The lab-on-a-chip approach is being applied to the development of glucose meters for the early diagnosis and prevention of diabetes. An optofluidic device has enabled detection of glucose in solution, requiring only a tiny droplet of sweat. A team of researchers from The Hong Kong Polytechnic University and Zhejiang University has reported integrating fiber optic glucose sensors into a microfluidic chip, a technique that could be used to create portable, high-performance, low-cost devices for the noninvasive measurement of glucose levels. (a) Schematic design of the optical fiber biosensor integrated microfluidic chip: 1 are two inlets, 2 is outlet, 3 is a spiral mixture, 4 are optical fibers and 5 is the embedded LPG sensor. (b) The mode coupling and optical resonance in the long-period grating biosensor. (c) Working mechanism of the multilayer film for glucose sensing and signal enhancement. Courtesy of Yin et al./Biomedical Optics Express, a publicaton of The Optical Society (OSA). While electrochemical glucose biosensors can be integrated into microfluidic channels to develop easy-to-handle, low-cost, and portable microfluidic chips, electroactive interference problems often appear in electrochemical sensors. Researchers from Hong Kong Polytechnic University say fiber optic sensors offer a solution to this issue, thanks to their immunity to electromagnetic interference. By combining a fiber optic biosensor with a microfluidic chip, professor A. Ping Zhang and colleagues created an interference-free optofluidic device for ultrasensitive detection of glucose levels. Their method involved fabricating an optical fiber long-period grating (LPG) with a period of 390 μm within a small-diameter optical fiber with a cladding diameter of 80 μm. Such fiber optic devices induce strong codirectional mode coupling through a resonant scattering process, the researchers said. The resulting central wavelength is very sensitive to changes of the refractive index (RI) of the surrounding media via the evanescent field of optical fiber cladding mode. To transform the fiber optic RI sensor into a glucose sensor, the team selected glucose oxidase as a sensing material that would react with glucose in solution. To support the sensing film and magnify refractive index change, a pH-responsive multilayer film of polyethylenimine and polyacrylic acid (PAA) was deposited on the side surface of the LPG sensor before immobilization of the sensing film. The PEI/PAA multilayer film "surveils the oxidation of glucose with the gluclose oxidase catalyst and responds to the reaction via swelling or contracting," Zhang noted. Experimental results revealed that the new fiber optic sensor is very sensitive on its own and can detect glucose oxidase concentrations as low as 1 nM. After integration into the microfluidic chip, the team reported that the sensor’s performance was further improved in terms of detection range and response time. Notably, no significant loss of biomolecular activity was observed during the experiments, implying that the layer-by-layer self-assembly technique renders a robust electrostatic absorption of glucose oxidase within the sensing film. The research was published in Biomedical Optics Express, a publication of The Optical Society (OSA) (doi: 10.1364/boe.7.002067).