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Design and Fabrication of Fiber-Optic Nanoprobes for Optical Sensing

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ABSTRACT

This paper describes the design and fabrication of fiber-optic nanoprobes developed for optical detection in single living cells. It is critical to fabricate probes with well-controlled nanoapertures for optimized spatial resolution and optical transmission. The detection sensitivity of fiber-optic nanoprobe depends mainly on the extremely small excitation volume that is determined by the aperture sizes and penetration depths. We investigate the angle dependence of the aperture in shadow evaporation of the metal coating onto the tip wall. It was found that nanoaperture diameters of approximately 50 nm can be achieved using a 25° tilt angle. On the other hand, the aperture size is sensitive to the subtle change of the metal evaporation angle and could be blocked by irregular metal grains. Through focused ion beam (FIB) milling, optical nanoprobes with well-defined aperture size as small as 200 nm can be obtained. Finally, we illustrate the use of the nanoprobes by detecting a fluorescent species, benzo[a]pyrene tetrol (BPT), in single living cells. A quantitative estimation of the numbers of BPT molecules detected using fiber-optic nanoprobes for BPT solutions shows that the limit of detection was approximately 100 molecules.

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SEM images of silver-coated nanofibers under different angles: a 40° and b 20°.
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Figure 5: SEM images of silver-coated nanofibers under different angles: a 40° and b 20°.

Mentions: Due to the inclination angle, the tip ends are shadowed from evaporation when the fiber tips are tilted away from the source. The effect of shadow evaporation angle is illustrated in Figure 4. A nanoaperture was formed on the tip end for optical excitation. The size of the nanoaperture is related to the angle between fiber axis and evaporation direction. For example, if the angle is less than 20°, most of the fibers are fully covered with metal and no aperture is visible using SEM. On the contrary, if the angle is higher than 30°, a larger area of the distal end of the fiber tip will be exposed (Figure 5). The optimal angle of inclination can be determined through characterization of nanoapertures under SEM. The SEM can determine the actual size of the tip aperture. However, having a nanometer-sized aperture does not guarantee a good near-field probe. The tip aperture, even though it may appear small on the SEM, could be a result of aluminum over-coating, and hence not be a functional light aperture for actual measurements. Therefore, a functional scan is necessary to reveal the near-field effect from the probe. Near-field scanning optical microscope (NSOM) enables functional analysis of the nanosensor probe by performing a scan on a standard sample, e.g., a Fischer pattern. The standard sample usually consists of patterns with size less than the diffraction limits (0.5 λ) that can be determined by an atomic force microscope. The nanosensor probe was attached to an NSOM system working as an NSOM probe. Typically, the aperture of the probe roughly determines the resolution of the image. In other word, the image quality thus represents the quality of the probe.


Design and Fabrication of Fiber-Optic Nanoprobes for Optical Sensing
SEM images of silver-coated nanofibers under different angles: a 40° and b 20°.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211233&req=5

Figure 5: SEM images of silver-coated nanofibers under different angles: a 40° and b 20°.
Mentions: Due to the inclination angle, the tip ends are shadowed from evaporation when the fiber tips are tilted away from the source. The effect of shadow evaporation angle is illustrated in Figure 4. A nanoaperture was formed on the tip end for optical excitation. The size of the nanoaperture is related to the angle between fiber axis and evaporation direction. For example, if the angle is less than 20°, most of the fibers are fully covered with metal and no aperture is visible using SEM. On the contrary, if the angle is higher than 30°, a larger area of the distal end of the fiber tip will be exposed (Figure 5). The optimal angle of inclination can be determined through characterization of nanoapertures under SEM. The SEM can determine the actual size of the tip aperture. However, having a nanometer-sized aperture does not guarantee a good near-field probe. The tip aperture, even though it may appear small on the SEM, could be a result of aluminum over-coating, and hence not be a functional light aperture for actual measurements. Therefore, a functional scan is necessary to reveal the near-field effect from the probe. Near-field scanning optical microscope (NSOM) enables functional analysis of the nanosensor probe by performing a scan on a standard sample, e.g., a Fischer pattern. The standard sample usually consists of patterns with size less than the diffraction limits (0.5 λ) that can be determined by an atomic force microscope. The nanosensor probe was attached to an NSOM system working as an NSOM probe. Typically, the aperture of the probe roughly determines the resolution of the image. In other word, the image quality thus represents the quality of the probe.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

This paper describes the design and fabrication of fiber-optic nanoprobes developed for optical detection in single living cells. It is critical to fabricate probes with well-controlled nanoapertures for optimized spatial resolution and optical transmission. The detection sensitivity of fiber-optic nanoprobe depends mainly on the extremely small excitation volume that is determined by the aperture sizes and penetration depths. We investigate the angle dependence of the aperture in shadow evaporation of the metal coating onto the tip wall. It was found that nanoaperture diameters of approximately 50 nm can be achieved using a 25° tilt angle. On the other hand, the aperture size is sensitive to the subtle change of the metal evaporation angle and could be blocked by irregular metal grains. Through focused ion beam (FIB) milling, optical nanoprobes with well-defined aperture size as small as 200 nm can be obtained. Finally, we illustrate the use of the nanoprobes by detecting a fluorescent species, benzo[a]pyrene tetrol (BPT), in single living cells. A quantitative estimation of the numbers of BPT molecules detected using fiber-optic nanoprobes for BPT solutions shows that the limit of detection was approximately 100 molecules.

No MeSH data available.