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Biomedical Optics & Medical Imaging

Micro- and nanocapillary glass technology for optical biosensing

A new class of photonic-crystal fiber with a hollow core and various lattice modifications may be an important component for biosensing applications.
30 June 2009, SPIE Newsroom. DOI: 10.1117/2.1200906.1631

Biosensor are devices used for detection of analytes that combine biological and physicochemical detector components. They include a sensitive biological element, which can be a biological material (e.g., tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, or nucleic acids), a biologically derived material, or a biomimic. We can create these sensitive elements through biological engineering. They also contain a transducer or detector, which works in a physicochemical manner (e.g., optically, piezoelectrically, or electrochemically) to transform the signal from the interaction of the analyte with the biological element into another signal that can be more easily measured and quantified. The third part consists of the associated electronics or signal processors that are primarily responsible for displaying the results in a user-friendly way.1 We are developing single-use capillary biosensors based on photonic-crystal fibers (PCFs) for use with blood samples to diagnose diseases such as AIDS (acquired immune deficiency syndrome) and hepatitis.

Figure 1. Cross section of a photonic-crystal fiber. The air-containing holes that run along the length of the fiber are important for the formation of photonic band gaps.

Photonic crystals (PCs) are artificial periodic dielectric media. A PCF is a 2D PC with a regularly repeating 3D structure on a scale of order optical wavelengths.2 When PCs include structural defects, they can have photonic band gaps that halt the propagation of light in a certain frequency range. We can create photonic band gaps using glass-air structural cladding, which has a high reflection factor. Such waveguides allow us to increase the efficiency of nonlinear optical interactions, to control the dispersion of waveguide modes, shift the wavelength of zero dispersion in the visible part of the spectrum, and establish an efficient refractive index for the cladding, and to transmit electromagnetic radiation with a high degree of localization.3–5

Light in PCFs behaves similarly to Bragg diffraction from crystalline materials, except that the effect can be seen in the visible regime. PCFs are unique since they can, in theory, enable extremely low-loss light transmission because of the photonic band-gap effect. That is, certain frequencies of light will not propagate in the material, and thus we can potentially construct efficient waveguides with novel properties for communications and sensing applications, such as biosensors. In PCFs, the light-guiding mechanism is fundamentally different from that of conventional optical fibers since it is due to a regular pattern of holes running along the length of the fiber (see Figure 1) rather than variations in the fiber's refractive index. This periodic arrangement of air-filled holes in the cladding is the main factor in the formation of waveguide modes.

The two main types of PCFs contain either glass or hollow cores.6,7 A PCF with a glass core has a high index of refraction and its cladding has the structure of a 2D PC with a low effective index of refraction. In this type of PCF, light propagates in the glass core (similarly as in a simple optical fiber) according to the law of total reflection. A PCF with a hollow core (see Figure 1)8,9 has a low index of refraction compared with that of its cladding. This type of fiber does not simulate a simple fiber, however, since the law of total reflection does not apply (because the index of refraction of the cladding is greater than that of the hollow core). However, certain light modes propagate through the hollow core due to the photonic band gaps, which localize and guide light.10,11

PCF manufacturing is based on classical micro- and nanocapillary glass technology.12 First, precision, thin-walled, round glass capillaries are stacked in a bundle, for example in the shape of a hexagon. The capillaries are heated to the temperature of glass softening and drawn to create hexagonal polycapillary structures. Several of these are then assembled into an hexagonal package and drawn again. We repeat this procedure until the desired variable periodicity of air channels is achieved. If we remove or replace one or several capillaries in the center of an initial package, we can create a core made of glass or air in the final structure. This technology allows us to manufacture micro- and nanostructured PCFs from different types of glass and produce PCFs with a great variety of configurations, stacking types (hexagonal, squared, and triangular, among others), and defect-arrangement topologies.

PCF-based capillary biosensors can be made of glass, quartz, silicon, polymers, or other substrates. These sensors contain channels and holes (in the cladding and/or core) that can be filled with samples and reagents. We have created hollow-core PCFs (see Figure 1) and filled them with various biological materials, including riboflavin (vitamin B2), cobalamin (vitamin B12), chlorophyll, erythrocytes (red blood cells), whole blood, and DNA. We used optical radiation in the hollow core to measure the optical spectra from different concentrations. Our detection limit was 10ng/ml.

We plan to investigate blood-filled PCFs with hollow cores to monitor changes in blood components, such as erythrocytes and leukocytes (white blood cells) associated with various diseases. We also intend to carry out immunological reactions in hollow-core PCFs to detect specific interactions with DNA molecules. The most important factor in increasing separation efficiency is minimization of the assay volume, which we can only realize with micro- and nanostructural elements. Therefore, the development of selective sensing devices for the complex analysis of biological fluids is extremely important to improve medical studies.

Julia Skibina
Saratov State University
SPE Nanostructured Glass Technology Ltd.
Saratov, Russia

Julia Skibina received her MS and PhD degrees in optics in 2000 and 2003, respectively, from Saratov State University. She is now a researcher in the Optics and Biomedical Physics Department and heads the Laboratory of NanoBioPhotonics in the Research-Educational Institute of Optics and Biophotonics. In addition, she is a scientific research leader at SPE Nanostructured Glass Technology Ltd.