Probing specific gravity in real-time with graphene oxide plasmonics

Specific gravity (SG), the ratio of the density of a substance to the density of a reference material, is a standard indicator of the concentration of an analyte in a given solution. SG is routinely used for product quality assessment in the food industry. However, currently available commercial SG meters, such as hand-held refractometers and density meters, are highly sensitive to humidity and temperature, and do not allow real-time measurements. For these reasons, SG detection is often time-consuming which leads to unwanted interruptions in food manufacturing processes. Therefore, highly sensitive, label-free, and real-time sensors for the detection of SG are urgently needed for food quality control. In this context, we develop a graphene oxide (GO)-coated gold (Au) surface plasmon resonance (SPR) sensor, for the first time, to measure the SG of food samples in real-time. The SG values of sample solutions are correlated with the refractive indices (RI) of these solutions, which are captured by the SPR measurements, with a sensitivity of 10⁵ SPR response units. Moreover, the use of a GO coating provides a strong enhancement of plasmonic resonances due to its optoelectronic properties, doubling the sensitivity of SPR response units per RI unit (2 × 10⁵) when compared to conventional Au SPR chips (1 × 10⁵). We also validate our sensor performance by measuring the SG in real food samples. Our results demonstrate a highly sensitive, efficient, high throughput, and reproducible approach for SG measurements in food industry settings, and open new opportunities to utilize improved SPR sensor technology for many other label-free analytical sensing applications.