Atomic Force Microscopy (AFM)
AFM at a glance
Atomic force microscopy (AFM) is an example of high resolution scanning probe microscopy, which allows the imaging and physicochemical analysis of material surfaces with nanometre resolution.
Right: The Bruker Dimension FastScan Bio hosted in the School of Pharmacy
Applications of AFM
- Surface topographical imaging in air and liquid.
- Force measurements (Surface energetics, electrostatics etc.)
- Hardness, Young’s Modulus, Phase mapping
- Functional probing (Biological system analysis, adhesion force dynamics)
- Micro-thermal analysis
How does AFM work?
In AFM a sample is scanned by a very sharp micron-sized tip mounted on a cantilever spring. The interaction between the tip and sample can be measured by monitoring the deflection of the cantilever. This is quantified by a laser signal focused on the cantilever tip and reflected onto a position sensitive photodiode. By plotting the deflection of the cantilever against its position on the sample, it is possible to map the sample's topography. Alternatively the height of the translation stage can be mapped while maintaining a constant force against the surface, where a feedback loop is initiated using piezoelectric input. In this way images on the nano-scale can be generated.
AFM can also be used to measure surface and interfacial forces. If the cantilever and sample are kept in fixed lateral positions, the substrate can be rastered up and down by the piezo translation stage it is carried upon, onto and off the cantilever by applying a voltage. When nearing contact the sample will very often attract the tip prior to a fixed level contact, causing a ‘jump in’ deflection. The piezo input then continues the sample movement until a pre-set value for the stage height (z-coordinate) is reached.
The stage then reverses, and the sample and tip will separate. Separation will occur only when the restoring force of the cantilever (spring constant) surpasses the interaction force of the tip and sample, and as such the negative deflection of the cantilever below the non-contact output can be used to quantify the interaction force by applying Hooke’s law.
By attaching functional materials of interest (pharmaceutical particles, proteins, aptamers etc.) to an AFM cantilever and using this ‘functionalised’ probe to make force measurements it is then possible to measure the interactive forces between that material and a given substrate. In so doing a multitude of systems can be analysed at a particulate to particulate and even molecule to molecule level. Other variations on AFM tip design such as the use of a resistive heater, allow for other modified probing experiments like micro-thermal analysis.
Our AFM Facilities
Multimode 8 Scanning Probe Microscope (Bruker)
- PeakForce™ tapping mode provide a direct force control with minimum peak force down to less than 100pN
- ScanAsyst™ mode works in conjunction with PeakForce™ to help provide self-optimising imaging and improve ease of use.
- Simultaneous high-resolution and well-defined force mapping can be obtained by PeakForce™ QNM™ (Quantitative Nanomechanical Measurement).
- Available modes and accessories: Contact; Tapping; Phase; Interleave; PeakForce; Multi-frequency; HarmoniX; Torsional Resonance; Lateral Force Microscopy; Force Microscopy; Magnetic Force Microscopy; MAC; Scanning Tunnelling Microscopy; Liquid; HSDC; Thermal tune; Heating stage; SAM (integrated on controller); 8 data channels
Dimension FastScan Bio (Bruker)
- Specialised 'life science' adaptations to the world's fastest high resolution AFM platform for detailed biological analyses.
- Designed for ''high resolution, live sample observations of interacting molecules, membrane proteins, DNA-protein binding, inter-cellular signalling and other dynamic biological processes.''
- Works 100s of times faster than conventional AFM, with PeakForce™ and Scanasyst™ modes available to help deliver high resolution imaging.
- Feature tracking and movie creation, real-time panning, zooming and scanning.
- Micro-volume fluid cell with controlled fluid exchange.
Dimension 3100 & 3000 AFMs (Bruker)
- ‘Tip down’ straight forward AFM apparatus, ideal for fundamental imaging and force investigation.
- Wide open access to tip and sample accommodates bulky or irregularly sized substrates.
- Available modes and accessories: Contact; LFM; Tapping; Phase; Interleave; FM; Force; Liquid; SAM; Motorised stage
EnviroScope AFM (Bruker)
- This special AFM allows observation of sample properties under precise temperature and humidity control.
- Available modes and accessories: Contact; Lateral Force Microscopy; Tapping; Phase; Interleave; FM; Liquid; SAM; Temperature control; Humidity control; Additional ADCs (6 channels in total)
ForceRobot® 300 (JPK Instruments)
- Fully automated molecular force spectroscope with high flexibility and automation.
- Users can design complex experimental parameters (fluids, temperatures, force ramp parameters, etc.) for an unlimited number of force measurements and operate in automation.
- Available modes and accessories: Force; Liquid; Thermal tune; Motorised stage; Flow control; Temperature control
TMX 200 Explorer microthermal analysis system (Bruker) & nano-TA2 (Anasys Instruments)
- Used as either micro-thermometers to obtain a temperature ‘map’, or a highly localised heater to obtain a conductivity/diffusivity ‘map’ of the sample surface.
- TMX 200 Explorer Atomic force microscope allows micrometre-scale localised thermal analysis.
- Nano-TA2 attachment for Multimode AFM can image the sample of interest with sub-30nm spatial resolution (in contact or intermittent contact modes) and identify regions to perform localised thermal analysis with sub 100nm resolution.
MFP-1D & MFP-3D (Asylum Research)
- The MFP-1D is a single axis force curve tracer designed specifically for soft samples sized 1nm - 2000 nm, with picoNewton force measurement capability.
- One of the MFP-1Ds is also combined with a Nikon TE300 ECLIPSE, achieving simultaneous molecular force measurements and fluorescence imaging.
- MFP-3D systems allow imaging and force investigations.
- One MFP-3D system is fitted with an ultra-high speed Video-AFM imaging facility capable of operation in liquid and ambient conditions.
Publications of Interest
Yaşayan G., Magnusson J.P., Sicilia G., Spain S.G., Allen S., Davies M., Alexander C., (2013). Multi-modal switching in responsive DNA block co-polymer conjugates. Phys. Chem. Chem. Phys. 15:16263-16274
Meeus J., Scurr D., Amssoms K., Davies M.C., Roberts C.J., Van Den Mooter G., (2013). Surface Characteristics of Spray-Dried Microspheres Consisting of PLGA and PVP: Relating the Inﬂuence of Heat and Humidity to the Thermal Characteristics of These Polymers. Molecular Pharmaceutics. 10:8:3213-3224
Rafati A., Boussahel A., Shakesheff K.M., Shard A.G., Roberts C.J., Chen X., Scurr D.J., Rigby-Singleton S., Whiteside P., Alexander M.R., Davies M.C., (2012). Chemical and spatial analysis of protein loaded PLGA microspheres for drug delivery applications. Journal of Controlled Release. 162: 321-329
Gao N., Yuying Y., Chen X., Mee D.J., (2012). Nanoparticle-Induced Morphology and Hydrophilicity of Structured Surfaces. Langmuir. 28:12256-12265
Hook A.L., Chang C.Y., Yang J., Scurr D.J., Langer R., Anderson D.G., Atkinson S., Williams P., Davies M.C., Alexander M.R., (2012). Polymer microarrays for high throughput discovery of biomaterials. Journal of Visualized Experiments. 59: e3636
Wu M., Kleiner L., Tang F.W., Hossainy S., Davies M.C., ROBERTS C.J., (2010). Surface characterization of poly(lactic acid)/everolimus and poly(ethylene vinyl alcohol)/everolimus stents. Drug Delivery. 17:6:376-384
Bouhroum A., Burley J.Cc, Champness N.R., Toon R.C., Jinks P.A., Williams P.M., Roberts C.J., (2010). An assessment of beclomethasone dipropionate clathrate formation in a model suspension metered dose inhaler. International Journal of Pharmaceutics. 391:1-2:98-106