Atomic force microscopy (AFM) is a high-resolution scanning probe technique that measures surface topography by sensing the forces between a sharp tip and the sample. Unlike electron microscopy, AFM can image non-conductive samples in ambient conditions or liquid, making it uniquely suited for biological and soft materials characterization.
Principles of AFM
An AFM consists of a sharp tip mounted at the end of a flexible cantilever. As the tip scans across the sample surface, tip-sample interactions cause the cantilever to deflect. A laser beam reflected from the cantilever surface onto a position-sensitive photodetector measures this deflection with sub-nanometer precision. A feedback loop adjusts the sample height or tip position to maintain a constant force or distance.
The tip radius is typically 1–20 nm for standard probes and below 1 nm for super-sharp tips. The force sensitivity can reach piconewton levels, enabling measurement of molecular interactions.
Imaging Modes
Contact mode maintains constant tip-sample contact during scanning. The cantilever deflection is kept constant by adjusting the z position, producing a topographic map. Contact mode provides high scan speeds but can damage soft samples or displace loosely bound material.
Tapping (intermittent contact) mode oscillates the cantilever near its resonance frequency while scanning. The tip contacts the sample briefly at the bottom of each oscillation. Changes in oscillation amplitude due to tip-sample interaction are used for feedback. Tapping mode reduces lateral forces and is the preferred mode for biological samples, polymers, and loosely adsorbed particles.
Non-contact mode oscillates the cantilever above the sample without contacting it, sensing attractive van der Waals forces. This mode avoids sample damage entirely but requires clean surfaces and stable conditions.
Force Spectroscopy
AFM can measure force-distance curves at single points or along a grid. The cantilever deflection is recorded as the tip approaches, contacts, and retracts from the sample. The resulting force curve reveals sample stiffness, adhesion, and deformation.
Force spectroscopy is used to measure molecular unbinding forces between ligand-receptor pairs, protein unfolding forces, and polymer elasticity. The technique can detect forces as low as a few piconewtons, comparable to single hydrogen bond strengths.
Sample Considerations
AFM requires relatively flat samples with surface roughness less than the z-range of the scanner, typically a few micrometers. Sample preparation involves mounting samples on glass slides, mica disks, or silicon wafers using adhesive or electrostatic attraction. Biological samples can be imaged in buffer solutions to maintain native conformation.
Immobilization is critical for stable imaging. Biomolecules can be adsorbed onto mica or functionalized surfaces, or tethered via specific linkages. Supported lipid bilayers and membrane proteins can be imaged in physiologically relevant conditions.
Advanced Techniques
High-speed AFM captures dynamic processes such as protein motion, enzyme activity, and cellular dynamics in real time. Scan rates of 10–50 frames per second are achieved with small cantilevers and optimized scanners.
Multiparametric imaging simultaneously records topography, stiffness, adhesion, and energy dissipation maps. Peak force tapping mode controls the maximum force applied at each pixel, enabling quantitative mechanical property mapping with minimal sample deformation.
Conductive AFM, Kelvin probe force microscopy, and magnetic force microscopy extend AFM capabilities to measure electrical, electrostatic, and magnetic properties at the nanoscale.
Applications
AFM is common in materials science to characterize surface roughness, thin film morphology, and nanoparticles. In biology, AFM images membrane proteins, DNA, and cytoskeletal structures at molecular resolution. In polymer science, AFM maps phase separation, crystallinity, and mechanical properties. In semiconductor manufacturing, AFM measures line edge roughness and critical dimensions.
Limitations
AFM has a limited scan area, typically 10–100 µm for high-resolution scanners. Image acquisition is slower than SEM, and tip artifacts can distort features. The technique images surfaces only and provides no direct chemical identification. Skilled operation is required for artifact-free imaging.
