Solvent-mediated amplification of polycrystalline biomolecular piezoelectricity Ciaran O'Malley and Sarah Guerin* Department of Physics and Bernal Institute, University of Limerick, Castletroy, Limerick, V94T9PX, Republic of Ireland Email: ciaran.omalley@ul.ie; sarah.guerin@ul.ie The piezoelectric effect is a property of certain crystalline materials to generate an electrical potential upon the application of mechanical stress, or conversely, undergoing mechanical deformation in the presence of a magnetic field. Piezoelectric sensors have found widespread use in industrial and consumer products including medical devices, consumer electronics, energy harvesters and automotive sensors 1 . Traditionally piezoelectric materials are based on inorganic solids with the vast majority utilizing perovskite lead zirconium titanite (PZT). PZT requires the use of lead oxide during its production and generates toxic waste both during PZT synthesis and through leeching after disposal. As a result, lead free alternatives are much desired but current alternatives are focused on ceramics containing niobium, bismuth and barium which come with their own environmental difficulties 2 . Recently biological based piezoelectric materials, based on amino acids and peptides have come under increasing interest as alternative materials. Biomolecular crystals possess many distinct advantages over traditional inorganics, being biocompatible, biodegradable and relatively easy to fabricate. Biomolecular crystals have also been shown the ability to possess piezoelectric responses approaching that of traditional PZT (350-550 pC/N) 2 . A prerequisite for the exhibition of piezoelectric activity in crystalline materials is the formation of non- centrosymmetric crystalline lattices due to charge separation upon application of mechanical force. With chiral molecules naturally existing in non-centrosymmetric space groups and nineteen of the twenty natural amino acids being chiral molecules, these exhibit a significant body of potential biocompatible piezoelectric materials with β-Glycine the first known biomaterial to approach the piezoelectric response performance of PZT 3 . In this study we focus on the growth of amino acid thin films through the drop cast method with the ability to modulate piezoelectric responses through variable solvent choice and composition. The piezoelectric response of polycrystalline organic film layers is greatly affected by crystalline morphology, directionality and thickness of layers while the presence of solvents and additives can have a large impact on the crystal morphology and polymorphism of amino acids 4 . Through close control over crystal growth conditions and by appropriate solvent system choice, we show the ability to alter crystalline morphology to an extent able to produce variable piezoelectric responses. For example, DL-alanine has been shown to give variable responses between 0 to 80pC/N. This work demonstrates a significant step forward into the development of green, biocompatible piezoelectric devices with a potentially facile synthetic process. References
1. Guerin, S. Mater. Res. 2022, 3 , 782-784. 2. Guerin et al., Growth Des. 2018, 18 , 4844. 3. Kim et al., Adv. Mater. 2020 , 32 , 1906989 4. Hod et al., CrystEngComm, 2011, 13 , 502-509
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