# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Metal Particles for LSPR-Based Sensing
Photolithography was combined with hole-mask colloidal lithography in order to form homogeneous nanoparticle arrays with particle diameters of 130 nm and a height of 30 nm. Among the biggest advantages for using these methods was increased sensitivity over colloidal particles by patterning nanoparticle arrays of more asymmetric dimensions. It was possible to fabricate microfluidic devices with many channels in comparison to commercial SPR devices. In the same study, glass and PDMS plates, which contained 96 spots of nanoparticle arrays, were fabricated for sensing of protein binding. By using the 96-spot plate, it was possible to simultaneously measure the binding curves of six different protein pairs. After specific antibodies were coupled to nanoparticle arrays, the plate was dried in the room temperature and the reference LSPR spectra were measured. Then, varying concentrations of investigated proteins were added into different plate spots. After drying, the LSPR spectrum was measured, and the shift in LSPR wavelength due to binding of protein was determined. As a result, binding curves were generated, and the limit of detection (LOD) values were calculated. Integration of an LSPR-based sensor and microfluidic chip was presented by [PERSON] et al. [41]. An antigen-antibody (biotin/anti-biotin) interaction system was used to demonstrate the biological sensing by continuously measuring the transmitted light through the surface of a gold nanoparticle-coated sensor. Gold particles with a diameter of 14-43 nm were synthesized by sodium citrate reduction of chloroauric acid (HAuCl4) [42]. After, silane layer was used as molecular glue for immobilization of gold nanoparticles on the substrates. Further modification with a self-assembled monolayer (SAM) of thiols was performed in order to carry out the biological experiments. After the immobilization of biotin on SAM, the binding of anti-biotin antibody was detected with a detection limit of 270 ng/mL. [PERSON] et al. [20] presented an LSPR-based biosensor for label-free detection of inflammation-related biomarkers (IgG and CRP). In order to produce Au nanostructures, a thin gold film was deposited on the glass slide using the metallic physical vapor evaporation (PVD) coating technique, which was followed by a rapid thermal annealing (RTA) process. After RTA treatment, the color of the fabricated Au nanostructures changed to dark purple as a result of the LSPR effect. Despite simple and rapid fabrication, formed structures featured poor homogeneity and broad size distribution. The biosensor was integrated into an automated four-channel microfluidic system. For detection of antigens, channels of microfluidic device were functionalized with specific antibodies. Various concentrations of pure IgG antigen or CRP antigen solution were introduced into a sensing device, and specific antibody-antigen binding was investigated by measuring the LSPR absorbance spectrum and monitoring LSPR wavelength shift. It was shown that by using presented sensors, it was possible to detect 10 ng/mL concentration of both IgG and CRP antigens only in 60 uL of sample and in 3.5 h assay time. Gold nanostructures, particularly spherical nanoparticles, are the most commonly used for LSPR-based sensing despite the fact that silver nanoparticles would give a better sensing performance. Gold nanoparticles are characterized by high stability, and even though silver particles show higher scattering efficiency, their long-time stability is relatively low. However, by forming gold-silver core-shell particles, it is possible to improve sensors' sensitivity and stability [31]. Sensing potential of core-shell nanostructures was reported by [PERSON] et al. [43]. In this research, five different compositions of core-shell nanoparticles were presented, and their sensitivities towards changes in refractive index were investigated. The experimental findings were compared to theoretical calculation and accordingly, experimental results were supported by calculation.