Promoting Piezoelectricity in Amino Acids by Fluorination.

Bioinspired piezoelectric amino acids and peptides are attracting attention due to their designable sequences, versatile structures, low cost, and biodegradability. However, it remains a challenge to design amino acids and peptides with high piezoelectricity. Herein, a high piezoelectric amino acid by simple fluorination in its side chain is presented. The three phenylalanine derivatives are designed: Cbz-Phe, Cbz-Phe(4F), and Cbz-pentafluoro-Phe. The effect of fluorination on self-assembly and piezoelectricity is investigated. Cbz-Phe(4F) can self-assemble into crystals with a C2 space group, while Cbz-Phe and Cbz-pentafluoro-Phe form aggregated self-assemblies. Moreover, Cbz-Phe(4F) crystals exhibit a remarkably higher piezoelectric coefficient ( $d_{33}^{eff}$ ) of ≈17.9 pm V^-1 than Cbz-Phe and Cbz-pentafluoro-Phe. When fabricated as a piezoelectric nanogenerator, it generates an open-circuit voltage of ≈2.4 V. Importantly, Cbz-Phe(4F) crystals as a flexible piezoelectric sensor for the classification of various nuts and their quality sorting, which includes those as small as individual pumpkin seeds with high sensitivity and accuracy of sorting and quality checks. When mounted onto soft grippers, the sensor performs the tactile self-sensing functions. This work provides a promising approach to designing high piezoelectric amino acids by simple fluorination, offering exciting prospects for advancements in bioinspired piezoelectric materials in the application of smart agriculture and soft robotics.