is capable of filtering 50μL of PBS containing ~6.3×10 counts of lentivirus particles in 10 minutes with 97% of capture efficiency, quantified by the cell infectious titration technique. INTRODUCTION Once a viral infectious disease breaks out, the detriment can grow exponentially without effective quarantine. In 1918, an influenza outbreak caused 50 million deaths globally. The HIV/AIDS pandemic has resulted in 36 million deaths thus far and the number is increasing [1]. The emergence of these pathogens and their subsequent spread have not only had a destructive impact on global health but have also caused economic losses and interruptions in trade and travel [2]. Therefore, rapid on-site detection and analysis are crucial. Viruses are small parasites and display in a variety of sizes and shapes. In general, most discovered viruses range from 20-300 nm in size and contain either an RNA or DNA genome surrounded by a protective protein coat or capsid [3]. Current diagnostic approaches include direct virus detection via isolation of the virus in cell culture, identification of viral nucleic acids or antigens, and serological tests detecting virus-specific antibodies. However, viral culture requires a minimum of two to ten days of enrichment to provide statistically relevant information [4]. Because immunological approaches depend on specific recognitions provided by antibodies or probes [5], it poses a challenge to target unknown viruses or unidentified strains. In this paper, we integrate a hand-held device to achieve virus capture and enrichment by size-based filtration. The active material is a VACNT forest with 97% porosity and the ability to selectively isolate virus from aqueous suspensions at the nanoscale. Via bottom-up synthesis processing, VACNT selectively grow on patterned iron catalyst thin films. First, we characterized VACNT material synthesis by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Second, we studied VACNT filtration properties of gap size and porosity and fitted the results using Darcy’s equation. Third, we integrated droplet-shaped VACNT pattern into a microfluidic device via a sealing PDMS cover on top. We characterized the device filtration performance using lentivirus and measured the capture efficiency by cell infectious titration. FABRICATION AND FILTRATION MECHANISM Bottom-up fabrication and device assembly Starting from a 4” prime silicon wafer, the silicon substrate was cleaned by piranha solution followed by subsequent sonication in acetone, IPA and DI water. In Fig.1A, The iron catalyst thin film was deposited by e-beam evaporation and patterned by photoresist lift-off process. The ion thin film is approximately 6.5nm thick when measured by atomic force microscopy (AFM). Due to its better biocompatibility [6], nitrogen-doped CNTs were synthesized inside an aerosol based CVD by using benzylamine as the carbon source with nitrogen doping. The benzylamine mist was generated by an ultrasonic neubulizer and was carried by argon/hydrogen gas into two furnaces in series at 825oC under a flow rate of 2.5 L/min. In Fig.1B, the VACNT selectively grew on a patterned catalyst thin film to form an enclosed droplet shape microfluidic chamber. The newly synthesized VACNT pattern was then sealed on top with s molded PDMS chamber via oxygen plasma enhanced bonding. We fabricated the PDMS cover by using an SU-8 mold and by puncturing two fluidic access ports, one inlet and one outlet. In Fig.1C, the inlet port is attached with a 100μL cylindrical reservoir made with silicone. The outlet port is connected to a flow-through collection tube and a vacuum source through a T-shaped adaptor. The negative pressure is regulated by a mechanical regulator and measured with a pressure gauge. Figure 1. VACNT device fabrication and assembly process. A: Illustration of fabrication process flow. B: Top view of VACNT selective growth into droplet pattern (top), and device assembled with PDMS cover (bottom). C: Picture of hand-held virus capture device with fluidic access connections. Filtration mechanism and operation The test sample is loaded into the inlet reservoir by pipetting and actuated through the filtration device by negative pressure generated at the outlet end. The droplet-shaped porous wall was sealed on top by PDMS permanent bonding. Fig.2 illustrates that the device performs dead-end filtration through the VACNT forest with nanoscale gap size and high porosity. During the filtration process, the suspension mixture containing particles of different sizes is 9781940470016/HH2014/$25©2014TRF 17 Solid-State Sensors, Actuators and Microsystems Workshop Hilton Head Island, South Carolina, June 8-12, 2014 transported through the VACNT microfluidic channel wall by the pressure driven flow. The particles with a size range similar to the gap size are trapped inside the membrane. Any larger particles are blocked and remain confined within the enclosed droplet chamber, while smaller particles flow through the VACNT membrane and are collected at the outlet. Figure 2. Schematic illustration of the virus isolation process inside the VACNT filtration device.