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The development and analysis of a microfluidic sample preconcentration system using a highly ion-conductive charge-selective polymer [poly-AMPS (2-acrylamido-2methyl-1-propanesulfonic acid)] is reported. The preconcentration is based on the phenomenon of concentration polarization which develops at the boundaries of the poly-AMPS with buffer solutions. A negatively charged polymer, poly-AMPS, positioned between two microchannels efficiently extracts cations through its large cross section, resulting in efficient anion sample preconcentration. The present work includes the development of a robust polymer that is stable over a wide range of buffers with varying chemical compositions. The sample preconcentration effect remains linear to over 3 mM (0.15 pmol) and 500/~M (15 fmol) for fluorescein and TRITC-tagged albumin solutions, respectively. The system can potentially be used for concentrating proteins on microfluidic devices with subsequent analysis for proteomic applications. 10.1021/ac101297t

We finely tuned the Au shell on a polystyrene microsphere of 2 [micro]m in diameter to achieve a strong surface enhanced raman scattering (SERS)-active platform so that the molecules on a single microspherical shell surface produce their own fingerprint SERS spectra. The proposed microshells can be easily and individually manipulated under a conventional optical microscope using a micropipet and act as a sensitive probe to obtain the SERS spectra of the monolayer of molecules on Pt as well as Au surfaces without any requirement of special surface morphology or modification for inducing SERS activity. Well-defined SELLS spectra can be obtained at a very short acquisition time of milliseconds, suggesting useful applications of the present system based on the decoding of the SERS-active barcodes on individually functionalized microshells. 10.1021/ac901904v

A microfluidic electroporator operating under a continuous low dc voltage (7 to ~ 15 V) is reported. The proposed electroporation microchip exploits the ionic conductivity of polyelectrolytic gel electrodes to precisely control the electric field that is applied to cells without bubble generation in the microchannel. In this study, pDADMAC (poly diallyldimethylammonium chloride) was used to efficiently apply the electric potential difference to the cells in the microchannels. Impedance analysis showed that the pDADMAC plugs could work as ionic conductors with a conductivity of approximately 16 S [m.sup.-1]. In accordance with the calculation using CFD-ACE, an input voltage of only 10 V could generate an electric field of 0.9 kV [cm.sup.-1] across the microchannel; this meets the requirements for electropermeation. The electropermeation of K562 human chronic leukemia cells was observed in the microchip from 7 V, and the efficiency increased up to 60% upon the application of an input voltage of 15 V with a viability of 80%. An amount of 105 cells could be transfected every minute under a constant potential difference. The transfection and expression of DNA plasmids were also successfully demonstrated in the suspension cell fine.

The enantiomeric recognition of amines by voltammetry using electroactive macrocyclic molecules, nitroazophenolic crown ethers, is reported. The oxidation potential of the nitroazophenol moiety in nitroazophenols with 18-crown-6 sensitively depends on the structure of alkyl amines. Based on this phenomenon, enantiomeric amines and even the quantitative assay of the R/S ratio in enantiomeric mixtures can be selectively recognized by using 18-crown-6 azophenol (3-H) with chiral centers. In the case of phenylglycinol, the association constants (K) of 3-H for the R and S forms have an R/S value of 3.5. The peak potential of the R form in square-wave voltammograms reproducibly differs from that of the S form by 32 mV, within which the peak potential linearly varies with the enantiomeric ratio. Free energy perturbation and molecular dynamics simulation provide deeper understanding of the enantiomeric recognition in this system. The theoretical analysis indicates that the free energy difference between diastereomeric complexes agrees well with the experimental results, and the [pi]-[pi] of charge--charge interaction plays a key role in enantiomeric recognition.

We report the fabrication and analytical functions of a biosensor based on a nanoneedle consisting of a multiwall carbon nanotube attached to the end of an etched tungsten tip. The devised electrode is the smallest needle-type biosensor reported to date. The nanoneedles prepared in this work are 30 nm in diameter and 2-3 [micro]m in length. Dopamine and glutamate, which are physiologically important neurotransmitters, were successfully detected using these nanoneedles. Bare nanoneedles detected dopamine in the range from 100 to 1000 [micro]M by differential pulse voltammetry, and enzyme-modified nanoneedles were able to respond to glutamate in the 100 500 [micro]M range by potentiostatic amperometry.

The nanoporous platinum oxide ([H.sub.1]-ePtO) as a hydrogen ion-selective sensing material is reported. Bare nanoporous platinum oxides exhibit near-Nernstian behavior (e.g., -55 mV/pH in PBS), ignorable hysteresis, a short response time, and high precision, which are remarkably better than those of flat platinum oxides. The electrode potential of a nanoporous platinum oxide responds exclusively to hydrogen ion, which implies its usefulness as a solid-state pH sensor. In the present study, the performance of nanoporous platinum oxide was investigated and compared with that of Ir[O.sub.x] in terms of selectivity and the influences of ionic strength, temperature, complexing ligands, and surfactants. HrePtO functions well as a pH-sensing solid-state material, and it is viewed as a promising alternative to Ir[O.sub.x]. Interference by redox couples was successfully suppressed by covering the [H.sub.1]-ePtO surface with a protective layer, e.g., an electropolymerized polyphenol thin film. Since the nanoporous platinum oxide with such a protective layer is particularly suitable for miniaturization and micropatterning, our findings suggest its usefulness in applications such as solid-state pH sensors embedded in chip-based microanalysis systems.

This paper reports a polyelectrolytic salt bridge-based electrode (PSBE), which is a key embedded unit in a microchip device that can size-selectively count microparticles and measure their velocities. The construction of salt bridges at specific locations within a microfluidic chip enables dc-driven electrical detection to be performed successfully. This is expected to be a competitive alternative to the optical methods currently used in conventional cell sorters. The PSBEs were fabricated by irradiating ultraviolet light over a patterned mask on the parts of interest, which were filled with an aqueous monomer solution containing diallyldimethylammonium chloride. A pair of such PSBEs was easily formed at the two lateral branches perpendicular to the main microchannel and was found to be very useful for dc impedometry. The human blood cells as well as the fluorescent microbeads passing between the two PSBEs produced impedance signals in proportional to their size. The information about the velocity of a microparticle was extracted from a doublet of the dc impedance signals, which were generated when cells or microbeads sequentially passed through two PSBE pairs separated from each other by a fixed distance. The plot of peak amplitude versus velocity of the moving microbeads and cells indicated only a slight correlation between the size and the velocity, which means that the peak amplitude of the dc impedance signals alone can provide information about the size of the cells in a mixture. The experimental results showed a screening rate of over 1000 cells [s.sup.-1] and a velocity of the cells of over 100 mm [s.sup.-1]. Compared with the previously suggested electrical detection system based on metal electrodes, the sensitivity and selectivity in cell detection were remarkably improved. In addition, the detection unit including the operating circuit became innovatively simple and the whole device could be miniaturized.

Roughness of nanoscopic dimensions can be used to selectively enhance the faradaic current of a sluggish reaction. Using this principle, we constructed mesoporous structures on the surfaces of pure platinum electrodes responding even more sensitively to glucose than to common interfering species, such as L-ascorbic acid and 4-acetamidophenol. Good sensitivities, as high as 9.6 [micro]A [cm.sup.-2] [mM.sup-1], were reproducibly observed in the presence of high concentration of chloride ion. The selectivities, sensitivities, and stabilities determined experimentally have demonstrated the potential of mesoporous platinum as a novel candidate for nonenzymatic glucose sensors.

Mesoporous platinum (Pt) has received much attention with regard to its potential applications. By an electrochemical method, a hexagonal liquid crystalline phase can be used as a template to fabricate mesoporous platinum.

The Redox-active monolayer of a novel calix[4]arene recognizing redox-inactive ionic species by voltammetry is reported. Calix[4]arene-disulfide-diquinone, which is not only redox-active but is also a highly selective ionophore for the [Ba.sup.2+] ion, spontaneously forms a stable and dense monolayer film on gold. The redox-active calixarene monolayer selectively recognizes [Ba.sup.2+] ion in aqueous media, and the voltammetric signals are proportional to the ionic concentration. A new voltammetric peak can be detected by square-wave voltammetry upon adding a dilute solution containing [Ba.sup.2+] ion having a concentration as low as 1.0 x [10.sup.-6] M. The Langmuir plot (1/[i.sub.p] vs 1/[[Ba.sup.2+]]) shows a linear slope in the range from 1.0 x [10.sup.-6] M to 1.0 x [10.sup.-4] M. This modified electrode does not show any significant interference from alkali and alkaline earth metal ions except for [Sr.sup.2+] and [Ca.sup.2+]. Only 100- and 500-fold concentrations of [Sr.sup.2+] and [Ca.sup.2+] ions, respectively, can lead to voltammetric responses comparable to that of [Ba.sup.2+].

Equilibrium partitioning of [HCIO.sub.4] between aqueous solutions and benzonitrile (BN) or nitrobenzene (NB) was measured and used to evaluate the pKa of the acid in the two organic solvents. The potential drop across the BN/ [H.sub.2]O interface was evaluated from the known potential drop across the NB/[H.sub.2]O interface and the voltammetrically measured formal potential of a ferrocenium/ferrocene redox couple confined within thin layers of the two organic solvents. The voltammetric reduction of tetrachloro-1, 4-benzoquinone in thin layers of BN was used to monitor changes in the concentration of protons in the layer during proton-consuming faradaic reactions. The rate of proton transfer from the aqueous to the nonaqueous phase across the BN/[H.sub.2]O interface was shown to be adequate to sustain proton-consuming reactions at the electrode/BN interface.

Blood cell counting typically requires complex machinery. Flow cytometers used for this purpose involve precise optical alignment, costly detectors, and pretreatment with fluorescent labels. Coulter countertype devices, which monitor ion current, are simpler. However, conventional Coulter counters provide only information about size, making it impossible to distinguish similarly sized lymphocytes from red blood cells (RBCs). Inspired by the fact that RBCs have an exceptionally high propensity to absorb light and convert it to heat, i.e., photothermal effect, this study proposes integrating photothermal phenomena into a microfluidic Coulter counting chip. Photothermal heat selectively amplifies the ion current from RBCs over other components including lymphocytes. The combination of ion current monitoring and the photothermal effect for RBC counting suggests an evolution toward versatile flow cytometers.

A practical method for C(sp 3 )-B bond activation was developed. Using a combination of alkyl trifluoroborates and N -iodosuccinimide (NIS), various C(sp 3 )-heteroatom bonds were readily generated in an efficient manner. Mechanistic studies revealed the bifunctional ability of NIS: mediating the formation of reactive halogenated intermediates and activating them via halogen bonding. This electrophilic activation of the reaction center enables the utilization of general heteroatom nucleophiles, which are used in a limited capacity in traditional 1,2-metalate rearrangements.

Photoelectrochemical cells represent one of the promising ways to renewably produce hydrogen (H 2 ) as a future chemical fuel. The design of a catalyst/semiconductor junction for the hydrogen evolution reaction (HER) requires various factors for high performance. In catalytic materials, an intrinsic activity with fast charge-transfer kinetics is important. Additionally, their thermodynamic property and physical adhesion should be compatible with the underlying semiconductor for favorable band alignment and stability during vigorous H 2 bubble formation. Moreover, catalysts, especially non-noble materials that demand a large amount of loading, should be adequately dispersed on the semiconductor surface to allow sufficient light absorption to generate excitons. One of the methods to simultaneously satisfy these conditions is to adopt an interfacial layer between the semiconductor and active materials in HER. The interfacial layer efficiently extracts the electrons from the semiconductor and conveys those to the catalytically active surface. We demonstrate Ag as a 3D interfacial nanostructure of patterned MoS x catalysts for photoelectrochemical HER. The nanostructured porous Ag layer was introduced by a simple chemical process, followed by photoelectrochemical deposition of MoS x to form MoS x /Ag nanostructures in cross-shaped catalyst pattern arrays. Ag modulated the surface electronic property of MoS x to improve the reaction kinetics as well as helped a charge transport at the Ag|p-Si(100) junction. The physically stable adhesion of catalysts was also achieved despite the ∼40 nm thick catalysts owing to the interfacial Ag nanostructure. This work contributes to further understand the complex multistep HER from light absorption to charge transfer to protons, helping to develop cost-effective and efficient photocathodes.

Low-cost catalysts with high activity and durability are necessary to achieve efficient large-scale energy conversion in photoelectrochemical cell (PEC) systems. An additional factor that governs the construction of photoelectrodes for PECs is the spatial control of the catalysts for efficient utilization of photogenerated charge carriers. Here, we demonstrate spatial decoupling of the light-absorbing and catalytic components in hierarchically structured Si-based photocathodes for the hydrogen evolution reaction (HER). By simply modifying a well-known metal-assisted chemical etching procedure, we fabricated a Si nanowire (NW) array-based photocathode with Ag-Pt catalysts at the base and small amounts of the Pt catalyst at the NW tips. This approach simultaneously mitigates the parasitic light absorption by the catalytic layers and recombination of charge carriers owing to the long transport distance, resulting in improved photoelectrochemical HER performance under simulated AM 1.5G illumination., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Catalyst-transfer polymerization has revolutionized the field of polymer synthesis due to its living character, but for a given catalyst system, the polymer scope is rather narrow. Herein we report a highly efficient Suzuki-Miyaura catalyst-transfer polymerization (SCTP) that covers a wide range of monomers from electron-rich (donor, D) to electron-deficient (acceptor, A) (hetero)arenes by rationally designing boronate monomers and using commercially available Buchwald RuPhos and SPhos Pd G3 precatalysts. Initially, we optimized the controlled polymerization of 3,4-propylenedioxythiophene (ProDOT), benzotriazole (BTz), quinoxaline (QX), and 2,3-diphenylquinoxaline (QXPh) by introducing new boronates, such as 4,4,8,8-tetramethyl-1,3,6,2-dioxazaborocane and its N -benzylated derivative, to modulate the reactivity and stability of the monomers. As a result, PProDOT, PBTz, PQX, and PQXPh were prepared with controlled molecular weight and narrow dispersity ( Đ < 1.29) in excellent yield (>85%). A detailed investigation of the polymer structures using 1 H NMR and MALDI-TOF spectrometry supported the chain-growth mechanism and the high initiation efficiency of the SCTP method. In addition, the use of RuPhos-Pd showing excellent catalyst-transfer ability on both D/A monomers led to unprecedented controlled D-A statistical copolymerization, thereby modulating the HOMO energy level (from -5.11 to -4.80 eV) and band gap energy (from 1.68 to 1.91 eV) of the resulting copolymers. Moreover, to demonstrate the living nature of SCTP, various combinations of D-A and A-A block copolymers (PBTz- b -PProDOT, PQX- b -PProDOT, and PQX- b -PBTz) were successfully prepared by the sequential addition method. Finally, simple but powerful one-shot D-A block copolymerization was achieved by maximizing the rate difference between a fast-propagating pinacol boronate donor and a slow-propagating acceptor to afford well-defined poly(3-hexylthiophene)- b -poly(benzotriazole).

Light addressable/activated electrochemistry (LAE) has recently attracted attention as it can provide spatially resolved electrochemical information without using pre-patterned electrodes whose sizes and positions are unchangeable. Here, we propose hematite (α-Fe 2 O 3 ) as the photoanode for LAE, which does not require any sort of surface modification for protection or facilitating charge transfer. As experimentally confirmed with various redox species, hematite is stable enough to be used for repetitive electroanalytical measurements. More importantly, it offers exceptionally high spatial resolution so that the "virtual electrode" is exactly as large as the light spot owing to the short diffusion length of the minority carriers. Quantitative analysis of dopamine in this study shows that the hematite-based photoanode is a promising platform for many potential LAE applications including spatially selective detection of oxidizable biomolecules.

For more efficient photoelectrochemical water splitting, there is a dilemma that a photoelectrode needs both light absorption and electrocatalytic faradaic reaction. One of the promising strategies is to deposit a pattern of electrocatalysts onto a semiconductor surface, leaving sufficient bare surface for light absorption while minimizing concentration overpotential as well as resistive loss at the ultramicroelectrodes for faradaic reaction. This scheme can be successfully realized by "maskless" direct photoelectrochemical patterning of electrocatalyst onto an SiO x /amorphous Si (a-Si) surface by the light-guided electrodeposition technique. Electrochemical impedance spectroscopy at various pHs tells us much about how it works. The surface states at the SiO x /a-Si interface can mediate the photogenerated electrons for hydrogen evolution, whereas electroactive species in the solution undergo outer-sphere electron transfer, taking electrons tunneling across the SiO x layer from the conduction band. In addition to previously reported long-distance lateral electron transport behavior at a patterned catalyst/SiO x /a-Si interface, the charging process of the surface states plays a crucial role in proton reduction, leading to deeper understanding of the operation mechanisms for photoelectrochemical water splitting.

Nanoporous electrified surfaces create a unique nonfaradaic electrochemical behavior that is sensitively influenced by pore size, morphology, ionic strength, and electric field modulation. Here, we report the contributions of ion concentration and applied ac frequency to the electrode impedance through an electrical double layer overlap and ion transport along the nanopores. Nanoporous Pt with uniform pore size and geometry (L2-ePt) responded more sensitively to conductivity changes in aqueous solutions than Pt black with poor uniformity despite similar real surface areas and enabled the previously difficult quantitative conductometry measurements at high electrolyte concentrations. The nanopores of L2-ePt were more effective in reducing the electrode impedance and exhibited superior linear responses to not only flat Pt but also Pt black, leading to successful conductometric detection in ion chromatography without ion suppressors and at high ionic strengths.

We devised an electrochemical redox cycling based on three-dimensional interdigitated array (3D IDA) electrodes for signal amplification to enhance the sensitivity of chip-based immunosensors. The 3D IDA consists of two closely spaced parallel indium tin oxide (ITO) electrodes that are positioned not only on the bottom but also the ceiling, facing each other along a microfluidic channel. We investigated the signal intensities from various geometric configurations: Open-2D IDA, Closed-2D IDA, and 3D IDA through electrochemical experiments and finite-element simulations. The 3D IDA among the four different systems exhibited the greatest signal amplification resulting from efficient redox cycling of electroactive species confined in the microchannel so that the faradaic current was augmented by a factor of ∼100. We exploited the enhanced sensitivity of the 3D IDA to build up a chronocoulometric immunosensing platform based on the sandwich enzyme-linked immunosorbent assay (ELISA) protocol. The mouse IgGs on the 3D IDA showed much lower detection limits than on the Closed-2D IDA. The detection limit for mouse IgG measured using the 3D IDA was ∼10 fg/mL, while it was ∼100 fg/mL for the Closed-2D IDA. Moreover, the proposed immunosensor system with the 3D IDA successfully worked for clinical analysis as shown by the sensitive detection of cardiac troponin I in human serum down to 100 fg/mL.

Calix[4]arene-triacid-monoquinone (CTAQ), a quinone-containing water-soluble ionophore, was utilized to investigate how proton-coupled electron transfer (PCET) reactions of quinones were influenced by redox-inactive metal ions in aqueous environment. This ionophoric quinone derivative captured a Ca(2+) ion that drastically altered the voltammetric behavior of quinone, showing a characteristic response to pH and unique redox wave separation. Spectroelectrochemistry verified significant stabilization of the semiquinone, and electrocatalytic currents were observed in the presence of Ca(2+)-free CTAQ. Using digital simulation of cyclic voltammograms to clarify how the thermodynamic properties of quinones were altered, a simple scheme was proposed that successfully accounted for all the observations. The change induced by Ca(2+) complexation was explained on the basis of the combined effects of the electrostatic influence of the captured metal ion and hydrogen bonding of water molecules with the support of DFT calculation.

We propose a new method for performing in-channel electrochemical detection under a high electric field using a polyelectrolytic gel salt bridge (PGSB) integrated in the middle of the electrophoretic separation channel. The finely tuned placement of a gold working electrode and the PGSB on an equipotential surface in the microchannel provided highly sensitive electrochemical detection without any deterioration in the separation efficiency or interference of the applied electric field. To assess the working principle, the open circuit potentials between gold working electrodes and the reference electrode at varying distances were measured in the microchannel under electrophoretic fields using an electrically isolated potentiostat. In addition, "in-channel" cyclic voltammetry confirmed the feasibility of electrochemical detection under various strengths of electric fields (∼400 V/cm). Effective separation on a microchip equipped with a PGSB under high electric fields was demonstrated for the electrochemical detection of biological compounds such as dopamine and catechol. The proposed "in-channel" electrochemical detection under a high electric field enables wider electrochemical detection applications in microchip electrophoresis.

The individual encapsulation of living cells has a great impact on the area of cell-based sensors and devices as well as fundamental studies in cell biology. In this work, living yeast cells were individually encapsulated with functionalizable, artificial polydopamine shells, inspired by an adhesive protein in mussels. Yeast cells maintained their viability within polydopamine, and the cell cycle was controlled by the thickness of the shells. In addition, the artificial shells aided the cell in offering much stronger resistance against foreign aggression, such as lyticase. After formation of the polydopamine shells, the shells were functionalized with streptavidin by utilizing the chemical reactivity of polydopamine, and the functionalized cells were biospecifically immobilized onto the defined surfaces. Our work suggests a biomimetic approach to the encapsulation and functionalization of individual living cells with covalently bonded, artificial shells.

Controlled orientation and reserved activity of biomolecules, when site-selectively immobilized in a highly integrated manner on a minimal time scale, are crucial in designing biosensors for the multiplex detection. Here, we describe a novel method for the orientation-controlled immobilization of biomolecules based on site-selective electrochemical activation of p-hydroxyazobenzene self-assembled monolayer (SAM) followed by one-step coupling of cysteinyl biomolecules. The p-aminophenol, a product of reductive cleavage of p-hydroxyazobenzene, was subsequently oxidized to yield p-quinoneimine which then conjugated with cysteinyl biomolecules through 1,4-Michael addition, thus obviating additional linker agents and the related time consumption. Using this method, we selectively activated the electrode surface and immobilized laminin peptide IKVAV, a neurite promoting motif. When we cultured hippocampal neurons on the electrode, the extended neurites were found only within the electrochemically activated area. Hence, the proposed method represents a new promising platform for the patterning of functional peptides, active proteins, and live cells.