Za3300

The porous structure of AuNP@cellulose nanofiber paper and AuNP@cellulose nanofiber/pulp paper were observed by a field-emission scanning electron microscope (FE-SEM, SU-8020, Hitachi High-Tech Science Corp., Tokyo, Japan). The AuNP contents were determined by an atomic absorption spectrophotometer (ZA3300, Hitachi High-Tech Science Corp., Tokyo, Japan). Transmittance, reflectance and absorbance spectra were measured by a UV-vis-NIR spectrophotometer (UV-3600 Plus, Shimadzu Corp., Kyoto, Japan) with an integrating sphere attachment (ISR-603, Shimadzu Corp., Kyoto, Japan). Absorption spectra were calculated from the transmittance and reflectance spectra as shown in Fig. S3.† The X-ray diffraction (XRD) patterns were recorded by an Ultima IV (Rigaku Corp., Tokyo, Japan), with a scanning angle (2θ) range of 5–80°. A digital thickness gauge (G2N-255M, Ozaki MFG. Co., Ltd., Tokyo, Japan) was used for the measurement of the thickness.

Soil pH was determined in a 1:5 water-soluble extract. Exchangeable Al³⁺ (exch. Al) was determined by an atomic absorption spectrophotometer (ZA3300, Hitachi, Japan) after 1 M KCl extraction. Total C and N were measured using a dry combustion method with an NC analyzer (SUMIGRAPH NC-TR-22, Sumika Chemical Analysis Service, Ltd., Japan). Inorganic N (NH₄⁺ and NO₃^–) was extracted from 2 g of soil (dry base) with 10 ml of 1 M KCl for 30 min on a shaker, and the suspension was filtered through filter paper (No. 5C, Advantec, Co., Ltd, Japan). NH₄⁺–N in the extract was analyzed using the modified indophenol blue method (Rhine et al., 1998 (link)) with a spectrophotometer (UV-1280, Shimadzu, Co., Ltd., Japan). NO₃^––N in the extract was analyzed by flow injection analysis using a flow-through visible spectrophotometer (S3250, Soma Optics, Ltd., Japan). All soil measurements were expressed on an oven-dry soil weight basis (105°C, 24 h).