Yang-Rae Kim

Graphene-incorporated chitosan substrata for adhesion and differentiation of human mesenchymal stem cells†

A simple method that uses graphene to fabricate nanotopographic substrata was reported for stem cell engineering. Graphene-incorporated chitosan substrata promoted adhesion and differentiation of human mesenchymal stem cells (hMSCs). In addition, we proposed that nanotopographic cues of the substrata could enhance cell-cell and cell-material interactions for promoting functions of hMSCs.

Time-Resolved Detection and Analysis of Single Nanoparticle Electrocatalytic Impacts

There is considerable interest in understanding the interaction and activity of single entities, such as (electro)catalytic nanoparticles (NPs), with (electrode) surfaces. Through the use of a high bandwidth, high signal/noise measurement system, NP impacts on an electrode surface are detected and analyzed in unprecedented detail, revealing considerable new mechanistic information on the process. Taking the electrocatalytic oxidation of H2O2 at ruthenium oxide (RuOx) NPs as an example, the rise time of current-time transients for NP impacts is consistent with a hydrodynamic trapping model for the arrival of a NP with a distance-dependent NP diffusion-coefficient. NP release from the electrode appears to be aided by propulsion from the electrocatalytic reaction at the NP. High-frequency NP impacts, orders of magnitude larger than can be accounted for by a single pass diffusive flux of NPs, are observed that indicate the repetitive trapping and release of an individual NP that has not been previously recognized. The experiments and models described could readily be applied to other systems and serve as a powerful platform for detailed analysis of NP impacts.

Redox-dependent spatially resolved electrochemistry at graphene and graphite step edges

The electrochemical (EC) behavior of mechanically exfoliated graphene and highly oriented pyrolytic graphite (HOPG) is studied at high spatial resolution in aqueous solutions using Ru(NH3)6(3+/2+) as a redox probe whose standard potential sits close to the intrinsic Fermi level of graphene and graphite. When scanning electrochemical cell microscopy (SECCM) data are coupled with that from complementary techniques (AFM, micro-Raman) applied to the same sample area, different time-dependent EC activity between the basal planes and step edges is revealed. In contrast, other redox couples (ferrocene derivatives) whose potential is further removed from the intrinsic Fermi level of graphene and graphite show uniform and high activity (close to diffusion-control). Macroscopic voltammetric measurements in different environments reveal that the time-dependent behavior after HOPG cleavage, peculiar to Ru(NH3)6(3+/2+), is not associated particularly with any surface contaminants but is reasonably attributed to the spontaneous delamination of the HOPG with time to create partially coupled graphene layers, further supported by conductive AFM measurements. This process has a major impact on the density of states of graphene and graphite edges, particularly at the intrinsic Fermi level to which Ru(NH3)6(3+/2+) is most sensitive. Through the use of an improved voltammetric mode of SECCM, we produce movies of potential-resolved and spatially resolved HOPG activity, revealing how enhanced activity at step edges is a subtle effect for Ru(NH3)6(3+/2+). These latter studies allow us to propose a microscopic model to interpret the EC response of graphene (basal plane and edges) and aged HOPG considering the nontrivial electronic band structure.

Impact of Surface Chemistry on Nanoparticle–Electrode Interactions in the Electrochemical Detection of Nanoparticle Collisions

The electrochemical detection of a single nanoparticle (NP) at a support electrode can provide key information on surface chemistry and fundamental electron transfer (ET) properties at the nanoscale. This study employs scanning electrochemical cell microscopy (SECCM) as a fluidic device to both deliver individual citrate-capped gold nanoparticles (AuNPs) and study the interactions between them and a range of alkanethiol-modified Au electrodes with different terminal groups, namely, -COOH, -OH, and -CH3. Single NP collisions were detected through the AuNP-mediated ET reaction of Fe(CN)6(4-/3-) in aqueous solution. The collision frequency, residence time, and current-time characteristics of AuNPs are greatly affected by the terminal groups of the alkanethiol. Methods to determine these parameters, including the effect of the instrument response function, and derive ET kinetics are outlined. To further understand the interactions of AuNPs with these surfaces, atomic force microscopy (AFM) force measurements were performed using citrate-modified Au-coated AFM tips and the same alkanethiol-modified Au substrates in aqueous solution at the same potential bias as for the AuNP collision experiments. Force curves on OH-terminated surfaces showed no repulsion and negligible adhesion force. In contrast, a clear repulsion (on approach) was seen for COOH-terminated surface and adhesion forces (on retract) were observed for both COOH- and CH3-terminated surfaces. These interactions help to explain the residence times and collision frequencies in AuNP collisions. More generally, as the interfacial properties probed by AFM appear to be amplified in NP collision experiments, and new features also become evident, it is suggested that such experiments provide a new means of probing surface chemistry at the nanoscale.

A BODIPY-functionalized bimetallic probe for sensitive and selective color-fluorometric chemosensing of Hg2+

A new BODIPY dye conjugate has demonstrated selective quenching by mercury over other metal ions. Coupling of this probe to Au-Fe(3)O(4) nanoparticles as well as platinum electrodes offered sensitive systems for suspension and surface based sensing, respectively.

Electrochemical signal amplification for immunosensor based on 3D interdigitated array electrodes.

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.

Enhanced electrochemical reactions of 1,4-benzoquinone at nanoporous electrodes

We report that the proton-coupled electron transfer kinetics of 1,4-benzoquinone was significantly enhanced in electrified nanopores in aqueous media. At nanoporous Pt and Au electrodes, the voltammetric behaviour of 1,4-benzoquinone at nanoporous electrodes was clearly distinct from that at flat surfaces. Proton transfer as well as electron transfer kinetics were facilitated in the nanopores by the confinement effect, which indicates all factors originated from the geometric features of nano-scale concave space surrounded by inner walls, suggesting how to utilize nanoporous electrodes for electrocatalysis.

Modulation of Quinone PCET Reaction by Ca2+ Ion Captured by Calix[4]quinone in Water

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.

Light-guided electrodeposition of non-noble catalyst patterns for photoelectrochemical hydrogen evolution

Hydrogen is in the lime light as a carbon-free alternative energy source due to its high energy conversion efficiency. Solar-driven water splitting is one of the most promising methods for renewable hydrogen production. However, commercialization of a photoelectrochemical hydrogen production system remains a great challenge. One of the emerging concerns is the development of an inexpensive and transparent catalyst, which does not obstruct the light pathways to the semiconductor electrode. Here we report a non-noble metal electrocatalyst for hydrogen evolution, Ni-Mo, which is directly patterned on amorphous Si (a-Si) by light-guided spatially selective electrodeposition without consecutive photolithography processes. A light pattern is illuminated onto the a-Si using a digital micromirror device to commence the photoelectrochemical deposition. The catalyst patterned by the proposed method not only admits sufficient light to a-Si but also enables long distance carrier transport along the inversion layer, as previously observed in crystalline Si (c-Si) photocathodes. This new electrodeposition method enables mask-free patterning on a-Si and is expected to expedite a lower cost, more efficient, and self-biasing integrated photoelectrochemical water-splitting device.

Gold Microshell Tip for In Situ Electrochemical Raman Spectroscopy

Tip fabrication by a new strategy is proposed for simultaneous acquisition of electrochemical (EC) signals from an ultramicroelectrode and spectroscopic information from surface-enhanced Raman scattering (SERS). The EC-SERS tip is prepared by carefully tuning a SERS-active gold microshell to maximize Raman scattering, mechanically attaching it to the end of a micropipet, and electrically connecting it to a ruthenium inner layer through electroless deposition.