br Introduction Biosensors bear a high potential

Introduction Biosensors bear a high potential for biomedicine in view of their ability to detect the mass, from the molecular level down to atomic one. The general principle of biosensing on the basis of determining the changes of mass is the detection of these changes from the accompanying change of the resonance frequency, beam deviation or resistance variation with the changes in the mass present on biosensor surface [16,10]. Microweighing by means of the quartz crystal microbalance (QCM) is used for these purposes most frequently, while biosensors based on cantilevers use the properties of piezo-resistive materials [16]. According to Sauerbray’s equation [21], if an object with definite mass is present on the surface, the change of resonance frequency is proportional to the total mass change. QCM-based microweighing can measure mass changes down to several nanograms and allows one to detect biomolecules when a receptor is immobilized on the surface. QCM-based biological identification in liquid covers the range from nM [11] to several hundred fM [13,25] and is used for continuous analysis of the substance under study. During the recent years, electrochemical and multichannel systems based on QCM have been developed [23]. The recent achievements were aimed at lowering the detection limit for quantitative analysis, and enhancement of selectivity, which requires surface modification. In addition, selective detection of definite biomarkers encounters some problems. For example, the ability to detect C-reactive protein in human blood is hindered because of non-specific adsorption of the proteins of human blood on sensor surface. The authors of [19] used electrode-free QCM analysis with the help of pten inhibitor at the resonance frequency of 182MHz. The C-reactive protein is essential biomarker in case of inflammation, with the threshold level of 30ng/ml. To solve this problem, they used mass analysis with the help of bispecific antibodies increasing the mass by attaching streptavidin molecules. The sensitivity of 0.1ng/ml was achieved for C-reactive protein in solution, which is substantially lower than the threshold level. Another problem of piezoelectric microweighing is the reduction of functionality in liquid. A circuit of quartz generator was proposed [8] to make a biosensor able to detect the DNA sequence in liquid; the quartz was completely immersed into the liquid. To make a highly sensitive system able to detect DNA complementarity for the concentration of 50ng/ml and higher, Miller’s effect was employed (an increase in the equivalent capacity of inverting amplifier) at the working frequency of 50MHz. Sensors based on cantilevers comprise another type of mass-sensitive biosensors. These biosensors measure quasi-static deviations of miniature mechanical elements caused by biomolecule binding with the functional groups attached to the surface of the element. These highly sensitive mass sensors are able to distinguish mass changes down to the level of single molecules. Cantilever-based biosensors measure mass change on the basis of the changes of resonance frequency of the excited piezoelectric film [9]. These biosensors are simple, highly sensitive and able to operate in the real time mode. Recent advances [1] were aimed at improvement of the sensitivity of surface and the development of more efficient methods to determine mass, independently of the position of an object on the cantilever. The applications for the devices of this kind are only starting to develop; examples are the measurements of conformation changes of proteins [24,21,4], DNA [26,7,17,20,22], RNA [27] and drug interactions [18]. A new method of QCM-based biosensing was proposed in [5], based on measurements of bond rupture forces (REVS). To break a complex characterized by moderate affinity, the acceleration exceeding the gravity by several orders of magnitude is necessary. It is possible to apply so large force to a particle under detection using the piezoelectric device. Unlike for usual methods involving QCM (for example, microbalance or resonance biosensor), in this case the QCM not only functions as a sensor but also plays an active part with respect to the particles attached to its surface. The piezoelectric properties of the QCM allow one to detect the excitation of substrate oscillations caused by bond rupture. These oscillations are converted into the electric signal.