Samriddha Dutta* and Ashok Mulchandani | University of California, Riverside Yu Shen | Lam Research Abstract: Identifying body fluids left at a crime scene is a fundamental piece of evidence in forensic science. Often, knowing what the body fluid is serves as the key in a criminal investigation. However, the identification of human body fluids has not always been an easy task because of their resemblance to other fluids or substances. Over the years, a variety of methods have been developed for the identification of commonly found body fluids at crime scenes, but most of these tests are only qualitative, suffer from low sensitivity and selectivity, and consume a significant quantity of valuable samples. To alleviate these drawbacks, this study developed a single-walled carbon nanotube (SWNT)-based chemiresistive nanobiosensor array for quantitative, selective, on-site identification of multiple body fluids, namely blood, semen, saliva, urine, and sweat, through detection of their protein biomarkers using corresponding antibodies. Chromatography paper was used as the substrate, and electrical leads were inkjet printed on its surface. The sensor array consisted of five individual sensing channels made by a simple wax-printing method, and they only required a microliter-sized sample, which was equally split into aliquots by the built-in paper microfluidics. For fabricating the chemiresistors, the researchers synthesized a dimethylformamide (DMF)-based PBASE (1-pyrenebutanoic acid succinimidyl ester)/SWNT ink where PBASE was attached to SWNTs through non-covalent π-π stacking interaction and the ink was deposited on the paper substrate by inkjet printing, keeping an optimal three-dimensional semiconductor density. The researchers identified two sets of unique biomarker proteins for each of the five body fluids from literature to be tested with this sensor. Individual antibodies were immobilized onto the SWNTs surface, and any specific antigen-antibody interaction near the interface of the SWNTs and the environment caused a significant increase in the measured electrical responses because of surface charge–induced gating effect, which gave the sensor response values. Furthermore, to facilitate easy, multiplexed sensing and reduce the time of fabrication, the researchers developed a water-based all-in-one SWNT-Antibody Bioink, containing both transducer (SWNT) and biological elements (antibody) that could be drop-casted or printed on the paper substrate to form the chemiresistor arrays. The described sensing methods showed promising results, with specific interactions giving an average response of >70% and non-specific interactions giving an average response of <10% in each case, thereby proving the specificity of the sensor. Quantitative and selective detection of the tested biomarker proteins could be demonstrated with the very low limit of detection values obtained. The paper-based chemiresistive biosensor system reported here was easy to fabricate and handle and designed for rapid, sensitive, selective detection of biomarkers. It also followed the ASSURED criteria (i.e., Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment-free and Deliverable to end users). This biosensing platform exhibited promising performance and potential for unprecedented on-site use during crime scene investigations and is expected to promote the future development and adoption of chemiresistors on paper substrates in other sensing scenarios.