Here, we present a surface acoustic wave (SAW) biosensor for the highly sensitive detection of cardiac troponin I, a gold standard biomarker for the diagnosis of myocardial infarction, using a triple transit echo (TTE) wave of a SAW rather than a conventional main wave. Compared with the main wave, the TTE wave is caused by reflections from output and input interdigitized transducers (IDTs) after traversing the propagation path three times between input and output IDTs. The SAW biosensor using a TTE wave signal showed enhanced sensitivity for measuring the viscosity of glycerol solution and the mass loading effect of immunoassay reaction due to much bigger modulation induced in the amplitude, phase, frequency, and time-delay of the TTE wave. Our results showed that the proposed SAW biosensor could quantitatively analyze cardiac troponin I. Detection limit values using the main wave and the TTE wave were 766 pg/ml and 24.3 pg/ml, respectively. Therefore, one can say that the sensitivity of the SAW biosensor substantially improved when the TTE wave is used.

We present a quartz crystal microbalance (QCM) immunosensor for highly sensitive detection of prostate-specific antigen (PSA) in a human serum immunoassay. In particular, in this study, we employed signal amplification using and enlarging gold nanoparticles. Because QCM measures the change of resonance frequency according to the mass change occurring on the sensor surface, we could quantitatively analyze PSA based on a tremendous increase in mass by sandwich immunoassay using AuNP-conjugated anti-PSA-detecting antibody enhanced with subsequent gold staining. The limit of detection of the PSA immunoassay in human serum without gold staining enhancement was 687 pg ml −1 but was 48 pg ml −1 with the gold staining-mediated signal amplification. That is, amplifying the signal resulted in increased sensitivity and reproducibility of immunoassay in a human serum.

We report here on novel biosensors for the highly sensitive and specific enzymatic analysis of glycated hemoglobin (HbA1c), which we measured primarily to identify three‐month average plasma glucose concentrations. Hydrogen peroxide (H2O2) was generated in this enzymatic measuring of glycated hemoglobin from oxidative cleavage of fructosyl valine by fructosyl amino acid oxidase, and we then used the generated hydrogen peroxide as a reducing agent of gold (III) ion to synthesize gold (0) as a nanoparticle. We were able to measure the gold nanoparticles obtained from this novel approach using colorimetry under a UV–VIS spectrophotometer. For this colorimetric detection, we analyzed 96‐well plates for glycated hemoglobin, and this system worked very well, with a detection limit of 0.142% and coefficient of variation below 10%. Finally, this method can measure glycated hemoglobin in patients with diabetes.

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