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Monitoring the presence of viruses and bacteria in environmental reservoirs such as wastewater, lake water etc. is a very useful tool for early detection and controlling the spread of diseases, while also helping estimate the severity of infections at a community level. While several DNA sensors have been demonstrated for nucleic acid sensing for a variety of applications, most such demonstrations require sophisticated instrumentation or assays that need to be controllably performed in a well-equipped laboratory. In this work, we demonstrate a low-cost, portable device to detect DNA in wastewater and lake water, based on the color change in samples due to the intercalation of DNA with methylene blue (MB) dye. The optical sensor consists of a phase-sensitive detection circuit constructed with low-cost semiconductor ICs, red LED, and a silicon PIN photodiode. The phase-sensitive detection circuit has a resolution of 68μV, and the sensor has a limit of detection of 25.68 nM for DNA sensing. The utility of this sensor is demonstrated through the detection of bacteria (E. coli) and viruses (bacteriophage phi6) from wastewater and lake-water samples, respectively.

The detection of Circulating Tumor Cells (CTCs) is of profound importance for cancer diagnosis and monitoring in diseased patients. In the presented work, an aptasensor for CTC detection was developed using AS1411 aptamer specific for nucleolin on cancer cells’ surface as receptor on Gold NanoParticles (GNPs) decorated U-bent optical fiber. The aptamer binds to target upon folding into a parallel G-quadruplex structure, and therefore, components of folding and running buffer were optimized to 50 mM tris buffer and 100 mM KCl with and without 5 mM Mg+2 based on Circular Dichroism spectroscopy (CD) and bright field microscopy respectively. Further, the binding of aptamer and MCF-7 cells to GNP coated U-bent fiber were validated by fluorescence microscopy. The aptasensor showed significantly higher absorbance for 104 cancer cells (retinoblastoma, meningioma, breast, cervical, and colon cancer cells) as compared to non-cancer cells (MCF-10A and WBCs) with a t-test p-value of 0.0012. On an optical bench, aptasensor was able to detect as low as 500 cells/mL. To develop a portable and small CTC detection system, aptasensor was integrated with a hand-held opto-electronic device μSens (custom developed in our lab) and was able to detect 50 cells/mL. This is the first time a portable sensing system based on U-bent optical fiber was tested for sensing cancer cells. The ability to detect diverse types of cancer cells by AS1411 aptamer is promising and paves the way for the detection of circulating tumor cells in blood samples, with relative ease and low cost.

Understanding the fluid flow on paperfluidic substrates is crucial due to an increasing interest in affordable sensors for diagnosis, environmental testing, food testing, etc. Charged dyes are the most commonly used model analytes for characterizing the fluid flow in paperfluidic devices. Here, we propose a framework to quantify the concentration-dependent loss during transport of four charged dyes (e.g., rhodamine B, methylene blue, tartrazine and amaranth). At concentrations lower than a critical value (Cc), electrostatic interaction of the dye molecules with the filter paper retards the flow of cationic dyes rhodamine B and methylene blue, resulting in 75%–99% sample loss. We determined Ccto be 20 μM for rhodamine B and 5 mM for methylene blue. The large difference in their Ccvalues is explained by the molecular structures of these dyes. While anionic dyes (e.g., tartrazine and amaranth) are not retarded during flow on filter paper due to negligible electrostatic interaction with the substrate, they form strong coffee ring patterns in the detection zone when the dye concentration is below Cc. We introduced a new parameter called the ‘coffee ring index’ (CRI) to quantify the non-uniform sample distribution in the detection zone, and from this, determined Ccto be 250 μM for amaranth and 1.25 mM for tartrazine. Finally, we demonstrated that an increase in the dye viscosity by adding 30% glycerol can lower Ccof tartrazine from 1.25 mM to 100 μM. These results provide useful guidelines for the paperfluidics community to design more effective sensors.