Kathryn Mayer - Academia.edu (original) (raw)

Papers by Kathryn Mayer

Materials Science and Engineering B-advanced Functional Solid-state Materials, 1991

ABSTRACT Ion-implantation of magnesium into extremely pure InP grown by the liquid-encapsulated C... more ABSTRACT Ion-implantation of magnesium into extremely pure InP grown by the liquid-encapsulated Czochralski method was carried out with magnesium concentrations [Mg] ranging from 1 × 1015 to 3 × 1020 cm−3 with a maximum energy of 400 keV. Low temperature photoluminescence and Raman scattering measurements were performed on these samples. A new state of excitions bound to what was presumed to be a new type of acceptor g and an emission attributed to acceptor-acceptor pairs [g-g] were found below the bound exciton emission series. Two novel emissions located far below the band-to-acceptor emissions were also observed, and these showed remarkable energy shifts towards the lower energy side with increasing [Mg]. With heavily implanted samples, a strong broad emission of a new type was observed in the vicinity of the band-to-acceptor emissions.

Acs Nano, 2008

Robust gold nanorod substrates were fabricated for refractive index sensing based on localized su... more Robust gold nanorod substrates were fabricated for refractive index sensing based on localized surface plasmon resonance (LSPR). The substrate sensitivity was 170 nm/RIU with a figure of merit of 1.3. To monitor biomolecular interactions, the nanorod surfaces were covered with a self-assembled monolayer and conjugated to antibodies by carbodiimide cross-linking. Interactions with a specific secondary antibody were monitored through shifts in the LSPR spectral extinction peak. The resulting binding rates and equilibrium constant were in good agreement with literature values for an antibody-antigen system. The nanorod LSPR sensors were also shown to be sensitive and specific. These results demonstrate that given a sufficiently stable nanoparticle substrate with a well defined chemical interface, LSPR sensing yields similar results to the surface plasmon resonance technique, yet with much simpler instrumentation.

Biophysical Journal, 2008

The electrostatic properties of biological membranes can be described by three parameters: the tr... more The electrostatic properties of biological membranes can be described by three parameters: the transmembrane potential, the membrane surface potential, and the membrane dipole potential. The first two are well characterized in terms of their magnitudes and biological effects. The dipole potential, however, is not well characterized. Various methods to measure the membrane dipole potential indirectly yield different values, and there is not even agreement on the source of the membrane dipole moment. This ambiguity impedes investigations into the biological effects of the membrane dipole moment, which should be substantial considering the large interfacial fields with which it is associated. Electrostatic analysis of phosphatidylcholine lipid membranes with the atomic force microscope reveals a repulsive force between the negatively charged probe tips and the zwitterionic lipids. This unexpected interaction has been analyzed quantitatively to reveal that the repulsion is due to a weak external field created by the internal membrane dipole potential. The analysis yields a dipole moment of 1.5 Debye per lipid with a dipole potential of 1275 mV for supported phosphatidylcholine membranes. This new ability to quantitatively measure the membrane dipole moment in a noninvasive manner with nanometer scale spatial resolution will be useful in identifying the biological effects of the dipole potential.

Nanotechnology, 2009

The strong cetyltrimethylammonium bromide (CTAB) surfactant responsible for the synthesis and sta... more The strong cetyltrimethylammonium bromide (CTAB) surfactant responsible for the synthesis and stability of gold nanorod solutions complicates their biomedical applications. The critical parameter to maintain nanorod stability is the ratio of CTAB to nanorod concentration. The ratio is approximately 740 000 as determined by chloroform extraction of the CTAB from a nanorod solution. A comparison of nanorod stabilization by thiol-terminal PEG and by anionic polymers reveals that PEGylation results in higher yields and less aggregation upon removal of CTAB. A heterobifunctional PEG yields nanorods with exposed carboxyl groups for covalent conjugation to antibodies with the zero-length carbodiimide linker EDC. This conjugation strategy leads to approximately two functional antibodies per nanorod according to fluorimetry and ELISA assays. The nanorods specifically targeted cells in vitro and were visible with both two-photon and confocal reflectance microscopies. This covalent strategy should be generally applicable to other biomedical applications of gold nanorods as well as other gold nanoparticles synthesized with CTAB.

Electrostatic potential was measured near supported zwitterionic lipid bilayer membrane surfaces ... more Electrostatic potential was measured near supported zwitterionic lipid bilayer membrane surfaces with atomic force microscope. In our recent work, two methods were developed to measure the surface charge density of the membrane surface, Fluid electric force microscopy (FEFM) which creates a two- dimensional map of a surface charge density with a corresponding topographic map simultaneously and quantitative measurement method which based on tip-sample force curve analysis.Both FEFM and tip-sample force curve analysis showed that the surface of a DOPC (dioleoylphosphatidylcholine) lipid bilayer carries a negative electrostatic potential.This is an interesting and surprising result, for the head group of DOPC is carrying zero net charge over a broad range of pH where both the choline and phosphate groups are ionized.Two sources are proposed to explain the origin of this negative charge.The bilayers could carry a net charge density due to the counterions from the electrolyte binding to the lipid head groups.Alternatively, the dipole density in the DOPC lipid head group layer could cause an effective surface potential outside the membrane region.To study the source of this negative potential, Charge densities of supported DOPC bilayers under different ion concentrations were measured and compared with both of these two charge mechanisms.

Biophysical Journal, 2007

The atomic force microscope (AFM) is sensitive to electric double layer interactions in electroly... more The atomic force microscope (AFM) is sensitive to electric double layer interactions in electrolyte solutions, but provides only a qualitative view of interfacial electrostatics. We have fully characterized silicon nitride probe tips and other experimental parameters to allow a quantitative electrostatic analysis by AFM, and we have tested the validity of a simple analytical force expression through numerical simulations. As a test sample, we have measured the effective surface charge density of supported zwitterionic dioleoylphosphatidylcholine membranes with a variable fraction of anionic dioleoylphosphatidylserine. The resulting surface charge density and surface potential values are in quantitative agreement with those predicted by the Gouy-Chapman-Stern model of membrane charge regulation, but only when the numerical analysis is employed. In addition, we demonstrate that the AFM can detect double layer forces at a separation of several screening lengths, and that the probe only perturbs the membrane surface potential by ,2%. Finally, we demonstrate 50-nm resolution electrostatic mapping on heterogeneous model membranes with the AFM. This novel combination of capabilities demonstrates that the AFM is a unique and powerful probe of membrane electrostatics.

Analytical Chemistry, 2009

Gold nanoparticles bound to substrates exhibit localized surface plasmon resonance (LSPR) in thei... more Gold nanoparticles bound to substrates exhibit localized surface plasmon resonance (LSPR) in their optical extinction spectra at visible and near-infrared wavelengths. The LSPR wavelength is sensitive to the surrounding refractive index, enabling a simple, label-free immunoassay when capture antibodies are bound to the nanoparticles. Gold bipyramids are nanoparticles with a penta-twinned crystal structure, which have a sharp LSPR because of their high monodispersity. Bipyramid substrates were found to have a refractive index sensitivity ranging from 288 to 381 nm/ RIU (-0.62 to -0.68 eV/RIU), increasing with the nanoparticle size and aspect ratio. In an immunoassay, the bipyramid substrates yielded higher sensitivity than nanorods and nanospheres. An immunoassay sensitivity constant which depends on both the optical properties of the nanoparticle and conjugation chemistry was found to be K LSPR ) 0.01 nm · µm 2 for gold bipyramids.

Noble metal nanoparticles exhibit localized surface plasmon resonance (LSPR), in which incident l... more Noble metal nanoparticles exhibit localized surface plasmon resonance (LSPR), in which incident light causes a collective oscillation of a nanoparticle's free electrons. This phenomenon results in unique optical properties, including enhanced electric fields near the particle surface and an extinction peak at the resonant wavelength. The LSPR extinction peak's location is sensitive to the refractive index of the surrounding medium, especially in the volume closest to the particle surface. This makes plasmonic nanoparticles ideal for biosensing: their refractive index sensitivity can be used to transduce molecular binding signals. A method has been developed to use the optical extinction of films of gold nanorods to track antibody-antigen interactions in real time, resulting in a label-free kinetic immunoassay based on LSPR. Also, this method has been adapted to scattering spectra of single gold bipyramids. The single-particle approach has allowed the label-free detection of single biomolecules with kinetics information. These methods have future applications to both molecular biology and clinical assays.

Journal of Physical Chemistry C, 2010

... JE Anderson † § , Kathryn M. Mayer † § , Robert D. Fraleigh † , Yi Yang † , Seunghyun Lee ‡ a... more ... JE Anderson † § , Kathryn M. Mayer † § , Robert D. Fraleigh † , Yi Yang † , Seunghyun Lee ‡ and Jason H. Hafner* † ‡ §. Department of Physics & Astronomy, Department of Chemistry, and Laboratory for Nanophotonics, Rice University, Houston, Texas 77005. J. Phys. Chem. ...

Nanotechnology, 2010

Noble metal nanoparticles exhibit sharp spectral extinction peaks at visible and near-infrared fr... more Noble metal nanoparticles exhibit sharp spectral extinction peaks at visible and near-infrared frequencies due to the resonant excitation of their free electrons, termed localized surface plasmon resonance (LSPR). Since the resonant frequency is dependent on the refractive index of the nanoparticle surroundings, LSPR can be the basis for sensing molecular interactions near the nanoparticle surface. However, previous studies have not yet determined whether the LSPR mechanism can reach the ultimate sensing limit: the detection of individual molecules. Here we demonstrate single molecule LSPR detection by monitoring antibody-antigen unbinding events through the scattering spectra of individual gold bipyramids. Both experiments and finite element simulations indicate that the unbinding of single antigen molecules results in small, discrete < 0.5 nm blue-shifts of the plasmon resonance. The unbinding rate is consistent with antibody-antigen binding kinetics determined from previous ensemble experiments. According to these results, the effective refractive index of a single protein is approximately 1.54. LSPR sensing could therefore be a powerful addition to the current toolbox of single molecule detection methods since it probes interactions on long timescales and under relatively natural conditions.

Materials Science and Engineering B-advanced Functional Solid-state Materials, 1991

ABSTRACT Ion-implantation of magnesium into extremely pure InP grown by the liquid-encapsulated C... more ABSTRACT Ion-implantation of magnesium into extremely pure InP grown by the liquid-encapsulated Czochralski method was carried out with magnesium concentrations [Mg] ranging from 1 × 1015 to 3 × 1020 cm−3 with a maximum energy of 400 keV. Low temperature photoluminescence and Raman scattering measurements were performed on these samples. A new state of excitions bound to what was presumed to be a new type of acceptor g and an emission attributed to acceptor-acceptor pairs [g-g] were found below the bound exciton emission series. Two novel emissions located far below the band-to-acceptor emissions were also observed, and these showed remarkable energy shifts towards the lower energy side with increasing [Mg]. With heavily implanted samples, a strong broad emission of a new type was observed in the vicinity of the band-to-acceptor emissions.

Acs Nano, 2008

Robust gold nanorod substrates were fabricated for refractive index sensing based on localized su... more Robust gold nanorod substrates were fabricated for refractive index sensing based on localized surface plasmon resonance (LSPR). The substrate sensitivity was 170 nm/RIU with a figure of merit of 1.3. To monitor biomolecular interactions, the nanorod surfaces were covered with a self-assembled monolayer and conjugated to antibodies by carbodiimide cross-linking. Interactions with a specific secondary antibody were monitored through shifts in the LSPR spectral extinction peak. The resulting binding rates and equilibrium constant were in good agreement with literature values for an antibody-antigen system. The nanorod LSPR sensors were also shown to be sensitive and specific. These results demonstrate that given a sufficiently stable nanoparticle substrate with a well defined chemical interface, LSPR sensing yields similar results to the surface plasmon resonance technique, yet with much simpler instrumentation.

Biophysical Journal, 2008

The electrostatic properties of biological membranes can be described by three parameters: the tr... more The electrostatic properties of biological membranes can be described by three parameters: the transmembrane potential, the membrane surface potential, and the membrane dipole potential. The first two are well characterized in terms of their magnitudes and biological effects. The dipole potential, however, is not well characterized. Various methods to measure the membrane dipole potential indirectly yield different values, and there is not even agreement on the source of the membrane dipole moment. This ambiguity impedes investigations into the biological effects of the membrane dipole moment, which should be substantial considering the large interfacial fields with which it is associated. Electrostatic analysis of phosphatidylcholine lipid membranes with the atomic force microscope reveals a repulsive force between the negatively charged probe tips and the zwitterionic lipids. This unexpected interaction has been analyzed quantitatively to reveal that the repulsion is due to a weak external field created by the internal membrane dipole potential. The analysis yields a dipole moment of 1.5 Debye per lipid with a dipole potential of 1275 mV for supported phosphatidylcholine membranes. This new ability to quantitatively measure the membrane dipole moment in a noninvasive manner with nanometer scale spatial resolution will be useful in identifying the biological effects of the dipole potential.

Nanotechnology, 2009

The strong cetyltrimethylammonium bromide (CTAB) surfactant responsible for the synthesis and sta... more The strong cetyltrimethylammonium bromide (CTAB) surfactant responsible for the synthesis and stability of gold nanorod solutions complicates their biomedical applications. The critical parameter to maintain nanorod stability is the ratio of CTAB to nanorod concentration. The ratio is approximately 740 000 as determined by chloroform extraction of the CTAB from a nanorod solution. A comparison of nanorod stabilization by thiol-terminal PEG and by anionic polymers reveals that PEGylation results in higher yields and less aggregation upon removal of CTAB. A heterobifunctional PEG yields nanorods with exposed carboxyl groups for covalent conjugation to antibodies with the zero-length carbodiimide linker EDC. This conjugation strategy leads to approximately two functional antibodies per nanorod according to fluorimetry and ELISA assays. The nanorods specifically targeted cells in vitro and were visible with both two-photon and confocal reflectance microscopies. This covalent strategy should be generally applicable to other biomedical applications of gold nanorods as well as other gold nanoparticles synthesized with CTAB.

Electrostatic potential was measured near supported zwitterionic lipid bilayer membrane surfaces ... more Electrostatic potential was measured near supported zwitterionic lipid bilayer membrane surfaces with atomic force microscope. In our recent work, two methods were developed to measure the surface charge density of the membrane surface, Fluid electric force microscopy (FEFM) which creates a two- dimensional map of a surface charge density with a corresponding topographic map simultaneously and quantitative measurement method which based on tip-sample force curve analysis.Both FEFM and tip-sample force curve analysis showed that the surface of a DOPC (dioleoylphosphatidylcholine) lipid bilayer carries a negative electrostatic potential.This is an interesting and surprising result, for the head group of DOPC is carrying zero net charge over a broad range of pH where both the choline and phosphate groups are ionized.Two sources are proposed to explain the origin of this negative charge.The bilayers could carry a net charge density due to the counterions from the electrolyte binding to the lipid head groups.Alternatively, the dipole density in the DOPC lipid head group layer could cause an effective surface potential outside the membrane region.To study the source of this negative potential, Charge densities of supported DOPC bilayers under different ion concentrations were measured and compared with both of these two charge mechanisms.

Biophysical Journal, 2007

The atomic force microscope (AFM) is sensitive to electric double layer interactions in electroly... more The atomic force microscope (AFM) is sensitive to electric double layer interactions in electrolyte solutions, but provides only a qualitative view of interfacial electrostatics. We have fully characterized silicon nitride probe tips and other experimental parameters to allow a quantitative electrostatic analysis by AFM, and we have tested the validity of a simple analytical force expression through numerical simulations. As a test sample, we have measured the effective surface charge density of supported zwitterionic dioleoylphosphatidylcholine membranes with a variable fraction of anionic dioleoylphosphatidylserine. The resulting surface charge density and surface potential values are in quantitative agreement with those predicted by the Gouy-Chapman-Stern model of membrane charge regulation, but only when the numerical analysis is employed. In addition, we demonstrate that the AFM can detect double layer forces at a separation of several screening lengths, and that the probe only perturbs the membrane surface potential by ,2%. Finally, we demonstrate 50-nm resolution electrostatic mapping on heterogeneous model membranes with the AFM. This novel combination of capabilities demonstrates that the AFM is a unique and powerful probe of membrane electrostatics.

Analytical Chemistry, 2009

Gold nanoparticles bound to substrates exhibit localized surface plasmon resonance (LSPR) in thei... more Gold nanoparticles bound to substrates exhibit localized surface plasmon resonance (LSPR) in their optical extinction spectra at visible and near-infrared wavelengths. The LSPR wavelength is sensitive to the surrounding refractive index, enabling a simple, label-free immunoassay when capture antibodies are bound to the nanoparticles. Gold bipyramids are nanoparticles with a penta-twinned crystal structure, which have a sharp LSPR because of their high monodispersity. Bipyramid substrates were found to have a refractive index sensitivity ranging from 288 to 381 nm/ RIU (-0.62 to -0.68 eV/RIU), increasing with the nanoparticle size and aspect ratio. In an immunoassay, the bipyramid substrates yielded higher sensitivity than nanorods and nanospheres. An immunoassay sensitivity constant which depends on both the optical properties of the nanoparticle and conjugation chemistry was found to be K LSPR ) 0.01 nm · µm 2 for gold bipyramids.

Noble metal nanoparticles exhibit localized surface plasmon resonance (LSPR), in which incident l... more Noble metal nanoparticles exhibit localized surface plasmon resonance (LSPR), in which incident light causes a collective oscillation of a nanoparticle's free electrons. This phenomenon results in unique optical properties, including enhanced electric fields near the particle surface and an extinction peak at the resonant wavelength. The LSPR extinction peak's location is sensitive to the refractive index of the surrounding medium, especially in the volume closest to the particle surface. This makes plasmonic nanoparticles ideal for biosensing: their refractive index sensitivity can be used to transduce molecular binding signals. A method has been developed to use the optical extinction of films of gold nanorods to track antibody-antigen interactions in real time, resulting in a label-free kinetic immunoassay based on LSPR. Also, this method has been adapted to scattering spectra of single gold bipyramids. The single-particle approach has allowed the label-free detection of single biomolecules with kinetics information. These methods have future applications to both molecular biology and clinical assays.

Journal of Physical Chemistry C, 2010

... JE Anderson † § , Kathryn M. Mayer † § , Robert D. Fraleigh † , Yi Yang † , Seunghyun Lee ‡ a... more ... JE Anderson † § , Kathryn M. Mayer † § , Robert D. Fraleigh † , Yi Yang † , Seunghyun Lee ‡ and Jason H. Hafner* † ‡ §. Department of Physics & Astronomy, Department of Chemistry, and Laboratory for Nanophotonics, Rice University, Houston, Texas 77005. J. Phys. Chem. ...

Nanotechnology, 2010

Noble metal nanoparticles exhibit sharp spectral extinction peaks at visible and near-infrared fr... more Noble metal nanoparticles exhibit sharp spectral extinction peaks at visible and near-infrared frequencies due to the resonant excitation of their free electrons, termed localized surface plasmon resonance (LSPR). Since the resonant frequency is dependent on the refractive index of the nanoparticle surroundings, LSPR can be the basis for sensing molecular interactions near the nanoparticle surface. However, previous studies have not yet determined whether the LSPR mechanism can reach the ultimate sensing limit: the detection of individual molecules. Here we demonstrate single molecule LSPR detection by monitoring antibody-antigen unbinding events through the scattering spectra of individual gold bipyramids. Both experiments and finite element simulations indicate that the unbinding of single antigen molecules results in small, discrete < 0.5 nm blue-shifts of the plasmon resonance. The unbinding rate is consistent with antibody-antigen binding kinetics determined from previous ensemble experiments. According to these results, the effective refractive index of a single protein is approximately 1.54. LSPR sensing could therefore be a powerful addition to the current toolbox of single molecule detection methods since it probes interactions on long timescales and under relatively natural conditions.