# Sensor Micro and Nanoparticles for Microfluidic Application
## 5 Microfluidic Systems for Biomedical Applications
### Paper-Based Bioanalytical Systems Using Sensor Particles  
Microfluidic paper-based analytical devices (\(\upmu\)PADs) demonstrate numerous advantages for fluid handling and analysis because they are simple, affordable, portable, biodegradable, automated with capillary action, offer a low reagent and samples consumption and are capable of providing semiquantitative results for various analytes. [PERSON] et al. [71] described a microfluidic \(\upmu\)PAD  
Figure 15: Fully integrated sensor for visual determination of influenza AH1N1 virus that incorporated a micro-RT-PCR with an immunohistochemical strip (ICS). Reprinted from [68] with permission from Elsevier. modified with silica nanoparticles (SiO\({}_{2}\)) for analysis of three different enzymatic reactions. The color homogeneity and the intensity of signal were improved by trapping SiO\({}_{2}\) nanoparticles modified with APTES within the cellulose structure in the paper device detection zone. Nanoparticle surface modification was necessary for enhancing the protein adsorption process and immobilization in the device. Trapped SiO\({}_{2}\) particles provided a solid surface for adsorption of selected enzymes; therefore, they prevented the washing away effect, which created color gradient. For the semiquantitative detection of lactate, glucose and glutamate in an artificial urine sample, first of all, 1 \(\upmu\)L of selected enzymes solution was dotted on the detection zone. After drying at room temperature for 15 min, a 1\(\upmu\)L drop of specific chromogenic agent solution was spotted on the detection zone and dried again. Finally, 10\(\upmu\)L solutions of lactate, glucose and glutamate were applied on the center channel by allowing the detection zones to be reached by capillarity forces. Analysis of analytes was performed in 30 min. As a result, the three analytes were detected with limits of detection (LODs) of 0.63 mM for lactate, 0.50 mM for glucose and 0.25 mM for glutamate. T.-T. [PERSON] et al. [72] developed a colorimetric assay for the determination of targeted tuberculosis (TB) with a gold nanoparticle-containing paper-based device. By using the SPR effect, color changes of a gold nanoparticle were monitored by using a spectrophotometer. Unmodified gold particles with a diameter of 13 nm were prepared by the citrate reduction method. In this study, single-stranded oligonucleotide sequences (ssDNA) and targeted double-stranded TB DNA (dsDNA) were mixed together with AuNPs for 30 min. After adding NaCl solution to the mixture, gold nanoparticles started to aggregate by inducing a color change (color turned bluer) due to hybridization of single-stranded DNA probe molecules and targeted double-stranded TB DNA. The hybridization caused the changes of the surface charge density; therefore, nanoparticles aggregated to various degrees and the concentration of the targeted TB DNA was determined. On the contrary, in the TB negative case, the color of the mixture remained reddish even after NaCl adding. When a mixture of AuNPs-ssDNA/dsDNA complexes was spotted onto the paper device, the blue/red ratio of testing zone was measured using a smartphone. The blue/red ratio correlated with AuNP aggregation degrees, which depended on the TB concentration. As a result, the detection limit of the paper-based sensor was \(1.95\times 10^{-2}\) ng/mL. In comparison to traditional clinical molecular diagnostics, this sensing platform was performed rapidly, approximately in 1 h after DNA extraction. Moreover, the amount of solution needed in \(\upmu\)PAD was only 200 \(\upmu\)L. However, the amount of NaCl in the sample mixture could strongly influence the gold nanoparticle-based colorimetric results and increase the variance between samples because of low concentration of analyte and low sample volume. [PERSON] et al. [73] presented a convenient, low cost and rapid method for producing nanoparticle-containing test papers. First, a thin gold film was deposited on the test paper. A laser-induced annealing method was applied in order to fabricate nanoparticles.