Investigation on Applicability and Suitability of Micro Cantilever Based Biosensors for DNA Detection

A micro cantilever is an extreme sensitive bio sensing transducer responding to detection of the target molecules in nano scale unit [1,2]. Generally, this device consists of a tiny horizontal beam which is fixed on the supporting material at one end of this beam. Another end is normally free for spatial movement according to the molecular interaction at biological recognition area on the beam surface. Micro cantilever-based biosensors have attracted much attention due to their small size, low cost, fast response, high sensitivity, and suitability for parallelization into arrays [3-5]. The biosensors are generally operated in two modes: static bending and dynamic resonance frequency shift [6]. The current methods for measuring micro cantilever bending involve optical, interferometer, piezo resistive, and capacitive detection technologies [7]. In the static case, label-free biosensors have provided common platforms for DNA hybridization [8], biotin-antibody binding [9], and BRAF mutation in RNA from melanoma cells [10]. Biological interaction between target molecules in analyzed samples and specific capture molecules immobilized on a surface of the beam induces structural bending. Deflection length of the micro cantilever beam can be measured by an optical device as found in Atomic Force Microscopy (AFM) application [11-15]. Optical-based detection offers very high sensitivity in determining a deflection length. However, this system requires great precision of the external readout device, laser source assembly, and position sensitive photo detector (PSD) which increases both cost and size of the whole device. A micro cantilever integrated with an embedded piezo resistive material serves as an inexpensive and portable micro cantilever biosensor platform [16,17]. Deflection length of this device is monitored by using the resistance change of piezo resistive material due to structural bending of piezo resistive material inside the beam [18,19]. Several previous studies developed piezo resistive micro cantilever-based DNA sensors to detect the short piece of synthetic DNA targets [8,2022].


Introduction
A micro cantilever is an extreme sensitive bio sensing transducer responding to detection of the target molecules in nano scale unit [1,2]. Generally, this device consists of a tiny horizontal beam which is fixed on the supporting material at one end of this beam. Another end is normally free for spatial movement according to the molecular interaction at biological recognition area on the beam surface. Micro cantilever-based biosensors have attracted much attention due to their small size, low cost, fast response, high sensitivity, and suitability for parallelization into arrays [3][4][5]. The biosensors are generally operated in two modes: static bending and dynamic resonance frequency shift [6]. The current methods for measuring micro cantilever bending involve optical, interferometer, piezo resistive, and capacitive detection technologies [7]. In the static case, label-free biosensors have provided common platforms for DNA hybridization [8], biotin-antibody binding [9], and BRAF mutation in RNA from melanoma cells [10]. Biological interaction between target molecules in analyzed samples and specific capture molecules immobilized on a surface of the beam induces structural bending. Deflection length of the micro cantilever beam can be measured by an optical device as found in Atomic Force Microscopy (AFM) application [11][12][13][14][15]. Optical-based detection offers very high sensitivity in determining a deflection length. However, this system requires great precision of the external readout device, laser source assembly, and position sensitive photo detector (PSD) which increases both cost and size of the whole device. A micro cantilever integrated with an embedded piezo resistive material serves as an inexpensive and portable micro cantilever biosensor platform [16,17]. Deflection length of this device is monitored by using the resistance change of piezo resistive material due to structural bending of piezo resistive material inside the beam [18,19]. Several previous studies developed piezo resistive micro cantilever-based DNA sensors to detect the short piece of synthetic DNA targets [8,[20][21][22].

DNA Detection
The detection of DNA is considered to be a milestone in the advancement of microcantilever biosensors. Experiments have shown that deflections of the DNA micro cantilever can be induced by many factors such as length and sequence [23], grafting density and hybridization density of molecules [24], buffer salt solution concentration [25], moisture concentration [26], time, and temperature [27]. The detection of nucleic acid hybridization is achieved using cantilever arrays where the micro cantilever surface is modified using oligonucleotide chain which reacts with the complementary single chain solution. The cantilever transduces this reaction process into the mechanical deflection. Compared to the commercial silicon nitride, the su8 polymer fabricated cantilever shows enhanced sensitivity for single stranded DNA by a factor of 6 [28][29][30][31][32][33][34]. Micro cantilevers have also been investigated for studying the secondary structure of DNA. A comprehensive table of applications of micro cantilever sensor in DNA detection is provided in Table 1. One such micro cantilever based mechanical resonance DNA detection using gold nano particle-modified probes has been demonstrated in [35]. The measurement of the mass change of a micro fabricated cantilever which is induced by DNA hybridization through the shift of the resonance frequency of the cantilever is the main focus. The attachment of gold nanoparticles on the cantilever reflects the hybridization which is then chemically amplified by gold nano particle-catalyzed nucleation of silver in a developing solution. The gold-thiol covalent bonding link the capture DNA strands on the cantilever as shown in Figure 1 & 2, following that the cantilever is dipped into the target DNA solution for hybridization. After that the DNA strands labelled with gold particles, are hybridized on the other end of target DNA through complementary interactions. When exposed to photographic developing solution the gold nano particles act as nucleating agent for the growth of silver. Detectable frequency shift is seen from the growth of silver particles by increasing the effective mass of the micro cantilever, which can be readily detected.

Materials
A wide range of materials have been investigated for micro cantilever sensors. Most popular materials include silicon and silicon based materials like silicon nitride and silicon dioxide because these materials resonate with high Q-values and have low energy dissipation. In addition to traditional silicon based materials the magneto elastic and piezoelectric materials exhibit unique properties of both actuation and sensing [36]. The piezoelectric materials occur in many forms such as single crystals (e.g. Quartz), piezo ceramic (PZT), thin film (e.g.,

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sputtered ZnO) and polymer materials such as poly vinyle dene chloride (PVDF and SU8). A comprehensive table of commonly   used materials for the sensing applications of materials is  provided below Table 2 [37,38].

Conclusion
This paper investigated the application of micro cantilever biosensors for DNA detection and it was observed that the unique structural features of micro cantilever provides several advantages such as high sensitivity, high throughput, high mass detection accuracy, small volume, and low cost. Given the current advancements in the field of Micro-Electro-Mechanical Systems (MEMS), in future micro cantilevers with ability of parallel detection of multiple species at the same time by patterning different capture DNA strands can be designed.