Larisa-Emilia Cheran

Electromagnetic excitation of high frequency acoustic waves and detection in the liquid phase

Excitation of acoustic waves in a quartz disk has been instigated by exposing the piezoelectric substrate to the electromagnetic field of a planar spiral coil placed in close proximity to the disk. It is argued that reciprocally induced magnetic and electric fields lead to secondary electric fields which couple with the piezoelectric tensor. A comparison of acoustic resonance envelopes recorded in air and in electrolyte demonstrates that spurious acoustic modes are damped in the liquid medium, and that the dielectric properties at the device–solution interface contribute to the exciting electric field. In agreement with previous acoustic device work, increased viscosity of the surrounding fluid leads to a rise in damping which manifests itself in changes in sensor frequency, amplitude and quality factor. With respect to work at high frequencies it is possible to operate the device at as high a harmonic as the 75th, although at this level of frequency resonance begins to break down because of a significant reduction in acoustic Q value. Finally, the use of the system in the flow-injection mode has also been demonstrated through the on-line detection of the adsorption of the protein, neutravidin, to the device surface. The glycosylated parent molecule is very widely employed as a linker for the immobilization of biological macromolecules in bioanalytical chemistry.

Protein microarray scanning in label-free format by Kelvin nanoprobe

Surface-immobilized protein species deposited in the microarray format have been detected by time-of-flight secondary ion mass spectrometry and by scanning Kelvin nanoprobe. The former method was used to examine the nature of protein deposition on amine-coated glass slides and gold substrates in preparation for Kelvin measurements. Both gallium and SF(5)(+) ion sources were employed to produce positive and negative ion spectra of amino acids and polypeptides. Scanning Kelvin technology has been used to detect antibody-antigen interactions in a label-free protocol through measurement of the surface potential of the biochemical pair on indium tin oxide, amine-treated slides and gold substrates. The results show that good inter-spot reproducibility can be achieved and that deposited areas can be examined for homogeneity at 100 nm resolution. This work represents the first report on surface potential detection in protein microarray technology.

Scanning Kelvin nanoprobe detection in materials science and biochemical analysis

The Kelvin nanoprobe is an extremely sensitive instrument capable of discerning subtle molecular interactions using vibrating electromagnetic and acoustic fields. It is based on the measurement of a fundamental material property, the work function. Modulation of this substrate parameter is caused by the adsorption or desorption of molecules, oxidation, corrosion, contamination, mechanical stress, illumination, temperature changes, electrostatic charging, surface treatment, attached dipolar structures and/or the immobilization of biomolecules. The present article explains the general principles of the method and offers an indication of the wide range of possible applications, with an emphasis on potential use in the biotechnological arena.

Work-function measurement by high-resolution scanning Kelvin nanoprobe

Nanoscience promises to transform todays world in the same way that integrated semiconductor devices transformed the world of electronics and computation. In the post-genomic era, the greatest challenge is to make connections between the structures and functions of biomolecules at the nanometre-scale level in order to underpin the understanding of larger scale systems in the fields of human biology and physiology. To achieve this, instruments with new capabilities need to be researched and developed, with particular emphasis on new levels of sensitivity, precision and resolution for biomolecular analysis. This paper describes an instrument able to analyse structures that range from tenths of a nanometre (proteins, DNA) to micron-scale structures (living cells), which can be investigated non-destructively in their normal state and subsequently in chemical- or biochemical-modified conditions. The high-resolution scanning Kelvin nanoprobe (SKN) measures the work-function changes at molecular level, instigated by local charge reconfiguration due to translational motion of mobile charges, dipolar relaxation of bound charges, interfacial polarization and structural and conformational modifications. In addition to detecting surface electrical properties, the instrument offers, in parallel, the surface topographic image, with nanometre resolution. The instrument can also be used to investigate subtle work function/topography variations which occur in, for example, corrosion, contamination, adsorption and desorption of molecules, crystallographic studies, mechanical stress studies, surface photovoltaic studies, material science, biocompatibility studies, microelectronic characterization in semiconductor technology, oxide and thin films, surface processing and treatments, surfaces and interfaces characterization. This paper presents the design and development of the instrument, the basic principles of the method and the challenges involved to achieve nanometric resolution and sub-millivolt sensitivity, for both the topographic imaging of surface micromorphology and surface potential and work-function determination.

Label-free detection of neuron–drug interactions using acoustic and Kelvin vibrational fields

Kelvin and acoustic fields of high-frequency have been employed in the non-invasive investigation of immortalized hypothalamic neurons, in order to assess their response to different concentrations of specific drugs, toxins, a stress-reducing hormone and neurotrophic factors. In an analytical systems biology approach, this work constitutes a first study of living neuron cultures by scanning Kelvin nanoprobe (SKN) and thickness shear mode (TSM) acoustic wave techniques. N-38 hypothalamic mouse neurons were immobilized on the gold electrode of 9 MHz TSM acoustic wave devices and gold-coated slides for study by SKN. The neurons were exposed to the neurochemicals betaseron, forskolin, TCAP, and cerebrolysin. Signals were collected with the TSM in real-time mode, and with the SKN in scanning and real-time modes, as the drugs were applied at biologically significant concentrations. With the TSM, for all drugs, some frequency and resistance shifts were in the same direction, contrary to normal functioning for this type of instrument. Possible mechanisms are presented to explain this behaviour. An oscillatory signal with periodicity of approximately 2 min was observed for some neuron-coated surfaces, where the amplitude of these oscillations was altered upon application of certain neurotrophic factors. These two new techniques present novel and non-invasive electrodeless methods for detecting changes at the cellular level caused by a variety of neuroactive compounds, without killing or destroying the neurons.

Surface immobilized biochemical macromolecules studied by scanning Kelvin microprobe

The measurement of work function is a particularly effective method for the characterization of surfaces because of the sensitivity of the parameter to interfacial structure, modification and overall chemistry. Accordingly, techniques for the analysis of work function offer a powerful tool for monitoring surface chemical changes, especially for situations involving the immobilization of new moieties at the interface. In the present paper, we describe the performance of a new, modified scanning Kelvin microprobe which is capable of the tandem measurement of contact potential and surface topography with resolutions of 1 mV and 10 nm, respectively. The lateral resolution is 1 micron. The instrument has been applied to the study of substrates modified by the attachment of biochemical macromolecules such as oligonucleotides and DNA. This preliminary work confirms the great potential of the technique in the study of biocompatibility, macromolecular structure and microarray devices.

Scanning Kelvin microprobe in the tandem analysis of surface topography and chemistry

This article presents the principles of operation, performance and applications of a scanning Kelvin microprobe for combined contact potential and surface topographical measurements. The new instrument uses a miniaturized vibrating probe for exploring, point by point, the surface of a sample, through measurement of the current emerging from the local ‘capacitor’ formed between the vibrating tip and the surface. In particular, the instrument is capable of simultaneously imaging the contact potential difference and topography of a scanned surface. This tandem characterization provides unique information regarding material properties. The lateral resolution is 1 µm, the topographic resolution is in the nanometer range and contact potential sensitivity is in the millivolt range. We also present preliminary studies of graphite, silicon, mica, metal and polymer surfaces.

Imaging TOF-SIMS analysis of oligonucleotide microarrays

Single strand thiolated oligonucleotide (25-mer) was printed onto chemically modified glass and silicon surfaces. Confirmation of the level of attachment attained in each case was effected through detection by conventional confocal fluorescence microscopy. Both positive-ion and negative ion imaging time-of-flight mass spectra were recorded for the visualization of micro-patterned oligonucleotide arrays. This represents the first report of such detection by this form of mass spectrometry on glass. Ultimately, we are interested in the possibility that imaging time-of-flight secondary ion mass spectrometry can discern the orientation and conformation of DNA strands present on the surface of a substrate.

Coupling of neurons with biosensor devices for detection of the properties of neuronal populations

The in vitro detection of the neural biophysical chemistry of populations of neurons is an important emerging area of research. This critical review describes the current methodologies, challenges and future prospects for this exciting field of research. There are different classes of techniques for the study of neuron-based systems. These include devices to measure inter-neuron contact and connectivity, microelectrodes for the determination of extracellular metabolic products, and sensors employed for the evaluation of complex neuron-small molecule interactions, toxicity, and mutagenicity of anti-tumor drugs. Since the neuron is an electrogenic cell and a complex biological entity capable of effecting recognition, the main emphasis of this article will be placed on devices based on nerve-cell networks that are able to electrically detect neuron-active compounds and specific pharmacological activity. Such neuron-based devices can be used to measure numerous neurological events with a high degree of sensitivity. Examples include the influence of different neuro-active compounds on neuronal function, the effects of neurotransmitters and neuro-modulators, changes in membrane potential, transmission effects that influence the propagation of the action potential, and the manner through which neuro-chemicals can influence ion channels. Moreover, these devices posses promising potential for the testing and development of novel neuron-active drugs and fundamental neurological research to further the understanding of brain activity. The inner workings of the human mind remain largely unknown and the key to comprehending it may rely on how molecules can initiate and influence synchronous neural oscillations, and the phenomenon of resonance in neural cells. The knowledge acquired in such detailed investigations can lead to the future development of regenerative medicines, neurochips and biocomputers, intelligent prosthetic devices and new applications that integrate neurobiology with molecular electronics (69 references).

Gigahertz surface acoustic wave probe for chemical analysis

This paper considers the propagation of high frequency 0.1–2.6 GHz surface acoustic wave pulses in aqueous solutions of pure water, glycerol and protein. The GHz frequency components of the pulse are used to provide the highest operating frequencies so far reported and also to construct the first acoustic absorption spectrum associated with the evanescent field. Acoustic generation is sourced from a single non-linear SAW device that provides a series of harmonic frequencies, simultaneously. The received power level is determined from digital samples of the received pulse waveform. The power leaked into glycerol solutions at the fundamental frequency was found to be 50% smaller for pulses, than for continuous acoustic waves, an effect that could be related to the equilibration of the evanescent field. Increasing the concentration of the glycerol solutions or time exposed to the protein (IgG) solution, showed that the power losses from the surface acoustic wave pulse were broadly consistent with the behaviour of transverse shear mode sensors. Atomic force microscope measurements of the bare device revealed that the morphology of the silica overlayer was uniformly granular, whereas adsorbed protein films formed non-contiguous islands. Confirmation of the presence of the IgG film was obtained from quantitative X-ray photoelectron spectroscopy. An 8 gigasample per second digitising oscilloscope running a fast Fourier transform routine captured the acoustic absorption spectrum, and revealed a smooth characteristic for the glycerol and IgG, although for the latter, frequencies beyond 500 MHz were associated with an irregular spectrum. These multiple frequency measurements of the solid–liquid interface provide evidence that when the penetration depth and film thickness are similar, disruption of the predicted exponential form of the evanescent wave occurs, as indicated by the fluctuations seen in the absorption spectrum recorded. These preliminary results have shown that multiple frequency operation of single non-linear SH-SAW devices is possible, and an evanescent interfacial absorption spectrum can be obtained. By extending the measurement technique it may be possible to obtain additional information about the structure and composition of the solid–liquid interface.