Development of calcium alginate gel sphere-based SERS substrates for minimally invasive sampling and identification of indigo on the surface of simulated murals and aged imitation cultural relic samples.

Abstract

Pigments in painted cultural relics serve as critical historical evidence, offering significant research value for understanding historical contexts and cultural development processes. However, the pigments, especially organic pigments (or dyes), have progressively degraded over time, resulting in severe fading that challenges their sensitive identification. To address this issue, we developed an innovative sampling methodology combined with surface-enhanced Raman spectroscopy (SERS) analysis for identifying indigo pigment in painted cultural relics. The methodology employs calcium alginate gel beads doped with either gold nanobipyramid (Au NBPs/CAGBs) or hybrid Au-Ag nanoprisms (Ag NPrs/Au NBPs/CAGBs), enabling minimally invasive sampling of indigo through controlled interactions with the mural surface. Subsequent SERS analysis of the retrieved substrates demonstrated enhanced molecular identification capabilities compared to conventional in situ non-destructive methods. Furthermore, microscopic examination and color difference quantification (ΔE < 1.5) revealed no observable surface alterations on tested mural samples before and after sampling, which confirmed the method's minimal invasiveness. Finally, the approach was successfully applied to imitation heritage objects, including Tang Tri-color ("Lady Spring Festival") replica ceramics and bottle-type simulated ceramic artifacts before and after aging, achieving effective sampling and pigment identification. This study establishes a novel paradigm for sensitive pigment analysis in cultural heritage conservation, with promising implications for future relic restoration and protection strategies.