2.5. Experimental Setup and Methodology

Figure 7 depicts the experimental setup used in the laboratory, which consisted of a broadband optical source (1520–1620 nm), the sensor, and an OSA (Yokogawa AQ6370C, Tokyo, Japan) connected with a circulator. The OSA has a resolution of 20 pm and can measure wavelengths ranging from 600 to 1700 nm. To ensure stability and to prevent contact between the sensor and the walls of the test sample tubes, the sensor was carefully inserted into a capillary tube. The samples were handled on a lifting platform, which permitted only vertical movement while maintaining the sensor in a fixed position.

Schematic configuration of the experimental setup.

Glucose aqueous solutions with concentrations ranging from 25 to 200 mg/dL were prepared through a dilution method in a laboratory-controlled environment at approximately 21 °C. This range was purposely chosen to mimic the glucose concentration levels found in human saliva. A magnetic stirrer (NAHITA, magnetic stirred, model n°690/1) was used to homogenize the aqueous solutions with glucose (G8270-100 g) supplied by Sigma Aldrich. The sensor was immersed in these solutions, and the optical spectra were captured with an OSA. The OSA was configured to operate on a linear scale with a 0.5 nm resolution. For each measurement, the tip was immersed for 150 s before processing the corresponding data.

For the RI characterization of the sensor, the glucose aqueous solutions were prepared in the same manner, and the RI was determined with an Abbe refractometer. The solutions ranged from 0.1 mg/mL to 50 mg/mL, which corresponded to a RI range between 1.3380 RIU and 1.3853 RIU.