Evaluation of repeated electrowetting on three different fluoropolymer top coatings

Degradation of the electrowetting effect by a repeated actuation is evaluated over an extended period (200 min) on electrowetting-on-dielectric samples for three popular fluoropolymer top coatings: Teflon, FluoroPel and Cytop. A conductive liquid droplet is tested in an air environment at electrowetting number Ew ≅ 0.34. A pulse train (6 s period and 50% duty cycle) of three different voltage types is used for the actuation: positive dc, negative dc and 1 kHz ac. For the dc actuations, electrowetting degrades gradually on Cytop but significantly faster on Teflon and FluoroPel under the tested conditions. For the ac actuation, electrowetting degrades gradually on all three materials in a similar fashion. © 2013 IOP Publishing Ltd

Micro and Nanoscale Fabrication and Characterization For Next-Generation Biosensors

Pressing performance demands require next-generation biosensors to detect target chemical and biological molecules with higher sensitivity, shorter response times, and lower detection limit. Micro- and nanoscale devices are attractive for a wide range of biosensor applications since at small scale, in addition to being more compact, the device may exhibit improved performance. The benefits include minimization of tissue damage for implantable devices, improved spatial resolution and sensitivity, as well as increased surface charge to mass ratio, which is important for the performance of our novel technology for nucleic acid detection described below. Borrowing from the processing technologies used in the semiconductor industry, we implemented micromachining techniques to fabricate devices at both the micro- and nanoscale. In this dissertation, we present our work on the fabrication and characterization of two next-generation biosensors. The first device we fabricated is a sequence-specific nucleic acid sensor based on the blockage of a nanopore. Current methods for nucleic acid detection generally rely on polymerase chain reaction (PCR) and fluorescent labeling, however, these methods render the devices slow, expensive, complex, and bulky. In order to address these limitations, a new sensor was fabricated from a single glass wafer, consisting of a glass nanopore in a thin glass membrane. For nanopore sensing, low frequency noise is critical since it limits the discrimination of signal change based on target analyte movement from the fluctuation of noise. To further our understanding of nanopores, we observed how different pore geometries affect noise characteristics, and then compared this newly developed glass nanopore to conventional Si-based nanopores. Based on the analysis, low-noise glass nanopores, suitable for sequence-specific nucleic acid detection, were fabricated. By scaling down the pore diameter to the nano-regime, 1 aM detection of 16S rRNA from Escherichia coli was demonstrated even in the presence of a million-fold background of RNA from Pseudomona putida. This new platform for the PCR-free, optics-free, label-free sequence-specific nucleic acid detection shows the potential to detect pathogens in body fluids, food, or water. In addition, we developed a new method to transfer enzyme to a microelectrode array on an implantable microprobe, which enables fabrication of better performing microprobes for the sensing of multiple neurochemicals in vivo. Monitoring the release of neurotransmitters in real-time offers valuable information necessary to understand neurological disorders and abnormal behaviors. We employed polydimethylsiloxane (PDMS) stamping to transfer enzyme onto microelectrode array microprobes. A model enzyme, glucose oxidase (GOx), was stamped onto the surface of disk electrodes to test the feasibility of PDMS stamping for biosensor fabrication. The model sensor showed a good combination of performance (29 μA/mM cm2 sensitivity and 4 μM detection limit) proving that PDMS stamping offers a simple and cost-effective enzyme deposition method for construction of electroenzymatic sensors. The next step was to add an alignment function to PDMS stamping to create microprobes with dual sensing (glucose and choline) capabilities for in vivo applications. Two different enzymes, GOx and choline oxidase (ChOx), were selectively transferred onto specific sites in a 4 microelectrode array by PDMS stamping with alignment using a microscope and a custom-built stage. The dual sensor showed improved consistency and performance including sensitivity to choline and to glucose (286 and 117 μA/mM cm2, respectively) as well as low detection limits (3 and 1 μM, respectively). This work demonstrated the ability to immobilize specific enzymes on selected microelectrodes in an array to give a high performance microprobe for simultaneous sensing of two analytes for neuroscience application

Enzyme Deposition by Polydimethylsiloxane Stamping for Biosensor Fabrication

High-performance biosensors were fabricated by efficiently transferring enzyme onto Pt electrode surfaces using a polydimethylsiloxane (PDMS) stamp. Polypyrrole and Nafion were coated first on the electrode surface to act as permselective films for exclusion of both anionic and cationic electrooxidizable interfering compounds. A chitosan film then was electrochemically deposited to serve as an adhesive layer for enzyme immobilization. Glucose oxidase (GOx) was selected as a model enzyme for construction of a glucose biosensor, and a mixture of GOx and bovine serum albumin was stamped onto the chitosan-coated surface and subsequently crosslinked using glutaraldehyde vapor. For the optimized fabrication process, the biosensor exhibited excellent performance characteristics including a linear range up to 2 mM with sensitivity of 29.4 ± 1.3 μA mM-1 cm-2 and detection limit of 4.3 ± 1.7 μM (S/N = 3) as well as a rapid response time of ~2 s. In comparison to those previously described, this glucose biosensor exhibits an excellent combination of high sensitivity, low detection limit, rapid response time, and good selectivity. Thus, these results support the use of PDMS stamping as an effective enzyme deposition method for electroenzymatic biosensor fabrication, which may prove especially useful for the deposition of enzyme at selected sites on microelectrode array microprobes of the kind used for neuroscience research in vivo

Microbiosensor fabrication by polydimethylsiloxane stamping for combined sensing of glucose and choline

High performance microprobes for combined sensing of glucose and choline were fabricated using microcontact printing (μCP) to transfer choline oxidase (ChOx) and glucose oxidase (GOx) onto targeted sites on microelectrode arrays (MEAs). Most electroenzymatic sensing sites on MEAs for neuroscience applications are created by manual enzyme deposition, which becomes problematic when the array feature size is less than or equal to ∼100 μm. The μCP process used here relies on use of soft lithography to create features on a polydimethylsiloxane (PDMS) microstamp that correspond to the dimensions and array locations of targeted, microscale sites on a MEA. Precise alignment of the stamp with the MEA is also required to transfer enzyme only onto the specified microelectrode(s). The dual sensor fabrication process began with polyphenylenediamine (PPD) electrodeposition on all Pt microelectrodes to block common interferents (e.g., ascorbic acid and dopamine) found in brain extracellular fluid. Next, a chitosan film was electrodeposited to serve as an adhesive layer. The two enzymes, ChOx and GOx, were transferred onto different microelectrodes of 2 × 2 arrays using two different PDMS stamps and a microscope for stamp alignment. Using constant potential amperometry, the combined sensing microprobe was confirmed to have high sensitivity for choline and glucose (286 and 117 μA mM cm-2, respectively) accompanied by low detection limits (1 and 3 μM, respectively) and rapid response times (≤2 s). This work demonstrates the use of μCP for facile creation of multianalyte sensing microprobes by targeted deposition of enzymes onto preselected sites of a microelectrode array