Quartz crystal microbalance (QCM) technique is widely used in in-liquid biochemical applications. The main challenges remain on the improvement of sensitivity and limit of detection, as well as multianalysis capabilities and reliability.
The improvement of sensitivity has been addressed in the last decades by increasing the sensor fundamental frequency; following the increment of the frequency/mass sensitivity with the square of frequency predicted by Sauerbrey. However, this sensitivity improvement has not been completely transferred in terms of limit of detection. The decrease on frequency stability due to the increase of the phase noise, particularly in oscillators, made impossible to reach the expected resolution.
A new concept of sensor characterization at constant frequency has been proposed in this thesis, based on the phase/mass sensitivity equation: (variation of phase)/(variation of mass) = -1/(mL), where mL is the liquid mass perturbed by the resonator in its oscillatory movement; this mass reduces proportionally with the frequency square root. 
The validation of the new concept is presented in this thesis. An immunosensor application for the detection of the very low molecular weight insecticide, Carbaryl, has been chosen for the validation and compared with an improved oscillator configuration proposed in this thesis.
The new characterization concept, particularly for biosensor applications, has the following advantages: 
a) the sensor is interrogated passively with an external source, which can be designed with high frequency stability an very low phase noise, even at very high frequencies, 
b) the sensor circuit can be made very simple with high level of integration capabilities, 
c) sensors working at the same fundamental resonance frequency could be characterized, in principle, with only one source, opening the possibility of working with sensor arrays for multianalysis detection.
   

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