Elsevier

Biosensors and Bioelectronics

Volume 30, Issue 1, 15 December 2011, Pages 328-332
Biosensors and Bioelectronics

Short communication
Tapered plastic optical fiber-based biosensor – Tests and application

https://doi.org/10.1016/j.bios.2011.09.024Get rights and content

Abstract

Cells detection is crucial in microbiological analysis of clinical, food, water or environmental samples. However, currently employed methods are time consuming. Plastic optical fiber (POF) biosensors consist in a viable alternative for rapid and inexpensive scheme for detection. In order to study the sensitivity of tapers for microbiological detection, geometric parameters are studied, such as the taper waist diameter since the formation of taper regions are the key sensing element in this particular type of sensors. In this study, a series of POF taper sensors were prepared using a specially developed tapering machine, and the dispersion of geometric dimensions is evaluated, aiming to achieve the best tapering characteristics which will provide a better sensitivity on the sensor response. The fiber tapers that presented the finest results were those constructed in U-shaped (bended) configurations, with taper waist diameters ranging from 0.40 mm up to 0.50 mm. These fiber tapers were used as the main section of the monitoring device, and when chemically treated as immunosensors for the detection of bacteria, yeast and erythrocytes.

Highlights

► Study of the waist diameter influence in tapered plastic optical fiber (POF) applied in biosensing. ► U-shaped POF applied in biosensing. ► Comparison of the biosensor's response to bacterial cells, yeasts and blood cells.

Introduction

Fiber-optic sensors have been widely investigated as sensors for chemicals, and physical properties. Among them, tapered sensors in glass multimode optical fiber are well established in applications such as gas detection, spectroscopy and detection of biological systems (Ferreira et al., 2001, Leung et al., 2008, Chua et al., 2011). However, this technique has not been broadly applied to polymeric optical fiber (POF) to be used as biological sensors, but mainly as physical sensors (Merchant et al., 1999, Fixe et al., 2004).

Fiber-optic sensors can be combined with antibodies which are able to recognize and bind to a defined antigen, which induces immediate environmental changes, such as the refraction index (RI), around the probe containing the antibody. Also, the large diameter of POFs facilitates installation and alignment, unlike their glass counterparts in which a few microns misalignment results in heavy losses. Other well-known advantages are the efficient light coupling owing to the large numerical aperture, high ductility, low cost of production and easy handling.

The main objective of this study is to determine the optimum dimensions of the POF taper that best suits a biosensor probe for cells detection. We analyzed the taper performance as an RI sensor making the numerical aperture and bending radii constant whilst the waist diameter varied for both straight and U-shaped fibers. Several tapers were manufactured by means of heating and traction, using a piece of equipment developed for this goal. The tapers were then evaluated and used as an immunosensor system, which detected bacteria, yeast and lamb erythrocytes.

The operating principle of the taper is based on evanescent field (EF), given by the well known equation (Mizaikoff, 2003):dp=λ2πnco2sin2θncl2where dp is the depth of penetration of the EF, λ is the wavelength of transmitted light, θ is the incident angle of the light ray at the core/cladding interface, nco and ncl are, respectively, the RI of the core and the cladding. The EF is enclosed into the cladding, but by decreasing the fiber diameter by the tapering process, the EF becomes exposed outside the fiber, and consequently it interacts with the measurand.

In literature, a variety of studies approach the application of straight silica optical fibers in the manufacture of tapered sensors; nevertheless the employment of U-shaped tapers in POF can afford several advantages, such as increased sensitivity, smaller taper length, economic use of reagents, an improved handling and fabrication, and a greater mechanical resistance (Ferreira et al., 2001, Frazão et al., 2008, Nazaré et al., 2011).

Section snippets

Materials and methods

The fibers used in this work are multi-mode, step index POF type Mitsubishi Eska GH 4001. They are POFs with a pure poly-methyl-methacrylate (PMMA) core and a doped low-index-of-refraction fluoropolymer as cladding. The fibers present 980 μm of core diameter and a 20-μm-thick cladding.

Due to the softening temperature of the PMMA be around 70–80 °C, it is very easy to draw it into a taper. Although it has been reported that in a hot drawing method on finished POF the fiber breaks before any

Results and discussion

The taper sensor was capable to measure 1.33–1.39 or 0.06 RIU as a full scale range, being 1.33 for pure water without any bacteria and 1.39 for a suspension saturated with bacteria. For measurements in the real world the samples will be somewhere between these two figures.

Fig. 2 shows the normalized output voltage of selected tapers under the influence of different RIs, for straight tapers and U-shaped tapers.

As expected, the output light power of the fiber containing the taper is inversely

Conclusion

The POF technology employed in the construction of the biosensor fulfills the requirements of ease handling and simple construction. It was observed that the geometry of the taper interferes in the behavior of the sensor. Both straight and U-shaped tapers were sensitive to the surrounding environment when produced in diameters between 0.40 and 0.50 mm. However, U-shaped tapers have shown better sensitivity and are more practical as they fit easier in smaller sample flasks. The manufacturing of

Acknowledgments

The authors would like to thank Petrobras for its financial support.

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