*Article* **Noble Metal Composite Porous Silk Fibroin Aerogel Fibers**

**Alexander N. Mitropoulos 1,2,\*, F. John Burpo 1,\*, Chi K. Nguyen 1, Enoch A. Nagelli 1, Madeline Y. Ryu 1, Jenny Wang 1, R. Kenneth Sims 3, Kamil Woronowicz 1 and J. Kenneth Wickiser 1**


 3 Department of Civil and Mechanical Engineering, United States Military Academy, West Point, NY 10996, USA; robert.sims@westpoint.edu

**\*** Correspondence: alexander.mitropoulos@gmail.com (A.N.M.); john.burpo@westpoint.edu (F.J.B.); Tel.: +1-845-938-3900 (F.J.B.)

Received: 11 February 2019; Accepted: 11 March 2019; Published: 18 March 2019

**Abstract:** Nobel metal composite aerogel fibers made from flexible and porous biopolymers offer a wide range of applications, such as in catalysis and sensing, by functionalizing the nanostructure. However, producing these composite aerogels in a defined shape is challenging for many protein-based biopolymers, especially ones that are not fibrous proteins. Here, we present the synthesis of silk fibroin composite aerogel fibers up to 2 cm in length and a diameter of ~300 μm decorated with noble metal nanoparticles. Lyophilized silk fibroin dissolved in hexafluoro-2-propanol (HFIP) was cast in silicon tubes and physically crosslinked with ethanol to produce porous silk gels. Composite silk aerogel fibers with noble metals were created by equilibrating the gels in noble metal salt solutions reduced with sodium borohydride, followed by supercritical drying. These porous aerogel fibers provide a platform for incorporating noble metals into silk fibroin materials, while also providing a new method to produce porous silk fibers. Noble metal silk aerogel fibers can be used for biological sensing and energy storage applications.

**Keywords:** biopolymer; silk fibroin; aerogel; fiber; nanomaterials; nanoparticles; noble metals; gold; platinum; palladium
