Chien-Fu Chen

Detection of Mercury(II) Ions Using Colorimetric Gold Nanoparticles on Paper-Based Analytical Devices

An on-field colorimetric sensing strategy employing gold nanoparticles (AuNPs) and a paper-based analytical platform was investigated for mercury ion (Hg(2+)) detection at water sources. By utilizing thymine-Hg(2+)-thymine (T-Hg(2+)-T) coordination chemistry, label-free detection oligonucleotide sequences were attached to unmodified gold nanoparticles to provide rapid mercury ion sensing without complicated and time-consuming thiolated or other costly labeled probe preparation processes. Not only is this strategys sensing mechanism specific toward Hg(2+), rather than other metal ions, but also the conformational change in the detection oligonucleotide sequences introduces different degrees of AuNP aggregation that causes the color of AuNPs to exhibit a mixture variance. To eliminate the use of sophisticated equipment and minimize the power requirement for data analysis and transmission, the color variance of multiple detection results were transferred and concentrated on cellulose-based paper analytical devices, and the data were subsequently transmitted for the readout and storage of results using cloud computing via a smartphone. As a result, a detection limit of 50 nM for Hg(2+) spiked pond and river water could be achieved. Furthermore, multiple tests could be performed simultaneously with a 40 min turnaround time. These results suggest that the proposed platform possesses the capability for sensitive and high-throughput on-site mercury pollution monitoring in resource-constrained settings.

Polymer Microchips Integrating Solid-Phase Extraction and High-Performance Liquid Chromatography Using Reversed-Phase Polymethacrylate Monoliths

Polymer microfluidic chips employing in situ photopolymerized polymethacrylate monoliths for high-performance liquid chromatography separations of peptides is described. The integrated chip design employs a 15 cm long separation column containing a reversed-phase polymethacrylate monolith as a stationary phase, with its front end seamlessly coupled to a 5 mm long methacrylate monolith which functions as a solid-phase extraction (SPE) element for sample cleanup and enrichment, serving to increase both detection sensitivity and separation performance. In addition to sample concentration and separation, solvent splitting is also performed on-chip, allowing the use of a conventional LC pump for the generation of on-chip nanoflow solvent gradients. The integrated platform takes advantage of solvent bonding and a novel high-pressure needle interface which together enable the polymer chips to withstand internal pressures above 20 MPa (approximately 2900 psi) for efficient pressure-driven HPLC separations. Gradient reversed-phase separation of fluorescein-labeled model peptides and BSA tryptic digest are demonstrated using the microchip HPLC system. Online removal of free fluorescein and enrichment of labeled proteins are simultaneously achieved using the on-chip SPE column, resulting in a 150-fold improvement in sensitivity and a 10-fold reduction in peak width in the following microchip gradient LC separation.

Covalently functionalized double-walled carbon nanotubes combine high sensitivity and selectivity in the electrical detection of small molecules.

Atom-thick materials such as single-walled carbon nanotubes (SWCNTs) and graphene exhibit ultrahigh sensitivity to chemical perturbation partly because all of the constituent atoms are surface atoms. However, low selectivity due to nonspecific binding on the graphitic surface is a challenging issue to many applications including chemical sensing. Here, we demonstrated simultaneous attainment of high sensitivity and selectivity in thin-film field effect transistors (TFTs) based on outer-wall selectively functionalized double-walled carbon nanotubes (DWCNTs). With carboxylic acid functionalized DWCNT TFTs, we obtained excellent gate modulation (on/off ratio as high as 4000) with relatively high ON currents at a CNT areal density as low as 35 ng/cm(2). The devices displayed an NH(3) sensitivity of 60 nM (or ~1 ppb), which is comparable to small molecule aqueous solution detection using state-of-the-art SWCNT TFT sensors while concomitantly achieving 6000 times higher chemical selectivity toward a variety of amine-containing analyte molecules over that of other small molecules. These results highlight the potential of using covalently functionalized double-walled carbon nanotubes for simultaneous ultrahigh selective and sensitive detection of chemicals and illustrate some of the structural advantages of this double-wall materials strategy to nanoelectronics.

Paper-based tuberculosis diagnostic devices with colorimetric gold nanoparticles

Abstract A colorimetric sensing strategy employing gold nanoparticles and a paper assay platform has been developed for tuberculosis diagnosis. Unmodified gold nanoparticles and single-stranded detection oligonucleotides are used to achieve rapid diagnosis without complicated and time-consuming thiolated or other surface-modified probe preparation processes. To eliminate the use of sophisticated equipment for data analysis, the color variance for multiple detection results was simultaneously collected and concentrated on cellulose paper with the data readout transmitted for cloud computing via a smartphone. The results show that the 2.6 nM tuberculosis mycobacterium target sequences extracted from patients can easily be detected, and the turnaround time after the human DNA is extracted from clinical samples was approximately 1 h.

Flow-through immunosensors using antibody-immobilized polymer monoliths.

High-sensitivity and rapid flow-through immunosensors based on photopolymerized surface-reactive polymer monoliths are investigated. The porous monoliths were synthesized within silica capillaries from glycidyl methacrylate and ethoxylated trimethylolpropane triacrylate precursors, providing a tortuous pore structure with high surface area for the immobilization of antibodies or other biosensing ligands. The unique morphology of the monolith ensures efficient mass transport and interactions between solvated analyte molecules and covalently immobilize antibodies anchored to the monolith surface, resulting in rapid immunorecognition. The efficacy of this approach is demonstrated through a direct immunoassay model using anti-IgG as a monolith-bound capture antibody and fluorescein-labeled IgG as an antigen. In situ antigen measurements exhibited a linear response over a concentration range between 0.1 and 50 ng/mL with 5 min assay times, while controllable injection of 1 μL volumes of antigen through the monolith elements yielded a mass detection limit of 100 pg ((∼700amol). These results suggest that porous monolith supports represent a flexible and promising material for the fabrication of rapid and sensitive immunosensors suitable for integration into capillary or microfluidic devices.

High-pressure on-chip mechanical valves for thermoplastic microfluidic devices.

A facile method enabling the integration of elastomeric valves into rigid thermoplastic microfluidic chips is described. The valves employ discrete plugs of elastomeric polydimethylsiloxane (PDMS) integrated into the thermoplastic substrate and actuated using a threaded stainless steel needle. The fabrication process takes advantage of poly(ethylene glycol) (PEG) as a sacrificial molding material to isolate the PDMS regions from the thermoplastic flow channels, while yielding smooth contact surfaces with the PDMS valve seats. The valves introduce minimal dead volumes, and provide a simple mechanical means to achieve reproducible proportional valving within thermoplastic microfluidic systems. Burst pressure tests reveal that the valves can withstand pressures above 12 MPa over repeated open/close cycles without leakage, and above 24 MPa during a single use, making the technology well suited for applications such as high performance liquid chromatography. Proportional valve operation is demonstrated using a multi-valve chemical gradient generator fabricated in cyclic olefin polymer.

Size-tunable copper nanocluster aggregates and their application in hydrogen sulfide sensing on paper-based devices

Polystyrene sulfonate (PSS), a strong polyelectrolyte, was used to prepare red photoluminescent PSS-penicillamine (PA) copper (Cu) nanoclusters (NC) aggregates, which displayed high selectivity and sensitivity to the detection of hydrogen sulfide (H2S). The size of the PSS-PA-Cu NC aggregates could be readily controlled from 5.5 μm to 173 nm using different concentrations of PSS, which enabled better dispersity and higher sensitivity towards H2S. PSS-PA-Cu NC aggregates provided rapid H2S detection by using the strong Cu-S interaction to quench NC photoluminescence as a sensing mechanism. As a result, a detection limit of 650 nM, which is lower than the maximum level permitted in drinking water by the World Health Organization, was achieved for the analysis of H2S in spring-water samples. Moreover, highly dispersed PSS-PA-Cu NC aggregates could be incorporated into a plate-format paper-based analytical device which enables ultra-low sample volumes (5 μL) and feature shorter analysis times (30 min) compared to conventional solution-based methods. The advantages of low reagent consumption, rapid result readout, limited equipment, and long-term storage make this platform sensitive and simple enough to use without specialized training in resource constrained settings.

Mixed-mode electrokinetic and chromatographic peptide separations in a microvalve-integrated polymer chip

A cycloolefin polymer chip supporting the concatenation of isoelectric focusing (IEF) and reversed-phase liquid chromatography (RPLC) is demonstrated for high throughput two dimensional peptide separations. A unique benefit of the mixed-mode platform is the ability of IEF to act as a highly concentrating electrokinetic separation mode for effective isolation of sample components prior to RPLC. The thermoplastic chip contains integrated high pressure microvalves, enabling uniform sample transfer from the IEF channel to multiple parallel RPLC channels, gradient elution from each RPLC column, and hydrodynamic isolation between the separation dimensions. The reusable system is shown to provide efficient 2-D separations together with facile interfacing with MALDI-MS, suggesting a new path towards effective peptide analysis from complex samples.

A microfluidic nanoliter mixer with optimized grooved structures driven by capillary pumping

It is known that surface tension-capillary pumping is an effective driving force in a microchannel, however a power-free mixer that uses only surface tension has not yet been achieved. In the present study, a power-free method is explored to perform mixing in a microchannel without any external active mechanisms such as pumps, valves or external energies like electrostatic or magnetic fields. The mixer is cost effective as the channel is designed to have no sidewalls with the liquid being confined to flow between a bottom hydrophilic stripe and a top-covered hydrophobic substrate. It is found from both theoretical analysis and experiments that for a given channel width, the flow rate solely due to capillary pumping can be maximized at an optimal channel height. The flow rate is in the order of nanoliters per second, for example, the flow rate is 0.65 nL s?1 at the optimal channel height 13 ?m, given the channel width 100 ?m. It is most crucial to this power-free mixing device that two liquid species must be well mixed before the liquids are transported to exit to a reservoir. For this purpose, asymmetric staggered grooved cavities are optimally arranged on the bottom substrate of the channel to help mixing two different liquid species. It is shown that maximum mixing occurs when the depth of the grooved structures is about two-thirds of the total channel height.

Microscale Patterning of Thermoplastic Polymer Surfaces by Selective Solvent Swelling

A new method for the fabrication of microscale features in thermoplastic substrates is presented. Unlike traditional thermoplastic microfabrication techniques, in which bulk polymer is displaced from the substrate by machining or embossing, a unique process termed orogenic microfabrication has been developed in which selected regions of a thermoplastic surface are raised from the substrate by an irreversible solvent swelling mechanism. The orogenic technique allows thermoplastic surfaces to be patterned using a variety of masking methods, resulting in three-dimensional features that would be difficult to achieve through traditional microfabrication methods. Using cyclic olefin copolymer as a model thermoplastic material, several variations of this process are described to realize growth heights ranging from several nanometers to tens of micrometers, with patterning techniques include direct photoresist masking, patterned UV/ozone surface passivation, elastomeric stamping, and noncontact spotting. Orogenic microfabrication is also demonstrated by direct inkjet printing as a facile photolithography-free masking method for rapid desktop thermoplastic microfabrication.