Label-free detection of prostate specific antigen (PSA) using a bridge-shaped PZT resonator

Abstract

This paper reports the results of the development and testing of a bridge-shaped resonator to detect different concentrations of prostate specific antigen (PSA) based on shifts in resonant frequency. The bridge-shaped PZT resonator was fabricated by Micro Electro Mechanical Systems techniques, including photolithography, wet and dry etching, and film deposition. The prototype consists of multiple layers comprised of Ta/Pt/PZT/Pt followed by a supporting layer of SiN_x. The resonant frequency of the bridge-shaped PZT resonator was measured in the air by a heterodyne laser doppler vibrometer, and the frequencies were 406 ± 3, 177 ± 2, and 121 ± 1 kHz for the dimensions of 50 μm × 250 μm, 100 μm × 500 μm, and 150 μm × 750 μm (width × length), respectively. Bridge-shaped PZT resonators of different geometries have been used to detect PSA over a wide range of concentrations from 10 pg/ml to 100 ng/ml, which includes the clinically relevant diagnostic range. Using the specific binding between PSA and PSA antibody, different concentrations of PSA can be detected through the change of resonant frequency caused by the mass of the bound PSA. The measured resonant frequency and the calculated mass of bound PSA were used to calculate mass sensitivity (S = Δf/Δm). Mass sensitivities of bridges with dimensions of 50 μm × 250 μm, 100 μm × 500, and 150 μm × 750 μm were 44.14, 18.90 and 8.31 Hz/pg, respectively. This shows that mass sensitivity increases as the dimensions of the bridge get smaller. The feasibility of detecting 10 pg/ml of PSA was confirmed, which is a much smaller amount than 4 ng/ml, which is the conventional cut-off point between a patient and a non-patient in clinical diagnostics.