Ring Ultramicroelectrodes for Current-Blockade Particle-Impact Electrochemistry

High-Throughput Single-Entity Electrochemistry with Microelectrode Arrays

We describe micro- and nanoelectrode array analysis with an automated version of the array microcell method (AMCM). Characterization of hundreds of electrodes, with diameters ranging from 100 nm to 2 μm, was carried out by using AMCM voltammetry and chronoamperometry. The influence of solvent evaporation on mass transport in the AMCM pipette and the resultant electrochemical response were investigated, with experimental results supported by finite element method simulations. We also describe the application of AMCM to high-throughput single-entity electrochemistry in measurements of stochastic nanoparticle impacts. Collision experiments recorded 3270 single-particle events from 671 electrodes. Data collection parameters were optimized to enable these experiments to be completed in a few hours, and the collision transient sizes were analyzed with a U-Net deep learning model. Elucidation of collision transient sizes by histograms from these experiments was enhanced due to the large sample size possible with AMCM.

Average collision velocity of single yeast cells during electrochemically induced impacts

We recorded current-time (i-t) profiles for oxidizing ferrocyanide (FCN) while spherical yeast cells of radius (rc ≈ 2 μm) collided with disk ultramicroelectrodes (UMEs) of increasing radius (re ≈ 12-45 μm). Collision signals appear as minority steps and majority blips of decreased current overlayed on the i-t baseline when cells block ferrocyanide flux (JFCN). We assigned steps to adsorption events and blips to bouncing collisions or contactless passages. Yeast cells exhibit impact signals of long duration (Δt ≈ 15-40 s) likely due to sedimentation. We assume cells travel a threshold distance (T) to generate collision signals of duration Δt. Thus, T represents a distance from the UME surface, at which cell perturbations on JFCN blend in with the UME noise level. To determine T, we simulated the UME current, while placing the cell at increasing distal points from the UME surface until matching the bare UME current. T-Values at 90°, 45°, and 0° from the UME edge and normal to the center were determined to map out T-regions in different experimental conditions. We estimated average collision velocities using the formula T/Δt, and mimicked cells entering and leaving T-regions at the same angle. Despite such oversimplification, our analysis yields average velocities compatible with rigorous transport models and matches experimental current steps and blips. We propose that single-cells encode collision dynamics into i-t signals only when cells move inside the sensitive T-region, because outside, perturbations of JFCN fall within the noise level set by JFCN and rc/re (experimentally established). If true, this notion will enable selecting conditions to maximize sensitivity in stochastic blocking electrochemistry. We also exploited the long Δt recorded here for yeast cells, which was undetectable for the fast microbeads used in early pioneering work. Because Δt depends on transport, it provides another analytical parameter besides current for characterizing slow-moving cells like yeast.

Ultramicro and ultrasensitive detection of lipopolysaccharides based on triple-signal amplification via ultrafast ATRP and an ultramicroelectrode

Highly sensitive testing of trace lipopolysaccharides (LPS) is very important due to their high toxicity to the human body. Here, an ultrasensitive electrochemical sensor requiring only 5 μL solution was developed for LPS detection via triple-signal amplification based on ultrafast atom transfer radical polymerization (UATRP) and a Au ultramicroelectrode (UME). Firstly, the Au UME was modified with gold nanoparticles (nAu) and an LPS aptamer (Apt) in turn. When the Apt recognized LPS, the ATRP initiator of 4-(bromomethyl)phenylboronic acid (BPA) could be tethered to the electrode by covalent cross-linking between the phenylboronic acid moiety and the cis-diol site of LPS. Then UATRP was conducted for 2.5 min with nitrogen-doped carbon quantum dots (N-CQDs) as the photocatalyst and methylacrolein (MLA) as the monomer. After the electroactive probes of Ag nanoparticles (AgNPs) were formed on the surface of poly(MLA) by the silver mirror reaction, the electrochemical sensor was successfully prepared. Under the optimal conditions, the sensor exhibited a lower detection limit and a wider linear range when it was compared with a similar assay for LPS. In particular, the LOD of 7.99 × 10-2 pg mL-1 was better than that of the limulus amoebocyte lysate (LAL)-based technique, which is the gold standard for LPS detection. In the end, the sensor reported in this paper showed good selectivity and satisfactory feasibility for LPS detection in real biological samples and food products. The results obtained from the drug, blood and potable water samples laid a strong foundation for its clinical applications and application in other fields.

Utilizing the Oxygen Reduction Reaction in Particle Impact Electrochemistry: A Step toward Mediator-Free Digital Electrochemical Sensors

The current blockade particle impact method opens a route toward highly parallelized single-entity electrochemical assays. An important limitation is, however, that a redox mediator must be present in the sample, which can detrimentally interfere with molecular recognition processes. Dissolved O2 that is naturally present in aqueous solutions under ambient conditions can in principle serve as a suitable mediator via the oxygen reduction reaction (ORR). Here, we demonstrate the validity of this concept by performing current blockade experiments to capture and detect individual microparticles at Pt microelectrodes using solely the ORR. The readout modality is independent of the absolute O2 concentration, allowing operation under varying conditions. We further determine how the trajectories of individual microparticles are influenced by the combination of electrophoresis and electroosmotic flows and how these can be utilized to provide continuous detection of cationic particles in water for environmental monitoring.

Electrochemical Redox Cycling Behavior of Gold Nanoring Electrodes Microfabricated on a Silicon Micropillar

We report the microfabrication and characterization of concentric gold nanoring electrodes (Au NREs), which were fabricated by patterning two gold nanoelectrodes on the same silicon (Si) micropillar tip. Au NREs of 165 ± 10 nm in width were micropatterned on a 6.5 ± 0.2 µm diameter 80 ± 0.5 µm height Si micropillar with an intervening ~ 100 nm thick hafnium oxide insulating layer between the two nanoelectrodes. Excellent cylindricality of the micropillar with vertical sidewalls as well as a completely intact layer of a concentric Au NRE including the entire micropillar perimeter has been achieved as observed via scanning electron microscopy and energy dispersive spectroscopy data. The electrochemical behavior of the Au NREs was characterized by steady-state cyclic voltammetry and electrochemical impedance spectroscopy. The applicability of Au NREs to electrochemical sensing was demonstrated by redox cycling with the ferro/ferricyanide redox couple. The redox cycling amplified the currents by 1.63-fold with a collection efficiency of > 90% on a single collection cycle. The proposed micro-nanofabrication approach with further optimization studies shows great promise for the creation and expansion of concentric 3D NRE arrays with controllable width and nanometer spacing for electroanalytical research and applications such as single-cell analysis and advanced biological and neurochemical sensing.

Inkjet-printed 3D micro-ring-electrode arrays for amperometric nanoparticle detection

Chip-based impact electrochemistry can provide means to measure nanoparticles in solution by sensing their stochastic collisions on appropriately-polarized microelectrodes. However, a planar microelectrode array design still restricts the particle detection to the chip surface and does not allow detection in 3D environments. In this work, we report a fast fabrication process for 3D microelectrode arrays by combining ink-jet printing with laser-patterning. To this end, we printed 3D pillars from polyacrylate ink as a scaffold. Then, the metal structures are manufactured via sputtering and laser-ablation. Finally, the chip is passivated with a parylene-C layer and the electrode tips are created via laser-ablation in a vertical alignment. As a proof of principle, we employ our 3D micro-ring-electrode arrays for single impact recordings from silver nanoparticles.