Nanosensor Fabrication 41 Fabrication of Nanofiber Probes

This section discusses the protocols and instrumental systems involved in the fabrication of fiber-optic nanoprobes. Two methods are generally used for preparing the nanofiber tips. The so-called "heat-and-pull" method is the most commonly used. This method consists of local heating of a glass fiber using a laser or a filament, and subsequently pulling the fiber apart. The shape of the nanofiber tips obtained depends on controllable experimental parameters such as the temperature and the timing of the procedure. The second method, often referred to as "Turner's method," involves the chemical etching of glass fibers. In a variation of the standard etching scheme, the taper is formed inside the polymer cladding of the glass fibers. The description of these fabrication methods is given in the following section.

The fabrication of nanosensors requires techniques capable of making reproducible optical fibers with a submicron-size diameter core. Figure 2 illustrates the experimental procedures for the fabrication of nanofibers using the heat-and-pull procedure [21]. Since these nanoprobes are not commercially available, investigators have to fabricate them in their own laboratories. Our laboratory uses the heat-and-pull procedure, which consists of pulling a larger silica optical fiber to produce the tapered nanotip fiber using a special fiber-pulling device (Sutter Instruments P-2000). This method yields fibers with submicron diameters. One end of a 600 /m silica/silica fiber is polished to a 0.3 /m finish with an Ultratec fiber polisher. The other end of the optical fiber is then pulled to a submicron length using a fiber puller. A scanning electron microscopy (SEM) photograph of one of the fiber probes fabricated for studies is shown in Figure 3. The distal end of the nanofiber is approximately 30 nm.

To prevent light leakage of the excitation light on the tapered side of the fiber, the sidewall of the tapered end is then coated with a thin layer of metal, such as silver, aluminum, or gold (100 nm thickness) using a thermal evaporation metal coating device. The coating procedure is schematically illustrated in Figure 4 [21]. The metal coating is only for the sidewall, and leaves the distal end of the fiber free for subsequent binding with bioreceptors. The fiber probe is attached on a rotating plate inside a thermal evaporation chamber [3, 19, 21]. The fiber axis and the evaporation direction formed an angle of approximately 45°. While the probe is rotated, the metal is allowed to evaporate onto the tapered side of the fiber tip to form a thin coating. The tapered end is coated with 300-400 nm of silver in a Cooke Vacuum Evaporator system using a thermal source at 10-6 torr. Since the fiber tip is pointed away from the metal source, it remains free from any metal coating. With the metal coating, the size of the probe tip is approximately 250-300 nm (Fig. 5).

Chemical etching using HF is the basis of the second method for fabricating optical nanofibers. There are two variations of the HF etching method: one method involving the use of a mixture of HF acid and organic solvent, known as Turner etching [25], and the second using only HF, known as tube etching [26-28]. In the Turner method, a fiber is placed in the meniscus between the HF and the organic overlayer, and over time, a small tip is formed, with a smooth, large-angled taper. This large taper angle provides much more light at the tip of the fiber, which in turn greatly increases the sensitivity of the nanosensors. The reproducibility of the Turner method is strongly affected by environmental parameters such as temperature and vibration because of the dual chemical nature of the etching process. To avoid this problem, a variation of the etching method was developed, which involves a tube-etching procedure. In this procedure, an optical fiber with a silica core and an organic cladding material is paced in an HF solution. The HF slowly dissolves the silica core, producing a fiber with a large taper angle and nanometer-sized tip. The HF begins first to dissolve the fiber's silica core, while not affecting

Nanofiber Tip Diameter ~ 40 nm

100 nm

Pulling

Pulling

Clamps

Laser Beam

Optical Fiber

Laser Heating Pulling Tapered Fiber

Pulling

Nanofiber Tip

Figure 2. The "heat-and-pull" method for the fabrication of nanofibers.

Figure 3. Scanning electron photograph of an uncoated nanofiber. The size of the fiber tip diameter is approximately 40 nm.

Laser

Tapered

Quartz tip

Quar tip

Tapered

Quar tip

Thermal evaporation

Metal source

Figure 4. Procedure for coating a fiber with silver. (A) Uncoated fiber tip. (B) Fiber coating using metal evaporation over the rotating tip. (C) Coated fiber. Adapted from [21], T. Vo-Dinh et al., Nature Biotech-nol. 18, 76 (2000). © 2000, Nature Publishing Group.

Metal coating

Quartz tip

Metal coating

Thermal evaporation

Metal source

Figure 4. Procedure for coating a fiber with silver. (A) Uncoated fiber tip. (B) Fiber coating using metal evaporation over the rotating tip. (C) Coated fiber. Adapted from [21], T. Vo-Dinh et al., Nature Biotech-nol. 18, 76 (2000). © 2000, Nature Publishing Group.

the organic cladding material. This unaffected cladding creates localized convective currents in the HF solution, which causes a tip to be formed. After some time, more of the silica core is dissolved, until it emerges above the surface of the HF solution. At this juncture, the HF is drawn up the cladding walls via capillary action, and runs down the silica core to produce a nanometer-sized tip. By varying the time of HF exposure and the depth to which the fiber is submerged in the HF solution, one can control the size of the fiber tip and the angle of the taper. Once the tip has been formed, the protruding cladding can be removed either with a suitable organic solvent or by simply burning it off. Nanotips fabricated using etching procedures, which can be designed to have sharp tips [13], have been used in NSOM studies to detect SERS-labeled DNA molecules on solid substrates at the subwavelength spatial resolution [11-13].

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