# Sensor Micro and Nanoparticles for Microfluidic Application
## 2 Construction Principles of Sensor Particles
### Metal Particles for LSPR-Based Sensing
Specific binding of molecules was sensed via plasmonic resonance shift of gold nanoparticles, which strongly depended on the changes in the environment's IR. The efficient RI sensing was performed with a nanoparticle-functionalized suspended core fiber (SCF). The surface of interior channel was modified with amino groups in order to create adhesive layers for self-assembled monolayers of Au nanospheres. The nanoparticle layer deposition (NLD) technique was used to coat Au nanoparticles along the entire fiber channel length. The RI sensitivity measurements of various RI oils showed that, in the presence of the oil, LSPR shifted towards a longer wavelength. It was determined that the refractive index sensitivity of sensor was 170 nm/RIU for an aqueous analyte. In comparison to the sensitivity of similar gold nanoparticles dispersed in water, the proposed nanoparticle-functionalized fiber was 1.5 times more sensitive. [PERSON] et al. [39] presented a AuNP-based biosensor chip for Aflatoxin B1 (AFB1) sensing. Colloidal AuNPs were synthesized using the seed-mediated method [40]. Surface functionalization of AuNPs was modified by the solvent extraction-based ligand exchange method. First of all, CTAB surfactant was partially removed from the aqueous phase of AuNPs to immiscible organic solvent of dichloromethane. Second, complete replacement of CTAB to lipoic acid was achieved through direct  
Figure 8: Schematic model of biosensing protocol. Adapted from [37]. Copyright © 2013 Elsevier. chemisorption. For chemical conjugation of AuNPs with cystamine self-assembly modified SPRi chips, carboxylic functional groups from lipoic acid were activated by using EDC-NHS crosslinkers. After the formation of a self-assembled monolayer (SAM), chemically modified AuNPs were grafted over a SAM Au chip surface for 12 h. Unreacted AuNPs were washed out, and further modifications of SPRi chip were performed directly within the flow cell. After the activation of surface carboxylic acid groups, anti-AFB1 antibodies were immobilized. AuNPs/SAM/Au chips were used for AFB1 detection from 0.01 to 50 nM with a limit of detection of 0.003 nM. Immunosensor responded linearly with various AFB1 antigen concentration due to the binding of AFB1 molecules to immobilized anti-AFB1 at the surface of the sensor. AuNPs acted as signal amplifier and provided a larger area for immobilization of anti-AFB1 antibodies. It was shown that the AuNP integrated sensor chip was three times more sensitive than the SAM/Au sensor chip (15.72 RU/nM). [PERSON] et al. [35] reported an integrated platform combining microfluidic chips and an LSPR method for multiplexed and label-free protein analysis. By using a specially designed optical bench, it was possible to measure the extinction spectrum of the sample in the various regions. The different concentrations of Anti-gp41 antibody, biomarker for HIV/AIDS diagnosis, were investigated in the microfluidic chip with immobilized gold nanoparticles. Nonspecific adsorption of biomolecules was prevented by AuNPs modification with PEG-thiol. It was found that with the increase in the target molecule concentration, the LSPR peak intensity also increased. When comparing results with standard enzyme-linked immunosorbent assay (ELISA), it was shown that detection limits of both methods were similar, but a label-free sensing technique was faster and simpler. In this LSPR-based immunoassay investigation, the importance and effect of linker molecule length were investigated. When linker molecules stand up on the surface, they create a certain distance between the investigated molecule and the surface. Glutaraldehyde was used as the linker between the gp41 antigen and the sensor surface. By applying different linkers, the different thicknesses can be produced. As a result, detection sensitivity was enhanced by using linker molecules with longer spacer arms the due to reduced steric hindrance. [PERSON] et al. [18] introduced a 7-channel microfluidic device, which contained nanoparticle arrays, for detection of streptavidin.