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Taking a solution-based approach to obtain transition metal dichalcogenide monolayers affords a number of advantages, including processing that is scalable, cost effective, and enables tuning monol...

MnO2 nanosheets and ultraviolet-visible (UV-Vis) absorbance spectroscopy are used to study glucose oxidase (GOx) kinetics. Glucose oxidation by GOx produces H2O2, which rapidly decomposes the nanosheets and reduces their absorption. This direct approach for monitoring glucose oxidation enables simpler, real time kinetics analysis compared to methods that employ additional enzymes. Using this approach, the present study confirms that GOx kinetics is consistent with the Michaelis–Menten (MM) model, and reveals that the MM constant increases by an order of magnitude with increasing buffer concentration. Since larger MM constants imply higher enzyme substrate concentrations are required to achieve the same rate of product formation, increasing MM constants imply decreasing enzyme performance. These results demonstrate the facility of using MnO2 nanosheets to study GOx kinetics and, given the widespread applications of enzymes with buffers, the important sensitivity of enzyme–buffer systems on buffer concentration.

Lock-in amplifiers (LIAs) are commonly used in chemistry laboratories to improve noise-to-signal ratios. Constructing and testing a suitably designed LIA provide undergraduate students with an exce...

MoS2 are two-dimensional (2D) materials that exhibit emerging photoluminescence (PL) at the monolayer level and have potential optoelectronic applications. Monolayers of MoS2 typically achieved by ...

MnO

The present study demonstrates the ability of excess, weakly amphiphilic n-alkanethiols (n = 4, 12, 18) and solvent composition to tune through a wide range of large-scale, macroscopic architectures formed by alkanethiol-capped Au nanoparticles (NPs). Both the alkanethiols and NPs are significantly hydrophobic species and compete for surface area at an air-water interface. When solutions of the two species are spread on a large (50 cm2) water surface in a Teflon well, a thin film forms and exhibits co-existing macroscopic regions with various distinct NP self-assembled architectures, namely a close packed monolayer, a network phase characterized by micron-sized pores (micropores) surrounded by quasi-linear bundles of nanoparticles, and finally aggregates. We hypothesize that the co-existence of various NP architectures results from fast, non-uniform evaporation across the large water surface. When solutions are instead deposited on a smaller (5 cm2) water surface contained within a Teflon ring to control the water surface curvature and the evaporation rate is slowed, we show for the first time that NPs form macroscopically uniform self-assemblies whose architectures can be tuned from monolayers → monolayers with micropores → extended micropore/NP bundle networks by varying excess alkanethiol concentration and solvent composition. We propose that competition between NPs and excess alkanethiols for water surface area, and alkanethiol self-assembly as well as solvent dewetting play important roles in the formation of the network phase, and discuss a potential mechanism for its formation.

Size-selective precipitation (SSP) is a powerful tool for obtaining monodisperse nanoparticles. Here we report a fast, high yield and tunable SSP procedure via non-solvent addition for producing nearly monodisperse, organic-soluble Au nanoparticles with standard deviations as low as σ < 7%. The addition of excess ligands and judicious choice of ligand head group significantly improve both precipitate yields and nanoparticle monodispersity.

MoS

Using a prototypical nanoparticle-molecule assembly, namely alkanedithiol-linked gold nanoparticle films, we observe hallmark signatures of the Kondo effect in conductance vs. voltage as well as temperature measurements. Its contribution to temperature dependence of conductance is much larger than those from all other temperature-dependant effects up to 300 K by >20-fold - much larger than previous reports of the Kondo effect using other platforms. We find that previous models of the Kondo effect describe our data even in this regime. Given the synthetic control available over nanoparticle properties such as surface area, shape, and chemical composition, our work points to combining flexibility afforded by molecule + nanoparticle assemblies as a powerful way to generate materials exhibiting strong spin-electron interactions.

We explore the magnetoconductance that can be generated at tunnel junction contacts with disordered normal (DN) materials. We use self-assembled 1,4-butanedithiol-linked gold nanoparticle films as a prototype DN material and different types of tunnel junction contacts, namely one native to a gold electrode/1,4-butanedithiol interface and another naturally occurring at aluminum/aluminum oxide layer. For control measurements, we also study normal metal and superconducting contacts above and below critical temperature Tc in place of tunnel junction contacts. We focus on a nanoparticle film regime where contact resistance is significant, i.e. when films are metallic and have low resistance. In this regime, superconducting contacts yield a zero-field magnetoconductance enhancement due to “reflectionless tunneling” below Tc. Normal metal contacts do not yield significant magnetoconductance. This is also true for superconducting contacts above Tc when the superconductor becomes a normal metal. When a tunnel junction is present between the normal metal and the DN material, a ~ 10% zero-field magnetoconductance suppression appears at temperatures below ~ 10 K. We propose a mechanism for the tunnel junction mediated magnetoconductance based on coherent back scattering in disordered granular materials, percolation and a local nature of current flow across planar tunnel junctions.

Strong electron-electron interactions experienced by electrons as they delocalize are widely believed to play a key role in a range of remarkable phenomena such as high Tc superconductivity, colossal magnetoresistance, and others. Strongly correlated electrons are often described by the Hubbard model, which is the simplest description of a correlated system and captures important gross features of phase diagrams of strongly correlated materials. However, open challenges in this field include experimentally mapping correlated electron phenomena beyond those captured by the Hubbard model, and extending the model accordingly. Here we use electrolyte gating to study a metal-insulator transition (MIT) in a new class of strongly correlated material, namely, nanostructured materials, using 1,4-butanedithiol-linked Au nanoparticle films (NPFs) as an example. Electrolyte gating provides a means for tuning the chemical potential of the materials over a wide range, without significantly modifying film morphology. On the insulating side of the transition, we observe Efros-Shklovskii variable range hopping and a soft Coulomb gap, evidencing the importance of Coulomb barriers. On the metallic side of the transition, we observe signatures of strong disorder mediated electron-electron correlations. Gating films near MIT also reveal a zero-bias conductance peak, which we attribute to a resonance at the Fermi level predicted by the Hubbard and Anderson impurity models when electrons delocalize and experience strong Coulomb electron-electron interactions. This study shows that by enabling large changes in carrier density, electrolyte gating of Au NPFs is a powerful means for tuning through the Hubbard MIT in NPFs. By revealing the range of behaviours that strongly correlated electrons can exhibit, this platform can guide the development of an improved understanding of correlated materials.

The present study evaluates the performance and potential utility of apertured impedance microchips (AIMs). The electronics-based, dielectric constant detection devices feature a novel layered architecture with aluminum metal/silicon-oxide/silicon layers, where the top metal layer has apertures and the middle oxide layer has wells. This layered yet open geometry enables molecules to have access to detection of electric fields induced by a voltage applied between the top metal/bottom silicon electrode layers. Our design rationale surmised that the AIM device's large footprint area and thin nanoscale oxide layer should enable impedance detection of molecules with high sensitivity in a variety of solvents. The present study confirms this hypothesis and explores the effects that simple surface modifications have on the device's response. Specifically, the affinity between various analytes and the device's surface is evaluated when: (a) the aluminum top electrode is hydrated and rendered hydrophilic by immersing the microchip in boiling water, and (b) the silicon substrate is stripped of oxide and rendered hydrophobic (hydrogen-terminated) by hydrofluoric acid (HF) wet etching. Both types of devices were incorporated into a high performance liquid chromatography (HPLC) system already equipped with an ultra-violet–visible (UV–vis) detector. A range of analytes was injected using both normal and reverse phase modes; and the signals generated by each microchip device and UV–vis detector were recorded simultaneously and compared. The microchip devices’ responses were found to vary for analytes according to the surface modification used. The second surface modification method of exposing bare silicon surface showed greater sensitivity than the first method for all the analytes. Both types of devices were remarkably sensitive to organic salts tested, i.e., their sensitivity was significantly greater than that of the UV–vis detector. To demonstrate the novel dielectric constant capability of AIM, as a case study, the HF-treated device was used to detect enantiomers of a synthesized chiral molecule separated by a chiral column using an insulating eluent, namely hexanes/isopropanol.

We have studied electronic transport in disordered granular films fabricated by sintering superconducting Nb nanoparticles. Measurements of resistance versus bias voltage and magnetic field reveal ...

To explore charge transfer between nanostructured assemblies and an electrode surface, we have performed cyclic voltammetry (CV) of films self-assembled layer-by-layer using gold nanoparticles (NPs) and l-(hexanethiol)-1′-(ethanethiol) ferrocene (FDT). Reflection–Absorption Fourier Transform Infrared Spectroscopy (RAIRS) and Ultraviolet–visible Spectroscopy (UV–vis) of films on aminosilane-functionalised glass slides were used to confirm self-assembly. CV of films on gold electrodes was used to monitor redox peaks of the films following self-assembly of each FDT and NP layer. Addition of a NP layer consistently resulted in a significant decrease in redox peak height, while addition of a subsequent FDT layer resulted in a net overall increase. Occasionally, redox activity dropped substantially but recovered with increasing number of NP–FDT layers. All samples exhibited an increase in redox peak width and a slight increase in peak separation with increasing FDT-linked NP film thickness for the first 4–5 layers. No systematic trend was observed from sample to sample for subsequent layers. These results are consistent with a disordered, porous film structure. Importantly, because of increasing surface area and nanoparticle hopping sites, charge transfer between outer layers and underlying electrodes in such devices is not significantly hampered by increasing number of deposited NP/FDT layers, despite addition of intervening insulating material.

We study the influence of Coulomb effects on conductance $(g)$ of 1,4-butanedithiol-linked gold nanoparticle (NP) films near a percolation insulator-to-metal transition. On the insulating side, $g\ensuremath{\sim}\mathrm{exp}[\ensuremath{-}{({T}_{\ensuremath{\circ}}/T)}^{1/2}]$, where $T$ is absolute temperature, a behavior predicted by Efros-Shklovskii's theory for charges optimizing pathways that accommodate Coulomb charging barriers. On the metallic side below $\ensuremath{\sim}20$ K, $g$ varies linearly with ${T}^{1/2}$. Such a correction to $g(T=0)$ is predicted by Altshuler-Aronov's theory for Fermi liquid metals when disorder mediates electron-electron $(e\text{\ensuremath{-}}e)$ Coulomb interactions. Remarkably, in the present system, the ${T}^{1/2}$ component of $g$ is significant compared to $g(T=0)$, and fitting to Boltzmann's transport theory yields elastic scattering lengths that are anomalously small\char22{}much smaller than the distance between atoms (Ioffe-Regel limit required for metals). Previous studies of materials such as fullerites, layered organic salts, and transition metal compounds have also reported such anomalously small scattering lengths and large ${T}^{1/2}$ components and attributed them to strong Coulomb mediated $e\text{\ensuremath{-}}e$ correlations, which we believe is likely the case in the present system as well. This study highlights a potential opportunity to use molecularly linked nanoparticle films as a platform to study strongly correlated electrons in a controlled fashion.

Digital computers use binary states, typically represented by 0 and 5 V, to store and process information at all stages of a calculation. If more states (ideally a continuum) were available in between, density of information could be dramatically increased. Here we show that self-assembled nanoparticle films can feature such continuous state or analogue information storage. Information provided by an arbitrary gate voltage is 'written' by trapping charges in local, gate-modified potentials when films are cooled below 175 K. The information is 'read' using the film's built-in ability to sense charge via Coulomb blockade. Application of a time-dependent, multi-step writing gate voltage generates conductance maps corresponding to multi-valued continuous information. As a proof of concept, we exploit this technique to store 'UT' in Morse code.

We describe a hybrid scanning tunneling–atomic force microscope (STM–AFM) capable of measuring current and force simultaneously under ambient conditions. In order to reduce meniscus forces, the microscope uses a sharp STM tip as a probe and an AFM cantilever as a sample substrate. This improvement allows use of correspondingly flexible cantilevers enhancing force detection sensitivity. Using the STM–AFM, we have been able to explore a number of phenomena that can occur in nanometer scale tunnel junctions in air, including a correlation between hysteretic changes in contact potential and rapid increases in current at large bias voltages.

We have investigated electrical properties of planar aluminum/aluminum oxide/silver tunnel junctions modified with phenyl-based azomethine oligomers. Normalized differential conductance, NDC (NDC=σV/σV=0, where σ=dI/dV), of the junctions increases with oligomer length. At a bias of 2 V, azomethines with three phenyl rings exhibit NDCs that are on average more than an order of magnitude greater than those of unmodified oxide junctions. Differential conductances of junctions modified with azomethines increase more rapidly with temperature than those of plain oxide junctions. Our results are consistent with a model in which both increased conjugated length of the sandwiched organic layer and a molecule/metal interface lead to a lowering of the barrier profile outside the aluminum oxide tunnel region.

We study electrical transport in strongly coupled, molecularly linked, gold nanoparticle (NP) films whose bulk dc conductances are governed by percolation phenomena. Films with fewer NPs exhibit current suppression below a threshold voltage, likely due to single-electron charging of NP clusters. In some cases, the thresholds are very large (∼1 V) and suppression persists to room temperature. The thresholds tend to decrease with increasing amounts of NPs in the film, and eventually, metal-like conductance is observed down to at least 10 K. The observed trend toward metal-like conductance, despite the presence of film disorder, is enabled by strong inter-NP coupling and increasing film connectivity. The latter is an inherent property of molecularly linked NP films due to both robust chemical inter-NP linkages provided by alkane dithiol linker molecules, coupled with the ability to grow chains of connected NPs to arbitrary lengths through cyclical Au/dithol treatments. In the case of small thresholds, our da...

According to the “orthodox” model for single electron tunneling, sudden changes in current–voltage characteristics of nanoparticle (NP)-tunnel junction (TJ) systems [“Coulomb blockade” (CB) and “Coulomb staircase” (CS) phenomena] arise fundamentally due to charge quantization. We have embedded NPs (∼2.5 nm in diameter) in the TJ of a hybrid scanning tunneling-atomic force microscope and have simultaneously measured current and forces generated in the system. We discuss an application to micromechanical switching actuated by single electrons. We also show that CB and CS phenomena are in fact associated with steplike changes in force, directly confirming the discrete charge nature of the phenomena.

We present the first measurement of the longitudinal density matrix of a matter-wave beam. Usinga unique interferometric scheme, both the amplitude and phase of off-diagonal density matrix elementswere determined directly, without the use of traditional tomographic techniques. The measureddensity matrix of a doubly amplitude modulated atomic sodium beam compares well with theoreticalpredictions.

We demonstrate the absence of off-diagonal elements for the density matrix of a supersonic Na atomic beam, thus showing that there are no coherent wave packets emerging from this source. We used a differentially detuned separated oscillatory field longitudinal interferometer to search for off-diagonal density matrix elements in the longitudinal energy/momentum basis. Our study places a stringent lower bound on their possible size over an off-diagonal energy range from 0 to 100 kHz.

Localized oscillating fields are beam splitters that can entangle internal and longitudinal momentum states in an atomic beam. Differentially detuned separated oscillatory fields and an am modulator constitute a {open_quotes}white fringe{close_quotes} longitudinal interferometer which rephases velocity averaging by a process analogous to half a spin echo. Differentially detuned separated oscillatory fields are used to produce a downstream coherence or rephase an upstream coherence in an atomic beam. {copyright} {ital 1998} {ital The American Physical Society}

Two separated oscillatory fields, if tuned to different frequencies, can generate or interrogate longitudinal momentum coherences in a beam of two-state particles. We demonstrate that use of differentially detuned separated oscillatory fields is an efficient method to determine the longitudinal density matrix of a particle beam.

Charge transport across a disordered normal-superconductor (DN-S) interface was studied using a macroscopic, molecularly linked Au nanoparticle film as the DN component. Low-temperature conductance versus voltage and magnetic field exhibit zero-bias and zero-field peaks, respectively. Importantly, the latter typically exhibit superimposed oscillations. Such oscillations are rarely seen in other DN-S systems and are remarkable given their robustness in these macroscopic films and interfaces. A number of observations indicate that conductance peaks and oscillations arise due to a 'reflectionless tunnelling' process. Scattering length scales extracted from the data using a reflectionless tunnelling picture are consistent with literature values. Factors resulting in the observation of oscillations in this system are discussed.

Interfaces between disordered normal (DN) materials and superconductors (S) are known to generate conductance peaks at zero-bias voltage (V) and magnetic field (B). Using molecularly linked Au nanoparticle films as the DN component, we find that superimposed on conductance peaks are oscillations that depend simultaneously on both V and B. Such correlated conductance oscillations are predicted by a "reflectionless tunneling" phenomenon but have not been observed in other DN-S systems. Length scales extracted from periods of conductance oscillation correlate well with film nanostructure.

Combining the advantages of nanogap devices and impedance spectroscopy can potentially provide a platform for dielectric spectroscopy with widely ranging applications—from fundamental studies at the nanoscale and surfaces to label free and selective sensors. The present study characterizes the impedance response of a microfabricated, silicon-based device with a large array of nanowells surrounded by annular, nanogap detection regions. Device impedance is measured versus frequency over 5 orders in a variety of organic solvents with dielectric constants ranging over 2 orders. The study finds two key results. First, an equivalent R/C circuit model is found to compare favorably with device impedance response over these wide ranges of parameters. Importantly, the model correlates with structure of the nanogap device, which suggests that such a structure-impedance response approach can help guide modeling of other devices geometries. Second, the model points to—and data confirm—correlation between nanogap device response and dielectric constant of materials in the nanogaps, particularly at low frequencies. In addition, the correlation is significantly modified by robust, silane functionalization of the devices due to a large surface-to-volume ratio of the nanogaps. These results demonstrate that nanogap impedance spectroscopy using microfabricated/silanized silicon devices is a robust and versatile platform for dielectric spectroscopy of materials on the nanoscale and on surfaces.

Films of butanedithiol interconnected nanoparticles can exhibit a percolation-driven insulating to metal transition. To explore properties of materials with interpolating behavior, we have measured conductance of these films with systematically varying thickness. Films below a certain threshold coverage exhibit thermally assisted conductance and conductance suppression near zero bias indicative of single-electron-charging barriers. In analogy with semiconductors, we show that these films permit transistor-type gating of film conductivity.

We report a metal to insulator transition (MIT) in disordered films of molecularly linked gold nanoparticles (NPs). As the number of carbons ($n$) of alkanedithiol linker molecules (${\mathrm{C}}_{n}{\mathrm{S}}_{2}$) is varied, resistance ($R$) at low temperature ($T=2\text{ }\text{ }\mathrm{K}$) and at 200 K, as well as trends in $R$ vs $T$ data at intermediate temperatures, all point to an MIT occurring at $n=5$. We describe these results in a context of a Mott-Hubbard MIT. We find that all insulating samples ($n\ensuremath{\ge}5$) exhibit a universal scaling behavior $R\ensuremath{\sim}\mathrm{exp} [({T}_{0}/T{)}^{\ensuremath{\nu}}]$ with $\ensuremath{\nu}=0.65$, and all metallic samples ($n\ensuremath{\le}5$) exhibit weaker $R\mathrm{\text{\ensuremath{-}}}T$ dependencies than bulk gold. We discuss these observations in terms of competitive thermally activated processes and strong, $T$-independent elastic scattering, respectively.

We have investigated the temperature dependence of the conductance of 1,4-butane dithiol linked Au nanoparticle films from $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At low temperatures $(Tl10\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, conductance becomes independent of temperature and exhibits strong nonlinearity with voltage, which we attribute to tunneling. At higher temperatures $(Tg20\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the conductance behaves as $g\ensuremath{\propto}\mathrm{exp}[\ensuremath{-}{({T}_{0}∕T)}^{1∕2}]$. Qualitatively, this is consistent with an Efros-Shklovskii variable range hopping model based on a competition between Coulombic and intercluster tunneling processes. However, we find that hopping distances are too large ($62--720\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) to be consistent with tunneling between clusters and tend to scale with cluster size. We propose a modified, ``quasilocalized hopping'' model based on competition between single-electron cluster charging and intracluster electron backscattering to explain this temperature dependence.

We investigate electrical characteristics of single-electron electrode/nanoisland/electrode devices formed by alkanedithiol assisted self-assembly. Contrary to predictions of the orthodox model for double tunnel junction devices, we find a significant ( approximately fivefold) discrepancy in single-electron charging energies determined by Coulomb blockade (CB) voltage thresholds in current-voltage measurements versus those determined by an Arrhenius analysis of conductance in the CB region. The energies do, however, scale with particle sizes, consistent with single-electron charging phenomena. We propose that the discrepancy is caused by a multibarrier junction potential that leads to a voltage divider effect. Temperature and voltage dependent conductance measurements performed outside the blockade region are consistent with this picture. We simulated our data using a suitably modified orthodox model.

Planar tunnel junctions were fabricated by self-assembling 1,1'- ferrocenedicarboxylic acid (FDCA) onto native oxides of thermally deposited aluminum films and subsequently depositing a second aluminum film. Junctions were characterized using Reflection-Absorption Fourier Transform Infrared Spectroscopy (RAIRS) and current-voltage (I-V) spectroscopy. Before deposition of the second aluminum film, RAIRS of FDCA and ferrocenecarboxylic acid (FCA) films revealed COO(-), C=O, and Fc ring stretching modes, indicating that both types of molecules can interact strongly with the oxide and remain intact. After deposition, systems exhibited prominent COO(-) modes and weakened C=O modes, indicating further reaction with aluminum/aluminum oxide. Fc ring modes persisted in FDCA systems but disappeared in FCA systems, suggesting that the second COOH group in the FDCA molecule can act as a protecting group for the ferrocene moiety. Cyclic I-V measurements of FDCA tunnel junction systems revealed very strong ( approximately 10-fold) hysteretic differential conductance switching that was both reversible and stable. Control measurements using as prepared junctions, as well as junctions containing 1,6-hexanedioic acid, 1,9-nonanedioic acid, 1,4-dibenzoic acid, or FCA revealed only very weak ( approximately 10%) differential conductance changes. We attribute FDCA junction switching to barrier profile modifications induced by oxidation/reduction of the functionally protected ferrocene moieties.

By using an atomic force microscope (AFM) cantilever as a substrate for a scanning tunneling microscope (STM), we can simultaneously probe currents I and forces F in a tunnel junction as a function of bias voltage V. Measurements performed using gold-nanoparticle (NP) coated cantilevers under ambient conditions reveal correlated kinks in both $I\ensuremath{-}V$ and $F\ensuremath{-}V$ curves. Changes in background charge distributions, which have been problematic in controlling single charges in such systems, can be readily detected by the hybrid STM-AFM. To test whether we can statistically attribute observed kinks to Coulomb staircase phenomena, we have performed measurements using NP's of different sizes. NP's 4.8 and 2.5 nm in diameter exhibit kinks spaced on average 0.22 and 0.52 V apart, respectively. These values are in good agreement with a metallic sphere\char21{}flat surface model for the STM tip\char21{}NP capacitance and the orthodox model for single-electron tunneling.

We have used a hybrid scanning tunneling \char21{} atomic force microscope to measure current and forces generated between nanotunnel junction (NTJ) electrodes. Cyclic bias voltage scans performed in air reveal hysteretic increases in force above a \ensuremath{\sim}1 V threshold. Current measurements exhibit simultaneous increases in magnitude and noise. Our results are consistent with hysteretic changes in contact potentials and indicate that, in addition to tunneling and electrochemistry, molecules can mediate widespread occurrences of ion trapping and sudden detrapping in NTJ's.

We have used a conventional scanning tunneling microscope (STM) as well as a combined scanning tunneling\char21{}atomic force microscope (STM-AFM) to probe tunneling characteristics of gold films in air. Both instruments yield current-voltage measurements that frequently exhibit sudden changes. In addition, STM and STM-AFM measurements reveal associated discrete changes in barrier height and in tip-sample forces, respectively. We propose that these phenomena can be attributed to charge trapping/detrapping events that are mediated by molecules present in the tunnel junction.

We present initial measurements of the dispersive index of refraction for sodium matter waves passing through argon. In addition, we describe a novel scheme for performing tomography on the longitudinal quantum state of particles in an atomic beam.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The present study evaluates high performance liquid chromatography (HPLC) detection based on a commercial conductance detector with a low cell constant of 0.005 cm−1 and a volume of 2 μL and a photodiode array UV-vis detector typically used in HPLC. When detecting a static NaCl solution, the conductance detector yields a limit of detection (LOD, 3 × noise) for NaCl of 39 parts per trillion. When flowing methanol through both conductance and UV-vis detectors and injecting benzoic acid/methanol, the signal-to-noise (S/N) ratio of the conductance detector is 8-fold higher than that of the UV-vis at its optimal wavelength. Using an HPLC with a C-18 column, flowing a 75 : 25 water : methanol solution, and using an acetylsalicylic acid (ASA)/water solution, the conductance detector yielded an ∼18-fold higher S/N ratio. It was found that HPLC system noise reduces the S/N ratio of the conductance detectors. The conductance detector detected non-chromophoric species generated by atmospheric CO2 as well as by decomposition of ASA. Conductance chromatograms yielded ASA peak heights and areas that varied linearly with the ASA concentration from 0.5 ppm to 10 ppm with linear correlation coefficients exceeding 0.999. In view of the sensitivity of conductance detection, its potential application as a sensitive tool for cleaning assessments for pharmaceutical equipment was confirmed by dispersing 50 μg of ASA on a 2′′ × 2′′ stainless steel sheet, swabbing the surface, dissolving the collected material in water and injecting the solution into the HPLC. The results in this study demonstrate that low cell constant conductance detection can be remarkably sensitive to ionized/charged species and thereby has potential to serve as an analytical tool for this important class of molecules.

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Interfaces between disordered normal materials and superconductors (S) can exhibit 'reflectionless tunnelling' (RT)-a phenomenon that arises from repeated disorder-driven elastic scattering, multiple Andreev reflections, and electron/hole interference. RT has been used to explain zero-bias conductance peaks (ZBCPs) observed using doped semiconductors and evaporated granular metal films as the disordered normal materials. Recently, in addition to ZBCPs, magnetoconductance oscillations predicted by RT theory have been observed using a novel normal disordered material: self-assembled nanoparticle films. In the present study, we find that the period of these oscillations decreases as temperature (T) increases. This suggests that the magnetic flux associated with interfering pathways increases accordingly. We propose that the increasing flux can be attributed to magnetic field penetration into S as [Formula: see text]. This model agrees remarkably well with known T dependence of penetration depth predicted by Bardeen-Cooper-Schrieffer theory. Our study shows that this additional region of flux is significant and must be considered in experimental and theoretical studies of RT.

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