The self-assembled carbon nanotube (CNT) and indium oxide nanoparticle (INP) multilayers are presented for the applications to electrochemical pH and biological sensing. The excellent electrochemical properties of the nanomaterial thin film made of layer-by-layer self assembly is exploited to design and fabricate sensors targeted for a facile and low-cost application.
The pH-sensitive conductance of the self-assembled CNT/INP chemoresistor and ion-sensitive field-effect transistor (ISFET) is studied, and its shift mechanisms are elucidated. There are two primary factors influencing the conductance of the semiconducting nanomaterial thin film: the direct protonation/deprotonation and the proximal ion effect. The CNT chemoresistor experiences the conductance change due to the molecular protonation/deprotonation of carboxylic groups. The effect of proximal ions demonstrates conventional semiconductor theory, where the pH increase corresponds to negative shift in gate voltage resulting in a higher conductance in p-type CNTs. The additional silica nanoparticle (SNP) layer adjusts the pH-sensitive conductance behavior, particularly from nonlinear to linear response, which is beneficial to the implementation of pH sensors. Indeed, the electrochemical properties of nanomaterial thin film are tunable by exploiting a different type of the nanomaterial, surface chemistry, and structure.
Glucose biosensors and immunosensors are designed and implemented based on the conductance shift mechanisms explored. The sensitivity of CNT chemoresistor and ISFET glucose sensors is 10.8 and 18-45 μA/mM, respectively, on a linear range of 0-10 mM with a response time of a few minutes. An INP chemoresistor sensor array is designed to address variant electrical properties of the nanomaterial films, allowing the statistical analysis of data with one-shot of sample delivery. The INP immunoglobulin G (IgG) ISFET sensor exhibits a resolution of 40 pg/ml, and the CNT conductometric H1N1 swine influenza virus (SIV) sensor demonstrates a detection limit of 180 viruses TCID50/ml with a specificity to non-SIVs.
The nanomaterial thin film electrochemical transducers are proven to be a potent candidate for the next-generation of the chemical and biological sensors possessing a high sensitivity and resolution. Due to a facile implementation and operation, nanomaterial biosensors could be used for domestic and clinical diagnosis, point-of-care detection, and a sensing component in lab-on-a-chip systems.