Surface Enhanced Raman Spectroscopy at Electrochemically Fabricated Silver Nanowire Junctions.

Here we describe enhanced Raman scattering at Au electrode 1 (E1)/Ag nanowire (NW)/4-aminothiophenol (4-ATP)/Au electrode 2 (E2) nanojunctions fabricated by combining self-assembly and metal electrodeposition at microgap electrodes (E1 and E2). In this method we assemble the 4-ATP on electrode E2 and electrodeposit Ag on the opposite electrode E1 of an Au interdigitated array (IDA) electrode device. The electrodeposited Ag grows in the form of NWs on E1 and makes nanoscale contact to E2 to form the junctions. The presence of the Ag NW leads to strong Raman scattering of the 4-ATP molecules within the nanojunction leading to estimated enhancement factors ranging from 10(3) to 10(6). Scanning electron microscopy (SEM) images provide insight into the morphology of the junctions. The magnitude of the Raman enhancement depends on the extent of contact between the Ag NW and the 4-ATP self-assembled monolayer (SAM). With this approach we could detect 4-ATP molecules diluted by a factor of 1000 with hexanethiol molecules within the junctions. Our approach is simple and fast with the potential to correlate electronic measurements of molecules with Raman spectroscopy data of the same molecules in a nanoscale junction for molecular electronics or chemiresistive sensing applications.

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.

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.

Ultrasensitive electroanalytical tool for detecting, sizing, and evaluating the catalytic activity of platinum nanoparticles.

Here we describe a very simple, reliable, low-cost electrochemical approach to detect single nanoparticles (NPs) and evaluate NP size distributions and catalytic activity in a fast and reproducible manner. Single NPs are detected through an increase in current caused by electrocatalytic oxidation of N(2)H(4) at the surface of the NP when it contacts a Hg-modified Pt ultramicroelectrode (Hg/Pt UME). Once the NP contacts the Hg/Pt UME, Hg poisons the Pt NP, deactivating the N(2)H(4) oxidation reaction. Hence, the current response is a "spike" that decays to the background current level rather than a stepwise "staircase" response as previously described for a Au UME. The use of Hg as an electrode material has several quantitative advantages including suppression of the background current by 2 orders of magnitude over a Au UME, increased signal-to-noise ratio for detection of individual collisions, precise integration of current transients to determine charge passed and NP size, reduction of surface-induced NP aggregation and electrode fouling processes, and reproducible and renewable electrodes for routine detection of catalytic NPs. The NP collision frequency was found to scale linearly with the NP concentration (0.016 to 0.024 pM(-1)s(-1)). NP size distributions of 4-24 nm as determined from the current-time transients correlated well with theory and TEM-derived size distributions.

Influence of the redox indicator reaction on single-nanoparticle collisions at mercury- and bismuth-modified Pt ultramicroelectrodes.

Single-Pt nanoparticles (NPs) can be detected electrochemically by measuring the current-time (i-t) response associated with both hydrazine oxidation and proton reduction during individual Pt NP collisions with noncatalytic Hg- and Bi-modified Pt ultramicroelectrodes (Hg/Pt and Bi/Pt UMEs, respectively). At Hg/Pt UMEs, the i-t response for both hydrazine oxidation and proton reduction consists of repeated current "spikes" that return to the background level as Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and deactivation of the redox reaction. Furthermore, at a Hg/Pt UME, the applied potential directly influences the interfacial surface tension (electrocapillarity) that also impacts the observed i-t response for single-Pt NP collisions for proton reduction that exhibits a faster decay of current (0.7-4 ms) to background levels than hydrazine oxidation (2-5 s). Because the surface tension of Hg is lower (-0.9 V), Pt NPs possibly react faster with Hg (amalgamate at a faster rate), resulting in sharp current spikes for proton reduction compared to hydrazine oxidation. In contrast, a stepwise "staircase" i-t response is observed for proton reduction for single-Pt NP collisions at a Bi/Pt UME. This different response suggests that electrostatic forces of negatively charged citrate-capped Pt NPs also influence the i-t response at more negative applied potentials, but the Pt NPs do not poison the electrochemical activity at Bi/Pt UMEs.

Electrochemical fabrication of metal/organic/metal junctions for molecular electronics and sensing applications.

A simple electrochemical approach was used for fabricating electrode/metal nanowire/(molecule or polymer)/electrode junctions for sensing or molecular electronics applications. The procedure for fabricating these molecule-based devices involves electropolymerization of phenol or chemisorption of alkanethiols on one set of electrodes (E1) and electrodeposition of Ag metal nano/microwires on a second electrode (E2) which is ∼5 µm away from E1. Under appropriate deposition conditions, Ag nanowires grow from E2 and cross over to E1, forming a E1/(molecule or polymer)/Ag nanowire (NW)/E2 junction. The junction resistance was controlled by (1) electrodepositing polyphenol of varied densities on E1 and (2) assembling alkanethiols of different chain lengths on E1. Ag NWs at high resistance E1/polyphenol/Ag NW/E2 junctions functionalized with Pd monolayer protected clusters (MPCs) responded fast and reversibly to H(2) concentrations as low as 0.11% in a nitrogen carrier gas by a resistance decrease, likely due to volume expansion of the Pd nanoparticles, demonstrating the use of these electrochemically fabricated junctions for gas sensing applications.

Hydrogen switches and sensors fabricated by combining electropolymerization and Pd electrodeposition at microgap electrodes.

Here we describe a simple electrochemical approach to fabricate devices which behave as hydrogen sensors and switches. Devices fabricated by the electrodeposition of Pd directly across a 5 microm gap interdigitated array (IDA) of gold electrodes behaved as "hydrogen sensors". These devices had initial currents on the 10(-3) A level at -0.3 V and exhibited fast and reversible decreases in current in the presence of H(2) concentrations in a N(2) carrier gas with an average detection limit of 400 ppm. The current decrease is due to the formation of the more resistive PdH(x) in the presence of H(2). Devices fabricated by polyphenol electropolymerization on one set of electrodes and Pd electrodeposition on the other set behaved as "hydrogen switches". These devices displayed very low baseline currents of 10-100 pA at -0.3 V due to the presence of polphenol in the Electrode1/Pd/Polyphenol/Electrode 2 junction, and the current increased a remarkable 7-8 orders of magnitude in the presence of > or = 1.0% H(2) due to volume expansion upon PdH(x) formation, which leads to a direct connection between Pd (as PdH(x)) and Electrode 2 through the porous 4-10 nm thick polyphenol insulating film. The response and recovery time for the "hydrogen sensor" ranged from 20 to 60 s while that for the "hydrogen switch" ranged from 10 to >100 s. The response and recovery time generally decreased for the "hydrogen switch" as the number of polyphenol electrochemical cycles decreased.

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.

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.

Electrochemical monitoring of single nanoparticle collisions at mercury-modified platinum ultramicroelectrodes.

Here, we report a potentiometric method for detecting single platinum nanoparticles (Pt NPs) by measuring a change in open-circuit potential (OCP) instead of the current during single Pt NP collisions with the mercury-modified Pt ultramicroelectrode (Hg/Pt UME). Similar to the current-time (i-t) response reported previously at Hg/Pt UMEs, the OCP-time (v-t) response consists of repeated potential transient signals that return to the background level. This is because Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and results in deactivation of the redox reaction. For individual Pt NP collisions the amplitude of the OCP signal reaches a maximum and decays to the background level at a slower rate compared to the comparable i-t response. Due to this, OCP events are broader and more symmetrical in shape compared to i-t "spikes." The collision frequency of Pt NPs derived from v-t plots (0.007 to 0.020 pM(-1) s(-1)) is in good agreement with the value derived from i-t plots recorded at Hg/Pt UMEs (0.016 to 0.024 pM(-1) s(-1)) under similar conditions and was found to scale linearly with Pt NP concentration. Similar to the current response, the amplitude of the OCP response increased with the NP's size. However, unlike the change in current in a i-t response, the change in OCP in a v-t response observed during single Pt NP collisions with Hg/Pt UME is larger than the estimated change in OCP based on the theory. Therefore, the Pt NP sizes derived from the v-t response did not correlate with the TEM-derived Pt NP sizes. In spite of these results the potentiometric method has great value for electroanalysis because of its significant advantages over the amperometric method such as a simpler apparatus and higher sensitivity.