Upcycling fish scales through heating for steganography and Rhodamine B adsorption application

A team of researchers from the National University of Singapore has managed to unlock fish scale waste as an economical alternative material with a multidimensional application using a single processing step. In this work, treated fish scale wastes are applied as biosorbents for Rhodamine B pollutants removal and to serve as a natural material with multidimensional security capable of micro- and macroscopic text and imagery transmission.

Seafood is a delicacy that many enjoy, and globally, 7.2-12 million tons of fish waste are projected to be discarded yearly. That makes fish scale waste an abundant natural resource that can be investigated for alternative upcycling as we strive towards sustainable living. Indeed, the primitive fish scale has inspired the development of next-generation bionic flexible armor, active camouflaging surfaces and transparent fish gelatin films for electroluminescence devices. Its extracts have applications ranging from biomedical (wound healing, bone and cornea repair, etc.) to flexible electronics, batteries and pollutant adsorption. With such a long list of amazing properties demonstrated by a single, tiny fish scale, it is surprising that there is still much to learn about this material.

 From fishes that glow in the dark to those that camouflage into their surroundings, there is so much more that we can learn from studying the optical properties (particularly fluorescence) of nautical biomatter. What started as a curiosity-driven project helmed by a high school student who is keen to venture into the wonder of science and research has turned out to be a fascinating and fruitful journey – one that dives deep to understand the underlying mechanism of the fluorescence properties and uncovers alternative applications for functionalized primitive fish scales.   

 A normal translucent fish scale will appear a royal blue color when viewed under ultra-violet (UV) light. By heating a piece of fish scale on a hotplate, the fluorescence shifted to cyan and became brighter. So why does this change occur?

 Fig. 1 Comparison of the fluorescence property of pristine and heated fish scales. Observation is carried out under bright field (BF) and UV excitation.

Fish scales are mainly made up of interlacing collagen, a protein that gives us a youthful appearance and hydroxyapatite (HAp), a mineral found in our bones and teeth. When heated, long collagen chains break down into smaller segments of fluorescing tyrosine-like cross-links that can be elongated as the scale becomes dehydrated. At the same time, CO₂ and H₂O molecules that are adsorbed and trapped within the fish scale are also released. Not only does this process create holes on the surface and within the fish scale (Fig. 2a-d). It also encourages substituting OH groups with intrinsic CO₃ impurities and allows heat to reach deep within the fish scales (Fig. 2e).

 When UV light shines on the broken downed collagen, light with a very short lifetime is emitted and can further excite the nearby HAp. Combining the effects of the UV light introduced onto the fish scales, the emitted light from the neighboring excited collagen and the presence of impurities within the HAp, strong cyan fluorescence is observed from heated fish scales (Fig. 2f).

Fig. 2 Characterization of pristine and heated fish scale. a Top and b-d side-view SEM images of the same piece (separated into two portions) of pristine and heated fish scale. e FTIR of pristine and heated fish scale. f Schematic of the proposed cascading effect that fluorescence from denatured collagen structure can have at the interface to the adjacent HAp structure.

 Further expanding on the simplicity of the treatment process, focusing a laser through the lens of a microscope (Fig. 3a) can then be used as a crafting tool to control the degree of fluorescence and the location and size of any pattern created on the fish scale. With good control, an otter image can be created on the piece of fish scale without it being seen under normal light. Once the fish scale is placed under UV light, voila! The otter image appears! (Fig. 3b) The process thus allows the fish scale to be readily transformed into an alternative material for hiding messages.

 On another note, holes on the surface and within the fish scale also double up as more diffusion paths and adsorption sites for the attachment of Rhodamine B, a common dye used in flow tracing and textile industries, which can cause oxidative stress on cells when ingested. Rhodamine B appears as pink under UV light and orange under green light. Setting up a dual syringe system (Fig. 3c) in which one syringe is filled with Rhodamine B, while the other is filled with heated fish scales, transferring the Rhodamine B into the syringe with fish scales allows the adsorption process. After 10 mins, 91% of the Rhodamine B pollutants are extracted, and the solution now appears transparent, while the heated fish scales take on a pink appearance under UV light. What makes these heated fish scales even more appealing as a cheaper alternative bio-adsorbent is the ability to recoverable and reuse these fish scales for multiple rounds of adsorption (Fig. 3d).

 As the Rhodamine B dye emits orange fluorescence under green excitation, Fig. 3e shows a cat sculpture created from a blend of heated fish scales and heated fish scales that had been immersed in Rhodamine B. No clear differences between BF and UV excitation are observed from the sculpture. Switching the illuminating source to green light reveals an orange-fluorescing, wave-particle sculpture hidden within the cat sculpture, thus highlighting the capability of using the treated fish scales to achieve a multidimensional level of hiding text and images at both micro and macroscopic levels.  

Fig. 3 Hiding messages and chemical adsorption properties of thermally treated fish scales. a A setup where a laser is focused into the microscope for controlled heat treatment of the fish scales. b An otter structure created using a focused laser beam is not observable under a bright field and appears under UV light. c Two syringe setup where Rhodamine B solution (appears pink under UV light) is pushed from one syringe into another with the fish scale adsorbent and is cleaned up by the heated fish scales. d Recovery of heated fish scale (R0) from Rhodamine B adsorption through multiple rounds of adsorption (R1, R3) and recovering (R2, R4) via ultra-sonication. e Sculpture of a cat made from heated fish scales under a bright field and UV light. Under green light, a hidden wave-particle sculpture is revealed within the cat sculpt.

 In summary, controlled heating of fish scales changes the intrinsic nature of collagen and HAp within the fish scale structure. These changes enhanced the fluorescence of fish scale waste and enabled the fluorescing fish scales to effectively remove 91 % of Rhodamine B pollutants within 10 mins and serve as natural material with multidimensional security capable of micro- and macroscopic text and imagery transmission. Thus, using a single processing step, fish scale waste is established as an economical alternative material with multidimensional application. Moving forward, we believe that uncovering alternative properties of natural waste materials sow and re-inventing them as a host of multiple applications is a viable route towards a sustainable and resource-constrained future.

 More details can be found via the following link: https://rdcu.be/doGzx