Single atoms and small clusters of atoms may accompany Au and Pd dendrimer-encapsulated nanoparticles.

We report the presence of small clusters of atoms (<1 nm) (SCs) and single atoms (SAs) in solutions containing 1-2 nm dendrimer-encapsulated nanoparticles (DENs). Au and Pd DENs were imaged using aberration-corrected scanning transmission electron microscopy (ac-STEM), and energy dispersive spectroscopy (EDS) was used to identify and quantify the SAs/SCs. Two main findings have emerged from this work. First, the presence or absence of SAs/SCs depends on both the terminal functional group of the dendrimer (-NH2 or -OH) and the elemental composition of the DENs (Au or Pd). Second, dialysis can be used to remove the majority of SAs/SCs in cases where a high density of SAs/SCs are present. The foregoing conclusions provide insights into the mechanisms for Au and Pd DEN synthesis and stability. Ultimately, these results demonstrate the need for careful characterization of systems containing nanoparticles to ensure that SAs/SCs, which may be below the detection limit of most analytical methods, are taken into consideration (especially for catalysis experiments).

Plastic-based lateral flow immunoassay device for electrochemical detection of NT-proBNP.

Here we report an easily fabricated, plastic-based lateral flow device for carrying out metalloimmunoassays. The device is called ocFlow to emphasize the open-channel design. We have shown that the ocFlow is capable of magnetic microbead (MµB)-based metalloimmunoassays for the detection of two types of immunoconjugates: a model composite (MC) and a sandwich immunoassay for the heart failure marker NT-proBNP. In both assays, Ag nanoparticles (AgNPs) were used as electrochemically detectable labels. NT-proBNP and MC concentrations as low as 750.0 pM and 10.0 pM, respectively, could be detected using the ocFlow device. Four key conclusions can be drawn from the results presented herein. First, immunoconjugates attached to the MµBs can be transported in the flow channel using combined hydrodynamic and capillary pressure passive pumping. Second, the ocFlow device is capable of on-chip storage, resolvation, and conjugate formation of both the MC and NT-proBNP composites. Third, electrochemical detection can be conducted on analytes suspended in serum by rinsing the electrodes with a wash buffer. Finally, and perhaps most significantly, the assay is quantitative and has a detection limit for NT-proBNP in the high picomolar range when the necessary reagents are stored on the device in a dry form.

Well-Defined Nanoparticle Electrocatalysts for the Refinement of Theory.

The relationship between experiment and theory in electrocatalysis is one of profound importance. Until fairly recently, the principal role of theory in this field was interpreting experimental results. Over the course of the past decade (roughly the period covered by this review), however, that has begun to change, with theory now frequently leading the design of electrocatalytic materials. Though rewarding, this has not been a particularly easy union. For one thing, experimentalists and theorists have to come to grips with the fact that they rely on different models. Theorists make predictions based on individual, perfect structural models, while experimentalists work with more complex and heterogeneous ensembles of electrocatalysts. As discussed in this review, computational capabilities have improved in recent years, so that theory is better represented by the structures that experimentalists are able to prepare. Likewise, synthetic chemists are able to make ever more complex electrocatalysts with high levels of control, which provide a more extensive palette of materials for testing theory. The goal of this review is to highlight research from the last ∼10 years that focuses on carefully controlled electrocatalytic experiments which, in combination with theoretical predictions, bring us closer to bridging the gap between real catalysts and computational models.

Interactions between Oligoethylene Glycol-Capped AuNPs and Attached Peptides Control Peptide Structure.

Peptide-functionalized nanoparticles (NPs) often rely on a well-defined peptide structure to function. Here, we report the attachment of model peptides to the ligand shell of AuNPs passivated with oligoethylene glycol (OEG). Specifically, peptides containing the repeating (LLKK)n motif plus either one or two reactive functional groups were covalently linked to OEG-capped, ∼5 nm AuNPs via the Cu+-catalyzed azide-alkyne cycloaddition reaction. This work builds on a previous study from our group in which an (LLKK)n peptide having two reactive functional groups was considered. Peptide attachment was confirmed by FTIR spectroscopy. Amino acid analysis was used to determine that 3-4 peptides were immobilized per AuNP. Circular dichroism spectroscopy revealed a structural change from random coil in solution to α-helical upon attachment to OEG-capped AuNPs. The key result of this study is that the nature of the capping layer on the AuNP surface influences peptide structure to a significant degree. Other important findings resulting from this work are that the AuNP-peptide conjugates reported here are water soluble and that the long axis of the helical peptides is oriented tangent to the AuNP surface. The latter point is important for applications involving biorecognition.

Orientation-Controlled Bioconjugation of Antibodies to Silver Nanoparticles.

Here we report on the use of heterobifunctional cross-linkers (HBCLs) to control the number, orientation, and activity of immunoglobulin G antibodies (Abs) conjugated to silver nanoparticles (AgNPs). A hydrazone conjugation method resulted in exclusive modification of the polysaccharide chains present on the fragment crystallizable region of the Abs, leaving the antigen-binding regions accessible. Two HBCLs, each having a hydrazide terminal group, were synthesized and tested for effectiveness. The two HBCLs differed in two respects, however: (1) either a thiol or a dithiolane group was used for attachment to the AgNP; and (2) the spacer arm was either a PEG chain or an alkyl chain. Both cross-linkers immobilized 5 ± 1 Abs on the surface of each 20-nm-diameter AgNP. Electrochemical results, obtained using a half-metalloimmunoassay, proved that Abs conjugated to AgNPs via either of the two HBCLs were 4 times more active than those conjugated by the more common physisorption technique. This finding confirmed that the HBCLs exerted orientational control over the Abs. We also demonstrated that the AgNP-HBCL-Ab conjugates were stable and active for at least 2 weeks. Finally, we found that the stability of the HBCLs themselves was related to the nature of their spacer arms. Specifically, the results showed that the HBCL having the alkyl chain is chemically stable for at least 90 days, making it the preferred cross-linker for bioassays.

Combined Experimental and Theoretical Study of the Structure of AuPt Nanoparticles Prepared by Galvanic Exchange.

In this article, experiment and theory are combined to analyze Pb and Cu underpotential deposition (UPD) on ∼1.7 nm Au nanoparticles (NPs) and the AuPt structures that result after galvanic exchange (GE) of the UPD layer for Pt. Experimental Pb (0.49 ML) and Pt (0.50 ML) coverages are close to values predicted by density functional theory-molecular dynamics (DFT-MD, 0.59 ML). DFT-MD reveals that the AuNPs spontaneously reconstruct from cuboctahedral to a (111)-like structure prior to UPD. In the case of Pb, this results in the random electrodeposition of Pb onto the Au surface. This mechanism is a consequence of opposing trends in Pb-Pb and Pb-Au coordination numbers as a function of Pb coverage. Cu UPD is more complex, and agreement between theory and experiment takes into account ligand effects (e.g., SO42- present as the electrolyte) and the electric double layer. Importantly, AuPt structures formed upon Pt GE are found to differ markedly depending on the UPD metal. Specifically, cyclic voltammetry indicates that the Pt coverage is ∼0.20 ML greater for Cu UPD/Pt GE (0.70 ML) than for Pb UPD/Pt GE (0.50 ML). This difference is corroborated by DFT-MD theoretical predictions. Finally, DFT-MD calculations predict the formation of surface alloy and core@shell structures for Pb UPD/Pt GE and Cu UPD/Pt GE, respectively.

Conjugation of an α-Helical Peptide to the Surface of Gold Nanoparticles.

We are interested in functionalizing gold nanoparticles (AuNPs) with proteins using a biomimetic approach in which an intermediate peptide "glue" directs the orientation of a protein relative to the AuNP surface. The first step toward this goal is described in the present article. Specifically, we show that ∼5 nm AuNPs can be functionalized with a mixed self-assembled monolayer (SAM) consisting of oligo(ethylene glycol) alkanethiols terminated with either hydroxyl or azide groups, and that the resulting materials are stable and soluble in water. The azide groups on the surface of the AuNPs can be subsequently linked to alkyne-functionalized peptides via a copper-catalyzed azide-alkyne cycloaddition (click) reaction. Analysis of the resulting material by Fourier transform infrared and circular dichroism spectroscopy demonstrates that the peptide is covalently linked to the SAM and that it exists in an α-helical conformation. In addition to our intended purpose of using these highly structured, biomimetic materials to orient proteins, they may also be useful for applications involving interactions between nanoparticles and cells.

Electrocatalytic Study of the Oxygen Reduction Reaction at Gold Nanoparticles in the Absence and Presence of Interactions with SnO x Supports.

Here we report that density functional theory (DFT) can be used to accurately predict how Au nanoparticle (NP) catalysts cooperate with SnO x ( x = 1.9 or 2.0) supports to carry out the oxygen reduction reaction (ORR). Specifically, dendrimers were used to encapsulate AuNPs and prevent their interactions with the underlying SnO x supports. After removal of the dendrimers, however, the AuNPs are brought into direct contact with the support and hence feel its effect. The results show that both SnO1.9 and SnO2.0 supports strongly enhance the electrocatalytic properties of AuNPs for the ORR. In the case of AuNP interaction with a SnO1.9 support, the number of electrons involved in the ORR ( neff) increases from 2.1 ± 0.2 to 2.9 ± 0.1 following removal of the dendrimers, indicating an increased preference for the desired four-electron product (water), while the overpotential decreases by 0.32 V. Similarly, direct interactions between AuNPs and a SnO2.0 support result in an increase in neff from 2.2 ± 0.1 to 3.1 ± 0.1 and a reduction of the overpotential by 0.28 V. These experimental results are in excellent agreement with the theoretically predicted onset potential shift of 0.30 V. According to the DFT calculations, the observed activity enhancements are attributed to the existence of anionic Au resulting from electron transfer from surface oxygen vacancies within the SnO x supports to the AuNPs. This theoretical finding was confirmed experimentally using X-ray photoelectron spectroscopy. Importantly, the calculations reported here were performed prior to the experiments. In other words, this study represents an unusual case of theory accurately predicting the electrocatalytic manifestation of strong metal support interactions.

Experimental and Theoretical Structural Investigation of AuPt Nanoparticles Synthesized Using a Direct Electrochemical Method.

In this report, we examine the structure of bimetallic nanomaterials prepared by an electrochemical approach known as hydride-terminated (HT) electrodeposition. It has been shown previously that this method can lead to deposition of a single Pt monolayer on bulk-phase Au surfaces. Specifically, under appropriate electrochemical conditions and using a solution containing PtCl42-, a monolayer of Pt atoms electrodeposits onto bulk-phase Au immediately followed by a monolayer of H atoms. The H atom capping layer prevents deposition of Pt multilayers. We applied this method to ∼1.6 nm Au nanoparticles (AuNPs) immobilized on an inert electrode surface. In contrast to the well-defined, segregated Au/Pt structure of the bulk-phase surface, we observe that HT electrodeposition leads to the formation of AuPt quasi-random alloy NPs rather than the core@shell structure anticipated from earlier reports relating to deposition onto bulk phases. The results provide a good example of how the phase behavior of macro materials does not always translate to the nano world. A key component of this study was the structure determination of the AuPt NPs, which required a combination of electrochemical methods, electron microscopy, X-ray absorption spectroscopy, and theory (DFT and MD).

Detection of Silver Nanoparticles by Electrochemically Activated Galvanic Exchange.

Here we report on the seemingly simple process of galvanic exchange (GE) between electrogenerated AuCl4- and silver nanoparticles (AgNPs). The results were obtained in the specific context of using AgNPs as labels for bioassays in paper fluidic devices. Results obtained from a combined electrochemistry and microscopy study indicate that the GE process results in recovery of only ∼5% of the total equivalents of Ag present in the system. This low value is a consequence of two factors. First, after an initial fraction of each AgNP undergoes GE, a Au shell forms around the remaining AgNP core preventing further exchange. Second, to simulate a true biological fluid, the experiments were carried out in a Cl--containing buffer. Consequently, some Ag+ formed during GE precipitates as AgCl, and it also serves to block additional GE. Following optimization of the GE process, it was possible to detect AgNP label concentrations as low as 2.6 fM despite these limitations.

Shape-controlled electrodeposition of single Pt nanocrystals onto carbon nanoelectrodes.

In this paper, we report the electrosynthesis and characterization of individual, shape-controlled Pt nanocrystals (NCs) electrodeposited on carbon nanoelectrodes (CNEs). Single Pt NCs were deposited onto the CNEs using an empirically developed square-wave potential program. Characterization by scanning electron microscopy indicates that the sizes of Pt NCs are remarkably reproducible (relative standard deviation = 6%). Electrochemically active surface areas, determined by Cu underpotential deposition and H adsorption/desorption analyses, are also reproducible. Selected area electron diffraction indicates that each Pt NC is comprised of just one single crystal (no grain boundaries). Although different square-wave potential programs lead to different types of crystals, the Pt NCs discussed here have a concave hexoctahedral geometry bound primarily by {13 6 2} surface facets. The results in this report represent a first step toward our ultimate goal of studying electrocatalysis at individual, shape-controlled, single-crystal nanoparticles.

Size Stability and H2/CO Selectivity for Au Nanoparticles during Electrocatalytic CO2 Reduction.

In this paper, we show that Au nanoparticles (AuNPs) stabilized with either citrate or by low-generation dendrimers rapidly grow during electrocatalytic reduction of CO2. For example, citrate-stabilized AuNPs and AuNPs encapsulated within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers (G6-OH DENs) having diameters of ∼2 nm grow substantially in size (to 6-7 nm) and polydispersity during just 15 min of electrolysis at -0.80 V (vs RHE). This degree of instability makes it impossible to correlate the structure of AuNPs determined prior to electrocatalysis to their catalytic function. In contrast to the G6-OH dendrimer, the higher generation G8-OH analogue stabilizes AuNPs under the same conditions that lead to instability of the other two materials. More specifically, G8-OH DENs having an initial size of 1.7 ± 0.3 nm increase to only 2.2 ± 0.5 nm during electrolysis in 0.10 M NaHCO3 at -0.80 V (vs RHE). Even when the electrolysis is carried out at -1.20 V, the higher-generation dendrimer stabilizes encapsulated AuNPs. This is presumably due to the compactness of the periphery of the G8-OH dendrimer. Although the G8-OH dendrimer nearly eliminates AuNP growth, the surface of the AuNP is still accessible for electrocatalytic reactions. The smaller, more stable G8-OH DENs strongly favor formation of H2 over CO. Some previous reports have suggested that AuNPs in the ∼2 nm size range yield primarily CO, but we believe these findings are a consequence of the growth of the AuNPs during catalysis and do not reflect the true function of ∼2 nm AuNPs.

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.

Tunability of the Adsorbate Binding on Bimetallic Alloy Nanoparticles for the Optimization of Catalytic Hydrogenation.

In this paper, we show that PtAu and PdAu random alloy dendrimer-encapsulated nanoparticles with an average size of ∼1.6 nm have different catalytic activity trends for allyl alcohol hydrogenation. Specifically, PtAu nanoparticles exhibit a linear increase in activity with increasing Pt content, whereas PdAu dendrimer-encapsulated nanoparticles show a maximum activity at a Pd content of ∼60%. Both experimental and theoretical results suggest that this contrasting behavior is caused by differences in the strength of H binding on the PtAu and PdAu alloy surfaces. The results have significant implications for predicting the catalytic performance of bimetallic nanoparticles on the basis of density functional theory calculations.

Faradaic Ion Concentration Polarization on a Paper Fluidic Platform.

We describe the design and characteristics of a paper-based analytical device for analyte concentration enrichment. The device, called a hybrid paper-based analytical device (hyPAD), uses faradaic electrochemistry to create an ion depletion zone (IDZ), and hence a local electric field, within a nitrocellulose flow channel. Charged analytes are concentrated near the IDZ when their electrophoretic and electroosmotic velocities balance. This process is called faradaic ion concentration polarization. The hyPAD is simple to construct and uses only low-cost materials. The hyPAD can be tuned for optimal performance by adjusting the applied voltage or changing the electrode design. Moreover, the throughput of hyPAD is 2 orders of magnitude higher than that of conventional, micron-scale microfluidic devices. The hyPAD is able to concentrate a range of analytes, including small molecules, DNA, proteins, and nanoparticles, in the range of 200-500-fold within 5 min.

Microelectrochemical Flow Cell for Studying Electrocatalytic Reactions on Oxide-Coated Electrodes.

Here, we report a new kind of microelectrochemical flow system that is well suited for studying electrode modifications, like thin films prepared by atomic layer deposition (ALD), that require substrates to have a two-dimensional form factor. The design provides a means for electrodes to be modified ex situ and then incorporated directly into the flow cell. The electrodes can be removed after testing and further modified or tested before being reincorporated into the flow cell. Using this cell, mass-transfer coefficients up to 0.011 cm/s and collection efficiencies up to 57 ± 10% have been achieved. Electrodes modified with an ultrathin layer of ALD Al2O3 and an overlayer of Pt dendrimer-encapsulated nanoparticles (DENs) have been incorporated into the flow cell and their electrocatalytic properties evaluated. Subsequently, the dendrimer was removed from the Pt DENs using a UV/O3 treatment, and this provided direct contact between the Al2O3 layer and the NPs. Finally, the product distribution for the oxygen reduction reaction (water vs H2O2) was evaluated in the presence and absence of Pt-Al2O3 support interactions.

Microfluidic Surface Titrations of Electroactive Thin Films.

We report the use of microfluidic surface titrations (MSTs) for studying electroactive self-assembled monolayers (eSAMs) and other thin films. The technique of MST utilizes a microfluidic generation-collection dual channel electrode (DCE) configuration to quantify the charge associated with electroactive thin films that might or might not be in direct contact with an electrode surface. This technique allows for quantitative measurement of surface coverages, Γ, as low as 30 pmol cm-2 for electrodeposited Cu thin films. Additionally, we show that it is possible to quantify Γ for ferrocene (Fc)-terminated alkylthiols in mixed-monolayer eSAMs. Interestingly, MSTs sometimes reveal a two-fold higher eSAM concentration compared to direct electrochemical measurements. This finding suggests that in these instances not all the constituent Fc-moieties of the eSAM are in sufficiently close proximity to the surface to be addressable via direct electrochemistry.

Structural Characterization of Rh and RhAu Dendrimer-Encapsulated Nanoparticles.

We report the structural characterization of 1-2 nm Rh and RhAu alloy dendrimer-encapsulated nanoparticles (DENs) prepared by chemical reduction with NaBH4. In contrast to previously reported results, in situ and ex situ X-ray absorption spectroscopic experiments indicate that only a fraction of the Rh3+ present in the precursors are reduced by NaBH4. Additional structural analysis of RhAu alloy DENs using extended X-ray absorption fine structure spectroscopy leads to a model in which there is significant segregation of Rh and Au within the nanoparticles. In Rh-rich alloy DENs, Au atoms are segregated on the nanoparticle surface.

Electron Transfer Facilitated by Dendrimer-Encapsulated Pt Nanoparticles Across Ultrathin, Insulating Oxide Films.

Electrocatalytic oxygen reduction at carbon electrodes fully passivated by Al2O3 is reported. Specifically, pyrolyzed polymer film (PPF) electrodes were prepared and then coated with pinhole-free Al2O3 layers ranging in thickness from 2.5 to 5.7 nm. All of these ultrathin oxide film thicknesses completely passivated the PPF electrodes, resulting in no faradaic current for either inner-sphere or outer-sphere electrochemical reactions. The electrodes could, however, be reactivated by immobilizing Pt dendrimer-encapsulated nanoparticles (DENs), containing an average of 55 atoms each, on the oxide surface. These PPF/Al2O3/Pt DEN electrodes were completely stable under a variety of electrochemical and solution conditions, and they are active for simple electron-transfer reactions and for more complex electrocatalytic processes. This approach for preparing well-defined oxide electrodes opens the door to a better understanding of the effect of oxide supports on reactions electrocatalyzed by metal nanoparticles.

A theoretical and experimental approach for correlating nanoparticle structure and electrocatalytic activity.

The objective of the research described in this Account is the development of high-throughput computational-based screening methods for discovery of catalyst candidates and subsequent experimental validation using appropriate catalytic nanoparticles. Dendrimer-encapsulated nanoparticles (DENs), which are well-defined 1-2 nm diameter metal nanoparticles, fulfill the role of model electrocatalysts. Effective comparison of theory and experiment requires that the theoretical and experimental models map onto one another perfectly. We use novel synthetic methods, advanced characterization techniques, and density functional theory (DFT) calculations to approach this ideal. For example, well-defined core@shell DENs can be synthesized by electrochemical underpotential deposition (UPD), and the observed deposition potentials can be compared to those calculated by DFT. Theory is also used to learn more about structure than can be determined by analytical characterization alone. For example, density functional theory molecular dynamics (DFT-MD) was used to show that the core@shell configuration of Au@Pt DENs undergoes a surface reconstruction that dramatically affects its electrocatalytic properties. A separate Pd@Pt DENs study also revealed reorganization, in this case a core-shell inversion to a Pt@Pd structure. Understanding these types of structural changes is critical to building correlations between structure and catalytic function. Indeed, the second principal focus of the work described here is correlating structure and catalytic function through the combined use of theory and experiment. For example, the Au@Pt DENs system described earlier is used for the oxygen reduction reaction (ORR) as well as for the electro-oxidation of formic acid. The surface reorganization predicted by theory enhances our understanding of the catalytic measurements. In the case of formic acid oxidation, the deformed nanoparticle structure leads to reduced CO binding energy and therefore improved oxidation activity. The final catalytic study we present is an instance of theory correctly predicting (in advance of the experiments) the structure of an effective DEN electrocatalyst. Specifically, DFT was used to determine the optimal composition of the alloy-core in AuPd@Pt DENs for the ORR. This prediction was subsequently confirmed experimentally. This study highlights the major theme of our research: the progression of using theory to rationalize experimental results to the more advanced goal of using theory to predict catalyst function a priori. We still have a long way to go before theory will be the principal means of catalyst discovery, but this Account begins to shed some light on the path that may lead in that direction.