Effects of surface energy and substrate on modulus determination of biological films by conical indentation

Micro-/nano-indentation has become prevalent in evaluating the mechanical characteristics of biological samples,such as cells and tissues.However,the existing contact models describing conical indentation ignore the joint effects of surface energy and substrate,and consequently cannot accurately extract the Young's modulus of biological samples deposited on substrate.Through finite element methods,we examine the conical indentation of biological films on substrates while taking surface energy into account.Based on the dimensional analysis,the explicit relationship between load and indentation depth is achieved for films with their moduli varying from 0.001 to 100 times that of the substrate.If the classical Sneddon's model was employed to analyze the load-depth data,the measured modulus could reach 18 times the real modulus for films on harder substrates,but only 4%of the real modulus for films on softer substrates.Meanwhile,in micro-/nano-indentations,neglecting the contribution of surface energy would result in an overestimation of the Young's modulus of films depending on the contact size.The analytical expression provided here can be utilized to precisely deduce the mechanical characteristics of biological films deposited on substrate from the load and indentation depth data of a conical indentation.

Interaction between B-Doped C60 Fullerene and Glycine Amino Acid from First-Principles Simulation

The possibility of formation of complexes between glycine and boron doped C60 （C59B） fullerene is investigated and compared with that of C60 fullerene by using the density functional theory calculations. It has been found that the binding of glycine to C59B generated the most stable complexes via its carbonyl oxygen active site, with a binding energy of-37.89 kcal/mol, while the glycine molecule prefers to bind to the pure C60 cage via its amino nitrogen active site, consistent with the recent experimental and theoretical studies. We have also tested the stability of the most stable Gly-C59B complex with ab initio molecular dynamics simulation, carried out at room temperature. These indicate that the B-doped C60 fullerenes seem to be more suitable materials for bindings to proteins than pure C60 fullerenes.

Comparative Study on Polarization of DNA and CdSe Quantum Dots

The polarizabilities of DNA in transverse direction and CdSe semiconductor quantum dots （QDs） deposited on mica surface are compared by means of electrostatic force microscopy （EFM）. We observe clear EFM-phase shift over CdSe QDs, while no obvious signal on DNA is detected, suggesting that DNA molecules is an electrical insulator.