Papers by Francis Zamborini
Analytical Chemistry, Jul 28, 2023
Journal of Physical Chemistry C, Mar 28, 2011
b S Supporting Information ' INTRODUCTION Some noble metal nanoparticles absorb light in the UVÀv... more b S Supporting Information ' INTRODUCTION Some noble metal nanoparticles absorb light in the UVÀvis region when the frequency of incident photons matches the collective oscillations of the conduction band electrons of the metal, which is known as localized surface plasmon resonance (LSPR). 1À5 The result is a strong absorption band(s) or increased scattering intensity at specific wavelengths for metals like Au and Ag when monitoring the optical properties in transmission mode or reflection/dark-field mode, respectively. It is well-known that the intensity and wavelength of maximum absorbance/scattering (λ max) depends on the size, shape, and composition of the metal nanoparticles. 1,2,4,6 It also depends on the refractive index of the environment surrounding the metal. 1,4,7 If the size, shape, and composition are constant for a given nanostructure throughout an experiment, then the LSPR peak intensity and λ max are sensitive to changes in the environment, which has been exploited for sensing applications. The optical properties of a metal nanostructure functionalized with a chemical receptor change if a molecule binds to the receptor and significantly alters the refractive index of the medium directly surrounding the metal nanostructure. LSPR spectroscopy has been exploited in this way for sensing a wide variety of analytes, including metal ions, 8,9 vapor molecules, 10 polymers, 11 and biomolecules. 1,12À24 The method is especially promising for biosensing applications because it is highly sensitive, simple, low cost, and label-free. 1,17,24 Reports of LSPR sensing of biological molecules, such as DNA 20,21,23 and proteins, 1,12À19,22,24 have increased tremendously over the past few years. Englebiene and co-workers first reported on the extinction changes of Au nanoparticles in solution upon antibody binding. 25 Other examples of solution-phase measurements include biopolymer adsorption kinetics, 26 ligandprotein interactions, 27 high throughput screening of proteins, 28 pH, 29 and ascorbic acid. 29 Recently, Yu and Irudayaraj used antibody-functionalized Au nanorods of varying aspect ratio for multiplex biosensing. 30 Most of the solution examples detect proteins in the μM to nM range. There are several studies on LSPR sensing with evaporated or chemically synthesized films of metal nanostructures. Chilkoti and co-workers synthesized and assembled Au nanoparticles 17,31 and Au nanorods 15 for sensing of streptavidin. Rubinstein and co-workers evaporated discontinuous Au films for sensing of avidin and antibodies specific to IgG and hCG antigens. 14,32
Langmuir, Oct 17, 2006
Here we describe the chemiresistive H 2-sensing properties of drop-cast films comprised of 3.0 nm... more Here we describe the chemiresistive H 2-sensing properties of drop-cast films comprised of 3.0 nm average diameter hexanethiolate-coated Pd monolayer-protected clusters (C6 Pd MPCs) bridging a pair of electrodes separated by a 23 µm gap. The gas-sensing properties were measured for 9.6-0.11% H 2 in a H 2 /N 2 mixture. The sensing mechanism is based on changes in the resistance of the film upon reaction of Pd with H 2 to form PdH x , which is known to be larger in volume and more resistive than pure Pd. As-prepared Pd MPC films are highly insensitive to H 2 , requiring O 3 and thermal treatment to enhance changes in film resistance in the presence of H 2. Exposure to O 3 for 15 min followed by activation in 100% H 2 leads to an increase in film conductivity in the presence of H 2 , with a detection limit of 0.11% H 2. When exposed to temperatures of 180-200°C, the conductivity of the film increases and a decrease in conductivity occurs in the presence of H 2 with a detection limit of 0.21%. The sensing behavior reverses after further heating to 260°C, exhibiting an increase in conductivity in the presence of H 2 as in O 3-treated films and a detection limit of 0.11%. The sensitivity of the variously treated films follows the order O 3 > high temp > low temp, and the response times at 1.0% H 2 range from 10 to 50s, depending on the treatment. FTIR spectroscopy, Raman spectroscopy, and atomic force microscopy provide information about the C6 monolayer, Pd metal, and film morphology, respectively, as a function of O 3 and heat treatment to aid in understanding the observed sensing behavior. This work demonstrates a simple chemical approach toward fabricating a fast, reversible sensor capable of detecting low concentrations of H 2 .
Langmuir, 1997
In-situ electrochemical scanning tunneling microscopy (ECSTM) was used to study the corrosion of ... more In-situ electrochemical scanning tunneling microscopy (ECSTM) was used to study the corrosion of naked and n-alkanethiol-modified Au(111) surfaces in basic CNsolutions. In these studies, the potential of a naked Au electrode is poised at a sufficiently negative value that no electrochemical etching takes place. Small positive potential excursions from the initial potential permit observation of the initial stages of corrosion. The data indicate that initial corrosion of naked Au(111) occurs only at high energy defect sites such as pits and step edges. At slightly higher overpotentials, pitting occurs in the middle of terraces while continuing at step edges. At even higher overpotentials, etching occurs rapidly and uniformly and the surface becomes very rough. Results from this experiment are compared to those obtained after coating the Au surface with a hexadecyl mercaptan self-assembled monolayer (SAM) to determine the extent to which the organomercaptan SAM inhibits Au corrosion. On the SAM-modified Au(111) surface the onset potential for a significant level of etching is shifted several hundred millivolts more positive than on the naked surface. Additionally, the rate of etching is significantly slower and the corrosion process is very different: etching initially occurs on terraces at defect sites within the monolayer instead of on step edges. We determined that potential, rather than time, is the primary factor that controls the rate of corrosion.
ChemElectroChem, Jul 15, 2016
Journal of Physical Chemistry C, Aug 23, 2019
Here we report that the peak oxidation potential (E p,ox) of electrode-attached Au nanospheres (N... more Here we report that the peak oxidation potential (E p,ox) of electrode-attached Au nanospheres (NSs) in anodic stripping voltammetry (ASV) experiments depends on the electrochemically measured surface area-to-volume ratio (SA/V). The SA/V in turn depends on the method of NS assembly onto the electrode, which results in different E p,ox values for the same-sized Au NSs that are assembled in different ways onto the same electrode material. The assembly methods tested on indium tin oxide-coated glass electrodes (glass/ITO) include electrostatic attachment to an amine-functionalized silane linker, electrophoretic deposition (EPD), direct drop-cast deposition, and drop-cast deposition after mixing with carbon black. The measured SA/V of same-sized NSs follows the order of silane linker > EPD > drop-cast with carbon black > direct drop-cast. The E p,ox decreases as the SA/V increases as controlled by the assembly method. The measured E p,ox for the Au NSs correlated to the measured SA/V better than the actual NS diameter. These findings reveal important information about what ultimately controls the oxidative stability of metal NSs and can help to explain previously described electrode effects on metal NS oxidation potentials. These results also provide guidelines for choosing an assembly method that optimizes the SA/V for performance and stability against oxidation or ripening (size increase).
Journal of Physical Chemistry C, Mar 11, 2019
Here, we describe the composition analysis of 5–7 nm diameter citrate-stabilized Cu/Au core/shell... more Here, we describe the composition analysis of 5–7 nm diameter citrate-stabilized Cu/Au core/shell nanoparticles with composition ratios of Cu1/Aux (x = 0.001–2) attached to amine-functionalized glass/indium-tin-oxide electrodes using anodic stripping voltammetry (ASV) in acidic KBr or KCl electrolyte solutions. One peak typically appears for Au oxidation, which is at more positive potential than the peaks for Cu. There are typically two peaks for Cu oxidation due to different bonding environments for Cu (Cu–Cu and Cu–Au bonds). The Au and Cu peak positions and size in the ASVs at different compositions are consistent with the formation of a Cu/Au core/shell structure. For low quantities of Au in the alloy (x < 1), the Au oxidation peak included the oxidation of trapped Cu in KBr solution, but there was no trapped Cu in the Au oxidation peak in KCl solution, making composition analysis straightforward in KCl by simply integrating the peaks for Cu and Au separately. Atomic rearrangement of Au due to strong ...
Journal of the American Chemical Society, Nov 3, 2020
Here, we describe the surprising reactivity between surface-attached (a) 0.9, 1.6, and 4.1 nm dia... more Here, we describe the surprising reactivity between surface-attached (a) 0.9, 1.6, and 4.1 nm diameter weakly stabilized Au nanoparticles (NPs) and aqueous 1.0 × 10 −4 M Ag + solution, and (b) 1.6 and 4.1 nm diameter weakly stabilized Au NPs and aqueous 1.0 × 10 −5 M PtCl 4 2− , which are considered to be antigalvanic replacement (AGR) reactions because they are not thermodynamically favorable for bulk-sized Au under these conditions. Anodic Stripping Voltammetry (ASV) and Scanning Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (STEM-EDS) mapping provide quantitation of the extent of Ag and Pt replacement as a function of Au NP diameter. The extent of the reaction increases as the Au NP size decreases. The percentage of Ag in the AuAg alloy following AGR based on ASV is 17.8 ± 0.6% for 4.1 nm diameter Au NPs, 87.2 ± 2.9% for 1.6 nm Au NPs, and an unprecedented full 100% Ag for 0.9 nm diameter Au NPs. STEM-EDS mapping shows very close agreement with the ASV-determined compositions. In the case of PtCl 4 2− , STEM-EDS mapping shows AuPt alloy NPs with 3.9 ± 1.3% and 41.1 ± 8.7% Pt following replacement with 4.1 and 1.6 nm diameter Au NPs, respectively, consistent with qualitative changes to the ASV. The size-dependent AGR correlates well with the negative shift in the standard potential (E 0) for Au oxidation with decreasing NP size.
Langmuir, Oct 24, 2019
Here, we describe the size-dependent, electrochemically controlled Ostwald ripening of 1.6, 4, an... more Here, we describe the size-dependent, electrochemically controlled Ostwald ripening of 1.6, 4, and 15 nm-diameter Au nanoparticles (NPs) attached to (3-aminopropyl)triethoxysilane (APTES)-modified glass/indiumtin-oxide electrodes. Holding the Au NP-coated electrodes at a constant negative potential of the dissolution potential in a bromide-containing electrolyte led to electrochemical Ostwald ripening of the different-sized Au NPs. The relative increase in the diameter of the NPs (D final /D initial) during electrochemical Ostwald ripening increases with decreasing NP size, increasing applied potential, increasing NP population size dispersity, and increasing NP coverage on the electrodes. Monitoring the average size of the Au NPs as a function of time at a controlled potential allows the measurement of the Ostwald ripening rate. Anodic stripping voltammetry and electrochemical determination of the surface area-to-volume ratio provide fast and convenient size analysis for many different samples and conditions, with consistent sizes from scanning electron microscopy images for some samples. It is important to better understand electrochemical Ostwald ripening, especially under potential control, since it is a major process that occurs during the synthesis of metal NPs and leads to detrimental size instability during electrochemical applications.
Journal of The Electrochemical Society, Dec 9, 2021
The electrophoretic deposition (EPD) of citrate-stabilized Au nanoparticles (cit-Au NPs) occurs o... more The electrophoretic deposition (EPD) of citrate-stabilized Au nanoparticles (cit-Au NPs) occurs on indium tin oxide (ITO)-coated glass electrodes upon electrochemical oxidation of hydroquinone (HQ) due to the release of hydronium ions. Anodic stripping voltammetry (ASV) for Au oxidation allows the determination of the amount of Au NP deposition under a specific EPD potential and time. The binding of Cr3+ to the cit-Au NPs inhibits the EPD by inducing aggregation and/or reducing the negative charge, which could lower the effective NP concentration of the cit-Au NPs and/or lower the electrophoretic mobility. This lowers the Au oxidation charge in the ASV, which acts as an indirect signal for Cr3+. The binding of melamine to cit-Au NPs similarly leads to aggregation and/or lowers the negative charge, also resulting in reduction of the ASV Au oxidation peak. The decrease in Au oxidation charge measured by ASV increases linearly with increasing Cr3+ and melamine concentration. The limit of detection (LOD) for Cr3+ is 21.1 ppb and 16.0 ppb for 15.1 and 4.1 nm diameter cit-Au NPs, respectively. Improving the sensing conditions allows for as low as 1 ppb detection of Cr3+. The LOD for melamine is 45.7 ppb for 4.1 nm Au NPs.
Current Opinion in Electrochemistry, Feb 1, 2019
Abstract Traditional anodic stripping voltammetry (ASV) involves electrodeposition (reduction) of... more Abstract Traditional anodic stripping voltammetry (ASV) involves electrodeposition (reduction) of metal ions from solution over some time scale onto a working electrode followed by stripping (oxidation) of the deposited metal in a second step, where the stripping potential and quantity of charge passed provide information about the metal identity and solution concentration, respectively. ASV has recently been extended to the analysis of metal nanoparticles (NPs), which have grown popular because of their fascinating properties tunable by size, shape, and composition. There is a need for improved methods of NP analysis, and because metal NPs can be oxidized to metal ions, ASV is a logical choice. Early studies involved metal NPs as tags for the detection of biomolecules. More recently, anodic stripping has been used to directly analyze the physical, chemical, and structural properties of metal NPs. This review highlights the stripping analysis of NP assemblies on macroelectrodes, individual NPs in solution during collisions with a microelectrode, and a single NP attached to an electrode. A surprising amount of information can be learned from this very simple, low-cost technique.
Journal of the American Chemical Society, Nov 11, 2008
Journal of the American Chemical Society, Oct 4, 2018
Here we report on the very low size stability of electrocatalytically-active 1.5 to 2.0 nm diamet... more Here we report on the very low size stability of electrocatalytically-active 1.5 to 2.0 nm diameter tetrakis(hydroxymethyl)phosphonium chloride-stabilized Au nanoparticles (THPC Au2nm NPs) chemically attached to glass/indium tin oxide electrodes. The potential for oxidative dissolution of THPC Au2nm NPs in the presence of bromide is about 250 mV negative of 4 nm diameter citrate-stabilized Au NPs (Cit Au4nm NPs) and 450 mV negative of bulk Au, which provides us with an easy method to assess the size stability using anodic stripping voltammetry (ASV). The THPC Au2nm NPs show a strong CO2 reduction wave at about-0.40 V (vs. RHE), which is non-existent for the Cit Au4nm NPs or bulk Au. The THPC Au2nm NPs are also comparatively more electroactive for the hydrogen evolution reaction (HER). In acid electrolyte, however, the potential for surface Au2O3 formation on THPC Au2nm NPs is significantly negative relative to bulk Au and a single cycle through the surface oxide and reduction waves leads to an increase in the NP size to about 4 nm. Similarly, the THPC Au2nm NPs undergo Ostwald ripening in the presence of bromide within 5 minutes at potentials well before oxidation, which increases
Analytical Chemistry, Jun 21, 2018
Here we report the electrochemical determination of the surface area-to-volume ratio (SA/V) of Au... more Here we report the electrochemical determination of the surface area-to-volume ratio (SA/V) of Au nanospheres (NSs) attached to electrode surfaces for size analysis. The SA is determined by electrochemically measuring the number of Coulombs of charge passed during the reduction of surface Au2O3 following Au NS oxidation in HClO4, while V is determined by electrochemically measuring the Coulombs of charge passed during the complete oxidative dissolution of all of the Au in the Au NSs in the presence of Brto form aqueous soluble AuBr4-. Assuming a spherical geometry and taking into account the total number of Au NSs on the electrode surface, the SA/V is theoretically equal to 3/radius. A plot of the electrochemically measured SA/V versus 1/radius for 5 different sized Au NSs is linear with a slope of 1.8 instead of the expected value of 3. Following attachment of the Au NSs to the electrode and ozone treatment, the plot of SA/V versus 1/radius is linear with a slope of 3.5 and the size based on electrochemistry matches very closely with those measured by scanning electron microscopy. We believe the ozone cleans the Au NS surface, allowing a more accurate measurement of SA.
Journal of the American Chemical Society, Nov 11, 2016
Here we describe size-dependent electrophoretic deposition (EPD) of citrate-stabilized Au nanopar... more Here we describe size-dependent electrophoretic deposition (EPD) of citrate-stabilized Au nanoparticles (NPs) onto indium-tin-oxide-coated glass (glass/ITO) electrodes as studied by linear sweep stripping voltammetry (LSSV) and scanning electron microscopy (SEM). LSSV allows both the determination of the Au NP coverage and NP size from the peak area and the peak potential, respectively. Two-electrode EPD in aqueous solutions of Au NPs plus H2O2 reveal that a minimum potential of 1.5 V is needed for significant deposition of 4 nm diameter Au NPs as opposed to 2.0 V for 33 nm diameter Au NPs. EPD at 0.4 V in a solution of Au NPs prepared with a short 5 min reaction time led to the successful capture of 1-2 nm diameter Au NPs with appreciable coverage. In all cases, deposition did not occur in the absence of H2O2. Three-electrode experiments with a real reference electrode revealed the same size selective deposition with potential and that the amount of Au deposited depends on the deposition time and H2O2 concentration. The deposition occurs indirectly by oxidation of H2O2, which liberates protons and neutralizes the citrate stabilizer, leading to precipitation of the Au NPs onto the glass/ITO electrode. Studies on pH stability show that larger Au NPs aggregate at lower pH compared to smaller Au NPs. More importantly, though, 4 nm diameter Au NPs are much more catalytic for H2O2 oxidation, which is the main reason for the size selective deposition.
Journal of the American Chemical Society, Sep 8, 2017
Here we describe the effect of aggregation on the oxidation of citrate-stabilized Au nanoparticle... more Here we describe the effect of aggregation on the oxidation of citrate-stabilized Au nanoparticles (NPs) attached electrostatically to amine-functionalized glass/ITO electrodes. When the Au NPs are attached to the electrode from a solution with pH greater than ~3.0, they are wellseparated on the electrode and oxidize in bromide-containing electrolyte at 0.698 V, 0.757 V, and 0.943 V (vs. Ag/AgCl) for 4 nm, 15 nm, and 50 nm diameter Au NPs, respectively, in line with their size-dependent oxidation behavior. In solutions below pH 3.0, the Au NPs aggregate in solution and attach to the electrode in the aggregated form. The solution UV-vis spectra and scanning electron microscopy (SEM) images of the electrodes show clear evidence of aggregation. The oxidation potential for aggregated 4 nm and 15 nm diameter Au NPs shifts positive by a maximum of 230 mV and 180 mV, respectively. The magnitude of the shift depends on the extent of aggregation, which was controlled by the solution pH and time. NP aggregation leads to a significant reduction in the surface area-to-volume ratio, which is likely responsible for the positive shift in the oxidation potential. The oxidation potential does not shift at all for aggregated 50 nm diameter Au NPs.
ACS Catalysis, Nov 2, 2012
Here, we describe the stability of solutions of various Pd and PdAg organic-protected nanoparticl... more Here, we describe the stability of solutions of various Pd and PdAg organic-protected nanoparticles (NPs) in the presence of H 2 and their selectivity and reactivity as catalysts for hydrogenation or isomerization of allyl alcohol. Pd and Pd 91 Ag 9 NPs stabilized with hexadecylamine (C16NH 2) ligands are stable against H 2-induced aggregation, whereas those stabilized with octylamines (C8NH 2) and dodecylamines (C12NH 2) precipitate within 1 h. The stability of C16NH 2 Pd NPs is comparable to that of hexanethiolate (C6S)-protected Pd NPs and mixed monolayer C6S/C8NH 2 (1/1) Pd NPs that were studied previously. The stability of C16NH 2 Pd NPs decreases as the alkylamine/Pd II ratio used in the synthesis decreases from 12:1 to 6:1 to 3:1. A bilayer or partial bilayer of C16NH 2 ligands forms around the Pd core for ratios greater than 6:1, which explains the higher stability of these NPs against aggregation. The various Pd and PdAg NPs catalyzed the hydrogenation and isomerization of allyl alcohol in the presence of H 2 with various selectivities and reactivities. C6S Pd NP catalysts are >95% selective toward the isomer; C8NH 2 /C6S Pd NPs are 60−75% selective toward the isomer, depending on the ligand ratio; and CnNH 2-coated Pd NPs generally produce a 1:1 or 3:2 ratio of the hydrogenation/isomerization products, with a few exceptions. The catalytic turnover frequency (TOF) is low for C6S Pd NPs becaue of the strong thiolate−Pd bond. The TOF increases with increasing chain length in the order C16NH 2 Pd > C12NH 2 Pd > C8NH 2 Pd and increases for Pd 91 Ag 9 alloys compared with pure Pd. The mixed ligand C8NH 2 /C6S Pd NPs exhibit TOFs similar to pure C8NH 2 Pd for low thiol content and similar to C6S Pd NPs for high thiol content. The 130/150 C8NH 2 /C6S Pd exhibits the optimal TOF for the mixed monolayer Pd NPs. C16NH 2 Pd 91 Ag 9 has the highest TOF of all the NPs studied due to the high stability afforded by the bilayer structure of the C16 chain and the high reactivity due to very little interference from the weak metal−amine interaction. Several of the Pd NPs that are stable in the presence of H 2 are not stable during the catalysis reaction (H 2 plus allyl alcohol), showing that the substrate also plays a role in NP stability.
Journal of the American Chemical Society, Apr 20, 2000
Journal of Physical Chemistry C, Feb 15, 2023
Langmuir, Jan 16, 2019
Here we describe the size-selective, hydroquinone (HQ)-mediated electrophoretic deposition (EPD) ... more Here we describe the size-selective, hydroquinone (HQ)-mediated electrophoretic deposition (EPD) of 4 nm and 15 nm diameter citrate-stabilized Au nanoparticles (NPs) onto a glass/ITO electrode. Protons liberated from HQ during electrochemical oxidation at the Au NP surface during collisions with the glass/ITO electrode leads to Au NP deposition through neutralization of the citrate stabilizer surrounding the Au NPs, protonation of the glass/ITO electrode, or some combination of the two. Interestingly, the 4 nm Au NPs deposit at about a 300-400 mV more negative potential than 15 nm diameter Au NPs because of faster HQ oxidation kinetics at the 4 nm NPs, leading to lower overpotentials. This allows for selective deposition of the 4 nm Au NPs over 15 nm Au NPs in a solution containing a mixture of the two by controlling the electrode potential. Controlled pH experiments indicate that significant NP deposition occurs on glass/ITO at a pH of ~3, giving insight into the local pH needed from HQ oxidation in order to deposit the Au NPs. Experiments performed at different ionic strength confirms that migration is a major mode of mass transport of the NPs to the glass/ITO. Long deposition times lead to films of
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Papers by Francis Zamborini