Visualization of the Local Catalytic Activity of Electrodeposited Pt–Ag Catalysts for Oxygen Reduction by means of SECM

Pt–Ag nanoparticle co‐deposits with different Pt–Ag ratios were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and investigated for their catalytic activity in electrocatalytic oxygen reduction by using cyclic voltammetry (CV), rotating disc electrode (RDE) and scanning electrochemical microscopy (SECM) in 0.1 M phosphate buffer (pH 7.0). The atomic composition of the Pt–Ag co‐deposits was studied by means of energy‐dispersive X‐ray analysis (EDAX). In combination with X‐ray diffraction (XRD), the presence of partly alloyed Pt and Ag on the GC surface was confirmed. Scanning electron microscopy (SEM) images indicate that the prepared Pt–Ag catalyst particles are homogenously dispersed over the GC surface. Their size and morphology depend on their composition. The electrocatalytic activity of Pt–Ag deposits with high Pt content was the highest, exceeding even that of electrodeposited Pt as evaluated by quantitative RDE analysis. The redox competition mode of scanning electrochemical microscopy (RC‐SECM) was successfully used to visualize the local catalytic activity of the deposited Pt–Ag particles. Semi‐quantitative assessment of the SECM results confirmed the same order of activity of the different catalysts as the RDE investigations.     

Surfactant-assisted one-pot synthesis of superparamagnetic magnetite nanoparticle clusters with tunable cluster size and magnetic field sensitivity

Magnetic nanoparticles (MNPs) have many potential biomedical applications. Improvements in their magnetic properties and solubility are necessary for these applications to realize their full potential. In this study, MNPs in the form of raspberry-like magnetite (Fe(3)O(4)) nanoparticle clusters, consisting of tiny Fe(3)O(4) particles with a diameter of approximately 20 nm, were prepared under hydrothermal conditions at 200 °C in the presence of 3,4-dihydroxyhydroxysinnamic acid (DHCA). The primary particles were connected by DHCA molecules to form the clusters, which were well dispersed in water media because a COOH group from DHCA appeared on their surfaces. The cluster size could be tuned from 50 to 400 nm without changing the primary particle size by controlling the reaction time. Therefore, all prepared clusters displayed superparamagnetic properties at room temperature. In addition, the sensitivity of Fe(3)O(4) to an external magnetic field could also be controlled by the cluster size.     

Topographic, electrochemical, and optical images captured using standing approach mode scanning electrochemical/optical microscopy

We developed a high-resolution scanning electrochemical microscope (SECM) for the characterization of various biological materials. Electrode probes were fabricated by Ti/Pt sputtering followed by parylene C-vapor deposition polymerization on the pulled optical fiber or glass capillary. The effective electrode radius estimated from the cyclic voltammogram of ferrocyanide was found to be 35 nm. The optical aperture size was less than 170 nm, which was confirmed from the cross section of the near-field scanning optical microscope (NSOM) image of the quantum dot (QD) particles with diameters in the range of 10-15 nm. The feedback mechanism controlling the probe-sample distance was improved by vertically moving the probe by 0.1-3 microm to reduce the damage to the samples. This feedback mode, defined as "standing approach (STA) mode" (Yamada, H.; Fukumoto, H.; Yokoyama, T.; Koike, T. Anal. Chem. 2005, 77, 1785-1790), has allowed the simultaneous electrochemical and topographic imaging of the axons and cell body of a single PC12 cell under physiological conditions for the first time. STA-mode feedback imaging functions better than tip-sample regulation by the conventionally available AFM. For example, polystyrene beads (diameter approximately 6 microm) was imaged using the STA-mode SECM, whereas imaging was not possible using a conventional AFM instrument.     

Analysis of diffusion-controlled stochastic events of iridium oxide single nanoparticle collisions by scanning electrochemical microscopy

We investigated the electrochemical detection of single iridium oxide nanoparticle (IrO(x) NP) collisions at the NaBH(4)-treated Pt ultramicroelectrode (UME) in a scanning electrochemical microscope (SECM) over an insulating surface. The NP collision events were monitored by observing the electrocatalytic water oxidation reaction at potentials where it does not take place on the Pt UME. These collisions occurred stochastically, resulting in a transient response ("blip") for each collision. The frequency of the collisions is proportional to the flux of NPs to the UME tip, and thus equivalent to the SECM current. A plot of collision frequency versus distance followed the theoretical approach curve behavior for negative feedback for a high concentration of mediator, demonstrating that the collisions were diffusion-controlled and that single-particle measurements of mass transport are equivalent to ensemble ones. When the SECM was operated with a Pt substrate at the same potential as the tip, the behavior followed that expected of the shielding mode. These studies and additional ones result in a model where the IrO(x) NP collision on the Pt UME is adsorptive, with oxygen produced by the catalyzed water oxidation causing a current decay. This results in a blip current response, with the current decay diminished in the presence of the oxygen scavenger, sulfite ion. Random walk and theoretical bulk simulations agreed with the proposed mechanism of IrO(x) NP collision, adsorption, and subsequent deactivation.     

Modifying the surface of Fe3O4/SiO2 magnetic nanoparticles with C18/NH2 mixed group to get an efficient sorbent for anionic organic pollutants

In this article, C(18)/NH(2) mixed group modified Fe(3)O(4)/SiO(2) magnetic nanoparticles (Fe(3)O(4)/SiO(2)/C(18)+NH(2) MNPs) were successfully synthesized and used for the extraction of perfluorinated compounds (PFCs) from large volume of water solution. The Fe(3)O(4)/SiO(2)/C(18)+NH(2) MNPs, about 25 nm in diameter, possess high extraction ability to the anionic organic pollutants due to the dual function of hydrophobic octadecyl group and cationic aminopropyl groups at low pH. More than 90% of the targets can be extracted from 500 mL of water solution with 0.1g of the MNP sorbent at pH 3. Twenty min is sufficient to reach adsorption equilibrium, and the targets can be desorbed from the sorbent readily with 12 mL of alkalized methanol after magnetic separation. Simplified extraction procedure could be achieved because of the superparamagnetism and high saturation magnetization of the sorbent (44 emu g(-1)). Therefore, preconcentration of trace level of PFCs from water solution can be performed by using this Fe(3)O(4)/SiO(2)/C(18)+NH(2) MNP sorbent which are stable for multiple reuses.     

Scanning Electrochemical Microscopy of Individual Catalytic Nanoparticles

Electrochemistry at individual metal nanoparticles (NPs) can provide new insights into their electrocatalytic behavior. Herein, the electrochemical activity of single AuNPs attached to the catalytically inert carbon surface is mapped by using extremely small (≥3 nm radius) polished nanoelectrodes as tips in the scanning electrochemical microscope (SECM). The use of such small probes resulted in the spatial resolution significantly higher than in previously reported electrochemical images. The currents produced by either rapid electron transfer or the electrocatalytic hydrogen evolution reaction at a single 10 or 20 nm NP were measured and quantitatively analyzed. The developed methodology should be useful for studying the effects of nanoparticle size, geometry, and surface attachment on electrocatalytic activity in real‐world application environment.     

Redox‐Filled Carbon‐Fiber Microelectrodes for Single‐Cell Exocytosis

Carbon‐fiber microelectrodes (CFEs) are the primary electroanalytical tool in single‐cell exocytosis and in vivo studies. Here we report a new study on the kinetic properties of electrolyte‐filled CFEs in single‐cell measurements and demonstrate that the addition of outer sphere redox species, such as Fe(CN)₆^3? and Ru(NH₃)₆³⁺, in the backfill electrolyte solution can greatly enhance the kinetic response of CFEs. We show that at 750 mV, a voltage normally applied for detection of dopamine, the presence of fast outer sphere redox species in the backfilling solution significantly enhances the kinetic response of CFEs toward fast dopamine detection at single PC12 cells. Moreover, we also demonstrate that the use of Fe(CN)₆^3? in the backfilling solution has enabled direct measurement of dopamine at applied voltages as low as 200 mV. This kinetic enhancement is believed to be due to faster electron‐transfer kinetics on the coupling pole as compared to the sluggish reduction of oxygen. We anticipate that such redox‐filled CFE ultramicroelectrodes will find many useful applications in single cell exocytosis and in vivo sensing.     

Development of adenosine sensor: effect of physiological buffers on activity and sensitivity in adenosine determinations by fast scan voltammetry

A new fast scan voltammetry (FSV) method was tested in the determinations of adenosine in physiological buffers at pH 7.4. The buffers can be used in the determinations of adenosine in vivo and include 7 x 10(-2) M phosphate, Krebs-Henseleit (K-H) and Hanks' Balanced Salts (HBSS). A new method of fabrication of carbon fiber electrodes (CFEs) by polishing, followed by electrochemical pretreatment (ECP) was developed for the determination. After the ECP of CFE, CFE background current was stable in FSV determinations even though an increase in the background current was observed after the ECP in the buffers at pH 7.4. The sensitivity in FSV determinations of adenosine at the pretreated electrodes was tested in the physiological buffers at the potential scan rate of 500 V s(-1) at pH 7.4. Buffer composition and pH was the same during the ECP of CFE and in the FSV determinations. The sensitivity in the FSV determinations of adenosine at the new CFEs was high, compared to that previously reported at CFEs prepared by other methods, and showed a limited dependence on buffer composition. However, a small increase in buffer pH above 7.4 resulted in a decrease in sensitivity in the determinations of adenosine. The decrease in sensitivity was associated with an additional increase in CFE background current at pH > 7.4. At pH 7.4 best sensitivity and limit of detection was obtained in 7 x 10(-2) M phosphate buffer, where the background current was lowest. The sensitivity was half that in K-H and HBSS. Standard deviation of measurements was ca. 1%. The results demonstrate the feasibility of sensitive FSV determinations of adenosine in physiological buffers at pH 7.4 at CFEs.     

Photosystem I patterning imaged by scanning electrochemical microscopy

We report the first directed adsorption of Photosystem I (PSI) on patterned surfaces containing discrete regions of methyl- and hydroxyl-terminated self-assembled monolayers (SAMs) on gold. SAM and PSI patterns are characterized by scanning electrochemical microscopy (SECM). The insulating protein complex layer blocks the electron transfer of the SECM mediator, thereby reducing the electrochemical current significantly. Uniformly and densely packed adsorbed protein layers are observed with SECM. Pattern images correlate with our previous studies where we showed that low-energy surfaces (e.g., CH3-terminated) inhibit PSI adsorption in the presence of Triton X-100, whereas high-energy surfaces (e.g., OH-terminated) enable adsorption. Therefore, a SAM pattern with alternating methyl and hydroxyl surface regions allows PSI adsorption only on the hydroxyl surface, and this is demonstrated in the resulting SECM images.     

Calcium Carbonate Mineralized Nanoparticles as an Intracellular Transporter of Cytochrome c for Cancer Therapy

A new intracellular delivery system based on an apoptotic protein‐loaded calcium carbonate (CaCO₃) mineralized nanoparticle (MNP) is described. Apoptosis‐inducing cytochrome c (Cyt c) loaded CaCO₃ MNPs (Cyt c MNPs) were prepared by block copolymer mediated in situ CaCO₃ mineralization in the presence of Cyt c. The resulting Cyt c MNPs had a vaterite polymorph of CaCO₃ with a mean hydrodynamic diameter of 360.5 nm and exhibited 60 % efficiency for Cyt c loading. The Cyt c MNPs were stable at physiological pH (pH 7.4) and effectively prohibited the release of Cyt c, whereas, at intracellular endosomal pH (pH 5.0), Cyt c release was facilitated. The MNPs enable the endosomal escape of Cyt c for effective localization of Cyt c in the cytosols of MCF‐7 cells. Flow cytometry showed that the Cyt c MNPs effectively induced apoptosis of MCF‐7 cells. These findings indicate that the CaCO₃ MNPs can meet the prerequisites for delivery of cell‐impermeable therapeutic proteins for cancer therapy.     

Liquid–Air Interface Self-Assembly of Nanoparticles Synthesized from Reaction Between Fe(dbm)<Subscript>3</Subscript> and Pt(acac)<Subscript>2</Subscript>

The liquid–air interface is demonstrated as a method to assemble nanoparticles synthesized from the reaction between iron (III) dibenzoylmethane (Fe(dbm)₃) and platinum acetylacetonate (Pt(acac)₂) into a long range monolayer. These surface-modified particles have average Fe to Pt atomic ratio of 0.77:1. The increase in surfactants further reduces the Fe:Pt ratio and increases the particle diameter to over 4 nm. The self-assembled pattern of FePt-based nanoparticles can be enhanced by dropping nanoparticle suspensions on the surface of diethelyne glycol (DEG). The concentrations of these nanoparticle suspensions in hexane from 0.2 to 0.4 mg/ml can be used without the agglomeration into multilayered islands. The voids in the self-assembled monolayer on the DEG-air interface are reduced to the minimum in the case of the lowest concentration.     

表面活性剂碳化法合成Fe3O4/C复合物及其电化学性能

以水热法合成的包覆油酸的α-Fe₂O₃粒子为前驱体, 在氩气下500 °C煅烧1 h, 得到Fe₃O₄/C纳米复合物. 用傅里叶变换红外(FTIR)光谱, X射线衍射(XRD), 扫描电镜(SEM), X射线能量散射(EDX)谱, 高分辨透射电镜(HRTEM), 元素分析, 循环伏安(CV)和恒流充放电测试等方法对材料的结构、形貌、成分及电化学性能进行了表征. 结果表明: 所制备的Fe₃O₄/C复合物呈长约200 nm, 粗约100 nm的纺锤形, 表面碳层厚约1-2 nm, 碳含量为1.956%(质量分数); 这种复合物作为锂离子电池负极材料具有很好的循环稳定性(在0.2C (1C=928 mA·g^-1)循环80次后具有691.7 mAh·g^-1比容量)和倍率性能(在2C循环20次后依然有520 mAh·g^-1比容量). 相对于未包覆的商业Fe₃O₄粒子, 复合物显著提高的电化学性能是由于碳包覆能防止粒子聚集, 提高导电性以及稳定固体电解质界面(SEI)膜.     

Local electron transfer rate measurements on modified and unmodified glassy carbon electrodes

A natural and artificial distribution of electron transfer activity on glassy carbon electrodes can be observed and quantified by the use of scanning electrochemical microscopy (SECM). A large (sevenfold) spread in rate constant is found for randomly sampled sites on polished, untreated glassy carbon surfaces. Direct-mode oxidation with the SECM tip was used to produce small regions of oxidized carbon on a polished surface. A large increase in electron transfer rate for the Fe(II/III) ion is observed on the locally oxidized carbon surface in comparison to the unoxidized region. Rate constant measurements made along a line profiles the transition from unoxidized to oxidized surfaces. SECM images of defect sites show reaction–rate variations. Rate constants measured at several locations of the defective surface allows discrimination between the kinetic and topographic components of the SECM image. Dedicated to the 80th birthday of Keith B. Oldham     

Distribution of magnetic nanoparticles in spherical polyelectrolyte brushes as observed by small-angle X-ray scattering

Magnetic nanoparticles (MNPs) with a size of about 2 nm are prepared in nanoreactors of spherical polyelectrolyte brushes (SPBs) consisting of a solid polystyrene (PS) core and a shell of linear poly(acrylic acid) (PAA) chains densely grafted onto the core by one end. The synthesized MNP are strongly adhered to PAA chains due to the intense interaction of chemical coordination with the carboxyl groups. The generation of MNP in SPB layer is legibly revealed by small-angle X-ray scattering (SAXS) due to the significant increase in electron density. The radial distribution of MNP in SPB is built by fitting SAXS data. Most of MNP are found to locate nearby the surface of PS core. Compared to dynamic light scattering and transmission electron microscopy, SAXS can observe the generation and distribution of MNP in SPB as well as the changes upon changing pH and salt concentration in real time. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1681–1688     

Chemical grafting of a DNA intercalator probe onto functional iron oxide nanoparticles: a physicochemical study

Spherical magnetite nanoparticles (MNPs, ～ 24 nm in diameter) were sequentially functionalized with trimethoxysilylpropyldiethylenetriamine (TMSPDT) and a synthetic DNA intercalator, namely, 9-chloro-4H-pyrido[4,3,2-kl]acridin-4-one (PyAcr), in order to promote DNA interaction. The designed synthetic pathway allowed control of the chemical grafting efficiency to access MNPs either partially or fully functionalized with the intercalator moiety. The newly prepared nanomaterials were characterized by a range of physicochemical techniques: FTIR, TEM, PXRD, and TGA. The data were consistent with a full surface coverage by immobilized silylpropyldiethylenetriamine (SPDT) molecules, which corresponds to ～22,300 SPDT molecules per MNP and a subsequent (4740-2940) PyAcr after the chemical grafting step (i.e., ～ 2.4 PyAcr/nm(2)). A greater amount of PyAcr (30,600) was immobilized by the alternative strategy of binding a fully prefunctionalized shell to the MNPs with up to 16.1 PyAcr/nm(2). We found that the extent of PyAcr functionalization strongly affects the resulting properties and, particularly, the colloidal stability as well as the surface charge estimated by ζ-potential measurement. The intercalator grafting generates a negative charge contribution which counterbalances the positive charge of the single SPDT shell. The DNA binding capability was measured by titration assay and increases from 15 to 21.5 μg of DNA per mg of MNPs after PyAcr grafting (14-20% yield) but then drops to only ～2 μg for the fully functionalized MNPs. This highlights that even if the size of the MNPs is obviously a determining factor to promote surface DNA interaction, it is not the only limiting parameter, as the mode of binding and the interfacial charge density are essential to improve loading capability.     

Mixed Polymer‐Coated Magnetic Nanoparticles as Forward Osmosis Draw Agents of Tuned Hydrophilicity

We recently reported a polymer‐coated magnetic nanoparticle (MNP) draw agent for the forward osmosis (FO) water desalination process. The water flux was found to increase when the polymer poly(sodium acrylate) (PSA) was anchored to the MNP surface as compared to the polymer (or polyelectrolyte solution) alone, due to the polymer chains being stretched out and most of the hydrophilic groups on the polymer contributing to water flux. We herein report the use of a secondary polymer poly(N‐isopropylacrylamide) PNIPAM to manipulate the PSA polymer conformation and influence inter‐ and intrachain interactions to enhance the efficiency of the FO draw agent. These PSA–PNIPAM‐coated MNPs generated a much higher water flux of ～11.66 LMH when compared to the 100 % PSA‐coated MNPs featuring a value of ～5.32 LMH under identical FO conditions. The osmotic pressure and water flux driven by the mixed polymer‐coated MNPs were found to be a strong function of the net polymer coverage on MNPs, that is, net available hydrophilic groups. Our new draw agent demonstrates potential for use in the water industry due to its improved efficiency and cost effectiveness as it uses only ～0.062 % (w/v) of the draw agent solution.     

Electron transport in two-dimensional arrays of gold nanocrystals investigated by scanning electrochemical microscopy

This article reports the use of the scanning electrochemical microscope (SECM) to investigate the electronic properties of Langmuir monolayers of alkane thiol protected gold nanocrystals (NCs). A substantial increase in monolayer conductivity upon mechanical compression of the Au NC monolayer is reported for the first time. This may be the room temperature signature of the insulator to metal transition previously reported for comparable silver NC monolayers. Factors influencing the conductivity of the monolayer NC array are discussed.     

Scanning electrochemical microscopy determination of organic soluble MPC electron-transfer rates

In this paper, we describe a novel method for measuring the forward heterogeneous electron-transfer rate constant (kf) through the thiol monolayer of gold monolayer protected clusters (MPCs) in solution using scanning electrochemical microscopy (SECM). Applying the equations for mixed mass-transfer and electron-transfer processes, we develop a new formula using only the diffusion coefficient and the tip radius and use it as part of a new method for evaluating SECM approach curves. This method is applied to determine the electron-transfer rates from a series of SECM approach curves for monodisperse hexanethiol MPCs and for polydisperse hexanethiol, octanethiol, decanethiol, dodecanethiol, and 2-phenyethylthiol gold MPCs. Our results show that as the alkanethiol length increases the rate of electron transfer decreases in a manner consistent with the previously proposed tunneling mechanism for the electron transfer in MPCs. However, the effective tunneling coefficient, Beta, is found to be only 0.41 A-1 for alkanethiol passivated MPCs compared to typical values of 1.1 A-1 for alkanethiols as self-assembled monolayers on two-dimensional gold substrates. Similar SECM approach curve results for Pt and Au MPCs indicate that the electron-transfer rate is dependent mostly on the composition of the thiol layer and not on differences in the core metal.     

Conductive-matrix-mediated alkaline Fe(III/VI) charge transfer: three-electron storage, reversible super-iron thin film cathodes

An extended conductive matrix facilitates a 100-fold enhancement in charge storage for reversible Fe(III/VI) super-iron thin films. These films were deposited, by electrochemical reduction of Na2FeO4, with an intrinsic high capacity 3 e- cathodic storage of 485 mAh g(-1). Whereas 3 nm Fe(III/VI) films exhibited a high degree of reversibility (throughout 100 charge/discharge cycles), thicker films had been increasingly passive toward the Fe(VI) charge transfer. Films were alternatively deposited on either smooth or on extended conductive matrixes composed of high-surface-area Pt, Ti, and Au and probed galvanostatically and via cyclic voltammetry. A 100 nm Fe(VI) cathode, on the extended conductive matrixes, sustained 100-200 reversible three-electrode charge/discharge cycles, and a 19 nm thin film cathode sustained 500 such cycles. With a metal hydride anode, full cell storage was probed, and a 250 nm super-iron film cathode film sustained 40 charge/discharge cycles, and a 25 nm film was reversible throughout 300 cycles. Fe(VI) salts exhibit higher cathodic capacity and environmental advantages, and the films are of relevance toward the next generation charge storage chemistry for reversible cathodes.     

Oxygen reduction activity of carbon-supported Pt-M (M = V, Ni, Cr, Co, and Fe) alloys prepared by nanocapsule method

Monodispersed Pt and Pt-M (M = V, Cr, Fe, Co, and Ni) alloy nanoparticles supported on carbon black (denoted as Pt/CB and Pt-M/CB) were prepared by the simultaneous reduction of platinum acetylacetonate and the second metal acetylacetonate within nanocapsules formed in diphenyl ether in the presence of carbon black. For the Pt/CBs, the average Pt diameters measured by scanning transmission electron microscopy (STEM) or X-ray diffraction (XRD) ranged from 2.0 to 2.5 nm, regardless of the catalyst-loading level from 10 to 55 wt % on CB. The alloy composition was found to be well-controlled to the projected value among the supported particles. The activities for the oxygen reduction reaction (ORR) at Nafion-coated catalysts in O2-saturated 0.1 M HClO4 solution were evaluated by using a channel flow electrode (CFE) cell at 30 degrees C. The area-specific ORR activities at Pt-M/CB were found to be 1.3 to 1.8 times higher than that at Pt/CB. The ORR activity increased in the order Pt/CB < Pt-Ni/CB < Pt-Fe/CB < Pt-Co/CB < Pt-V/CB < Pt-Cr/CB.