Segregation of micrometer-dimension biosensor elements on a variety of substrate surfaces

Fabrication of Biosensing Interface with Monolayers

Surface modification is recognized as one of the fundamental techniques to fabricate biosensing interfaces. This review focuses on the surface modification of carbon substrates (GC and HOPG) and silica with a close-packed monolayer, in particular. In the cases of carbon substrates, GC and HOPG, it was demonstrated that surface modification of carbon substrates with diazonium derivatives could create a close-packed monolayer similar to the self-assembled monolayer (SAM) formation with mercapto derivatives. Similarly, the potential of trialkoxysilanes to form a close-packed monolayer was evaluated, and modification with a close-packed monolayer tended to occur under milder conditions when the trialkoxysilanes had a longer alkyl chain. In these studies, we synthesized surface modification materials having ferrocene as a redox active moiety to explore features of the modified surfaces by an electrochemical method using cyclic voltammetry, where surface concentrations of immobilized molecules and blocking effect were studied to obtain insight for density leading to a close-packed layer. Based on those findings, fabrication of a biosensing interface on the silica sensing chip of the waveguide-mode sensor was carried out using triethoxysilane derivatives bearing succinimide ester and oligoethylene glycol moieties to immobilize antibodies and to suppress nonspecific adsorption of proteins, respectively. The results demonstrate that the waveguide-mode sensor powered by the biosensing interface fabricated with those triethoxysilane derivatives and antibody has the potential to detect several tens ng/mL of biomarkers in human serum with unlabeled detection method.

Functional polymers in protein detection platforms: optical, electrochemical, electrical, mass-sensitive, and magnetic biosensors

The rapidly growing field of proteomics and related applied sectors in the life sciences demands convenient methodologies for detecting and measuring the levels of specific proteins as well as for screening and analyzing for interacting protein systems. Materials utilized for such protein detection and measurement platforms should meet particular specifications which include ease-of-mass manufacture, biological stability, chemical functionality, cost effectiveness, and portability. Polymers can satisfy many of these requirements and are often considered as choice materials in various biological detection platforms. Therefore, tremendous research efforts have been made for developing new polymers both in macroscopic and nanoscopic length scales as well as applying existing polymeric materials for protein measurements. In this review article, both conventional and alternative techniques for protein detection are overviewed while focusing on the use of various polymeric materials in different protein sensing technologies. Among many available detection mechanisms, most common approaches such as optical, electrochemical, electrical, mass-sensitive, and magnetic methods are comprehensively discussed in this article. Desired properties of polymers exploited for each type of protein detection approach are summarized. Current challenges associated with the application of polymeric materials are examined in each protein detection category. Difficulties facing both quantitative and qualitative protein measurements are also identified. The latest efforts on the development and evaluation of nanoscale polymeric systems for improved protein detection are also discussed from the standpoint of quantitative and qualitative measurements. Finally, future research directions towards further advancements in the field are considered.

Microelectromechanical system-based diagnostic technology for cervical cancer

PURPOSE

To determine the correlations of human papillomavirus (HPV) infection and α6 integrin up-regulation to the tumorigenesis and development of cervical cancer, and develop an on-chip antibody-based detection system using the microelectromechanical system (MEMS) device for recognizing and capturing cervical cancer cells from a mixed cell population.

METHODS

We determined the efficiency of an antibody-based MEMS platform in recognizing and capturing cervical cancer cells. In addition, α6 integrin was used as a capture antibody bound to the channel surface. Channels 2 cm long × 50 cm wide with inlet and outlet diameters of 50 cm were formed in microfluidic polydimethylsiloxane (PDMS) chips.

RESULTS

The overall chip measured 2 cm × 1.5 cm × 0.5 cm. Using a syringe micropump, 20,000 normal human cervical epithelial cells (HCEC), 20,000 human cervical stromal cells (HCSC), and 20,000 human cervical cancer cells (HCCC) were suspended in PBS and flowed through the system at a rate of 20 mL/min. At this flow rate, more than 45% of cancer cells were captured, whereas less than 5% of normal cells were captured. In addition, we confirmed the specificity of this system in cell-type targeting using PCR.

CONCLUSIONS

MEMS is a sensitive and accurate method for capturing and enriching cells of interest. This technique is potentially useful in detecting cervical cancer at all stages, as well as other cancers with similar characteristics of cell surface antigen expression.

Surface modification of GC and HOPG with diazonium, amine, azide, and olefin derivatives

Surface modification of glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) was carried out with diazonium, amine, azide, and olefin derivatives bearing ferrocene as an electroactive moiety. Features of the modified surfaces were evaluated by surface concentrations of immobilized molecule, blocking effect of the modified surface against redox reaction, and surface observation using cyclic voltammetry and electrochemical scanning tunneling microscope (EC-STM). The measurement of surface concentrations of immobilized molecule revealed the following three aspects: (i) Diazonium and olefin derivatives could modify substrates with the dense-monolayer concentration. (ii) The surface concentration of immobilized amine derivative did not reach to the dense-monolayer concentration reflecting their low reactivity. (iii) The surface modification with the dense-monolayer concentration was also possible with azide derivative, but the modified surface contained some oligomers produced by the photoreaction of azides. Besides, the blocking effect against redox reaction was observed for GC modified with diazonium derivative and for HOPG modified with diazonium and azide derivatives, suggesting fabrication of a densely modified surface. Finally, the surface observation for HOPG modified with diazonium derivative by EC-STM showed a typical monolayer structure, in which the ferrocene moieties were packed densely at random. On the basis of those results, it was demonstrated that surface modification of carbon substrates with diazonium could afford a dense monolayer similar to the self-assembled monolayer (SAM) formation.

Two-component mixed and patterned films on carbon surfaces through the photografting of arylazides

Organic films have been grafted to glassy carbon surfaces by the photolysis of arylazides. Atomic force microscopy and electrochemical measurements reveal that the films are loosely packed. The methodology was expanded to prepare two-component thin films incorporating either a reactive tether species and a nonreactive background film or two different reactive tethers. Strategies were developed to generate both continuous mixed films and surfaces presenting patterns of two components. For patterning, the arylazide derivative was grafted onto previously modified glassy carbon surfaces. In this case, the first modification step is not limited to photografting, which increases the scope of the methods. For all grafted surfaces, the reactivity of tether species was confirmed by coupling electroactive targets to the tethers, followed by electrochemical monitoring. The ease of preparing surfaces with spatially controlled functionality offers promise for the design of sensing platforms on graphitic carbon substrates.

A simple approach to patterned protein immobilization on silicon via electrografting from diazonium salt solutions

A highly versatile method utilizing diazonium salt chemistry has been developed for the fabrication of protein arrays. Conventional ultraviolet mask lithography was used to pattern micrometer sized regions into a commercial photoresist on a highly doped p-type silicon (100) substrate. These patterned regions were used as a template for the electrochemical grafting of the in situ generated p-aminobenzenediazonium cation to form patterns of aminophenyl film on silicon. Immobilization of biomolecules was demonstrated by coupling biotin to the aminophenyl regions followed by reaction with fluorescently labeled avidin and visualization with fluorescence microscopy. This simple patterning strategy is promising for future application in biosensor devices.

Serial DNA immobilization in micro- and extended nanospace channels

That focused arrays, even with a small set of ligands, provide more data than single point experiments is well established in the DNA microarray research field, but microarray technology has yet to be transferred to fused silica microchips. Fused silica microchips have several attractive features such as stability to pressure, solvents, acids and bases, and can be fabricated with minute dimensions, making them good candidates for nanofluidic research. However, due to harsh bonding conditions, DNA ligands must be immobilized after fabrication, thus preventing standard microarray spotting techniques from being used. In this paper, we provide tools for serial DNA immobilization in fused silica microchips using UV. We report the synthesis of a new UV-linker which was used to covalently couple functional DNA oligos to the inside of channels in fused silica microchips. With some simple modifications to our mask aligner, we were able to transfer OHP mask patterns, which allows the creation of basically any pattern in the channels. The functionality of the oligos was measured through the binding of fluorophore-labeled complementary target oligos. We examined parameters influencing DNA immobilization, and carry-over between spots after consecutive immobilizations inside the same channel. We also report the first successful multiple immobilizations of functional DNA oligos inside single channels of extended nanospace depth (460 nm).

Biomolecular papain thin films growth by laser techniques

Papain thin films were synthesised by matrix assisted and conventional pulsed laser deposition (PLD) techniques. The targets submitted to laser radiation consisted on a frozen composite obtained by dissolving the biomaterials in distilled water. For the deposition of the thin films by conventional PLD pressed biomaterial powder targets were submitted to laser irradiation. An UV KrF* excimer laser source was used in the experiments at 0.5 J/cm(2) incident fluence value, diminished one order of magnitude as compared to irradiation of inorganic materials. The surface morphology of the obtained thin films was studied by atomic force profilometry and atomic force microscopy. The investigations showed that the growth mode and surface quality of the deposited biomaterial thin films is strongly influenced by the target preparation procedure.

Activity study of self-assembled proteins on nanoscale diblock copolymer templates

Novel methods for affixing functional proteins on surfaces with high areal density have the potential to promote basic biological research as well as various bioarray applications. The use of polymeric templates under carefully balanced thermodynamic conditions enables spontaneous, self-assembled protein immobilization on surfaces with spatial control on the nanometer scale. To assess the full potential of such nanometer-scale protein platforms in biosensing applications, we report for the first time the biological activity of proteins on diblock copolymer platforms. We utilized horseradish peroxidase, mushroom tyrosinase, enhanced green fluorescent protein, bovine immunoglobulin G, fluorescein isothiocyanate conjugated anti-bovine IgG, and protein G as model systems in our protein activity studies. When specific catalytic functions of HRP and MT, immobilized on selective domains of microphase-separated PS-b-PMMA, are evaluated over a long period of time, these enzymes retain their catalytic activity and stability for well over 3 months. By performing confocal fluorescence measurements of self-fluorescing proteins and interacting protein/protein systems, we have also demonstrated that the binding behavior of these proteins is unaffected by surface immobilization onto PS-b-PMMA diblock copolymer microdomains. Our polymer platforms provide highly periodic, high-density, functional, stable surface-bound proteins with spatial control on the nanometer scale. Therefore, our diblock copolymer-guided protein assembly method can be extremely beneficial for high-throughput proteomic applications.

Two-Dimensionally Self-Arranged Protein Nanoarrays on Diblock Copolymer Templates

Novel methods for creating protein arrays with two-dimensional control can significantly enhance basic biological research as well as various bioarray applications. We demonstrate that the structural variety and chemical heterogeneity of self-assembled, hexagonal polystyrene-b-poly(vinylpyridine) micelles can be successfully exploited as templates for easy and rapid fabrication of functional protein arrays over a large scale. Spontaneous formation of such polymeric template-guided protein molecules yields high-density protein arrays that exhibit repeat spacings in a nanoscopic dimension. The ensuing self-assembled protein molecules in the array maintain their natural conformation and activity over a very long time period. By tuning the size of the underlying block copolymer templates, our amphiphilic diblock copolymer-based approach to create high-density protein patterns also permits spatial control over two-dimensional repeat spacings of protein nanoarrays. These unique advantages of polystyrene-b-poly(vinylpyridine) templates make the spontaneously constructed protein nanoarrays highly suitable as functional protein sensor substrates. Therefore, our novel two-dimensional protein assembly method can be greatly beneficial for high-throughput proteomic assays and multiplexed high-density protein sensing applications.

Microfluidic-based diagnostics for cervical cancer cells

The use of biomarkers has facilitated the detection of specific tumor cells. However, the technology to apply these markers in a clinical setting has not kept pace with their increasing availability. In this project, we use an antibody-based microfluidics platform to recognize and capture cervical cancer cells. Because HPV-16 infection of cervical cells and up-regulation of alpha6-integrin cell surface receptors are correlated, we utilized alpha6-integrin as a capture antibody bound to the channel surface. Normal human glandular epithelial cells (HGEC), human cervical stromal cells (HCSC) and cervical cancer cells (HCCC) were suspended in PBS and flowed through the system. Greater than 30% of the cancer cells were captured while the capture of the normal cell types was less than 5%. The technique is sensitive and accurate. It is potentially useful in the detection of cervical cancer at all stages, as well as other of cancers with similar characteristics of cell surface antigen expression.

Phospholipid biotinylation of polydimethylsiloxane (PDMS) for protein immobilization

Polydimethylsiloxane (PDMS) surfaces can be functionalized with biotin groups by adding biotinylated phospholipids to the PDMS prepolymer before curing. The addition of beta-D-dodecyl-N-maltoside (DDM) in the solution blocks non-specific protein binding on these functionalized PDMS surfaces. We characterize the surface by measuring fluorescently labeled streptavidin binding. Single molecule tracking shows that the phospholipids are not covalently linked to PDMS polymer chains, but the surface functionalization is not removed by washing. We demonstrate the immobilization of biotinylated antibodies and lectins through biotin-avidin interactions.

Small-Volume Detection ofPlasmodiumfalciparumCSP Gene Using a 50-μm-Diameter Cavity with Self-Contained Electrochemistry

An electrochemical enzyme-linked immobilized DNA-hybridization assay for the detection of Plasmodium falciparum has been developed. The target molecule was a segment of the repeat sequence of the gene coding for the circumsporozoite (CSP) protein from the AF54087 gene. This analyte offers the possibility of specifically detecting P. falciparum. The assay involves attachment of a biotinylated primary DNA probe via its 5'-amine-terminus to the streptavidin-coated surface of microwells in a 96-well plate. The primary DNA probe (1(0)P, which was of two different sequences we call 1(0)P(a) and 1(0)P(b)) was used to capture the target (T, which was of two different sequences, T1 sequence 481-590 and T2 sequence 472-590 of AF54087 gene for the CSP gene) by hybridization to a complementary sequence on the target. On 1(0)P(a), 47 bases were complementary to T1 and T2 at 543-590, while on 1(0)P(b), 35 bases were complementary to T1 and T2 at 555-590. A secondary DNA probe that contained 36 bases with alkaline phosphatase (2(0)P-AP) label on the 3' end was hybridized to a complementary base sequence on the 5' end of the target. p-Aminophenol, which is enzymatically generated by the immobilized AP from p-aminophenyl phosphate (PAPP), is detected using electrochemistry. The peak current of cyclic voltammograms from a PAPP solution incubated inside the microwells modified with the complete assembly of the assay components gives a linear relationship with the concentration of the target (2-50 ng/mL, where P1 (P1a and P1b) and P2-AP concentrations are 50 ng/mL). A detection limit of 1.4 ng/mL (or 46 pM) of the DNA target was obtained. The signals of the assays were not significantly affected when performed in the presence of human hepatocytes, pig liver, or chicken serum indicating the viability of this assay in real clinical samples.

Micropatterning of biomolecules on glass surfaces modified with various functional groups using photoactivatable biotin

Biomolecule patterning by photolithographic methods has considerable advantages because a large number of different biomolecules can be assembled on a spatial area by a combinatorial method and complex biomolecule patterning can be created in situ in closed environments such as microfluidic channels. Here, a photobiotin was used as the photoactivatable reagent to create patterned arrays of biomolecules. The variability of photobiotin deposition on glass substrates modified with a variety of materials having carboxyl, lysine, aldehyde, amine groups, and BSA (bovine serum albumin) was characterized by subsequent derivatization with Cy3-labeled streptavidin. The fluorescence images of the photobiotin patterned glass surfaces showed that the BSA/aldehyde-coated glass could be considered as the most appropriate substrate to immobilize photobiotin, in view of the homogeneous immobilization of biomolecules with high density in defined regions and the reduction of nonspecific binding to the surface. In streptavidin equilibrium adsorption assays, the maximum amount of streptavidin-Cy3 bound to the BSA/aldehyde-coated glass surface continued to rise with increasing streptavidin-Cy3 concentration until 12.0 microg/mL was reached and the surface then became saturated. Also, a line array of biotin-labeled single-strand probe DNAs was created on the BSA/aldehyde-coated glass by photolysis of photobiotin through a slit-type mask and biotin/streptavidin/biotin chemistry, extended to a quantitative measurement of the concentrations of target DNA. The results of target DNA analysis showed linearity over a wide range from 0.5 ng/mL to 5 microg/mL and were reproducible.

Nanoscale protein patterning using self-assembled diblock copolymers

Novel methods for immobilizing proteins on surfaces have the potential to impact basic biological research as well as various biochip applications. Here, we demonstrate a unique method to pattern proteins with a nanometer periodicity on silicon oxide substrates using microphase-separated diblock copolymer thin films. We developed a straightforward and effective protein immobilization technique using the microphase-separated domains of polystyrene-block-poly(methyl methacrylate) to localize various model protein molecules such as bovine immunoglobulin G, fluorescein isothiocyanate conjugated anti-bovine immunoglobulin G, and protein G. The self-organizing nature of the diblock copolymer was exploited to produce periodically alternating, nanometer-spaced polymeric domains exposing the two chemical compositions of the diblock to surface. We demonstrate that the model proteins selectively self-organize themselves on the microdomain regions of specific polymer components due to their preferential interactions with one of the two polymer segments. This diblock copolymer-based, self-assembly approach represents a step forward for facile, nanometer-spaced protein immobilization with high areal density and could provide a pathway to high-throughput proteomic arrays and biosensors.

Wireless sensors powered by microbial fuel cells

Monitoring parameters characterizing water quality, such as temperature, pH, and concentrations of heavy metals in natural waters, is often followed by transmitting the data to remote receivers using telemetry systems. Such systems are commonly powered by batteries, which can be inconvenient at times because batteries have a limited lifetime and must be recharged or replaced periodically to ensure that sufficient energy is available to power the electronics. To avoid these inconveniences, a microbial fuel cell was designed to power electrochemical sensors and small telemetry systems to transmit the data acquired by the sensors to remote receivers. The microbial fuel cell was combined with low-power, high-efficiency electronic circuitry providing a stable power source for wireless data transmission. To generate enough power for the telemetry system, energy produced by the microbial fuel cell was stored in a capacitor and used in short bursts when needed. Since commercial electronic circuits require a minimum 3.3 V input and our cell was able to deliver a maximum of 2.1 V, a DC-DC converter was used to boost the potential. The DC-DC converter powered a transmitter, which gathered the data from the sensor and transmitted it wirelessly to a remote receiver. To demonstrate the utility of the system, temporal variations in temperature were measured, and the data were wirelessly transmitted to a remote receiver.

Single-feature inking and stamping: a versatile approach to molecular patterning

A technique for micrometer-scale patterning of multiple functional biological molecules on surfaces is demonstrated. The technique is referred to as single-feature inking and stamping (SFINKS). It combines elements of dip-pen nanolithography and microcontact printing. "Inked" atomic force microscopy probes are used to ink individual features of an elastomer stamp. From a single stamp, we printed three different probe ssDNA with <10 mum resolution and showed that they specifically hybridize the complementary DNA labeled with different fluorophores. As a further demonstration of SFINKS' versatility, we patterned a silane onto a silicon wafer consisting of four subpatterns separated by >100 mum and composed of 2 mum lines. We discuss why patterns such as these are impractical with available techniques. Furthermore, we comment on the prospects for multiple stamping after a single inking.

Label-free molecular interaction determinations with nanoscale interferometry

Quantification of protein-protein and ligand-substrate interactions is central to understanding basic cellular function and for evaluating therapeutics. To mimic biological conditions, such studies are best executed without modifying the proteins or ligands (i.e., label-free). While tools for label-free assays exist, they have limitations making them difficult to fully integrate into microfluidic devices. Furthermore, it has been problematic to reduce detection volumes for on-channel universal analyte quantification without compromising sensitivity, as needed in label-free methods. Here we show how backscattering interferometry in rectangular channels (BIRC) facilitates label-free studies within picoliter volumes. The simple and unique optical train was based on rectangular microfluidic channels molded in poly(dimethylsiloxane) and low-power coherent radiation. Quantification of irreversible streptavidin-biotin binding and reversible protein A-human IgG Fc molecular interactions in a 225 pL detection volume was carried out label-free and noninvasively. Detection limits of 47 x 10(-15) mol of biotin reacted with surface-immobilized streptavidin were achieved. In the case of reversible interactions of protein A and the Fc fragment of human IgG, detection limits were determined to be 2 x 10(-15) mol of IgG Fc. These experiments demonstrate for the first time that (1) high-sensitivity universal solute quantification is possible using interferometry performed within micrometer-sized channels formed in inexpensive PDMS chips, (2) label-free reversible molecular interaction can be studied with femtomoles of solute, and (3) BIRC has the potential to quantify binding affinities in a high-throughput format.

Microfluidic Immunoassay for Bacterial Toxins with Supported Phospholipid Bilayer Membranes on Poly(dimethylsiloxane)

We report a heterogeneous immunoassay for cholera toxin (CT) using supported bilayer membranes (SBMs) in a poly(dimethylsiloxane) (PDMS) microfluidic device. Phosphatidylcholine membranes assembled on plasma-oxidized PDMS by vesicle fusion bring about favorable surface properties, such as improved wettability and protein resistance. Contact angle measurements show that the lipid membranes can preserve hydrophilic surfaces for hours, whereas untreated substrates rapidly undergo hydrophobic recovery. Fluorescence recovery after photobleaching performed in situ reveals that the membranes have relatively high lateral mobility. Experimental data-fitting to theoretical models yields diffusion coefficients of 1.8 +/- 0.7 microm(2)/s on PDMS and 3.4 +/- 0.8 microm(2)/s on glass. Fluorescence studies utilizing tagged proteins show that SBMs reduce nonspecific adsorption of avidin and BSA on PDMS by 2-3 orders of magnitude, as compared to that on plasma oxidized surfaces. SBMs and their protein-resistant properties are not significantly affected by long flow times, indicating good membrane stability. These studies increase our understanding of the relationship between molecular level interactions and membrane properties, allowing for development of a rapid heterogeneous immunoassay for CT in PDMS microchips with cell surface receptor molecules. Using optimized sample injection and buffer washing conditions, microfluidic immunoassay of CT is complete within 25 min, and a dynamic range over 3 orders of magnitude with a detection limit of 8 fmol of toxin is achieved.

Submicron patterning of DNA oligonucleotides on silicon

The covalent attachment of DNA oligonucleotides onto crystalline silicon (100) surfaces, in patterns with submicron features, in a straightforward, two-step process is presented. UV light exposure of a hydrogen-terminated silicon (100) surface coated with alkenes functionalized with N-hydroxysuccinimide ester groups resulted in the covalent attachment of the alkene as a monolayer on the surface. Submicron-scale patterning of surfaces was achieved by illumination with an interference pattern obtained by the transmission of 248 nm excimer laser light through a phase mask. The N-hydroxysuccinimide ester surface acted as a template for the subsequent covalent attachment of aminohexyl-modified DNA oligonucleotides. Oligonucleotide patterns, with feature sizes of 500 nm, were reliably produced over large areas. The patterned surfaces were characterized with atomic force microscopy, scanning electron microscopy, epifluorescence microscopy and ellipsometry. Complementary oligonucleotides were hybridized to the surface-attached oligonucleotides with a density of 7 x 10(12) DNA oligonucleotides per square centimetre. The method will offer much potential for the creation of nano- and micro-scale DNA biosensor devices in silicon.

Covalent DNA immobilization on polymer-shielded silver-coated quartz crystal microbalance using photobiotin-based UV irradiation

The use of a commercial, silver-coated quartz crystal microbalance (QCM) as a disposable, low-cost, and reliable DNA sensor is presented. This is an incorporation of polymer-based silver electrode shielding and photochemistry-based surface modification for covalent DNA immobilization. To prevent undesired oxidation, the silver electrodes are coated with thin polystyrene films. The polymer surfaces are then modified by a photoreactive biotin derivative (photobiotin) under UV irradiation. The resulting biotin residues on the polymer-shielded surface react with a tetrameric avidin. Consequently a biotin-labeled DNA probe can be immobilized through a biotin-avidin-biotin bridge. A 14-mer single-stranded biotin-DNA probe and a 70-mer single-stranded DNA fragment containing complementary or noncomplementary sequences are used as a model system for DNA hybridization assay on the proposed sensors. The shielding ability of the polystyrene coatings after photo irradiation is investigated. The DNA probe binding capacity, hybridization efficiency, and kinetics are also investigated.