NANOMATERIALS MODIFIED CONDUCTING PAPER SENSORS FOR BIOMEDICAL APPLICATIONS

Abstract

Paper is a sustainable, flexible, affordable, and easy to use substrate for electrochemical biosensors. Recent advances in materials sciences have allowed researchers to incorporate nanomaterials into and modulate the porosity and surface roughness of paper to develop highly efficient paper-based biosensing devices. Such modified paper is referred to as conductive paper, owing to the conductivity imparted to it by nanomaterials. There are several ways of incorporating nanomaterials into paper and modulating its properties. One such method is the coating of paper by conductive inks. These specialized inks comprise conductive nanomaterials and binders, which can render the paper substrate conductive and ensure adherence of nanomaterials to it. These inks can be coated onto the paper either manually or using various printing techniques such as screen printing, which offer batch reproducibility and scalability to the fabrication process. Different kinds of nanomaterials have been utilized for the development of conductive inks, such as silver nanoparticles, carbon nanomaterials (graphene, carbon nanotubes), PEDOT:PSS, etc. Multiwalled carbon nanotubes (MWCNTs) have good conductivity and dispersibility in solvents, intrinsic adherence to plastic and paper, and mechanical strength. These can be for development of conductive ink towards the applications of printed and flexible electronics. Biosensors have sparked a renewed interest in the post-pandemic era. These fast, specific, sensitive, and reliable counterparts of the more traditional molecular assays can prove instrumental towards better access to health care and mass screening for several diseases. Electrochemical biosensors (that convert a biological signal into a current/resistance/voltage change) have proven more successful in terms of their quantitative nature, low power requirements, ease of use, and ability to be miniaturized. With an increasing focus on accessible health care and the rise of telemedicine, biosensing devices are being developed to be more vi flexible, wearable, and even skin-conformal. Thus, there has been a shift from the conventional metallic/semiconductor biosensing substrates to more sustainable, flexible, and ‘wearable’ options such as paper. The emergence of printable conductive inks has especially brought more attention to paper-based electrochemical devices. Several different types of paper-based electrochemical biosensors have been demonstrated recently for the detection of different disease biomarkers and pathogens. Evidently, there is an increased attention on the development of paper-based electrochemical devices for the screening and detection of infectious diseases including sexually transmitted infections (STIs). Neisseria gonorrhoeae is a bacterial pathogen in humans whose infection causes the STI, gonorrhea. It is a curable infection, however lack of awareness, absence of symptoms in most cases, and stigma associated with STIs have made it difficult to diagnose and treat this infection timely. There is urgent need to develop efficient point-of-care devices that are sensitive, easy to use, specific, and yield faster results as compared to the conventional techniques being utilized for N. gonorrhoeae detection. This thesis on “Nanomaterials modified conducting paper sensors for biomedical applications” deals with the fabrication and characterization of MWCNT-modified conducting paper and its application towards the development of electrochemical DNA biosensors for N. gonorrhoeae detection. Conductive ink formulations were prepared by dispersing carboxylated MWCNTs in solvent bases. The conductive inks were optimized for conductivity, rheology, and adherence to the paper substrates. The solvent base(s) comprised terpineol (Tp) and deionized water as solvents, carboxymethylcellulose (CMC) as binder and rheology modifier, and polysorbate 80 (PS80) as a stabilizer/emulsifier. The advantage of using inks over other methods such as dip coating and in situ nanomaterial synthesis is that it can prevent nanomaterial leaching, allow change in the hydrophilicity of the paper substrate, and enable batch reproduction and scalability. The number of coats required for desirable conductivity vii (without leaching) was optimized along with the annealing temperature and time. Magnetic bead-assisted DNA assays were designed for specific capture of the target DNA (porA pseudogene sequence of N. gonorrhoeae), and integrated to the cMWCNT@paper electrodes for electrochemical detection via CV and EIS. Screen printed electrodes (SPEs) were fabricated on paper using the conductive ink, which were further utilized for detection of gonorrheal target sequence as proof of concept. This work demonstrated the development of paper-based biosensing assays with potential of point-of-care (PoC) application. It has attempted to bring STI diagnostics to the forefront and promote the development of sensitive and reliable PoC devices for STIs including gonorrhea.