Combinable poly(dimethyl siloxane) capillary sensor array for single-step and multiple enzyme inhibitor assays

We describe a new method for fabricating a capillary-type sensor, called a combinable poly(dimethyl siloxane) (PDMS) capillary (CPC) sensor. The method for preparing the CPC simplifies enzyme inhibitor assays into a simple, single step assay. The sample inhibitor solution is introduced by capillary action. This triggers the spontaneous dissolution of physically adsorbed fluorescent substrates, and the substrate mixes with the inhibitor. This is followed by competitive reaction with insoluble enzyme to give a fluorescence response. CPC is composed of a convex-shaped PDMS stick containing reagents immobilized in an insoluble coating, and a concave-shaped PDMS stick containing reagents immobilized in a soluble coating. Since the concave-shaped PDMS has a deeper channel than the convex structure, combining these PDMS sticks is like closing the zipper of a "freezer bag". This allows easy fabrication of "thin and long" capillary structures containing different reagents inside the same capillary, without the need for precise alignment. This method allows the immobilization of two reactive reagents, such as enzyme and substrate required for a single step assay, which are typically very difficult to immobilize using commercially available conventional capillaries. Furthermore, by simply arraying various CPCs, the CPC sensor allows multiple assays. Here, we carried out a single-step enzyme inhibitor assay using the CPC. In addition, two independent CPCs were arrayed to demonstrate multiple assaying of a protease inhibitor.     

Development of an ultra-low volume flow cell for surface plasmon resonance detection in a miniaturized capillary electrophoresis system

A miniaturized capillary electrophoresis system coupled to a surface plasmon resonance (SPR) sensor on a microfluidic platform fabricated from PDMS is detailed. A previously described split-flow injection technique is first utilized to manipulate sample into the microfluidic chip, followed by separation within the fused-silica capillary and final off-capillary detection of analytes via SPR. Instead of using commercial SPR flow cells requiring relatively large detection volumes, samples of less than 1 nL volume are utilized. The interface between the CE system and SPR sensor made it possible to detect minute volumes of sample with minimal dispersion. The flow cell has the potential to be applicable to miniaturized flow-injection (FI) systems where submicroliter volumes of sample are frequently only available for analysis. The components present in solution, but not bound to the sensor surface, were also investigated. The sensitivity of the CE-SPR system was similar to that found in UV-spectrometric instruments and nonchromophoric components could also be measured.     

Solvent‐enhanced headspace sorptive extraction in the analysis of the volatile fraction of matrices of vegetable origin

The solvent‐enhanced headspace sorptive extraction technique aims at modifying PDMS polarity using a solvent to increase its concentration capability. In solvent‐enhanced headspace sorptive extraction, a PDMS tubing closed at both ends by small glass stoppers and filled with an organic solvent is suspended in the sample headspace for a fixed time. After sampling, the sampled analytes are recovered from the PDMS tubing by thermal desorption and online transferred to a GC–flame ionization detector or GC‐MS system for analysis. Cyclohexane, iso‐octane, ethyl acetate, acetone, acetonitrile and methanol were tested as PDMS modifiers to sample the volatile fractions of sage (Salvia lavandulifolia Vahl.), thyme (Thymus vulgaris L.) and roasted coffee. Ethyl acetate was found to be the most effective PDMS modifier for all matrices investigated; although to a lesser extent, cyclohexane also increased component recoveries with sage and thyme. Acetone, acetonitrile and methanol did not increase PDMS recovery, while isooctane was excluded because of its interaction with the polymer. The results show that solvent‐modified PDMS extends the range of sampled headspace components with different polarities, increases the recovery of many of them, improves sensitivity in trace analysis, speeds up recovery and gives repeatability comparable with that of unmodified PDMS.     

A Simple Method for Fabricating Patterned Curvilinear Microstructures in Poly(dimethylsiloxane) by Selective Wetting

The fabrication of patterned microstructures in poly(dimethylsiloxane) (PDMS) is a prerequisite for soft lithography. Herein, curvilinear surface relief microstructures in PDMS are fabricated through a simple three‐stage approach combining microcontact printing (μCP), selective surface wetting/dewetting and replica molding (REM). First, using an original PDMS stamp (first‐generation stamp) with linear relief features, a chemical pattern on gold substrate is generated by μCP using hexadecanethiol (HDT) as an ink. Then, by a dip‐coating process, an ordered polyethylene glycol (PEG) polymer‐dot array forms on the HDT‐patterned gold substrate. Finally, based on a REM process, the PEG‐dot array on gold substrate is used to fabricate a second‐generation PDMS stamp with microcavity array, and the second‐generation PDMS stamp is used to generate third‐generation PDMS stamp with microbump array. These fabricated new‐generation stamps are utilized in μCP and in micromolding in capillaries (MIMIC), allowing the generation of surface micropatterns which cannot be obtained using the original PDMS stamp. The method will be useful in producing new‐generation PDMS stamps, especially for those who want to use soft lithography in their studies but have no access to the microfabrication facilities.     

Screen-printed dissolved oxygen sensor based on cerium oxide-supported silver catalyst and polydimethylsiloxane film

A screen-printed dissolved oxygen sensor was fabricated using cerium oxide-supported silver catalyst and polydimethylsiloxane (PDMS) film. A PDMS film of 3 μm thickness showed good permeability for oxygen and impermeability for hydrogen peroxide. The calibration curve has shown a linear relationship with a correlation coefficient of 0.996 for the dissolved oxygen concentration. The sensitivity and detection limit of the present sensor were calculated at -158 μA mM(-1) and 8.4 μM, respectively.     

DNA-directed protein immobilization on mixed self-assembled monolayers via a streptavidin bridge

The simultaneous detection of multiple analytes is an important consideration for the advancement of biosensor technology. Currently, few sensor systems possess the capability to accurately and precisely detect multiple antigens. This work presents a simple approach for the functionalization of sensor surfaces suitable for multichannel detection. This approach utilizes self-assembled monolayer (SAM) chemistry to create a nonfouling, functional sensor platform based on biotinylated single-stranded DNA immobilized via a streptavidin bridge to a mixed SAM of biotinylated alkanethiol and oligo(ethylene glycol). Nonspecific binding is minimized with the nonfouling background of the sensor surface. A usable protein chip is generated by applying protein-DNA conjugates which are directed to specific sites on the sensor chip surface by utilizing the specificity of DNA hybridization. The described platform is demonstrated in a custom-built surface plasmon resonance biosensor. The detection capabilities of a sensor using this protein array have been characterized using human chorionic gonadotropin (hCG). The platform shows a higher sensitivity in detection of hCG than that observed using biotinylated antibodies. Results also show excellent specificity in protein immobilization to the proper locations in the array. The vast number of possible DNA sequences combine with the selectivity of base-pairing makes this platform an excellent candidate for a sensor capable of multichannel protein detection.     

Application of a capillary‐assembled microfluidic system for separation of cephalosporins

This paper demonstrates a simple and easy setting up of a fused‐silica capillary‐assembled microfluidic system (μCE). This system incorporates a split‐flow pressure injection of the sample into a microfluidic system made from PDMS and a short (～20 cm) length of fused‐silica capillary as a separation unit. The on‐capillary detection was carried out by fiber optic spectrometry. A mixture of six cephalosporin antibiotics was separated in the μCE system and the obtained results were compared to those achievable by conventional CE. The six components could be separated within 8.5 min with the number of theoretical plates around 10 000.     

Transient deflection response in microcantilever array integrated with polydimethylsiloxane (PDMS) microfluidics

We report the integration of a nanomechanical sensor consisting of 16 silicon microcantilevers with polydimethylsiloxane (PDMS) microfluidics. For microcantilevers positioned near the bottom of a microfluidic flow channel, a transient differential analyte concentration for the top versus bottom surface of each microcantilever is created when an analyte-bearing fluid is introduced into the flow channel (which is initially filled with a non-analyte containing solution). We use this effect to characterize a bare (nonfunctionalized) microcantilever array in which the microcantilevers are simultaneously read out with our recently developed high sensitivity in-plane photonic transduction method. We first examine the case of non-specific binding of bovine serum albumin (BSA) to silicon. The average maximum transient microcantilever deflection in the array is -1.6 nm, which corresponds to a differential surface stress of only -0.23 mN m(-1). This is in excellent agreement with the maximum differential surface stress calculated based on a modified rate equation in conjunction with finite element simulation. Following BSA adsorption, buffer solutions with different pH are introduced to further study microcantilever array transient response. Deflections of 20-100 nm are observed (2-14 mN m(-1) differential surface stress). At a flow rate of 5 μL min(-1), the average measured temporal width (FWHM) of the transient response is 5.3 s for BSA non-specific binding and 0.74 s for pH changes.     

Laser induced surface modification of polydimethylsiloxane as a super-hydrophobic material

In order to render the surface of polydimethylsiloxane (PDMS) super-hydrophobic without changing its bulk properties, a PDMS film without photosensitizer was exposed to CO₂ pulsed laser, at room temperature, as the excitation source. The modified surfaces have been studied by performing scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDXA) and attenuated total reflectance infrared (ATR-IR) spectroscopy. To evaluate the surface property, the water drop contact angle was measured. The dependence of ---Si---O---Si infrared peak intensity, O/Si ratio and water drop contact angle of the treated PDMS as a function of the number of laser pulses were studied. SEM micrographs and water drop contact angle variations show the uniform porosity and super-hydrophobic nature on the surface of PDMS. ATR-FTIR spectra show that the modified PDMS surface contains carbonate groups which enriched the oxygen content of the surface. EDXA analysis shows a higher percentage of oxygen on the surface of the modified PDMS. The hydrophobicity of the samples was found to depend upon the number of laser pulses, but with significant variation between the treated samples. The bulk mechanical properties of PDMS after being laser-treated did not change as shown by dynamic mechanical thermal analysis (DMTA).     

Electric detection of DNA with PDMS microgap electrodes and silver nanoparticles

In this work a novel microdevice sensor has been developed by plating gold on the PDMS surface to generate a sandwich-type gap electrode for DNA detection. The microdevice utilizes a gold band electrode-PDMS-gold band electrode configuration and the minimum detectable volume could be as low as 5 μL. The 20 μm PDMS-based gap was chemically modified with DNA capture probes and DNA sandwich hybrids were formed with the addition of DNA target and silver nanoparticle probes. To increase detection sensitivity, parallel detection zones have been developed in which the relevant resistances decrease substantially upon hybridyzation. By measuring the change in electrical conductivity, the DNA target in the concentration range of 1000-0.1 nM can be assayed and the limit of lowest detectable concentration was achieved at 0.01 nM.     

Probing photoelectrochemical processes in Au-CdS nanoparticle arrays by surface plasmon resonance: application for the detection of acetylcholine esterase inhibitors

The photoelectrochemical charging of Au-nanoparticles (NP) in a Au-nanoparticle/CdS-nanoparticle array assembled on a Au-coated glass surface is followed by means of surface plasmon resonance (SPR) spectroscopy upon continuous irradiation of the sample. The charging of the Au-NPs results in the enhanced coupling between the localized surface plasmon of the Au-NP and the surface plasmon of the bulk surface, leading to a shift in the plasmon angle. The charging effect of the Au-NPs is supported by concomitant electrochemical experiments in the dark. Analysis of the results indicates that ca. 4.2 electrons are associated with each Au-nanoparticle under steady-state irradiation. The photoelectrochemical charging effect of the Au-NPs in the Au-CdS NP array is employed to develop a SPR sensor for acetylcholine esterase inhibitors.     

Properties of poly(dimethylsiloxane)/hydrogel multicomponent systems

The properties of surface‐ and bulk‐modified poly(dimethylsiloxane) (PDMS) were examined. Laser‐induced surface grafting of poly(2‐hydroxyethyl methacrylate) (PHEMA) on PDMS and a sequential method for preparation of interpenetrating polymer networks of PDMS/PHEMA were, respectively, used for surface and bulk modifications. The hydrogel content and water‐uptake capability of the modified samples were also investigated. The modified PDMS samples were examined by performing attenuated total reflection/Fourier transform infrared spectroscopy, dynamic mechanical thermal analysis, scanning electron microscopy, and water contact‐angle measurements. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2145–2156, 2003     

Fabrication of bimetallic microfluidic surface-enhanced Raman scattering sensors on paper by screen printing

Au–Ag bimetallic microfluidic, dumbbell-shaped, surface enhanced Raman scattering (SERS) sensors were fabricated on cellulose paper by screen printing. These printed sensors rely on a sample droplet injection zone, and a SERS detection zone at either end of the dumbbell motif, fabricated by printing silver nanoparticles (Ag NPs) and gold nanoparticles (Au NPs) successively with microscale precision. The microfluidic channel was patterned using an insulating ink to connect these two zones and form a hydrophobic circuit. Owing to capillary action of paper in the millimeter-sized channels, the sensor could enable self-filtering of fluids to remove suspended particles within wastewater without pumping. This sensor also allows sensitive SERS detection, due to advantageous combination of the strong surface enhancement of Ag NPs and excellent chemical stability of Au NPs. The SERS performance of the sensors was investigated by employing the probe rhodamine 6G, a limit of detection (LOD) of 1.1 × 10⁻¹³ M and an enhancement factor of 8.6 × 10⁶ could be achieved. Moreover, the dumbbell-shaped bimetallic sensors exhibited good stability with SERS performance being maintained over 14 weeks in air, and high reproducibility with less than 15% variation in spot-to-spot SERS intensity. Using these dumbbell-shaped bimetallic sensors, substituted aromatic pollutants in wastewater samples could be quantitatively analyzed, which demonstrated their excellent capability for rapid trace pollutant detection in wastewater samples in the field without pre-separation.     

A prototypic system of parallel electrophoresis in multiple capillaries coupled with microwell arrays

We present a microfluidic system that can be directly coupled with microwell array and perform parallel electrophoresis in multiple capillaries simultaneously. The system is based on an array of glass capillaries, fixed in a polydimethylsiloxane (PDMS) microfluidic scaffold, with one end open for interfacing with microwells. In this capillary array, every two adjacent capillaries act as a pair to be coupled with one microwell; samples in the microwells are introduced and separated by simply applying voltage between two electrodes that are placed at the other ends of capillaries; thus no complicated circuit design is required. We evaluate the performance of this system and perform multiple CE with direct sample introduction from microwell array. Also with this system, we demonstrate the analysis of cellular contents of cells lysed in a microwell array. Our results show that this prototypic system is a promising platform for high-throughput analysis of samples in microwell arrays.     

A surface plasmon resonance immunosensor for detecting a dioxin precursor using a gold binding polypeptide

A surface plasmon resonance (SPR) based biosensor was developed for monitoring 2,4-dichlorophenol, a known dioxin precursor, using an indirect competitive immunoassay. The SPR sensor was fabricated by immobilizing a gold-thin layer on the surface of an SPR sensor chip with an anti-(2,4-dichlorophenol) antibody using a gold binding polypeptide (GBP) and protein G. The SPR response based on the antigen-antibody reaction in a flow system was measured by injecting a 2,4-dichlorophenol sample solution into the flow system in which the SPR sensor was located. In a direct immunoassay system using the modified sensor chip, no significant SPR angle shift less than 0.001° was observed when a 25 ppm of 2,4-dichlorophenol solution was injected. In order to improve the sensitivity of the SPR sensor, an indirect competitive immunoassay method was used in conjunction with the SPR sensor system using 2,4-dichlorophenol conjugated with bovine serum albumin (BSA). In the competitive assay, a 350 ppm 2,4-dichlorophenol-BSA conjugate solution containing 2,4-dichlorophenol at various concentrations (10-250 ppb) were injected into the SPR sensor system. The sensitivity of this indirect immunoassay was found to be extremely sensitive, compared to the direct one, and a detection limit of 20 ppb was estimated. Verification that the use of GBP for immobilizing the antibody on the sensor chip enhanced the sensitivity to 2,4-dichlorophenol was obtained by comparing the procedure with another modification, in which BSA was used instead of GBP for immobilizing the antibody on the sensor chip. The affinity constant of 2,4-dichlorophenol and its conjugate to the antibody were estimated form the SPR response.     

纳米银碗阵列结构的高效制备及荧光增强性能

报道了一种以自组装单层聚苯乙烯纳米微球阵列为模板, 通过真空热蒸镀银纳米粒子高效制备大面积银碗阵列结构的方法. 测试结果表明, 制得的银碗阵列结构为微纳米复合分级结构, 银碗由平均粒径为10 nm的银纳米粒子组成. 紫外-可见吸收光谱测试结果表明, 银碗阵列结构表面具有银纳米粒子的局域表面等离子体共振吸收峰. 将荧光分子N,N'-二正丁基喹吖啶酮(DBQA)分别蒸镀到普通银膜和银碗阵列结构表面并测试了荧光光谱. 结果表明, 在银碗阵列结构表面的荧光分子强度得到了显著增强, 说明制备的银碗阵列结构是优良的荧光增强基底.     

Inkjet-printed gold nanoparticle electrochemical arrays on plastic. Application to immunodetection of a cancer biomarker protein

Electrochemical detection combined with nanostructured sensor surfaces offers potentially low-cost, high-throughput solutions for detection of clinically significant proteins. Inkjet printing offers an inexpensive non-contact fabrication method for microelectronics that is easily adapted for incorporating into protein immunosensor devices. Herein we report the first direct fabrication of inkjet-printed gold nanoparticle arrays, and apply them to electrochemical detection of the cancer biomarker interleukin-6 (IL-6) in serum. The gold nanoparticle ink was printed on a flexible, heat resistant polyimide Kapton substrate and subsequently sintered to create eight-electrode arrays costing <0.2 euro per array. The inkjet-printed working electrodes had reproducible surface areas with RSD <3%. Capture antibodies for IL-6 were linked onto the eight-electrode array, and used in sandwich immunoassays. A biotinylated secondary antibody with 16-18 horseradish peroxidase labels was used, and detection was achieved by hydroquinone-mediated amperometry. The arrays provided a clinically relevant detection limit of 20 pg mL(-1) in calf serum, sensitivity of 11.4 nA pg(-1) cm(-2), and a linear dynamic range of 20-400 pg mL(-1).     

Surface modification of poly(dimethylsiloxane) with a perfluorinated alkoxysilane for selectivity toward fluorous tagged peptides

Poly(dimethylsiloxane) (PDMS) and similar polymers have proved to be of widespread interest for use in microfluidic and similar microanalytical devices. Surface modification of PDMS is required to extend the range of applications for devices made of this polymer, however. Here we report on the grafting of perfluorooctyltriethoxysilane via hydrolysis onto an oxidized PDMS substrate in order to form a fluorinated microchannel. Such a fluorinated device could be used for separating fluorous tagged proteins or peptides, similar to that which has been recently demonstrated in a capillary electrophoresis system or in an open tubular capillary column. The modified polymer is characterized using chemical force titrations, contact angle measurements, and X-ray photoelectron spectroscopy (XPS). We also report on a novel means of performing electroosmotic measurements on this material to determine the surface zeta potential. As might be expected, contact angle and chemical force titration measurements indicate the fluorinated surface to be highly hydrophobic. XPS indicates that fluorocarbon groups segregate to the surface of the polymer over a period of days following the initial surface modification, presumably driven by a lower surface free energy. One of the most interesting results is the zeta potential measurements, which show that significant surface charge can be maintained across a wide range of pH on this modified polymer, sufficient to promote electroosmotic flow in a microfluidic chip. Matrix-assisted time-of-flight mass spectrometry (MALDI-TOF MS) measurements show that a fluorous-tagged peptide will selectively adsorb on the fluorinated PDMS in aqueous solution, demonstrating that the fluorinated polymer could be used in devices designed for the enrichment or enhanced detection of fluorous-labeled proteins and peptides.     

Electrodeposition of Ag nanoparticles on graphenized pencil lead electrode as a sensitive and low-cost sensor for iodate determination

In this study, a simple, sensitive and low-cost iodate electrochemical sensor based on graphenized pencil lead electrode (GPLE) modified with Ag nanoparticles (AgNPs) was presented. The GPLE was simply prepared via electrochemical exfoliation of pencil lead electrode (PLE) by applying an optimized potential in acidic media. Afterward, silver nanoparticles were electrochemically deposited on the surface of GPLE using chronoamperometry technique. The fabricated electrode was carefully characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) techniques. Electrochemical behavior and also the electrocatalytic performance of the modified electrode toward the reduction of iodate were studied in details using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The fabricated sensor responds efficiently to iodate over the concentration range of 0.05 to 75 mM with a detection limit of 0.017 mM and sensitivity of 0.26 µA µM^?1 cm^?2. Remarkably enhanced electrocatalytic performance of the modified electrode was ascribed to the synergistic effect of graphene-like nanostructures with high surface to volume ratio, excellent conductivity and also the excessive electrocatalytic behavior of silver nanoparticles. The modified electrode was successfully employed for the determination of iodate in table and sea salt samples.     

Single etch patterning of stacked silver and molybdenum alloy layers on glass using microcontact wave printing

Stacked thin layers of silver alloy (AgPdCu) and MoCr layers on 10 x 15 cm2 glass substrates were patterned by microcontact wave printing and etching. Patterns of etch-resistant octadecanethiol self-assembled monolayers (SAMs) were wave printed with regular backplane stabilized PDMS stamps. Pattern development was achieved by etching both metal layers in a single step, employing a nitric acid-based etching bath. Trifluoroacetic acid and a nitrite salt were identified as essential bath components for a homogeneous etching process. Etch defects could be eliminated by the addition of a decanesulfonate, which stabilizes the SAM resist via a defect healing mechanism.