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Development of chemical and biochemical sensors is the current need of the society. In this report, we present our investigation on the development of a label-free fibre-optic biosensor based on evanescent wave absorbance to detect the presence of analytes such as bacteria, virus and some clinically important proteins. A simple UV-LED (280 nm) and photodetector combination along with a fibre probe was used for developing cost-effective, user-friendly and field applicable device. To improve the sensitivity of the detection technique, the probe design was modified and the U-bent probe was fabricated by simple procedure. Further, to overcome the problems for using UV light source in the fibre, the localized surface plasmon resonance of noble metal nanoparticles at visible wavelength was exploited as a sensing medium for the biochemical reactions. Our systematic studies in this regard presented in this communication may bring the excitement for developing the waterborne pathogen detection device for house-hold as well as field applications.

The localized surface plasmon resonance (LSPR) of silver nanoparticles during immobilization on the U-bent sensor fiber probe has been studied. The gradual wavelength shift of the peak resonance due to the increment of the number of nanoparticles on the fiber surface has been attributed to the plasmonic coupling effect. Since the immobilization is random, the shift of the resonance as well as the broadening of the absorption spectrum depends on the distribution of the nanoparticles. To get the insight of the cause of this spectral behavior, we have incorporated the two-particle coupling approximation and assumed that the particles form pairs with its nearest neighbors. The distribution of pairs was obtained from the microdimensional images of fiber surfaces. The collective response from the contributions of these pairs was calculated using the theoretical framework of discrete dipole approximation (DDA). The shifts of the resonance as well as the broadening of the calculated spectra were found to be in excellent agreement with those of the experimentally observed spectra. Thus, this methodology can be utilized for tuning and enhancing the sensitivity by exploiting the plasmonic coupling between the noble metal nanoparticles for designing and fabricating the LSPR based optical sensor.

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