**3. A novel surface stress-based biosensor**

Surface stress-based membrane biosensors, whose bottom surface can be sealed by the bio‐ logical solutions, are ideal for the development of novel surface stress based biosensors with capacitive readout. Capacitive readout has the advantage of easy and accurate detection as well as it is suitable for device miniaturization. However, it is not applicable in the cantile‐ ver biosensors due to faradaic currents in the electrolytic solutions.

Due to the advantage of membrane biosensors, a new biosensor is proposed by Micro Nano Sys‐ tem Research Center (MNSRC), which can be used to detect cells. It consists of micro-fluidics, sensitive membrane and capacitive readout unit. The sensitive element of this biosensor membrane (Figure 9) is formed by the Polydimethyl siloxane (PDMS) thin film with part gold coated on its surface and the trapezoidal structure in the silicon substrate. The surface stress changes when analyte species adhere to the probes which are immobilized on the membrane surface. The change of surface stress causes an out of plane deflection that alters capacitance.

The parameters of membrane (Table 1) have been decided based on the FE(finite element) analysis software ANSYS just like reference [86] and the fabrication techniques. The dimen‐ sion of one whole bio-sensor chip is 13×7mm2 (length×width).

**Figure 9.** Schematic diagram of membrane[86].

*2.2.2. Chemiluminescence*

102 State of the Art in Biosensors - General Aspects

*2.2.3. Radioactivity*

applications.

**3. A novel surface stress-based biosensor**

sion of one whole bio-sensor chip is 13×7mm2

ver biosensors due to faradaic currents in the electrolytic solutions.

Chemiluminescence or chemoluminescence is the phenomenon of light emission as the re‐

Chemiluminescence (CL) analysis promises high sensitivity with simple instruments and without any light source, so chemiluminescence has become an attractive detection method in µTAS in recent years [75-78]. On the other hand, many flow sensors based on CL reaction and molecular recognition using enzymes, which have great sensitivity in environmental, biomedical and chemical analysis, have been developed [79-84]. However, limited feature resolution because of signal bleeding and limited dynamic range is the reported drawbacks of this method [85]. Furthermore, sensing with chemiluminescence can be performed only

Radioactivity-based detection method is comparable to fluorescence method except that the labels are radioisotopes instead of fluorophores. Techniques using radioactive labels offer robust and reproducible protocols in applications that require ultimate sensitivity and/or resolution. Quantification of results is possible by the following exposure of signal to autora‐ diography film or reusable storage phosphor screens in automated imaging systems. How‐ ever, automated liquid handling of radioactivity is difficult due to the need for safe handling and disposal of radioactivity, and is generally limited to manual, low-throughput

Surface stress-based membrane biosensors, whose bottom surface can be sealed by the bio‐ logical solutions, are ideal for the development of novel surface stress based biosensors with capacitive readout. Capacitive readout has the advantage of easy and accurate detection as well as it is suitable for device miniaturization. However, it is not applicable in the cantile‐

Due to the advantage of membrane biosensors, a new biosensor is proposed by Micro Nano Sys‐ tem Research Center (MNSRC), which can be used to detect cells. It consists of micro-fluidics, sensitive membrane and capacitive readout unit. The sensitive element of this biosensor membrane (Figure 9) is formed by the Polydimethyl siloxane (PDMS) thin film with part gold coated on its surface and the trapezoidal structure in the silicon substrate. The surface stress changes when analyte species adhere to the probes which are immobilized on the membrane surface. The change of surface stress causes an out of plane deflection that alters capacitance.

The parameters of membrane (Table 1) have been decided based on the FE(finite element) analysis software ANSYS just like reference [86] and the fabrication techniques. The dimen‐

(length×width).

sult of a chemical reaction, which leads to limited emission of heat (luminescence).

once, unlike fluorescence-based methods which can be archived for future imaging.


**Table 1.** The membrane parameter [87]

Based on the parameters, the biosensors were fabricated [87].One biosensorchip contains two micro-membranes, one acting as the active membrane and the other as reference, as shown in Figure 10. The active membrane is sensitized to react with specific analytes. The selective biochemical reactions between the analytes and the membrane will induce a sur‐ face stress change that causes the membrane to deflect. The analytes will not present on the reference membrane, it is only used to remain sensitive to other environmental factors that can also result in a deflection of the active membrane, such as temperature variations (bi‐ metallic effect), laminar or turbulent flow around the membrane, vibrational (including acoustic) noise, non-specific binding (including the swell induced by solution) and so on. The differential signal is solely due to the interaction of the analytes with the membrane. This design has very sensitive surface stress measurements for analytes detection, which is one main novelty of the biosensor.

**Figure 10.** The schematic structure of biosensor(a: 3D schematic diagram of biosensor, b: Cross-section view of bio‐ sensor) [87]

One biosensor test systems was set up based on the fiber optic interferometer (FOI) debug‐ ged for the test [88]. Figure 11 shows the comparison of active signals and reference signals obtained from FOI-based biosensor test system. Lines 1 and 2represent the basic reference signals when 20µl pure medium (no E.coli) was loaded on the membranes. The voltage had a big change at the loading point because the membrane deflections were first mainly caused by the medium weight. With the evaporation of medium, the influence factors of weight and reflex became smaller and the signals increased again. The active signals shown by lines 3 and 4 were achieved when20µl medium with living and with dead E. coli (1.7 × 103 cells/µl)was loaded into the reservoirs, respectively. The primary deflections also came from the weight and the reflex of E. coli medium, so at the loading point the active signals became smaller as well.

**Figure 11.** The comparative signals between only medium and the medium containing living or dead E. coli[88]

For E.coli detection, the experimental results show that the conditions of E. coli can be de‐ tected based on the deflection tests. For the biological and medical applications, other kinds of cells or molecules can also be detected based on the biosensor test systems. The analysis of some pathological changes of cells or molecules can be carried out based on the alteration of membrane deflection. Furthermore, the deflection signal can be translated into electric signal though adding an electrode on the substrate, which is still promoting for portable sensitive biosensor by us.

#### **4. Conclusion**

Labeling of biomolecules with fluorescent or other similar tags for detection can result in sample losses during the labeling and purification process and occasional loss of functionali‐ ty, especially in proteins and other small molecules. Label-free detection technique can con‐ quer the disadvantage, and the optical interference method is easy and simply to operate and high precision to detect the deflection of membrane for getting the information/proper‐ ties of the biological analytes. Label-free detection measurements of biosensors are becom‐ ing a promising alternative approach to traditional label-based methods. Surface stressbased micro membrane biosensor is a relatively new class of sensor, this kind of biosensor allows the detection in opaque media and the miniaturization of the sensor to incorporate it in portable devices for point-of-use sensing. The technique is advantageous since the proc‐ ess does not require any labeling of target molecules or the addition of redox probes.

PDMS attracts extensive attention worldwide as a membrane of sensors. One kind of PDMS micro membrane biosensors was successfully fabricated through conquering many challeng‐ es, e.g., integration of PDMS processing with conventional micro-fabrication processes and the fabrication of perfect PDMS thin film.
