Musings on Biological Sensors

AFM cantilever probe modes

In stress sensing mode, the reaction from the molecules or sample occur on one side of the cantilever, when there is a change in the total surface free energy, this will the equate to a change in surface stress, which will cause the cantilever to bend. The magnitude and direction of the bending can then be measures using optical means or electrical means by use of lasers and piezo-resistors. The spring constant is inversely proportional to the sensitivity of the device.

In mass sensing mode, the cantilever is excited mechanically, this causes it to vibrate at it’s resonant frequency. The change is mass can be detected by the shift in resonant frequency once a biological molecule attached to the cantilever. Optical or electrical means can be used to measure these shifts.

The above two modes is similar to the contact and non-contact mode on an AFM probe, all methods cause the cantilever to bend either by deflection (Contact Mode), therefore contact mode is similar to stress sensing mode.

Mass sensing mode is similar to Non-contact mode, since here the resonant frequency of the cantilever is used to measure the deflection of the cantilever, where oscillation in the vertical direction are excited and a piezoelectric actuator can be used to pick up the oscillations.

(b) Mass of virus particle

k=0.006 N m-1

Therefore the mass will be the difference in mass changes over the two frequencies.

f1≈1.35 x 106 Hz

f2≈1.42 x 106 Hz

m1= 8.34×10-17 kg

m2= 7.54×10-17 kg

∆m≈8.02×10-18 kg which is the mass of the virus particle.

So a the virus is approximately 8.02 fg (femto grams).

(c)Cantilever modifications

Biding sites may be attached to the cantilever to attract a certain ligand to bind to the cantilever. Gold is a excellent coating to apply to a silicon based cantilever, since this provides a good surface for capture probes to bind to. The cantilever can then be used in stress/mass-sensing mode.

So in summary:

· Create a silicon based cantilever

· Coat with gold

· Attach capture probes to cantilever surface which will match the target ligand

(d)Feasibility and detection (Tuning In)

A cantilever will need to have a high resonant frequency as possible to minimise vibration from external noise. Therefore the lowest resonant frequency of the cantilever should be at least 10 times greater than the highest frequency present in the room. Therefore this specific cantilever’s resonant frequency should be greater than 200kHz.

The feasibility will be to see if the cantilever can vibrate with the range of the resonant frequency required to detect this mass size, which is:

k=0.006 N m-1

m=0.20 x 10-18 kg

f=2.76 * 107 Hz

f≈27.6 MHz
This is well out of the range my one order of magnitude, so the cantilever will not be able to detect a mass of 0.2 fg.

The maximum frequency detected is 1.42 x 106 Hz. It is like trying to tune a radio to listen to microwaves, well close!

However, I wonder if a cantilever has ever been designed to work in amplitude mode, like AM radios?


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