Quantitative analysis of sialic acid on erythrocyte membranes using a photothermal biosensor.

The quantitative analysis of sialic acid (SA) at an erythrocyte membrane is becoming an important clinical parameter in diagnosing cancer and diabetes. In spite of such clinical importance, there are only a few, very expensive, time consuming and complicated quantifying methods established. To solve this problem, we demonstrate a novel and direct measurement technique for SA exposed to the cell membrane using a photothermal biosensing system in which the hemoglobin molecules in the erythrocyte absorb a specific wavelength of photons (532 nm) and convert it to a temperature change. For measuring the quantity of SA, we first modified the sensor surface of a micro-scaled thermometer using phenylboronic acid (PBA) containing a self-assembled monolayer (SAM) to capture the SA-expressing erythrocytes. Second, the sensor surface was thoroughly washed, and when more SA was expressed, tighter association of erythrocytes to the biosensor was expected. Thirdly, blood sample changes in temperature, heated by the 532 nm wavelength laser, were measured by the bottom layer's micron sized platinum thermometer. The temperature changes from the erythrocytes captured on the sensor surface could be estimated by the amount of SA expressed on the erythrocyte membrane. This novel SA analysis system can solve the problems raised by conventional methods such as multiple enzyme reactions and a time consuming process. We expect that this system will help provide a new tool in the quantitative analysis of SA expression level for the diagnosis of diabetes and cancers.