1：Experimental Design and Method Selection:

The study involved modifying a gold thin film with a nanocrystalline cellulose (NCC)-based material to enhance the sensitivity of an SPR optical sensor for Zn2+ detection. The SPR system was based on the Kretschmann configuration.

2：Sample Selection and Data Sources:

Zn2+ solutions of various concentrations (0.01–10 ppm) were prepared by diluting a 1000 ppm standard zinc solution with deionized water.

3：01–10 ppm) were prepared by diluting a 1000 ppm standard zinc solution with deionized water. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: A custom-built SPR optical sensor system consisting of a He–Ne laser beam, optical chopper, polarizer, stepper motor, prism, photodiode detector, and lock-in amplifier. Chemicals were purchased from Sigma Aldrich.

4：Experimental Procedures and Operational Workflow:

The SPR signal was investigated using both modified and unmodified gold thin films in contact with deionized water and Zn2+ solutions. The SPR curve was taken after Zn2+ was injected and left for 10 minutes in the cell.

5：Data Analysis Methods:

The sensitivity of the SPR sensor was determined by deriving the gradient of the resonance angle shift against the Zn2+ concentrations graph. The binding affinity was calculated using the Langmuir isotherm model.