Tunable and Highly Accessible Plasmonic Gap Nanostructures on Flexible Film as a High-Performance Plasmonic Drug Sensor

Tunable plasmonic gap nanostructures on flexible substrates have enabled novel functionalities and been considered as highly promising plasmonic structures. It’s because manipulating the distance of nano-gaps allows regulation of the degree of concentration of the incident electromagnetic field into a small space to provide strong and optimal field localization in a spatially controlled manner. In addition, compared to rigid substrates, the soft, bendable, and stretchable properties of flexible devices render intimate contact with arbitrary surfaces for in-situ and on-site operation.<br/>Because the optical responses of these structures are easily affected by small changes in the size and morphology of the nano-gaps, precise control in the fabrication of plasmonic gap nanostructures on flexible substrates is crucial. In addition, the resulting nano-gap should not be sealed with two metal layers or filled with organic materials (such as surfactants or gap-directing molecules). If the nano-gaps are blocked, it will be difficult for the surrounding molecules to enter the nano-gaps.<br/>Various studies have aimed to create tunable and highly accessible plasmonic gap nanostructures on flexible devices. However, most of the previously presented plasmonic gap nanostructures on flexible devices are not feasible for precisely controlling a wide range of nano-gap distances, resulting in weak optical tunability. In other cases, the nano-gaps are filled with other materials required for the generation of the nano-gaps, resulting in low sensitivity for the identification of objective molecules.<br/>Here, we report a state-of-the-art strategy for creating precisely tunable and vacant plasmonic nano-gap structures on flexible substrates (TVPFs). First, the polyimide (PI) film was modified with a graphene layer via direct laser writing. Silver dendrite (SD) structures were electro-deposited on the surface of the graphene under an overpotential of -2 V (vs Ag/AgCl), owing to the high conductivity and flexibility of graphene. Then, protruding gold nano-islands were created on the surface of the SD via the galvanic replacement method. As the surface of the SD was not covered by any material, the fast redox kinetics between gold ions and SD enabled island growth, rather than layer-by-layer deposition, despite the similar lattice constants of gold and silver. By fine-tuning the amount of infused gold precursor, reaction time, and solvent volume, different morphological configurations of gold nano-islands were created, thus providing controllability of the nano-gap distance between the gold nano-islands. The presented TVPFs succeeded in fabricating of nano-gaps with sizes ranging from approximately 3 to 30 nm. Moreover, we successfully demonstrated that the degree of Raman signal enhancement in the TVPFs could be manipulated, corresponding to the reduced length of nano-gaps. Our TVPFs exhibited robustness under repeated mechanical deformation, stemming from the stable chemical bonding between SD and graphene. Moreover, the secondary gold nano-islands could endow TVPFs with the good stability of the TVPFs for an extended period by protecting the metal structure from oxidation.<br/>Subsequently, we demonstrated that TVPFs could detect 6-thioguanine (6-TG), a kind of well-known anti-cancer drug, on an arbitrary curved surface after coming in direct contact with the curved surface to capture the chemicals. Rapid and accurate detection of 6-TG is important due to its narrow safe dosage range and severe side reactions upon exceeding the appropriate dosage. Unfortunately, the very low concentration of 6-TG in patients’ plasma, along with the formation of supramolecular complexes with serum proteins, hinders the precise analysis of 6-TG. Experimental results showed that TVPFs could rapidly and sensitively detect 6-TG with a low limit of detection of 1 × 10^-13 M in buffer solution and 1 × 10^-11 M in 10 % serum solution.