Field-Effect Transistor-Based Biosensor for pH Sensing and IgG Detection

Introduction: Maintaining the balance of urine pH and urine metabolite concentrations is vital for a good health, with abnormal levels indicating various conditions such as infection, kidney disease, or metabolic disorders. Hence, this work aims to create a biosensor using field-effect transistors (Bio-FET) to measure urine pH and detect multiple metabolites, starting with Immunoglobulin G (IgG).

Methods: Our Bio-FET system comprises a microfabricated gold electrode, working as an Extended Gate (EG) and a readout circuit for signal detection and amplification. pH tests were conducted with six distinct solutions (10.34, 8.60, 7.40, 7.00, 4.00, and 2.91) applied onto the EG surface, resulting in I_ds – V_ref transfer curves performed within the linear region of a MOSFET, allowing us to create a calibration curve (I_ds – pH). For the IgG detection, an Anti-IgG was covalently bound to a mixed 11-Mercaptoundecanoic acid (MUA) and 11-Mercapto-1-undecanol (MUD) (ratio 1:2) Self-Assembled Monolayer (SAM) on the gold electrodes. Each step of the surface functionalization was characterized using ellipsometry. Preliminary tests were conducted to the Bio-FET response when exposed to two distinct solutions: (1) phosphate-buffered saline (PBS) solution at pH 7.40; and (2) PBS solution spiked with IgG (0.01 mg/mL).

Results: Regarding pH tests, the results showed a linear relationship, wherein elevated pH values were associated with increased current values, with a corresponding sensitivity of 2.20 µA/pH. The binding of target IgG with the specific antibodies immobilized on the EG surface triggered a positive vertical shift of the PBS-IgG solution transfer curve when compared to the one obtained when the PBS solution alone was used.

Conclusions: The Bio-FET system shows a good response both for pH measurement and IgG detection. However, regarding IgG detection, further tests using different PBS-IgG solutions are required to evaluate the limit-of-detection and linear range of the Bio-FET system as an IgG sensor.