UC San Diego

The coffee-ring effect (CRE) is a common occurrence in drying droplets, characterized by the formation of a ring-like deposit around the droplet’s edge due to differential evaporation rates. This phenomenon is widely observed in various applications, including enhancing detection in biological assays, where researchers have explored ways to utilize the CRE to improve analyte concentration. Reversing the CRE becomes crucial as it enables the concentration of biological analytes at the droplet's center, thereby enhancing detection sensitivity in subsequent assays. In this work, we discuss a simple method to reverse the CRE using airflow, creating a reverse evaporative gradient, and providing a potential solution for improving sensitivity in bioassays. The initial study focuses on utilizing the evaporative enrichment method to control fluid flow for solutes in liquids. The general principle of reversing the evaporative gradient is demonstrated through use of an IR laser, then accomplished in simpler terms using airflow. The simplicity and adaptability of this technique are showcased through applications in colorimetric detection of proteins and nucleic acids on paper in point of care settings. By employing airflow, the colorimetric substance can be concentrated, mitigating its uniform distribution. This concentration allows for the formation of distinct spot morphologies on the paper, enabling more precise detection of analytes compared to traditional methods. The next part of work explores the capabilities and implementation of the airflow method to concentrate various particles and biological analytes in liquid samples. Model analytes, such as λ-DNA, HeLa-S3 RNA, and heat-inactivated SARS-CoV-2, were included in spiked samples to assess the resulting reduction in cycle threshold values in PCR analysis. Additionally, co-enrichment of PEG was applied as a sample pre-treatment method to minimize interference from salt accumulation in downstream analysis. While the integration of airflow enrichment was initially explored in microfluidic systems, the method was also scaled up to larger volumes to assess its efficacy in common laboratory practices related to PCR-based detection.