# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Metal Particles for LSPR-Based Sensing
For preparation of core-shell NPs, first of all, Au nanoparticles, whose average diameters were 13 and 16 nm, were synthesized according to the Turkevich method [44]. Later, silver was deposited on the gold nanoparticle surface via an electroless deposition process using the solutions of reducing agent and silver salt. Formation of the Ag shell was proven by color change of the solution (from red to yellow). By varying the concentration of silver salt and reducing agent in the system, the desired thickness of silver shell was formed. It was found that Au-Ag core-shell particles with 2-4 nm shell thicknesses showed higher sensitivity than monometallic gold or silver nanoparticles. Therefore, the LSPR-based sensor could be improved by forming Au-Ag core-shell particles with a specific shell thickness. Due to the fact that optimal shell thickness depends on the range in which the refractive index changes, for different media, core-shell nanoparticles must be designed separately. For the regime interesting for biological application, where changes in refractive index varied from 1.33 to 1.37, the optimal thickness of Ag shell should be 4-5 nm on the 13 nm size gold nanoparticle core. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Application (Analyte)** & **Particle Type** & **Size** & **Fluidic System** & **Main Advantage** & **Ref.** \\ \hline Anti-gp41 antibody & AuNPs & - & Microfluidic chip & Fast and simple & [35] \\ \hline Bovine growth hormone & AuNPs & 125 nm & PDMS microfluidic chip & Low limit of detection (185 pM) & [37] \\ \hline Detecting small & & & Nanoparticle & Low sample volume & \\ quantities of specific & AuNPs & 34 nm & -functionalized suspended-core & and a real-time & [38] \\ target molecules & & & & fiber & analyte \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Examples of particles for localized surface plasmons resonance (LSPR)-based sensing. Due to the fact that the shape of the nanostructures influences the refractive index (RI) sensitivity, rough corners, sharp tips and edges enhance the local electrical field and can strongly affect the sensitivity. [PERSON] et al. [45] reported the synthesis of Au nanocubes for LSPR sensing application by using a PDMS microfluidic chip. Gold nanocubes were formed from growing gold seeds during a metal-catalyzed deposition of gold atoms, which were reduced by sodium borohydride. Growth of nanocubes was stopped and prevention from overgrowing was achieved by using centrifugation. In comparison to batch synthesis, it was found that Au seed particles, prepared using a single-phase or segmented flow microfluidic methods, were more homogenous and smaller. When sensitivity of the 80 nm Au nanoparticles and 78 nm Au nanocubes was compared, the huge effect of edges and corners was shown. Au nanocubes were 2 times more sensitive (with 202 nm/RIU) then the Au nanoparticles. [PERSON] et al. [46] presented an LSPR sensor based on silver-gold nanoprism immobilization on the optical fiber taper. For the investigation, silver nanoprisms with a size of \(40\pm 20\) nm and thickness of \(9\pm 2\) nm were formed by combining synthesis of seed particle and nanoprism growth. The silver nanoprisms were reinforced with a thin gold layer to protect the nanoprism edge against etching. In order to reach a higher sensitivity, fiber optical taper needed to be coated with a dense nanoprism monolayer. It was demonstrated, when density of particles was approximately 210 particle/\(\upmu\)m\({}^{2}\), that adjacent particles interacted with each other and boosted the sensitivity. By comparing sensitivities of a hybrid plasmonic-photonic system with bulk solution, it was found that by using tapers with densely immobilized gold-reinforced silver nanoprisms, sensor sensitivity up to 900 nm/RIU was achieved due to interparticle interactions. When interparticle distance between nanostructures increased, sensitivity of the sensor decreased to 385 nm/RIU, which was similar to the sensitivity of a bulk solution.