Reusable biosensors via in situ electrochemical surface regeneration in microfluidic applications

Reusable Cyclodextrin-Based Electrochemical Platform for Detection of trans-Resveratrol

A reusable sensor architecture, through the combination of self-assembled monolayers and cyclodextrin supramolecular interactions, is demonstrated for class recognition of hydrophobic analytes demonstrated with trans-resveratrol. The reloadable sensor is based on reversible immobilization of α-cyclodextrin on polyethylene glycol surface. α-cyclodextrins complexes with polyethylene glycols and causes the polymer chains to change their surface configuration. The reproducibility and stability of the sur-face, in the detection of nanomolar concentrations of trans-resveratrol, can be demonstrated by electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and Attenuated total reflectance-Fourier transform infrared spectroscopy. We propose that during sensor operation, α-cyclodextrin decouples from the poly-ethylene glycol surface to complex with trans-resveratrol in solution, and after use, the surface regeneration is conducted with a simple α-cyclodextrin soak. To test the nonspecific response, the sensor was also tested with trans-resveratrol spiked human urine.

Reusable surface plasmon resonance biosensor chip for the detection of H1N1 influenza virus

We developed a reusable magnetic surface plasmon resonance (SPR) sensor chip for detecting various target molecules repeatedly in a conventional SPR system. Here, ferromagnetic patterns on a SPR sensor chip were utilized to trap a layer of magnetic particles, and they were utilized as a solid substrate for SPR sensing in a conventional SPR system. After a sensing experiment, the used magnetic particles were removed by external magnetic fields, and a new layer of magnetic particles was immobilized to the SPR sensor chip for additional sensing measurements. Since magnetic particles were trapped on the ferromagnetic patterns, we could use our reusable SPR chip for SPR sensing measurements in a traditional SPR system without any applied magnetic fields. Significantly, ferromagnetic patterns on the sensor chip surface deflected the strong external fields, so that the large aggregation of magnetic particles on the sensor surface was reduced. We demonstrated using a single reusable SPR sensor chip to measure the nucleoprotein (NP) of H1N1 influenza virus solution ranging repeatedly for more than 7 times without significant signal degradation. Also, different target molecules could be repeatedly measured in a single SPR chip. Since our reusable SPR sensor chip can be repeatedly used in a conventional SPR system without any chemical processes for refreshment, the cost for SPR sensing should be significantly reduced. In this case, our reusable SPR sensor chip can be a major breakthrough and can be used for versatile practical applications of SPR sensors.

Triple functional small-molecule-protein conjugate mediated optical biosensor for quantification of estrogenic activities in water samples

Establishing biosensors to map a comprehensive picture of potential estrogen-active chemicals remains challenging and must be addressed. Herein, we describe an estrogen receptor (ER)-based evanescent wave fluorescent biosensor by using a triple functional small-molecule-protein conjugate as a signal probe for the determination of estrogenic activities in water samples. The signal probe, consisting of a Cy5.5-labelled streptavidin (STV) moiety and a 17β-estradiol (E₂) moiety, acts simultaneously as signal conversion, signal recognition and signal report elements. When xenoestrogens compete with the E₂ moiety of conjugate in binding to the ER, the unbound conjugates are released, and their STV moiety binds with desthiobiotin (DTB) modified on the optical fiber via the STV-DTB affinity interactions. Signal probe detection is accomplished by fluorescence emission induced by an evanescent field, which positively relates with the estrogenic activities in samples. Quantification of estrogenic activity expressed as E₂ equivalent concentration (EEQ) can be achieved with a detection limit of 1.05 μg/L EEQ by using three times standard deviation of the mean blank values and a linear calibration range from 20.8 to 476.7 μg/L EEQ. The optical fiber system is robust enough for hundreds of sensing cycles. The biosensor-based determination of estrogenic activities in wastewater samples obtained from a full-scale wastewater treatment plant is consistent with that measured by the two-hybrid recombinant yeast bioassay.

Vapor-Phase Deposition and Electronic Characterization of 3-Aminopropyltriethoxysilane Self-Assembled Monolayers on Silicon Dioxide

Although organosilanes, especially 3-aminopropyltriethoxysilane (APTES), are commonly used to functionalize oxide substrates for a variety of applications ranging from molecular/biosensors and electronics to protective layers, reliable and controlled deposition of these molecules remains a major obstacle. In this study, we use surface potential analyses to record and optimize the gas-phase deposition of APTES self-assembled monolayers (SAMs) and to determine the resulting change of the electrokinetic potential and charge at the solid?liquid interface when the system is exposed to an electrolyte. Using a gas-phase molecular layer deposition setup with an in situ molecule deposition sensor, APTES is deposited at room temperature onto ozone-activated SiO₂. The resulting layers are characterized using various techniques ranging from contact angle analysis, ellipsometry, fluorescence microscopy, X-ray photoelectron spectroscopy, and electrokinetic analysis to AFM. It turns out that adequate postdeposition treatment is crucial to the formation of perfect molecular SAMs. We demonstrate how a thick layer of APTES molecules is initially adsorbed at the surface; however, the molecules do not bind to SiO₂ and are removed if the film is exposed to an electrolyte. Only if the film is kept in a gaseous environment (preferable at low pressure) for a long enough time do APTES molecules start to bind to the surface and form the SAM layer. During this time, superfluous molecules are removed. The resulting modification of the electrokinetic potential at the surface is analyzed in detail for different states.

High-Resolution Analysis of Antibodies to Post-Translational Modifications Using Peptide Nanosensor Microarrays

Autoantibodies are a hallmark of autoimmune diseases such as lupus and have the potential to be used as biomarkers for diverse diseases, including immunodeficiency, infectious disease, and cancer. More precise detection of antibodies to specific targets is needed to improve diagnosis of such diseases. Here, we report the development of reusable peptide microarrays, based on giant magnetoresistive (GMR) nanosensors optimized for sensitively detecting magnetic nanoparticle labels, for the detection of antibodies with a resolution of a single post-translationally modified amino acid. We have also developed a chemical regeneration scheme to perform multiplex assays with a high level of reproducibility, resulting in greatly reduced experimental costs. In addition, we show that peptides synthesized directly on the nanosensors are approximately two times more sensitive than directly spotted peptides. Reusable peptide nanosensor microarrays enable precise detection of autoantibodies with high resolution and sensitivity and show promise for investigating antibody-mediated immune responses to autoantigens, vaccines, and pathogen-derived antigens as well as other fundamental peptide-protein interactions.

Heterotypic 3D tumor culture in a reusable platform using pneumatic microfluidics

The construction of a micro-platform capable of microscale control for continuous, dynamic, and high-throughput biomimetic tumor manipulation and analysis plays a significant role in biological and clinical research. Here, we introduce a pneumatic microstructure-based microfluidic platform for versatile three-dimensional (3D) tumor cultures. The manipulative potential of pneumatic microstructures in a fabrication-optimized microfluidic device can be stimulated to achieve ultra-repetitive (tens of thousands of times) and persistent (over several months) microfluidic control. We demonstrated that the microfluidic platform is reusable (dozens of times) for stable, reproducible, and high-throughput generation of tumors with uniform size. Various heterotypic and homotypic 3D tumor arrays can be produced successfully in the device based on robust pneumatic control. On-chip monitoring and analysis of tumor phenotypes and responses to different culture conditions and chemotherapies were also achieved in real-time in the microfluidic platform. The results indicate that fibroblasts cocultured with tumor cells positively promote the phenotypical appearance of heterotypic tumors. This microfluidic advancement offers a new methodological approach for the development of high-performance and non-disposable 3D culture systems and for tissue-mimicking cancer research. We believe that it could be valuable for various tumor-related research fields such as oncology, pharmacology, tissue engineering, and bioimaging.

Gecko gaskets for self-sealing and high-strength reversible bonding of microfluidics

We report in this work a novel reversible bonding technique for elastomeric microfluidic devices by integrating gecko-inspired dry adhesives with microfluidic channels which greatly enhances the bonding strength of reversibly sealed channels. The concept is applicable to nearly any elastomer and can be used to bond against any smooth surface which allows for van der Waals interactions. It does not require any solvents or glues or sources for plasma activation or thermal-compressive loading to aid the bonding process and is achievable at zero extra cost. We also demonstrate a quick fabrication technique involving soft master thermo-compressive molding of these microfluidic devices with thermoplastic elastomers. The resultant devices can be used for both pressure driven and non-pressure driven flows. We report the maximum contained pressure of these devices manufactured from two grades of styrene ethylene butylene styrene (SEBS) by conducting a burst pressure test with various substrates.

Towards maintenance-free biosensors for hundreds of bind/release cycles

A single aptamer bioreceptor layer was formed using a common streptavidin-biotin immobilization strategy and employed for 100-365 bind/release cycles. Chemically induced aptamer unfolding and release of its bound target was accomplished using alkaline solutions with high salt concentrations or deionized (DI) water. The use of DI water scavenged from the ambient atmosphere represents a first step towards maintenance-free biosensors that do not require the storage of liquid reagents. The aptamer binding affinity was determined by surface plasmon resonance and found to be almost constant over 100-365 bind/release cycles with a variation of less than 5% relative standard deviation. This reversible operation of biosensors based on immobilized aptamers without storage of liquid reagents introduces a conceptually new perspective in biosensing. Such new biosensing capability will be important for distributed sensor networks, sensors in resource-limited settings, and wearable sensor applications.

Synergizing nucleic acid aptamers with 1-dimensional nanostructures as label-free field-effect transistor biosensors

Since the introduction by Gold et al. in 1990, nucleic acid aptamers had evolved to become a true contender in biosensors for protein and cell detections. Aptamers are short strands of synthetically designed DNA or RNA oligonucleotides that can be self-assembled into unique 3-dimensional structures and can bind to different proteins, cells or even small molecules at a high level of specificity and affinity. In recent years, there had been many reports in literature in using aptamers in place of conventional antibodies as capture biomolecules on the surface. This is mainly due to the better thermal stability properties and ease in production. Consequently, also these characteristics allowed the aptamers to find use in field effect transistors (FETs) based upon 1D nanostructured (1D-NS) as label-free biosensing. In terms of designing label-free platforms for biosensors applications, 1D-NS FET had been an attractive option due to reported high sensitivities toward protein targets arising from the large surface area for detection as well as to their label-free nature. Since the first aptamer-based 1D-NS FET biosensor had surfaced in 2005, there had been many more improvements in the overall design and sensitivity in recent years. In this review, the latest developments in synergizing these two interesting areas of research (aptamers and 1D-NS FET) will be discussed for a range of different nanowire types as well as for the detection results.

Survey of the 2009 commercial optical biosensor literature

We took a different approach to reviewing the commercial biosensor literature this year by inviting 22 biosensor users to serve as a review committee. They set the criteria for what to expect in a publication and ultimately decided to use a pass/fail system for selecting which papers to include in this year's reference list. Of the 1514 publications in 2009 that reported using commercially available optical biosensor technology, only 20% passed their cutoff. The most common criticism the reviewers had with the literature was that "the biosensor experiments could have been done better." They selected 10 papers to highlight good experimental technique, data presentation, and unique applications of the technology. This communal review process was educational for everyone involved and one we will not soon forget.

Recycling and reusing patterned self-assembled monolayers for cell culture

Patterned self-assembled monolayers (SAMs) have been widely utilized for the study of cellular growth and behavior. While microcontact printing is a straightforward method of producing patterned substrates, the process is time consuming and requires the use of many techniques and specialized equipment. Here we present a method by which patterned substrates can be reused up to 15 times, saving both time and valuable resources.