Label—Free Integrated Optical Biosensors for Multiplexed Analysis

The need for real time, fast and frequent monitoring in health-care, pharmaceuticals research, environmental monitoring, the food industry, and homeland security has been rising, leading to extensive research in the development of Point of Care (POC) devices for diagnostics. POC devices need to be handy, fast, robust, highly sensitive to sensing analytes, selective against other interferents, and accurate. Since, in the real world scenario, the analytes are typically complex and useful analysis depends on sensing multiple parameters, multianalyte sensing has emerged to help users arrive at concrete conclusions or inferences. Research has been conducted for exploiting a plethora of transduction phenomena, such as electrochemical, impedometric, piezoelectric, magnetic, optical,  etc. Of  these, optical methods of sensing have the upper hand due to their resistance to electromagnetic interference, fast response, ease of parallel analyte sensing, low Limit of Detection (LOD), and, most importantly, easy translation of well-established techniques to in silico devices. Label-free biosensing provides the advantage of simplicity and cost reduction by avoiding complex steps prior to sensing by monitoring analytes in their unmodified state i.e. detecting analytes without any alteration. Further, sensing in ultra-small quantities has been possible due to the concurrent development of MEMS technology. All this has led to the development of Lab on Chip (LOC) devices which perform various separation, and detection, operation and analysis on small chips and are cost effective due to bulk fabrication. LOC devices integrating label—free optical sensing on small chips also give rise to the possibility of multi-analyte assays using small test samples. These facilitate on-site deployment of these devices for different applications. This review will present various optical waveguide-based devices, including those based on optical absorbance, evanescent wave absorbance and surface plasmon resonance. Various configurations of such devices that aid multianalyte on-chip assays will be explored.