Cultural relics serve as tangible remnants of human activities and invaluable cultural heritage passed down from ancient to modern societies. They encapsulate diverse aspects of human life and culture, encompassing social systems, economic activities, technological advancements, and ideological frameworks. Among these relics, paper artifacts stand out as crucial carriers of ancient art, culture, and historical narratives, representing irreplaceable cultural reservoirs. Under suitable environmental conditions such as optimal temperature and humidity, fungi secrete enzymes to hydrolyze these nutrients, facilitating their growth and reproduction. Notably, Trichoderma longibrachiatum, a species within the genus Trichoderma, can thrive, posing a significant threat to paper-based cultural relics. Current methods for detecting mold on paper cultural relics predominantly employ offline and online detection techniques. However, these detection methods often necessitate the use of large analytical instruments, which can potentially damage the artifacts and are time-intensive. Additionally, some methods require direct contact with the artifacts, posing further risks of harm. In this study, we propose the development of a reflective concave stepped inclined lens fiber optic sensor designed specifically for detecting mold growth on the surface of paper cultural relics. This sensor aims to effectively identify and monitor the presence and proliferation of Trichoderma longibrachiatum on paper artifacts, offering promising applications in mold control for paper-based cultural heritage preservation.

The reflective concave stepped inclined lens fiber optic sensor design features a central incident fiber and an arrangement of 6 and 12 receiving fibers in the inner and outer layers, respectively. The end faces of these receiving fibers adopt both flat and inclined plane structures. Firstly, the detection principle of fiber optic sensors is established, and the influence of sensor structural parameters (such as incident fiber radius, receiving fiber radius, and receiving fiber end face tilt angle) on the detection sensitivity of the sensors is explored. Next, based on the simulation outcomes, the optimal performance fiber optic sensor is fabricated using large core diameter single clad quartz fibers, with core and cladding materials comprising pure quartz and silicone rubber, respectively. The fibers possess core diameters of 400 and 300 μm, cladding diameter of 40 μm, a numerical aperture of 0.22±0.02, an operating temperature range spanning -50 to 250 ℃, and a spectral transmission range of 200?1100 nm. Experimental validation involves cultivating Trichoderma longibrachiatum on tissue paper and rough edge paper substrates lacking ink or dye, using a glycerol nutrient solution and various fungal spores to achieve the required concentrations of fungal spore suspensions. The growth of Trichoderma longibrachiatum is characterized using a super depth of field three-dimensional microscope, and the online nondestructive detection of the growth process of Trichoderma longibrachiatum is carried out using the fabricated fiber optic sensor.

The simulation results highlight the critical impact of outer receiving fiber inclination angle and incident/receiving fiber radii on sensor performance. With the outer receiving fiber at an inclination of 70° (Fig. 4) and the radii of the incident fiber and the receiving fiber of 200 and 150 μm respectively (Fig. 5 and Fig. 6), while keeping other parameters constant, the sensor receives the maximum light intensity. Experimental findings demonstrate that by positioning the optimal reflective fiber optic sensor 2.5 mm from the mold growth on the sample surface (Fig. 8), we can capture the maximum light intensity. Characteristic absorption peaks of Trichoderma longibrachiatum on tissue paper and rough edge paper, both before and after ink dyeing, are consistently observed at 270 nm (Fig. 10 and Fig. 12), with absorbance linearly correlating with mold growth height. Characterization using a super depth of field 3D microscope reveals denser Trichoderma longibrachiatum growth on ink-dyed paper surfaces, fueled by gum and organic matter within the ink, providing rich growth substrates for fungi. Sensor sensitivity for detecting Trichoderma longibrachiatum on cotton paper and rough edge paper before and after ink dyeing is quantified at 9.3×10^-4 AU/μm (Fig. 10, cotton paper before ink dyeing), 10.4×10^-4 AU/μm (Fig. 12, rough paper before ink dyeing), 10.4×10^-4 AU/μm (Fig. 14, cotton paper after ink dyeing), and 11.4×10^-4 AU/μm (Fig. 14, rough paper after ink dyeing), respectively. Compared to single-layer reflective fiber sensors with flat fiber end faces used for detecting Aspergillus niger and Aspergillus fumigatus, our sensor demonstrates approximately twice the detection sensitivity.

This study introduces a novel reflective fiber optic sensor for detecting the growth of Trichoderma longibrachiatum on paper cultural relics. Our experimental results demonstrate the sensor’s capability for online, non-contact detection and precise identification of fungal growth on paper artifacts both before and after ink dyeing processes. The sensor is straightforward to manufacture and offers effective support for mold prevention and control in cultural heritage conservation. Additionally, it broadens the application of fiber optic sensing technology in the field of cultural relic protection, contributing to the advancement of preservation technologies.