This paper presents a novel biosensor capable of continuously monitoring specific molecules (i.e., adenosine triphosphate - ATP) in the human serum by integrating the aptamer probes into a nanofluidic device. The advantage lies in its real-time signal regeneration of biosensor without the need of uploading the clean solutions for washing process since the ionic gate in the nanofluidic device could block or allow the target molecules flow through periodically.
Ion concentration polarization (ICP) is a transport phenomenon which is observed when an electric field is applied across the nanofluidic channels, for instance formed by using a nanoporous Nafion membrane. Once the ICP process is stabilized, an ion enrichment zone and a depletion zone are established at the both sides of the channels. These two effects have been utilized for a wide range of applications such as desalination , pre-concentration of target analyte species . ATP is an important biomolecule found in the living cells (intra/extracellular) and it relates directly to many physiological and pathophysiological events. Current techniques of secreted biomarker detections were mainly based on ion conductivity measurement, so the correlation between the signals and disease progress was limited. There were several binding assays based platforms developed before, but the contradiction between portability and functionality of signal regeneration remained. For example, aptamer assay was demonstrated for continuous real-time small molecules measurement , but the washing process was required by uploading a large amount of clean buffer solutions by professional operators for aptamer signal regeneration, which is highly demanded for applications in domestic health monitoring.
In response, we have designed an aptamer based nanofluidic device for the real-time and continuous ATP detection in a small amount of human serum (100 µl for each cycle) (Figure 1). Our proposed nanofluidic device could allow the real-time monitoring individual patient’s conditions so the proper therapy can be offered on time. The working mechanism of aptamer probes was studied in detail (Figure 2, 3). Unprecedentedly, we found that there is a strong relationship between the binding affinity of ligand-receptor and the detaching force caused by the hydrodynamic flow. The critical energy to detach the ATP from ATP-aptamer complex was calculated as 1.86 x 10-21 J and this energy can be used to settle the critical flowrate (i.e., 1 µl/min; kinetic energy E = 1.99 x 10-21 J). We demonstrated that the device could produce the clean buffer solutions and endure five washing cycles to regenerate the aptamer without significant decrease of signal (Figure 4).
In conclusion, by converting patient samples (i.e., human serum) to be clean solution for washing through applying electrical field to the device, the washing process could be approached without uploading buffer solutions. Accordingly, the dynamic signals of biomarkers could be measured in real time. With further miniaturization into a small wearable device for remote health condition monitoring, this nanofluidic device could improve current healthcare facilities significantly. The flexibility of this nanofluidic device could offer the possibility to integrate different receptors (i.e., aptamers) to monitor other secreted biomarkers in sweat, saliva.