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Papers by Mark Anderson

Research paper thumbnail of Development and Student Use of Web-Based Prelabs in Analytical Chemistry Courses

This paper describes the development and student use of Web-based prelabs in freshman-level Quant... more This paper describes the development and student use of Web-based prelabs in freshman-level Quantitative Analysis and senior-level Instrumental Analysis. The main goal of these prelabs is to help students prepare to do experiments or use instrumentation before they enter the lab. Typical Quantitative Analysis prelabs include a Beer's law tutorial and a tutorial and demonstration of a Spectronic 20 spectrophotometer. Spectroscopy prelabs in Instrumental Analysis include tutorials on the principles and instrumentation of atomic absorption, atomic emission, UV-Vis molecular absorption, and molecular fluorescence spectroscopy. The prelabs contain some or all of the following: textual descriptions, instrument schematics, graphical instrument tours, multiple-choice questions, slide shows, and video demonstrations. The programming tools used to deliver the prelabs include clickable-image maps, JavaScript, PERL scripts, and RealVideo streaming video. Student and instructor perceptions indicate that the prelab assignments help the students prepare for their lab work and help teach the principles of the experiments or instrumentation. The overall effectiveness of the prelabs is hindered by the difficulty in getting students to take greater responsibility for their own lab preparation and outcomes. Using these prelabs has made evident the challenge of teaching students to use all available learning aids effectively: lectures, textbook, lab work, and multimedia.

Research paper thumbnail of The Effect of Ion-Pairing on the Open Circuit Potential of 3-Mercaptoproprionic Acid Modified Gold Electrodes

ECS Transactions, 2010

Open circuit potential (OCP) measurement is used to monitor the charge of a monolayer of 3-mercap... more Open circuit potential (OCP) measurement is used to monitor the charge of a monolayer of 3-mercaptopropionic acid. OCP is determined by the concentrations of the reduced and oxidized forms of a redox couple in the solution adjacent to the modified interface. OCP values were monitored in the presence of the one-electron redox couple Fe(CN)63-/4- as a function of solution pH. The OCP measured at basic pHs was different from the acidic OCP due to differences in electrostatic interactions between the interface and the redox species as described by the Nernst equation. The influence of ion-pairing with the monolayer is studied by varying the identity and concentration of the electrolyte cations in the solution.

Research paper thumbnail of Selective Formation of a Symmetric Sc3N@C78 Bisadduct:  Adduct Docking Controlled by an Internal Trimetallic Nitride Cluster

Journal of the American Chemical Society, 2008

Research paper thumbnail of Electrochemical Glucose Sensors—Developments Using Electrostatic Assembly and Carbon Nanotubes for Biosensor Construction

Sensors, 2010

In 1962, Clark and Lyons proposed incorporating the enzyme glucose oxidase in the construction of... more In 1962, Clark and Lyons proposed incorporating the enzyme glucose oxidase in the construction of an electrochemical sensor for glucose in blood plasma. In their application, Clark and Lyons describe an electrode in which a membrane permeable to glucose traps a small volume of solution containing the enzyme adjacent to a pH electrode, and the presence of glucose is detected by the change in the electrode potential that occurs when glucose reacts with the enzyme in this volume of solution. Although described nearly 50 years ago, this seminal development provides the general structure for constructing electrochemical glucose sensors that is still used today. Despite the maturity of the field, new developments that explore solutions to the fundamental limitations of electrochemical glucose sensors continue to emerge. Here we discuss two developments of the last 15 years; confining the enzyme and a redox mediator to a very thin molecular films at electrode surfaces by electrostatic assembly, and the use of electrodes modified by carbon nanotubes (CNTs) to leverage the electrocatalytic effect of the CNTs to reduce the oxidation overpotential of the electrode reaction or for the direct electron transport to the enzyme.

Research paper thumbnail of Electrochemical Impedance Spectroscopy to Evalutate the Effects of pH Variations and Ionic Self-Assembly on Undecanoic Acid Monolayers Immobilized on Gold

Research paper thumbnail of Characterization of Carboxylic Acid-Terminated Self-Assembled Monolayers by Electrochemical Impedance Spectroscopy and Scanning Electrochemical Microscopy

Langmuir, May 22, 2008

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are u... more Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.

Research paper thumbnail of Electrostatic deposition of polycations and polyanions onto cysteine monolayers

Journal of Colloid and Interface Science, Mar 1, 2009

Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor th... more Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor the deposition of polyelectrolytes onto an interface modified with a monolayer of cysteine. When using an anionic redox probe, Fe(CN) −4 6 , the impedance of the cysteine modified electrode is a function of the solution pH; the charge-transfer resistance being higher at basic pH, and smaller under acidic conditions relative to the value found when the solution is buffered near the pI for cysteine. This behavior is consistent with the electrostatic interaction between the anionic redox probe and the net charge of the interface. After exposure of the cysteine-modified interface to 0.10 mol L −1 HCl solutions containing polystyrene sulfonate, the impedance of the modified interface increases. This behavior is consistent with the interface having a net negative charge, and suggests that the polyanion is confined to the interface. Quartz crystal microbalance measurements also show that 700 ± 20 ng of the PSS is deposited under the acidic conditions. After exposure to 0.10 mol L −1 NaOH solutions containing poly(diallyldimethyl)ammonium chloride, the impedance of the modified interface decreases, consistent with the electrostatic assembly of the polycation to the cysteine monolayer under these conditions. Quartz crystal microbalance measurements indicate that 770 ± 40 ng of the poly(diallyldimethylammonium chloride) are deposited under the basic conditions.

Research paper thumbnail of Effects of Potential on Ionic Self-Assembly of Carboxylic Acid Terminated Monolayers

Research paper thumbnail of Potential driven deposition of polyelectrolytes onto the surface of cysteine monolayers assembled on gold

Journal of Colloid and Interface Science, Feb 1, 2010

Electrochemical impedance spectroscopy and the quartz crystal microbalance measurements are used ... more Electrochemical impedance spectroscopy and the quartz crystal microbalance measurements are used to examine the ability of potential applied to a substrate to create, in situ, conditions favorable for the electrostatic deposition of polyelectrolytes onto a gold substrate modified by the self-assembly of cysteine. Cysteine is a zwitterionic compound that, when confined to a substrate, has the ability to establish either a net positive or a net negative interfacial charge, depending on the conditions. As such, cysteine modified interfaces could possibly be used as a versatile substrate for deposition of either cationic or anionic polyelectrolytes. The potential of zero charge of a gold electrode modified by self-assembly with cysteine in the presence of 0.10molL(-1) KCl and buffered at pH 5 is found by differential capacitance measurement to be -0.12(+/-0.02)V vs. Ag/AgCl. When -0.05V vs. Ag/AgCl is applied to the substrate (a potential positive of the PZC) in the presence of different polyelectrolytes, both impedance spectroscopy and quartz crystal microbalance data suggest the accumulation of anionic poly(sodium styrenesulfonate) along the cysteine modified interface. Conversely, when -0.40V vs. Ag/AgCl is applied to the substrate (a potential negative of the PZC), experimental results suggest the accumulation of cationic poly(diallydimethylammonium chloride).

Research paper thumbnail of Potential Driven Deposition of Poly(diallyldimethylammonium chloride) onto the Surface of 3-Mercaptopropionic Acid Monolayers Assembled on Gold

Langmuir, Oct 24, 2008

Electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM) measurements a... more Electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM) measurements are used to examine the ability of applied potential to drive the ionic self-assembly of poly(diallyldimethylammonium) chloride (PDDA) onto a substrate modified with a monolayer of 3-mercaptopropionic acid (3-MPA). The potential of zero charge (PZC) of the gold electrode modified with a monolayer of 3-MPA was found by differential capacitance measurements to be-0.12 ((0.01) V versus Ag-AgCl. Changing the substrate potential to values positive (-0.01 V vs Ag-AgCl) of the PZC induces interfacial conditions that are favorable for the electrostatic deposition of cationic polymers onto the surface of 3-MPA monolayers. This result is also consistent with experimental observations obtained when the 3-MPA-modified substrate is exposed to 0.10 mol L-1 NaOH solutions. When potentials equal or negative to the PZC are applied to the substrate, no significant accumulation of the PDDA is found by either QCM or EIS measurement. This result is consistent with results obtained when the 3-MPA modified substrate is exposed to 0.10 mol L-1 HCl solutions where no PDDA adsorption is expected because the monolayer is neutral under these conditions. Changes in the impedance and quartz crystal frequency obtained after potential is applied to the substrate are interpreted in terms of the applied potential creating interfacial conditions that are favorable for the deprotonation of the terminal carboxylic acid groups and the subsequent electrostatic assembly of the polycation onto the negatively charged monolayer.

Research paper thumbnail of Application of Square Wave Voltammetry for Electrochemical Detection in Gradient Elution HPLC

Journal of Liquid Chromatography, 1993

The utility of square-wave voltammetry with a conventionally sized indicator electrode for use in... more The utility of square-wave voltammetry with a conventionally sized indicator electrode for use in the detection of gradient elution HPLC applications is demonstrated. A redesigned electrochemical detector cell, based upon a type of wall-jet design, provides more efficient potential control and allows the voltammetric detection. The minimum detectable quantity and linear dynamic range for the new detector design were found to be 13pg and 5 orders of magnitude, respectively, comparable to other electrochemical detectors. In the separation of a test mixture containing hydroquinone. bromohydroquinone, catechol, resorcinol, and phenylhydroquinone baseline resolution of all five components is obtained in 12 minutes using a nonlinear solvent gradient. Importantly, no change in the chromatographic baseline is observed during the course of the separation. Gradient separations of the extracts of fennel seeds and mainstream cigarette smoke are also demonstrated with no apparent change in the baseline.

Research paper thumbnail of Electrochemical reduction of alkyl halides at vitreous carbon cathodes in dimethylformamide

Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1986

At vitreous carbon electrodes in dimethylformamide which contains tetramethylammonium perchlorate... more At vitreous carbon electrodes in dimethylformamide which contains tetramethylammonium perchlorate, cyclic voltammograms for 1-iododecane, 1-bromodecane, and 2-bromooctane consist of one irreversible reduction wave signaling formation of the alkyl carbanion, whereas ...

Research paper thumbnail of Application of polarization modulation Fourier transform infrared spectroscopy for the in situ study of spontaneously adsorbed monolayers

Vibrational Spectroscopy, 1993

... of polarization modulation Fourier transform infrared spectroscopy for the in situ study of s... more ... of polarization modulation Fourier transform infrared spectroscopy for the in situ study of spontaneously adsorbed monolayers Marilyn Gatin and ... REFERENCES 1 JD Swalen, DL Allara, JD Andrade, EA Chandross, S. Garoff, J. Israelachvili, TJ McCarthy, R. Murray, RF Pease, JF ...

Research paper thumbnail of Characterization of Carboxylic Acid-Terminated Self-Assembled Monolayers by Electrochemical Impedance Spectroscopy and Scanning Electrochemical Microscopy

Langmuir, 2008

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are u... more Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.

Research paper thumbnail of The Voltammetry of Sc3@C82

Journal of the American Chemical Society, 1997

This paper describes the first electrochemical data for a polymetallic endohedral metallofulleren... more This paper describes the first electrochemical data for a polymetallic endohedral metallofullerene. The voltammetric behavior of Scâ@Cââ resembles that for La@Cââ and Y@Cââ, indicating that the identity of the metal does not influence dramatically the energies of the metallofullerene molecular orbitals. The three Sc atoms, however, transfer more electrons from the encapsulated metals to the cage than the monometallic counterparts, accounting for the differences in the voltammetry for Scâ@Cââ. The similarity of the voltammetry of Scâ@Cââ with that of the monometallic fullerenes suggests that Scâ{sup +4}@Cââ⁴⁻ may describe the formal charges on the Sc trimer and the Cââ cage of these species. Metallofullerene electronic structure and physical properties should be better understood as more metallofullerenes become available for investigation. 31 refs., 1 fig., 1 tab.

Research paper thumbnail of Structure and Enhanced Reactivity Rates of the D5h Sc3N@C80 and Lu3N@C80 Metallofullerene Isomers:  The Importance of the Pyracylene Motif

Journal of the American Chemical Society, 2006

In this paper we report enhanced reactivity of the D5h isomers in comparison with the more common... more In this paper we report enhanced reactivity of the D5h isomers in comparison with the more common Ih isomers of Sc3N@C80 and Lu3N@C80 toward Diels-Alder and 1,3-dipolar tritylazomethine ylide cycloaddition reactions. Also, the structure of the D5h isomer of Sc3N@C80 has been determined through single-crystal X-ray diffraction on D5h-Sc3N@C80‚Ni(OEP)‚2benzene (OEP) octaethylporphyrin). The Sc3N portion of D5h-Sc3N@C80 is strictly planar, but the plane of these four atoms is tipped out of the noncrystallographic, horizontal mirror plane of the fullerene by 30°. The combination of short bond length and high degree of pyramidization for the central carbon atoms of the pyracylene sites situated along a belt that is perpendicular to the C5 axis suggests that these are the sites of greatest reactivity in the D5h isomer of Sc3N@C80. Consistent with the observation of higher reactivity observed for the D5h isomers, cyclic voltammetry and molecular orbital (MO) calculations demonstrate that the D5h isomers have slightly smaller energy gaps than those of the Ih isomers. The first mono-and bis-adducts of D5h Sc3N@C80 have been synthesized via 1,3-dipolar cycloaddition of tritylazomethine ylide. The NMR spectrum for the monoadduct 2b is consistent with reaction at the 6,6-ring juncture in the pyracylene unit of the D5h Sc3N@C80 cage and is the thermodynamically stable isomer. On the other hand, monoadduct 2a undergoes thermal conversion to other isomeric monoadducts, and three possible structures are proposed.

Research paper thumbnail of Electrochemistry of C82: relationship to metallofullerene electrochemistry

Journal of Electroanalytical Chemistry, 1996

Square-wave voltammetry of C,, in pyridine solutions containing O.lOM tetra-n-butyl ammonium perc... more Square-wave voltammetry of C,, in pyridine solutions containing O.lOM tetra-n-butyl ammonium perchlorate indicates that C,, is reduced in four one-electron steps prior to the onset of electrolyte decomposition. The first reduction occurs at potentials positive of the reported first reduction potentials of other empty cage fullerenes, and at approximately the same reported potential for the first reduction of the endohedral metallofullerenes La@C,, and Y&W,,. No oxidations were observed for the C,, under these conditions.

Research paper thumbnail of Electrostatic deposition of polycations and polyanions onto cysteine monolayers

Journal of Colloid and Interface Science, 2009

Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor th... more Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor the deposition of polyelectrolytes onto an interface modified with a monolayer of cysteine. When using an anionic redox probe, Fe(CN) −4 6 , the impedance of the cysteine modified electrode is a function of the solution pH; the charge-transfer resistance being higher at basic pH, and smaller under acidic conditions relative to the value found when the solution is buffered near the pI for cysteine. This behavior is consistent with the electrostatic interaction between the anionic redox probe and the net charge of the interface. After exposure of the cysteine-modified interface to 0.10 mol L −1 HCl solutions containing polystyrene sulfonate, the impedance of the modified interface increases. This behavior is consistent with the interface having a net negative charge, and suggests that the polyanion is confined to the interface. Quartz crystal microbalance measurements also show that 700 ± 20 ng of the PSS is deposited under the acidic conditions. After exposure to 0.10 mol L −1 NaOH solutions containing poly(diallyldimethyl)ammonium chloride, the impedance of the modified interface decreases, consistent with the electrostatic assembly of the polycation to the cysteine monolayer under these conditions. Quartz crystal microbalance measurements indicate that 770 ± 40 ng of the poly(diallyldimethylammonium chloride) are deposited under the basic conditions.

Research paper thumbnail of Electrostatic Assembly of a Redox Catalysis System for Detection of Glutamate

Electroanalysis, 2006

... Bioelectron. 2003, 18, 689. [28] NV Kulagina, L. Shankar, AC Michael, Anal. Chem. 1999, 71, 5... more ... Bioelectron. 2003, 18, 689. [28] NV Kulagina, L. Shankar, AC Michael, Anal. Chem. 1999, 71, 5093. ... Bioelectron. 2001, 16, 33. [38] PU Abel, T. von Woedtke, Biosens. Bioelectron. 2002, 17, 1059. [39] MR Anderson, R. Baltzersen, J. Colloid Interface Sci. 2003, 263, 516. ...

Research paper thumbnail of Modification of a Capillary for Electrophoresis by Electrostatic Self-Assembly of an Enzyme for Selective Determination of the Enzyme Substrate

Electroanalysis, 2007

ABSTRACT The integration of a separation capillary for capillary electrophoresis (CE) with an on-... more ABSTRACT The integration of a separation capillary for capillary electrophoresis (CE) with an on-column enzyme reaction for selective determination of the enzyme substrate is described. Enzyme immobilization is achieved by electrostatic assembly of poly(diallydimethylammonium chloride) (PDDA) followed by adsorption of a mixture of the negatively charged enzyme glucose oxidase (GOx) and anionic poly(styrenesulfonate) (PSS). The reaction of glucose with the GOx produces hydrogen peroxide which migrates the length of the capillary and is detected amperometrically at the capillary outlet. The enzyme reaction occurs during a capillary separation, allowing selective determination of the substrate in complex samples without the need for pre- or post-separation chemical modification of the analyte. The enzyme reactor is found to have an optimal response to glucose when a 5 : 1 mixture of PSS:GOx is used. Under these conditions the limit of detection for glucose is found to be between 5.0×10−4 and 1.3×10−3 M, dependent upon the inner-diameter of the capillary. The apparent Michaelis-Menten constant for the enzyme reaction was determined to be 0.047 (±0.001) M and 0.0037 (±0.0007) M for a 50 and 10 μm inner-diameter capillaries, respectively. These results indicate that the enzyme reaction is efficient, having enzyme kinetics similar to that of a reaction occurring in solution. This enzyme immobilization method was also applied to another enzyme, glutamate oxidase, yielding similar results.

Research paper thumbnail of Development and Student Use of Web-Based Prelabs in Analytical Chemistry Courses

This paper describes the development and student use of Web-based prelabs in freshman-level Quant... more This paper describes the development and student use of Web-based prelabs in freshman-level Quantitative Analysis and senior-level Instrumental Analysis. The main goal of these prelabs is to help students prepare to do experiments or use instrumentation before they enter the lab. Typical Quantitative Analysis prelabs include a Beer's law tutorial and a tutorial and demonstration of a Spectronic 20 spectrophotometer. Spectroscopy prelabs in Instrumental Analysis include tutorials on the principles and instrumentation of atomic absorption, atomic emission, UV-Vis molecular absorption, and molecular fluorescence spectroscopy. The prelabs contain some or all of the following: textual descriptions, instrument schematics, graphical instrument tours, multiple-choice questions, slide shows, and video demonstrations. The programming tools used to deliver the prelabs include clickable-image maps, JavaScript, PERL scripts, and RealVideo streaming video. Student and instructor perceptions indicate that the prelab assignments help the students prepare for their lab work and help teach the principles of the experiments or instrumentation. The overall effectiveness of the prelabs is hindered by the difficulty in getting students to take greater responsibility for their own lab preparation and outcomes. Using these prelabs has made evident the challenge of teaching students to use all available learning aids effectively: lectures, textbook, lab work, and multimedia.

Research paper thumbnail of The Effect of Ion-Pairing on the Open Circuit Potential of 3-Mercaptoproprionic Acid Modified Gold Electrodes

ECS Transactions, 2010

Open circuit potential (OCP) measurement is used to monitor the charge of a monolayer of 3-mercap... more Open circuit potential (OCP) measurement is used to monitor the charge of a monolayer of 3-mercaptopropionic acid. OCP is determined by the concentrations of the reduced and oxidized forms of a redox couple in the solution adjacent to the modified interface. OCP values were monitored in the presence of the one-electron redox couple Fe(CN)63-/4- as a function of solution pH. The OCP measured at basic pHs was different from the acidic OCP due to differences in electrostatic interactions between the interface and the redox species as described by the Nernst equation. The influence of ion-pairing with the monolayer is studied by varying the identity and concentration of the electrolyte cations in the solution.

Research paper thumbnail of Selective Formation of a Symmetric Sc3N@C78 Bisadduct:  Adduct Docking Controlled by an Internal Trimetallic Nitride Cluster

Journal of the American Chemical Society, 2008

Research paper thumbnail of Electrochemical Glucose Sensors—Developments Using Electrostatic Assembly and Carbon Nanotubes for Biosensor Construction

Sensors, 2010

In 1962, Clark and Lyons proposed incorporating the enzyme glucose oxidase in the construction of... more In 1962, Clark and Lyons proposed incorporating the enzyme glucose oxidase in the construction of an electrochemical sensor for glucose in blood plasma. In their application, Clark and Lyons describe an electrode in which a membrane permeable to glucose traps a small volume of solution containing the enzyme adjacent to a pH electrode, and the presence of glucose is detected by the change in the electrode potential that occurs when glucose reacts with the enzyme in this volume of solution. Although described nearly 50 years ago, this seminal development provides the general structure for constructing electrochemical glucose sensors that is still used today. Despite the maturity of the field, new developments that explore solutions to the fundamental limitations of electrochemical glucose sensors continue to emerge. Here we discuss two developments of the last 15 years; confining the enzyme and a redox mediator to a very thin molecular films at electrode surfaces by electrostatic assembly, and the use of electrodes modified by carbon nanotubes (CNTs) to leverage the electrocatalytic effect of the CNTs to reduce the oxidation overpotential of the electrode reaction or for the direct electron transport to the enzyme.

Research paper thumbnail of Electrochemical Impedance Spectroscopy to Evalutate the Effects of pH Variations and Ionic Self-Assembly on Undecanoic Acid Monolayers Immobilized on Gold

Research paper thumbnail of Characterization of Carboxylic Acid-Terminated Self-Assembled Monolayers by Electrochemical Impedance Spectroscopy and Scanning Electrochemical Microscopy

Langmuir, May 22, 2008

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are u... more Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.

Research paper thumbnail of Electrostatic deposition of polycations and polyanions onto cysteine monolayers

Journal of Colloid and Interface Science, Mar 1, 2009

Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor th... more Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor the deposition of polyelectrolytes onto an interface modified with a monolayer of cysteine. When using an anionic redox probe, Fe(CN) −4 6 , the impedance of the cysteine modified electrode is a function of the solution pH; the charge-transfer resistance being higher at basic pH, and smaller under acidic conditions relative to the value found when the solution is buffered near the pI for cysteine. This behavior is consistent with the electrostatic interaction between the anionic redox probe and the net charge of the interface. After exposure of the cysteine-modified interface to 0.10 mol L −1 HCl solutions containing polystyrene sulfonate, the impedance of the modified interface increases. This behavior is consistent with the interface having a net negative charge, and suggests that the polyanion is confined to the interface. Quartz crystal microbalance measurements also show that 700 ± 20 ng of the PSS is deposited under the acidic conditions. After exposure to 0.10 mol L −1 NaOH solutions containing poly(diallyldimethyl)ammonium chloride, the impedance of the modified interface decreases, consistent with the electrostatic assembly of the polycation to the cysteine monolayer under these conditions. Quartz crystal microbalance measurements indicate that 770 ± 40 ng of the poly(diallyldimethylammonium chloride) are deposited under the basic conditions.

Research paper thumbnail of Effects of Potential on Ionic Self-Assembly of Carboxylic Acid Terminated Monolayers

Research paper thumbnail of Potential driven deposition of polyelectrolytes onto the surface of cysteine monolayers assembled on gold

Journal of Colloid and Interface Science, Feb 1, 2010

Electrochemical impedance spectroscopy and the quartz crystal microbalance measurements are used ... more Electrochemical impedance spectroscopy and the quartz crystal microbalance measurements are used to examine the ability of potential applied to a substrate to create, in situ, conditions favorable for the electrostatic deposition of polyelectrolytes onto a gold substrate modified by the self-assembly of cysteine. Cysteine is a zwitterionic compound that, when confined to a substrate, has the ability to establish either a net positive or a net negative interfacial charge, depending on the conditions. As such, cysteine modified interfaces could possibly be used as a versatile substrate for deposition of either cationic or anionic polyelectrolytes. The potential of zero charge of a gold electrode modified by self-assembly with cysteine in the presence of 0.10molL(-1) KCl and buffered at pH 5 is found by differential capacitance measurement to be -0.12(+/-0.02)V vs. Ag/AgCl. When -0.05V vs. Ag/AgCl is applied to the substrate (a potential positive of the PZC) in the presence of different polyelectrolytes, both impedance spectroscopy and quartz crystal microbalance data suggest the accumulation of anionic poly(sodium styrenesulfonate) along the cysteine modified interface. Conversely, when -0.40V vs. Ag/AgCl is applied to the substrate (a potential negative of the PZC), experimental results suggest the accumulation of cationic poly(diallydimethylammonium chloride).

Research paper thumbnail of Potential Driven Deposition of Poly(diallyldimethylammonium chloride) onto the Surface of 3-Mercaptopropionic Acid Monolayers Assembled on Gold

Langmuir, Oct 24, 2008

Electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM) measurements a... more Electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM) measurements are used to examine the ability of applied potential to drive the ionic self-assembly of poly(diallyldimethylammonium) chloride (PDDA) onto a substrate modified with a monolayer of 3-mercaptopropionic acid (3-MPA). The potential of zero charge (PZC) of the gold electrode modified with a monolayer of 3-MPA was found by differential capacitance measurements to be-0.12 ((0.01) V versus Ag-AgCl. Changing the substrate potential to values positive (-0.01 V vs Ag-AgCl) of the PZC induces interfacial conditions that are favorable for the electrostatic deposition of cationic polymers onto the surface of 3-MPA monolayers. This result is also consistent with experimental observations obtained when the 3-MPA-modified substrate is exposed to 0.10 mol L-1 NaOH solutions. When potentials equal or negative to the PZC are applied to the substrate, no significant accumulation of the PDDA is found by either QCM or EIS measurement. This result is consistent with results obtained when the 3-MPA modified substrate is exposed to 0.10 mol L-1 HCl solutions where no PDDA adsorption is expected because the monolayer is neutral under these conditions. Changes in the impedance and quartz crystal frequency obtained after potential is applied to the substrate are interpreted in terms of the applied potential creating interfacial conditions that are favorable for the deprotonation of the terminal carboxylic acid groups and the subsequent electrostatic assembly of the polycation onto the negatively charged monolayer.

Research paper thumbnail of Application of Square Wave Voltammetry for Electrochemical Detection in Gradient Elution HPLC

Journal of Liquid Chromatography, 1993

The utility of square-wave voltammetry with a conventionally sized indicator electrode for use in... more The utility of square-wave voltammetry with a conventionally sized indicator electrode for use in the detection of gradient elution HPLC applications is demonstrated. A redesigned electrochemical detector cell, based upon a type of wall-jet design, provides more efficient potential control and allows the voltammetric detection. The minimum detectable quantity and linear dynamic range for the new detector design were found to be 13pg and 5 orders of magnitude, respectively, comparable to other electrochemical detectors. In the separation of a test mixture containing hydroquinone. bromohydroquinone, catechol, resorcinol, and phenylhydroquinone baseline resolution of all five components is obtained in 12 minutes using a nonlinear solvent gradient. Importantly, no change in the chromatographic baseline is observed during the course of the separation. Gradient separations of the extracts of fennel seeds and mainstream cigarette smoke are also demonstrated with no apparent change in the baseline.

Research paper thumbnail of Electrochemical reduction of alkyl halides at vitreous carbon cathodes in dimethylformamide

Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1986

At vitreous carbon electrodes in dimethylformamide which contains tetramethylammonium perchlorate... more At vitreous carbon electrodes in dimethylformamide which contains tetramethylammonium perchlorate, cyclic voltammograms for 1-iododecane, 1-bromodecane, and 2-bromooctane consist of one irreversible reduction wave signaling formation of the alkyl carbanion, whereas ...

Research paper thumbnail of Application of polarization modulation Fourier transform infrared spectroscopy for the in situ study of spontaneously adsorbed monolayers

Vibrational Spectroscopy, 1993

... of polarization modulation Fourier transform infrared spectroscopy for the in situ study of s... more ... of polarization modulation Fourier transform infrared spectroscopy for the in situ study of spontaneously adsorbed monolayers Marilyn Gatin and ... REFERENCES 1 JD Swalen, DL Allara, JD Andrade, EA Chandross, S. Garoff, J. Israelachvili, TJ McCarthy, R. Murray, RF Pease, JF ...

Research paper thumbnail of Characterization of Carboxylic Acid-Terminated Self-Assembled Monolayers by Electrochemical Impedance Spectroscopy and Scanning Electrochemical Microscopy

Langmuir, 2008

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are u... more Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.

Research paper thumbnail of The Voltammetry of Sc3@C82

Journal of the American Chemical Society, 1997

This paper describes the first electrochemical data for a polymetallic endohedral metallofulleren... more This paper describes the first electrochemical data for a polymetallic endohedral metallofullerene. The voltammetric behavior of Scâ@Cââ resembles that for La@Cââ and Y@Cââ, indicating that the identity of the metal does not influence dramatically the energies of the metallofullerene molecular orbitals. The three Sc atoms, however, transfer more electrons from the encapsulated metals to the cage than the monometallic counterparts, accounting for the differences in the voltammetry for Scâ@Cââ. The similarity of the voltammetry of Scâ@Cââ with that of the monometallic fullerenes suggests that Scâ{sup +4}@Cââ⁴⁻ may describe the formal charges on the Sc trimer and the Cââ cage of these species. Metallofullerene electronic structure and physical properties should be better understood as more metallofullerenes become available for investigation. 31 refs., 1 fig., 1 tab.

Research paper thumbnail of Structure and Enhanced Reactivity Rates of the D5h Sc3N@C80 and Lu3N@C80 Metallofullerene Isomers:  The Importance of the Pyracylene Motif

Journal of the American Chemical Society, 2006

In this paper we report enhanced reactivity of the D5h isomers in comparison with the more common... more In this paper we report enhanced reactivity of the D5h isomers in comparison with the more common Ih isomers of Sc3N@C80 and Lu3N@C80 toward Diels-Alder and 1,3-dipolar tritylazomethine ylide cycloaddition reactions. Also, the structure of the D5h isomer of Sc3N@C80 has been determined through single-crystal X-ray diffraction on D5h-Sc3N@C80‚Ni(OEP)‚2benzene (OEP) octaethylporphyrin). The Sc3N portion of D5h-Sc3N@C80 is strictly planar, but the plane of these four atoms is tipped out of the noncrystallographic, horizontal mirror plane of the fullerene by 30°. The combination of short bond length and high degree of pyramidization for the central carbon atoms of the pyracylene sites situated along a belt that is perpendicular to the C5 axis suggests that these are the sites of greatest reactivity in the D5h isomer of Sc3N@C80. Consistent with the observation of higher reactivity observed for the D5h isomers, cyclic voltammetry and molecular orbital (MO) calculations demonstrate that the D5h isomers have slightly smaller energy gaps than those of the Ih isomers. The first mono-and bis-adducts of D5h Sc3N@C80 have been synthesized via 1,3-dipolar cycloaddition of tritylazomethine ylide. The NMR spectrum for the monoadduct 2b is consistent with reaction at the 6,6-ring juncture in the pyracylene unit of the D5h Sc3N@C80 cage and is the thermodynamically stable isomer. On the other hand, monoadduct 2a undergoes thermal conversion to other isomeric monoadducts, and three possible structures are proposed.

Research paper thumbnail of Electrochemistry of C82: relationship to metallofullerene electrochemistry

Journal of Electroanalytical Chemistry, 1996

Square-wave voltammetry of C,, in pyridine solutions containing O.lOM tetra-n-butyl ammonium perc... more Square-wave voltammetry of C,, in pyridine solutions containing O.lOM tetra-n-butyl ammonium perchlorate indicates that C,, is reduced in four one-electron steps prior to the onset of electrolyte decomposition. The first reduction occurs at potentials positive of the reported first reduction potentials of other empty cage fullerenes, and at approximately the same reported potential for the first reduction of the endohedral metallofullerenes La@C,, and Y&W,,. No oxidations were observed for the C,, under these conditions.

Research paper thumbnail of Electrostatic deposition of polycations and polyanions onto cysteine monolayers

Journal of Colloid and Interface Science, 2009

Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor th... more Electrochemical impedance spectroscopy and the quartz crystal microbalance are used to monitor the deposition of polyelectrolytes onto an interface modified with a monolayer of cysteine. When using an anionic redox probe, Fe(CN) −4 6 , the impedance of the cysteine modified electrode is a function of the solution pH; the charge-transfer resistance being higher at basic pH, and smaller under acidic conditions relative to the value found when the solution is buffered near the pI for cysteine. This behavior is consistent with the electrostatic interaction between the anionic redox probe and the net charge of the interface. After exposure of the cysteine-modified interface to 0.10 mol L −1 HCl solutions containing polystyrene sulfonate, the impedance of the modified interface increases. This behavior is consistent with the interface having a net negative charge, and suggests that the polyanion is confined to the interface. Quartz crystal microbalance measurements also show that 700 ± 20 ng of the PSS is deposited under the acidic conditions. After exposure to 0.10 mol L −1 NaOH solutions containing poly(diallyldimethyl)ammonium chloride, the impedance of the modified interface decreases, consistent with the electrostatic assembly of the polycation to the cysteine monolayer under these conditions. Quartz crystal microbalance measurements indicate that 770 ± 40 ng of the poly(diallyldimethylammonium chloride) are deposited under the basic conditions.

Research paper thumbnail of Electrostatic Assembly of a Redox Catalysis System for Detection of Glutamate

Electroanalysis, 2006

... Bioelectron. 2003, 18, 689. [28] NV Kulagina, L. Shankar, AC Michael, Anal. Chem. 1999, 71, 5... more ... Bioelectron. 2003, 18, 689. [28] NV Kulagina, L. Shankar, AC Michael, Anal. Chem. 1999, 71, 5093. ... Bioelectron. 2001, 16, 33. [38] PU Abel, T. von Woedtke, Biosens. Bioelectron. 2002, 17, 1059. [39] MR Anderson, R. Baltzersen, J. Colloid Interface Sci. 2003, 263, 516. ...

Research paper thumbnail of Modification of a Capillary for Electrophoresis by Electrostatic Self-Assembly of an Enzyme for Selective Determination of the Enzyme Substrate

Electroanalysis, 2007

ABSTRACT The integration of a separation capillary for capillary electrophoresis (CE) with an on-... more ABSTRACT The integration of a separation capillary for capillary electrophoresis (CE) with an on-column enzyme reaction for selective determination of the enzyme substrate is described. Enzyme immobilization is achieved by electrostatic assembly of poly(diallydimethylammonium chloride) (PDDA) followed by adsorption of a mixture of the negatively charged enzyme glucose oxidase (GOx) and anionic poly(styrenesulfonate) (PSS). The reaction of glucose with the GOx produces hydrogen peroxide which migrates the length of the capillary and is detected amperometrically at the capillary outlet. The enzyme reaction occurs during a capillary separation, allowing selective determination of the substrate in complex samples without the need for pre- or post-separation chemical modification of the analyte. The enzyme reactor is found to have an optimal response to glucose when a 5 : 1 mixture of PSS:GOx is used. Under these conditions the limit of detection for glucose is found to be between 5.0×10−4 and 1.3×10−3 M, dependent upon the inner-diameter of the capillary. The apparent Michaelis-Menten constant for the enzyme reaction was determined to be 0.047 (±0.001) M and 0.0037 (±0.0007) M for a 50 and 10 μm inner-diameter capillaries, respectively. These results indicate that the enzyme reaction is efficient, having enzyme kinetics similar to that of a reaction occurring in solution. This enzyme immobilization method was also applied to another enzyme, glutamate oxidase, yielding similar results.