## 2 Chemo-Mechanical Sensing with Optical Detection
### Measurements of the Cantilever Response to TNT Vapours  
The cantilevers with standard Au reflective coating surface modified according to Section 2.2. showed huge response even to pure N\({}_{2}\), which was due the temperature induced bending. Even small variations of the temperature of the incoming gas caused a notable PSD signal. It was shown a long time ago that standard Au-coated AFM cantilevers, such as STMS-06 AU (Park Scientific, Sunnyvale, CA, USA), are very sensitive thermometers and bend for \(\sim\)0.2 \({}^{\circ}\)for 1 K temperature difference [21]. Figure 4: Optical detection principle and implementation. (**a**) A collimated diode laser light (\(\lambda\) = 635 nm, 5 mW) (A) passes through small aperture (B) and is focused at the backside of a cantilever (D) using \(f\) = 50 mm spherical lens (C). The reflected beam is directed onto a position sensitive photodiode (F) using a rotatable mirror (E). Distances d1, d2, and d3 are 48.3 mm, 34 mm, and 60.3 mm, respectively. (**b**) The optical detection system used for a precise detection of a functionalized (blue beam—A) and a reference (red beam—B) cantilever bending. The outer (gray—C) parts of the system are made of stainless-steel and designed to be as rigid as possible. The inner (yellow—D) temperature-controlled chamber is made of gold-plated copper, and thermally stabilized by several Peltier elements with an accuracy of a few mK. The temperature sensitivity of the z-deflection of the cantilever tip is estimated to \(\sim\)0.1 nm/1 mK, which is indeed the order of magnitude, observed in our experiments. Because of such high temperature sensitivity, Au-back-coated AFM cantilevers were abandoned in further experiments and cantilevers without any coating were used. The best results were acquired by using uMasch CSC12 tipless Si cantilevers (Innovative Solutions Ltd., Sofia, Bulgaria) with no coatings on either side of the cantilever. After being chemically functionalized (as described in Section 3.2.2.) the bottom side of the cantilevers was ion sputtered in an ultra-high vacuum with Ar\({}^{+}\) ions (15 min at 500 eV ion energy and ion current of 1 \(\upmu\)A/cm\({}^{2}\)). The sputtering process destroys and partially removes the active molecular layer, as checked by the XPS analysis of clean, functionalized, and sputtered surfaces. All measurements were performed using a homemade vapour generator described in Section 4.1. This vapour generator shows excellent temperature stability of output gas, good control of gas flows and mixture ratios, and is vital for reliable measurements. Prior to measurement, the vapour generator and the measuring system were extensively purged with clean nitrogen (4.6 quality) for several hours. The concentration of targeted molecules in the carrier gas was regularly checked by using a cold trap and doing NMR and gravimetric analysis. Figure 5 shows the response of a CSC12 cantilever with \(k\) = 0.03 N/m and functionalized with 3-trimethoxysilyl-propan-1-amine (APS) molecules on switching between a clean N\({}_{2}\) gas and a 50% saturated vapour pressure of TNT at room temperature. The maximum response from PSD of approximately 30 mV is achieved in about three minutes. The cantilever shows stable responses on TNT molecules over several hours of operation. In the same figure the temperature measured close to the cantilevers is plotted. The temperature is stable to about 1 mK during the time of the experiment and shows no change during switching between the two gases. Figure 5: Measured responses of CSC12 cantilevers functionalized with APS molecules (red) on switching between pure N\({}_{2}\) and mixture of N\({}_{2}\) and 50% vapour pressure of TNT at room temperature. Temperature changes in the system (blue) due to switching between the two gases are below 1 mK.