We report an innovative biosensing technique using a focused laser beam for the fabrication of a polymer nanostructure and the detection of its nanoscale growth. A nanoaggregate structure is formed by focusing a single beam of a continuous-wave (cw) green laser beam on an o-phenylenediamine (o-pD) solution dropped on a glass substrate. The backreflection intensity of the focused laser beam shows a temporal oscillation, whereas the size of the aggregate monotonously increases. Simple calculations based on the Fresnel equation qualitatively reproduce the experimental results, indicating that the backreflected laser oscillation occurs because of the interference between two beams reflected at the front and the back surfaces of the aggregate. Because the growth speed of the aggregate depends on light absorption by the oxidized o-PD, the backreflection oscillation curve can be used to monitor the oxidative reaction in the solution. We apply this phenomena to optical biosensing based on the oxidation of o-PD by the peroxidase enzyme reaction. A reliable quantification of glucose can be achieved by simply focusing a single laser beam and detecting its reflection intensity in a manner similar to the optical pickup unit of optical storage drives.

We propose a new methodology for optical biosensing with drop-casting fabrication of sensor chips and irradiation/detection of a single laser beam. Glucose sensor chips were fabricated by simply depositing a solution, including o-phenylenediamine (oPD), glucose oxidase (GOD), and horseradish peroxidase (HRP), on a glass substrate. A sample solution was dropped on the sensor chip and the concentration was detected by measuring the reflection intensity of the laser beam focused on the chip. A surface deformation induced by the laser irradiation on the chip was observed by electron microscopy, demonstrating that this sensor chip converts the enzyme reactions into morphological changes using the assistance of the laser beam. The mechanism of the laser-induced surface deformation was investigated by testing the effect of sodium azide and fluorescence spectroscopy.