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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.

No MeSH data available.


FIB-etched nanoprobe with aperture diameter of 200 nm.
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Figure 7: FIB-etched nanoprobe with aperture diameter of 200 nm.

Mentions: The first FIB milling process involved placing the metal-coated optical fiber tips horizontally, i.e., orthogonal to the focused ion beam and then cutting the tips (both the tapered silica fiber and the metal over-coating) such that an aperture could be developed at the tip. Milling of the nanoapertures using this process has an advantage that it is not time-consuming as several tips placed adjacent to each other can be cut with the same beam raster, it gives reliable nanoprobes with well-defined nanoapertures of circular geometry, and the length of the optical fiber nanoprobes can be longer, which can make coupling of light into the optical fibers easier. The second process involved positioning the fiber tips such that they faced the focused ion beam and then carrying out the milling of the nanoapertures at the tip. Although this process enables fabrication of nanoapertures of different geometries and sizes in a very controllable manner, it limits the length of the fiber-optic probe as only a certain length of the optical fiber probe can be placed vertically in the Hitachi FB2100 focused ion beam milling machine. By milling with a focused ion beam, an aperture with controllable shape and diameter as small as 200 nm was achieved (Figure 7). The angle of evaporation is not necessary in FIB, therefore reducing the chance of pin-hole formation. A clean aperture free from grains also facilitates the subsequent functionalization of bioreceptor molecules on the fiber distal end for biosensing applications. FIB processing is a promising technique in nanoprobe fabrication in addition to laser pulling and chemical etching.


Design and Fabrication of Fiber-Optic Nanoprobes for Optical Sensing
FIB-etched nanoprobe with aperture diameter of 200 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 7: FIB-etched nanoprobe with aperture diameter of 200 nm.
Mentions: The first FIB milling process involved placing the metal-coated optical fiber tips horizontally, i.e., orthogonal to the focused ion beam and then cutting the tips (both the tapered silica fiber and the metal over-coating) such that an aperture could be developed at the tip. Milling of the nanoapertures using this process has an advantage that it is not time-consuming as several tips placed adjacent to each other can be cut with the same beam raster, it gives reliable nanoprobes with well-defined nanoapertures of circular geometry, and the length of the optical fiber nanoprobes can be longer, which can make coupling of light into the optical fibers easier. The second process involved positioning the fiber tips such that they faced the focused ion beam and then carrying out the milling of the nanoapertures at the tip. Although this process enables fabrication of nanoapertures of different geometries and sizes in a very controllable manner, it limits the length of the fiber-optic probe as only a certain length of the optical fiber probe can be placed vertically in the Hitachi FB2100 focused ion beam milling machine. By milling with a focused ion beam, an aperture with controllable shape and diameter as small as 200 nm was achieved (Figure 7). The angle of evaporation is not necessary in FIB, therefore reducing the chance of pin-hole formation. A clean aperture free from grains also facilitates the subsequent functionalization of bioreceptor molecules on the fiber distal end for biosensing applications. FIB processing is a promising technique in nanoprobe fabrication in addition to laser pulling and chemical etching.

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.