no-instrument measurement in paper-based device

Paper-based devices (PBDs) have emerged as a new class of inexpensive devices with potential use in medical diagnostics, environmental applications etc. They are not only inexpensive but also proven to be sensitive, quantitative, and reliable in many applications.

The colorimetric detection is one of the popular methods of quantitation of analytes in PBDs. Color developing reagents are used on the micro-reaction chambers on paper. The color developing reagent then reacts with the analyte of interest when the later is applied specifically and quantitatively. Simple and widely available instrumentations like a office scanner or a camera (could be a mobile phone camera) are used to capture the color which is then quantitated by measuring the signal using image processing software.  If you don't have camera and/ or image processing software, don't worry. Scientists from Colorado State University have developed even a simpler method to quantitate analytes. This interesting work has recently been published in Lab on a Chip Journal.

The new technique relies on the length of the color developed. They call this as a simple distance-based detection. You just need a ruler to quantitate the analyte concentration: no camera, no scanner, no software.

How does this work?

1.

First of all create a paper-based device. In their work, they have used wax printing to make PBDs. Their design has a straight channel, like a thermometer. There is one circular reservoir at one end. This reservoir is used for sample addition and/or transfer of analyte to the detection zone (long conduit).

2.



Colorimetric detection reagents are deposited along the flow channel. They tested two different methods for this purpose: spray application or pipetting. Once the reagents are dry, the device is ready to use.

3.

Sample solution in water is then added to the sample reservoir which flows into the detection/flow channel. The analyte reacts with reagent and develops color. Once all of the analyte is consumed, the color development stops. Analyte quantification is then done by measuring the length of the colored region in the detection zone by a ruler.

Authors of this paper have demonstrated their technique by measuring three different analytes: nickel, reduced glutathion, and glucose.

This new measurement technique is claimed to be accurate and sensitive compared to conventional methods.

In this figure, you can see that higher the concentration of analyte-the longer is the color developed region in detection zone.

shallow region on microchannel for better separation

CZE technique, introduced in the 1960s, is used to separate charged analytes under the influence of an electric field. Charged analytes are separated on the basis of their size to charge ratio. Some of the applications of CZE include separation of DNA (e.g, in DNA fingerprinting), separation of proteins, analysis of basic drugs and related substances in pharmaceutical analysis etc.

Researchers from University of Wyoming have demonstrated that "shallow region" in a microfluidic separation duct enhances the resolving power of capillary zone electrophoresis (CZE). They have shown a 8 fold increase in resolving power while separating a mixture of amino acids utilizing this technique. 

Simple microfluidic device to have better separation in CZE
as described by Xia and dutta (copied from Anal Chem)

In their recent article published in Analytical Chemistry, Prof Dutta and his student Ling used pressure driven back flow (counter flow to electroosmotic flow in the microchannel) to achieve their goal of high resolution CZE. This back flow was produced by a ~0.5 um deep shallow region. When electric field is applied on the separation channel, a mismatch in electroosmotic flow generates pressure driven back flow. Even though this back flow introduces some band broadening, overall increase in resolving power was better than a device without shallow region.

Authors claim to have following advantages in their device:

1. This device omits the use of an external pump for pressure generation simplifying its fabrication and operation;

2. It can readily be integrated into any detection and/or downstream analysis method; and 

3. It is suitable for further miniaturization of the CZE technique to the submicrometer length scale.

link of article dx.doi.org/10.1021/ac302530y