NCAFM2023 Programme Booklet

Friday 1120 - 1140

NON-CONTACT IMAGING OF CHARGES IN LIQUIDS WITH LOW OSCILLATION DYNAMIC ATOMIC FORCE MICROSCOPY

Jaime Colchero 1 , Lisa Almonte 2

1 Centro de Investigación en Óptica y Nanofísica (CIOyN), Departamento de Física, Universidad de Murcia, Campus Espinardo, E-30100 Murcia Departamento de Física Aplicada y Unidad asociada CSIC, 2 Universidad de Alicante, Campus de San Vicente del Raspeig, E-03690 Alicante, Spain. Email: colchero@um.es

Biomolecules in physiological medium have a surface charge which fundamentally determines their function. These charges can be measured on a mesoscopic scale by different techniques which access the so-called Zeta potential. Measuring these charges on a nanoscale is much more challenging. Atomic Force Microscopy (AFM), with its high spatial and force resolution is a powerful tool and has proven its potential in this field. Determination of surface charge has been mainly achieved by acquiring and processing force spectroscopy curves [1-3]. Using a direct imaging method in liquids has been elusive. Electrostatic Force Microscopy as used in Vacuum and ambient conditions is very challenging due to the finite resistance of the liquid medium (see, however, [4]). In the present work we will discuss how the simultaneous measurement of amplitude, force and frequency shift (or phase) allows accessing the electrostatic force induced by the Electrical Double Layer -and thus the surface charge- without spectroscopy approaches. We will show that using careful adjustment of the excitation of the cantilever (small amplitude and correct phase) it is possible to acquire images at very low interaction forces (of the order of 10-25 pN) in the non-contact regime where essentially only electrostatic forces act. We will show that for samples with different local charge domains (“heterogeneous-charge” samples) correct imaging and measuring of electrostatic measurements cannot be performed in force mode or frequency modulation dynamic AFM (FM-DAFM) in the non-contact regime; instead low oscillation amplitude modulation (AM-DAFM) has to be used for stable SFM-imaging [4]. We present experimental data acquired in liquid environment on DNA and a mixed lipid bi-layer (1,2-dioleoyl-3-trimethylammonium-propane, DOTAP, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) on mica that supports our hypothesis (see figure).

Fig. Bi-lipid membrane imaged in liquid environment at (relatively) low oscillation amplitude (1nm) in the non contact regime using AM-DSFM. Images correspond –from left to right) to topography (total scale of gold colors: 8 nm), oscillation amplitude (grey, error signal), phase and normal force. The phase and the normal force images show the variation of tip-sample interaction due to local charge differences.

References [1] H.-J. Butt, Biophys. J., 1991, 60 (4), 777–785. [2] J. Sotres and A. M. Baró, 2010, Biophys. J. 98 (9), 1995–2004. [3] G. Gramse, M. A. Edwards, L. Fumagalli and G. Gomila, 2012, APL 101 (21), 213108. [4] L. Almonte and J. Colchero, 2017, Nanoscale 9 (8), 2903-2915.

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