# Sensor Micro and Nanoparticles for Microfluidic Application
## 5 Microfluidic Systems for Biomedical Applications
### Colorimetric Particle Sensor for Microbiology and Biomedical Measurements
In the case of FITC antibody captured amplified DNA complex with biotin antibody-immobilized gold nanoparticles by antigen-antibody interaction, a reddish color appeared within 1 min due to the gold nanoparticle accumulation on the test line. By comparing the ADLFIA technique to agarose gel electrophoresis, it was found that sensitivity of both methods was similar but detection using ADLIFA was simpler and could be identified only with the naked eye. Immunochromotographic strip (ICS) tests are widely applied for analysis of DNA and protein because of their simplicity, low cost, user-friendly format, rapid detection, requirement of small sample volume and good stability over different environmental conditions. [PERSON] et al. [68] presented an integrated microdevice for RT-PCR with colorimetric immunoblotting detection of influenza virus (AH1N1). Au NPs, labelled with anti-hapten antibodies, were employed for colorimetric measurements that were identified by the naked eye. Figure 15 illustrates a fully integrated microdevice, which was made from five-layered glass-polydimethylsiloxane (PDMS), incorporating a micro-RT-PCR chamber with an ICS. By combining an RT-PCR with an ICS technology, genetic analysis of influenza virus was performed rapidly and simply. To demonstrate the colorimetric detection sensitivity, the signal intensities measured by ICS and the agarose gel electrophoresis methods were compared. During ICS analysis, a minimum needed amount of viral RNA sample was 17.2 fg in order to see violet color signal at the test line. Meanwhile, for gel electrophoresis, at least 1.72 pg of the RNA sample was needed to generate a dim band. As a result, it was demonstrated that the ICS analysis was 100 times more sensitive. Moreover, ICS analysis was performed within 10 min and without any specific analytical devices. However, the agarose gel electrophoresis analysis needed additional equipment (UV illumination) and took longer than 1 h. It was successfully demonstrated that genetic analysis of influenza AH1N1 virus could be completed within 2.5 h by using a portable microdevice incorporating RT-PCR with an ICS. The color changes of AuNPs can be inhibited by adding aptamers. For this reason, complexes consisting of AuNPs, specific aptamer and aggregation reagent can be applied as biosensors in order to detect target analyte in a wide spectrum of applications. One of the examples was presented by [PERSON] et al. [69] for the realization of a microfluidic micropaper-based analytical device (\(\upmu\)PAD). In a four-channel microdevice, gold nanoparticles (from 20 to 40 nm size) were used as colorimetric nanosensors. Additionally, by applying specifically tailored aptamers, cocaine, codeine and methamphetamine drugs were captured and detected. The detection principle was based on salt-induced aggregation of gold nanoparticles, which produced a color change by indicating the presence of the target drug. During the experiment, lateral flow of MgCl\({}_{2}\) and sucrose solution was created by capillary action. Due to the formed flow, small analyte molecules were moved up hydrophilic channels to the aptamer area. In the case that the target drugs molecules were in the sample, aptamers bound to them. After, complexes of the target drug and aptamer were transported to the area of gold nanoparticles, where nanoparticles aggregated due to presence of sucrose and MgCl\({}_{2}\). As the result of aggregation, the color in the sample area visibly changed. In the absence of drug  
Figure 14: Schematic illustration of colorimetric biosensor working principle for rapid determination of _Escherichia coli_ O157:H7 by using gold nanoparticle aggregation and smart phone imaging. Reprinted from [66] with permission from Elsevier. molecules, the unbound aptamers were carried by the flow of MgCl\({}_{2}\) and sucrose to the gold particle area. Aptamers bound to gold nanoparticles; therefore, the aggregation was inhibited. One channel in the microfluidic device was without the aptamer area and was used as a control.