**3.2 Nanoparticles**

The optical fiber sensor structures also utilize the immobilization of nanoparticles over the sensing region to enhance the sensitivity by means of introducing the concept of localized surface plasmon resonance (LSPR) phenomenon.

*SEM image of SERS probe of a tapered optical fiber sensor structure: (A) tapered optical fiber, and (B) distribution of nanoparticles over the fiber [54].*

#### **Figure 10.**

*Analysis of tapered optical fiber sensor structure: (a) diameter analysis, and (b) transmitted spectra [55].*

### *Fiber Optics - Technology and Applications*

The characterization of nanoparticles can be done by using UV-spectrophotometer and by observing their distribution through transmission electron microscopy (TEM) images. TEM is also a technique that employs a focused beam of electrons to visualize the distribution of particles in nanometer dimensions. The UVspectrophotometer provides the resonance peak of the nanoparticles through the absorbance spectrum and is useful to confirm their initial synthesis. The resonance peak of all the nanoparticle is different and usually falls in the visible spectrum of white light. The peak resonance wavelength in absorbance spectrum of gold and zinc oxide nanoparticles appears at 519 nm and 370 nm for the particles size of less than 15 nm and 50 nm, respectively, and illustrated in **Figure 11**. The initial confirmation of nanoparticles can be carried forward to analyze their distribution which usually done by using capturing the microscopic image by using TEM. The TEM images of gold and zinc nanoparticles are illustrated in **Figure 12**. From the TEM images it can be concluded that the distribution of nanoparticles is uniform and easily visible. Further, the morphology of the nanoparticles or layered nanomaterials is also an important factor to assure the synthesis of nanoparticles,

**Figure 11.** *Absorbance spectrum of nanoparticles: (a) gold, and (b) zinc oxide [55].*

**Figure 12.** *TEM images of nanoparticles: (a) gold, and (b) zinc oxide [55].*

*Application of Fiber Optics in Bio-Sensing DOI: http://dx.doi.org/10.5772/intechopen.99866*

**Figure 13.** *AFM image of zinc oxide nanoparticles [55].*

and can be done by taking the images by using atomic force microscopy (AFM). An AFM image of zinc oxide nanoparticles is illustrated in **Figure 13**.

#### **3.3 Biomolecules**

The preparation of samples of targeted biomolecules is also an important factor which helps in increase the performance of sensor probe. The analysis of samples of the targeted biomolecules can be done by preparing them in different pH base solutions. The similar kind of approach has been used to analyze the validity of ascorbic acid (AA) samples and illustrated in **Figure 14**. The performed test was basically done to check the solubility of artificial samples of AA [55]. The analysis was done by dissolving the artificial sample of AA in different pH solutions and the samples of lowest and highest concentration were prepared. Then, the peak resonance wavelength was measured for the highest and lowest sample concentration and their difference is plotted with respect to each pH solution. For the reported work, it was concluded that the AA samples are highly soluble in phosphate buffer solution (PBS) whose pH is about 7.4.

#### **3.4 Sensing analysis**

The sensing analysis of the sensor probe can be done in several steps. The first step is to sense all the samples through the sensor probe. For each measurement

**Figure 14.** *Solubility test of ascorbic acid samples in different pH solutions [55].*

respective peak resonance wavelength can be recorded which is useful to plot the autocorrelation coefficient of the sensor probe. The autocorrelation curve is used to evaluate the linearity, regression coefficient, sensitivity and resolution of the sensor. Then, the analysis of sensor can be done in terms of stability, reusability, reproducibility and selectivity.

The stability of any optical fiber biosensor can be evaluated by measuring the base solution through a sensor probe more than 10 times. The results can be plotted in terms of number of measurements and peak resonance wavelength. Then, the standard deviation (SD) can be evaluated to observe the stability and for a good sensor SD is usually less than 0.1.

The reusability is an another important parameter to analyze the performance of optical fiber sensor. Reusability can be evaluated by measuring two different concentration of bio-molecules through the same sensor probe. The measurement of any concentration should need to be performed three times to attain higher accuracy. The sensor head must need to be rinsed properly after all the measurements by using base solution. Then, the results can be plotted in terms of recorded spectra or in terms of peak absorbance wavelength. The resonance wavelength for similar concentration should be same for each measurement to attain the higher reusability.

The reproducibility is also an another important factor to analyze the performance of any optical fiber sensor. The reproducibility test can be done by measuring the similar concentration of bio-samples through one sensor probe. The measurement must need to be done at least 5 times to attain the higher accuracy. The outcome of the measurements can be plotted in terms of recorded spectra and in terms of peak resonance wavelengths. The higher reproducibility of the probe can be claimed if the peak resonance wavelength for all the measurements is similar.

The selectivity or specificity of the optical fiber sensor is a crucial factor of an optical fiber biosensor which helps in to remove the interference of other biomolecules present in real liquid samples of human bodies. The higher specificity of any optical fiber sensor can be attained by functionalizing the sensor head with appropriate enzyme which oxidize only in the presence of targeted bio-samples. For instance, the AA oxidized only in the presence of ascorbate oxidase.

### **4. Conclusions**

This book chapter presents a brief discussion about the different optical fiber geometries which have been utilized for the development of different optical fiber sensors and biosensors. The mostly common used geometry of optical fibers is cladding less, tapered, interferometers, and gratings. The second section of the chapter presents the brief discussion about the presence of biochemical markers usually used in bio-sensing applications. The detection of biochemical markers is generally done in two phases such as in gas phase and in liquid phase. The third section of the chapter presents a brief discussion of the characterization and sensing process of the optical fiber based biosensors. The characterization of optical fiber sensor is done by capturing the images through TEM, SEM and AFM. The analysis of nanoparticles can be done by recording the absorbance spectrum by using UVspectrophotometer. The sensing analysis of the optical fiber sensor can be done by performing the stability, reusability, reproducibility and selectivity test of the sensor probe. The optical fiber based biosensors are emerging in current era and can be employed in various health care applications.
