Optical sensor design and development for identification of fluids using nanotags

Barcode technology is an important concept for product identification and tracking. However, conventional barcodes cannot be used for identification of liquids. In this thesis, design and characterization of an optical sensor for identification of liquids using nano-tags are presented. The proposed system uses Quantum dots (QDs) for tagging due to their wide absorption and narrow emission spectrum, high quantum efficiency and high resistance to photo-bleaching. Several light collection architectures are explored in the course of this thesis including imaging based system, high numerical aperture lens coupled to a single detector and fiber optic based system. This research is focused primarily on the fiber optic based system due to its distinct advantages in terms of cost, mobility and sensitivity. The proposed fiber optic sensor consists of a fiber optic probe, a detection unit and illumination unit and a signal processing unit. Through detailed optimization of each block in the system, quantum dots can be detected in sub-pM concentrations in transparent liquids such as water. On the other hand, detection of nano-tags in non-transparent liquids is rather challenging due to complex interactions between the medium and the quantum dots. In this thesis research, a novel background subtraction algorithm is implemented to overcome this problem. This algorithm achieved to extract pure quantum dot signal from the readings with a detection limit in the 100 pM range. The optical performance of the system is simulated by using Monte Carlo method for quantum dots dispersed liquids. The simulations results show close correlation with the measurement data. The results obtained are far beyond the state-of-the art and has potential to open new research directions to use quantum dots for in-line monitoring of various kinds of precious liquids.