Electrocatalytic Amplification Research Articles

Effects of Electrochemical Tailoring of Monolayers on a Catalytic Redox Entity: An ON–OFF Phenomenon Regulated by the Surrounding Medium

Here we report an ON–OFF effect on electrocatalytic amplification when the molecules are connected to each other on an electrode surface. When the molecular connection is achieved, the redox potential of the catalytic redox entity is significantly upshifted with concurrent experience of a more electron-withdrawing atmosphere, and the electron transfer to and from the catalytic center is accelerated, rendering the molecules with strong electroreducing character; however, the overall outcome is dictated by the surface charge. Outer sphere redox probes and X-ray photoelectron spectroscopy evidently revealed that a positive surface charge is preserved in acidic and neutral media after molecular connection which is in parallel with the corresponding electrocatalytic amplifications toward oxygen reduction reaction (ORR), suggesting the electrodics after molecular connection is extremely pH dependent. Surface charge present after molecular connection is significantly neutralized by the abundant hydroxyl groups i...

Electrocatalytic Amplification of Single Nanoparticle Collisions Using DNA-Modified Surfaces.

Here we report on the effect of DNA modification on individual collisions between Pt nanoparticles (PtNPs) and ultramicroelectrode (UME) surfaces. These results extend recent reports of electrocatalytic amplification (ECA) arising from collisions between naked surfaces, and they are motivated by our interest in using ECA for low-level biosensing applications. In the present case, we studied collisions between naked PtNPs and DNA-modified Au and Hg UMEs and also collisions between DNA-modified PtNPs and naked Au and Hg UMEs. In all cases, the sensing reaction is the catalytic oxidation of N2H4. The presence of ssDNA (5-mer or 25-mer) immobilized on the UME surface has little effect on the magnitude or frequency of ECA signals, regardless of whether the electrode is Au or Hg. In contrast, when DNA is immobilized on the PtNPs and the electrodes are naked, clear trends emerge. Specifically, as the surface concentration of ssDNA on the PtNP surface increases, the magnitude and frequency of the current transients decrease. This trend is most apparent for the longer 25-mer. We interpret these results as follows. When ssDNA is immobilized at high concentration on the PtNPs, the surface sites on the NP required for electrocatalytic N2H4 oxidation are blocked. This leads to lower and fewer ECA signals. In contrast, naked PtNPs are able to transfer electrons to UMEs having sparse coatings of ssDNA.

L-cysteine induced manganese porphyrin electrocatalytic amplification with 3D DNA-Au@Pt nanoparticles as nanocarriers for sensitive electrochemical aptasensor

We successfully applied the novel l-cysteine (RSH) induced manganese porphyrin (MnTPP) electrocatalytic amplification in electrochemical aptasensor for detection of thrombin (TB) with the 3D DNA-Au@Pt nanoparticles (DNA-Au@Pt NPs) as nanocarrier. A further discussion for the suggested mechanism of the reaction involved the MnTPP-catalyzed oxidation of thiols (RSH) to disulfides l-cystine (RSSR) was also discussed in detail in this work. In comparison with traditional H2O2 mediated MnTPP electrocatalytic amplification, the novel reaction with more stable catalytic substrate RSH possessed high catalytic amplification efficiency. In addition, the 3D DNA-Au@Pt NPs could provide abundant binding sites for immobilizing signal tags and enzymes. As a result, the electrochemical signal can be greatly enhanced by the RSH mediated MnTPP electrocatalytic reaction and 3D DNA-Au@Pt NPs. Under optimal conditions, the proposed aptasensor exhibited a wider linear range of 0.1pM–100nM with a more sensitive detection limit of 29fM for TB detection.

Increasing the Collision Rate of Particle Impact Electroanalysis with Magnetically Guided Pt-Decorated Iron Oxide Nanoparticles.

An integrated microfluidic/magnetophoretic methodology was developed for improving signal response time and detection limits for the chronoamperometric observation of discrete nanoparticle/electrode interactions by electrocatalytic amplification. The strategy relied on Pt-decorated iron oxide nanoparticles which exhibit both superparamagnetism and electrocatalytic activity for the oxidation of hydrazine. A wet chemical synthetic approach succeeded in the controlled growth of Pt on the surface of FeO/Fe3O4 core/shell nanocubes, resulting in highly uniform Pt-decorated iron oxide hybrid nanoparticles with good dispersibility in water. The unique mechanism of hybrid nanoparticle formation was investigated by electron microscopy and spectroscopic analysis of isolated nanoparticle intermediates and final products. Discrete hybrid nanoparticle collision events were detected in the presence of hydrazine, an electrochemical indicator probe, using a gold microband electrode integrated into a microfluidic channel. In contrast with related systems, the experimental nanoparticle/electrode collision rate correlates more closely with simple theoretical approximations, primarily due to the accuracy of the nanoparticle tracking analysis method used to quantify nanoparticle concentrations and diffusion coefficients. Further modification of the microfluidic device was made by applying a tightly focused magnetic field to the detection volume to attract the magnetic nanoprobes to the microband working electrode, thereby resulting in a 6-fold increase to the relative frequency of chronoamperometric signals corresponding to discrete nanoparticle impact events.

(Invited) Ultrasensitive Electroanalytical Detection and Study of Single Nanoparticle Catalysts

The continued interest in nanoparticles (NPs) is due to their numerous properties such as active components in sensors, optics, energy conversion and energy storage applications. The development of new analytical tools for characterizing NP properties in a fast and reproducible manner is sorely needed. Especially for those to correlate structure-function relationships of NPs and optimize NPs activity for in homogeneous and heterogeneous catalysis. The precise characterization of individual NPs in terms of their size, shape and composition would faciliate better fundamental understanding. The development of "electrocatalytic amplification" is one such method where single NPs are detected by measuring electrocatalytic current (or potential) due to the electrochemical processes (oxidation/reduction of the species present in solution) occurring on the surface of the NP whenever a NP collides with an inert UME (gold, platinum, platinum oxide and carbon) which otherwise cannot catalyze the reaction. Here we report the application of electrocatalytic amplification method employing inert (non-catalytic) UMEs (Hg, Bi, Ga) as electrodes for evaluating the catalytic activity of Pt NPs of different shapes (hexagones and cubes) for hydrazine oxidation. To precisely correlate structure function relationship of Pt NPs, Pt NPs without ligands referred as "naked Pt NPs" were used in this study. "Naked Pt NPs" were obtained by following the procedure reported previously where polyacrylate capped Pt NPs were synthesized first followed by a strong base treatment (potassium hydroxide) to remove the ligand (polyacrylate) off the NP's surface. From TEM the average size of polyacrlylate capped hexagonal and cubic Pt NPs was found to be 11.2 ± 2.8 nm and 11.8 ± 1.7 nm respectively. Hexagonal Pt NPs possess (100) and (111) facets where cubic Pt NPs possess (111) planes as preferential surface facets. Complete removal of polyacrylate ligand from the Pt NP's surface was confirmed by obtaining CVs, TGA and IR spectra of polyacrylate capped Pt NPs before and after KOH treatment. Also, Pt NPs retained their size and structure after base treatment. The integrated charge passed per spike during single NP collision for polyacrylate capped hexagonal and cubic Pt NPs concentrated over a range of (120-210) and (40-100) pC respectively. Whereas, for naked hexagonal and cubic Pt NPs the integrated charge passed per spike during single NP collision concentrated over a range of (180-400) and (60- 180) respectively. Naked Pt NPs displayed higher catalytic activity for hydrazine oxidation compared to polyacrylate capped Pt NPs of similar size and size.

Platinum-doped ceria based biosensor for in vitro and in vivo monitoring of lactate during hypoxia.

Measurements of lactate concentrations in blood and tissues are an important indication of the adequacy of tissue oxygenation and could be useful for monitoring the state and progress of a variety of diseases. This paper describes the fabrication, analytical characterization, and physiological application of an amperometric microbiosensor based on lactate oxidase and oxygen-rich platinum doped ceria (Pt-ceria) nanoparticles for monitoring lactate levels during hypoxic conditions. The Pt-ceria nanoparticles provided electrocatalytic amplification for the detection of the enzymatically produced hydrogen peroxide and acted as an internal oxygen source for the enzyme, enabling lactate monitoring in an oxygen depleted tissue. In vitro evaluation of the biosensor demonstrated high selectivity against physiological levels of ascorbic acid, a storage stability of 3 weeks, a fast response time of 6 s, and good, linear sensitivity over a wide concentration range. In vivo experiments performed by placing the biosensor in the hippocampus of anesthetized rats demonstrated the feasibility of continuous lactate monitoring over 2 h ischemia and reperfusion. The results demonstrate that Pt-ceria is a versatile material for use in implantable enzyme bioelectrodes, which may be used to assess the pathophysiology of tissue hypoxia. In addition to measurements in hypoxic conditions, the detection limit of this biosensor was low, 100 pM, and the materials used to fabricate this biosensor can be particularly useful in ultrasensitive devices for monitoring lactate levels in a variety of conditions.

Detection of Single Au Nanoparticle Collisions Using Electrocatalytic Amplification Method

Detection of single gold (Au) nanoparticle (NP) collisions on a carbon‐fiber (C‐fiber) ultramicroelectrode (UME) was carried out successfully using the electrocatalytic amplification method. The C‐fiber UME shows a low and quite stable background current for the hydrazine oxidation over a wide potential range. The electrocatalytic current transient generated by Au NP collision on the C‐fiber UME was also detected simultaneously without any pretreatment of the UME or deformation of the Au NP. The magnitude and frequency of the transient current from Au NP collisions agree well with the calculated values based on the size and concentration of the Au NP.

Electrocatalytic amplification of nanoparticle collisions at electrodes modified with polyelectrolyte multilayer films.

We report electrochemical catalytic amplification of individual collisions between ∼57 nm diameter Pt nanoparticles (Pt NPs) and 12.5 μm diameter Au ultramicroelectrodes modified with passivating, electrostatically assembled polyelectrolyte multilayer (PEM) films prepared by the layer-by-layer deposition method. Two key findings are reported. First, despite the thicknesses of the insulating PEM films, which range up to 5 nm, electrons are able to tunnel from the Pt NPs to the electrode resulting in electrocatalytic N2H4 oxidation at the PEM film-solution interface. These single-particle measurements are in accord with prior reports showing that the electrochemical activity of passive PEM films can be reactivated by adsorption of metallic NPs. Second, it is possible to control the frequency of the collisions by manipulating the net electrostatic charge present on the outer surface of the PEM thin film. These results, which demonstrate that chemistry can be used to control electrocatalytic amplification, set the stage for future sensing applications.

L-cysteine induced hemin/G-quadruplex concatemers electrocatalytic amplification with Pt-Pd supported on fullerene as a nanocarrier for sensing the spore wall protein of Nosema bombycis.

This work described a pseudobienzyme electrocatalytically amplified immunosensor for sensing the spore wall protein of Nosema bombycis by adopting L-cysteine as the electrocatalytic substance to generate H2O2in situ with the aid of hemin/G-quadruplex concatemers loaded onto C60@Pt-Pd. The hemin/G-quadruplex here is also acting as the probe for electrochemical signal output.

Single nanoparticle collisions at microfluidic microband electrodes: the effect of electrode material and mass transfer.

We report on the effect of convection on electrochemically active collisions between individual Pt nanoparticles (PtNPs) and Hg and Au electrodes. Compared to standard electrochemical cells utilizing Hg and Au ultramicroelectrodes (UMEs) used in previous studies of electrocatalytic amplification, microelectrochemical devices offer two major advantages. First, the PtNP limit of detection (0.084 pM) is ∼8 times lower than the lowest concentration measured using UMEs. Second, convection enhances the mass transfer of PtNPs to the electrode surface, which enhances the collision frequency from ∼0.02 pM(-1) s(-1) on UMEs to ∼0.07 pM(-1) s(-1) in microelectrochemical devices. We also show that the size of PtNPs can be measured in flowing systems using data from collision experiments and then validate this finding using multiphysics simulations.

Observation of Electrocatalytic Amplification of Iridium Oxide (IrOx) Single Nanoparticle Collision on Copper Ultramicroelectrodes

Recently, the observation of the electrocatalytic behavior of individual nanoparticles (NPs) by electrochemical amplification method has been reported. For example, the Iridium oxide (IrOx) NP collision on the Pt UME was observed via electrocatalytic water oxidation. However, the bare Pt UME had poor reproducibility for the observation of NP collision signal and required an inconvenient surface pre-treatment for the usage. In this manuscript, we has been investigated other metal electrode such as Cu UME for the reproducible data analysis and convenient use. The IrOx NP collision was successively observed on the bare Cu UME and the reproducibility in collision frequency was improved comparing with previous case using the NaBH4 pre-treated Pt UME. Also, the adhesion coefficient between NP and the Cu UME was studied for better understanding of the single NP collision system.

Molecularly imprinted magnetic nanoparticles for determination of the herbicide chlorotoluron by gate-controlled electro-catalytic oxidation of hydrazine

We have synthesized magnetic nickel hexacyanoferrate (NiHCF) nanoparticles and coated them with a polymer that was molecularly imprinted for the herbicide chlorotoluron. The NiHCF nanoparticles coated with the molecular imprint (MIP) were then applied to prepare a sensitive, selective, and easily renewable modified electrode. Chlorotoluron can be determined indirectly based on the change in the current resulting from the oxidation of hydrazine hydrate on the modified electrode because the cavities in the MIP are increasingly blocked after binding chlorotoluron. There is a linear relationship between the response currents and chlorotoluron in the 5 × 10−9 to 1 × 10−7 mol · L−1 concentration range, and the detection limit is 9.27 × 10−10 mol · L−1. This method represents an attractive platform for trace analysis of this pollutant because of (a) the advantage of “gate-controlled electro-catalytic amplification” effect, and (b) the enrichment of analyte on the magnetic particles. The renewal of the sensor was carried out by removing the disposal magnetic particles. Water samples were spiked with chlorotoluron and then assayed to give recoveries ranging from 97 % to 105 %.

Mediator-free triple-enzyme cascade electrocatalytic aptasensor with exonuclease-assisted target recycling and hybridization chain reaction amplification

The amplified sensitive detection of protein is essential to biomedical research as well as clinical diagnosis. Here, we developed an ultrasensitive mediator-free triple-enzyme cascade electrocatalytic aptasensor for thrombin detection on the basis of exonuclease-assisted target recycling and hybridization chain reaction (HCR) amplification strategy. The double strands constructed by the hybridization of thrombin binding aptamer (S1) with its complementary strand (S2) were firstly assembled on the electrode. Upon addition of target to the system, the S1 recognized thrombin and left off electrode to make space for assembly of hybrid-primer probe (H0). Then, the H0 triggered the HCR to form the multi-functional hemin/G-quadruplex DNAzyme nanowires. In the mediator-free triple-enzyme cascade electrocatalytic amplification system, the hemin/G-quadruplex DNAzyme nanowires here simultaneously played three roles: the redox probe, NADH oxidase and HRP-mimicking DNAzyme, respectively, which effectively avoided the fussy redox probe and enzyme labeling process, serving a useful alternative or supplement to conventional assays that typically suffer from complexity and poor sensitivity. Additionally, in order to improve the assembly amount of hemin/G-quadruplex DNAzyme nanowire, the exonuclease-assisted target recycling amplification was used for the continuous removal of S1. As a result, the proposed method can detect thrombin specifically with a detection limit as low as 20fM.

DNA analysis based on the electrocatalytic amplification of gold nanoparticles

The detection of deoxyribonucleic acid (DNA) has a very important significance in life sciences. The paper demonstrates a novel and effective electrochemical detection method of observing single nanoparticle (NP) collision events as signal label to amplify electrocatalytic activity for hydrazine electrooxidation in a sandwich-type DNA sensor. The Au NP was characterized by UV–vis spectrophotometer and transmission electron microscope (TEM). Hydrazine oxidation on DNA-modified Au NP was compared with that on Au NP by cycle voltammetry (CV). Via NaBH4 treatment the enhancement of electrocatalytic activity of DNA-modified Au NPs was proposed and applied to the sandwich-type DNA sensor. It is discussed by cyclic voltammetry, linear sweep voltammetry (LSV), and chronoamperometry (CA). The result showed that the Au NP of 3–4 nm diameters had large specific surface area and less prone to agglomeration with good electrocatalytic activity. Cyclic voltammetry displayed that the current of electrode increased 1.103–1.889 × 10−8 A after NaBH4 treatment in the different concentrations of target DNA. Target DNA could be detected at concentration ranging from 1 pM to 100 nM and the detection limit was 1 pM. It can be applied to the development of even more sensitive biosensors.

An ultrasensitive electrochemical aptasensor with autonomous assembly of hemin–G-quadruplex DNAzyme nanowires for pseudo triple-enzyme cascade electrocatalytic amplification

We for the first time fabricate a pseudo triple-enzyme cascade electrocatalytic electrochemical aptasensor by using the alcohol dehydrogenase (ADH) as well as the autonomously assembled hemin-G-quadruplex that simultaneously acted as an NADH oxidase and HRP-mimicking DNAzyme.

Highly sensitive electrochemical label-free aptasensor based on dual electrocatalytic amplification of Pt–AuNPs and HRP

In this work, a label-free electrochemical aptamer-based sensor (aptasensor) was constructed on account of the direct immobilization of redox probes on an electrode surface. For this proposed aptasensor, a gold nanoparticles (AuNPs)-coated electrode was firstly modified with redox probes-nickel hexacyanoferrates nanoparticles (NiHCFNPs) through chemisorption and electrostatic adsorption. Then, platinum-gold alloy nanoparticles (Pt-AuNPs) and horseradish peroxidase (HRP) were respectively assembled onto the modified electrode surface, which formed the multilayer films for amplifying the electrochemical signal of NiHCFNPs and immobilizing thiolated thrombin aptamers (TBAs). In the presence of target thrombin, the TBA on the multilayer could catch the thrombin onto the electrode surface, which resulted in a barrier for electro-transfer, leading to the decrease of the electrochemical signal of NiHCFNPs amplified by the Pt-AuNPs and HRP toward H(2)O(2). The proposed method avoided the redox probes labeling process, increased the amount of redox probes, and further amplified the electrochemical signal. Thus, the approach showed a high sensitivity and a wider linearity to thrombin in the range between 0.01 nM and 50 nM with a detection limit of 6.3 pM.

DNA biosensors based on metallo-intercalator probes and electrocatalytic amplification

Strategies for electrochemical sensing of DNA can be classified into label-free and label-based approaches, categories of which include enzyme-, nanomaterial- and redox labels that are attached to DNA either by covalent or non-covalent means. Metallointercalators represent one group of small molecule redox labels that non-covalently enter the groove of a DNA. The metallointercalator plays a dual-role in acting as a structure indicator (for hybridization) and a signal generator. Labeling is not needed, and electrochemical measurements can be carried out in a label-free solution of an electrolyte. However, such metallointercalators lack the option of catalytic signal generation as in the case of enzyme- and nanomaterial-based labels. Therefore, signal amplification becomes crucial. We first survey here recent progress in this area. A signal-amplifying system is presented that relies on the electroatalytic oxidation of a metallointercalator ruthenium(II)bipyridine/phenoxazine complex in the presence of electron donor species such as oxalate, DNA bases, or tripropylamine. Recent work on such DNA sensors is discussed. Results suggest that such metallointercalator-based DNA sensors represent a viable platform for developing high-throughput and automated PCR/lab-on-a-chip devices as well as visualized multifunctional DNA sensors.

Observation of Discrete Au Nanoparticle Collisions by Electrocatalytic Amplification Using Pt Ultramicroelectrode Surface Modification

After growing a thin layer of oxide (PtOx) by anodization of a Pt electrode, it changed from catalytically active for electrochemical NaBH4 oxidation into an inactive electrode. When held at a potential where the oxide film was maintained, collisions of individual 14 nm diameter Au nanoparticles (NPs) that catalyzed NaBH4 oxidation were successfully observed as discrete current pulses (spikes or blips) for each NP interaction with the modified Pt electrode via amplification from NaBH4 oxidation. The current response is affected by NP concentration and the applied potential.

Sensitive label-free electrochemical immunoassay by electrocatalytic amplification

A label-free electrochemical immunosensor for the detection of protein analytes without redox-active centers is for the first time developed based on the combination of gold nanoparticles and the mediated charge transport through the multilayer films that is related to an electrocatalytic process. Given the low detection limit, high sensitivity and selectivity, the success achieved here seems plausibly to serve as a significant step toward the development of versatile label-free immunoassay.

Single Nanoparticle Electrocatalysis: Effect of Monolayers on Particle and Electrode on Electron Transfer

The electrocatalytic properties of individual single Pt nanoparticles (NPs) can be studied electrochemically by measuring the current−time (i−t) responses during single NP collisions with a noncatalytic ultramicroelectrode (UME). The Pt NPs are capped with citrate ions or a self-assembled monolayer (SAM) of alkane thiols terminated with carboxylic acid that affect the observed i−t responses. By varying the length of the SAMs or the composition of a mixed monolayer, we have studied the effect of adsorbed molecules on the catalytic activity of Pt NPs at the single particle level through electrocatalytic amplification of single NP collisions. Collisions of single NPs were triggered and recorded as individual current steps whose amplitude represents the reactivity of single Pt NPs for the reaction of interest, here hydrazine oxidation, at a given electrode potential. The catalytic properties of Pt NPs are dependent not only on the nature of the interaction between the adsorbed monolayer and the catalytic NP surface, but also on the rate of electron transfer through the SAMs, governed by their length.