Immunoassay of cardiac biomarkers using a photodiode array biochip

We present a photodiode array (PDA) biochip using a bipolar semiconductor that can detect cardiac biomarkers such as myoglobin (Myo) and cardiac troponin I (cTnI) with sensitivity and a large dynamic range. The PDA chip, which consists of a 12 × 12 array of photodiodes with each photodiode having a diameter of 200 μm, acts as both a support for antibody immobilization and as a two-dimensional detector. In addition, the PDA chip system does not require complicated optical components and can be miniaturized, resulting in the construction of a portable microarray chip system. An immune-sandwich assay was carried out on the PDA chip, followed by gold nanoparticle-promoted silver enhancement for the improvement of sensitivity. The deposited silver metals blocked the light irradiated from light-emitting diodes, resulting in a decrease of the output voltage corresponding to the produced photocurrent. The decreased voltage was directly proportional to the concentrations of target antigens present in the sample solution, allowing the specific detection and the quantitative analysis of the cardiac biomarkers. The linear ranges of quantification of Myo and cTnI were from 10 to 104 ng/mL (R2 = 0.998, n = 3) and from 10 to 105 pg/mL (R2 = 0.992, n = 3), respectively. The detection limits of Myo and cTnI were determined to be 2.2 ng/mL and 4.0 pg/mL, respectively, which are lower than the clinical cutoff levels (Myo: 70-200 ng/mL; cTnI: 10-100 pg/mL).