Advances in materials that enable quantitative point-of-care assays
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Introduction Point-of-care (POC) assays are used for detecting analytes in environments that lack laboratory infrastructure.1 Quantitative POC assays are more challenging to perform than qualitative assays, yet they provide the most detailed—and oftentimes essential—diagnostic information (e.g., glucometers). Traditional approaches to quantitative POC assays are not suitable for use in remote villages, and therefore new technologies are needed.2–6 New materials that simplify standard POC chemistry will enable assays that are ideally suited to the most resource-poor environments in the world without compromising sensitivity and selectivity. Such a simplification can be achieved by (1) developing appropriate detection and signal amplification reagents, (2) designing inexpensive and readily available assay platforms, and (3) advancing batteries and electronics with a focus on low cost and simplicity of operation. Progress in these three areas requires close collaboration between organic and analytical chemists as well as materials scientists, with guidance from medical practitioners. Creative advances in materials chemistry also may enable entirely new types of quantitative POC assays that circumvent the tenets of traditional analytical chemistry; we will show one example of this type. This overview is organized by features that are common in traditional quantitative POC assays, with a focus on new
materials (such as paper that is patterned into hydrophilic microfluidic channels) that enable low-cost assays while still maintaining performance. Other materials in this context include micron-thick squares of metal electrodes embedded in paper, paper coated with paraffin wax, and organic materials coated onto paper that switch from hydrophobic to hydrophilic in response to specific analytes. The examples presented are primarily from our work, but other articles in this issue and published reviews7–10 provide summaries of alternative and often complementary approaches.
Defined sample volume Fixed volumes of sample are needed in quantitative assays to normalize the number of molecules of analyte in a sample that are exposed to the assay reagents. One of the most straightforward methods for obtaining a fixed volume of sample is to use (or create) an absorbent material that holds a defined quantity of sample when wet. In 2007, Whitesides and colleagues demonstrated that paper (an abundant and absorbent material) could be used for this purpose when the paper is patterned into defined hydrophilic and hydrophobic regions. This process creates microfluidic channels in paper, where the hydrophobic paper confi nes a sample into the hydrophilic channels (Figure 1a–b).11,15,16 These channels absorb relatively fixed volumes of water and are capable of
Scott T. Phillips, Department of Chemistry, The Pennsylvania State University; [email protected] Gregory G. Lewis, Department of Chemistry, The Pennsylvania State University; [email protected] DOI: 10.1557/mrs.2013.57
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • APRIL 2013 • www.
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