# Surface Acoustic Wave Sensor for C-Reactive Protein Detection
## 3 Results and Discussion  
Figure 5 plots the frequency response of SAW sensors with and without the deposition of the gold-sensing area on top of the SiO\({}_{2}\) guiding layer. The fabricated 100 double pairs IDT SAW structure generated the center frequency at 57.34 MHz. The insertion loss of the SAW sensor without  
Figure 4: The measurement system for the frequency response of SAW sensors. Figure 3: Schematic representation of C-reactive protein (CRP) and anti-CRP interaction process: (**a**) Au surface cleaning; (**b**) Self-assembled monolayer (SAM) on Au surface; (**c**) Activation of SAM; (**d**) Immobilization of anti-CRP; (**e**) Blocking the remaining non-specific binding of antibody active sites; (**f**) Evaluating the interaction between CRP and anti-CRP. the deposition of the gold-sensing area was -26 dB, as shown in Figure 5b. After depositing the gold-sensing area on the SAW sensor, the insertion loss of the SAW sensor was observed as \(-\)33 dB, which is a reduction of 7 dB, as shown in Figure 5a. It is known that traveling surface acoustic waves are very sensitive to variations on the sensor surface. The deposition of the gold-sensing area may cause a mass-loading effect on the SAW sensor. Thus, the insertion loss attenuated to \(-\)33 dB, possibly due to the mass-loading effect caused by the deposition of the gold-sensing area. There are several very commonly used guiding layers, such as polymethylmethacrylate (PMMA), polydimethylsiloxane (PDMS), SiO2, and zinc oxide (ZnO) [35; 36; 37]. The guide layer can isolate and passivate the SAW devices. Silicon dioxide is the most promising material as a guiding layer, because it offers low damping, sufficient low shear velocity, and excellent chemical and mechanical resistance [38]. We selected SiO2 as a guiding layer for the confinement of acoustic energy and passivation of IDTs. Using this guiding layer, the acoustic energy of the sensor is confined in the guiding layer, keeping the wave energy trapped tightly near the surface, which makes the SAW sensor very sensitive to its sensing area [39]. After the fabrication of the SAW sensor, the immobilization process was performed for CRP/anti-CRP interaction. The CRP/anti-CRP interaction can be detected as mass variations on the sensing area. These mass variations may produce changes in the amplitude or phase of the sensor response. Different concentrations (0.1 \(\upmu\)g/mL to 1 mg/mL) of standard CRP were tested to observe the frequency response of the SAW sensor as listed in Table 1. Measurements for each concentration were performed three times. The SAW sensor was cleaned by a large amount of PBS before every measurement. Interestingly, only the amplitude variation was observed with no apparent frequency shift. Generally, the mass-loading effect in SAW sensors may change the amplitude and the center frequency. Because the thickness of the SiO2 guiding layer is much smaller than the SAW wavelength [40], it can \begin{table}
\begin{tabular}{c c c} \hline \hline
**CRP Concentration (\(\upmu\)g/mL)** & **Average Peak Amplitude (dB)** & **Standard Deviation (dB)** \\ \hline
0.1 & \(-\)31.763 & 1.187 \\
1 & \(-\)33.726 & 3.93 \\
10 & \(-\)35.184 & 3.213 \\
100 & \(-\)39.5 & 1.706 \\
1000 & \(-\)41.413 & 0.884 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The averaged values of the peak amplitude and the related standard deviations with different CRP concentrations. Figure 5: The frequency response of SAW sensors (**a**) with and (**b**) without the deposition of gold-sensing area on top of the SiO2 guiding layer. be thought as an acoustically thin, elastic film in which the SAW-induced displacement is uniform across the film and varies only in the direction of propagation. The apparent change in insertion loss with no frequency shift in the proposed SAW sensor under different CRP concentrations is unclear now. However, a very small frequency shift was observed at a very high CRP concentration (1 mg/mL). There are several possible reasons for the lack of frequency shift. [PERSON] et al.