Electrocatalytic Amplification Research Articles

Ultra-sensitive analysis based on electrochemistry, electrokinetic and magnetic preconcentration of analytes

Abstract Some of the challenges with detection of ultra-low concentrations of analytes are to achieve sufficient sensitivity of the measurement and to direct the analyte species to the sensor (electrode) surface. This review describes various strategies that are available to address these challenges: method of electrocatalytic amplification, electrochemical measurements performed in combination with electrokinetic preconcentration of analytes, ultra-sensitive analysis utilizing increased surface area and also the manipulation by the magnetic force.

Fabrication of Glass-Insulated Ultramicrometer to Submicrometer Carbon Fiber Electrodes to Support a Single Nanoparticle and Nanoparticle Ensembles in Electrocatalytic Investigations.

We report a procedure to fabricate glass-insulated carbon fiber ultramicroelectrodes (CF UMEs) with sizes of 10 and 1 μm in diameter. CF UMEs of theses sizes are commonly insulated with polymer or epoxy resins, which typically have pinholes and are less stable and difficult to polish. Through judicious choice of polishing materials, the fabrication procedure reported here leads to CF UMEs with a surface geometry that, within experimental error, is close to that of an inlaid disk. We demonstrate how the hardness factor of commonly used polishing materials may alter the geometry of CF UMEs, in which carbon has a larger hardness factor compared to other metals (i.e., Pt or Au). Atomic force microscopy (AFM) and steady-state voltammetry (SSV) were used to characterize the CF UME surface and record the electrochemical response, respectively. These results were compared to theoretical values for an inlaid disk UME. Pt nanoparticle (NP) collision experiments using electrocatalytic amplification were used to deposit an exact number of Pt NPs on a CF UME surface. Surface roughness was found to significantly decrease the sticking of Pt NPs on the CF surface, compared to the theoretical collision frequency. However, decreasing surface roughness through judicious polishing led to good agreement between experimental and theoretical collision frequencies of NPs on a CF UME surface, leading to the fabrication of NP ensembles of UME dimensions (UME-NPE) and a single-nanoparticle UME (SNP-UME). These electrodes were used to record and analyze SSVs for the hydrogen evolution reaction (HER) in acidic media.

Electrocatalytic Reduction of Hydrogen Peroxide by Pd−Ag Nanoparticles Based on the Collisional Approach

AbstractParticles collision onto a surface or an interface is a novel electrochemical approach, which can be used to investigate the behavior of single entities. Benefiting from advantages including single particle/single event resolution, label‐free detection and high sensitivity, this technique has been widely employed in the study of electrocatalytic amplification of detection signal by nanoparticles. Herein, we report the investigation of the collisional electrocatalytic behaviors of novel Pd−Ag hyperbranched nanoparticles through the observation of catalysis of hydrogen peroxide (H2O2) oxidation. The newly prepared Pd−Ag hyperbranched nanoparticles have been characterized by cyclic voltammetry and surface techniques. They can obviously reduce the overpotential of the reduction of H2O2 and show the high electrocatalytic activity. When the nanoparticles collide onto the electrode via the Brownian random motion, electrocatalytic reaction occurs and results in significant current increases, which could provide information about nanoparticles and electrochemical reactions. Furthermore, concentration and potential dependence are also investigated in detail, which confirms the reliability and universality of this methodology as an ultra‐sensitive probe. Based on these results, it is clearly shown that such platform is promising in further applications for biosensing and single nanoparticle analysis.

Single‐Entity Electrochemistry: Fundamentals and Applications

Single‐Entity Electrochemistry: Fundamentals and Applications

Catalytic Amplification of Au144 Nanocluster Collisions by Hydrogen Evolution Reaction

AbstractQuantification of the charge‐transfer kinetics at single catalytic nanoparticles is both, challenging and crucial for fundamental understanding and practical application in catalysis and energy storage. We report here the quantification of catalytic activity of single Au144 nanoclusters towards hydrogen evolution reaction in mixed organic media. The electrocatalytic amplification by the proton reduction reaction was used to detect current spikes produced by collisions of single Au144 nanoclusters (1.6 nm in diameter) with a carbon ultramicroelectrode. Relatively short (ms‐scale) current spikes were recorded at the naturally hydrophilic carbon surface, while longer (seconds) collision current responses were observed at the hydrophobic carbon fibre electrode treated by acetone. The kinetics of the hydrogen evolution reaction at a single Au nanocluster was evaluated by measuring the amplitudes of current spikes at different overpotentials. The relationship between the current amplitude and the overpotential yielded a linear Tafel plot with the 114 mV/decade slope. This approach is potentially useful for quantifying the electron‐transfer kinetics at singe catalytic nanoclusters and larger nanoparticles.

Individual Modified Carbon Nanotube Collision for Electrocatalytic Oxidation of Hydrazine in Aqueous Solution

Collision at a single molecule level was achieved based on the nanoimpact of an individual pyrroloquinoline quinone (PQQ) modified multiwalled carbon nanotube (MWCNT) at the carbon fiber ultramicroelectrode (C UME). Electrocatalytic amplification of the current responses was observed when the PQQ-modified MWCNT collided with C UME in the presence of hydrazine (N2H4) in a Tris-HCl buffer solution, which was also supported by the conventional cyclic voltammetry and chronoamperometry techniques. The enhanced catalytic oxidation of N2H4 was due to the “addition-elimination” redox cycling mechanism of PQQ/PQQH2, where the oxidation of N2H4 occurred together with the reduction of PQQ under an external bias, and the formed PQQH2 intermediate would be reoxidized back to PQQ simultaneously. The average collision current, duration, and charge for PQQ-modified MWCNT at 1.0 V vs Ag/AgCl were 105 pA, 0.45 ms, and 49 fC, respectively. As a result, the turnover frequency of electrocatalytic oxidation of N2H4 by PQQ was ...

Electrochemical Dynamics of a Single Platinum Nanoparticle Collision Event for the Hydrogen Evolution Reaction

AbstractChronoamperometry was used to study the dynamics of Pt nanoparticle (NP) collision with an inert ultramicroelectrode via electrocatalytic amplification (ECA) in the hydrogen evolution reaction. ECA and dynamic light scattering (DLS) results reveal that the NP colloid remains stable only at low proton concentrations (1.0 mm) under a helium (He) atmosphere, ensuring that the collision events occur at genuinely single NP level. Amperometry of single NP collisions under a He atmosphere shows that each discrete current profile of the collision event evolves from spike to staircase at more negative potentials, while a staircase response is observed at all of the applied potentials under hydrogen‐containing atmospheres. The particle size distribution estimated from the diffusion‐controlled current in He agrees well with electron microscopy and DLS observations. These results shed light on the interfacial dynamics of the single nanoparticle collision electrochemistry.

Electrochemical Dynamics of a Single Platinum Nanoparticle Collision Event for the Hydrogen Evolution Reaction.

Chronoamperometry was used to study the dynamics of Pt nanoparticle (NP) collision with an inert ultramicroelectrode via electrocatalytic amplification (ECA) in the hydrogen evolution reaction. ECA and dynamic light scattering (DLS) results reveal that the NP colloid remains stable only at low proton concentrations (1.0 mm) under a helium (He) atmosphere, ensuring that the collision events occur at genuinely single NP level. Amperometry of single NP collisions under a He atmosphere shows that each discrete current profile of the collision event evolves from spike to staircase at more negative potentials, while a staircase response is observed at all of the applied potentials under hydrogen-containing atmospheres. The particle size distribution estimated from the diffusion-controlled current in He agrees well with electron microscopy and DLS observations. These results shed light on the interfacial dynamics of the single nanoparticle collision electrochemistry.

Ultrasensitive Electroanalysis: Femtomolar Determination of Lead, Cobalt, and Nickel.

We demonstrate the feasibility of attaining femtomolar limits of quantitation in electroanalysis. The method employed is based on electrocatalytic amplification, where small quantities of metal deposit performed on a carbon electrode causes a large increase in the observed current, for example, for the oxidation of water. We show calibration curves at the femtomolar level for cobalt, nickel, and lead ions on carbon ultramicroelectrodes (CUMEs), ca. 500 nm radii. The CUME was biased at a potential where the ion would deposit as the metal oxide, MOx, and a high concentration of species that is oxidized at the deposit is present in solution. Blips were observed in the amperometric i-t response, and their frequency scaled linearly with the concentration of ions at the femtomolar level. From these results, the limits of quantitation for cobalt, nickel, and lead ions were reported at 10 s of femtomolar level for the first time.

Pt Nanoparticle Collisions Detected by Electrocatalytic Amplification and Atomic Force Microscopy Imaging: Nanoparticle Collision Frequency, Adsorption, and Random Distribution at an Ultramicroelectrode Surface.

We demonstrate good agreement between the theoretical and experimental collision frequency of individual Pt nanoparticles (NPs) undergoing collisions at a Au ultramicroelectrode (UME) (5 μm radius) using electrocatalytic amplification provided by 15 mM hydrazine in 5 mM phosphate buffer (PB; pH 7) over 100 to 300 s. Dynamic light scattering (DLS) measurements demonstrated that Pt NP aggregation in this solution had the least impact on NP diffusion coefficient and concentration values, which are directly proportional to collision frequency. We show that the smaller, uniform current steps are indicative of NPs of metallic radii in agreement with those determined by transmission electron microscopy (TEM), with corresponding larger NP diffusion coefficient and concentration, in agreement with DLS results. These contribute to the larger NP collision frequency observed experimentally. Using atomic force microscopy (AFM) imaging, we show good agreement between the number of NPs imaged on the UME surface and the number of NP collisions that led to their adsorption, a spherical NP shape with a metallic radius size distribution comparable to that determined by TEM, and a random NP distribution on the UME surface. Through the Pt NP electroactive surface area, we show that all NPs on the UME surface after collision are attached and electrochemically active. Collectively, these results demonstrate for the first time that, within experimental error, every NP collision is successful and occurs through a sticking mechanism. Thus, collision experiments can be used to prepare small NP ensembles on a UME (i.e., UME-NPEs). In electrocatalysis, such UME-NPEs bridge the gap between classical ensemble studies on large platforms and isolated single NP investigations.

Detection of microRNA by Electrocatalytic Amplification: A General Approach for Single-Particle Biosensing.

Here we report a sensing scheme for detection of microRNA (miRNA) using electrocatalytic amplification (ECA). ECA is a method in which nanoparticles (NPs) that are catalytic for a specific electrochemical reaction collide with an inert electrode surface. Each collision results in a detectable current transient. In the present article, we show that this general approach can be extended to detection of miRNA. Specifically, PtNPs are modified with a single-strand DNA (ssDNA) shell that is complementary to the miRNA target. Next, the ssDNA:miRNA conjugate is formed, which passivates the PtNP surface. In the presence of an enzyme called duplex specific nuclease (DSN), however, a fraction of the surface-bound DNA is removed thereby exposing some of the PtNP surface. In other words, the electrocatalytic properties of the PtNPs are reactivated only if miRNA complementary to ssDNA is present. This methodology resolves a number of problems that have rendered ECA ineffective for biosensing applications. Moreover, the results suggest that the underlying chemistry is broadly applicable to nucleic acid sensing.

Specific Picomolar Detection of a Breast Cancer Biomarker HER‐2/neu Protein in Serum: Electrocatalytically Amplified Electroanalysis by the Aptamer/PEG‐Modified Electrode

AbstractSpecific and sensitive electroanalysis of blood‐circulating protein cancer biomarkers is often complicated by interference from serum proteins nonspecifically adsorbing at the biosensing interface and masking specific reactions of interest. Here, we have developed an electrocatalytically amplified assay for specific and sensitive analysis of human epidermal growth factor receptor‐2 (HER‐2/neu, a protein cancer biomarker over‐expressed in breast cancers) that allows us to avoid both the interference from bovine serum albumin (BSA) and electrocatalytic amplification of the signal stemming from the specific aptamer−HER‐2/neu binding. A HER‐2/neu‐specific thiolated aptamer sequence was co‐adsorbed on gold together with a C11 alkanethiol bearing two ethylene glycol (EG)2 head groups that prevented non‐specific adsorption of BSA. On such layers, electrochemical reduction of a ferricyanide redox indicator is inhibited and is shown to be electrocatalyzed by methylene blue electrostatically interacting with negatively charged HER‐2/neu. The electrocatalytic signal increased upon HER‐2/neu binding to the aptamer, which allowed 10−12–10−8 M HER‐2/neu detection in 1 % serum, being practically applicable for clinical testing. The developed strategy can be considered as general and applicable for the electroanalysis of other blood‐circulating proteins once the electrostatic compatibility between the protein and redox probe is established.

Cathodically Dissolved Platinum Resulting from the O2 and H2O2 Reduction Reactions on Platinum Ultramicroelectrodes.

The cathodic dissolution of platinum, resulting from the oxygen reduction reaction (ORR) or hydrogen peroxide reduction on platinum, has been investigated. Highly oxidizing hydroxyl radicals (OH•) are believed to be the species responsible for the platinum dissolution phenomenon. These radicals are produced from the ORR byproduct, hydrogen peroxide, through a 1 electron reduction pathway (H2O2 + e- → OH• + OH-). Platinum ultramicroelectrodes (UMEs) were polarized sufficiently negative to drive the ORR or H2O2 reduction on the platinum surface, mainly using square wave potential pulses but constant applied potential and cyclic voltammetry (CV) were also investigated. The dissolved platinum was detected using a femtomolar level detection technique which involves reducing platinum ions to platinum metal species followed by an electrocatalytic amplification of proton reduction on an inert carbon fiber electrode. This method has allowed the quantification of the amount of platinum metal dissolved into the solution, from which the rate of platinum dissolution could be determined. Additionally, the detection method demonstrates the platinum is dissolved into the solution as an ionic species and does not form metallic nanoparticles.

Direct Observation of the Collision of Single Pt Nanoparticles onto Single-Crystalline Gold Nanowire Electrodes.

We observed the collision of single Pt nanoparticles (NPs) onto an Au nanowire (NW) electrode by using electrocatalytic amplification. Previously, such observations had typically been performed by using a microscale disk-type ultramicroelectrode (UME). The use of a NW electrode decreased the background noise current and provided a shielding effect, owing to adsorption of the NPs onto the insulating sheath. Therefore, the transient current signal that was caused by the collision of single NPs could be more clearly distinguished from the background current by using a NW electrode instead of a UME. Furthermore, the use of a NW electrode increased the collisional frequency and the magnitude of the transient current signal. The experimental data were analyzed by using a theoretical model and a random walk simulation model.

Addressing Colloidal Stability for Unambiguous Electroanalysis of Single Nanoparticle Impacts.

Herein the problem of colloidal instability on electrochemically detected nanoparticle (NP) collisions with a Hg ultramicroelectrode (UME) by electrocatalytic amplification is addressed. NP tracking analysis (NTA) shows that rapid aggregation occurs in solution after diluting citrate-stabilized Pt NPs with hydrazine/phosphate buffers of net ionic strength greater than 70 mM. Colloidal stability improves by lowering the ionic strength, indicating that aggregation processes were strongly affected by charge screening of the NP double layer interactions at high cation concentrations. For the system of lowest ionic strength, the overwhelming majority of observed electrocatalytic current signals represent single NP/electrode impacts, as confirmed by NTA kinetic monitoring. NP diffusion coefficients determined by NTA and NP impact electroanalysis are in excellent agreement for the stable colloids, which signifies that the sticking probability of Pt NPs interacting with Hg is unity and that the observed NP impact rate agrees with the expected steady-state diffusive flux expression for the spherical cap Hg UME.

Observation of Single Pt Nanoparticle Collisions: Enhanced Electrocatalytic Activity on a Pd Ultramicroelectrode

Single Pt nanoparticle (NP) collisions on an electrode surface were detected by using an electrocatalytic amplification method with a Pd ultramicroelectrode (UME). Pd is not a preferred material for UMEs for the detection of single Pt NP collisions, because Pd shows similar electrocatalytic activity compared with Pt for hydrazine oxidation, thus resulting in a high background current level. However, a Pt NP colliding on the Pd UME shows greatly enhanced activity compared with a Pt NP on an inert UME, such as a Au UME, which is usually used for the detection of single Pt NP collisions. The use of an electroactive UME material instead of an inert one facilitated the study of single-NP activity on the various solid supports, which is important in many NP applications.

Combined Blip and Staircase Response of Ascorbic Acid-Stabilized Copper Single Nanoparticle Collision by Electrocatalytic Glucose Oxidation.

The current response of the collision of ascorbic acid-stabilized copper (Cu) single nanoparticles (NPs) on a gold (Au) ultramicroelectrode (UME) surface was observed by using an electrocatalytic amplification method. Here, the glucose oxidation electrocatalyzed by oxidized Cu NPs was used as the indicating reaction. In this system, the NP collision signals were obtained simultaneously by both direct particle electrolysis and electrocatalytic amplification. For example, when the applied potential was high enough for Cu NP oxidation, a blip response combined with a staircase response was observed as a current signal. The blip part in the single Cu NP collision signal indicates the self-oxidation of a Cu NP, and the staircase part indicates the steady-state electrocatalytic reaction by oxidized Cu NP.

Observation of Blip Response in a Single Pt Nanoparticle Collision on a Cu Ultramicroelectrode

The collision of a single Pt nanoparticle (NP) was observed by an electrocatalytic amplification method. A Au ultramicroelectrode (UME) is usually used for the detection of Pt single NP collisions. The transient current response of the colliding Pt NP with the Au UME was observed as a staircase response. However, the response of the transient current by NP collision is affected by various factors including the material of the UME. In this study, a Cu UME was employed for the detection of a single Pt NP collision to investigate the effect of the UME material on the current response. A different current response, namely a blip response, was obtained by using Cu UME unlike the other UMEs. The peak height and collision frequency were analyzed and compared with the previous results using Au, Hg, or Ni UME for a better understanding of the UME material's effect on single Pt NP collision.

Electrocatalytic amplification of DNA-modified nanoparticle collisions via enzymatic digestion.

We report a new and general approach that will be useful for adapting the method of electrocatalytic amplification (ECA) to biosensing applications. In ECA, individual collisions of catalytic nanoparticles with a noncatalytic electrode surface lead to bursts of current. In the work described here, the current arises from catalytic electrooxidation of N2H4 at the surface of platinum nanoparticles (PtNPs). The problem with using ECA for biosensing applications heretofore, is that it is necessary to immobilize a receptor, such as DNA (as in the case here) or an antibody on the PtNP surface. This inactivates the colliding NP, however, and leads to very small collision signatures. In the present article, we show that single-stranded DNA (ssDNA) present on the PtNP surface can be detected by selectively removing a fraction of the ssDNA using the enzyme Exonuclease I (Exo I). About half of the current associated with collisions of naked PtNPs can be recovered from fully passivated PtNPs after exposure to Exo I. Experiments carried out using both Au and Hg ultramicroelectrodes reveal some mechanistic aspects of the collision process before and after treatment of the ssDNA-modified PtNPs with Exo I.

Mechanistic aspects of hydrazine-induced Pt colloid instability and monitoring aggregation kinetics with nanoparticle impact electroanalysis

Here we investigate the mechanistic aspects of Pt nanoparticle (NP) aggregation in solutions typically used for detecting NP/electrode impacts by electrocatalytic amplification (ECA). We previously proposed a general mechanism for Pt colloid destabilization that involved the participation of both the hydrazine redox probe and the pH buffer species as coagulants. Herein the Pt NP coagulation and aggregation mechanisms were further investigated with microscopic kinetic NP concentration monitoring and zeta potential measurements using nanoparticle tracking analysis (NTA), as well as open circuit potential experiments with a citrate-treated polycrystalline Pt surface to assess electrical double layer potential. After considering the combined results of these experiments we propose that the colloidal stability of citrate-capped platinum nanoparticles involves much more than the typical physicochemical interactions predicted by DLVO theory. A structure based on intermolecular H-bonding in the citrate capping layer is the most plausible explanation for the exceptional stability of large Pt NPs in high ionic strength buffers. Thus, the mechanism of Pt NP aggregation includes specific reactive contributions from hydrazine. The catalytic decomposition of hydrazine, in particular, is thought to occur to some extent at the citrate-coated Pt surface while the citrate remains adsorbed. Evolved gases such as ammonia and possible surface bound intermediates from Pt-catalyzed decomposition of hydrazine may disrupt the stability of the citrate layer, causing colloidal instability and thus promoting Pt NP coagulation. In the closing section, we demonstrate nanoparticle impact electroanalysis by ECA detection as a method to quantify Pt NP concentration with adequate time resolution for monitoring the kinetics of Pt NP coagulation.