Introduction

Sensors for the discovery and identification of both simplest inorganic molecules and organic compounds (including macro- and bio-molecules in living organisms) are mainly used in pharmacology, bio-medicine and chemistry. In general, the progress in sensor technology is determined by our ability to decompose different molecules contained in a “big” volume over elementary “cubicles”, containing identical molecules with equal mass, mobility, optical spectrum, etc. Thus, in particular we are able to determine the presence of low concentration of analytes given the information about processes, say, in human cells. The most advanced current method of sensing is associated with mass spectrometry. This method has become established as the primary means for protein identification from complex mixtures of biological origin, uncovering the set of proteins that make up human cells. Moreover, due to new approaches based on MALDI imaging, the mapping of protein distribution inside of cells becomes routine procedure. Prospectively measuring the dynamics of map variations make predicting a living cells lifetime achievable. However, this technique is rather expensive and can’t be applied for very big molecules (with sizes of tens and hundreds nanometers). As an alternative, Ion mobility spectrometers allow classification of very big molecules, but to use this method one usually has to know in advance what kind of analytes need to be found. Laser induced fluorescence gives detailed information about analyte spectra, but strong background emission usually masks the low concentrated molecules of interest. Raman spectroscopy can also provide very useful information about molecular structure and outlines features of molecules with close chemical performances, but it too is masked by laser induced fluorescence which usually exceeds Raman emissions on several orders. As such, there is a need for new methods in detecting molecular structure, and for sensing inorganic molecules and organic compounds. This article proposes a new electrically-controlled (active) filter for the trapping of molecules with high polarizabilities. This new method of molecular sensing (active micro/nano-structured spectroscopy – AMOS) is based on our proprietary technology. The important features of AMOS are its ability to trap bio-molecules based on their polarizability. According to our estimation it is feasible achieving of sensitivity for detection of admixed molecules up better than part per trillion. Likewise, this filter can be a core element of ultra-sensitive analytical spectrometers, filtration of organic compounds in pharmacology, water extraction from air and oil, etc.

General description of AMOS, technologies to be used and advantages of ps-UV pulses

Among the different methods of sensing, there are separately staying adsorption methods. These methods are characterized by the physical adherence or bonding of ions and molecules onto the surface of another phase. The typical example is a solgel plate, which absorbs water from air. Absorbed material is analyzed, for example, following ionization (in IMS technology) or optical spectra analysis (Laser Induced Fluorescence, Raman or molecular spectroscopy). However the absorber used for molecules of interest accumulation usually is passive. Recently we have developed a new method of sensing based on a lasermade active filter consisting of dense packaging of micro-holes in dielectric plate matches to vias in structured metal or Indium Tin Oxide electrodes (see Figure 1). By controlling voltage on these electrodes and temperature of the filter it is possible to accumulate molecules and organic compounds near (or inside) of the holes. The physical reason for such accumulation is that the electrostriction interaction of neutral molecules and organic compounds occurs with a high voltage electrical field having a strong non-uniform distribution near and inside of the holes. Especially if such holes are blind (not drilled through a dielectric plate) the bottom of the holes has quasi-periodical spikes (like “stalagmites”) with a typical distance between them achieving up to λ/3, where λ is the laser wavelength in the UV band. For 213 nm, that distance can achieve 71 nm and the transverse size of correspond
ing spikes is two to three times less (about 25 nm). Due to high gradient of electrical field (sharp spatial change with nanometer scale) the electrostriction forces attract neutral molecules and organic compounds (in particular proteins) inside of the holes to allow bonding them around spikes. As more polarizability α has molecule as stronger interaction occurs. If the molecule velocity induced by electrostriction force (~ τα E2/m) is comparable to its thermal velocity (~ kT/m), then a molecules bonding becomes strong enough to fix that molecules to the filter. Here we use denotations: k is the Boltzmann constant, T is the filter temperature, τ is the time between molecules collisions (free running time), m is the molecule mass. However, if polarizability α is not very high the molecules can’t be fixed on the filter surface or inside of the hole. From the above, one will notice that the selective attraction of analyte to the filter’s holes facilitates optical spectral analysis with relatively low background emission. Thus, one of the important features of AMOS is its ability to trap the bio-molecules based on their polarizabilities, and furthermore the larger the polarizability of the molecule, the better the trapping performances of the AMOS as an analytical device. One important thing to note is that the number of big bio-molecules trapped into each micro-hole (spot) is discrete; correspondingly the molecular performances in individual spots, say, their dipole moments or scattering cross-sections, are also discrete (proportional to the number of molecules). If in the results obtained there are non-discrete numbers of molecules, then this means that there are some unusual features of trapped molecules. Thus, analyzing all spots (pixels), say, via measuring of response on electrical fi eld scanned over fi lter cross-section or spectrum of scattered light, we will be able to obtain additional information about the properties of the analyte molecules. Defi nitely in the case when there is a relatively low concentration of those molecules, vast majority of micro-holes (pixels) don’t have analyte molecules. Nevertheless there may be present spots with analyte molecules. In other words, if analyte molecules are available in any solution, they will also be trapped in certain micro-holes, which make their recognition easier. The most suitable instrument to execute the investigation of analyte molecules in these considered conditions is Raman scattering of the laser light. Raman lines unlike fl uorescence will provide more information about the geometrical shape of the molecule. In particular, the organic compounds containing the same elementary groups of molecules, but orientated in different directions with respect to the rest of molecules, have different Raman vibration and rotational modes. Recognition of these modes allows for the discovery of organic compounds containing certain defects, for example, a minor group of molecules originated from a parent bio-molecule, but bonded to a molecular body in an unusual manner (from the side or from anoth
er end). The processing of obtained results may uncover details of reproductive living organisms including cancer and other diseases at earlier stages. The use of molecular scattering, in particular so-called Rayleigh wing scattering where the broadening of frequency spectrum and polarization features of scattered light characterize the molecules, is extremely important. Also, we have mentioned the possibility of using surface enhanced Raman scattering (for some molecule complex having a conduction band). And fi nally regarding Coherent Anti-Stokes Raman Scattering, this method may provide a very high sensitivity especially if we know in advance what kind of analyte is expected. According to our estimation, this means achieving the sensitivity of about one part per trillion.

Technology of AMOS fabrication

The most suitable technology for AMOS fabrication is based
on UV picoseconds lasers where the lasers are used in two technology processes: 1) Controllable hole drilling in metal and dielectrics with an accuracy of ~0.1 μm. 2) Controllable removal of thin metal layers deposited on dielectric substrates.
Using these two technology processes we are able to deposit a thin metal fi lm on a fused silica surface and drill 5 to 10 μm holes though that fi lm at a depth of about 10100 μm. In the fi rst technology process, due to its relatively high aspect ratio (the depth of the hole to its diameter) the short wavelength UV laser beam (λ = 266 nm or 213 nm) touches the walls of the hole and is refl ected towards the hole bottom initiating light-plasma interaction causing the spatial instability of material removal process. The result of that instability is the development of a pattern with a period of about λ/2n (here n is refl ective index of dielectric substrate).

In the second technology process, a high energy UV laser beam focused by cylindrical optics ablates the surface of the substrate with a thin metal fi lm deposited on one of its faces. The period of patterning is about 5 – 1 20 μm. From another face of the same plate the patterning is to be made with the stripes having orthogonal orientation. Then the UV laser beam drills holes between the stripes in each spot. The voltage given to the structured metal fi lm provides an electrical fi eld up to several kV/cm. That voltage can be variable from spot to spot allowing variable electrical fi eld over the surface. Thus, one can control conditions and create traps for molecules that have different polarizabilities.

Features of AMOS and ways to the improve AMOS design

The performance of the AMOS depends on diameter and surface density of the holes, their depth, the ability of making spikes on the bottom, type of dielectric material and the polarization of the laser beam making the holes. We can separate several types of AMOS technology: 1. Simplifi ed version of AMOS based on metal-dielectric-metal “sandwich”. In that case the holes through metal and dielectric are made from opposite sandwich faces. The holes from one face are blind, and holes from opposite side are drilled only through metal. It allows collecting analytes on one side and provides optical spectral analysis on the other; 2. A Structured version of AMOS based on the ability to control voltage inside of individual holes; 3. A variation of the metaldielectric-metal “sandwich” with simpler (no blind) holes. The main goal of such a device is fi ltering molecules with high polarizability and ensuring their separation in a gaseous or liquid mixture from other molecules.

One new and interesting structures made by the UV laser ablation is the set micro circuits around the vias or blind holes (see Figure 2) The electrical current passes these micro-circuits with a typical diameter of 10-20 μm excites the magnetic fi eld inside of the hole. This creates a fi eld that selectively attracts atoms, molecular complex and micro-particles with high magnetic polarizability. This method can open up new possibilities for analysis of metal micro-particles in oil or cleaning oil from such micro-particles, by trapping of atoms of less-common metals, for isotope separation, etc. One could call it “Ω-technology” because each micro-circuit looks like the Ancient Greek letter Ω.

Expected applications and market prospects for AMOS

There are several important applications associated with the use of active fi lters including the fi ltering of analytes which would prove useful to the bio-medical analysis, pharmaceutical industry. The fi lters can also be adapted for ion mobility spectrometry, in separating selected molecules or organic compounds such as those required for the growing of crystals including organic ones. The fi lters can assist in water extraction from oil to provide improved performance of gasoline or kerosene. Likewise they can be used for high effi cient water extraction from air or alternatively air extraction from water.

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Author thanks Dr. Sabatino Nacson, Adolf Kleiner and Mrs. Maria Konchalina for fruitful discussion and help.
Dr. Guerman Pasmanik is the lead researcher for Amos Photonics Research Technology, a small technology company, developing and selling optical equipment globally.

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