To generate force curve in scanning probe microscopy, the cantilever probe is to go up and down against the sample as illustrated below. You can get the information of the sample as hardness, adhesiveness and folding force of the protein molecule etc. by monitoring the caniltever deflection.
Soft cantilever (small spring constant) is requested to measure a weak force act in soft samples as bio samples. OLYMPUS achieves the small spring constant less than 10pN/nm with B lever of the Bio-Lever.
The left chart is the thermal vibration spectrum of the B lever at room temperature. The resonant frequency is 12 kHz and the spring constant is estimated at 3pN/nm by thermal method.
In force curve measurements, thermal noise of the cantilever itself should be small. As Brawnian motion of particles in water, that is well known phenomina , the cantilever vibrates thermally. This is a mechanical noise which worsens the S/N ratio of the force measurement.
Short cantilever is the solution for lowering the noise level. When the spring constant is the same, the shorter cantilever shows less thermall noise in the band width of the SPM instrument.
The A lever of the OLYMPUS Bio-Lever (60µm long) is the shortest cantilever avairable in the market and shows the spring constant of 30pN/nm (typ.).
As a practical application of lower noise measurements to single molecule force measurements, graph ( right ) shows a measurement of the B-S transition in lambda digest DNA in PBS solution. The noise level is obviously lower than the step in the middle of the curve of around 55pN.