An investigation into the use of zinc oxide nanowiresensors in the detection of micro-RNA cancer biomarkers

Cancer is a complex disease characterised by genes which encode oncogenic and tumour-suppressor proteins. microRNAs (miRNA), a group of small noncoding RNAs that regulate gene expression, have been shown to participate in a number of essential biological process including cell proliferation control, hematopoietic B-cell lineage fate, B-cell survival, brain patterning, pancreatic cell insulin secretion and adipocyte development [1]. Abnormal expression, that is, the loss, amplification and mutations of miRNA genes has been identified in a wide variety of cancers including B-Cell Chronic Lymphocytic Leukemia (B-CLL) [2], breast carcinoma [3], primary glioblastoma [4], hepatocellular carcinoma [5], papillary thyroid carcinoma [6], lung cancer [7], colon carcinoma [8], and pancreatic tumours [9]. Presently, medical diagnostic tests, by and large, are performed in laboratories equipped with benchtop analyzers and operated by trained lab technicians. Although these systems have a high throughput, in most cases patients wait a number of days to receive their test results [10]. Being able to perform diagnostic tests at or near the site where patients encounter the health care system; and receiving the results within the time frame of a consultation with a healthcare professional (approximately 15 minutes [11]), would be extremely beneficial. It would provide actionable information that can lead to several changes in patient management. With respect to cancer diagnostics and treatment, this would reduce the need for multiple patient visits; enabling the prompt treatment of the illness in a more targeted fashion. Point of Care (PoC) devices are diagnostic devices which rapidly provide actionable information for patient care at the time and location of an encounter with the health care system. They are becoming more prevalent. The most commonly found type of PoC device is the Lateral Flow Immunoassays (LFIA) [12] [13]. However, LFIA conventionally provide qualitative results (i.e., yes or no) which are of little use when trying to gauge changes in concentration as would be needed in detecting the loss or amplification miRNA strands. Furthermore, LFIA suffers from difficulties due to varying consistency of the flow rate and from non-uniform dispersion of the sample to label [10]. Field Effect Transistor (FET) biosensors, a promising class of PoC devices, have been shown to able to distinguish between iv different concentrations of molecular analyte [14]. This function would be vital in cancer diagnosis revolving around detection of the abnormal expression of miRNA. This is because cancerous cells typically manifest a deviation in miRNA concentration from the normal range. These FETs are made with established semiconductor techniques and technologies meaning that, they can be readily integrated with other electronic systems. This would enable on chip signal processing and the instantaneous electronic transmission of results from remote areas to a centralised hub. The goal is to leverage the advantages in semiconductor technologies to develop a PoC device for cancer diagnostics. This is to enable cancers to be caught and treated earlier thus reducing the need for invasive or debilitating treatments like surgery or chemotherapy. In pursuit of this goal, the preliminary step was to fabricate FETs capable of detecting changes in miRNA concentration. The FETs fabricated for this purpose were Zinc Oxide Nanowire Field Effect Transistors (NWFETs) arrays. ZnO is an ideal material with which to fabricate these NWFETs because it is naturally a n-type semiconductor [15], thus eliminating the need for a high temperature doping process steps. ZnO has a large and direct band-gap (3.37 eV [16]) which enables it to sustain large electric fields; withstand higher breakdown voltages; generate lower levels of noise; and operate at high temperatures and levels of power [17]. The ZnO NWFETs were passivated with stack high-κ dielectrics. The stack layer consists of a layer of Hafnium dioxide sandwiched between two Aluminium oxide layers which has been shown to diminish threshold voltage drift effectively [18]. Once fabricated, the ZnO NWFETs were first tested to observe how well they functioned as transducers of ionic charge. The ZnO NWFETs were seen to be excellent transducers of ionic charge with a shift in gate voltage per pH of 117 mV/pH. This shift in gate voltage per pH is comparable to largest known value of 220 mV/pH recorded by Knopfmacher’s single Silicon NWFET with a Dual Gate [19]. It is also twice as large as the Nernst limit (59 mV/pH). Following the pH-sensing experiment, a microDNA(miDNA) detection investigation was conducted. miDNA are the stable biological equivalent of miRNA and thus can serve as proxy of miRNA detection. The result of the investigation was compelling. The ZnO NWFETs were found to have a 43.88% Sensitivity to one order of magnitude changes in miDNA concentration (10 nM, 100 nM and 1 µM). Subsequently, the same investigation was carried out with miRNA as the analyte. In this instance the ZnO NWFETs were found to have a 5.07% Sensitivity to one order of magnitude changes in miRNA concentration of (10 nM, 100 nM and 1 µM). These results irrevocably demonstrate that ZnO NWFETs are capable of detecting changes in miRNA concentration. Thus, making ZnO NWFETs a suitable candidate for the development of a PoC device with which to conduct cancer diagnostics