Naturally Bred Korean Silkworm Cocoon Showing Strong Green Fluorescence and Its Textile-Based Environmental Sensing Applications

doi:10.21203/rs.3.rs-3357345/v1

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Naturally Bred Korean Silkworm Cocoon Showing Strong Green Fluorescence and Its Textile-Based Environmental Sensing Applications

https://doi.org/10.21203/rs.3.rs-3357345/v1

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Mulberry silkworms have provided many valuable byproducts to humans for 5,000 years. However, numerous possibilities for their utilization still exist owing to the countless varieties of silkworms. Here, we report the application of Yeonnokjam silk (pistachio silk, PS), one of the characteristic silk varieties in Korea, as a smart fabric environmental sensor. Unlike the previously reported extrinsically modified silk, this new silk textile emits fluorescence under external stimuli. PS cocoons were exposed to different concentrations of HCl vapors. Due to the large surface-to-volume ratio of the cocoon textile, a very low concentration (5 ppm) of HCl vapor can be detected by reading the decrease in the fluorescence intensity. At a lethal concentration (3000 ppm), an instantaneous fluorescence response is obtained. Additionally, the fluorescent pigment in PS can respond to temperature and can be extracted using ethanol, indicating that PS exhibits temperature and ethanol-sensing capabilities. The findings of this study prove that natural PS can be used in bio-optical applications.

Physical sciences/Materials science/Biomaterials

Biological sciences/Biotechnology/Biomaterials/Bioinspired materials

Smart textiles are attractive for environmental and biological applications owing to their fascinating functions. Such textile platforms are sensitive to external stimuli, such as pH, pressure, mechanical stress, temperature, light, and electrical signals^1–3. Porous structures with large surface-to-volume ratios, present in smart textiles, enhance the interaction between the textile and external stimuli as well as increase the flexibility and permeability of the textile material^4,5. In addition, incorporating light manipulation functions into a textile platform facilitates wireless, fast, and sensitive sensing operations^6,7. Especially, fluorescent textiles can transform their color and exhibit light-illuminating patterns and pictures upon exposure to light. Therefore, fluorescence-emitting smart textiles provide protection and are suitable for developing flexible displays, sensory systems, and security barcodes^8,9. However, most pigments and dyes used to manufacture fluorescent textiles are highly toxic and environmentally persistent and are thus hazardous to humans and ecosystems^10–12. Although nanomaterials, such as quantum dots and nanowires, have been used in previous studies, the potential risk is still an ongoing issue^13–15. Additionally, natural textile materials are viable for biocompatible and eco-friendly smart textile applications^16,17.

The mulberry silkworm, called Bombyx mori, has been providing numerous byproducts to humans for almost five millennia^18,19. High-quality natural fabrics obtained from silkworm cocoons are well-known products²⁰. Its pupae have been used as a source of proteins and oils, and silkworms have been used in medicines in East Asian countries because of their health-promoting constituents²¹. Interestingly, recent studies have proven that silk proteins, extracted from Bombyx mori cocoons, offer biocompatibility, biodegradability, and optical transparency, along with micro/nanofabrication and other functionalities^22–25. Owing to such advantageous properties, mulberry silkworms have been used to develop various optical and electrical devices for biomedical and environmental applications and have thus opened new possibilities in biomedicine and biology^26–28. However, the diversity of silk species remains hitherto unexplored^29,30. Notably, there are over 340 pure breeds of silkworms in the National Academy of Agriculture Sciences (NAAS), Korea alone³¹. Nevertheless, only selected varieties have been studied thus far, indicating that the potential of silk materials remains largely untapped to date.

Fabrication of fluorescent silk textiles can be accomplished through several means. For example, a dye can be added to a silk solution, which can then be electrospun to form nanofibers^32–34. Else ways, inorganic nanoparticles (quantum dots) when embedded in silk fibers can facilitate colored silk materials³⁵. Feeding methods involving dye-containing diets and generic modifications can be applied to silkworms to achieve inherent modifications³⁶. Although all these methods can be successfully applied to obtain fluorescent silk, each method exhibits certain disadvantages. The feeding method does not ensure dye addition into silk^37,38. Transgenic methods exhibit low efficiencies and are expensive as well as complex^39,40. Post-processing methods involving additional chemical and physical processes are also expensive, and avoiding the use of toxic materials like quantum dots remains a challenge⁴¹. Further, although mulberry leaves can be used to develop fluorescent textiles because of the autofluorescence (natural fluorescence) displayed by its constituent nutrients, the fluorescence intensity is extremely weak for practical applications⁴².

In this study, we observed a strong fluorescence from the pistachio silk (PS) variety and assessed its applicability in textile-based environmental sensing. PS silkworms naturally generate fluorescent pigments, such as flavonoids, which are rich in the sericin cladding layer of the silk fiber. PS cocoon mats exhibit clear green fluorescence under ultraviolet (UV) excitation, and the fluorescence intensity is comparable to those of conventional biological dye-doped silk layers. Since the functionality of flavonoids changes under stimuli, we investigated the feasibility of using a fluorescent PS cocoon fiber as an environmental sensor. The PS cocoon mats were exposed to different environments, including HCl vapors, rising temperatures, and ethanol (EtOH) in water. Under all these conditions, the fluorescence intensity decayed, and the decay rate was proportional to the exposure time and concentration of the analytes. In addition, the analytes were detected with high sensitivity because of the large surface-of-volume ratios of the fiber mats, indicating the feasibility of using natural cocoon fibers, which produce natural pigments, as a sensing platform.

Collection of Silkworm Cocoons. All the silkworm cocoons used in this study were collected from the National Academy of Agriculture Sciences, Korea. The WJ is the F1 hybrid of Jam 123 and Jam 124 PBs (relatively large white cocoons). The GS is the F1 hybrid of Jam 311 and Jam 312 PBs (yellow cocoons). The PS, our target variety, is also the F1 hybrid of Jam 315 and Jam 316 PBs (light-green cocoons).

Preparation of Silkworm Cocoon Samples. Pristine cocoon fibers were prepared by drying and cleaning the cocoons with an N₂-gun. These fibers were then cut into 1 cm x 1 cm pieces and flattened. Degummed fibers were prepared by boiling the dried cocoons for 30 min in an aqueous solution of 0.02 M Na₂CO₃, which removed globular sericin proteins. Then, the extracted silk fibroin fibers were rinsed three times with deionized water for 20 min, stored in 1 cm x 1 cm molds, and dried for 24 h. These dried degummed fibers were then flattened.

An aqueous silk fibroin solution was prepared by dissolving the degummed fibers in a 9.3 M LiBr solution and incubating them in a vacuum oven at 60 ℃ for 4 h. These solutions were dialyzed against deionized water using dialysis cassettes (Cellu-Sep T1, Membrane Filtration Products, MWCO 3.5K) at room temperature for 48 h. The obtained solutions were then twice centrifuged for 20 min at − 1 ℃ and 9000 rpm to remove impurities, and finally, silk fibroin aqueous solutions with an approximate concentration of 6 wt% were obtained.

Silk fibroin films were prepared by drop-casting the aqueous silk solution on arbitrary 3 cm x 3 cm substrates. These coated substrates were then dried at normal room temperature inside a fume hood for 24 hours. The dried silk fibroin films were peeled off, and the resulting free-standing films were used in the experiments.

Preparation of HCl Vapor Environment. To prepare an HCl vapor environment on a ppm scale, liquid HCl was dropped in a beaker with a micropipette. The beaker was then sealed for 2 h at room temperature to let the liquid HCl vaporize completely inside the beaker. For a concentration of 1 ppm (v/v), the ratio of the volume of HCl vapor to that of the beaker was maintained at 1 µL/L. The HCl vapor environments were prepared at different ppm scales, such as 5, 10, 50, 100, 300, 1300, and 3000 ppm.

Optical Measurements and Characterizations. The fluorescence spectra of the Yeonnokjam silk pristine fibers were measured using a visible/near-infrared spectrometer (USB-2000 + VIS-NIR-ES, Ocean Optics, USA, spectral resolution: 1.2 nm). The samples were optically illuminated by a 365-nm continuous-wave LED (M365LP1 UV Mounted LED, Thorlabs Inc., USA). To excite a large area on the samples, the illuminating spot of the LED was expanded using a plano-convex optical lens (f = 10 cm). High-quality PS pristine fibers were prepared with spatial homogeneities in diameter and fluorescence emissions. The exact distance between the spectrometer detector tip and the sample was adjusted, the focused beam size was maintained, and the detection angle was fixed by placing the detector tip normally to the sample. Field-emission SEM (Apreo S Hivac, Thermo Fischer Scientific Inc.) was used to examine the morphological and structural characteristics of the co-coon samples.

Pigment Extraction and Separation from PS, AY, and GS Cocoons. The pigments in the PS, AY, and GS cocoons were separated by paper chromatography. The PS, AY, and GS cocoons were cut into small pieces and separately extracted using EtOH via heating at 75 ℃ for 2 h (at 1400 rpm) with a cocoon-to-solvent ratio of 1:10 w/v. The EtOH extracts of the PS, AY, and GS co-coons were further heated at 80 ℃ for 2 h (at 1400 rpm) to obtain their concentrated solutions, which were poured into different vials. Next, filter papers were immersed in these vials to absorb the pigment solutions, and therefore the pigment molecules, via the capillary force. These immersed filter papers were then exposed to UV light to observe the sky-blue, strong green, and weak fluorescence intensities emitted by the GS (due to the presence of carotenoids), PS (due to the flavonoid quercetin 5-O-glucoside), and AY pigments, respectively.

Three varieties of silk cocoons, white jade (WJ, known as Baegokjam in Korea), golden silk (GS), and PS (known as Yeonnokjam in Korea), were provided by the National Academy of Agricultural Sciences in Korea. Figures 1a-1c and Fig. 1a₁, b₁, and c₁ show the images of the WJ, GS, and PS cocoons and threads, respectively. Under daylight, the PS, WJ, and GS cocoons exhibit light green, white, and yellow colors, respectively. Because of the nutrients absorbed by the mulberry leaves, the silk fibers of all the varieties display fluorescence under UV excitation (Fig. 1a₂, b₂, and c₂)⁴³. The WJ silk thread shows a weak blue fluorescence, whereas the fluorescence intensities of the colored silk cocoons increase; notably, the PS cocoons exhibit a strong green fluorescence (upper right of Fig. 1c). A microscopy image of a single PS-cocoon thread, captured under UV light, confirms the emission of a strong and uniform green fluorescence from the fiber surface, indicating the existence of fluorescent materials in sericin (Fig. 1c₃). Scanning electron microscopy (SEM) images show that the surface of a PS cocoon is smooth and contains microfibers (diameter: ~16 µm), resulting in a large surface-to-volume ratio, which increases the sensitivity of the PS cocoon (Fig. 1d).

To investigate the fluorescence quantitatively, we constructed a spectrometer set-up as shown in Fig. 1e. The UV light emitted by a 365-nm light-emitting diode (LED), mounted on the spectrometer set-up, was collimated, and focused on the silk cocoon samples. The emitted fluorescent light was collected by an optical fiber and sent to the spectrometer to record fluorescent spectra. Figure 1f shows the fluorescent spectra of the WJ, GS, and PS cocoon mats. Interestingly, the PS mat exhibits a strong green fluorescence with a central wavelength of 552 nm and full width at half maximum of 65 nm, whereas the fluorescence intensities of the WJ and GS cocoon mats are negligible (magnified plot in the right panel of Fig. 1f). Because silk fiber is composed of two proteins (fibroin in the core and sericin in the sheath), identifying the location of the fluorescent pigment is essential. The PS fiber mat was fully degummed to remove the outer sericin layer and excited by the UV LED. As shown in Fig. 1g, the fluorescence totally disappears after the degumming, indicating that the pigment molecules are mainly present inside the sericin layer⁴⁴. The reflectance spectra of the GS and PS cocoons indicate the existence of color pigments in both samples (Fig. S1). However, the colors expressed by the GS and PS cocoons can be attributed to absorption and fluorescence, respectively. In addition, the PS fluorescence was compared to those of the commercial fluorescent dyes, i.e., riboflavin and sodium fluorescein, which are widely used in biological and medical applications, and their gains are sufficient to induce lasing. We prepared separate silk fibroin films containing 1% riboflavin and sodium fluorescein and demonstrated the strong optical gain of the dye-containing silk fibroin film for lasing. The fluorescence emitted from the PS cocoon mat was relatively weaker than that emitted by sodium fluorescein, but comparable to that of riboflavin (Fig. 1h). Notably, qualitative analysis revealed that the quantum yield of the fluorescent pigment in the PS sericin was comparable to that of a commercial dye.

The PS is the progeny of Japanese-originated green silk (Jam 315) and Chinese-originated white silk (Jam 316) cross-species and can therefore simultaneously carry two genes Ga and Gb, which complement each other to produce a light green color. Another independently inherited Gc gene, which is dominant in white cocoons, produces green cocoons with a stronger color. Silkworms produce two main pigments, viz. flavonoids, which impart green color to the cocoons, and carotenoids, which control the yellow and pinkish colors of cocoons (such as the GS)^45,46. A green silk species (Antheraea yamamai, AY) was prepared and its fluorescent spectra were measured for comparison (Fig. S2). The green silk cocoon exhibited a sky-blue fluorescence due to the existence of biliverdin⁴⁷, but the intensity was much weaker than that emitted by the PS cocoon (Fig. S3). The pigments in the PS, AY, and GS co-coons were extracted using EtOH, and the resulting pigment solutions were absorbed on filter papers. The capillary force drove the upward movement of the pigment molecules and EtOH (Fig. S4). Under UV light, the GS pigment showed a sky-blue fluorescence due to the presence of carotenoids⁴⁶. The fluorescence intensity emitted by the AY pigment was extremely weak and could not be observed with our naked eyes⁴⁷. The PS pigment exhibited a strong green fluorescence, confirming the existence of quercetin 5-O-glucosides, which are the major constituents of cocoon flavonoids⁴⁵.

PS cocoons can be utilized as large fluorescent gas sensors because of their large surface-to-volume ratios. HCl vapor was adopted as an analyte because it is widely used to produce organic compounds but is corrosive and harmful to humans. The lowest concentrations (LCLo) of HCl vapors, lethal for an individual human, are 1300 and 3000 ppm for 30-min and 5-min exposures, respectively⁴⁸. The RD₅₀ (i.e., exposure concentration that decreases the respiratory rate by 50%) test is another standard method used as an indicator of the hazards of vapors. The 10-min RD₅₀ value for mice has been reported to be 309 ppm⁴⁹. Moreover, the permissible HCl-vapor exposure limit for an 8-h time-weighted average (TWA-PEL) in working spaces is 5 ppm⁵⁰. Safety limits are very low compared to lethal concentrations because they are set by considering a long-time exposure over many hours. To examine the gas-sensing response of the fluorescent PS, the PS cocoons were placed in a beaker containing HCl vapors (Fig. 2a), whose concentration was controlled by modulating the evaporated content of an HCl droplet. Figure 2b shows the absorption and emission spectra of the PS cocoons before (solid) and after (dotted) exposure to HCl vapors. Due to the exposure to a strong acid (300 ppm for 2 min), the absorbance and fluorescence intensities simultaneously deteriorate, and this feature can be used for fluorescent chemosensing. The effect of the HCl vapor exposure could be recognized with the naked eye. As shown in Fig. 2c, the PS mats that were cut to express letters revealed a bright green fluorescence under UV light. After the HCl vapor exposure, the daylight color and fluorescence gradually faded as evident from Fig. S5. The GS mats also showed a similar fading upon HCl-vapor exposure (Fig. S6); however, the rates at which the fluorescence intensity and absorbance decreased were relatively lower than those of the PS mats, and a weak fluorescence was detected.

For quantitative analysis, the PS cocoon mats were exposed to different concentrations of HCl vapors (from 5 to 3000 ppm). A distinct fluorescence decay (fading) is evident even at a concentration of 5 ppm (TWA-PEL limit) as shown in Fig. 3a. The fluorescence intensity decreases by 25% for a 5-min exposure, and this intensity decay can be easily detected by a simple photodetector. These results indicate the feasibility of using PS cocoon mats as cost-effective and compact sensing systems. Further, the fluorescence decay rate increases with the increasing HCl vapor concentration. For a 5-min exposure, the fluorescence ratios decrease to 65% and 79% at 100 and 300 ppm, respectively (Fig. 3b and Fig. S7c), and at a concentration of > 1000 ppm, the fluorescence completely vanishes/fades after 5 min (Fig. 3c and Fig. S7d). Figure 3d shows that the fading rate, determined by the ratio of the relative peak intensity change and exposure time, is high in the as-exposed stage and becomes constant. At 5 ppm, the fading rate is 15.0%p/min for 30 s and 3.6%p/min afterward, whereas the rates are 40.8%p/min and 10.0%p/min at 100 ppm. Notably, the fluorescence decays linearly and vanishes completely at all the HCl vapor concentrations, although the fading rate depends on the HCl concentrations.

To investigate the versatility of fluorescence sensing, the PS cocoon mats were exposed to different environments, and temperature and EtOH content, which play a crucial role in biological and environmental research, were analyzed as the sensing parameters. The sensing performance of a material is determined by its ability to detect the decay in fluorescence under external stimuli (Fig. 4a). Especially, the fluorescent pigment found in PS cocoons is a kind of flavonoid, and thus, these cocoons can efficiently respond to temperature changes and EtOH in water. Similar to the response of the cocoons under HCl exposure, the fluorescence of the cocoons diminished with a linear decay rate with the increasing heating time and/or immersion time in the EtOH solution at different concentrations of EtOH in water. Figure 4b shows the thermally induced changes in fluorescence exhibited by the PS cocoons upon heating where the temperature was increased slowly from 20 to 40 ℃. We observed that the transition to the fluorescence de-cay started at ~ 35 ℃, and above 40 ℃, the fluorescence decay followed a linear pattern up to 200 ℃; the total fluorescence decay was ~ 75% at 200 ℃ (Fig. 4b and Fig. S8). To investigate the EtOH sensing response, the PS cocoon mats were immersed in an EtOH-water solution with different EtOH concentrations. The fluorescence intensity rapidly faded during the first 60 s, and the highest decay rate was observed at an EtOH concentration of 100%. However, as the cocoon immersion time in EtOH increased, the fluorescence decayed linearly; a decay of ~ 80% was observed in the pure EtOH case after 30 min of immersion (Fig. 4c, Fig. S9, and Fig. S10). These results collectively suggest that because of the presence of the fluorescent pigment in PS, PS cocoons can be used as sensors under various biological and environmental stimuli. These sensing abilities of PS can be attributed to the changes in the flavonoid activity induced by various agents.

In summary, we demonstrated the feasibility of using light-green PS cocoons as fluorescent environmental sensors. The PS cocoon mats exhibited a strong green fluorescence, whose intensity was comparable to those emitted by commercial fluorescent dyes. We found out that the fluorescent pigment molecules were a kind of flavonoid and existed in the outer sericin cladding layer. Especially, the PS cocoon mat was instantly responsive and highly sensitive to HCl vapors. A low concentration (a few ppm) of HCl vapors was sufficient to induce a detectable change in the fluorescence intensity, and at a lethal concentration (3000 ppm), an instantaneous decrease in the fluorescence intensity was observed. In addition to chemosensing, the fluorescence from the PS cocoons responded to temperature and EtOH concentration in water. The decrease in the fluorescence intensity was strongly correlated to the changes in temperature and EtOH concentrations. Although further studies are required to understand the intrinsic light-emitting properties of the flavonoid-based pigment in PS cocoons, fabrics and health-promoting dietary supplements incorporated with the natural fluorescent pigment can act as biocompatible and environmentally friendly optical sensing materials.

Competing interests

The authors declare no competing interests.

Acknowledgments

The authors acknowledge the support of the Rural Development Administration (PJ016130, Research Program for Agricultural Science and Technology Development of the National Academy of Agricultural Science).

Author contributions

S.K. designed the research; R.K.J. conducted the experiments, measured the optical properties, and generated the figures, S.W.K. provided the resources; R.K.J. and S.K. analyzed the data and wrote the manuscript. All authors reviewed the manuscript.

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No competing interests reported.

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Naturally Bred Korean Silkworm Cocoon Showing Strong Green Fluorescence and Its Textile-Based Environmental Sensing Applications

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