# Sensor Micro and Nanoparticles for Microfluidic Application
## 5 Microfluidic Systems for Biomedical Applications  
### Colorimetric Particle Sensor for Microbiology and Biomedical Measurements  
Colorimetric methods are simple, fast, inexpensive and are widely used for determining the concentration of analyte or identifying chemical compound in a solution through the color changes of the reagent. For biosensing purposes, colorimetric assays are often based on usage of gold and silver nanoparticles. Due to the fact that the surface plasmon resonance properties of noble metal particles are strongly influenced by the interparticle distance, the difference between aggregates and individual particles can be simply visualized by the color change in particle absorption spectra. The aggregation of AuNPs (normally 10-50 nm in size) causes a color change from red to blue, and for AgNPs--from yellow to brown due to the interparticle coupling effect. This recognizable change of color creates the ability to design colorimetric sensors for target analytes [33]. [PERSON] et al. [66] developed an innovative microfluidic and colorimetric sensor for determination of _E. coli_ O157:H7. The presented biosensor was based on the aggregation of AuNPs and imaging by using a smart phone. Gold nanoparticles were synthesized by the hydrothermal reduction method when chloroauric acid was reduced with trisodium citrate in boiling water. The detection principle of the biosensor is shown in Figure 14. The biosensor microfluidic chip consisted of two mixing channels--the separation and detection chambers. In the beginning of the experiment, the sample containing the target _E. coli_ was mixed in the first mixing channel with magnetic nanoparticles (MNPs) containing capture antibodies and the polystyrene microparticles (PSs) functionalized with the catalases as well as the detection antibodies against _E. coli_. After mixing, the formed complexes of MNP-bacteria-PS were localized in the separation chamber by applying A magnet. Then, H\({}_{2}\)O\({}_{2}\) was introduced into the channel, it was catalyzed by catalases that were incorporated on MNP-bacteria-PS complexes, and therefore, broke down into oxygen and water. After injection of the crosslinking agents and gold nanoparticles into the mixing channel, all reactants were mixed, and the final mixture was collected in the detection chamber. In the presence of _E. coli_, aggregation of gold nanoparticles was prevented, and the solution color become red. In case of the sample without _E. coli_, MNP-bacteria-PS complexes were not formed; therefore, H\({}_{2}\)O\({}_{2}\) was not catalyzed and the aggregation of the AuNPs took place resulting in the solution color change into blue. Color change was measured via smart phone imaging. The concentration of the _E. coli_ in chicken samples was determined with good sensitivity and specificity. However, it was determined that the limit of detection was 50 CFU/mL and did not meet the criteria for practical applications, which was 1 CFU/mL. Moreover, the efficiency of the immune reaction was not satisfying enough. By applying the active micromixers or at least smaller-sized passive mixers, the detection system could be improved. [PERSON] et al. [67] applied a microfluidic continuous-flow reverse transcription PCR (RT-PCR) chip in order to amplify influenza virus RNA and used amplified DNA detection lateral flow immuno-assay (ADLFIA) for amplified DNA detection. FITC labelled and biotinylated primers were used for the amplification of Virus RNA. In the proposed colorimetric assay, gold nanoparticles were labelled with biotin antibodies by a physical adsorption process. PEG dissolved in the buffer and bovine serum albumin (BSA) were added in order to block non-coated gold nanoparticle surfaces. In order to perform the detection, the amplified DNA and biotin antibody-immobilized gold nanoparticles were mixed together and absorbed by a capillary effect for ADLFIA, in which FITC antibody was immobilized on the test line and mouse IgG antibody was immobilized on the control line.