K. Dukenbayev

Near-field scanning optical microscopy using polymethylmethacrylate optical fiber probes

We report the first use of polymethylmethacrylate (PMMA) optical fiber-made probes for scanning near-field optical microscopy (SNOM). The sharp tips were prepared by chemical etching of the fibers in ethyl acetate, and the probes were prepared by proper gluing of sharpened fibers onto the tuning fork in the conditions of the double resonance (working frequency of a tuning fork coincides with the resonance frequency of dithering of the free-standing part of the fiber) reported earlier for the case of glass fibers. Quality factors of the probes in the range 2000-6000 were obtained, which enables the realization of an excellent topographical resolution including state-of-art imaging of single DNA molecules. Near-field optical performance of the microscope is illustrated by the Photon Scanning Tunneling Microscope images of fluorescent beads with a diameter of 100nm. The preparation of these plastic fiber probes proved to be easy, needs no hazardous material and/or procedures, and typical lifetime of a probe essentially exceeds that characteristic for the glass fiber probe.

Scanning near‐field optical microscope based on a double resonant fibre probe montage and equipped with time‐gated photon detection

We discuss scanning near‐field optical microscope based on original double resonant montage of a fibre probe onto the tuning fork and proprietary electronics capable of fast and precise measurements of the resonant frequency and the quality factor of sensor dithering. Special emphasis is given on the pulsed excitation/gated detection of optical signal. This option as well as the possibility of fast scanning facilitates a lot the problem of single fluorescence centres detection. To illustrate the performance of this microscope, we present first true single‐molecule fluorescence resonance energy transfer scanning near‐field optical microscope images of single CdSe nanocrystals on glass slide surface and observation of an optical ‘pseudoresolution’ of densely packed 100‐nm‐diameter transfluorescent spheres in noisy conditions.

Scanning near-field optical microscopy-based study of local dynamics of receptor-ligand interactions at the single molecule level

A scanning near-field optical microscope (SNOM)—based modification of the method to study the dynamics of single molecule receptor—ligand interactions exploiting the fluorescence imaging by total internal reflection fluorescence microscopy is introduced. The main advantage of this approach consists in the possibility to study the single molecule interaction dynamics with a subwavelength spatial resolution and a submillisecond time resolution. Additionally, due to the much smaller irradiation area and some other technical features, such a modification enables to enlarge the scope of the receptor—ligand pairs to be investigated and to improve the temporal resolution. We briefly discuss corresponding experimental set up with a special accent on the SNOM operation in liquid and present some preliminary results of related investigations.

Detection of various Thrombin concentrations using etched fiber Bragg gratings functionalized with DNA aptamer

The response of etched fiber Bragg grating (EFBG) functionalized with 29-mer DNA aptamer to the different concentrations of Thrombin protein has been investigated. Etched FBGs are an efficient technology for detection of refractive index, and have been demonstrated also for biosensors applications. EFBGs have a simpler manufacturing approach comparing to other methodologies and are based on a low-cost device; their fabrication can be achieved by simple chemical etching, without requiring fusion splicing. During the test we assessed its feasibility for small variations of thrombin concentrations (10μg/ml, 20μg/ml, 40μg/ml and 80μg/ml). In particular, we performed experiments of chemical etching with hydrofluoric acid, which progressively depletes the fiber cladding exposing the core to the outer medium. Additionally, unstriped not etched FBGs were also used as a control for temperature pattern compensation. Before functionalization, EFBG was calibrated with different sucrose and ethanol solutions that validated the sensitivity to refractive index change. EFBG was further silanized with 3-Aminopropyl-triethoxysilane (APTES) in order to immobilize Thrombin binding aptamer on the silica surface of the fiber. The change of Bragg wavelength when functionalized EFBG is exposed to different concentrations of Thrombin using Micron Optics Hyperion si255-x55 sensing system was demonstrated. A small yet detectable sensitivity (several tens of nanomolars) even between small protein variations allows hypothesizing a future use of this kind of functionalized fiber for biosensor development.

An etched chirped fiber Bragg grating for measurement of refractive index and temperature pattern

In this work, partially etched chirped fiber Bragg grating (pECFBG) for the real-time multi-parameter measurement of temperature and refractive index is proposed. The sensor is fabricated by wet-etching a portion of a linearly chirped FBG with linear chirp profile. Obtained CFBG has two active areas: the unetched part of the grating that can be used either as a uniform temperature sensor, or to detect thermal gradients experienced through the grating length; the etched part, besides having a similar thermal sensitivity, is exposed to refractive index sensing through the variations of external refractive index. Overall, the pECFBG structure behaves as a compact sensor with multi-parameter capability, that can both measure temperature and refractive index on the same grating, but also spatially resolve temperature detection through the grating section. The results have been validated through both a model and experimental setup, showing that the mutual correlation algorithm applied to different spectral parts of the grating is able to discriminate between uniform and gradient-shaped temperature profiles, and refractive index changes. The reflected spectra showed a clear correlation between the RI change of the surrounding media and spectral shift with temperature variations.

Partially etched chirped fiber Bragg grating (pECFBG) for joint temperature, thermal profile, and refractive index detection

In this work, a partially etched chirped fiber Bragg grating (pECFBG) is introduced, as a compact sensor for multi-parametric measurement of temperature, thermal gradients over the active length, and refractive index. The sensor is fabricated by wet-etching a portion of a 14-mm linearly chirped FBG with linear chirp profile. The resulting device has two active areas: the unetched part of the grating (2 mm) can be used either as a uniform temperature sensor, or to detect thermal gradients experienced through the grating length by means of a spectral reconstruction technique; the etched part (12 mm), besides having a similar thermal sensitivity, is exposed to refractive index sensing through the introduction of a sensitivity to external refractive index. Overall, the pECFBG structure behaves as a compact sensor with multi-parameter capability, that can both measure temperature and refractive index on the same grating, but also spatially resolve temperature detection through the grating section. The results have been validated through both a model and experimental setup, showing that the mutual correlation algorithm applied to different spectral parts of the grating is able to discriminate between uniform and gradient-shaped temperature profiles, and refractive index changes.

Luminescence studies on photostimulable phosphor CsBr:Eu2+

Photostimulable phosphor CsBr:Eu2+ is prepared through a solid-state reaction. The effect of annealing atmosphere on photoluminescence and photostimulated luminescence was investigated. Optimum luminescence intensity was obtained when samples were prepared at 350°C in air atmosphere. The effect of irradiation of gamma and neutron had resulted in the formation of optically stimulable traps with different trap depths. The role of monovalent and divalent dopants on thermoluminescence dosimetric properties has been discussed.

Scanning near-field optical microscope based on double-resonant fiber probe montage and its operation in liquids

The operation of the scanning near-field optical microscope based on the double-resonant montage of a fiber probe onto the tuning fork (working frequency of the latter, that is 32 kHz, coincides with the second resonance frequency of the bending oscillations of the free standing part of a fiber beam) in liquid is reported. It is shown that due to the peculiarities of the probe montage (initially large, around 3,000 - 5,500 quality factor of the dithering and long projection of the fiber beam beyond the tuning fork body) and microscope electronics, this SNOM is very fit to work in liquids. Quality factor of the sensor drops down to the values around 300 - 600 when the probe tip is submerged on the depth of 0.2 - 0.3 mm, thus remaining large enough to enable high quality imaging with rather small acting force value laying in the subnanoNewton region. We also discuss the joint liquid recipient construction which connects the liquid cell containing a sample with the large water reservoir via a flexible tube. This reservoir is placed onto separate Z-stage and hence the water level in the cell can be regulated independently from the sample position which facilitates the SNOM operation a lot.