Guest Molecule-Mediated Energy Harvesting in a Conformationally Sensitive Peptide–Metal Organic Framework

The apparent piezoelectricity of biological materials is not yet fully understood at the molecular level. In particular, dynamic noncovalent interactions, such as host–guest binding, are not included in the classical piezoelectric model, which limits the rational design of eco-friendly piezoelectric supramolecular materials. Here, inspired by the conformation-dependent mechanoresponse of the Piezo channel proteins, we show that guest–host interactions can amplify the electromechanical response of a conformationally mobile peptide metal–organic framework (MOF) based on the endogenous carnosine dipeptide, demonstrating a new type of adaptive piezoelectric supramolecular material. Density functional theory (DFT) predictions validated by piezoresponse force microscopy (PFM) measurements show that directional alignment of the guest molecules in the host carnosine–zinc peptide MOF channel determines the macroscopic electromechanical properties. We produce stable, robust 1.4 V open-circuit voltage under applied force of 25 N with a frequency of 0.1 Hz. Our findings demonstrate that the regulation of host–guest interactions could serve as an efficient method for engineering sustainable peptide-based power generators.


Crystal preparation.
The Car_Zn MOF crystals in different solvent were prepared following literature protocol 1 . In a typical synthesis procedure, the weighed carnosine powder was dissolved in water, to a concentration of 80 mM. A stock solution of carnosine (2 mL, 80 mM), Zn(NO 3 ) 2 (1 mL, 160 mM), solvent (4 mL, DMF, IPA, EtOH, Acetone or MeCN), and water (2 mL) were mixed in a 20 mL scintillation vial under vigorous sonication.
The vial was heated at 100 °C for 4 h with ramping rate of 1 °C /min and was cooled down to room temperature at 1 °C /min. The resulting crystal was washed several times with ethanol and deionized water, then dried at 60 °C for 12 hours.

Characterization
Scanning electron microscopy (SEM). The silicon substrate was ultrasonically cleaned with acetone followed by isopropanol, and water for 5 minutes. Then, 10 μL Car_Zn MOF crystal samples were dropcasted on a silicon substrate and heated for 10 minutes. The Quorum SC7620 sputter coater system was used to sputter gold on the crystal samples at 15 mA for 45 seconds to improve the conductivity of the sample surface. SEM images were collected using Zeiss Gemini 300 (Zeiss, Germany) with an operating voltage of 3 kV. Images were collected in Inlens and secondary electron mode by using an Everhart-Thornley detector, with probe current of 237 pA and system vacuum lower than 9×10 -7 mbar. X-ray crystallography. Crystals suitable for diffraction were coated with Paratone oil (Hampton Research), mounted on loops and flash frozen in liquid nitrogen. Single crystal X-ray diffraction data measurements for Car-Zn·(IPA), Car-Zn·(EtOH), and Car-Zn·(acetone) were performed using a Bruker Kappa ApexII system with MoKα radiation ( =0.71073Å). Data were collected and processed using the Apex3 suite of programs (Bruker 2018). Single crystal X-ray diffraction data measurements for Car-Zn·(MeCN) were performed using a Rigaku XtaLabPro system with MoKα radiation ( =0.71073Å). Data were collected and processed using the CrysAlisPro suite of programs (RigakuOD 2018). The structures were solved by direct methods using SHELXT-2016/4 and refined by full-matrix least squares against F2 with SHELXL-2016.

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Powder X-ray diffraction (PXRD). Car_Zn MOF crystal powder samples were deposited on a quartz zerobackground sample holder. The diffraction patterns were collected using a D8ADVANCE diffractometer (Bruker, Germany) equipped with a linear detector LYNXEYE XE. Data collection was performed at room temperature with a scan range 2θ of 10-40°.
Optical microscopy. The Car_Zn·(EtOH) single crystals casted on a glass slide were directly observed using a Nikon Eclipse Ti-E fluorescence microscope at bright field channels.
Density functional theory (DFT) calculations. DFT electromechanical properties were predicted from periodic DFT 2 calculations on the single crystals using the VASP 3 code. Electronic structures were calculated using the PBE functional 4 with Grimme-D3 dispersion corrections 5 and projector augmented wave (PAW) pseudopotentials 6 . The crystal structure was optimised using a plane wave cut-off of 600 eV with a 4x4x4 k-point grid. A finite differences method was used to calculate the stiffness tensor, with each atom being displaced in each direction by ± 0.01 Å, and piezoelectric strain constants and dielectric tensors were calculated using Density Functional Perturbation Theory 7 (DFPT), with a plane wave cut-off of 1000 eV and k-point sampling of 2x2x2. Young's moduli were derived from the stiffness and its inverse compliance matrix components. Values are presented as an average of three calculation methods: the orthorhombic and triclinic approximations of Nye 8 , and the Voigt-Reuss-Hill method 9,10 . All three methods give reasonable match with known experimental values for organic and inorganic crystals [11][12][13] , so an arithmetic mean over the three analysis methods was used. Crystal structures were visualised using VESTA 14 . Using the piezoelectric charge coefficients, e ij , calculated directly by VASP, and the elastic stiffness constants, c kj , we could calculate the more useful piezoelectric strain coefficient, d ik , using the relationship = To obtain the voltage constants g ik , we divided the corresponding piezoelectric strain constant d ik, by the relevant dielectric constant ε ii . These constants are measured in Vm/ N.

AFM nano-indentation experiments.
Atomic force microscopy (AFM) nano-indentation experiment was carried out using a commercial AFM F corresponds to the force, corresponds to the depth of the crystal pressed by the cantilever tip, corresponds to the radius of the tip, is the Young's modulus of the crystals and ν is the Poisson ratio (ν = 0.3). The point stiffness was determined as the normal force divided by the deformation of the sample and calculated from the force-displacement curves after deducting the deformation of the cantilever. For each sample, more than 5 regions were randomly selected to perform the experiment and the data collected using the JPK software. At least three cantilevers were used in the experiments to exclude the tip-to-tip dependency.

Piezoelectric characterization.
Piezoresponse force microscopy (PFM) was measured using an NT-MDT Ntegra Spectra operating in contact PFM mode [15][16][17] . Generally, in this mode, the atomic force microscopy probe is in contact with the sample, and an AC voltage is applied to generate a piezoelectric response within the sample which is then