Embedded nanoparticle silk fibroin hydrogel as surface enhanced Raman scattering (SERS) substrates for detection of methylene blue in water

Surface Enhanced Raman Scattering (SERS) is a powerful and attractive analytical detection technique capable of amplifying Raman signal of target molecules near or at the surface of plasmonic metal nanoparticles due to resonance and charge transfer effect. Silk fibroin (SF), a protein extracted from Bombyx mori cocoon has stirred interest for use as a SERS matrix due to ease of processing and workability into different material shapes for hosting SERS active materials. Water stabilized plasmonic nanoparticles (Nps) namely: Au, and Ag synthesized by facile green procedure and another synthesized form of Ag nanoparticles via Lee Meisel protocol (referred to here as Ag∗) were prepared, and characterized by transmission electron microscopy, dynamic light scattering, and UV–visible spectroscopy. Firstly, SERS detection activity tests of the Nps were performed in suspension over methylene blue (MB) as the model organic pollutant. The SF-AuNp, SF-AgNp, SF-AgNps∗ hydrogels were then prepared by previously developed enzyme cross-linking methodology. SEM, FTIR, and UV–vis spectroscopy were used to characterize the Nps containing hydrogels. SERS detection activity over MB was then extended to hydrogels containing Nps. Executed SERS test over MB analyte and under 785 nm excitation recorded 1.56 μM, 15.63 μM, and 15.63 μM respectively as concentration detection levels reached with Au-Nps, AgNps, AgNps∗ suspensions, while 0.27 μM, 0.27 μM, and 0.17 μM were respectively achieved in the case of SF-AuNps, SF-AgNps, and SF-AgNps∗ hydrogels SERS activity performance evaluation. From a general look, the best performing SF-AgNps∗ with a signal amplification factor of about 7.4 for tested Nps suspension, achieved over 400 signal amplification for the tested SF-AgNps∗ hydrogel material representing almost 60 times fold of enhancement obtained in comparison to Nps tested in solution. The inherent adsorption capability of the SF-Nps hydrogels in comparison to the suspension test, facilitated through the concentration of MB by the SF-Nps hydrogels matrix for detection, represents a promising strategy in the development of efficient environmental detection systems.