Smartphone-based dynamic measurements of electro-optically modulated lossy-mode resonance and its biosensing applications

Abstract

In this work, we discuss an application of a smartphone-based spectrometer for dynamic analysis of optical response of an electro-optically modulated lossy-mode resonance (LMR) optical fiber sensor. The LMR was achieved when a section of a multimode optical fiber core was coated with a fluorine-doped tin oxide (FTO) thin film. The FTO, thanks to its electrical properties, has been simultaneously applied as an electrochemical electrode. It has been shown that when an electrical potential is applied to the obtained dual-domain sensor, the distribution of charge carriers in the FTO changes and it can be dynamically measured optically using the smartphone-based spectrometer with a time resolution below 0.5 s. Moreover, when charge carriers' accumulation is induced at the FTO surface, the changes on the sensor surface, such as biomaterial binding, can be identified with the device. The optical label-free biosensing performance of the system has been verified using streptavidin and green fluorescent protein (GFP) as receptor and target, respectively. The outcome of optical and electrical measurements, acquired simultaneously, were compared. It has been found that the system can detect concentration of GFP as low as 1 ng/mL. It must be noted that the smartphone-based dynamic optical measurements were possible with only the device, a low-cost detachable adapter, and a custom-made software application. Electrical measurements using a smartphone with such low-cost modifications are hardly possible. The setup can be further simplified using just a battery for setting the potential difference, thus establishing electro-optical modulation of the sensor. Such a system may be considered as suitable for a broad spectrum of sensors and their label-free biosensing applications.