Label-Free Electrochemical Aptasensors Based on Photoresist Derived Carbon for Cancer Biomarker Detection
Authors: Forouzanfar Shahrzad , Alam Fahmida , Pala Nezih , Wang Chunlei 
Published: December 2020

Cancer biomarkers are substances found in the human fluid, tissue, and bone marrow that can indicate the presence of cancer diseases in the human body. In the past decades, various cancer biomarkers have been discovered for early detection of cancers and risk assessment of reoccurring cancers. Among the discovered cancer biomarkers, platelet-derived growth factor-BB (PDGF-BB) is an essential biomarker for early detection of cancer diseases. This biomarker plays a significant role in the development of multiple solid malignant tumors (e.g., breast, brain, pancreatic, prostate, and ovarian) and lymphatic metastasis. A feasible means for early detection of cancers is developing point-of-care biosensors based on synthetic DNA or RNA bio-recognizers (i.e., aptamers), which are referred to as aptasensors. Various advanced technologies have been investigated for developing cancer biomarker aptasensors, including optical, piezoelectric, and electrochemical based techniques. Among the developed technologies, label-free electrochemical aptasensors are highly potent means for this purpose since they can reduce the cost and complications of sample preparation, and the electrochemical cells can be efficiently miniaturized and integrated with available lab-on-chips and MEMS technologies. However, the sensing performance of the label-free electrochemical cancer aptasensors, including the limit of detection, selectivity, and dynamic range requires further enhancements to be at similar levels to benchtop testing. In the light of the importance of PDGF-BB detection and demanded enhancements for label-free electrochemical aptasensors, we have developed label-free electrochemical aptasensors based on photoresist derived carbon microelectrodes. The active electrode of this aptasensor was synthesized via well-established C-MEMS (carbon microelectromechanical systems) fabrication technology. C-MEMS platforms have distinguishing features such as low background capacitance, high stability when they exposed to different physical/chemical treatments, biocompatibility, and good electrical conductivity. Furthermore, the surface of C-MEMS can be modified effectively via electrochemical processes as well as depositing nanomaterials. The applied C-MEMS fabrication in this study includes photopatterning of SU-8 25 negative photoresist-based microelectrodes and carbonization of the developed microelectrodes at high temperatures and oxygen-free tube furnaces. The carbonized microelectrodes were functionalized utilizing oxygen plasma oxidation pretreatment to introduce carboxyl groups to the surfaces of the carbon microelectrodes. The PDGF-BB affinity aptamers were covalently immobilized via amid binding of amino-tag terminated aptamers and carboxyl groups covered carbon surfaces. The Fourier-transform infrared spectroscopy (FTIR) confirmed the successful carboxyl group functionalization of the C-MEMS microelectrodes and covalent immobilization of affinity aptamers via amide biding. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used for characterizing the C-MEMS based aptasensors in different stages of development and sensing performances. The turn-on sensing strategy was deployed via measuring the charge transfer resistance (RCT) from EIS Nyquist plots, which yielded to wide sensing linear range of 0.005 – 50 nM with a high sensitivity of 14.82 × 103 Ω.(Log (M))-1 and low limit of detection of 1.9 pM (S/N=3). The turn-off sensing strategy was applied via measuring capacitance from CV curves, which conceded to wide linear response range of 0.01 – 50 nM with a high sensitivity of 29.97 mF.cm-2.(Log (M))-1 and low limit of detection of 7 pM (S/N=3) toward the PDGF-BB. The developed label-free electrochemical aptasensor exhibited good selectivity, stability, and repeatability, which is highly promising for future lab-on-chip and point-of-care cancer diagnosis technologies.