## 2 Chemo-Mechanical Sensing with Optical Detection  
### Principles of Operation  
The invention of the Atomic Force Microscope (AFM) [11] has triggered great interest in using micromechanical devices (MEMS) for quick and reliable detection of small concentrations of target molecules in air and solutions. AFM cantilevers are primarily used for surface imaging, where an atomically sharp tip senses the tip-surface interaction at the sub-nano-newton level. Besides surface imaging, the AFM cantilevers are, in principle, very sensitive force sensors that are able to detect forces in the 10\({}^{\text{-}12}\) Newton (pN) range, and we can measure the cantilever deflections below nm. This makes it possible to measure single molecular forces, and the forces due to the molecules adsorbed on functionalized surface of the cantilever [12]. For this purpose, commercially available, micro fabricated silicon cantilevers, which are typically 100-350 \(\upmu\)m long, 20-25 \(\upmu\)m wide, and 0.5-1 \(\upmu\)m thick (Figure 1a) and with force constants ranging from 0.03 to 1.75 N/m, are functionalized on one of the two sides with a thin molecular layer that shows an affinity to the target molecules present in the environment. The surface coverage of the functionalized surface by target molecules is increased in the continuous process of surface adsorption and desorption, and generates either compressive or tensile surface stress which eventually results in a bending of the cantilever. This small bending can be measured either optically or electrically. In optical detection, a focused laser beam is reflected from the cantilever and the cantilever bending causes a deflection of the reflected beam [12]. This deflection is measured very precisely using a quadrant photodiode detection scheme (Figure 1b), a principle invented for the Atomic Force Microscope (AFM) microscope [11]. Another possibility is to measure a change in the resonant frequency of the cantilever due to the mass loading [13,14]. The detection sensitivity of chemically functionalized MEMS is in the range of 1:10\({}^{\text{\text{\  
cantilevers were immersed in the thiol solutions for 24 h at 25 \(\,\)C. After the completion of the modification, the cantilevers were rinsed with absolute ethanol and blown dry with argon before use. The X-Ray Photoelectron Spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) analyses were carried out on the PHI-TFA XPS spectrometer produced by Physical Electronics, Inc. (Chanhassen, MN, USA). The analysed area was 0.4 mm in diameter and the analysed depth was about 3-5 nm. Sample surfaces were excited by X-ray radiation from a monochromatic Al source. The surface composition was quantified from XPS peak intensities considering the relative sensitivity factors provided by the instrument manufacturer. Two different locations were analysed on each sample, and the data were averaged. The surface composition and chemical bonding of the modified surfaces were characterized by the XPS technique. Figure 3 shows typical survey XPS spectra from the Au surface modified with 4-mercaptoethanol acid. The peaks of carbon, oxygen, sulphur, and gold were identified in the acquired XPS spectra. Figure 3: The representative X-Ray Photoelectron Spectroscopy (XPS) spectra analysis of the surfaces of the cantilevers modified with 4-mercaptoethanol acid (MBA). Figure 2: Chemical modification of gold-coated cantilever surface with: (**a**) 4-mercaptoethanol acid (MBA), (**b**) 6-mercaptoethanol acid (MNA), and (**c**) 2-aminoethanethiol (AET). In accordance with the molecular structures of 4-mercaptoethanol acid, the C and O peaks increased and the Au peak decreased compared to the unmodified Au surface; in addition S peaks appeared after the modification. These results confirmed successful bonding to the Au surface. The inserted spectrum in Figure 3 is high-energy-resolution S 2p XPS spectrum.