Gennadii E. Kotkovskii

A laser desorption ion-mobility increment spectrometer for detection of ultralow concentrations of nitro compounds

A compact instrument that implements for the first time a method for analyzing substances, in which laser desorption of analyzed molecules and laser ionization of an air sample are combined with the ionmobility increment spectroscopy, is described. Pulse radiation of the fourth harmonic of a portable (2.6 kg) GSGG: Cr3+: Nd3+ laser (λ = 266 nm) is used. The detection limit of the developed laser desorption spectrometer of the ion-mobility increment is 40 pg for trinitrotoluene (TNT), and the linear dynamic range for TNT is 0.1–20.0 ng. The results from detection of nitro compounds are presented: trinitrotoluene, cyclotrimethylene trinitramine (RDX), and octogen (HMX). It is shown that laser desorption of nitro compounds from a metal is accompanied by their decomposition on the surface and emission of fragments. An ion signal is obtained for nitro compounds that were ionized outside the spectrometer.

A laser ion-mobility spectrometer

The process of ion-packet broadening in a longitudinal laser spectrometer of ion mobility is studied. The contributions of the diffusion, Coulomb, and other broadening mechanisms are compared. The resolution of the developed spectrometer was measured (R ∼ 45) in atmospheres of both purified air and pure nitrogen. The dependence of the spectrometer resolution on the drift voltage was studied. The recorded spectra of a number of molecules of explosives with an extremely low pressure of saturated vapors indicate a high sensitivity of the developed spectrometer (no worse than 10−14 g/cm3).

Photoluminescence of CdSe/ZnS quantum dots in a porous silicon microcavity

It is known that manufacturing and applications of photonic crystals is currently an area of much interest. One of the focuses of special attention in this area is various microcavity (MC) devices. Porous silicon is one of the most promising materials for manufacturing such devices because it is simple to prepare, its optical parameters are precisely controllable, and it has an enormous surface area. This allows to inject different kinds of luminophores into porous silicon MC devices. Apparently, semiconductor quantum dots (QDs) are among the most interesting of them. QDs are characterized by a wide absorbance spectrum, large absorption cross-section, high quantum yield, and excellent photostability. To date, there have been few studies on QD injection into porous silicon photonic structures. In addition, many structures used lack the desired characteristics; the depth of QD penetration also remains a question. This is the first study to analyze the photoluminescence spectrum and kinetics of QDs in a high-quality porous silicon MC. A drastic narrowing of the luminescence spectrum has been observed after QD injection. We have found that the MC morphology considerably affects the penetration of QDs. The kinetics of photoluminescence has also been investigated. Measurements have shown a decrease in the QD characteristic photoluminescence decay time after QD injection into a porous silicon MC compared with the QD photoluminescence decay time in a toluene solution. However, we have not observed a significant difference between the photoluminescence decay times of QDs in an MC and in single-layer porous silicon.

Intracavity laser field asymmetric ion mobility spectrometer for highly efficient detection of organics

A new optical multipass scheme for laser field asymmetric ion mobility spectrometry (FAIMS) has been proposed and implemented. The scheme and the gas-analyzer system developed include both intracavity multiphoton laser ionization and FAIMS detection of the analyte. The computation and experimental results show multifold growth of the laser intensity in the FAIMS ion source, as well as the amplitude of ion signal. The gas phase detection threshold achieved is estimated to be 10?14 to 10?15?g?cm?3. Further development of the proposed approach is discussed.

A laser spectrometer of field-asymmetric ion mobility

A method for analyzing a substance has been experimentally tested. The method combines the field-asymmetric ion mobility spectrometry and laser ionization of molecules under atmospheric pressure. Pulsed radiation of the fourth harmonic of an YAG: Nd3+ laser (λ = 266 nm) and a spectrometer with a cylindrical analysis camera were used. The results of detecting nitrocompounds—trinitrotoluene, cyclotrimethylenetrinitramine (hexogen, RDX), etc.—are presented. The experimental detection limits of the spectrometer are 5 × 10−15 g/cm3 (cyclotrimethylenetrinitramine) and ≤3 × 10−15 g/cm3 (trinitrotoluene).

Surface ligands affect photoinduced modulation of the quantum dots optical performance

Changes of optical properties of the solutions of CdSe/ZnS quantum dots (QDs) covered with the trioctylphosphine oxide (TOPO) ligands under the pulsed ultraviolet (UV) laser irradiation are observed. The fluorescence quantum yield (QY) of QDs decreases by more than an order of magnitude when the radiation dose approaches 2 × 10-15 J per particle. This process is accompanied by a blue shift of both fluorescence and the first excitonic absorption peaks. The fluorescence quenching becomes less pronounced when the overall TOPO content in the solution is increased. When ТОРО ligands are replaced with n-hexadecylamine (HDA), QY and spectral properties are not changed at the same irradiation conditions. We assume that the above changes of the optical properties are associated with photooxidation of TOPO ligands by excited QD. Such process is less probable for the HDA ligand due to its different energy structure.

Formation of anions of nitroaromatic compounds in gases during UV laser irradiation

The mechanism of formation of negative ions of trinitrotoluene, dinitrotoluene, and para-nitrotoluene in the gas phase under the action of laser irradiation was studied using ion mobility spectrometry. The gas mixture was ionized by the fourth harmonic radiation of a YAG:Nd3+ laser. The irradiance was varied within 2–5 MW/cm2. It was demonstrated that, although the test compounds are characterized by a high cross section of absorption of laser radiation the formation of their anions occurs largely through the multiphoton ionization of organic admixtures present in the gas mixture with the subsequent attachment of electrons formed. In a nitrogen medium, the attachment of electrons occurs directly, whereas in air, the O2− immediate plays an important role. Laser radiation causes the dissociation of the molecules under study, especially marked for para-nitrotoluene.

Resonance energy transfer in nano-bio hybrid structures can be modulated by UV laser irradiation

A method for targeted variation of the radiation properties of quantum dots (QDs) to control the efficiency of resonance energy transfer in nanocrystal assemblies and nano-bio hybrid materials has been developed. The method is based on strong ultraviolet (UV) laser irradiation of QDs and allows the extinction and luminescence spectra to be controlled and the luminescence quantum yield and decay kinetics to be varied. Water-soluble QDs have been synthesized and used for analyzing the effect of energy transfer from semiconductor nanocrystals on the photocycle of the photosensitive protein bacteriorhodopsin (bR) in bR–QD complexes. The UV irradiation mode has been selected in a way permitting the modulation of QD optical parameters without modification of their structure or physico-chemical properties. It is concluded that the QD interaction with bR accelerates its photocycle, but this acceleration is determined by electrostatic interactions, rather than Forster resonance energy transfer from QDs to bR. The method of UV laser irradiation of fluorescent semiconductor QDs has proven to be an efficient technique for variation of nanocrystal optical properties without affecting their structure, as well as for fine modulation of the energy transfer processes in the nanocrystal assemblies and nano-bio hybrid materials.

Silicon photonic structures with embedded polymers for novel sensing methods

At present time research and development of a new generation of optical sensors using conjugated polymers, in particular sensors of explosives are actively underway. Nevertheless, the problems of the sensitivity, selectivity, and stability of such sensors are still of great interest. One of the ways to solve the problem is the creation of luminescence sensors based on photonic crystals with a high specific surface area, which have significant sorption ability and allow to effective modulate emission properties of luminophores. In this paper, porous silicon microcavities with embeded organic polyphenylenevinylene- (PPV) and polyfluorene- (PF) type polymers were created. It was shown that polymer infiltration in porous silicon microcavities leads to modification of their luminescence properties, which is expressed in narrowing of the emission spectrum and changing of its directional pattern. It was demonstrated that such structures exhibit sensitivity to saturated vapors of trinitrotoluene. The structures proposed can be treated as a basis for development of new type of sensors used for detection of vapors of nitroaromatic compounds.

An excimer-based FAIMS detector for detection of ultra-low concentration of explosives

A non-contact method for analyzing of explosives traces from surfaces was developed. The method is based on the laser desorption of analyzed molecules from the surveyed surfaces followed by the laser ionization of air sample combined with the field asymmetric ion mobility spectrometry (FAIMS). The pulsed radiation of the fourth harmonic of a portable GSGG: Cr3+ :Nd3+ laser (λ = 266 nm) is used. The laser desorption FAIMS analyzer have been developed. The detection limit of the analyzer equals 40 pg for TNT. The results of detection of trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX) are presented. It is shown that laser desorption of nitro-compounds from metals is accompanied by their surface decomposition. A method for detecting and analyzing of small concentrations of explosives in air based on the laser ionization and the FAIMS was developed. The method includes a highly efficient multipass optical scheme of the intracavity fourthharmonic generation of pulsed laser radiation (λ = 266 nm) and the field asymmetric ion mobility (FAIM) spectrometer disposed within a resonator. The ions formation and detection proceed inside a resonant cavity. The laser ion source based on the multi-passage of radiation at λ = 266 nm through the ionization region was elaborated. On the basis of the method the laser FAIMS analyzer has been created. The analyzer provides efficient detection of low concentrations of nitro-compounds in air and shows a detection limit of 10-14 - 10–15 g/cm3 both for RDX and TNT.