Shift Δf Research Articles

Electromagnetically induced transparency-like metamaterials for detection of lung cancer cells.

A biosensor based on electromagnetically induced transparent (EIT) metamaterials (MMs) is proposed owing to the low loss and high Q-factor. The theoretical sensitivity of the biosensor based on EIT-like MMs were evaluated up to 248.8 GHz/RIU (RIU, Refractive Index Unit). In experiments, the cancer cells A549, as an analyte, are cultured on EIT-like MMs surface. The results show that when the cell concentration increases from 0.5 × 105 to 5 × 105 cells/ml, the frequency shift Δf could change from 24 to 50 GHz. Moreover, the coupled oscillators model is applied to explain the effect of the refractive index of analyte in simulations and the cell concentration in experiments on the EIT-like MMs. The fitting results exhibit that the refractive index of analyte and cell concentration significantly affect the radiative damping of the bright mode resonator γ1. The proposed EIT-like MMs biosensors show great potentials for cell measurement because any change that results in the lineshape variation in EIT-like MMs can only be attributed to the change of external dielectric environment due to the suppression of radiative losses.

Real‐Time Detection of Nanoparticles Interaction with Lipid Membranes Using an Integrated Acoustical and Electrical Multimode Biosensor

AbstractUnderstanding interactions of nanoparticles with biomembranes is critical in nanomedicine and nanobiotechnology. The underlying mechanisms still remain unclear due to the fact that there are no reliable tools to follow such complex processes. In this work, the interactions between gold nanoparticles (AuNPs) and the supported lipid bilayer (SLB) are monitored in situ by a multimode biosensor integrating a quartz crystal microbalance with dissipation function (QCM‐D) and a field effect transistor (FET). Real‐time responses of frequency shift (Δf), dissipation (ΔD), and ion current (ΔI) are simultaneously recorded to provide complementary information for AuNPs translocation across the SLB. The combined mass loading, mechanical and electrical measurements reveal the dynamics of the particle–membrane interactions as well as the formation of transient pores or permanent defects in the membrane. AuNPs with different diameters, surface charge, and ligand properties are used to study their translocation behaviors, including adsorption on or desorption from the membrane surface, diffusion into or penetration through the lipid bilayer. This multimode sensing approach provides insights into the mechanism of the particle–membrane interactions and suggests a method of label‐free screening of nanomaterials' interaction with model membranes in a real‐time manner.

The terahertz electromagnetically induced transparency-like metamaterials for sensitive biosensors in the detection of cancer cells

A kind of novel biosensor based on the electromagnetic induced transparency like (EIT-like) metamaterials (MMs) have been proposed. It demonstrates that the symmetry-breaking double-splits ring resonators can realize the EIT-like plasmonic resonance, the according transparency window occurs at 1.67 THz. The coupled oscillators model illustrates that with the increase of asymmetry degree of double splits, the coupling between bright and dark mode is enhanced. Consequently, the non-radiative damping γ2 grows from 1.45 to 1.85 THz and coupling coefficient κ from 3.46 to 4.49 THz2, while the radiative damping γ1 decreases from 11.5 to 9 THz. Such EIT-like MMs were evaluated in simulation as the refractive index sensors, which the theoretical sensitivity was calculated to 455.7 GHz/RIU (RIU, Refractive Index Unit) under 11 μm-thick analyte layer. Meanwhile, the dependence of full width at half maximum (FWHM) on analyte thickness was also studied. In experiments, it is found that the frequency shift Δf increases from 50 to 90 GHz when the oral cancer cells (HSC3) concentration improves from 1 × 105 to 7 × 105 cells/ml. The maximum experimental sensitivity approaches 900 kHz/cell ml−1 at 7 × 105 cells/ml. Additionally, the apoptosis of cancer cells under the effect of anti-cancer drug was investigated. It shows that with the increase of anti-cancer drug concentration from 1 to 15 μM and the extension of drug action duration from 24 to 72 h, the Δf changes from 140 to 70 GHz and 140–40 GHz, respectively. Besides, the corresponding FWHM also increases from 237.9 to 305.4 GHz and 237.8–337.6 GHz. The results measured by MMs biosensors and biological method exhibit a relatively good agreement, showing a great potential for cells measurement with the sensitive biosensors based on the EIT-like MMs.

Corrigendum: Counterion Specificity of Polyelectrolyte Brushes: Role of Specific Ion‐Pairing Interactions

In the above paper, the y-axis label of Figure 1 was incorrect. The y-axis label should read “Δf / Hz”and the corrected figure is shown below as Figure 1. Salt-concentration (Csalt) dependence of the frequency shift (Δf) of PMETAC brushes in the presence of different types of counterions with Na+ as the common cation. Here, the overtone number (n) is 3. The authors apologize for this oversight.

Resistivity change in Joule heat energy dissipation detected by noncontact atomic force microscopy using a silicon tip terminated with/without atomic hydrogen

The mechanical energy dissipation (DJ) of an oscillating atomic force microscopy (AFM) cantilever, which originated from Joule heat, was analyzed by noncontact (nc)-AFM, in relation to the electrostatic force interaction between a Si tip on the cantilever and a sample of Si(111)-(7×7). DJ and the resonant frequency shift (Δf) of the cantilever were simultaneously measured with respect to the tip–sample distance and tip–sample bias voltage, using a Si tip terminated with atomic hydrogen (H) and that without H termination. From the linear relationship between DJ and Δf due to the electrostatic force, the surface resistances around the tip and the sample were evaluated to be of the order of GΩ, through which the current passed. The current was induced by cantilever oscillation, resulting in Joule heat generation. This study demonstrates that the simultaneous measurements of DJ and Δf lead to novel nanoscale characterization of the surface resistances of the tip and the sample.

Urea-functionalized SBA-15 hybrids: Post-grafting synthesis, high-performance organophosphorus sensing and their response mechanism

There is an urgent need for effective and portable gas sensors toward organophosphorus (OP) vapors in the areas of anti-terrorism and pesticides residue monitoring. Herein, we report a series of urea-functionalized SBA-15 (UXs/SBA-15: U1/SBA-15, U2/SBA-15 and U3/SBA-15) prepared by post-grafting methods and their applications in trace-level dimethyl methylphosphonate (DMMP, lab simulate of toxic OP agents) sensing by means of quartz crystal microbalance (QCM) transducers. U3/SBA-15 based QCM sensors exhibited excellent responses and remarkable selectivity toward DMMP vapors. A frequency shift (ΔF) of −50 Hz was achieved when the DMMP vapors reached 30 ppb. And the limit of detection (LOD) of QCM sensor modified with U3/SBA-15 was found at 12.25 ppb. Moreover, the simulated adsorption enthalpies (ΔH) of UXs and DMMP molecules were calculated with Gaussian 09 to investigate the sensing mechanism. The results indicated the existence of chemisorption between UXs and DMMP molecules caused by hydrogen-bonding interactions, among which a minimum ΔH (−71.96 kJ mol−1) between U3 and DMMP molecule was found, resulting a maximum response to DMMP compared with U1-DMMP (−52.53 kJ mol−1) and U2-DMMP (−57.01 kJ mol−1). Therefore, the well-defined UXs/SBA-15 lead to a great potential on the detection of OP vapors.

Controlling the electric charge of gold nanoplatelets on an insulator by field emission nc-AFM

Charging of 2D Au nanoplatelets deposited on an insulating SiO2 substrate to or from the tip of a non-contact atomic force microscope (nc-AFM) is demonstrated. Charge transfer is controlled by monitoring the resonance frequency shift Δf(V) during the bias voltage ramp V applied to the tip-back electrode junction. The onset of charge transfer is revealed by a transition from a capacitive parabolic behavior to a constant Δf(V) region for both polarities. An analytical model, based on charging by electron field emission, shows that the field-emitted current saturates shortly after the onset of the charging, due to the limiting effect of the charge-induced rise of the Au platelet potential. The value of this current plateau depends only on the rate of the bias voltage ramp and on the value of the platelet/SiO2/back electrode capacitance. This analysis is confirmed by numerical simulations based on a virtual nc-AFM model that faithfully matches the experimental data. Our charging protocol could be used to tune the potential of the platelets at the single charge level.

Composition-Dependent Alterations in Thickness and Physical Properties of Lipid Bilayer FILM Revealed by Frequency Modulation Atomic Force Microscopy

Assembly of microdomains and membrane viscosity due to these physical properties of membrane have been highlighted as important factors supporting proper functions of membrane proteins. Although advanced imaging technologies including scanning probe microscopy and super-resolution optical microscopy have revealed dynamics of microdomains and visco-elastic properties of cell membranes, it is still difficult to link modifications of nanoscale membrane structure and specific diseases. In this study, we aimed to obtain high-resolution structural information of supported planner membrane with different fatty acid lengths using liquid dynamic-mode AFM and FM-AFM (Frequency Modulation Atomic Force Microscope). In FM-AFM, the cantilever is moved in a non-contact state, maintaining the cantilever frequency shift (Δf) is constant. This enables a highly sensitive force detection, 20 times better S/N than with existing methods, thereby markedly improving the image resolution. We used supported phosphatidylcholine membranes with fatty acids ranged from PC 12:0 to PC 22:6. For saturated PCs, membrane thickness increased along with the number of carbon from 4.32 nm (PC12:0) to 6.37nm (PC18:0), but diarachidoylphosphatidylcholine (PC20:0) showed reduced membrane thickness: PC20:0 thickness had 6.09 nm, that is ≈0.3 nm thinner than PC18:0. Unsaturation of fatty acid further reduced membrane thickness to 4.12 nm for PC20:4, suggesting that structural arrangement of PCs with fatty acid chain length and unsaturation level occurred. These data demonstrated that our imaging modality can detect membrane thickness with minimal membrane compression by scanning probe while maintaining high detection sensitivity. We will also present modulations of mechanical properties of membrane in the functions of carbon chain length and the level of unsaturation.

Ib-AMP4 insertion causes surface rearrangement in the phospholipid bilayer of biomembranes: Implications from quartz-crystal microbalance with dissipation

Most antimicrobial peptides exert their rapid bactericidal activity through a unique mechanism of bacterial membrane disruption. However, the molecular events that underlie this mechanism remain partly unresolved. In this study, the frequency shift (ΔF) obtained through quartz-crystal microbalance with dissipation (QCM–D) indicated that the initial binding of Ib-AMP4 within the lipid membrane started at a critical Ib-AMP4 concentration that exceeded 100μg/ml. Circular dichroism measurements provided evidence that Ib-AMP4 occurs in a β-sheet configuration which is adapted for insertion into the lipid membrane. Monolayer experiments and the value of dissipation alteration (ΔD) obtained through QCM–D showed that the pressure increased within the phospholipid bilayer upon peptide insertion, and the increase in pressure subsequently forced the bilayer to wrinkle and form pores. However, D continued to increase, indicating that the membrane surface underwent a dramatic morphological transition: the membrane surface likely became porous and uneven as Ib-AMP4 projected from the external surface of the lipid bilayer. Intensive peptide insertion, however, soon plateaued 1min after the addition of Ib-AMP4. This behaviour corresponded with the results of bactericidal kinetics and liposome leakage assays. A sudden decrease in D accompanied by a negligible decrease in F occurred after replacing the Ib-AMP4 solution with HEPES buffer. This result implied that the bilayer surface rearranged and that poration and wrinkling decreased without further peptide insertion. Transmission electron microscopy results indicated that pore formation occurred during Ib-AMP4 insertion but eventually subsided. Therefore, the mode of action of AMP in bacterial membranes could be elucidated through QCM–D.

A novel quartz crystal microbalance sensor array based on molecular imprinted polymers for simultaneous detection of clenbuterol and its metabolites

For the rapid and robust detection of both parent clenbuterol (CLB) and its metabolites in swine urine samples, a novel quartz crystal microbalance (QCM) sensor array for CLB detection based on molecularly imprinted polymers (MIPs) was developed in this investigation. At first, clenbuterol and the structural analogs of its metabolites, 4-Aminohippuric acid (AHA) and 4-hydroxymandelic acid (HMA), were chosen as molecular templates. Through computational molecular modeling, the optimum ratio between the functional monomer and molecular template was selected. The surface imprinting method was applied to modify QCM electrode surface to graft a thin MIP film. The grafting polymer was characterized by Fourier-transformed infrared spectrometry (FTIR) and atomic force microscopy (AFM), respectively. After then, an array system composed of three sensors was employed to test the responses with different solutions and the principal component analysis (PCA) was adopted to analyze the corresponding data. As a result, for the designed sensor to clenbuterol, a linear equation y=100.07x-722.96 (R2=0.9928) was found between the sensor frequency shift ΔF and negative logarithm of clenbuterol concentration (-lgC). The limitation of detection (LOD) was 3.0ng/mL, which is lower than the Codex Alimentarius Commission regulations residue limit 10μg/L. The corresponding data of the three template solutions were analyzed by PCA, obtaining 100% recognition. The result demonstrated the feasibility that the developed method could be applied to detect whether the livestock was feed with CLB nutrient redistribution agent by checking the urine samples.

Turn-on fluorescence detection of cyanide with large stokes shift: Activation of latent intramolecular Br⋯H[sbnd]C hydrogen bonding

A specific recognition of CN− by activation of latent intramolecular Br⋯HC hydrogen bonding was developed for the first time. The visually colorimetric and “turn-on” fluorescent detection with large stokes shift (ΔF ≈ 137 nm) can be attributed to the formation of semi-rigid hydrogen-bonded complexes by intermolecular hydrogen bonding followed by activated intramolecular Br⋯HC hydrogen bonding.

(Invited) Quantification and Characterization of Adsorbed BTA on Copper Surfaces Under Conditions Relevant to Barrier CMP

In the formation of copper interconnects by CMP, barrier polishing is typically carried out using alkaline slurries containing copper corrosion inhibitors. Benzotriazole (BTA) has been the inhibitor of choice in such slurries. The effect of BTA on copper corrosion has been extensively studied in the past few decades, and there is general agreement that the adsorbed film formed in acidic solutions is a multilayer film with a chemisorbed first layer. However, simultaneous real time measurement of the amount of BTA adsorbed and characterization of nature of the adsorbed film (rigid/viscoelastic) has received very little attention. The objective of this work was to measure the extent of BTA adsorption onto PVD Cu films and characterize the nature of the adsorbed film in real time using a quartz crystal microbalance with dissipation monitoring (QCMD) capability. Quartz crystals (fundamental frequency of 5 MHz) coated with ~300 nm PVD copper layer were used in the experiments. To mimic the conditions during barrier polishing, BTA dissolved in alkaline solutions was pumped through the flow cell containing the crystal, and the change in crystal oscillation frequency and dissipation data were collected at a time resolution of fraction of a second for several minutes. This was followed by rinsing with DI or solutions containing selected surfactants to determine extent of desorption of BTA. As an example, adsorption of BTA from a solution of pH 10 is shown in figure 1. The measured frequency shift (Δf) and dissipation shift (ΔD) at different overtones (multiples) of crystal fundamental frequency are plotted as a function of time. At the time marked with t1, a sharp decrease of frequency may be seen, which indicates rapid adsorption of BTA. The data points for all frequencies fall on one curve indicating that the adsorbed BTA film on copper is quite rigid in nature. This is confirmed by very little measured change in the ΔD values. At the end of adsorption marked with t2, deionized water was introduced into the system, and the positive shift of frequency is indicative of BTA removal and/or etching of copper in DI water. In the presentation, results of adsorption and desorption experiments carried out under different experimental conditions will be presented. The disruption of adsorbed BTA layer by selected chemicals will be addressed. Figure 1

Adaptive photodetectors based on charge gratings in the problems of frequency-modulated optical signal detection

Nonstationary photovoltage excitation by frequency-modulated light in an adaptive photodetector based on GaAs is studied. To observe the effect, the crystal is exposed to light beams with the relative frequency shift Δf(t). Linear frequency modulation Δf(t) = At is used in the experiments. As a result of such illumination, a pulsed electrical signal is induced in the crystal. The pulse appears at the deceleration of the interference pattern motion, and its duration is controlled by the frequency variation rate A and the time of the charge grating formation. The possibility of using the effect in the systems for measuring velocities and accelerations of moving objects is shown.

Distance dependence of atomic-resolution near-field imaging on α-Al2O3 (0001) surface with respect to surface photovoltage of silicon probe tip

Recently, we achieved atomic-resolution optical imaging with near-field scanning optical microscopy using photon-induced force detection. In this technique, the surface photovoltage of the silicon-tip apex induced by the optical near field on the surface is measured as the electrostatic force. We demonstrated atomicresolution imaging of the near field on the α-Al2O3 (0001) surface of a prism. We investigated the spatial distribution of the near field by scanning at different tip-sample distances and found that the atomic corrugation of the near-field signal was observed at greater distances than that of the atomic force microscopy signal. As the tip-sample distance increased, the normalized signal-to-noise ratio of the near field is in a gradual decline almost twice that of the frequency shift (Δf).

Virtual reality visual feedback for hand-controlled scanning probe microscopy manipulation of single molecules.

Controlled manipulation of single molecules is an important step towards the fabrication of single molecule devices and nanoscale molecular machines. Currently, scanning probe microscopy (SPM) is the only technique that facilitates direct imaging and manipulations of nanometer-sized molecular compounds on surfaces. The technique of hand-controlled manipulation (HCM) introduced recently in Beilstein J. Nanotechnol. 2014, 5, 1926–1932 simplifies the identification of successful manipulation protocols in situations when the interaction pattern of the manipulated molecule with its environment is not fully known. Here we present a further technical development that substantially improves the effectiveness of HCM. By adding Oculus Rift virtual reality goggles to our HCM set-up we provide the experimentalist with 3D visual feedback that displays the currently executed trajectory and the position of the SPM tip during manipulation in real time, while simultaneously plotting the experimentally measured frequency shift (Δf) of the non-contact atomic force microscope (NC-AFM) tuning fork sensor as well as the magnitude of the electric current (I) flowing between the tip and the surface. The advantages of the set-up are demonstrated by applying it to the model problem of the extraction of an individual PTCDA molecule from its hydrogen-bonded monolayer grown on Ag(111) surface.

Tip induced mechanical deformation of epitaxial graphene grown on reconstructed 6H–SiC(0001) surface during scanning tunneling and atomic force microscopy studies

The structural and mechanical properties of an epitaxial graphene (EG) monolayer thermally grown on top of a 6H–SiC(0001) surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM). Experimental STM, dynamic STM and AFM images of EG on 6H–SiC(0001) show a lattice with a 1.9 nm period corresponding to the (6 × 6) quasi-cell of the SiC surface. The corrugation amplitude of this (6 × 6) quasi-cell, measured from AFM topographies, increases with the setpoint value of the frequency shift Δf (15–20 Hz, repulsive interaction). Excitation variations map obtained simultaneously with the AFM topography shows that larger dissipation values are measured in between the topographical bumps of the (6 × 6) quasi-cell. These results demonstrate that the AFM tip deforms the graphene monolayer. During recording in dynamic STM mode, a frequency shift (Δf) map is obtained in which Δf values range from 41 to 47 Hz (repulsive interaction). As a result, we deduced that the STM tip, also, provokes local mechanical distortions of the graphene monolayer. The origin of these tip-induced distortions is discussed in terms of electronic and mechanical properties of EG on 6H–SiC(0001).

Surface potential imaging with atomic resolution by frequency-modulation Kelvin probe force microscopy without bias voltage feedback

We investigated the capability of obtaining atomic resolution surface potential images by frequency-modulation Kelvin probe force microscopy (FM-KPFM) without bias voltage feedback. We theoretically derived equations representing the relationship between the contact potential difference and the frequency shift (Δf) of an oscillating cantilever. For the first time, we obtained atomic resolution images and site-dependent spectroscopic curves for Δf and VLCPD on a Si (111)-7 × 7 surface. FM-KPFM without bias voltage feedback does not involve the influence of the FM-KPFM controller because it has no deviation from a parabolic dependence of Δf on the dc-bias voltage. It is particularly suitable for investigation on molecular electronics and organic photovoltaics, because electron or ion movement induced by dc bias is avoided and the electrochemical reactions are inhibited.

Shifts in Photon Spectral Power Densities within Schumann (7.7 to 7.8 Hz) Values in Microtubules during Complex Magnetic Field Exposures May Reflect an Information Interface with Universal Energies

The specific diameter of microtubules was shown to be a primary solution when magnetic energy was set equal to Casimir energy. To discern if this spatial containment could be foci for information photon emissions were measured from preparations of microtubules (MTs) while they were exposed in sequential 4 min intervals to various patterns of weak magnetic fields whose intensities ranged from 3 to 10 μT. Calculations from the median mass of a tubulin dimer, its summed charges and the applied magnetic field as well as the change in magnetic moment derived from the energy of the hydrogen line when applied to our experimental fields predicted a dynamic shift (Δf) between 0.03 and 0.21 Hz. Spectral power densities (SPD) indicated marked enhancements in photon numbers during periods of magnetic field exposures within the 7.6 to 7.8 Hz increment. The total SPD units for the shift were 10-18 to 10-17 J per s. Five of the eight patterns elicited a split spectrum of power within this range. Separate factor analyses of the SPDs of the serial values that composed the points of the actual field patterns indicated those that evoked the split-spectrum (Δf = 0.05 to 0.13 Hz) displayed significantly higher loadings on the same factor compared to those that did not. If this shift in photon energy reflects a phase modulation of the coherence frequency (8 MHz) of MTs, the increment of energy per MHz frequency would be within the energy of the neutral hydrogen line. These results suggest that the intrinsic structure or information from specific intensity magnetic fields when applied to MTs is reflected in photon energy densities vacillating around the fundamental Schumann Resonance that could be an interface between Casimir and magnetic sources.

Binding interaction and gelation in aqueous mixtures of poly(N-isopropylacrylamide) and hectorite clay.

The binding interaction between poly(N-isopropylacrylamide) (PNIPAm) chains and hectorite clay platelets was directly detected by quartz crystal microbalance with dissipation (QCM-D) to explore the cross-linking mechanism in the strong nanocomposite hydrogel (NC gel), which is in situ polymerized with NIPAm in the clay suspension. PNIPAm chains were allowed to be adsorbed on the gold surface of the QCM electrode. A large frequency shift Δf in the QCM as introducing the clay indicated that a large amount of clay platelets were adsorbed on the deposited PNIPAm layer. The relationship between the dissipation shift ΔD and Δf revealed that the adsorption included two steps of fast initial buildup and following densification of the clay platelets. In dilute aqueous mixtures, the PNIPAm chain and clay formed aggregates as observed from the hydrodynamic diameter. A gelation state diagram was established for concentrated aqueous PNIPAm-clay mixtures. The Raman spectrum pointed out the conformation change of the PNIPAm chains in aqueous solutions when the clay was added, which would be caused by the adsorption of PNIPAm chains to the clay platelets.

Application of electrochemically deposited nanostructured ZnO layers on quartz crystal microbalance for NO2 detection

The research was fixed on sensing behavior of ZnO nanostructured (NS) films to NO2 concentrations in the environment. The ZnO NS layers are deposited by electrochemical method on quartz resonators with Au electrodes. The sorption properties of ZnO layers were defined by measuring the resonant frequency shift (Δf) of the QCM-ZnO structure for different NO2 concentrations. The measurements were based on the correlation between the frequency shift of the QCM and additional mass loading (Δm) on the resonator calculated using Sauerbrey equation for the AT-cut quartz plate. Frequency – Time Characteristics (FTCs) of the samples were measured as a function of different NO2 concentrations in order to define the sorption abilities of ZnO layers. The experiments were carried out on a special set up in a dynamical regime. From FTCs the response and the recovery times of the QCM-ZnO structure were measured with varying NO2. Frequency shift changed from 23 Hz to 58Hz when NO2 was varied in the range of 250ppm – 5000ppm. The process of sorption was estimated as reversible and the sorption as physical. The obtained results demonstrated that QCM covered with the electrochemically deposited nanostructured ZnO films can be used as application in NO2 sensors.