**7. Assembly of living photonic probe by OFTs for bio-probing and detection**

Recently, using OFTs, a fully biocompatible living photonic probe for subwavelength probing of localized fluorescence from leukemia single-cells in human blood has been created [13]. The high-aspect-ratio living photonic probe based on a yeast cell (1.4 μm in radius) and Lactobacillus acidophilus (*L. acidophilus*) cells (2 μm in length and 200 nm in radius) is formed at the tip of a tapered optical fiber by optical trapping (**Figure 8a**). In the assembly, the authors have precisely moved the fiber to approach a yeast cell. Benefited from the spherical shape of the yeast,

the trapping laser beam was focused into a tiny region and exerted a strong optical force on a *L. acidophilus* cell that traps it behind the yeast. With this alignment, the trapping laser beam propagates through the *L. acidophilus* cell and exert an optical force on other *L. acidophilus* cells, which were orderly bound together by optical binding effect and finally formed the living photonic probe. **Figure 8b** shows a formed probe assembled with a yeast and five *L. acidophilus* cells. To view the light propagation, after assembly of the probe, the trapping laser remained on, and a visible illumination light was launched into the probe. **Figure 8c**–**e** show the illumination light propagating along the tapered fiber. At the output port of the probe, a tiny light spot was observed with full width at half maximum (FWHM) of 345, 282, and 248 nm for the illumination wavelengths of 644, 532, and 473 nm, respectively.

As a benefit of the highly focused effect of the living cells, the living photonic probe can also deliver subwavelength excitation light to biological samples, and detect optical signals with a subwavelength spatial resolution. Moreover, within human blood, selective probing of the localized fluorescent signals on single leukemia cell surface can be realized via the precise manipulation of the living photonic probe. Due to the high biocompatibility and resolution, these photonic probes hold great promises for biosensing and imaging in bio-microenvironment. Furthermore, the living photonic probe can be integrated in the available near-field scanning optical microscopy, functioning as a biocompatible and non-invasive scanning probe for near-field imaging of living cells. **Figure 9**, as an example, shows the use of the living photonic probe in probing localized fluorescence of leukemia cells in human blood [13]. **Figure 9a**–**d** shows the spot excitation capability by manipulating the living photonic probe to approach the cell membrane. As shown in **Figure 9a**,

#### **Figure 8.**

*Assembly of living biophonic probes for bio-probing [13]. (a) Schematic illustration for assembly of living photonic probe by OFTs. (b) Image of a formed living photonic probe. (c)-(e) images showing light propagation along the formed living photonic probes. Light spots can be observed at the end of each photonic probes.*

*Optical Fiber Tweezers for the Assembly of Living Photonic Probes DOI: http://dx.doi.org/10.5772/intechopen.98845*

#### **Figure 9.**

*Living photonic probe for single-cell probing and detection [13]. (a-d) Excitation and detection of local fluorescence from a leukemia cell in human blood by manipulating the living photonic probe to scan a cell. (e,f) Flexibility testing of the probe by pushing the probe against the leukemia cell membrane. (g-i) Touching and punching of the cell directly using a tapered optical fiber tip, to compare the flexibility of the living photonic probe.*

there was no fluorescent signals when the distance between the living photonic probe and the surface of a leukemia cell was 3 μm. But the fluorescent signal was detected with a distinct fluorescent spot observed at the cell membrane when the probe was in contact with the cell (**Figure 9b**). The fluorescent signals at other locations were also detected by scanning the cell surface via precisely moving the probe (**Figure 9c** and **d**). Flexibility and deformability of the living photonic probe have also been demonstrated by interacting with biospecimens. As shown in **Figure 9e** and **f**, the living photonic probe was forced against a leukemia cell, then the living photonic probe was bent to an angle θ of 15° without puncture to the cell membrane. A certain degree of the deformability of the probe has no obvious influence on the scanning capabilities. For comparison, the authors pushed a fiber probe with a sub-micrometer tip, which is commonly used in scanning probe microscopes, against the leukemia cell (**Figure 9g**). As a result of the relatively large dimension and rigid structure, the fiber probe could easily insert into the cell (**Figure 9h**), and rupture the cell membrane (**Figure 9i**).

#### **8. Conclusions**

In this chapter, we reviewed the trapping and assembly of biological cells using OFTs, and finally extended the trapping capability for the assembly of living photonic probes such as cell-based biophotonic waveguides, cell-based periodical structures, cell-based structures in vivo, and living photonic probe for bio-probing and detection. These living photonic probes exhibit extremely high biocompatibility for further biological applications in bio-environment. As a benefit of the

light focusing ability of the cells, the biocompatible living photonic probes allow the trapping, manipulation, sensing, and diagnostics in vivo. Furthermore, the living photonic probes assembled using OFTs offer an biophotonic bridge between optical and biological worlds with natural materials. With the advantages of its biocompatibility, the living photonic probes are envisioned to provides a new opportunity for direct sensing and detection of biological signal and information in biocompatible microenvironments.
